THEORY FOR THE 1ST-ORDER SPIN-WAVE INSTABILITY THRESHOLD IN FERROMAGNETIC INSULATING THIN-FILMS

A theory for the first-order Suhl and the parallel pumping instability in thin films is presented. Significant differences for the critical microwave field and wave vector to former calculations occur, which di scuss the problem in terms of bulk spin-waves neglecting boundary cond itions. A coupling matrix C(kk') is introduced, which describes the co uplings between the modes and the driving microwave field. For bulk st anding spin-waves C(kk') is always diagonal. For the true discrete sta nding modes of a thin film C(kk') changes only in case of 1. Suhl inst ability and if the wavevector has a non vanishing component perpendicu lar to the film plane. Here the diagonal bulk couplings have to be rep laced in part by off diagonal terms, describing couplings between mode s, which perpendicular wave vector component k(perpendicular-to) diffe rs by pi/d (d = film thickness). The decisive quantity, which decides if the finite thickness of the film is of importance or if the film ca n be treated as a bulk system, is the frequency difference deltaomega( k) of the coupled modes. For deltaomega(k) much smaller than the spin- wave damping eta(k) the bulk approximation is correct. For deltaomega( k) much greater than eta(k) two experimental situations for 1. Suhl in stability are discussed: For a perpendicular to the film plane magneti zed film the critical microwave field is by pi/2 bigger than in the bu lk case. In an in-plane magnetized film the critical spin-waves propag ate always in the film plane, as only here C(kk') remains identical to the bulk case.