Effect of Co and Mg doping at Cu site on structural, magnetic and dielectric properties of α-Cu₂V₂O₇

We have studied the effect of doping of both magnetic (Co) and nonmagnetic (Mg) ions at the Cu site on phase transition in polycrystalline α-Cu₂V₂O₇through structural, magnetic, and electrical measurements. X-ray diffraction reveals that Mg doping triggers an onset ofα- toβ-phase structural transition in Cu_2-MgV₂O₇above a critical Mg concentrationx_c= 0.15, and both the phases coexist up tox= 0.25. Cu₂V₂O₇possesses a non-centrosymmetric crystal structure and antiferromagnetic ordering along with a non-collinear spin structure in theαphase, originated from the microscopic Dzyaloshinskii-Moriya interaction between the neighboring Cu spins. Accordingly, a weak ferromagnetic (FM) behavior has been observed up tox= 0.25. However, beyond this concentration, Cu_2-MgV₂O₇exhibits complex magnetic properties. A clear dielectric anomaly is observed in α-Cu_2-MgV₂O₇around the magnetic transition temperature, which loses its prominence with the increase in Mg doping. The analysis of experimental data shows that the magnetoelectric coupling is nonlinear, which is in agreement with the Landau theory of continuous phase transitions. Co doping, on the other hand, initiates a sharpαtoβphase transition around the same critical concentrationx_c= 0.15 in Cu_2-CoV₂O₇but the FM behavior is very weak and can be detected only up tox= 0.10. We have drawn the magnetic phase diagram which indicates that the rate of suppression in transition temperature is the same for both types of doping, magnetic (Co) and nonmagnetic (Zn/Mg).