Probing the magnetic transitions in exchange-biased FePt3/Fe bilayers -: art. no. 054411

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 L1(2) phase which develops antiferromagnetic (AF) order below T-N1=160 K. Cooling through T-N1, 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 [1 (1) over bar0] direction. Both the biasing and reduction in anisotropy are consistent with the FePt3 moments lying in the (1 (1) over bar0) plane with the most likely spin directions being the out-of-plane [111] and [11 (1) over bar] axes. A second magnetic transition is observed at T-N2=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 T-N2 in similarly grown FePt3 films on MgO (110). Possible origins of the second magnetic transition in the coupled structure are discussed.