Spintronic concepts and materials are well known for their applications in data storage, magnetic memory and hybrid logic devices. Besides, they can also bring novel approaches for the realization of microwave components such as rf signal sources or rf detectors. These applications are based on the fact that a spin polarized current can counteract the natural damping forces and induce large angle steady state (non-damped) excitations in the free layer of a magneto-resistive device.
While most of the experimental results can be explained within a picture where the free layer of the magneto-resistive device, is excited independently, see Fig. 1a,b, there are a number of observations that cannot be captured within this picture. For instance our experiments on spin valve nano-pillars, reveal non-continuous features such as gaps and kinks in the frequency field dependence, see Fig. 1c.
With the aid of macrospin simulations it could be shown that these deviations result from the dynamic dipolar interaction of the free layer steady oscillations with the damped oscillations of the polarizing layer, see Fig. 1a. In fact the free layer steady oscillation frequency is down-shifted when increasing the spin polarized current density. When this frequency, or one of its higher harmonics, gets close to the frequency of one of the damped modes, dipolar interaction fields radiated by the oscillating free layer, can start pumping the damped mode. This generates a new hybridized mode in a certain range of current and fields, where the steady state and damped modes are frequency locked resulting in strong deviations in the frequency-field dispersion and even discontinuous jumps. A further interesting observation, accompanying this non-linear interaction, is that it can also lead to a reduction of the linewidth of the steady state mode, see Fig. 1d,e.
These results have provided a first insight into the effect of interlayer coupling on the non-linear dynamics of spin torque driven excitation. They are of interest to further exploit interaction effects between layers within a magneto-resistive stack to improve the microwave performances for applications.
This work has been realized at INAC/SPINTEC in collaboration with CEA/LETI.
Fig. 2. Experimental assembly for microwave signal measurements: the chip containing several devices is placed into the gap of an electromagnet. Electrical contact to the device is made via a microwave probe and transmission lines (see inset) into which the device is embedded. The SEM picture shows a single nanopillar device.
Source: Non-linear mode interaction between spin torque driven and damped modes in spin torque nano-oscillators
M. Romera, E. Monteblanco, F. Garcia-Sanchez, B. Delaët, L. D. Buda-Prejbeanu and U. Ebels
Contact: Ursula EBELS ( )
Last update : 02/08 2016 (1152)