Multiferroicity with coexisting isotropic and anisotropic spins in Ca${}_3$Co${}_{2-x}$Mn${}_x$O${}_6$

Abstract

We study magnetic and multiferroic behavior in Ca${}_3$Co${}_{2-x}$Mn${}_x$O${}_6$ ($x\sim$0.97) by high-field measurements of magnetization ($M$), magnetostriction ($L$($H$)/$L$), electric polarization ($P$), and magnetocaloric effect. This study also gives insight into the zero and low magnetic field magnetic structure and magnetoelectric coupling mechanisms. We measured $M$ and $\hat{I}”$$L$/$L$ up to pulsed magnetic fields of 92 T, and determined the saturation moment and field. On the controversial topic of the spin states of Co${}^{2+}$ and Mn${}^{4+}$ ions, we find evidence for $S$ = 3/2 spins for both ions with no magnetic field-induced spin-state crossovers. Our data also indicate that Mn${}^{4+}$ spins are quasi-isotropic and develop components in the $ab$-plane in applied magnetic fields of 10 T. These spins cant until saturation at 85 T whereas the Ising Co${}^{2+}$ spins saturate by 25 T. Furthermore, our results imply that mechanism for suppression of electric polarization with magnetic fields near 10 T is flopping of the Mn${}^{4+}$ spins into the $ab$-plane, indicating that appropriate models must include the coexistence of Ising and quasi-isotropic spins.