The origin of the effective magnetic fields at the nuclei of magnetic materials which have been determined by M\"ossbauer, nuclear magnetic resonance, electron paramagnetic resonance, specific heat, and nuclear polarization methods is investigated theoretically by means of the exchange polarization mechanism. Exchange-polarized iron series Hartree-Fock calculations were carried out for (a) free ions and neutral atoms, (b) ions in a (crude) crystalline field (as in a salt), and (c) spin densities and configurations which conform with energy band and neutron magnetic scattering observations for the ferromagnetic metals. The effective field data for metals, ferrites, rare-earth garnets, and salts are then discussed and it is shown that the dominant contribution to the effective field (in almost every case) arises from the (exchange) polarization of the core electrons by the spin density of the unpaired outer electrons. For the transition metals, the role of the conduction electrons is analyzed including some new contributions not previously considered. The data for ions like ${\mathrm{Fe}}^{3+}$ and ${\mathrm{Mn}}^{++}$ may be understood mainly on the basis of the core polarization term but such factors as covalent bonding, charge transfer, crystal field effects (such as distortions from cubic symmetry) must also be included. For ions like ${\mathrm{Fe}}^{++}$ and ${\mathrm{Co}}^{++}$ the (large) field due to unquenched orbital angular momentum must also be considered and several cases in which the orbital field dominates are discussed. The exchange polarization method and the accuracy of the analytic spin-polarized Hartree-Fock functions are discussed with regard to the sensitivity of the internal field to orbital descriptions, the effect of crystalline environments, and to expansion and contraction of the spin density. Each factor is investigated in detail by means of accurate exchange-polarized calculations. In conjunction with these studies a restricted Hartree-Fock calculation for ${\mathrm{Mn}}^{++}$ was carried out (and is reported as an Appendix) which is more accurate than existing calculations and indicates the accuracy of earlier analytic Hartree-Fock calculations.

Origin of effective fields in magnetic materials

The recoilless emission and resonant absorption of low energy γ-rays by solids (Mossbauer effect) are reviewed, with particular reference to the present and future applications to solid state and nuclear physics.

http://iopscience.iop.org/article/10.1088/0034-4885/25/1/311/pdf

The mössbauer effect

The observation of nuclear magnetic resonance in ferromagnetic cobalt is reported. The resonance frequency for finely divided face-centered-cubic material has been measured in the intermediate temperature range. The frequency extrapolated to 0°K is 217.2 Mc/sec, and the temperature dependence in this range is in general agreement with that of the magnetization. This frequency implies a hyperfine field of 217 500 oe, which is in good agreement with the field deduced from specific heat measurements on hexagonal cobalt. The agreement in the two structures indicates that there is no dipolar contribution to the hyperfine field. The theoretical implications of this observation are discussed. The resonance line is inhomogeneously broadened with a half width of 400 kc/sec. A pattern of beats is observed at high passage rates which makes it possible to determine a spin-spin time of 25 μsec. By varying the modulation frequency under conditions of intermediate saturation the spin lattice relaxation time is measured ...

Nuclear Resonance in Ferromagnetic Cobalt

All data available in the literature for the impurity hyperfine fields in Fe, Co, Ni and Gd matrices are collected and tabulated. The literature search was carried out for all references listed in Physics Abstracts on papers published till the end of December 1978. The present work is an extension of similar earlier compilations [1–6]. The data are arranged in ten columns as explained below:

Table of hyperfine fields for impurities in Fe, Co, Ni, Gd and Cr

https://escholarship.org/content/qt2sz3k3hk/qt2sz3k3hk.pdf?t=p0t19s

RECOIL-FREE RESONANT ABSORPTION IN Au$sup 19$$sup 7$

A strong resonance was detected at room temperature in a sample of finely divided face-centered-cubic cobalt (Co/sup 59/) metal. The resonance frequency was measured to be 213.1 Mc/sec in zero applied field. The observed frequency indicates a magnetic field of 213,400 oersteds at the site of the cobalt nuclei. This very sizeable magnetic field arises from both core and conduction electron contributions. (auth)

A. M. Portis

Papers

Observation of Nuclear Resonance in a Ferromagnet