Influence of the crystallographic order onto magnetization reversal in granular uniaxial Co films

CIC nanoGUNE Seminars

Speaker

Olatz Idigoras, nanoGUNE

When

2011/03/07
12:00

Place

nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian

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It is well known that magnetic properties depend strongly on the crystallographic structure of materials due to the magneto-crystalline anisotropy [1]. Fundamentally, the magneto-crystalline anisotropy is caused by the quantum-mechanical spin-orbit interaction [1,2], which leads to the fact that some crystallographic orientations in crystals become more preferable. Along these preferable crystallographic orientations it is easy to magnetize the sample, which is the reason why these orientations are typically referred to as easy axes (EAs). Correspondingly, directions that are very difficult to magnetize are called hard axes (HAs). In materials without long-range crystallographic order, there is not any preferential orientation and the magnetic properties do not show any orientational dependence either, while systems with long-range crystallographic order can have one or more preferential axes of magnetization. Systems with only one preferential orientation are called uniaxial systems. Despite many studies on the crystallographic influence onto magnetic properties, most of the work has been focused on either (nearly) perfect crystallographic samples or highly disordered samples. There are only very few studies, in which the range in between strong crystalline alignment and highly disordered samples has been explored in a well-defined manner. Here, we have studied the influence of crystallographic order onto magnetic properties and magnetization reversal in granular uniaxial thin Co-films. In order to allow for a well-defined variation of the crystallographic alignment, a reproducible processing sequence was developed that enabled the continuous modification of epitaxial growth quality for (10 _1_ 0) Co-films grown by ultra high vacuum (UHV) sputter deposition. We observed that while in epitaxial samples of high crystallographic alignment the magnetization reversal is dominated by uniform magnetization states, non-uniform intermediate stable or meta-stable states exist in the case of partially epitaxial (10 _1_ 0) Co samples that exhibit a substantial level of inter- granular misalignment. Moreover, we also observe an anomaly, in which conventional HA behavior disappears upon passing a crystallographic misalignment threshold. When the magnetic field is applied along the HA or at angles very close to the HA in such anomalous samples, one observes surprisingly large values of remanent magnetization and coercive field. This behavior is associated with a frustrated magnetic state, which arises from the competition between ferromagnetic exchange and misaligned uniaxial anisotropies of adjacent grains and has been modeled by means of a two-grain Stoner Wohlfarth model. Experimentally, the existence of this frustrated state has been corroborated by means of Kerr microscopy. [1] S. J. Blundell, Magnetism in Condensed Matter, Oxford University Press, New York, 2001. [2] P. Bruno, Phys. Rev. B **39** , 865 (1989); P. Gambardella et al., Science **300** , 1130 (2003).