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We investigate the high-frequency dynamics of dysprosium and cobalt gratings fabricated at the surface of a GaAs/Al0.33Ga0.67As heterojunction. We detect the collective and localized spin-wave modes of the grating by measuring the photovoltage and the photoresistance induced in the two-dimensional electron gas (2DEG). The magnetic excitations couple to the 2DEG through their stray magnetic field. We perform a spectroscopy of dipolar-exchange spin waves as a function of microwave power, temperature, the tilt angle of the applied magnetic field, and by varying the structural and material parameters to change the strength of dipolar interactions. The data reveal two types of spin waves. Dipolar magnetization waves propagate across the grating through the magnetostatic interaction between the stripes. We derive an analytical expression of their dispersion curve and obtain a good fit of the ferromagnetic resonance broadening from first principles. The second type is dipolar edge spin waves which manifest through a series of sharp resonances at lower magnetic field. These waves are confined near the pole surfaces and interact very little with neighboring stripes. We calculate the eigenfrequencies of the quantized modes and obtain a qualitative explanation of the low-field resonances. The fit yields a value of the exchange stiffness constant of dysprosium, A=1.5 x 10(-12) J m(-1). Our experiments show that photovoltage measurements in hybrid semiconductor-ferromagnetic structures provide a sensitive and noninvasive tool for probing the spin waves of small magnets (10-500 nm).
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