Data Storage Systems Center

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.

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.

Magnetic annuli, or rings, have the ability to produce various magnetization configurations, which make them attractive for magnetic memory applications [1], [2]. The vortex state is of particular interest, since it forms a fully closed magnetic flux path, with no demagnetization field. Magnetic rings in the vortex state also provide an effective vehicle for studying spin transfer torque, due to their lack of stray field [3], [4]. 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. Further evidence indicates that the spin transfer torque can either assist or act against the Oersted field by as much as 30% depending on the chirality of the reference layer in the CPP-GMR ring. 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. The resulting data shows the symmetry of the Oersted contribution, as well as the asymmetry of the spin torque contribution.

[1] J.-G. Zhu, Y. Zheng, and G. A. Prinz, J. Appl. Phys. 87, 6668 (2000) [2] M. T. Moneck and J.-G. Zhu, J. Appl. Phys. 99, 08H709-1 (2006) [3] J. C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996) [4] L. Berger, Phys. Rev. B 54, 9353 (1996)

MRAM has been hailed for its potential to become the first universal memory due to its nonvolatility, unlimited cycleability, and fast switching speeds. However, years of research have shown that scalable low power MRAM designs can be difficult to achieve. In this study, we present an annular vertical MRAM design capable of both scalability and low power operation [1],[2],[3]. The design is based on a CPP GMR magnetic multilayer stack composed of NiFe (20 Å)/CoFe (10 Å)/Cu (35 Å)/[CoFe (10 Å)/Cu(3-5 Å)]x4/CoFe (110 Å) where the 3-5 Å Cu layers are designed to enhance the CPP GMR effect [3]. By injecting a vertical current through a CPP GMR ring with 600 nm outer diameter and 300 nm inner diameter, switching can be achieved in two operational modes. The first mode is achieved when both the reference and storage layers are in a fully circular, or vortex state [1],[3]. In the second mode, the NiFe/CoFe composite storage layer forms a pair of trapped domain walls by initializing the ring with a linear in-plane magnetic field [2]. By moving the pair of trapped domain walls with the Amperean field generated by an injected current, low power switching can be achieved. It has been demonstrated that the trapped domain state can lower the switching current 3 to 6 times as compared to the fully circular switching mode. In addition, evidence of the spin transfer effect has been observed in the fully circular mode. The spin transfer torque can both assist and act against the switching, depending on the orientation of the circular reference layer.

[1] J.-G. Zhu, Y. Zheng, and G. A. Prinz, J. Appl. Phys. 87, 6668 (2000)
[2] X. Zhu and J.-G. Zhu, U.S. Patent No. 6,956,257 B2 (18 October 2005)
[2] M. T. Moneck and J.-G. Zhu, J. Appl. Phys. 99, 08H709-1 (2006) 

Magnetic tunnel junctions that utilize perpendicular magnetic anisotropy have attracted growing attention due to their potential for higher storage densities in future high capacity magnetic memory applications[1], [2]. In this project, we present an experimental demonstration of magnetic tunnel junctions composed of perpendicularly magnetized Co/Pt multilayer electrodes and an AlOx tunnel barrier. The emphasis has been on how to maximize the thickness of the Co layers adjacent to the tunnel barrier while still magnetized perpendicularly for possible spin torque utilization in future applications. It is found that the thickness ratio between the Co and Pt layers and the number of bilayers were significant parameters to customize the magnetic properties. The difference between the switching fields of the soft and the hard layers can be adjusted by the number of repeats of the Co/Pt bilayrers. The measured hysteresis shows virtually zero exchange coupling between the two layers through the tunnel barrier. Measured TMR of the fabricated submicron size tunnel junctions ranges from 10 to 15% at room temperature.

[1] N. Nishimura, T. Hiral, A. Koganei, T. Ikeda, K. Okano, Y. Sekiguchi, and Y. Osada, J. Appl. Phys., 91, 5246 (2002) [2] I. Yoo, D. Kim, Y. Kim, J. Appl. Phys., 97, 10C919 (2005)