Antiferromagnetic Spin State Research Articles

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

Tunable Spin Textures in a Kagome Antiferromagnetic Semimetal via Symmetry Design.

Kagome antiferromagnetic semimetals such as Mn3Sn have attracted extensive attention for their potential application in antiferromagnetic spintronics. Realizing high manipulation of kagome antiferromagnetic spin states at room temperature can reveal rich emergent phenomena resulting from the quantum interactions between topology, spin, and correlation. Here, we achieved tunable spin textures of Mn3Sn through symmetry design by controlling alternate Mn3Sn and heavy-metal Pt thicknesses. The various topological spin textures were predicted with theoretical simulations, and the skyrmion-induced topological Hall effect, strong spin-dependent scattering, and vertical gradient of spin states were obtained by magnetotransport and magnetic circular dichroism (MCD) spectroscopy measurements in Mn3Sn/Pt heterostructures. Our work provides an effective strategy for the innovative design of topological antiferromagnetic spintronic devices.

Photonic-Crafting of Non-Volatile and Rewritable Antiferromagnetic Spin Textures with Drastic Difference in Electrical Conductivity.

Antiferromagnetic spintronics is an emerging field of non-volatile data storage and information processing. The zero net magnetization and zero stray fields of antiferromagnetic materials eliminate interference between neighbor units, leading to high-density memory integrations. However, this invisible magnetic character at the same time also poses a great challenge in controlling and detecting magnetic states in antiferromagnets. Here, two antiferromagnetic spin states close in energy in strained BiFeO3 thin films at room temperature are discovered. It can be reversibly switched between these two non-volatile antiferromagnetic states by a moderate magnetic field and a non-contact optical approach. Importantly, the conductivity of the areas with each antiferromagnetic textures is drastically different. It is conclusively demonstrated the capability of optical writing and electrical reading of these newly discovered bistable antiferromagnetic states in the BiFeO3 thin films.

Spin current distribution in antiferromagnetic zigzag graphene nanoribbons under transverse electric fields

The spin current transmission properties of narrow zigzag graphene nanoribbons (zGNRs) have been the focus of much computational research, investigating the potential application of zGNRs in spintronic devices. Doping, fuctionalization, edge modification, and external electric fields have been studied as methods for spin current control, and the performance of zGNRs initialized in both ferromagnetic and antiferromagnetic spin states has been modeled. Recent work has shown that precise fabrication of narrow zGNRs is possible, and has addressed long debated questions on their magnetic order and stability. This work has revived interest in the application of antiferromagnetic zGNR configurations in spintronics. A general ab initio analysis of narrow antiferromagnetic zGNR performance under a combination of bias voltage and transverse electric field loading shows that their current transmission characteristics differ sharply from those of their ferromagnetic counterparts. At relatively modest field strengths, both majority and minority spin currents react strongly to the applied field. Analysis of band gaps and current transmission pathways explains the presence of negative differential resistance effects and the development of spatially periodic electron transport structures in these nanoribbons.

Enhanced stability and induced magnetic moments of silicene by substitutional doping of nickel

Effective controlling of charge and magnetic states is critical to the use of magnetic nanostructures in quantum information devices, but still a challenging task. Substitutional doping of transition metal atoms provides an effective route to tune the magnetism of silicene and to enhance the stability of silicene. Here, by using density-functional theory calculations we explore the structure, stability, electronic, and magnetic properties of Ni-doped silicene. We find that magnetic states show a unique dependence on their site registration on the silicene. Specifically, Ni atoms present antiferromagnetic spin states when Ni atoms in the same lattice substitute two silicon atoms. In contrast, Ni atoms exhibit ferromagnetic spin states once located in different sublattices. Given that the pristine silicene is intrinsically nonmagnetic and unstable in air, through the coupling of ferromagnetic elements with silicene may provide an alternative avenue for widening their applications as spintronics.

Large uniaxial magnetostriction with sign inversion at the first order phase transition in the nanolaminated Mn2GaC MAX phase

In 2013, a new class of inherently nanolaminated magnetic materials, the so called magnetic MAX phases, was discovered. Following predictive material stability calculations, the hexagonal Mn2GaC compound was synthesized as hetero-epitaxial films containing Mn as the exclusive M-element. Recent theoretical and experimental studies suggested a high magnetic ordering temperature and non-collinear antiferromagnetic (AFM) spin states as a result of competitive ferromagnetic and antiferromagnetic exchange interactions. In order to assess the potential for practical applications of Mn2GaC, we have studied the temperature-dependent magnetization, and the magnetoresistive, magnetostrictive as well as magnetocaloric properties of the compound. The material exhibits two magnetic phase transitions. The Néel temperature is TN ~ 507 K, at which the system changes from a collinear AFM state to the paramagnetic state. At Tt = 214 K the material undergoes a first order magnetic phase transition from AFM at higher temperature to a non-collinear AFM spin structure. Both states show large uniaxial c-axis magnetostriction of 450 ppm. Remarkably, the magnetostriction changes sign, being compressive (negative) above Tt and tensile (positive) below the Tt. The sign change of the magnetostriction is accompanied by a sign change in the magnetoresistance indicating a coupling among the spin, lattice and electrical transport properties.

Canted magnetic ground state of quarter-doped manganites R0.75Ca0.25MnO3 (R = Y, Tb, Dy, Ho, and Er)

Polycrystalline samples of the quarter-doped manganites R0.75Ca0.25MnO3 (R = Y, Tb, Dy, Ho, and Er) were studied by x-ray diffraction and AC/DC susceptibility measurements. All five samples are orthorhombic and exhibit similar magnetic properties: enhanced ferromagnetism below T1 (∼80 K) and a spin glass (SG) state below TSG (∼30 K). With increasing R3+ ionic size, both T1 and TSG generally increase. The single crystal neutron diffraction results on Tb0.75Ca0.25MnO3 revealed that the SG state is mainly composed of a short-range ordered version of a novel canted (i.e. noncollinear) antiferromagnetic spin state. Furthermore, calculations based on the double exchange model for quarter-doped manganites reveal that this new magnetic phase provides a transition state between the ferromagnetic state and the theoretically predicted spin-orthogonal stripe phase.

Spin transport properties for iron-doped zigzag-graphene nanoribbons interface

By using the first-principles method based on the density-functional theory, the spin transport properties for the systems consisting of iron-doped zigzag-edged graphene nanoribbons (ZGNRs) with iron doping at the interface, where the connection is realized between electrodes and the central scattering region, are investigated theoretically. The ribbon widths of ZGNRs are four zigzag C chains (4 ZGNRs), and the length of scattering region is N unit cells (here, N=4, 6, 8, 10). Results show that -spin current is obviously greater than the -spin current under the ferromagnetic (FM) configuration, which is the spin filtering effect. The reason of spin filtering effect cames from two aspects: a) The symmetry-dependent transport properties which arise from different coupling rules between the up and * subbands around the Fermi level, that are dependent on the wave-function symmetry of the two subbands; b) the distribution of molecular orbit within the bias windows, location, or delocalization. While for antiferromagnetic (AFM) spin state, both and spin currents are very small and both the positive and negative bias regions originate from the existence of band gap; therefore, no obvious spin filtering effect can be obtained. For antiparallel (AP) magnetism configuration, spin filtering effect also can be obtained at high bias. Next, we also investigate the other models: the ribbon width of ZGNRs is five (six) zigzag C chains, namely, 5 ZGNRs (6 ZGNRs), and the scattering region is 6 unit cells length. The currents in 6 ZGNRs are less than that of 5 ZGNRs obviously, and this difference is revealed to arise from different couplings between the conducting subbands around the Fermi level, which is dependent on the symmetry of the systems. However, both of the two models show the similar characteristic: spin filtering effect. The spin current is obviously greater than the -spin current with the whole bias under the ferromagnetic (FM) configuration, The analysis on the electronic structure, transmission spectra, the molecular projected self-consistent Hamiltonian (MPSH) which have been modified by the electrodes, local density (LDOS) and the spin density give an insight into the observed results for the systems. These results indicate that the iron doping at interface between electrodes and central scattering region for ZGNRs can modulate effectively the spin electrons. It is of important significance for developing high spin polarization filtering device based on GNRs.

LOCV CALCULATION OF DENSITY DEPENDENCE OF NUCLEAR SYMMETRY ENERGY AT DIFFERENT SPIN STATES

In this paper, we use the lowest order constrained variational method to study the behavior of nuclear symmetry energy versus density and spin states of neutrons and protons with AV18 potential. The dependency of incompressibility at saturation density on isospin asymmetry, ferromagnetic spin states and antiferromagnetic spin states is also investigated.

Observation of an isosceles triangular electronic structure around the excess iron atoms in Fe1+δTe

We present scanning tunneling microscopy and spectroscopy studies around an individual excess Fe atom, working as a local perturbation, in the parent material of the iron-chalcogenide superconductor Fe${}_{1+\ensuremath{\delta}}$Te. Spectroscopic imaging reveals an isosceles triangular electronic structure around the excess Fe atoms. Its spatial symmetry reflects the underlying bicollinear antiferromagnetic spin state and the structural monoclinic symmetry. These findings provide important clues to understanding the role of the excess Fe atoms, which complicate the understanding of the phenomena occurring in iron-chalcogenide materials.

Theoretical Study of Properties of Goethite (α-FeOOH) at Ambient and High-Pressure Conditions

Magnetic, electronic, structural, vibrational, and mechanical properties of mineral goethite (α-FeOOH) at ambient conditions and their dependence on isostatic volume compression (corresponding pressure range 0–80 GPa) have been studied by means of spin-polarized density functional theory (spDFT), modified spDFT+U method, which includes a Hubbard-type on-site repulsion term U, and hybrid DFT functional MSE06. The antiferromagnetic high spin ground state has been confirmed as the most stable at low-pressure conditions. Calculated Fe magnetic moments differ from experiment by about 0.3–0.4 μB. The antiferromagnetic high spin state is stable over the whole compression range in the spDFT+U and MSE06 calculations. For the spDFT, a collapse from a high spin to low spin state has been induced by compression. In comparison to experiment, the spDFT method strongly underestimates electronic band gap, and the spDFT+U results are in a reasonable agreement (with a difference of 0.1–0.5 eV), while the hybrid MSE06 functional gives an overestimated value. The spDFT+U calculated band gap narrows upon increasing volume compression. Structural parameters calculated by all three methods are in very good agreement with experimental data with an error below 1–1.5%. The analysis of structural parameters has shown that the compression proceeds mainly through the compression of structural channels containing hydrogen bonds. Very narrow splitting in a range of ∼20 cm–1 has been found for the calculated OH stretching modes. The predicted bulk modulus of 114.3 GPa (spDFT+U) is in accord with an experimental value of 111 GPa. The calculated elastic constants document an anisotropic character of the single crystal structure of goethite. In general, the spDFT+U provides reliable results for all studied properties, while the MSE06 overestimates some of them (e.g., band gap).

Resistive transition, anomalous magnetocapacitance, and ferroelectric phenomena observed in charge ordered canted antiferromagnetic La0.5Ba0.5FeO3 perovskite

We report multifunctional properties of the charge disproportionate correlated electronic system La0.5Ba0.5FeO3 synthesized by conventional solid state reaction route. The bulk sample undergoes semiconductor–metal like transition at TSM = 102 K showing appreciable magnetoresistance (∼20%) around TSM even at a low field (∼0.5 T). A crossover of electrical conduction mechanism from Mott’s variable range hopping to small polaron hopping occurs around 210 K, well above TSM. Analysis of the transport, magnetic susceptibility, and magnetization data in the light of de Gennes proposal indicates the formation of canted antiferromagnetic spin order preceded by charge ordering in La0.5Ba0.5FeO3. Nonsaturation behavior and nonlinearity of the M-H curve at low temperature and at high field (∼5 T), confirms the canted spin order. The observed resistive transition is considered to be associated with the onset of weak ferromagnetic ordering in the canted antiferromagnetic spin state of the materials. Moreover, novel ferroelectric like behavior showing electric field-polarization hysteresis loop below the charge ordering temperature along with anomalous magnetocapacitance response representing magnetoelectric coupling is also observed. In the low temperature regime spin lattice coupling might be responsible for exhibiting magnetocapacitance in La0.5Ba0.5FeO3. Such multifunctional character implies fundamental as well as technological importance of this sample.

Electron Spin Resonance Study of Nitroxide Radical Adsorption at Cupric Ions in the Metal−Organic Framework Compound Cu3(btc)2

The Cu(II) pairs in the paddle-wheel building blocks of the metal−organic framework compound Cu3(btc)2 give rise to an antiferromagnetic spin state with an electron spin resonance (ESR)-silent S = 0 ground state. However, the adsorption of di-tert-butyl nitroxide (DTBN) radicals leads to the formation of an unusual nitroxide ESR spectrum and later, upon thermal treatment of the samples, to distinct paramagnetic Cu(II) centers, whose ESR signals can be observed at temperatures below 70 K. Various scenarios for the suppression of the antiferromagnetic coupling of the Cu(II) ions in the paddle-wheel units by interaction with the nitroxide and the subsequent formation of these S = 1/2 copper centers are discussed.

Continuous Wave and Pulsed Electron Spin Resonance Spectroscopy of Paramagnetic Framework Cupric Ions in the Zn(II) Doped Porous Coordination Polymer Cu3−xZnx(btc)2

In the parent metal−organic framework Cu3(btc)2 material the Cu(II) pairs in the paddle wheel building blocks of the framework give rise to an antiferromagnetic spin state with an electron spin resonance (ESR) silent S = 0 ground state. The thermally excited S = 1 state of the Cu(II) pairs can be observed for temperatures above 80 K by ESR spectroscopy but give rise to an exchanged narrowed resonance line preventing the exploration of any structural details in the environment of the paddle wheel units. However, magnetically diluted paramagnetic binuclear Cu−Zn clusters can be formed by substitution of Cu(II) ions by Zn(II) at low doping levels, as already known for zinc-doped copper acetate monohydrate. Indeed, ESR, hyperfine sublevel correlation spectroscopy (HYSCORE) and pulsed electron nuclear double resonance (ENDOR) verify the successful incorporation of zinc ions at cupric ion sites into the framework of the resulting Cu3−xZnx(btc)2 coordination polymer. The formation of such paramagnetic binuclear ...

High-Field Study of Strong Magnetoelectric Coupling in Single-Domain Crystals of BiFeO3

Magnetic and dielectric properties of single-domain crystals of BiFeO 3 were studied in pulsed magnetic fields up to 55 T. At low temperatures, metamagnetic transitions accompanied with sharp changes in electric polarization ( P ) were observed at around 18 T. The angular dependence of the transition field coincides with that of the transition from the cycloidal state to the canted antiferromagnetic spin state studied in the framework of the Landau–Ginzburg theory incorporated with the Lifshitz-like invariant. The parasitic P component caused by the cycloidal spin structure amounts to 210±30 µC/m 2 in terms of the projected component on the pseudocubic principal axis, which can be controlled by applying magnetic fields at least up to 500 K. This result indicates that the microscopic magnetoelectric coupling in BiFeO 3 is not weak: In fact, it is rather strong as compared to that in the canonical multiferroic material TbMnO 3 .

Unusual temperature behavior of the entropy of the antiferromagnetic spin state in nuclear matter with an effective finite range interaction

The unusual temperature behavior of the entropy of the antiferromagnetic (AFM) spin state in symmetric nuclear matter with the Gogny D1S interaction, being larger at low temperatures than the entropy of nonpolarized matter, is related to the dependence of the entropy on the effective masses of nucleons in a spin polarized state. The corresponding conditions for comparing the entropies of the AFM and nonpolarized states in terms of the effective masses are formulated, including the low and high temperature limits. It is shown that the unexpected temperature behavior of the entropy of the AFM spin state at low temperatures is caused by the violation of the corresponding low-temperature criterion.

Magnetoelectric exchange bias systems in spintronics

Magnetoelectric switching of perpendicular exchange bias is observed in a Co∕Pt multilayer attached to single crystalline magnetoelectric antiferromagnetic Cr2O3(111). The exchange bias field HEB can be set to positive or negative values by applying an electric field Efr either parallel or antiparallel to the magnetic freezing field Hfr while cooling to below the Néel temperature. Based on this result, the antiferromagnetic spin state can be used as a medium for data storage. The authors propose magnetic random access memory cells and magnetic logic devices, which are purely voltage controlled.

Intrinsically coupled stripes within the CuO2 planes of the high-Tc materials

Analysis of the electronic state of the CuO2 planes of high-Tc materials has been performed with special regard to the influence of the coulomb interactions separated after moments. Using this analysis to derive the basic structure of the electronic states within the CuO2 planes of the high-Tc materials, different symmetry breaking effects were revealed. First of all, a commensurate charge and bonding fluctuation state (CBF) with the period (2a,2b) is established which exists collinearly with the antiferromagnetic spin state. It is concluded that the CBF state and the antiferromagnetic spin state are results of the same electronic renormalizations. Furthermore, the existence of localized topological hole states under hole doping is established. As a natural consequence of this, the local symmetry is broken. It is proven that a quadrupolar polarization induced attractive hole–hole interaction can exist between such topological hole states. This interaction creates an ordered topological hole structure which leads to a global symmetry breaking. This highly ordered topological hole structure, which will be referred as bonded holes (b-holes), can be characterized as parallel one-dimensional electronic states (stripes) along particular ⋯Cu–O–Cu⋯ bonding directions which are intrinsically coupled to each other. The highly ordered topological b-hole state exists undisturbed for hole concentrations nh in the range of 0.125 holes/copper⩽nh⩽0.25 holes/copper. The total correlation energy per b-hole |ECtot| was deduced to be ⩾160meV. In addition to b-holes, holes which are not intrinsically bonded exist in the concentration range of nh>0.125 holes/copper. These non-bonded holes are termed as free holes (f-holes). The inevitable consequence is an electronic two fluid behaviour (b-holes, f-holes) within the hole concentration range of 0.125 holes/copper<nh⩽0.25 holes/copper. The characteristics of the electronic state deduced here, enable a more complex understanding of various experimental results, and gives in particular insight into the foundations of the 1/8 problem and the creation of checkerboard structures.

Antiferromagnetism of nuclear matter in the model with effective Gogny interaction

The possibility of ferromagnetic (FM) and antiferromagnetic (AFM) phase transitions in symmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with effective Gogny interaction. It is shown that, at some critical density, nuclear matter with the D1S effective force undergoes a phase transition to the AFM spin state (opposite directions of neutron and proton spins). The self-consistent equations of spin-polarized nuclear matter with the D1S force have no solutions corresponding to FM spin ordering (the same direction of neutron and proton spins) and, hence, the FM transition does not appear. The AFM spin polarization parameter is found for zero and finite temperature. It is shown that the AFM spin polarization parameter of partially polarized nuclear matter at low enough temperatures increases with temperature. The entropy of the AFM spin state for some temperature range is larger than the entropy of the normal state. Nevertheless, the free energy of the AFM spin state is always less than the free energy of the normal state, and the AFM spin-polarized state is preferable for all temperatures below the critical temperature.

Finite temperature effects in antiferromagnetism of nuclear matter

The influence of the finite temperature on the antiferromagnetic (AFM) spin ordering in symmetric nuclear matter with the effective Gogny interaction is studied within the framework of a Fermi liquid formalism. It is shown that the AFM spin polarization parameter of partially polarized nuclear matter for low enough temperatures increases with temperature. The entropy of the AFM spin state for some temperature range is larger than the entropy of the normal state. Nerveless, the free energy of the AFM spin state is always less than the free energy of the normal state and, hence, the AFM spin polarized state is preferable for all temperatures below the critical temperature.