Relative-thickness dependence of exchange bias in bilayers and trilayers

We consider the models of ferromagnetic (FM)/antiferromagnetic (AFM) bilayers and trilayers and perform a modified Monte Carlo method to study their exchange bias (EB) properties at low temperature after field cooling on increasing one component thickness at the expense of the other one. The results indicate that EB is insensitive to the thickness variations as the FM layer is thicker than the AFM one. Otherwise, it has a steep increase with the decrease of FM thickness, but the purely inverse proportion is no longer valid due to the dual influences of FM and AFM thicknesses. EB in trilayers should be approximately twice larger than that in bilayers because there is a double interfacial area in the trilayers compared with the bilayers, but the dispersed FM/AFM distributions may break this relation as a result of thermal destabilization. Moreover, EB is independent of FM/AFM stacking sequences probably because of the ideal interface between them. It has been clarified unambiguously that such control of EB through varying the FM/AFM dimensions in heterostructures is attractive for spintronics applications.     

Room temperature exchange bias in multiferroic BiFeO<Subscript>3</Subscript> nano- and microcrystals with antiferromagnetic core and two-dimensional diluted antiferromagnetic shell

Exchange bias (EB) of multiferroics presents many potential opportunities for magnetic devices. However, instead of using low-temperature field cooling in the hysteresis loop measurement, which usually shows an effective approach to obtain obvious EB phenomenon, there are few room temperature EB. In this article, extensive studies on room temperature EB without field cooling were observed in BiFeO₃ nano- and microcrystals. Moreover, with increasing size the hysteresis loops shift from horizontal negative exchange bias (NEB) to positive exchange bias (PEB). In order to explain the tunable EB behaviors with size dependence, a phenomenological qualitative model based on the framework of antiferromagnetic (AFM) core-two-dimensional diluted antiferromagnet in a field (2D-DAFF) shell structure was proposed. The training effect (TE) ascertained the validity of model and the presence of unstable magnetic structure using Binek’s model. Experimental results show that the tunable EB effect can be explained by the competition of ferromagnetic (FM) exchange coupling and AFM exchange coupling interaction between AFM core and 2D-DAFF shell. Additionally, the local distortion of lattice fringes was observed in hexagonal-shaped BiFeO₃ nanocrystals with well-dispersed behavior. The electrical conduction properties agreed well with the space charge-limited conduction mechanism.     

Recent advances in exchange bias of layered magnetic FM/AFM systems

The exchange bias(EB) has been investigated in magnetic materials with the ferromagnetic(FM)/antiferromagnetic(AFM) contacting interfaces for more than half a century.To date,the significant progress has been made in the layered magnetic FM/AFM thin film systems.EB mechanisms have shown substantive research advances.Here some of the new advances are introduced and discussed with the emphasis on the influence of AFM layer,the interlayer EB coupling across nonmagnetic spacer,and the interlayer coupling across AFM layer,as well as EB related to multiferrioc materials and electrical control.     

Exchange bias and asymmetric reversal in nanostructured dot arrays

The size dependence of exchange bias field HE and coercivity Hc was studied by measuring exchange biased Fe-FeF2 dot arrays in comparison with an unstructured exchange biased Fe-FeF2 bilayer. The domain sizes in the ferromagnet (FM) and the antiferromagnet (AFM) play an important role for exchange bias (EB), and thus interesting phenomena may be expected when the size of an EB system becomes comparable to these sizes. We observe drastic changes of HE and Hc in nanostructured Fe-FeF2, which are unexpected because they appear even at a structure size which is too large for matching with AFM or FM domain size to play a role. We propose that under certain conditions the hysteresis loop is affected differently in the two branches of the reversal by shape anisotropy due to patterning. This is possible because the EB induces a reversal asymmetry already in the unpatterned bilayer system.     

Tuning exchange bias through zero field cooling from different remanent states above blocking temperature in Ni50Mn36Sb14 alloy

Changing remanent states above blocking temperature (T_B) in Ni₅₀Mn₃₆Sb₁₄ alloy has been proven to be an effective way of tuning the value and sign of exchange bias (EB) field. The hysteresis loops at 5 K exhibit double shifted shape, indicating that there are two opposite EB signs resulting from an imprint of domain pattern of ferromagnetic (FM) regions into anti‐ferromagetic (AFM) ones during cooling. All the results demonstrate that the interfacial spin configuration plays a crucial role on the origin of EB, while the high cooling field not only induces a single FM domain state above T_B but also tunes the fractions of FM and AFM interactions through martensitic transition. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Exchange bias in ferromagnet/antiferromagnet bilayers

Since the exchange bias （EB） effect was discovered in the Co/CoO core-shell nanoparticles, it has been extensively studied in various ferromagnet （FM）/antiferromagnet （AFM） bilayers due to its crucial role in spintronics devices. In this article, we review the investigation of the EB in our research group. First, we outline basic features of the EB, including the effects of the constituent layer thickness, the microstructure and magnetization of the FM layers, and we also discuss asymmetric magnetization reversal process in wedged-FM/AFM bilayers. Secondly, we discuss the mechanisms of the positive EB and the perpendicular EB. Thirdly, we demonstrate the hysteretic behavior of the angular dependence of the EB and analyze the EB training effect. Finally, we discuss the roles of the rotatable anisotropy in the two phenomena.     

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 in Layered GdBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co₂O_{5 + δ} cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo₂O_5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H _EB , blocking temperature T _B , exchange coupling energy J _i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co³⁺ and Co⁴⁺ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.     

Interfacial roughness and temperature effects on exchange bias properties in coupled ferromagnetic/antiferromagnetic bilayers

Monte Carlo simulations have been used to study the relationship between the exchange bias properties and the interface roughness in coupled ferromagnetic/antiferromagnetic (FM/AFM) films of classical Heisenberg spins. It is shown that the variation of the exchange bias field versus the AFM anisotropy strongly depends on the FM/AFM interface. Unlike the flat interface, a non-monotonic dependence is observed for the roughest FM/AFM interface. This is explained by canted magnetic configurations at the FM/AFM interface, which appear after the first reversal due to the magnetic frustration. The temperature dependence of the exchange field is also dependent on the roughness. While the exchange field is roughly constant for the flat interface, a decrease is observed for the roughest interface as the temperature increases. This has been interpreted as a significant decrease of the effective coupling between the FM and the AFM due to the disordering of the moments at the FM/AFM interface because of the combination of magnetic frustration and temperature activation.     

Dynamic magnetic behavior of the mixed spin （2, 5/2） Ising system with antiferromagnetic/antiferromagnetic interactions on a bilayer square lattice

Using the mean-field theory and Glauber-type stochastic dynamics, we study the dynamic magnetic properties of the mixed spin （2, 5/2） Ising system for the antiferromagnetic/antiferromagnetic （AFM/AFM） interactions on the bilayer square lattice under a time varying （sinusoidal） magnetic field. The time dependence of average magnetizations and the thermal variation of the dynamic magnetizations are examined to calculate the dynamic phase diagrams. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and the effects of interlayer coupling interaction on the critical behavior of the system are investigated. We also investigate the influence of the frequency and find that the system displays richer dynamic critical behavior for higher values of frequency than that of the lower values of it. We perform a comparison with the ferromagnetic/ferromagnetic （FM/FM） and AFM/FM interactions in order to see the effects of AFM/AFM interaction and observe that the system displays richer and more interesting dynamic critical behaviors for the AFM/AFM interaction than those for the FM/FM and AFM/FM interactions.     

The magnetoresistive effect induced by stress in spin-valve structure

Using a method of free energy minimization, this paper investigates the magnetization properties of a ferromagnetic (FM) monolayer and an FM/antiferromagnetic (AFM) bilayer under a stress field, respectively. It then investigates the magnetoresistance (MR) of the spin-valve structure, which is built by an FM monolayer and an FM/AFM bilayer, and its dependence upon the applied stress field. The results show that under the stress field, the magnetization properties of the FM monolayer is obviously different from that of the FM/AFM bilayer, since the coupled AFM layer can obviously block the magnetization of the FM layer. This phenomenon makes the MR of the spin-valve structure become obvious. In detail, there are two behaviors for the MR of the spin-valve structure dependence upon the stress field distinguished by the coupling (FM coupling or AFM coupling) between the FM layer and the FM/AFM bilayer. Either behavior of the MR of the spin-valve structure depends on the stress field including its value and orientation. Based on these investigations, a perfect mechanical sensor at the nano-scale is suggested to be devised experimentally.     

铁磁和反铁磁双层膜中铁磁共振的研究

采用微磁学理论研究了铁磁/反铁磁双层膜中的铁磁共振现象.本模型将铁磁薄层抽象为一个单晶,具有立方磁晶各向异性和单轴磁晶各向异性,而反铁磁层视为厚度趋近于半无穷,且只有单轴磁晶各向异性.推导出了该系统的铁磁共振频率和频率谱宽度的解析式.数值计算表明,铁磁共振模式分两支,取决于立方磁晶各向异性.而界面的交换耦合,是磁易轴具有单向性的起因.     

外应力场下铁磁/反铁磁双层膜系统的铁磁共振性质

从系统能量出发，采用Smith和Beljers理论方法研究了铁磁/反铁磁双层膜系统在外应力场下的铁磁共振现象.本模型中铁磁薄层具有单轴磁晶各向异性和立方磁晶各向异性，而反铁磁层非常薄因而其能量可忽略.推导出了该系统的铁磁共振频率和频谱宽度的解析式.结果表明：外应力场仅在低磁场下对具有立方磁晶各向异性系统的铁磁共振有影响，且区分弱磁场和强磁场的临界场依赖于外应力场的方向. 关键词： 铁磁/反铁磁双层膜 交换偏置 铁磁共振 应力场     

铁磁/反铁磁双层膜系统中交换各向异性及其厚度依赖性

研究铁磁/反铁磁双层膜系统中交换偏置场和矫顽场的冷却磁场依赖性.结果表明,随着冷却磁场的增加,交换偏置场由负值向正值转变.在转变点附近,矫顽场有-个特别的增强,并达到最大值.结果同相关实验-致.研究铁磁层和反铁磁层厚度对交换偏置场和矫顽场的影响.发现,正负交换偏置场和矫顽场随着铁磁层厚度的增大而减小,但随反铁磁层厚度的变化关系复杂.在正交换偏置场的情形,随反铁磁层厚度的增大,交换偏置场增强,矫顽场减弱;在负交换偏置场的情形,随反铁磁层厚度的增大,交换偏置场减弱,矫顽场增强.     

外应力场下铁磁/反铁磁双层膜系统中的自旋波

采用自由能极小的方法研究了铁磁/反铁磁双层膜系统在外应力场下的一致进动自旋波性质，即铁磁共振现象. 本模型中铁磁层很薄可看成单畴结构，但具有单轴磁晶各向异性和立方磁晶各向异性；而反铁磁层仅具有单轴磁晶各向异性，但其厚度趋于半无穷. 推导出了该系统的铁磁共振频率和频谱宽度的解析式. 结果表明，外应力场和界面交换耦合或反铁磁磁强度仅在弱磁场下对系统的铁磁共振有影响，且系统的铁磁共振行为按磁场强度可分为两支，其区分弱磁场和强磁场的临界场依赖于外应力场的方向. 另一方面，应力场方向的改变可借助于反铁磁层磁畴变化对铁磁层磁晶各向异性轴有影响. 关键词： 铁磁/反铁磁双层膜 界面耦合强度 铁磁共振 应力场     

Influences of Crystal-Field and Interlayer Coupling Interactions on Dynamic Phase Diagrams of a Mixed-Spin(3/2,2) Bilayer System

Influences of crystal-fields(D_A and D_B) and interlayer coupling interactions(J_3) on dynamic magnetic critical behaviors of a mixed-spin(3/2, 2) bilayer system under an oscillating magnetic field are investigated by the Glauber-type stochastic dynamics based on the mean-field theory. For this purpose, dynamic phase diagrams are constructed in the reduced temperature and magnetic field amplitude plane for the ferromagnetic/ferromagnetic(FM/FM),antiferromagnetic/ferromagnetic(AFM/FM) and AFM/AFM interactions in detail. We observe that the influences of D_A, D_B and J_3 interactions parameters on the behavior of the dynamic phase diagrams are very much.     

The hysteresis loops of FM/AFM two-layer Bethe lattice

The distinct hysteresis loops (HLs) of ferromagnetic/antiferromagnetic (FM/AFM) two-layer Bethe lattice with the Ising spins of the top layer having only FM interactions and the bottom ones having only AFM interactions with the interlayer coupling is either FM or AFM type are studied by using a pairwise approach. The sublattice magnetizations are studied by increasing and decreasing the external magnetic field (H) to obtain the HLs. The shapes of the HLs are strongly dependent on the competitions among the system parameters and on the phase configurations. The HLs are formed only when the AFM-type interactions are involved. The small loops of hysteresis are also formed because of the reentrant behavior in the FM region.     

Simulating a perpendicular exchange-biased structure with a partially covering nonmagnetic insertion at the FM/AFM interface

The existence of minority spins, opposite to the perpendicular exchange bias, and majority spins aligned with this bias, and the dependence of the perpendicular exchange-bias field on the imprint effect, caused by the partially covering spacer at the ferromagnetic/antiferromagnetic (FM/AFM) interface, have been studied using Ising-type simulations. The present investigation suggests that the main factors influencing this phenomenon were dependent on the FM/AFM interface morphology, the balance between FM/AFM coupling and the FM-spins coupling, and the numerical balance between minority and majority spins. It was also determined that the imprint phenomenon can be used to enhance the perpendicular exchange-bias for small partial insertions at the FM/AFM interface. An erratum to this article is available at.     

Exchange bias in nanostructures

The phenomenology of exchange bias and related effects in nanostructures is reviewed. The types of systems discussed include: lithographically fabricated ferromagnetic (FM)—antiferromagnetic (AFM) nanostructures, chemically surface modified FM particles, FM particles embedded in an AFM matrix, controlled core–shell particles, nanoparticles with surface effects and coupled AFM–AFM systems. The main applications of exchange biased nanostructures are summarized. Finally, the implications of the nanometer dimensions on some of the existing exchange bias theories are briefly discussed.     

Effects of Cu dilution in IrMn on the exchange bias of CoFe/IrMn bilayers

The effect of nonmagnetic dilution in metallic antiferromagnets (AFMs) on the exchange bias (EB) has been investigated from a structural, magnetic, and Monte Carlo simulation point of view in bilayers of CoFe/(IrMn)1-xCux. Dilution by Cu atoms throughout the volume of the AFM IrMn gives rise to an enhanced EB field (HEB) for 5 K相似文献