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A NEW MEASUREMENT OF THE GYROMAGNETIC RATIO FOR IRON BY A MECHANICAL METHOD
It is well known that the determination of the gyromagnetic ratio for ferromagnets has played a prominent role in the history of the development of physical knowledge. In particular, it showed that almost the entire magnetic moment of ferromagnets is associated with the spin of the electron, and not with the orbital motion of electrons around the nucleus. However, careful measurements of the gyromagnetic ratio for various ferromagnets, carried out by mechanical methods, led to values somewhat exceeding \(m/e\) and differing for different materials. At the same time, recent measurements of the gyromagnetic ratio for ferromagnets, performed with the aid of microwave techniques, led to values somewhat smaller than \(m/e\). Kittel\(^1\) indicated that these deviations from the value \(m/e\) can be explained if one assumes that some small—about 10%—part of the magnetic moment of a ferromagnet is due to the orbital moments of the atoms. Since this conclusion, fundamental for the theory of ferromagnets, requires further experimental verification, and since there are significant discrepancies among the measurement data of different authors, new careful measurements of the gyromagnetic ratio for pure iron by both mechanical and microwave methods are of undoubted interest.
Such mechanical measurements were carried out by the author of the reviewed work\(^2\) according to a scheme, to a known extent analogous to the scheme of the Einstein–de Haas experiment.
The investigated specimen of pure iron (99.94% iron), made in the form of a cylinder 1.5 cm in diameter and 22 cm long, was suspended on a unifilar suspension, forming a torsion pendulum. A 14-layer coil of fine wire was wound on the surface of the specimen, through which a direct current was passed. Torsional oscillations were imparted to the pendulum, and the change in the amplitude of the oscillations was measured as a result of the change in the angular momentum of the specimen when the direction of the current was changed (and consequently the magnetization of the specimen), which was carried out at the moment when the pendulum passed through the equilibrium position.
The gyromagnetic ratio in this case can be determined by independently measuring the change in the amplitude of the torsional pendulum oscillations and the magnitude of the magnetic moment of the specimen at the given strength of the current flowing around it. In doing so, it is essential to introduce a correction for the magnetic moment and the angular momentum of the electron current itself that magnetizes the specimen. To eliminate external interference, the measurements were carried out in a special room at a depth of 10 feet underground; the earth’s magnetic field was compensated by means of a carefully developed electromagnetic device with an accuracy up to \(10^{-5}\) oersted. The torsional oscillations were observed with the aid of an optical lever, the distance from the mirror to the scale being 52 feet. The position of the pointer was determined with an accuracy of up to \(0.1\) mm.
The magnetic moment of the specimen was measured simultaneously with the measurements of the amplitude of its oscillations.
As a result of statistical processing of numerous measurements, the author obtained for the gyromagnetic ratio in the case of pure iron the value
\[ G = (1.0278 + 0.0014)\frac{m}{e}, \]
which is in good agreement with Barnett’s earlier and less accurate measurements\(^3\):
\[ G = 1.031\frac{m}{e}. \]
R. G.
CITED LITERATURE
- C. Kittel, Phys. Rev. 76, 743 (1949).
- G. S. Skott, Phys. Rev. 82, 542 (1951).
- S. J. Barnett, Phys. Rev. 66, 224 (1944).