Emission spectrum of an oscillator based on MTJ (left), and extrapolated spectrum from the signal by removing frequency variations. We note that the width of the resonance peak is strongly reduced. The inset images are reconstructed from time recordings and cover a temporal duration of 200 ns.
With original measurements on high quality samples, Spintec laboratory, in collaboration with Leti and Hitachi, has just revealed one of the reasons why the performance of radio frequency spin oscillators still remains limited. Our studies show that the quality of the oscillators is inherently very good, and is reduced by the frequency fluctuations.
The development of radio frequency oscillators is one of the applications of spintronics: the building blocks are magnetoresistive elements (see inset), whose resistance may, under certain stimuli conditions, vary periodically at a frequency of several gigahertz, thus generating RF signals. Early research used spin valves that provide low transmission power. The development of oscillators based on magnetic tunnel junctions (MTJ) has recently boosted the power by a factor of 1000, which makes these devices much more promising.
An oscillator worthy of its name must emit a periodic signal at a well-defined frequency. Quantitatively, we characterize the frequency width of the emission peak (or line) as such: if the peak is narrow, the generated frequency is very precise. The spectrum analyzer is the device that can measure this directly in the frequency domain. For our samples, we find a width too large to qualify for competition with conventional oscillators. Where does this width come from?
To answer the question, we took a closer look to these oscillations by time domain spectroscopy. Indeed, the spectrum analyzer, by construction, averages the signal over a long timeframe. To perform reliable measures in real time, which are highly sensitive, we had to have oscillators of very high quality with strong signals. Collaboration with Leti and Hitachi has provided access to samples based on MTJ with a very high quality factor. Time measurements show that the oscillation frequency varies over time, with periods of stability (or coherence time) of around 80ns, during which the signal frequency is found to be very stable and well-defined. The digital processing signal was used to evaluate an "intrinsic" line width of less than 1MHz, i.e., better than a factor of 20, compared to that of the real oscillator (Fig.).
These results are extremely encouraging because they demonstrate that the quality of the spin oscillators is inherently very good. Their reduced performance stems mainly from frequency fluctuations that can be compensated by a suitable electronic device of the phase lock loop type.
(a) MTJ, consisting of two ferromagnetic layers (red and blue) separated by an insulating layer (green). The red layer is "hard": it has a fixed magnetic moment mp and serves as a reference. The blue layer is "soft": it has a magnetic moment m, which can vary under the action of a magnetic field or a spin-polarized current. In crossing the junction, the current first polarizes as it goes through the hard layer and then transmits its polarization as a couple to the soft layer. (b) The resistance R of the stack depends on the angle between m and mp. It displays sinusoidal variations between a minimum RP when the magnetizations are parallel and a maximum RAP when m and mp are 180°. If the magnetization m rotates regularly - this is called sustained precession - it then generates a sinusoidal output signal.
Magnetoresistive components are nanoscale multilayers of ferromagnetic and nonmagnetic materials, whose resistance depends on the relative direction of magnetization of ferromagnetic layers. "Spin valves" alternate ferromagnetic layers with metal layers, whereas "magnetic tunnel junctions" or MTJ (see Fig.) alternate ferromagnetic layers with insulating layers. By combined application of a magnetic field and a spin polarized current (whose couple brings energy to compensate for magnetic losses), we can obtain sustained oscillations of magnetization and therefore an electrical resistance oscillating at a frequency of several gigahertz. This becomes the core of a radio frequency oscillator.
Further reading: D. Houssameddine, et al., Physical Review Letters 102 (2009) 257202
Maj : 20/02/2014 (969)