Kiril Krezhov

Microstructure and Magnetic Behaviour of Nanosized Fe3O4 Powders and Polycrystalline Films

Summary. The object of investigation were the magnetic interactions in nanostructured Fe3O4 assemblies of two kinds (powder and film) where particles of similar size present nearly uniform domains in a close to planar arrangement with spacings sufficient for magnetic interactions. We discuss the use of the soft-chemistry method, i.e. the modified ‘ferrite plating’ (MFP) technique, for the synthesis of polycrystalline films of magnetite with nanosized crystallites.

Weak antiferromagnetism in YFeO3 and HoFeO3

Abstract We have performed neutron diffraction measurements of the weak antiferromagnetic component of the noncollinear spin arrangement of Fe3+ ions in YFeO3 and HoFeO3 by means of the SNIM-2 neutron spectrometer at the IBR-2 pulsed reactor of JINR, Dubna. Single crystals were used. The ratio of the week antiferromagnetic component Ab to the basic antiferromagnetic component Ga was found A b G a = 0.014 for YFeO3 and A b G a = 0.011 for HoFeO3 within error limits of about 10%.

Induced antiferromagnetism in HoFeO3

We present results from neutron diffraction studies of single crystalline HoFeO3 in a pulsed magnetic field by means of the spectrometer SNIM-2 at JINR-Dubna. The weak antiferromagnetism induced by the applied field in the Fe3+ and Ho3+ ion sublattices is measured in the temperature range 79-154 K. The temperature dependence of the antiferromagnetic susceptibility of the holmium subsystem is derived and found to obey the Curie-Weiss law.

Neutron powder diffraction study of a system of half-hole-doped bismuth-based calcium manganites at room temperature

The structural effects of partial replacement of diamagnetic Bi 3+ for potential charge ordering and magnetic ordering phenomena in Bi 0.25 R 0.25 Ca 0.5 MnO 3 (R = La, Nd, Ho) have been studied. The analysis of the room-temperature data suggests that in bismuth-based calcium perovskites the lone pair character of 6s 2 Bi 3+ orbitals is constrained rather than dominant. At 10 K the ground magnetic state of all compounds was specified as CE-type antiferromagnetic.

Influence of the Bi3+ electron lone pair in the evolution of the crystal and magnetic structure of La1−xBixMn2O5 oxides

La(1-x)Bi(x)Mn2O5 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1) oxides are members of the RMn2O5 family. The entire series has been prepared in polycrystalline form by a citrate technique. The evolution of their magnetic and crystallographic structures has been investigated by neutron powder diffraction (NPD) and magnetization measurements. All the samples crystallize in an orthorhombic structure with space group Pbam containing infinite chains of Mn(4+)O6 octahedra sharing edges, linked together by Mn(3+)O5 pyramids and (La/Bi)O8 units. These units become strongly distorted as the amount of Bi increases, due to the electron lone pair of Bi(3+). All the members of the series are magnetically ordered below TN = 25-40 K and they present different magnetic structures. For the samples with low Bi content (x = 0.2 and 0.4) the magnetic structure is characterized by the propagation vector k = (0,0,1/2). The magnetic moments of the Mn(4+) ions placed at octahedral sites are ordered according to the basis vectors (Gx, Ay, 0) whereas the Mn(3+) moments, located at pyramidal sites, are ordered according to the basis vectors (0, 0, Cz). When the content of Bi increases, two different propagation vectors are needed to explain the magnetic structure: k1 = (0,0,1/2) and k2 = (1/2,0,1/2). For x = 0.6 and 0.8, k2 is predominant over k1 and for this propagation vector (k2) the magnetic arrangement is defined by the basis vectors (Gx, Ay,0) and (Fx, Cy, 0) for Mn(4+) and Mn(3+) ions, respectively.

Magnetic properties of nanosized MgFe2O4 powders prepared by auto-combustion

Targets were prepared to be used for magnetron sputtering and laser ablation and their microstructural and magnetic properties were investigated. The base material was nanosized MgFe2O4 powder produced by citrate auto-combustion synthesis. The auto-combusted powders were annealed at temperatures in the range 600 - 1000 o

Radiation Dosimeter Based on Metal-Oxide-Semiconductor Structures Containing Silicon Nanocrystals

MOS structures containing silicon nanocrystals in the gate dielectric have been tested as dosimeters for ionizing radiation. Before irradiation the nanocrystals have been charged with electrons by applying a pulse to the gate electrode. The γ-irradiation with doses in the range 0-100 Gy causes approximately linear variation of the flatband voltage, resulting in sensitivities of ~ 2.5 mV/Gy. At higher doses the sensitivity decreases because of decrease of the oxide electric field.

Study of quasi-monophase Y-type hexaferrite Ba2Mg2Fe12O22 powder

We present the structural and magnetic properties of a multiferroic Ba2Mg2Fe12O22 hexaferrite composite containing a small amount of MgFe2O4. The composite material was obtained by auto-combustion synthesis and, alternatively, by co-precipitation. The Ba2Mg2Fe12O22 particles obtained by co-precipitation have an almost perfect hexagonal shape in contrast with those prepared by auto-combustion. Two magnetic phase transitions, responsible for the composites multiferroic properties, were observed, namely, at 183 K and 40 K for the material produced by auto-combustion, and at 196 K and 30 K for the sample prepared by co-precipitation. No magnetic phase transitions in these temperature ranges were observed for a MgFe2O4 sample, which shows that the magnesium ferrite does not affect the multiferroic properties of this type of multiferroic metarials.

Characterization of Y-type hexaferrite Ba 2 Mg 2 Fe 12 O 22 powders

Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf> is a Y-type hexagonal ferrite known for a relatively high temperature (∼200 K) of the magnetic transition to a spiral spin arrangement and an easy magnetization axis lying in a plane perpendicular to the c crystal axis. The multiferroicity exists in the absence of an external magnetic field H; a longitudinal-conical spin arrangement sets below 50 K; and the direction of the electric polarization P can be governed by relatively low magnetic fields (<0.02 T). The structural, magnetic and ferroelectric parameters have been mostly studied on monocrystals and, in spite of the attractive possibilities of designing promising composite multiferroic structures, the information on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders is scarce. We report on Ba<inf>2</inf>Mg<inf>2</inf>Fe<inf>12</inf>O<inf>22</inf>powders studied by X-ray and neutron powder diffraction, and magnetometry. The samples were prepared by two different techniques: sonochemical co-precipitation and sol-gel auto-combustion. The structures retained the rhombohedral crystal symmetry, but mixed occupancies of the 14 (Mg, Fe) cation sites was a new feature. The magnetizations at a magnetic field of 60 kOe at 300 K were 22.78 emu/g for the sample prepared by sol-gel auto-combustion, and 24.95 and 25.06 emu/g for the samples obtained by sonochemical co-precipitation and annealed at 1170 °C and 1200 °C, respectively.

Effects of Substitution in Barium Hexaferrites BaFe12-xXxO19 (X=Co,Ti; Sc)

The short and long-range atomic and magnetic order in BaFe12-xCoxTixO19 (x=0.4, 0.7, 0.8) and BaFe10.4Sc1.6O19 prepared by soft chemistry routes were studied and compared with parent BaFe12O19 prepared by solid-state reaction. For BaFe10.3Co0.85Ti0.85O19 neutron diffraction and magnetic measurements revealed that (Co2+, Ti4+) substitution causes significant distortions in the local oxygen surrounding of ferric cation sites, while the grain-size effect on the structural parameters is considerably smaller. The thermal expansion coefficient exhibits a strong anisotropy. The refined magnetic moments for x=0.45 and 0.7 based on the five-cation sublattice block-type collinear ferrimagnetic structure of uniaxial type known as Gorter type for BaFe12O19, are considerably lower than the theoretical spin only moments, especially for the 4e and 12k sites, indicating for x = 0.7 a local noncollinearity with short-range ordering. For x = 0.85, at 10 K a block-type conical magnetic structure sets in. For BaFe10.4Sc1.6O19, combined neutron diffraction, field-dependent 57Fe Mössbauer studies and magnetic measurements show that the collinear block-type structure remains effective at 300 K and below it down to about 190 K. In the 190 – 1.6 K range we observe a temperature dependent incommensurate complex block-type conical structure.