NEUTRON DIFFRACTION STUDIES OF MAGNETIC ORDERING IN EXCHANGE-BIASED Fe3O4 /CoO SUPERLATTICES. Yumi Ijiri, Julie A. Borchers, Ross W. Erwin, NIST, Gaithersburg, MD and Pieter J. van der Zaag, R. M. Wolf, Philips Research Laboratories, Eindhoven, The Netherlands.,


Exchange biasing antiferromagnetic layers have been of much recent interest due to their role in improving magnetoresistive heads in magnetic storage applications. However, the mechanisms by which the antiferromagnet affects the rest of the device are not well understood. Unlike bulk magnetization or surface microscopy methods, neutron diffraction can probe the antiferromagnetic layer directly to determine its magnetic properties. As such, we have used neutron scattering techniques to examine the magnetic ordering in a prototypical series of MBE-grown Fe3O4/CoO superlattices of different thicknesses and compared them with device characteristics such as the blocking temperature. In particular, the intensity of the antiferromagnetic CoO (111) magnetic peak has been measured as a function of temperature, and the resulting ordering has been compared to that of bulk CoO (TN = 291 K). The CoO ordering temperature increases with decreasing CoO concentration due to local exchange interactions with the Fe3O44 layers (TC= 858 K for bulk). In contrast, the field dependent blocking temperature, which usually tracks this ordering, decreases with decreased CoO thickness for this antiferromagnetic-ferrimagnetic exchange-biased system. Additional reflections, namely the Fe3O44(220) peak as well as the structural (222) peak, have been examined to understand these effects in more detail. From these measurements, we find that the in-plane coherence lengths differ for the Fe3O44 and CoO layers, suggesting possible frustration at the Fe3O4/CoO interfaces. These results indicate the intricate manner in which the antiferromagnet influences the rest of the device structure.