Microwave magnetic-envelope dark solitons in yttrium iron garnet thin films

Abstract

Dark soiitons of magnetostatic surface waves in magnetic films have been observed for the first time. The experiments were conducted at 5.19 GHz on 7.2 /im single-crystal yttrium iron garnet films. The dark soiitons were excited by 15 ns wide **off" pulses in a high power cw microwave signal applied to the film. The characteristic soliton narrowing effect in the output pulses was observed as the input power was increased above the 0.5-1 W threshold levels. The shape of the ''dark" pulse agrees with the |tanh| functional dependence predicted from theory. Direct measurements of the carrier signal showed a phase shift of close to 180° at the center of the dark soliton, also in agreement with theory.PACS numbers: 75.30.Ds, 76.50.+g, 85.70.Ge Envelope soiitons are nonlinear wave packets which preserve their shape without dispersive spreading. In recent years, soliton excitations have been realized in many physical systems [1-3]. One well-known example is the optical-envelope soliton in optical fiibers [4,5]. Envelope soiitons for spin waves at microwave frequencies have been observed in yttrium iron garnet (YIG) thin films for various magnetic field and propagation combinations, including forward-volume wave [6,7], surface wave [8], and backward-volume wave [9] configurations.The envelope of nonlinear spin wave packets propagating in ferromagnetic thin films has been found to be best described by the nonlinear Schrodinger (NLS) equation [10,11]. It is well known that the NLS equation has two different types of solutions which correspond to bright and dark soiitons, depending on the relative signs of the dispersion coeflficient and the nonlinearity coeflficient in the equation [12,13]. Bright soliton solutions exist when the product of these two coefficients is negative, while dark soliton solutions exist when the product is positive. All of the magnetic experiments to date have been for bright soiitons, that is, for normal propagating wave packets.This Letter reports the first observation of microwave magnetic-envelope dark soiitons and the first experimental verification of the 180° phase shift in the carrier signal at the center of the pulse for any category of NLS dark soliton. The experiments were done at 5.19 GHz on in-plane magnetized single-crystal YIG films. Both the output signal envelope and the actual carrier signal output were measured. For input power levels above a certain threshold in the range of 0.5-1 W, the output signal pulses showed a narrowing which is characteristic of dark soiitons and the carrier showed at 180° phase shift over the central minimum region, also characteristic of dark soiitons. These results are in quantitative agreement with the theoretical dark soliton solutions for the NLS equation [13].The propagating spin waves were excited in the magnetic thin film by applying cw microwave power to a magnetostatic wave (MSW) delay line structure using a microstrip transducer [14]. The width of the dark pulse was controlled by chopping the cw signal with a fast microwave switch. The cw si...

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