IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 3, MARCH 2014 3100704

Acoustically Assisted Magnetic Recording: A NewParadigm in Magnetic Data Storage

Weiyang Li, Benjamin Buford, Albrecht Jander, and Pallavi Dhagat

School of Electrical Engineering and Computer Science, Oregon State University, Corvallis, OR 97331 USA

A new paradigm in energy-assisted magnetic recording, acoustically assisted magnetic recording (AAMR), is investigated. In thisapproach, a surface acoustic wave (SAW) is applied to a magnetostrictive recording medium to temporarily lower its coercivity belowthe write field. Akin to other energy-assisted recording technologies, AAMR provides a strategy to enable writing on high-coercivitymedia required for thermal stability in high-density disk drives. Using a contact recording tester, it is demonstrated that the writecurrent needed to record data on a galfenol film is reduced in the presence of SAWs. Further, it is shown that standing and focusedacoustic waves can, respectively, be used to lower the coercivity in selected regions on the medium.

Index Terms— Magnetic recording, surface acoustic wave.

I. INTRODUCTION

MAGNETIC recording has been the dominant solutionfor mass information storage for several decades. To

meet the demands of ever increasing storage capacity, the bitsize and magnetic grain volume of the recording medium mustbe reduced. Correspondingly, to avoid data loss due to thermalinstability, the coercivity of the medium must be increased,leading to the challenge of recording with a practicable writefield. For high-density recording, there are thus conflictingrequirements of high coercivity for storage and low coercivityfor writeability. Several technical solutions have been proposedto surmount this challenge: heat-assisted magnetic recording(HAMR) [1] uses a laser to heat the bit above its Curie temper-ature and temporarily lower the coercivity during writing. Harddisk drives using HAMR have recently been demonstrated [2];however, reliability of the near field optical transducers usedfor focusing the laser remains a concern. A less developedapproach, microwave-assisted magnetic recording [3], seeks tointegrate a spin torque oscillator in the write head to reducethe coercivity of the medium through magnetic resonance.

Here, we demonstrate a new technology, acousticallyassisted magnetic recording (AAMR), which uses a surfaceacoustic wave (SAW) to modulate the coercivity of the record-ing medium by the inverse magnetostrictive (Villari) effect[4]–[6]. In the proof-of-concept experiments discussed here,the transducer for generating the acoustic waves is fabricatedon the recording film substrate. For practical application in ahard disk drive, it may be possible to integrate a non-contactSAW transducer with a conventional recording head to focusacoustic energy under the write pole.

II. EXPERIMENTS

In our experiments, galfenol (FeGa) is chosen as the record-ing medium owing to its low coercivity, high magnetostric-

Manuscript received July 17, 2013; revised September 6, 2013; acceptedSeptember 30, 2013. Date of current version March 14, 2014. Correspondingauthor: P. Dhagat (e-mail: dhagat@eecs.oregonstate.edu).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2013.2285018

Fig. 1. Schematic of the experimental device. A surface acoustic wave of20 μm wavelength is generated within the 3 mm acoustic aperture when theinterdigitated transducer is excited.

tion coefficient of up to 400 ppm [7], and ease of deposi-tion by sputtering [8]. The substrate is piezoelectric quartz(ST-X cut) to facilitate the excitation of SAWs using interdig-itated electrodes as described in [9] and [10].

A schematic of the experimental device is shown in Fig. 1.The device consists of an interdigitated transducer (IDT) andreflector fabricated on both sides of the galfenol recordingmedium. When the transducer is excited with an ac source,temporally and spatially periodic stress is induced in thesubstrate by the piezoelectric effect. This stress wave travelsin a mode where its energy is localized near the substrateand is hence known as a surface acoustic wave. The reflectors(electrically shorted transducers) create a resonant cavity forthe acoustic waves.

The IDT is designed with an acoustic aperture of 3 mm and287 electrode pairs. The electrode pitch is 10 μm. The IDTmost efficiently excites surface acoustic wave when the SAWwavelength coincides with the electrode period. Accordingly,given a SAW propagation velocity of 3158 m/s in ST-X cutquartz, the transducer generates a SAW of 20 μm wavelengthwhen driven at ∼158 MHz. The driving point impedanceof the IDT at the operating frequency is 50 �, matched tothe source impedance of the instrumentation available. Thereflector, designed with 476 shorted electrodes also at a 10μm pitch, is placed 12.5 μm from the transducer.

Both the transducers and the reflectors are patterned bystandard lithography in a thin film of dc sputtered aluminum.The measured aluminum thickness is 110 nm. Subsequently,galfenol is sputtered and patterned by lift-off into a rectangle

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3100704 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 3, MARCH 2014

Fig. 2. Kerr image of the galfenol film showing (a) magnetization imprintedby a credit card (b) acoustically assisted erasure of the magnetization.A magnetic field lower than the coercivity of the unstrained galfenol filmis needed to erase the magnetization when acoustic power is applied. (Theseimages are taken with an yttrium-iron-garnet viewer placed on the galfenolfilm to enhance the Kerr contrast).

4.96 mm long and 20 mm wide (i.e., considerably wide com-pared with the acoustic aperture). The galfenol film thicknessis 57 nm. Sputter deposition is performed at 2.4 mTorr Arpressure and 200 W dc power, using an Fe81.6Ga18.4 alloytarget.

The device configuration seen in Fig. 1 allows for bothstanding as well as traveling SAWs to be excited. For standingwaves, both transducers are driven synchronously. The result-ing counter-propagating acoustic waves superpose to generatestrain periodic in 10 μm and amplified by the quality factorof the resonant cavity. To generate traveling waves, only oneIDT is powered. The other IDT and reflectors are covered withsilicone absorbers to avoid reflections. The traveling SAW hasa 20 μm wavelength.

As stated earlier, an acoustic wave propagating through themagnetostrictive recording film modulates its coercivity viastrain. A first proof of this principle is demonstrated in theerasure of a magnetic recording with the assistance of a SAW.A magnetization pattern is imprinted on galfenol by placing acredit card in contact with the film. When a traveling SAW isapplied, the pattern is erased by a magnetic field lower thanthe coercivity of the unstrained galfenol film. Fig. 2 shows theoriginal and erased magnetization patterns, imaged by Kerrmicroscopy. Only the pattern in the path of the acoustic waveis seen to be erased. The coercivity of the unstrained galfenolfilm, as measured on a vibrating sample magnetometer, is6.71 kA/m. The erased recording is imaged under a fieldof 5.96 kA/m and an acoustic power of 0.67 W/mm. (Here,acoustic power is the power delivered to the interdigitatedtransducer divided by the acoustic aperture.)

Next, the temporary change in coercivity effected by theacoustic wave is quantified by observing the field at whichthe net magnetization vanishes in a galfenol film subjected toacoustic waves of varying power. The film is first saturated in afield of 28 kA/m and then strained by a traveling acoustic waveof preset power. A reverse magnetic field, increasing in stepsof 0.1 kA/m, is simultaneously applied and the correspondingmagnetization in the film imaged via Kerr microscopy. Thefield at which roughly equal areas of bright and dark domainsappear [Fig. 3(b)] is identified as the coercivity under theapplied acoustic power. The measurement is repeated for

Fig. 3. Coercivity versus acoustic power. The Kerr images, obtained as theapplied field and acoustic power are varied, correspond to the (a) saturated,(b) demagnetized, and (c) reverse magnetized states of the galfenol film.

Fig. 4. Illustration of the contact recording tester. A floppy-disk head is usedto record data tracks at varying acoustic power. A magnetoresistive head, notshown in the sketch, is used for readback.

varying acoustic power and the resulting coercivity plotted inFig. 3. As expected, the higher the applied acoustic power,the lower is the coercivity or field needed to demagnetizethe film. The inset images typify the magnetization behaviorobserved: the saturated film (Fig. 3a) remains unaffected untilthe combination of the applied field and acoustic power issufficient to create reversal domains (Fig. 3b). Thereafter, thefilm is magnetized in the opposite direction (Fig. 3c).

The preceding experiments verified the inverse magne-tostrictive effect in acoustically strained galfenol. We nextdemonstrate AAMR using a contact tester [11] as illustrated inFig. 4. It employs an inductive floppy-disk head and a magne-toresistive hard drive head to, respectively, write and read data.Both heads can be translated relative to the recording mediumusing computer-controlled micropositioners. The galfenol filmis saturated before recording the data tracks shown in Fig. 5(a).Each track is written while applying acoustic waves at agiven power. The waves travel perpendicular to the downtrackdirection of the data track. Here, for proof of AAMR, standingwaves are excited to obtain maximal strain in the film andhence, assistance in recording. The write head is stepped fromleft to right as the amplitude of the alternating write current(or equivalently, write field) is gradually increased. Transitionsbegin to be recorded when the write field exceeds the coerciv-ity of the film under the given strain. Increasing acoustic powerfurther lowers the coercivity resulting in the medium being

LI et al.: ACOUSTICALLY ASSISTED MAGNETIC RECORDING 3100704

Fig. 5. (a) Data tracks recorded at different acoustic power. Recording is from left to right with gradually increasing write current. Progressively lowercurrent (or equivalently, write field) is needed to record the data as acoustic power is increased. (b) Corresponding reconstructed readback signals.

Fig. 6. (a) Schematic of coercivity modulation by a standing acoustic waveand the expected striped pattern in the film magnetization upon applyinga reverse field. (b) Kerr image of the 10 μm period stripes obtained withstanding waves of 1.33 W/mm power and a reverse field of 5.8 kA/m appliedperpendicular to wave propagation. (c) Illustration of a curved transducer forfocusing acoustic waves. (d) Kerr image of magnetization reversed at the focalspot.

writeable with smaller write fields. The corresponding read-back signals [Fig. 5(b)], reconstructed by integrating acrossthe width of each track, additionally emphasize this trend inwriteability with increasing acoustic power.

It is noted that in these experiments, the full recordingfilm in the acoustic path experiences strain. To localize theacoustic strain selectively on the recording medium, interfer-ence between the waves can suitably be tailored. In proof ofthis principle, spatial addressing using standing and focusedacoustic waves is demonstrated.

The galfenol film is saturated, as before, in a field of28 kA/m. A standing acoustic wave is then established in thefilm. The coercivity at the antinodes, where the acoustic strainis maximum, is periodically lowered as shown in Fig. 6(a).The coercivity at the nodes, where the strain is zero, remainsunaffected. Thus, when a reverse field is applied only themagnetization around the antinodes is re-oriented, creatinga striped pattern as shown in Fig. 6(b). The stripes appear

with a 10 μm period and a width determined by both theamplitude of the standing waves and applied reverse field.The image included here [Fig. 6(b)] corresponds to standingwaves of 1.33 W/mm power and a reverse field of 5.8 kA/m.If the field were increased, the coercivity at the nodes wouldeventually be overcome and the film uniformly magnetizedin the opposite direction.

In a similar experiment, an isolated spot on the recordingmedium is selectively written by focusing the SAWs using acurved IDT [Fig. 6(c)]. The converging waves induce a largestrain at the focal point, allowing for the local magnetization tobe reversed by a small field. For a tight focus, the curvatureof the IDT is designed to match the anisotropy in acousticvelocity of the quartz substrate [12], [13]. Shown in Fig. 6(d) isa spot reversed with 1 W acoustic power and 4.2 kA/m field ina previously saturated galfenol film. The spot size, dependenton the SAW wavelength and IDT geometry in addition to theapplied field, is ∼3 μm.

III. CONCLUSION

To conclude, we have demonstrated the basic principles ofAAMR, showing that acoustic waves can be used to temporar-ily and locally lower the coercivity of a recording medium.For experimental convenience, a recording medium with lowcoercivity is used. However, high-coercivity materials such asFePt, FePd, and SmCo, being investigated for thermally stable,high-density data storage, also exhibit large magnetostriction[14], [15] and may be well suited for AAMR.

The results reported here are obtained using acoustic wavesof 20 μm wavelength, limited by the resolution of lithographiccapabilities available. Shorter wavelength acoustic waves,attainable by more advanced lithography techniques, willprovide higher peak strain and hence, aid to writeability. Addi-tionally, a smaller focus for localizing the strain will be pos-sible, enabling higher density recording. SAWs with 220 nmwavelengths have been experimentally demonstrated [16]. Theacoustic spectrum extends well beyond 100 GHz [17]. Thus,in principle, sub-100 nm resolution with acoustically assistedrecording should be possible.

3100704 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 3, MARCH 2014

The acoustic transducer, in this paper, is realized on therecording film substrate. For application in a hard disk drive,a non-contact transducer may be integrated with the recordinghead to generate acoustic waves in the medium using air-coupling [18], Lorentz force [19] or magnetostrictive [20], [21]transduction.

In sum, AAMR presents a new paradigm for magnetic datastorage. Experimental results shown here advocate its promise.However, needless to say, significant additional research anddevelopment will be required in materials selection, transducerdesign and system integration for AAMR to be technologicallyviable. It is hoped that our work has paved the way forcontinued investigation of this novel concept.

ACKNOWLEDGMENT

This work was supported by the National Science Founda-tion under Grant 0645236. The authors would like to thankHGST, a Western Digital company for the magnetoresistiveheads used in these experiments.

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