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In this work, we investigated the effects of postdeposition heat treatment on structural and magnetic properties of CoFe2O4 nanoparticles produced by pulsed laser deposition. Structural analysis by X-ray diffraction, transmission electron microscopy (TEM) and Mössbauer spectroscopy indicate the formation of a single phase cobalt ferrite nanoparticles with the size ranging from 4.3 to 33.3 nm depending on the annealing temperature. The magnetic properties of the samples were investigated in a wide temperature range (50–400 K). Noticeable effects of the cubic magnetocrystalline anisotropy on the magnetization process of nanoparticles were observed for samples annealed at 450 °C and 600 °C, while for samples as-deposited and annealed at 300 °C the magnetization properties were dominated by a uniaxial effective anisotropy. ΔM technique was used to investigate the magnetic interaction among the nanoparticles. Only demagnetizing interactions were observed for the sample annealed up to 300 °C, while for the samples treated at 450 °C and 600 °C, both magnetizing and demagnetizing interactions were observed. The results are discussed considering the evolution of the nanoparticles’ nanostructure with anneals and its effects on the magnetic properties. [ABSTRACT FROM AUTHOR]

57Fe Mössbauer spectroscopic studies of iron ludwigite Fe3O2BO3 performed between 4 and 450 K allow the discussion of magnetic spin arrangements and the dynamics of electronic configurations of iron. The observed magnetic transitions are related to charge ordering. [ABSTRACT FROM AUTHOR]

57Fe Mössbauer spectroscopic studies of iron ludwigite Fe3O2BO3 performed between 4 and 450 K allow the discussion of magnetic spin arrangements and the dynamics of electronic configurations of iron. The observed magnetic transitions are related to charge ordering. [ABSTRACT FROM AUTHOR]

The ludwigite Co5Sn(O2BO3)2 was studied using x-ray diffraction, Mossbauer spectroscopy, and magnetic and thermodynamic measurements. This material belongs to a family of oxyborates which presents low-dimensional subunits in the form of three-leg ladders in its structure. The subunits confer to these materials a strong anisotropy in their exchange interactions that provide to the ludwigites several interesting magnetic properties, from partial ordering to spin-glass states. Despite being doped by nonmagnetic ions, Co5Sn(O2BO3)2 has long-range magnetic order below 82 K which is, surprisingly, the highest critical temperature found sofar in the ludwigites. This record can be explained by the absence of double-exchange interactions, usually present in the ludwigites and that gives rise to strong competition. In this paper we study the magnetic and structural properties of Co5Sn(O2BO3)2 and compare our results with those obtained in other cobalt ludwigites. [ABSTRACT FROM AUTHOR]

There are two known ludwigites containing a single transition metal element, Fe3O2BO3 and Co3O2BO3. The structure of these materials has low-dimensional units in the form of three-legged ladders (3LL) that confer to each of them unique magnetic and electronic properties. Fe3O2BO3 presents a staggered charge density wave (CDW) transition in the ladders near room temperature and two magnetic transitions. It has remained a mystery why the other compound Co3O2BO3 behaves so conventionally, with a single magnetic transition and no CDW in spite of similar structural and electronic configurations. Neutron diffraction results presented here in this system finally unravel these differences. Far from a trivial explanation, we uncover a coexistence of low and high spin Co ions in well-defined octahedral sites. Our results allow one to solve the contrasting behavior of the Fe and Co ludwigites in terms of a subtle and unique charge ordering mechanism occurring at the microscopic level of the rungs of the 3LL. [ABSTRACT FROM AUTHOR]

This paper presents an extensive study of the structural, magnetic, and thermodynamic properties of the heterometallic ludwigite Co4.76 Al1.24(O2 BO3)2. This material orders ferrimagnetically at 57 K. Despite the fact that, in Co4.76 Al1.24 (O2 BO3)2, one quarter of the Co ions of the parent compound Co3 O2 BO3 is replaced by nonmagnetic Al3+ ions, the TC of the former (57 K) is larger than that of the latter (42 K). We obtain the phase diagram of Co4.76 Al1.24 (O2 BO3)2 in an external magnetic field. We find a line of second-order phase transitions where the system presents a metamagnetic transition (from 57 to 25 K). This line ends at a tricritical point at 25 K and 4.3 T, below which the transition to the paramagnetic phase is first order (from 25 to 3 K). In the magnetic phase at low temperatures specific heat measurements in zero and finite magnetic fields show that the low-energy excitations are magnetic (magnons) and elastic (phonons) excitations with two-dimensional character. These measurements also reveal the magnetocaloric properties of this material. [ABSTRACT FROM AUTHOR]