In a magnetic tunnelling junction (MTJ) with perpendicular magnetic anisotropy (PMA), offset field (Ho) of the free layer is usually controlled by using a synthetic antiferromagnetic (SAF) coupling structure, which is composed of an antiferromagnetic coupling (AFC) layer sandwiched by two ferromagnetic (FM) layers. However, Ho increases significantly as the size of MTJ devices shrinks to accommodate high density. In addition, magnetostatic field in PMA SAF structure tends to destabilize the antiferromagnetic (AFM) alignment of the SAF layers, in contrast to the in-plane anisotropy SAF, where the closed flux forms stable AFM magnetic configuration. Here, we present a double SAF structure to control Ho, while maintaining high magnetic stability of the reference layer (RL). The double SAF consists of FM1/AFC/FM2/AFC/FM3 multilayer. An AFM layer like PtMn is added to further stabilize the magnetic configuration of the double SAF. As the magnetization of other FM layers (FM1 and FM2) is aligned oppositely, the magnetostatic field acting on the RL (FM3) layer is significantly reduced due to cancellation effect from its adjacent layers. Both simulation and experimental results demonstrate that the double SAF layers provide high stability for the RL in addition to the reduction of Ho. Our results on MTJ devices show that the AFM pinned double SAF has the highest RL stability. The RL switch rate decreases as the thickness of the CoFe inserted layer between AFM and the pinned layer (Co/Pt multilayer) increases due to improved exchange coupling.

While investigating uniformity of magnetic tunnel junction (MTJ) stacks we find experimentally and analytically that variation in the resistance area product (RA) is more important to monitor as compared to the tunnel magnetoresistance (TMR), which is less sensitive to MTJ variability. The standard Current In-Plane Tunneling (CIPT) method measures both RA and TMR, but the usefulness for uniformity mapping, e.g. for tool optimization, is limited by excessive measurement time. Thus, we develop and demonstrate a fast complementary static magnetic field method focused only on measurement of RA. We compare the static field method to the standard CIPT method and find perfect agreement between the extracted RA values and measurement repeatability while the static field method is several times faster. The static field CIPT method is demonstrated for 200 mm wafer mapping showing radial as well as asymmetrical variations related to the MTJ deposition conditions.

Magnetic and structural properties of Co/Pt multilayers with varying Co (t(Co)) and Pt (t(Pt)) thickness grown on various seed layers have been examined. It is challenging to grow Co/Pt multilayers as a top electrode with high perpendicular magnetic anisotropy (PMA) for magnetic tunnel junctions which usually comprise of CoFeB/MgO/CoFeB. We show that a thicker Pt layer of 1.2 nm is necessary to attain effective anisotropy (K-eff) up to 0.14 MJ/m(3). On the other hand, Co/Pt multilayers with high PMA have been achieved for proper textured seed layers of Ru and Pt. In the case of Ru seed layer, a higher K-eff = 0.45 MJ/m(3) can be achieved for t(Co) = 0.5 nm and t(Pt) = 0.2 nm. This can be attributed to the lower lattice mismatch (similar to 4%) within the multilayers and hence a more coherent CoPt (111) structure. Finally, we note that the film roughness could play an important role in influencing the PMA of the multilayers. The highest K-eff similar to 0.6 MJ/m(3) is achieved for Pt seed layer. (C) 2015 AIP Publishing LLC.

We examine the effect of junction sizes on the magnetization reversal process and spin-transfer torque switching of the MgO-based CoFeB magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA). From the magnetic field transport measurements, it was found that the miniaturization of MTJs inherently enhances the switching asymmetry and the PMA of the soft layer. Our micromagnetic simulations confirmed that the dipolar field from the hard layer is responsible for the switching asymmetry and the increase in perpendicular shape anisotropy induces improvement of the PMA. It was further revealed that this additional anisotropy gained from the smaller MTJ sizes is not sufficient to sustain the thermal stability to meet the long-term information storage at the state-of-the-art complementary-metal–oxide semiconductor technology node. The pulsed spin-transfer torque measurements showed that a higher current density is needed to switch the magnetization of the soft layer in MTJ with smaller lateral dimensions, which is attributed to the increase in PMA.

We report on a study of shape effect on the flux guide (FG) reader output at an ultrahigh recording density, using micromagnetic simulations. A FG with a sharp, protruding reading edge is ineffective in magnetization reversal but is less susceptible to adjacent track interference. By using an FG with an acute, tapered angle and an elongated spade shape, a reversible magnetization can be achieved and the amplitude of the readback signal can be improved. The findings indicate that the shape of the FG can be tailored for effective control of the magnetoresistance signal for ultrahigh data storage density in hard disk drives.

We study the magnetization reversal process of tri-layer readers using micromagnetic simulations. The magnetoresistance response of tri-layer readers has been shown to be distinctive from the conventional spin valve giant magnetoresistance sensors. We look into the challenges for future high storage density when scaling down the read sensor size. The possible sensor reversal process control by engineering the sensor dimensions and material properties has been systematically investigated. We further observe an interesting dependence of magnetization states on the media bit transition length, which could be attributed to the small but finite physical separation between the two coupled ferromagnetic films. The findings in this work are useful for the design of tri-layer sensors for ultrahigh storage density in hard disk drives.