0741-3106 (c) 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/LED.2019.2962876, IEEE Electron Device Letters Shuvam et al.: Magnetic field tunable Ferromagnetic Shape Memory Alloy based Piezo-Resonator 1 Abstract—In this letter, we have performed a comprehensive analysis of the influence of the magnetic field on BAW resonator consisting of a highly magnetostrictive layer and AlN thin film. The fundamental resonant frequency of the fabricated BAW resonator is about ~4.22 GHz. In the presence of a magnetic field, we studied the effect on the resonator parameters such as resonant frequency, acoustic velocity, and coupling coefficient. For the magnetic field of strength 1200 Oe, the resonant frequency significantly shifts by ~360 MHz. Resonant frequency increases and electromechanical coupling coefficient (kt) decreases with the increase in the DC magnetic field. The maximum acoustic velocity of ~7350 m/sec was observed at the magnetic field of 1500 Oe when applied parallel to the surface. Agilent Advanced Design Software (ADS) was used to extract the equivalent Modified Butterworth-Van Dyke circuit parameters (Rm, Cm, and Lm) of the resonator. Further, in the presence of the magnetic field, we obtained the variation in values of Rm, Cm, and Lm of the resonator structure. Such tunable resonators can be useful and vital in dealing with varying frequency bands for sustainable growth in wireless communication and magnetic field sensor applications. Index Terms— Ferromagnetism, Piezoelectric, Sensors, Thin Films. I. Introduction N the current era, Bulk Acoustic Wave (BAW) devices have practically substituted Surface Acoustic Wave (SAW) devices due to a wide range of applications of BAW in the wireless domain for modern electronics [1]. BAW resonators also play a crucial role in RF components in various kinds of sensors and communication devices operating at a higher frequency. With recent developments and advancement in technology, there is a need to tune the resonant frequency with some external parameters like the electric field, temperature, magnetic field etc. [2]. This increase the importance of the BAW device without altering the complicated fabrication process. The frequency tuning is highly crucial for filters, phase shifter, and resonators as it prevents any deviation in their standard resonant frequency due to ageing, environmental factors, thermal mismatch etc. [3]. Magnetoelectric composites-based BAW resonators have achieved substantial research owing to its ability to control the reflection spectrum by an external magnetic field [4, 5]. Shuvam Pawar, and Davinder Kaur are with the Indian Institute of Technology Roorkee, Roorkee, India 247667 (e-mail: dkaurfph@iitr.ac.in). Jitendra Singh is with the CSIR- Central Electronics Engineering Research Institute, Pilani Rajasthan India 333031. Project supported by Department of Science & Technology (DST), India, under Nano Mission with Reference No. SR/NM/NT-1065/2015 and Defense Research Development Organization (DRDO) under EP & IPR 2018 with Reference No. ERIP/ER/99011650/M/01/1739 (G). Magnetoelectric is the class of materials in which magnetic and electric responses are interlinked [6]. In layered structures, the strain transfer mechanism is responsible for the coupling between ferromagnetic and piezoelectric films. They are a strong candidate for the design and development of innovative electronic devices [7-9]. The strength of magnetodielectric coupling in these structures depends on factors such as interface quality, morphology, and functional properties of individual layers [10]. Therefore, these layered structures require attention for their use in futuristic magnetic field tunable microwave devices like filters, resonators, phase shifters. Ferromagnetic Shape memory alloys (FSMAs) are the class of materials used in magnetic actuation applications because of their giant magnetostrictive response [11]. But little work is reported on the combination of FSMAs alloy with piezoelectric materials [12, 13]. Our previous results show excellent ME coupling between NiMnIn alloy and AlN piezoelectric layers [6]. Furthermore, BAW devices based on AlN are in more demand due to its high acoustic velocity, high stiffness and CMOS compatibility [14, 15]. In this work, we have studied the effect of magnetic field on the resonant frequency of AlN/NiMnIn layered based BAW resonator. The resonant frequency, acoustic velocity, coupling coefficient and circuit components were related to the change in young modulus of the ferromagnetic layer. Such tunable devices can be vital in the applications of magnetic field sensing at room temperature. II. EXPERIMENTAL DETAIL The NiMnIn/AlN thin film-based BAW device was fabricated on a (100) p-type silicon substrate using DC magnetron sputtering. Initially, a ferromagnetic Ni50Mn35In15 layer of thickness ~750 nm was sputtered on a silicon substrate at 500 ̊ C for 6 min with deposition pressure of 10 mTorr. For the excitation of the longitudinal/ transversal BAW resonator, a c-axis wurtzite structured layer of AlN (~800 nm) were grown on NiMnIn at an elevated temperature of 400 ̊ C. The sputtering pressure of 4 mTorr was maintained in a mixed nitrogen and argon atmosphere with flow rates of 6 sccm and 14 sccm respectively. The top electrode (NiMnIn) of the BAW resonator was deposited through a stencil mask with circular patterns of radius 300 μm. The films were grown ex-situ and no annealing process was employed. The phase formation and crystallographic structure of the layered structure were explored by X-ray diffractometer (Bruker advanced D8) using Cu Kα (1.54 Å) source. The Field Emission Scanning Electron Microscopy (FESEM) imaging was performed for cross-sectional and interface study of the layered BAW device. The complete frequency spectra of the layered BAW resonator were measured with the preferential excitation of a magnetic field. The strength of the magnetic field is changed by adjusting the distance Magnetic field tunable Ferromagnetic Shape Memory Alloy based Piezo-Resonator Shuvam Pawar, Jitendra Singh and Davinder Kaur I