Strain relaxation studies of the Fe₃O₄/MgO (100) heteroepitaxial system grown by magnetron sputtering

Abstract

PACS numbers:68.55Jk: Structure and morphology; thickness; crystalline orientation and texture 81.15Cd: Deposition by sputtering 75.50Bb: Fe and its alloys Detailed strain relaxation studies on epitaxial magnetite, Fe 3 O 4 , films on MgO (100) substrates grown by magnetron sputtering reveal the accommodation of strain up to 600 nm thickness, a thickness much above the critical thickness (t c ) predicted by theoretical models. The results are in agreement with the suggestion that the excess strain in Fe 3 O 4 /MgO (100) hetero-epitaxy is accommodated by the presence of antiphase boundaries. The compressive strain generated by the antiphase boundaries compensate for the tensile strain within the growth islands, allowing the film to remain fully coherent with the substrate. Contrary to the earlier findings, magnetisation decreases with an increase in film thickness. This vindicates the view that the structure of the antiphase boundaries depend on the growth conditions. Key words: Magnetite; Hetero-epitaxy; Strain relaxation; Magnetron sputtering; Antiphase boundary Introduction Magnetite, Fe 3 O 4 , is one of the transition metal oxides studied most extensively owing to its high Curie temperature (T C ), metal-insulator transition at low temperatures (~120 K) [1] and half-metallic nature [2]. Epitaxial films of magnetite hold out promise for device fabrication. Strain in epitaxial magnetite films has a strong influence on the magnetic and magneto-transport properties. Though there exist reports on the growth and characterisation of thin films of magnetite on different substrates [3][4][5][6][7][8][9][10][11], the strain relaxation behaviour in Fe 3 O 4 /MgO films grown by sputtering technique has not been investigated in detail. The lattice mismatch between the film and substrate is − 0.344%. In our previous study [12] on the strain relaxation behaviour of Fe 3 O 4 /MgO (100) hetero-epitaxial films grown by oxygen assisted molecular beam epitaxy (MBE), it was shown that the films remain in a fully strained state up to a much greater thickness than that predicted (~70 nm) by critical thickness models based on misfit strain [13,14]. The anomalous strain-relaxation behaviour was attributed [12] to the presence of antiphase boundaries (APBs). The APBs are the natural defects occurring during growth in Fe 3 O 4 /MgO hetero-epitaxial system [4,5]. The oxygen atoms in both Fe 3 O 4 and MgO are arranged in a facecentred cubic (fcc) lattice. The lattice constant of Fe 3 O 4 (a=8.3987 Å) is nearly twice that of MgO (a=4.213 Å) due to the specific arrangement of Fe atoms at the vacant interstitial tetrahedral (A) and octahedral (B) sites. During the growth, the islands nucleate randomly and when they coalesce they can be shifted or rotated with respect to each other, forming antiphase boundaries. At some of these boundaries the angle subtended by the cation-anion-cation interaction is 180° and hence the superexchange interaction between the cations mediated by oxygen is antiferromagnetic. This results in a picture where la...

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