Strain-dependence of the structure and ferroic properties of epitaxial Ni1−xTi1−yO3 thin films grown on sapphire substrates

Abstract

Polarization-induced weak ferromagnetism has been predicted a few years back in compounds MTiO 3 (M = Fe, Mn, Ni) (Fennie, 2008). We set out to stabilize this metastable, distorted perovskite structure by growing NiTiO 3 epitaxially on sapphire Al 2 O 3 (001) substrate, and to control the polar and magnetic properties via strain. Epitaxial Ni 1 − x Ti 1 − y O 3 films of different Ni/Ti ratios and thicknesses were deposited on Al 2 O 3 substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy shows that substrate-induced strain has a marked effect on the structure and crystalline quality of the films. Physical property measurements reveal a dependence of the Néel transition and lattice polarization on strain, and highlight our ability to control the ferroic properties in NiTiO 3 thin films by film stoichiometry and thickness.

Strain-Dependence of the Structure and Ferroic Properties of Epitaxial NiTiO3Thin Films Grown on Different Substrates

“…Al 2 O 3 substrate was chosen as a nonmagnetic, nonpolar alternative with lattice parameters close to those of NiTiO 3 , but still providing a large positive (5.7%) lattice mismatch and thus compressive strain. Sapphire substrates have been used previously to grow epitaxial FeTiO 3 films [29] and, more recently, epitaxial NiTiO 3 films [9,11,30]. The potential benefit to using sapphire substrate may be the considerable compressive strain it provides, which may mimic a high-pressure compression employed in the synthesis of bulk LNO-type FeTiO 3 [5] and MnTiO 3 [7].…”

“…Recently, we have demonstrated that epitaxial NiTiO 3 films can be grown on sapphire substrate by PLD [9] and that such films exhibit both lattice polarization and weak ferromagnetism [10]. Furthermore, the properties of NiTiO 3 / Al 2 O 3 films were successfully manipulated by applied strain [11]. While the latter reports confirmed that our epitaxial NiTiO 3 films were indeed of the LNO-type structure, the lattice mismatch between the film(s) and the sapphire substrate resulted in less-than-high-quality films in terms of crystallinity, most likely affecting the observed physical properties 2 Advances in Condensed Matter Physics adversely.…”

Atomic Layer Deposition of oriented nickel titanate (NiTiO3)

“…Although FeTiO3 has been synthesized with the LiNbO3 structure at high pressure, [137,138] this polymorph has not been unambiguously observed for NiTiO3. [139][140][141] Determining if the symmetry is R−3c or R3c is hard with X-ray diffraction alone. The difference in unit cell parameters between such polymorphs is small and effects from strain and off-stoichiometry is likely to a If not removed, the native SiO2 layer on Si inhibits epitaxial growth.…”

Phase and Orientation Control of NiTiO3 Thin Films

“…lattice polarization was observed, and possibly weak ferromagnetism, indicating the presence of the LiNbO3 structure [11][12][13]. Ferroelectricity and weak ferromagnetism have indeed been observed in bulk samples of NTO as well [14], but the report did not discuss the crystal symmetry of the samples.…”

“…Ferroelectricity and weak ferromagnetism have indeed been observed in bulk samples of NTO as well [14], but the report did not discuss the crystal symmetry of the samples. Thin films of NTO have previously been made by various deposition techniques, including aerosol-assisted CVD [2], sol-gel methods [3,[15][16][17][18], dip-coating [19] and RF-sputtering [4,5], as well as with the previously mentioned pulsed laser deposition [10][11][12]. Especially for the latter technique, compositional control seems to be challenging.…”