Stripe Head Research Articles

Characteristics and performance of a 1 kbit Bloch line memory prototype

The design of a Bloch line memory prototype is presented. The different basic functions are studied experimentally. Two aspects of minor loop initialization are considered: memory clearing and bias field stability. Minor loop stability margins depend on material parameters and the groove design. The propagation of line pairs as well as line clusters is studied. The maximum line displacement is 2 mu /pulse. Bit propagation depends on the fall time of the vertical field pulse. At short fall time, pinning points hamper bit propagation. Preliminary results of the input/output gate show that the basic functions such as stripe head stretching, line control, and chopping are compatible. The major loop operation, including bubble propagation and stretching, is demonstrated. >

Propagation of Vertical Bloch Line Pairs around Stripe Head by Single Phase Current

This note reports on a new method to propagate vertical Bloch line (VBL) pairs, which are the information units in the recently proposed VBL memory, around stripe heads by single phase current. The method reported has simpler conductor patterns than the method to propagate by two circuit currents. The bias field margin for the propagation has been obtained by computer simulation and the reliability of the method has been confirmed.

Controlling Bloch lines and domain chopping for multiple stripes aligned in parallel

Controlling vertical Bloch line (VBL) position and stripe domain chopping in multiple stripe domains, aligned in parallel by a period of 12 μm, are studied experimentally using a 5-μm bubble garnet film. Patterned high coercive-force CoPt films aligned in parallel are used for stabilizing the stripe domain. During the stripe stretching process, the VBL motion at the stripe head was controlled by the dc conductor current to get two different wall states: one with a VBL at the stripe head and the other without VBL. Stripe chopping experiments for VBL detection have been performed using hairpin and meandering conductors. We found that the meandering conductor presents excellent characteristics; the bias field margin and the chopping pulse amplitude margin are 58 Oe±6% and 133 mA±11%, respectively, for one stripe. VBL generation by the meandering conductor has also been studied using the same sample.

Computer simulation of vertical Bloch line propagation around stripe domain head

Vertical Bloch line pair propagation at the stripe domain head is studied by computer simulation. Several numerical techniques in the calculation are adopted to save computation time. The time step and the grid points interval are varied through the iterative computation. The side wall of the stripe domain is assumed to be straight to simulate a long domain with small number of grid points. Simulation results reveal that a thickness step inside the stripe domain is effective to control the wall motion. A half of the film thickness is chosen for the step depth. The VBL pairs at the stripe head can be propagated one bit period by applying a collapse bias pulse.

Operation of a VBL memory write gate

A write gate for the VBL memory was designed and fabricated on an as-grown garnet wafer supporting 5 μm bubbles. Pictures of the transient stripe during the write operation were taken showing the effectiveness of the gate design in controlling the stripe head motion and the generation of the VBL's in the head, There are two regions of operational margin for the expanding current, one is between 23 and 48 mA, with 500 to 700 nsec pulse width, the other is between 68 and 92 mA. with 300 to 700 nsec pulse width. The operation of the write gate is independent of the in-plane field.

Bloch lines generation in moving stripe heads

The generation of vertical Bloch lines (VBL’s) at the head of a moving stripe domain with and without an in-plane field was studied. For an as-grown sample, the velocity saturation is independent of in-plane field direction and the same velocity as an implanted sample when the in-plane field direction is the same as the stray field in the implanted layer. When the in-plane field direction was reversed, the wall saturation velocity is higher. Overshoot starts when the wall velocity approaches the saturation velocity and increases with drive field as the stripe head becomes asymmetrically distorted. Eventually, the stripe will move at an angle. The presence and motion of VBL’s was inferred from the shape of the stripe head during and at the end of the applied pulse field, the bending of the stripe, and the presence of overshoot. In the ion-implanted film, after the generation of a pair of VBL’s on the stripe head, the positive (negative) VBL was pushed into the straight part of the stripe when the in-plane field direction is opposite (parallel) to the stray field direction in the implanted layer at the stripe head. In the as-grown film, which kind of VBL is pushed away seems random and independent of the in-plane field direction and amplitude.

磁気バブル伸長型検出器でのストライプ磁区動特性

Stripe domin head dynamics for a strong drive field in a current stretcher bubble detector have been studied for three different component garnet films with different dynamic properties. It has been observed that the stripe head velocity is sometimes very small in the beginning of stretching in a hard bubble suppressed region and that there are two streching modes with fast rise and slow rise times in an unimplanted stretching region. The stripe head mobilities in the fast rise mode are approximately 1/3 the plane wall mobilities. The mobilities in the slow rise mode are mostly proportional to Gilbert’s damping parameters. These two modes are discussed as originating from vertical Bloch lines at the stripe head by using Slonczewski’s velocity equation for a adomain wall with Bloch lines. Saturation velocities obtained for two modes are 42 m/s, which is nearly equal to plane wall peak velocity.

Stretching of stripe domains in ion-implanted channels

The dynamic behavior of stripe domains stretching under the influence of a surrounding hairpin conductor and ion-implanted channel is investigated using high speed optical sampling techniques. Experiments were conducted on 2μm bubble supporting material. Various channel widths, ranging from 1.5μm to 3.0μm were used. The direction and magnitude of the applied in-plane field were also varied. An initial linear mobility μ 1 is observed followed by a sharp drop in velocity and another linear region with mobility μ 2 1 . This second region is accompanied by an overshoot phenomenon in which the stripe head is observed to move after the termination of the drive field. In addition, deflection of the stripe head, presumably by gyroscopic forces, is observed in the high drive region, particularly in the wider channels. These results suggest the formation of vertical wall structures during the stretching process. Optimal phase angles for stretching and de-stretching(collapse) in the absence of a permalloy sense element have been found that are based on measurements of the critical bias field for bubble-stripe transition as a functlon of in-plane field angle.

Velocity asymmetry during stripe head propagation in ion-implanted bubble garnet films

Stripe head propagation has been investigated in ion-implanted garnet films using high speed photography with 10 nsec laser pulse illumination. For a given in-plane field applied along the stripe, the stripe head velocity increases linearly with drive until a peak drive is reached, beyond which the velocity drops. The peak velocity increases with in-plane field magnitude and depends on which of the two directions parallel to the stripe that the in-plane field is applied. Numerical calculations of the dynamic wall structure are made to predict the peak velocity as a function of in-plane field magnitude and direction. The effect of the implanted layer is taken into account by using a surface pinning condition. Results show that velocity breakdown occurs when a horizontal Bloch line (HBL) is nucleated. The velocity asymmetry with respect to in-plane field direction occurs because the HBL nucleates at opposite surfaces for the two in-plane field directions. The implanted layer inhibits HBL nucleation by pinning the surface spins, thus producing a higher peak velocity.

Measurement of stripe head velocity using magneto-resistance signal

Stripe head velocity has been measured by monitoring the signal from a straight magneto-resistive element as a stripe expands and contracts along the element. The MR element provides a path for the stripe as well as indicating its position versus time. This new measurement technique allows continuous monitoring of the stripe velocity as drive and in-plane field are varied. An ion-implanted garnet film of composition Y1.95Sm0.09Lu0.09Ca0.87Ge0.87Fe4.13O 12 with a thickness of 2.0 μm was measured using an MR element 0.04 μm thick, 4 μm wide, and 150 μm in length. The in-plane field H1 is approximately perpendicular to the stripe head velocity. Two types of velocity behavior are observed depending on the direction of H1×V. For H1×V pointing toward the implanted surface, the velocity versus drive depends on the H1 magnitude. For H1<50 Oe, the velocity versus drive is linear up to a velocity instability above which the velocity tends to saturate. The instability is assigned as a horizontal Bloch line motion. For H1≳100 Oe, the velocity increases to a much higher value which is in agreement with the terminal velocity formula, (π/2) γΔH1. For 50<H1<100 Oe, the velocity curve shows the horizontal Bloch line instability; but at higher drive the velocity suddenly increases up to the terminal velocity. For H1×V pointing toward the nonimplanted surface, the velocity curve has a weak instability at low drive followed by a concave shape.

Low drive stripe head velocity in bubble garnet material

By observing the motion of an unpinned stripe head with a sampling optical microscope, domain wall motion in Y1.57Eu0.78Tm0.65Ga1.05Fe3.95O12 bubble garnet has been investigated at constant drive in the low field range from well below the Slonczewski critical field, through the Walker critical field, and up to the region previously investigated by radial expansion. A low field mobility of 400 cm/sec Oe was measured compared to 1020 cm/sec Oe predicted theoretically. The velocity increases smoothly as the drive field increases with no evidence of any peak velocity as predicted theoretically. A saturation velocity of about 7 m/sec was observed in good agreement with the radial expansion data considering the large difference in the mode of motion. Overshoot was observed in the stripe head motion which varied somewhat with drive and seemed to be essentially constant for the long propagations involved. Since the overshoot could easily be made less than 10% of the total distance, stripe head propagation is a good method for materials characterization.