Magnetoresistance in Iron and Cobalt to 150 kOe

Abstract

Magnetoresistance anisotropy and field dependence have been studied in single crystals of iron and cobalt in fields up to 150 kOe in the temperature range 1-4.2 K. The crystals of iron and cobalt had residual resistance ratios up to $\ensuremath{\sim}4600$ and $\ensuremath{\sim}400$, respectively. The relation $\frac{\ensuremath{\Delta}\ensuremath{\rho}}{{\ensuremath{\rho}}_{0}}=a{B}^{n}$ has been studied for both metals and in the case of iron the exponent $n$ approaches values in the range 1.8-1.9 for fields up to 90 kOe and then decreases to the range 1.3-1.5 at 150 kOe. This decrease in the exponent has been observed for all field and current directions measured suggesting that extensive magnetic breakdown may occur. A preliminary measurement on iron to 215 kOe shows $n$ decreasing to less than 1 for some field directions. Cobalt shows saturation behavior for most field and current directions but values of $n>1$ are observed for certain specific directions. The magnetoresistance rotation curves for iron exhibit sharp minima consistent with narrow bands of open orbits in the 100 > and 110 > directions. Minima are also observed which indicate open orbits in the 310 >, 410 >, and 530 > directions. The behavior of cobalt suggests open orbits parallel to the basal plane and possibly along the $c$ axis. Both metals show Shubnikov-de Haas oscillations which appear to be related to small pockets of the Fermi surface corresponding to ${10}^{\ensuremath{-}3}$-${10}^{\ensuremath{-}4}$ electrons per atom. In iron oscillations with frequencies in the range 1-6.2 MG have been observed while in cobalt three oscillations with frequencies of 1.07, 3.57, and 11.63 MG have been observed. The low-frequency oscillation in cobalt has been studied as a function of angle and indicates that two distinct branches exist as the field is rotated away from the $c$ axis. Oscillations are observed on only one branch at a time and the behavior suggests the possible presence of magnetic breakdown. The data on both iron and cobalt have been compared to models of the Fermi-surface topology in these metals.