Control of magnetization by an electric field : room temperature ferrimagnetic thin films of the magnetoelectric Ga2-xFexO3

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 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. The Ga3+ and Fe3+ cations are distributed on four types of sites labeled Fe1, Fe2, Ga1 and Ga2.

• Room temperature multiferroic thin films

with a low polarization though, compared to what is expected (0.2 vs. 25 µC/cm2)

Figure 2 : Ferroelectric loop measured on a GFO1.4 thin film – The polarization is about 0.2 µC/cm2, i.e. a hundred times smaller than expected [Thomasson et al. J. Appl. Phys. 113, 214101 (2013)]

• A robust polar material

The polarization of the films has been determined both by the non-destructive Resonant X-ray scattering and by HR TEM.

Figure 3 : (i) REXS spectra simulated for outwards or inwards polarized GFO films demonstrating the efficiency of the technique to discriminate between the two polarization states [Lefevre et al. Small Methods 1, 1700234 (2017)]; (ii) A chemically induced tail-to-tail polarization domain wall as observed by HR-TEM [Homkar et al. Physical Review Materials 3, 124416 (2019)]

• A complex ferrimagnetic behaviour

with a non-zero and strongly anisotropic magnetic orbital moment – Indication of a strong magnetoelectric coupling.

Figure 4 : XMCD spectra measured on a GFO1.4 thin film in grazing (GI) and normal (NI) incidence. The opposite sign of the integrated signals evidences an important anisotropy of the orbital moment [Preziosi et al. Phys. Rev. B 103, 184420 (2021)]

We are currently working on the possibility to make use of the magnetoelectric character of GFO in spin Hall effect promoted spin-orbit-torque-based devices. (projet ANR “MISSION”).

Read more in:

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