Intralayer Exchange Coupling Research Articles

Spin-wave resonance frequency and quantum fluctuation of the (anti)ferri-(anti)ferrimagnetic double-layer superlattice

The Hamiltonian of a four-sublattice simple cubic superlattice was solved by applying the linear spin-wave theory and the retarded Green's function technique. This study focused on the spin-wave resonance frequency, energy gap, sublattice magnetization, and quantum fluctuations of the system. The results show that the spin quantum number, interlayer (intralayer) exchange couplings, and intralayer anisotropy have a significant influence. In the ground state, the magnetizations of the sublattices are smaller than their spin quantum numbers, indicating the existence of quantum zero-point vibrations in the (anti)ferrimagnetic-(anti)ferrimagnetic bilayer system.

Compensation temperature and hysteresis behaviors of the ferrimagnetic bilayer zigzag graphyne nanoribbon with RKKY interaction

In this study, a bilayer zigzag graphyne nanoribbon with RKKY interaction is described using a ferrimagnetic mixed-spin (2, 5/2) Ising model. The compensation behaviors and hysteresis characteristics of the system generated by various physical parameters, such as crystal fields of sublattices, interlayer exchange coupling and the intralayer exchange coupling are explored using Monte Carlo simulation. In the research results, we found that among the 30 possible spin configurations that may occur in the ground state of the system, only 21 spin configurations are stable. At the same time, under certain parameters, the system exhibit interesting behaviors such as compensation phenomena and multi-loop hysteresis. This can provide a theoretical reference for multi-state magnetic recording.

Magnetization Process in Bilayer Honeycomb Spin Lattice

The magnetization process of the AA-stacked bilayer honeycomb spin lattice in the out-of-plane applied field is examined in the framework of the Ising model and mean field approximation. Competition between different kinds (antiferromagnetic: AF, or ferromagnetic: FM) of intra-layer and inter-layer spin exchange couplings could lead to the first order magnetization process, characterized by sharp jumps in magnetization curves occurring in critical external fields (spin-flop and spin- flip fields). There is only the spin-flip field at very low temperature and its magnitude depends not only on the intra-layer frustration level but also exchange coupling between spin layers. During the magnetization process, the initial AF bilayer honeycomb spin lattice may undergo ferrimagnetic or week-ferromagnetic states before reaching full ferromagnetic saturation state by the spin-flip field. The results of this work can be applied for an explanation of the spin-flip phenomenon registered in the AF bilayer CrI3.

Influence of proton irradiation on the magnetic properties of two-dimensional Ni(II) molecular magnet

The influence of 1.9 MeV proton irradiation on structural and magnetic properties has been explored in the two-dimensional (2D) NiSO4(1,3-phenylenediamine)2 coordination ferrimagnet. The X-ray powder diffraction and IR spectroscopy revealed that the octahedrons with Ni ion in the center remain unchanged regardless of the fluence a sample received. In contrast, proton irradiation greatly influences the hydrogen bonds in the flexible parts in which the 1,3-phenylenediamine is involved. Dc magnetic measurements revealed that several magnetic properties were modified with proton irradiation. The isothermal magnetization measured at T = 2.0 K varied with the proton dose, achieving a 50% increase in magnetization in the highest measured field µ0Hdc = 7 T or a 25% decrease in remanence. The most significant change was observed for the coercive field, which was reduced by 90% compared to the non-irradiated sample. The observed results are accounted for the increased freedom of magnetic moments rotation and the modification of intralayer exchange couplings.

Compensation temperatures and hysteresis loops of the ferrimagnetic bilayer kagome lattice with RKKY interaction

In this study, a bilayer Kagome lattice with RKKY interaction is described using a ferrimagnetic mixed spin-3/2 and spin-2 Ising model. Interesting compensation temperature and hysteresis loops of the system generated by various physical parameters, such as the crystal fields of sublattices (Da, Db), the interlayer exchange coupling (JR) and the intralayer exchange coupling (Jb) are explored using Monte Carlo simulation. The results demonstrate that the compensation temperature (Tcomp), weaker Da and Jb, and stronger Db, are favourable for the emergence of the compensation temperatures. Besides, under certain physical parameters, the system may exhibit triple-loop hysteresis behavior and multiple magnetization plateaus.

Field-Tunable Berezinskii-Kosterlitz-Thouless Correlations in a Heisenberg Magnet.

We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless (BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the molecular-based bulk material [Cu(pz)_{2}(2-HOpy)_{2}](PF_{6})_{2}. At zero field, a transition to long-range order occurs at 1.38K, caused by a weak intrinsic easy-plane anisotropy and an interlayer exchange of J^{'}/k_{B}≈1 mK. Because of the moderate intralayer exchange coupling of J/k_{B}=6.8 K, the application of laboratory magnetic fields induces a substantial XY anisotropy of the spin correlations. Crucially, this provides a significant BKT regime, as the tiny interlayer exchange J^{'} only induces 3D correlations upon close approach to the BKT transition with its exponential growth in the spin-correlation length. We employ nuclear magnetic resonance measurements to probe the spin correlations that determine the critical temperatures of the BKT transition as well as that of the onset of long-range order. Further, we perform stochastic series expansion quantum MonteCarlo simulations based on the experimentally determined model parameters. Finite-size scaling of the in-plane spin stiffness yields excellent agreement of critical temperatures between theory and experiment, providing clear evidence that the nonmonotonic magnetic phase diagram of [Cu(pz)_{2}(2-HOpy)_{2}](PF_{6})_{2} is determined by the field-tuned XY anisotropy and the concomitant BKT physics.

Anomalous Second Harmonic Generation from Atomically Thin MnBi2Te4

MnBi2Te4 is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature-dependent SHG measurements on MnBi2Te4 flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi2Te4 is unexpectedly unrelated to both magnetic state and layer number. We suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi2Te4-vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry-breaking in van der Waals centrosymmetric magnets.

Magnetic Interactions and Band Gaps of the (CrO2)2/(MgH2) n Superlattices

Lattice superlattices constructed with different materials such as ferromagnets and insulators at atomic scale provide an ideal platform for exploring many emergent physical phenomena. In the present work, a new type of superlattices composed of ferromagnetic half-metal CrO2, with a thickness of two atomic layers, together with insulating MgH2 are constructed. Systematic theoretical studies on the (CrO2)2/(MgH2) n (n = 2, 3, 4, 5, 6) superlattices are carried out based on first-principles density-functional theory calculations. These superlattices are ferromagnetic semiconductors with similar intra-layer magnetic exchange couplings between Cr ions. As the thickness of the MgH2 layer increases, the magnetic exchange interaction between inter-layer Cr ions shows oscillating decaying behavior, while the energy band gaps show a small increase. The understanding of magnetic couplings in these superlattices provides a pathway for constructing new ferromagnetic semiconductors.

Spin-wave resonance frequency and low-temperature properties of an antiferromagnetic graphene-like bilayer system

We studied the spin-wave resonance (SWR) frequency and low-temperature magnetization, internal energy, and specific heat of an antiferromagnetic graphene-like bilayer system with two sublattices by applying the retarded Green's function method and the linear spin-wave approximation. The numerical results show that two SWR frequencies in the system can be adjusted by changing the longitudinal magnetic field, interlayer (intralayer) exchange coupling, and intralayer anisotropy. These parameters greatly influence the magnetization, internal energy, and specific heat at low temperatures. Crossovers between the two sublattice magnetizations were observed for the different intralayer exchange couplings, which are affected by the quantum and thermal fluctuations.

Critical and compensation temperature in a ferrimagnetic mixed spin Ising trilayer nano-graphene superlattice

An effective field method has been employed to study the magnetic properties of a ferrimagnetic mixed-spin Ising trilayer nanographene structure. The system is formed by alternate layers of two different spins atoms A (spin-1) and B (spin-½) arranged in a particular fashion A–B–A for which the nearest-neighbor spins of each layer are coupled ferromagnetically (JA and JB), whereas the cross atom interaction A-B (interlayer) is antiferromagnetic (JAB). We examined the effects of interlayer (or intra-layer) exchange coupling and single-ion anisotropy on the magnetization curves of the system. We found that the antiferromagnetic inter-layers (JAB < 0) coupling and the single-ion anisotropy have a dominant factor in the occurrence of the single or the double compensation temperature. We also deeply examined the occurrence of hysteresis loops and their various unusual forms. The results obtained are discussed in detail as a function of the main parameters of the magnetic Hamiltonian. Finally, we noticed that our results are in line with other recently reported theoretical and numerical studies.

Pressure-Tuned Intralayer Exchange in Superlattice-Like MnBi2Te4/(Bi2Te3)n Topological Insulators.

The magnetic structures of MnBi2Te4(Bi2Te3)n can be manipulated by tuning the interlayer coupling via the number of Bi2Te3 spacer layers n, while the intralayer ferromagnetic (FM) exchange coupling is considered too robust to control. By applying hydrostatic pressure up to 3.5 GPa, we discover opposite responses of magnetic properties for n = 1 and 2. MnBi4Te7 stays at A-type antiferromagnetic (AFM) phase with a decreasing Néel temperature and an increasing saturation field. In sharp contrast, MnBi6Te10 experiences a phase transition from A-type AFM to a quasi-two-dimensional FM state with a suppressed saturation field under pressure. First-principles calculations reveal the essential role of intralayer exchange coupling from lattice compression in determining these magnetic properties. Such magnetic phase transition is also observed in 20% Sb-doped MnBi6Te10 because of the in-plane lattice compression.

Elevating the magnetic exchange coupling in the compressed antiferromagnetic axion insulator candidate EuIn2As2

The magnetic topological materials have attracted much attention recently for their potential realization of various novel quantum states. However, the onset of magnetization in these materials usually occurs at low temperatures, impeding further applications. Here, by means of high pressure, we have significantly increased the magnetic transition temperature in an antiferromagnetic axion insulator candidate $\mathrm{Eu}{\mathrm{In}}_{2}{\mathrm{As}}_{2}$. Both crystal and magnetic structures remain the same with pressure up to 17 GPa. The N\'eel temperature can be monotonously increased from 16 K (ambient pressure) to 65 K (14.7 GPa). This is mainly attributed to the enhancement of intralayer ferromagnetic exchange coupling by pressure. With increasing pressure up to 17 GPa, a crystalline-to-amorphous phase transition occurs, which impedes further enhancement of the N\'eel temperature. Our results show that high pressure is an effective pathway to greatly enhance the magnetic transition temperature in topological materials. It is helpful for the realization of novel quantum states at elevated temperatures.

Monte Carlo study of magnetization plateaus and thermodynamic properties of a nano-graphene with a sandwich-like structure in a longitudinal magnetic field

Abstract Monte Carlo simulation has been used to study magnetization plateaus and thermodynamic properties of the sandwich-like nano-graphene with mixed spin-3/2 and spin-5/2 in a longitudinal magnetic field. The effects of the single-ion anisotropy, the intralayer and interlayer exchange couplings as well as the longitudinal magnetic field on the magnetization, internal energy and specific heat of the system are discussed in detail. Multiple magnetization plateaus behaviors at low temperatures have been found, originating from the competition among various physical parameters. In particular, the results show that the anisotropy, exchange coupling and longitudinal magnetic field play significant roles in controlling the internal energy and specific heat of the system.

Electronic structure and magnetic properties of few-layer Cr2Ge2Te6: the key role of nonlocal electron–electron interaction effects

Atomically-thin magnetic crystals have been recently isolated experimentally, greatly expanding the family of two-dimensional materials. In this Article we present an extensive comparative analysis of the electronic and magnetic properties of , based on density functional theory (DFT). We first show that the often-used approaches fail in predicting the ground-state properties of this material in both its monolayer and bilayer forms, and even more spectacularly in its bulk form. In the latter case, the fundamental gap decreases by increasing the Hubbard-U parameter, eventually leading to a metallic ground state for physically relevant values of U, in stark contrast with experimental data. On the contrary, the use of hybrid functionals, which naturally take into account nonlocal exchange interactions between all orbitals, yields good account of the electronic gap as measured by ARPES. We then calculate all the relevant exchange couplings (and the magneto-crystalline anisotropy energy) for monolayer, bilayer, and bulk with a hybrid functional, with super-cells containing up to 270 atoms, commenting on existing calculations with much smaller super-cell sizes. In the case of bilayer , we show that two distinct intra-layer second-neighbor exchange couplings emerge, a result which, to the best of our knowledge, has not been noticed in the literature.

Spin dynamics of antiferromagnetically coupled bilayers—the case of Cr2TeO6

Understanding the dynamics of interacting quantum spins has been one of the active areas of condensed matter physics research. Recently, extensive inelastic neutron scattering measurements have been carried out in an interesting class of systems, Cr2(Te, W, Mo)O6. These systems consist of bilayers of Cr3+ spins (S = 3/2) with strong antiferromagnetic inter-bilayer coupling (J) and tuneable intra-bilayer coupling (j) from ferro (for W and Mo) to antiferro (for Te). In the limit when , the system reduces to weakly interacting quantum spin-3/2 dimers. In this paper, we discuss the low-temperature magnetic properties of Cr2TeO6 systems where both intra-layer and inter-layer exchange couplings are antiferromagnetic, i.e. . Using linear spin-wave theory we obtain the magnon dispersion, sublattice magnetization, two-magnon density of states, and longitudinal spin–spin correlation function.

Magnetic and thermodynamic properties of a ferromagnetic mixed-spin (1/2, 1, 3/2) three-layer film superlattice

Using the Monte Carlo simulation, we have studied the magnetic and thermodynamic properties of a ferromagnetic three-layer film mixed-spin (1/2, 1, 3/2) system. We have discussed the influence of intralayer and interfacial exchange couplings, film thickness, magnetic atom concentration and temperature on the magnetization of the superlattice system, magnetic susceptibility, internal energy and specific heat of the system. The phase diagrams in various parameters planes are obtained. Loads of interesting magnetic behaviors have been found, such as double-peak and triple-peak phenomena in the susceptibility and specific heat curves as well as obvious finite size effects for small layer thickness. Through a comparison, there is qualitatively a good agreement between our results and those of other theoretical and experimental studies.

Quantum effects on a graphene-like material with four-sublattice

A Heisenberg model is formed by a graphene-like material with four-sublattice, which the intralayer exchange coupling of upper layer is ferromagnetic and that of bottom layer is antiferrimagnetic. The retarded Green’s function and the linear spin-wave approximation are introduced to study the magnetic properties of the system. The effects of various parameters in the ground state properties of the system, such as the spin-wave spectra, energy gap and the magnetization have been studied. Two energy gaps are found in the spin-wave spectra. The antiferrimagnetic couplings and spin quantum numbers play important roles on energy gaps and sublattice magnetization.

Hybrid Design for Advanced Magnetic Recording Media: Combining Exchange-Coupled Composite Media with Coupled Granular Continuous Media

How far can we miniaturize magnetic recording media? Reducing grain size and layer thickness nearly to the atomic level requires finer handling than in typical micromagnetic calculations. The authors develop atomistic simulation of spin dynamics to study magnetization reversal in complex granular media, to optimize performance and push areal density above 1 Tbit/in${}^{2}$. They demonstrate the importance of magnetostatic interactions and inter- and intralayer exchange couplings, and visualize switching at the atomic level to reveal the reversal mechanism. This study transfers knowledge from a university setting directly to industry and real-world problems.

Realization of a spin-12spatially anisotropic square lattice in a quasi-two-dimensional quantum antiferromagnetCu(en)(H2O)2SO4

Theoretical and experimental studies of a quasi-two-dimensional quantum antiferromagnet $\text{Cu}(\mathrm{en}){({\mathrm{H}}_{2}\mathrm{O})}_{2}{\mathrm{SO}}_{4}$ $(\mathrm{en}={\mathrm{C}}_{2}{\mathrm{H}}_{8}{\mathrm{N}}_{2})$ were performed. ab initio calculations of exchange interactions confirmed that the system represents a realization of a spatially anisotropic zigzag square lattice. Corresponding quantum Monte Carlo calculations of thermodynamic quantities were realized and the results were applied in the analysis of experimental susceptibility, magnetization, and specific heat studied at temperatures ranging from nominally 300 mK up to 8 K and magnetic fields up to 14 T. The analysis of experimental results provided the estimates of intralayer exchange couplings, $J/{k}_{B}=3.5\ifmmode\pm\else\textpm\fi{}0.2$ K and ${J}^{\ensuremath{'}}=0.35J$. Theoretical analysis of spin symmetries in $\text{Cu}(\mathrm{en}){({\mathrm{H}}_{2}\mathrm{O})}_{2}{\mathrm{SO}}_{4}$ structure predicted the presence of symmetric exchange anisotropies (out-of plane and in-plane spin anisotropy) and a spin-flop transition within the easy plane induced by the magnetic field applied along the easy axis. Isothermal magnetization measurements indicated the expected transition in the field 200 mT applied along the $b$ axis which was finally identified as the easy axis lying within the easy plane $bc$. Magnetic phase diagrams with saturation fields about 6.5 T show nearly identical behavior in all studied directions. Differences appear only in weak magnetic fields as a result of the presence of weak exchange anisotropies $\ensuremath{\approx}{10}^{\ensuremath{-}3}J$. The present analysis suggests that $\text{Cu}(\mathrm{en}){({\mathrm{H}}_{2}\mathrm{O})}_{2}{\mathrm{SO}}_{4}$ can be a model system for exploring the interplay of quantum fluctuations, exchange anisotropies, and magnetic field in the two-dimensional lattice space.

Monte Carlo simulation of magnetic properties of a nano-graphene bilayer in a longitudinal magnetic field

Monte Carlo simulation has been used to study the magnetic properties of a nano-graphene bilayer which consists of the upper layer A with spin-3/2 and the bottom layer B with spin-5/2. The effects of the single-ion anisotropy, the intralayer exchange coupling and the longitudinal magnetic field on the magnetization, the susceptibility, the blocking temperature and hysteresis loops of the mixed-spin nano-graphene bilayer system have been examined detailedly. In particular, the variations of the blocking temperature with different intralayer exchange couplings, single-ion anisotropies, and the longitudinal magnetic field are obtained for the present system. Many multiple hysteresis loop behaviors have also been found, depending on the combinations of both the upper and bottom layer magnetizations in the longitudinal magnetic field. Through a comparison, our results obtained are according well with other theoretical researches and experimental results.