Current-Driven Excitations in Symmetric Magnetic Nanopillars

A typical experiment on current-driven excitation of a ferromagnet usually involves two single-domain thin film magnets separated by a nonmagnetic spacer. One magnet is ‘hard’ and used to polarize the current while the spacer is thin enough for the polarized current to get through and excite the second ‘free’ magnet. The free magnetic layer is generally thin compared to the hard one thus marking an intrinsic asymmetry of the phenomenon - for initially parallel magnetizations of the two magnets the current-driven excitation occurs only when electrons flow from the free magnet to the fixed one. Here we study the current-driven excitations in magnetic nanopillars with comparable thicknesses of the two magnetic layers.

Schematic drawing of a magnetic nanopillar: pillar sequence - two Co layers (F₁ and F₂) are separated by a Cu spacer. At room temperature and in magnetic fields B up to 6 T we have measured the current-voltage (I-V) characteristics of pillars with various thickness ratios t(F₁)/t(F₂).

In contrast to all the previous observations where current of only one polarity is capable of exciting a multilayer system saturated by an externally applied magnetic field, we observe that both polarities of the applied current trigger excitations in a symmetric multilayer. This may indicate that in symmetric structures the current propels high-frequency magnetic oscillations in all magnetic layers. We argue, however, that only one layer is excited in our multilayers but, interestingly, currents of opposite polarities excite different layers. This hypothesis is supported by modeling the spin accumulation in symmetric magnetic multilayers.

Variation of the pillar resistance R=V/I as a function of dc bias current I for a sample with t(F₁)/t(F₂)=4. Step increase in R at a certain critical bias current I_c(B) (positive) correspond to the onset of the current-induced excitations. Open symbols in the insert show I_c vs B.

Variation of the pillar resistance R=V/I as a function of dc bias current I for a sample with t(F₁)/t(F₂)=1.5. Step increases in R at a certain critical bias current I_c(B) (positive and negative) correspond to the onset of the current-induced excitations. Open symbols in the corresponding inserts show I_c vs B.

Black trace shows variation of the pillar resistance R=V/I as a function of dc bias current I for a symmetric sample: t(F₁)/t(F₂)=1. Step increases in R at a certain critical bias current I_c(B) (positive and negative) correspond to the onset of the current-induced excitations. Open symbols in the corresponding inserts show I_c vs B. Blue trace shows dV/dI(I) along with R(I) to spotlight variations in R.