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 - Neel - Brown- type and rather hints at collective freezing into a spin glass phase below T-g similar or equal to 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. (C) 2003 American Institute of Physics.