Magnetoelectric materials, thanks to the coupling existing between their magnetic and electric properties, allow the manipulation of magnetization by an electric field. Such materials currently receive considerable interest for they open new perspectives in terms of memory devices. Indeed, a magnetoelectric memory would combine the best of both the FeRAMs and MRAMs worlds. At the present time, the development of magnetoelectric memories appears to be limited by the scarcity of magnetoelectric materials and even more by the scarcity of magnetoelectric materials presenting a magnetic order at room temperature.
Gallium ferrite Ga2-xFexO3 as an alternate material for the magnetoelectric manipulation of magnetization
The magnetoelectric manipulation of magnetization has recently been proved experimentally at room temperature with a ferroelectric antiferromagnet, BiFeO3 (BFO).This is the only material considered so far in the literature presenting both a magnetoelectric coupling and a magnetic order at room temperature. However, since the magnetic order is antiferromagnetic, the actual use of BFO requires the use of an extra layer to which it is magnetically coupled. This makes the fabrication of devices more complicated and the device itself subject to failures. It is therefore essential to make efforts in developping magnetoelectric materials presenting a non zero magnetization at room temperature. With this aim in view, we have considered gallium ferrite Ga2-xFexO3 (0.8 < x < 1.4) (GFO) with great interest. It indeed appears as the perfect alternative material to BFO in magnetoelectric memories : it is ferrimagnetic above room temperature for x=1.4 and pyroelectric with a strong magnetoelectric coupling.
Figure 1 : Crystallographic structure of GFO together with its net electric polarization (P) and magnetization (M) directions. GFO adopts an orthorhombic structure, crystallizing in the space group Pc21n with a = 0.87512 ± 0.00008 nm, b = 0.93993 ± 0.00003 nm and c = 0.50806 ± 0.00002 nm.6 The Ga3+ and Fe3+ cations are distributed on four types of sites labeled Fe1, Fe2, Ga1 and Ga2.
While its bulk properties had been well established, GFO had been very little studied in thin films. Two important points had in particular to be addressed : 1. the possibility to obtain, as had been done in bulk, room temperature ferrimagnetic films by tuning the Ga/Fe ratio, and 2. the possibility to deposit high crystalline quality GFO thin films on a conducting electrode. This last question conditions the actual use of GFO thin films in magnetoelectric devices. We have addressed both points and performed the elaboration of room temperature ferrimagnetic GFO thin films on conducting Pt(111) electrodes by pulsed laser deposition (PLD).
Growth Ga2-xFexO3 thin films : room temperature ferrimagnetic films
An optimization of the growth conditions of GFO thin films by PLD allowed us to obtain perfectly b-axis oriented Ga2-xFexO3 films on yttrium-stabilized zirconia (001) (YSZ) substrates, for all x values between 0.8 and 1.4. The films present a strong anisotropy, the out-of-plane direction (GFO b axis) being a hard magnetization direction. The Curie temperature increases with x, as for the bulk, and a value superior to room temperature was obtained for x=1.4 (TC = 370 K). The in-plane coercive field reaches the relatively high value of about 3200 Oe at 5 K for x=1.4. A hysteresis cycle is still observed when performing M-H measurements on the x=1.4 sample at room temperature, as awaited from the Curie temperature of 370 K for this composition. The observed saturation magnetization is of 90 emu/cm3 and the coercive field of ca. 600 Oe at 300 K.
Figure 2 : (Top) Magnetization versus temperature for Ga2-xFexO3 thin films with 0.8<x0.6Fe1.4O3 thin film at 5 K and at 300 K (in-plane measurements)</x
Growth of GFO on a conducting bottom electrode : towards potential magnetoelectric applications.
Pure b-axis growth of high crystalline quality GFO thin films has been obtained on two different conducting bottom electrodes : ITO (001) buffered YSZ (001) and Pt (111) buffered YSZ (111). The GFO thin films deposited on single crystalline Pt(111)/YSZ(111) electrodes exhibit excellent crystallization and the number of GFO in-plane variants is reduced from six to three. The measured leakage current on this latter electrode decreases by 4 orders of magnitude in comparison to the use of an ITO bottom electrode. Growth on a single crystalline Pt(111) electrode therefore leads to substantial improvement of both the crystalline quality and the dielectric properties of the GFO films.
We have shown that it was possible to grow room temperature ferrimagnetic films of the promising magnetoelectric material Ga2-xFexO3 on conducting electrodes. The films are perfectly (0k0) oriented and show a low surface roughness. This work opens new perspectives for the integration of a room temperature ferrimagnetic magnetoelectric material in spintronic devices.
Read more in:
“Room temperature ferrimagnetic thin films of the magnetoelectric Ga2-xFexO3”
M. Trassin, N. Viart, G. Versini, S. Barre, G. Pourroy, J. Lee, W. Jo, K. Dumesnil, C. Dufour, and S. Robert
Journal Of Materials Chemistry, 19 (2009) 8876