Injection locking of tunnel junction oscillators to a microwave current

Injection locking of a spin transfer nano-oscillator, based on an in-plane magnetized magnetic tunnel junction and generating the frequency f(0), to an external signal of varying frequency f(e) is studied experimentally and with macrospin simulations. It is shown, that if the driving signal has the form of a microwave current, the locking effect is well-pronounced near f(e) congruent to 2f(0), but is almost completely absent near f(e) congruent to f(0), confirming predictions of analytical theory. It is also shown that noise plays an important role in the locking process, causing the linewidth of the locked oscillation to substantially exceed that of the driving signal. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3587575]