S. Tyagi

Temperature and field dependence of microwave losses in manganite powders

We report systematic measurements of the temperature and field dependence of microwave losses in micron-size powders (∼3 μm) of the manganites Nd0.7Sr0.3MnO3, La0.7Ca0.3MnO3, La0.8Ba0.2MnO3, and La1.4Ca1.6Mn2O7. We fully confirm previous findings of a large increase in zero-field absorption, in La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3, accompanying the onset of ferromagnetism and a concomitant colossal low-field magnetoimpedance (up to 80% at 600 Oe at room temperature). An oversimplified set of computations renders plausible support for interpreting these observations as arising from absorption in the spin system. Possible applications of these novel phenomena are also described.

Electron spin resonance measurements in La1−xSrxMnO3

Abstract We present here the results of an investigation into the electron spin resonance in single crystals of La1 − xSrxMnO3 with x = 0.1, 0.2, 0.3. It turns out that the linewidth (ΔH) and the intensity (I) can be expressed as ΔH = ΔH(θ) + b(T − θ) and I(T) ∝ (T − θ)−1, respectively, where θ is the appropriate Curie-Weiss constant. Various models are discussed.

Ferromagnetic resonance and intrinsic properties of La0.67Ba0.33MnOz

Systematic magnetization and ferromagnetic resonance (FMR) measurements have been performed on thin‐film samples of La0.67Ba0.33MnOz (TC∼340 K). It is found from the FMR data that as‐ grown films show broad lines whose width Γ increases to nearly 1 kOe as T reduced to 77 K. The increase roughly follows the increase in the magnetization suggesting magnetic inhomogeneity as the main cause for the rise in Γ. However, when films were properly annealed in O2, we obtained narrow lines (Γ∼200 Oe at 10 GHz). Γ was effectively independent of temperature and linearly dependent upon microwave frequency, both of which are strong signs that, we have succeeded in obtaining a uniform, homogeneous manganite sample. Having gotten a uniform material, we have measured the intrinsic properties of this system.

Giant microwave magneto‐impedance in a single crystal of La0.7Sr0.3MnO3: The effect of ferromagnetic antiresonance

We show that at about 340 K a large (≂50%) change in the microwave absorption of a single crystal of La0.7Sr0.3MnO3 is caused by the application of a field of just a few tens of mT. This large magneto‐impedance occurs at 10 GHz but is absent at 36 GHz, and we demonstrate that these results follow from the frequency and field dependence of the dynamic permeability of a ferromagnetic conductor.

Microwave absorption and resistively shunted Josephson junctions in high temperature CuO superconductors

Abstract We report that the field dependence of the magnetoabsorption (virgin curve) in all pristine granular CuO type (HTSC) superconductors follows a one-parameter expression. This “universal” result is combined with previous measurements on field and temperature dependences of microwave absorption in HTSC to demonstrate that a simple model based on resistively shunted Josephson junctions is adequate to qualitatively account for almost all the observations on powders, pellets, thin films and single crystals.

Ferromagnetic resonance in a crystal of La0.7Sr0.3MnO3

A single crystal of La0.7Sr0.3MnO3 was grown by the floating zone method. The ferromagnetic resonance linewidth at 8.5 GHz has a minimum of 50 Oe at 340 K and increases with lowering temperature. This is indicative of some residual inhomogeneity. The anisotropy of the resonance field was studied as a function of angle in the sample plane, and we find a small uniaxial anisotropy, Han≅230 Oe at 77 K.

Interphase exchange coupling in Fe∕Sm–Co bilayers with gradient Fe thickness

©2005 American Institute of Physics. The electronic version of this article is the complete one and can be found online at: http://link.aip.org/link/?JAPIAU/98/063908/1

Fluxon dynamics and field induced microwave absorption in c-axis oriented thin films of Y1Ba2Cu3O7

Abstract We report onn the temperature and DC magnetic field dependence of microwave absorption in c-axis oriented Y1Ba2Cu3O7 thin films. Whereas the zero-field transition curves are by no means ideal, the field dependence is well represented by conventional fluxon dynamics and yields the first determination of the ratio of restoring to viscous forces.

Calculation of the critical cooling rate for amorphous Pd77.5Si16.5Cu6

The viscosity of the molten alloy Pd77.5Si16.5Cu6 has been determined from 744 to 1000 °C using an oscillating cup viscometer. Using this data (together with other pertinent physical data available for this alloy) in existing crystallization theory indicates that it could be made amorphous at cooling rates as low as 5–10 Ks−1. This analysis assumes strictly homogeneous nucleation. Containerless solidification will be one good testing ground for the results of this analysis.

Internal friction and shear modulus of certain metallic glasses

Abstract We have measured the temperature dependence of the internal friction and shear modulus of metallic glass alloys Fe 40 Ni 40 P 14 B 6 , Fe 29 Ni 49 P 14 B 6 Si 2 , Fe 80 B 20 and Fe 78 Mo 2 B 20 in the glassy and crystalline phases using a torsional pendulum technique. The internal friction in the glassy phase remains approximately constant from room temperature to a temperature which depends on the annealing treatment, and the rises exponentially with temperature. The internal friction measurements were made at two different frequencies (∼0.2 Hz and ∼1.0 Hz) and the activation energies for the relaxation processes estimated by the frequency shift method. The shear modulus increases by about 30% when the amorphous alloys are crystallized. The amorphous alloys Fe 80 B 20 and Fe 78 Mo 2 B 20 exhibit Invar type behaviour which disappears upon crystallization.