---
title: 'Co1/4TaSe2: Layered Altermagnetic Dichalcogenide'
url: https://www.emergentmind.com/topics/co1-4tase2
type: topic
---

# Co1/4TaSe2: Layered Altermagnetic Dichalcogenide

Co$_{1/4}$TaSe$_2$, also written Co$_{0.25}$TaSe$_2$ or $1/4$-CoTaSe$_2$, is an intercalated transition-metal dichalcogenide derived from the 2H-TaSe$_2$ host by insertion of cobalt into the van der Waals gap. It crystallizes in the hexagonal $P6_3/mmc$ space group (No. 194), forms a $2\times 2$ in-plane enlargement of the 2H-TaSe$_2$ unit cell, and exhibits type-A antiferromagnetic order with reported Néel temperatures of $T_N = 173\,\mathrm{K}$ and $178\,\mathrm{K}$ in two complementary studies. A neutron and bulk-characterization study established its stoichiometry, magnetic structure, anisotropy, and itinerant character, while a subsequent ARPES/DFT study identified it as a layered altermagnetic material through momentum-dependent spin splitting at the Fermi surface and its suppression above $T_N$ [2408.10421; 2508.12985].

## 1. Composition, nomenclature, and crystal structure

Co$_{1/4}$TaSe$_2$ is a layered 2H-polytype TaSe$_2$ intercalate in which cobalt occupies the van der Waals gap. The stoichiometry was established by a combination of single-crystal X-ray diffraction, powder X-ray diffraction Rietveld refinement, elemental EDS, and XPS. Single-crystal refinement at 250 K found full occupancy on all refined sites, giving a stoichiometric composition with Co, Ta, and Se occupancies equal to unity on their respective crystallographic positions. The powder diffraction pattern was fit well by a hexagonal 2H structure, with room-temperature lattice constants from pXRD of $a = 6.8835(5)\,\text{\AA}$ and $c = 12.4791(8)\,\text{\AA}$, essentially matching the single-crystal values $a = 6.8828(1)\,\text{\AA}$ and $c = 12.4535(3)\,\text{\AA}$ at 250 K [2408.10421].

The refined unit cell at 250 K is characterized by $a = 6.8828(1)\,\text{\AA}$, $c = 12.4535(3)\,\text{\AA}$, and $V = 510.92(2)\,\text{\AA}^3$ with $Z=8$. In the refined atomic model, cobalt occupies the $2a$ Wyckoff site at $(0,0,1/2)$, tantalum occupies the $2b$ site at $(0,0,1/4)$ and the $6h$ site, and selenium occupies the $12k$ and $4f$ sites. The intercalated Co forms a triangular sublattice in the van der Waals gap, corresponding to the usual $2a_0 \times 2a_0$ superlattice motif that often appears at $x = 1/4$ in Nb- and Ta-based intercalated TMDs [2408.10421].

The ARPES study describes the same structure as consisting of 1H-TaSe$_2$ layers separated by intercalated Co atoms occupying the $2a$ Wyckoff site, preserving the parent space group while producing a $2\times 2$ in-plane enlargement of the 2H-TaSe$_2$ unit cell. In that work, the bulk hexagonal Brillouin zone is discussed in terms of the $k_z=0$, $k_z=\pi/2c$, and $k_z=\pi/c$ planes, with high-symmetry points $\Gamma, M, K$, $\Gamma', M', K'$, and $A, L, H$ labeled accordingly [2508.12985].

## 2. Magnetic ground state and ordered moment

Bulk probes and neutron diffraction identify Co$_{1/4}$TaSe$_2$ as an A-type antiferromagnet. Magnetic susceptibility, specific heat, and neutron powder diffraction detect antiferromagnetic order below $T_N = 173\,\mathrm{K}$, while the ARPES/DFT study reports a sharp susceptibility anomaly at $T_N = 178\,\mathrm{K}$ [2408.10421; 2508.12985]. In both accounts, the magnetic structure consists of ferromagnetic alignment within each Co layer and antiferromagnetic coupling between adjacent layers along the $c$ axis.

Neutron powder diffraction gives the decisive magnetic structural determination. Below $T_N$, new low-$Q$ magnetic reflections appear, specifically indexed as $(101)$ and $(111)$, and the magnetic propagation vector is $\mathbf{k} = (0,0,0)$, so the magnetic unit cell coincides with the chemical unit cell. Magnetic symmetry analysis using k-SUBGROUPGMAG led to the best refinement in magnetic space group $P6_3^\prime/m^\prime m^\prime c$ (No. 194.268). The refined order is collinear, with all spins along [001], consistent with susceptibility and magnetization identifying the $c$ axis as the easy axis [2408.10421].

The ordered moment is substantially reduced relative to the naive high-spin Co$^{2+}$ expectation. The neutron refinements give $1.08(11)\,\mu_B$ per Co at 120 K and $1.35(11)\,\mu_B$ per Co at 10 K. The ARPES/DFT study likewise cites neutron-scattering evidence for a local Co moment of about $1.35\,\mu_B$ rather than the full high-spin $d^7$ value. DFT further indicates only a small opposite-sign polarization induced in the TaSe$_2$ layers, about $10\%$ of the Co moment [2408.10421; 2508.12985].

A secondary feature of the magnetization data is a weak in-plane ferromagnetic component. For in-plane fields, the low-temperature magnetization rises slightly, and subtraction of an antiferromagnetic background from the $M(H)$ curves at 2 K and 200 K isolates a very small soft ferromagnetic signal that saturates at $\Delta M = 0.000183\,\mu_B$ per formula unit. The principal magnetic structure nevertheless remains collinear antiferromagnetic [2408.10421].

## 3. Valence state, itinerancy, and exchange picture

XPS identifies cobalt as Co$^{2+}$ and supports a metallic intercalated compound. Ta $4f$ peaks at 24.7 eV and 25.5 eV, with satellite structure, were assigned to mixed Ta$^{3+}$/Ta$^{4+}$ states typical of metallic, conducting layered dichalcogenides. The Co $2p$ peaks at 777.9 eV and 793.1 eV, together with satellite features about 6.5 eV above the main $2p_{3/2}$ line, were interpreted as evidence for Co$^{2+}$; the satellite structure was taken as consistent with high-spin Co$^{2+}$, although some surface oxidation to CoO could contribute. Se $3d$ binding energies at 53.8 eV and 54.7 eV match the expected values for Se in a chalcogenide environment [2408.10421].

Several measurements point to itinerant antiferromagnetism rather than a simple local-moment insulator. The Curie-Weiss analysis of $\chi_{||ab}^{-1}$ gave $\Theta_{\mathrm{CW}} = -3914(17)\,\mathrm{K}$ and a Curie constant $C = 2.05(1)\,\mathrm{K\,emu\,Oe^{-1}\,mol^{-1}}$, corresponding to an effective moment $\mu_{\mathrm{eff}} = 4.05(1)\,\mu_B$, slightly larger than the spin-only high-spin Co$^{2+}$ value for $S = 3/2$, namely $\mu_{\mathrm{eff}} = 3.88\,\mu_B$. The same study emphasizes that simple Curie-Weiss behavior is inadequate and that a substantial temperature-independent Pauli contribution from itinerant electrons must be present [2408.10421].

The reduced ordered moment is accompanied by reduced magnetic entropy. After subtraction of a Debye-Einstein phonon background, the integrated magnetic entropy is only about $26\%$ of the expected value for $S=3/2$. Specific heat also shows a clear second-order anomaly at $\sim 173\,\mathrm{K}$, and the low-temperature fit $C_p/T = \gamma + \beta T^2 + \delta T^4$ yields $\gamma = 4.38(8)\,\mathrm{mJ\,mol^{-1}\,K^{-2}}$, $\beta = 0.21(3)\,\mathrm{mJ\,mol^{-1}\,K^{-4}}$, and $\delta = 6.2(2)\times10^{-4}\,\mathrm{mJ\,mol^{-1}\,K^{-6}}$. From $\beta$, the estimated low-temperature Debye temperature is about $310(1)$ K, while a full Debye-Einstein fit gives $\Theta_D = 190(5)\,\mathrm{K}$ and $\Theta_E = 313(4)\,\mathrm{K}$ with weights $n = 1.22(15)$ and $a = 2.07(6)$ [2408.10421].

The electronic-structure interpretation centers on strong Co $3d$ and Ta $5d_{z^2}$ hybridization near the Fermi level. The earlier study argues that cobalt donates electrons into the Ta-derived conduction band and that magnetic exchange is mediated by itinerant carriers, likely through an RKKY-like mechanism. The ARPES/DFT study adds that the Co moments are centered primarily on Co $3d$ orbitals, with only a small opposite-sign polarization in the TaSe$_2$ layers. This suggests that the magnetism is neither purely localized on Co nor purely host-derived, but is mediated by hybridized Co–Ta electronic states [2408.10421; 2508.12985].

## 4. Altermagnetic symmetry and band topology

The central symmetry claim is that Co$_{1/4}$TaSe$_2$ satisfies the defining condition for altermagnetism: opposite spin sublattices are related by a crystal symmetry, but not by simple inversion or translation. In this compound, the two Co sites are related by mirror symmetry. According to the ARPES/DFT study, this symmetry enforces nodal spin-degenerate planes while allowing momentum-dependent spin splitting away from them [2508.12985].

This distinguishes Co$_{1/4}$TaSe$_2$ from a conventional antiferromagnet whose bands remain Kramers-degenerate everywhere because of translation-plus-time-reversal symmetry. Instead, the mirror-related Co sublattices and the hexagonal crystal symmetry generate momentum-selective spin polarization with nodal planes in the $\Gamma$-$K$-$H$-$A$ plane as well as in the $k_z=0$ and $k_z=\pi/c$ planes. The resulting band structure is described as a $g$-wave altermagnetic band structure [2508.12985].

The earlier neutron and bulk-characterization study had already noted that the symmetry and weak in-plane ferromagnetic component make Co$_{0.25}$TaSe$_2$ a candidate for altermagnetic behavior according to the Amcheck symmetry analysis, although that work primarily described the ordered state as a simple A-type antiferromagnet with $\mathbf{k}=(0,0,0)$ [2408.10421]. The later ARPES/DFT results supply the direct momentum-space evidence required to move from candidacy to identification as a layered altermagnetic material [2508.12985].

The symmetry class is stated to be analogous to that of recently studied MnTe and CrSb systems, but realized here in a layered intercalated TMD. A plausible implication is that Co$_{1/4}$TaSe$_2$ occupies an intermediate conceptual position between bulk altermagnets and van der Waals magnetic materials, because its altermagnetic order is embedded in a structurally layered host [2508.12985].

## 5. ARPES and DFT evidence for momentum-dependent spin splitting

The decisive electronic-structure evidence comes from the comparison of ARPES and DFT in the ordered phase. At $T=7$ K, deep in the ordered phase, the measured Fermi surface at $h\nu=55$ eV is in excellent agreement with DFT for the $k_z \approx \pi/2c$ plane, where maximal altermagnetic splitting is expected. The observed Fermiology consists of a small electron-like pocket around $\Gamma'$, petal-like pockets extending along $\Gamma'$-$K'$, and dog-bone pockets near $M'$ [2508.12985].

The DFT overlays show red and blue curves corresponding to opposite-spin bands, and the spin splitting follows a characteristic $g$-wave pattern: the opposite-spin Fermi surfaces alternate with sixfold symmetry, are degenerate along $\Gamma'$-$K'$, and separate most strongly as one rotates toward $\Gamma'$-$M'$. The splitting is clearest on the dog-bone pocket, where ARPES and DFT indicate a momentum separation of the two crossings of about $0.01\,\text{\AA}^{-1}$. Momentum distribution curve analysis along $\Gamma'$-$M'$ resolves two peaks separated by $\sim 0.09\,\text{\AA}^{-1}$ in the 55 eV data [2508.12985].

The $k_z$ selectivity of the splitting is a critical point. Measurements at lower photon energy, $h\nu=48$ eV, corresponding to $k_z \approx 0$, show the splitting vanishing, consistent with the symmetry-imposed nodal structure of the altermagnetic bands. The authors present this $k_z$-selective spin splitting as one of the strongest pieces of evidence that the observed band reconstruction is not a generic exchange splitting, but the momentum-dependent altermagnetic splitting predicted by theory [2508.12985].

Because the measured Fermi surface geometry, the degeneracy along symmetry-protected directions, and the selective splitting away from those directions all match the DFT calculation, the ARPES/DFT comparison directly links the bulk magnetic symmetry to the low-energy quasiparticle structure. This is the key experimental basis for classifying Co$_{1/4}$TaSe$_2$ as a layered altermagnet rather than only as an itinerant A-type antiferromagnet [2508.12985].

## 6. Temperature dependence, transport response, and research context

Temperature-dependent ARPES shows that the ordered-state reconstruction collapses on heating through the Néel transition. Comparing spectra at 7 K and 200 K across $T_N=178$ K, the Fermi surface broadens and the petal features along $\Gamma'$-$K'$ lose intensity above the transition. In the dispersion along $\Gamma'$-$M'$, the low-temperature spectrum shows the two-peaked structure associated with altermagnetic splitting, while the high-temperature spectrum collapses into a narrower, largely unsplit band crossing $E_F$ [2508.12985].

Two reconstruction regions are emphasized. Near $\Gamma'$, the small hole-like pocket is suppressed and likely shifts downward away from the Fermi level. Near $M'$, the altermagnetic doublet weakens, and the high-temperature spectrum indicates closing of the magnetic gap and reduction of spectral splitting. The EDC stacks show redistributions of spectral weight, with the leftmost EDCs losing Fermi-level intensity above $T_N$ and the remaining lines showing thermal broadening plus suppression of the ordered-state splitting. These changes are interpreted as the collapse of altermagnetic spin splitting and associated magnetic reconstructions upon entering the paramagnetic phase [2508.12985].

Transport data from the earlier study are consistent with an itinerant reconstructed Fermi surface. The in-plane resistivity is metallic but relatively poor conducting, with residual resistivity ratio $RRR = \rho_{300\,\mathrm{K}}/\rho_{2\,\mathrm{K}} = 1.97$ and residual resistivity $\rho_0 = 144.3\,\mu\Omega\cdot\text{cm}$. The resistivity is roughly linear above $T_N$ and then drops more rapidly below about 150 K, attributed to reduced spin-disorder scattering once antiferromagnetic order develops. Magnetoresistance becomes appreciable only below about 120 K, reaching roughly $4\%$ at 2 K, and Kohler scaling fails. Hall resistivity is positive, implying holelike carriers, with no strong anomalous Hall contribution; carrier densities estimated from the Hall slope are approximately $1.4\times10^{22}\,\mathrm{cm^{-3}}$ at 2 K, $2.8\times10^{23}\,\mathrm{cm^{-3}}$ at 170 K, and $4.0\times10^{23}\,\mathrm{cm^{-3}}$ at 200 K [2408.10421].

The broader significance assigned to Co$_{1/4}$TaSe$_2$ is tied to its coexistence of layered crystal structure, type-A antiferromagnetism, and directly observed altermagnetic spin splitting. The ARPES/DFT study highlights the van der Waals character of the material as especially significant for heterostructures, interfaces, and moiré-based platforms, while the earlier study places it in the context of other magnetic-ion intercalated TMDs, contrasting it with ferromagnetic Fe$_{0.25}$TaSe$_2$ and Ni$_{0.25}$TaSe$_2$ and with more complex sulfide analogues. A common misconception would be to treat Co$_{1/4}$TaSe$_2$ as either a conventional Kramers-degenerate antiferromagnet or a simple local-moment cobalt compound; the combined neutron, transport, ARPES, and DFT results instead define it as a metallic, hybridized, layered A-type antiferromagnet whose low-energy bands exhibit momentum-dependent altermagnetic spin splitting [2408.10421; 2508.12985].

Source: https://www.emergentmind.com/topics/co1-4tase2