The Magnetic Spin Structure of Fe50Pt50-xRhx Films: A Neutron Diffraction Study

In the work presented here the structural and magnetic properties of Fe50Pt50-xRhx alloys with different Rh concentration (x=5, 10, 17.5 and 25) and thicknesses (100nm, 200nm, 300nm and 500nm) have been analyzed with macroscopic measurements and magnetically sensitive probes with resolution on the atomic range. Compared to previous studies on bulk Fe50P50-xRhx alloys with macroscopic measurements the magnetically sensitive investigations on the thin films allow together with structure factor calculations not only the determination of general magnetic properties but for the first time the development of a detailed model of the magnetic configuration. The structural analysis and macroscopic magnetic measurements were carried out by X-ray diffraction and MOKE measurements, respectively, indicating that the films are well chemically ordered in a bct structure and show different magnetic phases depending on the composition. A more detailed analysis of the magnetic properties has been carried out by unpolarized and polarized neutron diffraction. The measurements showed that in the 200nm thick Fe50Pt50-xRhx films a transition between a ferromagnetic (FM) order in the in-plane direction and an antiferromagnetic (AF) order in the out-of-plane direction takes place with increasing Rh concentration. Additionally a temperature driven AF-FM transition can be observed in the film with x=10. Measurements with the application of an external magnetic field induce a FM order in the out-of-plane direction in the films with low Rh concentration while the AF ordered films with high Rh content are not affected by the field. The thickness of the Fe50P40Rh10 films has in a range from 100nm to 500nm almost no influence on the fundamental magnetic properties. Only the AF-FM transition of the thicker films is more rapid with increasing temperature. The application of detailed structure factor calculations allow to develop a model of the magnetic configuration of the Fe50Pt50-xRhx alloys. The magnetic moments in the films with low Rh concentration are FM ordered with a dominant alignment in the out-of-plane direction while in the films with high Rh concentration they are AF ordered and are dominantly orientated in the in-plane direction. The different magnetic configurations can be described by a phenomenological model considering two major contributions: The first term is a bilinear anisotropic Hamiltonian with direction dependent exchange constants and the second term includes the