---
title: 'Chromium Thiophosphate (CrPS4): Low-Symmetry Magnet'
url: https://www.emergentmind.com/topics/chromium-thiophosphate-crps4
type: topic
---

# Chromium Thiophosphate (CrPS4): Low-Symmetry Magnet

Chromium thiophosphate, \(\mathrm{CrPS_4}\), is a layered van der Waals magnetic semiconductor in which \(\mathrm{Cr^{3+}}\) ions in distorted sulfur octahedra form a low-symmetry rectangular network, producing strongly anisotropic magnetism, field-tunable antiferromagnetic order, localized \(d\)-orbital optical excitations, and multiple spin-dependent transport phenomena [2006.12539][2511.17403]. Across bulk, few-layer, and heterostructure realizations, CrPS\(_4\) has become a reference system for studying A-type antiferromagnetism, quasi-one-dimensional intralayer correlations, room-temperature linear dichroism, magnon transport, and electrically tunable tunneling anisotropic magnetoresistance in an air-stable 2D material [2308.15396][2604.05572][2410.18428].

## 1. Crystal chemistry and structural symmetry

CrPS\(_4\) is a ternary chromium thiophosphate composed of Cr, P, and S, and its crystal architecture is layered: the layers are strongly bonded in plane and stacked out of plane with weak van der Waals coupling, enabling mechanical exfoliation to few-layer and monolayer thicknesses [2008.02466]. Structurally, the Cr ions reside in slightly distorted \(\mathrm{CrS_6}\) octahedra, while P occupies tetrahedral coordination in \(\mathrm{PS_4}\) units; this low-symmetry coordination is central to both the magnetic anisotropy and the optical anisotropy of the material [2010.05442][2604.05572].

The crystallographic symmetry has been discussed in two closely related forms. Earlier structural work placed CrPS\(_4\) in monoclinic \(C2/m\), with representative room-temperature lattice parameters \(a = 10.871\,\text{\AA}\), \(b = 7.254\,\text{\AA}\), \(c = 6.140\,\text{\AA}\), and \(\beta = 91.88^\circ\) [2008.02466]. Later neutron powder diffraction on the ordered low-temperature phase identified a non-centrosymmetric monoclinic \(C_2\) structure with \(a = 10.8510(3)\,\text{\AA}\), \(b = 7.2560(2)\,\text{\AA}\), \(c = 6.1058(3)\,\text{\AA}\), and \(\beta = 92.036(4)^\circ\) at 4 K [2006.12539]. The literature therefore treats the \(C2/m\) to \(C_2\) distinction as a genuine structural issue rather than a purely notational one; low-symmetry optical work explicitly emphasizes that earlier studies placed the compound in \(C2/m\), whereas more recent x-ray studies suggest the lower \(C2\) symmetry that naturally explains the pronounced in-plane anisotropy [2604.05572].

Within each layer, CrPS\(_4\) differs from the honeycomb geometry typical of many \(MPX_3\) magnets. The Cr ions form a rectangular 2D lattice, and the dominant magnetic couplings run along a single crystallographic direction, producing quasi-one-dimensional chains inside an otherwise quasi-two-dimensional layer [2006.12539]. High-resolution structural studies further showed unequal Cr–Cr spacings along the basal-plane axes and preferential cleavage along diagonal Cr rows, providing direct crystallographic signatures of this anisotropy in exfoliated flakes [2008.02466].

## 2. Magnetic ground state and exchange hierarchy

The magnetic ion in CrPS\(_4\) is nominally \(\mathrm{Cr^{3+}}\) with \(3d^3\) and \(S=3/2\), and the low-temperature ordered moment is close to the spin-only expectation: neutron powder refinement gives \(m_{\mathrm{Cr}} = 2.8(1)\,\mu_B\) per Cr at 3–4 K [2006.12539]. Neutron diffraction established that the bulk ground state is a layered antiferromagnet with propagation vector
\[
\mathbf{k}=(0,0,\tfrac{1}{2}),
\]
so the magnetic unit cell is doubled along \(c\); spins are ferromagnetically aligned within each layer, antiferromagnetically stacked between adjacent layers, and lie in the \(a\)–\(c\) plane with a dominant \(c\)-axis component at zero field [2006.12539]. In the later few-layer literature this is described as A-type antiferromagnetism: each monolayer is ferromagnetic, while neighboring monolayers are oppositely magnetized [2308.15396].

The exchange network is unusually anisotropic. In a local-moment Hamiltonian,
\[
\mathcal{H}=\sum J_{1}\,\mathbf{S}_i\!\cdot\!\mathbf{S}_j+\sum J_{2}\,\mathbf{S}_i\!\cdot\!\mathbf{S}_k+\sum J_{3}\,\mathbf{S}_i\!\cdot\!\mathbf{S}_l+\sum J_{c}\,\mathbf{S}_i\!\cdot\!\mathbf{S}_m-\sum D_z(S_i^z)^2,
\]
the fitted inelastic-neutron-scattering parameters are \(J_1=-2.96(4)\,\text{meV}\), \(J_2=-2.09(5)\,\text{meV}\), \(J_3=-0.51(4)\,\text{meV}\), \(J_c=+0.16(5)\,\text{meV}\), and \(D_z=0.0058(5)\,\text{meV}\), using the sign convention in which negative \(J\) denotes ferromagnetic coupling [2006.12539]. This immediately defines the hierarchy \(|J_1|,|J_2| \gg |J_3| \gg |J_c|\): CrPS\(_4\) is quasi-two-dimensional because interlayer coupling is an order of magnitude weaker than the strongest in-plane couplings, and it is simultaneously quasi-one-dimensional because the strongest ferromagnetic exchange runs along chain-like bonds inside each layer [2006.12539][2605.16541]. The single-ion anisotropy is correspondingly small, giving a fitted spin gap \(\Delta \approx 0.13(3)\,\text{meV}\) and nearly Heisenberg behavior with weak easy-axis anisotropy [2006.12539].

The modern neutron result superseded an earlier bulk interpretation based on magnetization, specific heat, and ESR that proposed a C-type antiferromagnetic ground state with in-plane antiferromagnetic coupling and ferromagnetic interlayer coupling [1602.08174]. That earlier model was useful as a first phenomenology, but later neutron diffraction and spin-wave analysis fixed the bulk structure as ferromagnetic layers stacked antiferromagnetically and resolved the microscopic exchange constants directly [2006.12539].

## 3. Field response, spin reorientation, and reduced dimensionality

CrPS\(_4\) does not exhibit a single simple magnetic transition. In bulk neutron diffraction, additional magnetic Bragg peaks appear below about 40 K, but the intensities of different reflections evolve differently; the ordered state develops through a spin reorientation between 36 and 40 K without changing \(\mathbf{k}\), producing what was described as a “non-trivial magnetic transition” [2006.12539]. The origin of that two-stage evolution remains open: the neutron study discussed either a dimensional crossover from 2D in-plane correlations to full 3D order or a weak structural effect that changes local anisotropy, while noting that no strong structural anomaly was resolved in the lattice constants [2006.12539].

The low anisotropy also makes the spin structure highly field-tunable. With field along the easy axis, a spin-flop occurs near \(\mu_0H_c \approx 0.8\,\text{T}\): the zero-field structure with spins largely along \(c\) reorients into a state with spins predominantly along \(b\), while the propagation vector remains \((0,0,\tfrac12)\) [2006.12539]. Few-layer and nanoscale probes observe the same basic metamagnetic sequence. In an 8 nm flake, dynamic cantilever magnetometry resolved three regimes for \(H \parallel c\): low-field A-AFM, an intermediate canted AFM region beyond a spin-flop near \(\mu_0H_{\rm sf}\sim 0.8\) T at 5 K, and a field-polarized state above \(H_{\rm s}\sim 7.6\) T [2404.08521]. For \(H \perp c\), the same study found an M-shaped \(\Delta f(H)\) at low temperature, a hump developing near \(25\) K, and a negative W-shaped response by \(30\)–\(35\) K, which it interpreted as a continuous temperature-driven reorientation from out-of-plane toward in-plane anisotropy [2404.08521].

Real-space neutron total-scattering has now sharpened that picture. Magnetic PDF analysis resolved the local spin direction and showed that the polar angle of the local moment rotates from \(\theta \approx 12^\circ\) away from \(c\) at 2 K to \(\theta \approx 58^\circ\) at 200 K, while ferromagnetic intrachain correlations persist far above \(T_N = 37\,\text{K}\) [2605.16541]. Combining those correlations with a DFT-derived spin Hamiltonian showed that single-ion and exchange-anisotropy channels are renormalized differently as temperature increases: the single-ion terms remain local, whereas the exchange-anisotropy contribution scales with the decay of intersite correlations, rotating the effective easy axis [2605.16541]. Above \(T_N\), the continued experimental rotation exceeds the dominant-chain model prediction, explicitly identifying the limits of that minimal description rather than invalidating the correlation-driven mechanism itself [2605.16541].

In the atomically thin limit, CrPS\(_4\) preserves its layered antiferromagnetic character but acquires a pronounced even–odd effect. Wide-field NV magnetometry directly imaged strong stray field in trilayers and negligible stray field in bilayers, consistent with uncompensated net magnetization in odd-layer A-type stacks and compensation in even-layer stacks; the reconstructed trilayer magnetization at 5 K is spatially averaged at roughly \(20\,\mu_B/\text{nm}^2\), and the trilayer ordering temperature is reduced to \(T_N \approx 26\) K [2308.15396]. Because all intralayer couplings in bulk are ferromagnetic while the interlayer coupling is small and antiferromagnetic, this suggests that removing \(J_c\) in the monolayer limit favors ferromagnetic order within a single CrPS\(_4\) layer [2006.12539].

## 4. Electronic structure and optical spectroscopy

CrPS\(_4\) is a semiconductor, but the relevant optical energy scales depend strongly on the probe. Earlier few-layer optical work reported an optical bandgap near \(1.4\) eV and a dominant photoluminescence peak at \(1.31 \pm 0.01\) eV, both associated with localized \(\mathrm{Cr^{3+}}\) \(d\)–\(d\) transitions [2010.05442]. Differential-reflectance analysis in another study gave \(E_g \simeq 2.00\) eV and identified two weak sub-gap absorption features at 1.54 and 1.65 eV [1910.05164]. More recent optical anisotropy work therefore framed the bandgap as being on the order of \(\sim 1.5\)–2 eV while emphasizing that the 1.37–2.48 eV spectral window is dominated by intra-ionic \(\mathrm{Cr^{3+}}\) transitions rather than simple band-edge absorption [2604.05572]. A plausible implication is that different experiments are emphasizing different parts of the same correlated ligand-field and charge-transfer landscape.

Momentum-resolved photoemission and DFT+\(U\) have now supplied the missing band-structure framework. The valence band is dominated by Cr \(3d\) and S \(3p\) states, and the gap is of ligand-to-metal charge-transfer type: the valence-band maximum has strong S \(3p\) weight with significant Cr \(3d\) admixture, whereas the lowest unoccupied states are predominantly Cr \(3d\) [2511.17403]. The \(t_{2g}\) manifold remains comparatively weakly hybridized and carries the local moments responsible for magnetic order, while the \(e_g\) manifold hybridizes strongly with ligand states and forms bonding/antibonding structures that relax dipole selection rules [2511.17403]. Soft-x-ray XMCD and RIXS reach a closely related conclusion from a different direction: CrPS\(_4\) is more covalent than a purely ionic \(\mathrm{Cr^{3+}}\) picture, exhibits hybridization between crystal-field and charge-transfer excitations, and shows measurable spin polarization on both S and P ligands, consistent with extended superexchange paths of the form Cr–S–P–S–Cr [2602.16083].

These orbital ingredients organize the optical spectra. In the 1.6–1.9 eV range, polarized reflectivity, photocurrent, and PLE identify two principal \(\mathrm{Cr^{3+}}\) crystal-field transitions, \(T_1\) near \(1.6\)–\(1.7\) eV and \(T_2\) near \(1.76\)–\(1.8\) eV [2604.05572]. CrPS\(_4\) also exhibits strong NIR photoluminescence. One line of work assigned room-temperature peaks at 1.32 and 1.43 eV to spin-allowed intra-\(d\)-shell transitions of \(\mathrm{Cr^{3+}}\) in an octahedral-like ligand field [1910.05164]. Another showed that the PL pathway bifurcates between fluorescence from \(^4T_2 \rightarrow\, ^4A_2\) at about \(1.13\) eV and phosphorescence from \(^2E \rightarrow\, ^4A_2\) at about \(1.36\) eV, with the branching controlled by thickness, temperature, and defect density through activated reverse intersystem crossing [2206.04194]. In that three-level Cr\(^{3+}\) model, thinner and more defective flakes shorten both biexponential decay times and increase the fast-component weight, consistent with defects lowering the barrier for reverse intersystem crossing [2206.04194].

Two additional spectroscopic phenomena are distinctive. First, below \(T_N \approx 38\) K, photoluminescence develops a Fano-type asymmetric line shape caused by interference between a broad continuum of \(\mathrm{Cr^{3+}}\) intra-\(d\) transitions and a discrete impurity-related level attributed to excess phosphorus; the discrete channel becomes optically active only in the antiferromagnetic phase and is further enhanced by magnetic field [1910.05164]. Second, CrPS\(_4\) displays pronounced room-temperature in-plane optical anisotropy. In reflection, the linear dichroism reaches about \(+50\%\) near \(1.77\) eV and about \(-20\%\) near \(1.68\) eV, implying a sign reversal within a \(\sim 100\) meV window; in photocurrent, the linear dichroism reaches \(\sim 60\%\) near 1.63 eV at zero bias, and scanning photocurrent mapping shows a 3-fold enhancement along the \(b\)-axis compared with the \(a\)-axis [2604.05572]. These responses are explicitly tied to the low-symmetry monoclinic lattice and polarization-dependent coupling to the \(T_1\) and \(T_2\) \(\mathrm{Cr^{3+}}\) transitions [2604.05572].

## 5. Transport, magnonics, and tunneling phenomena

Bulk transport establishes CrPS\(_4\) as a semiconductor with thermally activated resistivity. The in-plane resistivity follows \(\rho(T)=\rho_0\exp(E_a/k_BT)\), with \(\rho(300\,\text{K}) \approx 1.2\times10^3\,\Omega\cdot\text{cm}\) and activation energy \(E_a \approx 0.166\) eV in early single-crystal measurements [1602.08174]. More recent device studies shifted attention from charge transport itself to spin and magnon transport through this insulating antiferromagnet.

A central development is non-local magnon transport. In unconventional Pt/CrPS\(_4\) devices operating in the collinear state at in-plane fields \(>7\) T and \(T=25\) K, the same contact can act as injector and gate or the gate can be placed outside the nominal transport channel, yet still modulate diffusive magnon transport of incoherent magnons [2409.02621]. In this platform, electrically generated magnon signals are modulated by up to \(25\%\)/mA and thermally generated signals by up to \(16\%\)/mA, with the gate current altering the magnon chemical potential through the spin Hall effect and the magnon temperature through Joule heating [2409.02621]. CrPS\(_4\) therefore supplies a vdW antiferromagnetic realization of gate-tunable magnon transistors rather than merely a passive magnon medium.

Vertical tunneling devices reveal a complementary regime. In tetralayer graphite/CrPS\(_4\)/graphite spin-filter junctions, conductance is low in the low-field A-type antiferromagnetic state, jumps at the spin-flop near \(\sim 7\) kG, and saturates at higher field as the layers align ferromagnetically, yielding tunneling magnetoresistance of about \(100\%\) at 10 K and \(V_{\text{bias}} = 300\) mV [2308.15396]. Few-layer CrPS\(_4\) tunnel junctions go further: out-of-plane and in-plane field sweeps reveal layer-dependent and anomalous tunneling-magnetoresistance oscillations, especially in 4-layer devices, together with bias- and gate-controllable tunneling anisotropic magnetoresistance whose sign can reverse as bias or gate voltage is tuned [2410.18428]. First-principles transport calculations in that work attribute the TAMR sign reversal primarily to the energy dependence of the difference between tunneling currents for out-of-plane and in-plane spin orientations rather than to a wholesale change of the magnetic easy axis [2410.18428].

CrPS\(_4\) also functions as an interfacial antiferromagnet in exchange-bias heterostructures. In pristine CrPS\(_4\)/Fe\(_3\)GeTe\(_2\) devices, anomalous Hall loops show exchange bias of about \(12.5\) mT at 5 K, vanishing above about 20 K and even reversing sign around 15 K; in oxidized CrPS\(_4\)/(O-FGT)/FGT heterostructures, the exchange bias becomes non-monotonic with temperature and persists to about 140 K because CrPS\(_4\) interplays with ferrimagnetic Fe\(_3\)O\(_4\) and FeO-like oxide layers [2303.13167]. The same “fully uncompensated” character of a single CrPS\(_4\) monolayer that makes it attractive for exchange bias is precisely the property that underlies odd-layer uncompensated magnetization in the few-layer NV measurements [2308.15396][2303.13167].

## 6. Defects, proximity effects, and spin-optical control

Because CrPS\(_4\) combines a layered antiferromagnetic host with optically and electronically addressable excitations, it has become a useful platform for engineered proximity and impurity coupling. In bulk-CrPS\(_4\)/monolayer-WSe\(_2\) heterostructures, type-II band alignment and spin-polarized charge transfer allow WSe\(_2\) defect luminescence to act as an optical transducer of the CrPS\(_4\) surface magnetic order; below \(T_N \approx 37\) K, the heterostructure exhibits zero-field splitting and finite polarization contrast in the WSe\(_2\) defect PL, enabling optical readout of the surface layer and, by extension, the Néel vector orientation [2408.11986]. The same study reported that in the A-type AFM regime the circularly polarized CrPS\(_4\) transition remains nearly constant with small field variations, whereas the WSe\(_2\) response changes because the spin-polarized transfer channel is field-sensitive [2408.11986].

Rare-earth doping introduces an even sharper spin-optical probe. In Yb\(^{3+}\)-doped CrPS\(_4\), exchange coupling between isolated Yb\(^{3+}\) dopants and the Cr spin lattice produces large exchange splittings in narrow \(f\)–\(f\) photoluminescence lines below \(T_N\), and those splittings track the field-driven spin-flop with exceptional sensitivity [2507.10889]. In that system the spin-flop at \(B_{\text{sf}} \approx 0.9\)–1.0 T modulates the Yb\(^{3+}\) luminescence energies and exchange splittings strongly enough that an additional 405 nm optical beam can drive a reversible, optically induced spin-flop through photothermal heating of the CrPS\(_4\) lattice and spin system [2507.10889]. This places CrPS\(_4\) among the few 2D magnets where a metamagnetic phase boundary can be both read out and traversed optically.

Across these diverse experiments, two unresolved issues recur. One is crystallographic: the relationship between the \(C2/m\) and \(C_2\) descriptions remains an important part of the low-symmetry phenomenology rather than a closed historical footnote [2006.12539][2604.05572]. The other is magnetic: the spin reorientation between 36 and 40 K, and more generally the temperature dependence of the easy axis above and below \(T_N\), is now microscopically constrained but not exhausted by minimal dominant-chain models [2006.12539][2605.16541]. Even with those open questions, the accumulated evidence defines CrPS\(_4\) rather precisely: it is a low-symmetry, quasi-one-dimensional-in-a-layer, A-type antiferromagnetic semiconductor whose ligand-field excitations, exchange hierarchy, and weak anisotropy make its spin structure unusually susceptible to dimensional reduction, field, gating, defects, and optical control.

Source: https://www.emergentmind.com/topics/chromium-thiophosphate-crps4