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Mossbauer study and magnetic measurements were carried out on the stoichiometric FeO, prepared with the high pressure (∼5.5GPa) synthesis technique. Well known defects (Koch-Cohen clusters) in FeO are detected even in the stoichiometric specimen, prepared at high temperatures.

For the determination of the magnetic structure, elimination of the magnetic domains is indispensable and the appropriate stress will be effective when the crystal has a structural transformation at T N . The effect of uniaxial stress cooling on the neutron magnetic scattering in a ZnCr 2 O 4 single crystal was studied. The pressure axis was set parallel to the [1 0 0] or [0 0 1], parallel or perpendicular to the diffraction plane, (0 0 1). The [1 0 0] pressure cooling increases one half-integer spots along the [1 0 0], whereas decreases others. The [0 0 1] pressure cooling increases two half-integer spots in the (0 0 1) plane, whereas decreases one half-integer spots.

RMn 2 O 5 ( R = Y and rare earth) shows successive magnetic and ferroelectric phase transitions at about 45, 40, 39, 20 and 10 K. We have reinvestigated the magnetic structure of YMn 2 O 5 at the commensurate phase ( T = 25 K ) using a single crystal four-circle diffractometer in order to discuss the mechanism of magnetoelectric interaction and the origin of ferroelectricity. We also observed the lattice modulation vectors ( q L ) to compare the magnetic propagation vectors ( q M ) by synchrotron X-ray diffraction. Improved magnetic structure data are compared with the theory recently proposed.

In a series of oxides RFe 2 O 4 (R = rare earth between Dy and Lu, or Y), equal numbers of Fe 2 + and Fe 3 + are located on a layer composed of two triangular nets stacked together. These oxides have several phase transitions where the arrangement of Fe 2 + and Fe 3 + ions transforms from one type to another. Unique dielectric properties of these oxides have been explained in connection with the charge ordering. Recent developments on the structure of the ordered phases are reviewed with comments on their correlation with the observed dielectric dispersion

We have studied magnetic orderings of a ferroelectric TmMn 2 O 5 by single-crystal neutron diffraction and dielectric measurements. Below T N1 = 44 K, TmMn 2 O 5 shows the sequence of magnetic phas...

We have investigated Mossbauer spectra of single crystal YMn 2 O 5 (with 1.5 mol % 57 Fe), which is simultaneously ferroelectric (the ferroelectric Curie temperature T CE =39 K and the second dielectric transition temperature T C2 =19 K) and antiferromagnetic ( T N =45 K) at low temperatures. In the temperature range of T C2 < T < T CE , the spectrum is composed of two different hyperfine sextets, indicating that the magnetic moments of Mn 3+ ions are in two different states. A half of them are oriented parallel to the a -direction, which is probably the easy direction, while the other half are forced to be oriented along the bisector of the b - and c -direction. A few evidences indicate that both A and B moments rotate at T C2 to form an incommensurate helical spin structure with bunching. We present a remark on a possible relation between the ferroelectric transition and the magnetic long-range order in this oxide.

Neutron diffraction measurements have been performed on a single crystal of TbMn 2 O 5 to elucidate the interplay between magnetic ordering and ferroelectricity. On cooling below T N1 = 42 K, the system enters a lock-in commensurate magnetic (CM) state with the magnetic propagation vector q = (1/2,0,1/4) from a two-dimensionally modulated (2D) incommensurate magnetic (ICM) one with the temperature-dependent q = ( q x ,0, q z ). Unlike isomorphous YMn 2 O 5 and ErMn 2 O 5 where a 1D-ICM phase with q = ( q x ,0,1/4) appears and separates both 2D-ICM and CM phases, the CM phase coexists with the 2D-ICM one at temperatures between T = 36.5 and 37.8 K, associated with a kink and a sharp peak in the dielectric constant along the b axis. On further cooling, the system reenters an ICM state at T = 22.4 K whose q is pinned at (20/41,0,5/16) at low temperatures. The comparison between the temperatures of dielectric anomalies and magnetic phase transitions indicates that the formation of the CM ordering is related t...

Magnetic ordering around the ferroelectric transition temperature T C1 of weakly ferroelectric YMn 2 O 5 has been reinvestigated by single-crystal neutron diffraction and dielectric measurements. Besides a two-dimensionally modulated incommensurate magnetic (2D-ICM) phase with the propagation vector q = ( q x ,0, q z ) and a lock-in commensurate magnetic (CM) one with q = (1/2,0,1/4) reported previously, we observed a 1D-ICM phase characterized by q = ( q x ,0,1/4) with q x ∼0.492 that appears in a narrow temperature interval of ∼1 K and separates the 2D-ICM and CM phases. Corresponding to the 2D-ICM-to-1D-ICM and 1D-ICM-to-CM phase transitions at T = 40.8 K and 40.0 K, a sharp peak and a shoulder in the dielectric constant along the b -axis were detected. These results suggest that the ferroelectricity appears simultaneously with the magnetic phase transition from the 2D-ICM to 1D-ICM phase in YMn 2 O 5 .

We report results of dielectric and neutron diffraction measurements in zero field on magnetic ordering of weakly ferroelectric ErMn 2 O 5 . Around the ferroelectric transition temperature of T C1 ∼38 K below T N1 = 44 K, we observed successive magnetic phase transitions from a two-dimensionally-modulated (2D) incommensurate magnetic (ICM) phase with the propagation vector q = ( q x ,0, q z ) to another 1D-ICM one with \(\mathbf{q} = (q_x,0,\frac{1}{4})\) at T = 39.1 K, and then to a commensurate magnetic (CM) phase with \(\mathbf{q} = (\frac{1}{2},0,\frac{1}{4})\) at T = 37.7 K. These two magnetic phase transitions (39.1 K and 37.7 K) were found to be associated with an anomaly in the dielectric constant along the b axis. In addition, we found that in the case of a rapid-cooling procedure a low-temperature ICM state characterized by \(\mathbf{q} = (\frac{1}{2},0,0.265)\) shows up and coexists with the CM phase below T N2 = 11.5 K where evolution of magnetic order of the Er ions is inferred from measureme...

Compounds in the pseudo-binary BaTiO3-Ba(Fe1/2Ta1/2)O-3 system have been synthesized at 1500degreesC in air and characterized by X-ray and electron diffraction as well as impedance analysis and Mossbauer spectroscopy. The symmetry of Ba(Fe1/2Ta1/2)O-3 is found to be trigonal with the space group P3m1. Two solid solutions exist in the BaTi1-xFex/1Tax/1O3 system at room temperature. The first (SS1) is in the composition range 0 less than or equal to x less than or equal to 0.07 and is tetragonal, P4mm, while the second broader solid solution, (SS2) 0.12 less than or equal to x less than or equal to 1, is trigonal and has the space group P3ml. The conductivity becomes higher with reduced amount of Ti in the structure. The activation energy of conductivity differs between the two solid solutions, 0.80eV for SS1 and 0.30eV for SS2, which indicates different mechanisms. The maximum of the real part of the pemittivity recorded for compounds in SS1 decreases towards room temperature with increasing amount of Fe/Ta doping and a high dielectric constant of around 7000 is found for BaTi(0.94)Fc(0.03)Ta(0.03)O(3). (C) 2004 Elsevier Inc. All rights reserved.

Low frequency dielectric dispersion was observed in a mixed valence oxide Fe 2 BO 4 . It suggests that the fluctuation of possible charge ordering of Fe 2+ and Fe 3+ is working. A few evidences of the ordering were obtained by Mossbauer spectroscopy.

We report the low temperature charge ordered structure and magnetic relaxation in nearly stoichiometric YFe 2 O 4 in which the charge frustration effect is considered to be dominant. The superlattice reflections have revealed the presence of four different phases with a triple, double, quadruple and septuplex charge-superstructure in the temperature range from room temperature to 130 K. The observed successive transitions have been proposed by a previous X-ray powder diffraction study. A slow relaxation of the superstructure was found at 130 K. Time dependence of the magnetic moment was observed at about 240 K to suggest a similar relaxation.

Rare earth (R) manganese oxides RMn 2 O 5 are the ferroelectric crystals with Curie temperature between 25 and 39 K. They are at the same time antiferromagnetic below the Neel temperature slightly above their ferroelectric transition temperature. By applying Mossbauer spectroscopy to the samples with a small amount of 57 Fe, we concluded that Mn 3+ ions in ferroelectric YMn 2 O 5 and HoMn 2 O 5 are classified into two groups with different magnetic environments. We suggest a possibility that the long-range magnetic structure induces the spontaneous polarization.

In order to develop useful lead-free ferroelectric materials based on BaTiO 3, Ba 1 m x La x Ti 1 m x Cr x O 3 ceramics were synthesized and their dielectric properties were studied. The crystal structure of Ba 1 m x La x Ti 1 m x Cr x O 3 changes in the sequence of tetragonal, cubic, rhombohedral and orthorhombic as x increases. Curie temperature T C decreases largely as x increases. The samples except for x = 0.003 show broad peak of high dielectric constant with small dielectric loss. The frequency dispersion like in relaxors is observed below room temperature with x = 0.08.

From magnetic, dielectric and magnetoelectric measurement, it is concluded that hexagonal HoMnO 3 , ErMnO 3 and YbMnO 3 undergo a magnetic phase transition at a low temperature where a magnetic lon...

The Sr1−xLaxTi1−xCrxO3 (0≤x≤1) system was synthesized at 1500°C for about 60 h. Their structures were analyzed by the use of powder X-ray diffraction data and Rietveld analysis software GSAS (General Structure Analysis System). Three solid solutions are found: cubic solid solution Sr1−xLaxTi1−xCrxO3 (0≤x≤0.168, space group Pm 3 m), rhombohedral solid solution Sr1−xLaxTi1−xCrxO3 (0.23≤x≤0.67, space group R 3 c) and orthorhombic solid solution Sr1−xLaxTi1−xCrxO3 (0.92≤ x≤1, space group Pnma). The conductivities of this series were measured mainly from 25 to 450°C. It was found that this series changed from dielectric material SrTiO3 to semiconductor LaCrO3 continuously. The highest conductive sample was found to be Sr1−xLaxTi1−xCrxO3 with x=0.92, whose conductivity at room temperature was about 5×10−3Ω−1cm−1.

Dielectric constant and magnetic susceptibility have been measured on a single crystal of ZnCr 2 O 4 , which is a typical example of magnetically frustrated systems. Two characteristic features are observed in the temperature dependence of dielectric constant. The first is a jump of dielectric constant at the Neel temperature, where an antiferromagnetic transition presumably of the first order occurs. The other feature is dielectric dispersion appearing below 70 K, where the presence of antiferromagnetic short-range order have been demonstrated by a neutron diffraction and a temperature dependence of magnetic susceptibility. The jump indicates the presence of the lattice distortion to relax the spin frustration which otherwise prevents magnetic long-range ordering. The origin of the dispersion is presumably related to the fluctuation of interfacial charge accompanied by magnetic fluctuation.

The series Ba 1− x La x Ti 1− x Cr x O 3 (0≤ x ≤1) was synthesized at 1400°C for about 60 h. Their structure was carefully analyzed by the use of powder X-ray diffraction and Rietveld analysis software GSAS (General Structure Analysis System). Four solid solutions are found in this series: tetragonal solid solution Ba 1− x La x Ti 1− x Cr x O 3 (0≤ x ≤0.029), cubic solid solution Ba 1− x La x Ti 1− x Cr x O 3 (0.0365≤ x ≤0.600), rhombohedral solid solution Ba 1− x La x Ti 1− x Cr x O 3 (0.700≤ x ≤0.873), and orthorhombic solid solution Ba 1− x La x Ti 1− x Cr x O 3 (0.956≤ x ≤1). There are corresponding two-phase regions between the adjacent two solid solutions. The detailed lattice parameters are presented. The relationship between the lattice parameters and the composition of the solid solutions is developed.

YFe 2 O 4 m i ( i < 0.005), which is a member of charge-frustrated system RFe 2 O 4 was synthesized and investigated by the low-temperature synchrotron powder X-ray diffraction. The structure between 225K and 250K is the same with the three-dimensional charge ordered state of LuFe 2 O 4 . Below the charge ordering temperature, at least five different phase transitions were detected. Since the balance of competing interaction between ionic charge essentially dominates the presence of multi phases in frustrated system, the low oxygen deficiency in YFe 2 O 4 m i is considered as the origin of the multi phases in low temperature.

We studied systematically the crystal structure of simultaneously ferroelectric and antiferromagnetic RMn 2 O 5 (R = Pr-Lu and Y, except Tm) oxides, based on powder X-ray diffraction and Rietveld analysis. We propose a simple picture of the change of the structure with the size of a rare earth ion.

Heat capacities of ferroelectromagnetic crystals YMnO3, ErMnO3, and LuMnO3 were measured in a temperature range of 2–300 K and in some magnetic fields between 0 and 9 T. Heat capacity anomalies were observed at around 71, 79, and 84 K in YMnO3, ErMnO3, and LuMnO3 crystals, respectively, due to an antiferromagnetic phase transition. Schottky anomalies were observed in ErMnO3 crystal due to population changes among the energy levels of erbium ions. In the low temperature region and in zero magnetic field, a heat capacity term proportional to temperature was observed in YMnO3 and LuMnO3 crystals possibly due to an unusual magnetic excitation.

We grew epitaxially single phase ferromagnetic EuO film on SrTiO 3 substrate by inserting BaO buffer layer with an orientation of EuO[100]//BaO[100]//SrTiO 3 [110] and EuO(001)//BaO(001)//SrTiO 3 (001). Compared with EuO film on MgO substrate, high quality of the film was obtained, demonstrated by appearance of Kikuchi line in RHEED pattern with less thickness, Laue oscillations around EuO(002) Bragg peak, lower full width of half maximum of rocking curve for EuO(002), and lower coercive field. The growth rate of EuO was between 8.0×10 -2 and 2.6×10 -1 A· s -1 , depending on the growth condition. Saturation magnetic moment 6.84(±0.31)µ B was obtained per europium atom. The oxygen deficiency of EuO film was estimated as less than 0.03% from the temperature dependence of resistivity. The highest resistivity detected was 2.55×10 11 Ω· cm at 83.8 K.

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.

Ferromagnetic EuO films were epitaxially grown on MgO and SrTiO 3 substrates aligned with EuO[100]//MgO[100], EuO(001)//MgO(001) and EuO[100]//SrTiO 3 [110], EuO(001)//SrTiO 3 (001). The single pha...

The formation of three-dimensional magnetic ordering has been studied on a quasi-one-dimensional magnet ${\mathrm{CoNb}}_{2}{\mathrm{O}}_{6}$ by mean-field calculations as well as neutron scattering measurements down to $T=1.5\mathrm{K}$ under magnetic fields up to ${H}_{\ensuremath{\parallel}c}\ensuremath{\sim}600\mathrm{Oe}.$ Measurements of a deviation of the magnetic Bragg scattering function from the delta function in the ordered state reveal a surprisingly rich variety of the magnetic formation arising from an isosceles triangular arrangement of the magnetic chain with competing interchain interactions in the $a\ensuremath{-}b$ plane. The competing interactions result in quasidegenerate ground states with different propagation wave numbers along the ${b}^{*}$ direction in the sinusoidally amplitude-modulated incommensurate magnetic (IC) phase. Our mean-field calculations qualitatively reproduce the complicated ${H}_{\ensuremath{\parallel}c}\ensuremath{-}T$ magnetic phase diagram and give evidence for a high degeneracy of ground states by calculating the ${H}_{\ensuremath{\parallel}c}\ensuremath{-}T$ dependence of the free energy curve in the propagation wave number space. In addition, a partial cancellation of the exchange field at the apex site from the base sites on the isosceles triangular lattice leads to a quasi-long-range ordering along the a axis in both IC and antiferromagnetic states where the correlation length along the a axis depends on the propagation wave number along the ${b}^{*}$ direction.

Magnetic excitations in the Ising ferromagnetic chain CoNb 2 O 6 have been studied by submillimeter wave electron spin resonance in strong transverse magnetic fields. A distinct non-linear field dependence of the resonance branches and also zero field splittings have been observed. A large residual intensity of the spin-cluster resonance was found which was caused by the strong mixing among the different n -fold spin-clusters. In addition to a series of n -fold spin-cluster excitations, we also discovered a new set of spin-cluster excitation which can be attributed to non-integer n such as n ∼1.5 or n ∼2.5 for the first time. This effect is explained by the change of the quantization axis of the system in strong transverse magnetic fields. These new aspects of spin-cluster excitation show the significant effect of transverse magnetic fields on the magnetic excitation in an Ising ferromagnetic chain.

Transmission measurements of far-infrared (FIR) polarized radiation have been performed in CoNb 2 O 6 single crystal. We have observed an absorption line at 9.7 cm -1 with a broad band from 9 to 25 cm -1 which should be attributed to the phonon of the lowest frequency below 30 cm -1 from the lattice dynamical analysis. As the temperature is decreased below 20 K and also as the magnetic field is applied up to 6 T, the absorption line with the band was intensively studied in magnetic fields as well as the various temperatures by using polarized FIR radiation field E parallel to the b - and c -axis. From the study, the band is attributed to the spin-cluster excitation (SCE) in the Ising-like chain. On the other hand, the absorption line also exhibits the variation as similar as SCE in magnetic fields. It concludes that the absorption line at 9.7 cm -1 is the hybridized mode of the B 3u mode of the phonon of the lowest frequency and SCE.

We have investigated magnetization, neutron scattering, resistivity, thermoelectric power and Hall effect in LaCo 1- x Ni x O 3 ( x =0.05 and 0.1) single crystals. At low temperatures, they show spin-glass behavior with a strong ferromagnetic correlation. Around 500 K, spin state transition from the intermediate spin state ( S =1) to the high spin state ( S =2) accompanied by an insulator-to-metal transition is observed as in undoped LaCoO 3 . These magnetic properties of LaCo 1- x Ni x O 3 are very similar to those previously reported for La 1- x Sr x CoO 3 . The low-temperature transport properties of both LaCo 1- x Ni x O 3 and La 1- x Sr x CoO 3 exhibit the characteristics of an insulator near the insulator-to-metal transition. The sign of the carrier is verified to be positive in both systems from the thermoelectric power measurement. However, the sign of the Hall coefficient of LaCo 1- x Ni x O 3 is negative. These transport properties are discussed in terms of small polaron hopping mechanism.

Oxides of the group RMn2O5 (R=a rare earth or Y) are simultaneously ferroelectric and antiferromagnetic at low temperatures. The specific features of the phase transitions in this oxide are reviewed with a particular emphasis on the possible interactions between dielectric and magnetic properties.

We made the measurements of dielectric constant, pyroelectricity, magnetoelectric effect and magnetization of SmMn2O5 single crystals. From the results, we conclude that SmMn2O5 is ferroelectric below 35.5 K, and that it undergoes another phase transition at 28.8 K. The change in both electric and magnetic structures are supposed to occur at the lower transition temperature.

LuMn2O5 with non-magnetic rare earth Lu is a ferroelectric crystal with the Curie temperature at 34 K, where a weak spontaneous polarization appears along the b-direction. It has characteristic features of improper ferroelectrics. We suppose that a possible order parameter is magnetic order.

We made dielectric, magnetic and magnetoelectric studies of a hexagonal ErMnO3. Our purpose is to make clear the magnetic-electric interaction phenomena expected in this oxide with both electric and magnetic long range ordering. The magnetic-electric interaction was demonstrated as the anomalies of dielectric constant along the polar axis at magnetic transitions. An additional spontaneous polarization and a large magnetoelectric effect were detected at low temperatures where a long range ordering of Er3+ moments was present.

Based on the measurements of pyroelectricity, dielectric constant, magnetization and magnetoelectric effect, we conclude that TmMn2O5 is ferroelectric below the Curie temperature of 36 K. There are two other transitions at 26 K and 6 K. The phase below 6 K undergoes a transition into another phase at about 5 kOe, possibly the reorientation of the Tm3+ moments. The dielectric properties change remarkably at this transition, indicating a strong interaction between electric and magnetic orders. Magnetic phase diagram at low temperatures is determined from the results of magnetic and dielectric measurement.

We report the magnetic field dependence of dielectric constant and magnetoelectric effect of TmMn2O5 and YbMn2O5 single crystals. An anomalous change of magnetoelectric effect occurs at 5 kOe (TmMn2O5) or at 18 kOe (YbMn2O5) at 4.2 K, when an magnetic field is applied along the c-direction. The anomaly in YbMn2O5 is attributed to a transition between different magnetic structures. In TmMn2O5 an anomaly was also observed in dielectric constant. We propose a structural transition caused by a magnetic field.

From the measurements of dielectric constant, pyroelectricity, magnetoelectric effect and magnetization, we conclude that ErMn2O5 is ferroelectric below 37 K. In addition, magnetoelectric susceptibility increases remarkably with decreasing temperature below approximately 10 K, accompanied with the growth of magnetic long-range ordering of Er3+. We suppose that this magnetic ordering brings a change in the crystal structure, resulting in the change in spontaneous polarization. A magnetic-field-induced transition occurs between 10 and 30 kOe in this low-temperature phase.

We report the results of dielectric, pyroelectric, magnetoelectric and specific heat measurements of HoMn2O5 single crystals, concluding that this oxide is ferroelectric below 38 K. The direction of the spontaneous polarization is along the b-direction of the orthorhombic structure. The presence of the ferroelectric transition is demonstrated by a peak in the temperature dependence of dielectric constant and also a shoulder in specific heat. The transition is accompanied by an appearance of spontaneous polarization and the quadratic (in magnetic field) magnetoelectric effect, which can be switched by electric field applied during cooling the crystal through the transition temperature. The crystal undergoes another phase transition around 22 K. HoMn2O5, like other RMn2O5 oxides, has several features typical to improper ferroelectrics, such as small spontaneous polarization, a low peak height of dielectric constant at the transition temperature, etc. We suppose that the ferroelectricity of RMn2O5 i...

The pressure dependence of the 100 K spin-state transition in LaCoO 3 is investigated through magnetization measurement under pressures of up to 18.2 kbar. The energy gap, Δ, between the low-spin ground state and the excited magnetic state (intermediate spin) increases remarkably with increasing pressure. The linear part of the pressure dependence of Δ, which is dominant below 5 kbar, is interpreted consistently by the volume expansion due to the spin-state transition under ambient pressure. In the higher pressure region, a quadratic pressure dependence is dominant, which suggests a very steep volume dependence of the energy of the excited magnetic state under the highly compressed condition.

We report low frequency dielectric dispersions of ErFe2O4 in which equal numbers of Fe2+ and Fe3+ ions occupy the sites on a triangular lattice. The first dispersion observed between 200 K and 300 K is explained qualitatively on a model that spontaneous polarization is present in the crystal below the Verwey transition. The second dispersion at higher temperatures is characterized by a few glass-like features. It was interpreted by successive freezing of the local dipole moments accompanied by short-range ordering of Fe2+ and Fe3+ ions.

We made measurements of dielectric constant, pyroelectric current, magnetic susceptibility and magnetoelectric effect of single crystals of YbMn 2 O 5 . From the results, we concluded that there were two phase transitions besides antiferromagnetic transition at approximately 40 K. The first transition at 32 K is characterized by a peak of dielectric constant, a kink of magnetoelectric polarization and an appearance of pyroelectricity below this temperature. It was considered to be electric in nature. The second one is at 6 K. It is also accompanied by another peak of dielectric constant, a steep rise of pyroelectricity and anomalies of both magnetic and magnetoelectric susceptibilities. We suppose that the ordering of Yb 3+ magnetic moments begins to develop at this temperature, involving a change in the crystal structure and then in spontaneous polarization. The low temperature phase has a magnetic field-induced transition which occurs at about 19 kOe at 4.2 K.

In a series of papers, we have reported the results of dielectric, magnetic and magnetoelectric studies of (R = rare earth between Nd and Lu, Y or Bi), proposing the possibility of ferroelectricity in these oxides, but the direct demonstration of ferroelectricity was lacking. In this paper we present the results of pyroelectric current measurements of and single crystals between 4.2 and 273 K. These results indicated the existence of spontaneous polarization at low temperatures. The spontaneous polarization was switched by an applied electric field. Its possible direction is along the b axis of an orthorhombic crystal. The supposed temperature dependence of spontaneous polarization was complicated except for the case of , suggesting a ferrielectric origin of the polarization.

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.

Magnetization, M , of EuS and Eu 1- x Sr x S ( x ≤0.4) was measured below 90 K in an external magnetic field up to 150 kOe. It is shown that M in the paramagnetic region is well expressed by a Brillouin function for S =7/2, with an effective field H +λ M . Even at temperatures below asymptotic Curie temperature, magnetization approaches the Brillouin function with increasing field strength. Moreover, it is found that magnetization scaled by ( T - T c ) b can be expressed as a universal function of magnetic field scaled by ( T - T c ) b + c . Here, T c is the ferromagnetic Curie point. Scaling exponents, b and c , are very different from those in the critical region. The exponent c is about 1.15 and independent of x whereas b is 0.14 for EuS and decreases rapidly with increasing x, proportionally to T c .

We report neutron scattering as well as ac susceptibility studies on the formation of magnetic ordering in a quasi-1D ferromagnetic chain system CoNb 2 O 6 in magnetic fields up to 600 Oe. At T =1.5 K, a noncollinear ferrimagnetic (FR) phase with up-up-down spin arrangement along the b axis is field-induced in the magnetic field above ∼300 Oe. Interestingly, the pronounced 2D magnetic character previously found in the noncollinear antiferromagnetic phase disappears in the FR phase. This is direct evidence that the 2D magnetic character is due to the cancellation of interchain exchange fields at an apex site of a 2D isosceles-triangular lattice where quasi-1D ferromagnetic chains lie.

A possibility of ferroelectricity has been proposed in some low temperature phases of the oxides of the series RMn2O5 (R = a rare earth from Nd to Lu, Y or Bi). YMn2O5 is unusual in this series, because Y3+ is non-magnetic. We made dielectric, magnetic and magnetoelectric measurements on single crystals of this oxide to detect two electric transitions at 39 K and 19 K. These transitions are accompanied with anomalies in the dielectric constant and the magnetic and the magnetoelectric susceptibilities. Particularly, the sign of the magnetoelectric susceptibility is switched below 19 K, depending on the sense of the electric field applied during cooling the sample down to 4.2 K. This fact indicates that YMn2O5 possesses spontaneous polarization at least below 19 K.

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.

We made dielectric, magnetic and magnetoelectric measurements on single-crystal samples of TbMn2O5, which was an antiferromagnet with the Neel temperature of approximately 40 K. From the results, we concluded that there were two groups of phase transitions at lower temperatures. The first group are three successive transitions at 29.3, 27.2 and 25.6 K, whose natures are probably electric. The crystal is polar below this temperature range, because the polarity of the magnetoelectric effect can be controlled by electric field cooling. Another transition exists at 8 K, where time inversion symmetry breaks. It is related to the ordering of Tb3+ magnetic moments.