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
- Add to calendar
- iCal
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).