Frequency-independent Peak Research Articles

Interchain interactions induced multiferroicity in SrFeF5

SrFeF5 is a typical spin-chain compound with strong intrachain and moderate interchain interactions. It has been predicted to be multiferroic due to the cycloidal spin structure. Herein, we provide clear experimental evidence of multiferroicity of SrFeF5 by a systematic investigation of its magnetic, dielectric, and pyroelectric properties. The complicated interactions between neighboring Fe ions lead to the following three magnetic transitions. The divergence between field cooling (FC) and zero field cooling (ZFC) M(T) curves at TN1∼ 80 K originates from the antiparallel alignment of neighboring spins along the chains due to the strong intrachain interactions. The sudden increase in magnetization with further divergence between the FC and ZFC M(T) curves is observed at TN2 ∼ 42 K, which originates from the formation of cycloidal spin structure due to the contribution from moderate interchain interactions. An upturn of magnetization with further decreasing temperature at TN3 ∼ 6 K is due to the contribution from the much weaker interchain interactions. Exchange bias starts to emerge at around 30 K, reaches a maximum at 20 K, and further increases below 10 K. A frequency independent peak of dielectric constant is observed at around TN2, due to the cycloidal spin structure induced spontaneous polarization. This work helps to understand the multiferroic properties in the spin-chain fluorides.

Impedance spectroscopy and conduction mechanism of ferroelectric rich [formula omitted] magnetoelectric composite

In the present work, structural and dielectric properties of 0.7PbZr0.52Ti0.48O3−0.3CoFe2O4 composite prepared by a mixed oxides route were explored. X-ray diffraction analyses confirmed the formation of distinct CoFe2O4 and PbZr0.52Ti0.48O3 phases in the prepared composite. Temperature dependent dielectric permittivity behavior displayed three distinct peaks: a frequency independent peak at high temperature (T = 663K) is identified as structural phase transition of PbZr0.52Ti0.48O3 and two thermally activated frequency dependent peaks which showed a Debye- and relaxor-like behaviors at low and intermediate temperatures, respectively. The mechanism for these relaxations is elucidated on the basis of conduction associated with the single- and doubly-ionized oxygen vacancies. Impedance spectroscopy analysis was used to estimate the activation energy (Ea) of constituent phases and we found that Pb(Zr0.52Ti0.48)O3 phase has a dominant role (i.e., EPZT>ECFO) in describing the relaxation/conduction mechanism of prepared composite. Moreover, the values of Ea calculated from the dielectric, impedance and conductivity data were found to be comparable which clearly indicate that the relaxation and conduction processes in the prepared composite are related to same origin.

Coexistence of ferromagnetism and spin glass state in YbNi2 nanoparticles

Abstract We report changes on the magnetic behavior of the Heavy-Fermion ferromagnetic YbNi 2 alloy when reducing the size of the particles to the nanometer scale by high-energy ball milling. The milling process induces a reduction on the particle size of the bulk alloy down to 10 (2) nm and a lattice strain of 1.8 (2)%, for 30 h of milling time, as calculated from the X-ray diffraction data. The ferromagnetic transition at T C = 10.5 K in the bulk alloy tends to disappear whereas other magnetic transitions arise at lower temperatures with the milling process. The AC magnetic susceptibility of the 30 h milled sample shows a frequency independent peak at 3 K, in a wide range of frequencies, consistent neither with superparamagnetic nor simple freezing of the magnetic moments, and as expected for a ferromagnetic behavior. However, from the specific heat measurements in the 30 h milled sample, a peak at 3.6 K is shifted respect to that observed in the AC magnetic susceptibility, as found in spin glasses. Additionally, the specific heat on nanometric samples show reduced magnetic contributions respect to the bulk alloy with Δ c max around 0.45 J/molK. The results are explained by the influence of magnetic inhomogeneities and disorder of the alloys enhanced by the milling process in the ensemble of nanoparticles.

Functional properties of percolative CoFe2O4-PbTiO3 composite ceramics

The structural, microstructural, electrical and magnetic characteristics of CoFe2O4-PbTiO3 composite ceramics with various amounts of magnetic phase are reported. The magnetization as a function of magnetic field and temperature showed a “dilution effect” when decreasing the ferrite amount, while the magnetic Curie temperature remains in the range of about 750 K for all the compositions, with small shifts due to the possible doping. The maximum value of magnetostriction response of 122 ppm, was obtained for the composition with 85% amount of ferrite at a field of 8 kG. The variation of dielectric properties with frequency and temperature in the range of (300–800)K is characterised by a frequency independent peak derived from the ferroelectric-to-paraelectric phase transition of PbTiO3 component and two frequency dependent thermally activated relaxations: one in the range of (300÷420)K with an activation energy of 0.5–0.6eV and one in the paraelectric state at (420÷650)K, with an activation energy of ∼1.2÷1.4eV which were described in terms of charge defects in the composite system.

Defects-assisted ferromagnetism due to bound magnetic polarons in Ce into Fe, Co:ZnO nanoparticles and first-principle calculations.

Zn0.94TM0.03Ce0.03O [Zn0.94Fe0.03Ce0.03O (ZFCeO) and Zn0.94Co0.03Ce0.03O (ZCCeO)] nanoparticles were synthesized by a sol-gel process. Elemental analysis of these nanoparticles detects the weight percentage of Zn, Co, Fe, Ce and O in each sample. The Rietveld refinement of the X-ray diffraction pattern obtains the occupancy of dopant atoms, Wurtzite ZnO structure, crystallinity and lattice deformation with doping. The Ce doping into ZFO and ZCO form nanoparticles than nanorods was observed in pure ZnO, ZFO and ZCO samples that described due to chemical and ionic behavior of Ce, Fe, Co and Zn ions. The Raman active modes have peak broadening, intensity changes and peak shifts with metal doping that induces lattice defects. Photoluminescence spectra show blue-shifts at near-band edges and defects that influence broad visible emission with Ce doping. An enhancement in ferromagnetism in the magnetic hysteresis at 5 K is measured. The zero-field cooling and field cooling at H = 500 Oe and T = 300-5 K could confirm antiferromagnetic interactions mediated by defect carriers. The bound magnetic polaron at defect sites is responsible for the observed ferromagnetism. The ac magnetic susceptibility measurements determine the antiferromagnetic to ferromagnetic transition with some magnetic clustered growth in the samples and reveal a frequency independent peak that shows the Neel temperature. Weak room temperature ferromagnetism and optical quenching in ZFCeO are described by valance states of Fe and Ce ions, respectively. Using first-principle calculations, we studied the occupancy of Ce (replacing Zn atoms) in the Wurtzite structure.

Lattice dynamics and broad-band dielectric properties of multiferroic Pb(Fe1/2Nb1/2)O3 ceramics

Complex dielectric properties of Pb(Fe1/2Nb1/2)O3 ceramics were investigated in a broad frequency range from 100 Hz up to 90 THz. A broad dielectric anomaly was observed near the temperature of the ferroelectric phase transition (TC1 = 376 K). Below 1 MHz, the anomaly is strongly influenced by conductivity of the sample, but higher frequency data taken up to 81 MHz reveal a broad and frequency independent peak at TC1 typical for a diffuse ferroelectric phase transition. Surprisingly, dielectric permittivity measured at 37 GHz exhibits a peak shifted by 25 K above TC1, which indicates polar nanoregions with dynamics in microwave frequency region. A dielectric relaxation, which appears in THz region below 700 K, slows down towards TC1 and again hardens below TC2 = 356 K. This central mode drives both phase transitions, so they belong to order–disorder type, although the polar phonons exhibit anomalies near both phase transitions. In the paraelectric phase, infrared reflectivity spectra correspond to local Fm3¯m structure due to short-range chemical ordering of Fe and Nb cations on the B perovskite sites. Moreover, each polar phonon is split due to two different cations on the B sites. Recently, Manley et al. [Nat. Commun. 5, 3683 (2014)] proposed a new mechanism of creation of polar nanoregions in relaxor ferroelectrics. They argued, based on their inelastic neutron scattering studies of PMN–PT, that the TO1 phonon is split and interaction of both components gives rise to so called Anderson phonon localization, which can produce regions of trapped standing waves and these waves induce polar nanoregions in relaxors. We cannot exclude or confirm this mechanism, but we show that the splitting of polar phonons is a common feature for all complex perovskites with relaxor ferroelectric behavior and it can be also observed in canonical ferroelectric BaTiO3, where the soft mode is split in paraelectric phase due to a strong lattice anharmonicity.

Dynamic Mechanical Analysis of Phase Transformations and Anelastic Relaxation in Stabilized Zirconias

Elastic properties of polycrystalline stabilized zirconias, namely Yttria Stabilized Zirconia (YSZ) and Scandia, Ceria Stabilized Zirconia (SCSZ), were studied by Dynamic Mechanical Analyzer (DMA) in air. Changes of storage modulus E′, loss modulus E″ and mechanical damping (tanδ) with temperature were simultaneously monitored at various frequencies. In 8 mol% (8YSZ) and 10 mol% (10YSZ) Yttria Stabilized Zirconia, a drop in storage modulus of ∼37% was observed in the 30°C to 400°C temperature range, together with two frequency dependent damping peaks. While the first tanδ peak can be attributed to the anelastic relaxation of (Y′ZrVO••)• elastic (and dielectric) dipoles, the second one is probably the result of local ordering of oxygen vacancies and/or relaxation of (2Y′ZrVO••)x dipoles. For cubic SCSZ, four damping peaks were observed in the similar temperature regime accompanied by significant nonlinear drop of the storage modulus. The first tanδ is attributed to anealstic relaxation of (Sc′ZrVO••)•. It was also found that stress induced phase transformation from cubic to rhombohedral structure is the reason for the frequency dependent third peak in the tanδ spectra while the fourth frequency independent peak was found to be the result of the phase transformation from rhombohedral to cubic structure.

Ferroelectric quantum criticality

Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO3 and KTaO3. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO3 and 3K in KTaO3 believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that in ferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.

Dielectric properties of the spin-1/2 dimer compounds Ba3Cr2O8 and Sr3Cr2O8

We have investigated the dielectric and magnetic properties of the spin-1/2 compounds Ba3Cr2O8 and Sr3Cr2O8. For Ba3Cr2O8, the real part dielectric constant ε′(T) exhibit a frequency-independent peak near the Jahn-Teller transition temperature TJT = 70 K. However, no anomaly in the ε′(T) curve is observed near TJT = 275 K for the isostructural compound Sr3Cr2O8. The difference in the ε′(T) behaviors may be attributed to the different onset temperatures of the phonon mode splitting in the samples. Dielectric relaxation analysis revealed that extrinsic contribution due to Maxwell–Wagner effect dominated at high temperatures for both compounds. The magnetization data reveal the occurrence of antiferromagnetic interactions between the Cr5+ ions and the χ(T) curves can be described by the Bleaney–Bowers equation for both compounds.

Coexistence of magnetic order and spin-glass-like phase in the pyrochlore antiferromagnet Na3Co(CO3)2Cl

We present comprehensive investigations on a pyrochlore antiferromagnet Na${}_{3}$Co(CO${}_{3}$)${}_{2}$Cl. Performed dc magnetization indicates a broad maximum at around 4 K. The field dependence of the peak temperature of this maximum follows the de Almeida--Thouless line, suggesting a spin-glass-like phase transition. The ac susceptibility measurements determine the glassy transition temperature to be 4.5 K and reveal a frequency-independent peak at 17 K. The temperature dependence of the specific heat shows a sharp peak at 1.5 K and a broad hump at around 5 K, which are attributed to a long-range magnetic phase transition and a spin-glass-like freezing process, respectively. The average crystallographic structure of Na${}_{3}$Co(CO${}_{3}$)${}_{2}$Cl has been determined using neutron-powder diffraction. No obvious site disorder has been detected within the experimental resolution. The diffuse neutron scattering with polarization analysis indicates short-range spin correlations characterized by dominating antiferromagnetic coupling between nearest neighbors and weak ferromagnetic coupling between next-nearest neighbors. The long-range magnetic order below 1.5 K is evidenced by the magnetic reflections observed at 50 mK and can be well explained with an all-in--all-out spin configuration. Inelastic neutron scattering of Na${}_{3}$Co(CO${}_{3}$)${}_{2}$Cl exhibits collective magnetic excitations at 3.5 K, suggesting that the spin-glass-like transition temperature ${T}_{\mathrm{g}}$ $=$ 4.5 K does not correspond to a complete spin-glass freezing as expected in canonical spin glasses. The peak observed in magnetic susceptibility at 17 K is attributed to the onset of an intermediate partially ordered phase transition, qualitatively consistent with the theoretical predictions for pyrochlore antiferromagnets with weak ferromagnetic next-nearest-neighbor interactions.

Co4(OH)2(C10H16O4)3 Metal–Organic Framework: Slow Magnetic Relaxation in the Ordered Phase of Magnetic Chains

Reported here are the synthesis and structural and topological analysis as well as a magnetic investigation of the new Co(4)(OH)(2)(C(10)H(16)O(4))(3) metal-organic framework. The structural analysis reveals a one-dimensional inorganic subnetwork based on complex chains of cobalt(II) ions in two different oxygen environments. Long alkane dioic acid molecules bridge these inorganic chains together to afford large distances and poor magnetic media between dense spin chains. The thermal dependence of the χT product provides evidence for uncompensated antiferromagnetic interactions within the cobaltous chains. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.4 K while both neutron diffraction and heat capacity measurements give evidence of long-range order (LRO) below this temperature. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ(τ)/k(B) = 67(1) K for the [10-5000 Hz] frequency range. Moreover, in nonzero dc fields the ac susceptibility signal splits into a low-temperature frequency-dependent peak and a high-temperature frequency-independent peak which strongly shifts to higher temperature upon increasing the bias dc field. Heat capacity measurements have been carried out for various applied field values, and the recorded C(P)(T) data are used for the calculation of the thermal variations of both the adiabatic temperature change ΔT(ad) and magnetic entropy change ΔS(m). The deduced data show a modest magnetocaloric effect at low temperature. Its maximum moves up to higher temperature upon increasing the field variation, in relation with the field-sensibility of the intrachain magnetic correlation length.

Interpolation Method and Frequency Independent Peak Amplitude Measurement of Orthogonal – Periodic Signals

A flash interpolator circuit inherent amplitude measurement algorithm, AMM, is described. A flash interpolation circuit converts a pair of periodic and orthogonal sine-signals into a stream of periodic – phase shifted sinusoidal signals, the amplitudes of which can be combined to produce useful information about the peak amplitude of the input sinesignals, independently of the signal's frequency. The proposed technique of amplitude measurement is suitable for automatic gain control, AGC, and the systems for automatic signal conditioning, where a flash interpolator is already a part of the integrated motion control system. An interpolation factor of 4 is shown to be sufficient for measuring amplitudes with an accuracy of 5.8 %. The patents regarding signals conditioning and interpolation are reviewed. The interpolator architecture, combined with an amplitude measuring system, has been designed, integrated as a part of the interpolator ASIC, evaluated and analyzed. The SoC ASIC for incremental optical encoders is designed and processed in 0.35 μm CMOS technology. Keywords: Interpolation, amplitude measurement, orthogonal signals, flash interpolator, automatic gain control, encoder, opto-sensor, signal conditioning, quadrature signals, multiplier, automatic signal, DC output signal, optical channel, Resistivity, interpolation factor

Dynamic study of the internal magnetic order of Mn3O4 nanoparticles

The dynamic magnetic properties of Mn3O4 nanoparticles with mean diameter 〈ϕ〉 = 15 nm have been investigated by frequency and dc-field (HDC) dependence of the in phase (χ′) and out of phase (χ″) ac-susceptibility. The studies were performed in non-interacting and interacting systems of Mn3O4 nanoparticles diluted in a polymer with concentrations 1.5 and 17.6%. The ac-susceptibility of the non-interacting system, measured with HDC = 0 Oe, presents only one maximum located at TC = 42 K associated to the paramagnetic (PM)–ferrimagnetic (FiM) transition. In contrast, the susceptibility of the interacting system shows two anomalies. One frequency independent peak associated to TC and a second low temperature maximum was observed in χ″, located at TP. The position of the TP maximum shifts to higher temperature when the frequency increases. The relation between the relaxation time and TP was well described by the Vogel–Fulcher law. When the susceptibility was measured with HDC = 20 kOe, the PM–FiM transition was observed in both systems. Remarkably, in the non-interacting system, the low temperature anomaly is evidenced by the magnetic field. This anomaly is present as a well-defined maximum, which shifts to higher temperature when the frequency increases.

Multiferroicity and magnetoelectric coupling in half-doped manganite La0.5Ca0.5MnO3

We present experimental evidences to verify the peculiar multiferroics predicted in the half-doped manganite La0.5Ca0.5MnO3. The dielectric constant exhibits a frequency-independent peak just at the antiferromagnetic charge ordering transition temperature TCO, indicating a close correlation between the ferroelectriclike transition and charge ordering. In addition to a large magnetodielectric effect, the dynamic magnetoelectric coupling measurements provide solid evidences to an enhanced magnetoelectric coupling below TCO. Moreover, the magnetic field dependence of the coupling coefficient shows a close correlation with the magnetization hysteresis loop, which reveals that a large magnetoelectric coupling only persists in the charge ordering state.

Development of perovskite structure and electrical properties of Pb(Zr1/2Ti1/2)O3-Pb(Ni1/3Nb2/3)O3system

Phase pure perovskite structure (1- x )Pb(Zr 1/2 Ti 1/2 )O 3 - x Pb(Ni 1/3 Nb 2/3 )O 3 (PZT-PNN, x = 0.05-0.40) ferroelectric ceramics were prepared by conventional solid-state reaction method via the columbite precursor technique. The PZT-PNN ceramics sintered at 1225 °C for 2 h exhibit rather homogenous microstructure and high relative density. With the increase of the Pb(Ni 1/3 Nb 2/3 )O 3 (PNN) content, the crystal structure of PZT-PNN changes gradually from tetragonal perovskite structure to rhombohedral one, where the morphotropic phase boundary (MPB) composition locates at a region of x = 0.15-0.20 determined by X-ray diffraction (XRD) and dielectric measurements. Furthermore, the dielectric response peaks of PZT-PNN also changes from narrow, sharp and almost frequency independent peaks of normal ferroelectrics to broad, diffused and apparent frequency dependent peaks of relaxor ferroelectrics, accompanied by the decrease of the temperature of dielectric maximum ( T m ) with the increase of the PNN content. PZT-PNN with the MPB composition exhibits integral excellent electrical properties, where 0.80PZT-0.20PNN exhibits the maximum value of dielectric constant e m 26750 at 280.6 °C, the remanent polarization P r is 25.17 µC/cm 2 , coercive field E c is 5.82 kV/cm, and piezoelectric constant d 33 reaches 281 pC/N.

Effect of internal friction on transformation twin dynamics in perovskiteSrxBa1−xSnO3(x=0.6,0.8)

The dynamics of transformation twins in ${\mathrm{Sr}}_{x}{\mathrm{Ba}}_{1\ensuremath{-}x}\mathrm{Sn}{\mathrm{O}}_{3}$ $(x=0.6,0.8)$ perovskite has been studied by dynamical mechanical analysis in three-point bend geometry. This material undergoes phase transitions from orthorhombic to tetragonal and cubic structures on heating. The mechanical loss signatures of the transformation twins include relaxation and frequency-independent peaks in the orthorhombic, and tetragonal phases, with no observed energy dissipation in the cubic phase. The macroscopic shape, orientation, and relative displacements of twin walls have been calculated from bending and anisotropy energies. The mechanical loss angle and distribution of relaxation time are discussed in terms of bending modes of domain walls.

Effect of transformation twins on the anelastic behavior of polycrystalline Ca 1− xSr xTiO 3 and Sr xBa 1− xSnO 3 perovskite in relation to the seismic properties of Earth's mantle perovskite

The mechanical loss of Ca 1− x Sr x TiO 3 as a function of temperature has been studied up to 1000 °C using dynamical mechanical analysis. The loss spectrum reveals a relaxation peak in the tetragonal phase and negligible mechanical loss in the cubic and orthorhombic phases. The peak is accompanied by a softening of the storage modulus, caused by the formation of mobile transformation twin domain walls. The small value of internal friction in the orthorhombic phase is associated with a negative volume strain, resulting in domain walls that are strongly pinned by lattice defects. The volume changes and mechanical loss in Ca 1− x Sr x TiO 3 are compared with those in Sr x Ba 1− x SnO 3, a system which shows a positive volume strain. In contrast to Ca 1− x Sr x TiO 3, Sr x Ba 1− x SnO 3 displays a thermally activated peak in the orthorhombic phase. A frequency-independent peak attributed to the tetragonal–orthorhombic transition is also seen. The positive volume strain in Sr x Ba 1− x SnO 3 results in domain wall structures that are less strongly pinned by lattice defects.

An investigation on electrical properties of microwave treated natural ilmenite (FeTiO3)

It has been observed that microwaves of 2.45 GHz heat dielectric materials. On subjecting natural ilmenite to microwave irradiation, the mineral is observed to heat, with a surface temperature in proportion to the irradiation time. With irradiation times from 40 to 240 seconds (increased in steps of 40 sec), the surface temperature measured on the samples were between 280 and 520 K. Electrical measurements made on the samples before and after irradiation show that the electrical properties are modified by the microwave irradiation. The real conductivity (σ′), dielectric constant (ɛ′) and the dielectric loss (ɛ″) plotted against frequency generally showed universal dielectric behaviour [1 and references therein] similar to that observed in other systems studied in the literature but using conventional heating techniques. Plots of σ′, ɛ′ and ɛ″ against the surface temperature of the sample showed frequency independent peaks around 460 K. The experimental dielectric loss (ɛ″) results fit a peak function of the form: $$\varepsilon^{\prime\prime} = \varepsilon_0 + \frac{A}{\omega_0 T {\sqrt{2\pi}}} \exp \left(-\frac{\big(\ln\big(\frac{T}{T_p}\big)\big)^2}{2\omega_0^2}\right) $$ where ɛ0, A, and ω0 are constants. The fitting of the dielectric loss results gives a frequency independent peak temperature (Tp) as 464 ± 5 K. Plotting the dielectric (γ′) and dielectric loss (γ″) exponents against temperature also gives an estimate of Tp close this value. The possibility of a ferroelectric transition in the samples is investigated by comparison with previous results obtained from synthetic ilmenite samples as it is the case with well known ferroelectric systems such as BaTiO3 [2].

Reentrance in theMn(tetracyanoethylene)x⋅y(CH2Cl2)high-Tcmolecule-based ferrimagnet

Results of static and dynamic magnetic studies of the compound ${\mathrm{Mn}(\mathrm{TCNE})}_{x}\ensuremath{\cdot}y({\mathrm{CH}}_{2}{\mathrm{Cl}}_{2})$ (TCNE =tetracyanoethylene) are reported. Curie-Weiss analysis of the high-temperature susceptibility data indicates a relatively strong exchange. The ac susceptibility, measured as a function of frequency and temperature, reveals frequency-independent peaks in both the in-phase and out-of-phase components of the susceptibility near 75 K. Additional peaks in both components, which are frequency dependent, are observed near 10 K. Weak irreversibilities, including a remanent moment and a bifurcation of the field-cooled (FC) and zero-field-cooled (ZFC) dc magnetizations are observed below 75 K. These irreversibilities become more pronounced below $\ensuremath{\sim}10\mathrm{K}.$ The presence of frequency dependence in the ac susceptibility and field dependence of the FC/ZFC magnetizations near 10 K but not at 75 K strongly suggest reentrance. Static scaling analysis at the high-temperature transition and dynamic scaling analysis at the low-temperature transition provide clear evidence for ordering with a spontaneous moment at 75 K and as a spin glass below 10 K. Possible mechanisms for this behavior are discussed.

Dielectric properties of Gd 2CuO 4

The complex dielectric response of the Gd 2CuO 4 samples measured from 20 Hz to 1 MHz and from 20 K to 300 K reveals very high real part of the dielectric constant ε′>10 4 at 300 K and >5×10 3 above 200 K. The large polarizability results from a thermally activated process with a very small activation energy of 0.065 eV and extra additional polarization. We observed one frequency-dependent and one frequency-independent peak in dissipation factor D( T, ω) curves and corresponding features in imaginary part of the dielectric constant ε″ curves. The first peak occurs at T∼230 K and is thought to be due to the occurrence of antiferromagnetic transition in the sample and the second one occurs over the range of 50 to 120 K and is related to the change of conductivity mechanism.