Features of magnetization reversal in trilayer nanostructures

The process of magnetization reversal in an ultrathin magnetic trilayer is analyzed. It is shown that the shape of magnetization hysteresis loops and the giant magnetoresistance essentially depend on the relative magnitudes of magnetic parameters of the top and bottom layers. Hysteresis loops are found for characteristic relative magnitudes of the parameters. Analysis is performed of the dependence of the shape of hysteresis loops on the magnitude of interlayer exchange. A phase diagram is constructed, which determines the regions of existence of characteristic hysteresis loops for different relative magnitudes of the uniaxial anisotropy constant and exchange constant J ₁.     

Hysteresis loops and susceptibility of a transverse Ising nanowire

In this work, the magnetization, susceptibility, and hysteresis loops of a magnetic nanowire are described by the transverse Ising model using the effective field theory within a probability distribution technique. The effects of the exchange interaction between core/shell and the external fields on the magnetization and the susceptibility of the system are examined. Some characteristic phenomena are found in the thermal variations, depending on the ratios of the physical parameters in the shell and the core.     

Magnetization reversal in granular thin films

The paper discusses the physics of magnetization reversal in granular magnetic films. It gives an overview of the key physical properties that determine the collective and macroscopically observable magnetization reversal behavior. In particular, the multitude of observable hysteresis loops is reduced to three key physical quantities, namely the single grain switching field distribution D(h_s), the inter-granular exchange coupling constant J_ex, and the magnetostatic interaction constant J_ms. By varying the relative influence of these quantities, many different shapes of hysteresis loops can occur, which is documented by experimental examples. The regime of partially and strongly correlated reversal is discussed in detail, and minor loop measurements are presented that show scaling behavior for strongly correlated magnetization reversal in the vicinity of hysteresis loop criticality.     

铁磁球/反铁磁纳米体系磁滞回线的Monte Carlo模拟

利用经典Heisenberg模型和Monte Carlo方法研究外磁场和反铁磁磁晶各向异性、交换相互作用对铁磁球均匀嵌入到反铁磁基体中的铁磁/反铁磁纳米体系磁滞回线的影响.模拟结果显示,外加反向最大磁场不同时,磁滞回线形状不同.当磁场正向增加时,体系的磁化强度会产生一个跃变,但跃变高度与反向场最大值无关.反铁磁磁晶各向异性越大,体系的交换偏置现象越明显,且磁化强度回到饱和值所需的外磁场越大.随着反铁磁基体交换相互作用的增大,在正向和负向磁场区域还可能出现新的磁滞现象.     

Theoretical Investigation of Influence of Mechanical Stress on Magnetic Properties of Ferromagnetic/Antiferromagnetic Bilayers Deposited on Flexible Substrates

Effect of mechanical stress on magnetic properties of an exchange-biased ferromagnetic/antiferromagnetic bilayer deposited on a flexible substrate is investigated.The hysteresis loops with different magnitudes and orientations of the stress can be classified into three types.The corresponding physical conditions for each type of the loop are deduced based on the principle of minimal energy.The equation of the critical stress is derived,which can judge whether the loops show hysteresis or not.Numerical calculations suggest that except for the magnitude of the mechanical stress,the relative orientation of the stress is also an important factor to tune the exchange bias effect.     

Influence of magnetic field sweep rate on the hysteresis loops of Ni0.8Fe0.2/Fe0.5Mn0.5 exchange bias bilayer

The influence of the magnetic field sweep rate on the hysteresis loops of exchange bias Ni0.8Fe0.2/Fe0.5Mn0.5 bilayers has been investigated with a vibrating sample magnetometer. It was found that the sweep rate of 13.6 kA/4πms is high enough to bring about obvious changes in the hysteresis loops of the exchange bias bilayer. High sweep rate in the magnetization reversal stage enlarges the coercivity of the sample, while high sweep rate in the saturation state reduces the coercivity. The above phenomena were attributed to magnetic viscosity in the ferromagnetic layer enhanced by the interface exchange interaction and domain magnetization reversals assisted by thermal fluctuation in the antiferromagnetic layer respectively.     

Effects of the trimodal random field on the magnetic properties of a spin-1 Ising nanotube

In this work, the hysteresis behavior of a nanotube, consisting of a ferromagnetic core of spin-1 atoms surrounded by a ferromagnetic shell of spin-1 atoms with ferro-or anti-ferromagnetic interracial coupling is studied in the presence of a random magnetic field. Based on a probability distribution method, the effective-field theory has been used to investigate the effects of the random magnetic field, the interfacial coupling constant, and the temperature on the hysteresis loops of the nanotube. Some characteristic behaviors have been found, such as the existence of double or triple hysteresis loops for appropriate values of the system parameters. The remanent magnetization and the coercive field, as functions of the temperature, are examined.     

Monte Carlo study of the magnetic properties and magnetocaloric effect of an AFM/FM BiFeO_3/Co bilayer

The magnetic properties and magnetocaloric effect of an antiferromagnetic/ferromagnetic(AFM/FM)BiFeO_3/Co bilayer with mixed-spin(5/2,3/2) have been studied based on Monte Carlo simulation.The magnetization, susceptibility, and critical temperature are investigated under various exchange couplings and an external magnetic field. In particular, the influence of exchange couplings and an external magnetic field on the magnetic entropy change, adiabatic temperature change, and the relative cooling power(RCP) are studied. The simulation results indicated that the decrease of the exchange coupling and the increase of external magnetic fields can cause an increase of magnetic entropy change, adiabatic temperature change, and RCP. In addition, the hysteresis loops of the system are presented for different exchange couplings and temperatures.     

Hysteresis loops and adiabatic Landau-Zener-Stückelberg transitions in the magnetic molecule {V6}

We have observed hysteresis loops and abrupt magnetization steps in the magnetic molecule {V(6)}, where each molecule comprises a pair of identical spin triangles, in the temperature range 1-5 K for external magnetic fields B with sweep rates of several Tesla per millisecond executing a variety of closed cycles. The hysteresis loops are accurately reproduced using a generalization of the Bloch equation based on direct one-phonon transitions between the instantaneous Zeeman-split levels of the ground state (an S=1/2 doublet) of each spin triangle. The magnetization steps occur for B approximately 0, and they are explained in terms of adiabatic Landau-Zener-Stückelberg transitions between the lowest magnetic energy levels as modified by an intertriangle anisotropic exchange of order 0.4 K.     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Magnetization reversal in submicron disks: exchange biased vortices

Submicron, circular, ferromagnetic-antiferromagnetic dots exhibit different magnetization reversal mechanisms depending on the direction of the magnetic applied field. Shifted, constricted hysteresis loops, typical for vortex formation, are observed for fields along the exchange bias direction. However, for fields applied close to perpendicular to the exchange bias direction, magnetization reversal occurs via coherent rotation. Magnetic force microscopy imaging together with micromagnetic simulations are used to further clarify the different magnetic switching behaviors.     

Hysteresis of the Magnetic Particle in a Dipolar Ising Model

Zero-temperature Monte Carlo simulations are used to investigate the hysteresis of a magnetic particle in a dipolar Ising model.The magnetic particle is described in a systemm of permanent dipoles,and the dipoles are located in a cubic lattice site.The effects of the shape and the size of the particle on the hysteresis loop at zero temperature are obtained.For strong exchange interactions,the shapes of magnetic hysteresis loops approach rectangle.For weak exchange interactions,the effects of the size and the shape of the particle on the loops are more remarkable than those of strong exchange interactions case.The slope of the hysteresis loop decreases with the increase of the ratio of the semi major axis to the semi minor axis of the ellipsoidal magnetic particle,and there is an increase of the slope of the hysteresis with the decrease of the size of the magnetic particle.The effects of the shape and size of the particle on the coercive force at zero temperature are also investigated.     

Microscopic nature of ferro- and antiferromagnetic interface coupling of uncompensated magnetic moments in exchange bias systems

Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.     

The magnetic properties of disordered Fe-Al alloy system

Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation functions, the magnetic properties of disordered Fe-Al alloys on the basis of a site-diluted quantum Heisenberg spin model are examined. We calculated the critical temperature and the hysteresis loops for this system. We find a number of characteristic phenomena. In particular, the effect of concentration c of magnetic atoms and the reduced exchange anisotropic parameter η on both the critical temperature and magnetization profiles are clarified.     

Magnetization reversal in exchange biased Co/CoO patterns

The influence of patterning on exchange bias has been investigated using arrays of micron-sized Co/CoO dots with different lateral confinement and length-to-width ratio. The patterned samples show higher coercive and exchange bias fields than a continuous Co/CoO bilayer. As in unpatterned film, magnetization reversal mechanisms on opposite sides of the hysteresis loops of the microstructured samples are different. However, with the increase of lateral confinement and shape anisotropy of the dots, the asymmetry in the magnetization reversal starts to differ from that observed in continuous Co/CoO films.     

On the effect of an elastic shear stress upon the magnetization of ferromagnetic sheets

When an elastic shear stress and a cyclical magnetic field, parallel to each other, are applied in the plane of a ferromagnetic sheet, magnetization changes perpendicular to the field are induced in the sample. “Transverse” hysteresis loops, i.e. transverse magnetization plotted as a function of the longitudinal field, were studied in various materials. The characteristic shape of the loop and the differences in sign and magnitude of the “transverse” magnetization have been qualitatively explained in terms of magnetic domain theory. Work supported by G.N.S.M. (CNR).     

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively.     

Magnetization and polarized neutron reflectivity experiments on patterned exchange bias structures

Patterned Co/CoO thin film structures have been investigated by magnetization and polarized neutron reflectivity measurements in order to study the influence of finite size and shape anisotropy effects on the magnetization reversal. An anomaly was found in the upper branch of the hysteresis loops, probably caused by incomplete bias in the patterned structures. The asymmetry in magnetization reversal mechanism commonly found in the two branches of the hysteresis loops of unpatterned Co/CoO layers is altered in the patterned structures, consistent with the existence of interfacial domains.     

Long-distance super-exchange and quantum magnetic relaxation in a hybrid metal–organic framework

The hybrid metal–organic framework [(CH3)2NH2]Fe(HCOO)3with a perovskite-like structure exhibits a variety of unusual magnetic behaviors at low temperatures. While the long-distance super-exchange through the Fe-O–CH-O–Fe exchange path leads to a canted antiferromagnetic ordering at TN～ 19 K, a second transition of magnetic blocking develops at TB～ 9 K. The stair-shaped magnetization hysteresis loops below TBresemble the behaviors of resonant quantum tunneling of magnetization in single-molecular quantum magnets. Moreover, the magnetic relaxation also exhibits several features of resonant quantum relaxation, such as the exponential law with a single characteristic relaxation time, and the nonmonotonic dependence of relaxation rate on the applied magnetic field with a much faster relaxation around the resonant fields. The origin of quantum tunneling behaviors in the [(CH3)2NH2]Fe(HCOO)3metal–organic framework is discussed in terms of magnetic phase separation due to the modification of hydrogen bonding on the long-distance super-exchange interaction.     

Inverted near-surface hysteresis loops in heterogeneous (nanocrystalline/amorphous) Fe<Subscript>81</Subscript>Nb<Subscript>7</Subscript>B<Subscript>12</Subscript> alloys

Inverted hysteresis loops were observed for the first time in the near-surface layers of heterogeneous (nanocrystalline/amorphous) Fe₈₁Nb₇B₁₂ alloys. In particular, a negative residual magnetization is observed when a positive magnetic field applied in the sample plane is decreased to zero. The inverted hysteresis is qualitatively explained within the framework of a two-phase model, according to which the heterogeneous alloys contain two dissimilar phases exhibiting uniaxial magnetic anisotropy and featuring antiferromagnetic exchange interaction.