Synthesis and properties of barium ferrigermanate Ba<Subscript>2</Subscript>Fe<Subscript>2</Subscript>GeO<Subscript>7</Subscript>

The magnetic susceptibility and specific heat of single crystals of the Ba₂Fe₂GeO₇ barium ferrigermanate are investigated. It is revealed that the temperature dependence of the magnetic susceptibility exhibits a kink at a temperature T = 8.5 K. The number of nonequivalent positions of Fe³⁺ ions and their occupancies are determined using Mössbauer spectroscopy. It is shown that the Fe³⁺ ions located in tetrahedral positions T2 are ordered incompletely, which is inconsistent with the results obtained previously. An assumption is made regarding the possible ground magnetic state of the Ba₂Fe₂GeO₇ compound.     

Long-wavelength magnetic excitations in the Zn0.55Mn0.45Fe2O4 ferrite

The small-angle neutron scattering energy spectra of the Zn_0.55Mn_0.45Fe₂O₄ ferrite are analyzed at different temperatures (both below and above T _C ? 390 K) and scattering angles. The thermal expansion coefficient α(T) is measured in the temperature range 80–600 K. It is revealed that inelastic neutron scattering is governed not only by spin waves of the Holstein-Primakoff type but also by the substantial contribution of additional long-wavelength magnetic excitations. The physical nature of these low-energy magnetic excitations is discussed.     

Magnetic study 2D-antiferromagnets Ba2NiF6 and Ba2FeF6 by neutron diffraction and mossbauer spectroscopy

The 2D-antiferromagnetism of tetragonal Ba₂NiF₆ and Ba₂FeF6 — whose structure derives from that of K₂NiF₄ — has been studied by magnetization measurements and by powder neutron diffraction. The magnetic cell is 2a, 2$\text{a}$,$\text{c}$ and the magnetic moments (μ_Ni²⁺ = 1.9μB and μ_Fe²⁺ = 3.46μB) lie along c. This has been confirmed by Mossbauer spectroscopy for Ba₂FeF₆.     

Mössbauer study of Lu2Fe3O7: A two-dimensional antiferromagnet on a triangular lattice

Mössbauer studies reveal that there are two kinds of Fe³⁺ spins with completely different characteristics in Lu₂Fe₃O₇: one is an Ising-like property and the other is a Heisenberg-like property in a two-dimensional antiferromagnet on a triangular lattice. The former spin orders ferrimagnetically along thec-axis at around 220 K. The latter spin is lying in thec-plane and a corresponding hyperfine magnetic field is observed at temperatures below 50 K. At very low temperatures, however, the latter spin has a component parallel to thec-axis and couples with the former spin. This finding is consistent with the modulated profile of the magnetic scattering in neutron diffraction and the result of a magnetization measurement.     

Magnetic excitations of HoAl2 in a magnetic field

The ground state spin wave excitation energies of single crystalline HoAl₂ have been studied at T= 4.2 K in external magnetic fields up to 7 T by means of inelastic neutron scattering. The results have been interpreted in terms of a cubic crystalline electric field using the parameters determined by magnetization measurements and an exchange interaction with the exchange parameters taken from the zero field measurements.     

Magnetic structure simulation of Fe/V superlattices with a variable thickness of iron layers

The model of the magnetic structure of Fe/V superlattices is discussed. The discrepancy in estimation of the critical temperature for the Fe₂/V _n /Fe₃/V _n superlattice obtained by neutron scattering and the magneto-optical Kerr effect is caused by inhomogeneity of the magnetization distribution in a finite superlattice.     

Anisotropic magnetization and susceptibility of Ba2Zn2Fe12O22 (Zn2Y)

The anisotropy of the magnetization and the susceptibility of the hexagonal ferrite Ba₂Zn₂Fe₁₂O₂₂ have been determined experimentally between 5 K and T_r = 367 K. The susceptibility is ascribed to the paraprocess. The measurements confirm for the first time the basic concept of the model for the anisotropy of the magnetization proposed by Callen and Callen.     

Electronic and magnetic properties of the iron borate Fe2BO4

Polycrystalline Fe₂BO₄ was prepared by solid state reactions and its electronic and magnetic properties were investigated by Mössbauer spectroscopy and magnetization measurements. The Mössbauer spectra of Fe₂BO₄ below 270 K indicate the presence of Fe²⁺ and Fe³⁺ sites in the structure, in a ratio 1 : 1. Above this temperature electron delocalization sets in between the divalent and trivalent iron ions and Fe^2.5+ states are observed. The temperature dependence of the Mössbauer spectra and magnetization measurements clearly show the onset of magnetic order below 155 K.     

Effects of magnetic anisotropy and exchange in Tm2Fe17

Neutron diffraction experiments have been carried out to study the magnetocrystalline anisotropy of two (2b and 2d) Tm sublattices and four (4f, 6g, 12j, and 12k) Fe sublattices in ferrimagnetic compound Tm₂Fe₁₇ (space group P6₃/mmc). We have determined the temperature dependence of the magnitude and orientation of magnetization for each of the thulium and iron sublattices in the range (10?C300) K. A spontaneous rotation (at about 90 K) of the Tm and Fe sublattice magnetizations from the c-axis to the basal plane is accompanied by a drastic change in the magnetization magnitude, signifying a large magnetization anisotropy. Both Tm sublattices exhibit an easy-axis type of the magnetocrystalline anisotropy. The Fe sublattices manifest both the uniaxial and planar anisotropy types. The sublattice formed by Fe atoms at the 4f position reveals the largest planar anisotropy constant. The Fe atoms at the 12j position show a uniaxial anisotropy. We find that the inelastic neutron scattering spectra measured below and above the spin-reorientation transition are remarkably different.     

Magnetic properties of SiO2-coated iron oxide nanoparticles studied by polarized small angle neutron scattering

The structural and magnetic properties of non-coated and SiO₂-coated iron oxide (Fe₃O₄) nanoparticles (NPs) were investigated by a polarized small-angle neutron scattering (P-SANS) method. Measurement of the P-SANS allowed us to obtain nuclear and magnetic scattering cross sections of the NPs under applied magnetic field. The analysis of the scattering intensity provided the structural parameters and the spatial magnetization distribution of the non-coated and the SiO₂ coated core–shell NPs. The measured radius of both NPs and the shell thickness of the core–shell NPs were in consistent with those measured by the transmission electron microscopy. In comparison, the magnetic core radii of both NPs were 0.12–0.6 nm smaller than the nuclear radii, indicating the magnetization reduction in the surface region of core Fe₃O₄ in both NPs. However, the reduced magnetization region, which is the surface spin canting region, of the SiO₂-coated NPs was relatively narrower than that of the non-coated NPs. We suggest that the SiO₂ coating on the Fe₃O₄ NPs may stabilize the spin order of atoms and prohibit the oxidation or defect formation at the surface region of the Fe₃O₄ NPs, and enhance the corresponding magnetization of the Fe₃O₄ NPs by the reduction of the spin canting layer thickness.     

Magnetic structure and space group of the garnet Ca3Fe2(GeO4)3

Ca₃Fe₂(GeO₄)₃, is paramagnetic at room temperature with the cubic space group Ia3d. At 4.2 K the compound is magnetically ordered. From powder neutron diffraction data a pair of collinear, enantiomorphic magnetic structures is derived, the magnetic space group of which is I_p4₁ 22 or I_p4₁ ‘22’, respectively.     

Magnetization anisotropy of the Tm- and Fe-subsystems in Tm2Fe17

Magnetic and neutron diffraction measurements were carried out in order to study the spontaneous and induced spin-reorientation (SR) transition of the “easy axis–easy plane” type in the poly and single-crystalline samples of the hexagonal Tm₂Fe₁₇. We have determined the temperature dependence of the lattice parameters and the angle between the c-axis and the magnetic moment of the Tm-subsystem. We also find that the SR transition is accompanied by a large (about 20%) magnetization change of the Tm subsystem. In order to induce such a SR transition with the external magnetic field, μ₀H_cr=5 T is necessary to be applied along the hard-magnetization direction (the a-axis) at 4.2 K. The H_cr value decreases with an increasing temperature. The magnetization measurements demonstrate that at 10 K the saturation magnetization along the easy-magnetization direction (the c-axis) is smaller than that along the hard-magnetization direction. Based on this observation, we believe that Fe-subsystem of Tm₂Fe₁₇ is likely to have magnetization anisotropy.     

Low temperature heat capacity and magnetic excitation of HoAl2 in a magnetic field

The specific heat of single crystalline HoAl₂ in magnetic fields up to 7.5 T has been measured for the temperature range 1.5–16 K. In addition the energy of a magnetic excitation in a magnetic field of 5 T at 4.2 K has been determined by inelastic neutron scattering. The results have been interpreted with a cubic crystalline electric field and an exchange interaction using the same parameter set B₄=-0.85×10^-4 meV, B₆=+0.71× 10^-6 meV and T_C=31.5 K previously obtained by magnetization measurements.     

Magnetization harmonics for YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7 − <Emphasis Type="Italic">x</Emphasis></Subscript> textured polycrystals above the superconducting transition temperature

The temperature dependences of the third and other higher magnetization harmonics for YBa₂Cu₃O_{7 ? x} textured polycrystals are experimentally investigated in the temperature range 77–120 K. It is revealed that the magnetization of the YBa₂Cu₃O_{7 ? x} textured polycrystals exhibits a nonlinear behavior up to temperatures considerably higher than the superconducting transition temperature. The observed nonlinearity of the magnetization of the YBa₂Cu₃O_{7 ? x} textured polycrystals is attributed to the pseudogap state that appears for this compound at a temperature T ～ 102 K.     

Specific features of neutron scattering upon heat treatment of the Co67Fe31V2 alloy

This paper reports on the results obtained from small-angle neutron scattering investigations during annealing of two plates of the Co₆₇Fe₃₁V₂ alloy under heating the first and second samples to 360 and 500°C, respectively, as well as in applied weak magnetic fields (H ≈ 3 Oe). To elucidate the role of the vanadium impurity, measurements have also been performed on the Co₆₈F₃₂ alloy. It has been revealed using neutron diffraction that, at temperatures below 360°C, the Fe and Co atoms begin to undergo ordering in the samples, which results in a change in the space group of the crystal structure. With an increase in the temperature, a fine-dispersed phase is formed in the Co₆₇Fe₃₁V₂ alloy, which leads to a qualitative change in the small-angle neutron scattering pattern. It has been found that, after annealing and magnetization (H > 4 kOe) of the second sample, there is a strong difference in the integrated intensities of scattered neutrons for opposite directions of their spins in the primary beam.     

Magnetism in a UNi2/3Rh1/3Al single crystal

We report a magnetization, magnetostriction, electrical resistivity, specific heat and neutron scattering study of a UNi_2/3Rh_1/3Al single crystal, a solid solution of an antiferromagnet UNiAl and a ferromagnet URhAl. The huge uniaxial magnetic anisotropy confining the principal magnetic response to the c axis in the parent compounds persists also for the solid solution. The magnetization curve at 1.6 K has a pronounced S shape with an inflection at 12 T. The temperature dependence of magnetic susceptibility exhibits a maximum around 10 K and is magnetic history dependent at lower temperatures where the resistivity increases linearly with decreasing temperature. The low-temperature ρ(T) anomaly is removed in a magnetic field applied along c, which yields a large negative magnetoresistance amounting to m46 zin 14T (at 2 K). The C/T values exhibit a minimum around 12 K and below 8 K they become nearly constant (about 250 mJ mol^?1 K^?2), which is strongly affected by magnetic fields. Neutron scattering data confirm a non-magnetic ground state of UNi_2/3Rh_1/3Al. The bulk properties at low temperatures are tentatively attributed to the freezing of U magnetic moments with antiferromagnetic correlations. The additional intensities detected on top of nuclear reflections in neutron diffraction in a magnetic field applied along c are found to be proportional to the field-induced magnetization, which reflects field-induced ferromagnetic coupling of U magnetic moments. This scenario is corroborated also by finding low-temperature magnetostriction data that also scale with the square of magnetization.     

Spin glass properties of CsNiFeF6

Neutron diffraction shows, that CsNiFeF₆ has no magnetic long range order at 2 K, well below the temperature, at which magnetic resonance occurs (6.5 K). Diffuse neutron scattering proves the existence of magnetic short range order. S-shaped magnetization curves are observed at low temperatures. Peaks in the field-susceptibility are explained with a simple short range order model.     

A note on exchange and crystal field interactions in R2Fe14B compounds: Yb2Fe14B

The R₂Fe₁₄B phase has been found to exist for R=Yb. The magnetic properties presented in this paper complete the characterization of the compounds in this series for which the Stevens α_J coefficient of the R³⁺ ion is positive. ⁵⁷Fe Mössbauer spectroscopy establishes the existence of a magnetization reorientation at 115 K of the type observed in Er and Tm compounds associated with a small Fe magnetization anisotropy. From the neutron diffraction measurements obtained at 4.2 K with and without an applied magnetic field, the easy direction of magnetization was found to be along the [100] direction, in the basal plane of the tetragonal structure. These results show that in all compounds where α_J>0 for the R³⁺ ion, the easy direction of magnetization in the plane is determined by the second order crystal field terms and rare earth-Fe exchange interactions and is independent of the sign of the 4th order crystal field terms.     

Atomic and magnetic ordering in Fe3−xVxSi alloys

Alloys of composition Fe_3?xV_xSi with x = 0.25, 0.50, 0.75, 0.90 and 1.00 were studied by neutron diffraction to obtain site distributions and information on magnetic ordering. Vanadium atoms were found to substitute almost completely on the Fe site with 8Fe near neighbors, with little evidence for other types of disorder. Ferromagnetic scattering was observed for x = 0.25, 0.50 and 0.75 in an amount consistent with the observed magnetization, but the sample with x = 0.90 showed no evidence of magnetic ordering down to 4K.     

Magnetic anisotropy and magnetostriction of Lu2Fe17 single crystal

The magnetic properties of Lu₂Fe₁₇ single crystal have been studied by means of magnetization, susceptibility and magnetostriction measurements. The unusual magnetic behavior with two magnetic phase transitions has been observed in magnetic fields up to 50 Oe. The magnetostriction of the Lu₂Fe₁₇ compound has the maximum at temperature T≈285 K at which the paraprocess makes the main contribution to the magnetization.