Material Links Magnetism and Electrons in New Ways

Researchers are revisiting the electronic structure of chromium thiophosphate, a layered semiconducting antiferromagnet exhibiting recently discovered and intriguing properties such as gate-tunable metamagnetism and oscillating magnetoresistance. Giuseppe Buccoliero, Rachel Nickel, Roberto Sant, and colleagues, in a collaborative effort between the European Synchrotron Radiation Facility and the Dipartimento di Fisica, Politecnico di Milano, have employed X-ray magnetic circular dichroism and resonant inelastic X-ray spectroscopies to unravel the complex magnetoelectronic coupling within this material. Working with colleagues at Université Grenoble Alpes, CNRS, Grenoble INP, and Institut NEEL, including Tristan Riccardi and Johann Coraux, alongside Kurt Kummer and Daniel A. Chaney from the European Synchrotron Radiation Facility, the team reveals extended superexchange paths involving phosphorus and sulphur atoms that mediate interactions between chromium spins across antiferromagnetic, canted, and ferromagnetic phases. This research elucidates the electronic states underpinning these transitions and offers valuable insights for manipulating the metamagnetic behaviour of CrPS4.

Imagine building with magnetic Lego bricks, carefully arranging them to create a stable structure. Controlling magnetism at this atomic level is the challenge, and new work with chromium thiophosphate reveals how electronic connections influence magnetic behaviour. Understanding these interactions allows precise manipulation of a material’s magnetic properties, potentially leading to advanced devices.

Scientists have long investigated chromium-based semiconductors for their unique spin and optical properties, driven by electronic transitions within chromium’s d molecular orbitals. Research into chromium thiophosphate, a layered semiconducting antiferromagnet, reveals a complex interaction between magnetism and electronic structure. Recent discoveries of gate-tunable metamagnetism, unusual magnetoresistance, and distinctive luminescence prompted a detailed examination of its underlying electronic behaviour and its connection to magnetic order.

Employing advanced X-ray techniques alongside quantum calculations, researchers have mapped the complex connections between magnetic order and electronic states in CrPS4. Investigations reveal extended interactions between chromium spins, mediated by phosphorus and sulfur atoms, across the material’s different magnetic phases: antiferromagnetic, canted, and ferromagnetic.

These interactions are not simply direct exchanges but involve pathways through the surrounding ligand atoms. For decades, the standard band-structure picture described chromium thiophosphate’s electronic structure with chromium eg and t2g levels separated by several electronvolts, within a sizable charge-transfer gap. However, this model failed to account for the observed behaviour.

The new work highlights the importance of hybridization between crystal-field and charge-transfer transitions, offering a more complete picture of the material’s electronic field. Detailed measurements using X-ray magnetic circular dichroism and resonant inelastic X-ray scattering, combined with theoretical calculations, have begun to unravel these complexities.

These techniques allow researchers to probe the electronic excitations within the material, identifying those states that actively participate in the sequence of magnetic orders as a function of applied magnetic field. The work elucidates the anisotropy of the chromium-ligand hybrid orbitals, providing insights into how these orbitals contribute to the material’s magnetic response.

Inside CrPS4, the crystal structure features layers of octahedrally coordinated chromium atoms bridged by phosphorus-sulfur tetrahedra. Below 38 K, the material adopts an antiferromagnetic order, where spins align within layers but oppose between them. Applying a magnetic field induces transitions to canted and then fully ferromagnetic states, but CrPS4 exhibits these transitions at remarkably low fields, considerably lower than similar materials like manganese or nickel thiophosphates.

By mapping the magnetic phase diagram and modelling the local chromium environment, researchers have established a foundation for understanding the material’s behaviour. The findings clarify the role of metal-ligand covalency in driving the metamagnetic response, allowing future work to focus on engineering magnetic order through careful ligand coordination. Beyond fundamental materials science, these insights could pave the way for new spintronic devices and tunable magnetic materials with tailored properties.

Probing Chromium Thiophosphate Magnetism via X-ray Spectroscopy and Computational Analysis

X-ray magnetic circular dichroism, encompassing both absorption and resonant inelastic X-ray spectroscopies, underpinned the investigation of magnetoelectronic coupling within chromium thiophosphate (CrPS4). These techniques, sensitive to the polarization of X-rays, were selected for their ability to probe the electronic structure and magnetic properties simultaneously.

Total electron yield measurements were performed at beamline I20, utilising circularly polarised X-rays to induce and monitor magnetic responses. Resonant inelastic X-ray scattering (RIXS) was employed to map out electronic excitations. Computational modelling was integrated to interpret the experimental data, including full ligand-field and charge-transfer terms for detailed comparison with the observed X-ray absorption spectra.

The analysis focused on the geometry dependence of the measurements, with total fluorescence yield (TFY) and normal incidence (NI) geometries providing complementary information. GI geometry, probing π-character states, was contrasted with NI, enhancing sensitivity to σ-character transitions. To fully characterise the magnetic interactions, element-specific measurements extended beyond chromium to the sulfur and phosphorus K-edges, utilising circularly polarised light to detect induced spin polarization on the ligand atoms.

Linear muffin-tin orbital (LMTO) simulations were used to predict the induced spin on both sulfur and phosphorus, providing a theoretical framework for interpreting the observed XMCD signals. These simulations accounted for the distinct hybridization paths between the metal d manifold and the ligand p orbitals, explaining the differing XMCD profiles. By combining experimental techniques with advanced computational methods, the research revealed a complex network of magnetic interactions mediated by the ligand atoms.

Low-field magnetic transitions and quasi-one-dimensional structural characteristics of chromium thiophosphate

At 38 K, chromium thiophosphate (CrPS4) undergoes a transition to antiferromagnetic order, exhibiting alternating ferromagnetic layers. Magnetometry measurements reconstructing the magnetic phase diagram revealed a spin-flop transition at a field of μ0Hflop, inducing a canted antiferromagnetic (CAFM) state, followed by a fully polarized ferromagnetic (FM) state at a higher field, μ0Hflip.

These transitions occur at remarkably low critical fields when contrasted with other MPS3 compounds like MnPS3, which requires approximately 5 T for a spin-flop transition at 5 K, or NiPS3, needing around 6 T with an in-plane field. Analysis of the crystal structure indicates that CrPS4 crystallizes in a monoclinic C2 structure, featuring edge-sharing CrS6 octahedra bridged by PS4 tetrahedra along the a axis.

These octahedra connect into quasi-one-dimensional chains along the b axis, establishing pronounced in-plane structural anisotropy. Researchers investigated the d, d and charge-transfer transitions within CrPS4 using X-ray magnetic circular dichroism (XMCD) and resonant inelastic X-ray scattering (RIXS). RIXS, MCD, an element-specific probe of electronic excitations in magnetic materials, was utilised to identify electronic states participating in the sequence of magnetic orders as a function of the magnetic field.

The crystal-field splitting in an octahedral environment within the Cr 3d manifold is approximately 1.8 eV, but this value is modified by trigonal distortions, resulting in a splitting of 2.2 eV. The RIXS, MCD spectra show a clear evolution of the Cr 3d excitations upon application of a magnetic field, indicating a strong coupling between the electronic structure and the magnetic order.

The intensity of the dxy, dxy excitation decreases as the field increases, while the intensity of the dxz, dxz excitation increases, reflecting the change in magnetic symmetry. Extended superexchange paths involving both sulfur and phosphorus atoms were revealed, coupling ferromagnetically spins localized on chromium ions. At 7 T, the RIXS, MCD spectra show a new peak appearing at 2.5 eV, attributed to a charge-transfer excitation from the Cr 3d states to the S 3p states.

This peak is only observed in the CAFM and FM phases, providing evidence for the involvement of charge-transfer excitations in the metamagnetic transitions. By analysing the anisotropy details of the Cr, ligand hybrid orbitals, the work clarifies the role of metal, ligand covalency in driving the metamagnetic response of CrPS4.

Tunable magnetism and extended interactions in layered chromium thiophosphate

Chromium thiophosphate is now revealing itself as a surprisingly adaptable material. Controlling magnetism at the nanoscale has been a central challenge in materials science, hampered by the difficulty of finding substances that respond predictably to external stimuli. This research, detailing the complex interaction between magnetism and electronic behaviour within chromium thiophosphate, offers a new degree of control over its magnetic properties.

Rather than simply observing magnetism, scientists are now able to tune it via external gating, opening possibilities for devices where magnetic states can be switched and manipulated with precision. The significance extends beyond fundamental physics, as chromium thiophosphate’s layered structure lends itself to potential two-dimensional devices. The extended interactions between chromium atoms, mediated by phosphorus and sulphur, are key to understanding and engineering its metamagnetic properties.

Fully realising this potential requires addressing limitations in material synthesis and device fabrication. The precise mechanisms governing the transition between different magnetic states, antiferromagnetic, canted, and ferromagnetic, remain areas for further investigation. The broader effort will likely focus on creating heterostructures incorporating chromium thiophosphate with other materials, where entirely new functionalities might emerge. The path from laboratory demonstration to practical application is still long, but this work represents a definite step towards a future where magnetism is not a fixed property, but a programmable one.