Effective Exchange Interaction Research Articles

Symmetry in magnetic and vibrational spectra of multi-element spinel ferrite

We fabricated by solid-state ceramic technique single-phase mixed spinel ZnxMg0.3−xCu0.7Al0.3Fe1.7O4 with x index from 0.05 to 0.25, step 0.05 for the first time. The goal was to study the effect of cation doping on internal structure and magnetic properties of the material. The ferrites werefirst characterized by X-ray diffraction to confirm their purity. Magnesium and Zinc cations showed tetrahedral site preference, while Aluminum cations preferred the octahedral site. The effective magnetic moment and exchange interaction increased with the Zinc content. The pure single phase structures allowed us to investigate the vibrational spectra based on the symmetry conditions. We found only two infrared bands, while based on symmetry calculation of the factor group, four bands are predicted. The number of atoms and their relative arrangement in the crystal are the main criteria to define the active vibrational bands as predicted by factor group theory and the magnetic response of the material.

Ground States Phase Diagrams and Magnetizations Properties of Ferrimagnetic Model on Decorated Hexagonal Nanolattice: Monte Carlo Study

The ground states phase diagrams and magnetizations properties of ferrimagnetic model on decorated hexagonal nanolattice have been studied via Monte Carlo simulations. The shape of the phase diagrams depends of the values of the magnetic parameters given in the Hamiltonian. The phase diagrams show some key features: coexistence between regions, points where two, three, four and five and states can coexist. This richness of the ground state phase diagrams can serve as a guide for exploring interesting regions of the model’s finite temperature phase diagrams. The transition temperatures have been deduced for different values of exchange interactions between the atoms of spins moments(S). The effect of crystal field and exchange interactions on magnetization has been investigated. The magnetic coercive field and remanent magnetization are obtained for different parameters such as: exchange couplings and temperatures values. The superparamagnetic behavior has been found at the transition temperature.

Effect of exchange and dipolar interlayer interactions on the magnonic band structure of dense Fe/Cu/Py nanowires with symmetric and asymmetric layer widths

We present a systematic experimental and theoretical investigation of the magnonic band structure in dense arrays of both asymmetric and symmetric cross-section trilayered Fe(10 nm)/Cu( $t$)/Py(10 nm) nanowires (NWs). The Cu spacer thickness ($t$) is varied in the range between 0 and 10 nm. The frequency dispersion of collective spin-wave excitations in the above trilayered NW arrays has been studied by the Brillouin light-scattering technique while sweeping the wave vector perpendicularly to the nanowire length over four Brillouin zones of the reciprocal space. The experimental results have been quantitatively reproduced by an original numerical model that includes a two-dimensional Green's function description of the dipole field of the dynamic magnetization and exchange coupling between the layers. We found that, depending on $t$, the Py and Fe magnetic layers within the same nanowire are coupled by either the interlayer exchange or dipolar interactions. This has an impact on both the magnetization reversal and the collective dynamical properties of the artificial crystal. In particular, it is possible to stabilize a magnetization configuration where the layer magnetization vectors point in the same or in the opposite direction over a field range that varies with the Cu thickness. In addition, several modes are detected whose propagation properties (i.e., stationary or dispersive) through the array depend on static magnetization configuration as well as on the relative phase (in-phase or out-of-phase) of dynamic magnetizations between the two layers within the same nanowire.

Experimental observation of magnetic dimers in dilutedYb:YAlO3

We present a comprehensive experimental investigation of Yb magnetic dimers in Yb$_{0.04}$Y$_{0.96}$AlO$_3$, an Yb-doped Yttrium Aluminum Perovskite (YAP) YAlO$_3$ by means of specific heat, magnetization and high-resolution inelastic neutron scattering (INS) measurements. In our sample, the Yb ions are randomly distributed over the lattice and $\sim 7$\% of Yb ions form quantum dimers due to nearest-neighbor antiferromagnetic coupling along the $c$-axis. At zero field, the dimer formation manifests itself in an appearance of an inelastic peak at $\Delta \approx 0.2$~meV in the INS spectrum and a Schottky-like anomaly in the specific heat. The structure factor of the INS peak exhibits a cosine modulation along the $L$ direction, in agreement with the $c$-axis nearest-neighbor intra-dimer coupling. A careful fitting of the low-temperature specific heat shows that the excited state is a degenerate triplet, which indicates a surprisingly small anisotropy of the effective Yb-Yb exchange interaction despite the low crystal symmetry and anisotropic magnetic dipole contribution, in agreement with previous reports for the Yb parent compound, YbAlO$_3$ [arXiv:1904.11513, arXiv:1902.04112], and in contrast to Yb$_2$Pt$_2$Pb [arXiv:1606.01309, arXiv:1907.01067]. The obtained results are precisely reproduced by analytical calculations for the Yb dimers.

XANES investigation of novel lanthanide-doped CuCr0.99Ln0.01S2 (Ln = La, Ce) solid solutions

The study of the valence state, local environment structure and magnetic properties of novel type CuCr0.99Ln0.01S2 (Ln = La, Ce) solid solutions was carried out using X-ray absorption spectroscopy, finite difference method simulations and static magnetic susceptibility measurements. The good agreement between experimental and calculated data indicates that cationic substitution does not lead to significant changes in the copper, chromium and sulfur local environment character and electronic density distribution. The copper atoms were found to be in Cu+ oxidation state, the sulfur atoms—in S2− oxidation state and the chromium atoms—in Cr3+ state. The cationic substitution of chromium by lanthanum and cerium in CuCrS2 does not significantly affect the effective magnetic moment and exchange interactions character. The lanthanum-doped CuCr0.99Ln0.01S2 solid solution demonstrates the Seebeck coefficient value 4 times greater than for CuCrS2-matrix at 500 K.

Magnetoelastic interaction in the two-dimensional magnetic material MnPS3 studied by first principles calculations and Raman experiments

We report experimental and theoretical studies on the magnetoelastic interactions in MnPS3. Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm−1, when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can be explained by the symmetry of their corresponding normal coordinates which involve the virtual modification of the super-exchange angles associated with the leading antiferromagnetic (AFM) interactions.

Clarification of the Relationship between the Magnetic and Conductive Properties of Infinite Chains in Trioxotriangulene Radical Crystals by Spin‐Projected DFT/Plane‐Wave Calculations

AbstractApproximate spin projection method and density functional theory with plane‐wave‐based calculations are combined to uncover and clarify the relationship between the magnetism and electronic conductivity of trioxotriangulene (TOT) radical crystals. The presented scheme exhibits an exceptionally high accuracy. It is clearly different from other techniques that have been conventionally used to calculate the magnitude of the effective exchange interaction (J) and the band structure of organic radicals. This enhanced performance is explained by these facts: i) the proposed scheme allows one to estimate band structures with electronic correlation between whole molecules in the crystal, ii) no crystal cutting‐induced artifacts and errors are introduced, and iii) both spin polarization and spin contamination are treated correctly. Consequently, the discrepancy between the experimental J value and the estimated value by the scheme equals 11.1 cm−1. The scheme can be applied to the DFT‐D method, which is mostly considered for dispersion force. The collaboration allows discussions on the relationship between the magnetic and electronic properties, and the geometric structures. Moreover, it gives useful insights for the design of new functional organic materials. For example, the results of Br3‐TOT revealed by the approach better explains the previously reported but nonclarified properties of these species.

Effect of a Soft Magnetic Phase on the Processes of Magnetization Reversal of a Hard/Soft Magnetic Bilayer

Magnetization reversal of a hard/soft magnetic bilayer in an external magnetic field has been studied by the Monte Carlo method. The magnetization reversal curves of a magnetic bilayer have been built at various thicknesses of the soft magnetic layer. The effects of the intralayer exchange interaction and the anisotropy constant of the soft magnetic layer on the magnetization reversal have been studied, too. The phase diagrams of the magnetic bilayer are presented.

Majorana-Kondo interplay in T-shaped double quantum dots

The transport behavior of a double quantum dot side-attached to a topological superconducting wire hosting Majorana zero-energy modes is studied theoretically in the strong correlation regime. It is shown that Majorana modes can leak to the whole nanostructure, giving rise to a subtle interplay between the two-stage Kondo screening and the half-fermionic nature of Majorana quasiparticles. In particular, the coupling to the topological wire is found to reduce the effective exchange interaction between the two quantum dots in the absence of normal leads. Interestingly, it also results in an enhancement of the second-stage Kondo temperature when the normal leads are attached. Moreover, it is shown that the second stage of the Kondo effect can become significantly modified in one of the spin channels due to the interference with the Majorana zero-energy mode, yielding the low-temperature conductance equal to $G=G_0/4$, where $G_0 = 2e^2/h$, instead of $G=0$ in the absence of the topological superconducting wire. We also identify a nontrivial spin-charge ${\rm SU}(2)$ symmetry present in the system at a particular point in parameter space, despite lack of the spin nor charge conservation. Finally, we discuss the consequences of a finite overlap between two Majorana modes, as relevant for short Majorana wires.

Surface plasma wave in spin-polarized semiconductor quantum plasma

AbstractThe possibilities of surface plasma wave (SPW) on a metal-vacuum interface in semiconductor quantum plasma by considering the effects of Coulomb exchange (CE) interaction and the spin-polarization has been explored. The dispersion for the SPW has been setup using the modified quantum hydrodynamic (QHD) model taking into account the Fermi pressure, the quantum Bohm force, the CE, and the electron spin. The optical gain of SPW has been evaluated. It is found that CE effects and spin-polarization increases the wave frequency and enhances the gain during the stimulated emission.

Large anisotropic magnetocaloric effect in ferromagnetic semimetal PrAlSi

We report on the large anisotropic magnetocaloric effect of PrAlSi, a ferromagnetic semimetal of current interest with a probable topologically nontrivial electronic structure. The maximum magnetic entropy change amounts to −ΔSM=22.6J/kgK near the Curie temperature TC=17.8K under a magnetic field change of μ0ΔH=5T along the magnetic easy c axis. A highly relevant feature is the small crystalline electric field splitting of the Pr3+ (J=4) multiplet, which appears to be comparable with the effective exchange interaction in the magnetic ordering. This leads to the full saturation moment of the Pr3+ ion in small fields and an accumulation of magnetic entropy in the vicinity of the magnetic ordering. The weakly first-order nature of the ferromagnetic transition and the subsequent reentrant spin-glass transitions below TC are features further enhancing the magnetocaloric effect. Given the strong magnetic anisotropy, a large rotating magnetocaloric effect becomes achievable in this material.

Enhanced Ferromagnetism and Tunable Magnetism in Fe3GeTe2 Monolayer by Strain Engineering.

Recent discovery of intrinsic ferromagnetism in Fe3GeTe2 (FGT) monolayer [Deng, Y.; Nature 2018, 563, 94-99; Fei, Z.; Nat. Mater. 2018, 17, 778-782] not only extended the family of two-dimensional (2D) magnetic materials but also stimulated further interest in the possibility to tune their magnetic properties without changing the chemical composition or introducing defects. By means of density functional theory computations, we explore strain effects on the magnetic properties of the FGT monolayer. We demonstrate that the ferromagnetism can be largely enhanced by the tensile strain in the FGT monolayer due to the competitive effects of direct exchange and superexchange interaction. The average magnetic moments of Fe atoms increase monotonically with an increase in biaxial strain from -5 to 5% in FGT monolayer. The intriguing variation of magnetic moments with strain in the FGT monolayer is related to the charge transfer induced by the changes in the bond lengths. Given the successful fabrication of the FGT monolayer, the strain-tunable ferromagnetism in the FGT monolayer can stimulate the experimental effort in this field. This work also suggests an effective route to control the magnetic properties of the FGT monolayer. The pronounced magnetic response toward the biaxial strain can be used to design the magnetomechanical coupling spintronics devices based on FGT.

Magnetic hedgehog lattices in noncentrosymmetric metals

The magnetic hedgehog lattice (HL) is a noncoplanar magnetic texture with a periodic array of magnetic monopoles and anti-monopoles. Despite phenomenological and numerical studies thus far, there remain open issues on the microscopic origin, especially with respect to the recent experimental findings of two different types of HLs even at zero magnetic field. Here, we study the stability of the HLs for an effective spin model with long-range interactions arising from itinerant nature of electrons. By variational calculations and simulated annealing, we find that the HLs are stabilized in the ground state at zero magnetic field by the synergetic effect of the anti-symmetric exchange interactions generated by the spin-orbit coupling and the multiple-spin interactions generated by the spin-charge coupling. We also clarify the phase diagram in the magnetic fields, which includes topological phase transitions with pair annihilation of the monopoles and anti-monopoles depending on the field directions.

Dipole-dipole and exchange interaction effects on the magnetization relaxation of two macrospins: Compared

The magnetization response including dipole–dipole interactions of a pair of macrospins following the sudden alteration of a dc magnetic field is calculated from the stochastic Gilbert-Landau-Lifshitz equation by reducing the overall task to an infinite hierarchy of differential-recurrence relations in the time domain for the statistical moments (averaged products of spherical harmonics in this case), which is exactly solved in the frequency domain by matrix continued fractions. The greatest relaxation time and dynamic susceptibility are then compared with the corresponding results for two exchange-coupled spins using the same exact method. For a given interaction parameter dipole–dipole coupling in the circularly symmetric configuration considered has a much more pronounced effect on the magnetization dynamics than exchange coupling principally manifested by a greater increase in the relaxation time.

Magnetocaloric effect in the Nd2TIn8 (T=Rh, Ir) intermetallics

The magnetocaloric effect of Nd2TIn8 (T = Rh, Ir) intermetallics was studied theoretically. Both compounds have an antiferromagnetic interaction at low temperatures and crystallize in the same tetragonal Ho2CoGa8 structure. We considered a Hamiltonian spin model for two coupled magnetic sublattices that takes into account the effects of crystalline electrical field and exchange interaction in the mean-field approximation. The estimated crystal field parameters explain the magnetocrystalline anisotropy in these compounds and a comparison of the theoretical value of the magnetocaloric effect with the experimental one in Nd2RhIn8, and Nd2IrIn8 shows good agreement.

Effect of AFM and FM exchange interaction on magnetic anisotropy properties of single domain SmFeO3 at nanoscale

Multifunctional Samarium orthoferrites (SmFeO3) nano-particles have been studying extensively due to their applications in digital memory, ultrafast switching, temperature dependent spin valves and sensor applications. Magnetic properties of SFO were investigated by varying particle size, initial magnetic state of material and applied field under Zero Field Cooling (ZFC) and Field Cooling (FC) protocol. It was found that the remnant magnetization, M−H behavior and coercive field are extremely dependent on particle size. It was also found that the magnetic behavior of single domain SFO nanoparticle is completely different to that of multi domain bulk due to uniaxial magneto crystalline anisotropy. The magnetization vs temperature (M−T) behavior of nano-particle depends on unidirectional anisotropy along with uniaxial magneto-crystalline anisotropy of SFO. The Sm3+ spin ordering in nano-SFO compensates to that of Fe3+ spin ordering at higher temperature (~13 K) compare to that of bulk (~4K). The Sm3+ spin unidirectional magnetic anisotropy is more effective in between ~13 K to ~65 K and Fe3+ spin unidirectional anisotropy is effective at higher temperature beyond ~65 K. Magnetically poled nano-particles show completely different behavior to that of non-poled nano-SFO particles at low temperature. In nutshell, the manuscript shows why one can’t predict the magnetic behavior of the SFO without knowing its particle or grain size, initial magnetic state of the particle, procedure of measurement, the magnitude of applied field and temperature etc., since all these affects the magneto crystalline anisotropy as well as unidirectional anisotropy of SFO.

Micromagnetic simulations of reversal magnetization in core ((Nd<sub>0.7</sub>, Ce<sub>0.3</sub>)<sub>2</sub>Fe<sub>14</sub>B)-shell (Nd<sub>2</sub>Fe<sub>14</sub>B) type

The effects of core size, shell thickness and shell distribution on the coercivity of single-grain core ((Nd<sub>0.7</sub>,Ce<sub>0.3</sub>)<sub>2</sub>Fe<sub>14</sub>B)-shell (Nd<sub>2</sub>Fe<sub>14</sub>B) magnets are studied by programming and modeling them through using the C++ language. All the micromagnetic simulations are carried out via object oriented micro magnetic framework (OOMMF). The results show that the coercivity decreases with the increase of core size when the shell thickness is constant. It is considered that for the grain, the increase in the size of the core leads the average magnetocrystalline anisotropy field to increase and the total demagnetization energy to increase, thereby contributing to the magnetization reversal occurring under a smaller external field. When the core size is unchanged, as the shell thickness increases gradually, the coercivity first increases and then decreases. The analysis of the position of the nucleation point shows that the reason why the coercivity increases in the early period is mainly that the nucleation point is located at the core-shell junction and belongs to the core. As the thickness of the shell increases, the exchange interaction effect between the magnetic moment of the shell and the one of the nucleation point is strengthened, so a larger external field is needed in the nucleation process. As for the decrease of the coercivity in the later period, the main reason is that the nucleation points are exactly the vertices of the shell (also the vertices of the grain), and the increase of the shell thickness conduces to increasing the total demagnetization energy, so the nucleation points can be formed under a smaller external magnetic field. With core size and shell volume kept unchanged, when the shell is distributed on the two easy-axis planes (i.e. the planes perpendicular to the easy axis) of the core, the coercivity of the magnet reaches a largest value. It is because that the nucleation points are located at the vertices of the shell (also the vertices of the grain), of which the magnetocrystalline anisotropy field is larger, and the demagnetization field is smaller. Via magnetocrystalline anisotropy field, the demagnetization energy, nucleation point, etc, the changes of coercivity in above cases can be explained.

Влияние магнитомягкой фазы на процессы перемагничивания магнитотвердого/магнитомягкого бислоя

Investigation of the magnetization reversal of a hard/soft magnetic bilayer in an external magnetic field was performed by the Monte Carlo method. The magnetization reversal curves of the magnetic bilayer are plotted for various values of the thickness of the soft magnetic layer. Also the effect of the intralayer exchange interaction and the anisotropy constant on the magnetization reversal processes was studied. The phase diagrams of the magnetic bilayer are plotted.

Effect of biquadratic exchange and anisotropy on the critical temperature of antiferromagnet

The existence of biquadratic exchange had been well established by Anderson, Huang and Allen. Early investigations indicated that the biquadratic exchange term in spin Hamiltonian was found to have significant effects on the magnetic properties of compounds containing iron-group ions or rare-earth metal ions. Recently, studying the anisotropic magnetic excitation of iron-based superconducting materials indicated that the experimental phenomenon cannot be explained well only by a simple collinear Heisenberg antiferromagnetic model. However, if the nearest neighbor biquadratic spin coupling is introduced, the experiment can be well explained. Similar results had also been obtained in the investigation of the superconducting material FeSe. As a result, exploring the effect of biquadratic exchange interactions on the magnetic properties of magnetic systems is significance. In this paper, we use the double-time Green's function method to study the properties of the spin-1 Heisenberg antiferromagnets with biquadratic exchange interactions and anisotropy on a three-dimensional lattice. We derive the equation of motion of the Green's function by a standard procedure. In the course of this, the higher order Green functions have to be decoupled. For the higher order Green functions of different lattice points, a Tyablikov or random phase approximation decoupling are used to decuple. For the higher order Green functions of the same lattice points, we adopt the Anderson-Callen decoupling to decouple. Based on the above procedures, the analytic expressions of the magnetization and critical temperature are obtained. The effects of biquadratic exchange interactions and anisotropy on the critical temperature are discussed in detailed. Our results show that the critical temperature increases with the increase of the anisotropy for a given biquadratic interaction. Regardless of the value of the anisotropy, the critical temperature always decreases with the increase of the biquadratic interaction. As the value of anisotropic parameter <inline-formula><tex-math id="M1">\begin{document}$\eta = 0$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20200077_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="10-20200077_M1.png"/></alternatives></inline-formula>, there is an obvious linear relationship between the critical temperature and the biquadratic interaction. However, when the anisotropy of the system becomes weaker, there is no such linear relationship between them. By comparing similar ferromagnetic models, it is found that the results of this paper are significantly different from those of the ferromagnetic model. When the biquadratic interaction is equal to 0, our results agree with other theoretical results.

Current driven domain wall dynamics in ferrimagnetic strips explained by means of a two interacting sublattices model

The current-driven domain wall dynamics along ferrimagnetic elements are here theoretically analyzed as a function of temperature by means of micromagnetic simulations and a one dimensional model. Contrarily to conventional effective approaches, our model takes into account the two coupled ferromagnetic sublattices forming the ferrimagnetic element. Although the model is suitable for elements with asymmetric exchange interaction and spin-orbit coupling effects due to adjacent heavy metal layers, we here focus our attention on the case of single-layer ferrimagnetic strips where domain walls adopt achiral Bloch configurations at rest. Such domain walls can be driven by either out-of-plane fields or spin transfer torques upon bulk current injection. Our results indicate that the domain wall velocity is optimized at the angular compensation temperature for both field-driven and current-driven cases. Our advanced models allow us to infer that the precession of the internal domain wall moments is suppressed at such compensation temperature, and they will be useful to interpret state-of-the art experiments on these elements.