Naoshi Ikeda

Charge Frustration and Dielectric Dispersion in LuFe2O4

We report the dielectric anomaries at the phase transitions of Lu(Fe 2+ Fe 3+ )O 4 , which is considered as a charge and spin frustrated system. Dielectric constant shows a hump, just above the transition from the high temperature two-dimensional to the low temperature three-dimensional charge ordering. A few glassy behaviors were observed at this transition. The presence of spontaneous polarization in the low temperature phase was confirmed by the measurement of pyroelectricity. A step-wise increase of the dielectric constant with increasing temperature was observed between 150 K and 250 K as in other RFe 2 O 4 . These phenomena are explained by the presence of the polar domains with the ordering of Fe 2+ and Fe 3+ and the motions of the antiphase boundaries between them.

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.

Incommensurate charge ordering in charge-frustrated LuFe2O4 system

The charge ordering process of a mixed valence LuFe 2 O 4 system has been studied, focusing attention on the frustration effect associated with the charge configuration. It is revealed that the system undergoes successive phase transitions following the sequence of disorder→2D CDW→3D CDW. In the 2D-CDW state, the charge density wave defined by the wave vector (1/3, 1/3), is formed in the hexagonal layer. On the nodal lines of the CDW, the charge order is still undetermined due to the charge frustration. From a dielectric viewpoint, this phase is viewed as random stacking of 2D ferroelectric layers. In the 3D-CDW state, antiphase stacking of the 2D ordered layer develops. At the same time, the in-plane charge order exhibits a doubly modulated long-period structure. From a dielectric viewpoint, this phase is characterized by an incommensurate antiferroelectric phase. Relevance of the charge ordering mechanism to the observed anomalously large dielectric constant in LuFe 2 O 4 is discussed.

Spin-glass behavior in amorphous BiFeO3

Short‐range antiferromagnetic (speromagnetic) behavior was observed for amorphous BiFeO3 by static magnetic measurements, ac susceptibility (χac) measurement, and 57Fe Mossbauer spectra measurements. The magnetic behavior is classified into three temperature ranges: (i) paramagnet for T≳220 K; (ii) local clustering of spins for 20 K<T<220 K; and (iii) speromagnet in which all spins are frozen into random directions for T<20 K. In the spin freezing state the magnetization shows irreversibility and the χac−T curve has a cusp. The variation of the spin freezing temperature against measuring magnetic field and measuring frequency indicate that the speromagnetic order is very similar to that of spin glass.

Anisotropy of Dielectric Dispersion in ErFe2O4 Single Crystal

Dielectric dispersion was observed in a single crystal with the composition of ErFe 2 O 3.95 below magnetic transition temperature. It is characterized by a large dielectric constant of the order of 10 4 , and a Debye-type frequency dependence like in the polycrystalline samples of the same substance which we reported in a previous paper. Large anisotropy exists in the strength e s - e ∞ and the characteristic relaxation frequency of the dispersion. The strength is higher within the c -plane than along the c -axis, while the characteristic frequency is much larger along the c -axis than within the c -plane. The dielectric dispersion in the polycrystalline sample cannot be explained by a simple average of the single crystal results. The observed dispersion is qualitatively discussed with a model of domain wall motion on an assumption that ErFe 2 O 4 is a polar crystal.

Incommensurate charge ordering in mixed valence system LuFe2O4

Abstract Charge ordering of a mixed valence system LuFe2O4 has been studied by neutron scattering technique, focusing attention on the charge frustration effect on triangular lattice. It is revealed that the system undergoes successive phase transitions following the sequence of disorder → two-dimensional charge density wave state (2D-CDW) → three-dimensional charged density wave state (3D-CDW). In 2D-CDW state, the charge density wave defined by the wave vector ( 1 3 1 3 ) is formed in the hexagonal layer.

Effect of Pr substitution on superconductivity in (PrxY1−x)Ba2Cu4O8

Abstract The compounds of (Pr x Y 1−x )Ba 2 Cu 4 O 8 (x=0, 0.1 and 0.6) were prepared in the oxygen atmosphere of 33 atm using a electrical furnace, where the materials were annealed at 930°C for 48 h. Magnetic and electrical measurements were performed: Transition temperatures for compounds of x=0, 0.1 and 0.6 were 80 K, 75 K and 44 K, respectively. The critical content X c , at which superconductivity disappears, was estimated to be 0.9. Effective moment and the Pauli paramagnetic component of Pr 0.6 Y 0.4 Ba 2 Cu 4 O 8 were 3.05±0.15 and (1.3±0.2) x 10 -6 cm 3 /g, respectively.

Possibility of magnetoelectric effect in antiferromagnetic RFe2O4

Abstract We report the magnetoelectric effect of a polycrystalline sample of a triangular Ising antiferromagnet ErFe2O4-δ, in which the existence of short range charge ordering was inferred from the neutron and dielectric studies. The sign of the magnetoelectric signal was reversed at 77 K, when the electric or magnetic field was applied in the opposite directions during cooling the specimen down to 77 K. The sign of the signal was independent of the direction of the static bias magnetic field in the range above 150 K. The signal was maximum at 183 K. We suggest a relation between the origin of the magnetoelectric effect and the dielectric relaxation due to an electron hopping between Fe3- and Fe2+ sites.

Magnetic Relaxation in Diluted Triangular Antiferromagnet LuFeMgO4

We observed a few magnetic relaxation phenomena in a LuFeMgO 4 single crystal, that is, the difference in the magnetization between field-cooled and zero-field-cooled specimens, the appearance of thermoremanent magnetization, and the frequency dependence of the cusp temperature of AC susceptibility. The observed relaxation was interpreted in terms of thermal fluctuations and freezing of the magnetic clusters which are supposed to exist in this substance.

A Simple Alternating Current Magnetometer of the Automatic Balancing Type

An improvement of a Hartshorn type alternating current magnetometer is reported. The use of a dual channel oscillator, a two-phase lock-in amplifier and a personal computer enabled us to obtain a simple feedback system for automatic balancing of the bridge. The resolution of this system is about 3×10-7 emu of magnetic moment or higher at the field of 0.8 Oe in amplitude and 159 Hz in frequency.