Dielectric anomaly in MNO near the magnetic phase transition

The electronic and magnetic properties of MnO and other transition-metal oxides have remained the focus of major attention for many years [1]. In this work, we report the discovery of an anomaly in the low-frequency (10 kc) dielectric constnat K' of MnO near its Neel temperature TN ≃ 118 K. The measurements were made in a single crystal of MnO, stressed along [111], with the electric field E ‖ [111] axis, using a three-terminal capacitance bridge. In the temperture range of 6K to 250K, both K' and K'' (magnitude ∼ 10-2) were frequency-independent up to 100 kc. The data for K' near TN for stress P = 20 bars (a stress sufficient to eliminate T domains [1]) are shown in the figure. For P = 0, the observed decrease in K' was about ½ of the value for P ≃ 20 bars. It is evident that the anomaly in K' near TN is associated with magnetic ordering. This anomaly is not due to a magnetoelectric effect since no significant changes in K' were observed near TN with an applied magnetic field of up to 8 kOe along [111]. A magnon-phonon coupling usually results in an increase in K' [2] rather than the decrease observed here. It is suggested that considerable lattice contraction along [111] below TN observed in MnO [4] may be the source of the observed anomaly in K' since energy for the corresponding long-wavelength phonon mode is expected to increase due to contraction along [111], thus resulting in a decrease in K'. The first order nature of the transition is evident from the behavior of dK'/dT near TN.