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A Unified Symmetry Classification of Magnetic Orders via Spin Space Groups: Prediction of Coplanar Even-Wave Phases

Ziyin Song, 1, 2 Ziyue Qi, 1, 2 Chen Fang, 1, 3 Zhong Fang, 1, 2 and Hongming Weng 1, 4, ∗

arXiv:2512.08901v2 [cond-mat.mtrl-sci] 14 Dec 2025

1 Beijing National Laboratory for Condensed Matter Physics, and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Kavli Institute for Theoretical Sciences, Chinese Academy of Sciences, Beijing 100190, China 4 Condensed Matter Physics Data Center, Chinese Academy of Sciences, Beijing 100190, China

Spin space groups (SSGs) impose fundamentally different constraints on magnetic configurations in real and reciprocal spaces. As a consequence, the correspondence between real-space and momentum-space spin arrangements is far richer than traditionally assumed. Building on the complete enumeration of SSGs, we develop a systematic, symmetry-based framework that classifies all possible spin arrangements allowed by these groups. This unified approach naturally incorporates conventional magnetic orders, altermagnetism, and p-wave magnetism as distinct symmetry classes. Crucially, our classification predicts a variety of novel magnetic phases, highlighted by the discovery of the coplanar even-wave magnet: a state that is non-collinear in real space but hosts a collinear even-wave spin polarization in k-space. Analysis of a minimal model reveals that this phase is characterized by non-quantized spin polarization and exhibits a novel mechanism for symmetry-enforced zero polarization on non-degenerate bands. Extending the framework from bulk crystals to layer SSGs appropriate for two-dimensional systems, we further predict layered counterparts and provide symmetry guidelines for designing bilayer coplanar odd-wave and even-wave magnets. We further validate this finding through first-principles calculations and propose CoCrO 4 as a promising candidate for its experimental realization, thereby demonstrating the completeness and predictive power of the SSG-based classification of magnetic orders.

I. INTRODUCTION

Symmetry lies at the heart of condensed matter physics. For magnetic materials, the presence of ordered moments breaks time-reversal symmetry, and their crystal symmetry is commonly described by magnetic space groups (MSGs)[1–3]. However, in the weak spin-orbit coupling (SOC) regime relevant for many magnets, the spin and lattice degrees of freedom decouple to a good approximation, since the crystal-field splitting, electronelectron interactions, and kinetic hopping amplitudes are all typically much larger than SOC. In this limit, it is natural to regard spin rotations as independent of spatial operations, which gives rise to the more general framework of spin space groups (SSGs)[4–6]. Many magnetic and transport properties are therefore governed primarily by SSG symmetries, with SOC acting merely as a perturbation[7, 8]. Beyond these aspects, SSGs also refine the symmetry classification of band topology, enabling symmetry-enforced topological phases that are not captured by the conventional MSG description[6, 9–15].

This SSG framework has recently attracted significant interest due to the discovery of altermagnetism[16–23], pwave magnetism[24–28], and other unconventional magnetic phases[13, 29–33]. Together with conventional ferromagnetism and antiferromagnetism, these phases exhibit distinct spin arrangements in real and reciprocal

∗ [email protected]

spaces, denoted by S(r) and S(k), respectively. Remarkably, altermagnets display zero net magnetization but exhibit spin splitting in reciprocal space, whereas pwave magnetism, which is in fact one type of coplanar odd-wave magnetism[33], features coplanar spin arrangements in real space and collinear arrangements in reciprocal space. Despite their apparent differences, these magnetic phases share a common origin: their structures are fully dictated by how SSG operations constrain the spin

fully dictated by how SSGoper
configurations S(r) and S(k).

An SSG operation is written as {U s ||U r }, where U s ∈ O(3) acts in spin space and U r is a real space operation containing both a rotational part and a translation. When det(U s ) = +1, U s is a proper spin rotation. When det(U s ) = −1, it is assumed that it contains time reversal symmetry T and is antiunitary, i.e., U s ∼ det(U s )U s · T . The action of {U s ||U r } on the spin configuration in real and reciprocal spaces reads

{Us | |Ur }S(r) = Us S (U −1 r r), (1) {Us | |Ur }S(k) = Us S (det(Us ) U −1 r k), (2)

where the key distinction is the appearance of the factor det(U s ) in reciprocal space. This additional sign causes certain symmetry operations to act differently on S(r) and S(k), thereby allowing distinct combinations of realspace and momentum-space spin textures. Altermagnetism and p-wave magnetism arise precisely from this fundamental difference.

Recently, all SSGs have been enumerated[34–36], providing a complete catalogue of possible symmetry groups.

2 However, the implications of these groups for constraining spin configurations—particularly for relating S(r) and S(k) within a unified framework—remain underdeveloped.

In this work, we present a comprehensive analysis of how SSG symmetries constrain spin arrangements in real and reciprocal spaces. Within this symmetry-based framework, we show that the recently proposed altermagnetic and p-wave magnetic phases naturally emerge as distinct SSG symmetry classes. Moreover, we demonstrate that additional unconventional magnetic textures, including coplanar even-wave phase, are also predicted within this framework.

The paper is organized as follows. In Sec. II, we analyze the SSG constraints on S(r) and S(k) for both threedimensional bulk crystals and layered (two-dimensional) systems, and develop a general symmetry-based classification. Section III introduces a minimal model for the coplanar d-wave magnetic phase and compares it with existing magnetic classes. In Sec. IV, we propose a candidate material realization for this new phase. Finally, Sec. V presents the discussion and concluding remarks.

II. SYMMETRY CONSTRAINTS ON SPIN

ARRANGEMENTS FROM SPIN SPACE GROUPS In this section, we analyze how SSGs impose constraints on magnetic configurations in both real and reciprocal space. We show how these distinct constraints form the basis of a systematic classification of magnetic orders, as summarized in the flowchart in Fig. 1.

We first examine the constraints imposed by SSGs on real-space magnetic configurations. These constraints fall into two distinct categories:

1. Local onsite constraints: If an onsite symmetry

leaves an atomic position r invariant (U r −1 r = r), it imposes a local condition on the magnetic moment,

S(r) = Us S(r), (3)

as expressed in Eq. 1. At a general Wyckoff position, only spin-only operations appear in the site symmetry. These can restrict the spins to be collinear or coplanar, whereas the presence of pure time-reversal symmetry forces S(r) = 0, yielding a nonmagnetic configuration.

2. Global constraints from the point group

of the spin part: The set of all spin rotations {U s } appearing in the SSG forms a point group in spin space (hereafter referred to as the point group of the spin part, P s , to avoid confusion with the spin point group). Whether this group admits a nonzero invariant vector determines whether a uniform magnetization is symmetry allowed: polar groups permit a net magnetic moment, whereas nonpolar groups forbid it.

This provides the most general formulation of the FM/AFM dichotomy[8]: polar P s correspond to ferromagnetic-type orders, whereas nonpolar P s correspond to antiferromagnetic-type orders.

Combining these onsite and spin-part constraints yields a systematic classification of real-space spin arrangements, including nonmagnetic, collinear, coplanar, and non-coplanar configurations, with an additional distinction based on whether a net magnetization is allowed.

We now turn to reciprocal space, where the constraints imposed by SSGs become richer and often more decisive in determining the magnetic phase. At a general momentum k, the spin texture S(k) is constrained by the symmetries of its little group. These onsite symmetries may be unitary or antiunitary. The relevant unitary onsite operations take the general form {U s ||E | τ } with

det(Us ) = +1. Antiunitary onsite symmetries can ap- pear as {Us | |P | τ } with det(U s ) = −1, where the k-

inversion induced by time reversal is compensated by the spatial inversion P , making the operation onsite for k.

Since these transformations impose local constraints on the spin texture, S(k) must be a simultaneous invariant of all spin parts of the onsite symmetries:

S(k) = Us S(k), {Us | |E | τ } ∈ SSG, det(Us ) = +1 (unitary), {Us | |P | τ } ∈ SSG, det(Us ) = −1 (antiunitary). (4)

By combining the reciprocal-space constraints with those in real space, we obtain a complete and systematic classification of magnetic orders. Building on the full enumeration of SSGs, this group-theoretical framework naturally reproduces all known collinear magnetic phases—including conventional antiferromagnetism and altermagnetism—while also revealing a broader landscape of coplanar orders. In particular, for coplanar SSGs, the reciprocal-space constraints admit a rich variety of spin textures, such as odd-wave magnetism[33] and other nontrivial phases that we will discuss later.

A. Collinear configurations

A collinear SSG (1,421 in total) is defined by a spinonly subgroup that preserves a single axis (without loss of generality, we choose the z axis). This subgroup contains a unitary rotation C nz and an antiunitary mirror M x . The operation C nz protects the collinear configuration in both real and reciprocal spaces, while the antiunitary M x imposes an even-wave constraint on the spin texture:

Sz (k) = Sz (−k), (5)

ensuring that the spin polarization is symmetric under k → −k.

The classification of collinear SSGs is determined by their point group of the spin part, P s , together with the 3

FIG. 1. Flowchart illustrating the SSG-based classification of magnetic orders. Starting from a given SSG 𝒢s , real space constraints (left) determine the local spin configuration and whether a net magnetization is allowed, while reciprocal space constraints (middle), formulated separately for bulk (3D) and layer (2D) systems, fix the allowed spin textures S(k). Com- bining these operations yields the possible reciprocal space spin configurations (spin-polarized, coplanar, non-coplanar, or non-magnetic) and thus the corresponding magnetic phases.

symmetry constraints at a generic k point in reciprocal space:

1. P s = 1: No SSG operation reverses the spin

direction. The absence of spin-flipping symmetries permits a net magnetization, resulting in a collinear ferromagnet (FM) with an even-wave spin-polarized Fermi surface. This class contains 230 SSGs, identical to the number of space groups.

2. P s = m (generated by M z ): The presence of M z (a

spin-flip operation, equivalent to C 2x or C 2y in the presence of spin-only mirrors M x or M y ) forbids a net magnetization, leading to collinear antiferromagnetism (AFM). This AFM class further divides according to k-space degeneracy:

(a) Conventional AFMs: These SSGs con-

tain a degeneracy-enforcing constraint such as {M z ||P |τ } (an antiunitary operation involving inversion P ) or {C 2x(y) ||E|τ }. These symmetries guarantee spin degeneracy at every k point, enforcing a vanishing reciprocal-space spin texture, S(k) = 0. This class contains 769 SSGs. We note that although unitary and antiunitary degeneracy-enforcing mechanisms differ in other physical consequences (e.g., Berry curvature[37]), we do not distinguish them here since our focus is solely on the vanishing spin texture. (b) Altermagnets (AMs): These SSGs also

have P s = m (hence zero net magnetization) but lack the symmetries {M z ||P |τ } or {C 2x(y) ||E|τ } that enforce degeneracy. Their absence allows a spin-split band structure in which the nonzero spin polarization S(k) alternates across the Brillouin zone, producing the characteristic even-wave patterns (d-wave, g-wave, etc.). This class contains 422 SSGs.

Thus, this SSG-based classification naturally divides collinear orders into FM and AFM categories, with the AFM category further subdivided into conventional AFMs and altermagnets according to their k-space symmetry. This classification of collinear SSGs has been established in previous works[34–36], and here we demonstrate how it follows directly from our general SSG-based framework.

B. Coplanar configurations

Coplanar SSGs (24,788 in total within 12× supercell[35]) are characterized by a spin-only antiunitary symmetry, M z , which confines spins to the xy plane. This M z symmetry has two key consequences. First, it dictates the wave parity of the k-space spin texture:

S(k) = Mz S(−k). (6) Since Mz flips S z (Mz S z = −Sz ) but preserves S x,y (Mz S x ,y = Sx ,y ), this implies S z (k) = −S z (−k) (odd- wave) while S x,y (k) = S x,y (−k) (even-wave). Second,

for every unitary operation {U s ||U r |τ }, a corresponding antiunitary counterpart {M z U s ||U r |τ } exists, enriching the k-space constraints.

The point group of the spin part P s for coplanar SSGs can be C nz , m, or C nv . The resulting k-space spin textures are classified by their effective dimensionality (0D, 1D, 2D, or 3D) based on the onsite symme-

ality (0D, 1D, 2D, or 3D) based on the onsite symme- try, such as {Cn z ||E|τ }, {C2 x(y)| |E|τ }, {Mz ||P |τ }, and {Mx (y)| |P |τ }.

1. Non-magnetic (0D)

A non-magnetic (0D) spin texture (S(k) = 0) implies a fully spin-degenerate band structure. This is enforced by two primary mechanisms:

1. PT-like symmetry: The presence of a P T -like

PT-like symmetry: The presence of a P T -like symmetry, such as {C2 z ||P |τ } (noting C2 z Mz =

−1), enforces spin degeneracy at every k point (2,380 such SSGs).

4

2. Conflicting constraints: A non-magnetic state

can also arise from mutually contradictory con-

straints. For example, two orthogonal C2 rotation onsite symmetry, {C2 | |E|τ1 } and {C ′ 2 | |E|τ2 } , may

enforce S(k) to lie along two different, perpendicular axes, for which the only solution is S(k) = 0 (474 SSGs).

2. Spin-polarized (1D)

A 1D (spin-polarized) k-space texture occurs when SSG stabilizers constrain S(k) to a single, fixed axis. This class reveals two physically distinct patterns:

  1. z-axis polarization (odd-wave): The stabilizer

z-axis polarization (odd-wave): The stabilizer operation {C n z ||E|τ } (with n ≥ 2) constrains S(k)

to the z axis. Combined with Eq. (6), this implies

to the z axis. Combined with Eq. (6), this implies an odd-wave z-polarized texture Sz (k) = −Sz (−k). (7)

Such coplanar odd-wave patterns were recently proposed in Ref. [33], and in our enumeration, are realized by 17,830 distinct coplanar SSGs. These SSGs can be further classified according to the symmetry of their spin polarization in reciprocal space into p-

of their spin polarization in reciprocal space into p-
wave[24] (17,049 SSGs), f -wave[33] (637), h-wave

(116), j-wave (14), and l-wave (14) subclasses. The derivation of these wave patterns from the underlying SSG symmetries is given in the Supplementary Material S6.

2. In-plane polarization (Even-wave): The stabi-

lizer {C 2x(y) ||E|τ } constrains S(k) to lie along the x (y) axis, i.e., within the coplanar plane. From Eq. (6), this in-plane polarization must be evenwave:

Sx (y) (k) = Sx (y) (−k). (8)

This pattern, featuring a symmetry-enforced evenwave spin polarization, is a distinct phase predicted by our classification. Our SSG enumeration shows that this class contains 517 polar SSGs, which can be understood as s-wave-like phases. The number 517 exactly matches that of type-IV MSGs, since such SSGs are generated by combining a pure space group with the spin translation operation {C 2x(y) ||E|τ }. In addition, there are 1,173 nonpolar SSGs in this class, among which 939 real-

polar SSGs in this class, among which 939 real-
ize d-wave, 198 realize g-wave, and 36 realize i-

wave even-spin textures. These numbers are exact within our classification scheme, where the SSG is restricted to be at a twofold supercell of the atomic space group. In Section III and the Supplementary Material, we will discuss in detail this coplanar even-wave phase (which includes the k-space d-wave and higherorder moments), including its minimal model for different wave numbers and its many-to-one mapping with the altermagnetic phase.

3. Spin-coplanar (2D)

A 2D (spin-coplanar) texture in k space is realized when S(k) is constrained to a plane but not to a line. This arises from antiunitary mirror-like symmetries of the form {M ||P |τ }. We identify two types:

1. In-plane coplanar: The operation {M z ||P |τ }

(actually a pure inversion symmetry combined with the spin-only M z ) enforces S(k) to lie within the xy-plane for all k. This results in a coplanar texture in both real and reciprocal space (605 SSGs: 160 polar; 445 nonpolar).

2. Perpendicular-plane coplanar: The operation

Perpendicular-plane coplanar: The operation {Mx (y) | |P |τ } enforces S(k) to lie within a plane

perpendicular to the real-space spin plane (e.g., the xz- or yz-plane). This is a nontrivial case where the real-space and k-space coplanar planes are mutually orthogonal (814 SSGs: 252 polar; 562 nonpolar).

4. Spin non-coplanar (3D)

Finally, if an SSG lacks any of the k-space onsite symmetries that constrain the dimensionality (0D, 1D, or 2D), the spin arrangement in reciprocal space is unconstrained. At a generic k point, S(k) is free to point in any direction, resulting in a generic non-coplanar (3D) spin texture. This constitutes the most general case (995 SSGs: 262 polar; 733 nonpolar).

C. Non-coplanar configurations

Finally, we consider non-coplanar SSGs. These groups are defined by the absence of any non-trivial spin-only symmetry. Consequently, their real-space spin textures lack an intrinsic characteristic direction, such as the collinear axis or the coplanar plane found in the other classes.

Despite this lack of real-space constraint, the reciprocal-space spin texture S(k) can still be highly constrained by k-space stabilizers. Our SSG enumeration finds that these non-coplanar SSGs can produce a variety of k-space spin arrangements, including:

1. Non-magnetic (0D): A total of 12,838 SSGs en-

force a non-magnetic S(k) = 0 texture. As in the 5 coplanar case, this is typically protected by a P T - like symmetry or by contradictory onsite symmetry constraints.

2. Spin-polarized (1D): A total of 127,146 SSGs en-

force a collinear k-space spin texture, protected by an onsite symmetry operation such as {C n ||E|τ }.

3. Spin-coplanar (2D): A total of 8,323 SSGs en-

force a coplanar S(k) texture, protected by an on-

force a coplanar S(k) texture, prot site symmetry such as {M ||P |τ }.

4. Spin-non-coplanar (3D): A total of 8,982 SSGs,

which lack any of the k-space onsite symmetry constraints required for 0D, 1D, or 2D arrangements.

This demonstrates that our symmetry framework, based on the distinction between real-space and k-space constraints, can be applied comprehensively to all magnetic classes, including this most general non-coplanar case.

D. Extension to layered magnetic orders

In this subsection, we show that the unified symmetry classification of magnetic orders developed above can also be applied to layered magnetic orders, i.e., systems with an open boundary condition along one direction (taken to be the z axis without loss of generality). For such layered structures, the appropriate symmetry group reduces from the 230 three-dimensional space groups to the 80 layer groups, and the corresponding SSG becomes a layer SSG constructed from these 80 layer groups[38] and supercells along the z axis are not allowed.

Our classification relies on real-space and reciprocalspace constraints imposed by the SSG. For layer SSGs, the real-space constraints remain unchanged, and magnetic orders can still be classified as non-magnetic, collinear, coplanar, or non-coplanar, with or without a net magnetization. However, in reciprocal space the constraints at a generic in-plane momentum k = (k x , k y ) become richer. In particular, we now have

S(k) = Us S(k), {Us | |E | τ } ∈ SSG, det(Us ) = +1 (unitary), {Us | |Mz | τ } ∈ SSG, det(Us ) = +1 (unitary), {Us | |C2 z | τ } ∈ SSG, det(Us ) = −1 (antiunitary), {Us | |P | τ } ∈ SSG, det(Us ) = −1 (antiunitary). (9)

Within this framework, the dimensionality of the spin texture can be classified in exactly the same way as for three-dimensional SSGs. The conclusions of Sec. II carry over essentially unchanged: one simply extends Eq. (4)

by allowing the role of {Us | |E | τ } to be played by ei- ther {Us | |E | τ } or {Us | |M z | τ }, and similarly allow- ing {Us | |P | τ } to be replaced by either {Us | |P | τ } or

{U s ||C 2z | τ }. In particular, layer SSGs again support 0D, 1D, 2D, and 3D spin textures in k space.

For collinear orders, this construction reproduces the family of layered altermagnets, which break spin-flip

family of layered altermagnets, which break spin-flip symmetry via the layer operations {M z |τ }, {P |τ }, {E|τ },

and {C 2z |τ }. These four operations are precisely the layer counterparts generated by the replacements discussed above, and this family has been analyzed previously in Refs. [39–41].

We highlight that this layer-based symmetry classification makes it particularly convenient to design coplanar odd-wave (including p-wave) magnets[33] and coplanar even-wave magnets in bilayer structures. In the purely three-dimensional setting, a collinear spin texture in reciprocal space can only be protected by a symmetry of the form {C 2 ||E | τ }, i.e., a spin rotation combined with a pure translation, which can be rather rare in real materials. In contrast, in the layered setting one can construct a bilayer system from two coplanar layers that are related by a symmetry of the form {C 2 ||M z }, where the C 2 axis is either perpendicular or parallel to the coplanar plane. The former realizes a coplanar odd-wave magnetic phase, while the latter realizes a coplanar even-wave magnetic phase. This design principle is exploited explicitly in the Supplementary Material S4 and S5, where we construct concrete bilayer models for even-wave phases.

III. MODEL OF COPLANAR D-WAVE

MAGNETISM In this section, we present a minimal model for a coplanar d-wave magnetic phase. We highlight its critical differences from coplanar odd-wave magnetism, which can share a similar real-space spin configuration, and from d-wave altermagnetism, which exhibits a similar k-space symmetry. The construction of minimal models for other coplanar even-wave magnetic phases is presented in Supplementary Materials S4.

A. Symmetry and Model Hamiltonian

We consider a model based on the real-space configuration shown in Fig. 2(a), which features four magnetic atoms per unit cell. The magnetic moments are (α, 0, β), (−α, 0, β), (α, 0, −β), and (−α, 0, −β), respectively. This arrangement yields zero net magnetization and a spin configuration that is coplanar in the xz-plane, consistent with the spin-only antiunitary symmetry {M y ||E}.

Despite the real-space coplanarity, the reciprocal-space spin texture is constrained by the SSG to be collinear and z-polarized. The symmetry constraints in the reciprocal space are:

{C2 z ∥t} : Sx ( kx , ky ) = 0; Sy (kx , ky ) = 0 {My ∥E} : Sz (kx , ky ) = Sz (−kx , −ky ) {C2 x∥ My } : Sz (kx , ky ) = −Sz (kx , −ky )

(10)

6 Spin polarization (k-space) Collinear (r-space) Coplanar (r-space)

Coplanar (r-spa ε↑ ( k) = ε↓ ( −k) ε↑ ( k) = ε↑ ( −k)

Spin polarization direction odd-/even-wave Collinear altermagnet Coplanar odd-wave Coplanar even-wave

ollinear altermagnet Coplanar odd-wave Coplanar even-wave {Cn z ||E} {C2 z ||E|τ } {C2 z ||E|τ } {Cn z ||E} × × × {Mz ||E} {Mx /y ||E} × {Mz ||E} × {Mx ,y ||E} × {Mx /y ||E} S(k) ∥ S(r) (z axis) S(k) ⊥ S(r) S(k) ∥ S(r) plane (yz/xz plane) S(k) = S(−k) S(k) = −S(−k) S(k) = S(−k)

TABLE I. Comparison of the symmetry properties of collinear altermagnets, coplanar odd-wave magnets, and coplanar evenwave magnetic orders. For clarity, we adopt a coordinate convention in which the spin polarization in momentum space is chosen along the z axis for all three phases: the real-space moments are then collinear along z for the altermagnet, coplanar in the xy plane for the coplanar odd-wave phase, and coplanar in a plane containing the z axis (e.g. xz or yz) for the coplanar

in the xy plane for the coplanar odd-wave phase, and coplanar in a plane containing the z axis (e.g. xz or yz) for the coplanar even-wave phase. FIG. 2. (a) Real-space lattice model for the coplanar d-wave magnet, fulfilling the required SSG symmetries. The dashed box indicates the magnetic unit cell. (b) Calculated band structure along high-symmetry lines and spin polarization (⟨Sz ⟩) projected onto the bands. (c) Spin polarization (⟨Sz ⟩) of four bands along the Γ − M path. (d) Spin-polarized Fermi surface at µ = −2 eV, showing the characteristic d-wave anisotropy. (e) Magnetic structure of CoCrO4 exhibiting the coplanar magnetic configuration. (f ) First-principles band structure of CoCrO4 and spin polarization (⟨Sx ⟩ ) projected onto the bands along the high-symmetry path.

The first constraint, {C 2z ∥t}, enforces a pure z-axis spin polarization in k-space. The second, {M y ∥E}, dictates an even-wave parity under inversion. The third, {C 2x ∥M y }, enforces an odd parity under reflection k y → −k y . This momentum-space symmetry is precisely that

−ky . This moment of a d-wave state.

We then construct a minimal effective Hamiltonian for the nonmagnetic sites that respects these symmetries and the detailed derivation is given in Supplementary Material S1:

ℋ(k) = 2t0 cos kx 2 σ0 τ 1 + 2t0 cos ky σ0 τ 0 + [t1 cos kx + t2 cos ky ] σ1 τ 3 + t3 sin kx 2 σ1 τ 2 + t4 sin ky σ2 τ 3

(11)

where σ i and τ i are Pauli matrices for the spin and sublattice degrees of freedom, respectively. The calculations are performed using the hopping parameters t 0 = −0.6,

are performed using the hopping paramet t1 = 0.5, t2 = 0.7, t3 = 0.4, and t4 = 0.6.

7

The resulting band structure and spin polarization pro- jected onto the bands are presented in Fig. 2. Fig. 2(b) shows the band structure along high-symmetry paths. Along the Γ − X path, the bands are doubly degenerate and the total spin polarization Sz (k) is zero, as the de- generacy is protected by the {C2 x | |My } symmetry. The X − M path is non-degenerate, yet the expectation value of the spin polarization is still enforced to be zero by the {Mz ||My } symmetry. The M − Γ path is non-degenerate and exhibits a finite spin polarization along the z-axis. Fig. 2(c) plots the expectation value of spin polariza- tion ⟨S z ⟩ for these four bands along the Γ−M path, where the expectation values of ⟨Sx ⟩ and ⟨Sy ⟩ are zero and therefore not shown. Finally, Fig. 2(d) displays the spin- polarized Fermi surface at µ = −2 eV, which shows the characteristic d-wave anisotropy of this magnetic phase. Further analyses and model Hamiltonians for a coplanar d-wave phase with C4 symmetry, as well as coplanar g- and i-wave phases, are presented in Supplementary Ma- terial S4 and S5. B. Distinction from Other Phases In Table I, we compare the symmetry properties of collinear altermagnets, coplanar odd-wave, and coplanar even-wave magnetic orders. For coplanar odd-wave mag- netism, the k-space spin polarization is perpendicular to the real-space spin plane, whereas for the coplanar even- wave, it is parallel to the real-space spin plane. Conse- quently, their respective spin-only symmetries result in different k-space parities: coplanar odd-wave magnetism exhibits an odd-wave pattern, while the coplanar even- wave exhibits an even-wave pattern. Thus, the coplanar even-wave is a new phase that simultaneously shows sim- ilarities with collinear altermagnetism (even-wave par- ity) and coplanar odd-wave magnetism (coplanar in real space, collinear in k-space). We further show that copla- nar even-wave SSGs admit a many-to-one mapping onto altermagnetic SSGs, as detailed in Supplementary Mate- rial S3. It is crucial to emphasize another fundamental differ- ence between this coplanar even-wave phase and collinear altermagnetism, even though both exhibit z-axis spin po- larization and even-wave parity in k-space. (This dis- tinction also applies when comparing coplanar odd-wave magnetism with altermagnetism.) In the collinear altermagnet, spin is a good quantum number. The Hamiltonian contains only σ0 and σ 3 terms, and the eigenstates are pure spin-up or spin-down, i.e., ⟨σz ⟩ = ±1. In our coplanar model (the same as in coplanar odd- wave magnetism), the Hamiltonian explicitly contains σ1 and σ2 terms. Consequently, spin is not a good quantum number, and the spin rotation symmetry Cn z is broken. Although the spin expectation value ⃗M = (⟨σx ⟩ , ⟨σy ⟩, ⟨σz ⟩) is polarized along z (i.e., Mx = My = 0 and Mz /| ⃗ M | = 1 due to the {C 2 z ∥t} symmetry), the local eigenstates are not pure spin states. As shown in Fig. 2(c), the spin polarization varies continuously along the Γ − M path, approaching zero smoothly as it reaches the M point. Moreover, this coplanar model exhibits a crucial dis- tinction from collinear altermagnetism in how its spin polarization vanishes. In collinear altermagnets, S z (k) vanishes only on high-symmetry paths where spin degen- eracy is protected. Our model shows this conventional behavior along the Γ − X path [Fig. 2(b)], where S z = 0 because the degeneracy is protected by the symmetry {C2 x| |My }. However, a novel mechanism is observed along the X −M path. Here, the bands are clearly non-degenerate, yet the spin polarization S z (k) is still forced to be zero. This is a direct consequence of the onsite sym- metry {Mz ||My }. As this symmetry is anti-unitary and squares to +1 (not −1), it does not protect band degen- eracy. Instead, it reverses the spin polarization operator (Sz → −Sz ), forcing its expectation value to vanish along the entire X −M path. This explains why the spin polar- ization smoothly reaches zero precisely at the M point, as observed in Fig. 2(c). IV. MATERIAL REALIZATION: COCRO4 Our SSG-based framework provides a practical work- flow for material classification: starting from an experi- mentally determined magnetic structure, one can identify its SSG and assign it to one of the symmetry classes in our scheme. By analyzing the magnetic structures in the MAGNDATA database[42], we find 73 entries (including CeNiAsO proposed in Ref [24]) whose SSGs and mag- netic configurations realize coplanar odd-wave magnets, defined as states with coplanar local moments, zero net magnetization, and a collinear spin polarization in mo- mentum space perpendicular to the spin plane. In this search we did not require all magnetic atoms to be related by SSG operations (i.e., magnetically inequivalent sites are allowed). The MAGNDATA identifiers and chemical formulas of these 73 candidates are listed in the Supple- mentary Material S7. In addition, we identify cobalt chromate, CoCrO4 , as a promising candidate material for coplanar even- wave magnet. According to neutron diffraction studies, CoCrO4 is isostructural with CrVO4 and crystallizes in the orthorhombic space group Cmcm (No. 63). The magnetic Co2 + ions are located at the Wyckoff positions (0, 0, 0), (0, 0, 1/2), (1/2, 1/2, 1/2), and (1/2, 1/2, 0) within the unit cell[42–44]. Crucially, the experimentally determined magnetic structure is non-collinear. As depicted in Fig. 2(e), the arrangement of the magnetic Co2 + spins (represented by the red arrows on the blue Co atoms) is characterized by a combination of Ax and Gz modes. (The blue spheres rep-

8 resent Co atoms, the purple spheres represent Cr atoms, and the green spheres represent O atoms.) This superposition results in a coplanar spin texture where all magnetic moments lie in the xz-plane (at an angle of 55 ◦ with the x-axis). This real-space structure corresponds to a coplanar SSG whose spin-only point group is gener-

to a coplanar SSG w ated by {My ||E}.

The full SSG is generated by operations including

The full SSG is generated by operations including {E||P }, {C2 y ||C2 z |0, 0, 1 2 }, {E||C 2 x} , and {C 2x | |E| 1 2 1 2 0}.

in Sec. II. Specifically, the operation {C 2x ||E| 12 12 0} constrains the reciprocal-space spin arrangement to be polarized along the x-axis (S y (k) = S z (k) = 0).

It is crucial to justify the use of the SSG framework here. First, this SSG is fully compatible with the nonmagnetic space group (Cmcm), as every spatial operation in Cmcm is paired with a specific spin operation to form an element of the SSG. Second, the conventional Magnetic Space Group (MSG) for this structure is P bcn (BNS No. 60.417), which contains only one-fourth of the symmetry operations present in the full SSG. The MSG framework is therefore insufficient, and the SSG provides a richer, more accurate description. Finally, this approach is physically appropriate, as CoCrO 4 is composed of 3d elements (Cr, Co) where SOC is weak, validating the decoupling of spin and spatial operations.

To confirm this prediction, we performed firstprinciples calculations and the computational details are given in Supplementary Material S2. Fig. 2(f) shows the calculated band structure and spin polarization of CoCrO 4 along the R(0, π 2 , π 2 ) to Γ(0, 0, 0) to −R(0, − π 2 , − π 2 ) path. The bands are clearly spinpolarized. As required by the SSG symmetry, the spin polarization is purely along the x-axis (polarization components along other axes are zero and not shown). Furthermore, the plot demonstrates the even-wave symmetry of this polarization, as S x (k) = S x (−k) (i.e., the polarization is identical on the Γ → R and Γ → −R segments). This even parity is a direct consequence of the spin-only symmetry {M y ||E}, as discussed in Sec. II.

We also note that the spin polarization is separately forced to zero on the k y = 0 and k z = 0 planes. This is an additional constraint enforced by the liitle group

is an additional constraint enforced by the liitle group symmetries {My ||C2 z |0, 0, 12 } and {Mz ||C2 z |0, 0, 12 }, re-

conventional classifications.

Using the complete enumeration of SSGs, we have shown that our framework provides a unified understanding of known magnetic orders. It naturally incorporates conventional ferromagnetism and antiferromagnetism, as well as the recently discovered unconventional phases of altermagnetism and p-wave magnetism, as distinct symmetry classes.

More importantly, our systematic approach predicts a variety of novel magnetic states. We focused on a new predicted phase, the coplanar even-wave magnet, which is characterized by a coplanar spin arrangement in real space but hosts a collinear, even-wave spin polarization in reciprocal space. We presented a minimal model for this phase, highlighting its unique physical properties, such as the non-quantized nature of its spin polarization and a novel mechanism for symmetry-enforced zero po-

and a novel mechanism for symmetry-enforced zero po- larization (Sz (k) = 0) on non-degenerate bands. Fur-

thermore, we identified the existing material CoCrO 4 , with its coplanar magnetic structure, as a promising candidate for its experimental realization.

Looking forward, the classification framework presented here provides a powerful roadmap for the discovery and identification of new unconventional magnetic materials. While we focused on the coplanar even-wave phase, our analysis revealed many other exotic possibilities, particularly within the coplanar and non-coplanar SSGs (e.g., states with k-space coplanar spin configuration), which represent a new frontier for theoretical and experimental exploration.

Investigating the physical consequences of these new magnetic orders is a critical next step. For instance, the unique interplay of non-collinear real-space spins and collinear, even-wave momentum-space spins in the coplanar even-wave phase may lead to novel magnetotransport and optical responses, distinct from both altermagnets and p-wave magnets. Finally, we call for further experimental investigation of our candidate material, CoCrO 4 , using techniques such as spin-polarized ARPES, to directly visualize the predicted d-wave spin polarization in its Fermi surface and confirm its status as the first coplanar even-wave magnet.

ACKNOWLEDGMENTS

V. CONCLUSIONS AND OUTLOOK

In this work, we have developed a comprehensive and systematic classification of magnetic orders based on SSG symmetries. Our framework is built upon the fundamental principle that SSGs impose distinct constraints on spin configurations in real space (S(r)) and reciprocal space (S(k)). This discrepancy, which originates from the det(U s ) factor in the momentum-space transformation, allows for a rich variety of magnetic phases beyond This work was supported by the Science Center of the National Natural Science Foundation of China (Grants No. 12188101 and No. 12325404), the National Key R&D Program of China (Grants No. 2022YFA1403800, No. 2023YFA1607400, No. 2024YFA1408400 and No. 2023YFA1406704), the National Natural Science Foundation of China (Grants No. 12274436, No. 11925408, No. 11921004), and H.W. acknowledges support from the New Cornerstone Science Foundation through the XPLORER PRIZE.

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11

Supplementary Material

CONTENTS

I. Introduction 1 II. Symmetry Constraints on Spin Arrangements from Spin Space Groups

A. Collinear configurations B. Coplanar configurations

1. Non-magnetic (0D) 2. Spin-polarized (1D) 3. Spin-coplanar (2D) 4. Spin non-coplanar (3D) C. Non-coplanar configurations

D. Extension to layered magnetic orders

2 2 3 3 4 4 4 4 5

III. Model of Coplanar d-wave Magnetism

A. Symmetry and Model Hamiltonian B. Distinction from Other Phases

5 5 7

IV. Material Realization: CoCrO 4 7 V. Conclusions and Outlook 8

Table 2 · 2 columns × 8 rows · p.11–11 · CSV
Acknowledgments8
References9
S1. Derivation of the Minimal Model11
S2. Details of First-Principles Calculations13
S3. Many-to-One Mapping from Coplanar Even-Wave to Altermagnetic Phase13
S4. d-wave model with C4 symmetry14
S5. Models for the coplanar g-and i-wave phases15
S6. Determination of the wave pattern for spin-polarized phases16
S7. candidates of odd-wave magnet in MAGNDATA database17

S7. candidates of odd-wave magnet in MAGNDATA database 17 S1. DERIVATION OF THE MINIMAL MODEL In this section, we show how to derive the minimal tight-binding Hamiltonian for the coplanar d-wave magnetic phase. The model is constructed on a two-sublattice (A and B) basis, spanned by the direct product of spin and

phase. The model is constructed on a two-sublattice (A sublattice degrees of freedom: Ψ = (cA ↑, cA ↓, cB ↑, cB ↓) T .

The minimal model must satisfy the following three SSG symmetry constraints that define the coplanar d-wave 12 phase (as analyzed in the main text):

{C2 z ∥t} : Sx ( k) = 0; Sy (k) = 0 (Collinear Sz polarization in k-space) {My ∥E} : Sz (k) = Sz (−k) (Even-wave parity) {C2 x∥ My } : Sz (k) = −Sz (kx , −ky ) (Symmetry for d-wave shape)

(S1)

The SSG operations that generate these constraints are represented by the following matrices in the (σ i ⊗ τ j ) basis (where σ and τ denote spin and sublattice degrees of freedom, respectively):

Generator {C2 z ∥t} {My ∥E} {C2 x∥ My } Matrix 𝒰 −iσ3 τ 1 σ0 τ 0 κ −iσ1 τ 0 k change k → k k → −k k → (kx , −ky )

TABLE S1. Operators of the SSG generators in the basis of the Hamiltonian, and the corresponding change in momentum

TABLE S1. O k = (kx , ky ).

The Hamiltonian H(k) must be invariant under all SSG operations U:

H(k) = 𝒰H(k′ )𝒰 −1 , (S2)

where k ′ is the momentum after the operation (e.g., k ′ = −k for {M y ∥E}). For anti-unitary operations (U = Aκ),

where k′ is the momentum after the operation the invariance requires H(k) = AH(k′ )∗ A− 1 .

We systematically consider terms of the form σ i τ j f (k) that survive these constraints. Due to the minimal model assumption, we only consider sin k i and cos k i dependencies ( k 2 x with τ 1 and τ 2 ).

Constraints Imposed by {C 2z ∥t}:

The operation is unitary (𝒰 = −iσ3 τ 1 ) and leaves k invariant. The invariance condition that H(k) commutes with 𝒰 : [𝒰, H(k)] = 0. • Surviving Terms (Commuting): σ0 τ 0 , σ0 τ 1 , σ1 τ 2 , σ1 τ 3 , σ2 τ 2 , σ2 τ 3 , σ3 τ 0 , σ3 τ 1 . • Forbidden Terms (Anti-Commuting): σ0 τ 2 , σ0 τ 3 , σ1 τ 0 , σ1 τ 1 , σ2 τ 0 , σ2 τ 1 , σ3 τ 2 , σ3 τ 3 .

Constraints Imposed by {M y ∥E}: This operation is anti-unitary (U = σ 0 τ 0 κ) and transforms k → −k. The invariance condition H(k) = AH(−k) ∗ A −1

This operation is anti-unitary simplifies to H(k) = H(−k)∗ .

For simplicity, we test one of the surviving terms, H 1 (k) = f (k)σ 2 τ 3 .

  1. The condition is f (k)σ 2 τ 3 = (f (−k)σ 2 τ 3 ) ∗ = f (−k)(σ 2 τ 3 ) ∗ .

2. Since σ 2 τ 3 is a purely imaginary matrix, (σ 2 τ 3 ) ∗ = −σ 2 τ 3 .

  1. The condition becomes f (k)σ 2 τ 3 = f (−k)(−σ 2 τ 3 ), which implies f (k) = −f (−k).

This requires f (k) to be an odd function of k (e.g., sin k x or sin k y ).

Constraints Imposed by {C 2x ∥M y }: This final operation is unitary (U = −iσ 1 τ 0 ) and transforms k → (k x , −k y ). We now test the terms that survived the first two constraints, such as H 1 (k) = f (k)σ 2 τ 3 where f (k) must be odd.

1. The invariance condition is H 1 (k) = U H 1 (k x , −k y )U −1 .

  1. The matrix σ 2 τ 3 anti-commutes with U = −iσ 1 τ 0 , meaning U σ 2 τ 3 U −1 = −σ 2 τ 3 .

3. The condition becomes f (kx , ky )σ2 τ 3 = f (kx , −ky )(−σ2 τ 3 ) , which implies f (kx , ky ) = −f (kx , −ky ).

Therefore, the only surviving term of this form is sin k y σ 2 τ 3 .

Following this systematic constraint analysis for all surviving matrix bases, we can construct the full minimal model for the coplanar d-wave magnetic phase as presented in the main text.

13 S2. DETAILS OF FIRST-PRINCIPLES CALCULATIONS The first-principles calculations were performed using the Vienna ab initio simulation package (VASP), utilizing the generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) type for the exchange-correlation potential. We explicitly did not take spin-orbit coupling (SOC) into account, as its minor influence on the band structure does not alter the symmetry-derived conclusion, and its inclusion often complicates the convergence process.

In the self-consistent field (SCF) calculation, a Monkhorst-Pack (5 × 5 × 5) k-point mesh and an energy cutoff of 600 eV were used. The self-consistent iterations converged reliably, achieving an energy difference criterion within 1 × 10 −6 eV. Constraints on the magnetic moments were not applied, allowing the magnetic configuration to be determined purely by the self-consistent potential.

The resulting local magnetic moment of the Co ion converged to a magnitude of 2.384 µ B , with the moment direction lying at an angle of 58.55 ◦ with respect to the x-axis. This calculated moment is in very good agreement with experimental results, demonstrating the robustness and reasonableness of the material structure used for our magnetic model.

S3. MANY-TO-ONE MAPPING FROM COPLANAR EVEN-WAVE TO ALTERMAGNETIC PHASE In this section, we demonstrate a formal many-to-one mapping between the SSGs of the coplanar even-wave phase and those of the altermagnetic (collinear) phase. This mapping is based on the key insight that both phases can share an identical set of reciprocal-space symmetry constraints, even though their real-space spin arrangements are fundamentally different (coplanar vs. collinear).

In our definition, the coplanar even-wave phase is characterized by: (1) An anti-unitary spin-only symmetry defining the plane (e.g., {M y ||E} for the xz-plane). (2) An onsite symmetry (e.g., {C 2 ||τ }) that constrains the k-space spin polarization to an axis parallel to that plane.

A corresponding collinear SSG can be constructed by formally mapping the generators. The unitary onsite symmetry that protects the in-plane polarization in the coplanar case (e.g., {C 2 ||τ }) plays the same functional role as the unitary spin-only rotation ({C nz ||E}) that defines the axis in the collinear case. The anti-unitary spin-only operation (e.g., {M y ||E}) dictates the even-wave parity in both phases.

This mapping can be illustrated using the generators of our coplanar d-wave model (LHS) and their counterparts in a corresponding collinear SSG (RHS):

  1. {C 2z ∥t} (Coplanar stabilizer protecting S z ) → {C nz ∥E} (Collinear spin-only op protecting S z )

2. {My ∥E} (Coplanar spin-only, even-wave parity) → {My ||E} (Collinear spin-only, even-wave parity) 3. {C2 x∥ My } (Coplanar k-space constraint) → {C2 x∥ My } (Collinear k-space constraint)

One can verify that the set of generators on the right-hand side, which has the same k-space symmetry actions as the left, constitutes a valid collinear SSG.

This mapping can also be understood using the International Notation. An example of a coplanar even-wave SSG is C 1 m 2 010 c 2 010 m|(1, 1, 1; 2 100 ) m 1. Here, 2 100 constrains the x-polarization, and 2 010 acts as a spin-flip. This SSG maps to the collinear SSG C 1 m −1 c −1 m ∞m 1, which notably lacks P T symmetry and is therefore an altermagnetic phase. The translational parts of the operators can be omitted in this mapping because the definition of this coplanar phase already requires that they do not flip the spin, simplifying the correspondence.

Using this mapping scheme, one can unambiguously find a corresponding collinear (altermagnetic) SSG for every coplanar even-wave SSG. However, the mapping from the altermagnetic phase back to the coplanar even-wave phase is not a simple one-to-one correspondence. Because this mapping is defined purely by k-space symmetry, a single altermagnetic SSG may correspond to multiple, distinct coplanar SSGs, or none at all.

This one-to-many correspondence arises because different operations can be functionally equivalent in the collinear case but non-equivalent in the coplanar case. For example, two distinct spin operations involving C 2x and C 2y might both act as simple spin-flips on a z-polarized collinear state. However, these operations are non-equivalent within the coplanar spin group, thus generating two distinct coplanar SSGs that map to the same altermagnetic SSG.

The one-to-none correspondence occurs when an altermagnetic SSG is built from a non-symmorphic space group that is incompatible with the definition of a coplanar even-wave phase. For instance, the space group P 2 1 2 1 2 1 (No. 19) can form an altermagnetic collinear SSG (P −1 2 1 1 2 1 1 2 1 ). However, the structure of its non-symmorphic operations cannot be paired with the spin operations required to satisfy the definition of a coplanar even-wave SSG, resulting in an altermagnetic phase with no coplanar counterpart.

14 FIG. S1. (a) Real-space lattice model for the coplanar d-wave magnet with C4 symmetry, fulfilling the required SSG symmetries. The green and yellow planes show two layers of magnetic atoms related by the SSG symmetry {C2 z ||Mz }. (b) Calculated band structure along high-symmetry lines and spin polarization ⟨Sz ⟩ projected onto the bands for the C4 model. (c) Spin-polarized Fermi surface for the C4 model at µ = −2 eV, showing the characteristic d-wave anisotropy compatible with C4 symmetry. (d, e, f ) Schematics of various even-wave spin-polarized Fermi surface patterns: (d) a d-wave pattern (corresponding to the main text model), (e) a g-wave pattern, and (f ) an i-wave pattern, generated by models detailed in the Supplementary Material.

S4. D-WAVE MODEL WITH C4 SYMMETRY In the main text, we have shown a minimal model for the coplanar d-wave phase, and here we show another minimal model for the coplanar d-wave phase with C 4 symmetry. This model is based on a layered structure, as illustrated in Fig. S1(a), where two layers are related by the SSG operations {C 2z ||M z }.

We consider a two-layer, two-sublattice model (four-band model), with the following SSG symmetry constraints:

{C2 z ∥Mz } : Sx ( kx , ky ) = 0; Sy (kx , ky ) = 0 {My ∥E} : Sz (kx , ky ) = Sz (−kx , −ky ) {C2 x∥ C4 z } : Sz (kx , ky ) = −Sz (−ky , kx ) {E∥Mx } : Sz (kx , ky ) = Sz (−kx , ky )

(S3)

Similar to the analysis in Sec. S1, the SSG generators act on the Hamiltonian basis as summarized in Table S2:

Generator {C2 z ∥Mz } {My ∥E} {C2 x∥ C4 z } {E∥Mx } Matrix 𝒰 −iσ3 τ 1 σ0 τ 0 κ −iσ1 τ 0 σ0 τ 0 k change k → k k → −k k → (−ky , kx ) k → (−kx , ky )

TABLE S2. Operators of the SSG generators in the basis of the Hamiltonian, and the corresponding change in momentum

TABLE S2. O k = (kx , ky ).

The resulting minimal Hamiltonian H(k) that satisfies all the above SSG constraints is:

15

ℋ(k) = t0 ( cos kx + cos ky )σ0 τ 0 + t1 ( cos kx + cos ky )σ0 τ 1 + t2 ( cos kx + cos ky )σ1 τ 3 + t3 sin(kx + ky ) sin(kx − ky )σ2 τ 2 + t4 sin(kx + ky ) sin(kx − ky )σ3 τ 0 + t5 sin(kx + ky ) sin(kx − ky )σ3 τ 1

(S4)

Here we must introduce the second order of k. Since it is not sufficient to describe the symmetry in this four bands model if only with the first order of k. Figure S1(b) and (c) show the resulting spin-polarized band structure and Fermi surface for parameters t 0 , t 1 , t 2 , t 3 , t 4 , t 5 = −0.8, −1, 0.5, 0.8, 0.6, 0.5. The results confirm the expected even-wave spin polarization and the characteristic d-wave anisotropy under C 4 symmetry.

S5. MODELS FOR THE COPLANAR G- AND I-WAVE PHASES In altermagnetic systems, the magnetic order may exhibit higher even-parity form factors such as g- and i-wave patterns. In this section, we construct explicit tight-binding models realizing coplanar spin structures with g- and i-wave symmetry, and we present their spin-polarized Fermi surfaces, which display the characteristic symmetry patterns dictated by the corresponding SSGs.

Coplanar g-wave phase For the g-wave coplanar state, we consider the SSG generated by {C2 z ||Mz }, {My ||E}, {E||C4 z } and {C2 z ||Mx } .

A minimal four-band Hamiltonian respecting these generators is

ℋ(k) = t0 ( cos kx + cos ky )σ0 τ 0 + t1 ( cos kx + cos ky )σ0 τ 1 + t2 ( cos kx + cos ky )σ1 τ 3 + t3 f 1 ( kx , ky )σ2 τ 2 + t4 f 1 ( kx , ky )σ3 τ 0 + t5 f 1 ( kx , ky )σ3 τ 1

(S5)

where f1 ( kx , ky ) = sin k x sin ky (cos kx −cos ky ) is chosen to satisfy the symmetry constraints f1 ( kx , ky ) = −f1 ( −kx , ky ) and f1 ( kx , ky ) = −f1 ( −ky , kx ) . Coplanar i-wave phase For the coplanar i-wave phase, we consider the SSG generated by {C 2 z ||Mz }, {My ||E}, {E||C 6 z } and {C 2z ||Mx } .

Here the model is naturally expressed in the hexagonal reciprocal basis (k 1 , k 2 ). The four-band Hamiltonian takes the form

ℋ(k) = t0 f 2 ( k1 , k2 ) σ0 τ 0 + t1 f 2 ( k1 , k2 ) σ0 τ 1 + t2 f 2 ( k1 , k2 ) σ1 τ 3 + t3 f 3 ( k1 , k2 ) σ2 τ 2 + t4 f 3 ( k1 , k2 ) σ3 τ 0 + t5 f 3 ( k1 , k2 ) σ3 τ 1

(S6)

f 2 (k 1 , k 2 ) = cos k 1 + cos k 2 + cos(k 1 + k 2 ) f 3 (k 1 , k 2 ) = [cos k 1 − cos(k 1 + k 2 )] × (cos k 2 − cos k 1 ) × (cos(k 1 + k 2 ) − cos k 2 ) (S7)

where the factors are Figures S1(e) and (f) display the resulting spin-polarized Fermi surfaces for the parameter set t 0 , t 1 , t 2 , t 3 , t 4 , t 5 = −0.8, −1, 0.5, 0.8, 0.6, 0.5. The emergent Fermi-surface textures exhibit the characteristic g- and i-wave symmetry patterns analogous to those of the corresponding altermagnetic states. Together with Figs. S1(c) and (d), these models provide explicit realizations of coplanar magnetic textures protected by distinct SSG symmetries.

16 S6. DETERMINATION OF THE WAVE PATTERN FOR SPIN-POLARIZED PHASES In this section, we explain how to determine the precise wave character of a spin-polarized phase, i.e., whether its k-space spin texture belongs to the p-, f -, h-, . . . family for odd-wave phases, or to the d-, g-, i-wave family for even-wave phases.

The s-wave case is special: an s-wave spin texture is constant in k space and therefore does not require vanishing net magnetization. It corresponds to a ferromagnetic-type configuration, such as a collinear ferromagnet or the polar coplanar even-wave phase discussed in the main text.

For all higher waves, the wave pattern characterizes the symmetry of the spin polarization in reciprocal space. We focus on symmetry-enforced spin-polarized phases, where the spin at a generic k point is constrained to lie along a fixed axis (e.g., the z axis). In this case the spin polarization can be treated as a scalar function s(k). The symmetry operations {U s ||R|τ } of the SSG act on this function as

s(k) = + s(RT k), for Us preserve the spin direction, − s(RT k), for Us that flip the spin direction,

so s(k) carries a one-dimensional representation of the SSG in which spin-preserving operations have character +1 and spin-flipping operations have character −1.

As a concrete example, consider the d-wave coplanar model in the main text. The spin-only antiunitary generator

{My ||E} preserves the spin direction while inverting momentum, so it enforces s(k) = + s(−k). On the other hand, the generator {C 2 x| |M y } flips the spin and maps k → (kx , −ky ), which enforces s(k) = − s(k x , −k y ). The monomial k x k y

satisfies both constraints, k x k y = (+1) (−k x )(−k y ) = (−1) k x (−k y ), and therefore transforms according to exactly this one-dimensional representation. This is what we mean when we say that the spin texture has d-wave symmetry.

More generally, we define the wave number l as the degree of the polynomial in (k x , k y , k z ) that transforms according to the above one-dimensional representation. Explicitly, we consider polynomials of the form

f (l) (k) = i+j+k=Nl ci jk kix k jy kkz , (S9)

with integer exponents i, j, k ∈ Z ≥0 and total degree i + j + k = l. For a given SSG, we determine the smallest integer l > 0 for which there exists a nontrivial set of coefficients {c ijk } such that f (l) (k) transforms as

f (l) (k) = + f (l) (RT k), f (l) (k) = − f (l) (RT k),

for all spin-preserving operations , for all spin-flipping operations . (S10) In group-theoretical terms, this is equivalent to asking whether the representation of the SSG on the space of degree-l polynomials contains a one-dimensional subrepresentation with characters ±1 as specified above. Practically, this reduces to solving a linear function for the coefficients {c ijk }, and is therefore straightforward to implement numerically. The resulting l agrees with the "characteristic spin-group integer" defined in terms of spin-degenerate nodal surfaces crossing the Γ point in Ref. [16].

We apply this procedure to the bulk coplanar odd-wave and even-wave classes discussed in the main text. For coplanar odd-wave SSGs, we find

• p-wave: 17,049 SSGs (leading with l = 1), • f -wave: 637 SSGs (leading with l = 3), • h-wave: 116 SSGs (leading with l = 5), • j-wave: 14 SSGs (leading with l = 7), • l-wave: 14 SSGs (leading with l = 9), while for coplanar even-wave SSGs we find

• d-wave: 939 SSGs (leading with l = 2), • g-wave: 198 SSGs (leading with l = 4), • i-wave: 36 SSGs (leading with l = 6).

17 S7. CANDIDATES OF ODD-WAVE MAGNET IN MAGNDATA DATABASE In this section, we list candidate odd-wave magnets identified in the MAGNDATA database [42]. We find 73 entries (including CeNiAsO, previously proposed in Ref.[24]) whose SSGs and magnetic configurations realize coplanar oddwave magnetism of the p- and f -wave types; higher-order h-, j-, and l-waves, are not found in currently known materials. We also note that the material search and the definition of these phases have been independently proposed in Ref.[33].

TABLE S3. List of candidate materials, their SSG numbers[35], MAGNDATA numbers, and corresponding even-wave symmetry.

Table 5 · 2 columns × 13 rows · p.17–17 · CSV
Material SSG number MAGNDATA Wave MaterialSSG number MAGNDATA Wave
Ba3 MnNb2 O9 164.3.2.2.P 1.0.8 f -wave CsFeCl3194.3.2.2.P 1.0.14 f -wave
EuIn2 As2 194.3.4.3.P 1.0.32 p-wave CsMnBr3194.3.4.6.P 1.0.35 f -wave
RbFeCl3 194.3.2.2.P 1.0.40 f -wave RbNiCl3194.3.4.6.P 1.0.41 f -wave
CsNiCl3 194.3.4.6.P 1.0.42 f -wave Ba3 CoSb2 O9194.3.4.6.P 1.0.44 f -wave
Ba3 CoSb2 O9 194.3.4.6.P 1.0.45 f -wave Ba3 Nb2 NiO9164.6.2.3.P 1.13 f -wave
PrMn2 O5 26.2.2.7.P 1.19 p-wave HoAuGe8.2.1.6.P 1.34 p-wave
NdNiO3 26.2.2.32.P 1.44 f -wave HoNiO34.2.1.2.P 1.48 p-wave
Cs2 CoCl4 14.2.2.4.P 1.51 p-wave GdMn2 O526.2.2.7.P 1.54 p-wave
TmPtIn 25.2.2.27.P 1.67 p-wave NaNdFeWO61.2.1.2.P 1.68 p-wave
BiMn2 O5 26.2.2.30.P 1.74 p-wave DyMn2 O526.2.2.7.P 1.76 p-wave
DyFe4 Ge2 25.2.2.2.P 1.98 p-wave TbMn2 O56.2.1.8.P 1.108 p-wave
HoMn2 O5 6.2.1.8.P 1.109 p-wave YBaFe4 O74.2.2.2.P 1.124 p-wave
TmPdIn 174.2.3.1.P 1.163 f -wave Tm5 Ni2 In46.2.1.8.P 1.17 p-wave
GeCu2 O4 119.4.2.3.P 1.185 f -wave CoNb2 O618.2.2.5.P 1.224 p-wave
Ca2 Cr2 O5 46.2.1.6.P 1.227 p-wave NiPS35.2.1.4.P 1.231 p-wave
VCl2 164.6.2.3.P 1.237 f -wave VBr2164.6.2.3.P 1.238 f -wave

HoMn2 O 5 6.2.1.8.P 1.109 p-wave YBaFe4 O 7 4.2.2.2.P 1.124 p-wave TmPdIn 174.2.3.1.P 1.163 f -wave Tm5 N i2 I n4 6.2.1.8.P 1.17 p-wave

18.2.2.5.P 1.224 p-wave GeCu 2 O 4 119.4.2.3.P 1.185 f -wave CoNb 2 O 6

GeCu2 O 4 119.4.2.3.P 1.185 f -wave CoNb2 O 6 18.2.2.5.P 1.224 p-wave Ca2 C r2 O 5 46.2.1.6.P 1.227 p-wave NiPS3 5.2.1.4.P 1.231 p-wave

VCl 2 164.6.2.3.P 1.237 f -wave VBr 2 164.6.2.3.P 1.238 f -wave SmFe 3 (BO 3 ) 4 155.2.1.2.P 1.266 f -wave CeNiAsO 11.2.2.2.P 1.272 p-wave

SmFe3 ( BO3 ) 4 155.2.1.2.P 1.266 f -wave CeNiAsO 11.2.2.2.P 1.272 p-wave Yb2 C u2 O 5 7.2.2.5.P 1.28 p-wave DyMn2 O 5 26.2.2.7.P 1.324 p-wave

Yb 2 CoMnO 6 4.2.1.2.P 1.328 p-wave Lu 2 CoMnO 6 4.2.1.2.P 1.33 p-wave LuMnO 3 31.2.2.11.P 1.34 f -wave Sr 2 FeO 3 Cl 129.2.4.5.P 1.38 f -wave Sr 2 FeO 3 Br 129.2.4.5.P 1.381 f -wave Ca 2 FeO 3 Cl 129.2.4.5.P 1.382 f -wave Ca 2 FeO 3 Br 129.2.4.5.P 1.383 f -wave Sr 2 FeO 3 F 129.2.4.5.P 1.385 f -wave 129.2.4.5.P 1.387 f -wave Sr 2 FeO 3 F 129.2.4.2.P 1.386 f -wave Sr 2 FeO 3 F

Sr2 F eO3 F 129.2.4.2.P 1.386 f -wave Sr2 F eO3 F 129.2.4.5.P 1.387 f -wave Cu4 O 3 119.4.2.3.P 1.418 f -wave Gd2 B aCuO5 26.2.2.7.P 1.443 p-wave Er2 P t 31.2.2.7.P 1.444 p-wave Sr2 C uO2 C u2 S 2 139.4.2.5.P 1.456 f -wave Li2 M nGeO4 7.2.2.8.P 1.484 p-wave CsFe(MoO4 ) 2 147.6.2.4.P 1.499 p-wave

140.2.2.32.P 1.518 p-wave DyBe 13 140.2.2.32.P 1.517 p-wave TbBe 13 157.2.6.1.P 1.525 f -wave InMnO 3 157.2.6.1.P 1.524 f -wave InMnO 3 TbC 2 47.2.2.8.P 1.533 p-wave GeNi 2 O 4 166.2.2.2.P 1.562 p-wave GeCo 2 O 4 166.2.2.2.P 1.564 p-wave DyMn 2 O 5 6.2.1.2.P 1.599 p-wave LuNiO 3 4.2.1.2.P 1.657 p-wave DyGa 3 8.2.1.2.P 1.658 p-wave

LuNiO3 4.2.1.2.P 1.657 p-wave DyGa3 8.2.1.2.P 1.658 p-wave Ba3 C oNb2 O 9 164.6.2.3.P 1.665 f -wave KFe(PO3 F )2 147.12.2.5.P 1.669 p-wave

NiCr 2 O 4 24.2.2.2.P 1.688 p-wave CsCrF 4 6.2.1.8.P 1.709 p-wave Ba 3 NiTa 2 O 9 164.6.2.3.P 1.725 f -wave RuCl 3 1.2.1.2.P 1.726 p-wave Cu 2 MnSiS 4 7.2.2.5.P 1.73 p-wave Cu 2 FeSiS 4 7.2.2.8.P 1.731 p-wave Cu 2 MnGeS 4 7.2.2.5.P 1.733 p-wave ErAuIn 189.2.6.1.P 1.747 f -wave

Cu2 M nGeS4 7.2.2.5.P 1.733 p-wave E TbAuIn 189.2.6.1.P 1.748 f -wave