Role of surface spins on magnetization of Cr2O3 coated γ-Fe2O3 nanoparticles.

Effect of surface spins in chromium oxide (Cr 2 O 3 ) coated maghemite (γ-Fe 2 O 3 ) nanoparticles (13 nm) as prepared by microwave plasma technique have been studied in detail. The temperature dependent zero field cooled/field cooled (ZFC/FC) measurements revealed the blocking temperature at T B  = 75 K. Simulated ZFC/FC curves exhibited large value of effective anisotropy of Cr 2 O 3 coated γ-Fe 2 O 3 nanoparticles as compared to bulk γ-Fe 2 O 3 but less than bare γ-Fe 2 O 3 nanoparticles. Bloch's law was fitted on M S -T data and revealed the values of Bloch's constant B = 3.523 × 10 −4  K −b and Bloch's exponent b = 1.10. The higher value of B than in bulk is due to weaker exchange coupling J (B ̴ 1/J) on the surface of nanoparticle due to disorder surface spins, while lower value of b is due to no spin wave excitation in presence of large energy band gap at nanoscale. Kneller's law fit on H C -T data deviated in all temperature range which is due to strong surface anisotropy, core-shell interactions and superparamagnetism. Interparticle interactions and spin glass behavior were investigated by using different physical laws for f-dependent ac susceptibility and they confirmed the presence of spin glass behavior which is due to disordered frozen surface spins and random interparticle interactions. [ABSTRACT FROM AUTHOR]