Daniele Mauri

A Three-Terminal Approach to Developing Spin-Torque Written Magnetic Random Access Memory Cells

Using a self-aligned fabrication process together with multiple-step aligned electron beam lithography, we have developed a nanopillar structure where a third contact can be made to any point within a thin-film multilayer stack. This substantially enhances the versatility of the device by providing the means to apply independent electrical biases to two separate parts of the structure. Here, we demonstrate a joint magnetic spin-valve (SV)/tunnel junction structure sharing a common free layer nanomagnet contacted by this third electrode. A spatially nonuniform spin-polarized current flowing into the free layer via the low-resistance SV path can reverse the magnetic orientation of the free layer as a consequence of the spin-torque (ST) effect, by nucleating a reversal domain at the spin injection site that propagates across the free layer. The free layer magnetic state can then be read out separately via the higher resistance magnetic tunnel junction (MTJ). This three-terminal structure provides a strategy for developing high-performance ST magnetic random access memory (ST-MRAM) cells, which avoids the need to apply a large voltage across a MTJ during the writing step, thereby enhancing device reliability, while retaining the benefits of a high-impedance MTJ for read-out.

Spin torque, tunnel-current spin polarization, and magnetoresistance in MgO magnetic tunnel junctions.

We employ the spin-torque response of magnetic tunnel junctions with ultrathin MgO tunnel barrier layers to investigate the relationship between spin transfer and tunnel magnetoresistance (TMR) under finite bias, and find that the spin torque per unit current exerted on the free layer decreases by < 10% over a bias range where the TMR decreases by > 40%. This is inconsistent with free-electron-like spin-polarized tunneling and reduced-surface-magnetism models of the TMR bias dependence, but is consistent with magnetic-state-dependent decay lengths in the tunnel barrier.

A Dual Free Layer Sensor With Side Shields

We use a hybrid finite element code to explore the design space of a dual free layer (or trilayer) magnetic sensor to understand the effect of the stripe height (SH) to track width (TW) ratio on the bias point, as well as the noise. Transfer curves and noise properties are found to be strongly sensitive to the SH/TW ratio compared to a standard single free layer sensor. Side shields can change the eigenmodes of the coupled system and modify the noise cancellation.

Advanced Dual-Free-Layer CPP GMR Sensors for High-Density Magnetic Recording

All-metal current-perpendicular-to-plane (CPP) giant magnetoresistance (GMR) read sensors with a shield-to-shield spacing (S2S) of 16-21 nm and a narrow track width of down to 25 nm were fabricated using ferromagnetic CoFeMnSi Heusler-alloy-based spin valves. Room temperature GMR ratios from these read sensors are obtained of up to 6% and 14-24% (ARA = 7.1-12.0 mΩμm2) at S2S = 16 and 21 nm, respectively. Studies and results of electron transport and CPP GMR support the sustainability and scalability of the CPP GMR process for Tb/in2 the areal density of magnetic recording. A universal parameter defined as magnetic resistivity for a sensor device, ΔRA/S2S in ohm micrometers, is proposed to gauge the practically and rationally applicable CPP GMR for the read sensor process. The investigation of the CPP GMR operation range and micromagnetic simulation demonstrates the feasibility of the CPP GMR read sensors at S2S = 21 nm for sustaining 1.0 Tb/in2 and of those at S2S = 16 nm for marginally supporting 2.0 Tb/in2 the areal density of magnetic recording, The future path to and potential of the technology for ever increasing areal density beyond 2.0 Tb/in2 are addressed with emphasis on the importance of further enhancing the CPP GMR for process margin improvement.

Shield Design for Enhanced Reader Resolution

We propose a shield design that enhances linear resolution at a given shield-to-shield spacing. Patterning tabs in the cross-track direction of the shields at the read gap introduces a transverse anisotropy that can be further enhanced by using a high-moment material in the shield near the sensor. We use a finite element micromagnetic code to explore the performance of a read head embodying this design. We study the dependence of resolution on both the thickness and the moment of the shield tabs. The increased shield rotation improves the sensors linear resolution by more than 2% in absolute terms.

Influence of Parasitic Capacitance on Single and Dual 2-D Magnetic Recording Read Head Performance

The parasitic capacitance of the reader affects performance through signal-to-noise ratio (SNR) in single readers and through crosstalk in dual 2-D magnetic recording readers. In a single reader, the majority of the capacitance is not from shield-to-shield capacitance but instead from lead-to-shield capacitance. The low-pass filtering of the reader circuit leads to an equalized SNR that depends on the parasitic capacitance. In a dual reader, the capacitance between the outer shields must be considered in addition to the capacitance between the inner shields in order to model accurately the crosstalk. Finite-element electromagnetic models and lumped element circuit models are validated with RF experiments on real heads.

Suppression of spin pumping with insulating layers

The damping constant of thin magnetic layers is a combination of an intrinsic part related to the material itself and an extrinsic contribution related to the spin pumping effect. Here a thermal fluctuation of the magnetization is creating a spin current that leads to a loss of angular momentum and increase in damping constant. For many applications (e.g. magnetic read heads) it is desirable to have magnetic layers with lowest damping constant as possible to reduce thermal fluctuations and noise. To reduce the spin pumping the layer next to the magnetic film should have small spin flip scattering rate and high rate of momentum scattering. Insulating layers possess these properties but are problematic to use in a CPP type sensor because of the restriction of current flow. In a sufficiently thin tunnel barrier however the RA can be very low. Here we show data on damping constant of a magnetic layer sandwiched between two tunnel barriers and examine how the extrinsic damping constant varies with the RA of the barrier. >10% reduction of damping constant can be achieved with RA values that are 10 times smaller than typically used in a magnetic read head. We also characterize the magnetic properties (Ms, Hk, magnetostriction) as a function of barrier RA. B.K. and T.M acknowledge support by NSF-CAREER Award No. 0952929.

Magnetic Heusler alloys and CPP GMR: Technology breakthrough and potential application in magnetic recording

Magnetic Heusler alloys that benefit from their half-metal characteristics have recently seen significant progresses in material researches and process development. As a result, current perpendicular to plane (CPP) giant magnetoresistance (GMR) has been proportionally enhanced, at least but not limited, by an order of magnitude in devices that contain such magnetic Heusler alloys and all-metal layer stacking. Amongst a wide selection of ferromagnetic Heusler alloys, Co2MnSi and its variations show good process compatibility and high spin polarization that yields large CPP GMRs in spin valves. Recent experiments in Heusler alloy based spin valve structures epitaxially-grown on MgO (001) substrates have shown the room temperature ΔR/R can be as large as 75% in the CoMnFeSi Heusler alloy based pseudo spin valves grown on MgO (001) substrates. As a major application, CPP GMR reader technology has been extensively investigated in the last few years in response for the demand for increasing areal density in magnetic recording. One of recent industrial efforts shows that ΔR/R of 18 %, ΔRA= 9.0 mΩ μm2, is achievable in the reader sensors fabricated using the same CoMnFeSi Heusler alloy based and antiferromagnetically pinned spin valves grown on AlTiC wafers. First and most important, this implication of these results is that the advance of technology provides large potential to the CPP GMR in future reader sensor development to accommodate all the requirements for SNR improvement and solution to spin torque effect induced instability in devices. Second, a large compromise in the CPP GMR is observed when the film stack or the reader sensor gap is reduced in thickness. This originates from the nature of stack-structure-dependent electron transport and process imperfectness and constraints in reader sensor building. With strict requirement for high areal density recording at 1TB/in2 and beyond, for the time being, dealing with this compromise with the scaling down of the reader sensor gap will be a major challenge and the focus of effort to better shape this technology as a success. This talk will briefly review and discuss recent magnetic Heusler material and reader sensor development and limiting factors that might affect the use of such magnetic material in device fabrication and operation.