Spin reorientation in CoV2O4 thin film: A first principles study

Spinel vanadates (AV2O4) are a class of materials where an interesting interplay between spin and orbital degrees of freedom on a frustrated lattice is manifested in the multiple structural and magnetic phase transitions. CoV2O4 is unique among the spinel vanadates as it shows no or very weak cubic to tetragonal structural phase transition but two magnetic (paramagnetic to collinear ferrimagnetic and collinear to noncollinear ferrimagnetic) phase transitions at lower temperatures in its bulk form. In a recent experiment (Thompson et al., Phys. Rev. Mater. 2 (2018) 104411), an epitaxial thin film of CoV2O4 grown on the SrTiO3 (001) substrate, is observed to have an orthorhombic structural phase accompanied by a noncollinear magnetic state where the spin moments of Co and V are reoriented from [001] (seen in the bulk form) to the [110] direction. In this work, we have explored the mechanism behind this complex spin-reorientation and noncollinear magnetic ground state by investigating the electronic structure, magnetic, and structural properties of CoV2O4 in its orthorhombic phase and compared with the corresponding properties of the cubic phase using the first principles density functional theory. Our GGA+U+SO calculations on orthorhombic CoV2O4 reveal that spin moments prefer to lie in the ab-plane with the crystallographic b-direction being the preferred direction in the collinear ferrimagnetic state where it has a larger band gap and orbital moment (of V) as compared to the cases with spin moments aligned parallel to a and c-directions. We further observe competing magnetic exchange interaction values in orthorhombic phase when the spins lie in the ab-plane giving rise to more frustration and hence more canting in the non-collinear magnetic ground stateas compared to the cubic phase, consistent with experiment. With the help of source-free exchange–correlation (XC) functional we were able to capture the non-collinear magnetic ground state and the spin-reorientation in this system. We also discussed the possible orbital ordering in the orthorhombic phase of CoV2O4.

Tailoring magnetism in spinel vanadate CoV2O4epitaxial thin films.

Near itinerant cubic bulk CoV2O4is at variance with other spinel vanadates by not showing orbital ordering down to low temperature, albeit it displays fragile anomalies related to spin, and lattice structure, signaling a spin/orbital glass transition around 95 K. We investigate tetragonal-like epitaxial CoV2O4films on SrTiO3and (La0.3Sr0.7)(Al0.65Ta0.35)O3substrates that exhibit pronounced signature of spin reorientation transition from toa/bplane around 90 K unlike its bulk counterpart. Using in-plane and out-of-plane magnetic measurements, we demonstrate the intricate link between Co2+and V3+sublattice magnetizations that give rise to anisotropic magnetic switching. In-plane magnetic measurements reveal a wasp-waist shapedM(H) loop below reorientation transition temperature, while the out-of-plane follows antiferromagnet-likeM(H) response. The wasp-waist shaped feature could be linked to in-plane spin-canted (anti)ferromagnetism induced by canting away of V-spins away from antiferromagnetically coupled Co-spin direction below reorientation transition temperature. Further, we uncover the evidence for slow relaxation over a period of ∼104 s at 20 K and memory effect that indicates the possible existence for magnetic glassy phase in the low temperature regime. Using epitaxial strain as a control knob, our results inspire future study to manipulate orbital states, spin texture and itinerant electron character in tailored CoV2O4films away from cubic lattice symmetry.