Magnetic properties of europium chromites (EuCrO3)

Abstract

Rare earth chromites exhibit a diversity of interesting chemical and physical properties depending on the rare earth ion. EuCrO3 shows a weak spontaneous magnetic moment below a Néel temperature of 181 K; attributed to a slight canting of the Cr3+ magnetic moments that are otherwise antiferromagnetically aligned. We have synthesized chromites by using a combustion reaction method applying urea as fuel. X-ray diffraction (XRD), Fourier transformed Infrared Spectroscopy (FT-IR), Scanning and Transmission Electron Microscopy (SEM and TEM), Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) were used to study the samples. The magnetization was measured by using a vibrating sample magnetometer (VSM) and a Physical Properties Measurement System (PPMS) at a wide range of temperature (5 K to 300 K). Rietveld analysis of the XRD data yielded to an orthorhombic structure (Pnma) for EuCrO3, with an average crystallite size about 65 nm. Then, EuCrO3 was doped with iron forming a mixed chromite Eu1−xFexCrO3 (0 ≤ x ≤ 1.0). The presence of iron proved to be a source of microstrain in the system, due to the substitution of Eu3+ ions by Fe3+ in the perovskite structure. In fact, single phase EuCrO3 is paramagnetic at room temperature. However, above x>0.2, an enhance in the exchange interaction among the magnetic ions. The magnetic properties of Eu1−xFexCrO3 is accounted for by taking into consideration the lattice distortion introduced by the iron substitution in the EuCrO3 perovskite structure mainly due to the large difference in the ionic radius of Eu3+ and Fe3+ and in their magnetic moments. One of the most important contributions of our work has been focus on the relation between the bond angle of Cr3+-O2−-Cr3+ and the Néel temperature. This angle is responsible for the weak magnetic behavior of the rare earth chromites. We have found that as a product of the induced stress the Néel temperature is modified, so does the bonding angle. This behavior has been also found analyzing the shift towards higher values of the Hopkinson peak. To conclude, a termal treatment at 800°C for the Eu1−xFexCrO3 made at different times, shown a reduction in the microstrain of the system accompanied by an inhibition of the ferromagnetic-like character of the as-prepared samples.