V. Sikolenko

Dzyaloshinskii-Moriya interaction and spin reorientation transition in the frustrated kagome lattice antiferromagnet

Magnetization, specific heat, and neutron scattering measurements were performed to study a magnetic transition in jarosite, a spin-52 kagome lattice antiferromagnet. When a magnetic field is applied perpendicular to the kagome plane, magnetizations in the ordered state show a sudden increase at a critical field H c , indicative of the transition from antiferromagnetic to ferromagnetic states. This sudden increase arises as the spins on alternate kagome planes rotate 180 ° to ferromagnetically align the canted moments along the field direction. The canted moment on a single kagome plane is a result of the Dzyaloshinskii-Moriya interaction. For H H c , the Zeeman energy overcomes the interlayer coupling causing the spins on the alternate layers to rotate, aligning the canted moments along the field direction. Neutron scattering measurements provide the first direct evidence of this 180 ° spin rotation at the transition.

Electromechanical and magnetic properties of BiFeO3-LaFeO3-CaTiO3 ceramics near the rhombohedral-orthorhombic phase boundary

BiFeO3-LaFeO3-CaTiO3 ceramics have been studied by X-ray diffraction, magnetization measurements, and piezoresponse force microscopy (PFM). The compositional ranges of the polar, antipolar, and non-polar phases have been estimated. PFM measurements testify gradual decrease of piezoelectric response in Bi0.85−xLa0.15CaxFe1−xTixO3 system with Ca/Ti content increase, except a narrow concentration region near polar-antipolar phase boundary where piezoelectric signal shows maximum value. It is found that increase of dopant concentration leads to apparent decrease of the off-center Bi-O displacement and, consequently, causes a reduction of piezoelectric response. It is concluded that notable remanent magnetization in polar and non-polar structural phases is a result of the Dzyaloshinsky-Moria interaction.

Magnetic properties of La1−xSrxCoO3 (x = 0.15 and 0.3)

Neutron diffraction, transversal field μSR and magnetization studies of La1−xSrxCoO3 with x = 0.15 and 0.3 has been performed. Long-range ferromagnetic ordering has been found only in the x = 0.3 sample. Magnetization measurements indicate the inhomogeneous magnetic state of this system. Magnetization data confirm well defined long-range ordering for the x = 0.3 sample with , whereas for the x = 0.15 compound this temperature marks the onset of magnetic order within ferromagnetic clusters. μSR data confirm that the transition in the ferromagnetic phase takes place in the large fraction of the x = 0.3 sample, but not in all of the volume. This data also provide evidence for the absence of long-range magnetic ordering for the x = 0.15 sample and show that even short-range ordering takes place only in part of the sample volume. Thus, all these data are in agreement with the phase separation scenario of the concentration phase transition in cobaltites.

Crystal structure, magnetic and electrical properties of the Nd1?xBaxCoO3 system

The properties of Nd1?xBaxCoO3 (0?x?0.24) solid solutions were studied at non-ambient temperatures by x-ray and neutron powder diffraction methods as well as magnetization and resistivity measurements. All the samples have an orthorhombically distorted perovskite structure (Pbnm space group). It was found that compositions with x?0.06 exhibit paramagnetic behaviour down to liquid helium temperature, and those with 0.08?x?0.16 are spin glasses, whereas both long-range ferromagnetic order (?K) and metallicity above TC seem to occur simultaneously at x?0.18. According to neutron diffraction data at different temperatures (T = 3?540?K), there are no crystal structure anomalies associated with orbital ordering of the Co3+ ions in the intermediate-spin state for NdCoO3. This is in agreement with gradual thermal excitation of the Co3+ ions from low-?to intermediate-?or high-spin state in a wide temperature range up to a metal?insulator transition (~600?K). The possible mechanisms of magnetic interactions are discussed.

Magnetic and piezoelectric properties of the Bi1 − xLaxFeO3 system near the transition from the polar to antipolar phase

The crystal structure, piezoelectric and magnetic properties of the Bi1 − xLaxFeO3 solid-solution system near the structural transition between the rhombohedral and orthorhombic phases (0.15 ≤ x ≤ 0.2) have been investigated. The regions of existence of the polar rhombohedral and orthorhombic phases have been determined, and the sequence of structural transitions as a function of the lanthanum ion concentration and temperature has been studied. The maximum piezoelectric signal is found for the solid solution with the composition x = 0.16, which has a single-phase rhombohedral structure. The relation between the type of crystal structure distortions and the increase in the magnetization upon the concentration-driven structural transition from the polar to antipolar phase has been established.

Very large magnetoresistance and spin state transition in Ba-doped cobaltites

Structural, magnetization, and magnetotransport measurements have been performed for anion deficient La0.5Ba0.5Co1− x Fe x O3−δ ( x ≤ 0.4 ) and La1− y Ba y CoO3−δ ( 0.5 ≤ y ≤ 0.6 ) perovskites. It has been found that the iron doped compositions with x ≤ 0.23 are predominantly ferromagnetic. The Curie point and magnetization are slightly larger in insulating phase (x = 0.1) than in metallic one (x = 0.05). Magnetoresistance ratio rises with lowering temperature and increasing iron content reaching three orders of magnitude in predominantly antiferromagnetic state ( x ≥ 0.25 ). The rise of the barium content in La1− y Ba y CoO3−δ series above y = 0.5 leads to stabilization of antiferromagnetic phase and strong enhancement of the magnetoresistance. Antiferromagnetic ordering is accompanied by increase in resistivity. The lack of the low field intergrain magnetoresistance is in agreement with a weak spin polarization of the charge carriers. Magnetoresistance is associated with antiferromagnet–ferromagnet (from mixed high/low into intermediate spin state) transition induced by magnetic field. It is suggested that ferromagnetism is originated from superexchange interaction via oxygen.

Neutron diffraction studies of La1 xSrxCoO3 magnetic structure at x=0.15 and 0.3

Abstract Neutron powder diffraction and zero-fields μSR studies of La 1− x Sr x CoO 3 with x =0.15 and 0.3 has been performed. Only compound with Sr concentration x =0.3 shows ferromagnetically ordered state at low temperature. Values of Co magnetic moments have been refined.

Mössbauer study of partially ordered cobaltites Nd2CoMnO6+δ with perovskite structure

The properties of Nd2CoMnO6+? (? = 0, 0.12) samples were investigated by M?ssbauer, ?SR and magnetization measurements. Our experimental data were interpreted in the following model. The samples consist mainly of clusters of cobalt and manganese ions with different degrees of order. The valence state of these ions is predominantly Co2+/Mn4+. Increasing the oxygen content leads to an enhancement of the disordered fraction of the sample which contains mainly three-valent ions. The highly ordered stoichiometric (? = 0) compound exhibits both pronounced ferromagnetic and weak spin-glass-like components. Ionic disordering leads to a partial destruction of the ferromagnetic component and to strong enhancement of the spin-glass-like one in non-stoichiometric (? = 0.12) samples. The spin-glass-like component transforms into the ferromagnetic one as a first-order metamagnetic phase transition in an external magnetic field.

Metamagnetic behaviour in TbCo0.5Mn0.5O3.06 perovskite

Investigation of the crystal structure, magnetic ordering and magnetic properties of the compound TbCo0.5Mn0.5O3.06 has been carried out by neutron powder diffraction and magnetization measurements. The Co and Mn ions are shown to have a partial NaCl-type crystallographic ordering in the B-sublattice of the ABO3 perovskite. On the basis of the magnetization study, metamagnetic behaviour has been revealed. The critical fields of the metamagnetic transition have been found to decrease with the approach to the Curie point at 98 K. The neutron diffraction experiments suggest that the compound TbCo0.5Mn0.5O3.06 consists of ferromagnetic and spin-glass-like phases. In the absence of an external magnetic field, a small ferromagnetic component appears in the Tb sublattice. The magnetic moments are directed antiparallel to the ferromagnetic component of the Co/Mn sublattice due to a negative f–d exchange polarization. Below 10 K a short-range antiferromagnetic order starts to develop in the Tb sublattice. The spin-glass-like phase associated with the random Co/Mn distribution transforms into the ferromagnetic phase under an external magnetic field, while in the Tb sublattice long-range magnetic order, being a superposition of the F, A, C, and G components, develops.

Neutron scattering studies of the structure and lattice dynamics of a solid solution of hydrogen in -manganese

A solid solution of hydrogen in -Mn, -MnHx with xD 0:073, was prepared at 623 K and a hydrogen pressure of 0.85 GPa. A profile analysis of the neutron diffraction patterns measured at 225 K and at 300 K showed that hydrogen in-MnH0:073 randomly occupies the interstitial positions 12e (0, 0, 0.538) of the -Mn host lattice. An inelastic neutron scattering study of -MnH0:073 at 90 K revealed a pronounced peak at 6.4 meV and a rather broad optical hydrogen band with peaks at 73, 105 and 123 meV. The splitting of the optical band into three different modes agrees with the low symmetry, 2, of the 12e hydrogen sites. A tentative interpretation is given for the peak at 6.4 meV.