Influence of the Competition Between Dipolar and Exchange Interactions on the Magnetic Structure of Single-Wall Nanocylinders. Monte Carlo Simulation

In this work, we address the Metropolis Monte Carlo simulation of single-wall magnetic nanocylinders with zig-zag edges on the basis of a three-dimensional classical Heisenberg model with short-range nearest neighbors ferromagnetic exchange interactions and long-range dipolar interactions. A study of the magnetic properties (thermal dependence of the magnetization per site, specific heat, and magnetic susceptibility) with different strengths of dipolar interaction relative to the exchange energy is carried out. Our results reveal a strong dependence of the low-temperature magnetic structure of the nanotubes with the strength (??) of the dipolar interaction. Comparisons with the case without considering dipolar interaction are carried out and a summary of our results in terms of the ??-dependence of the low-temperature magnetization is also presented and discussed.     

Monte Carlo Study of Magnetic Properties in Semiconducting MnTe

The magnetic properties of antiferromagnetic MnTe alloys have been studied by Monte Carlo simulations within Ising model framework. The considered Hamiltonian includes nearest-neighbor interactions, external magnetic field, and crystal field. Magnetizations and magnetic susceptibility versus temperature are computed for a fixed size. The blocking temperature is established, and the magnetic hysteresis cycle is deduced for different values of temperature. The remanent magnetization and the coercive field are obtained. In addition, the magnetization versus the first exchange interaction (J ₁) with a fixed value of the second exchange interaction (J ₂) and the third second interaction (J ₃) is estimated. The magnetization versus the crystal field is deduced.     

Experiment and Theoretical Study of Critical Behavior in Magnetic Multilayers

Ni/NM multilayers (with noble metal NM=Au,Ag and Cu) were prepared by the electron beam evaporation method under ultra high vacuum conditions. The magnetic properties of Ni/NM multilayers are examined as a function of Ni layer thickness t _Ni. The temperature dependence of the spontaneous magnetization M(T) is well described by a T ^3/2 law in all multilayers. A spin-wave theory has been used to explain the temperature dependence of the magnetization and the approximate values for the bulk Exchange interaction J _b and surface exchange interaction J _s for various Ni layer thicknesses have been obtained. In the other hand we have used the high-temperature series expansion technique, to analyze the phase transition and the critical phenomena of a ferromagnetic a two-component multilayer, through three models: Ising, XY and Heisenberg. The critical reduced temperature ?? _c (??) is studied as function of the thickness of constituents in the unit cell of the multilayer. In the two-component multilayer ?? _c (??) is studied as function of the exchange interaction in each material and within the interface J _s ,J _b and J _??, respectively. A?critical value of the surface exchange interaction in the film and interface exchange interaction in the multilayer above which the surface and the interface magnetism appears is obtained. The dependence of the reduced critical temperature on the thickness of the film and the unit cell of multilayer has been investigated. The effects of an amorphous magnetic surface on the critical properties of the film of simple cubic lattice have been studied. A number of characteristic behaviors, such as the possibility of the existence of a critical length of the unit cell thickness at which the temperature of the multilayer remains insensitive to the exchange coupling within interface, are reported. In a defined range of the exchange interactions, the values of ?? are comparable to the universal ones and are independent of the film thickness. The asymmetry of the structure and the competition of the effects of the exchange coupling are important for the magnetic properties of the system.     

Small-Angle Neutron Scattering by the Magnetic Microstructure of Nanocrystalline Ferromagnets Near Saturation

The paper presents a theoretical analysis of elastic magnetic small-angle neutron scattering (SANS) due to the nonuniform magnetic microstructure in nanocrystalline ferromagnets. The reaction of the magnetization to the magnetocrystalline and magnetoelastic anisotropy fields is derived using the theory of micromagnetics. In the limit where the scattering volume is a single magnetic domain, and the magnetization is nearly aligned with the direction of the magnetic field, closed form solutions are given for the differential scattering cross-section as a function of the scattering vector and of the magnetic field. These expressions involve an anisotropy field scattering function, that depends only on the Fourier components of the anisotropy field microstructure, not on the applied field, and a micromagnetic response function for SANS, that can be computed from tabulated values of the materials parameters saturation magnetization and exchange stiffness constant or spin wave stiffness constant. Based on these results, it is suggested that the anisotropy field scattering function S_H can be extracted from experimental SANS data. A sum rule for S_H suggests measurement of the volumetric mean square anisotropy field. When magnetocrystalline anisotropy is dominant, then a mean grain size or the grain size distribution may be determined by analysis of S_H.     

Magnetic Properties of Mg x Cu1−x Cr2O4 Spinels are Studied by Different Theoretical Methods

The external magnetic field induced the reorientation of magnetization of a ferromagnetic (or antiferromagnetic) treated within the framework of many-body Green’s function theory by considering all components of the magnetization. We present a new method for the calculation of expectation values in terms of the eigenvalues and eigenvectors of the equations of motion matrix for the set of Green’s functions. Magnetization and magnetic susceptibility are investigated when an external magnetic field is applied in (x-z)-plane. The mean field theory is applied to calculate the nearest neighbour and the next-neighbour super-exchange J ₁(Cr?Cr) and J ₂(Cr?(Mg(Cu)?O)?Cr), respectively, for the Mg _x Cu_1?x Cr₂O₄ in the range of 0 ≤ x ≤ 1. The intra-planar and the inter-planar interactions are deduced. The high-temperature series expansions (HTSEs) are derived for the magnetic susceptibility and two-spin correlation functions for a Heisenberg ferromagnetic model on the B-spinel lattice. The calculations are developed in the framework of the random-phase approximation (RPA). The magnetic-phase diagram is deduced. A spin-glass phase is predicted for intermediate range of concentration. The spin glass is obtained. The obtained results are comparable with those obtained by magnetic measurements. The critical exponents associated with the magnetic susceptibility (γ) and the correlation lengths (ν) have been deduced. The obtained values are comparable to those of 3D Heisenberg model.     

Spin Compensation Temperatures in the Monte Carlo Study of a Mixed Spin-3/2 and Spin-1/2 Ising Ferrimagnetic System

The magnetic properties of mixed-spin S ^′ = 3/2 and S = 1/2 ferrimagnetic system have been studied by Monte Carlo simulations. The ground-state phase diagrams of mixed spin-3/2 and spin-1/2 Ising system are given. The critical and compensation temperatures have been found with different values of reduced exchange interactions. The variation of total magnetization with reduced exchange interactions of mixed spins is given with different temperatures and crystal fields. We have also given the variation of total magnetization with crystal field for different reduced exchange interactions and different temperatures. The magnetic hysteresis cycle is found to have different values of reduced exchange interactions, temperatures, and crystal fields. The multiple hysteresis and the superparamagnetic phase are established.     

Electronic and Magnetic Properties of MnAu Superlattices

Self-consistent ab initio calculations, based on the Density Functional Theory (DFT) approach and using full potential linear augmented plane wave (FLAPW) method, are performed to investigate both electronic and magnetic properties of the MnAu superlattices. Polarized spin and spin-orbit coupling are included in calculations within the framework of the antiferromagnetic state between two adjacent MnAu superlattices. Obtained data from ab initio calculations are used as input for the Monte Carlo simulations to compute other magnetic parameters. On the other hand, and within the framework of Monte Carlo simulations, we examine the magnetic properties in the binary MnAu superlattices modelized by the Mn ion with spin moment, S=5/2. The considered Hamiltonian takes into account the nearest neighbor and second nearest interactions, and an external magnetic field h. The magnetization of the MnAu superlattice is calculated versus temperature for a fixed size. The magnetic hysteresis cycle is established for T=120 K. The effect of magnetization versus the second exchange interaction (J _c ) are established with absence and presence of magnetic field (h).     

Neutron Diffraction Investigation of the DyFe11Ti Magnetic Structure and Its Spin Reorientations

In this article, we report on the magnetic structure of DyFe₁₁Ti and its thermal evolution as probed by neutron powder diffraction. A thermodiffraction technique was used to follow the temperature dependence of the magnetic moments, as well as their orientation. The Dy and Fe moments were coupled to each other in an antiparallel manner to form a ferrimagnet, where the easy magnetization direction at 2 K was the [110] axis in the basal (a, b) plane. This magnetic structure underwent two successive spin reorientation phenomena with increasing temperature. A large Dy magnetic moment of 9.7 Bohr magneton (μ_B) was obtained at low temperatures, and the magnitude decreased rapidly to 7.5μ_B at 200 K. The largest Fe magnetic moment was observed on the Fe 8i position. A ThMn₁₂-type crystal structure was preserved in the studied temperature range, despite the large changes of the magnetic structure. A sharp tilt was observed at the first-order spin reorientation, T_SR1; the angle between the easy magnetization axis and the crystal c axis was reduced from 90° at 2 K to about 20° at 200 K (where c is the easy axis above 200 K); and the Dy and Fe magnetic moments maintained an antiparallel coupling.     

Magnetic Studies in Sputtered Ni/Ti Multilayers

Magnetic properties of Ni/Ti multilayers, prepared by the DC triode sputtering method, have been studied by magnetic measurements. Both metal layers are crystalline with a (111) fiber structure when they are thicker than 20 Å. The magnetization decreases with a decrease in Ni layer thickness t _Ni and the analysis of the results at 5 K indicates the presence of a dead Ni layer about 13 Å thick. The effective anisotropy K _eff of Ni/Ti multilayers is obtained using a torque magnetometer. Spin-wave theory has been used to explain the temperature dependence of the magnetization. Approximate values for bulk exchange interaction J _b, surface exchange interaction J _S and interlayer coupling strength J _I for various Ni layer thicknesses have been obtained.     

Magnetism of Nano-Graphene with Defects: A Monte Carlo Study

The effect of the defects on magnetic properties of a bilayer Ising ferromagnetic antiferromagnetic model is studied by Monte Carlo simulations, for a nano-graphene lattice with spins that can take the values σ=3/2 and S=5/2. We consider two ferromagnetic and antiferromagnetic bilayers with N=42 spins, with a random number of defects. We only consider the nearest-neighbor interactions between the site i and j on each layer. The effects of the defects on magnetization are investigated for fixed temperature, crystal field, and magnetic field values. The thermal dependency of each layer magnetization is calculated for fixed defect rate values K ₃ and K ₅, the crystal field, and external magnetic field. The magnetization hysteresis loops for several rate defects are also investigated as a function of the external magnetic field.     

Magnetic Properties of Nanosized Ba2Mg2Fe12O22 Powders Obtained by Auto-combustion

We studied the magnetic properties of nanosized Ba₂Mg₂Fe₁₂O₂₂ powder obtained by citrate auto-combustion synthesis. The powder consists of agglomerates with mean crystallite size of 100?nm. The magnetic properties of the powder were investigated at 4.2?K and at room temperature. The values measured of the magnetization M at a magnetic field of 60?kOe are 22.78?emu/g and 30.47?emu/g at room temperature and 4.2?K, respectively. The magnetic phase transition at 183 K is related to the ferromagnetic-to-spiral spin order and is a precondition for this material??s exhibiting multiferroic properties.     

Magnetocaloric effect in Gd(1−y)DyyAl2

In this work we calculate the magnetic properties and the magnetocaloric effect in Gd_1−yDy_yAl₂ (y = 0, 0.25, 0.5, 0.75 and 1). The model Hamiltonian includes contributions from the Zeeman effect, the crystalline electrical field anisotropy and the exchange interactions among Gd–Gd, Gd–Dy and Dy–Dy ions. To obtain a composite with ΔS_T as constant as possible in the temperature range from T = 60 K to 170 K, the appropriate concentrations of the five compounds investigated were calculated using the Smaili and Chahine method. The magnetization and ΔS_T dependences on temperature in the composite were simulated and compared with the partial contributions of the single magnetic component materials. Also, the magnetic field dependence on magnetization was investigated in Gd_0.25Dy_0.75Al₂, where the discontinuous spin reorientation transitions were predicted for magnetic fields lower than 2 T, applied along <110> direction.     

Ferromagnetism and Anti-ferromagnetism in Nano-films with Alternate Crystal Fields: Monte Carlo Study

Using Monte Carlo simulations, the magnetic properties of the Ising model based on a nano-film formed by alternating layers are studied. The effect of a reduced crystal field with the anti-ferromagnetic and ferromagnetic reduced exchange interactions is deduced for fixed system sizes: N= 9 and 10 layers. Indeed, the anti-ferromagnetic reduced exchange interaction increases when one increases of the number layers, for odd or even values of N. The total magnetization increases with increasing of the anti-ferromagnetic and the ferromagnetic reduced exchange interactions. It is found that the total magnetization remains almost constant for large values of layer numbers (N).     

Investigations on the Magnetization Switching Dynamics in Spin Valve Multilayers Under Periodic Applied Magnetic Field

Magnetization switching dynamics in a spin valve nanopillar, induced by spin transfer torque in the presence of a periodic applied field is investigated by solving the Landau–Lifshitz–Gilbert–Slonczewski equation. Under steady state conditions, the switching of magnetization occurs in the system, above a threshold current density value J _c. A general expression for the critical current density is derived and it is shown that this further reduces when there is magnetic interface anisotropy present in the free layer of the spin valve. We also investigated the chaotic behavior of the free layer magnetization vector in a periodically varying applied magnetic field, in the presence of a constant DC magnetic field and spin current. Further, it is found that in the presence of a nonzero interfacial anisotropy, chaotic behavior is observed even at much smaller values of the spin current and DC applied field.     

Dilution Effect on Nanographene Magnetic Properties

The magnetic properties of a diluted ferromagnetic–antiferromagnetic Ising model on a nanographene lattice, constituted of pure ferromagnetic layer with spin S=5/2 and diluted antiferromagnetic layer with a spin σ=3/2, have been studied using Monte Carlo simulations. The considered Hamiltonian takes into account nearest-neighbor interactions, crystal field, and external magnetic field. The calculated magnetizations versus the dilution of the bonds in block A depend on the temperature, crystal field Δ, and external magnetic field h. The saturation magnetization and the magnetic remanent are also obtained for the two blocks.     

Strongly Surface State Carrier-Dependent Spin–Orbit Torque in Magnetic Topological Insulators

The topological surface states (TSS) in topological insulators (TIs) can exert strong spin–orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy-efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation-doped Cr-Bi_xSb_2−xTe₃ (Cr-BST) film is quantitatively determined via both transport and optic approaches, where consistent results are obtained. A large spin Hall angle of ≈90 in the modulation-doped Cr-BST film is demonstrated at 2.5 K, and the spin Hall angle drastically decreases to 0.3–0.5 as the temperature increases. Moreover, by tuning the top TSS carrier concentration, a competition between the top and bottom TSS in contributing to SOT is observed. The above phenomena can account for the large discrepancies among the previously reported spin Hall angle values and reveal the unique role of TSS in generating SOT.     

High-Field and Multi-Frequency ESR in the Quasi Two-Dimensional Triangular-Lattice Antiferromagnet CuCrO2

We have performed high-field and multi-frequency electron spin resonance (ESR) and magnetization measurements in magnetic fields up to about 53 T on single crystals of a delafossite triangular-lattice antiferromagnet CuCrO₂, which has been proposed to show an out-of-plane 120° spiral spin structure below about 24 K. We calculated ESR resonance modes and magnetization curves assuming an Ising-like Heisenberg Hamiltonian with a spiral spin structure by a mean-field theory. The experimental results can be well reproduced using the following parameters, the intralayer exchange constant J/k _B=26.9 K, the easy-axis anisotropy D/k _B=?0.47 K, and the in-plane g-factor g _ab =2.0.     

Magnetic characterization of MnPt/CoFe bilayers using the MOKE technique

The performance of magnetoresistive devices (spin valves, tunnel junctions), made of two ferromagnetic (FM) layers and separated by a non-magnetic spacer, rely on the existence of two well separated resistance states. For this to occur, one of the FM layer is deposited just adjacent to an antiferromagnetic (AFM) layer. Due to the exchange interaction at the AFM/FM interface, the reversal of the magnetization (M) of such FM-pinned layer occurs at a high applied magnetic field. The magnetization of the other FM layer reverses almost freely when a small magnetic field is applied. Here we study the exchange bias effect in the MnPt (t)/CoFe (50 Å) system, using the Magneto-Optical Kerr Effect (MOKE) and domain imaging techniques. The exchange (H_E) and coercive (H_c) fields increase with increasing AFM thickness, saturating for t > 200 Å (H_E ≈ 670 Oe and H_c ≈ 315 Oe). Furthermore, we observe that the value of the exchange field is almost independent of the applied magnetic field sweeping rate (up to ≈ 300 kOe/s). Domain imaging allowed us to conclude that magnetization reversal in the studied system proceeds essentially by coherent magnetic moment rotation.     

Magnetic and electrical properties of hot-pressed Ni-Zn-Li ferrites

The magnetic properties of Ni-Zn ferrites have been upgraded by preparing hot-pressed Ni_0.4Zn_0.6–2xLi_xFe_2+xO₄ ferrites wherein 2xZn²⁺ ions have been substituted byxLi¹⁺ andxFe³⁺ ions. This results in an increase of saturation magnetization, Curie temperature and dielectric constant, whereas resistivity and initial permeability are reduced. The values of saturation magnetization, Curie temperature and dielectric constant are improved due to hot pressing in which grain growth and densification are controlled simultaneously. The variations of saturation magnetization and Curie temperature can be explained by the preferential site occupancy of Li¹⁺ and Ni²⁺ ions at the tetrahedral site, sublattice magnetization with canted spin structure, and magnetic exchange interactions. The inferences drawn from the bulk magnetic properties of these ferrites conform with the conclusions drawn from variations of internal magnetic field, obtained from Mössbauer studies of these samples. The decrease in d.c. resistivity due to substitution of Li¹⁺ ions is attributed to increased hopping due to formation of Fe²⁺ and Ni³⁺ ions.