Multiferroic are a special class of smart materials that exhibit both ferroelectric and magnetic properties and have generated great interest for a variety of applications, ranging from spintronic devices to novel photovoltaic devices, and from cryogenic-free highly-sensitive magnetic sensors to innovative non-volatile memories. However, obtaining materials with ferroelectric and magnetics properties that are sufficiently strong and robust at or above room temperature for potential integration into novel devices remain elusive.
As a result, several strategies have been pursued in the quest of thin films of novel multiferroic materials with good multiferroic properties at room temperature. One such strategy is to grow films or modify one material known to be multiferroic at low temperature with the goal to increase it Curie temperature above room temperature. Another one is to engineer a composite material with one component ferroelectric and a second component is magnetic at the temperature of operation. Yet another approach is to create a polar structural distortion in a magnetic material to induce piezoelectricity and possibly ferroelectricity.
In addition, novel integrated devices require that these materials be produced in the form of thin films of high quality. Fortunately, in particular when synthesized via deposition processes far from equilibrium, thin films synthesis do provide for a number of parameters controlling their properties, such as strain engineering to stabilize otherwise metastable phases.
An example of the first strategy presented here is the growth and characterization of epitaxial thin films and nanostructures of room temperature ferroelectric and ferrimagnetic Bi 2 FeCrO 6 (BFCO) synthesized by pulsed laser deposition (PLD), as well as our current understanding of their properties .
The second scheme will be exemplified by presenting two material systems and exhibiting spontaneous in-situ formation of a multiferoic nanocomposite, namely (i) maghemite (g-Fe 2 O 3 )/BFO nanocomposite epitaxial films and (ii) tetragonal tungsten bronze Ba 2 LnFeNb 4 O 15 (TTB-Ln) / BaFe 12 O 19 / LnNbO 4 , with Ln = Eu and Sm . Macroscopic ferroelectric P-E and magnetic M-H hysteresis loops as well as microscopic electromechanical behavior probed by piezoelectric force microscopy (PFM) and magnetic force microscopy (MFM) are used to characterize the room temperature spontaneous polarization and magnetic properties of the composite thin film systems synthesized by PLD.
The third approach will be illustrated by PLD-grown epitaxial thin films of metastable epsilon ferrite
(ε-Fe 2 O 3 ) and ε-Al x Fe 2-x O 3 stabilized by epitaxial strain. epsilon ferrite is magnetoelectric and potentially multiferroic at room temperature is . In addition ε-Fe 2 O 3 exhibits a large magnetic anisotropy at room temperature and a ferromagnetic resonance (FMR) frequency at room temperature in the THz range in the absence of any magnetic field, which is of interest for short-range wireless communications and ultrafast computer non-volatile magnetic memories. In particular we verified that the synthesized epitaxial thin films of ε-Fe 2 O 3 indeed exhibit a high magnetic anisotropy and a FMR in the THz range as expected.
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