Helicon relaxation of electron spins

Volume 26A. number

PHYSICS

5

HELICON

Lawrence

Radiation

RELAXATION

LETTERS

OF

21 December

It is sho\\n that relaxation of electron spins by thermally process at 101~ temperatures in the lighter metals

Relaxation of nuclear spins by helicons has been observed when a radio frequency field is applied at the nuclear Larmor frequency [ 11. This letter points out that at low temperatures electron spin relaxation in metals may proceed by interaction with the rotating magnetic field of thermally excited helicons. This mechanism produces a relation TIT = con&. We use the long-wavelength dispersion relation for helicons =

c2wcq2cos e/w2

(1)

P and q is the helicon wave vecwhere 0 = qz/lq/ tor. The relaxation time due to random magnetic fields in the solid may be written in terms of their spectral density S(wL) at the electron Larmor frequency, WL:

& = &,,(WL) where y is the electron have

ratio.

-=1 Tl

5

($)$

(T>”

Livermore,

*

California,

USA

1967

excited

helicons

competes

a-ith the phonon

sume AWL >> kT. Present work [2,3] in sodium gives Tl x 10-S set for samples with low impurity concentration, in the region T < lOoK. Yafet’s mechanism [4] (relaxation by electronphonon scattering accompanied by a spin flip) adequately accounts for this data. [Eq. (4) gives much longer times for sodium.] This process is insensitive to II, so it should give T1 N 10S5 in lithium as well. In contrast, note that eq. (4) gives Tl xn#. In lithi urn we find TlT = 1.5 x x 10-5 sec°K for B = 300 G. Thus we would expect the helicon and phonon processes to compete in the lightest metals. The electron-phonon scattering mechanism produces a rise in Tl faster than l/T as T falls below - 30°K, so the two processes may be distinguished.

We References

S(WL) = (2V) -‘~(a,ct)tiw,~[6(W-W,)+6(WW~)] n, is the (Bose-Einstein) occupation helicons. Eqs. (l), (2) and (3) yield

SPINS

1968

The author would like to thank G. Dunifer and J. Benford for helpful conversations.

+$JwL)l

gyromagnetic

ELECTRON

G. BENFORD Unir~ersity of California,

Laboratory.

Received

wq

29 January

(3) number for

[l - (w,7)-f]

1. B.Sapoval, Phys. Rev. Letters 17 (1966) 241. 2. F. Vescial, N. S.Vander Ven and R. T. Schumacher, Phys. Rev. 134 (1964) A1286. 3. G.Dunifer and S.Schultz, to be published. 4. Y.Yafet. Solid State Physics 14 (1963) 1.

(4)

WC

where T is the relaxation

time (wc7 > 1). We as-

* Work performed under the auspices Atomic Energy Commission.

of the U.S.

*****

199