A sound idea: Manipulating domain walls in magnetic nanowires using surface acoustic waves

Abstract

We propose a method of pinning and propagating domain walls in artificial multiferroic nanowires using electrically induced surface acoustic waves. Using finite-element micromagnetic simulations and 1D semi-analytical modelling, we demonstrate how a pair of interdigitated acoustic transducers can remotely induce an array of attractive domain wall pinning sites by forming a standing stress/strain wave along a nanowire's length. Shifts in the frequencies of the surface acoustic waves allow multiple domain walls to be synchronously transported at speeds up to 50 ms À1 . Our study lays the foundation for energy-efficient domain wall devices that exploit the low propagation losses of surface acoustic waves to precisely manipulate large numbers of data bits. V C 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4932057] Domain walls (DWs) in magnetic nanowires 1 have great technological potential through the development of "racetrack" memory devices. 2 In these devices, DWs separate magnetically bi-stable domains, the orientations of which represent binary data. Synchronously moving the DWs transports data along the nanowires, thus facilitating read/write operations.A major challenge in the development of racetrack memory has been finding efficient methods of transporting DWs. Although DWs in soft-ferromagnetic nanowires can be propagated at modest applied fields $10 Oe, 3 neighbouring DWs travel in opposite directions, making synchronous data transport impossible unless complex "ratcheted" nanowires 4,5 or field pulses with intricate spatial 6 or temporal profiles 7 are employed. Moving DWs via spin-torque effects is a more attractive approach, since neighbouring DWs travel uni-directionally. However, current-induced DW transport is inefficient and unreliable in soft ferromagnetic systems, 8,9 with complex multilayer nanowires exploiting spin-orbit effects [10][11][12] or containing antiferromagnetically coupled layers 13 being required to obtain fast and reliable DW motion at typical experimental current densities.Previously, we have shown that artificial multiferroic systems, where magnetostrictive nanowires are coupled to electrically contacted piezoelectric layers, offer alternative routes to obtaining synchronous DW motion. 14 In this approach, electrically induced strains in the piezoelectric layer produce local variations in the nanowire's magnetic anisotropy by the Villari effect. These can then be utilized to both pin and synchronously propagate DWs. The approach is attractive because it is voltage rather current driven, and thus is expected to be power efficient. Furthermore, in contrast to both field-and current-induced approaches, where interactions are impulse-based, in the multiferroic approach DWs are constantly confined within a stress-induced potential well, offering precise control of the DWs' positioning. However, the necessity of fabricating arrays of electrical contacts makes for complex device designs when compared to the simple, two-terminal configurations required for current-induced motion...

Magnetic skyrmion bubble motion driven by surface acoustic waves

“…A possibility to drive a domain wall by electrically induced surface acoustic waves (SAWs) has been demonstrated recently. 27,28 Such studies pave the way for various applications in magnetic memory and logic devices. 26,32,33 In this work, we explore a skyrmion bubble dynamics induced by counter-propagating SAWs.…”

“…23,24 Since the modulation of anisotropy by strain is observed in many ferromagnetic materials, strain can also be used to control magnetization dynamics. [25][26][27][28][29][30][31] Thus, one can also electrically control magnetization dynamics by combining piezoelectric and magnetoelastic effects. A possibility to drive a domain wall by electrically induced surface acoustic waves (SAWs) has been demonstrated recently.…”

“…For the piezoelectric substrate we use the speed of sound 2114 m/s which corresponds to PZT. 27 In a long nanowire, only the edge repulsion along the transverse direction should be considered which leads to the force corresponding to parabolic potential in the y-direction. The edge repulsion along the y-direction will suppress the side motion due to the Magnus force.…”

Vector network analyzer measurement of the amplitude of an electrically excited surface acoustic wave and validation by X-ray diffraction

“…Surface acoustic waves have also been proposed as an alternative method for synchronously propagating and pinning domain walls in magnetic nanowires for applications in racetrack memory devices. 11,12 It was found that the velocity of propagation of the domain walls depends on the amplitude of the SAW. In the area of microfluidics, recent research demonstrates that SAWs provide an effective means to manipulate fluids (fluid mixing, [13][14][15] translation, [16][17][18] jetting, 19 and atomization 20 ) and manipulate particles (handling, 21 focusing, 22 separation, 23,24 sorting, 25,26 concentration, [27][28][29][30] and reorientation 31 ) in lab-on-a-chip devices for applications in chemistry, biology, and medicine.…”

Mechanical back-action of a spin-wave resonance in a magnetoelastic thin film on a surface acoustic wave

“…Among these applications are the acoustic manipulation and readout of magnetic memory elements [1][2][3][4] , acoustic driving of magnetic domain walls 5 , the acoustic generation of resonant spin-wave excitations [6][7][8][9][10][11][12][13][14] , and magnetic field detectors. 15, 16 Some of the interest rests on the point that acoustical wavelengths range in the sub-micron to micron scale at the gigahertz frequencies typical of spin-wave resonance.…”