Yuankai Zheng

The study of origin of interfacial perpendicular magnetic anisotropy in ultra-thin CoFeB layer on the top of MgO based magnetic tunnel junction

A comprehensive microstructure study has been conducted experimentally for identifying the origin or mechanism of perpendicular magnetic anisotropy (PMA) in the ultra-thin (10 A) CoFeB layer on the top of magnetic tunnel junction (MTJ). The high resolution transmission electron microscopy reveals that the feature of crystal structure in 10 A-CoFeB layer is localized in nature at the CoFeB-MgO interface. On the other hand, the strain-relaxed crystalline structure is observed in the thick CoFeB (20 A) layer at the CoFeB-MgO interface, associated with a series of dislocation formations. The electron energy loss spectroscopy further suggests that the local chemical stoichiometry of the ultra-thin 10 A-CoFeB layer is notably changed at the CoFeB-MgO interface, compared with an atomic stoichiometry in a thick 20 A-CoFeB layer. The origin of PMA mechanism is therefore identified experimentally as an interface effect, which can be attributed to a change of local atom bonding or lattice constant of the transition ...

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.

The uniformity study of non-oxide thin film at device level using electron energy loss spectroscopy

Electron energy loss spectroscopy (EELS) has been widely used as a chemical analysis technique to characterize materials chemical properties, such as element valence states, atoms/ions bonding environment. This study provides a new method to characterize physical properties (i.e., film uniformity, grain orientations) of non-oxide thin films in the magnetic device by using EELS microanalysis on scanning transmission electron microscope. This method is based on analyzing white line ratio of spectra and related extended energy loss fine structures so as to correlate it with thin film uniformity. This new approach can provide an effective and sensitive method to monitor/characterize thin film quality (i.e., uniformity) at atomic level for thin film development, which is especially useful for examining ultra-thin films (i.e., several nanometers) or embedded films in devices for industry applications. More importantly, this technique enables development of quantitative characterization of thin film uniformity a...

A TEM Structural Study of the Origin of Perpendicular Magnetic Anisotropy in Ultra-thin CoFeB Film

Perpendicular magnetic anisotropy (PMA) at ferro-magnetic transition metal/insulator interfaces has attracted intensive interest in the context of developing various spintronic devices. Magnetized out-ofplane magnetic tunnel junctions (p-MTJ) are now developed for spin transfer torque (STT) magnetic random access memories (STT-MRAM) applications where the strong perpendicular anisotropy originating from the CoFe/MgO interface allows to maintain the thermal stability of the storage layer magnetization down to at least 20nm technological node. MTJ stack possessing ferromagnetic layer with a large PMA is expected to provide novel features such as high thermal stability and low switching energy consumption. Ultra-thin CoFeB (CFB) films in contact with MgO show a large PMA, which is originated from the interface integration, and have been recently studied in MTJs’ stack [1]. Several studies tried to optimize the PMA and to study its dependence on the thickness of CFB layer [1], on the buffer layer [2] as well as the oxidation condition at the interface and the annealing temperature. Although PMA has been widely studied, the origin of its very large value in CFB/MgO system has not yet been fully unveiled. In this work, we have conducted a detailed TEM characterization for experimentally clarifying the PMA origin in ultra-thin CFB layer on the top of MTJs. Such study would certainly deepen our understanding of the mechanism of PMA in CFB thin film in MTJ stacks.

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.

A New Method to Characterize Non-oxide Thin Film Uniformity at Device Level using Electron Energy Loss Spectroscopy

High quality thin films become critically important as the dimensions in hard disk magnetic recording heads and media continue to shrink down to nanometer/angstrom. It is increasingly challenging to characterize film properties at the device level using conventional methodology developed for large area full films, such as x-ray diffraction, during fabrication process due to limited spatial resolution and sensitivity. Transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) has the unique advantage in providing angstrom level spatial resolution in location specific analysis. EELS technique has been well known for characterizing chemical bonding and valence state of transition metal oxide materials by studying the white line ratio and extended energy loss fine structure (EXELFS).