2938 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007 Micromagnetic Modeling of Magnetization Reversal in Nano- Contact Devices Giovanni Finocchio 1 , Ozhan Ozatay 2 , Luis Torres 3 , Mario Carpentieri 1 , Giancarlo Consolo 1 , and Bruno Azzerboni 1 Dipartimento di Fisica della Materia e Tecnologie Fisiche Avanzate, University of Messina, Messina 981 Cornell University, Ithaca, New York, 14853-2501 USA Departamento de Fisica Aplicada, Universidad de Salamanca, Salamanca, Spain This paper deals with micromagnetic model of magnetization reversal in nano scale-point contact devices dr jection of a spin-polarized current. A computational study of the magnetization reversal in the nanosecond regime wil considering the influence of the current density distribution below the aperture region on the reversal time. F a strong dependence of the reversal time on the current distribution has been observed. Finally, results of mi show that the reversal time versus current behaviour (at T = 0K ) is monotonic, very different from the switching processes in standard spin valves and magnetic tunnel junctions with uniform current injection. Index Terms—Nanosecond regime, nano-point contacts, spin polarized current. I. I NTRODUCTION AND N UMERICAL M ODEL T HERE is an increasing interest in the spin transfer torque driven magnetization dynamics and reversal, due to several potential technological applications: MRAMs [1], nano-oscilla- tors [2], and radio-frequency detectors [3]. For memory applications the most difficult task is to achieve magnetization reversal in the nanosecond regime (switching time less than 5 ns [4]–[6]) with low critical currents by de- creasing both the duration of the pulse current and its amplitude [4].There have been several attempts in pursuit of this goal. One good example involves taking advantage of the presence of a misalignment between the free layer (FL) and the pinned layer (PL) (e.g., by exchanged biasing the PL at an angle with respect to the easy axis of the structure). This approach proved to be successful in decreasing the reversal time for the same applied current; but,at the same time, it also reduces the magnetoresistance signal atthe reading step [5]. Another experimental strategy to decrease the switching time has been presented in [6], where a dc precharging current excites the magnetization to a precession trajectory thereby accelerating the reversal induced by a subsequent current pulse. The draw- back is that the current pulse has to be of the same polarity of the precharging current to speed up the reversal. This limit can be overcome by applying a small ac magnetic field (it plays the same role of the dc precharging current) together with the spin-polarized current (SPC). With this approach it is possible to achieve magnetization reversal with shorter pulsewidths as compared to the application of the same DC SPC level alone [7] for both parallel to antiparallel and antiparallel to parallel reversals. In magnetic tunnel junction (MTJ) devices, current induced magnetization reversal in the nanosecond regime is quite chal- lenging due to the risk of high voltage breakdown of the tunnel Digital Object Identifier 10.1109/TMAG.2007.892326 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. barrier. In MgO-based low-resistance magnetic tunnel jun (MTJs), it is possible to reach pulse current levels high eno to achieve magnetization reversal without exceeding the b down voltage of the barrier due to high current polarization available in these devices [8]. Another way to reduce the am- plitude of the current pulse required for reversal is by inje a nonuniform current in a nanomagnet. This can be achiev a nanoscale-point contact device (for details about nanofab tion techniques see [9]). In thispaper,we present a micromagnetic study of the magnetization reversal processes in the nanosecond regim a nano scale-point contactdevice where, a nano-aperture is inserted into a nanopillar [see Fig. 1(a)] [9]. We have studied nanopillar spin valves (210 nm 150 nm elliptical cross sec- tion)with the following layer composition Py(5nm) (FL/Cu (8)/Py(20) (PL) with a 30 nm diameter nanohole located at center of the FL. The thickness of the Al O is 3.5 nm.The simulations have been performed by numerically solving t Landau–Lifshitz–Gilbert–Slonczewski equation [10], where the Oersted field (due to the current flowing through the a ture) and the magnetostatic field (due to the coupling with PL) are added to the standard micromagnetic effective fiel [11].We do not consider magnetocrystalline anisotropy sin it is expected to be very low in Py. We also use a saturation magnetization A/m, a damping , and an exchange constant of J/m.The expression of the polarization function is , where is the angle between the magnetization of the PL and the FL, and have been computed considering the experimen data (the values of the critical currents and asymmetry be them) of a similar structure [9], their values are 0.54 and spectively. Fig. 1(b) shows the static loop computed by solving the Brown equation. For the dynamic simulations, we apply a constant external field of 35 mT along the -axis in order to compensate the average value of the magnetos coupling with the PL computed by a 3-D simulation of the whole structure. Furthermore, we consider the magnetization of the PL to be fixed along the positive axis. In our calculations, we use a time step of 31 fs. Calcula- tions performed with shorter time steps also gave similar 0018-9464/$25.00 © 2007 IEEE