Observation of low temperature metastable states in complex CaMn₇O₁₂

The low temperature magnetic behaviour of the multiferroic quadruple perovskite CaMn₇O₁₂ is investigated. The magneto-caloric effect in this material is presented for the first time. Along with the established magnetic transitions, T _N1 ∼ 90 K and T _N2 ∼ 45 K, an anomaly at 20 K is observed in our dc magnetization, ac susceptibility and dielectric measurements; below which, an inverse magnetocaloric effect is also observed in our  −ΔS _M (T) plots. The neutron scattering measurements show minimal change between 10 K and 30 K in static correlations, but a clear change in energy and linewidth of the magnetic excitations is evident. The results suggest that only dynamic correlations change across T _M ∼ 20 K. The existence of multiple magnetic interactions below 45 K, with significant coupling between them, is demonstrated using an Arrott plot analysis of our magnetic data. Compatible conclusions are drawn from magnetocaloric plots. The peak change in isothermal magnetic entropy (−ΔS _M) is 1.3 JK⁻¹ kg⁻¹ and the value of refrigeration capacity in CaMn₇O₁₂ is 34.5 J · kg⁻¹ at 7 T. (paper)