Superparamagnetic resonance in antiferromagnetic nanoparticles: systematic features in the temperature dependence

Antiferromagnetic (AF) nanoparticles (NP) of dimensions $\approx $ 5 nm usually acquire significant magnetic moment even below T$_{N}$ due to uncompensated surface spins. This moment follows superparamagnetism (SPM) above the blocking temperature T$_{B} \quad <$ T$_{N}$. In this work, we will discuss some unique features in the temperature dependence of the electron magnetic resonance (EMR) spectra which have been reported in NP of ferrihydrite, NiO and $\gamma $Fe$_{2}$O$_{3}$ [1]. As T is lowered towards T$_{B}$ the resonance field H$_{r}$ decreases and the linewidth $\Delta $H increases so that $\delta $H$_{r} \quad \approx $ ($\Delta $H)$^{n}$ with n $\approx $ 3 is observed. This is in line with the variation expected from the demagnetizing fields of non-spherical particles [2]. From the temperature variation of the line intensity I = ($\Delta $H)$^{2}$ h (h = peak-to-peak height), the location of T$_{B}$(m) and T$_{B}$(EMR) has been obtained [1]. For T $<$ T$_{B}$(m), as H$_{r}$ approaches zero, the EMR line becomes highly asymmetrical, which is explained from the combined contributions of resonance at +H$_{r}$ and -H$_{r}$. [1]. Seehra et al, IEEE Trans. Magn; \underline {37}, 2207 (2001); Seehra et al, J. Appl. Phys. \underline {97}, 10J609 (2005); Dutta et al, Phys. Rev. B \underline {70}, 174428 (2004). [2]. Nagata {\&} Ishihara, J. Magn. Magn. Mater. \underline {104-107}, 1571(1992).