Universal hysteresis scaling for incommensurate magnetic order in dysprosium

We study the scaling properties of magnetic minor hysteresis loops in a polycrystalline dysprosium metal, varying temperature and magnetic-field amplitudes. We observe irreversibility-related hysteresis loss in the helical antiferromagnetic phase, which is related with remanent flux density as a power law with the same scaling exponent of 1.25±0.05 as that in ferromagnetic materials. In contrast to hysteresis scalings in ferromagnets associated with 180° Bloch walls, the observed law is governed by spiral walls which separate helical domains with oppositely rotating spins.     

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.     

一维钻石链反铁磁Ising模型磁化的模拟

采用Monte Carlo模拟方法对自旋为1/2的一维钻石链反铁磁Ising系统的磁化行为进行研究.在这个系统中,反铁磁的交换作用和三角结构导致存在自旋阻挫.重点研究不同的反铁磁自旋交换作用对系统磁行为和自旋构型的影响.模拟磁化曲线中M=M_S/3磁化平台,得到平台宽度随不同的自旋交换相互作用强弱的变化关系,以及出现磁化平台时格点的自旋构型;给出亚稳态存在的条件及亚稳态时的微观构型.研究磁滞回线随温度的变化关系.结果表明,随着温度的升高,磁滞回线逐渐减小,最终消失.     

Magnetic properties of a mixed spin-3/2 and spin-2 Ising octahedral chain

Using an effective field theory with correlations, magnetic properties of an octahedral chain described by a mixed spin Ising model are investigated. Unique phenomena such as multiple hysteresis loops, saturation magnetization, and reverse flip of the magnetization plateaus occur when certain typical parameters are applied. These results may be helpful to further investigate the magnetic properties of one-dimensional systems and could potentially be utilized in the design of spin devices.     

Origins of the magnetomechanical effect

A hypothesis is presented to explain the mechanism by which externally applied stresses can affect the magnetic properties of ferromagnetic materials. Experiments have revealed coincident points in the second and fourth quadrants on stressed hysteresis loops of mild steel. The results are presented along with an explanation of this effect. An atomic level theory of the origins of the magnetomechanical effect is introduced whereby spin–spin and spin–orbit coupling interact with magnetic moments to alter the magnetocrystalline anisotropy and exchange energies.     

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.     

Modeling of exchange bias in the antiferromagnetic (core)/ferromagnetic (shell) nanoparticles with specialized shapes

Zero-field-cooled (ZFC) and field-cooled (FC) hysteresis loops of egg- and ellipsoid-shaped nanoparticles with inverted ferromagnetic (FM)-antiferromagnetic (AFM) core-shell morphologies are simulated using a modified Monte Carlo method, which takes into account both the thermal fluctuations and energy barriers during the rotation of spin. Pronounced exchange bias (EB) fields and reduced coercivities are obtained in the FC hysteresis loops. The analysis of the microscopic spin configurations allows us to conclude that the magnetization reversal occurs by means of the nucleation process during both the ZFC and FC hysteresis branches. The nucleation takes place in the form of “sparks” resulting from the energy competition and the morphology of the nanoparticle. The appearance of EB in the FC hysteresis loops is only dependent on that the movements of “sparks” driven by magnetic field at both branches of hysteresis loops are not along the same axis, which is independent of the strength of AFM anisotropy. The tilt of “spark” movement with respect to the symmetric axis implies the existence of additional unidirectional anisotropy at the AFM/FM interfaces as a consequence of the surplus magnetization in the AFM core, which is the commonly accepted origin of EB. Our simulations allow us to clarify the microscopic mechanisms of the observed EB behavior, not accessible in experiments.     

Exchange bias and asymmetric hysteresis loops from a microscopic model of core/shell nanoparticles

We present Monte Carlo simulations of hysteresis loops of a model of a magnetic nanoparticle with a ferromagnetic core and an antiferromagnetic shell with varying values of the core/shell interface exchange coupling which aim to clarify the microscopic origin of exchange bias observed experimentally. We have found loop shifts in the field direction as well as displacements along the magnetization axis that increase in magnitude when increasing the interfacial exchange coupling. Overlap functions computed from the spin configurations along the loops have been obtained to explain the origin and magnitude of these features microscopically.     

Magneto-Coulomb effect in carbon nanotube quantum dots filled with magnetic nanoparticles

Electrical transport measurements of carbon nanotubes filled with magnetic iron nanoparticles are reported. Low-temperature (40 mK) magnetoresistance measurements showed conductance hysteresis with sharp jumps at the switching fields of the nanoparticles. Depending on the gate voltage, positive or negative hysteresis was observed. The results are explained in terms of a magneto-Coulomb effect: The spin flip of the iron island at a nonzero magnetic field causes a shift of the chemical potential induced by the change of Zeeman energy; i.e., an effective charge variation is detected by the nanotube quantum dot.     

Ferroelectricity of polycrystalline GdMnO3 and multifold magnetoelectric responses

The multiferroic behaviors of polycrystalline GdMnO₃ are investigated by focusing on the ferroelectric response to the spin ordering sequence and external magnetic field. The polarization current shows sensitive response to both the Mn cycloidal spin order and Gd antiferromagnetic (AFM) order. The complicated magnetoelectric behaviors suggest that the Mn cycloidal spin order can be modulated by the Gd AFM order at low temperature via the Gd–Mn spin interaction. Due to the possible disorder and defects in polycrystalline nature, polycrystalline GdMnO₃ may accommodate the cycloidal spin order in addition to the A-type AFM order at Mn sites, as illustrated by simulation based on the two-orbit double exchange model and measured hysteresis loops of polarization against magnetic field, indicating the switching of the ferroelectric domains coupled with the magnetic domains in response to magnetic field.     

Micromagnetic simulations of 200-nm-diameter cobalt nanorings using a Reuleaux triangular geometry

Using micromagnetic simulations, we investigated the magnetic states and switching processes of Co nanorings with lateral dimensions of 200 nm. We propose a special geometry of nanorings that adopts different Reuleaux triangular shapes. Reuleaux's triangles (RT) combine both the equilateral triangle and circular geometries. We studied the magnetic spin configurations of individual nanorings by varying the thickness and geometry of the nanomagnets. Our results demonstrated that in most nanomagnets exhibiting a thickness of less than 4 nm, there exists an onion-type state, which precedes either a twisted, double twisted, or cardioid state, when studying the magnetization reversal process. The hysteresis loops and magnetic states found in these RTs are compared with circular nanorings.     

Comparative study of coherent terahertz emission from Fe/Pt ferromagnetic heterostructures

The ultrafast spin dynamic of in-plane magnetized Fe/Pt films was investigated by terahertz emission spectroscopy. The amplitude of the emitted terahertz wave is proportional to the intensity of the exciting laser beams.Both the amplitude and polarity of the terahertz wave can be adjusted by modifying the external magnetic field.The dependency of the amplitude on external magnetic fields is coincident to the hysteresis loops of the sample.Also, the polarity of the terahertz wave is reversed, as the magnetization orientation is reversed. The superdiffusive transient spin current with an inverse spin Hall effect is attributed to the main mechanism of the terahertz emission.     

永磁性大块金属玻璃Nd60Al10Fe20Co10低温磁性的研究

研究了Nd6 0 Al1 0 Fe2 0 Co1 0 大块金属玻璃磁性随温度的变化关系 ,结果表明Nd6 0 Al1 0 Fe2 0 Co1 0 在室温下表现为永磁性 ,随着温度的降低 ,矫顽力和磁滞回线形状都有很大的变化 .交流磁化率在 18K左右出现尖峰而且峰值温度随频率变化 ,表明该大块非晶体系中存在自旋玻璃态     

Calculation of Magnetic Configuration and Resistance for Nanomagnets

Based on a classical Heisenberg lattice model with dipole interaction and the method of spin dynamic simulation,the magnetic configurations (MC),hysteresis loop (HL) and magnetic resistance (MR) of the nanomagnets with different geometries,such as circle,square and rectangle,are studied for different directions of applied field.In the case of perpendicular field to the plane,the magnetization and MR are reversible and have not hysteresis.When the field is applied in the plane,the HL is irreversible and is qualitatively well agreeable with the current experimental results.The MR loop is also irreversible and appears two peaks distributed at two sides around zero field.The peaks of magnetic resistance are relative to the vortex state of similar configuration.Large easy-axis anisotropy will suppress the MC anisotropy,and the large magnetoresistance effect disappears.     

Hysteresis characteristics of an analytical vector hysteron

The analytical way to describe uniaxial single-domain particles, using new symmetry considerations on the magnetic anisotropy, as a vector hysteresis unit (vector hysteron), is presented. The main characteristics of the vector hysteron such as the longitudinal hysteresis loops, transversal hysteresis loops and rotational hysteresis loops are presented. An extension of the vector hysteron and a vector hysteron that can be applied to the ferroelectric hysteresis are also presented.     

Effects of the Copper and Oxygen atoms of the CuO-plane on magnetic properties of the YBCO by using the effective-field theory

We investigate the effects the Copper and Oxigen atoms of the CuO-plane on magnetic properties in the YBCO within the frame work of the Ising model by using the effective field theory. In particular, we study the thermal behaviors of magnetizations of CuO-plane, YBCO and their components with taking the antiferromagnetic spin orientations. We find that the system undergoes a second-order phase transition and only the magnetization of CuO-plane illustrates the N-type compensation behavior. We also investigate the hysteresis loops behavior of CuO-plane, YBCO and their components and present the superconducting phase diagram. The phase diagram contains two different Meissner and four different vortex and a normal states.     

磁泡材料的磁滞迴线和泡阵磁畴的剩磁

本文采用一种用光电倍增管作为检测装置的技术,可以自动绘出透明磁性介质的磁滞迴线,测得了不同磁泡外延膜样品,在不同磁畴形状等条件下的磁迴线;解释了曲线的特点,并从实验上证实,具有泡阵磁畴的磁泡膜是处于一种特殊的剩磁状态。 关键词：     

The remagnetization process in spin-valve structures

The process of magnetization reversal in 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 the bottom layers. New types of hysteresis loops are found for characteristic relative magnitudes of the parameters. Analysis of the dependence of the shape of hysteresis loops on the magnitude of interlayer exchange is performed. The phase diagram which determines the regions of existence of characteristic hysteresis loops for different relative magnitudes of the uniaxial anisotropy constant and exchange constant J₁ is constructed.     

Complex anisotropy and magnetization reversal on stepped surfaces probed by the magneto-optical Kerr effect

So-called split hysteresis loops have been measured for ultrathin ferromagnetic films grown on stepped surfaces. Since the shape of the loops is sensitive to the direction in which the magnetic field is applied with respect to the steps, the sample orientation against the field is particularly important. We performed systematic magneto-optical Kerr effect studies for 15 and 58 ML of Fe grown on Au(1,1,13). In view of the complex magnetic anisotropy of such systems we discuss representative hysteresis loops taken at sample orientations misaligned from the field (and laser beam) direction. In particular, the presence of a so-called low field component to the hysteresis loops is discussed and its reversed polarity is explained.     

Tension-induced multistability in inextensible helical ribbons

We study the nonmonotonic force-extension behavior of helical ribbons using a new model for inextensible elastic strips. Unlike previous rod models, our model predicts hysteresis behavior for low-pitch ribbons of arbitrary material properties. Associated with it is a first-order transition between two different helical states as observed in experiments with cholesterol ribbons. Numerical solutions show nonuniform uncoiling with hysteresis also occurring under controlled tension. They furthermore reveal a new uncoiling scenario in which a ribbon of very low pitch shears under tension and successively releases a sequence of almost planar loops. Our results may be relevant for nanoscale devices such as force probes.