Surface Induced Magnetic Phase Transition: MnPt₃ vs. MnAu₃

Soon C. Hong

 Department of Physics, University of Ulsan, Ulsan 680-749, Republic of Korea.

During the last decades, a great deal of research has been made on the novel magnetic phenomena and magnetic materials in reduced dimensions. Ultrathin Fe films on Cu and Rh(001) shows an AFM state differently from its bulk, being accompanied by strain induced structural phase transition. It is a typical example that a subtly different environment results in different magnetism. There have been many reports on the magnetic properties of elemental, ferromagnetic ultrathin films, while relatively minor attention has been drawn to the magnetic behaviour of magnetic (in their bulk) alloy films.

MnPt₃ bulk alloy is ferromagnetic with its Curie temperature (Tc), ~380 K and magnetic moments, 3.60  B for Mn and 0.17 or 0.26  B for Pt at 77K, while MnAu3 bulk alloys is antiferromagnetic with its Neel temperature (TN), ~140 K and effective magnetic moment, 4.06  B. Besides, MnPt₃ ordered alloy has recently drawn much attention because it shows very large magneto-optical Kerr effect (MOKE) with respect to blue light, and hence is a promising candidate for high density magneto-optical storage media. Furthermore when Mn atoms are deposited on some metal substrates (Cu, Au, Ag, Pd, and Pt) it is found that the magnetic surface alloys form after mild annealing. For the recently synthesized MnPt₃ type surface alloy, they did not observe any surface magneto-optical Kerr effect (SMOKE) signal.

In this study we studied the magnetism of MnPt₃ and MnAu₃ systems (bulk, surface, and surface alloy) to investigate the surface and size effects on magnetism. The full-potential linearized augmented plane wave (FLAPW) method was used with general gradient approximation for exchange-correlation potential. Some several possible magnetic structures have been taken account and calculations on their total energy have been carried out. We will discuss surface induced magnetic transitions (FM to AFM for MnPt₃ and AFM to FM for MnAu₃) observed in calculations and experiments based on calculated single-particle energy spectra.