Mark Rubinstein

Two-component model of polaronic transport

A phenomenological two-component polaron model, originally formulated to describe the temperature-dependent transport properties of La2/3Ca1/3MnO3(LCMO), has been applied to treat the resistivity and Hall effect of EuO and ZrTe5.

Investigation of the insulating phase of magnetite by NMR and the Mössbauer effect

Abstract New low temperature NMR and Mossbauer effect data are presented and are interpreted as further evidence against the validity of the Verwey model of magnetite.

Ferromagnetic-resonance field shift in an exchange-biased CoO/Ni80Fe20 bilayer

The temperature dependence of the ferromagnetic resonance field shift has been investigated in a CoO/Permalloy bilayer between room temperature and 4 K. The field shift is found to consist of two components: a unidirectional exchange-bias field shift and a uniaxial anisotropic field shift. These field shifts are shown to be equal to the conventional exchange bias and anisotropy fields, provided the rigid spin rotation hypothesis is correct. We have also measured the temperature variation of the exchange bias and the magnetic anisotropy fields from hysteresis loops obtained from SQUID-magnetometry, and found incomplete agreement with the parameters obtained from the ferromagnetic resonance spectra. Analogies with spin-glass phenomena indicate that this incomplete agreement results from the breakdown of the rigid rotation model (or equivalently, the surface anisotropy model) results due to the large rotation angles encountered in a magnetic moment reversal. Notwithstanding, we show that the results of rotating the sample by 180° in the plane of the film yields changes in the resonance field shift which are in agreement with hysteresis loop parameters. A connection between results observed in exchange bias materials and those in spin-glass materials is suggested.

Electron spin resonance study of NiO antiferromagnetic nanoparticles

The electron spin resonance (ESR) spectra of antiferromagnetic nanoparticle NiO specimens have been investigated as a function of temperature at x-band (microwave) frequencies. Below the nominal Neel temperature, the x-band resonances arising from the bulk antiferromagnets, including NiO particles with diameters greater than 100 A, all vanish due to the emergence of large molecular exchange fields. The ESR resonance signals of 60 A antiferromagnetic nanoparticles, however, persist to the lowest temperatures. These nanoparticle resonance lines shift to lower fields rapidly as the temperature is decreased, while the lineshapes broaden and distort.

Magnetic properties of electrodeposited copper-nickel composition-modulated alloys

Abstract Pulsed potentiostatic deposition offers a new technology for the production and stabilization of composition-modulated alloy (CMA) structures. Using this new technology, a series of Cu-Ni-modulated structures with layer thickness in the to 6 nm range were prepared. The first transmission electron micrographs of electrodeposited CMA are presented, showing the existence of sharp interfaces. Magnetic properties of the CMA structures were measured by vibrating sample and SQUID magnetometers, and by ferromagnetic resonance.

FMR of powder La0.7Ca0.3MnO3

We report the observation of ferromagnetic antiresonance and resonance in powder La0.7Ca0.3MnO3 at microwave frequencies of 22 GHz, 32 GHz and 36 GHz in the temperature range 77K–350 K. Using different grain sizes we are able to place an upper limit on the resistivity of the grain and estimate the grain resistivity ϱgrain ≤ 1 m Ω cm in the neighbourhood of the peak resistivity where the reported bulk resistivity values are ϱbulk ≈20 m Ω cm − 50 m Ω cm. This indicates that the grains appear to be atleast an order of magnitude more conducting than the bulk.

Magnetization and magnetic aftereffect in textured Ni/Cu compositionally-modulated alloys

Abstract The magnetic properties of Ni/Cu compositionally-modulated alloys with [100], [110] and [111] textures were measured by magnetometry and ferromagnetic resonance. These alloys were found to exhibit a pronounced magnetic aftereffect.

Anomalous magnetoconductivity of epitaxial Nd0.7Sr0.3MnO3 and Pr0.7Sr0.3MnO3 films

Abstract In Nd 0.7 Sr 0.3 MnO 3 films, ranging in peak resistivity from 1000 Ω·cm to ≤1 Ω·cm, and exhibiting large magnetoresistance effects, the zero-field resistivity above the peak temperature T p exhibits thermally activated behavior with an activation energy of 0.115±0.005 eV. At T well below T p , the magnetoconductivity, rather than the magnetoresistance, is linear in B and this linear dependence is observed over a wide B range even when the magnetization is essentially saturated. A cross-over from the linear relation to σ increasing as B 2 is observed near and above T p . A tentative model is proposed.

Ferromagnetic resonance in nanocrystalline Fe73.5CuNb3Si13.5B9 (Finemet)

Abstract The ferromagnetic resonance linewidth of Fe 73.5 CuNb 3 Si 13.5 B 9 melt-spun ribbons has been investigated as a function of annealing temperature. Fe 73.5 CuNb 3 Si 13.5 B 9 possesses an ultrafine grain structure which can be altered by suitable annealing to exhibit a combination of excellent soft magnetic characteristics and high saturation induction. It is a ferromagnetic metal consisting of crystallites whose anisotropy axes are randomly oriented and which may interact with each other via exchange or dipolar fields. When annealed at moderate temperatures, Fe 73.5 CuNb 3 Si 13.5 B 9 is characterized by an extremely low coercive field, and is useful in magnetic cores and memories. At a critical temperature (∼600°C), the grain size rapidly increases with annealing temperature. This onset of crystallization is accompanied by a relatively abrupt increase in the FMR linewidth, the magnetic anisotropy field, and the coercive field. We extend the scaling arguments of Herzer to explain these obviously related phenomena.

Normal-state resistivity of the high- T c compound Y 1 Ba 2 Cu 3 O 7 − δ

The resistivity of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-//sub delta/ has been measured in an oxygen atmosphere from 90 to 800 K. Above 300 K, the slope of the resistivity versus temperature curve increases from its initial constant value. Around 625 K an anomaly occurs and resistivity remains essentially constant with increasing temperature and becomes hysteretic during cooling. We correlate the data above 500 K with the gain and loss of oxygen observed by thermogravimetric analysis. Such an explanation is consistent with recent density-of-states calculations.