Temperature-driven flexomagnetic effects in thin Cr2O3 films

Antiferromagnetic ordering, being prevalent over ferromagnetic one in nature, is very sensitive to the lattice structure. For example, in the absence of magnetostatics, the stress fields can be responsible for the domain formation in easy-plane antiferromagnets, while the hydrostatic pressure provides a possibility to manipulate the phase transition temperature between magnetically ordered and disordered phases [1]. Phenomena related to the strain gradient being allowed in the majority of magnetic symmetry classes are much less explored [2].Here, we provide a theoretical and experimental evidence of flexomagnetism in the uniaxial room-temperature antiferromagnet Cr2O3 [3]. In the experiment, high-quality Cr2O3 thin films grown on sapphire substrate are considered. Their magnetic state is accessed by a combination of magnetotransport measurements and Nitrogen vacancy (NV) magnetometry, which allows one to address both the uncompensated magnetization at the film surface and the interior of the film. We found a gradual transition from antiferro- to paramagnetic state by thickness with heating, which is substantially enhanced in comparison with bulk Cr2O3. To explain this observation, we provide a systematic analysis of sources of magnetization and symmetry analysis regarding the presence of a sizeable strain gradient along the film thickness. The latter enables (i) the net uniform bulk magnetization along the film thickness, which cannot be directly detected by NV magnetometry, and (ii) distribution of the Neel temperature along the film thickness. The gradual change of the magnetic phase transition temperature along the sample breaks the compensation of antiferromagnetic sublattices. The respective magnetization is proportional to the Neel vector and changes its direction betwen antiferromagnetic domains contributing to the stray fields and being detectable by NV magnetometry. We provide a theoretical description of this strain-gradient-driven effects in thin Cr2O3 fi