  Spin torque oscillators are conventionally described by the
Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation. However, at the onset of
oscillations, the predictions of the conventional LLGS equation differ
qualitatively from experimental results and thus appear to be incomplete. In
this work we show that taking charge conservation into account leads to a
previously-overlooked self-induced torque, which modifies the LLGS equation. We
show that the self-induced torque originates from the pumping current that a
precessing magnetization drives through a magnetic tunnel junction. To
illustrate the importance of the self-induced torque, we consider an in-plane
magnetized nanopillar, where it gives clear qualitative corrections to the
conventional LLGS description.
