Noncontact Atomic Force Microscopy - Yale School of Engineering ...

P.II-35
Enhancement of the Exchange-bias Effect based on Quantitative
Magnetic Force Microscopy Results.
N. Pilet 1 , M.A. Marioni 1 , S. Romer 1 , N. Joshi 2 , S. Özer 2 and H.J. Hug, 1,2
1 Empa, Swiss Federal Institute for Materials Testing and Research, CH-8600 Duebendorf, Switzerland,
2 Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland
Exchange biasing (EB) causes a lateral shift of the magnetization vs. field curve in
antiferromagnetic-ferromagnetic (AF-F) thin-film structures. It is largely believed that
EB arises from linking the magnetization of the F to that of (pinned) uncompensated
spins (UCS) in the AF. In the present work we are able to measure the local density of
UCS on the scale of 20 nm, using quantitative magnetic force microscopy (qMFM). With
quantitative results at this lateral resolution, it becomes possible to resolve and measure
the UCS underneath each F domain at scales comparable to the grain size. The average
density of UCS around 20% of that expected for a full monolayer of spins is considerably
higher than expected from the measured exchange field and local values may reach
100%. We found that UCSs coupling antiparallel to the F magnetization stabilize its
orientation, i.e. they are biasing [1,2]. Locally, UCS oriented parallel to the F
magnetization were found. These have an antibiasing effect and are hence expected to
weaken the exchange field. We hypothesized that such anti-biasing arises from
conflicting exchange interaction between the F-layer and AF-grains, and between AFgrains
themselves. Exchange-bias systems grown with suppressed inter-granular coupling
in the AF-layer proved our hypothesis to be correct. These systems showed an exchangebias
effect enhanced by 20%. High resolution MFM revealed a more homogenous pattern
of UCS with a correspondingly higher density. The evolution of the F-domains in an
applied field was strikingly different.
Figure 1: . (A) UCS density calculated from measured MFM data and the instrument calibration
function. Positive UCS (i.e. parallel to the cooling field) are colored yellow, negative UCS red.
(B) UCS with overlayed contours of the F domains at H = 0; (C) Idem for H = 200mT. With few
exceptions (such as pointed out by the arrows) the UCS align antiparallel to the domain
magnetization at H=0.
[1] I. Schmid, P. Kappenberger, O. Hellwig, M. Carey, E. Fullerton and H. J. Hug, EPL 81 (2008) 17001
[2] I. Schmid, M. Marioni et al. submitted.
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