M. A. Bashir

Microwave-assisted three-dimensional multilayer magnetic recording

Layer-selective writing of two layer bit patterned media is demonstrated by performing micromagnetic finite element simulations. Selectivity is achieved by controlling the frequency of an oscillating magnetic field in the gigahertz range, applied in addition to the head field. Generation of the microwave field by means of a wire next to the tip of a single pole head is proposed. The Oersted field from the alternating current induces magnetic oscillations in the pole tip which create a high frequency field that is superimposed to the perpendicular write field. The amplitude of the ac field component is in the order of 0.1 T.

Microwave-Assisted Magnetization Reversal in Exchange Spring Media

Presented here are micromagnetic simulations of the behavior of single phase media and exchange spring media for data storage devices under the influence of a microwave field. A reduction of the switching field by about a factor of two can be found in both single phase and exchange spring media when the microwave field reaches an amplitude of 12% of the remanent coercivity without microwave assist. It is shown here that the switching time for exchange spring media is less than that for single phase media due to the influence of soft upper layer which helps in reversing the magnetization along the opposite direction. The optimum microwave frequency that leads to the most effective reduction of the switching field depends on the angle of the applied field. At higher field angle the frequency band for successful microwave assist becomes smaller. In exchange spring media the frequency that leads to a maximum switching field reduction is smaller than in single phase media.

Switchable Cell Trapping Using Superparamagnetic Beads

Ni81Fe19 microwires are investigated as the basis of a switchable template for positioning magnetically labeled neural Schwann cells. Magnetic transmission X-ray microscopy and micromagnetic modeling show that magnetic domain walls can be created or removed in zigzagged structures by an applied magnetic field. Schwann cells containing superparamagnetic beads are trapped by the field emanating from the domain walls. The design allows Schwann cells to be organized on a surface to form a connected network and then released from the surface if required. As aligned Schwann cells can guide nerve regeneration, this technique is of value for developing glial-neuronal coculture models in the future treatment of peripheral nerve injuries.

Thermally induced adjacent track erasure in exchange spring media

Repeated writing causes adjacent track erasure. During multiple write cycles, the grains of neighboring tracks are exposed to the write field, which will lower the energy barrier. Grains in an adjacent track are likely to reverse their magnetization when the cumulated field expose time exceeds the transition time. The energy barrier of a grain subject to the write field is computed as a function of distance from the track center. For the same perpendicular writer and the same intergranular exchange field, the number of passes before erasure is orders of magnitudes higher in exchange spring media than in single phase media.

The incorporation of the Cauchy stress matrix tensor in micromagnetic simulations

A method of simulating complex deformational changes of magnetoelastic systems has been developed. This involves incorporation of the complete Cauchy stress matrix tensor into a finite element micromagnetic code. Finite element modeling was used to precompute the stress on each mesh-element in the studied model. This stress-map is then imported into a finite element micromagnetic code to solve the resultant changes in magnetization. The veracity of this method is demonstrated by comparing simulations of the Villari effect and a hybrid piezoelectric-piezomagnetic system to experimental observations.

Exchange bias interactions in polycrystalline/amorphous bilayers

For technologically relevant systems of polycrystalline antiferromagnetic layer coupled to an amorphous ferromagnetic layer, quantitative models and micromagnetic simulations are challenging due to inherent structural differences. We present a numerical study, performed using a surface integral technique with finite element micromagnetic simulations, that can incorporate structural and magnetic parameters such as grain crystallography, mixed spin-interface coupling and granular stability, arising from grain volume and anisotropy. We show that this model is in good agreement with experimental results for exchange bias and coercive fields as well as the training effect.

Exchange Coupled Bit Patterned Media Under the Influence of RF-Field Pulses

In this work, we have studied the switching of single phase material and exchange spring bit patterned islands under the influence of a microwave field. Micromagnetic simulations shows that the reduction of the switching field owing to a microwave assist field is most effective for an optimal value of the Gilbert damping constant of 0.05. The microwave assist field changes the angle dependence of the switching field. With increasing field angle the switching field can be reduced to 25% of the switching field at zero field angle.

In situ electron holography of the dynamic magnetic field emanating from a hard-disk drive writer

The proliferation of mobile devices in society accessing data via the “cloud” is imposing a dramatic increase in the amount of information to be stored on hard disk drives (HDD) used in servers. Forecasts are that areal densities will need to increase by as much as 35% compound per annum and by 2,020 cloud storage capacity will be around 7 zettabytes corresponding to areal densities of 2 Tb/in2. This requires increased performance from the magnetic pole of the electromagnetic writer in the read/write head in the HDD. Current state-of-art writing is undertaken by morphologically complex magnetic pole of sub 100 nm dimensions, in an environment of engineered magnetic shields and it needs to deliver strong directional magnetic field to areas on the recording media around 50 nm × 13 nm. This points to the need for a method to perform direct quantitative measurements of the magnetic field generated by the write pole at the nanometer scale. Here we report on the complete in situ quantitative mapping of the magnetic field generated by a functioning write pole in operation using electron holography. The results point the way towards a new nanoscale magnetic field source to further develop in situ transmission electron microscopy.

Tailoring Domain-Wall Dynamics With Uniaxial Anisotropy in Nanowires

Micromagnetic modeling is used to show that faster domain walls and higher Walker breakdown fields are achieved using materials with an in-plane uniaxial anisotropy weaker than the shape anisotropy. The domain-wall velocity increases by more than 30% if the uniaxial anisotropy direction changes from the long axis to the short axis of the wire. The superposition of uniaxial and shape anisotropies can be interpreted as a change in the effective wire width. Nonuniform uniaxial anisotropy is averaged over the wire thickness, suggesting that the domain-wall dynamics can be tailored by controlling the material anisotropy during fabrication.

Dynamic wetting in microfluidic droplet formation

The extent to which the carrier fluid wets the walls of a microchannel is crucial in the droplet formation process for segmented flow microfluidic applications and can be influenced by the use of surfactants. Surfactants dynamically modify the microchannel surface leading to stabilization of the two phase interface, affecting the droplet formation process. An experimental study of the influence of hydrophobic surfactant (Span 80) during the formation of water-inoil droplets in a T-shaped microchannel geometry is presented and the wetting properties of the microchannel walls were characterized. The range of data to be analyzed on the microscale is estimated from the macroscopic interfacial tension and contact angle measurements. The critical micelle concentration (CMC) level at the microscale was estimated by observing the trend of droplet length variation with concentration of surfactant in a microchannel. Microchannels used in this work were fabricated using softlithography methods and bonded using a custom-made plasma bonding setup that does not require an ultra high vacuum chamber and hence saves the fabrication cost.