Nat. Nanotech. https://doi.org/cxqd (2018)

Spintronics often relies on the conversion of charge and spin currents to create new devices, but the role of heat dynamics can be important too. Now, Minori Goto and collaborators have made the first heat-based microwave amplifier with substantial gain, based on the spin dynamics in a device usually employed in memory technology.

The time-dependent Joule heating associated with an oscillating current through a magnetic tunnel junction makes the magnetic anisotropy vary in time. The spins in the free layer of the magnetic tunnel junction rotate — following the magnetic anisotropy — and so the magnetoresistance effect changes the resistance of the junction. Therefore, the heat converts the d.c. part of an input current to an a.c. output with the frequency of the magnetic rotation, amplifying the oscillating part of the input. By choosing appropriate materials, the authors made a device where gain in the device exceeds unity.