Re-entrant Spin Glass and Magnetocaloric Effect in Frustrated Double Perovskite Ho2CoMnO6 Flat Nanorod

In the present work, we have synthesized single-phase flat nanorods of double perovskite Ho2CoMnO6 (HCMO) by following a hydrothermal route. The sample crystallizes into a monoclinic structure with a P21/n space group. Expansion of MnO6 octahedra and hence increase in unit cell volume as compared to its bulk counterpart is observed. Unlike the single ferromagnetic transition in the bulk HCMO sample, here magnetization versus temperature measurements show three magnetic transitions corresponding to ferromagnetic (FM), antiferromagnetic (AFM) and re-entrant spin glass (SG) ordering at 182, 97 and 31 K respectively. The FM transition (Tc) which is much higher than the bulk is attributed to super-exchange interaction in Co2+- O - Mn4+ networks and the AFM transition is due to exchange interactions across Co2+- O – Co2+ and Mn4+- O - Mn4+ networks, as well as due to the presence of Co3+ and Mn3+ ions, which is proven by the XPS analysis. The glassy magnetic phase is confirmed by magnetic relaxation and aging effect studies. A low value of coercivity (Hc) and saturation magnetization (MS) is observed for HCMO flat nanorods (8.52 µB/f.u) as compared to that of bulk HCMO (14.9 µB/f.u). It may be due to the presence of higher oxidation states of ions as well as the introduction of AFM ordering and frustration in the system. A study of magnetocaloric effect gives a change in entropy value of 12.4 J/K.Kg at low temperature for 7 T.