Emergence of the Stoner-Wohlfarth astroid in thin films at dynamic regime

Abstract

The Stoner-Wohlfarth (SW) model is the simplest model that describes adequately the magnetization reversal of nanoscale systems that are small enough to contain single magnetic domains. However for larger sizes where multi-domain effects are present, e.g., in thin films, this simple macrospin approximation fails and the experimental critical curve, referred as SW astroid, is far from its predictions. Here we show that this discrepancy could vanish also in extended system. We present a detailed angular-dependent study of magnetization reversal dynamics of a thin film with well-defined uniaxial magnetic anisotropy, performed over 9 decades of applied field sweep rate (dH/dt). The angular-dependent properties display a gradual transition from domain wall pinning and motion-like behaviour to a nucleative single-particle one, as dH/dt increases. Remarkably, in the high dynamic regime, where nucleation of reversed domains is the dominant mechanism of the magnetization reversal (nucleative regime), the magnetic properties including the astroid become closer to the ones predicted by SW model. The results also show why the SW model can successfully describe other extended systems that present nucleative regime, even in quasi-static conditions.

An extraordinary chiral exchange-bias phenomenon: engineering the sign of the bias field in orthogonal bilayers by a magnetically switchable response mechanism

“…The MOKE system used in this study combines simultaneous acquisition of rotations of polarization and changes of reflectivity for a given sample orientation with respect to a variable external magnetic field, allowing full angular studies. 33 The sample was placed on a eucentric goniometer head to ensure a fixed plane of reflection upon sample rotation. The whole head could be rotated in the complete angular range 0-360° by a stepping motor in steps of 0.5°.…”

“…3c) agrees well with that of a thin film with a well-defined uniaxial anisotropy. 33 This may be understood considering that the strength of the magnetostatic coupling between both soft and hard magnetic layers is diminished with increasing the separation layer between both of them. On this basis, the maximum coercivity for a sufficiently thick spacer is foreseen to lie along H = 0° since for such situation the CoFeB layer will not be subjected to the magne- Please do not adjust margins Please do not adjust margins tostatic field of the SmCo5 layer, and thus will follow the uniaxial anisotropy that was induced during its growth (analogous configuration to that shown in Fig.…”

“…Independently of the thickness of the spacer layer, the loop shift has 360° symmetry while coercivity has 180° symmetry, which is the well-known result of the unidirectional nature of the exchange bias phenomenon. 33,42 However, with increasing the thickness of the spacer, the magnetostatic coupling between the CoFeB and the SmCo5 layers is diminished and, accordingly, the uniaxial anisotropy of CoFeB begins to play a relevant role as it may be observed by the presence of two maxima (minima) in the angular region H = 0-180° (H = 180-360°). As a consequence, the cosine angular dependence of the bias field, 42 related to the unidirectional anisotropy, typically observed in EB systems, becomes frustrated by an increased influence of the uniaxial anisotropy of the CoFeB layer (Fig.…”

Correlation between crystal structure and magnetism in PLD grown epitaxial films of ε-Fe2O3 on GaN

“…The shape of the in-plane VSM magnetization curves has been described in the present work following the Stoner Wolhfarth (SW) approach [25] of weakly interacting magnetic particles, which is the simplest way to describe magnetization reversal in the systems consisting of nanoscale objects that are small enough to contain single magnetic domains [26,27]. The columns in ε-Fe 2 O 3 film can be considered as such weakly interacting objects as they are crystallographically non-coherent to the neighbors, have differently oriented easy magnetization axes at 120 deg to each other and are small enough to be single domain.…”

Towards magnetic control of magnetite

“…In turn, the coercivity is not a direct measure of the strength of anisotropy. It is modified by defects and may present strong dynamical effects [35], but its angular dependence can provide information on the dynamic effective magnetic anisotropy symmetry, and its evolution with thickness can indicate general trends, if any. Apart of the angular dependence measurements for specific thicknesses and substrates mentioned below, we are not aware of the aforementioned studies that are required, nor general trends have been identified yet.…”