Integrated Thin-Film Magnetoelectric Waveguide With Tun-Able Resonance Frequency

Magnetoelectrics offer the ability to electrically control magnetic material and device properties. This feature has the potential to provide numerous benefits to electronic systems and motivate the integration of magnetic devices into mainstream electronics [1]– [3]. Most work on magnetoelectrics thus far, however, has involved either specialized crystalline multiferroic materials [1], [4] or bulk piezoelectric/magnetic composite layers [7]– [13]. Here, we attempt to bridge the gap between magnetoelectricity and silicon-integrated electronics by producing the first fully-integrated thin-film magnetoelectric resonant waveguides for tunable radio frequency (RF) systems [14]. Tunability of these devices is achieved through electric field control of magnetic permeability in order to change the phase velocity and resonance frequency of coplanar waveguides. The devices were fabricated on a conventional silicon substrate and incorporate thin-film magnetoelectric composites, made up of both magnetic and piezoelectric material layers for strain-control of magnetic anisotropy. The fabricated devices achieved reversible tunability of the resonance frequency with a large converse magnetoelectric coupling coefficient of up to 24 mG-cm/V using just thin films. The tunable waveguide device, shown in Fig. 1, was designed as an RF quarter-wavelength resonator, whose resonance is based on the total effective permeability and permittivity of the material stack. To provide the capability of tuning the resonance frequency, a thin-film magnetoelectric composite was created using a magnetostrictive ferromagnetic layer on top of a piezoelectric/electrodes combined layer, with the silicon substrate on the bottom. A coplanar waveguide on the surface of the structure carries the wave such that its propagation is influenced by the properties of the underlying magnetoelectric stack. Electric field control of piezoelectric strain couples to the magnetic layer for strain control of magnetization and permeability, therefore influencing the propagating phase velocity and resonance frequency of the waveguide. Fig. 1a illustrates the simplified integrated structure as well as the electric field lines from the interdigitated electrodes controlling the piezoelectric. Fig. 1b shows scanning electron microscope images of the cross-section of the fabricated devices, produced by focused-ion beam. Platinum interdigitated electrodes (IDE) [11] apply an approximately in-plane electric field, as shown in Fig. 1a, to the piezoelectric PNZT and take advantage of the $mathrm {d}_{33}$ strains, which are typically larger than $mathrm {d}_{31}$ strains. The ferromagnetic material, Co 43 Fe 4 B 14 At%) [12] was selected for its high magnetostriction value of $lambda mathrm {s}sim 55 times 10 ^{-6}$ and high ferromagnetic resonance frequency $( sim 2$ GHz for a square film). These two properties allow for a large degree of electrical control (via strain) of magnetization and a material resonance frequency well separated from that of the device resonance. In addition, the magnetic film was laminated and patterned into a narrow bar parallel to the wave propagation direction, such that the shape anisotropy contributed to aligning the magnetization uniformly along the length direction, with permeability measured along the orthogonal (width) direction. Fig. 2 presents measurement results from the magnetoelectric resonator device. The polarization measurements around the remanence point, indicate that the polarization (and related strain) is fairly linear and reversible, despite some hysteresis. The corresponding RF resonance shifts for the magnetoelectric waveguide follow a similar trend but with opposite polarity. This is due to the fact that the strain causes a rotation of the magnetization and an increase in the permeability with electric field; permeability is inversely related to the phase velocity and resonance frequency, therefore leading to a decrease in the resonance frequency with applied electric field. Neglecting contributions due to stray fields or temperature effects, we extract an effective magnetoelectric coefficient for the composite device equal to approximately 24 mG-cm/V, which is comparable with many other multiferroic and magnetoelectric composites. Acknowledgement Research for this project was conducted with government support under FA955011-C-0028 and awarded by the Department of Defense, Air Force Office of Scientific Research, National Defense Science and Engineering Graduate (NDSEG) Fellowship, 32 CFR 168a. Part of this work was performed using the Stanford Nanofabrication Facility (SNF) and the Stanford Nano Shared Facilities (SNSF) at Stanford University.