Observation of unequal activation volumes of wall-motion and nucleation processes in Co/Pd multilayers

We report the difference in activation volumes of wall-motion and nucleation processes in Co/Pd multilayers. Each activation volume was estimated from field dependence of wall-motion speed and nucleation rate, obtained by real-time domain imaging using a magneto-optical Kerr effect microscope. Delicate analysis shows that the two activation volumes are generally unequal and the ratio between the volumes varies noticeably from 0.9 to 1.1 with change of the multilayered structure. We found that the inequality in the two activation volumes has a crucial influence on magnetization reversal behavior: a process having a smaller activation volume dominates.     

Activation volumes of wall-motion and nucleation processes in Co-based ferromagnetic multilayer films

Activation volumes of the wall-motion and nucleation processes in Co-based multilayer films were characterized from time-resolved domain evolution patterns. These activation volumes were both sensitive to the multilayer structure as well as the film preparation condition. The two activation volumes were generally unequal with each other and the inequality directly influenced on magnetization reversal behavior.     

Microscopic origin of asymmetric magnetization reversal of Co/Pt multilayers with perpendicular magnetic anisotropy

We have comprehensively investigated asymmetric magnetization reversal behaviors of (x-Å Co/7.7 Å Pt)₅ multilayers (x = 3.1 and 4.7) with perpendicular magnetic anisotropy. Our direct observation of magnetic domain structures by means of magneto-optical microscopy reveals that the asymmetry arises both from nucleation and wall-motion processes. An asymmetric nucleation behavior is observed, which could be originated from the preexisting non-reversed domains which might have a reproducible or random spatial distribution, controllable by tuning the field profile. An asymmetric wall-motion behavior stemming from asymmetric stripe domain evolution is also observed.     

Complete magnetizing field relating with magnetization reversal dynamics

We report the experimental finding that a complete magnetizing field H_M exists in magnetization reversal dynamics of ferromagnetic thin films, which is much larger than the apparent magnetic saturation field measured from the major hysteresis loop. Magnetization reversal dynamics contrastingly changes from nucleation dominated to wall-motion dominated according to an initial magnetization state magnetized by a field below H_M, whereas it is basically unchanged when the field is larger than H_M. The complete magnetizing field is found to be 1.5–2.0 times larger than the apparent magnetic saturation field and 6–10 times smaller than the anisotropy field in Co/Pd multilayer thin films.     

Compressed exponential form for disordered domain wall motion in ultra-thin Au/Co/Au ferromagnetic films

Magnetization reversal in ultra-thin Au/Co/Au films deposited on single crystal silicon (1 0 0) was investigated using Kerr microscopy. In the considered ultra-thin Co films, with a thickness between 0.7 and 1 nm, the coercivity and magnetic anisotropy decrease with decrease in cobalt layer thickness and the magnetization reversal dynamics is dominated by disordered domain wall motion. An analysis of the observed magnetization reversal dynamics is proposed, starting from the Fatuzzo-Labrune model. We show that the relaxation curves of these samples are well described by a function obtained by a technical transformation of Fatuzzo-Labrune model in the regime dominated by domain wall motion.     

Magnetic relaxation measurements of exchange biased (Pt/Co) multilayers with perpendicular anisotropy

Magnetic relaxation measurements were carried out by magneto-optical Kerr effect on exchange biased (Pt/Co)5/Pt/FeMn multilayers with perpendicular anisotropy. In these films the coercivity and the exchange bias field vary with Pt spacer thickness, and have a maximum for 0.2 nm. Hysteresis loops do not reveal important differences between the reversal for ascending and descending fields. Relaxation measurements were fitted using Fatuzzo’s model, which assumes that reversal occurs by domain nucleation and domain wall propagation. For 2 nm thick Pt spacer (no exchange bias) the reversal is dominated by domain wall propagation starting from a few nucleation centers. For 0.2 nm Pt spacer (maximum exchange bias) the reversal is strongly dominated by nucleation, and no differences between the behaviour of the ascending and descending branches can be observed. For 0.4 nm Pt spacer (weaker exchange bias) the nucleation density becomes less important, and the measurements reveal a much stronger density of nucleation centers in the descending branch.     

Magnetic reversal processes and critical thickness in FePt/α-Fe/FePt trilayers

Magnetic reversal processes of a FePt/α-Fe/FePt trilayer system with in-plane easy axes have been investigated within a micromagnetic approach. It is found that the magnetic reversal process consists of three steps: nucleation of a prototype of domain wall in the soft phase, the evolution as well as the motion of the domain wall from the soft to the hard phase and finally, the magnetic reversal of the hard phase. For small soft layer thickness L^s, the three steps are reduced to one single step, where the magnetizations in the two phases reverses simultaneously and the hysteresis loops are square with nucleation as the coercivity mechanism. As L^s increases, both nucleation and pinning fields decrease. In the meantime, the single-step reversal expands to a standard three-step one and the coercivity mechanism changes from nucleation to pinning. The critical thickness where the coercivity mechanism alters, could be derived analytically, which is found to be inversely proportional to the square root of the crystalline anisotropy of the hard phase. Such a scaling law might provide an easy way to test the present theory. Further increase of L^s leads to the change of the coercivity mechanism from pinning to nucleation.     

Intrinsic distribution of magnetic anisotropy in thin films probed by patterned nanostructures

We demonstrate that the switching field distribution (SFD) in arrays of 50 nm to 5 microm Co/Pd elements, with perpendicular anisotropy, can be explained by a distribution of intrinsic anisotropy rather than any fabrication related effects. Further, simulations of coercivity and SFD versus element size allow the distribution of intrinsic anisotropy to be quantified in highly exchanged coupled thin films where the reversal mechanism is one of nucleation followed by rapid domain wall motion.     

Coercivity mechanisms in nanostructured permanent magnets

Coercivity mechanism in permanent magnets has been debated for many years.In this paper, various models of the coercivity mechanism are classified and re-examined by the comparison and contrast.Coherent rotation and curling models can reveal the underlying reversal mechanism clearly based on isolated grains with elliptic shapes.By contrast, the numerical methods consider inter-grain interactions while simulating the evolution of the spins and hysteresis loops with complicated shapes.However, an exact simulation of magnetic reversal in permanent nanomagnets requires many meshes to mimic the thin domain wall well.Nucleation and pinning are the two main coercivity mechanisms in permanent magnets.The former signifies the beginning of the magnetic reversal, whilst the latter completes it.Recently, it is proposed that the large difference between the intrinsic magnetic properties of the nucleation centers and those of the main phase can result in a large pinning field(self-pinning), which has the attributes of both traditional nucleation and pinning.Such a pinning explains the experimental data of permanent magnets very well, including the enhancement of the coercivity by the grain boundary pinning.     

Thermal activation of magnetization in Pr_2Fe_(14)B ribbons

The aftereffect field of thermal activation,which corresponds to the fluctuation field of a domain wall,is investigated via specific measurements of the magnetization behavior in Pr2Fe14B nanocrystalline magnets.The thermal activation is a magnetization reversal arising from thermal fluctuation over an energy barrier to an equilibrate state.According to the magnetic viscosity and the field sweep rate dependence of the coercivity,the calculated values of the fluctuation field are lower than the aftereffect field and in a range between those of domain walls and individual grains.Based on these results,we propose that the magnetization reversal occurs in multiple ways involving grain activation and domain wall activation in thermal activation,and the thermal activation decreases the coercivity by~0.2 kOe in the Pr2Fe14B ribbons.     

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 dynamics in Au/Co/Au(111) ultrathin films: Effect of roughness of the buffer layer

We present a study of the magnetization reversal dynamics in ultrathin Au/Co/Au films with perpendicular magnetic anisotropy, for a Co thickness of 0.5, 0.7 and 1 nm. In these films, the magnetization reversal is dominated by domain nucleation for t_Co=0.5, 0.7 nm and by domain wall propagation for t_Co=1 nm. The prevalence of domain nucleation for the thickness range 0.5-0.7 nm is different from results reported in the literature, for the same system and for the same thickness range, where the magnetization reversal took place mainly by domain wall motion. We attribute this difference to the effect of roughness of the Au buffer layer on the morphology of the magnetic layer.     

Tailoring the magnetic properties of Fe asymmetric nanodots

Asymmetric dots as a function of their geometry have been investigated using three-dimensional (3D) object oriented micromagnetic framework (OOMMF) code. The effect of shape asymmetry of the disk on coercivity and remanence is studied. Angular dependence of the remanence and coercivity is also addressed. Asymmetric dots are found to reverse their magnetization by nucleation and propagation of a vortex, when the field is applied parallel to the direction of asymmetry. However, complex reversal modes appear when the angle at which the external field is applied is varied, leading to a non-monotonic behavior of the coercivity and remanence.     

3D calculation of hysteresis loops,magnetic orientations and reversal processes for exchange-spring bilayers with perpendicular anisotropy

Hysteresis loops and magnetic reversal processes have been determined by a three dimensional (3D) micromagnetic model for exchange-coupled Nd₂Fe₁₄B/α-Fe bilayers and carefully compared with a popular one-dimensional (1D) micromagnetic model. It is found that the calculated hysteresis loops, the critical fields and the magnetic phase diagrams agree well with the results given by the 1D model. However, the calculated nucleation mode is a quasi-curling one where the magnetic moment exhibits a curling in the film plane and varies in the thickness direction, in contrast with the reported quasi-coherent mode. The calculated spatial distribution of the magnetization orientation in the thickness direction at various applied fields signifies a three-step magnetic reversal process, which includes nucleation, growth and displacement of the domain wall as well as the rotation and the reversal of magnetization in the hard phase. The magnetic reversal of the hard phase is much slower than that given by the 1D model, leading to a more slant hysteresis loop near the coercivity point.     

Nucleation of magnetisation reversal, from nanoparticles to bulk materials

We review models for the nucleation of magnetisation reversal, i.e. the formation of a region of reversed magnetisation in an initially magnetically saturated system. For small particles, models for collective reversal, either uniform (Stoner–Wohlfarth model) or non-uniform like curling, provide good agreement between theory and experiment. For microscopic objects and thin films, we consider two models, uniform (Stoner–Wohlfarth) reversal inside a nucleation volume and a droplet model, where the free energy of an inverse bubble is calculated, taking into account volume energy (Zeeman energy) and surface tension (domain wall energy). In macroscopic systems, inhomogeneities in magnetic properties cause a distribution of energy barriers for nucleation, which strongly influences effects of temperature and applied field on magnetisation reversal. For these systems, macroscopic material parameters like exchange interaction, spontaneous magnetisation and magnetic anisotropy can give an indication of the magnetic coercivity, but exact values for nucleation fields are, in general, hard to predict. To cite this article: J. Vogel et al., C. R. Physique 7 (2006).     

GdFeCo磁光薄膜中RE-TM反铁磁耦合与激光感应超快磁化翻转动力学研究

使用飞秒时间分辨抽运-探测磁光克尔光谱技术,研究了激光加热GdFeCo磁光薄膜跨越铁磁补偿温度时稀土-过渡金属(RE-TM)反铁磁交换耦合行为和超快磁化翻转动力学. 实验观察到由于跨越铁磁补偿温度、净磁矩携带者交换而引起的磁化翻转反常克尔磁滞回线以及在同向外磁场下,反常回线上大于和小于矫顽力部分的饱和磁化强度不同,显示出GdFeCo中RE与TM之间的非完全刚性反铁磁耦合. 在含有Al导热底层的GdFeCo薄膜上观测到饱和磁场下激光感应磁化态翻转及再恢复的完整超快动力学过程. 与剩磁态的激光感应超快退磁化过 关键词： 补偿温度 磁化翻转 反铁磁耦合 GdFeCo     

Substrate clamping effects on irreversible domain wall dynamics in lead zirconate titanate thin films

The role of long-range strain interactions on domain wall dynamics is explored through macroscopic and local measurements of nonlinear behavior in mechanically clamped and released polycrystalline lead zirconate-titanate (PZT) films. Released films show a dramatic change in the global dielectric nonlinearity and its frequency dependence as a function of mechanical clamping. Furthermore, we observe a transition from strong clustering of the nonlinear response for the clamped case to almost uniform nonlinearity for the released film. This behavior is ascribed to increased mobility of domain walls. These results suggest the dominant role of collective strain interactions mediated by the local and global mechanical boundary conditions on the domain wall dynamics. The work presented in this Letter demonstrates that measurements on clamped films may considerably underestimate the piezoelectric coefficients and coupling constants of released structures used in microelectromechanical systems, energy harvesting systems, and microrobots.     

Domain wall creep in epitaxial ferroelectric Pb(Zr(0.2)Ti(0.08)O(3) thin films

Ferroelectric switching and nanoscale domain dynamics were investigated using atomic force microscopy on monocrystalline Pb(Zr(0.2)Ti(0.8))O(3) thin films. Measurements of domain size versus writing time reveal a two-step domain growth mechanism, in which initial nucleation is followed by radial domain wall motion perpendicular to the polarization direction. The electric field dependence of the domain wall velocity demonstrates that domain wall motion in ferroelectric thin films is a creep process, with the critical exponent mu close to 1. The dimensionality of the films suggests that disorder is at the origin of the observed creep behavior.     

Magnetization reversal processes in FeSm thin films

We have investigated the in-plane magnetization reversal in FeSm thin films and discovered that it can be controlled through an induced anisotropy. For films with an induced easy direction, reversal is ultra fast and can be characterized approximately using the Fatuzzo model. In films with no pronounced induced easy axis, the reversal is much slower and can be described using a logarithmic model. We have also investigated the short time (1–50 s) dependence of the remanent coercivity and fitted to logarithmic equations. For films with no pronounced easy axis, the time dependence of the coercivity correlates with the film thickness, indicating that the switching volume scales with thickness. For films with an induced easy direction, the time dependence of the coercivity is essentially constant, independent of film thickness, indicating no scalable switching volume.     

Dependence of magnetic characteristics on sputtering-power and substrate-temperature in amorphous Tb40(FeCoV)60 films

The amorphous Tb₄₀(Fe₄₉Co₄₉V₂)₆₀ films were deposited at different sputtering powers and substrate temperatures. The microstructural and magnetic characteristics were investigated by means of field emission scan electron microscope, magnetic force microscope and vibrating sample magnetometer. Our results show that with increasing sputtering power, out-of-plane coercivity decreases monotonically while saturation magnetization has a maximum value of 231 kA/m for the sample prepared at 50 W. The as-deposited alloy films are amorphous, whereas the coercivity and saturation magnetization are strongly dependent on the substrate temperature. An out-of-plane hysteresis loop with coercivity below 22 mT and saturation magnetization over 290 kA/m is obtained combining dc power and substrate temperature. The dominant mechanism of room temperature coercivity appears to be domain wall pinning, rather than nucleation under all conditions measured. The variation of saturation magnetization is similar to that of perpendicular magnetic anisotropy with either sputtering power or substrate temperature according to the difference of magnetic domain structure.