What are the requirements ? Integration of STOs into conventional RF circuitry means that the STO has to meet certain specifications. The most important are output power and phasenoise. In this section we consider only the output power. In technical terms, the STO has to deliver 0dBm, for instance when its output is connected to a mixer that is used in frequency synthesis or in a receiver chain.
A quick estimate1 of the maximum output power Pout, see Fig. 1, yields in the case of
Spin valves Pout(dBm) = -70 to -90 dBm when considering a magneto-resistance ratio (MR) of 1-10%, a current IDC of 1mA, an impedance matched resistance R of 50 Ohm (b=1 ideal!). This corresponds roughly to the average values reported in literature. However, it should be noted, that Pout is proportional to I², so for instance for vortex oscillators where currents of tens of mA are used this can increase by a factor of 20 dB.
Magnetic tunnel junctions Pout(dBm) = -45 dBm when considering a magneto-resistance ratio (MR) of 100%, a current IDC of 1mA, a resistance R of 100 – 1000 Ohm (b=0.5 to 0.05). This is about 30 to 40 dB higher than for spin valves. Hence the interest to study the spin torque driven excitations in magnetic tunnel junctions.
1 : Please note that these estimates are just a rough guide and that they neglect any additional losses due to imperfect circuitry (ie electrode capacitance). Futhermore, in the experiment the power is integrated over the peak.
What are the challenges ? First experiments on spin torque driven excitations have been performed on spin valves for the simple reason that they are much easier to prepare. In fact, in order to use magnetic tunnel junctions, one can see relatively easily that the resistance area (RA) product has to be well below RA=5Ωµm², since the critical current (or the corresponding voltage) to induce the steady state oscillations has to be well below the breakdown voltage of the tunnel barrier (less than 1V). Low RA means an extremely thin MgO barrier while maintaining good structural properties. Magnetic tunnel junctions with appropriate properties for studying spin torque driven dynamics became first available by end of 2007 [Nazarov].
Magnetic tunnel junctions realized by Hitachi GST : For studying spin torque driven excitations in magnetic tunnel junctions we have been collaborating with Hitachi GST, who provided tunnel junction devices with MgO barriers of different RA values : 1, 1.5 and 2 Ωµm² and of different sizes (45 nm to 150 nm circular dots and ellipses). Only for RA=1 and 1.5 Ωµm² the steady state was reached. The samples are composed of an in-plane magnetized synthetic antiferromagnetic pinned layer (polarizer) and an in-plane magnetized free layer.
In 2008 we have characterized the excitation spectra of these devices for RA=1.5 Ωµm² and observed that it is possible to reach an integrated output power of -50 dBm. These experiments were among the first to confirm that it is possible to obtain high output power combined with low linewidth of 10 MHz for magnetic tunnel junction oscillators.
These devices have been used by our group for a number of detailed studies in particular concerning the linewidth, see corresponding sections ‘Origin of linewidth broadening’.
- 1) the devices (on the same wafer) can be divided roughly into two categories :High-TMR
- (HTMR) devices with a TMR of 70-100% and
Low-TMR (LTMR) devices with a TMR of below 50%, and with an overall lower resistance in the parallel and antiparallel state. We suspect that pinholes form in the barrier, although low temperature studies confirm that the overall resistance is tunnelling-like.
- 2) Steady state excitations occur mainly in the AP state and in negative current (electrons flowing from free to pinned layer).
- 3) The steady state excitation spectra at constant current and varying field yield 3-4 branches for both HMTR and LTMR devices, where the Kittel-like increase of the frequency f with field is interrupted by small jumps, see Fig. 2.
- 4) The steady state excitation spectra at constant field and varying current are quite different. While for HTMR we observe the ‘standard’ frequency redshift, as expected for this configuration, for the LTMR devices we observe a frequency blueshift, see Fig. 3
- 5) The linewidth is minimum at the center of the f vs field branches and increases at the transition region, most likely due to telegraph noise like transitions between the two branches, see Fig. 2. In the center of the branch, the linewidth decreases with increasing current, see Fig. 3. The lowest linewidth obtained for these nanopillar devices at room temperature is around 10 MHz.
Magnetic tunnel junctions realized at LETI and PTA : In 2009 the LETI/DIHS Laboratory has acquired an IBS sputtering deposition tool from SPTS (formerly AVIZA) to realize in a joint venture effort high quality magnetic tunnel junction devices. The current performances can be summarized as follows : best TMR of 70% at RA= 2 Ωµm² (CAPRES wafer level characterization), see Fig. 4.Using our PTA platform, we have realized nanopillars of 100 nm. Preliminary dynamic studies show that that the steady state regime is reached with a microwave emission linewidth of 40 MHz, see Fig. 4. Due to redeposition the output power cannot yet be estimated properly. More to follow…..
 Appl. Phys. Lett. 93, 022505 (2008), D. Houssameddine, et al.
•Dimitri Houssameddine, PhD microwave characterization
•Michael Quinsat, PhD microwave characterization
•Alex Zeltser, Danielo Mauri, Jordan Katine, Hitachi GST, San José (MTJ development and nanofabrication)
•Marie Thérès Delaye, PTA nanofabrication
•Karin Garcia, Postdoc, PTA nanofabrication
•Juan Sierra, Postdoc, microwave characterization
[Nazarov] J. Appl. Phys. 101, 7A503 (2008), A. V. Nazarov et al.