Magnetoelectric Device Feasibility Demonstration – Voltage Control of Exchange Bias in Perpendicular Cr2O3 Hall Bar Device Zhengyang Zhao1, Will Echtenkamp2, Mike Street2, Christian Binek2* and Jian-Ping Wang1* 1
2
Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455 Department of Physics & Astronomy and the Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, NE 68588 Email:
[email protected],
[email protected]/Phone: (612) 625-9509
Introduction Several emerging mechanisms using voltage to control the magnetism have recently been proposed because of their possible applications in energy-efficient spintronic devices [1]. Among others, one promising way to reach this goal is to use voltage to control the exchange bias of the magnetoelectric (ME) antiferromagnet Cr2O3 (Fig. 1) [2]. In this work, we demonstrate the ME effect in the device level using a bilayer thin film structure Cr2O3/[Co/Pd]n. Experimental Process Two Cr2O3 ME stacks were deposited on Al2O3 substrates using pulsed laser deposition (PLD) and molecular beam epitaxy (MBE) techniques, with the structures: substrate/Pd(20nm)/Cr2O3(300nm)/Pd(2nm)/[Co(0.6nm) /Pd(1nm)]2 (Sample 1) and substrate/Pd(20nm)/Cr2O3(300nm)/Pd (1.2nm)/[Co (0.6nm)/Pd(1nm)]2 (Sample 2). X-ray diffraction (XRD) pattern shown in Fig. 2(a) indicates the good quality of the samples. The perpendicular exchange bias of the samples can be seen in the perpendicular magnetic hysteresis loops in Fig. 2(b). The film stacks were fabricated into Hall bar devices using photolithography and an ion-milling process at Minnesota Nano Fabrication Center, as shown in Fig. 3. A voltage can be applied between the top electrode of the Hall bar and the bottom Pd layer, which can change the boundary magnetization of Cr2O3 and therefore control the exchange bias of the system. A SiO2 insulating layer was put between the top electrodes and Cr2O3, in order to extenuate the leakage current across the Cr2O3 layer. Device Results and Discussion We utilized the magnetoelectric field cooling (MEFC) processes [3] to examine the voltage control of exchange bias in our samples, during which the samples were heated to 320K (greater than TN of Cr2O3) and then cooled to room temperature (lower than TN of Cr2O3) as both the electric field (voltage) EMEFC and magnetic field HMEFC were applied. Then the out-of-plane magnetization curve was obtained by measuring the anomalous Hall effect (AHE). As can be seen from the results, the hysteresis loop shifts as the cooling voltage changes for both Sample 1 and Sample 2 (Fig. 4(a) and (b)). The exchange bias can be significantly controlled by the cooling voltage, reduced from 200 Oe to almost 0 as VMEFC changes from 0V to -5V (Fig. 4(c)). The EH product, the product of applied EMEFC and HMEFC, required for the reversal of magnetization was at least 1200 kOe kV/cm for our samples. The isothermal switching of the exchange bias field could be further performed [2]–[4], and the integration of the ME system into perpendicular MTJ devices is being investigated. With a well-optimized film [5], the voltage required for switching can be as low as 1V (for a 300nm-thick film) and a leakage current density of 10-5A/cm2 can be expected. Assuming the ME layer thickness is 50nm and the top electrode area of a device is 10-4 μm2, then the energy dissipated associated to the magnetization switching would be ~ 3 aJ, which is much lower than the state-of-the-art STT and SOT switching [6], [7]. This is promising for both logic and memory applications. Two remaining key challenges are 1) to grow thin ME film (
