The magnetic Curie temperature and exchange coupling between cations in tetragonal spinel oxide Mn_(2.5)M_(0.5)O_4 (M = Co, Ni, Mn, Cr, and Mg) films

摘要

Mn_3O_4 is a Jahn-Taller tetragonal ferrite that has a relatively low Curie temperature (T_c) of ～43 K due to weak coupling between the canting spins. In this study, we fabricated a series of 100-nm-thick Mn_(2.5)M_(0.5)O_4 (M = Co, Ni, Mn, Cr, and Mg) films via oxygen-plasma-assisted molecular beam epitaxy and measured the structural and magnetic properties of these films. These films show single phase quality, and the c-axis lattice parameter of pure Mn_3O_4 is 0.944 nm, with a c/a ratio ～1.16, consistent with the bulk values. The replacement of Mn by M (M = Co, Ni, Cr, and Mg) changes the lattice parameters, and the c/a ratio varies between 1.16 and 1.06 depending upon the cation distribution of the films. The magnetic Curie temperatures of these films also vary in the range of 25-66 K in that Ni and Co enhance the T_c whereas Mg reduces the T_c (Cr shows no effect on the T_c). These changes to the T_c are related to both the element electronic state and the cation distributions in these compounds. As a non-collinear spin configuration can induce electrical polarization, the present study provides a systematic way to enhance the magnetic transition temperature in tetragonal spinel ferrites.