Magnetic and Mössbauer spectroscopy (MS) measurements have been performed on a single crystal of Co2.25Fe0.75O2BO3 with ludwigite structure. Two magnetic transitions (TN=115 K and TC=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 Co3O2BO3 and Fe3O2BO3 compounds have been done for comparison. In Fe3O2BO3 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 Co3O2BO3 and Co2.25Fe0.75O2BO3. 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 Co2.25Fe0.75O2BO3 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 Co3O2BO3 were observed from magnetic hysteresis cycles measured below TC. The increase of coercive field is caused by the increase of defects upon Co substitution by Fe.

The financial support of Spanish MINCYT, Grant No. MAT08/1077. Also this study was supported by the Russian Foundation for Basic Research (Project No. 09-02-00171-a), the Federal Agency for Science and Innovation (Rosnauka) (Project No. MK-5632.2010.2), and the Physical Division of the Russian Academy of Science (Program “Strongly Correlated Electrons,” Project No. 2.3.1).

12 páginas, 17 figuras, 1 tabla.-- PACS number(s): 75.50.Gg, 75.50.Vv, 75.30.Gw.-- et al.

Peer reviewed