Shah Valloppilly¹, Christian Schanzer², Robert Georgii^1,3 and Peter Böni^1
1 Physics Department E21, Technische Universität München, 85747 Garching, Germany
² Laboratory for Neutron Scattering ETHZ & PSI, CH-5232 Villigen PSI, Switzerland
³ ZWE FRM-II, Technische Universität München, 85747 Garching, Germany

Fe/Cr thin films and multilayers remain as one of the most extensively investigated systems in thin film magnetism. Some pioneering discoveries in thin film magnetism like interlayer exchange coupling, giant magneto-resistance (GMR) etc. have been made on this system. Complex magnetic structure in Cr render intriguing magnetic properties in bulk as well as in thin films especially when Cr is used as a sandwich layer between two ferromagnet layers. Earlier models of interlayer exchange in Fe/Cr multilayers were based on an oscillatory RKKY-type exchange coupling and quantum-well behaviour of the electrons in the Cr spacer layer. Later, calculations based on the commensurate and incommensurate spin density waves (SDW) in Cr, and direct observation of SDW by neutron scattering suggested that Fe/Cr multilayers display more rich variety of magnetic phenomena.

A variety of experimental techniques have been employed for the investigations that extend from bulk to layer resolved magnetometry like in Polarized Neutron Reflectometry (PNR). PNR reveals the vectorial layer magnetization and is very sensitive to the parallel and perpendicular components of magnetization with respect to the neutron polarization. Thus, bilinear and bi-quadratic types of layer configurations can be easily resolved with depth sensitivity. The origin of bi-quadratic exchange has been attributed to intrinsic properties of the spacer layer, dipolar fields resulting from rough surfaces, super-paramagnetic impurities within the spacer, spacer thickness fluctuations etc. It has also been pointed out that the bi-quadratic exchange coupling has a dependence on the SDW.

The objective of the present work is to examine the magnetic configurations of the layers as a function of magnetic field and temperature of epitaxially grown [Fe(4 nm)/Cr(1.5 nm)]10 on MgO single crystal by PNR and corroborate with bulk magnetization results and to find a model to explain the temperature dependence of hysteresis behaviour. Epitaxial, Fe (4 nm)/Cr (1.5 nm)]10 multilayers were grown by magnetron sputtering on a MgO(100) single crystal wafer of 1 x 1 cm. Polarized neutron reflectometry with full polarization analysis was implemented at the MIRA reflectometer by using flipper coils in conjunction with polarizing benders. Due to small size of the sample, a clear total reflection region was not obtainable marred by the large footprint of the beam. Nevertheless, clear effects of PNR are immediately visible in the corrected data as illustrated for a typical case, T=10K and H=500 Oe in Fig. 1(left). The splitting of non spin-flip (NSF) channels + + and - -, and the presence of half order Bragg peak in the spin-flip (SF) channel + - and - + arising from the double periodicity of the magnetic lattice are indicative of the PNR with polarization analysis at MIRA. The experiments were performed around the magnetic (half-order) peak and its intensity as a function of applied magnetic field and temperature was measured. Fig. 1(right) summarizes the SF scattering intensity (integrated intensity of the half order peak) at selected fields and temperatures. In general, at remanence after exposing the sample to a high negative field, the Fe layers are found to order in an antiferroamgnetic configuration as indicated by a high NSF intensity and a low SF intensity. By the application of a small magnetic field of 50 Oe, the NSF intensity drops to low values and the SF intensity increases. With further high magnetic fields, the SF intensity reaches a maximum at a certain field and thereafter decreases gradually. The highest SF scattering indicates the highest deviation of magnetization from the collinear configuration and its field dependence indicates the evolution of magnetization in an exchange coupled magnetization reversal process. Clearly, the magnitude and behaviour of the field dependence of SF intensity at various temperatures, suggest the role of Cr on interlayer exchange and its temperature dependence.