ARTICLE pubs.acs.org/JPCC
Magnetic Properties of Self-Assembled Epitaxial Nanocomposite CoFe2O4:SrTiO3 and CoFe2O4:MgO Films Stacy M. Baber,† Qianglu Lin,† Guifu Zou,‡ Nestor Haberkorn,‡ Scott A. Baily,‡ Haiyan Wang,§ Zhenxing Bi,§ Hao Yang,^ Shuguang Deng,† Marilyn E. Hawley,‡ Leonardo Civale,‡ Eve Bauer,‡ T. Mark McCleskey,‡ Anthony K. Burrell,‡ Quanxi Jia,*,‡ and Hongmei Luo*,† †
Department of Chemical Engineering, New Mexico State University, Las Cruces, New Mexico 88003, United States Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Department of Electrical and Computer Engineering, Texas A&M University, College Station, Texas 77843, United States ^ Jiangsu Key Laboratory of Thin Films, School of Physical Science and Technology, Soochow University, Suzhou 215006, P. R. China ‡
ABSTRACT: Self-assembled epitaxial nanocomposite CoFe2O4:SrTiO3 and CoFe2O4:MgO thin films were grown on LaAlO3 substrates by a chemical solution approach of polymer-assisted deposition. X-ray diffraction and transmission electron microscopy analyses indicated that both phases in the composites were epitaxial with respect to the major axes of the substrate. Compared to the single-phase ferrimagnetic spinel CoFe2O4 films and the bilayer CoFe2O4/SrTiO3 and CoFe2O4/MgO films, the composite films exhibited reduced coercivity and enhanced magnetic anisotropy.
1. INTRODUCTION Spinel ferrite materials such as cobalt ferrite (CoFe2O4 or CFO) have been extensively investigated due to their unique magnetic properties and potential technological applications.1 15 Particularly, the structural and magnetic properties of bulk CFO have been widely studied. However, it is well-known that the strain introduced by the heteroepitaxy can lead to anomalous magnetic properties of the materials.1 Many efforts have been devoted to the growth of epitaxial CFO films in order to study the strain induced magnetic anisotropy of the material. Several techniques such as pulsed laser deposition (PLD),1 7 molecular beam epitaxy,8 11 sputtering,12,13 and sol gel14,15 have been employed to grow CFO thin films. Indeed, epitaxial CFO films have been grown on single crystal (001)-oriented MgO and SrTiO3 (STO) substrates.1,3,7 11 Furthermore, experimental results have shown that the magnetic properties such as the coercivity of CFO films can be tuned by the microstructure, the grain size, and the strain introduced from the lattice mismatch between the film and the substrate.10,11For instance, the CFO films are under compressive strain when they are epitaxially grown on STO substrates but are under tensile strain when they are grown on MgO substrates. Different magnetic properties of CFO films grown on different substrates have been demonstrated.10 Recently, new functionalities have been achieved through interfacing different oxides at the nanoscale by forming selfassembled vertically aligned nanocomposite films. A number of reports have shown that one material can spontaneously form nanodots and/or nanopillars (nanorods) embedded in a matrix of another material during the thin-film growth.16 22 For example, composites CFO:BaTiO3, CFO:PbTiO3, CFO:Pb(Zr,Ti)O3, and CFO:BiFeO3 have been prepared.20 36 The properties of these nanocomposites depend strongly on the morphologies and r 2011 American Chemical Society
elastic interaction between the two phases.20 23 Elastic interactions arise in multiphase nanostructures due to epitaxy, resulting in formation of coherent or semicoherent interphase boundaries and the film/substrate interface.23 A phase-field modeling has demonstrated that the elastic interactions control the magnetic and electric responses of the self-assembled multiferroic nanostructures.23 In this work, we report the growth of self-assembled epitaxial nanocomposite CFO:STO and CFO:MgO films on single-crystal (001)-oriented LaAlO3 (LAO) substrates using a chemical solution approach of polymer-assisted deposition (PAD).37 40 In these composite films, the heteroepitaxial strain arises not only from the lattice mismatch between the CFO and the LAO substrate but also between the CFO and the STO or the CFO and the MgO phases. In comparison with single phase epitaxial CFO and bilayer CFO/STO or CFO/MgO films on LAO substrates, such nanocomposite films show reduced coercivity and enhanced magnetic anisotropy.
2. EXPERIMENTAL SECTION To grow films by PAD, we prepared the precursor solutions by mixing individual aqueous solutions of Co, Fe, Sr, Ti, or Mg bound to polymers. Briefly, the Co precursor solution was prepared by dissolving 2 g of polyethyleneimine (PEI, from SigmaAldrich, average Mn ≈ 60,000, Mw ≈ 750,000. PEI is a branched chain polymer having a ratio of 1:2:1 of primary/secondary/ tertiary amines with a branching site every 3 to 3.5 nitrogen atoms and a general backbone of (CH2CH2NH)x. The nitrogen atoms must be protonated to achieve cationic charge.) in 40 mL Received: July 17, 2011 Revised: August 20, 2011 Published: November 28, 2011 25338
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Figure 2. AFM phase images of (a) composite CFO:STO, (b) bilayer CFO/STO, (c) composite CFO:MgO, and (d) bilayer CFO/MgO (1 μm 1 μm). Figure 1. XRD patterns of CFO and composites CFO:STO and CFO: MgO on LAO substrate. (a) θ 2θ scan, (b) ϕ-scans of composite CFO: STO films from (202) reflections of LAO, (202) of STO, and (404) of CFO.
water. The cobalt chloride (2 g) was then added into the solution. To prepare Fe, Sr, and Mg precursor solutions, 2 g of ethylenediaminetetraacetic acid (EDTA) and 2 g of PEI were dissolved in 40 mL water first. Following that, 2 g of iron(III) chloride, strontium nitrate, or magnesium chloride was added. For the Ti precursor solution, small aliquots of the titanium solution (made by slowly adding 2.5 g titanium tetrachloride to a mixture of 2.5 g of 30% peroxide in 30 mL water) were added into the solution containing 1 g of PEI, 1 g of EDTA, and 30 mL of water (maintaining the pH at 7.5). The solutions were separately filtered in an Amicon filtration unit that is designed to pass materials with molecular weight of less than 30 000 g/mol, to remove any unwanted nonbound ions and to concentrate the solution. The concentrations of Co2+, Fe3+, Sr2+, Ti4+, and Mg2+ after such a filtration process were 295, 209, 157, 408, and 233 mM, as determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). These solutions were mixed in the desired molar ratio needed to synthesize single phase CFO, bilayer CFO/STO and CFO/MgO, and composite CFO:STO and CFO:MgO films (CFO:STO (or CFO:MgO) = 1:1).39,40 These solutions were spin-coated onto LAO substrates at 2000 rpm for 30 s. All the precursor films were annealed at 550 C for 2 h to remove the polymers completely38 and at 950 C for 1 h in flowing oxygen to crystallize the oxides and achieve epitaxial growth of the films. About 50 nm thick films were obtained from one spin-coat. Thicker films can be achieved by multiple spin-coats. For the magnetic property measurements, a CFO layer thickness of around 150 nm was prepared. X-ray diffraction (XRD) was used to characterize the crystal structure of the films. The microstructure of the films was analyzed by atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). The magnetic properties
Figure 3. Cross-section HRTEM image of a composite CFO:STO film on LAO substrate. The inset is the SAED pattern taken from the interface between the film and the substrate.
of the films were evaluated by a superconducting quantum interference device magnetometer (SQUID).
3. RESULTS AND DISCUSSION We found that all the films (single-phase, bilayer, and composite) prepared by PAD are epitaxial with respect to the substrate. In other words, CFO, STO, and MgO can be epitaxially grown on the LAO substrate. Furthermore, CFO can be epitaxially grown on STO and MgO in the case of bilayer films. Figure 1a shows the typical XRD θ 2θ scans for single-phase CFO and composite CFO:STO and CFO:MgO films grown on LAO substrates. Both phases (CFO and STO or CFO and MgO) grow with their c axis normal to the substrates as clearly seen from the diffraction patterns. Compared to the lattice parameter of bulk CFO (a = 0.838 nm = 0.419 2 nm), the lattice parameter of the CFO on the LAO is slightly smaller for single-phase CFO (a = 0.830 nm), bilayer CFO/STO (a = 0.834 nm), and composite CFO:STO (a = 0.823 nm) films. This is understandable by considering the compressive strain due to the smaller lattice parameters from 25339
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Figure 4. Magnetization versus applied magnetic field (M H) hysteresis loops with magnetic field parallel and perpendicular to the substrate surface at 5 and 300 K for single phase CFO film (a and b), bilayer CFO/STO (c), bilayer CFO/MgO (d), composite CFO:STO (e and f), and composite CFO: MgO (g and h) on LAO substrates, respectively (e h, the magnetization values are normalized with respect to the magnetic content in the composites).
either the LAO (a = 0.379 nm) or STO (a = 0.3905 nm). However, the lattice parameter of the CFO is slightly larger in bilayer CFO/MgO (a = 0.842 nm) and composite CFO:MgO (a = 0.844 nm). The MgO (a = 0.4216 nm) controls the overall lattice strain (tensile strain) in this case. Figure 1b shows the typical ϕ scans from reflections of LAO {202}, CFO {404}, and STO {202} for composite CFO:STO films. An average value for the full width at half-maximum (fwhm) of 1.5 for CFO and 0.9 for STO, in comparison with a value of 0.7 for the single-crystal
LAO substrate, indicates the composites having good epitaxial quality. The epitaxial relationships between the films and the substrates can be described as (004)CFO||(002)STO,MgO||(002)LAO and [404]CFO||[202]STO,MgO||[202]LAO. Figure 2 shows the AFM images of composite CFO:STO and CFO:MgO and bilayer CFO/STO and CFO/MgO films. The morphology of composite CFO:STO (Figure 2a) is similar to that of the single phase CFO film (not shown) and bilayer CFO/ STO film (Figure 2b). An average surface feature size of around 25340
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Table 1. Coercivities of Single Phase CFO, Bilayer CFO/ STO and CFO/MgO, and Composite CFO:STO and CFO: MgO at 5 and 300 K in-plane
out-of-plane
in-plane
out-of-plane
300 K (Oe)
300 K (Oe)
5 K (Oe)
5 K (Oe)
CFO
600
600
9000
9000
CFO/STO
530
530
CFO/MgO
650
650
CFO:STO CFO:MgO
330 210
260 280
7600 910
6100 870
50 nm was observed for composite CFO:STO with a root-meansquare (rms) surface roughness of 8 nm. However, the morphology of composite CFO:MgO (Figure 2c) changed dramatically, where MgO with an average feature size of 10 nm is dispersed in the CFO matrix. The reduced feature size leads to a smaller rms value of about 2 nm. However, the bilayer CFO/MgO film (Figure 2d) shows much larger surface feature sizes ranging from 20 to 60 nm. Cross-sectional HRTEM was also used to study the microstructure of the composite CFO:STO film. As shown in Figure 3, self-assembled two phase materials are clearly seen. The interface between the film and the substrate is flat and clean without any indication of intermixing. There are high density and ordered strain contours at the interface between CFO and LAO. This indicates the formation of high density misfit dislocations at the interface, which is caused by the large lattice mismatch of 9% between CFO and LAO. There is only one strain contour/ misfit dislocation identified in the neighboring STO/LAO interfaces because of the relatively smaller lattice mismatch of 3% between STO and LAO. The corresponding selected area electron diffraction (SAED) pattern taken from the interface (see insert in Figure 3) confirms the epitaxial growth of CFO and STO on LAO, evidenced by the distinct diffraction dots from the CFO (and/or STO) film and the LAO substrate. The epitaxial growth of each phase and the epitaxial relationship between the film and the substrate determined from the SAED patterns are consistent with the XRD analysis. Figure 4 shows the magnetization versus applied magnetic field (M H) hysteresis loops with the magnetic field parallel and perpendicular to the substrate surface for the single phase CFO (a, 300 K; b, 5 K), bilayer CFO/STO (c, 300 K), bilayer CFO/ MgO (d, 300 K), composite CFO:STO (e, 300 K; f, 5 K) and composite CFO:MgO (g, 300 K; h, 5 K), respectively. For the single phase CFO or bilayer CFO/STO and CFO/MgO films, the in-plane and out-of-plane coercivities are the same, and the hysteresis loops have similar shapes at either 300 or 5 K, indicating small magnetic anisotropy. However, composite CFO: STO and CFO:MgO films have different coercivities and remanent magnetization values, indicating a relatively large magnetic anisotropy. Table 1 summarizes the coercivity values of the films illustrated in Figure 4. It is known that the magnetic anisotropy of CFO films grown on a lattice mismatched substrate can come from (1) magnetocrystalline effects (i.e., due to the crystal structure) and (2) magnetoelastic effects due to strain from the lattice mismatch between the film and the substrate.1 The magnetic isotropy represents a balance between magneto-crystalline anisotropy and strain anisotropy.2 For single phase CFO thin films, magnetic anisotropy is strongly related to the film thickness.3,5,6 Bilayer CFO/STO and CFO/MgO films can be considered as single phase CFO on STO
or on MgO, respectively. Because of the mismatch between the CFO film and the LAO substrate (the CFO and the STO; or the CFO and the MgO), dislocations at the interface between the film and the substrate or the film and the other layer partially relax the lattice mismatch strain. The CFO film grown on the LAO substrate or on the STO layer is under compression in the plane while under tension perpendicular to the film surface. However, the CFO on MgO layer is under tension in the plane while under compression perpendicular to the film surface. The stress anisotropy and the magneto-crystalline anisotropy are similar in magnitude for the relaxed CFO, resulting in little difference in magnetism between the in-plane and the out-of-plane directions. In contrast to the single phase CFO, the composites show enhanced anisotropy and reduced coercivity. It is noted that at 5 K the coercivity of the composite CFO:MgO is much lower than both that of the single phase CFO and the composite CFO: STO. For the composites, the strain can come not only from the substrate but also from the second-phase. The strain induced by these two phases in the composites can potentially affect the magnetic anisotropy of the CFO phase. Enhanced magnetic anisotropy was observed for CFO nanopillars embedded in a BaTiO3 or BiFeO3 matrix on a (001) STO substrate,20 22 while smaller magnetic anisotropy was observed for BFO nanopillars embedded in a CFO matrix on a (111) STO substrate.21 The reduced coercivity in composites is also observed in other composites such as CFO:PZT (lead zirconate titanate).18,19
4. CONCLUSIONS In summary, we have demonstrated that a chemical solution technique can be used to grow self-assembled epitaxial nanocomposite CFO:STO and CFO:MgO films. The strain in the two-phase composites significantly affects the magnetic properties of CFO. As compared to magnetically isotropic CFO films, the composite CFO:STO and CFO:MgO films show magnetic anisotropy and reduced coercivity. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (H.L.);
[email protected] (Q.J.).
’ ACKNOWLEDGMENT H.L. gratefully thanks the support from the NSF/CMMI NanoManufacturing Program (NSF 1131290) and Interdisciplinary Research Grant (IRG) from NMSU. H.W. acknowledges the support from the NSF/DMR Ceramic Program (NSF 0709831 and 1007969). This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences user facility at Los Alamos National Laboratory (contract DE-AC52-06NA25396) and Sandia National Laboratories (contract DE-AC04-94AL85000). We also acknowledge the support from the Los Alamos National Laboratory LDRD Program. ’ REFERENCES (1) Suzuki, Y.; Hu, G.; van Dover, R. B.; Cava, R. J. J. Magn. Magn. Mater. 1999, 191, 1. (2) Hu, G.; Choi, J. H.; Eom, C. B.; Harris, V. G.; Suzuki, Y. Phys. Rev. B 2000, 62, R779. (3) Lisfi, A.; Williams, C. M. J. Appl. Phys. 2003, 93, 8143. (4) Chopdekar, R. V.; Suzuki, Y. Appl. Phys. Lett. 2006, 89, 182506. 25341
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