A d etailed in v estig atio n is m ad e o f th e coefficient C in th e en erg y CK2 of a sp in w ave of sm all w ave v ecto r K in a ferro m ag n etic cubic m etal. I t is assum ed t h a t th e spins of all particles, w ith in th e energy b a n d sy stem considered, are alin ed in th e g ro u n d sta te . A n e x a c t ex p re s sion is o b tain ed for C on th e basis of one plausible assu m p tio n , t h a t th e inverse lifetim e o f a spin w ave te n d s to zero fa ste r th a n K 2 as K ->

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... T his expression is ex am in ed in d etail for a one-band m odel w ith sh o rt-ran g e in ter-p a rtic le in tera c tio n s a n d it consists of th re e term s. T he first tw o te rm s are closely re la te d to th e well know n r.p .a . form ula for G alth o u g h th e second te rm is m odified in a n im p o rta n t w ay. I n p a rtic u la r th is m odification is show n to ta k e acco u n t of tw o-p artic le correlations of th e ty p e considered b y K an am o ri, b u t a d a p te d to th e ferrom agnetic situ a tio n , a n d of m agnon-electron scatterin g . T he th ird te rm in C is com pletely new b u t is g enerally e x p ected to be sm aller th a n th e second term . I n a n A ppendix co n ta c t is m ade w ith N ag a o k a 's w ork on spin w ave energies in a n e a rly half-filled ban d . I n t r o d u c t i o n [ 373 ] 24-2