The temperature dependence of the domain structure of uniaxial single crystals with high-anisotropy field

The temperature dependence of the domain structure of single crystals of uniaxial ferrimagnetic oxides with high-anisotropy field in the SrO(6 - x)Fe2O3xAl2O3compositional series, withx = 1.8and 2.4, was investigated. The domain configurations of thermally and/or ac field demagnetized states and the changes in dc and ac fields were observed by means of the colloid technique using a suspension in paraffine oil. The temperature dependence of the wall energy density between room temperature and the Curie point (270°C) of the compound withx = 1.8was computed using the temperature dependence of the saturation magnetization and the domain width for the Kittel-like domain structure. It has been found that the room temperature domain structure of the compound withx = 2.4depends upon the mode of demagnetization, i.e., thermally or in the ac field. The ac demagnetized states are more stable than the thermally demagnetized ones. The samples exhibit a room temperature memory of the previous magnetization, which decreases with rising temperature. This memory is completely lost after heating at 400°C. The peculiarities of the temperature dependence of the domain structure and magnetic behavior on thermal cycling are explained by considering the existence of magnetic inhomogeneities within the crystal.     

Switching windows for a nanostructured cell of synthetic ferrimagnets

The magnetization switching window of nanostructured synthetic ferrimagnets with lateral dimension of 160 nm x 80 nm under combined in-plane magnetic fields along the longitudinal and transverse directions is investigated by numerical calculation using an analytical equation for the total energy. The considered total energy equation precisely accounts for the magnetostatic energy, which is significantly large in nanostructured magnetic cells. Due to the complex magnetization alignment of synthetic ferrimagnets, a different switching criterion based on the reversibility of magnetization process is used, instead of the simple criterion frequently used for single magnetic layers. Synthetic ferrimagnets with various thickness asymmetries are considered, and switching windows are calculated both in static and dynamic conditions. The static switching windows show a smaller dependence on the thickness asymmetry than the dynamic switching windows do. The dynamic switching window at a large thickness asymmetry resembles that of a single magnetic layer. The results are discussed in terms of energy profiles that can be obtained by locating the lowest energy path linking the two stable states from the total energy surface.     

Accidental printing effect of magnetic recording tapes using ultrafine particles of acicular γ-Fe2O3

Five kinds of acicular γ-Fe2O3powder of which axial length varies from 0.17 to 0.80 μm with almost the same axial ratio are manufactured. The variance of coercive force and printing effect as the average axial length of the particles varies is shown. Coercive force of which particle axial length is about 0.35 μm shows maximum value. The printing effect increases according to the decrease of axial length. The printing effect increases with contacting time. The temperature dependence of coercive force and remanent magnetization are measured from 25 to 200°C. These two properties decrease as the temperature increases, but the decreasing inclination becomes steeper as the axial length is smaller. The relationship between the printing effect and the decreasing inclination of coercive force is nearly linear. From the temperature dependence of the printing effect, the activation energy of the printing effect can be calculated. The activation energy of the smallest particle is about 0.05 eV and the others are about 0.15 eV. This measured activation energy is discussed in connection with the relaxation equation.     

On the analysis of magnetization and Mössbauer data for ferritin

Experimental data for antiferromagnetic nanoparticles are often analyzed as if the particles were ferromagnetic. However, due to the volume dependence of the magnetization resulting from uncompensated spins, such analysis will yield erroneous results. This is demonstrated as we analyze ac and dc magnetization data as well as M?ssbauer spectra obtained for ferritin. The values of the median energy barrier obtained from the different data are in very close agreement when a distribution of volumes and a volume dependence of the magnetization are taken into account. However, when the volume dependence of the magnetization is neglected, erroneous values of the anisotropy energy barrier and the attempt time τ(0) are obtained.     

Use of the asymmetric planar hall resistance of an Fe film for possible multi-value memory device applications

Systematic planar Hall measurements have been performed on a ferromagnetic Fe film grown on a standard (001) GaAs substrate at room temperature. The angular dependence of the planar Hall effect revealed the presence of both four-fold (cubic) and two-fold (uniaxial) anisotropies in the 7 nm thick Fe film. The dominance of the four-fold symmetric anisotropy, however, provided four magnetic easy axes near the (100) direction, which results in a two step switching phenomenon in the magnetization reversal process. An interesting asymmetric hysteresis loop was observed in the planar Hall resistance (PHR) when the turning point of the field scan is set at the value in the region of the second transition. The intermediate resistance states appearing in the asymmetric PHR loop were understood in terms of mutli-domain structures formed during the second switching of magnetization. Such multi-domain structure of the Fe film showing robust time stability provided additional Hall resistance states, which can be used for multi-valued memory device applications.     

Pinning and vortex dynamics in superconducting (K,Ba)BiO 3 single crystals

Pinning and vortex dynamics have been investigated in the 3-dimensional copper free (K, Ba)BiO₃superconductor (T_c 31K) by magnetization and transport measurements up to 30 Tesla. The magnetization curves present a pronounced fishtail effect which persists for time scales down to 10^–4s (pulsed field measurements). We show that it is an intrinsic feature of the critical current which can in part be well described by the collective pinning theory. Furthermore, this system presents evidence for a vortex liquid /glass transition for vanishingly small currents. As the current density is increased, dissipation in the glass state is dominated by creep effects. The temperature and current dependence of the activation energy is discussed.     

Effect of Hyperbolic Band Structure on the Energy Loss Rate of Hot Electrons with Non-equilibrium Phonon Distribution in the Extreme Quantum Limit at Low Temperatures in n-Hg0.8Cd0.2Te

The energy loss rate of hot electrons with the non-equilibrium phonons in narrowgap semiconductors with hyperbolic band structures has been investigated in the extreme quantum limit condition in the low temperature region. The calculation is done for n-Hg_0.8Cd_0.2Te sample considering electron scattering by acoustic phonons via piezoelectric coupling to be the dominant loss mechanism. The value of the energy loss rate with hyperbolic band is compared with the results of parabolic and non-parabolic band structures and at the same time all the results are also compared with the experimentally observed data. It is found that with the inclusion of hyperbolic band structure, the value of energy loss rate is found to be close to the experimental values. The dependence of energy loss rate on magnetic field and lattice temperature has been studied. Using the experimental value of the energy loss rate, the phonon life time is evaluated. The value of the phonon life time is found to be of the order of the phonon boundary relaxation time indicating that phonon boundary scattering is the dominant phonon dissipation mechanism. The dependence of the phonon life time on magnetic field, and lattice temperature has also been studied. The phonon life time is also found to decrease with increase in electron temperature.     

On the mechanism of electrical conduction in glow discharge polysilazane films

The electrical properties of thin films of glow-discharge-polymerized hexamethylcyclotrisilazane were investigated in order to determine the mechanism of electrical conduction. Current-voltage characteristics and activation energy measurements were obtained for samples 0.6–3.5 μm thick in symmetric metal- polymer-metal and in asymmetric metal-polymer-semiconductor (MIS) sandwich configurations. The results of the field dependence, temperature dependence and sample thickness dependence of the conductivity, supported by the results of thermally stimulated current measurements in the field-dependent regime, suggest that electrode-limited (Schottky) conduction is dominant in the material investigated. The smaller than expected differences in conduction in both directions for MIS structures are interpreted in terms of surface states.     

Domain theory model for magnetic thin films

A domain-theoretic model for external-field-driven magnetization processes in thin-film microelements is described. The film is mathematically described by a dynamic grid of quadrilateral domains whose magnetization directions and vertex locations are variables with which to minimize the total free energy. The magnetostatic interaction between uniformly (in-plane) magnetized polygonal domains is essentially treated exactly. Predicted results for reversal mechanisms and field-induced metastable remanent domain configurations in Permalloy microelements are in good qualitative agreement with Bitter pattern observations     

Effect of neodymium doping on structural,electrical and magnetic properties of multiferroic GdMn<Subscript>2</Subscript>O<Subscript>5</Subscript>

Neodymium doped gadolinium manganate with general composition (Nd_0.1Gd_0.9Mn₂O₅) was prepared by co-precipitation method. Microstructural and compositional analysis has been carried out by X-ray diffraction and scanning electron microscopy. The optical studies have been carried out by Raman and FTIR. The electrical properties studied include dielectric constant, dielectric loss, ac conductivity and activation energy in the temperature range 20–400 ?°C. The shift in the dielectric peak towards higher temperature side with increasing frequency indicates frequency dispersion and suggesting the relaxational behaviour of the material. Frequency dependence of ac conductivity obeys the universal power law. The value of activation energy depends on increase in frequency. The room temperature magnetic behaviour has been analyzed from the magnetic field dependent magnetization curve. The grown material exhibits the paramagnetic behavior at room temperature.     

Magnetic properties of β-FeSi2 single crystals

We investigated temperature and magnetic field dependences of the magnetization of β-FeSi₂ single crystals in the temperature range of 5-300 K in magnetic fields up to 15 kOe. The temperature dependence of the magnetic susceptibility of the Cr- and Ni-doped sample can be explained by temperature-dependent contributions due to paramagnetic centres and due to the carriers excited thermally in the extrinsic conductivity region. The values of the paramagnetic Curie temperature as well as activation energy of the donor and acceptor levels are estimated.     

Pinning Energies and Relaxation Rates in Laser Ablated Al-Doped YBCO Superconducting Thin Films

Systematic magnetization measurements were performed on laser ablated Al-doped YBCO thin films in the temperature range from 4 to 80 K and fields up to 12 T. The magnetization as a function of logarithmic time in the temperature range from 10 to 80 K generally showed a linear behavior. This type of performance allows us to determine the temperature and field dependence of the apparent pinning potential on the basis of the Kim–Anderson model for thermal activation of flux motion. The relaxation rate values are calculated from the time-dependent magnetization curves. The relaxation rate dependence on temperature, magnetic field, and doping concentrations has been investigated for different Al concentrations and applied fields up to 5 T. A strong nonlinearity was observed for the relaxation rate dependence on both temperature and applied fields. The apparent pinning potential values, U _o, are estimated from the dependence of relaxation rates temperature curves using U _o=k _B T/S expression. The dependence of U _o on both temperature and fields is elaborated and discussed in detail. The pinning potential, U _o, increases with temperature up to 40 K, however, above 40 K, a rapid decrease of U _o has been observed. Besides, a noticeable decrease in U _o as the applied field increases up to 0.1 T for temperatures below 40 K, and it is almost field independent for temperatures exceeding 40 K.     

Lifetime measurements of several S, P, and D states of thallium in a glow discharge by single-step and two-step laser-excited fluorescence

The lifetimes of several states in a thallium see-through hollow cathode discharge, or galvatron, are obtained to characterize its potential as an atomic line filter. The lifetimes of the thallium 6(2)D(3/2), 6(2)D(5/2), and 7(2)S(1/2) states are measured by time-resolved single-step laser-excited fluorescence by use of a 276.787 nm laser pulse or a 535.046 nm laser pulse and measuring the resulting fluorescence waveform at the appropriate wavelength. Values of 6.4 +/- 0.1, 7.5 +/- 1.1, and 7.7 +/- 0.2 ns were obtained for the 6(2)D(3/2), 6(2)D(5/2), and 7(2)S(1/2) states, respectively, which agree with values obtained by previous authors, as well as calculated values. No current dependence was observed for each of these states. The lifetime of the long-lived thallium 6(2)P(3/2) degrees metastable state was measured by two-step laser-excited fluorescence at various applied currents. The metastable level was pumped by a 276.787 nm laser pulse, and a temporally delayed 535.046 nm laser pulse interrogated the population of the metastable state. Relating the fluorescence intensity to the population of the metastable state as a function of delay time yielded a decay curve for the 6(2)P(3/2) degrees metastable state. Values of 2.1 +/- 0.2, 2.8 +/- 0.1, 3.1 +/- 0.3, 3.8 +/- 0.4, and 4.8 +/- 0.6 micros were found for applied currents of 14.0, 12.0, 10.0, 8.0, and 6.0 mA, respectively. The resulting lifetimes for the 6(2)P(3/2) degrees metastable state clearly show a dependence on the applied current and are expected to be due to collisions with the wall of the cathode, as well as a contribution due to collisions with electrons.     

Two carriers in vertically coupled quantum dots: magnetic field effect

We study a two-charge-carrier (two holes or two electrons) quantum dot molecule in a magnetic field. In comparison with the electron states in the double quantum dot, the switching between the hole states is achieved by changing both the inter-dot distance and magnetic field. We use harmonic potentials to model the confining of two charge carriers and calculate the energy difference delta E between the two lowest energy states with the Hund-Mulliken technique, including the Coulomb interaction. Introducing the Zeeman effect, we note a ground-state crossing, which can be observed as a pronounced jump in the magnetization at a perpendicular magnetic field of a few Tesla. The ground states of the molecule provide a possible realization for a quantum gate.     

Femtosecond laser spectroscopy of spins: Magnetization dynamics in thin magnetic films with spatio-temporal resolution

Based on the Magneto-Optical Kerr Effect (MOKE), we have developed an experimental set-up that allows us to fully characterize the magnetization dynamics in thin magnetic films by measuring all three real space components of the magnetization vector M. By means of the pump-probe technique it is possible to extract the time dependence of each individual projection with sub-picosecond resolution. This method has been exploited to investigate the temporal evolution of the magnetization (modulus and orientation) induced by an ultrashort laser pulse in thin epitaxial iron films.According to our results, we deduced that the initial, sub-picosecond demagnetization is established at the electronic level through electron-magnon excitations. The subsequent dynamics is characterized by a precessional motion on the 100 ps time scale, around an effective, time-dependent magnetic field. Following the full dynamics of M, the temporal evolution of the magneto-crystalline anisotropy constant can be unambiguously determined, providing the experimental evidence that the precession is triggered by the rapid, optically-induced misalignment between the magnetization vector and the effective magnetic field.These results suggest a possible pathway toward the ultrarapid switching of the magnetization.     

Water diffusion and microstructure of hydrated cement pastes

Water migration in Portland cement paste follows closely the theoretical diffusion equation. The diffusion coefficient decreases with increasing curing time of the paste specimen at first, but after 14 days the value becomes nearly constant. The temperature dependence of the diffusion coefficient indicates that diffusion involves an activation process, but the characteristic energy depends on the temperature range. The behaviour of aluminous cement is similar, but the value of the diffusion coefficient and its temperature dependence are about double those for Portland cement. Stereoscan electron micrographs of aluminous cement show a temperature sensitive microstructure and this probably causes the change in the diffusion activation energy. No comparable clear change can be seen in Portland cement structure.     

Magnetization of EuS–PbS Multilayers with Antiferromagnetic Interlayer Coupling

SQUID magnetometry is applied to study the temperature and magnetic field dependence of magnetization M(T, H) of semiconductor EuS–PbS ferromagnetic multilayers grown on insulating KCl(100) and conducting n-PbS(100) monocrystalline substrates. For low external magnetic fields (of the order of 10 Oe) and PbS spacer layers thinner than about 2 nm, we observe in EuS–PbS–EuS trilayers the strongly nonmonotonic temperature dependence of magnetization with almost zero total magnetic moment below the Curie temperature. The application of the magnetic field of the order of 100 Oe restores the regular monotonic increase of magnetization with decreasing temperature. To explain this M(T,H) dependence we present a model that considers the competition of three (temperature dependent) contributions to the total magnetic energy of the trilayer: the antiferromagnetic interlayer interaction energy, the Zeeman energy, and the energy of in-plane magnetocrystalline anisotropy.     

Dynamic conductivity of ferroelectric domain walls in BiFeO₃

Topological walls separating domains of continuous polarization, magnetization, and strain in ferroic materials hold promise of novel electronic properties, that are intrinsically localized on the nanoscale and that can be patterned on demand without change of material volume or elemental composition. We have revealed that ferroelectric domain walls in multiferroic BiFeO(3) are inherently dynamic electronic conductors, closely mimicking memristive behavior and contrary to the usual assumption of rigid conductivity. Applied electric field can cause a localized transition between insulating and conducting domain walls, tune domain wall conductance by over an order of magnitude, and create a quasicontinuous spectrum of metastable conductance states. Our measurements identified that subtle and microscopically reversible distortion of the polarization structure at the domain wall is at the origin of the dynamic conductivity. The latter is therefore likely to be a universal property of topological defects in ferroelectric semiconductors.     

Temporal Stability of Magnetization of ε-In<Subscript>0.24</Subscript>Fe<Subscript>1.76</Subscript>O<Subscript>3</Subscript> Nanoparticles

The kinetics of spontaneous demagnetization in nanoparticles of the exotic epsilon-phase of indium-doped iron(III) oxide (ε-In_0.24Fe_1.76O₃) has been studied using the method of accelerated testing of magnets for temporal stability in a magnetization-reversal field. Time dependences of the magnetization of nanoparticles measured in a wide range of magnetic fields exhibited rectification in semilogarithmic coordinates. The dependence of the magnetic viscosity on the magnetic field has been measured and used for determining the fluctuation field and activation volume. A relationship between the magnetic viscosity and magnetic noise caused by random thermoinduced magnetization reversal in separate nanoparticles is established.