Uniaxial magnetic anisotropy in Co2.25Fe0.75O2BO3compared to Co3O2BO3and Fe3O2BO ludwigites

Abstract

Magnetic and M\"ossbauer spectroscopy (MS) measurements have been performed on a single crystal of Co${}_{2.25}$Fe${}_{0.75}$O${}_{2}$BO${}_{3}$ with ludwigite structure. Two magnetic transitions (${T}_{N}=115 $K and ${T}_{C}=70 $K) were traced from the ac susceptibility temperature dependence. The MS spectra as a function of temperature clearly show the onset of magnetic ordering below 115 K. Magnetization measurements on the parent Co${}_{3}$O${}_{2}$BO${}_{3}$ and Fe${}_{3}$O${}_{2}$BO${}_{3}$ compounds have been done for comparison. In Fe${}_{3}$O${}_{2}$BO${}_{3}$ the anisotropy of the different phases has been determined, showing that the anisotropy axis changes from the $a$ to the $b$ axis in the low-temperature antiferromagnetic phase. High magnetic uniaxial anisotropy has been detected for both Co${}_{3}$O${}_{2}$BO${}_{3}$ and Co${}_{2.25}$Fe${}_{0.75}$O${}_{2}$BO${}_{3}$. From the angle-dependent magnetization measurements it is found that in both compounds the easy axis of magnetization is the $b$ [010] axis, where an antiferromagnetic component is superimposed on the main ferromagnetic component. In the $c$ direction the behavior is purely antiferromagnetic. In Co${}_{2.25}$Fe${}_{0.75}$O${}_{2}$BO${}_{3}$ a strong reduction of the remanent magnetization and a very strong increase in coercive field along the $b$ axis with respect to those found in Co${}_{3}$O${}_{2}$BO${}_{3}$ were observed from magnetic hysteresis cycles measured below ${T}_{C}$. The increase of coercive field is caused by the increase of defects upon Co substitution by Fe.