Liquid Phase Deposition of Barium Hexaferrite Thin Films - The

Jan 2, 2014 - Advanced Materials Research Institute, University of New Orleans 2000 ... Ceramic Society 2016 99 (10.1111/jace.2016.99.issue-3), 860-86...
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Liquid Phase Deposition of Barium Hexaferrite Thin Films Amin Yourdkhani,† Daniela Caruntu,‡ Armando K. Perez,† and Gabriel Caruntu*,‡ †

Advanced Materials Research Institute, University of New Orleans 2000 Lakeshore Drive, New Orleans, Louisiana 70148, United States ‡ Department of Chemistry, Science of Advanced Materials Program, Central Michigan University, Mount Pleasant, Michigan 48858, United States ABSTRACT: In this study we demonstrate for the first time that magnetic films with a complex crystal structure, such as hexagonal barium ferrite BaFe12O19, can be successfully fabricated in aqueous solutions at 60 °C by the liquid phase deposition method (LPD). Highly uniform films with a variable thickness were deposited on (001) Si substrates at 60 °C and analyzed by X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and magnetic force microscopy. As expected, the as-deposited films are amorphous and consist of a mixture of metal oxide/ oxyhydroxide intermediates which can be converted into the desired hexagonal barium ferrite phase upon heat treatment at 950 °C in air. The molar ratio of the metal cations in the treatment solution was found to play a crucial role on the composition of the films, with chemically pure BaFe12O19 films forming in solution when the Fe3+/Ba2+ molar ratio is n = 11. The films are constructed by close-packed spheroidal particles and have out-of-plane coercive fields which increase from 134.9 to 215.7 kA/m when their thickness varied from 100 to 524 nm. The in-plane coercivity values are slightly smaller than the out-of-plane coercivity, indicating the existence of a randomly oriented magnetic anisotropy. With their easy preparation and magnetic properties comparable to those of the bulk materials, these structures are of interest for implementation in data storage and microwave applications.

1. INTRODUCTION M-type barium hexaferrites (BaFe12O19) have garnered an increasing interest, both scientific and technologic, due to their unique magnetic properties which make them suitable in data storage and electronics. Bulk barium hexaferrite has been traditionally used in the design of permanent magnets as a result of its fairly large crystalline anisotropy and high intrinsic coercivity. Barium ferrite thin films with a high saturation magnetization and large uniaxial magnetocrystalline anisotropy along the c-axis of the hexagonal unit cell are excellent materials for ultrahigh density magnetic recording,1−3,3 whereas their low ferromagnetic resonance line width (FMR) made them attractive for the design of passive microwave components, such as isolators, inductors, circulators, phase shifters, and miniature antennas operating in a wide range of frequencies (1−100 GHz).4,5 In addition to their excellent magnetic properties, barium ferrite ceramics were found recently to exhibit large ferroelectricity at room temperature, as a result of the structural distortion of the FeO6 octahedra, which make these materials very good candidates for the design of multiferroic materials. Along with the study of the relationship between the structural and morphological features of these materials and their macroscopic properties, an important effort has been made for the controlled growth of high quality hexaferrite films by both chemical and physical methods such as sputtering,6,7 metallorganic decomposition,8 liquid phase epitaxial deposition,5,9,10 pulsed laser deposition,11,12 and sol− gel13,14 techniques. In general, the chemical synthesis of © XXXX American Chemical Society

stoichiometric barium ferrite is challenging due to the relatively high temperatures required to crystallize the hexaferrite structure, the complexity of the unit cell, which contains 64 ions distributed over 11 different crystallographic sites, and the higher thermodynamic stability of α-Fe2O3 than that of the barium ferrite, which leads oftentimes to its formation as an impurity and thereby alters significantly the properties of the material. This task is even more challenging in the case of thin films structures, because it is well-known that in polycrystalline layers the orientation of the constituting grains is dependent on the thickness of the film which, in turn, can dramatically influence their magnetic properties. Specifically, it has been suggested that below a critical thickness polycrystalline Bahexaferrite films grown by sputtering and sol−gel methods are constructed by randomly oriented or columnar grains whereas thicker films annealed above 850 °C have grains oriented parallel to the plane of the film which will lead to the reorientation of the magnetization easy axis parallel to the plane of the film, thereby influencing significantly their magnetic properties.7,15 The liquid phase deposition (LPD) is a soft solution chemical method which has been proposed for the deposition of high quality ceramic oxides due to its simplicity, low energy consumption, and lack of a need for expensive vacuum Received: September 26, 2013 Revised: December 29, 2013

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equipment.16−19 Chemical reactions in the liquid phase deposition method involve the slow hydrolysis of metal fluoro-complexes in supersaturated solutions at temperatures below 80 °C whereby the fluoride ions in the inner coordination sphere of the fluoro-anion are progressively replaced by OH− ions and/or water molecules. MFn(n − 2m) − + mH 2O ⇌ 1MOm + nF− + 2mH+

(1)

H3BO3 + 4HF ⇌ BF4 − + H3O+ + 2H 2O

(2)

followed by rinsing with deionized water and drying under ambient conditions. Since the glass melts at temperatures above 600 °C, only the thin films deposited on silicon were annealed at 950 °C, and thin films deposited on glass substrates obtained under similar conditions were used as control samples for thickness measurements. The iron source used in the deposition of the barium hexaferrite thin films was a solution obtained by dissolving 0.25 g of FeO(OH) in 50 mL of 1 M NH4F·HF aqueous solution. FeOOH was obtained by precipitating an aqueous solution of Fe(NO3)3·7H2O with a diluted solution of ammonia. The solid product was subsequently filtered, washed, and dried at room temperature in open atmosphere to ensure the oxidation of the Fe2+ ions. A volume of 10 mL of the Fe-containing solution was mixed with 40 mL of 0.5 M H3BO3 and subsequently diluted with 70 mL of deionized water before adding 10 mL of Ba(NO3)2 solution to prevent the instant precipitation and yield a clear treatment solution. (001) silicon substrates buffered by LaNiO3 were suspended vertically and kept in the treatment solution at 60 °C for different deposition times to control the film thickness. The pH of the solutions was varied in the range of 5.2−5.8. After their removal from the reaction solution, the films were carefully rinsed, cleaned ultrasonically with distilled water, and dried under flow of nitrogen gas. To ensure the conversion of the metal hydroxides/oxyhydroxides into the hexagonal barium hexaferrite structure and the complete crystallization of the films, the thin film samples were annealed in open air at a temperature of 950 °C for 6 h followed by a natural cooling to room temperature. b. Characterization of the Ba-Hexaferrite Thin Films. The composition and microstructure of the barium ferrite films were investigated with a LEO 1530VP field emission scanning electron microscope (FE-SEM) operating in low vacuum mode at an accelerating voltage of 200 kV and equipped with an energy dispersive (EDS) detector. X-ray diffraction experiments were performed in grazing incidence mode (2°) with a Philips X’Pert System equipped with a curved graphite single-crystal monochromator (CuKα radiation). Patterns were recorded in a step scanning mode in the 15−60° 2θ range with a step of 0.02° and a counting time of 10 s. The angular range was restricted to 15−60° due to a very strong reflection of the Si substrate which appears around 68° in 2θ and obscures the peaks of the metal oxide films. The thickness of the films was measured by cross-sectional observation using scanning electron microscope. AFM images of the thin films were collected at room temperature with an Asylum Research MFP3D atomic force microscope working in tapping mode and using commercial Si3N4 cantilevers with a force constant of 0.7 N m1−. Magnetic force microscopy (MFM) experiments were performed with a variable-field magnetometer (VFM) provided by Asylum Research, Inc. using a low coercivity tip. The magnetic properties of the deposited film were measured with a vibration sample magnetometer at room temperature by applying a magnetic field which varied from H = −2.2 to + 2.2 T. Room temperature Raman scattering studies were performed with a Thermo-Fisher DXR dispersive Raman spectrometer in a conventional backward geometry by using the λ = 532 nm line. The spectral resolution was 3 cm−1. The scattered light was analyzed with a triple monochromator coupled with an optical microscope, which allows the incident light to be focused on the sample as a circular spot of about 2 μm in diameter.

In general, the chemical precipitation of metal oxide films in aqueous solutions requires a very slow hydrolysis of the metal ions in order to prevent the spontaneous bulk precipitation of the oxide or hydroxide products. Boric acid has been conventionally used as a scavenger for the fluoride ions due to its ability to form the stable [BF4]− complex ion (1), which causes the equilibrium reaction 1 to proceed to the right side with formation of the metal oxide. Moreover, the [BF4]− ion is water-soluble, which allows its easy elimination from the film surface upon rinsing the resulting coatings and prevents the contamination of the metal oxide thin film structures with F−, which can be otherwise detrimental to the stoichiometry and the properties of the films.20,21 The liquid phase deposition method was first proposed by Deki et al.16 for the fabrication of silica thin films and was extended to simple and compositionally graded oxides such as TiO2,22 ZrO2,23 SnO2,24 Fe2O3,25 NiO,24 and Si1−xTixO2,26 respectively. Recently, the liquid phase deposition method was used for the fabrication of multicomponent oxide films, such as dielectric ferroelectric perovskites ATiO3 (A = Ba, Sr, and Pb).27−29 Using a similar approach, our group synthesized for the first time mirrorlike magnetic oxides, such as spinel ferrites AFe2O4 (A = Zn, Ni, and Co)30,31 and used these structures as building block for the design of hierarchically complex structures, such as bilayered perovskite/spinel ferrite nanocomposites,32 which demonstrates the versatility of this method to for the fabrication of metal oxide structures with variable selectable topology. In an attempt to expand the liquid phase deposition method to oxides with a more complex structure, we report here on our initial results on the deposition of M-type barium ferrite films at temperatures below 80 °C. The reliable deposition of multicomponent thin film ceramics by the liquid phase deposition method offers a wide variety of technological and environmental advantages along with opening the door toward the implementation of this simple methodology into silicon-based electronics, thereby enabling the design of functional devices with controllable characteristics and programmable properties.

2. EXPERIMENTAL PROCEDURE a. Chemical Deposition of Ba-Hexaferrite Thin Films. All experiments were performed in open atmosphere by using a magnetic hot plate (IKA Works, Inc.) equipped with a temperature controller and a pH electrode. Ba-hexaferrite thin films were deposited from treatment solutions prepared by dissolving reagent grade purity chemicals (Alfa Aesar) in deionized water (18 MΩ) obtained from a Barnstead Nanopure water purification system. The films were deposited on highly doped (001) silicon wafers (ρ = 0.05 Ω·cm) and commercial nonalkali glass plates (Corning no. 7059). Prior to each deposition, the substrates were degreased upon sonication for 15 min in a solution of acetone and ethanol (50:50% wt.) B

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Figure 1. Top view FE-SEM images of the as-deposited film at different reaction times: after 1 h (a), 2 h (b), and 4.5 h (c), respectively.

of the deposition vessel were annealed at 950 °C for 6 h (at a heating rate of 200 °C/h) without any precalcination steps, as typically required for barium ferrite obtained by coprecipitation methods.34 In this study, the optimization of the synthesis conditions allowed us to obtain well-crystallized phases below 1000 °C. This proved to be critical for avoiding the interfacial diffusion of silicon from the silicon substrate to the barium ferrite film, which can reduce the value of the saturation magnetization and increase the microwave loss of the films.35,36 Another possible major shortcoming associated with the chemical synthesis of barium ferrites with a magnetoplumbite-type crystal structure is the limited control of the stoichiometry of these materials, expressed by the Fe/Ba molar ratio, which can lead to the formation of secondary phases, such as BaFe2O4 and/or the antiferromagnetic Ba2Fe6O11, which will inevitably affect the physical properties of the obtained products. Xu and co-workers suggested that when barium ferrite is obtained from mixtures containing the metal precursors in a stoichiometric ratio, that is, Fe/Ba = 12, the product contained α-Fe2O3 (hematite) as a secondary phase.34,37 Furthermore, when the initial solution had an excess of barium the resulting hexagonal ferrite contained BaFe2O4.38 Therefore, the optimization of Fe to Ba ratio is a key requirement for the fabrication of stoichiometric hexagonal barium ferrites because both α-Fe2O3 and BaFe2O4 phases are nonmagnetic and their presence in the final products drastically influences the magnetic properties of the hexagonal ferrite, thereby limiting their technological applications.39,40 To address this issue and determine the ideal molar ratio of the salt precursors in the treatment solution, leading single phase hexagonal ferrite film deposition experiments were carried out by varying the molar ratio between the iron and barium salts in the treatment solution between 10 and 12, respectively. As seen in Figure 2, the X-ray diffraction pattern of thin films obtained from treatment solutions with a Fe3+/Ba2+ molar ratio n = 12 and 10 corresponds to a highly crystalline hexagonal ferrite phase but it also contains secondary phases, which have been

3. RESULTS AND DISCUSSION The visual inspection of the treatment solution during the deposition process indicated that the solution remains clear for about 1 h before turning progressively turbid due to the formation of small nuclei resulting from the hydrolysis of the cationic species in solution. These nuclei grow to eventually form small grains which will attach to any solid surface, including the substrate and the inner walls of the beaker, eventually leading to the formation of uniform, scratch-free metal hydroxide/oxide/oxyhydroxide films. The FE-SEM images of the films collected at different stages of the deposition process (Figure 1) suggested that the films form via the formation of three-dimensional islands on the surface of the substrate, followed by their growth and coalescence to eventually yield a continuous ceramic film (Volmer−Weber mechanism).33 As seen in Figures 1b and c the as-deposited films present cracks, presumably due to the existence of stresses during the film formation. The growth rate of the film at 60 °C was 50 nm/h, and the thickness of the films can be controlled from 50 nm to 1 μm by varying the deposition time. It is worth mentioning that diluting the treatment solution before addition of Ba(NO3)2 solution is key for the formation of a clear treatment solution and preventing the instant precipitation of the hydroxyl-containing metal species, which yield a bulk solid instead of a thin film. The deposited barium ferrite films adhere well to silicon substrates and present excellent mechanical properties. Powder X-ray diffraction data coupled with FT-IR and EDX experiments (not shown) indicated that the asdeposited films are amorphous in nature and consist of mixtures of barium hydroxide and ferric oxyhydroxide. Whereas for spinel-type ferrite thin films obtained by liquid phase deposition the conversion of the metal hydroxide/ oxyhydroxide intermediates into the corresponding oxide is performed at a relatively low temperature (t < 600 °C), in the case of hexagonal ferrites, their much more complex crystal structure will require substantially higher energies to stabilize the crystalline form of the as-deposited amorphous film. Consequently, the films and powders collected from the walls C

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Figure 2. Representative XRD patterns of powdered barium hexaferrite samples collected from the beakers in which the thin film deposition was performed. The patterns correspond to treatment solutions with different Fe/Ba ratios and are compared with the simulated pattern for bulk barium hexaferrite (PDF no. 7-0276).

Figure 3. Room temperature Raman spectrum of the powder obtained upon scratching the film. Peaks at the bottom of the figure are simulated peaks obtained upon deconvoluting the experimental Raman spectrum.

identified as α-Fe2O3 (hematite labeled by H in Figure 2) and BaO·Fe2O3 (labeled by M), respectively. Interestingly, for a molar ratio of the Fe3+/Ba2+ ions of n = 11, the XRD patterns of both the powder collected from the beaker walls and/or obtained by scratching the films (Figure 1) revealed the existence of a single phase, identified as BaFe12O19. Subsequent EDX analysis of the powders indicated that the Fe/ Ba molar ratio in the films is close to 12:1, which suggests that stoichiometric barium ferrite films can be obtained by the proposed methodology only when the treatment solution is rich in Ba2+ ions. This result corroborates well the assumption that (a) the two ions present different hydrolysis rates in solution and (b) films with a stoichiometric Fe/Ba composition form only when the much slower hydrolysis rate of the Ba2+ ions (pKa = 13.5) compared to that of the smaller Fe3+ cations (pKa = 2.2) is compensated by an excess amount of divalent ions in solution.41 We believe that the mechanism of deposition of hexagonal ferrites by the liquid phase deposition is similar to that observed for spinel-type ferrites; that is, once solutions become supersaturated the ionic products will largely exceed the values of the solubility products, thereby leading to the spontaneous precipitation of small nuclei of metal oxides/ hydroxides/oxyhydroxide, which are responsible for the observed turbidity of the solution. The subsequent growth of the nuclei results in the formation of grains which will minimize their high surface energy by attaching to any available solid surface with the formation of uniform films. Upon a heat treatment at 950 °C in air, these mixtures of hydroxide precursors are converted into the crystalline hexagonal ferrite.30 M-type barium ferrite crystallizes in the space group P63/ mmc with the Fe3+ ions are located in three octahedral sites, labeled by Fe(1), Fe(4), and Fe(5), one tetrahedral (Fe(3)), and one bipyramidal (Fe(2)), respectively.42 The 64 ions present in the unit cell lead to the existence of 189 optical modes (k = 0) of which 42 are Raman active (11A1g + 14E1g + 17E2g) and 30 IR active modes (13A2u + 17E1u), whereas the remaining 54 modes are silent (3A1u + 4A2g + 13B1g + 4B1u + 3B2g + 12B2u + 15E2u).43 The room temperature experimental Raman spectrum of the hexagonal barium ferrite film is presented in Figure 3. The spectrum features 11 well-defined peaks, located at 173, 201, 280, 318, 401, 455, 504, 514, 606, and 674 cm−1 ascribed to the different phonon modes in the crystal in good agreement

with previous experimental results and the theoretical predictions made by group theory.44 The peaks in the Raman spectrum of the barium ferrite film are well-defined and relatively sharp, thereby indicating a high degree of crystallinity of the ferrite, in excellent agreement with the X-ray diffraction data. No bands characteristic of Si(001) single-crystal substrate or impurity-related phonons associated with the presence of secondary phases, such as α-Fe2O3, γ-Fe2O3, and/or BaFe2O4, were observed experimentally, which indicates that the conversion of the amorphous metal oxide/oxyhydroxide intermediated into the Ba−M ferrite phase is complete during the postsynthesis annealing process and furthermore confirms the high quality of the barium ferrite films obtained by the liquid phase deposition method.45 The bands located at 173, 280, and 318 cm−1 correspond to the E1g vibrational modes, whereas the bands observed at 201 and 606 cm−1 have the E2g symmetry. According to Kreisel et al.46 the presence of optical phonons with the E1g symmetry in the spectrum of barium ferrite indicates the polycrystalline nature of the film and/or the presence of a randomly oriented texture, which confirms well our findings by X-ray diffraction. Finally, the Raman bands observed at 401, 455, and 514 cm−1 correspond to the A1g vibration, the former being associated with vibrational modes within the Fe(5)O6 octahedra, whereas the latter corresponds to the vibrations within the Fe(1)O6 and Fe(5)O6 octahedra, respectively. Finally, the Raman band observed at 674 cm−1 was ascribed to the vibrational modes within the bipyramidal Fe(2)O5 unit of the barium ferrite crystal.43 The magnetic hysteresis loops of the powder collected from a treatment solution corresponding to the Fe/Ba molar ratio n = 11 are shown in Figure 4. The powdered sample presents a robust magnetic response at room temperature with a saturation magnetization (Ms) and coercive field (Hc) of 63 emu/g and 402 kA/m, values which are a little lower than the predicted theoretical values (Ms = 72 emu/g; Hc = 533 kA/m), but much higher than those generally reported in the literature for bulk and microcrystalline barium ferrite.44,47,48 Interestingly, the coercivity of the powder obtained by liquid phase deposition is higher than that reported for barium ferrite powders obtained by the conventional ceramic approach (Hc = 174 kA/m), a milling process (Hc = 366 kA/m48), or coprecipitated in aqueous solutions and annealed at 860 °C (Hc = 392 kA/m) and 1000 °C (Hc = 318 kA/m), respectively. D

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a value which is comparable to that of the anisotropy field Hk = 2Ku/Ms = 17.3 kOe, where Ku and Ms are the anisotropy constant and the saturation magnetization and for BaFe12O19 single crystal have the values of 0.33 × 106 J/m3 and 380 kA/ m, respectively. The comparison of the values of the coercive field of the films and their effective anisotropy field reveals that the latter ones are about three times larger than the coercivity, a situation which can be associated with a curling mechanism involved in the magnetization reversal process.49 The thickness dependences of the in-plane and out-of-plane values of the coercivity (Hc) and squareness (SQ = Mr/Ms) of barium hexaferrite films are presented in Table 1. The Table 1. Magnetization Parameters of Barium Hexaferrite Thin Films with Various Thicknesses Figure 4. Magnetic hysteresis loop of the Ba-ferrite powder obtained from a treatment solution with the Fe/Ba molar ratio n = 11 annealed at 850 °C for 6 h in open atmosphere.

deposition time (h) thickness (nm) HcII (kA/m) Hc⊥ (kA/m) SQII SQ⊥

As seen in Figure 4, the sample cannot be saturated by magnetic fields as high as 1.8 T, which indicate the existence of a large magnetocrystalline anisotropy in powdered barium hexaferrite sample. We also investigated the room temperature magnetic properties of the barium ferrite films with different thicknesses by applying the magnetic field both parallel and perpendicular to the plane of the films. The M vs. H plots presented in Figure 5 clearly indicate that the magnetic properties of the films are strongly dependent on their thickness, which can be controlled by adjusting the deposition time. Films saturate at fields close to 17.1 kOe (or 1361 kA/m),

4.5 100 148.2 134.9 0.50 0.50

5.5 183 193.3 189.1 0.57 0.55

7.5 450 225.9 198.9 0.58 0.46

9 524 282.6 215.7 0.61 0.41

coercivity generally increases with the thickness of the films. Specifically, the in-plane and out-of plane values of the coercivity for the 100 and 183 nm thick films are comparable, with values around 1.84 kOe (144.3 kA/m) indicating a random distribution of the magnetization, whereas for the 450 and 524 nm thick films an increase of the out-of plane value of the coercivity was noted, which is indicative of the presence of a higher magnetic anisotropy. This change in coercivity is more

Figure 5. Room temperature field-dependent magnetization curves the Ba-ferrite films with different thicknesses: 100 (a), 183 (b), 450 (c), and 524 nm (d), respectively. The insets represent close-ups of the magnetic hysteresis loops which evidence the slight differences between the coercivity of the films measured with the magnetic field parallel and perpendicular to the film plane. E

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Figure 6. Top view FESEM images of barium hexaferrite thin films deposited at different period of time: (a) 4.5, (b) 5.5, (c) 7.5, and (d) 9 h (inset is higher magnification image).

substantial when the thickness of the films varied from 450 to 524 nm, which can be attributed to the existence of an increasing fraction of constituent particles aligned parallel to the c-axis of the crystal. It can also be noted that the coercivities of the films are significantly smaller than those of the powdered material, whereas for a given sample the coercivity is consistently higher for the in-plane direction than those measured with the magnetic field perpendicular to the film, thereby suggesting the existence of shape anisotropy in the barium ferrite films. The Mr/Ms values are close to 0.5, which indicate that the constituent grains of the barium ferrite films possess a single magnetic domain structure. As others pointed out, the critical size for the existence of a single domain magnetic structure in spheroidal grains is given by the formula dcritical = (72(AKu)1/2)/(μoM2s ).50,51 For barium ferrite single crystal, Ku = 0.33 × 106 J/m3 (uniaxial anisotropy constant), Ms = 380 kA/m (saturation magnetization), and A = 1.7 × 10−11 /m (exchange energy), which lead to a value of dc around 1 μm. However, because of the acicular shape of the grains constructing the barium ferrite films, this value is expected to be below the critical grain size predicted theoretically for the single crystal, which corroborates well the experimental results from electron microscopy and magnetic measurements. Moreover, unlike the in-plane value of the squareness, which increases from 0.50 to 0.61, the out-of-plane squareness decreased with the thickness of the films, which suggests that the hard axis of magnetization of the films lies in the plane of the film. These results confirm that the magnetic properties of the barium films obtained by liquid phase deposition are strongly dependent on their thickness and can be controlled by

varying the reaction parameters, such as the deposition time. The magnetic properties of the mirrorlike films and their variation with the thickness of the film and the orientation of the magnetic field prompted us to investigate their microstructure by field effect scanning electron microscopy (FESEM). As seen in Figure 6, the barium ferrite films deposited during 9 h and annealed at 950 °C present a granular morphology being constructed by acicular particles with an average length of 380 nm and a width of 45 nm. The formation of the rod-shaped particulate films is presumably ascribed to the intrinsic structural anisotropy of the barium ferrite crystal: barium hexaferrite has a crystallographic cell containing six barium hexaferrite formulas with a large c/a of 3.9431 (PDF 007-0276) and the uniaxial crystal anisotropy constant K = 330 kJ/m3. The inset image in Figure 6d clearly shows the morphology of rodshaped particles parallel to the plane of the film. The grains generally have a random orientation, which is in good agreement with the small difference between the in-plane and out-of plane values of the coercivity measured experimentally. It is also interesting to note that the randomness of the grains increases with the thickness of the films; that is, the rod-shaped particles of the 524 nm thick film are more disordered than in the case of the 450 nm thick film. These experimental observations are also in excellent agreement with the magnetic properties of the films since it has been found that thick films present a higher fraction of particles oriented in the plane of the film. It can be also noted that the films present cracks, presumably due to internal stresses developed during the F

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Figure 7. (a) Topography of the 524 nm thick barium hexaferrite thin film. (b) and (c) MFM images of the film subjected to the action of a magnetic field H = 2000 Oe (b) and H = −2000 Oe (c), respectively. The scale bar corresponds to 2 μm.

and, therefore, the magnetization of an only a small fraction of the ferrite grains will be reversed by the external magnetic field. To conclude, barium hexaferrite thin film structures were synthesized for the first time by the liquid phase deposition (LPD) method using FeOOH and Ba(NO3)2 as metal precursors. The films are highly crystalline, and their composition is extremely sensitive to the concentration of the cations in the treatment solution. Hexaferrite barium films possess a granular morphology with the magnetization lying in the film plane, and their magnetic properties can be easily modulated by changing their thickness, which, in turn, can be controlled by adjusting the deposition time. With their high values of coercivity, these barium ferrite films can have potential applications in the design of permanent magnets, data storage, power and frequency converters, and the microwave technology.

natural cooling process as a result of the difference between the thermal coefficients of the silicon substrate and the film. To better understand the relationship between the microstructure and the magnetic properties of the films, we studied the dynamics of the magnetic domains by using magnetic force microscopy (MFM). To this end, an in-plane magnetic field, varying from −2000 Oe to +2000 Oe, was applied parallel to the plane of the film using a variable field module (VFM) from Asylum Research. MFM images were collected when the low coercivity tip scanned the sample in the nap mode with the delta-height of 25 nm. As seen in the AFM image (Figure 7a), the 524 nm thick barium ferrite films present a uniform surface being constructed by grains with a size comparable to that observed from FE-SEM experiments. Some islands with a higher height (bright areas) have been also observed, confirming that films grow by a Volmer−Weber-type mechanism. The islands on the film were used as the reference to compare the same area when the magnetic field was switched. The MFM images (Figures 7b and 7c) show areas with a different contrast (purple and orange) which indicate the presence of both strip and cluster-type magnetic domains with different orientations of the magnetization; that is, upward (purple), downward (yellow), as well as intermediate directions between these two extreme orientations. Interestingly, the AFM and MFM images are completely different, which suggest that the magnetic domain configuration of the barium ferrite films does not match with the topography. Upon reversing the inplane magnetic field, it can be easily seen that grains whose magnetization lies in the plane of the film will now have the magnetization pointing to an opposite direction (circled areas in Figures 7b and 7c), whereas those with an out-of plane orientation of the magnetization did not change their magnetic state. This observation agrees well with the experimental observation relative to the orientation of the magnetization parallel to the plane of the film. It is important to note that the experiment was limited by the maximum magnetic field attainable with the variable field module (VFM), which is relatively small compared to the coercive field of barium ferrite,

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AUTHOR INFORMATION

Corresponding Author

*Phone: (989)-774-3863. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by Central Michigan University start-up funds and the National Science Foundation (NSF) CAREER Award No. 1157300.

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REFERENCES

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