Unusual ferrimagnetism in CaFe₂O₄

Ferrimagnetism is one of the magnetic states where the populations or amplitudes of collinear antiparallel spins are different and result in a net magnetization. Such a state is, so far, known to arise when two antiparallel sublattices are composed of (i) different magnetic elements, (ii) different valences of a magnetic element, and/or (iii) different numbers of a magnetic element(s).

In this study, we discovered a ferrimagnetic state due to a distinct origin from the above; two antiparallel sublattices are composed of (iii) the same number of (i) a single magnetic element (ii) having the same nominal valence but sitting on different crystallographic sites. We used X-ray magnetic circular dichroism together with resonant X-ray diffraction, enabling site-specific investigation of magnetism. As a result, we successfully demonstrate the interesting ferrimagnetic state in CaFe₂O₄. Two different Fe³⁺ sites having the same number of antiparallel spins (S = 5/2) and slightly different local coordination environments are found to host a net magnetization that has a significantly large coercive field despite the expected small magnetic anisotropy of the high-spin configuration of Fe³⁺. The newly found ferrimagnetic state is switchable between two magnetic domain states as in ferromagnets and possesses the characteristic features of antiferromagnets, such as the robustness for an external magnetic field and high resonant frequency, which have recently motivated researchers to develop spintronics based on antiferromagnets. Hence, our result can be relevant to future spintronics applications.