Metamagnetic transitions in R2CoMnO6 (R=Yb, Lu) magnetoelectric double perovskites

Abstract

R2CoMnO6 compounds, with R = Yb or Lu, adopt the crystal structure of a double perovskite with an almost perfect ordering of Co and Mn atoms in the B-position. The crystal structure is strongly distorted and the competitive magnetic interactions between neighbor atoms and next-neighbor atoms lead to the development of an E-type antiferromagnetic ordering at low temperature. This feature differentiates these compounds from La2CoMnO6 where a collinear ferromagnetic order is observed. Magnetic transition in distorted R2CoMnO6 samples is coupled to a large anomaly in the real part of the electric permittivity suggesting that these materials are very close to improper multiferroics. We have found that the dielectric anomaly disappears under a magnetic field of 5 T revealing a negative magnetocapacitance for these compounds. In order to identify the causes of this behavior, neutron diffraction patterns were collected in different conditions of temperature and external magnetic field for three selected samples: Yb2CoMnO6, YbLuCoMnO6 and Lu2CoMnO6. Above a critical value the external magnetic field induces similar changes in the neutron patterns of the three samples: The E-type magnetic peaks vanish whilst the intensity of some nuclear peaks increases. This fact reveals that these samples undergo a metamagnetic transition from E-type antiferromagnet into collinear ferromagnet. The critical field to produce this transition depends on the composition (higher for Lu than for Yb) and on the temperature (decreases as temperature increases). Measurements of the isothermal magnetization as function of magnetic field also show turning points at the critical field where the phase transition is produced. These measurements show hysteresis loops revealing spontaneous magnetization and an increase of coercivity as the temperature decreases. The combination of both hysteresis loops and neutron diffraction patterns allowed us to outline the H-T phase diagram that reveals the magnetic states present in these samples.