While surface acoustic wave (SAW) sensors have been used to measure temperature, pressure, strains, and low magnetic fields, the capability to measure bipolar fields and high fields is lacking. In this paper, we report magnetic surface acoustic wave sensors that consist of interdigital transducers made of a single magnetostrictive material, either Ni or ${\mathrm{TbFe}}_2$, or based on exchange-biased ($\mathrm{Co}/\mathrm{IrMn}$) multilayers. By controlling the ferromagnet magnetic properties, high-field sensors can be obtained with unipolar or bipolar responses. The issue of hysteretic response of the ferromagnetic material is especially addressed, and the control of the magnetic properties ensures the reversible behavior in the SAW response.

Here, we present an integrated ultra-high vacuum apparatus—named MBE-Cluster —dedicated to the growth and in situ structural, spectroscopic, and magnetic characterization of complex materials. Molecular Beam Epitaxy (MBE) growth of metal oxides, e.g., manganites, and deposition of the patterned metallic layers can be fabricated and in situ characterized by reflection high-energy electron diffraction, low-energy electron diffraction, Auger electron spectroscopy, x-ray photoemission spectroscopy, and azimuthal longitudinal magneto-optic Kerr effect. The temperature can be controlled in the range from 5 K to 580 K, with the possibility of application of magnetic fields H up to ±7 kOe and electric fields E for voltages up to ±500 V. The MBE-Cluster operates for in-house research as well as user facility in combination with the APE beamlines at Sincrotrone-Trieste and the high harmonic generator facility for time-resolved spectroscopy.

Wireless surface acoustic wave (SAW) sensors constitute a promising solution to some unsolved industrial sensing issues taking place at high temperatures. Currently, this technology enables wireless measurements up to 600-700 °C at best. However, the applicability of such sensors remains incomplete since they do not allow identification above 400 °C. The latter would require the use of a piezoelectric substrate providing a large electromechanical coupling coefficient K ² while being stable at high temperature. In this article, we investigate the potentiality of stoichiometric lithium niobate (sLN) crystals for such purpose. Raman spectroscopy and X-ray diffraction attest to the fact that sLN crystals withstand high temperatures up to 800 °C, at least for several days. In situ measurements of sLN-based SAW resonators conducted up to 600 °C show that the K ² of these crystals remains high and stable throughout the whole experiment, which is very promising for the future achievement of identifiable wireless high-temperature SAW sensors.

A temperature compensated magnetic field sensor based on the combination of CoFeB ferromagnetic thin films and Quartz/ZnO Love waveguide platform is developed and optimized. The Love wave is a shear horizontal guided wave and therefore provides an optimal interaction with magnetization in the magneto-elastic thin film resulting in higher acoustic wave magneto-elastic coupling compared to the conventional Rayleigh wave based devices. ST-cut Quartz was chosen as substrate, ZnO as insulating layer for Love wave generation and temperature coefficient of frequency (TCF) compensation and CoFeB as the magnetostrictive layer sensitive to magnetic field. Experimental results show a magneto-acoustic sensitivity of 15.53 MHz T ⁻¹ with almost zero TCF.

The piezoelectric response of ZnO thin films in heterostructure-based devices is strictly related to their structure and morphology. We optimize the fabrication of piezoelectric ZnO to reduce its surface roughness, improving the crystalline quality, taking into consideration the role of the metal electrode underneath. The role of thermal treatments, as well as sputtering gas composition, is investigated by means of atomic force microscopy and x-ray diffraction. The results show an optimal reduction in surface roughness and at the same time a good crystalline quality when 75% O₂ is introduced in the sputtering gas and deposition is performed between room temperature and 573 K. Subsequent annealing at 773 K further improves the film quality. The introduction of Ti or Pt as bottom electrode maintains a good surface and crystalline quality. By means of piezoelectric force microscope, we prove a piezoelectric response of the film in accordance with the literature, in spite of the low ZnO thickness and the reduced grain size, with a unipolar orientation and homogenous displacement when deposited on Ti electrode.

This work aims at studying the interaction between surface acoustic waves (SAW) and micro-structured magnetostrictive layers under a magnetic field with a perspective to develop magnetic field sensors. The impact of the competition between the strong intrinsic magnetic anisotropy of the magnetic material and the shape anisotropy of the interdigitated transducer (IDT) fingers introduced by the micro-structuration is investigated. Therefore, the macroscopic and microscopic magnetic properties of the IDT and their influence on the magneto-acoustic response are studied. A SAW resonator with the IDTs made of the magnetostrictive thin film was elaborated and the magnetic surface acoustic wave (MSAW) response under a magnetic field was performed and discussed. Depending on the energy balance, the anisotropy gets modified and a correlation with the MSAW sensitivity to an externally applied magnetic field is made.