Ac Magnetization Research Articles

Top-down and bottom-up approaches to obtain magnetic nanoparticle of Fe3O4 compound: Pulsed laser deposition and chemical route

Our goal in this work was to prepare nanoparticles (NPs) of Fe3O4 using two approaches: top-down and bottom-up. In the former two wavelengths (1064 and 532 nm) were used in the pulsed laser deposition (PLD) technique to produce NPs from α-Fe bulk, with subsequent heat treatment to produce Fe3O4, and in the latter the NPs were produced by chemical co-precipitation route (CR).Transmission Electron Microscopy (TEM) revealed that the as-deposited NPs exhibit a spherical morphology with core/shell structure and with diameters of (8 ± 4) nm (1064 nm) and (16 ± 5) nm (λ = 532 nm) for PLD approach, while for the CR, diameters of 8.0 ± 0.3 and 18.0 ± 0.3 nm were obtained for pure and SiO2-coated samples, respectively. X-Ray diffraction technique with Rietveld method was used to quantify the phases of oxidized NPs, and the percentages obtained were: 58% of Fe3O4 and 42% of α-Fe (532 nm) and 85% of Fe3O4 and 15% of α-Fe (1064 nm). For the samples obtained by CR, no spurious phase was observed in the diffractograms, only a very broad peak of SiO2 was observed around 25°, which indicates that the NPs were coated with SiO2, and this layer is amorphous. From Rietveld refinement, we found that the SiO2-coated sample has the highest Fetet-O-Feoct bond angle, which indicates a greater magnetic coupling between Fe atoms for these samples. Measurements of AC magnetization in different frequencies were carried out for all NPs and a detailed analysis of the dependence of the χ′ peak with T(K) helped to find out the relaxation time of the sample coated with SiO2 is 8.7 × 10−6 s, while for the other samples it was on the order of 10−9 s. Based on the results, it was possible to conclude that the coated sample presents a super spin-glass behavior, while in the other samples a magnetic interaction between NPs and oriented ferromagnetic clusters may be present.

Sn2Pd: a possible superconducting material with topological surface states

In this article, we report the detailed magneto transport measurements of topological semimetal candidate, Sn2Pd. Single crystal of Sn2Pd is synthesized through self-flux method. Phase purity and crystalline morphology are confirmed through powder x-ray diffraction pattern and field emission scanning electron microscopy image. Signatures of superconducting transition are seen in both transport and magneto transport measurements, which are further supported by the AC magnetization studies. Sn2Pd is found to show superconductivity onset at below 2.8 K, but not the zero resistivity down to 2 K. Although, isothermal magneto resistivity measurements below superconducting onset (2.8 K) clearly exhibited superconductor like behavior, but is not observed in heat capacity measurements, indicating the same to be of weak nature. Magneto transport measurements performed in normal state of Sn2Pd show the presence of weak antilocalization effect, which is confirmed by modelling of low field magneto-conductivity through Hikami Larkin Nagaoka formalism. Here, it is worth mentioning that the present article is the first report on magneto transport measurements of Sn2Pd, which show the same to be a topological material with weak superconducting phase below around 2.8 K.

Conventional type-II superconductivity in locally noncentrosymmetric LaRh2As2 single crystals

We report on the observation of superconductivity in $\mathrm{La}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$, which is the analog without $f$ electrons of the heavy-fermion system with two superconducting phases $\mathrm{Ce}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$ [S. Khim et al., Science 373, 1012 (2021)]. A zero-resistivity transition, a specific-heat jump, and a drop in magnetic ac susceptibility consistently point to a superconducting transition at a temperature of ${T}_{c}=0.28$ K. The magnetic-field temperature superconducting phase diagrams determined from field-dependent ac-susceptibility measurements reveal small upper critical fields ${\ensuremath{\mu}}_{0}{H}_{c2}\ensuremath{\approx}12$ mT for $H\ensuremath{\parallel}ab$ and ${\ensuremath{\mu}}_{0}{H}_{c2}\ensuremath{\approx}9$ mT for $H\ensuremath{\parallel}c$. The observed ${H}_{c2}$ is larger than the estimated thermodynamic critical-field ${H}_{c}$ derived from the heat-capacity data, suggesting that $\mathrm{La}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$ is a type-II superconductor with Ginzburg-Landau parameters ${\ensuremath{\kappa}}_{GL}^{ab}\ensuremath{\approx}1.9$ and ${\ensuremath{\kappa}}_{GL}^{c}\ensuremath{\approx}2.7$. The microscopic Eliashberg theory indicates superconductivity to be in the weak-coupling regime with an electron-phonon-coupling constant ${\ensuremath{\lambda}}_{e\ensuremath{-}ph}\ensuremath{\approx}0.4$. Despite a similar ${T}_{c}$ and the same crystal structure as the Ce compound, $\mathrm{La}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$ displays conventional superconductivity, corroborating the substantial role of the $4f$ electrons for the extraordinary superconducting state in $\mathrm{Ce}{\mathrm{Rh}}_{2}{\mathrm{As}}_{2}$.

Growth Effect of Crystallites in Fe82Si8B10 Amorphous Ribbon

Fe82Si8B10 amorphous ribbon which containing 82% of Fe and 18% of B and Si were prepared using rapid solidification technique. The samples were constantly annealed for 1 hour in the temperature range of 350°C-600°C. X-ray diffraction analysis (XRD) displays a large peak which is the diffusion hallow suggesting the amorphousity in its origin and formation of crystalline phase with the increment of annealing temperature at 450oC. The magnetic ordering of the ribbon is determined as temperature functions by both AC and DC magnetization. At 450°C, the saturation magnetization (Ms) was found 143 emu/gm and with better value of real part of initial permeability at 450°C as well as better relative quality factor (RQF) was observed at the same temperature. The results are explained as due to higher heating rates than this Si diffuse in amorphous and lost the soft magnetic properties. Journal of Engineering Science 13(1), 2022, 1-8

Tuning AC magnetic properties of FeCoNi1+xCu1-xAl (0 ≤ x ≤ 1.0) high-entropy alloys by adjusting Ni and Cu content

In this work, the AC magnetic properties and microstructure of FeCoNi1+xCu1-xAl (0 ≤ x ≤ 1.0) high-entropy alloys (HEAs) are studied. The results show that the total loss (Ps), AC coercivity (AC Hc), and AC applied field (AC Hm) are improved by adjusting the amounts of Ni and Cu. When x = 0.1, Ps, AC Hc, and AC Hm of the alloy at 50 Hz decrease by 40 %, 41 %, and 43 %, respectively. When x = 0.7, the alloy has optimised magnetic properties, and Ps, AC Hc, and AC Hm of the alloy at 50 Hz decrease by 77 %, 78 %, and 82 %, respectively. The Ps is divided into hysteresis loss (Ph) and eddy current loss (Pe). When f ≤ 100 Hz, Ph is the dominant contributor to Ps, while when f ≥ 850 Hz, Pe is the dominant contributor. At the same frequency, Ph is closely connected with AC Hc, and AC Hm, and Pe is inversely proportional to ρ. X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the alloys consist of a mixture of face-centred cubic (FCC) and body-centred cubic (BCC) phases. As x increases, the elemental distributions of FCC and BCC phases clearly change. Transmission electron microscopy (TEM) shows that there are many Cu-rich nano-precipitates in the BCC phase. Compared with the alloy where x = 0, nano-precipitates of the alloy where x = 0.1 have shorter lengths, lower distribution densities, and more uniform size. Our work provides an effective approach to explore novel soft magnetic materials.

Magnetizing current of a Large Power Transformer and its Harmonic Spectrum in Normal and GIC conditions

A measurement of the harmonic spectrum of magnetizing current was performed. This paper analyzes the results of the measurement of harmonic spectrum of magnetizing current in standard noload test at various values of induction. Magnetizing current harmonic spectrum during single-phase noload test was measured as well as the impact of combined AC and DC magnetization on core behavior and harmonic spectrum. A mathematical model of transformer core is introduced. The calculated results are presented.

Field-Angle Dependence Reveals Odd-Parity Superconductivity in CeRh2As2

CeRh2As2 is an unconventional superconductor with multiple superconducting phases and Tc=0.26 K. When H∥c, it shows a field-induced transition at μ0H*=4 T from a low-field superconducting state SC1 to a high-field state SC2 with a large critical field of μ0Hc2=14 T. In contrast, for H⊥c, only the SC1 with μ0Hc2=2 T is observed. A simple model based on the crystal symmetry was able to reproduce the phase diagrams and their anisotropy, identifying SC1 and SC2 with even and odd parity superconducting states, respectively. However, additional orders were observed in the normal state which might have an influence on the change of the superconducting state at H*. Here, we present a comprehensive study of the angle dependence of the upper critical fields using magnetic ac susceptibility, specific heat, and torque on single crystals of CeRh2As2. The experiments show that the state SC2 is strongly suppressed when rotating the magnetic field away from the c axis and it disappears for an angle of 35°. This behavior agrees perfectly with our extended model of a pseudospin triplet state with d→ vector in the plane and hence allows us to conclude that SC2 is indeed the suggested odd-parity state.Received 11 January 2022Revised 13 April 2022Accepted 5 May 2022DOI:https://doi.org/10.1103/PhysRevX.12.031001Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Open access publication funded by the Max Planck Society.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhase diagramsQuantum phase transitionsSuperconductivityPhysical SystemsHeavy-fermion systemsLow-temperature superconductorsStrongly correlated systemsTechniquesAC susceptibility measurementsMagnetization measurementsSpecific heat measurementsCondensed Matter, Materials & Applied Physics

A benchtop induction-based AC magnetometer for a fast characterization of magnetic nanoparticles

In this study, we report a development of a benchtop induction-based AC magnetometer to realize a simple, wideband, and sensitive AC magnetometer for bio-sensing applications and characterization of magnetic nanoparticles (MNPs). We investigate the inductance and parasitic capacitance of six different pickup coil geometries and compare their sensitivity and usable frequency range. In the pickup coil design, the number of turns and coil section separation are varied from 200 to 400 turns, and 1 to 4 sections, respectively. We find that the usable frequency range is greatly affected by the pickup coil’s inductance due to the self-resonance phenomena compared to their parasitic capacitance. A low noise instrument amplifier circuit (AD8429, Analog Devices, USA) was integrated and fabricated on a printed circuit board to amplify the weak signal from the pickup coil. We also implement a generalized Goertzel algorithm to achieve fast signal amplitude and phase extractions at a frequency. The developed magnetometer shows a sensitivity of 10−8 Am2/√Hz at 6 Hz and a frequency range of up to 158 kHz. Using the developed AC magnetometer, we demonstrate the viscosity effect on the frequency response of thermally blocked, single-core nanoparticles (SHP30, Ocean Nanotech, USA) in glycerol solutions. The excitation frequency is swept from 5 Hz to 158 kHz at a field amplitude of 0.55 mTpp within the acquisition time of 5 min (51 points). As a result, the viscosity change is confirmed by the peak shifting in the imaginary magnetization curve towards lower frequency values when the wt/V% of the glycerol solution is increased. The hydrodynamic size and the average anisotropy energy ratio σ are estimated to be 60.6 nm and 25, respectively, from the complex AC magnetization. It can be expected that the developed AC magnetometer can be a valuable tool in providing a fast and reliable assessment of MNPs for bio-sensing applications.

Dependence of hysteresis loss of immobilized magnetic nanoparticles on the easy-axis angle: Effect of easy-axis alignment

Magnetic nanoparticles (MNPs) have been widely studied for hyperthermia applications. We studied the hysteresis loss of immobilized MNPs with partially aligned easy axes. For this purpose, we investigated how the AC magnetization (M–H) curve of MNP depended on the easy-axis angle of magnetization, β, via numerical simulations of the Fokker–Planck equation. We clarified the dependences of the coercive field, Hc, and the hysteresis loss, A, on β. We, thus, obtained analytical expressions for Hc(β) and A(β) that explain simulation results for a wide range of MNP parameters and excitation conditions. The angle dependences were also discussed on the basis of magnetic moment reversal over the anisotropy energy barrier. We then examined the hysteresis loss of an MNP sample with partial alignment of the easy axes and obtained an expression for the loss by combining A(β) and a distribution function for β. We quantitatively clarified the relationship between the loss and the degree of easy-axis alignment. The loss of immobilized MNPs can be increased by a factor of 2.2 by using easy-axis alignment relative to the case of randomly oriented easy axes. Finally, we examined the alignment of easy axes induced by an AC field in suspended MNPs and showed that the loss of immobilized MNPs with partially aligned easy axes can be used to estimate the loss for suspended MNPs.

Magnetization AC losses of a like-quasi-isotropic conductor using accelerated three-dimensional T-A model

An accelerated T-A formulation based on a like-quasi-isotropic conductor, composed of a quasi-isotropic conductor and helical tapes, is developed and presented. This formulation reduces the degree of freedom of solution and is suitable for calculating of magnetization losses of large magnets. We use simulation and experiment methods to study the composition of the magnetization losses of a like-quasi-isotropic conductor and find a way to reduce the AC losses.

Structural, electronic and magnetic properties of La1.5Ca0.5(Co0.5Fe0.5)IrO6 double perovskite

In this work, we report the synthesis and investigation of structural, electronic, and magnetic properties of La1.5Ca0.5(Co0.5Fe0.5)IrO6. Our polycrystalline sample forms as a single-phase double perovskite in monoclinic P21/n space group. Co and Ir are most likely in bivalent and tetravalent oxidation states, respectively, while Mössbauer spectroscopy indicates that Fe is in a trivalent state. The ac and dc magnetization data suggest a ferrimagnetic behavior resulting from the presence of two antiferromagnetic sublattices at Co/Fe and Ir sites. The large coercive field HC≃ 32 kOe observed at 10 K, comparable to that of other double perovskites of interest for hard magnets, is discussed in terms of the structural distortion and the spin and orbital magnetic moments of the transition metal ions.

Achieving superconductivity with higher Tc in lightweight Al–Ti–Mg alloys: Prediction using machine learning and synthesis via high-pressure torsion process

Aluminum (Al) and titanium (Ti) are superconducting materials but their superconducting transition temperatures (Tc) are quite low as 1.20 and 0.39 K, respectively, while magnesium (Mg) never exhibits superconductivity. In this study, we explored new superconductors with higher Tc in the Al–Mg–Ti ternary system, along with the prediction using machine learning. High-pressure torsion (HPT) is utilized to produce the superconducting states. While performing AC magnetization measurements, we found, for the first time, superconducting states with Tc=4.0 and 7.3 K for a composition of Al:Ti = 1:2. The magnetic anomalies appeared more sharply when the sample was processed by HPT at 573 K than at room temperature, and the anomalies exhibited DC magnetic field dependence characteristic of superconductivity. Magnetic anomalies also appeared at ∼55 and ∼93 K, being supported by the prediction using the machine learning for the Al–Ti–O system, and this suggests that Al–Ti oxides play an important role in the advent of such anomalies but that the addition of Mg could be less effective.

J model for studying AC magnetization loss in 3D cable structures

Flux penetration and AC loss are among the key problems in the application of high-temperature superconductors (HTSs). Through simulation, the value and distribution of AC loss in HTS equipment can be evaluated and optimized in the design stage, so as to ensure the reliability and stability of the equipment. At present, there are many algorithms for AC loss calculation that can be simplified to a two-dimensional model, but for the case of three-dimensional structure such as cable in a spiral structure, there are still some problems such as difficult meshing, difficult convergence, and low efficiency. In this paper, an upgrade of the two-dimensional J model to a three-dimensional J model is proposed, permitting effective calculation of the flux penetration and AC loss in the CORC cable. The reliability of the method is verified by comparing the calculated results with the analytic model and with experimental results. At present, this method can efficiently calculate a 3D CORC model with a total of 15 superconducting tapes in five layers. Also discussed is the feasibility of upgrading to calculate larger models.

Slow-Relaxation Behavior of a Mononuclear Co(II) Complex Featuring Long Axial Co-O Bond.

Co(II) mononuclear complex with different coordination geometry would display various of field-induced single-ion magnet (SIM) behaviors. Here, we identify a field-induced single-ion magnet in a mononuclear complex Co(H2DPA)2·H2O (H2DPA = 2,6-pyridine-dicarboxylic acid) by the hydrothermal method. The long axial Co-O coordination bond (Co1‧‧‧O3) can be formed by Co1 and O3. Therefore, Co(II) ion is six-coordinated in a distorted elongated octahedron. AC magnetization susceptibilities show that the effective energy barrier is up to 43.28 K. This is much larger than most mononuclear Co(II). The distorted elongated octahedron caused by the axial Co-O coordination bond is responsible for the high effective energy barrier. The distribution of electron density in Co1 and O3 atoms in the long axial bond would influence the magnetic relaxation process in turn. Our work deepens the relationship between the effective energy barrier and the weak change of ligand field by long axial bonds, which would facilitate constructing SIM with high energy temperature.

Investigation of using CNT and Cu/CNT Wires for Replacing Cu for Power Electronics and Electrical Applications

The demand for power electronics increases continuously with the technological development. The next-generation power electronic converter circuits and electrical power system demands for sustainable, highly efficient and higher functionality material which should outperform Cu. In future electrical systems, Cu transmission cables and windings would be replaced by Carbon Nanotubes (CNTs) and Cu/CNT composite. This paper presents the investigation of using CNT and Cu/CNT wires for replacing Cu for power electronics and electrical applications. Conducting wire made of Cu, Cu/CNT composite and CNT are considered. Frequency domain electromagnetic analysis is carried out to obtain the performance parameters such as magnetic flux density, current density, impedance, voltage, power, resistance losses, inductance and AC resistance at the current of 1 A supplied with the frequency of 50 Hz. Finite Element Modelling (FEM) simulation is carried out using COMSOL Multiphysics. The frequency of the supply current is also varied from 50 Hz to 5 MHz. The analysis shows that Cu/CNT is performing close to Cu in terms of electromagnetic parameters. Thermal analysis is also carried out by varying the current from 1 A to 35 A. CNT conductors produces lowest temperature and perform better in terms of electro-thermal parameters.

Hybrid laser-arc welding of laser- and plasma-cut 20-mm-thick structural steels

It is already known that the laser beam welding (LBW) or hybrid laser-arc welding (HLAW) processes are sensitive to manufacturing tolerances such as gaps and misalignment of the edges, especially at welding of thick-walled steels due to its narrow beam diameter. Therefore, the joining parts preferably have to be milled. The study deals with the influence of the edge quality, the gap and the misalignment of edges on the weld seam quality of hybrid laser-arc welded 20-mm-thick structural steel plates which were prepared by laser and plasma cutting. Single-pass welds were conducted in butt joint configuration. An AC magnet was used as a contactless backing. It was positioned under the workpiece during the welding process to prevent sagging. The profile of the edges and the gap between the workpieces were measured before welding by a profile scanner or a digital camera, respectively. With a laser beam power of just 13.7 kW, the single-pass welds could be performed. A gap bridgeability up to 1 mm at laser-cut and 2 mm at plasma-cut samples could be reached respectively. Furthermore, a misalignment of the edges up to 2 mm could be welded in a single pass. The new findings may eliminate the need for cost and time-consuming preparation of the edges.

Entropic topography associated with field-induced quantum criticality in a magnetic insulator DyVO4

Exploration of low temperature phase transitions associated with quantum critical point is one of the most mystifying fields of research which is under intensive focus in recent times. In this work, through comprehensive experimental evidences, we report the possibility of achieving quantum criticality in the neighborhood of a magnetic field-tuned tricritical point separating paramagnetic, antiferromagnetic and metamagnetic phases in a magnetic insulator, DyVO4. Magnetic susceptibility and heat capacity indicate to the presence of a long-range second order antiferromagnetic transition at TN ~ 3.2 K. Field variation of Magnetic susceptibility and heat capacity, along with differential magnetic susceptibility and DC field dependent AC susceptibility gives evidence of the modification of the antiferromagnetic structure below the tricritical point; implying the presence of a field-induced first order metamagnetic transition which persists down to 1.8 K. Further, the magnetic field dependence of the thermodynamic quantity − dM/dT, which is related to magnetic Gruneisen parameter, approaches a minimum, followed by a crossover near 5 kOe to a maximum; along with a hyperbolic divergence in temperature response of dM/dT in the critical field regime. Temperature response of heat capacity at 5 kOe also shows a deviation from the conventional behavior. Entropic topography phase diagram allows tracking of the variation of the entropy, which indicates towards the emergence of the peak at quantum critical point into a V-shaped region at high temperatures. Our studies yield an inimitable phase diagram describing a tricritical point at which the second-order antiferromagnetic phase line terminates followed by a first order line of metamagnetic transition, as the temperature is lowered, leading to metamagnetic quantum critical end point.

Superconductivity and Phase Diagram in the Nanostructured Eutectic Ga-Ag Alloy

Here, we present a study of superconductivity in the Ga-Ag alloy embedded into porous template with pore diameter of 10 nm. The composition was close to the eutectic point in the gallium-rich range. We measured DC and AC magnetizations for temperatures from 1.8 to 10 K and magnetic fields up to 6 T. Three superconducting transitions were found at temperatures 7.05, 6.08, and 2.65 K in contrast to the bulk counterpart. Upper critical field lines were obtained. Activation barriers were evaluated from the AC data. The temperatures of the superconducting transitions were ascribed to emergence of β- and iota-Ga and of an intermetallic different from bulk Ag3Ga2. Keywords: nanostructured Ga-Ag eutectic alloy, superconducting transitions, segregated phases, DC and AC susceptibility.

Improvement of Charpy impact toughness by using an AC magnet backing system for laser hybrid welding of thick S690QL steels

The study deals with the influence of the heat input and the resulting cooling times on the microstructure and Charpy impact toughness of single-pass laser hybrid welded 20-mm thick high-strength steel S690QL. The main focus is on the change of the mechanical properties over the entire seam thickness. The cooling times were measured in-situ using a pyrometer and an optical fibre in three different depths of the seam where Charpy impact test specimens were also later taken. Thereby, three different heat inputs from 1.3 kJ/mm to 2 kJ/mm were investigated. Despite the observed decreased values of both t8/5-cooling time and the Charpy impact toughness in the root part of the seam, the required impact toughness of 38 J/cm2 could be reached in dependance on applied heat input, especially at the heat input of 1.6 kJ/mm.

Paramagnetic 7Li NMR Shifts and Magnetic Properties of Divalent Transition Metal Silylamide Ate Complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co).

7Li NMR shifts and magnetic properties have been determined for three so-called ate complexes [LiM{N(SiMe3)2}3] (M2+ = Mn, Fe, Co; e.g., named lithium-tris(bis(trimethylsilylamide))-manganate(II) in accordance with a formally negative charge assigned to the complex fragment [M{N(SiMe3)2}3]-, which comprises the transition metal). They are formed by addition reactions of LiN(SiMe3)2 and [M{N(SiMe3)2}2] and stabilized by Lewis base/Lewis acid interactions. The results are compared to those of the related "ion-separated" complexes [Li(15-crown-5)][M{N(SiMe3)2}3]. The ate complexes with the lithium atoms connected to the 3d metal atoms manganese, iron, or cobalt via μ2 nitrogen bridges reveal strong 7Li NMR paramagnetic shifts of about -75, 125, and 171 ppm, respectively, whereas the shifts for the lithium ions coordinated by the 15-crown-5 ether are close to zero. The observed trends of the 7Li NMR shifts are confirmed by density-functional theory calculations. The magnetic dc and ac properties display distinct differences for the six compounds under investigation. Both manganese compounds, [LiMn{N(SiMe3)2}3] and [Li(15-crown-5)][Mn{N(SiMe3)2}3], display almost pure and ideal spin-only paramagnetic behavior of a 3d5 high-spin complex. In this respect slightly unexpected, both complexes show slow relaxation behavior at low temperatures under applied dc fields, which is especially pronounced for the ate complex [LiMn{N(SiMe3)2}3]. Dc magnetic properties of the iron complexes reveal moderate g-factor anisotropies with small values of the axial magnetic anisotropy parameter D and a larger E (transversal anisotropy). Both complexes display at low temperatures and, under external dc fields of up to 5000 Oe, only weak ac signals with no maxima in the frequency range from 1 to 1500 s-1. In contrast, the two cobalt complexes display strong g-factor anisotropies with large values of D and E. In addition, in both cases, the ac measurements at low temperatures and applied dc fields reveal two, in terms of their frequency range, well separated relaxation processes with maxima lying for the most part outside of the measurement range between 1 and 1500 s-1.