Magnetoelectric surface acoustic wave characteristics of the ScAlN/FeGa heterostructure with negative Poisson's ratio

Magnetoelectric (ME) heterostructures of magnetostrictive/piezoelectric laminates and films contains enormous opportunities in spintronic memory, gyromagnetic-field sensor, transformer, energy harvester, electrical field tunable RF/microwave devices, etc. Recently, the traditional static and dynamic ME effects have been extended to high frequency ME effects, [1] such as electrical field induced ferromagnetic resonance frequency shift and magnetic field induced acoustic resonance frequency shift of surface acoustic wave (SAW) [2] or bulk acoustic wave (BAW) [3]. The former is related to the magnetic anisotropy of the magnetic layer mediated by piezoelectric stress or ferroelectric domain transformation, and has very important applications in tunable RF/microwave devices. The later can be attributed to the piezoelectricity and elastic anisotropy of the piezoelectric layer alternated by magnetoelastic modulus (ΔE) effect, and can be used in ultra sensitive DC magnetic field detection and NEMS antenna. [3], [4] In current work, we have investigated the magnetoelectric SAW characteristics of a novel piezoelectric/magnetostrictive heterostructure, i.e., ScAlN/FeGa laminates. Sc doping in AlN can result in more than 400% increase in piezoelectric constant. [5] Galfenol (Fe 1-x Ga x with $12 < \mathrm {x}<33)$ alloys has giant magnetostriction up to 400 ppm under a low driven field between 150 and 250 Oe. Of particular interest is that Galfenol is also auxetic in the $< 110 >\{100\}$ direction. A negative Poisson ratio of as low as -0.7 has been reported under either elastic load or magnetic field. [6], [7] Fig. 1 (a) and (b) shows the magnetic field dependent modulus of Fe 81.7 Ga 18.3 along the $< 100 >\{100\}$ direction and $< 110 >\{100\}$ direction, measured by a mechanical resonance/antiresonance method. As can be seen in Fig. 1(a), a giant (ΔE) about 25% and a maximum dE/dH close to 600 MPa/Oe were obtained in the $< 100 >\{100\}$ direction. Due to the higher elastic modulus, the (ΔE) effect along the $< 110 >\{100\}$ direction is only $\sim 3$%, and the corresponding (dE/dH $) _{max}$ is 80 MPa/Oe (Fig. 1b). Fig.2(a) shows the phase velocity dispersion of the ScAlN/(100) Fe 81.7 Ga 18.3 half space calculated by a scattering matrix method. [2] The SAW was designed to propagate along the [110] direction. As can be seen, only the basic Rayleigh mode was found due to the softening of the ScAlN layer by the FeGa substrate with the lower modulus. The phase velocity increases smoothly from 3009 m/s to 3296 m/s with increasing $\mathrm {f}* \mathrm {H}_{ScAlN}$ up to 1.58 GHz$* \mu \mathrm {m}$. But then the curve is broken between 1.58 and 4.6 GHz$* \mu \mathrm {m}$, indicating that even the basic Rayleigh wave mode cannot be excited. In this sense, there is a cutoff modulus for metallic magnetostrictive substrate, which sets an upper limit on the product of piezoelectric layer thickness and the SAW center frequency. For example, the upper limit of $\mathrm {f}* \mathrm {H}_{AlN}$ for AlN/FeSiB heterostructure is only 0.18 GHz$* \mu \mathrm {m}$, since the lowest Young's modulus of FeSiB is $\sim 60$ GPa upon applying magnetic field. This is why the [100] direction was not selected for SAW design, even that the (ΔE) effect is very large. However, due to the contribution of negative Poisson's ratio, Galfenol alloys with high Ga concentration all have very large f*HScAlN greater than 0.9 GHz$* \mu \mathrm {m}$, even though the modulus Fe 81.7 Ga 18.3 of is as low as 68 GPa, as shown in Fig. 2(b). Note that the modulus and Poisson's ratio data come from Ref. [6]. Finally, we have deposited highly c-axis oriented Sc 0.29 Al 0.71N film on a polished Fe 81.7 Ga 18.3 alloy with a 50nm-thickness Ti buffer layer. The full width at half maxiumu of the (002) rocking curve is $3.6 ^{o}$. A one-port SAW resonator was then patterned on the ScAlN surface via a lift-off photolithography techniques. The measured wavelength was $15.76 \mu \mathrm {m}$. The characterization of the SAW resonator was performed by measuring S11 parameters as a function of frequency using a vector network analyzer (Agilent N5230A) and a microwave micro-prober. It was found that the resonance frequency $f_{r}$ appeared at $208.87 \sim 218.75$ MHz, corresponding to a phase velocity in the range of $3292 \sim 3448\mathrm {m} /\text{s}$.