T. W. Clinton

Near-field microwave probe for local ferromagnetic resonance characterization

A local ferromagnetic resonance (FMR) technique using a thin-film microwave probe fabricated on coaxial transmission line is presented. Some key features of this probe are its micron-scale size coupled with broadband FMR characterization and mobility: a small area of any location on a magnetic medium can be probed. Permeability of Permalloy (Py) films has been measured, where the authors extract physical parameters, such as FMR frequency, anisotropy field, damping, and spin wave exchange constant, in excellent agreement with independent measurements. Samples of arbitrary shape and size can be measured with a spatial resolution that scales with the size of the probe “tip.”

Measurement of Ga implantation profiles in the sidewall and bottom of focused-ion-beam-etched structures

Ga implantation during focused-ion-beam (FIB) milling of metallic magnetic materials for magnetic recording heads was examined using Auger electron spectroscopy (AES). The Ga concentration profiles were characterized in two directions: in the depth direction (parallel to the incident beam) and in the lateral direction (perpendicular to the incident beam). The sample for AES analyses in the depth direction was prepared by FIB etching of a plated Ni78.5Fe21.5 film surface. The AES depth profile showed a maximum implanted Ga concentration of ∼20 at. % at a distance of ∼15 nm, and the tail of the implantation profile reached more than 100 nm. The sample for the lateral direction analyses was prepared by cutting a cross-sectional specimen out of the air-bearing surface of a working recording head. The lateral profile showed a maximum implanted Ga concentration of 47 at. % at the surface, with a lateral implantation depth of ∼10 nm. These results were compared with results predicted by a Monte Carlo implantatio...

The effect of media background on reading and writing in perpendicular recording

The effect of stray fields from the recording layer on the reading and writing process has been studied for a perpendicular recording system. The system consists of a single pole writer with a wide return pole, a conventional spin-valve reader and a double layer recording medium. A writer- and reader-induced asymmetry is observed in the recording process for a dc erased background. The asymmetry is interpreted as stray fields of the dc background giving rise to reader saturation and an offset in the current driven inductive write field.

Voltage-dependent ferromagnetic resonance in epitaxial multiferroic nanocomposites

We demonstrate electrical control of the ferromagnetic resonance (FMR) in multiferroic nanostructures. A series of heteroepitaxial BiFeO3–NiFe2O4 nanocomposites of varying thickness are characterized using a microwave probe with magnetic and electric sensitivity. We apply an electric field to a sample and observe voltage-driven shifts in the FMR frequency, reflecting a change in magnetic anisotropy. The voltage dependence of the FMR linewidths is even more pronounced, indicating the electric polarization can induce relatively large magnetic nonuniformity in the material. These characteristics may lead to a class of rf filters where both frequency and bandwidth are electrically tunable.

Comparison of perpendicular and longitudinal magnetic recording using a contact write/read tester

We have performed longitudinal and perpendicular magnetic recording measurements using a contact write/read tester, which scans a magnetic recording head in contact with the recording media. Our tester has a demonstrated positioning resolution of <5 nm. Perpendicular transitions in the range 125-425 kfci were recorded on double-layer perpendicular media with a high moment soft underlayer. The perpendicular bits were imaged using both a giant magnetoresistance (GMR) read head on the contact tester and using magnetic force microscopy (MFM). Longitudinal bit transitions with linear densities in the range 25 to 725 kfci were recorded, and the signal-to-noise ratios of both the perpendicular and longitudinal systems were measured.

Cutoff wavelength of ridge waveguide near field transducer for disk data storage.

The electromagnetic eigenmodes of and light transmission through a C-aperture to the far field, and to a storage medium, have been studied based on the full vectorial finite difference method. It is found that the cutoff wavelength of C-aperture waveguides in a gold film is much longer than that in a perfect electric conductor, and the fundamental mode is confined in the gap and polarized with the electric field along the gap. The light transmission resonance through C-apertures to far field and to a storage medium occurs at wavelengths below the waveguide cutoff wavelength. Measurements on the fabricated C-apertures confirm the mode confinement and transmission resonance.

Comparison of a near-field ferromagnetic resonance probe with pump-probe characterization of CoCrPt media

A near-field microwave technique is used to locally probe ferromagnetic resonance (FMR) in a series of CoCrPt alloys with varying perpendicular anisotropy (5kOe<Hk<15kOe). We observe broad FMR linewidths Δf on the order of a gigahertz and higher, indicating significant damping and sample inhomogeneities. These results are compared to time-domain measurements of the same samples using a pump-probe technique, where we observe relaxation times τ, considerably shorter than 500ps, using a damped sinusoid to model the magnetization dynamics. We find the local FMR measurements to yield time scales consistent with the pump-probe data, using the Fourier relationship, Δf=1∕πτ. Thus, this near-field technique is capable of quantitative characterization of high-anisotropy and highly damped magnetic systems, something that has not been demonstrated before with a local FMR technique.

Magnetic Configurations and Phase Diagrams of Sub-100-nm NiFe Nanorings

Using the micromagnetics package MAGPAR, we study the ground states of NiFe nanorings with sub-100-nm lateral dimensions, in zero external field. We solve the Landau-Lifschitz-Gilbert equation for three different initial magnetizations (in plane, out of plane, and vortex) to obtain the lowest energy state. Plotting the total energy as a function of thickness, along with its corresponding magnetostatic and exchange energies, we are able to identify various phase transitions and derive phase diagrams as a function of thickness and outer radius, normalized to the exchange length, for rings with different inner to outer radius ratios. We discuss the results in terms of shape anisotropy and its effect on the magnetostatic and exchange energies. We also compare the results of our numerical method to the phase diagram of a nanodot and the phase diagrams of rings obtained by analytical models. Finally, we present a new ground state configuration, the helix, found along the boundary between the vortex and out of plane phases, where the magnetization is vortex like, but with moments canted along the z direction

Effects of transition curvature on perpendicular magnetic recording performance

The effect of magnetic transition curvature has been studied for a perpendicular recording system. The curvature of the medium transitions is varied in a controlled manner by altering the write poles using a focused ion beam. Tracks are written using these write heads and are characterized with a magnetic force microscope, which demonstrates the ability to manipulate transition shape and even correct the curvature. The effect of transition curvature on the perpendicular recording performance is characterized by spinstand measurements and bit error rate simulations. These results show a significant performance loss, demonstrating that transition curvature degrades perpendicular recording performance.

Fabrication and characterization of focused-ion-beam trimmed write heads for perpendicular magnetic recording

A focused ion beam (FIB) has been used to trim write heads for perpendicular magnetic recording using untrimmed HGA-level longitudinal heads. The ion-beam imaging of the write head during FIB processing was minimized to limit exposure of the active magnetic material at the ABS to a 30 keV Ga+ ion dose of less than 1014 Ga+/cm2 (≈10−13 C/μm2) (the GMR reader was never exposed), which is significantly below levels where magnetic properties have been observed to degrade [W. M. Kaminsky et al., Appl. Phys. Lett. 78, 1589 (2001)]. The corresponding recording characteristics and spatial profiles of written tracks have been measured on a spin stand and a magnetic force microscope (MFM). Recording performance, such as SNR, and pulse shape of transitions, for example, as a function of head design and FIB processing is discussed, which compares very favorably to the performance of untrimmed heads. The MFM images reveal curvature in the magnetic transitions (transition smile) when writing with a single-pole writer w...