Randall Law

Reduction of switching current by spin transfer torque effect in perpendicular anisotropy magnetoresistive devices (invited)

Spin transfer torque-based magnetic random access memory with perpendicular magnetic anisotropy (PMA) provides better scalability and lower power consumption compared to those with in-plane anisotropy. Spin transfer torque switching in magnetoresistive spin valves with PMA is investigated. The hard layer is made of (Co/Pd) multilayer, whereas the soft layer is a lamination of (CoFe/Pd) and (Co/Pd). By the insertion of an in-plane spin polarizer adjacent to the perpendicular anisotropy free layer, thus creating a modified-dual spin valve, a significant reduction of about 40% in the current density required for spin torque transfer switching was observed. By using a spin polarized current with different pulse widths down to 10 ns, the barrier energy EB in 100-nm-diameter devices was found to be reduced from 1.1 to 0.43 eV. Besides the reduction of switching current density in a device with PMA, the new structure shows a clear increase in magnetization switching speed as revealed by micromagnetic simulation.

Spin transfer switching enhancement in perpendicular anisotropy magnetic tunnel junctions with a canted in-plane spin polarizer

We have studied spin transfer switching (STS) in a magnetic tunnel junction with perpendicular magnetic anisotropy for the reference and free layers using the Landau–Lifshitz–Gilbert formalism. We propose a multilayer structure in which the insertion of an additional spin polarizer with in-plane anisotropy can enhance the STS efficiency and switching speed of the device. It is revealed that a canted spin polarizer with an angle between 40° and 80° out of the film plane in the correct direction enhances the STS efficiency more than a fixed in-plane or perpendicular polarizer. Furthermore, we show that the spin transfer torque exerted on the in-plane polarizer layer by the free layer automatically tilts the in-plane polarizer in the direction that enhances STS for both magnetization states of the free layer.

Reduction in critical current for spin transfer switching in perpendicular anisotropy spin valves using an in-plane spin polarizer

We describe a strategy to reduce spin transfer switching (STS) currents in CoFe/Pd-based perpendicular anisotropy single spin valves (SSVs) by the insertion of an in-plane spin polarizer, thus creating a modified-dual spin valve (m-DSV). For SSV devices, concurrent STS of both magnetic layers was observed for positive currents, making the parallel-to-antiparallel (P→AP) transition impossible. In m-DSV devices, we observed a 60% reduction in the energy barrier for AP→P transitions and a 40% reduction in JcAP→P with 10 ns STS current pulses compared to SSV devices. Furthermore, the m-DSV structure enabled the soft layer to switch independently from the hard layer via STS.

Spin transfer torque switching for multi-bit per cell magnetic memory with perpendicular anisotropy

A novel multi-bit dual pseudo spin valve with perpendicular magnetic anisotropy is investigated for spin transfer torque (STT) switching. The structure consists of two free layers and one reference layer, and all are based on Co/Pd multilayer. STT switching of the multi-bit device shows distinct four resistance levels. The selection of intrinsic properties of each ferromagnetic layer can be controlled for distinct separation of the resistance levels as well as the respective STT switching current. Reversible transitions between different states can be achieved by a pulsed current, in which its critical value is found to be linearly dependent on pulse duration.

Patterned media with composite structure for writability at high areal recording density

Writability in bit-patterned media (BPM) is a critical issue for high areal densities. In this study, magnetization reversal for multilayer of (Co/Pd) nanodots was investigated using magnetic force microscopy. We observed an increase of more than 15 times in switching field (Hsw) in BPM over that of continuous films. An exchange coupled structure made of a thin Co layer with in-plane magnetization and high perpendicular anisotropy layer of (Co/Pd) multilayer to reduce the switching field is proposed. When the Co layer is thinner than 2 nm, its magnetization is aligned perpendicular to the film plane due to the exchange coupling with the 15 nm thick (Co/Pd) multilayer. The thin Co layer helps in reducing the Hsw of (Co/Pd) by almost 50% and also its distribution by 57% as measured from remanence magnetization.

Effect of film texture on magnetization reversal and switching field in continuous and patterned (Co/Pd) multilayers

We studied the reversal properties of perpendicular anisotropy (Co/Pd) multilayers with different crystallographic textures. In case of continuous films, an increase in the coercivity and reduction in the switching field distribution (SFD) were observed as the growth is improved. From magnetic force microscopy, a stripe-type domain configuration was observed in films deposited at low gas pressure while a bubble-type domain was observed in high pressure deposited films. In patterned films, the SFD did not vary significantly for samples with different textures although a 2 kOe increase in the switching field was measured. In patterned structures, the controllability of SFD may not be related to the improvement of film crystallographic growth as was observed for unpatterned films. The results from this study indicate that local variation in the intrinsic film properties plays a major role in the SFD.

Magnetoresistance and Switching Properties of Co–Fe/Pd-Based Perpendicular Anisotropy Single- and Dual-Spin Valves

Switching characteristics and magnetoresistance of spin valves with perpendicular anisotropy based on Co-Fe/Pd multilayers deposited by ultrahigh vacuum (UHV) sputtering have been studied. In unpatterned thin films without exchange bias, high current-in-plane giant magnetoresistance (CIP-GMR) of 9.7% and 15.2% in single-spin valves (SSVs) and dual-spin valves (DSVs) was measured, a significant improvement over previous work. We describe the effects of a Ta seed layer and postdeposition annealing on the perpendicular anisotropy and magnetoresistance of Co-Fe/Pd spin valves, which can be attributed to improvements in the fcc (111) orientation of Pd and the formation of Co-Pd alloys at the Co-Fe/Pd interfaces, respectively. We also show that the coercivity of the layers can be tuned by varying the Co-Fe alloy composition in the multilayers, and describe the minor loops of perpendicular DSVs that exhibit four distinct resistance states, which is a potential structure for multistate storage devices.

Magnetization reorientation in antiferromagnetically coupled Co films and (Co/Pd) multilayers

Magnetization reversal of antiferromagnetically coupled (AFC) Co thin film and (Co/Pd) multilayers has been investigated. For 5 A thick Co layer, a high exchange coupling field (Hex) of 7.8 kOe at room temperature and 8.5 kOe at 5 K was measured from the shift in the hysteresis loop. This high value of Hex was accompanied by a magnetization reorientation from in-plane to out-of-plane of the thin Co. When Co thickness increases, the magnetization reorientation was not possible. This unusual high Hex can be used to stabilize AFC structures when the patterning to nanoscale size is needed.

Multistate storage in dual spin valves with perpendicular magnetic anisotropy

We report the tunable separation of four distinct resistance levels in dual spin valves (DSV) with perpendicular magnetic anisotropy based on Co/Pd and CoFe/Pd multilayers. An optimal giant magnetoresistance (GMR) of 15.2% in the current in-plane geometry was obtained. By varying the spin filter layer thicknesses at the interfaces of one Cu spacer layer from 2 to 6 A, a linear dependence of GMR across the selected spacer layer was demonstrated without affecting the GMR contribution across the second Cu spacer layer in the DSV. Using this strategy, the intermediate resistance levels in a four-state perpendicular DSV can be adjusted independently, thus creating flexible platform for multistate storage.

Effect of Different Seed Layers on Magnetic and Transport Properties of Perpendicular Anisotropic Spin Valves

The effect of Ta, Cr and Cr90Ru10 seed layers on the magnetoelectronic properties of Co/Pd multilayers based pseudo-spin-valves (PSV) with perpendicular magnetic anisotropy (PMA) was investigated. Giant magnetoresistance curves of the PSV show the degradation of PMA with thicker Cr and CrRu seed layers, but no degradation was observed for Ta up to 50 A. It appears that after the deposition of an intial amorphous wetting layer up to 20 ¿, Cr and CrRu seed layers start to form three-dimensional clusters that do not favor the fcc (111) orientation of the Pd seed layer and Co/Pd multilayers. This resulted in a significant reduction of the PMA for thicker Cr and CrRu seed layers, such that well-separated switching of the magnetic layers cannot be achieved.