Crystalline orientation of BiMnO₃ thin films grown by rf-sputtering

Abstract

BiMnO₃ thin films were deposited by means of rf-sputtering onto [1 0 0] oriented SrTiO₃ substrates. X-ray diffraction measurements revealed epitaxial growth with [0 0 l]_m orientation in the monoclinic structure representation, equivalent to the ${[111]}_{\text{pc}}$ direction of the pseudocubic cationic lattice. This [0 0 l]_m orientation was obtained in a wide deposition parameters range. Preliminary magnetization measurements suggest that these films do not present ferromagnetism below the bulk Curie temperature, $T_{\text{C}}=105$ K.

Introduction

Multiferroic [1], [2] materials are scarce and combine at least two of the properties of ferromagnetism, ferroelectricity and ferroelasticity. These materials are intensively studied at present because they offer the possibility of realizing a whole range of new applications. Therefore big interest exists on its synthesis in thin film form which also would allow for its integration on micro- or nano-sized applications.

Bulk BiMnO₃ is an insulating multiferroic perovskite material which in bulk form presents both ferroelectricity below $T_{\text{E}}\sim 550$ K, and ferromagnetism below $T_{\text{C}}\sim 105$ K, with an interaction between them [3]. Unfortunately it is difficult to synthesize in bulk form because it requires high pressures (about 6 GPa) [4] and thus only recently single-crystalline samples have been obtained [5]. For this reason measurements performed using only either polycrystalline samples or thin films can be found in the literature.

The BiMnO₃ structure was initially identified [6], [7], as being a triclinically distorted pseudocubic (pc) perovskite lattice, with the following cell parameters: $a=c=3.935$ Å, $\alpha =\gamma =91$°$28^{\prime }$. However, recent detailed neutron and electron diffraction measurements at room temperature [8], as well as high-temperature X-ray diffraction [9], determined that the BiMnO₃ structure is monoclinic (m), space group $C$2, with unit cell parameters: $a=9.5323$ Å, $b=5.6064$ Å, $c=9.8535$ Å and $\beta =110.667°$. A unit cell of the monoclinic structure representation is shown in Fig. 1. It can be seen that there are three inequivalent Mn atoms arranged in parallel layers in the monoclinic [1 0 0]_m and [0 1 0]_m directions. The relation between these two possible ways of visualizing this structure consists in ${[-202]}_{\text{m}}\simeq {[010]}_{\text{pc}}\text{,}{[040]}_{\text{m}}\simeq {[-202]}_{\text{pc}}\text{,}{[111]}_{\text{m}}\simeq {[100]}_{\text{pc}}={[001]}_{\text{pc}}\text{,}{[004]}_{\text{m}}\simeq {[111]}_{\text{pc}}$, etc.