R. V. Coleman

Electronic structure of magnetic 3d metals: Ground state, Fermi surface and photoemission properties

We describe various electronic and magnetic properties of the ferromagnetic 3d metals Ni, Co and Fe and related metals, including ground‐state properties, Fermi surfaces and both one‐electron and many‐electron aspects of photoemission and optical absorption processes. Experimental de Haas‐Van Alphen results for the spin‐polarized Fermi surfaces and angle‐resolved photoemission results for the exchange‐split energy‐band dispersions for Ni, Co and Fe are summarized. Single‐particle energy‐band descriptions of these Fermi surfaces and band dispersions, as well as various ground‐state properties (lattice constant, cohesive energy, bulk modulus, magnetic moment, hyperfine field, etc.) are given in terms of the density‐functional theory of Hohenberg, Kohn and Sham. In general, these properties can be understood quite well within the single‐particle picture. Also discussed are troublesome questions concerning the exchange splitting, band dispersions and satellite structure of Ni. Various optical and photoemissio...

Magnetoresistance in Iron Single Crystals

Magnetoresistance in single crystals of iron has been studied in fields up to 50 kOe at temperatures from 1°–300°K. Iron whiskers 40–700 μ in diameter grown by the hydrogen reduction fo FeCl2 and having residual resistance ratios ρ300°K/p4.2°K from 200 to 2000 have been used for the experiments. In low fields the magnetoresistance shows a behavior related to the development of a net magnetization through domain growth and spin rotation. At low temperatures the predominant low‐field behavior consists of a large negative magnetoresistance which develops for both transverse and longitudinal field orientations. This effect is present at 77°K and increases by an order of magnitude at 4.2°K. Strong hysteresis effects are also observed at 4.2°K. Correlation of the low‐field magnetoresistance with the domain structure is discussed for both 〈100〉 and 〈111〉 axial whiskers. At high fields and low temperatures the magnetoresistance is in general agreement with theory and indicates that open orbits may exist for certa...

Galvanomagnetic Properties of Single‐Crystal Iron Films and Whiskers

Galvanomagnetic effects have been measured and compared in single‐crystal iron films 0.1 to 5 μ in thickness and in iron whiskers 10 to 500 μ in thickness. Measurements have been made in a temperature range 1° to 300°K and in magnetic fields to 80 kOe, although in this paper we discuss primarily the low‐field negative magnetoresistance observed at helium temperatures. The negative magnetoresistance develops when the mean free path of the electron becomes long. In the case of whiskers with RRR≈1000 the effect can represent a 90% reduction in resistance at 4.2°K, while with films having RRR≈10–50 the effect is reduced to 1%. The negative magnetoresistance in 〈100〉 axial whiskers is very sensitive to measuring current, while the negative magnetoresistance in 〈111〉 axial whiskers is not as sensitive, although both cases show substantial deviations from Ohms law. Analysis of the longitudinal and transverse data on the films indicates that the reverse galvanomagnetic effect is making a substantial contribution...

Magnetoresistance in Cobalt Whiskers

Measurements of magnetoresistance in cobalt whiskers with residual resistance ratios up to 400 have been made in the temperature range 1°–4.2°K and in magnetic fields up to 80 kOe. Field dependence curves give evidence of magnetic breakdown above 65 kOe for certain field and current orientations. Shubnikov‐deHaas oscillations in the longitudinal magnetoresistance indicate the existence of small pockets of the Fermi surface that contribute directly to the oscillatory magnetoresistance with periods of 9.3×10−7 G−1, 2.8×10−7 G−1, and 8.6×10−8 G−1, respectively. The larger period of 9.3×10−7 G−1 is in approximate agreement with the period observed by Reed and Fawcett in the transverse oscillatory magnetoresistance above 70 kOe. The temperature dependence of resistivity in the range 1°–4.2°K at H=0 is given by ρ∝αT2 with α=15×10−12 Ω cm/deg2. At 50 kOe the coefficient of the T2 coefficient increases by 33%.

Resistivity of Iron from 0.3° to 4.2°K

Measurement of the temperature dependence of resistance and magnetoresistance in single crystals of iron have been made in the temperature range 0.3° to 4.2°K. Samples with residual resistivity ratios of 1000–4000 were used. In the temperature range 1° to 4.2°K measurements have been made in applied magnetic fields up to 50 kOe. The data have been fitted by computer to expressions for the resistance and in all cases both T and T2 terms are present. In the range 1° to 4.2°K a strong linear term is present as was previously reported.1 Below 1°K the T2 term appears to become more dominant and at temperatures less than 0.8°K the data appear to fit an exponential down to the lowest points measured. The coefficients are strongly affected by applied magnetic fields up to 1200 Oe. However, for 50 kOe fields the changes are much smaller than might be expected. The data will be discussed in terms of electron‐magnon scattering, electron‐electron scattering, and other possible contributions to the temperature depende...

Magnetoresistance of Nickel and Iron Single-Crystal Films

Magnetoresistance measurements on single-crystal nickel and iron films have been made at temperatures of 4.2 to 295 K in fields of up to 50 kOe. The films were prepared by vacuum deposition onto heated rock salt and mica substrates producing films with (100) and (111) planes respectively, parallel to the film surface. The films varied in thickness from 500 A to 5 μ and had resistivity ratios, ρ295/ρ4.2, varying from 9 to 55. Transverse magnetoresistance curves taken at 295 K show crystalline anisotropies similar to those observed in bulk samples. At lower temperatures, the normal positive magnetoresistance is obtained, but anisotropies associated with the Fermi surface topology are not as marked as with bulk samples, due to the lower electron mean free paths of the electrons in film specimens. Both longitudinal and transverse magnetoresistance hysteresis curves were obtained for (100) nickel and iron films. An increased coercive force is obtained as the temperature is reduced for both nickel and iron films.

Anomalous Tunneling in Normal Metal Junctions using Ferromagnetic Films

Measurements of tunnel current flow through normal metal junctions made with the ferromagnetic metals Fe, Ni, and Co have been carried out in the range 1°–300°K. The insulating layer has been an oxide of the above metals or aluminum oxide and the second metal either aluminum or silver. In the case of Fe‐FeOx‐Al a giant zero‐bias anomaly corresponding to a resistance maximum has been observed at helium temperatures and this persists up to room temperature. Co‐CoOx‐Al also shows a similar but smaller anomaly while Ni‐NiOx‐Al shows an anomalous behavior of reverse sign. Data on the dynamic resistance as a function of voltage and temperature are presented in this paper. The effects of magnetic fields up to 50 kOe have also been studied.