Qunwen Leng

Head and Media Challenges for 3 Tb/in Microwave-Assisted Magnetic Recording

A specific design for microwave assisted magnetic recording (MAMR) at about 3 Tb/in2 (0.47 Tb/cm2 or 4.7 Pb/m2) is discussed in detail to highlight the challenges of MAMR and to contrast its requirements with conventional perpendicular magnetic recording (PMR). In particular, it has been determined that MAMR-optimized media should have: higher damping than todays PMR media upon which ferromagnetic resonance measurements are reported, very low intergranular exchange coupling, and somewhat stronger layer to layer exchange coupling. It was found that with exchange-coupled composite type media (i.e., graded anisotropy with controlled exchange), a spin torque oscillator in the write gap of a wider shielded pole cannot adequately define the written track at high track density. Adequate lateral field gradient, however, is achieved by modifying the pole tip geometry. Other details of this conceptual design are also discussed.

Frequency-selective control of ferromagnetic resonance linewidth in magnetic multilayers

We report on a frequency-specific linewidth broadening of the ferromagnetic resonance (FMR) mode of a NiFe free layer within a magnetic multilayer stack. The FMR studies reveal a significant broadening of the FMR linewidth of the free layer at frequencies where this resonance is degenerate with FMR modes stemming from other layers within the multilayer stack. By pinning part of the magnetic multilayer to an antiferromagnet, we tailor a ferromagnetic linewidth behavior that is anisotropic for a specific frequency.

Damping in free layers of tunnel magneto-resistance readers

Damping properties in the free layers of tunnel magneto-resistance (TMR) readers were reported. The study used a free layer sample consisting of the same stacking structure as in TMR readers, involved comprehensive ferromagnetic resonance (FMR) measurements, and included numerical analysis of the FMR data. The results indicate that the free layer shows a Gilbert damping constant of 8.1 × 10−3. When the free layer is magnetized with a field which is not normal to the film plane, two-magnon scattering also occurs, although its contribution to the relaxation is at least 2.5 times smaller than the Gilbert contribution.

Influence of Boron Diffusion on Transport and Magnetic Properties in CoFeB/MgO/CoFeB Magnetic Tunnel Junction

Influence of boron concentration in CoFeB on the transport properties of CoFeB (B 20% and B 16%)/MgO/CoFeB magnetic tunnel junction (MTJ) was investigated. Boron distribution was studied by using X-ray photoelectron spectroscopy (XPS). High-resolution transmission electron microscope was utilized for analysis of the texture and interface quality. The MTJ with the boron diluted CoFeB (B 16%) pinned layer shows 10% higher MR than the CoFeB (B 20%). HRTEM shows that the MgO/CoFeB interface for the sample with diluted CoFeB layer has a better epitaxial MgO/CoFeB growth. Higher boron concentration in the as-deposited CoFeB stays high after annealing but boron content in the MgO increases as well. These results suggest that that MR improvement is directly related to the distribution of the boron in CoFeB/MgO/CoFeB MTJs after annealing.

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.

Modulation of interlayer exchange coupling strength in magnetic tunnel junctions via strain effect

Interlayer exchange coupling of two ferromagnetic electrodes separated by a thin MgO tunnel barrier is investigated using magneto-optical Kerr effect. We find that the coupling field can be reduced by more than 40% as the thickness of a top Ta capping layer increases from 0.5 to 1.2 nm. In contrast, a similar film stack with an additional 3 nm Ru capping layer displays no such dependence on Ta thickness. Transmission electron microscopy study shows that the oxidation of the exposed Ta capping layer induces changes in the crystalline structures of the underlying films, giving rise to the observed reduction of the interlayer coupling field.

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.

Optimization of tunneling magnetoresistance of MgO based tunnel junctions by tuning the stage impedance for radio frequency sputtering of the barrier

In this article, we describe a method to maximize the tunneling magnetoresistance of magnetic tunnel junctions with MgO tunneling barrier by optimizing the stage impedance for the RF MgO deposition. The impedance can be varied continuously using a matchbox that is in place to apply a RF bias to the stage. It is measured by means of a network analyzer connected to a dummy wafer on the stage. For positive stage reactance, both resistance area product and tunneling magnetoresistance are observed to drop, related to the stage impedance resonating with the plasma sheath capacitance. At high negative stage reactance, resputtering is minimized and tunneling magnetoresistance maximized.