Antiferromagnetic Spin Research Articles

Thermal and Coherent Spin Pumping by Noncollinear Antiferromagnets.

Antiferromagnets attract much interest because of their potential for spintronic applications and open fundamental physics questions, but especially noncollinear antiferromagnets remain relatively unexplored. Here, we formulate the thermal and coherent pumping of spins in noncollinear antiferromagnets|normal metal bilayers. We find that the spin current polarization is a vector with components along both the Néel vector and net magnetic moment. The spin mixing conductance for the coherent spin pumping is a tensor with elements depending on the degree of noncollinearity and interface spin configuration. We explain the controversial sign problem of the antiferromagnetic spin Seebeck effect by interface effects and suggest that interface engineering may enhance the spin pumping efficiency.

Antiferromagnetic artificial neuron modeling of the withdrawal reflex.

Replicating neural responses observed in biological systems using artificial neural networks holds significant promise in the fields of medicine and engineering. In this study, we employ ultra-fast artificial neurons based on antiferromagnetic (AFM) spin Hall oscillators to emulate the biological withdrawal reflex responsible for self-preservation against noxious stimuli, such as pain or temperature. As a result of utilizing the dynamics of AFM neurons, we are able to construct an artificial neural network that can mimic the functionality and organization of the biological neural network responsible for this reflex. The unique features of AFM neurons, such as inhibition that stems from an effective AFM inertia, allow for the creation of biologically realistic neural network components, like the interneurons in the spinal cord and antagonist motor neurons. To showcase the effectiveness of AFM neuron modeling, we conduct simulations of various scenarios that define the withdrawal reflex, including responses to both weak and strong sensory stimuli, as well as voluntary suppression of the reflex.

Two-dimensional coherent spectrum of high-spin models via a quantum computing approach

We present and benchmark a quantum computing approach to calculate the two-dimensional coherent spectrum (2DCS) of high-spin models. Our approach is based on simulating their real-time dynamics in the presence of several magnetic field pulses, which are spaced in time. We utilize the adaptive variational quantum dynamics simulation algorithm for the study due to its compact circuits, which enables simulations over sufficiently long times to achieve the required resolution in frequency space. Specifically, we consider an antiferromagnetic quantum spin model that incorporates Dzyaloshinskii-Moriya interactions and single-ion anisotropy. The obtained 2DCS spectra exhibit distinct peaks at multiples of the magnon frequency, arising from transitions between different eigenstates of the unperturbed Hamiltonian. By comparing the one-dimensional coherent spectrum with 2DCS, we demonstrate that 2DCS provides a higher resolution of the energy spectrum. We further investigate how the quantum resources scale with the magnitude of the spin using two different binary encodings of the high-spin operators: the standard binary encoding and the Gray code. At low magnetic fields both encodings require comparable quantum resources, but at larger field strengths the Gray code is advantageous. Numerical simulations for spin models with increasing number of sites indicate a polynomial system-size scaling for quantum resources. Lastly, we compare the numerical 2DCS with experimental results on a rare-earth orthoferrite system. The observed strength of the magnonic high-harmonic generation signals in the 2DCS of the quantum high-spin model aligns well with the experimental data, showing significant improvement over the corresponding mean-field results.

Heteroleptic Ni(II) complexes containing chelating benzoato ligand with sharp bite angle: syntheses, crystal structures, and magnetism

From the system Ni(OH)2 – 4,4′-dmbpy – HBz (HBz = benzoic acid; 4,4′-dmbpy = 4,4′-dimethyl-2,2′-bipyridine) under various experimental conditions three complexes were isolated, namely light blue [Ni(4,4′-dmbpy)(Bz)2(H2O)] (1), violet [Ni(4,4′-dmbpy)2(Bz)](Bz)·2HBz (2), and pink microcrystalline product [Ni(4,4′-dmbpy)3](Bz)2·3.5H2O (3). The crystal structures of 1 and 2 were determined, that of 2 at two temperatures (100 and 277 K). The crystal structure of 1 is formed of neutral molecules, in which the Ni(II) central atom is hexa-coordinated by one chelating diamine ligand, one chelating and one monodentate benzoato ligands, and one aqua ligand. The crystal structure of 2 is ionic and in the complex cation the Ni(II) atom is hexa-coordinated by two chelating diamine ligands and one chelating benzoato ligand. The positive charge of the complex cation is counterbalanced by a benzoate anion, and two additional molecules of benzoic acid are present in the asymmetric unit. Magnetic properties of 1 and 2 were studied in the temperature range 1.8 – 300 K and confirmed strong axial single-ion anisotropy (D/kB = -7.78 K for 1, D/kB = -6.52 K for 2) of the Ni(II) ions in line with the deformed coordination spheres of the central atoms. In addition, antiferromagnetic spin dimers of Ni(II) ions mediated by hydrogen bonds are present in 1 with exchange coupling J/kB = -2.19 K. The obtained experimental results were supported by ab initio and DFT calculations.

An antiferromagnetic spin phase change memory

The electrical outputs of single-layer antiferromagnetic memory devices relying on the anisotropic magnetoresistance effect are typically rather small at room temperature. Here we report a new type of antiferromagnetic memory based on the spin phase change in a Mn-Ir binary intermetallic thin film at a composition within the phase boundary between its collinear and noncollinear phases. Via a small piezoelectric strain, the spin structure of this composition-boundary metal is reversibly interconverted, leading to a large nonvolatile room-temperature resistance modulation that is two orders of magnitude greater than the anisotropic magnetoresistance effect for a metal, mimicking the well-established phase change memory from a quantum spin degree of freedom. In addition, this antiferromagnetic spin phase change memory exhibits remarkable time and temperature stabilities, and is robust in a magnetic field high up to 60 T.

Adsorption of Gadolinium Bisphthalocyanine on Atomically Flat Surfaces: Comparison of Graphene and Hexagonal Boron Nitride from DFT Calculations

We studied the noncovalent interactions of gadolinium bisphthalocyanine (GdPc2) with cluster models for graphene and hexagonal boron nitride (hBN) of variable size by using the PBE functional of the generalized gradient approximation in conjunction with Grimme’s dispersion correction and a DND double numerical basis set (that is, PBE-D2/DND). We found that in terms of the bonding strength, changes in the Gd-N bond lengths, the charge and spin of the Gd central ion, and the spin of the GdPc2 molecule, the behaviors of the graphene- and hBN-based model systems are rather similar. As expected, when increasing the size of the graphene and hBN cluster models, the strength of the interaction with GdPc2 increases, in which the bonding with the hBN models is usually stronger by a few kcal/mol. One of the main questions addressed in the present work was whether a change in the antiferromagnetic spin alignment to a ferromagnetic one, which is typical for GdPc2, is (at least theoretically) possible, as it has been observed previously for a number of graphene models when a smaller basis set DN was employed. We found that the use of a larger DND basis set dramatically reduces the occurrence of ferromagnetic adsorption complexes but does not exclude this possibility completely.

High-Pressure Induced Continuous Structural Evolution of Kagome Antiferromagnet MgMn3(OH)6Cl2: A Structural Analogue to Quantum Spin Liquid Herbertsmithite.

MgMn3(OH)6Cl2 serves readily as the classical Heisenberg kagome antiferromagnet lattice spin frustration material, due to its similarity to herbertsmithite in composition and crystal structure. In this work, nanosheets of MgMn3(OH)6Cl2 are synthesized through a solid-phase reaction. Low-temperature magnetic measurements revealed two antiferromagnetic transitions, occurring at ∼8 and 55 K, respectively. Utilizing high-pressure synchrotron radiation X-ray diffraction techniques, the topological structural evolution of MgMn3(OH)6Cl2 under pressures up to 24.8 GPa was investigated. The sample undergoes a second-order structural phase transition from the rhombohedral phase to the monoclinic phase at pressures exceeding 7.8 GPa. Accompanying the disappearance of the Fano-like line shape in the high-pressure Raman spectra were the emergence of new Raman active modes and discontinuities in the variations of Raman shifts in the high-frequency region. The phase transition to a structure with lower symmetry was attributed to the pressure-induced enhancement of cooperative Jahn-Teller distortion, which is caused by the mutual substitution of Mn2+ ions from the kagome layer and Mg2+ ions from the triangular interlayer. High-pressure ultraviolet-visible absorption measurements support the structural evolution. This research provides a robust experimental approach and physical insights for further exploration of classical geometrical frustration materials with kagome lattice.

Antiferromagnetic magnonic charge current generation via ultrafast optical excitation

Néel spin-orbit torque allows a charge current pulse to efficiently manipulate the Néel vector in antiferromagnets, which offers a unique opportunity for ultrahigh density information storage with high speed. However, the reciprocal process of Néel spin-orbit torque, the generation of ultrafast charge current in antiferromagnets has not been demonstrated. Here, we show the experimental observation of charge current generation in antiferromagnetic metallic Mn2Au thin films using ultrafast optical excitation. The ultrafast laser pulse excites antiferromagnetic magnons, resulting in instantaneous non-equilibrium spin polarization at the antiferromagnetic spin sublattices with broken spatial symmetry. Then the charge current is generated directly via spin-orbit fields at the two sublattices, which is termed as the reciprocal phenomenon of Néel spin-orbit torque, and the associated THz emission can be detected at room temperature. Besides the fundamental significance on the Onsager reciprocity, the observed magnonic charge current generation in antiferromagnet would advance the development of antiferromagnetic THz emitter.

Magnetic phases of XY model with three-spin terms: interplay of topology and entanglement

Magnetic and topological properties along with quantum correlations in terms of several entanglement measures have been investigated for an antiferromagnetic (AFM) spin-1/2 XY model in the presence of transverse magnetic field and XZX−YZY type of three-spin interactions. Symmetries of the spin Hamiltonian have been identified. Under the Jordan–Wigner transformation, the spin Hamiltonian converted into spinless superconducting model with nearest neighbor (NN) hopping and Cooper pairing terms in addition to next NN Cooper pairing potential. Long range AFM order has been studied in terms of staggered spin–spin correlation functions, while the topological orders have been characterized by winding numbers. Magnetic and topological phase diagrams have been prepared. Faithful coexistence of magnetic and topological superconducting phases is found in the entire parameter regime. Boundaries of various quantum phases have been marked and positions of bicritical points have been identified.

Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3

We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe3. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe3 is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across TN, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above TN. These results suggest that the first-order AF phase transition in FePSe3 may arise from the competition between C3 symmetric AF and C1 symmetric FM spin fluctuations around TN, in place of a conventional second-order AF phase transition.

Giant impurity effect on anomalous Hall effect of Mn3Sn.

Mn3Sn is an anomalous Hall effect (AHE) antiferromagnet that exhibits the hysteretic AHE in antiferromagnetic (AFM) phase at room temperature. We report that whisker Mn3Sn crystals grown by the flux method exhibit a non-hysteretic AHE at mid-to-low temperatures when the whisker Mn3Sn is surrounded by a thin layer of ferromagnetic Mn2-xSn. These crystals exhibit a hysteretic AHE above 275K due to the spin alignment of the inverse triangular lattice, which is similar to other crystals. However, upon cooling the crystal, it exhibits a non-hysteretic AHE with a spiral AFM spin structure at 100-200K. We concluded that the non-hysteretic AHE is induced at the interface of Mn2-xSn/Mn3Sn. We believe that the scalar-spin chirality in the spiral AFM phase of Mn3Sn, modulated by Mn2-xSn through the magnetic proximity effect, produces the AHE. This discovery opens a new avenue for tailoring the AHE by magnetic layers.

The Interplay of Electronic Configuration and Anion Ordering on the Magnetic Behavior of Hydroxyfluoride Diaspores.

We report a new nickel hydroxyfluoride diaspore Ni(OH)F prepared using hydrothermal synthesis from NiCl2·6H2O and NaF. Magnetic characterization reveals that, contrary to other reported transition-metal hydroxyfluoride diaspores, Ni(OH)F displays weak ferromagnetism below the magnetic ordering temperature. To understand this difference, neutron diffraction is used to determine the long-range magnetic structure. The magnetic structure is found to be distinct from those reported for other hydroxyfluoride diaspores and shows an antiferromagnetic spin ordering in which ferromagnetic canting is allowed by symmetry. Furthermore, neutron powder diffraction on a deuterated sample, Ni(OD)F, reveals partial anion ordering that is distinctive to what has previously been reported for Co(OH)F and Fe(OH)F. Density functional theory calculations show that OH/F ordering can have a directing influence on the lowest energy magnetic ground state. Our results point toward a subtle interplay between the sign of magnetic exchange interactions, the electronic configuration, and anion disordering.

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.

Electric-field-induced multiferroic topological solitons.

Topologically protected spin whirls in ferromagnets are foreseen as the cart-horse of solitonic information technologies. Nevertheless, the future of skyrmionics may rely on antiferromagnets due to their immunity to dipolar fields, straight motion along the driving force and ultrafast dynamics. While complex topological objects were recently discovered in intrinsic antiferromagnets, mastering their nucleation, stabilization and manipulation with energy-efficient means remains an outstanding challenge. Designing topological polar states in magnetoelectric antiferromagnetic multiferroics would allow one to electrically write, detect and erase topological antiferromagnetic entities. Here we stabilize ferroelectric centre states using a radial electric field in multiferroic BiFeO3 thin films. We show that such polar textures contain flux closures of antiferromagnetic spin cycloids, with distinct antiferromagnetic entities at their cores depending on the electric field polarity. By tuning the epitaxial strain, quadrants of canted antiferromagnetic domains can also be electrically designed. These results open the path to reconfigurable topological states in multiferroic antiferromagnets.

Classical spin-liquid behavior in interactionally distorted antiferromagnetic spin systems on the kagome lattice

The influence of an interaction distortion of the antiferromagnetic system on the kagome lattice on its magnetic and thermodynamic properties is investigated in the framework of the antiferromagnetic J1−J2−J3 spin-1/2 Ising system with three different nearest-neighbor antiferromagnetic interactions on the star kagome-like recursive lattice. The exact solution of the model is found and the phase diagram is determined and analyzed. It is shown that, depending on the model parameters, three antiferromagnetic phases can be identified, which are separated from the paramagnetic phase by the second order phase transitions. The magnetic and entropy properties of all ground states of the model are determined. Three different ground states that arise from the paramagnetic phase in the zero-temperature limit are identified. Due to the frustration, each of these ground states is highly macroscopically degenerated with different values of the residual entropy. In addition, since the paramagnetic phase in the vicinity of these ground states also exhibits high degeneracy with the presence of strong correlations, related to the proximity of the second-order phase transitions, these ground states together with their immediate paramagnetic surroundings can be considered as regions with the classical spin-liquid behavior. It is also shown that the typical anomalous (Schottky-type) behavior of the specific heat with the existence of a two-peak (in general multi-peak) structure in its temperature dependence in the vicinity of the highly macroscopically degenerated ground states can be suppressed by the presence of the second-order phase transitions.

Using Kondo entanglement to induce spin correlations between disconnected quantum dots

We investigate the entanglement between the spins of two quantum dots that are not simultaneously connected to the same system. Quantum entanglement among localized spins is a crucial property for the advancement of quantum computing and quantum information. Generating and controlling an entangled state between quantum dots have garnered significant attention in recent years for this reason. In this study, we demonstrate that information about the spin orientation of a quantum dot can be preserved, utilizing Kondo entanglement, within a reservoir of electrons. Subsequently, this information can be transmitted to another dot after the initial dot has been decoupled from the reservoirs. We employ a double quantum dot system in a parallel geometry to establish the initial state, where each dot interacts with reservoirs of different symmetries. A specific phase in the couplings is chosen to induce antiferromagnetic spin correlation between the dots. The time evolution of the initial state is analyzed using the time-dependent density matrix renormalization group method. Our findings reveal that a partially entangled state between the dots can be achieved, even when they are not simultaneously connected. This entangled state arises transiently and dissipates in the stationary state. The stability of the state observed during the transient phase is demonstrated. To comprehend the details of these phenomena, we employ a canonical transformation of real space.

Giant gate modulation of antiferromagnetic spin reversal by the magnetoelectric effect

In this study, using the Pt/Cr2O3/Pt epitaxial trilayer, we demonstrate the giant voltage modulation of the antiferromagnetic spin reversal and the voltage-induced 180° switching of the Néel vector in maintaining a permanent magnetic field. We obtained a significant modulation efficiency of the switching field, Δμ0HSW/ΔV (Δμ0HSW/ΔE), reaching a maximum of −500 mT/V (−4.80 T nm/V); this value was more than 50 times greater than that of the ferromagnetic-based counterparts. From the temperature dependence of the modulation efficiency, X-ray magnetic circular dichroism measurements and first-principles calculations, we showed that the origin of the giant modulation efficiency relied on the electric field modulation of the net magnetization due to the magnetoelectric effect. From the first-principles calculation and the thickness effect on the offset electric field, we found that the interfacial magnetoelectric effect emerged. Our demonstration reveals the energy-efficient and widely applicable operation of an antiferromagnetic spin based on a mechanism distinct from magnetic anisotropy control.

Controlling magnetically induced shape memory behavior through volume proportions in Heusler-type Fe47-xMn24+xGa29 structures

Here we demonstrate that a ferromagnetic shape memory can be tuned conveniently by volume proportions within Heusler-type Fe47-xMn24+xGa29 (x = 8, 6, 4, 2, and 0 at%) alloys over an extended temperature range. Preliminary X-ray diffraction experiments indicate that the Fe47-xMn24+xGa29 alloys crystallize into a B2 cubic structure with space group Pm3¯m for all compositions. Samples with compositions of x = 0–4 at% show the co-existence of two cubic structures (B2 and L21). The temperature dependence of magnetization M(T) measurements on Fe47-xMn24+xGa29 alloys under cooling-heating processes showed a moderate to an exceptionally large temperature hysteresis in a wider temperature range of 70–340 K, corresponding to the first-order diffusionless martensitic-to-austenite phase transformations. The M(T) curves obtained at various magnetic fields demonstrated that the magnitude and direction of temperature hysteresis is modified by volume proportions of Fe-Mn constituents. At 5, 150 and 300 K, hysteresis loop shows ferromagnetic (FM) behavior and the coercivity and remanence values vary with temperature and Mn content due to large exchange bias effects. Further ac magnetic susceptibility of Fe47-xMn24+xGa29 measurements show a cusp with a maximum below 70 K corresponding to an antiferromagnetic (AFM) transition. Thermal shift of the AFM transition is attributed to the dominant AFM-FM interactions pertaining to the pinning efficiency at the interface between Fe and Mn. These findings provide a comprehensive understanding of AFM spin structure and ferromagnetic shape memory behavior in Fe47-xMn24+xGa29 across cryogenic to 350 K temperatures.

Magnetoelectric Properties of Aurivillius-Layered Perovskites

In the present work, we have synthesized rare-earth ion modified Bi4−xRExTi2Fe0.7Co0.3O12−δ (RE = Dy, Sm, La) multiferroic compounds by the conventional solid-state route. Analysis of X-ray diffraction by Rietveld refinement confirmed the formation of a polycrystalline orthorhombic phase. The morphological features revealed a non-uniform, randomly oriented, plate-like grain structure. The peaks evident in the Raman spectra closely corresponded to those of orthorhombic Aurivillius phases. Dielectric studies and impedance measurements were carried out. Asymmetric complex impedance spectra suggested the relaxation of charge carriers belonging to the non-Debye type and controlled by a thermally activated process. Temperature-dependent AC conductivity data showed a change of slope in the vicinity of the phase transition temperature of both magnetic and electrical coupling natures. Based on the universal law and its exponent nature, one can suppose that the conduction process is governed by a small polaron hopping mechanism but significant distortion of TiO6 octahedral. The doping of the A-sites with rare-earth element ions and changes in the concentrations of Fe and Co ions located on the B-sites manifested themselves in saturated magnetic hysteresis loops, indicating competitive interactions between ferroelectric and canted antiferromagnetic spins. The magnetic order in the samples is attributed to pair-wise interactions between adjacent Fe3+–O–Fe3+, Co2+/3+–O–Co3+/2+, and Co2+/3+–O–Fe3+ ions or Dzyaloshinskii–Moriya interactions among magnetic ions in the adjacent sub-lattices. As a result, enhanced magnetoelectric coefficients of 42.4 mV/cm-Oe, 30.3 mV/cm-Oe, and 21.6 mV/cm-Oe for Bi4−xDyxTi2Fe0.7Co0.3O12−δ (DBTFC), Bi4−xLaxTi2Fe0.7Co0.3O12−δ (LBTFC), and Bi4−xSmxTi2Fe0.7Co0.3O12−δ (SBTFC), respectively, have been obtained at lower magnetic fields (<3 kOe). The strong coupling of the Aurivillius compounds observed in this study is beneficial to future multiferroic applications.

Collinear-spin machine learned interatomic potential for Fe7Cr2Ni alloy

We have developed a machine learned interatomic potential for the prototypical austenitic steel Fe7Cr2Ni, using the Gaussian approximation potential (GAP) framework. This GAP can model the alloy's properties with close to density functional theory (DFT) accuracy, while at the same time allowing us to access larger length and time scales than expensive first-principles methods. We also extended the GAP input descriptors to approximate the effects of collinear spins (spin GAP), and demonstrate how this extended model successfully predicts structural distortions due to antiferromagnetic and paramagnetic spin states. We demonstrate the application of the spin GAP model for bulk properties and vacancies and validate against DFT. These results are a step towards modeling the atomistic origins of ageing in austenitic steels with higher accuracy. Published by the American Physical Society 2024