This work studies the influence of crystallographic alignment onto magnetization reversal in partially epitaxial Co films. A reproducible growth sequence was devised that allows for the continuous tuning of grain orientation disorder in Co films with uniaxial in-plane anisotropy by the controlled partial suppression of epitaxy. While all stable or meta-stable magnetization states occurring during a magnetic field cycle exhibit a uniform magnetization for fully epitaxial samples, non-uniform states appear for samples with sufficiently high grain orientation disorder. Simultaneously with the occurrence of stable domain states during the magnetization reversal, we observe a qualitative change of the applied field angle dependence of the coercive field. Upon increasing the grain orientation disorder, we observe a disappearance of transient domain wall propagation as the dominating reversal process, which is characterized by an increase of the coercive field for applied field angles away from the easy axis for well-ordered epitaxial samples. Upon reaching a certain disorder threshold level, we also find an anomalous magnetization reversal, which is characterized by a non-monotonic behavior of the remanent magnetization and coercive field as a function of the applied field angle in the vicinity of the nominal hard axis. This anomaly is a collective reversal mode that is caused by disorder-induced frustration and it can be qualitatively and even quantitatively explained by means of a two Stoner-Wohlfarth particle model. Its predictions are furthermore corroborated by Kerr microscopy and by Brillouin light scattering measurements.
REFERENCES
The main reason to use M0 instead of the saturation magnetization value (Ms) for the normalization is the fact that Ms is a somewhat elusive experimental quantity, because only along the easy or the hard axis one can actually achieve M = Ms.79 For all other directions, this is not possible and one would have to extrapolate the data to estimate Ms in these directions. So, given that the specific value of Ms does not contribute any substantial insight to the subject of this study, we have preferred to use M0 as a reliable experimental magnetization value for the data normalization.
Each magnetic state image displays the difference between the positive remanent magnetization state and the negative remanent magnetization state in order to increase the contrast.
Even though it would be more precise to devise a model with a more complex angular distribution,32 it is not necessary in order to understand the HA anomaly, since the two grain SW model is able to explain all key characteristics of the experimentally observed behavior.
In Eqs. (4) and (5), E defines the volume integrated energy over the two particles and thus all the coefficients including the field are given in energy units.
Note that the abrupt frequency shift around 50° observed in the inset of Fig. 11(b) arose from a dust particle that was illuminated while rotating. It is not reproducible when changing the illumination area.