Probing the magnetic transitions in exchange-biased FePt3/Fe bilayers
Abstract
Using magnetometry and ferromagnetic resonance (FMR), we have investigated the magnetic properties of exchange-biased FePt3 (110)/Fe (211) bilayer films epitaxially grown onto MgO (110). The Fe layer exhibits a large uniaxial anisotropy, the magnitude of which is quantitatively accounted for by epitaxial strains. The FePt3 layer is chemically ordered in the L12 phase which develops antiferromagnetic (AF) order below TN1 = 160 K. Cooling through TN1, the Fe layer becomes exchange biased and its anisotropy is reduced as a result of exchange coupling to the AF-ordered FePt3. Negative exchange bias is observed for cooling fields directed along the FePt3 in-plane [001] and out-of-plane [110] directions, whereas small positive bias is observed when cooling along the in-plane [110] direction. Both the biasing and reduction in anisotropy are consistent with the FePt3 moments lying in the (110) plane with the most likely spin directions being the out-of-plane [111] and [111] axes. A second magnetic transition is observed at TN2 = 100 K. This transition is reflected in the temperature dependence of the coercive field, exchange bias, and FMR resonance and linewidth. Such a transition has only been observed for slightly Fe-rich FePt3 bulk alloys as a reorientation into a second AF phase. However, our films are slightly Pt rich and neutron scattering did not indicate evidence of a transition at TN2 in similarly grown FePt3 films on MgO (110). Possible origins of the second magnetic transition in the coupled structure are discussed.