Magnetization Increases with Temperature

Magnetization increases with temperature for antiferromagnetic nanoparticles. This odd experimental finding, made a few years ago, is now explained, for the first time, by physicists at the Technical University of Denmark. The experimental behavior is odd for two reasons: first because antiferromagnets, whose tiny neighboring magnetic moments generally line up in an alternating down and up pattern, are supposed to sustain no significant net magnetization of their own in an applied field; and second because magnetism itself, which arises at the microscopic level from the aligned magnetic moments of many atoms (the atoms act as tiny bar magnets), should tend to decline as the disruptive action of higher temperatures takes effect.

The Danish physicists explain why "thermoinduced magnetization" is missing in bulk antiferromagnetic samples (which accounts for their being nonmagnetic), but become more noticeable in dots with size below 10 nm. Steen Morup (morup@fysik.dtu.dk) and Cathrine Frandsen (fraca@fysik.dtu.dk) argue that antiferromagnetic nanoparticles might be engineered into a new class of material, one in which magnetization can be switched quickly and without energy loss, making it valuable for use in high-frequency electronic devices. (Morup and Frandsen, Physical Review Letters, 28 May 2004.)