Thickness dependence of anomalous magnetic behavior in epitaxial Fe3O4(111) thin films: Effect of density of antiphase boundaries

Abstract

We study the magnetic behavior of Fe 3 O 4 ͑111͒ thin films with thicknesses between 5 nm and 50 nm. The films are epitaxially grown on ␣-Al 2 O 3 ͑0001͒ single crystals by atomic-oxygen-assisted molecular beam epitaxy. The Fe 3 O 4 ͑111͒ thin films exhibit high structural order with sharp interfaces and low roughness and exhibit a Verwey transition for thicknesses above 8 nm. However, the samples have magnetic properties that deviate from the bulk ones. The magnetic moment varies between 2.4 B for 5-nm-thick film and 3.2 B for 50-nm-thick film in a field of 1.2 T, which is lower than that of bulk samples (4.1 B /Fe 3 O 4 formula). Still the magnetic saturation is never reached, even in fields as large as 2 T. The thinner the film, the slower the approach to saturation. Structural analysis, performed using high-resolution transmission electron microscopy, reveals the presence of antiphase boundaries (APB's), the density of which decreases when the thickness increases. Using a model of ferromagnetic domains separated by antiferromagnetically sharp interfaces, we show that the slow approach to saturation observed in the films as a function of thickness is driven by the APB density.

Origin of the giant magnetic moment in epitaxialFe3O4thin films

“…A decrease in T V for decreasing Fe 3 O 4 thicknesses is observed, which has been previously reported by other authors, 7,19,33 and explained as a slight nonstoichiometry of the films 33,34 or strain due to the epitaxial growth on the substrate. 7 Recently, it has been suggested 8,19 that in ultrathin films where the APBs domain size is small, the long-range order of the Fe 2+ and Fe 3+ ions at the octahedral sites, necessary for the Verwey transition to occur, can be inhibited, 35 producing a decrease in T V . Indeed, this decrease in the Verwey transition temperature as a function of Fe 3 O 4 film thickness ͑t͒ can be fitted to the phenomenological expression given by Eq.…”

“…[17][18][19] The magnetite films with thicknesses below 20 nm exhibit increasing magnetization values as the thicknesses are reduced, up to values significantly larger than the bulk magnetic moment. The saturation magnetization values shown in Fig.…”

“…[17][18][19] The magnetization of our Fe 3 O 4 films can be modeled as a layer with M S which decreases as the film thickness decreases. The corrected magnetization data follow the same phenomenological law as that obtained for the Verwey Temperature,…”

“…These antiferromagnetic exchange interactions occurring across the APBs have been identified to be responsible for the impossibility to saturate the film magnetization in moderate magnetic fields. [17][18][19] The measured magnetization is, therefore, necessarily lower than the bulk one, due to this decrease in the magnetization at the APBs.…”

“…2͒ and its characteristic metal-insulator transition at 120 K ͑Verwey transition͒. [3][4][5] Differences between some properties of bulk magnetite and epitaxial thin films have been observed, such as an increased resistivity with decreasing film thickness, [6][7][8] negative magnetoresistance, [9][10][11] decreased and broadened Verwey transition temperature ͑T V ͒, 7,12,13 superparamagnetism in ultrathin films, 14,15 decreased saturation magnetization, [16][17][18][19] and anomalous, planar, and ordinary Hall effects. [20][21][22] This anomalous behavior has often been associated to the film microstructure and the presence of structural growth defects called antiphase boundaries ͑APBs͒.…”