Thomas C. Anthony

Magnetoresistance of symmetric spin valve structures

A spin valve configuration is presented in which an unpinned ferromagnetic film is separated from exchange-pinned ferromagnetic films on either side by two nonmagnetic spacers, thereby creating a symmetric spin valve structure. The symmetric spin valve is shown to increase the magnetoresistance by 50% over the values of individual spin valves. The increase is attributed to a reduction of spin-independent outer boundary scattering and doubling the number of spin-dependent scattering interfaces. The magnetoresistance and coupling fields of the spin valves that comprise the symmetric structure have been measured as a function of Cu spacer thickness. Oscillatory antiferromagnetic to ferromagnetic coupling was observed in standard spin valves, which had /spl Delta/R/R as high as 13%. >

Magnetoresistance values exceeding 21% in symmetric spin valves

We report values of the giant magnetoresistance (GMR) effect exceeding 21% in symmetric spin valves, the highest values ever reported for such structures. The key elements in this achievement are the use of a Co/Cu/Co/Cu/Co multilayer in which the center Co layer is substantially thicker than the outer Co layers and the use of the antiferromagnetic insulator NiO at the top and bottom to pin the adjacent Co layers magnetically. The relative Co layer thicknesses suggest that some specular scattering of conduction electrons may occur at the metal/insulator interfaces and may enhance the GMR.

Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)

We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses.

Thermal stability of magneto‐optic quadrilayers

The thermal stability of sputtered TbFeCo thin films in magneto‐optic quadrilayer structures containing Si3N4, SiO, and SiO2 dielectrics has been examined. The observed changes in coercivity upon annealing are attributed to two parallel mechanisms: structural relaxation in the amorphous magnetic alloy and preferential oxidation of Tb at the TbFeCo/SiO and TbFeCo/SiO2 interfaces. Kinetic modeling has revealed that a spectrum of activation energies is required to explain the relaxation data, whereas a single activation energy of 1.65 eV describes the oxidation process.

Magnetic Recording Head Materials

The materials used in magnetic recording heads have recently received a tremendous amount of attention. This has been the result of a fortunate set of circumstances. Ever-increasing demands for information storage, especially for graphics-intensive applications, have necessitated unprecedented increases in disk-drive areal densities. Combined with this are recent discoveries in the area of magnetoresistive materials, enabling the design and fabrication of much more sensitive recording heads. The end result is a flurry of activity that has come to dominate the field of magnetics. This article will explore choices for magnetoresistive read head materials, with an emphasis on the materials challenges. The recording heads that are used in high-performance disk drives typically consist of separate magnetoresistive read and inductive write heads (see Figure 1) where previously a single inductive head performed both functions. Separation of the two heads allows each to be optimized for their individual function, an essential factor in enabling disk drives to contain gigabytes of storage. The write head is the simpler of the two, consisting of a U-shaped ferromagnet surrounding a set of coils. The ends of the ferromagnet are the magnetic poles defining the write gap. When current passes through the coils, a field bridges the gap, setting the orientation of the magnetization in the media. Information is stored by changing the polarity of the current in order to write a pattern of magnetic domains in the media. The materials used in write poles will be reviewed in the section, Write Head Materials.

Spin-dependent tunneling junctions with AlN and AlON barriers

We report on ferromagnetic spin-dependent tunneling (SDT) junctions with NiFe/AlN/NiFe and NiFe/AlON/NiFe structures. Good barriers were formed by plasma nitridation and oxy-nitridation of Al films. Tunneling magnetoresistance ratios (TMR) up to 18% were observed at room temperature. The devices exhibit lower resistance-area products than those seen in reference junctions with Al2O3 barriers. The degradation in TMR at higher bias voltages is found to be less than that found in standard alumina junctions. AlN and AlON could thus be alternate materials for the tunnel barrier in SDT junctions.

Dual stripe magnetoresistive heads for high density recording

The design and recording performance of dual stripe magnetoresistive read/inductive write heads with read widths of 4 /spl mu/m and write widths of 4.5 /spl mu/m are described. A linear density of 75 kfci (D50) was measured in heads with shield-to-shield spacing of 420 nm and 70 nm of dielectric separating the two magnetoresistive stripes. Large signal amplitude, linear cross-track profile, and good second harmonic suppression are observed in accordance with theoretical expectations. Readback waveforms contain little baseline shift and the ratio of positive to negative peak amplitudes is very close to unity. Stable signals are seen for heads with and without exchange stabilization. Conductor topography in the read head is replicated in the write head and can adversely affect cross-track behavior. Non-planarity of the write head must be considered in the design of shared pole magnetoresistive heads. >

Exploration of magnetization reversal and coercivity of epitaxial NiO {111}/NiFe films

We have grown epitaxial NiO {111} films of thicknesses ranging from 60 to 1200 A, deposited 45 A NiFe films on these NiO substrates, and made measurements of exchange field and coercivity, of the effective uniaxial anisotropy, of rotational hysteresis, and of the training effect on these films. We find that the large coercive fields, ∼500 Oe, observed in these epitaxial systems can be understood using a model in which the magnetization reversal process is by rotation, with the coercive field determined by the effective uniaxial anisotropy of the system. This effective anisotropy is in turn determined by the anisotropy of the NiO and depends on NiO thickness.

Three-dimensional atom probe field-ion microscopy observation of Cu/Co multilayer film structures

Focused ion-beam milling has been used to fabricate field-ion specimens from a multilayer film structure containing 100 repetitions of a (Cu2 nm/Co2 nm) bilayer deposited directly onto a planar substrate. The as-deposited films showed a magnetoresistance ratio of ∼5% over a 250 Oe range at room temperature, and a coercivity of ∼60 Oe. The magnetic data suggest that the films are coupled ferromagnetically. Successful field-ion specimen preparation has allowed the observation of these layers by field-ion imaging and three-dimensional atom probe compositional analysis. Examination of the multilayer images reveals that, in some regions, the layers are nonparallel, but the interfaces are chemically quite sharp, with a diffuse interface region of ∼3 atomic layers. In addition, in some areas adjacent cobalt layers appear to be in contact. The fact that the layers are wavy suggests that the ferromagnetic coupling may be a result of Neel “orange peel” type magnetostatic coupling between adjacent cobalt layers. The...

Impact of new magnetoresistive materials on magnetic recording heads (invited)

Advances in magnetoresistive materials have recently enabled magnetic recording heads to achieve higher levels of performance. This article describes why higher signal outputs are necessary for improvements to be made in areal density. The requirements for recording at an areal density of 16 Mb/mm2 (10 Gb/in.2) are discussed with regards to both the channel and the head design. Increased output from new multilayer magnetoresistive materials is required to counteract the decrease in output due to the reduction in the size of the head geometry. An areal density of 16 Mb/mm2 is shown to be feasible with spin valve recording heads using materials with magnetoresistance ratios of 10%. Fabrication issues relating to the manufacturing of these materials are shown to be more stringent than previously required.