Morphology and magnetic properties of Fe3O4 nanodot arrays using template-assisted epitaxial growth
© Guan et al.; licensee Springer. 2015
Received: 6 November 2014
Accepted: 16 December 2014
Published: 6 January 2015
Arrays of epitaxial Fe3O4 nanodots were prepared using laser molecular beam epitaxy (LMBE), with the aid of ultrathin porous anodized aluminum templates. An Fe3O4 film was also prepared using LMBE. Atomic force microscopy and scanning electron microscopy images showed that the Fe3O4 nanodots existed over large areas of well-ordered hexagonal arrays with dot diameters (D) of 40, 70, and 140 nm; height of approximately 20 nm; and inter-dot distances (Dint) of 67, 110, and 160 nm. The calculated nanodot density was as high as 0.18 Tb in.−2 when D = 40 nm. X-ray diffraction patterns indicated that the as-grown Fe3O4 nanodots and the film had good textures of (004) orientation. Both the film and the nanodot arrays exhibited magnetic anisotropy; the anisotropy of the nanoarray weakened with decreasing dot size. The Verwey transition temperature of the film and nanodot arrays with D ≥ 70 nm was observed at around 120 K, similar to that of the Fe3O4 bulk; however, no clear transition was observed from the small nanodot array with D = 40 nm. Results showed that magnetic properties could be tailored through the morphology of nanodots. Therefore, Fe3O4 nanodot arrays may be applied in high-density magnetic storage and spintronic devices.
Fe3O4 has been extensively studied because of its high Curie temperature, and because it has been predicted to have a half metallicity and a full spin polarization [1, 2], these properties make Fe3O4 a promising material for applications in data storage and spintronic devices such as memories or magnetic sensors . Epitaxially grown Fe3O4 nanofilms have been extensively investigated and have been reported to contain a type of natural growth defect, namely, antiphase boundaries (APBs) caused by strains on the film from mismatch between the substrate and the film [4, 5]. The presence of APBs leads to unusual magnetic properties of epitaxial Fe3O4 films, such as nonsaturation of magnetization in high magnetic fields . Moreover, an epitaxial Fe3O4 nanodot array is a possible candidate to meet the requirements of ultrahigh-density storage and reduce the size of spintronic devices. In addition, the magnetic property dependence on nanodot size is worth investigating. However, to our knowledge, up to now, no studies on epitaxial Fe3O4 nanoarrays have been reported.
One popular top-down method, namely, the lithography technique, has been used to pattern films into nanoarrays [7, 8]. However, this top-down method has a complex procedure and is cost prohibitive for mass production. In addition, covalent oxides of Fe3O4 being fragile are easily damaged during etching. In another technique, Fe3O4 nanoparticles are synthesized first by chemical methods and then self-assemble on a substrate to form nanodot arrays [9, 10]. However, in this case, the particles have random orientations, easily aggregate, and have weak bonding force with the substrate, which make the properties quite different from those of epitaxial nanodot arrays.
Recently, porous anodized aluminum (PAA) templates have been used to fabricate large areas of metal and oxide nanodot arrays because of their low production cost, pore size controllability, and ease of fabrication, which is called the bottom-up method [11–15]. The PAA-based method is a direct approach for growing epitaxial nanodot arrays, in which the atoms passing through the pore directly arrive at the substrate. After removing the PAA template, an epitaxial growth of an ordered nanodot array is obtained.
In this paper, we used the bottom-up patterning technique, precisely controlled the fabrication parameters, and ultimately obtained large-area epitaxial Fe3O4 nanodot arrays on a single crystal substrate of SrTiO3. The magnetic property dependence on dot size and the morphology of nanoarrays were investigated and subsequently compared with their corresponding films. The density and size of the nanodots controlled by the PAA template could tailor magnetic properties including coercivity (Hc), squareness, magnetic anisotropy, and Verwey transition temperature (Tv).
Ultrathin PAA templates with various pore diameters and inter-pore distances were prepared through a typical two-step anodization process [16, 17]. After the second anodization, the top surface of the PAA template was spin coated with a layer of polymethylmetacrylate (PMMA). The aluminum and thin nonporous barrier layer were then removed by immersing the template in an acid mixture of CuCl2 and HCl. A thin PMMA layer prevents the ultrathin ceramic membrane from undergoing mechanical deformations, such as folding, cracking, or ripping. After complete removal of aluminum, the template was transferred onto the SrTiO3 substrate and immersed in C3H6O at 60°C to dissolve the PMMA coating. Prior to the removal of PMMA, the templates were further thinned by immersing in H3PO4 for different durations to obtain the desired template thickness. In this work, three sizes of the PAA template with the same thickness of 200 nm were fabricated. The average pore diameter and inter-pore distance of the templates were (a) approximately 38 and approximately 67 nm, (b) approximately 67 and approximately 115 nm, and (c) approximately 137 and approximately 160 nm, respectively.
Fe3O4 was subsequently deposited on the PAA/SrTiO3 (001) substrate by laser molecular beam epitaxy (LMBE) using a KrF excimer laser (λ = 248 nm) with a repetition rate of 10 Hz and energy of 300 mJ on a ceramic target of Fe3O4. The PAA template was then removed using NaOH solution at 35°C, so that large-area Fe3O4 nanodot arrays were obtained. The substrate temperature was chosen between 400°C and 700°C, and the chamber oxygen pressure was 10−4 Pa. Some samples were annealed at 700°C in situ for 2 h before cooling to room temperature.
The crystal structure was characterized by X-ray diffraction (XRD) using Cu Kα radiation (λ = 0.15406 nm). The morphology of the PAA templates and nanodot arrays was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Magnetization measurements were performed by a superconducting quantum interference device magnetometer (SQUID).
Results and discussion
The Ms of the film (310 emu/cm3) is smaller than that of the bulk (480 emu/cm3), which was attributed to the presence of APBs. The lattice constant (0.8398 nm) of the Fe3O4 film grown on SrTiO3 (0.3905 nm) is more than twice that of the substrate, indicating that the film undergoes a compressive strain. This strain induces some defects such as dislocations or APBs during film growth. Presences of APBs are the main reason for the lower saturation magnetic value compared with the bulk Fe3O4. Moreover, the Ms of the nanodot arrays is smaller than that of the film. This characteristic could be explained by the accumulation of spin canting around the nanoparticle surface and the exchange coupling between the nanodots that reduce magnetic moment .
From Figure 5, the in-plane hysteresis loop of the film displayed an almost rectangular shape with S = 0.96 and a large Hc of 450 Oe, which is better than that reported for Fe3O4 films with comparable thickness . The sample with D = 140 nm had a large Hc of 400 Oe and was easily saturated. The magnetization reversal may be dominated by the domain wall movement, similar to the film. However, the S of this sample is 0.50, which was half that of the film. The reduced S may be due to magnetization relaxation from dot edges. The sample with D = 70 nm exhibited smaller S = 0.28, large Hc of 300 Oe, and unsaturation at an external field of 2,000 Oe. At S = 0.05, Hc dropped to 50 Oe and unsaturation of the sample with D = 40 nm exhibited more or less soft magnetic properties similar to those of the bulk. The sample with D = 40 nm may reverse their magnetization by coherent rotation because of a single domain size of 38 nm for Fe3O4. On the other hand, the sample with D = 70 nm with its relatively high Ms and Hc maybe in a transition state so that reversal is partly due to wall movement, impeded by defects, and rotation, which should be further investigated in the future.
In summary, well-ordered large-area arrays of epitaxial Fe3O4 nanodots were obtained on the SrTiO3 (100) substrate by LMBE at an elevated growth temperature (700°C) using ultrathin PAA templates as shadow templates. An Fe3O4 film was also prepared using LMBE. The AFM and SEM images showed that Fe3O4 nanodot arrays had large areas with well-ordered hexagonal arrays with D of 40, 70, and 140 nm; height of around 20 nm; and Dint of 67, 115, and 160 nm. The nanodot density can be calculated to be as high as 0.18 Tb in.−2 when D = 40 nm. The film or nanodot array exhibited magnetic anisotropy. The anisotropy of the arrays and the S weakened with decreasing dot size. Magnetization reversal may be dominated by the domain wall movement of the film and of the sample with D = 140 nm, while the sample with D = 40 nm may reverse their magnetization by coherent rotation because of its single domain size of 38 nm. On the other hand, the sample with D = 70 nm exhibited smaller S, large Hc, and unsaturation at the external field of 2,000 Oe. It demonstrated that the sample with D = 70 nm may undergo a magnetization reversal transition, which should be further investigated. The Tv values of the film and the samples with large dots (D = 70 and 140 nm) were close to those of the bulk, whereas the Tv of the small dot sample with D = 40 nm was undetected. Based on the above analysis, magnetic anisotropy, S, and Tv of Fe3O4 could be tailored by the morphology of nanodots. To the best of our knowledge, no study has shown epitaxial Fe3O4 nanodots to date. These results facilitate a deeper understanding of the micromagnetization inside nanostructures of Fe3O4, and the output could be a candidate for ultrahigh-density data storage and spintronic devices.
The work is financially supported by National High Technology Research and Development Program of China (863 program, No. 2014AA032904), NSFC (Nos. 51025101, 11274214, and 61434002), and the Special Funds of Shanxi Scholars Program, the Ministry of Education of China (Nos. IRT 1156 and 20121404130001).
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