Ferromagnetism and crystal lattice stability of bcc and fcc iron

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Measurements have been made of relative marker movements in Au and Cu in the presence of temperature gradients of the order of 1200°K/cm. These experiments yielded results which indicate that a net vacancy current is established in these metals under appropriate experimental conditions. The magnitude and direction of the observed effects are consistent with kinetic theory predictions in conjunction with previously determined vacancy energies. A three-dimensional extension of existing kinetic theory is developed and important factors which do not appear in one-dimensional treatments are discussed. Porosity development in Cu was found under certain conditions and this may be a visual demonstration of the existence of a thermal diffusion effect in Cu.

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The existence of magnetic transitions in alloys of Fe in Au and Cu has been shown by using the Mössbauer effect. There is a significant difference in the internal-field distribution (and hence alignment of the atomic spins) between the Cu and the Au alloys. In the former a continuous distribution exists, whereas, in the latter a unique (or very nearly unique) internal field occurs in the dilute alloys. The results for the Cu alloys are consistent with an indirect interaction of the Ruderman-Kittel-Yosida type of spins localized at the iron atoms. The internal-field distribution appears to develop a minimum at zero field and a rather broad maximum which shifts gradually to higher fields with decreasing temperature. The nearly unique internal field in the dilute Au-Fe alloys seems to exclude an explanation on the basis of a Ruderman-Kittel-Yosida exchange interaction. A spiral static spin density wave as the mechanism for antiferromagnetically ordering the localized spins (or ferromagnetic order for the case in which the spin density wave vector is zero) seems possible. More concentrated Au-Fe (>16 at. % Fe) alloys show a much more rapid increase in the magnetic transition temperature than the more dilute alloys. Their behavior is ferromagnetic and can be described quite well with a nearest-neighbor interaction using the average coordination number method suggested by Sato, Arrott, and Kikuchi. The appropriate nearest-neighbor exchange interaction energy is J≃2.9×10−2 eV.

The stored energy release in copper has been measured in the temperature range 20°-60°K following irradiation with 1.2-Mev electrons. A differential temperature measurement was made between an irradiated specimen and an unirradiated standard. The specimens were immersed in liquid helium during irradiation; subsequent heating of the specimen was carried out in vacuum. A value of the total energy release of 2.5X 10~2 cal/g was observed for an integrated flux of 9X10 17 e/cm 2 . The stored energy-resistivity ratio obtained is (5.4±0.8) cal/g per micro-ohm-cm. The energy associated with a Frenkel pair is calculated to be (5.4±0.8) ev for a value of 3.6 micro-ohm-cm per atomic percent Frenkel defects.

Copper wires were cold worked at room temperature to approximately a 15% reduction in area and were then irradiated at temperatures between 100° and 150°C with 1.25-Mev electrons. The residual resistivity was observed to decrease as a function of exposure at temperatures above 100°C. The higher the temperature at which the irradiation was performed, the greater was the rate of resistivity decrease. From these data, it is concluded that one of the primary defects produced by electron irradiation becomes mobile in the temperature range, 100°–150°C. It is suggested that interstitials and vacancies produced by the irradiation initiate a process which results in the annihilation of dislocations. From an analysis of the temperature dependence of the rate of decrease, a value for the activation energy for vacancy migration in copper has been deduced: 1.28±0.10 ev.

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