Isao Kagomiya

Magnetic and ferroelectric properties of multiferroic RMn2O5

The magnetic and ferroelectric properties of multiferroic RMn2O5 (R = Y, Tb, Ho, Er, Tm) are reviewed based on recent neutron diffraction and dielectric measurements. Successive phase transitions of magnetic and dielectric ordering were found to occur simultaneously in this system. The characteristic magnetic ordering of the system exhibits an incommensurate–commensurate phase transition, and again transitions to an incommensurate phase. Special attention is given to the magnetic structure in order to discuss the mechanism for the introduction of ferroelectric polarization. For all the compounds examined, the spin configuration for Mn4+ and Mn3+ ions in the commensurate magnetic phase, where spontaneous electric polarization occurs, was determined to be a transverse spiral spin structure propagating along the c-axis. By contrast, the alignment of the induced 4f moment of R3+ ions showed variation, depending on the character of each of the elements. Corresponding responses to external fields such as a magnetic field, hydrostatic pressure etc at low temperature are strongly dependent on the rare earth element present in the RMn2O5 system. The so-called colossal magnetoelectric effect in this system can be easily interpreted by the phase transition from the magnetic incommensurate and weak ferroelectric phase to the commensurate and ferroelectric phase.

Reinvestigation of Simultaneous Magnetic and Ferroelectric Phase Transitions in YMn2O5

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 .

Magnetic Structure of NiCr2O4 Studied by Neutron Scattering and Magnetization Measurements

The magnetic ordering of the normal spinel ferrimagnet NiCr 2 O 4 below T C = 74 K was reinvestigated by neutron scattering and magnetization measurements on a powder specimen. We found another magnetic transition at T S = 31 K besides T C in both experiments. The ordering of a ferrimagnetic (longitudinal) component and that of an antiferromagnetic (transverse) component occur at T C and T S , separately. A new magnetic structure model of NiCr 2 O 4 below T S with a spontaneous magnetization of about 0.3 µ B /formula is proposed based on experimental neutron scattering intensity.

2D-to-1D Modulated-to-Lock in Successive Magnetic Phase Transitions Associated with Ferroelectricity in ErMn2O5

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...

Microwave Dielectric Properties of YAG Ceramics

This study has focused on relation between the sintering condition and microwave dielectric properties for YAG ceramics. Both dielectric constant (ϵ r ) and quality factor (Q · f) of the prepared samples were increased with increasing sintering temperature from 1400 to 1650°C, suggesting that the densification is closely related to the ϵ r and the Q · f. The ϵ r was independent of the grain growth resulting from longer sintering duration time at 1650°C because of no difference in the volume fraction of pores. On the other hand, the Q · f was responsible for the grain growth, indicating that the decrease of the grain boundary resulting in higher Q · f. The Q · f value of the sample sintered at 1650°C for 24 hrs reveals 440000 GHz, which is approximately a half, compared to that of the single crystal.

Paraelectric ceramics/metal dual composites SrTiO3∕Pt system with giant relative permittivity

Dielectric properties of SrTiO3–Pt (platinum) composites prepared using conventional ceramic method were investigated. Dense complex ceramics were obtained without chemical reaction between SrTiO3 and Pt during sintering processes. The relative permittivity (er) of SrTiO3 was increased by the addition of Pt particles nearly up to the percolation threshold of 27.8vol% as predicted using normalized percolation theory. The maximum er of 2150 at 1MHz was obtained for the composite including 27vol% Pt due to the increased electric field around Pt particles. Using finite difference time domain method, the increase of electric field in the direction of applied electric field was confirmed visually in the vicinity of the embedded metal particles.

Quality Factor of Forsterite for Ultrahigh Frequency Dielectrics Depending on Synthesis Process

Forsterite is a candidate ultrahigh frequency dielectric for wireless communications. The quality factor Q of forsterite (Mg2SiO4) for microwave dielectrics is affected by calcination conditions. The shape of calcined powder particles remains the same as that of fine grains of raw silica materials. This is one of the key points for high Q, because of the realization of a high green density. From the observation of the section of calcined grains by definition (EPMA), it was clarified that Si ions remain in the core part of the grains. The remaining silica affected Q by forming a glassy phase and enstatite (MgSiO3).

Spiral Spin Structure in the Commensurate Magnetic Phase of Multiferroic RMn2O5

Crystal and magnetic structure analyses have been performed for single crystals of multiferroic materials R Mn 2 O 5 ( R = Y, Ho, Er) using the neutron diffraction technique. For all the compounds, the magnetic structure in the commensurate magnetic phase, where spontaneous electric polarization occurs, was determined to be a transverse spiral spin structure propagating along the c -axis. The results demonstrate that the spin configuration for Mn 4+ and Mn 3+ ions is essentially the same in all three materials, suggesting that the ferroelectricity of the commensurate magnetic phase originates from the spin configurations of Mn ions. By contrast, the alignment of the induced 4 f -moment of Ho 3+ ions is quite different from that of Er 3+ ions, which might give a rich variety of magnetic field response for magnetic and dielectric properties in the R Mn 2 O 5 system.

Pressure-Dependent Raman Scattering Spectrum of Piezoelectric (Li,Na,K)NbO3 Lead-Free Ceramics

High-pressure Raman scattering experiments have been carried out for (Na0.5K0.5)NbO3 and Lix(Na0.5K0.5)1-xNbO3 (abbreviated as LNKN-100x) ceramics using a diamond-anvil cell (DAC) in the backscattering geometry. Appreciable spectral changes occurred between 2.04 and 3.41 GPa in (Na0.5K0.5)NbO3 ceramics, which was a sign of the orthorhombic (O) to tetragonal (T) phase transition. The Raman modes of T-phase LNKN-6 and LNKN-7 were less affected by increasing pressure, indicating them to be particularly stable against hydrostatic pressure. In LNKN-5.5, however, a pressure-induced phase transition from a polymorphic O–T phase mixture to a single O phase, similar to the case of LNKN-3, was observed at around 2.84 GPa, since the hydrostatic compression might favor the dense O phase rather than the T phase. Accompanying this result, it was newly found that a further important role of Li substitution in LNKN ceramics is the introduction of tensile stress in the crystal structure.

Synthesis of High-Quality Forsterite

To establish a process that produces high-quality forsterite stably, calcining and sintering conditions were investigated chiefly and two kinds of silica with different forms, and grain sizes were used as starting raw materials. On the basis of the quality factor (Qf) for forsterite, the sintered samples prepared using powders calcined for 10–24 h, were found to be more stable than those in the case of 2–4 h, and in the case of 24 h of calcination, the samples showed a single phase of forsterite with fine grains. Silica with an amorphous form and a small grain size of 0.25 µm brought a higher Qf value and a wider permissible temperature range of sintering than silica with a crystalline form and a coarse grain size of 0.82 µm. Concerning the sintering temperature, the sample sintered above 1400 °C showed a high Qf value. The Qf value of the sample calcined at 1175 °C for 24 h and sintered at 1450 °C for 2 h using fine-grain amorphous silica of 0.25 µm size, was improved to 219,200 GHz.