Anisotropic Spin Research Articles

Fluctuation-induced piezomagnetism in local moment altermagnets

It was recently discovered that, depending on their symmetries, collinear antiferromagnets may break spin degeneracy in momentum space, even in the absence of spin-orbit coupling. Such systems, dubbed altermagnets, have electronic bands with a spin-momentum texture set mainly by the combined crystal-magnetic symmetry. This discovery motivates the question of which novel physical properties derive from altermagnetic order. Here we show that one consequence of altermagnetic order is a fluctuation-driven piezomagnetic response. Using two Heisenberg models of d-wave altermagnets, a checkerboard one and one for rutiles, we determine the fluctuation-induced piezomagnetic coefficients considering temperature-induced transversal spin fluctuations. We establish in addition that magnetic fluctuations induce an anisotropic thermal spin conductivity. Published by the American Physical Society 2024

Surface energy and elementary excitations of the XYZ spin chain with integrable open boundary fields

We study the thermodynamic limit of the anisotropic XYZ spin chain with non-diagonal integrable open boundary conditions. Although the U(1)-symmetry is broken, by using the new parametrization scheme, we exactly obtain the surface energy and the excitation energy of the system, which has solved the difficulty in the inhomogeneous T − Q relation. With the boundary parameters in the regions making the Hamiltonian Hermitian, we have obtained the distribution patterns of the zeros of the eigenvalue of the transfer matrix for the ground state and the excited ones. We find that the surface and excitation energies depend on the parities of sites number N, due to the long-range Neel order in the bulk. The easy-axis and thermodynamic limit for all the regions of boundary parameters are studied. We also obtain the physical quantities in the thermodynamic limit of boundary XXZ model by taking the trigonometric limit.

Electronic structure of zaykovite Rh3Se4, prediction, and analysis of physical properties of related materials: Pd3Se4, Ir3Se4, and Pt3Se4.

In this work, we explore the electronic properties and chemical bonding in the recently discovered mineral zaykovite, the first natural rhodium selenide Rh3Se4. We comprehensively studied the bulk electronic structure, hybridization of rhodium and selenium orbitals, and the influence of spin-orbit interaction on the electronic spectrum, as well as inspected its topological properties. In addition, we investigated the surface electronic structure of zaykovite and revealed the anisotropic Rashba-type spin splitting in the surface states. In addition, using calculations of the phonon spectra and enthalpy of formation, we predicted the family of similar selenides based on other 4d and 5d transition metals such as Ir, Pd, and Pt. The structural and electronic properties of these materials are discussed.

Visualizing the quantum phase transition by using quantum steering ellipsoids in the anisotropic spin XY model

Abstract Quantum steering ellipsoids (QSEs) can serve as a useful geometric tool for describing both the strength and type of quantum correlations between two subsystems of a compound system. By employing the quantum renormalization-group method, we focus on investigating the relation between QSEs and the quantum phase transition (QPT) in the anisotropic spin XY model. The results indicate that the QPT is well visualized in terms of the shape of the QSE, i.e. it is an oblate spheroid in the spin-fluid phase and a needle in the Néel phase. Meanwhile, after several iterations of renormalization, the QSE volume V undergoes a contraction mutation, and can develop two saturated values at the critical points associated with the QPT, which correspond to two different phases: the spin-fluid phase and the Néel phase. We also find that the QSE is closely associated with quantum entanglement in the model, i.e. the volume of the QSE between blocks is more than 4π/81 when the system is in the spin-fluid phase, which indicates that the system must be entangled. Furthermore, the nonanalytic and scaling behaviors of the volume of the QSE have been analyzed in detail, and the results convince us that the quantum critical properties are connected with the behavior of the QSE.

Ground state magnetic structure and magnetic field effects in the layered honeycomb antiferromagnet YbOCl

YbOCl is a representative member of the van der Waals layered honeycomb rare-earth chalcohalide RChX (R = rare earth; Ch = O, S, Se, and Te; and X = F, Cl, Br, and I) family reported recently. Its spin ground state remains to be explored experimentally. We grew high-quality single crystals of YbOCl and conducted comprehensive thermodynamic, elastic, and inelastic neutron scattering experiments down to 50 mK. The experiments reveal an antiferromagnetic phase below 1.3 K which is identified as a spin ground state with an intralayer ferromagnetic and interlayer antiferromagnetic ordering. By applying sophisticated numerical techniques to a honeycomb (nearest-neighbor)–triangle (next-nearest-neighbor) model Hamiltonian which accurately describes the highly anisotropic spin system, we are able to simulate the experiments well and determine the diagonal and off-diagonal spin-exchange interactions. The simulations give an antiferromagnetic Kitaev term comparable to the Heisenberg one. The experiments under magnetic fields allow us to establish a magnetic field–temperature phase diagram around the spin ground state. Most interestingly, a relatively small magnetic field (∼0.3 to 3 T) can significantly suppress the antiferromagnetic order, suggesting an intriguing interplay of the Kitaev interaction and magnetic fields in the spin system. The present study provides fundamental insights into the highly anisotropic spin systems and opens a window to look into Kitaev spin physics in a rare-earth-based system. Published by the American Physical Society 2024

Giant Anisotropic Magnetoresistance in Few-Layer α-RuCl3 Tunnel Junctions.

The spin-orbit-assisted Mott insulator α-RuCl3 is proximate to the coveted quantum spin liquid (QSL) predicted by the Kitaev model. In the search for the pure Kitaev QSL, reducing the dimensionality of this frustrated magnet by exfoliation has been proposed as a way to enhance magnetic fluctuations and Kitaev interactions. Here, we perform angle-dependent tunneling magnetoresistance (TMR) measurements on ultrathin α-RuCl3 crystals with various layer numbers to probe their magnetic, electronic, and crystal structures. We observe a giant change in resistance, as large as ∼2500%, when the magnetic field rotates either within or out of the α-RuCl3 plane, a manifestation of the strongly anisotropic spin interactions in this material. In combination with scanning transmission electron microscopy, this tunneling anisotropic magnetoresistance (TAMR) reveals that few-layer α-RuCl3 crystals remain in the high-temperature monoclinic phase at low temperatures. It also shows the presence of a zigzag antiferromagnetic order below the critical temperature TN ≃ 14 K, which is twice the one typically observed in bulk samples with rhombohedral stacking. Our work offers valuable insights into the relation between the stacking order and magnetic properties of this material, which helps lay the groundwork for creating and electrically probing exotic magnetic phases such as QSLs via van der Waals engineering.

Anisotropic Spin Fluctuations Induced by Spin-Orbit Coupling in a Misfit Layer Compound (LaSe)1.14(NbSe2).

Spin-orbit coupling (SOC) has significant effects on the superconductivity and magnetism of transition metal dichalcogenides (TMDs) at the 2D limit. Although 2D TMD samples possess many exotic properties different from those of bulk samples, experimental characterization in this field is still limited, especially for magnetism. Recent studies have revealed that bulk misfit layer compounds (MLCs) with (LaSe)1.14(NbSe2)n = 1,2 exhibit an Ising superconductivity similar to that of heavily electron-doped NbSe2 monolayers. This offers an opportunity to study the effect of SOC on the magnetism of 2D TMDs. Here, the possible SOC effect in (LaSe)1.14(NbSe2) is investigated by measuring nuclear magnetic resonance (NMR) and electrical transport. It is found that the LaSe layer not only functions as a charge reservoir for transferring electrons into the NbSe2 layer but also remarkably influences the local electronic environment around the 93Nb nuclei. More importantly, the significant SOC induces both a weak antilocalization (WAL) effect and anisotropic spin fluctuations in noncentrosymmetric NbSe2 layers. The present work contributes to a deep understanding of the role of the SOC effect in 2D TMDs and supports MCLs as an intriguing platform for exploring exotic physical properties within the 2D limit.

Optically Generated Spin Current in Two-Dimensional Metallic 2H-MX2 (M = Nb and Ta and X= S and Se): Role of Anisotropic Spin Splitting

Optically Generated Spin Current in Two-Dimensional Metallic 2H-MX₂ (M = Nb and Ta and X= S and Se): Role of Anisotropic Spin Splitting

Probing the weak limit of magnetocrystalline anisotropy through a spin‒flop transition in the van der Waals antiferromagnet CrPS4

The influence of magnetocrystalline anisotropy (MCA) on antiferromagnetism is elucidated through the characterization of the spin‒flop transition. However, due to a lack of suitable candidates for investigation, a detailed understanding of the preservation of the spin‒flop transition in the presence of low MCA energy remains elusive. In this study, we introduce CrPS4, which is a two-dimensional van der Waals antiferromagnet, as an ideal system to explore the exceedingly weak limit of the thermally-evolved MCA energy. By employing a uniaxially anisotropic spin model and fitting it to the experimental magnetic properties, we quantify the MCA energy and identify the discernible spin configurations in different magnetic phases. Notably, even at the limit of extremely weak MCA, with a mere 0.12% of the interlayer antiferromagnetic exchange interaction at T = 33 K, which is slightly below the Néel temperature (TN) of 38 K, the spin‒flop transition remains intact. We further establish a direct correlation between the visualized spin arrangements and the progressive reversal of magnetic torque induced by rotating magnetic fields. This analysis reveals the essential role of MCA in antiferromagnetism, thus extending our understanding to previously undetected limits and providing valuable insights for the development of spin-processing functionalities based on van der Waals magnets.

Structure, control, and dynamics of altermagnetic textures

We present a phenomenological theory of altermagnets, that captures their unique magnetization dynamics and allows modeling magnetic textures in this new magnetic phase. Focusing on the prototypical d-wave altermagnets, e.g., RuO2, we can explain intuitively the characteristic lifted degeneracy of their magnon spectra, by the emergence of an effective sublattice-dependent anisotropic spin stiffness arising naturally from the phenomenological theory. We show that as a consequence the altermagnetic domain walls, in contrast to antiferromagnets, have a finite gradient of the magnetization, with its strength and gradient direction connected to the altermagnetic anisotropy, even for 180° domain walls. This gradient generates a ponderomotive force in the domain wall in the presence of a strongly inhomogeneous external magnetic field, which may be achieved through magnetic force microscopy techniques. The motion of these altermagentic domain walls is also characterized by an anisotropic Walker breakdown, with much higher speed limits of propagation than ferromagnets but lower than antiferromagnets.

Realizing Altermagnetism in Fermi-Hubbard Models with Ultracold Atoms.

Altermagnetism represents a type of collinear magnetism, that is in some aspects distinct from ferromagnetism and from conventional antiferromagnetism. In contrast to the latter, sublattices of opposite spin are related by spatial rotations and not only by translations and inversions. As a result, altermagnets have spin-split bands leading to unique experimental signatures. Here, we show theoretically how a d-wave altermagnetic phase can be realized with ultracold fermionic atoms in optical lattices. We propose an altermagnetic Hubbard model with anisotropic next-nearest neighbor hopping and obtain the Hartree-Fock phase diagram. The altermagnetic phase separates in a metallic and an insulating phase and is robust over a large parameter regime. We show that one of the defining characteristics of altermagnetism, the anisotropic spin transport, can be probed with trap-expansion experiments.

High-field/high-frequency electron spin resonances of Fe-doped β−Ga2O3 by terahertz generalized ellipsometry: Monoclinic symmetry effects

We demonstrate detection and measurement of electron paramagnetic spin resonances (EPR) of iron defects in β−Ga2O3 utilizing generalized ellipsometry at frequencies between 110 and 170 GHz. The experiments are performed on an Fe-doped single crystal in a free-beam configuration in reflection at 45∘ and magnetic fields between 3 and 7 T. In contrast with low-field, low-frequency EPR measurements, we observe all five transitions of the s=5/2 high-spin state Fe3+ simultaneously. We confirm that ferric Fe3+ is predominantly found at octahedrally coordinated Ga sites. We obtain the full set of fourth-order monoclinic zero-field splitting parameters for both octahedrally and tetrahedrally coordinated sites by employing measurements at multiple sample azimuth rotations. The capability of high-field EPR allows us to demonstrate that simplified second-order orthorhombic spin Hamiltonians are insufficient, and fourth-order terms as well as consideration of the monoclinic symmetry are needed. These findings are supported by computational approaches based on density-functional theory for second-order and on ligand-field theory for fourth-order parameters of the spin Hamiltonian. Terahertz ellipsometry is a way to measure spin resonances in a cavity-free setup. Its possibility of varying the probe frequency arbitrarily without otherwise changing the experimental setup offers unique means of truly disentangling different components of highly anisotropic spin Hamiltonians. Published by the American Physical Society 2024

Existence of long-range magnetic order in Heisenberg spin nanoribbons with edge modification

Long-range magnetic order appears on a side decorated Heisenberg spin nanoribbon at nonzero temperature, although no spontaneous magnetization exists in a one- or two-dimensional isotropic Heisenberg model at any nonzero temperature according to the Mermin–Wagner theorem. By use of the spin Green’s function method, we calculated the magnetizations of Heisenberg nanoribbons decorated by side spins with single-ion anisotropy and found that the system exhibits a nonzero transition temperature, whether the decorated edge spins of the system link together or separate from each other. When the width of the nanoribbon achieves infinite limit, the transition temperatures of the system tend to the same finite constant eventually whether one edge or both edges are decorated by side spins in the nanoribbon. The results reveal that the magnetism of a low-dimensional spin system is different from that of a three-dimensional spin system. When the single-ion anisotropy of edge spins in a Heisenberg spin nanoribbon can be modulated by an electric field experimentally, various useful long-range magnetic orders of the system can be obtained. This work can provide a detailed theoretical basis for designing and fabricating next-generation low-dimensional magnetic random-access memory.

Spin squeezing generated by the anisotropic central spin model

Spin squeezing generated by the anisotropic central spin model

Low-Energy Spin Excitations in Detwinned FeSe

Abstract Antiferromagnetic spin fluctuation is regarded as the leading driving force for electron pairing in high-T c superconductors. In iron-based superconductors, spin excitations at low energy range, especially the spin-resonance mode at E R ∼ 5k B T c, are important for understanding the superconductivity. Here, we use inelastic neutron scattering (INS) to investigate the symmetry and in-plane wave-vector dependence of low-energy spin excitations in uniaxial-strain detwinned FeSe. The low-energy spin excitations (E < 10 meV) appear mainly at Q = (±1, 0) in the superconducting state (T ≲ 9 K) and the nematic state (T ≲ 90 K), confirming the constant C 2 rotational symmetry and ruling out the C 4 mode at E ≈ 3 meV reported in a prior INS study. Moreover, our results reveal an isotropic spin resonance in the superconducting state, which is consistent with the s ± wave pairing symmetry. At slightly higher energy, low-energy spin excitations become highly anisotropic. The full width at half maximum of spin excitations is elongated along the transverse direction. The Q-space isotropic spin resonance and highly anisotropic low-energy spin excitations could arise from dyz intra-orbital selective Fermi surface nesting between the hole pocket around Γ point and the electron pockets centered at M X point.

Characterization of partially accessible anisotropic spin chains in the presence of anti-symmetric exchange

We address quantum characterization of anisotropic spin chains in the presence of anti-symmetric exchange, and investigate whether the Hamiltonian parameters of the chain may be estimated with precision approaching the ultimate limit imposed by quantum mechanics. At variance with previous approaches, we focus on the information that may be extracted by measuring only two neighboring spins rather than a global observable on the entire chain. We evaluate the Fisher information (FI) of a two-spin magnetization measure, and the corresponding quantum Fisher information (QFI), for all the relevant parameters, i.e. the spin coupling, the anisotropy, and the Dzyaloshinskii–Moriya (DM) parameter. Our results show that the reduced system made of two neighboring spins may be indeed exploited as a probe to characterize global properties of the entire system. In particular, we find that the ratio between the FI and the QFI is close to unit for a large range of the coupling values. The DM coupling is beneficial for coupling estimation, since it leads to the presence of additional bumps and peaks in the FI and QFI, which are not present in a model that neglects exchange interaction and may be exploited to increase the robustness of the overall estimation procedure. Finally, we address the multiparameter estimation problem, and show that the model is compatible but sloppy, i.e. both the Uhlmann curvature and the determinant of the QFI matrix vanish. Physically, this means that the state of the system actually depends only on a reduced numbers of combinations of parameters, and not on all of them separately.

Direct and Inverse Spin Splitting Effects in Altermagnetic RuO2.

Recently, the altermagnetic materials with spin splitting effect (SSE), have drawn significant attention due to their potential to the flexible control of the spin polarization by the Néel vector. Here, the direct and inverse altermagnetic SSE (ASSE) in the (101)-oriented RuO2 film with the tilted Néel vector are reported. First, the spin torque along the x-, y-, and z-axis is detected from the spin torque-induced ferromagnetic resonance (ST-FMR), and the z-spin torque emerges when the electric current is along the [010] direction, showing the anisotropic spin splitting of RuO2. Further, the current-induced modulation of damping is used to quantify the damping-like torque efficiency (ξDL) in RuO2/Py, and an anisotropic ξDL is obtained and maximized for the current along the [010] direction, which increases with the reduction of the temperature, indicating the present of ASSE. Next, by way of spin pumping measurement, the inverse altermagnetic spin splitting effect (IASSE) is studied, which also shows a crystal direction-dependent anisotropic behavior and temperature-dependent behavior. This work gives a comprehensive study of the direct and inverse ASSE effects in the altermagnetic RuO2, inspiring future altermagnetic materials and devices with flexible control of spin polarization.

Compensated Ferrimagnets with Colossal Spin Splitting in Organic Compounds.

The study of the magnetic order has recently been invigorated by the discovery of exotic collinear antiferromagnets with time-reversal symmetry breaking. Examples include altermagnets and compensated ferrimagnets, which show spin splittings of the electronic band structures even at zero net magnetization, leading to several unique transport phenomena, notably spin-current generation. Altermagnets demonstrate anisotropic spin splitting, such as d-wave, in momentum space, whereas compensated ferrimagnets exhibit isotropic spin splitting. However, methods to realize compensated ferrimagnets are limited. Here, we demonstrate a method to realize a fully compensated ferrimagnet with isotropic spin splitting utilizing the dimer structures inherent in organic compounds. Moreover, based on abinitio calculations, we find that this compensated ferrimagnet can be realized in the recently discovered organic compound (EDO-TTF-I)_{2}ClO_{4}. Our findings provide an unprecedented strategy for using the dimer degrees of freedom in organic compounds to realize fully compensated ferrimagnets with colossal spin splitting.

Interfacial Magnetic Anisotropy Controlled Spin Pumping in Co60Fe20B20/Pt Stack

Controlled spin transport in magnetic stacks is required to realize pure spin current-driven logic and memory devices. The control over the generation and detection of the pure spin current is achieved by tuning the spin to charge conversion efficiency of the heavy metal interfacing with ferromagnets. Here, we demonstrate the direct tunability of spin angular momentum transfer and thereby spin pumping, in CoFeB/Pt stack, with interfacial magnetic anisotropy. The ultra-low thickness of the CoFeB thin film by tilting the magnetization from in-plane to out-of plane direction due to interfacial anisotropy from higher thickness of CoFeB thin film. The ferromagnetic resonance measurements are performed to investigate the magnetic anisotropy and spin pumping in CoFeB/Pt stacks. We clearly observe tunable spin pumping effect in the CoFeB/Pt stacks with varying CoFeB thicknesses. The spin current density, with varying ferromagnetic layer thickness, is found to increase from 1.10[Formula: see text]MA/m2 to 2.40[Formula: see text]MA/m2, with increasing in-plane anisotropy field. Such interfacial anisotropy-controlled generation of pure spin current can potentially lead to next-generation anisotropic spin current-controlled spintronic devices.

Dephasing effects on nonclassical correlations in two-qubit Heisenberg spin chain model with anisotropic spin–orbit interactions

Dephasing effects on nonclassical correlations in two-qubit Heisenberg spin chain model with anisotropic spin–orbit interactions