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Magnetoelectric Ultra-low-power Spin-wave Transducers

Periodic Reporting for period 1 - MUST (Magnetoelectric Ultra-low-power Spin-wave Transducers)

Reporting period: 2018-04-01 to 2020-03-31

Electric-field control is an enabling solution for many emerging applications of magnetism, which have been so far rendered uncompetitive due to large power dissipation. Hence, magnetoelectric (ME) effect has seen a renaissance as an effective way to manipulate magnetization state in a magnetic medium by electric field/voltage. Almost all studies on magnetoelectricity have addressed the DC or low-frequency AC behaviour and only few studies have examined the coupling of GHz AC electric fields to ultrafast magnetisation dynamics, such as ferromagnetic resonance (FMR) or spin waves (SWs). Thus, the implementation of MUST has investigated and laid the groundwork for realizing magnetoacoustic spin wave (SW) transducer to enable device for magnonics logic. The targeted breakthrough of MUST is to quantify ME effect in a scaled structure and the demonstration of magnetoacoustic SW transducers with large bandwidth, small size that can be integrated into a complementary metal-oxide semiconductor (CMOS) microelectronics environment. The devices and structures within MUST will be designed to be relevant to applications in spintronic and magnonic logic devices.
To achieve this goal, over the course of 2 years, starting from April 01, 2018 and ending on March 31, 2020, the principal investigator along with his supervisor and team colleagues, have researched different aspects of ME effect. Due to the nature of complexity, the project has been modified and adjusted to address two key objectives:
1. To quantify magnetoelectric effect in different multiferroic composites with a view to tailor their properties, enabling ME spin wave emission and propagation.
2. To demonstrate a scaled ME spin wave transducer in the proposed ‘fringe capacitor device’ geometry in MUST for a scalable magnonic device application.

To meet these objectives, the work packages in MUST have been divided into 3 categories (WP1: Quantification, WP2 & WP3: Demonstration) where focus has been on the first two (WP1, WP2) concentrating on material and device fabrication before addressing spin wave emission.
The work in WP1 was comprised in two individual tasks (T). T1, Magnetoelectric composite deposition and characterizations using IMEC nanofabrication facility in 200mm clean room to optimize the device material properties; T2, Developing a reliable method to quantify the ME effect in different ME composite in scaled device geometry.
A particular focus was given on understanding and defining individual material components in a ME composite. For the piezoelectric component to generate strain, an in-house optimized recipe for the growth of PZT or Lead Zirconate Titanate (Pb[Zr(x)Ti(1-x)]O3) was used by using SOLMATE’s pulsed laser deposition tool. To deposit different magnetic materials (Ni, NiFe, FeGa, CoFeB), sputtering method was optimized and ferromagnetic resonance, vibrating sample magnetometry were used to characterise them. To complement these studies, additional characterization methods like peak force microscopy, atomic force microscopy, X-ray diffraction, sheet resistance measurement, scanning electron microscopy etc. were also performed.
The task T2 is one of the key findings of MUST. To evaluate a ME coupling in a ME composite, MUST devised a model based on anisotropic magnetoresistance measurement by four probe technique in a fringe capacitor geometry. This method allows to probe the ME effect by biasing two dc pads when the sample is rotated under a constant saturating external magnetic field and quantify how much magnetoelectric anisotropy field is generated per voltage. This parameter gives an estimation of how strong the ME coupling is in a particular ME composite.
In the WP2, knowledge gained from WP1, was used to fabricate fringe capacitor type micron size devices and were characterised by two very sensitive optical technique, a. Brillouin Light Scattering (BLS) and Femto-second Laser Time Resolved Magneto-optic Kerr Effect (TR-MOKE). By BLS, spin wave band by ME transduction was measured. The key observations were: 1. SW band modes were successfully generated by ME effect which correspond to phonon/acoustic modes. 2. SW modes are highly localized. By TR-MOKE, magnetization dynamics induced by ME effect were probed in time domain by using a pico-second pulse pump-probe technique in a CoFeB/PZT composite. The key finding are: 1. Magnetization dynamics were mapped in time and spatial domain and a complex magnetization dynamics were observed which could be mimicking the strain/phonon propagation in the PE domain, 2. The magnetization mode seemed to have a wavelength of ~ 1μm and velocity ~4200ms-1 which matches the phonon velocity in the magnetic domain. This has a potential implication of phonon-magnon coupling which can enhance the ME coupling in a composite.
MUST has managed not only to address most of its key research goals but also the training and public outreach goals that have been set. Additionally, it has developed an important theoretical model by COMSOL multiphysics to determine ME coupling effect in several multiferroic composites which has been incorporated with micromagnetics. This will further lead the investigation in the quest of realizing more optimized ME transducer. It goes without saying that a complete ME transducer capable of demonstrating universal magnonics logic is still underway, even though the project has run its course. But, it has managed to address the early key bottlenecks of realizing such complex heterostructures into a competitive devices, invented to quantify key device parameters along with modelling; and thus it remains well in progress within the research team of IMEC.
Magnetoelectric fringe capacitor device geometry