Magnetic properties of strained multiferroic CoCr2O4: a soft X-ray study

TL;DR

This study investigates the magnetic properties of strained CoCr2O4 thin films using resonant soft X-ray techniques, revealing enhanced coercivity, persistent multiferroic order under strain, and modifications to existing magnetic models.

Contribution

It provides new insights into the magnetic behavior of strained CoCr2O4, including the coexistence of ferroelectric and ferromagnetic moments and modifications to the spin model for magnetic spinels.

Findings

Co and Cr magnetic moments appear at ~90K with opposite signs

Co coercive field is two orders of magnitude higher than in bulk

Multiferroic conical magnetic order persists under strain

Abstract

Using resonant soft X-ray techniques we follow the magnetic behavior of a strained epitaxial film of CoCr2O4, a type-II multiferroic. The film is [110]-oriented, such that both the ferroelectric and ferromagnetic moments can coexist in plane. X-ray magnetic circular dichroism (XMCD) is used in scattering and in transmission modes to probe the magnetization of Co and Cr separately. The transmission measurements utilized X-ray excited optical luminescence from the substrate. Resonant soft X-ray diffraction (RSXD) was used to study the magnetic order of the low temperature phase. The XMCD signals of Co and Cr appear at the same ordering temperature Tc~90K, and are always opposite in sign. The coercive field of the Co and of Cr moments is the same, and is approximately two orders of magnitude higher than in bulk. Through sum rules analysis an enlarged Co2+ orbital moment (m_L) is found, which can explain this hardening. The RSXD signal of the (q q 0) reflection appears below Ts, the same ordering temperature as the conical magnetic structure in bulk, indicating that this phase remains multiferroic under strain. To describe the azimuthal dependence of this reflection, a slight modification is required to the spin model proposed by the conventional Lyons-Kaplan-Dwight-Menyuk theory for magnetic spinels. Lastly, a slight increase in reflected intensity is observed below Ts=27K when measuring at the Cr edge (but not at the Co edge).