Search

A detailed electron spin resonance (ESR) study in a broad temperature interval is carried out to investigate the nature of the magnetic phases in La0.5Ca0.5MnO3. The incommensurate-charge ordering phase (ICO) that coexists with ferromagnetic-metallic (FMM) clusters is mainly paramagnetic and not antiferromagnetic.

Research describing the effect of annealing temperature on transition metal perovskites is presented. In particular the existence of an electronic phase separation characteristic within the material is investigated.

Arrays of cobalt nanowires produced by electrodeposition, were studied for structural and magnetic properties.

A new study investigates the magnetoresistance effects in magnetic/insulator composites.

Noninteracting and interacting systems of particle that show different anisotropies are studied using extensive Monto Carlo simulations. The findings show that a direct relation between linearity and noninteractions exists only for systems of uniaxial particle at low temperatures, while particles with cubic anisotropy always show positive deviation of the Henkel plot in the whole range of temperatures.

The colossal magnetoresistance (CMR) response of polycrystalline mixed valence manganite La0.67Ca0.33MnO3 has been tuned by changing grain size using a sol-gel technique. It was observed that below the critical diameter of 150 nanometers, CMR disappears while intergrain magnetoresistance (MR) remains measurable until 1.2 magnetic phase transition temperature. High temperature MR observed at low fields has potential magnetoresistive device applications.

Research was conducted to examine the evolutions of the uniaxial torque and magnetization with the applied magnetic field which permits a discrimination of the sources of in-plane magnetic anisotropy in amorphous ribbons. A simple model was developed for an inhomogeneous material to determine the magnetic anisotropy sources. The model was applied to explain the different behavior seen in the evolution of the uniaxial torque amplitude with the applied magnetic field of two commercial Co-based amorphous alloys.

Research was conducted to examine the impact of cubic anisotropy constants on the hysteresis loops of single-domain particles using the Monte Carlo simulation method which is very effective in the study of simple magnetic models. Coercivity, remanence and the influence of the second term of the anisotropy energy were investigated. Results indicate that the second term has a major effect on the static magnetic behavior when the first anisotropy constant is negative.

Herein, we report on the preparation, structure, and magnetic characterization of FeCoCu nanowire arrays grown by DC electrodeposition inside self-assembled ordered nanopores of anodic aluminum oxide templates. A systematic study of their structure has been performed both in asprepared samples and after annealing in the temperature range up to 800 °C, although particular attention has been paid to annealing at 700 °C after which maximum magnetic hardening is achieved. The obtained nanowires have a diameter of 40 nm and their Fe0.28Co0.67Cu0.05 composition was confirmed by energy dispersive X-ray spectroscopy (EDS). Focused ion-beam lamellas of two samples (as-prepared and annealed at 700 °C) were prepared for their imaging in the high-resolution transmission electron microscopy (HRTEM) perpendicularly to the electron beam, where the obtained EDS compositional mappings show a homogeneous distribution of the elements. X-ray diffraction analysis, and selected area electron diffraction (SAED) patterns confirm that nanowires exhibit a bcc cubic structure (space group Im-3m). In addition, bright-dark field images show that the nanowires have a polycrystalline structure that remains essentially the same after annealing, but some modifications were observed: (i) an overall increase and sharpening of recrystallized grains, and (ii) an apparent shrinkage of the nanowires diameter. Obtained SAED patterns also show strong textured components with determined (111) and (112) crystalline directions parallel to the wires growth direction. The presence of both directions was also confirmed in the HRTEM images doing Fourier transform analyses. Magnetic measurements show strong magnetic anisotropy with magnetization easy axis parallel to the nanowires in as-prepared and annealed samples. The magnetic properties are tuned by suitable thermal treatments so that, maximum enhanced coercivity (∼2.7 kOe) and normalized remanence (∼0.91 Ms) values are achieved after annealing at temperature of 700 °C. The contribution of the changes in the crystalline structure, induced by the heat treatment, to the magnetic hardening of the FeCoCu nanowires is discussed. © 2014 AIP Publishing LLC. [ABSTRACT FROM AUTHOR]

The measurement of the magnetic entropy change of the series of ferromagnetic perovskites La2/3(Ca(sub 1-x)Sr(sub x))1/3MnO3 with x = 0, 0.05, 0.15, 0.25, 0.50, 0.75 and near their Curie temperatures is discussed. The evolution of magnetic entropy change with x exhibits a steep decline between x = 0.05 and x = 0.15.

Research into spin dynamics in the Mn-site substituted colossal magnetoresistance system is presented. It was found that the structural changes prompted by the low doping levels of Co used in this research were negligible.

The magnetic and electrical properties of R(sub 1-x)Sr(sub x)CoO(sub 3) materials (R=La, Pr, Nd, Gd) (0 less than or equal to x less than or equal to 0.50) are studied. Two main parameters are altered upon substitutions at the rare-earth site that are the ionic radius, and the magnetic site of the R(super 3+) ion.

We have synthesized two divanadates M2V2O7 (M2+ = Co2+ and Cu2+), that are known to show different and complex magnetic arrangements at low temperatures and have studied their dielectric behavior. We have observed a change in the slope of the dielectric constant at the magnetic transition temperature, result which confirms the existence of magnetodielectric coupling in these two divanadates. The origin of the magnetodielectric coupling seems to be different in the Cu- and Co-compounds, although in both cases it is related to their frustrated magnetic arrangement. In Cu2V2O7, the dielectric anomaly arises from a spin canting due to the antisymmetric exchange which is allowed by structural symmetry. In the Co-divanadate, the structural symmetry does not allow antisymmetric exchange but the compound shows correlation between the behavior of the dielectric constant and inverse magnetization, which points to a dependence of its electrical polarization with the pair correlation function of neighboring magnetic spins. [ABSTRACT FROM AUTHOR]

We present a detailed investigation on the magnetodielectric properties of Fe2OBO3, where we have found a magnetodielectric effect around 150% at room temperature and ν=2 kHz, that cannot solely arise from magnetoresistive extrinsic contributions. In addition, we find a crossover of the magnetodielectric response around TN, that we attribute to changes in the electronic microstructure of this compound that take place at that temperature. For TN the dielectric constant experiences a jump at a temperature dependent critical field that we relate to a partial melting of the charge ordering state. [ABSTRACT FROM AUTHOR]

We have investigated the propagation of terahertz (THz) surface plasmon polaritons (SPPs) on gratings formed by grooves structured in silicon wafers. These gratings exhibit a stop gap where SPPs are Bragg scattered. We observe a strong effect of the groove depth on the characteristics of the stop gap. To quantify the scattering strength of these structures we obtain the SPP attenuation length by measuring the transmission as a function of the number of grooves. We also determine the effective refractive index of the Bragg gratings using two different approaches: by measuring the transmittance through the gratings at different angles of incidence and from the phase of the transmitted THz pulses. The two approaches give results in good agreement. [ABSTRACT FROM AUTHOR]

The magnetic anisotropy induced in amorphous magnetic ribbons by applying a magnetic field to the melt during the solidification process is analyzed. Using a model based on the magnetization curve and on the evolution of the uniaxial torque with the applied magnetic field, the volume fractions of the sample affected by the different magnetic anisotropies are obtained for the ribbons of composition Fe80B20 and Co70Mn5Fe1Mo1Si14B9. The results are compared with those obtained in the ribbons of the same composition but with anisotropies induced by the static magnetic annealing. From these results, the different sources of the field-quenching-induced magnetic anisotropy in those amorphous magnetic ribbons are analyzed. [ABSTRACT FROM AUTHOR]

We study the nonlinear current-voltage (I - V) characteristics and analyze the voltage-dependent tunneling conductance in nanoparticles of La[SUB2/3]A[SUB1/3]MnO[SUB3] (A=Ca, Sr). The powders were prepared by different wet-chemical routes and low calcination temperatures were used to obtain an average particle size D≈30 nm. The data are comprehensively explained in terms of the tunneling picture, which allows one to estimate the height of the grain boundary insulating barrier (ϕ) for each sample. For constant D, our results show that the sample preparation route is mainly responsible for the value of ϕ in nanoparticles, while the Coulomb gap in the Coulomb blockade regime is ∼3 times higher for Sr- than for Ca-doping. We also show that a small fraction of the barriers contribute to the nonlinear transport, and the current is mainly carried through low-resistive percolated paths. In addition, despite the different barrier strengths, the low-field magnetoresistance (LFMR) is similar for all samples, implying that ϕ is not the fundamental parameter determining the LFMR. [ABSTRACT FROM AUTHOR]

We report the effect of progressive reduction of the annealing temperature on the magnetocaloric effect (MCE) of La[sub 2/3]Ca[sub 1/3]MnO[sub 3-δ] nanoparticles synthesized by the sol-gel technique. With this method, we are able to obtain particle diameters ranging form 60 to 500 nm. The peak in the MCE at the ferromagnetic to paramagnetic phase transition is strongly reduced as annealing temperature does, due to loss of the intrinsic first-order magnetic phase transition. This opens up a new way in which to tune the intrinsic properties of mixed-valence manganites, most of which are associated with the first-order character of the magnetic transition. [ABSTRACT FROM AUTHOR]

Throughout a complete electron spin resonance (ESR) and magnetization study of La[sub 0.5]Ca[sub 0.5]MnO[sub 3], we discuss about the nature of the complex phase-segregated state established in this compound below T∼ 210 K. Between T[sub N]... T... T[sub c], the ESR spectra shows two lines characteristic of two different magnetic phases. From the resonance field (H[sub r]) derived for each line, we argue that the incommensurate-charge-ordering phase (ICO) which coexists with ferromagnetic-metallic (FMM) clusters in this temperature interval, is mainly paramagnetic and not antiferromagnetic. The FMM/ ICO ratio can be tuned with a relatively small field, which suggests that the internal energy associated with those phases is very similar. Below T[sub N], there is an appreciable ferromagnetic (FM) contribution to the magnetization and the ESR spectra indicates the presence of FM clusters in an antiferromagnetic matrix (canted). Our results show that ESR could be a very useful tool to investigate the nature of the phase-separated state now believed to play a fundamental role in the physics of mixed valent manganites. [ABSTRACT FROM AUTHOR]

This work focuses on the structural and magnetic characterization of arrays of cobalt nanowires, with diameters in the range 100-400 nm and lengths of L≤6 μm, produced by electrodeposition in polycarbonate membrane pores. Diffraction patterns of Co nanowires indicate that Co is stabilized in the hcp structure with a preferential (0001) texture oriented close to the perpendicular direction of the wires axes. Magnetic measurements at room temperature reveal that magnetization undergoes a change in the easy axis as the length of the wires increases. Clearly, the easy axis of the shortest wires is oriented along the axis of the wire. In the case of the longest wires, a crossover of the easy axis is observed, from the perpendicular to the parallel direction to the wire for the shortest. This magnetic behavior is explained through a simple magnetostatic model, which takes into account the dipolar interactions among wires, the demagnetizing field, and the magnetocrystalline anisotropy of Co. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The ac magnetic susceptibility studies of two samples of Gd2CuO4 show the presence of magnetic domains with coherent lengths that depend on the thermal treatment. The two maximas at 20 K and 280 K, for the sample annealed at 1,080 degrees centigrade, are not dependent on the frequency. The sample sintered at 850 degrees centigrade shows a maxima at 210 K, which is frequency-dependent. These indicate the dynamic nature of the domains.

Use of the microemulsion method for Ag coating on Co particles helps develop ultrafine Co nanoparticles whose magnetic properties depend on the annealing temperature. The Ag undergoes crystallization in the face centered cubic phase of the particles. The size of the particles depends on the water droplets of microemulsion. The ultrafine Co particles exhibit a coercivity of about 600 Oe.

Solid state reaction methods, water-in-oil micromulsion and sol-gel methods are used to prepare Gd(2-x)Ce(x)CuO4 which is characterized by transmission electron microscopy and photon correlation spectroscopy. The characterization reveals that the internal magnetic field is related to the WF of the copper moments. The magnetic field is inversely proportional to the diameter.

Magnetic relaxation measurements facilitated the study of high-temperature relaxation in vacancy-doped polycrystalline magnetites when subjected to temperatures ranging from 200 to 800 Kelvin. Significant relaxation peaks were observed at various temperature ranges. The peak observed between 400 and 600 Kelvin was in accordance with a dislocation relaxation.

The external field-induced ferromagnetic characteristics of Gd2CuO4 were investigated using ac susceptibility measurements. The results showed that a weak ferromagnetism at temperatures well above the Neel temperature was present. In addition, it was proposed that the large staggered magnetic susceptibility observed was associated with strong 2D magnetic correlations and was probably responsible for the saturation of the weak ferromagnetic component above the Neel temperature.

The magnetodielectric properties of [Fe.sub.2]OB[O.sub.3] are examined, where a magnetodielectric effect is found around 150% at room temperature. The studies have shown that for T

The nonlinear current - voltage (I-V) characteristics are studied and the voltage-dependent tunneling conductance in nanoparticles of La2/3A1/3MnO3 (A=Ca,Sr) are analyzed. It is further shown that a distribution of grain boundary conductances is a fundamental integration, and that a small fraction of poor contacts determine the transport properties.

A complete characterization of the magnetotransport and magnetoelastic properties of two classical examples of half doped manganites, Nd0.5Sr0.5MnO3 (NSMO) and La0.5Ca0.5MnO3 (LCMO), was carried out by electrical resistivity, magnetoresistance, thermoelectric power, electron spin resonance (ESR), and thermal expansion measurements. ESR results showed that this technique could be a very useful tool to investigate the multiphase microscopic state characteristic of manganites.

Magnetite nanoparticles about 10 nm sized were synthesized by the polyol method. Zero-field-cooled (ZFC)-FC measurements showed a blocking temperature ~170K and the absence of the Verwey transition. They were subsequently consolidated by spark plasma sintering at 750 °C for 15 min, leading to a high density (92% of the theoretical density), solid body, with grains in the 150 nm range. X-ray diffraction patterns exhibited a spinel single phase with cell parameters corresponding to the magnetite structure. Magnetic measurements showed a decrease of coercivity from 685 Oe (54.5 kA/m) at 118K to 90 Oe (7.2 kA/m) at 139 K. ZFC measurements at 25 Oe presented a three-fold magnetization increase as temperature increased; a small transition between 116 and 117.5 K, followed by a larger one from 117.6 to 124 K. The first transition can be associated with a complex crystallographic transition and delocalization of Fe2+-Fe3+, while the second one can be attributed to spin reorientation due to the magnetocrystalline anisotropy constant (K1) change of sign as previously observed only in magnetite single crystals. [ABSTRACT FROM AUTHOR]

Torque magnetometry is the most direct way to measure the in-plane magnetic anisotropy in magnetic amorphous ribbons. Vitrovac 6150 is an amorphous alloy which shows falling torque amplitude. The unusual behaviour of the torque amplitude is explained by the existence of orthogonal magnetic anisotropies in different volumes. The longitudinal anisotropy originates with the oriented surface roughness and directional order. The transverse anisotropy originates with only surface roughness.

From data of initial magnetization isotherms, the magnetic entropy change ΔS[sub M] of the series of ferromagnetic perovskites La[sub 2/3](Ca[sub 1-χ]Sr[sub χ])[sub l/3]MnO[sub 3], with χ=0, 0.05, 0.15, 0.25, 0.50, 0.75 and 1, has been measured near their Curie temperatures. The results go from 3.7 Jkg[sup -l]K[sup -1] for La[sub 2/3]Ca[sub l/3]MnO[sub 3] to 1.5 J kg[sup -1] K[sup -1] for La[sub 2/3]Sr[sub l/3]MnO[sub 3]. Nevertheless, the evolution of ΔS[sub M] with χ is not monotonic, it shows a steep decrease between χ = 0.05 and χ = 0.15. This point corresponds to a tolerance factor t=0.92, at which the system changes from orthorhombic (Pbnm) to rhombohedral (R&3macr;c) structure, and the magnetic phase transition from first to second order. Provided that rhombohedral symmetry forbids static long-range cooperative Jahn-Teller distortions, present in orthorhombic samples, we interpret our results as the strong increase of lattice effects in them. The anomalous thermal expansion at the Curie temperature, due to these lattice effects, is thought to underlie the jump in ΔS[sub M] with χ. [ABSTRACT FROM AUTHOR]

The weak ferromagnetic transition of T[sup ′]-phase Gd[sub 2]CuO[sub 4] single crystals has been studied by dc-magnetization measurements. Weak ferromagnetism is generated in this material by a very slight canting of the Cu moments from a perfect antiferromagnetic structure. This canting enables the inspection of the magnetic state of the Cu sublattice by pure magnetic methods. The critical exponents of the transition are calculated, γ=1.32(2), β=0.34(1). Nevertheless, a rapid evolution of β to a 2D XY behavior is observed below the critical temperature. It is interpreted as evidence of a transition in the 2D system, namely CuO[sub 2] planes, that behave as 3D at the critical point due to the effect of the interlayer interaction along the c-axis. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on a dynamical magnetic susceptibility study on La1-xSrxCoO3 perovskite samples obtained by a coprecipitation method. Preparation of the samples; Dc magnetic measurements.

Presents a study that examined the current-voltage curves in yttrium-barium-copper-oxygen (YBaCuO) rings of fixed size but of different oxygen content. Information on the current voltage curves of liquid-nitrogen temperature of YBaCuO rings; Discussion on current-voltage curves.

Presents a study that described the synthesis of Gd[sub2-x]Ce[subx]CuO[sub4] (with X=0.00, 0.05, 0.10 and 0.15) using a sol-gel technique and water-in-oil microemulsions. Description of techniques to obtain particles of different sizes; Magnetization and ac susceptibility measurements; Discussion on the differences between the zero-field-cooling and the field-cooling magnetization curves.

Focuses on a study which observed weak ferromagnetism in gadolinium[sub 2] copper oxide[sub 4] which was induced by the external field above the Néel temperature T[sub N] alternating current measurements. Comparison of the shift of the electron-spin-resonance line of the Gadolinium ions as a function of temperature T, deltaH[sub r](T) with the static internal field H[sub t](T); Methodology of the study; Results and discussion.

Presents information on a study which examined the ferromagnetism and spin-glass-like behavior of a polycrystalline with tetragonal structure. Description of a tetragonal crystal structure; Methodology of the study; Results and discussion.