Magnetic properties of superparamagnetic, nanocrystalline cobalt ferrite thin films deposited at low temperature

Accepted Manuscript Magnetic Properties of Superparamagnetic, Nanocrystalline Cobalt Ferrite Thin Films Deposited at Low Temperature Neelima Sangeneni, KM Taddei, Navakanta Bhat, SA Shivashankar PII: DOI: Reference:

S0304-8853(18)31066-7 https://doi.org/10.1016/j.jmmm.2018.06.038 MAGMA 64058

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Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

10 April 2018 11 June 2018 13 June 2018

Please cite this article as: N. Sangeneni, K. Taddei, N. Bhat, S. Shivashankar, Magnetic Properties of Superparamagnetic, Nanocrystalline Cobalt Ferrite Thin Films Deposited at Low Temperature, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.06.038

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1

Magnetic Properties of Superparamagnetic, Nanocrystalline Cobalt Ferrite Thin Films Deposited at Low Temperature Neelima Sangeneni1, K M Taddei2, Navakanta Bhat1 and S A Shivashankar1 1 2

Centre for Nano Science and Engineering, Indian Institute of Science, Bengaluru-560012, India Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN-37831

Bulk cobalt ferrite, being a hard ferrite, shows high magnetization, high resistivity and high coercivity. If thin films of cobalt ferrite can be deposited at a low enough temperature and if its coercivity can be reduced, cobalt ferrite will make a very good candidate for use as a magnetic core of an integrated inductor in RF-CMOS ICs. Though polycrystalline and epitaxial thin films of cobalt ferrite have been made by various techniques, there are no reports of thin films of superparamagnetic cobalt ferrite. In this work, nanocrystalline cobalt ferrite thin films, which are superparamagnetic as deposited, have been prepared in the soluti on medium at ~190oC, using microwave irradiation. The as-prepared films have a saturation magnetization (MS) of 401 emu/cc and coercivity (H C) of 19 Oe at room temperature for a crystallite size of 2 nm. The cobalt ferrite powder obtained as a by-product during the same process has MS of 50 emu/g and HC of 5 Oe at room temperature, making it superparamagnetic. The as-prepared films were annealed in air at 300oC for 5 min and 10 min. Annealing for 10 min results in an increase in crystallite size to 36 nm, M S increases from 401 emu/cc to 545 emu/cc, and H C increases from 19 Oe to 860 Oe. The change in magnetic properties can be directly associated with change in the crystallite size and degree of crystallographic inversion, as determined by neutron diffraction and deduced from X-ray photoelectron spectroscopy. Index Terms— Cobalt ferrite, Ferrite thin Films, Magnetic materials, Superparamagnetism.

I. INTRODUCTION Magnetic thin films with high saturation magnetization (MS) and low coercivity (HC), as well as high resistivity, have become important for GHz RF-CMOS applications. Such films can be used as the magnetic core of integrated inductors in RF-CMOS ICs to enhance the inductor density and to render them capable of operating in the GHz regime [1]. Spinel ferrites satisfy the requirements of high magnetization and high resistivity, provided the deposition of thin films of such ferrites can be made compatible with today’s CMOS back-end of the line processing, i.e., at a temperature not higher than about 400oC. Progress has recently been reported in this direction by demonstrating the performance of solutionprocessed thin films of nanocrystalline zinc ferrite [2], [3]. Being a normal spinel, bulk zinc ferrite is not ordinarily ferrimagnetic. But, if the crystallite size is made sufficiently small, zinc ferrite exhibits ferrimagnetic behavior [4]. An inverse spinel ferrite like cobalt ferrite, which has a large magnetisation, might be expected to be a better candidate than nanocrystalline, ferrimagnetic zinc ferrite. Bulk cobalt ferrite has high permeability, high saturation magnetization (MS), high electrical resistivity, high coercivity (HC), and high magnetocrystalline anisotropy [5]. It is an inverse spinel ferrite, with inversion parameter ranging from 0.75 to 0.89 [6]. Concas et al. [6] have reported that the inversion parameter of cobalt ferrite depends on how it is synthesized, leading to different magnetic properties. Different factors, including site preference of cations based on crystallite size [7], the method of preparation, temperature of synthesis [8], and post-synthesis thermal treatment [9] influence how Co2+ and Fe3+ ions arrange themselves in the octahedral and tetrahedral sites of the spinel structure.

Although bulk cobalt ferrite has high MS, it also has a high HC, which would lead to hysteresis losses in high-frequency applications. Due to cubic anisotropy [10], cobalt ferrite has a high remnant magnetization (Mr), leading to a high Mr/MS value of 0.83. Reduction in crystallite size reduces the volume (V) which, in turn, reduces the anisotropy energy (KAV, where KA is the anisotropy energy constant), which favours superparamagnetism [11]. Although there is a slight reduction in saturation magnetization above the blocking temperature, superparamagnetism (SPM) results in coercivity reducing to zero [12]. Different investigators have reported different particle sizes ranging between 5 nm and 22 nm for the onset of superparamagnetism [13]-[17]. Superparamagnetic cobalt ferrite powder has been synthesized using alkalide reduction [13], the sol-gel method [14], thermal decomposition [18], hydrothermal method [19], and solid-state co-precipitation reaction [20]. Komarneni et al. [21] showed how ferrites of fine crystallite size could be obtained rapidly through microwave-hydrothermal synthesis, though they were unable to obtain phase-pure cobalt ferrite powder. Caillot et al. [22] have also reported the synthesis of cobalt ferrite powder using the microwave-hydrothermal process; the final product had some impurities, but a high MS and a high coercivity (1000 Oe). Thin films of cobalt ferrite have been deposited using several deposition techniques. Avazpour et al. [23] used the sol-gel technique to spin-coat such films, followed by annealing at up to 650oC, obtaining MS ranging from 176-237 emu/cc and a high coercivity (1.4 – 1.8 T). Pulsed laser deposition (PLD) [24], [25] has also been used to deposit cobalt ferrite thin films. Ragunathan et al. [24] found, in such films, MS≈275 emu/cc and HC≈1800 Oe, when the deposition temperature was 600oC. Tanaka et.al [26] used the evaporation technique in O2 plasma to obtain cobalt ferrite films at 500oC, followed

2 by annealing at 700oC, obtaining Ms≈300 emu/cc and coercivity of ≈2500 Oe. Using an electrochemical method, Lokhande et al. [27] obtained cobalt ferrite films with MS and HC being 298 emu/cc and 1.02 T, respectively. The films prepared by Lie et al. [28] using atomic layer deposition (ALD) required annealing at 600oC to 800oC to become polycrystalline. Barbosa et al. [29] used electrophoresis to deposit cobalt ferrite thin films, which were annealed at temperatures ranging from 400 to 600oC, yielding high coercivity, between 1 – 3 T. Wang et al. [30] used RF sputtering as the deposition method; the resulting films were annealed at 500 – 1200oC, yielding MS ranging from 10 to 350 emu/cc, and a very high coercivity (9 T). Using molecular beam epitaxy (MBE), Ramos [31] obtained cobalt ferrite films at room temperature, with Ms of 315 emu/cc and HC of ~2200 Oe. All the above deposition methods, except MBE, [31] require temperatures (> 400oC) incompatible with CMOS processing, or yield films with high HC at room temperature. Furthermore, there is no report in the literature on thin films of superparamagnetic cobalt ferrite. The present effort was aimed at the deposition of polycrystalline, nanostructured cobalt ferrite thin films (CFTF) at low temperatures, in the solution medium, using microwave irradiation [32]. The goal has been to achieve high MS and low coercivity, making the films a candidate material for the lowloss inductor core that is desirable in RF-CMOS circuits operating in the GHz regime. We report the deposition of uniform, adherent, CFTF at a low temperature (~190oC) on silicon substrate. The as-deposited films are found to be superparamagnetic at room temperature, due to their nanostructure, small particle size, and the altered degree of crystallographic inversion (relative to the bulk material). Annealing in air at moderate temperatures restores bulk-like magnetic behavior in the films. II. EXPERIMENT A. Method Metalorganic precursors [18], i.e., cobalt (II) acetylacetonate (Co(acac)2) and iron (III) acetylacetonate (Fe(acac)3) [Sigma-Aldrich], were used as received. To synthesize the ferrite, 0.5 mmol of cobalt (II) acetylacetonate and 1 mmol of Fe (III) acetylacetonate were dissolved in a solvent mixture of AR-grade ethanol (13.5 ml) and 1-decanol (22.5 ml). The resulting solution was stirred until it became clear and transparent. The solution was transferred to an 80ml, sealable glass vessel, which forms a hydrothermal reactor when under microwave irradiation. A p-type Si(100) wafer piece to be used as substrate was first RCA-cleaned and dried using a N2 gun. This substrate was immersed in the solution, which was then irradiated in a single-mode microwave reactor (Discover, CEM Corp, USA, 2.45 GHz, 300 W) for 20 min. The temperature reached was ~190oC, the pressure rising to 200 psi. The Si substrate was then removed from the solution, rinsed, sonicated in ethanol, and dried. A deposit on the substrate was visible. The solution was then centrifuged for 15 min at 7500 rpm. The powder that precipitated (labelled CFP) was washed with ethanol, and dried overnight.

The as-deposited films (labelled CFTF) were annealed subsequently in air at 300oC to obtain two samples: (CFTF_300_5) annealed for 5 min, and (CFTF_300_10) for 10 min. The ramp-up rate for the annealing was 200oC/min and the ramp-down rate 150oC/min. B. Characterization The crystallinity and composition of the samples were examined using X-ray powder diffraction (XRD, Rigaku SmartLab, Cu-Kα radiation). All the diffraction patterns were obtained in the glancing angle-mode (GIXRD) by varying 2θ from 10o to 70o in a continuous scan, with the scan step of 0.02o. The incident angle (ω) was kept at 0.5o to avoid contribution from the substrate. Film morphology and crosssection were examined by field-emission scanning electron microscopy (FESEM, Zeiss Ultra 55). In addition to XRD and SEM, particle size was confirmed using transmission electron microscopy (TEM, FEI Titan, operating voltage = 300 kV). The crystalline phase and site occupancy of the cobalt ferrite powder were determined using neutron powder diffractometry (HB-2A, Oak Ridge National Laboratory, USA). Powder diffraction data collected on this instrument are ideally suited for the Rietveld method. The diffraction data were obtained at one of the principal wavelengths (115 – 1.54 Å) and analyzed using the GSAS2 programme, employing the Rietveld refinement technique. Raman spectroscopy was employed, seeking qualitative evidence for crystallographic inversion (LabRAM-HR (UV), λ=532 nm). The chemical composition and core-level binding energies were determined using X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, AlKα radiation at 1486.3 eV, 117 W). Survey spectra were collected using a pass energy of 160 eV, and high-resolution spectra of the elements (Co-2p, Fe-2p, C-1s, O-1s and Si-2p) were collected using a pass energy of 20 eV, with step size of 0.1 eV. Charging effects were corrected by setting the C-1s peak at 285.0 eV and shifting the entire spectrum accordingly. Gaussian peak fitting was used. Magnetic measurements were conducted using a SQUID magnetometer (Quantum Design MPMS) in fields up to 7 T. The M-H measurements were made in steps of 11 Oe at 30 K and 300 K. Zero field-cooled (ZFC) and field-cooled (FC) measurements were carried out from 5 K to 400 K, at fields of 50 Oe, 1000 Oe, and 5000 Oe. III. RESULTS AND DISCUSSION Microwave-assisted synthesis resulted in the simultaneous formation of both a thin film on Si(100) and powder material. Both the film and powder were analyzed as detailed above and found to display the same material properties. While the data on the films are shown hereafter, the data for the powder are shown in the ESI, except that the powder neutron diffraction data are shown below. The XRD patterns of the as-prepared and annealed thin films are shown in fig 1. The patterns are consistent with those of cubic cobalt ferrite (ICSD file no. 94871). As expected, the peaks are broader in the as-prepared film than in the annealed films. From the Scherrer formula applied to the (311) peak, the average crystallite is deduced to be 8 nm in the as-prepared film, and 40 nm in the film annealed for 10 min at 300oC.

3 average crystallite size ranging from 8 nm in the sample CFTF to 36 nm in CFTF_300_10. The cross-sectional SEM image (fig 3) shows that the thickness of the as-deposited cobalt ferrite thin film on silicon substrate is ~820 nm. The image confirms that the film is reasonably uniform in thickness and that it adheres well to the substrate. In order to further confirm the crystallite size, TEM was employed. Figure 4(a) and 4(b) show the high-resolution TEM bright field and dark field images of the as-prepared cobalt ferrite thin film.

Fig 1: XRD patterns of as-prepared (CFTF) and annealed (CFTF_300_5, CFTF_300_10) samples SEM micrographs of fig 2 show the morphology of the as-prepared and annealed films. The images confirm that they are comprised of nanocrystalline material, the

(a)

Grain size is calculated from the zoomed-in dark field image. We get the mean grain size as 2.4 nm and standard deviation as 0.87 nm (figure 4(c)). Since TEM provides a more accurate measurement of crystallite size than other techniques, we consider the average crystallite size in the as-prepared thin film to be 2 nm. Figure 4(d) shows the SAED pattern of the film, with diffused continuous rings, confirming the nano-polycrystalline nature of the material deposited. The rings are indexed based on the d-spacing obtained from ICSD 94871. It is observed that both in SAED and XRD the highest intensity is seen for (311) plane.

(b)

(c)

Fig 2: SEM of (a) CFTF as-prepared, average crystallite size of 8 nm (b) CFTF_300_5 with average crystallite size ~24 nm and (c) CFTF_300_10 with average crystallite size ~36 nm.

Figure 3: Cross-sectional SEM of the as-prepared film

4

(a)

(c )

(b)

(d)

Fig 4: TEM images of the as-prepared cobalt ferrite thin film (a) bright field image (b) dark field image (c) grain size distribution obtained from the zoomed in dark field image (d) SAED image showing nano-polycrystallinity. A. Magnetic measurements on the as-prepared film The M-H data on CFTF is shown in fig 5. The saturation magnetization is ~550 emu/cc and ~400 emu/cc, respectively at 30 K and 300 K, the corresponding coercivities being 7565 Oe and 19 Oe. It is to be noted that, at 300 K, which is well below the Curie temperature of (bulk) cobalt ferrite, the coercivity is smaller by more than two orders of magnitude, whereas saturation magnetization falls by only 27%, relative to the corresponding values at 30 K.

Fig 5: M-H curve at 30 K and 300 K of CFTF

5 The Mr/M s ratio – the squareness of the hysteresis loop is calculated to be 0.016 at 300 K, which is much lower than 0.83, the value for bulk cobalt ferrite [9] (M r is the retentivity or remanence). This signifies that, although the anisotropy constant (K A) is higher in smaller crystallites [33], the anisotropy energy (K AV) is less than the thermal energy (kBT) at 300 K due to the crystallite volume (V) going down drastically in nanometric material. This implies that the film is superparamagnetic (SPM) at room temperature. The very low measured value of H C confirms the same. The saturation magnetization of the film is shown in emu/cc. The volume of the film was calculated by multiplying the area of the substrate with the film thickness on both sides of the substrate (obtained by SEM). In both the samples, i.e., CFTF (Fig 5) and CFP (ESI, Fig.S2), the M S is found to be lower than in bulk cobalt ferrite. This reduction in M S has been previously observed in small particles (1-10 nm) [18] and is related to the well-known changes in the degree of inversion in the spinel structure due to nanostructuring [6], [35], [36]. An additional factor influencing the observed reduction in M S could be the existence of spin-canting at the crystallite surface, originating from competing interactions between A and B sub-lattices when symmetry breaking and oxygen vacancies occur at the surface. That the magnetisation does not saturate fully even at high fields is evidence of spin-canting [18]. The M-T curves obtained under zero-field-cooled (ZFC) and field-cooled (FC) conditions, at different fields, are shown in fig 6. The blocking temperature (T B) is observed to be ~160 K. The higher the applied field, the smaller the thermal energy needed to overcome the barrier between

(a)

(b)

the two easy-axis orientations. Thus, the blocking temperature decreases with increasing applied field [34]. There are no significant differences in the magnetization between FC and ZFC conditions, above the blocking temperature, with the field increasing. This is consistent with SPM behaviour [34]. It is to be observed that the ZFC curve is narrow, implying that crystallite size distribution is not very broad. Torres et al. [17] have shown that, in a sample with narrow crystallite size distribution, as evidenced by TEM, the ZFC curve is narrow. This is well supported by the TEM analysis of the as-prepared thin film sample, wherein the deviation from the mean grain size is 0.87 nm (figure 4(c)). A. Effect of annealing on magnetic characteristics Figures 7(a) and 7(b) show the change in the magnetic characteristics of the CFTF when subjected to annealing: there is a rise in both the saturation magnetization and coercivity. Upon annealing for 10 min, M S increases to 545 emu/cc and coercivity to 860 Oe. As expected, annealing leads to an increase in grain size (fig 2) and hence to a reduction in the total area of grain boundaries, resulting in an increase in M S, which becomes comparable to that of bulk cobalt ferrite [5].The observed increase in coercivity is also related to the increase in grain size. As the grain size increases beyond 10 nm, there is a gradual departure from superparamagnetic behaviour, towards ferrimagnetic order. These results suggest that the properties of CFTF could be “tuned” to suit different applications.

(c)

Fig 6: FC & ZFC curves of CFTF at (a) 50 Oe (b) 1000 Oe and (c) 5000 Oe

6

(a)

(b)

Fig 7(a): M-H at 300 K for CFTF_300_5, the film annealed at 300oC for 5 min, (b) M-H at 300 K for CFTF_300_10, the film annealed at 300oC for 10 min Table 1 summarizes the magnetic characteristics of as-prepared and annealed cobalt ferrite films Sample

ParticleSize (nm)

MS (emu/cc)

HC (Oe)

CFTF

~2

401

19

CFTF_300_5

24

423

600

CFTF_300_10

36

545

860

B. Degree of crystallographic inversion in the films Bulk cobalt ferrite has the inverse spinel structure, with inversion parameter ranging from 0.75 to 0.89 [6]. Concas et al. [6] have reported that the inversion parameter of cobalt ferrite depends on how it is synthesized, leading to different magnetic properties. While Raman (ESI Fig S5) and XPS analysis were performed on the CFTF to obtain a qualitative measure of the degree of crystallographic inversion, neutron diffraction was performed on the CFP to obtain a more quantitative measure of the degree of crystallographic inversion in the powder. Rietveld refinement [38] of neutron diffraction data was employed to calculate the degree of inversion in CFP. During refinement, firstly the global parameters, such as background, scale factors, diffractometer zero correction, were refined [25]. Later the structural parameters such as lattice parameters, profile shape, width parameters, preferred orientation, asymmetry, isothermal parameters, atomic coordinates, and site occupancies were refined in sequence [39]. Parameters such as the ‘goodness of fit’ χ2 and the R factors were analyzed and the fitting was done till these parameters reached their minimum value. The site occupancies of the cations in the two interstitial sites

(tetrahedral and octahedral sites) are constrained so as to preserve the stoichiometric composition of the materials. Fig 8 shows the Rietveld refinement of the data. The Rw obtained is 3.682 % and χ2 is 2.49. The lattice parameters are seen to be 8.38904 Å. The degree of inversion is calculated as 0.38 and hence the chemical formula turns out to be (Co0.54 Fe0.46)[Co0.38Fe1.62]O4 .

Fig 8: Neutron diffraction data after Rietveld refinement Since the facility for thin film neutron diffraction was not available, the neutron diffraction measurements had to be

7 done on the powder sample. However as both the powder and thin film have been processed using the same protocol, we expect the structure of the cobalt ferrite in the two samples to be close enough. To analyze the cobalt ferrite thin film further, the change in the degree of crystallographic inversion due to nanostructuring in CFTF was also estimated qualitatively using high-resolution XPS of cobalt and iron (fig 9). Zhongpo et al. [35] and Wang et al. [36] specify the binding energy (BE) of Co 2+ and Fe3+ ions in both the tetrahedral and octahedral sites. These BE values were deduced for the present samples using the spectra in fig 9, by subtracting a Shirley background from the Co2p and Fe2p spectra. The peak positions obtained for Co2p 3/2, Co2p1/2, Fe2p3/2, and Fe2p1/2 are consistent with those reported by Zhongpo et al. [35] and Wang et al. [36]. On analyzing the Co2p3/2 spectrum further, it is found to be comprised of peaks at 779.5 eV and 780.9 eV, which correspond to Co2+ ions in octahedral and tetrahedral sites, respectively. Based on the area under each of these peaks, it is deduced that 64% of the Co2+ ions occupy tetrahedral sites, with 36% of them in octahedral sites. XPS shows Fe2p3/2 peaks at 710.3 eV and 712.6 eV, which correspond to Fe 3+ ions in octahedral and tetrahedral sites, respectively. Based on the area under each peak, it is deduced that 26% of the Fe 3+ ions occupy tetrahedral sites, with 73% of them in octahedral sites.

xFe x)[Co xFe 2-x]O4,

wherein (Co1-xFex) and [CoxFe 2-x] denote cations in the tetrahedral (A) and octahedral (B) sites, respectively. In the above formula, x denotes the degree of inversion. Based on the XPS analysis, we get the formula for the as-prepared thin film as (Co0.64Fe0.52)[Co0.36Fe1.46]O4 . Thus the degree of inversion in the as-prepared thin film is 0.36 (Table 2). It is to be noted that, although the residual std is close to 1, the chemical formula does not add up to CoFe 2 O4 ; this is because XPS provides only a qualitative estimate of the degree of inversion. Yet, the degree of inversion obtained for a film from XPS is consistent with neutron diffraction data of the powder sample, the latter being a more accurate and quantitative method of determining the degree of crystallographic inversion. The degree of inversion in CFTF (as estimated by XPS data) is observed to be smaller than in bulk cobalt ferrite, an inverted spinel in which the degree of inversion ranges from 0.75 to 0.89 [6]. Ranajit et al. [4] argue that microwave-assisted synthesis, with its fast reaction and growth rates, can cause the site occupancy of metal ions (degree of inversion) to be different from that in nanostructured spinel oxides prepared by other methods.

Cation distribution in the cobalt ferrite structure can be described through the chemical formula (Co1-

Fig 9: XPS spectra of CFTF showing a degree of inversion of 0.36(residual std being close to 1) Table 2 Degree of inversion in as-prepared CFTF

Atomic% in Tetrahedral (A) sites

Atomic% in Octahedral (B) sites

Fe3+

26.09

73.91

Co2+

64.27

35.73

8 In cobalt ferrite, the degree of inversion is found to have dependence on the sintering temperature [40] - the lower the sintering temperature, the lower the degree of inversion. Chandramohan et al. [37] also observe that the sample which is prepared at low temperature and has crystallite size of 6 nm (DME) shows a low degree of crystallographic inversion. The sample CFTF was prepared at ~190oC, which is quite low. Hence, when the sample is annealed, the degree of inversion increases, and so does magnetization. C. Degree of inversion in annealed samples Concas et al. [6] observed how a change in the inversion parameter brings about a change in saturation magnetization, because of the associated change in magnetic interactions. To examine how the degree of inversion is altered by annealing the cobalt ferrite films, XPS data were used (as noted above) and the results compared with the as-prepared sample. XPS analysis of the annealed samples shows a change in the degree of inversion (ESI, fig S3 & ESI, figure S4). The occupancy of Co2+ in octahedral sites changes from ~36% to ~71% and then to ~78%, as one moves from the as-prepared sample to the samples annealed at 300oC for 5 min and 10 min. It is also to be noted that the satellite peak at ~786 eV, which is some 4-6 eV higher in binding energy than the Co2p3/2 signal, is seen to intensify more as

the duration of annealing is increased. This peak is characteristic of the majority of high-spin Co2+ cations occupying the octahedral sites in the spinel lattice of cobalt ferrite. This supports the claim that the degree of inversion increases when the as-prepared film is annealed. The degree of inversion in CFTF_300_5 and CFTF_300_10 is found to nudge close to that of bulk cobalt ferrite. This is reflected in the measured higher value of M S in these samples (fig 7). It is to be noted that, although HC is higher in the annealed films, it is still considerably lower than that of bulk cobalt ferrite [4]. Conversely, in the as-prepared film, the degree of inversion is low, meaning that a smaller fraction of Fe 3+ ions occupy both the A and B sites than in bulk cobalt ferrite, thereby reducing magnetization density. It is also to be noted that as the sample is annealed, a small percentage of Fe2+ is seen in the samples [41]. While the 5 min anneal shows the proportion of Fe 2+ to be 5 %, the 10 min anneal increases it to 13 %. This could contribute to the change in M s as well [42]. Table 3 summarizes the results of the thin films obtained under different conditions, comparing them with bulk (i.e., ceramic) cobalt ferrite [6].

Table 3: Summary of characterization of the thin films with different annealing protocols

Sample

Annealing protocol

Particle Size (nm)

MS (emu/cc)

HC (Oe)

Inversion parameter

CFTF CFTF-300-5 CFTF-300-10

As-prepared At 300 oC, 5 min At 300 oC, 10 min

2 24 36

401 423 545

19 600 860

0.36 0.71 0.78

IV. CONCLUSION Nanocrystalline cobalt ferrite films of ~820 nm thickness have been deposited in the solution medium at ~190oC through microwave-assisted reactions. The as-prepared films comprise of very small crystallites (~2 nm) and are superparamagnetic at room temperature. This is corroborated by the measured very low Mr/M S of 0.016 and blocking temperature of ~160 K. Neutron diffraction and XPS analyses show that the degree of crystallographic inversion changes from 0.36 in the as-deposited material to ~0.78 – close to the bulk value - when annealed in air at 300oC for 10 min, due to grain growth. The change in MS and HC that results from different annealing protocols implies that magnetic characteristics could be “tuned” to suit applications. The superparamagnetic cobalt ferrite films with considerable M S, deposited at low temperature, can be a candidate material as magnetic core for inductors in GHz RF-CMOS circuits. ACKNOWLEDGMENT The authors thank Varadharaja Perumal, Manasa Jain, B.N. Suma, M.L. Pradeep Kumar, and A.S. Ashwini Kumari of the Micro Nano Characterization Facility, CeNSE, IISc, where all the characterization was carried out. The authors also thank Satyam Suwas, Anuj Bisht, Rajasekar and K.L. Ganapathi for their useful inputs regarding TEM, Rietveld analysis and XPS results. Funding by the Ministry of Electronics and Information Technology (MeitY), Government of India, under the Centre for Excellence in Nanoelectronics project, is gratefully acknowledged. A portion of this research used resources at the High Flux Isotope, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory. REFERENCES [1] C. Yang, F. Liu, T. Ren, L. Liu, H. Feng, A.Z. Wang and H. Long, “Fully integrated ferrite-based inductors for RF ICs”, Sens. Actuators A.2006, 130–131, 365–370. [2] Ranajit Sai, K.J. Vinoy, Navakanta Bhat, and S.A. Shivashankar, “CMOS-compatible and scalable deposition of nanocrystalline zinc ferrite thin film to improve inductance density of integrated RF Inductor”, IEEE Transactions on Magnetics. July 2013, Vol.49 No.7. [3] Ranajit Sai, Suresh D Kulkarni, Masahiro Yamaguchi, Navakanta Bhat, and S.A. Shivashankar, “An integrated X-band inductor with Nano ferrite-film core”, IEEE Magnetics Letters. 2017, Volume PP, Issue 99, DOI: 10.1109/LMAG.2017.2655495. [4] Ranajit Sai, Suresh D Kulkarni, K.J. Vinoy, Navakanta Bhat and S.A. Shivashankar, ZnFe2O4: rapid and sub-100 oC synthesis and annealtuned magnetic properties, J. Materials Chemistry. 2012. [5] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, Wiley, 2nd edition, Pg. 993, 1976. [6] G. Concas, G. Spano, C. Cannas, A. Musinu, D. Peddis and G. Piccaluga, Inversion degree and saturation magnetization of different nanocrystalline cobalt ferrites, J. Magnetism and Magnetic Materials. 2009, 321 1893 – 1897. [7] K. Maaz, Arif Mumtaz, S.K. Hasanain, Abdullah Ceylan, Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route, J .Magnetism and Magnetic Materials. 2007, 308 289-295. [8] Sheena Xavier, Smitha Thankachan, Binu P Jacob, E.M. Mohammed, Effect of sintering temperature on the structural and magnetic

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Supplementary Data: Cobalt ferrite powder (CFP) was obtained during the same process as CFTF. After the Si substrate was removed from the solution, the solution was centrifuged for 15 mins at 7500 rpm. The powder that precipitated was washed with ethanol and acetone and dried overnight (CFP). Since both CFTF and CFP are processed simultaneously, we expect to see the same material characteristic for both. Fig S1 shows the XRD patterns of the as-prepared cobalt ferrite powder (CFP). The pattern is consistent with that of cubic cobalt ferrite (ICSD file no. 94871).

Fig S2 shows the M-H curve of the CFP. It is observed that the M S and HC at 300 K are 50 emu/g and 5 Oe, respectively.

Fig S2: M-H curve of CFP at 30 K and 300 K

Fig S1: XRD of cobalt ferrite powder (CFP)

Figures S3 and S4 show the XPS spectra of Co2p and Fe2p for the annealed samples – CFTF_300_5 and CFTF_300_10 respectively. The accompanying tables show the proportions of Co2+ and Fe3+ present in the octahedral and tetrahedral sites.

11

Tetrahedral (A)

Octahedral(B)

Fe3+

70.24 %

24.48 %

Co2+

28.33 %

71.67 %

Figure S3: XPS data for the sample (CFTF_300_5) annealed at 300 oC for 5 mins. This deconvolution was done using casaxps.

Tetrahedral(A)

Octahedral(B)

Fe3+

63.37 %

22.75 %

Co2+

21.73 %

78.27 %

Figure S4: XPS data for the sample (CFTF_300_10) annealed at 300oC for 10 mins. This deconvolution was done using CasaXPS.

Chandramohan et al. [37] prepared cobalt ferrite powders in different particle sizes (6 nm to 470 nm) and compared their Raman spectra. The cobalt ferrite particles were prepared using 2 different methods: Solid state method with controlled cooling (SS(RT)) resulting in particle size

of 470 nm and double microemulsion method resulting in a particle size of 6 nm. They observed a marked difference in the relative intensity (I v) of the phonon modes at 624 cm-1 and 695 cm-1. They report that, for cobalt ferrite particles of a single domain (DME - 6 nm crystallites,

12 with significant change in inversion w.r.t. bulk cobalt ferrite), the Iv is 1.1; however, for particle sizes closer to bulk (SS(RT) - 470 nm), the Iv value is greater than 2.34. The Raman spectrum of the as-prepared film, CFTF, is shown in fig S5, together with those of [37]. The relative intensity, I v, of CFTF is found to be 1.2, close to the value of the sample in [37] wherein the crystallite size is 6 nm (DME). Thus, Raman data are further evidence that the asdeposited film comprises of nanostructured cobalt ferrite with an altered degree of inversion (tending more towards a normal spinel) vis-à-vis bulk cobalt ferrite.

Fig S5: Raman spectra of (a) SS(RT) (470 nm) (b) Cobalt ferrite thin film (CFTF) (c) DME (6 nm) from Chandramohan et al. [37]

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