MAGNETIC PHASE TRANSITIONS IN TRANSITIONAL METAL OXIDES STUDIED WITH PERTURBED ANGULAR CORRELATIONS

Abstract

The Perturbed Angular Correlation (PAC) method with radioactive tracer nuclei is well-suited to study the critical behaviour of magnetic hyperfine fields. Here we present the results of PAC measurements with 111 Cd probes, concerning the antiferromagnetic phase transitions in CuO, Co 3 O 4 and MnO. The measured Larmor frequencies were fitted with a Curie-Weiss type temperature dependence ωL (T) /ωL(0) = (1- T/T N)β. In CuO the PAC probe is subject to a high electric field gradient (efg) with the coupling constant νQ = 434(2) MHz and the asymmetry parameter η = 0.42(1). Therefore, combined electric/magnetic hyperfine interaction is observed below T N. Similar to neutron diffraction experiments, a second magnetic transition was observed at T 1 = 213 K . The analysis of the PAC data gave T N = 224(2) K and β = 0.33(1) for T 1 ≤ T ≤ T N. In the spinel Co 3 O 3, the measured critical parameters for 111 Cd at cation sites with tetrahedral oxygen coordination are T N = 39(1) K and β = 0.34(2). This site has a nearly cubic oxygen coordination and therefore a vanishing efg. In Mn 1−x O (defective NaCl structure, x ≤ 0.15), the critical parameters were found to sensitively depend on the annealing conditions. They will be discussed in comparison with those in Fe 1−x O .