IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-21, NO. 5 , SEPTEMBER 1985 1785

Low Power Microwave Bubble Generator

D. J. Seagle, J. 0. Artrnan, and S. H. Charap Department of Electrical and Computer Engineering,

Carnegie-Mellon University, Pittsburgh, Pennsylvania i52i3

Abstract In this paper we discuss experimental results, dwelling particularly on the frequency sensitivity of ,the minimum microwave field necessary for bubble generation. A broad- band coplanar waveguide circuit was developed to provide leveled microwave power to a 4 pm by 4 pm shorting region producing highly local and intense microwave magnetic fields. In the samples employed, extremely low microwave fields generate bubbles, implying power dissipation in the pW range., We feel that when the system is scaled up to the large anisotropy field values of conventional mag,netic bubble devices, the power dissipation will increase dramatically. However, our experimentation has revealed that microwave pulses shorter than 25 nsec may be used to generate bubbles. Due to the anticipated low average power dissipation, such a device may have an advantage over conventional bubble generators.

1 Introduction

The practicality of a microwave-induced magnetic bubble generator as an alternative to a conventional generator in a magnetic bubble memory device has been discussed previously'. Past experiments have dealt with excitation at the small-signal ferromagnetic. resonance frequency of the thin film', 2; this frequency, however, has no special significance in this connection since the effective anisotropy field drops off as the cosine of the precession angle. There have been reports' of precession angles as high as 44 a , implying a resonance frequency change of as much as 30% as the precession angle increases from 0 " . We have previously reported on a theory for magnetic bubble generation which predicts a minimum in the microwave power necessary to generate a bubble at frequencies substantially lower than the small signal FMR value3' '. Here we report on experimentation with a broad-band microwave magnetic bubble generator which has been used to optimize bubble generation with respect to frequency.

It should also be noted that the effect of the microwave damping parameter, a , on the magnetic response should not be too consequential. The magnitude of the susceptibility is significantly affected by the magnitude of a only when the internal field is within several line widths of resonance.

2 Apparatus

In Fig. 1 we show the apparatus employed in our experiments. The Hewlett Packard 8350B sweep oscillator was set to CW in a frequency range from 100 M H z to 1 GHz. Microwave power could be set from -2 dBm to +20. dBm and pulse duration could be timed manually or electronically as fast as 10 msec. At other times, the sweep oscillator was set to the CW mode and pulses as short as 25 nsec in duration were obtained with a PIN attenuator. The PIN attenuator was a TTL-compatible PIN diode switch controlled by the Hewlett Packard 8111A pulse generator.

The water cooled coils, powered by a Crown DC 300A amplifier, were capable of supplying a 1 kOe magnetic field perpendicular to the plane of the sample. The top coil had a hole cut in the center large enough to allow the lowering of a microscope objective above the sample. Domain observation by Faraday rotation was performed simultaneously with microwave excitation; this was facilitated by a 100 W mercury arc la,mp, an analyzer and polarizer. The return beam was focused on a Dage 650 TV camera with a Silicon Intensified Target. The image was viewed on a monitor and recorded on a VCR.

In Fig. 2 we show a picture of a 1 inch square I. D. minibox with a microwave circuit. The external connector, a female SMA adaptor with a cylindrical center pin, is used to launch the microwave signal onto coplanar waveguide (CPW). Electrical continuity was insured by silver painting the pin to the center conductor of the CPMi and the outer conductors to the sides of the minibox. The CPW was fabricated by evaporating a 0.5 pm layer of aluminum onto a 1 inch square Corning 7059 glass substrate matched to the inner dimensions of the minibox. Standard photolithographic techniques were employed to develop the appropriate circuit pattern into AZ-1350J photoresist. The circuit was etched with PAWN (80 parts Phosphoric Acid, 5 parts Acetic Acid, 10 parts Water, 1 part Nitric Acid) and the photoresist removed. The dimensions of the CPW were chosen to establish a 50 fl characteristic impedance taking account of the dielectric constant, 6,. = 5.84, of the glass substrate. The transmission line terminates in a short circuit near the center of the minibox.

The details of the short circuit are shown in Fig. 3. The circuit tapers down to a small region where a square 4 pm by 4 pm aluminum region shorts the center conductor of the CPW to the t w o grounded sides. In the several millimeters length region over which the transmission line tapers, the characteristic impedance deviates from 50 R. This is inconsequential since this region is much smaller than a

0018-9464/85/0900-1785$01.00@1985 IEEE

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Mercury 7 Sweeper 0 Monitor

L - Pol or i zer

Pulse ......... T V Camera Analyzer

1 Coi 1

.............. Opt i col Path

I Goussmeter I Electrical Connection

I Transmission Line Figure 1: Apparatus for Microwave Induced

Magnetic Bubble Generation.

quarter-wavelength at all frequencies of interest. We may then model this region by quasi-static circuit theory approximations; that is, a short circuit being fed by a current source. The current, I , forced through the short, is related to the incident microwave power, P, through the characteristic impedance, Zo, by;

For a width, 1 of 4 pm, 1 milliwatt of microwave power produces a 5 Oe amplitude microwave magnetic field.

A very small piece of the sample was placed, magnetic film down, on the shorted region of the circuit. Optical focusing established that the air gap between sample and circuit in the short region was within a several micron tolerance. Initial experiments were performed to determine sample collapse and nucleation fields. The sample was saturated and the field returned half-way between collapse and nucleation. The basic experiment was initiated with the signal generator set to CW and microwave power set as low as possible (-2 dBm). The microwave source was turned on manually for several seconds to see if a bubble raft would form. If not, the power level was increased and the experiment. repeated. We report the minimum microwave field necessary to generate bubbles as determined in this way.

I = J'. (1) 20

Assuming this current to be distributed uniformly across the width of the short, we may estimate the magnitude of the strongly-interacting, circularly-polarized component of the magnetic field, h immediately above it.

CP'

Side View View

Sample Substrate + Thin Film Sample

Coplanar Waveguide -

Substrate (Glass) Thicknesses

not to

I Brass Minibox Support I Scale

Figure 2: Shorted CPW Line in a minibox. Figure 3: Details of the short circuit.

3 Results

Initial experiments were performed on a La-Ga substituted yttrium-iron garnet. From standard FMR diagnostics, a value for 7 of 2.77 MHz/Oe was established. The excess anisotropy field, Hk, was found to be somewhere between -125 Oe to +145 Oe; the exact value was difficult to establish because of the complicated line structure of the saturated resonances as seen in Fig. 4. Bubbles of diameter 5 pm were barely visible for this sample because of low Faraday rotation. Bubble generation results for this sample are shown in Fig. 5.

On another sample with larger Faraday rotation, a test was performed to determine the minimum pulse length of microwave excitation needed to create a single bubble. In general this time was less than the minimum measurable value of 25 nsec characteristic of our apparatus. In, such a microwave experiment, one or two bubbles would be generated and then expelled to a distance of four or five bubble diameters from the region. If one bubble was formed, another pulse would send a second bubble out of the region in a direction opposite to the first bubble.

When a bubble raft was created by applying continuous microwave power, the bubbles had a tendency to move to one side of the circuit. When the field was reversed the direction of the bubble movement was likewise reversed.

4 Comparison of Theory to Results

The solid line in Fig. 5 represents our analytical prediction of the minimum microwave field necessary to induce magnetization reversal4 under the assumption that the excess anisotropy field, H,, is 120 Oe. The specific value of H , is not known; a lower Hk value would cause the predicted minimum to move to a lower frequency. What the comparison demonstrates is that there is a minimum in field at a specific frequency as predicted by the theory. It is uncertain why at lower frequency the microwave field necessary to generate bubbles tends to reduce.

An estimate of the power dissipated in the bubble generation region at 330 M H z yielded a value of 32 p W under the assumption of a 0.1 fl per square resistivity for the aluminum. Considering a 25 nsec pulse duration and a 1 M H z bit generation rate, an average power dissipation of 0.8 pWatts is anticipated. This extremely small value is expected to rise considerably when the effect is measured on conventional bubble materials, these have excess anisotropy fields well in excess of 1 kOe. The average power dissipation may still be well within tolerable limits; such a device, therefore, may be an attractive alternative to conventional generators when heating constraints are significant.

5 Acknowledgments

D. J. Seagle has been supported by an IBM Fellowship. Additional support was provided by the National Science Foundation grant #ESC-8304719.

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0.1 0.2 0.3 0.4 0.5 Applied Magnetic Field in KG

Figure 4: Saturated FMR Line Structure.

0 1 , 6 Id0 260 360 460 2 Frequency (MHz)

0.6

~

io

Figure 5: Minimum Microwave Field Necessary to Generate Bubbles Plotted as a Function of Frequency. Experimental Points denoted by Asterisks. Applied Field, Ha is 38 Oe. Solid Line Represents Theory for HI, = 120 Oe.

References

H. DEtsch, “Stability and Dynamics of Microwave ‘Generated Ring Domains”, AIP Con ference Proceedings, No. 29,1978, pp. 78-83.

A. M. Mednikov, S. I. OI’Khovskii, V. G. Redko, V. I. Rybak, V. P. Sondaevskii and G. Chirkin, “Formation and motion of magnetic domains in a microwave magnetic field”, Sow. Phys. Solid State, VOI. 19,1977, pp. 698-699.

J. 0. Artman, S. H. Charap, and D. J. Seagle, “Microwave Generation of Bubble Domains in Magnetic Thin Films”, IEEE Trans. Magn., Vol. MAG-19,1983, pp. 1814-1816.

D. J. Seagle, S. H. Charap, and J. 0. Artman, “Bubble Generation by Microwaves: Analytical”, J . A p p l . Phys. , Vol. 55, No. 6 , March 1984, pp. 2578-2580.