Magnetic phase diagram for the spinel ferrite series MgFe_2-xCr_xO₄ and Mg_1-xZn_xFe_1.5Cr_0.5O₄ as determined by Moessbauer spectroscopy

The spinel series MgFe_2-xCr_xO₄ (0.0≤x≤2.0) (MFC) and Mg_1-xZn_xFe_1.5Cr_0.5O₄ (0.0≤x≤1.0) (MZFC) are progressively diluted on the A-sites by non-magnetic Mg²⁺ and/or Zn²⁺ ions as x increases. The different magnetic states as a function of temperature and composition could be determined by Moessbauer spectroscopy in conjunction with high external magnetic fields. From the spectra it was derived that only for the spinel samples MFC with x≤0.6 and MZFC with x≤0.1 a pure ferrimagnetic behavior exists down to 4 K. In the composition range 0.8≤x≤1.2 for MFC and 0.2≤x≤0.7 for MZFC a marked canting of the A- and B-site spins was observed below a characteristic temperature T_f indicating a re-entrant semi-spin-glass behavior. Finally, for the samples with x>1.2 (ZFC) and x>0.7 (MZFC) an incomplete alignment of the spins remained up to the paramagnetic transition temperature T_g pointing to pure semi-spin glasses. Together with the temperature dependence of the average magnetic hyperfine field the complete magnetic phase diagram could be derived.