College Park, Maryland June 6 - 10 , 2004
M2-A5 (11:45 AM): Polarized neutron reflectivity on CoFeOx exchange springs for spin valve applications
K. V. O'Donovan (NIST Center for Neutron Research; University of Maryland, College Park and University of California, Irvine), J. A. Borchers (NIST Center for Neutron Research), S. Maat, M. J. Carey, B. A. Gurney (Hitachi Global Storage Technologies)
Modern hard disk reading heads employ spin valves, which are multicomponent magnetic films with two independent ferromagnetic layers. Most spin valves use a metallic antiferromagnet, such as PtMn, to pin one of the ferromagnetic layers. Use of an insulating biasing layer can be advantageous because it pins without shunting the current, which leads to higher giant magnetoresistance (GMR) values, thus providing greater readback sensitivity to the recorded data. One promising candidate is ferrimagnetic cobalt ferrite (CoFe2O4), which pins the neighboring ferromagnetic layer through an exchange-spring mechanism.1 Recently, we grew a CoFe2O4/Co/Cu/Co/NiFe spin valve that shows high GMR, excellent pinning and good free-layer properties. To optimize the performance of this structure, we have examined the field-dependent switching of the individual layers using polarized neutron reflectometry (PNR). Measurements of the reflectivity from the front and back of these films can be used to discern noncollinear magnetism, in which the direction of magnetization changes by several degrees over a depth of a few nanometers. Our measurements on a CoFe2O4(50.0 nm)/CoFe10(6.0 nm)/Ta(10.0 nm) sample confirm that the hard ferrimagnetic CoFe2O4 layer and the soft ferromagnetic CoFe layer are coupled together in applied fields ranging from 0 to 900 mT. In intermediate field ranges near 54 mT, fits to the data indicate that a twist is evident in the exchange-spring as expected and it is confined mostly to the CoFe2O4 layer. This surprising result contrasts with the usual expectation that the magnetic twist reside in the hard layer in the field region of reversibility. Fits suggest that the twist gradually collapses as the field is increased.
1 E.F. Kneller and R. Hawig, IEEE Trans. Magn., 27, 3588 (1991).
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