Microwave Magnetoresistance and Microwave Absorption in Sr₂FeMoO₆

We report the direct current (dc) magnetoresistance and microwave magnetoresistance (MWMR) as a function of frequency (f = 0.5–3 GHz), along with microwave absorption (f = 0.01–4.2 GHz) in Sr2FeMoO6 (SFMO) synthesized by a wet chemical method. Magnetization data indicates that the onset temperature of ferromagnetism in this sample is above 400 K. The resistivity of SFMO shows semiconducting behavior under dc magnetic fields of H = 0 and 30 kOe. The MWMR in this highly resistive SFMO is determined by coating a layer of silver paint on the upper surface of the sample to guide the microwave (MW) signal and measuring the magnetoimpedance of the silver paint. As H is increased from 0 to 2.2 kOe, the MWMR smoothly decreases for f < 1.2 GHz, whereas it changes sign from negative to positive and exhibits a peak at H = Hres for higher frequencies, and the peak migrates to a higher field value as f increases. The magnitude of MWMR at the peak reaches a maximum value of 17.9% for 3 GHz at a low field of H = 900 Oe, a value much larger than the room temperature dc magnetoresistance at the same field. We argue that the observed MWMR of the Ag layer reflects the spin dynamics in SFMO. The field dependence of MW power absorption measured using a vector network analyzer shows features similar to MWMR, which confirms that MWMR is caused by the current-induced resonant excitation of the exchange-coupled spins (ferromagnetic resonance) in the sample. The microwave current-driven ferromagnetic resonance reported here in polycrystalline SFMO is compared with a similar phenomenon reported recently in ferromagnetic nonmetal bilayer thin films.