FeRh is widely studied because of its novel temperature-dependent antiferromagnetic (AFM) to ferromagnetic (FM) phase transition. This AFM-FM phase transition, which is accompanied by a significant change in resistivity, occurs at a critical temperature that can be fine-tuned over a wide range through substitutional doping, strain, and patterning.[1,2] Moreover, the temperature dependence of the transition provides a means to manipulate the state via Joule heating. Recent reports, based on ultrafast pump-probe measurements, show the AFM-FM transition occurs on a sub-picosecond timescale, thus devices operating in switching applications have the potential to operate in excess of 100’s GHz provided adequate thermal dissipation is achieved. In this work we demonstrate high-speed switching of FeRh wires, giving rise to a dynamic memristive device based on a metamagnetic transition. The thermally-induced AFM-FM transition was evaluated using two-terminal devices consisting of an FeRh wire and Ti/Au contacts. We identified geometrical dependencies in the AFM-FM transition temperature, which scaled with both current density and wire length. Pulsed I-V measurements were used to investigate the dynamic Joule heating effects, including the device switching speed and resulting power switching losses accompanying the AFM-FM transition. The lower bound of our device switching time, measured to be near 300 nanoseconds, was limited by measurement equipment limitations, not the material system. The performance of this rudimentary device is comparable to other phase change memory technologies with more intricate device architectures. FeRh could be the basis for a very fast, phase-change approach to future computing.

High-Speed Switching of FeRh Memristors

Compact, ultra-low power magnetic field sensors are desirable for applications in multiple fields from biomedical engineering to military sensors [1,2,3]. Through direct coupling of magnetostrictive and piezoelectric strain, an on-chip sensor can be designed by exploiting the interface between thin films which can generate a charge when under an external magnetic field bias that depends not only on the magnitude of the measured field, but also the magnetic field direction enabling vector sensing capabilities. Magnetoelectric MEMS-scale heterostructures deposited in layers on silicon wafers and etched to create beams and cantilevers operating at the eletromechanical resonance were shown to work in a stress reconfigurable mode [4,5]. In this work we demonstrate magnetic field and directional response of FeC/AlN MEMS double-clamped heterostructure resonators. Through coupled strain engineering we are able to exploit the magnetic anisotropy and create a preferred axial response along the easy axis length of the suspended heterostructure beam. Resonant frequency response to the external magnetic field produces a sensitive measure of the angular dependence depending on the field vector components along the long and short axes. Figure 1a illustrates the heterostructure design of the double clamped ME MEMS structure and Figure 1b depicts a set of beams fabricated in a rosette pattern between Helmholtz coils used to apply a magnetic field to study the resonance response. The resonant peak shift with applied external magnetic field can be seen in Figure 2a where the butterfly loop has a linear slope df/dH near coercive field at 100G of 35 Hz/mT or 47 Hz/mT depending on the loop leg denoting the best bias response working point for this sensor. Polar plot of the resonance peak vs field at 15° increments can be seen in Figure 2b showing the directional response of the double clamped ME MEMS structures. Limit of detection measurements and further resonant peak angular dependence will be presented and deposition conditions will be discussed to illustrate the necessary conditions required to optimize creation of a high-quality MEMS sensor.

**References:**

[1] M. Hämäläinen, R. Hari, R.J. Ilmoniemi, and O. V. Lounasmaa, Reviews of Modern Physics 65, 413–497 (1993), DOI: 10.1103/RevModPhys.65.413 [2] J. W. Mooney, S. Ghasemi-Roudsari, E. Reade Banham, and B. T. H. Varcoe; Biomed. Phys. Eng. Express 3 (2017) 015008, DOI: 10.1088/2057-1976/3/1/015008 [3] T. R. Clem, Naval Engineers Journal 110, 139–149 (1998), DOI: 10.1111/j.1559-3584.1998.tb02393.x [4] S. P. Bennett, J. W. Baldwin, M. Staruch, and P. Finkel; Applied Physics Letters 111, 252903 (2017), DOI: 10.1063/1.5011728 [5] M. Staruch, et al.; Physical Review Applied 11, 034028 (2019); DOI:10.1103/PhysRevApplied.11.034028

**Images:**

![](https://assets.underline.io/uploads/markdown_image/1/image/50e45a3a8b51d1152352d15c4537b1ae.jpg)
Figure 1: (a) A top view and side view of magnetoelectric arched beam resonator, consisting of a layered film stack on a silicon wafer consisting of Si3N4 support, Pt back electrode with Ta adhesion layer, AlN piezoelectric layer, and Fe0.7Co0.3 top electrode and magnetostrictive layer. (b) Rosette pattern of beams fabricated on a silicon chip between Helmholtz coils, with rotation for angular dependence measurements.

![](https://assets.underline.io/uploads/markdown_image/1/image/d02c2d26681eeb936087c33b918db727.jpg)
Figure 2: (a) Frequency shift of resonance peak vs bias magnetic field applied parallel to long axis of the beam. (b) Polar plot of the magnetic field response at 15° intervals for 0, 200, 300, 500, and 1000 Gauss.

 Directional Magnetic Field Response of FeCo/AlN Heterostructure Magnetoelectric Resonators

**Session chair:** Stephane Mangin, Université de Lorraine


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High-Speed Switching of FeRh Memristors

Directional Magnetic Field Response of FeCo/AlN Heterostructure Magnetoelectric Resonators

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