Cooperative versus superparamagnetic behavior of dense magnetic nanoparticles in Co80Fe20/Al2O3 multilayers

Spin glasslike cooperative freezing is evidenced at low temperatures in a magnetic nanoparticle system prepared as a discontinuous metal–insulator multilayer [Co80Fe20(0.9 nm)/Al2O3(3 nm)]10. The relaxational behavior clearly deviates from Arrhenius–Néel–Brown-type and rather hints at collective freezing into a spin glass phase below Tg≃61 K. Holelike memory imprinting during a stop-and-wait magnetization procedure corroborates the collective nature of the frozen state. Consequences for future magnetic terabit storage devices are briefly discussed.