Ground-state Magnetic Configuration Research Articles

Janus MXene nanosheets with a strain-induced reversible magnetic state transition for storing information without electricity.

The application of strain induces a transition in the ground-state magnetic configuration of Janus TiVC MXene from A-AFM to FM. A new system and method of solid-state disk information storage without electricity is developed based on the as-discovered reversible magnetic state transition in TiVC, which can achieve efficient storage of information in extremely harsh conditions.

Stacking and layer dependence of magnetic properties in Ti2C and Fe2C

Magnetic MXenes are becoming an important family of materials for exploring 2D magnetism. However, investigations into the interdependence of layer thickness, stacking patterns and magnetism in these materials, from a microscopic point of view, are still lacking. In this work, we have used density functional theory based calculations to understand the effects of layer thickness and stacking on the magnetic properties in two magnetic MXenes, Ti2C and Fe2C, in their monolayer and bilayer forms. The ground-state magnetic structures, magnetic moments, magnetic exchange interactions, magnetic transition temperatures and magnetic anisotropy energies are calculated and analysed using their electronic structures and standardised models. We find that in both systems an increase in layer thickness (monolayer to bilayer) affects the ground-state magnetic configuration, which is driven by the changes in the magnetic exchange interactions. While the effects of the stacking pattern are rather weak in Ti2C, they are substantial, both qualitatively and quantitatively, in Fe2C. The computed results are analysed from their electronic structures. The results suggest that fascinating physical effects can be obtained in Fe2C by tuning the layer thickness and stacking patterns, making it more suitable for device applications.

Contrasting magnetic properties of polymorphic TbPt3

In this work, we report the contrasting magnetic behaviour of a bimorphic intermetallic compound TbPt 3 that undergoes a structural phase transformation from cubic AuBe 5 -type structure to the cubic AuCu 3 -type structure on annealing at high temperature for a very short time. The magnetic ground state configuration of TbPt 3 changes from ferromagnetic (FM) in AuBe 5 -type crystalline phase to antiferromagnetic (AFM) in AuCu 3 -type phase. However, the AFM ground state of the AuCu 3 -type compound is unstable, as a moderate magnetic field can induce the FM character through metamagnetic transformation. To understand the said crystallographic as well as magnetic transformations, structural and magnetic ab-initio calculations have been carried out for both phases. From theoretical calculations we have proposed that although the AuBe 5 -type structure of TbPt 3 irreversibly transforms to the AuCu 3 -type TbPt 3 by a very short heat treatment, the application of moderate magnetic field on AuCu 3 -type structure may result in recovering the magnetic properties of AuBe 5 -type structure of TbPt 3 . As the two different polymorphs exist simultaneously at ambient condition, TbPt 3 thus provides us a great opportunity to study directly the effect of crystal structure on its physical properties and vice versa. • TbPt 3 compound shows polymorphic structural phase transformation on annealing at high temperature. • Ground state magnetic configurations of TbPt 3 changes from FM to AFM under structural transformation. • Antiferromagnetic TbPt 3 compound exhibits field induced metamagnetic transition to ferromagnetism.

Magnonic control of the superconducting spin valve by magnetization reorientation in a helimagnet

We propose a method to control a bilayer superconducting spin valve (SSV) which does not perturb its superconducting state and is suitable for energy saving cryogenic electronics. This SSV consists of a superconducting layer and a helimagnetic layer of B20 family compounds, namely, Nb and spiral antiferromagnet MnSi. Thanks to unique properties of MnSi—broken inversion symmetry and cubic crystal lattice—there are a few ground state magnetic configurations with different directions of the magnetic spiral, divided by a potential barrier. Superconductivity in such a bilayer is controlled by the reorientation of the spiral vector in the MnSi layer, which leads to a change in the critical temperature of the Nb layer due to the proximity effect. The switching is proposed to be carried out by a several hundred ps in duration magnetic field pulse of several kOe in magnitude. Such a pulse does not destroy the superconducting state of the Nb layer by itself but leads to the excitation of magnons in the MnSi layer, which triggers the reorientation process of the magnetic spiral. After the completion of this process, the Nb layer switches into a normal state. Inverse switching returns the spiral to the initial state, opening the valve and turning on the superconducting state. The system can be switched there and back by a magnetic field of opposite signs along one direction in the layers plane, which allows an easy control. The switching time is estimated as several nanoseconds, which coincides with the scales of the STT-MRAM recording time.

Yu-Shiba-Rusinov states and ordering of magnetic impurities near the boundary of a superconducting nanowire

We theoretically study the spectrum induced by one and two magnetic impurities near the boundary of a one-dimensional nanowire in proximity to a conventional $s$-wave superconductor and extract the ground state magnetic configuration. We show that the energies of the subgap states, supported by the magnetic impurities, are strongly affected by the boundary for distances less than the superconducting coherence length. In particular, when the impurity is moved towards the boundary, multiple quantum phase transitions periodically occur in which the parity of the superconducting condensate oscillates between even and odd. We find that the magnetic ground state configuration of two magnetic impurities depends not only on the distance between them but also explicitly on their distance away from the boundary of the nanowire. As a consequence, the magnetic ground state can switch from ferromagnetic to antiferromagnetic while keeping the inter-impurity distance unaltered by simultaneously moving both impurities away from the boundary. The ground state magnetic configuration of two impurities is found analytically in the weak coupling regime and exactly for an arbitrary impurity coupling strength using numerical tight-binding simulations.

Helical structures in layered magnetic superconductors due to indirect exchange interactions mediated by interlayer tunneling

Motivated by the recent discovery of helical magnetic structure in RbEuFe$_{4}$As$_{4}$, we investigate interlayer ordering of magnetic moments in materials composed of spatially-separated superconducting and ferromagnetically-aligned layers. We consider the interplay between the normal and superconducting indirect exchange interaction mediated by tunneling between the conducting layers. We elaborate a recipe to evaluate the normal interlayer interaction via two-dimensional density of states of an isolated layer and demonstrate that for bands with small fillings, such interaction is typically ferromagnetic and short-range. The nearest-layer interaction is proportional to the ratio of the interlayer hopping and in-plane band width squared. On the other hand, the superconducting contribution always gives antiferromagnetic interaction and may extend over several layers when the interlayer hopping energy exceeds the superconducting gap. The frustration caused by the interplay between the normal and superconducting parts may lead to spiral ground-state magnetic configuration. The four-fold in-plane anisotropy may lock the rotation angle between the moments in the neighboring layers to $90^\circ$, as it was observed in RbEuFe$_{4}$As$_{4}$

Ab-initio study of electronic and magnetic properties of CoIrMnZ (Z = Al, Si, Ga, Ge) Heusler alloys

Using first principles calculations based on density functional theory, we study electronic and magnetic properties of CoIrMnZ (Z = Al, Si, Ga, Ge) Heusler systems. Based on the electronic structure calculations, CoIrMnSi and CoIrMnGe are predicted to be half-metallic. For the studied systems, the ground state magnetic configurations are found to be ferromagnetic with magnetic moment consistent with the Slater-Pauling rule. The calculated ferromagnetic transition temperatures are much higher than the room temperature for all the cases. The effect of spin orbit coupling has been discussed on the electronic structure for all the systems studied here. Furthermore, we construct junctions with MgO along (001) direction. The MnZ terminations are found to be energetically more favourable compared to the CoIr terminations. After discussing the tunnelling conductance, we find that CoIrMnSi/MgO/CoIrMnSi could be a promising magnetic tunnelling junction (MTJ) candidate.

Magnetic and structural properties of Ni-substituted magnetoelectric Co4Nb2O9

The magnetic and structural properties of polycrystalline Co$_{4-x}$ Ni$_x$ Nb$_2$ O$_9$ (x=1,2) have been investigated by neutron powder diffraction, magnetization and heat capacity measurements, and density functional theory (DFT) calculations. For x=1, the compound crystallizes in the trigonal P$\bar{3}$c1 space group. Below T$_N$ = 31 K it develops a weakly non-collinear antiferromagnetig structure with magnetic moments in the ab-plane. The compound with x=2 has crystal structure of the orthorhombic Pbcn space group and shows a hard ferrimagnetic behavior below T$_C$ =47 K. For this compound a weakly non-collinear ferrimagnetic structure with two possible configurations in ab plane was derived from ND study. By calculating magnetic anisotropy energy via DFT, the ground-state magnetic configuration was determined for this compound. The heat capacity study in magnetic fields up to 140 kOe provide further information on the magnetic structure of the compounds.

Non-collinear spin DFT study of the ground state magnetic structure, optical and electronic properties of the hexagonal LuFeO3 multiferroic

Hexagonal LuFeO3 compound is a multiferroic material whose ground state magnetic structure, band gap, and optical absorption spectra are matters of debate in the literature. In the present paper, these properties have been investigated by the employment of the calculations based on non-collinear spin density functional theory, with the exchange and correlation effects treated by local spin density approximation (LSDA) including effective Hubbard (Ueff) correction (LSDA + Ueff). It was concluded that the results with Ueff = 4.0 eV correlate to the experimental facts in the best manner. The ground state magnetic configuration is found to be Γ1 + Γ2, characterized by weak ferromagnetism along the c-axis (0.07 μB/Fe-atom). The calculated band gap is found to be 1.03 eV, supporting those experimental works which conclude that the gap value should be ∼1.1 eV. The optical absorption spectrum is computed and found to be in reasonable agreement with experimental data. Its edge (up to 5 eV) is generated by electronic transitions between the occupied and unoccupied Fe 3d- and the O 2p-states, with energy transfer occurring within the basal planes of the hexagonal crystal structure. The controversial lowest energy peak (centered between 1 and 2 eV) is generated by absorption of the light polarized perpendicularly to the hexagonal c-axis.

Half-metallicity in zigzag phosphorene nanoribbons with magnetic edges

Phosphorene nanoribbons with magnetic edges are potentially attractive for nanoelectronic and spintronic applications. Here, the attention is focused on electronic and magnetic properties of these materials. Using a self-consistent computational method it is shown that, in general, two types of configuration with spontaneous edge magnetization can be responsible for lifting the spin degeneracy, and consequently for the appearance of the half-metallic behavior. Namely, in addition to the well-known configuration with edge states ferromagnetically coupled along each edge and aligned in parallel between the two edges, there is another one (much less explored) with edge states ferromagnetically aligned on only one edge. The ground state magnetic configurations strongly depend on the chemical potential position (electron filling factor), and can be controlled by means of the external perpendicular gate voltage, and by changing the nanoribbon width. It is also shown that the magnetic edge configurations responsible for the half-metallicity can be stabilized with ferromagnetic substrates, due to the proximity effect.

Topological Magnon Bands and Unconventional Superconductivity in Pyrochlore Iridate Thin Films.

We theoretically study the magnetic properties of pyrochlore iridate bilayer and trilayer thin films grown along the [111] direction using a strong coupling approach. We find the ground state magnetic configurations on a mean field level and carry out a spin-wave analysis about them. In the trilayer case the ground state is found to be the all-in-all-out (AIAO) state, whereas the bilayer has a deformed AIAO state. For all parameters of the spin-orbit coupled Hamiltonian we study, the lowest magnon band in the trilayer case has a nonzero Chern number. In the bilayer case we also find a parameter range with nonzero Chern numbers. We calculate the magnon Hall response for both geometries, finding a striking sign change as a function of temperature. Using a slave-boson mean-field theory we study the doping of the trilayer system and discover an unconventional time-reversal symmetry broken d+id superconducting state. Our study complements prior work in the weak coupling limit and suggests that the [111] grown thin film pyrochlore iridates are a promising candidate for topological properties and unconventional orders.

Cation Substitution Effect on a Molecular Analogue of Perovskite Manganites

A recently synthesized Mn-containing molecule Ce2IIICeIVMn8IIIO8(O2CPh)18(HO2CPh) (Ce3Mn8III) structurally resembles the repeating unit of a bulk perovskite manganite. This resemblance brings forth the intriguing possibility of studying physical properties and effects that appear in both the molecule and the far more complex bulk perovskites, for example, effects of cation substitution, which in bulk manganites may lead to phase transition from metallic-ferromagnetic to insulating-paramagnetic as well as the colossal magnetoresistance effect. We investigate divalent and trivalent cation substitution in Ce3Mn8III molecules and its effects on ground-state magnetic configuration using first-principles-based approaches. One MnIII ion changes its valence to MnIV after trivalent (including La, Gd) cation substitution, while four MnIII ions become MnIV upon divalent (including Ca, Sr, Ba, and Pb) cation substitutions, all accompanied by vanishing local Jahn–Teller distortion around MnIV. The valence state of MnIV induced by cation substitutions can hop among Mn sites in molecule, and the calculated energy barrier for such state hopping is 0.25 to 0.6 eV/molecule. In addition, the charging energies of the Ce3Mn8III molecule and its derivatives are found to be strongly dependent on the spin direction of added electron.

Electronic structures and crystal field splitting of antiperovskite XNMn3 (X = 3d and 4d elements)

Antiperovskites have diverse novel physical properties and potential applications in fields of superconducting techniques, magneto-electronic devices, etc. Nevertheless, their fundamental electronic structures, such as ground state magnetic configurations and crystal field splitting, are still not yet fully understood. We thus utilize first principles calculations combining on-site Coulomb interaction of the Mn-3d orbitals to investigate the electronic and magnetic properties of the XNMn3 (X=Ni, Cu, Zn, Ga, As, Rh, Pd, Ag, In, Sn, Sb). Our results show that the Mn atoms prefer to align antiferromagnetic configuration in the ground state. Two nonequivalent Mn atoms (NEMAS) are identified in Mn6N octahedral unit. The Mn-3d orbitals with longer Mn-N bond length in the octahedron split into one doublet and three singlets, and the 3d orbitals of other Mn atoms with shorter Mn-N bond length split in five singlets, i.e. the degeneracy is completely relieved. These findings certainly offer an insight into clarifying the magnetic and electronic properties of these antipervoskites.

Tellurium-bridged two-leg spin ladder inBa2CuTeO6

We present single-crystal growth and magnetic property studies of tellurium-bridged copper spin-$\frac{1}{2}$ system ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$. The spin-exchange interaction among copper spins via $\mathrm{Cu}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Te}\text{\ensuremath{-}}\mathrm{O}\text{\ensuremath{-}}\mathrm{Cu}$ super-superexchange route leads to a novel two-leg spin ladder system. Spin susceptibility $\ensuremath{\chi}(T)$ data indicate that the triclinic ${\mathrm{Ba}}_{2}{\mathrm{CuTeO}}_{6}$ undergoes a stepwise crossover for exchange couplings revealed by a broad maximum of $\ensuremath{\chi}(T)$ near ${T}_{max}\ensuremath{\sim}75$ K and an anisotropic cusp in $\frac{d\ensuremath{\chi}}{dT}(T)$ at ${T}_{N}\ensuremath{\sim}15$ K to signify a three-dimensional (3D) antiferromagnetic long-range ordering (LRO). The 3D LRO has been suggested from the anisotropic behavior of $\ensuremath{\chi}(T)$ with strong $c$-axis spin anisotropy and the signature of spin-flop transition from the isothermal magnetization below ${\mathit{T}}_{N}$. Analysis of magnetic heat capacity (${C}_{m}$) at ${T}_{N}\ensuremath{\sim}15$ K indicates that most of the spin entropy $(\ensuremath{\sim}92%)$ has already been released above ${T}_{N}$, which supports the picture of consecutive spin entropy reduction upon cooling with Te-bridged two-leg spin ladder system with strong intraladder and interladder couplings. Theoretical $\mathrm{DFT}+U$ calculations have been performed to search for the ground-state magnetic configuration and also to evaluate exchange-coupling constants that support the magnetic model deduced from the combined spin susceptibility and crystal structure symmetry analysis.

Classical and quantum aspects of spin interaction in3dchains on aCu3N-Cu(110) molecular network

We present a study of the magnetic states and exchange coupling in transition-metal Mn, Fe, and Co atomic chains deposited on a self-corrugated $\mathrm{C}{\mathrm{u}}_{3}\mathrm{N}$-Cu(110) molecular network by means of first-principles calculations based on the density functional theory. The various adsorption sites on a bumping area of a self-corrugated $\mathrm{C}{\mathrm{u}}_{3}\mathrm{N}$ layer are investigated where the atomic chains are formed at the initial stage of nanowire growth. We demonstrate, by calculating the ground-state magnetic configurations, that the exchange coupling, magnetic order, and anisotropies in atomic chains depend sensitively on their chemical composition and adsorption sites on the $\mathrm{C}{\mathrm{u}}_{3}\mathrm{N}$ network. We find that the exchange interactions in atomic chains could lead to ferromagnetic or antiferromagnetic coupling of atomic spins depending on the position of the chain on the surface. The classical spin dynamics is investigated by means of the kinetic Monte Carlo method based on transition-state theory. Moreover we evaluate the Heisenberg--Dirac--Van Vleck quantum spin Hamiltonian for calculations of the magnetic susceptibility, in order to demonstrate the existence of quantum entanglement in the antiferromagnetic atomic chains at low temperatures.

Zigzag nanoribbons of two-dimensional silicene-like crystals: magnetic, topological and thermoelectric properties

The effects of electron–electron and spin–orbit interactions on the ground-state magnetic configuration and on the corresponding thermoelectric and spin thermoelectric properties in zigzag nanoribbons of two-dimensional hexagonal crystals are analysed theoretically. The thermoelectric properties of quasi-stable magnetic states are also considered. Of particular interest is the influence of Coulomb and spin–orbit interactions on the topological edge states and on the transition between the topological insulator and conventional gap insulator states. It is shown that the interplay of both interactions also has a significant impact on the transport and thermoelectric characteristics of the nanoribbons. The spin–orbit interaction also determines the in-plane magnetic easy axis. The thermoelectric properties of nanoribbons with in-plane magnetic moments are compared to those of nanoribbons with edge magnetic moments oriented perpendicularly to their plane. Nanoribbons with ferromagnetic alignment of the edge moments are shown to reveal spin thermoelectricity in addition to the conventional one.

Multiferroicity in doped hexagonalLuFeO3

Neutron scattering techniques are used to investigate the crystalline and magnetic structure of LuFe$_{0.75}$Mn$_{0.25}$O$_{3}$ in bulk polycrystalline form. We find that the crystalline structure is described by the hexagonal P6$_{3}$cm space group similar to that of thin-film LuFeO$_{3}$, and that the system orders antiferromagnetically below T$_{N}$ = 134 K. Inelastic neutron scattering reveals nearest-neighbor superexchange parameters that are enhanced relative to LuMnO$_{3}$. The observation of significant diffuse scattering above T$_{N}$ demonstrates the frustrated nature of the system; comparisons with similar materials suggest the ground state magnetic configuration is sensitive to local crystallographic distortions.

Magnetism driven by non-metal interstitials from first-principles prediction: The case of hydrogen- and fluorine-doped calcium monoxide with rock-salt structure

Our first-principles calculations based on density functional theory confirmed the formation of sp-ferromagnetic states of calcium monoxide with interstitial nonmagnetic F or H atoms. The hydrogen and fluorine interstitials in the oxides were found to be spin polarized and are more stable in the antiferromagnetic state and the ferromagnetic state, respectively. For H-doped CaO, no considerable charge transfer takes place and the spin remains localized on the impurity. For F-doped oxide, the observation may be attributed to the p–p interaction and the charge transfer between the interstitial atom and the neighboring O atoms. We demonstrate that H-doped compound is a potential n-type antiferromagnet, while F-doped material is a potential p-type ferromagnet. The different dopants would induce different magnetic couplings, thus show different ground-state magnetic configurations. The mechanism for the magnetism should be useful for understanding d0 magnetic semiconductors or insulators. The present potential d0 diluted magnetic materials, at least some of them, may be useful in spintronics.

First-Principles Study of Pressure-Induced Structural and Magnetic Phase Transitions of Binary Ferromagnets: MnSn and MnSb

First-principles calculations within density functional theory have been performed to study the structural and magnetic phase transitions of two 3d compounds (MnSn and MnSb) under pressure. We consider the rocksalt (B1), cesium chloride (B2), zinc-blende (B3), nickel arsenide (B81) modifications of MnSn and MnSb. Both spin-polarized and nonspin-polarized calculations are carried out for ferromagnetic (FM) and paramagnetic (PM) states, which can be used to distinguish the ground-state magnetic configuration with increasing pressure. The crystal structure preferences and the possible phase transitions among them have been studied. We report FM B81⟶14GPaFM B2⟶145GPaPM B2 phase transition for MnSn, FM B81⟶23GPaPM B2 for MnSb. Phonon band dispersions, density of phonon states, and elastic stiffness constants are given and used to analyze the lattice dynamical and mechanical stability for the pressure-induced phase. Electronic band structures and density of states at different pressures are given to discuss the detailed electronic and magnetic properties.

Atomic H interaction with the γ‐U (100) surface

Theoretical analysis of the adsorption of a H atom on the (100) surface of bcc γ‐U represented by a 5 layer slab indicates an exothermic reaction with the bridge site as the preferred site with a chemisorption energy of 3.80 eV at the relativistic level of theory with spin–orbit coupling (SOC) included. The ground state magnetic configuration of the UH system is found to be nonmagnetic, in agreement with experimental results for UH3. The change in work function of the slab with the addition of the H atom is at a maximum when the adatom is bound to the top site and is 0.61 eV. H bound to the lattice in interstitial regions was also found to be exothermic in nature with the interstitial bridge site being the most stable at 2.13 eV at the nonmagnetic state with SOC included. Two potential wells were found under the surface. The first with a barrier of 0.41 eV and the second with a barrier of 0.71 eV indicating preferred H bonding with the γ‐U surface and an aversion to further diffusion into the bulk.