Magnetic Properties of CoFe2O4 Thin Films

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Magnetic Properties of CoFeO Thin Films Synthesized by Radical Enhanced Atomic Layer Deposition Calvin D. Pham, Jeffrey Chang, Mark A. Zurbuchen, and Jane P. Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08097 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

Magnetic Properties of CoFe2O4 Thin Films Synthesized by Radical Enhanced Atomic Layer Deposition Calvin D. Pham(a)‡, Jeffrey Chang(a)‡, Mark A. Zurbuchen(b), (c), and Jane P. Chang(a)* (a)

Department of Chemical and Biomolecular Engineering, University of California, Los Angeles,

California, 90095 (b)

Department of Electrical Engineering, University of California, Los Angeles, California, 90095

(c)

Department of Materials Science and Engineering, University of California, Los Angeles,

California, 90095 KEYWORDS: atomic layer deposition, ALD, RE-ALD, plasma, thin films, magnetic oxides, cobalt ferrite, CoFe2O4, epitaxial ABSTRACT: A radical enhanced atomic layer deposition (RE-ALD) process was developed for growing ferrimagnetic CoFe2O4 thin films. By utilizing bis(2,2,6,6-tetramethyl-3,5-heptanedionato) cobalt (II) (Co(TMHD)2), Fe(TMHD)3, and atomic oxygen as the metal and oxidation sources respectively, amorphous and stoichiometric CoFe2O4 films were deposited onto SrTiO3 (001) substrates at 200 °C. The RE-ALD growth rate obtained for CoFe2O4 is ~2.4 Å/supercycle, significantly higher than the values reported for thermally activated ALD processes. Microstructural characterization by Xray diffraction (XRD) and transmission electron microscopy (TEM) indicate that the CoFe2O4 films

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annealed between 450 °C and 750 °C were textured polycrystalline with an epitaxial interfacial layer, which allows strain-mediated tuning of the magnetic properties given its highly magnetostrictive nature. The magnetic behavior was studied as a function of film thickness and annealing temperature — saturation magnetization (Ms) ranged from 260 to 550 emu/cm3 and magnetic coercivity (Hc) ranged from 0.2 to 2.2 kOe. Enhanced magnetic anisotropy achieved in the thinner samples while the overall magnetic strength improved after annealing at higher temperatures. The RE-ALD CoFe2O4 films exhibit magnetic properties that are comparable to both bulk crystal and films grown by other deposition methods — with thickness as low as ~7 nm — demonstrating the potential of RE-ALD for the synthesis of high-quality magnetic oxides with large scale processing compatibility.

INTRODUCTION Magnetic materials are of great importance in enabling applications such as microelectronics, antennas, spintronics, and magnetic memory storage. For instance, recent advancements in magnetictunnel junctions (MTJ), magnetic topological insulators, and magnetoelectric multiferroic composites highlight how suitable magnetic materials can lead to enhancements in tunneling magnetoresistance (TMR)1,2, ferromagnetic/antiferromagnetic exchange coupling3 and magnetoelectric (ME) coupling4 effects. Due to their magnetic strength and magnetic softness, magnetic metals such as CoFe5, FeGaB6, and CoFeB2 are often the materials of choice in the aforementioned devices and applications by interfacing with other functional oxides such as MgO1, PMN-PT7 and BiFeO34, respectively. However, the stability of such metal/oxide interfaces can impede long-term reliability and processing constraints due to oxidation of the metals at the interface, which in turn degrades overall functionality1. Alternatively, magnetic oxides could provide a better interfacial stability while retaining the applicable magnetic behaviors. CoFe2O4 is a ferrimagnetic ternary oxide with an inverse MgAl2O4 (spinel)-type structure with magnetic Curie temperature (Tc) ~870 K, saturation magnetization (Ms) ~400 emu/cm3, magnetic ACS Paragon Plus Environment

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coercive field (Hc) ~3-5 kOe, and first-order magnetocrystalline anisotropy constant (K1) ~10 × 106 erg/cm3.8,9 Because of its robust magnetic properties and high magnetic anisotropy10, there is a wealth of scientific reports of studies of CoFe2O4 in bulk, thin film, and nano-scale forms.11-14 In addition, the high magnetostrictive behavior in CoFe2O4 (λs < −120 ppm15,16) makes it suitable for applications such as actuators, sensors, and strain-coupled multiferroic composites17,18. CoFe2O4 has been synthesized in powder19, polycrystalline16, and single-crystal20 forms, as well as thin films using pulsed laser deposition8, magnetron sputtering21, sol-gel deposition14, and chemical vapor deposition22. But, none of the aforementioned growth methods are ideal for uniformity and conformality control at the nm-scale. Therefore, atomic layer deposition (ALD) is being pursued to offer highly-quality and conformal layer growth at low temperatures. ALD is an industrially scalable thin film deposition technique that consists of alternating self-limiting surface half-reactions between a cation precursor and an anion precursor.23,24 It is this self-limiting nature of surface reactions that allows the precursor molecules to uniformly saturate the surface with a single monolayer and conformally coat itself on to the surface even when the features are not planar. As a result, ALD can achieve high-quality and conformal thin film deposition with precise thickness control, making it advantageous compared to other growth methods. ALD of oxides is achieved by using oxidation agents after the organometallic precursors react with the surface. Common oxidation agents include O225, H2O26, O327, and oxygen plasmas28, each offering different processing windows and corresponding control over film properties. Here we report the ALD of complex oxides with metalorganic precursors that contain respective metal cations introduced into the process in an alternating manner to achieve an overall complex oxide growth 3



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with the target stoichiometry. There have been only a few demonstrated examples of growth via ALD for CoFe2O429-31, and no reports of plasma or radical assisted ALD thus far. Therefore, the presented work was focused on the first reported successful synthesis of CoFe2O4 films via a radical enhanced ALD (RE-ALD) approach and the characterization of their structural and magnetic properties.

EXPERIMENTAL METHOD A detailed overview of the experimental apparatus used in this work for the deposition of metal oxide thin films using β-diketonate precursors and atomic oxygen has been described previously, so only a brief description is presented here32-34. The multi-beam system used in this study consists of a 10” outer diameter stainless steel main chamber with a load-lock assembly for clean sample insertion and removal without exposing the entire system to atmosphere. Affixed to the main chamber ports are several components: a coaxial microwave cavity radical beam source used to introduce highly reactive atomic oxygen to the sample, a six-precursor doser array used to introduce evaporated organometallic precursor fluxes to the sample, a temperature-controlled sample stage, and a hot-filament ion gauge to monitor the chamber pressure. Pressure in the growth chamber was maintained between 1×10-6 Torr at base and 2×10-5 Torr during operation by a CTI 4000 L/s cryogenic pump. The beams of the atom source and the precursor dosers converged onto a heated sample stage upon which the substrate was mounted. Atomic oxygen was produced from the atom source using a 2.46 GHz Sairem microwave power supply at 25 W and ~0.6 sccm O2 gas.35 The metal β-diketonate precursors used were Tris(2,2,6,6-tetramethyl-3,5heptanedionato) iron (III), [Fe(TMHD)3] (99.9%-Fe, Strem Chemicals, Inc.) and Co(TMHD)2 (99.9%4


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Co, Alfa Aesar, a Johnson Matthey Company). These precursors are solid at room temperature and were sublimed at 130 °C and 120 °C, respectively. Depositions were carried out from 190-230 °C. Silicon (001) substrates were used during process optimization, followed by SrTiO3 (001) (MTI Corp.) for process-optimized depositions. For films deposited on SrTiO3, specimens were rapid thermal annealed (RTA) in order to facilitate crystallization inside an AccuThermo AW 610 RTP furnace using an oxygen flow of 5 sccm and a temperature of 450-750 °C. Film thicknesses were estimated with a J.A. Woollam M-88 spectroscopic ellipsometer with data modeled using a Lorenz oscillator fit that was calibrated by cross-section SEM imaging of selected samples after the synthesis process. The films’ composition and chemical bonding states were analyzed using an Axis Ultra DLD (Kratos Analytical Ltd.) X-ray photoelectron spectroscopy (XPS) instrument with a monochromatized Al-Kα source (1486.6 eV). Since the XPS measurements were conducted exsitu, the ambient carbon contamination at film surfaces (~15%) were quantified but excluded from the film composition calculations.34 The XPS spectra for Fe 2p and Co 2p both featured shake-up satellite peaks, verifying their oxidation states and therefore were not labeled in the presented XPS results. The crystal phase was determined, and the texture was characterized using an X'Pert Pro (PANalytical B.V.) Powder X-ray Diffractometer (XRD) using Cu-Kα radiation. Cross-sectioned lamella for TEM analysis were prepared by the standard trench-and-pluck focused ion beam (FIB) approach, with successive finishing at 30, 10, and 5 kV using an FEI Nova 600 FIB. An FEI Titan transmission electron microscope (TEM) was used for cross-section microstructural imaging, as well as selected-area electron diffraction (SAED) at 300 keV. Atomic force microscope (AFM) and magnetic force microscope (MFM) images were acquired using magnetic MESP probes by a Bruker Dimension® Icon® Atomic 5



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Force Microscope with ScanAsystTM. Magnetic properties were investigated using a MPMS (Quantum Design Inc.) Superconducting Quantum Interference Device (SQUID) magnetometer, between ±3 T.

RESULTS AND DISCUSSION For the ALD sequences, a single ALD cycle consisted of four repeating steps: a 90 s metalorganic precursor dose, 5 s pump-down to prevent gas-phase reactions, 20 s exposure of atomic oxygen, and finally a 5 s pump-down, denoted by M(TMHD)x:O. The sequence was repeated until the desired film thickness was reached. To synthesize stoichiometric CoFe2O4, the composition of the films was controlled by sequentially pulsing the precursors in a supercycle that includes a numbers of cycles Fe(TMHD)3:O and b numbers of cycles of Co(TMHD)2:O.34 To develop the protocol for the ternary oxides, it was first necessary to study the growth of the binary oxides Fe2O3 and CoO. Synthesis of Binary Oxides To confirm the initial synthesis of the constituent oxides CoO and Fe2O3, XPS was used. Figure 1 shows the results for the films deposited at 200 °C on Si (001), grown using 100 ALD cycles. XPS spectra were calibrated at 284.8 eV for C 1s. As expected, the high-resolution XPS spectra for both Co 2p and Fe 2p displayed the spin orbital doublets 2p3/2 and 2p1/2. For CoO, seen in Figure 1 (a), peak positions for Co 2p3/2 and 2p1/2 were approximately 779.4 eV and 794.4 eV, respectively. Co2+ and Co3+ oxidation states could not be deconvoluted satisfactorily due to inadequate energy resolution – however when removing adventitious oxygen species, the atomic ratio Co/O was determined to be ~48:52, indicating that the film was fully oxidized and comprised mostly of CoO. For Fe2O3, seen in Figure 1 (b), XPS peak positions for Fe 2p3/2 and 2p1/2 were approximately 710.7 eV and 724.0 eV, respectively. 6


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In addition, the detailed spectra also featured shake-up satellite peaks at 718.9 eV and 732.8 eV, as typically expected for the Fe3+ oxidation state36. After confirming the initial deposition of the binary oxide films, it was then possible to optimize the processing parameters. Film thicknesses were characterized using a combination of SEM cross-section micrographs and ellipsometry measurements. Deposition rates for CoO and Fe2O3 were characterized as a function of the deposition temperature (190-230 °C), as shown in Figure 1 (c).

(a)

(b) Co 2p

(c) 1.2

Fe 2p

2p3/2

2p3/2

Intensity (a.u.)

2p1/2

Deposition Rate (Å/cycle)

2p1/2

Intensity (a.u.)

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CoO

1.0

0.8

0.6

Fe2O3 0.4

810 800 790 780

Binding Energy (eV)

740

730

720

710

190

200

Binding Energy (eV)

210

220

230

Temperature (°C)

Figure 1. High resolution XPS spectra for (a) Co 2p from the CoO film and (b) Fe 2p peak from the Fe2O3 film grown by RE-ALD on Si (001) substrates. (c) Temperature dependence of ALD deposition rate for CoO (squares) and Fe2O3 (circles)) films. All films were grown on Si (001) substrates at 200 °C. The CoO deposition rate increased from 0.5 to 0.8 Å/cycle from 190-220 °C, before sharply increasing to 1.1 Å/cycle at 230 °C. The compositions of the CoO films as a function of the deposition temperatures were determined by XPS analysis, showing a consistent Co:O ratio of ~48:52 from 190220 °C. Although an increase in growth rate with temperature is generally attributed to surface precursor decomposition and CVD-like growth, XPS analysis indicates that the carbon content did not 7



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substantially increase until the temperature reached 230 °C. While Co(TMHD)2 has a decomposition temperature of 250 °C under ambient conditions and ~310 °C under vacuum37, this increase in carbon content is possibly due to the partial decomposition of Co(TMHD)2 and is expected to get higher with increasing temperature. When comparing to the work using Co(TMHD)2 and O3 to grow Co3O4 films, the resultant growth rate was about 0.2 Å/cycle on Si (001) substrates at 200 °C, and it also increased gradually with increasing temperature within a similar temperature range.38 The decomposition temperature for uncontrolled CVD growth was determined to be about 310 °C, likely due to precursor decomposition and a different reaction pathway with O3. In this work, the use of atomic oxygen enabled the synthesis of CoO instead of Co3O4, thereby allowing the formation of high-quality CoFe2O4. Moreover, compared to the use of O338, the use of atomic oxygen yields a ~250% higher growth rate at 200 °C, as well as a much lower ALD processing temperature (190-220 °C). The effects observed above with atomic oxygen were likely facilitated by a different reaction mechanism at the surface, such as enhanced ligand removal. For Fe2O3, the deposition rate was relatively consistent, increasing slightly from 0.4 to 0.5 Å/cycle within the range of temperatures studied. Similarly, XPS was used to characterize the compositions of the Fe2O3 films, showing a constant Fe:O ratio ~43:57 with no obvious trend corresponding to the growth rate. However, it was observed that for a substrate temperature higher than 250 °C, precursor decomposition on the surface occurred, leading to CVD-like film growth. A similar phenomenon was also observed with the Fe(TMHD)3 and O3 co-reactant chemistry, yielding a growth window of 160-310 °C27. The discrepancies in the maximum allowable growth temperature are believed to be due to the fact that atomic oxygen (250 °C) are more reactive than O3 (310 °C).28 In terms of the growth rates, the 8


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previously reported growth rate within the temperature window on thermal ALD process was around 0.1 Å/cycle, which is substantially lower than the RE-ALD process in this work.27 Since the growth of the constituent binary oxides were both relatively stable at 200 °C, the RE-ALD synthesis of CoFe2O4 films was conducted at this temperature. Due to ALD’s self-limiting nature, it was necessary to determine the effect of precursor pulse time upon the growth rate, as it coincides with ALD’s ability for conformal deposition over non-planar (3-D or not in line-of-sight) surfaces. The growth rates of CoO and Fe2O3 as a function of the precursor pulse time are shown in Figure 2 (a). The data indicate that a 90 s precursor pulse time for both Co(TMHD)2 and Fe(TMHD)3 was sufficient to achieve saturation, and was kept constant as a process parameter for the remainder of the experiments. To demonstrate a linear growth rate, the thicknesses of the CoO and Fe2O3, films were measured as a function of number of cycles, as shown in Figure 2 (b). According to the fitted curves, the growth rate of CoO and Fe2O3 were ~0.7 Å/cycle and ~0.5 Å/cycle, respectively, with no apparent nucleation delays for precursor absorption. When comparing the growth rates to literature values using the same TMHD-based precursors at similar temperature ranges with ozone in a thermal ALD process, the growth rates were 0.2 Å/cycle for Co3O438 and 0.1 Å/cycle for Fe2O327. We note that, the RE-ALD process yields substantially higher growth rates.

9



(a) 0.7 0.6

(b) 12 10

CoO

0.5

Thickness (nm)

Growth Rate (Å/cycle)

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Fe2O3

0.4 0.3 0.2

CoO 0.7 Å/cycle

8

Fe2O3

6

0.5 Å/cycle 4 2

0.1

Deposition Temperature: 200 °C

Deposition Temperature: 200 °C 0.0 0

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20

40

60

Pulse Time (Seconds)

80

0

0

50

100

150

200

Cycle Number

Figure 2. (a) Effect of ALD precursor pulse time on growth rate for CoO (squares) and Fe2O3 (circles) films grown on Si (001). (b) CoO/Si (001) and Fe2O3/Si (001) film thicknesses plotted as a function of the number of ALD cycles, showing a linear growth profile. Growth rates were obtained by the slope of linear regression-fit lines. Synthesis and Structure of CoFe2O4 Films Ternary CoFe2O4 films were deposited by pulsing the precursors sequentially in a a[Fe(TMHD)3:O] + b[Co(TMHD)3:O] manner while controlling the values of a and b to achieve the desired Fe/Co = 2 stoichiometry, known to have the maximum magnetostriction coefficient as well as magnetic properties.20 If the CoFe2O4 films were non-stoichiometric, depending on the level of deviation, the overall magnetic strength as well as coercivity would decrease due to the antiferromagnetic nature of the CoO and Fe2O3 impurities.31 The cation composition of the resultant CoFe2O4 by the co-deposition method on Si (001) substrates was measured by XPS. Figure 3 (a) shows the ternary film composition as a function of the precursor pulsing ratio, demonstrating the straightforward ability to control stoichiometry by ALD. Film stoichiometry of Fe/Co = 2 was achieved at a:b = 5:1 ALD cycling ratio. According to the XPS characterization of the CoFe2O4 films fabricated, the averaged stoichiometric number for Co and Fe is ~1.03 and ~1.97 respectively, with a standard deviation σ of 1.3 %, which is at 10


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the resolution of the XPS analysis and thus indicates highly reproducible and well-controlled composition with RE-ALD.Detailed spectra for Co 2p and Fe 2p are shown in Figure 3 (b) and (c), respectively, confirming consistency with our calibration for binary oxides.

(a)100

(b)

Fe 2p

2p3/2

80 60

20 0 0

2p3/2 2p1/2

CoFe2O4 2.4Å/supercycle 100 200 300 400 500

Supercycle number

40

2p1/2

Intensity (a.u.)

40

Intensity (a.u.)

60

Thickness (nm)

80

(c) Co 2p

100

Fe Content Fe/(Fe+Co) (%)

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a [Fe(TMHD)3:O]

20

b [Co(TMHD)2:O]

Deposition Temperature: 200 °C 0

0

20

40

60

80

a/(a+b) Cycle Percentage (%)

100

810 800 790 780

Binding Energy (eV)

740

730

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710

Binding Energy (eV)

Figure 3. (a) Iron cation percentage Fe/(Fe+Co) (%) in CoFe2O4 films as a function of the ALD dosing ratio between the two metalorganic precursors for the growth on Si (001) substrates. The inset in (a) shows the stoichiometric CoFe2O4 thicknesses on SrTiO3 (001) as a function of the supercycle numbers. The CoFe2O4 growth rate was obtained by the slope of a linear regression-fit line. Detailed XPS spectra for (b) Co 2p and (c) Fe 2p. With the process parameters optimized, stoichiometric CoFe2O4 films were deposited onto single crystal SrTiO3 (001) substrates and the XPS analysis also showed a Fe/Co = 2 stoichiometry. The growth curve of the stoichiometric CoFe2O4 films is shown in the inset of Figure 3 (a), showing a linear growth profile with ~2.4 Å/supercycle. Each supercycle is consisted of 5 a cycles and 1 b cycle. The growth rate achieved is much higher than the rates for thermally activated ALD processes using O3 as co-reactants (~0.4-0.5 Å/cycle).30,31 To improve film crystallinity, the as-deposited samples were annealed by RTA immediately after growth for 1 minute under an oxygen environment over a range of temperatures (450-750 °C). Although 11



CoFe2O4 (cubic inverse spinel, space group: Fd3m, a = 8.396 Å, PDF: 022-1086) has a large (~7%) lattice mismatch with a doubled unit cell of SrTiO3 (cubic, space group: Pm-3m, a = 3.9 Å, PDF: 0860178), epitaxial growth of CoFe2O4 has been reported using SrTiO3 (001) substrates, on both buffered8 and un-buffered substrates13,21, and the growth was reported as epitaxial, with a cube-on-cube orientation. Shown in Figure 4 (a), the XRD patterns for 50 nm thick films indicate the CoFe2O4 films crystallized with a preferred crystal orientation of [001] surface-normal, as judged by the intense 004 CoFe2O4 peak dominating among the other CoFe2O4 peaks. At higher annealing temperatures, the peaks are sharper and more intense, indicating an improved crystallinity at higher annealing temperatures, as well as an increased crystal grain size from ~23 nm to ~30 nm. (As determined by the Scherrer equation.) Although the CoFe2O4 film annealed under 750 °C shows better crystallinity, drastic increase in surface roughness was observed using AFM analysis (not shown). Therefore, the following study focuses on the annealing condition of 650 °C, which yielded a smoother surface, with the aim of integrating CoFe2O4 into multiferroic composites and next-generation electronic devices. RTA 650 °C

~90 nm

Log Intensity (a.u.)

STO 002

Cu Kβ

CFO 004

(b)

50 nm films STO 001

(a)

Log Intensity (a.u.)

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RTA 750 °C RTA 650 °C RTA 550 °C RTA 450 °C

~50 nm

~20 nm

CFO A.D. STO Sub.

10

20

~7 nm

30

40

50

42

2θ (°)

43

2θ ( °)

44

Figure 4. X-ray diffraction patterns for (a) CoFe2O4 films (~50 nm) grown on SrTiO3 (001), as prepared and annealed from 450 °C to 750 °C. The CoFe2O4 004 peak 12


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dominates in intensity for annealed films. (b) Short-range scans showing CoFe2O4 004 reflections for samples of different film thickness. Peak shifts (dotted lines) indicate altered CoFe2O4 strain states. (PDF: CoFe2O4 022-1086, SrTiO3 086-0178). For a more detailed view of the local microstructure, high-resolution TEM was used to characterize the CoFe2O4 film annealed at 650 °C. The CoFe2O4 film is of a polycrystalline nature at the top, with an epitaxial CoFe2O4 001 layer (~ 5-10 nm) near the substrate, shown in Figure 5 (a) and (b). It is believed that this effect is due to the relatively large lattice mismatch between the substrate and film. By comparing the selected area electron beam diffraction (SAED) patterns at the CoFe2O4/SrTiO3 interface (Figure 5 (c)) and bulk (Figure 5 (d)), the epitaxial to polycrystalline transition was verified by the change from the well-ordered diffraction pattern at the interface to the polycrystalline diffraction rings away from the interface. Unlike the fully relaxed CoFe2O4 films obtained by thermal ALD30, the XRD spectra of the CoFe2O4 films deposited via RE-ALD at different thicknesses, Figure 4 (b), indicate strained CoFe2O4 films. The CoFe2O4 004 peak position shifts to a lower value with decreasing thickness, which correlates to an increase in the lattice parameter along a surface-normal axis. This confirms the growth of strained CoFe2O4 films and the strain relaxation process along with increasing thickness. Since a shift is observed in the CFO 004 reflection of the 20-nm film (larger than the epitaxial layer thickness), it is believed that the interfacial strain also influences a portion of the polycrystalline region, given that the 50-nm film is fully relaxed. This is possibly due to the grain boundary formation in the polycrystalline region, allowing the strain to further relax at larger thicknesses, which are 50 nm and 90 nm in our case. We hypothesized that the observed difference is possibly due to the reactive atomic oxygen, which enables denser interfacial bonding with a more effective surface ligand removal.

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Nonetheless, because CoFe2O4 is magnetostrictive, one can attain desired CoFe2O4 magnetic behaviors by selecting suitable strain/thickness conditions. s s ss s ss s ss ss s s

Figure 5. (a) Cross-sectional TEM image of CoFe2O4 film crystallized after 650 °C RTA on SrTiO3 (001) substrate. The substrate is at the bottom in all cross-sectional TEM images. (b) High magnification TEM micrograph of the CoFe2O4 film, showing oriented polycrystalline growth away from the interface. Arrows indicate the interface. (c) SAED pattern collected from near the film-substrate interface. Note the sharp film peaks and their alignment with the substrate peaks. (d) SAED pattern collected in the bulk, indicating transition to a polycrystalline film.

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Magnetic Properties After crystallizing the RE-ALD CoFe2O4 films by RTA at 650 °C, their magnetic properties were characterized using a SQUID magnetometer to assess the effect of film thickness and annealing temperature, and to determine the correlation between structure and material properties. Shown in Figure 6 and summarized in Figure 7, four film thicknesses were studied in detail; room temperature magnetic hysteresis loops were obtained after the subtraction of linear diamagnetic and paramagnetic contributions from the substrate and the SQUID sample holder. It was observed that as film thickness decreases from 90 to 7 nm, the in-plane saturation magnetization (Ms) increases while the out-of-plane Ms decreases in general. On the other hand, in-plane coercivity (Hc) slightly increases from 1.4 kOe to 1.6 kOe while the out-of-plane Hc decreases drastically from 1.8 kOe to 0.3 kOe. Because the difference between the in-plane and out-of-plane Hc values correlates to the degree of magnetic anisotropy, it is concluded that a thinner film results in a stronger magnetic anisotropy for these RE-ALD CoFe2O4 films. (a) 600

Magnetization (emu/cm3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b)

In-plane Out-of-plane

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-200

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~90 nm

-600 -20 -10

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~50 nm

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15




(c)

(d) 600

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0

0

-200

-200

-400

-400

~20 nm

-600 -20 -10

0

10

20


~7 nm

-600 -20 -10

0

10

20


Figure 6. Room-temperature magnetic hysteresis loops for CoFe2O4 films annealed at 650 °C with thicknesses of (a) ~90 nm, (b) ~50 nm, (c) ~20 nm, and (d) ~7 nm. 800

1500

600

Ms (emu/cm3)

2000

Hc (Oe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1000

400

500

200 Soild symbols: In-plane Hollow symbols: Out-of-plane

0

100

80

60

40

20

0

0

Thickness (nm) Figure 7. Coercivity (black squares) and saturation magnetization (red circles) vs. film thickness for out-of-plane (open symbols) and in-plane orientation (solid symbols) of the applied magnetic field.

Using Ms ~400 emu/cm3 for bulk CoFe2O4 as a reference, in-plane Ms for the 7 and 20-nm thick films are enhanced, at 558 and 475 emu/cm3, respectively. On the other hand, the in-plane Ms for the 54 and 90-nm thick films were lower, at 368 and 355 emu/cm3, respectively. This contrast between samples of 16


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different thicknesses can be explained by a competition of magnetic contributions from the strain due to substrate-film lattice-mismatch versus the top portion of the film that developed away from the interface21. The surface morphology of epitaxial CoFe2O4 films grown on SrTiO3 substrates by PLD was reported previously to consist of microscopic pyramidal features that contributed to the surface roughness. When compared to the AFM of the CoFe2O4 film grown by RE-ALD in this study (Figure 8 (a)), there was a similar of surface morphology. In addition, it has been reported that the grain boundaries in CoFe2O4 thin films has a negative effect on the Ms values due to the decrease in magnetocrystalline anisotropy.39 As a result, 7 and 20-nm thick CoFe2O4 films exhibited improved Ms values due to the reduced polycrystalline character compared to the 50 and 90-nm films. In terms of the change in Hc, the thinner films have a drastically smaller Hc in the out-of-plane direction compared to the in-plane direction, whereas the thicker films exhibit comparable Hc values. This indicates that both the magnetic anisotropy and softness were affected both by the film-substrate strain state as well as magnetic shape anisotropy, all of which favor in-plane anisotropy at lower thicknesses. In other words, the energy required to flip the magnetic spins along the out-of-plane direction is lower for thinner films. MFM measurements on the 90-nm and 7-nm films were conducted to further investigate this effect, in Figure 8 (b) and (c), both showing grain-like magnetic domain structures. It is noteworthy that the 7-nm film exhibits larger out-of-plane domains, which correlates to a lower domain boundary energy and thus explains the low out-of-plane Hc value. The magnetic response of the thicker films was relatively more isotropic, which is considered to be due to the lower influence of film-substrate strain. 2.2

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Figure 8 (a) AFM surface topography of a 90 nm ALD CoFe2O4 film after 650 °C annealing in oxygen for 1 minute, showing pyramid-like surface topography. Inset shows an isometric 3D map of the same data. The vertical scale is 35 nm. (b) MFM phase image of the same CoFe2O4 film, showing grain-like magnetic domain structures. (c) MFM phase image of the 7-nm CoFe2O4 film, showing larger magnetic domains relative to the other film Shown in Figure 9 and summarized in Figure 10 is the effect of annealing temperatures ((a) 450, (b) 550, (c) 650, and (d) 750 °C) upon the room temperature magnetic hysteresis loops for a set of films with a 20 nm thickness. Different annealing temperatures yield different Ms and Hc values. Both inplane and out-of-plane Ms values are lower at 550 °C, but exhibited an increasing trend for higher RTA temperatures, reaching 544 emu/cm3 and 427 emu/cm3, respectively. Meanwhile, the in-plane Hc values increase to 1.6 kOe for the 650 ºC film, but decrease to 1.1 kOe for the 750 ºC film, while the out-ofplane values show an increasing trend with RTA temperature, reaching 2.2 kOe for the 750 ºC film.

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(a) Magnetization (emu/cm3)

600

(b)

In-plane Out-of-plane

600

400

400

200

200

0

0

-200

-200

-400

-400

-600

Annealed at 450°C -20 -10

0

10

-600

20


(c)

0

10

20


(d)

600

600

400

400

200

200

0

0

-200

-200

-400

-400



-600


0

10

-600

20



0

10

20


Figure 9. Room-temperature magnetic hysteresis loops for 20-nm thick CoFe2O4 films annealed at (a) 450 °C, (b) 550 °C, (c) 650 °C, and (d) 750 °C. 4000 600

Soild symbols: In-plane Hollow symbols: Out-of-plane

400

2000 200 1000

0 400

500

600

700

Annealing Temperature (°C) 19


0 800

Ms (emu/cm3)

3000

Hc (Oe)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Figure 10. Coercivity (black squares) and saturation magnetization (red circles) vs. annealing temperature for out-of-plane (open symbols) and in-plane orientation (solid symbols) of applied magnetic field. When correlating the magnetic properties with annealing temperature, the Ms and Hc for the CoFe2O4 films generally increase with increasing annealing temperature. The RE-ALD process used here consist of alternating layers of CoO and Fe2O3, which are individually antiferromagnetic and weakly ferromagnetic, respectively. Crystallization of the films to form domains of CoFe2O4 likely caused the spin-orbit coupling interactions between Co2+ and Fe3+ ions that we observed. Observation of the CoFe2O4 004 XRD peak increases in relative intensity and decreases in FWHM in 2θ with increasing annealing temperature, which indicates growth of the crystal grains. The increase in Ms as a function of processing temperature has also been reported elsewhere for growth of CoFe2O4 thin films.40 An anomalously high saturation magnetization value was found for the 450 °C film. A possible explanation could be that the residual strain from the substrate lattice mismatch relaxes as the polycrystalline domains form with increasing RTA temperature. Due to the large magnetostriction coefficient of CoFe2O4, induced lattice strain from the underlying substrate has a substantial effect on the magnetic properties of the epitaxial thin films, when compared to bulk powder or crystals.8,9,21,41,42 Although the RE-ALD CoFe2O4 films are not fully epitaxial, as indicated by TEM, the XRD indicates that the films were textured-polycrystalline, and the microstructural data correlates with the magnetic properties of the epitaxial films in a similar manner. In general, it is expected that magnetic anisotropy has contributions from several factors: strain, crystal structure, and shape. It was previously reported, using epitaxial CoFe2O4 films grown on MgO (100) substrates, that increasing the film thickness or the growth temperature caused the magnetic domains to 20


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relax and thus the influence of lattice strain diminished.12 In this work, the magnetic properties were found to drastically change as a function of the thickness and annealing temperature. For instance, a thinner CoFe2O4 film results in an enhanced magnetic anisotropy; that is, an increased difference between the in-plane and out-of-plane Hc values. A higher annealing temperature leads to an increase in both Ms and Hc. Moreover, we note that for the RE-ALD films that the Ms was anisotropic, despite the fact an isotropic Ms for magnetic materials is normally expected between different axes for saturated hysteresis curves. The hysteresis curves for the films were clearly saturated, indicated by the overlapping of the forward and reverse curves at high fields. One possibility for the observed phenomenon is the epitaxial strain arising from the interfacial lattice mismatch between the RE-ALD CoFe2O4 film and the SrTiO3 substrate.9,41,42 Suitable substrate selections with a minimal degree of lattice mismatch is expected to eliminate the formation of the polycrystalline top layer, achieving ALDgrown epitaxial CoFe2O4 thin films. Table 1 shows that the RE-ALD grown CoFe2O4 films have comparable magnetic properties relative to other reported literature values, from bulk and thin films, which are synthesized by more wellestablished techniques. RE-ALD is a synthesis technique that can produce films with desirable magnetic properties, as we demonstrate here with CoFe2O4 thin films with thicknesses as low as 7 nm, illuminating a path for the large-scale synthesis of high-quality and ultra-thin CoFe2O4 thin films for important technological applications.

Table 1. Comparison of properties of CoFe2O4 synthesized by different methods. (Hc: magnetic coercivity, Ms: saturation magnetization, t: film thickness) Hc (kOe)

Bulk

MBE

PLD

Sputter

This work

~0.3

0.5-12

0.5-5.2

~0.5-3.4

~0.2-2.2

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Ms (emu/cm3) t (nm)

~400 -

140-500 ~80-120

~420-490 ~200-500

~50-523 ~100-1000

~290-550 ~7-90

Crystal quality

-

epitaxial

epitaxial

polycrystalline or epitaxial

textured polycrystalline

Reference

-

39,41,42

8,9,40,43

21,44

-

CONCLUSIONS In conclusion, stoichiometric CoFe2O4 thin films synthesized by RE-ALD were demonstrated and showed great promise for technological applications. Via XRD and TEM analysis, the microstructure was determined to be textured polycrystalline transitioning from an initial epitaxial layer. The roomtemperature magnetic properties, as studied as a function of annealing temperature and film thickness, yielded several significant results. The Ms ranges from 260 to 550 emu/cm3 and the Hc ranges from 0.2 to 2.2 kOe. At lower thicknesses, the induced lattice strain from the substrate is more prominent and results in an increased magnetic anisotropy. For higher annealing temperatures, the CoFe2O4 crystallites grew, and both the Ms and Hc values increased. By comparing the magnetic behaviors with the films produced by other techniques, we have demonstrated RE-ALD to be a viable technique for the synthesis of high-quality, ultra-thin CoFe2O4 films with superior growth rates and large-area processing capability.

AUTHOR INFORMATION Corresponding Author *Electronic Mail: [email protected]

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Author Contribution ‡C. D. Pham and J. Chang contributed equally to this work

ACKNOWLEDGEMENTS The authors would like to acknowledge the UCLA Molecular Instrumentation Center and the SPM facility at the Nano and Pico Characterization Lab at the California NanoSystems Institute for the use of characterization instruments. This work was supported in part by the FAME Center, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.

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Magnetic Properties of CoFe2O4 Thin Films

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