Ferromagnetic resonance in amorphous nanoparticles

We present the results of ferromagnetic resonance (FMR) measurements on noninteracting amorphous nanoparticle systems (Co0.25Ni0.75)65B35 (CNB2) and (Fe0.25Ni0.75)50B50 (FNB2) in the temperature range 10-300 K. The high-temperature results show symmetric lineshapes that indicate a low value of effective anisotropy. When cooling the linewidth increases and the lineshapes lose their symmetric shape. The lineshape can be interpreted as due to slightly elongated particles. In CNB2, we observe a sharp intensity maximum at T=60 K, followed by a linewidth maximum and resonance field minimum at T=50 K. The same tendency is observed in FNB2, with an intensity maximum near T∼40 K, with an increment in the linewidth and simultaneous decrease of the resonance field down to the lowest temperature, T=10 K. Previous magnetization measurements on these systems indicate large surface effects in the same temperature region of the observed anomalies. This picture, together with simulations, permits us to explain the FMR behaviour