Spintransfer dynamics in magnetic tunnel junctions with an outofplane magnetized free layer and inplane polarizer
Spintransfer dynamics in magnetic tunnel junctions with an outofplane magnetized free layer and inplane polarizer
Kowalska, E.; Sluka, V.; Fowley, C.; Aleksandrov, Y.; Bernert, K.; Lindner, J.; Fassbender, J.; Deac, A. M.
Spintorque nanooscillators (STNOs) are novel spintronics devices which may be exploited in order to design energyefficient, frequencytunable receivers and transmitters for wireless technology purposes [13]. Indeed, such devices are considerably smaller than conventional oscillators, having lateral dimensions in the range of a few tens of nanometers. Moreover, it has been demonstrated that STNOs can generate signals with high quality factors (in metallic pointcontacts) and output powers in order of μW (in MgObased magnetic tunnel junctions (MTJs) nanopillars). These levels are compatible with applications, thus justifying the interest which they have attracted as potential mobile InformationTelecommunication Technology devices.
To date, most studies focusing on spintransfer driven dynamics have been carried out on devices with both the free and the reference layers magnetized inplane. In this configuration, under applicationdesirable conditions (i.e., close to zero applied field), steadystate precession mainly occurs on clamshell trajectories centered on the direction defined by the inplane shape anisotropy. Consequently, only a fraction of the full magnetoresistance amplitude translates into the radiofrequency output power. However, it has been demonstrated that devices utilizing an inplane (IP) magnetized polarizer (also acting as readout layer) and outofplane (OOP) magnetized free layer allow for the full paralleltoantiparallel resistance variation to be exploited in the limit of 90° precession angle [1]. In this particular geometry, it has been shown that steadystate precession can only be sustained if the spintransfer torque exhibits an asymmetric dependence on the angle between the free and the polarizing layer [1].
Nevertheless, it has been very recently demonstrated experimentally that spintransfer driven dynamics can also be sustained in similarly designed MgObased MTJs, in spite of the fact that such devices do not exhibit any asymmetry in the spintorque angular dependence [45].
Here, we explore potential mechanisms for sustaining steadystate precession in MgOBased MTJs with IP polarizing and OOP free layer. To this end, we analytically solve the LandauLifshitzGilbertSlonczewski (LLGS) equation for a nanopillar MTJ with circular crosssection, under a constant applied current (Fig. 1). We take into account both the inplane and the fieldlike spintorque terms, whose magnitude is determined not by the current, but by the corresponding voltage across the barrier. Steadystate precession can be sustained if the inplane spintorque term and the damping torque compensate over a full precession period.
Our results show that the stable dynamics occur only for negative current, for electrons flowing from the free to the reference layer (Fig. 2). According to our calculations, at small finite fields, the precession angle increases gradually from around zero to 90° with increasing current. High output powers (in the limit of 90° precession) can be obtained for relatively low values of applied current and field, which is beneficial from the point of view of STNOs application.
References:
[1] W. H. Rippard, A. M. Deac, M. R. Pufall, et al., Physical Review B 81, 014426 (2010).
[2] A. M. Deac, A. Fukushima, H. Kubota, et al., Nature Physics 4, 308 (2008).
[3] S. I. Kiselev, J. C. Sankey, I. N. Krivorotov, et al., Nature 425, 380 (2003).
[4] T. Taniguchi, H. Arai, S. Tsunegi, et al., Applied Physics Express 6, 123003 (2013).
[5] H. Kubota, K. Yakushiji, A. Fukushima, et al., Applied Physics Express 6, 103003 (2013).
Keywords: spintorque nanooscillator (STNO); MgObased magnetic tunnel junctions; tunnel magnetoresistance (TMR); spin dynamics

Poster
INTERMAG 2014  IEEE International Magnetics Conference, 04.08.05.2014, Dresden, Germany
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