Kay Kohn

Magnetic ordering of rare earth ions and magnetic-electric interaction of hexagonal RMnO3 (R=Ho, Er, Yb or Lu)

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...

Structure Relations of Hexagonal Perovskite-Like Compounds ABX3 at High Pressure

Phase stability relations among four hexagonal perovskite-like structures as well as the cubic perovskite structure have been studied for several oxides (BaMnO 3 and SrMnO 3 ) and fluorides (CsMnF 3 , RbNiF 3 and TlNiF 3 ) at high pressure. A series of high pressure transformations are found to occur in the order of the packing sequence along the hexagonal c axis (or cubic 111 axis) of ( a b ), ( a b a b c b c a c ), ( a b a c ), ( a b c a c b ) and ( a b c ) with increasing pressure. This order is corresponding with the increasing order of the proportion of the cubic close-packed layers in the hexagonal close-packed structure. It is suggested that the tolerance factor of the perovskite structure and the Coulomb repulsive force play an important role in determining the crystal structure and its order in the series of phase transformations at high pressure.

Crystal chemistry of MSeO3 and MTeO3 (M = Mg, Mn, Co, Ni, Cu, and Zn)

Abstract New selenites and tellurites MgSeO3, MnSeO3, CoSeO3, NiSeO3, CuSeO3, MnTeO3, CoTeO3, and NiTeO3 were synthesized under high pressures and temperatures. All the compounds are isomorphous and their crystal system is orthorhombic. Structure analyses were carried out for all the selenites and CoTeO3. The structure is described as a salt of M2+ and SeO32− or TeO32− ions or as a distorted perovskite. In these compounds an Se or Te atom is closely linked to three oxygen atoms to form a flattened trigonal pyramid. The features of this coordination are discussed. At low temperature, magnetic order appears in all the compounds containing iron group ions, among which CuSeO3 is a ferromagnet with the Curie temperature of 26°K.

Pyroelectric effect and possible ferroelectric transition of helimagnetic and

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.

Pressure dependence of the 100 K spin-state transition in LaCoO3

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.

Ferromagnetic dense Kondo behavior of Ce-Si system

Abstract In order to clarify the nonmagnetic-magnetic transition found in Ce-Si system, magnetization measurements have been made to reveal a large reduction of the magnetic moment of Ce atom. Taking into account the results and also the reduction of the magnetic entropy, we propose a dense Kondo model to interpret our Ce-Si system. Kondo temperatures are deduced from various physical quantities and a comparison is made between them. Our Ce-Si system is the first example of a ferromagnetic dense Kondo system.

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.

Structure and Ferroelectricity of RMn 2 O 5

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.

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 .