Manipulating the magnetic structure of Co core/CoO shell nanoparticles: implications for controlling the exchange bias.
We present an experimental study of the effects of oxidation on the magnetic and crystal structures of exchange biased epsilon-Co/CoO core-shell nanoparticles. Transmission electron microscopy measurements reveal that oxidation creates a Co-CoO interface which is highly directional and epitaxial in quality. Neutron diffraction measurements find that below a Néel temperature TN of approximately 235 K the magnetization of the CoO shell is modulated by two wave vectors, q1=(1/2 1/2 1/2)2pi/a and q2=(100)2pi/a. Oxidation affects the q1 component of the magnetization very little, but hugely enhances the q2 component, resulting in the magnetic decompensation of the core-shell interface. We propose that the large exchange bias effect results from the highly ordered interface between the Co core and CoO shell, and from enhanced core-shell coupling by the uncompensated interface moment.
University of Michigan, Ann Arbor, Michigan 48109-1120, USA.
S E
SE
Inderhees
J A
JA
Borchers
K S
KS
Green
M S
MS
Kim
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K
Sun
G L
GL
Strycker
M C
MC
Aronson