Session N25: Low Dimensional Magnetism: 2D Magnets

Structural Properties and Magnetic Phase Transition of Self-Intercalated CrTeX Compound

Magnetic transition-metal dichalcogenides (TMDs) have been of particular interest due to their unique magnetic properties and layered structure that can be useful in the field of spintronics. Self-intercalation of transition metals in the van der Waals (vdW) gaps of the TMDs can alter the structural and magnetic properties of the material to create new, interesting compounds. We report a self-intercalated compound CrTe_1.64 that forms a three-dimensional (2x2x2) superstructure with inequivalent occupation of the Cr atoms intercalated in the vdW gaps. This structure is confirmed using X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) measurements. In addition to this, magnetic measurements show an out-of-plane easy-axis with Curie temperature (T_c) of approximately 195 K and an emergent antiferromagnetic ordering just above the T_c. This research shows that self-intercalation of magnetic TMDs can provide a method of tuning magnetic and structural properties of the material.   

Understanding and tuning magnetism in layered Ising-type antiferromagnet FePSe3 for potential 2D magnet

Recent developments in two-dimensional (2D) magnetic materials have motivated the search for new van der Waals magnetic materials. In particular, layered Ising-type magnetic materials have been intensively investigated because of their strong magnetic anisotropy which makes them a suitable candidate for 2D magnets. Fe-based MPX₃ compounds such as FePS₃ and FePSe₃ exhibit Ising-type magnetic order. However, though FePS_3­ has been well studied, FePSe₃ was overlooked for long time. In this work, we explored the magnetic properties of FePSe₃ to establish the strategies to engineer magnetic anisotropy and exchange interactions. Our study reveals distinct effects between chalcogen S and metal Mn substitutions on magnetic properties of FePSe₃. In particular, Mn substitution leads to continuous rotation of moments from out-of-plane towards the in-plane direction while moment orientation remains unchanged upon S-Se substitution. Furthermore, the magnetic ordering temperature displays non-monotonic evolution with both chalcogen and metal substitutions, which is found to be attributed to different mechanisms. This provides a better understanding of materials with Ising-type magnetism, shedding light on the study of other Ising-type magnetic systems as well as discovering novel 2D magnets for potential applications in spintronics.   

Spin excitations in a XY-type antiferromagnet NiPS3

Magnons are collective spin excitations in magnetic crystals with long-range magnetic order. Magnons can be used to carry and transport information with low energy consumption and high speed. The recent popularly studied van der Waals (vdW) antiferromagnets with magnon in the terahertz range can be used as a good platform for ultrafast spintronics. Here, we performed magneto-Raman measurements on bulk NiPS₃ to detect its spin excitations including magnons. By polarization-resolved Raman measurement, we found two lower-energy magnon modes below 50 cm^-1, which is consistent with inelastic neutron scattering results and related theoretical calculations. The magnetic field dependence of these two magnon modes shows distinct behavior under the magnetic field applied along different crystal axes. Our results reveal the spin excitation in NiPS₃ and establish NiPS₃ as a desirable candidate for studying fundamental magnon physics.   

Above room-temperature ferromagnetism in van der Waals material Fe3GaTe2

The emergence of two-dimensional van der Waals magnets has introduced new prospects for vdW heterostructure and spintronic applications in the past decade. Recently a new member Fe3GaTe2 was found and stood out for its persistent ferromagnetism above room temperature. In this presentation, we will discuss our successful synthesis of single-crystalline Fe3GaTe2, as well as the following bulk characterization, transport, and magneto-optical measurements performed on our single crystal and exfoliated thin flakes. The magnetization and transport studies on the bulk Fe3GaTe2 crystals reveal a high Curie temperature of 355K and the thermodynamic investigations suggest the presence of a strong electron-electron correlation. Layer-dependent magneto-optical studies on thin Fe3GaTe2 flakes further unveil its robust ferromagnetism for room-temperature applications in 2D limits. Our work illuminates the remarkable magnetic properties of Fe3GaTe2, establishing it as a suitable vdW platform for realizing controllable 2D magnetism and spintronic applications in everyday environments.   

Out-of-plane surface magnetization in Fe5GeTe2 induced by magnetic coupling with NiPS3

In this talk, we present our recent comprehensive investigation on Fe₅GeTe₂/NiPS₃ heterostructure with element-specific measurements. Fe₅GeTe₂ is a van der Waals ferromagnet which was reported to possess a thickness- and temperature- dependent spin-reorientation transition. NiPS₃ is an in-plane van der Waals antiferromagnet with a weak uniaxial anisotropy along the a-axis of its monoclinic lattice. Micron-sized Fe₅GeTe₂/NiPS₃ heterostructures were transferred onto membrane window TEM grids for the characterizations with non-microscopic X-ray techniques. Through element-specific X-ray magnetic circular dichroism (XMCD), X-ray magnetic linear dichroism (XMLD), photoemission electron microscopy (PEEM) and local probe electronic transport measurements, we directly observed the XMLD and tri-axial in-plane antiferromagnetic domains in NiPS₃, and investigated the interfacial coupling effects on the ferromagnetic properties of Fe₅GeTe₂. We observed an interfacial-coupling induced magnetic anisotropy (PMA) in Fe₅GeTe₂. This extrinsic interfacial magnetic anisotropy influences the Fe₅GeTe₂ magnetizations within a small thickness near the interface, resulting in a spin texture along the perpendicular direction of the film. The discovery of spin texture along the perpendicular direction is a unique property of van der Waals magnetic thin films due to their weak interlayer coupling. Our work shows another novel and unique characteristics of van der Waals magnetic materials.   

Optical Detection of Antiferromagnetic Resonance in van der Waals Materials

Measurements of antiferromagnetic dynamics in thin samples is challenging due to a small number of spins which have no static magnetization. One of the ideal candidates for measuring the weak dipole field fluctuations from antiferromagnetic dynamics are single-spin color centers such as the NV center in diamond. Here we present optical detection of antiferromagnetic resonance in the van der Waals material CrSBr by the method of NV relaxometry in a bulk sample. Strongly driven uniform mode antiferromagnetic resonance populates finite wavevector magnon states via magnon scattering processes. Gapless magnetic field fluctuations produced by two-magnon processes are amplified by increasing the magnon population to levels detectable by an ensemble of NV centers on the surface. Field fluctuations at the NV electron spin resonance frequencies relax the spin polarization resulting in a decrease of photoluminescence. These measurements allow estimation of the strength of field fluctuations emanating from antiferromagnetic spinwaves and pave the way for measurements of antiferromagnetic dynamics in ultrathin materials.   

Tunneling current-controlled magnetic states in few-layer CrI3

Controlling two-dimensional (2D) magnetism is pivotal for developing novel spintronic devices utilizing 2D van der Waals (vdW) materials. While advancements such as giant tunneling magnetoresistance, electrostatic gating-induced magnetization transition, and twisted angle modulated magnetism in 2D vdW magnets have been moted, the influence of tunneling current in these magnet-based tunnel junctions remains under-explored. Here, we report on a novel discovery of tunneling-current-induced unidirectional spin state transition and stochastic switching in a few-layer CrI₃. At a moderate tunneling current, we can deterministically switch the spin configurations between spin parallel and spin antiparallel states. This switching is attributed to the nonequilibrium spin accumulation in graphene electrodes. Furthermore, we detected stochastic switching between these SAP and SP states, with the probabilities tunable by the bias current. Our findings mark a significant advancement in controlling magnetic phase transitions in 2D vdW magnets.   

Enhancement of Magnetic Anisotropy Energy in Heavy Metal/layered Magnet Heterostructure Induced by Strong Spin Orbit Coupling

Layered ekchromium telluride (Cr_1+xTe₂) is known as itinerant ferromagnet with self-intercalated Cr inside the Van der Waals gap based on 1T CrTe₂. Recently Chromium telluride (Cr_1+xTe₂) family draw a large interest due to their unique physical properties such as magnetic Skyrmions [1] and topological Hall effect [2] etc. As strong potential candidate to realize 2D-spintronics, numerous researches had done to understand the physical properties of Chromium telluride (Cr_1+xTe₂). However, manipulation of magnetic properties of Chromium telluride (Cr_1+xTe₂) had not been performed yet. Here, we investigate the controlling of the magnetic properties of Cr₂Te₃ ultra-thin films forming heterostructure with heavy metal to induce interface spin-orbit coupling. With complementary transport and spectroscopic evidence, we will address the origin of observed manipulation of magnetic properties of ultra-thin film system.

[1] Nature Communications, 13, 3965 (2022)

[2] ACS Nano, 16, 8974 (2022)   

Magnetism of the Gd(0001) surface revisited: DMI, Frustrating Exchange, Strain

There is a long standing effort of the scientific community to understand the magnetism of the Gd(0001) surface that produced over time  a lot of scientific insights with partly contradictory results. Combining spin-polarized scanning tunneling microscopy (SP-STM) investigations on thickness-dependent domain structures of Gd/W(110) with first-principles investigations based on density functional theory (DFT) we reach a converging picture. Here we report on the observation of a DMI-driven spin-spiral state at the surface of epitaxial Gd(0001) films grown on W(110). We confirmed the existence of a spin reorientation transition (SRT) [1]  from in-plane to out-of-plane magnetized films at a critical thickness Θ_crit ≈100 +- 20 AL [2].  In the vicinity of this SRT, by means of SP-STM we identify striped regions with a periodicity of about 2 nm.  The application of an external magnetic field induces a rearrangement of the stripes,  thereby unambiguously confirming its magnetic origin. We explain the results as a combination of the Dzyaloshinskii-Moriya interaction (DMI) [3,4]  and the competing exchange interaction at the Gd surface whose competition is controlled by strain.

[1] A.  Berger et al., Phys. Rev. B 52, 1078 (1995).

[2] P.  Härtl et al., Phys. Rev. B 105, 174431 (2022).

[3] I. E. Dzyalosinskij, Sov. Phys. JETP 5, 1259 (1957)

[4] T. Moriya, Phys. Rev. 120, 91 (1960).   

Magnetic properties of lanthanide adatoms on graphene

Individual magnetic adatoms on surfaces may exhibit long-lived spin quantum states that can be efficiently manipulated and detected, making such systems promising materials for ultrahigh-density magnetic information storage and quantum logic devices. Rare-earth adatoms on graphene are of particular interest since strong spin-orbit coupling of lanthanides may lead to a large anisotropy barrier while the absence of a nuclear spin in ¹²C is expected to increase the coherence time of adatom spin states. Further, the properties of the system may be controlled by gating. Here, we study electronic structure and magnetic properties of rare-earth adatoms on graphene using combination of density functional theory (DFT), ab initio multireference quantum chemistry methods, and effective spin Hamiltonian technique. Preferred adsorption sites and atomic coordinates are determined using DFT calculations in the supercell geometry.  The resulting structures are then used to build cluster models, and low-energy electronic spectra and magnetic interactions are calculated using quantum chemistry methods. Ab initio effective spin Hamiltonians are then constructed and different mechanisms for magnetic relaxation and decoherence are discussed.   

First principles studies of Fe adatoms on two-dimensional ferroelectric In2Se3 layer

Magnetic adatoms on surfaces may function as single-atom magnets with significant magnetization and coercivity at low temperatures [1]. Such systems can have potential applications in high-density magnetic memory and quantum information science technologies if adatom magnetization can be reliably controlled and detected. In this work, we investigate structural, electronic, and magnetic properties of Fe atoms adsorbed on a surface of two-dimensional ferroelectric α-In₂Se₃ using ab initio electronic structure methods and microscopic models. Preferred adsorption sites are identified for both In₂Se₃ electric polarization directions, and hopping energy barriers are computed. The interplay between adatom properties, the adsorption site, and the In₂Se₃ polarization is then investigated. We demonstrate that the difference in geometry between the competing adsorption sites significantly changes hybridization between the Fe orbitals and In₂Se₃ states. Consequently, the adatom electronic structure, magnetic moment, and magnetic anisotropy show a strong dependence on the adsorption site and can be effectively controlled by switching the electric polarization of the In₂Se₃ layer. This behavior can be potentially utilized to efficiently manipulate the adatom spin states with voltage.

[1] F.  Donati,  S.  Rusponi,  S.  Stepanow,  et al. Science 352, 318 (2016).   

Data-driven Exploration of New Two-Dimensional Magnets Using Graph Neural Networks

Two-dimensional (2D) magnets have a transformative potential in spintronics applications. In this study, we employ Graph Neural Networks (GNNs) to discover novel 2D magnetic materials. Using data from the Materials Project database and the Computational 2D materials database (C2DB), we train three GNN architectures on a dataset of approximately 1,200 magnetic monolayers with energy above hull less than 0.3 eV. Our Crystal Diffusion Variational Auto Encoder (CDVAE) generates around 11,000 material candidates. Subsequent training on two Atomistic Line Graph Neural Networks (ALIGNN) achieves a 93% accuracy in predicting magnetic monolayers and a mean average error of 0.04 eV for energy above hull prediction. After narrowing down candidates based on magnetic likelihood and predicted energy above hull, and constraining the atom count in the monolayer to four or fewer, we identified 160 potential materials. These are validated using density-functional theory (DFT) to confirm their magnetic and energetic favorability. Our approach offers a methodical way to explore and predict potential 2D magnetic materials, contributing to ongoing computational and experimental efforts aimed at the discovery of new 2D magnets.   

Ferromagnetic and Antiferromagnetic Magnons in Cr2C MXenes

The synthesis of 2D materials has boosted the study of magnetic properties in two dimensions¹. As an example, Cr-based structures (e.g., CrI₃) allow for the realization of different magnetic orderings², and topological magnons³. In this work, we study the MXenes Cr₂C magnetic material via the spin-spin Heisenberg Hamiltonian including exchange interactions between first, second and third neighbors and Zeeman interaction. Previous DFT calculations showed that different Cr₂C functionalization leads to different magnetic orderings, including one ferromagnetic (FM) and three antiferromagnetic (AFM) configurations². Here we calculate the magnon dispersion relations for all four magnetic orderings using the Holstein-Primakoff and obtain magnons group velocities larger than the ones in CrI₃. In addition, we also study the corresponding magnon thermal Hall conductivity as a function of temperature and magnetic field. 

References 

¹Gong, Cheng, and Xiang Zhang. "Two-dimensional magnetic crystals and emergent heterostructure devices." Science363.6428 (2019): eaav4450. 

²Yogendra Limbu, Hari Paudyal, Denis R. Candido, Michael E. Flatté and Durga Paudyal. To be submitted. 

³Costa, António T., et al. "Topological magnons in CrI₃ monolayers: an itinerant fermion description." 2D Materials 7.4 (2020): 045031.