Magnetic resonance at 20 GHz of an amorphous metal

MAGNETIC R E S O N A N C E AT 20 GHz OF AN A M O R P H O U S M E T A L A. M. S T E W A R T School of Physics, University of New South Wales, Sydney, Australia

Electron spin resonance measurements on Metglas 2826A show that the linewidth above the Curie point has a linear dependence on temperature. This can be interpreted by m e a n s of a localised m o m e n t model.

There is currently much interest in the magnetic properties of amorphous metals due to their many possible applications [1] and also to the desire to gain an understanding of the nature of the amorphous state itself [2]. In this paper I report the results of electron spin resonance experiments at 20 G H z on the amorphous metal Metglas 2826A, whose composition is Fe32Ni36CrlaP]2B 6. This material is ferromagnetic with a Curie temperature of 254 K [3]. Measurements at 10 G H z have been carried out by Bhagat et al. [4]. The purpose of the present work was to investigate the frequency dependence of the resonance parameters. The measurements were made with a conventional homemade reflection K-band spectrometer on samples of Metglas alloy provided by the manufacturer (The Allied Chemical Company). The specimens were of approximate size 2 x 2 × 0.05 mm3; they were attached to the wall of the microwave cavity with grease, with the glossy side outwards, and oriented so that both the steady and microwave fields lay within the plane of the specimen. No precautions were taken to shield [4] the edges of the specimen from the microwave field. The absorption derivative line was of asymmetric shape due to the skin effect in metals. The A / B ratio, as defined in fig. 1, is plotted against temperature in that figure for all the readings taken. This ratio was found to depend on temperature, on magnet angle with respect to the plane of the specimen, on spectrometer tuning and on other unidentified factors. It is seen that there is a tendency for it to increase as the temperature increases. The lineshape in the paramagnetic regime could be accurately computer fitted to a Lorentzian. In the ferromagnetic regime the line was more Gaussian, with one extra broader low field resonance appearing, due, no doubt, to the lack of microwave shielding of the edges of the specimen where demagnetising fields would be likely to be varying

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strongly. However, by assuming that the lineshape was an arbitrary mixture of Lorentzian absorption and dispersion curves a unique value of half-power half-width DH and resonant applied field B 0 could be associated with each A / B ratio. The values of D H and geff, given by geff = h°~/I-taBo, are shown in figs. 1 and 2. To obtain the intrinsic g factor of the material we use the relation [5] (ho~)2 -- (g/~B)Z[Bo + (Ny - Nx)/LoM] X [B o + (Nx - N,)/to/], where M is the magnetisation at a field of B o [3] and the N are the demagnetising factors with B o in the z direction. The specimen is not a perfect infinite plane, and using the demagnetising factors for an oblate ellipsoid [6], I estimate that Nx = N z

Journal of Magnetism and Magnetic Materials 15-18 (1980) 1417-1418 @North Holland

1417

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A. M. Stewart/Magnetic resonance of an amorphous metal

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bottlenecked Korringa relaxation rate [7] for localised moments in metals. We shall use this theory, due to the lack of any other, even though the large electronic specific heat at low temperature indicates that Metglas 2826A is an itinerant ferromagnet [8]. For the case of extreme bottlenecking [7]

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= 0.015; Ny is obtained from N x + Ny + N z = 1. We thus get the relation g - - g~f~[1 + 0 . 9 5 5 t t o M / B o ]

where "/d is the gyromagnetic ratio of the transition metal local moments, Xd is their susceptibility, Xs is the susceptibility of a broad s-like conduction band, and TsL is the spin lattice relaxation time between this band and the lattice. The paramagnetic moment of 2826A is 3.0#B per atom [3], and if we assume the conduction band to have a density of states of 0.25 e V - l a t o m - lspin- ] (corresponding to one electron per atom and a Fermi energy of 3 eV) we find that T S L ~ 10 -~3 s. This seems a reasonable value for this quantity, being about 400 times longer than the resistivity relaxation time [8], and confirms the usefulness of a localised moment model at high temperatures.

,/2.

References In fig. 2, g is plotted against temperature. The value at 77 K is in agreement with the X band value of 2.16 [4]; however, the g factor appears to decrease to a value of 2.08 _+ 0.02 in the paramagnetic regime. The linewidth in fig. 1 is substantially greater, in the ferromagnetic region, than that seen by Bhagat et al. [41, again due to lack of shielding of the edges of the specimen. In the paramagnetic region the linewidth varies linearly with temperature with a slope of 8.5 x 10 -4 T / K . This is suggestive of a

[1] J. Appl. Phys. 49 (3) (1978). [2] A. M. Stewart, The Australian Phys. 15 (1978) 135. [3] E. Figueroa, L. Lundgren, O. Beckman and S. M. Bhagat, Solid State Commun. 20 (1976) 961. [4] S. M. Bhagat, S. Haraldson and O. Beckman, J. Phys. Chem. Solids 38 (1977) 593. [5] C. Kittel, Solid State Physics (Wiley, New York, 1971). [6] R. M. Bozorth, Ferromagnetism (Van Nostrand, Princeton, 1951). [7l H. Hasegawa, Progr. Theor. Phys. (Kyoto) 21 (1959) 483. [81 A. M. Stewart and W. A. Phillips, Phil. Mag. B 37 (1978) 561.