Magnetodielectric coupling in frustrated spin systems: the spinels MCr2O4 (M = Mn, Co and Ni)

We have studied the magnetodieletric coupling of polycrystalline samples of the spinels MCr2O4 (M = Mn, Co and Ni). Dielectric anomalies are clearly observed at the onset of the magnetic spiral structure (T-s) and at the `lock-in' transition (T-f) in MnCr2O4 and CoCr2O4, and also at the onset of the canted structure (T-s) in NiCr2O4. The strength of the magnetodielectric coupling in this system can be explained by spin-orbit coupling. Moreover, the dielectric response in an applied magnetic field scales with the square of the magnetization for all three samples. Thus, the magnetodielectric coupling in this state appears to originate from the (PM2)-M-2 term in the free energy.