Ned Tabat

Atomic-scale analysis of CoFe/Cu and CoFe/NiFe interfaces

Internal interfaces in metallic multilayers grown on planar silicon substrates have been chemically analyzed with atomic resolution using three-dimensional atom probe microscopy. The structure studied was a NiFe/CoFe/Cu/CoFe multilayer grown with (111) texture. Atom probe measurements across the NiFe/CoFe interfaces yield widths of 1.1±0.2 nm for NiFe grown on CoFe and 1.7±0.2 nm for CoFe grown on NiFe. The widths of interfaces between CoFe and Cu layers vary as well, with values of 0.82±0.10 nm for CoFe grown on Cu, but only 0.47±0.15 nm for Cu grown on CoFe. In addition, the Fe concentration is enriched at the interface where Cu is grown on CoFe, and depleted where CoFe is grown on Cu. These results indicate that the Fe segregates to the surface during the deposition of CoFe so that the composition at the top of this layer is Fe rich.

Atom probe analysis of planar multilayer structures

Atom probe field ion microscopy has been used to analyze a planar-deposited layered structure in plan view. The specimens were prepared with a newly developed method that involves a combination of photolithography and focused ion-beam milling. A multilayer structure consisting of {Ta/CoFe/(Cu/CoFe)15/Ru/(CoFe/Ru)5/Ru/NiFe} was sputter deposited for use as a test stack. The corresponding thicknesses of these layers were 7/13(3/3)/50/(3/1)/50/150 nm. The nanometer-scale periodicity of the Cu/CoFe stack is readily apparent in transmission electron microscopy images of a field ion specimen fabricated from this material, suggesting that the specimen preparation procedure does not lead to destruction of the multilayer structure. Atom probe analysis of the bulk NiFe layer and the Ru/NiFe interface revealed the distribution of impurity atoms in the film, and these may affect the magnetic properties of the multilayers. Whereas a uniform distribution of C, N and Ar was observed, segregation of O was observed in the...

Noise and magnetic domain fluctuations in spin-valve GMR heads

Random telegraph noise (RTN) can be observed in the voltage across some spin valve heads over particular ranges of bias current, magnetic field and temperature. The detailed dependence of the RTN signal on these parameters demonstrates that the noise is due to thermally activated magnetic domain fluctuations. Further, a comparison of the effects of applied magnetic field and bias current provides information about the physical location of the magnetic domains within the head.

Tunable ferromagnetic resonance peak in tunneling magnetoresistive sensor structures

Noise properties of submicron scale tunneling magnetoresistive (TMR) sensors were investigated at frequencies up to 3 GHz. Noise spectral density was measured as a function of frequency, applied field, and bias current. Noise spectral density versus frequency dependence exhibits a pronounced peak, tunable over a wide frequency range. This peak appears to originate from current-driven precession of magnetization. The peak center frequency can be as low as 200 MHz and has a strong dependence on applied field and bias current. The damping constant α of the main precession mode in the TMR sensor free layer was found to be in the range of 0.05–0.18. It is shown that the magnetic state of a magnetoresistive sensor depends on the bias current and may be characterized by noise properties. The magnetoresistive element can operate as a source of high-frequency radiation with 1 nW emitting power from a 0.1 μm2 junction and signal to noise ratio of 10 dB.

Atom probe analysis of roughness and chemical intermixing in CoFe/Cu films (invited)

Three-dimensional atom probe analyses of the interfaces between CoFe and Cu layers has shown that both roughness and chemical intermixing can occur independently. Interfaces formed by the deposition of Cu onto CoFe mimic the roughness present in previously deposited interfaces, but have a very small amount of interfacial mixing. In contrast, interfaces formed by the deposition of CoFe onto Cu are less rough, but more chemically intermixed. The region of chemical intermixing formed when CoFe is deposited onto Cu (0.7–1.0 nm) is approximately two times larger than that when Cu is deposited onto CoFe (0.3–0.5 nm).

Perpendicular media: alloy versus multilayer

Properties and performance for alloy and multilayer perpendicular recording media designs utilizing a soft magnetic underlayer are compared. Among samples considered here, grain size and grain size dispersion are more highly refined for alloy media deposited at high substrate temperature, and are beginning to approach those now available in longitudinal recording. Multilayer media made at ambient temperature typically sacrifice film density and surface smoothness for interface quality. Although microstructural development and the manufacturing process for multilayer media are less mature versus alloy, multilayer media remain attractive due to their high anisotropy potential and the ease with which H/sub n/ and H/sub c/ can be controlled. For thermally stable alloy media made on a pilot production sputtering machine, a spin-stand areal density of 61 Gb/in/sup 2/ has been demonstrated at 350 Mb/s data rate with an on-track bit-error-rate reference level of 1e-6. Using the same media, a working perpendicular drive has been demonstrated at 32 Gb/in/sup 2/ and 500-800 Mb/s data rate.

Time-dependent fields and anisotropy dominated magnetic media

We use a single dipole approximation to analyze the behavior of anisotropy dominated magnetic nanoparticles subjected to an external rf field. Analysis of the steady state oscillations and their stability reveals the optimal conditions for the magnetization reversal by a combination of constant frequency rf field and a dc field.

Physical principles of microwave assisted magnetic recording

While the basic physics of Microwave Assisted Magnetization Reversal (MAMR) phenomenon is well established both theoretically and experimentally, its application in a practical magnetic recording environment was so far studied primarily with the help of micromagnetic recording models. In this work, we instead attempt to use analytical formulation and simple numerical models to understand the main challenges as well as benefits that are associated with such a system. It appears that the main difference between the previously introduced theory [G. Bertotti et al., Phys. Rev. Lett. 86, 724 (2001); K. Rivkin et al., Appl. Phys. Lett. 92, 153104 (2008); S. Okamoto et al., J. Appl. Phys. 107, 123914 (2010).] and recording environment is that both the RF and DC magnetic fields are applied at a substantial angle to the anisotropy axis. While the associated symmetry breaking prevents one from describing the reversal process explicitly, it is possible to approximate the solutions well enough to satisfactorily match numerical models both in the case of wire and Spin Torque Oscillator generated RF fields. This approach allows for physical explanation of various effects associated with MAMR such as high gradient of writeable anisotropy and reduction of track width, and offers a clear guidance regarding future optimization of MAMR recording.

Side-Track Erasure Processes in Perpendicular Recording

In perpendicular recording, substantial erasure of the stored data patterns could occur during writing process. Among all those erasure processes, the sidetrack erasure (STE) is one of the critical erasure issues. To sort out the intrinsic characteristics of the STE process and deepen our understanding of the underlying physics in the erasure processes, in this work, we have experimentally investigated the general attributes of STE process in various situations and quantified some of its distinctive signatures as well as some of its origins. Particularly, some STE behaviors have been characterized thoroughly by employing both the amplitude and BER based STE measurement methods in combination with other unique characterization techniques, in order to unambiguously reveal some of inherent features of the STE processes.

Ion beam deposition of permanent magnet layers for liftoff processes

Thin film permanent magnet layers of Cr/CoCrPt were prepared by ion beam deposition in a Veeco IBD-350 tool. The magnetic properties were measured as a function of deposition angle, deposition energy, assist energy, and the underlayer and permanent magnet thicknesses. It was found that higher deposition energies and angles (from normal) resulted in larger coercivities. Increasing the assist energy also helped increase the coercivity. The coercivities were in the range of 1400–2100 Oe depending on the deposition conditions, for a Cr-50 A/CoCrPt-250 A film. The squareness, S (the ratio of remnant magnetization to saturation magnetization) and Mrt (remnant magnetization and film thickness product) for these films were about 0.85 and 1.3 memu/cm2, respectively. It was found that underlayers of Cr as thin as 25 A could be used, without significant degradation of magnetic properties. The coercivity had a broad peak between 100 and 200 A of CoCrPt, above which the coercivity gradually decreased. θ–2θ x-ray diffr...