Heating and cooling with electron spins

Heating and cooling with electron spins. In thermoelectricity, a thermal gradient between electrical conductors generates an electrical current (the Seebeck effect) and vice versa (the Peltier effect). Conventionally, those effects arise from the coupling of charge and heat flows. But because an electrical current can be parsed into charge and spin components, researchers hope to exploit the spin degrees of freedom in order to ameliorate or take advantage of the buildup of heat in ever shrinking electronic devices. Bart van Wees and colleagues at the University of Groningen report the latest advance in that effort: a demonstration of the spin-dependent Peltier effect in a heterostructure composed of a 15-nm-thick strip of copper sandwiched between two layers of nickel–iron permalloy. The researchers drove a constant electrical current through the heterostructure and measured the temperature of the lower alloy layer using a thermocouple. To distinguish the spin-dependent effect from the (constant) charge-dependent one, they switched the relative magnetization of the alloys from parallel to antiparallel and remeasured the temperature. When the magnetizations were antiparallel, polarized spins accumulated at the copper interfaces, which cooled the device by 3 mK (at 1 mA) and allowed the researchers to determine the different amounts of heat carried by spin-up and spin-down electrons. The admittedly small effect can be switched on and off magnetically and, more significantly, may be dramatically enhanced to a few kelvin by using nonmetallic materials or those with much larger spin-dependent Peltier coefficients. (J. Flipse et al., Nat. Nanotech. 7, 166, 2012.)