Nanomagnetism and strain effects in magnetoelectric antiferromagnet Cr2O3

Concepts of the future of spintronics are often based on antiferromagnetic materials because of their attraction for low-energy operating and high-speed devices. However, an absence of significant net magnetization also results in challenges for manipulation and readout of the magnetic state. In this respect, room-temperature magnetoelectric easy-axis Cr2O3 is of special interest [1].

In a single-crystal Cr2O3 domain wall is a metastable excitation, which can be induced by a magnetoelectric poling procedure and being pinned by specially designed surface defects. In contrast, thin films usually have a granular structure with significant amount of crystal defects acting as pinning sites for domain walls. Concentration and structure of the defects can be controlled by the film fabrication procedure, keeping the net magnetoelectric effect present for the sample [2]. Furthermore, low-defect films grown at sapphire can be persistently strained [3]. An out-of-plane magnetic moment formed by one of antiferromagnetic sublattices at the c-plane of Cr2O3 provides a possibility for the Hall magnetometry detecting magnetization direction by the Hall resistance measured in Pt capping layer [4].

The Neel temperature of bulk Cr2O3 of 35C is the strong limiting technological factor for practical applications. There are theoretical and experimental demonstrations that the compressive strain induced by doping or procedure of growing at sapphire substrate leads to the substantial broadening of the antiferromagnetic phase by temperature increasing the Neel temperature above 100C [3,5]. Furthermore, strained thin films can possess an inhomogeneous strain along the thickness, enabling net magnetization of flexomagnetic origin and vertical gradient of the Neel temperature. The latter provides a local flexomagnetic response which is controlled by the direction of the antiferromagnetic order parameter and scales with the sample’s temperature [3].

We anticipate that Cr2O3 provides a flexible material platform for the fundamental and applied physics by demonstration of unique crystal-symmetry-assisted effects.