Ionic Liquid Gating Modulation of Diluted Magnetic Semicon-ductor (Zn, Mn)O Thin Films

Ionic liquid (IL) gating on functional oxide has drawn significant attention, since it can provide reversible changes in carrier concentration $(\sim 10 ^{14}$ cm
^-3
at the interface, permitting the manipulation of electrical and magnetic properties via a low voltage [1]–[3]. In this work, we demonstrate the electric-field manipulation of transport properties in the dilute magnetic oxide (DMS) of Zn
0.98
Mn
0.02
O (MZO) using an electric-double-layer transistor (EDLT) geometry through IL of N,N-diethy1-N-(2-methoxyethy1)-N-methylammonium (DEME +) and bis(trifluoromethylsulfony1)-imide (TFSI-). By the application of different gate voltages $(V_{g})$ through the top electrolyte, the accumulated and depleted charge carrier in the MZO channel lead to a reversible control in the transport phenomena at the interface. 10 nm-thick MZO thin films were deposited on (0001) Al
2
O
3
single crystal substrates by pulsed laser deposition. The growth of MZO films was conducted at 300 °C with an oxygen pressure of $5 \mathrm {x}10 ^{-4}$ Pa. To study electrical-field manipulation of MZO devices, the films were patterned into Hall bar patterns (channel width: $50 \mu \mathrm {m}$, channel length: $110 \mu \mathrm {m})$ by photolithography and wet etching using dilute HCl as etchants. Au (50 nm)/Ti (5 nm) coplanar electrodes for IL and contact electrodes for MZO were prepared by electron beam evaporation. Prior to the gating experiment, IL was baked at 80 °C in a high vacuum chamber to get rid of water contamination. A drop of IL was placed on the top of the as-grown film, serving as the top gate electrode. The devices were immediately cooled down to 230 K for the gating process. By applying different $V_{g}( -2$, 0 and 2 V), the charge carriers were accumulated and depleted in the channel surface. Then the devices were immediately cooled down to 180 K (below the freezing point of IL at 230 K) before $V_{g}$ was removed. After the transport measurements, the devices were heated up to 230 K before changing the $V_{g}$ for another measurement. Fig. 1 illustrates the profile of the longitudinal resistance $(R_{xx})$ of MZO EDLTs with alternating $V_{g}$ between -2 and 2 V at 230 K. $R_{xx}$ increases (decreases) sharply upon the application of $V_{g} \quad = -2\mathrm {V}(2\mathrm {V})$, which is consistent with the scenario of accumulated (depleted) electron charge carrier at the MZO interfaces [4]. Such modulations of $R_{xx}$ are due to electron charge movement in MZO rather than the contribution of gate current: the drain-source current is higher than the gate-source current by at least two orders of magnitude. Magnetotransport behavior of MZO at 10 K after the application of different $V_{g}( -2$, 0 and 2 V) are shown in Fig. 2, which shows the magnetoresistance (MR) with out-of-plane applied field for MZO EDLT. Here MR is defined as MR $=( \mathrm {R}_{xx}(\mathrm {H})- \mathrm {R}_{xx}(0))/ \mathrm {R}_{xx}(0)$, where $\mathrm {R}_{xx}(H)$ and $\mathrm {R}_{xx}(0)$ are the $R_{xx}$ values with external magnetic fields of H and zero, respectively. The peak positive MR increases from 0 to 1.8% and the negative-MR (measured at 9 T) decreases from –4.5% to -0.6% when $V_{g}$ increases from -2 to 2 V. Enhancement in positive MR in the low field regime $(< 1\mathrm {T})$ implies that the ferromagnetic state of MZO is enhanced, as the electron carrier concentration in MZO increases upon switching the $V_{g}$ from -2 to 2 V [5]. The present results, therefore, demonstrate controllable movement of anions and cations in IL by electric-field effect plays an important role in the manipulation of magnetism in the MZO. Financial support by RGC, HKSAR (PolyU 153015/14P) and PolyU (1-ZE25) are acknowledged.