Data Storage Systems Center

We describe an experimental demonstration of current-induced magnetic reversal of nanopillars with perpendicular magnetic anisotropy integrated with an in-plane synthetic antiferromagnet (SAF) structure. The perpendicular anisotropy is induced by using a Co/Pd multilayer and the SAF is implemented with a Co/Ru/Co trilayer. The film was patterned with e-beam lithography and ion-milled through to form pillars with an elliptical cross section of 50 x 100 nm2. The resistance measurement showed a decreasing trend as the magnitude of injected current is increased, which may be attributed to the alignment of the in-plane SAF layer to out-of-plane direction, resulting in the three layers pointing in the same direction. A GMR ratio of 0.8% was obtained, and the critical switching current is typically less than 2 mA. These devices may be useful for the future implementation of perpendicular spin torque oscillators, where the SAF layer will serve as a sensing structure so that the oscillation can be readout electrically as resistance oscillation.

The spin-torque-transfer effect at nano-magnetic devices with perpendicular magnetic anisotropy (PMA) has attracted extensive interest, fueled by its promising advantages towards high density MRAM and oscillator application [1], [2]. We previously demonstrated the spin-torque-transfer switching in nano-pillar magnetoresistive devices composed of Co/Pt multilayers with the PMA [3]. In order to investigate the feasibility of the devices towards the high-frequency oscillator application, we integrated the oscillator device by combining an in-plane-magnetized analyzer on top of the perpendicularly magnetized oscillating layer. This perpendicular oscillator device is anticipated to provide the detectible magnetization precession in which the precessional frequency can be tuneable by spin-polarized current. The spectrum of the emitted microwave signal in the spin-torque nano-oscillator with perpendicular anisotropy will be presented.

Transfer of angular momentum from a spin-polarized current carrying electrons to the magnetization of a ferromagnetic film can counteract the dampening in the system, leading to a coherent steady-state precession in the free layer. Under optimized conditions, this emission can have a high spectral purity, and a wide frequency range tunable by the current and the magnetic field. Such a system converts energy from a d.c. electrical current into high-frequency precession, offering the possibility of using it in microwave applications such as an on-chip high-frequency oscillator.

Magnetic annuli, or rings, have the ability to produce various magnetization configurations, which make them attractive for magnetic memory applications [1], [2], [3]. In this study, we present NiFe/CoFe/Cu/CoFe pseudo-spin-valve current-perpendicular-to-plane (CPP) giant magnetoresistive (GMR) rings with 600 nm outer diameter and 200 nm inner diameter. It is shown that by directly injecting a vertical current, single-step vortex-vortex magnetic switching is achieved due to contributions from an Oersted field and spin transfer torque [3], [4]. The total contribution of the spin torque and the Oersted field to the switching are separated and quantified, and the mechanisms contributing to the magnitude of each are discussed. Furthermore, it is shown that it is possible to trap a pair of domain walls within the NiFe/CoFe storage layer by initializing the rings with a linear in-plane magnetic field. The presence of the trapped domains leads to a significant decrease in the required switching current, thus enabling a low power switching mode.

[1] J.-G. Zhu, Y. Zheng, and G. A. Prinz, J. Appl. Phys. 87, 6668 (2000) [3] X. Zhu and J.-G. Zhu, IEEE Trans. Magn. 39, 2854 (2003) [3] M. T. Moneck and J.-G. Zhu, J. Appl. Phys. 99, 08H709-1 (2006) [4] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) [5] L. Berger, Phys. Rev. B 54, 9353 (1996)

In this project we are developing a model that describes the electric field-driven resistance switching behavior demonstrated by perovskite oxides. Although many different perovskite oxide possess such switching nature, in this study we are focusing on SrTiO3 as the function layer of our switches. In our model, we identify mobile oxygen vacancies as the key feature that leads to the observed switching behavior. Annealing SrTiO3 in a reducing environment introduces oxygen vacancies inside the material and they act as electron donors. We investigate how an applied bias affects the motion of the oxygen vacancies inside the functional layer and the effect of that on the energy barrier at the interface between the functions layer and the metal contact. We show our current simulation results using our mobile oxygen vacancy model and highlight the plans for future simulation efforts.

Spin-transfer induced magnetization switching at nano-magnetic devices with perpendicular magnetic anisotropy (PMA) has attracted extensive interest, fueled by its promising advantages towards high density MRAM and oscillator application [1], [2]. In this study, authors demonstrate spin-transfer switching at diverse nano-pillar CPP-GMR spin-valve devices composed of Co/Pt multilayer electrodes with PMA. We focus on obtaining highly spin-polarized current, which in turn can be used for facilitating the spin-transfer induced magnetization switching. It turns out that well-defined magnetization switching can be achieved when thick Co layers are adopted as adjacent layer next to Cu spacer. In the optimized film stack, two different Co/Pt multilayers have been tailored to possess different switching field with narrow distribution. Detrimental effect of surface roughness originated from thick Cu bottom lead has been minimized through the insertion of Ta layer. The nano-pillar devices sized from 90 to 150 nm diameter have been fabricated by using e-beam lithography and subsequent ion milling, RIE, and CMP processes. In the fabricated devices, well-defined single step switching has been achieved both at perpendicular field and injected current applied magnetoresistance loops. [1] S. Mangin, D. Ravelosona, J. A. Katine, M. J. Carey, B. D. Terris and E. E. Fullerton, Nature Materials, 5, 210 (2006). [2] D. Houssameddine, U. Ebels, B. Delaët, B. Rodmacq, I. Firastrau, F. Ponthenier, M. Brunet, C. Thirion, J.-P. Michel, L. Prejbeanu-Buda, M.-C. Cyrille, O. Redon and B. Dieny, Nature Materials, 6, 447 (2007).

In this project we are developing a model that describes the electric field-driven resistance switching behavior demonstrated by perovskite oxides. Although many different perovskite oxide possess such switching nature, in this study we are focusing on MIM’ heterostructures where Cr doped SrZrO3 serve as the function layer. In our model, we identify mobile oxygen vacancies as the key feature that leads to the observed switching behavior. We look at various experimental results such as the effect of using different types of contact metals in our heterostructure, varying thickness of the function layer and changing Cr doping level. We show some preliminary simulation results using our mobile oxygen vacancy model that reproduces the switching behavior found in experiments. We also comment on the inadequacies and the shortcomings of our current simulation approach and highlight changes undertaken for future simulation efforts.