Evolution of morphology and magnetic properties of α-Fe2−xCrxO3 nanoparticles in dependence on Cr concentration

Abstract

Hematite, α-Fe2O3, one of the most widely studied magnetic compounds known to mankind since ancient times, has amazing properties and continues to puzzle researchers to this day. In this work, we found exceptionally strong changes in the morphology and magnetic properties of hematite nanoparticles upon doping with rather low concentrations of chromium. Nanoparticles α-Fe2−xCrxO3 with x varied from zero to 0.15 were prepared by solvothermal method using FeCl3·6H2O, CrCl3·6H2O, and 1,2-diaminopropane, (CH2)3(NH2)2, as precursors and temperature 180 °C for 6 h. The X-ray diffraction data showed the presence of only one crystal phase α-Fe2O3 for all x meanings with growing average crystalline size as the x increases. For the parent α-Fe2O3 powdered samples, the transmission electron microscopy revealed very long narrow nanorods with rounded ends that transform into spindles of increasing volume and cross section diameter as the Cr concentration increases. Vibrating sample magnetometry (VSM) and magnetic circular dichroism (MCD) study of the nanoparticles behavior in dependences on temperature and magnetic field revealed many unusual peculiarities. Among them there are strong increase of NPs magnetization with x increase (usually opposite picture is observed), the Morin transition temperature TM dependence on magnetic field, VSM sees the Morin transition in samples with small x only while MCD – in all samples, magnetization field dependences also are different when measured with VSM and MCD, and so on. Qualitative considerations were proposed to explain the observed phenomena.