College Park, Maryland June 6 - 10 , 2004
M2-A4 (11:30 AM): Neutron scattering studies of the spin structure and magnetic domains in semiconductor superlattices
H. Kepa (Department of Physics, Oregon State University, Corvallis, OR 97331, USA; Institute of Experimental Physics, Warsaw University, ul. Hoza 69, Warsaw, Poland), A.Yu. Sipatov (Kharkov State Polytechnic University, 21 Frunze St., Kharkov 310002, Ukraine), P. Sankowski, P. Kacman (Institute of Physics PA S and ERATO Spintronics Project, Al. Lotnikow 32/46, 02-668 Warsaw, Poland), C. F. Majkrzak (NIST Center for Neutron Research), T. M. Giebultowicz (Department of Physics, Oregon State University, Corvallis, OR 97331, USA)
Neutron diffraction and reflectivity experiments performed on a number of magnetic semiconductor superlattices (SL), in search for interlayer coupling (IC), are reported. Antiferromagnetic (AFM) EuTe/PbTe SL and ferromagnetic (FM) EuS-based multilayers, with narrow-gap semiconductor (PbS) as well as insulating (YbSe) diamagnetic spacers, were studied. Pronounced interlayer magnetic correlations have been revealed in EuTe/PbTe by neutron diffraction, the only tool which enables such observation in AFM systems. In the FM multilayers it was proven by neutron reflectivity experiments that consecutive EuS layers are antiferromagnetically coupled. In order to determine the strength of the AFM IC in EuS-based systems, the intensity of the first magnetic SL Bragg peak vs. applied external magnetic field was measured. Finally, the in-plane anisotropy and the domain structure were studied by polarized neutron reflectivity. Despite the fourfold symmetry of the growth plane, a preferred orientation of domain magnetization directions along one of the two possible in-plane axes ( and  for EuS/PbS and EuS/YbSe, respectively) was found.
The IC strength J and the anisotropy constant K were obtained by least-square fitting of the peak intensity vs. magnetic field experimental data to model data. The model is based on minimizing the total magnetic energy of the SL as a function of the magnetization directions. To obtain a good fit, it was necessary to take into account the interfacial roughness by assuming a Gaussian spread of J. For both EuS/PbS and EuS/YbSe SL, the best fit was obtained for the directions of the in-plane easy axes, which agree with those determined by polarized neutron reflectivity. A tight-binding theoretical model was proposed, which explains the IC phenomena in both, AFM and FM, IV-VI semiconductor SLs in terms of electronic band structure effects. This is the only model which predicts the AFM IC between EuS layers, as seen in the experiment. The model also predicts smaller strength and shorter range of IC when the EuS layers are separated by YbSe instead of PbS, in qualitative agreement with the fitted J values.
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