Asymmetric Magnetization Reversal Probed by Recoil Loop Measurements inan Exchange Biased La0.67Sr0.33MnO3/La0.33Ca0.67MnO3 Bilayer Film

We exploit the recoil loop measurements to study La0.07 Sr0.33 Mn O3/La0.33 Ca0.67 Mn O3 bilayer film. the asymmetric magnetization reversal in an exchange-biased It is found that the recoil curve encloses a marked area only in the second quadrant of the hysteresis loop, and the recoil susceptibility in the descending branch of the major loop is evidently higher than that in the ascending branch. The study indicates that the exchange anisotropy of a unidirectional nature and an orientation deviated from the easy axis of the ferromagnetic layer plays a crucial role in creating the reversal asymmetry.     

Numerical simulation of magnetization process in antiferromagnetic–ferromagnetic bilayer with compensated interface

The properties of antiferromagnetic (AFM)–ferromagnetic (FM) bilayer have been studied using self-consistent mean-field approximation for Heisenberg Hamiltonian. The perpendicular exchange coupling has been revealed in a bilayer with a compensated interface. For a uniform AFM film a symmetrical hysteresis loop has been calculated, because the transverse instability develops within the AFM film at certain critical value of external magnetic field. On the other hand, shifted hysteresis loop with a finite exchange bias field has been obtained for a non-uniform AFM film consisting of various AFM domains with perpendicular directions of the easy anisotropy axes.     

Asymmetric magnetization reversal behavior and noncollinear anisotropies in exchange-bias system

The effect of noncollinearity between unidirectional and uniaxial anisotropies on asymmetric magnetization reversal of ferromagnet/antiferromagnet (FM/AFM) bilayer has been investigated. The results show the emergence of noncollinear anisotropies comes from the competition among applied magnetic field, magnetic anisotropy and exchange coupling in FM/AFM interface. The noncollinearity can lead to the asymmetry of hysteresis loop of FM/AFM bilayer. However, when the magnetic field is applied along the uniaxial anisotropy axis of FM layer, the hysteresis loop of FM/AFM bilayer is always symmetry independence of the noncollinear angle. Our results indicate that the asymmetry not only originates from the noncollinearity but also depends on the applied magnetic field orientation. Moreover, the asymmetry of hysteresis loop is always along with the appearance of unequivalence for magnetization reversal of FM/AFM bilayer, and there is a periodicity of π with orientation of applied field for its periodicity independence of the angle of the noncollinearity between the uniaxial and unidirectional anisotropies. The results can help us to open additional avenues to tailor the future advance magnetic device.     

MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

FMR studies in the TbFeCo/GdFeCoSi bilayers

Bilayers, TbFeCo/GdFeCoSi, made by sputtering on glass substrate with buffer and capping layers were studied by measuring the hysteresis loop and by ferromagnetic resonance (FMR). When the field H was applied along the film normal, a double H_C hysteresis loop related to the two sublayers was observed. In ferromagnetic resonance measurements, a peculiar out-of-plane angular dependence of FMR spectrum was obtained. When scanning field H was 0-637 kA/m less than the anisotropy field of TbFeCo sublayer, two FMR peaks were observed. One peak was characteristic of uniaxial and unidirectional anisotropy. The anisotropy constants were obtained by fitting the data with the theory of FMR, and this peak was considered to be related to the low anisotropy GdFeCoSi layer. The second peak appeared only when the dc field H was orientated in a limited angular range around 180°. This peak was considered to be related to an uncoupled interfacial GdFeCoSi sublayer near Al capping layer. However, when H was scanned between 0-1114 kA/m, only one peak is observed due to magnetization reversal of TbFeCo layer with uniaxial anisotropy.     

Thermally activated magnetization reversal in magnetic tunnel junctions

In this paper, the magnetization reversal of the ferromagnetic layers in the IrMn/CoFe/AlO_x/CoFe magnetic tunnel junction has been investigated using bulk magnetometry. The films exhibit very complex magnetization processes and reversal mechanism. Thermal activation phenomena such as the training effect, the asymmetry of reversal, the loop broadening and the decrease of exchange field while holding the film at negative saturation have been observed on the hysteresis loops of the pinned ferromagnetic layer while not on those of the free ferromagnetic layer. The thermal activation phenomena observed can be explained by the model of two energy barrier distributions with different time constants.     

Study of magnetization reversal and anisotropy of single crystalline ultrathin Fe/MgO(001) film by magneto-optic Kerr effect

The magnetization reversal process of Fe/MgO(001) thin film is investigated by combining transverse and longitudinal hysteresis loops. Owing to the competition between domain wall pinning energy and weak uniaxial magnetic anisotropy,the typical magnetization reversal process of Fe ultrathin film can take place via either an "l-jump" process near the easy axis, or a "2-jump" process near the hard axis, depending on the applied field orientation. Besides, the hysteresis loop presents strong asymmetry resulting from the variation of the detected light intensity due to the quadratic magneto-optic effect. Furthermore, we modify the detectable light intensity formula and simulate the hysteresis loops of the Kerr signal. The results show that they are in good agreement with the experimental data.     

Soft X-ray resonant magnetic scattering studies on Fe/CoO exchange bias system

We have used soft X-ray resonant magnetic scattering (XRMS) to search for the presence of an effective ferromagnetic moment belonging to the antiferromagnetic (AF) layer which is in close contact with a ferromagnetic (F) layer. Taking advantage of the element specificity of the XRMS technique, we have measured hysteresis loops of both Fe and CoO layers of a CoO(40 Å)/Fe (150 Å) exchange bias bilayer. From these measurements we have concluded that the proximity of the F layer induces a magnetic moment in the AF layer. The F moment of the AF layer has two components: one is frozen and does not follow the applied magnetic field and the other one follows in phase the ferromagnetic magnetization of the F layer. The temperature dependence of the F components belonging to the AF layer is shown and discussed.     

Field sweep rate dynamics in magnetic tunnel junctions

The dynamical behavior of magnetic tunnel junctions (MTJs) was investigated by varying the magnetic field sweep rate from 0.01 mT/s to 10 T/s in a magneto optical Kerr effect set-up. The bias fields of the pinned and free ferromagnetic electrodes were found to drastically decrease above a field sweep rate of 1 T/s. This decrease in the bias fields coincides with a change in the magnetization reversal process from domain wall motion at low-field sweep rates to domain nucleation at high-field sweep rates. The nucleation of inverse domains in the ferromagnetic layer changes the interfacial spin structure of the antiferromagnetic layer and therefore the magnitude of the exchange bias effect. Furthermore, the nucleation of domains induces a discontinuous magnetic charge density at the tunnel barrier interfaces and this reduces the interlayer coupling between the two ferromagnetic electrodes of the MTJ.     

Brillouin light scattering study of the magnetic hysteresis loop in Fe-Ni/Si(100) film with induced magnetic anisotropy

The magnetic hysteresis of Fe₅₇Ni₄₃/Si(100) with magnetic anisotropy induced by an external field has been studied by Brillouin light scattering (BLS). The results are compared with those of the magneto-optic-Kerr-effect (MOKE) measurement and the vibrating sample magnetometer (VSM). The BLS results show that the sample film has strong in-plane anisotropy. The angle between the magnetization and a 4.6 G applied magnetic field H reaches a maximum value of 45° when H lies along the hard axis. The coercivity and magnetic anisotropy field for the film obtained by the BLS are compared with the values obtained by the VSM and MOKE measurement.     

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.     

Magnetocaloric effect in DyCu2

The magnetocaloric effect of the rare-earth intermetallic compound DyCu₂ is explored through magnetization measurements. DyCu₂ is paramagnetic at the room temperature but becomes antiferromagnetic below 27 K (Neel temperature). Strong temperature and field dependence of magnetization in DyCu₂ at and around the Neel temperature lead to a large magnetocaloric effect. An appreciable magnetocaloric effect persists well above the Neel temperature probably because of the presence of short-range ferromagnetic correlations in the paramagnetic state of DyCu₂. This along with the absence of magnetic hysteresis lead to a large effective refrigerant capacity of 194 J/kg below 44 K, which makes the material important as a potential magnetic refrigerant for the cryogenic liquefier cycles.     

Direct experimental investigation of spin-reorientation phase transitions in an antiferromagnetic CoNiCu/Cu superlattice

The characteristic features of phase transitions induced by an external magnetic field and of the corresponding changes in the relative orientations of the spins in the ferromagnetic CoNiCu layers of a multilayer film, which are coupled by an antiferromagnetic exchange interaction via nonmagnetic Cu interlayers, are studied using a magnetooptic method for visualizing the fringing fields. It is established that the magnetization reversal process in this nanocomposite material proceeds by a spin-flop orientational phase transition on account of the formation and motion of specific domain walls as well as by incoherent rotation of the spins toward the applied field. It is shown that, depending on the direction of the external magnetic field with respect to the easy axis, asymmetric canted phases also arise as a result of such transitions. Pis’ma Zh. Tekh. Fiz. 64, No. 11, 778–782 (10 December 1996)     

Effect of illumination on magnetization and microwave absorption of La1−xCaxMnO3 (x<0.2) films

Light-induced changes of the hysteresis loops of magnetization and microwave absorption are investigated in low-doped La_1−xCa_xMnO₃ (x<0.2) thin films. The width of the hysteresis loops decreases clearly under illumination with visible or near-infrared light at temperatures below 50 K. The microwave conductivity has a minimum value at magnetic fields corresponding to the magnetization reversal and is shifted towards weaker fields under illumination. These effects show complex nonexponential time evolution and dependence on strength of the magnetic field. The results can be explained by assuming that small ferromagnetic metallic regions exist within the insulating ferromagnetic phase of the sample, and that these regions are expanded by optically induced charge transfer between Jahn–Teller split e_g states of neighboring Mn³⁺ ions. Decrease of the Mn³⁺ XPS core level spectrum is observed in the samples under illumination with a HeNe laser.     

Anisotropic pinned/biased magnetization in SrRuO3/SrMnO3 superlattices

The exchange coupling at the interfaces of magnetic superlattices consisting of ferromagnetic SrRuO3 and antiferromagnetic SrMnO3 grown on (001) oriented SrTiO3 is studied with in-plane and out-of-plane orientations of the cooling magnetic field, with respect to the substrate plane. The magnetization of the in-plane, field cooled hysteresis loop is lower than the corresponding in-plane zero-field-cooled hysteresis loop. The out-of-plane field cooled hysteresis loop is shifted, from the origin, along the graphical magnetization axis. We attribute this irreversible rotation of the moment to the pinning/biasing of spin in the SrRuO3 layer in the vicinity of interfaces by the antiferromagnetic SrMnO3 layer.     

小尺寸正方形铁磁薄膜边界效应对磁化翻转过程的影响

用自旋动力学方法系统地研究磁偶极相互作用表现的边界效应对小尺寸正方形铁磁薄膜的磁化翻转过程的影响.在确定的磁偶极相互作用强度下,针对不同的单轴各向异性强度和不同的磁化角(外磁场与易轴间的夹角),具体给出矫顽场与磁化角及单轴各向异性强度之间的依赖关系和-些有代表性的磁滞回线,并给出磁化翻转过程中-些有代表性的微观磁结构.模拟结果表明:磁偶极相互作用表现的边界钉扎作用与单轴各向异性场之间的竞争决定磁滞回线的形状和矫顽场的大小,从而在不同磁化角情况下会导致不同的矫顽场机理.本文提出可有效地描述正方形铁磁性薄膜复杂微观磁畴结构的形成与演变的五磁畴模型.这种五磁畴模型既能直接揭示单轴各向异性正方形铁磁薄膜的几何特性和物理特性,也方便于磁化翻转过程的分析.     

Effect of the surface anisotropy and the shape on the magnetization behavior in an egg-shaped nanoparticle

Using the classical Heisenberg model and Monte Carlo simulation, we compute the magnetization behavior of a ferromagnetic nanoparticle with an egg-shape in an external magnetic field along the symmetry axis. The particle is in a single-domain state with a surface anisotropy axis perpendicular to the surface of the particle. At low temperature, it is found that exchange bias appears in the hysteresis loop, which does not exist in the spherical and ellipsoidal nanoparticles. The bias field produced by the frozen spins on the surface of the egg-shaped nanoparticle may be the reason to produce the phenomenon of the exchange bias.     

Asymmetric reversal modes in ferromagnetic/antiferromagnetic multilayers

Experimentally an asymmetry of the reversal modes has been found in certain exchange bias systems. From a numerical investigation of the domain state model evidence is gained that this effect depends on the angle between the easy axis of the antiferromagnet and the applied magnetic field. Depending on this angle the ferromagnet reverses either symmetrically, e.g., by a coherent rotation on both sides of the loop, or the reversal is asymmetric with a nonuniform reversal mode for the ascending branch, which may even yield a zero perpendicular magnetization.     

Large vertical loop shifts in mechanically synthesized (Mn,Fe)2O3−t nanograins

Magnetic properties of zero field cooled (ZFC) and field cooled (FC) sample of (Mn,Fe)₂O_3−t nanograins have been investigated by magnetometry (up to 70 kOe) and Mössbauer spectroscopy (up to 60 kOe) in the temperature interval 4.2–300 K. Large horizontal (up to 0.8 kOe) and vertical (up to 80%) shifts of the magnetization hysteresis loops are observed in the FC regime. The obtained results are discussed in terms of exchange interaction between an antiferromagnetic core and a spin-glass-like state of the nanograins boundaries. It is shown that hysteresis loop shifts (horizontal and vertical) depend on the field cooling magnitude, an effect that can be understood by the change of the boundary magnetic structure induced by the external magnetic field. The vertical magnetization shift is described by a phenomenological model, which takes into account the magnetic interaction between the spin-glass like boundary spins and the applied field.