- Current flowing through a magnetic material can alter its magnetization
by spin torque, whereby the spins of the electrons flowing in the
current exert a torque on the magnetization. This mechanism can induce
high-frequency precession of the magnetization. The effect can be used
to make high-frequency wireless devices for future mobile phones,
devices that are significantly smaller and consume less power than the
current state-of-art current technology based, for example, on standard
quartz crystal resonators. In this work, we demonstrate that a very
tiny magnetic tunnel junction device, in which two magnetic layers
(CoFeB) are separated by an oxide barrier (MgO) can emit strong
microwave signals with GHz frequencies. The novelty of our results is
that we are able to demonstrate microwave emission from both the top
magnetic layer, which is called free layer, and the bottom magnetic
layer which is called fixed layer. We show that the precession frequency
of the free layer changes linearly with bias voltage due to the linear
variation of perpendicular component of spin torque. In contrast, the
precession frequency of the fixed layer changes quadratically as a
result of heating effects. By changing the applied field magnitude it is
possible to control which layer is excited and hence to manipulate the
behavior of the frequency with bias voltage. Thus our work provides an
important step towards making next generation wireless devices.
