arXiv subjects

A. F. Hebard

Publications and source records attributed to A. F. Hebard.

26 records · Page 2Linked to original sources

arXiv2008

Ultra-Thin Silver Films obtained by Sequential Quench-Anneal Processing

We have used the two-step growth technique, quench condensing followed by an anneal, to grow ultra thin films of silver on glass substrates. As has been seen with semiconductor substrates this process produces a metastable homogeneous covering of silver. By measuring the in situ resistance of the film during growth we are able to see that the low temperature growth onto substrates held at 100 Kelvin produces a precursor phase that is insulating until the film has been annealed. The transformation of the precursor phase into the final, metallic silver film occurs at a characteristic temperature near 150K where the sample reconstructs. This reconstruction is accompanied by a decrease in resistance of up to 10 orders of magnitude.

cond-mat.mtrl-sci↗

arXiv2007

Weak localization correction to the anomalous Hall effect in polycrystalline Fe films

In situ transport measurements have been made on ultrathin ($<$100 Å thick) polycrystalline Fe films as a function of temperature and magnetic field for a wide range of disorder strengths. For sheet resistances $R_{xx}$ less than $\sim 3kΩ$, we find a logarithmic temperature dependence of the anomalous Hall conductivity $σ_{xy}$ which is shown for the first time to be due to a universal scale dependent weak localization correction within the skew scattering model. For higher sheet resistance, granularity becomes important and the break down of universal behavior becomes manifest as the prefactors to $σ_{xx}$ and $σ_{xy}$ decrease at different rates with increasing disorder.

cond-mat.dis-nn↗

arXiv2002

Magnetocapacitance: A probe of spin-dependent potentials

The magnetic field dependence of the capacitance of Pd-AlOx-Al thin-film structures has been measured. The observed quadratic dependence of capacitance on magnetic field is consistent with a theoretical model that includes the effect of a spin-dependent electrochemical potential on electron screening in the paramagnetic Pd. This spin-dependent electrochemical potential is related to the Zeeman splitting of the narrow d-bands in Pd. The quantitative details depend on the electronic band structure at the surface of Pd.

cond-mat.mtrl-sci↗

arXiv2002

Coexistence of glassy antiferromagnetism and giant magnetoresistance (GMR) in Fe/Cr multilayer structures

Using temperature-dependent magnetoresistance and magnetization measurements on Fe/Cr multilayers that exhibit pronounced giant magnetoresistance (GMR), we have found evidence for the presence of a glassy antiferromagnetic (GAF) phase. This phase reflects the influence of interlayer exchange coupling (IEC) at low temperature (T < 140K) and is characterized by a field-independent glassy transition temperature, Tg, together with irreversible behavior having logarithmic time dependence below a "de Almeida and Thouless" (AT) critical field line. At room temperature, where the GMR effect is still robust, IEC plays only a minor role, and it is the random potential variations acting on the magnetic domains that are responsible for the antiparallel interlayer domain alignment.

cond-mat.mtrl-sci↗

arXiv2002

Unconventional carrier-mediated ferromagnetism above room temperature in ion-implanted (Ga, Mn)P:C

Ion implantation of Mn ions into hole-doped GaP has been used to induce ferromagnetic behavior above room temperature for optimized Mn concentrations near 3 at.%. The magnetism is suppressed when the Mn dose is increased or decreased away from the 3 at.% value, or when n-type GaP substrates are used. At low temperatures the saturated moment is on the order of one Bohr magneton, and the spin wave stiffness inferred from the Bloch-law T^3/2 dependence of the magnetization provides an estimate Tc = 385K of the Curie temperature that exceeds the experimental value, Tc = 270K. The presence of ferromagnetic clusters and hysteresis to temperatures of at least 330K is attributed to disorder and proximity to a metal-insulating transition.

cond-mat.mtrl-sci↗

arXiv2001

Spin-dependent electrical transport in ion-beam sputter deposited Fe-Cr multilayers

The temperature dependence of the electrical resistivity and magnetoresistance of Xe-ion beam sputtered Fe-Cr multilayers has been investigated. The electrical resistivity between 5 and 300 K in the fully ferromagnetic state, obtained by applying a field beyond the saturation field (H_sat) necessary for the antiferromagnetic(AF)-ferromagnetic(FM) field-induced transition, shows evidence of spin-disorder resistivity as in crystalline Fe and an s-d scattering contribution (as in 3d metals and alloys). The sublattice magnetization m(T) in these multilayers has been calculated in terms of the planar and interlayer exchange energies. The additional spin-dependent scattering Δρ(T) = ρ(T,H=0)_AF - ρ(T,H=H_sat)_FM in the AF state over a wide range of temperature is found to be proportional to the sublattice magnetization, both Δρ(T) and m(T) reducing along with the antiferromagnetic fraction. At intermediate fields, the spin-dependent part of the electrical resistivity (ρ_s (T)) fits well to the power law ρ_s (T) = b - cT^αwhere c is a constant and b and αare functions of H. At low fields α\approx 2 and the intercept b decreases with H much the same way as the decrease of Δρ(T) with T. A phase diagram (T vs. H_sat) is obtained for the field- induced AF to FM transition. Comparisons are made between the present investigation and similar studies using dc magnetron sputtered and molecular beam epitaxy (MBE) grown Fe-Cr multilayers.

cond-mat.mtrl-sci↗

arXiv2001

Ordered low-temperature structure in K4C60 detected by infrared spectroscopy

Infrared spectra of a K4C60 single-phase thin film have been measured between room temperature and 20 K. At low temperatures, the two high-frequency T1u modes appear as triplets, indicating a static D2h crystal-field stabilized Jahn-Teller distortion of the (C60)4- anions. The T1u(4) mode changes into the known doublet above 250 K, a pattern which could have three origins: a dynamic Jahn-Teller effect, static disorder between "staggered" anions, or a phase transition from an orientationally-ordered phase to one where molecular motion is significant.

cond-mat.mtrl-sci↗

arXiv2000

Unambiguous determination of the g-factor for holes in bismuth at high B/T

Magnetotransport has been investigated in high purity bismuth crystals in static magnetic fields as high as 20 T and temperatures as low as 25 mK. This high B/T ratio permits observation of pronounced Shubnikov-de Haas oscillations over a wide field range and up to fields where most of the carriers are in the lowest Landau level. For transport currents in the bisectrix or binary directions and field in the perpendicular trigonal direction, we have observed doublet splittings centered on each Shubnikov-de Haas oscillation. These splittings exhibit a quadratic dependence on field and disappear before the last oscillation. Our observations allow us to conclude unambiguously that already when the Landau level index is as high as 2 the carriers are fully polarized and that the g-factor for holes with the field in the trigonal direction is 35.3(4).

cond-mat↗