Effect of Partial Replacement of Mn with Cr on Exchange Bias Behavior and Negative Magnetization in Nanocrystalline Ho2CoMnO6 Double Perovskite

Abstract

In the present work, we have reported a detailed study on the structural and magnetic properties of Ho2CoMn1-xCrxO6 (x = 0–0.5) double perovskite. Rietveld analysis of observed x-ray diffraction patterns revealed that the examined samples are crystallized in the P21/n space group with a monoclinic structure. With Cr substitution, the lattice parameters got reduced. The particle size is also observed to be decreased. Magnetic analysis reveals a single ferromagnetic (FM) transition (TC1) at 83 K for x ≤ 0.2. It is attributed to the superexchange interaction in Co2+ - O2− - Mn4+ networks. Whereas, for x ≥ 0.3, we observed two transitions namely TC1 and TC2. Here TC2 is attributed to the result of antiferromagnetic (AFM) interactions in Cr3+- O2− - Cr3+ and/or Cr3+- O2− - Mn4+ networks. TC2 decreases from 77 K for x = 0.3 to 63 K for x = 0.5 sample, whereas no appreciable variation in TC1 is observed. In addition, the intermediate compounds such as x = 0.2 and 0.4 show another transition at 120 K and is attributed to the charge ordering temperature (TCO). Magnetization reversal (MR) has been observed in x = 0.5 at a lower applied field with a compensation temperature of 7 K. The competition between the paramagnetic (PM) moment of Ho3+/Mn3+ ions under the influence of the negative internal field caused by AFM ordered ions and the FM components of Co2+ and Mn4+ ions is attributed to this MR. At 4 K, a bipolar switching of magnetization is demonstrated for x = 0.5. After Cr substitution, the hysteresis (M−H) loops show a decrease in saturation magnetization (MS) and coercivity (Hc). Exchange bias field of − 114 Oe and + 208 Oe are observed for x = 0.0 and 0.5 samples respectively. They are attributed to the competition between canted magnetic moments of RE ions and FM moments of TM ions, where, the former one is dominating for x = 0.5 and vice versa for x = 0.0 samples.