Kiiti Siratori

Band Structure in the High Temperature Phase of Fe3O4

Band structure of the ferrimagnetic Fe 3 O 4 was calculated in the high temperature cubic phase by means of self-consistent APW method. Though the obtained band structure corresponds closely with the ionic Neel model, it was shown that the itinerant electron model is more adequate than the localized electron model. The minority spin d e bands of B site iron have an electron Fermi surface at the \( \varGamma \) point and hole surfaces around the W point. Experimental data including photoelectron emission, optical reflectivity, neutron scattering and transport phenomena were discussed qualitatively in relation to the calculated band structure.

Two-dimensional spin ordering in YFe2O4

Abstract A neutron diffraction study was performed on a single crystal of a new compound YFe2O4 below its Neel point. In a slightly oxygen deficient crystal, the elastic magnetic scattering takes the form of Bragg line along the c axis at ( n 3 , n 3 , l) (n ≠ 3m) in the hexagonal lattice. This fact indicates the two-dimensional long range order of a commensurate sinusoidal spin structure.

Dielectric Relaxation and Hopping of Electrons in ErFe2O4

We report the frequency and the temperature dependence of the dielectric constant of polycrystalline ErFe 2 O 4-δ with different degrees of oxygen deficiency δ. A large dielectric relaxation, with the order of magnitude of 10 4 and nearly of the Debye type, is observed in the magnetically ordered states within the temperature and frequency ranges of 150 K to 290 K and 1 kHz to 3 MHz, respectively. The dispersion is larger in the sample with more oxygen deficiency. From the temperature dependence of the characteristic frequency, we conclude that the elementary process of the dispersion is related to electron hopping between Fe 2+ and Fe 3+ ions. A large value of the dielectric constant is consistent with the existence of spontaneous polarization in the magnetic phases of this oxide. We propose that the motion of the polarization domain boundaries is a determining factor of low-frequency dielectric constant.

Low-Temperature Phase Transitions and Magnetic Properties of YFe2O4

Two-step phase transitions are found in stoichiometric YFe 2 O 4 at low temperatures, about 230 K and 190 K during cooling. A hexagonal lattice at room temperature is distorted to be monoclinic and subsequently triclinic. No parasitic ferrimagnetism is found in YFe 2 O 4.00 , in contrast to oxygen-deficient specimens such as YFe 2 O 3.94 .

Mössbauer Study of RFe2O4

Mossbauer spectra of RFe 2 O 4 (R=Y, Ho, Er, Tm or Yb) were measured between 200 and 500 K, in the paramagnetic region. Amalgamation, from two quadrupole doublets due to different ionic states of Fe into one doublet, was observed and successfully analysed by a stochastic theory, except the stoichiometric Y and Er compound. In the latter case, in which Verwey transition takes place at low temperature, the analysis failed because of the existence of other set of absorptions. When the stochastic theory was applicable, seven parameters were deduced at each temperature: isomer shift, quadrupole splitting and width for both states of Fe and the frequency of the fluctuation. It is concluded that Fe atoms fluctuate between two ionic states, neither 2+ nor 3+ in its literal meaning but mixtures of them: itinerant model is more appropriate than the ionic model for 3d electrons of iron in these oxides.

Iron Vacancy Ordered γ-Fe2O3(001) Epitaxial Films: The Crystal Structure and Electrical Resistivity

We report the structural characterization and transport properties of highly-ordered epitaxial γ-Fe 2 O 3 thin films grown by a pure ozone-assisted molecular beam epitaxy method. X ray diffraction ...

Conductivity and specific heat anomalies at the low temperature transition in the stoichiometric YFe2O4

Abstract Anomalies were found in electrical conductivity and specific heat at the two-step transition in the stoichiometric YFe2O4. It is concluded that the transition is a new type of Verwey transition: i.e., ordering of ionic charge, Fe2+ and Fe3+, accompanied by lattice distortion and antiferromagnetic spin ordering.

A Method of Controlling the Sense of the Screw Spin Structure

Considering the magnetic symmetry of a screw spin structure, it was shown that the sense of the screw spin structure can be controlled by a magnetoelectric cooling. It was confirmed experimentally in ZnCr 2 Se 4 by means of a polarized neutron diffraction.

Neutron scattering study of ZnCr2Se4 with screw spin structure

Neutron scattering experiments have been carried out on a single crystal and powdered specimens of ZnCr 2 Se 4 , a helimagnet, above and below the transition at T N =21.2 K. Below T N , crystal belongs to D 4h 19 . Atomic parameters of Se can be expressed by a single parameter, u =0.3847. Rather small magnetic moment (1.71 µ B at 0 K) and the linear dependence of the length of the screw vector, Q 0 , on the lattice deformation were observed. Anisotropic critical scattering ( J x / J y ≃3) was measured around (4- Q 0 , 0, 0) above T N and Q dependence of the exchange interaction constant, J ( Q ), near Q 0 was discussed. Recent studies by Plumier et al. were critically referred.

Magnetic Properties of Amorphous Fe-Nd Alloys*

Melt-spun amorphous Fe 1- x Nd x alloys (0.11≤ x ≤0.60) were investigated by the magnetization measurement up to 143 kOe and the Mossbauer spectroscopy. The coercive field at 4.2 K increases abruptly at around x =0.30 with increasing x , amounting to 53 kOe at x =0.40. The hyperfine field of 57 Fe at 77 K is independent of x and about 300 kOe on an average, suggesting that the magnetic moment of Fe is about 2.0 µ B /atom throughout this system. On this assumption and the measured magnetization at 4.2 K, it is estimated that the average Nd moment is about 3.2 µ B /atom at the low x limit. This value corresponds to the free ion value of Nd 3+ . The Nd moment decreases with increasing x to one half, 1.6 µ B /atom. A ferromagnetic cluster model with large random anisotropy is proposed for the high coercivity alloys of this system. Magnetization curves at 4.2 K are well explained by the model, by an adjustment of the parameters for the anisotropy energy of the cluster and the inter-cluster coupling. The concen...