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Effect of substrate temperature on antiphase boundaries and spin polarization of thin Fe3O4 film on Si (100)
Thin Solid Films 693 (2020) 137698
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Effect of substrate temperature on antiphase boundaries and spin polarization of thin Fe3O4 film on Si (100)
T
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Li Suna, , Dongmei Bana, Er Liub, Xiaoyan Lia, Hongyan Penga, Zhongyu Yaoa, Zhaocong Huangc, Ya Zhaic, Hongru Zhaid a
College of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c College of Physics, Southeast University, Nanjing 210096, China d National Laboratory of Solid Microstructures, Nanjing University, Nanjing 210093, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetite Verwey transition Antiphase boundaries Magnetoresistance Spin polarization
Polycrystalline Fe3O4 films with the thickness of 27 nm were grown at different temperature (Ts) on Si (100) substrate by Laser-Molecular Beam Epitaxy to study the influence of density of antiphase boundaries (APBs) and the spin polarization of Fe3O4. Films prepared with Ts in the range of 500 °C - 650 °C were proved to be of good crystallization, preferred orientation, and desirable values of saturation magnetization (a little smaller one for film prepared at Ts = 500 °C). The lowest density of APBs was verified by the measured bulk like electronphonon (s-p) coupling constant for film prepared at Ts = 550 °C, along with the highest value of spin polarization and the best crystalline quality as the most obvious inflection in the curves of lnρ vs. 1000/T around Verwey transition. This optimum value of Ts can be interpreted by the APBs thermally activated migration process with an activation energy. Films prepared with Ts = 500 °C, 600 °C, and 650 °C still have a reasonable value of the s-p coupling constant compared to recently reported results, while larger magnetoresistance accompanied by the superparamagnetic behavior was observed for films prepared at lower Ts (Ts = 400 °C and 450 °C).
1. Introduction As an ancient and fascinating material, the intriguing structural, electrical and magnetic properties of Magnetite (Fe3O4) [1, 2], in particular, the 100% theoretical spin polarization and high Curie temperature (~860 K) make it a ideal candidate for spintronics application such as spin valves and spin tunnel junctions [3-7] recently pursued [813]. However, the antiphase boundaries (APBs) which can influence the spin polarization and degrade electrical and magnetic properties of Fe3O4 films [14, 15] due to the antiferromagnetic coupling between adjacent structure domains [17, 18], are easily formed in Fe3O4 during film growth owing to the uncontrollable formation of growth defects like stacking faults. Various preparation parameters, such as oxygen partial pressure [19], subtract temperature (Ts) [18, 20-22], and nature of the substrate [16, 23, 24], as well as other factors, like post processing [25], even new approach of in-situ electric field assisted method [26] were investigated to reduce the APBs. Thus, engineering the APBs is one of the essential topics for preparing Fe3O4 film with good quantity.
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It has been reported that the density of APBs can be reduced by using high oxygen partial pressure during depositing of Fe3O4 films [19] or post annealing Fe3O4 films at temperature up to 900 °C [25]. However, the effect of Ts on the density of APBs was systematically studied only on epitaxial Fe3O4 films deposited on TiN buffered Si (100), in which the λ (electron-phonon (s-p) coupling constant) values for T2g (3) mode of Fe3O4 were decreased from 0.86 to 0.72 with increasing Ts from 400 °C to 600 °C, indicating the decreases of APBs densities [21]. Considering the conclusion that Fe3O4 films grown on Si substrates are free from the APBs [17], it is interesting to study the density of APBs for Fe3O4 film on Si by controlling the Ts. As a powerful method to evaluate the APBs for Fe3O4 films, the evolution of λ for T2g (3) mode in Fe3O4 films deposited on Si with different Ts due to the change of APBs is still an open issue, even though intensive studies have been performed on Fe3O4/Si films. For example, Singh et al. [19] claim that the optimum Ts is 500 °C for the Fe3O4 film on Si (100) by reactive pulsed DC magnetron sputtering, given the observed strongest T2g and A1g Raman mode. Using Ts in the range of 150 °C −250 °C, Kumar et al. [20] revealed the optimum Ts for preparing Fe3O4 films by reactive ion
Corresponding author at: College of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China E-mail address: [email protected] (L. Sun).
https://doi.org/10.1016/j.tsf.2019.137698 Received 26 March 2019; Received in revised form 9 November 2019; Accepted 13 November 2019 Available online 15 November 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
Thin Solid Films 693 (2020) 137698
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beam sputtering with low antiferromagnetic competitions is 175 °C, as indicated by the signature feature of the earlier room temperature magnetization. In this paper, the effect of Ts on APBs of single phase Fe3O4 films grown on Si (100) was systematically investigated by studying λ for T2g (3) mode in Raman spectra. At the same time, the spin polarization, which is much lower than the theoretical value and critical for the application of Fe3O4 in spintronics was also discussed from the magnetoresistance measurements. Benefiting from the almost broadest range of Ts (in a range of 350 °C - 700 °C) studied up to now, some interesting results were also obtained in the magneto-transport measurements, which would also provide useful information for preparing Fe3O4 film with desirable properties. 2. Experimental details The Fe3O4 thin films with the thickness of 27 nm were deposited using a α-Fe2O3 target on Si (100) at different Ts with intervals of 50 °C from 350 °C to 700 °C by Laser-Molecular Beam Epitaxy, which will be referred henceforth as F350, F400, F450, F500, F550, F600, F650, and F700, respectively. Before deposition, the Si (100) substrate with the naturally oxidized SiO2 layer was chemically cleaned successively with acetone, ethanol, and distilled water in an ultrasonic bath. All the samples were deposited with a based pressure of 5.0 × 10−7 Pa and a deposition rate of 0.15 Å/s. During deposition, the laser under constant energy mode with a pulse energy of 210 mJ and pulse repetition rate of 2 Hz were utilized, and the distance between the sample and the target was 4.5 cm. The standard configuration of Rigaku Ultima IV (185 mm) X-ray diffraction (XRD) was employed for ex situ investigations of the structural quality using Cu radiation (= 0.15418 nm) operated at 40 kV and 40 mA in air. The goniometer holds the sample horizontally and the sample is scanned by the X-ray generator and detector mounted on the arm of the goniometer. The phase purity of the samples was checked using Renishaw inVia Raman Microscope with He-Ne laser (632.8 nm) as an excitation source. The magnetic and transport properties (standard four-probe configuration) were measured by the 7404 Lakeshore vibrating sample magnetometer (VSM) equipped with a magnetoresistance (MR) option. All the above measurements were performed at room temperature. The resistivity vs. temperature measurements were performed in a physical property measurement system by Quantum Design.
Fig.1. X-ray diffraction patterns of Fe3O4 films deposited on Si (100) with different Ts.
while only about 20 nm for samples prepared with lower and higher Ts, and the smallest grain size of 14 nm was obtained for the film prepared at Ts = 350 °C with an obvious signature of superparamagnetic (SP) behavior occurred as discussed in the following magnetic properties. The Raman measurement which is sensitive to the different phases of iron oxides [18] on account of their local symmetry was performed on all samples to confirm the presence of Fe3O4 pure phase, as shown in Fig. 2(a). It is notable that only the strongest A1g mode of Fe3O4 was observed for samples prepared with low (350 °C and 400 °C) and the highest (700 °C) Ts, while three modes of A1g, T2g (2), and T2g (3) match to those of pure Fe3O4 films [19] were observed for films prepared with Ts in the range of 450 °C to 650 °C, even though the peak of T2g (2) mode merged into the Si Raman peak which is too strong to neglect. None of the Raman spectra revealed any signature of the corresponding modes of γ-Fe2O3 phase for which the peaks are expected to be observed at 350 cm−1, 500 cm−1, and 700 cm−1 [19, 28]. Considering the smaller intensities of A1g mode, which is link to the structural properties of Fe3O4 film as its sensitivities to the oxygen stoichiometry and disorder, we argue that samples F350, F400, and F700 have the minimum formation of Fe3O4 and poor crystalline quality, which is consistent with the XRD results. It should be mention that the Raman active modes at 306 cm−1 for Fe3O4 was assigned as Eg by O. N. Shebanova et al. [29] as the incident and scattered beams are polarized in a parallel way. It is believe that the Eg mode will be present along with the T2g modes in the depolarized Raman spectrum, unless a single crystal is in a specific orientation. Therefore, as defined in many
3. Results and discussions 3.1. Structural characterization Fig. 1 shows the X-ray diffraction patterns of Fe3O4 thin films grown at different Ts. For sample F350, a peak corresponding to (311) crystalline orientation was observed, which indicates the polycrystalline nature of Fe3O4 thin films. By increasing Ts to 450 °C, the intensity of diffraction peak is enhanced, and another 3 peaks corresponding to (111), (222) and (333) appear, implying the good crystallization of Fe3O4 thin film with Ts of 450 °C. As further increasing Ts to 650 °C, only the peaks corresponding to (111), (222), and (333) can be observed, which indicate the sample have good crystallization and preferred orientation. For Fe3O4 film prepared at the highest Ts of 700 °C, the intensities of peaks (111), (222), and (333) are rapidly reduced along with the reappearance of the strong peak (311). The results suggest that samples prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C have good crystallization and preferred orientation, while the broadening peaks in films with lower Ts of 350 °C and 400 °C and higher Ts of 700 °C indicate the poor crystallinity and the possibility of partial existence of other Fe-O phase(s) [19, 27]. Using the Scherrer formula, the average crystallite/grain size of these films were calculated to be 25 nm for films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C, 2
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Fig. 2. (color on line) (a) Raman spectra of Fe3O4 films deposited with different Ts recorded in the range 100cm−1–800cm−1 at room temperature. The Lorentzian fitted curves (solid smooth lines) of the peaks due to (b) A1g and (c) T2g (3) modes.
published articles [17, 19, 21, 30], the peak at around 306 cm−1 was marked as T2g (3). To further explore the structural and electronic properties of films prepared with different Ts, the λ for A1g Raman mode and λ for T2g (3) were estimated using Allen's formula [30]. For an mth phonon, the full width of half maximum (FWHM) and frequency are related to
Γ/ ω2 = (2π / gm) λm N (EF )
which further confirm the crystallinity improvement or variation in the films. Moreover, the almost constant value of λ for A1g mode are indicates a similar structure in the Fe3O4 films with Ts in the range of 400 °C - 650 °C. The small fluctuation in the peak position of T2g (3) mode as listed in Table 1 may be due to the surface oxidation of the prepared Fe3O4 films during exposure in air or in the course of the Raman experiment [29]. At the same time, the weak signal due to the thin film structure also affects the accuracy of simulation. Except the very large λ of T2g (3) Raman mode for sample F450 which implies high density of APBs, λ are varying in the range of 0.59–0.89 for films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C. The smallest value of 0.59 for film prepared with Ts = 550 °C is comparable to the bulk value (λ=0.51) of Fe3O4 [32] and much smaller than the reported value of film grown on MgO or TiN [17, 21, 24], which indicates the effective reduction of APBs and confirms the conclusion that Fe3O4 grown on Si is almost free from APBs. It should be mention that the value of 0.59 is a little bit larger than the previously reported value 0.48, which can be ascribed to the thinner thickness of our films [17]. With the decreasing or increasing of the Ts from 550 °C, the coupling constant λ always increases (listed in Table 1), implying an increasing density of APBs, which can be understood in terms of APBs migration approach reported by Eerenstein et al. [33]. The APBs migration process is a thermally activated process with activation energy. And this is the case of the
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In which, the value of shape parameters ω (peak position), Γ (FWHM) of A1g and T2g (3) modes listed in Table 1 were obtained by fitting Raman spectra using a sum of Lorentzian as shown in Fig. 2(b)(f), gm is the degeneracy of the mth mode, and N (EF) is the density of states at Fermi level. The value of N (EF) =3 states/eV per Fe [1] at room temperature was used for estimating λ (tabulated in Table 1), and the λ for T2g (3) arising from the symmetric and asymmetric bending of oxygen with respect to Fe ions is used to evaluate the present of APBs in films [31]. When an APB is created in the Fe3O4 films, antiferromagnetic coupling is formed in the films by the super-exchange interactions at cationic/ionic sites, which enhances the localization of electron at Fe site, thus giving rise to an increased s-p coupling constant [17, 26]. In Table 1, the peak position of A1g shifted towards the value of pure Fe3O4 phase (668 cm−1) [32] for Ts in the range of 400 °C - 650 °C,
Table 1 The resistivity at room temperature ρRT, the deduced parameters from Raman spectra using Allen's formula, and the spin polarization estimated from the value of MR for Fe3O4 films deposited with different Ts. Ts ( °C)
350 400 450 500 550 600 650 700
A1g ω (cm−1)
Γ (cm−1)
λ
T2g ω (cm−1)
Γ (cm−1)
λ
ρRT (mΩ cm)
662.1 667.5 666.4 666.8 666.8 666.5 665.6 663.8
59.73 40.25 39.02 36.97 38.26 38.77 37.94 36.85
0.072 0.048 0.047 0.044 0.046 0.046 0.045 0.044
– – 309.1 302.6 304.0 303.1 308.8 –
– – 180.97 41.90 34.53 42.69 53.38 –
– – 3.016 0.73 0.59 0.74 0.89 –
142.9 53.7 35.6 10.6 7.9 8.3 10.6 159.0
3
P (%) – – – 8.17 9.07 7.79 7.24
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Fig. 3. (color on line) (a) Representative in-plane (solid black lines) and out-of-plane (red dash-dotted lines) hysteresis loops for Fe3O4 films deposited with Ts = 550 °C. (b) The squareness (Mr/Ms), (c) saturation magnetization (Ms), and coercivity (Hc) for Fe3O4 films deposited with different Ts. Insert: In-plane hysteresis loops for Fe3O4 films deposited with Ts = 350 °C.
obtained for film prepared with the highest Ts of 700 °C, along with an abrupt increase of Hc which could be ascribed to the change in the texture of Fe3O4 as shown in the results of XRD [36].
optimum value of Ts. As the Ts increased, the APBs will increase via the thermally activated aggregation of the ionic defects at the grain-surfaces/inter-granular regions. And the corresponding relatively large s-p coupling constants for samples with Ts = 500 °C, 600 °C, and 650 °C are still comparable to the previous reported value 0.81 of polycrystal Fe3O4 film on Si [25], but smaller than 0.98 for epitaxial Fe3O4 films deposited on MgO [24]. It can be concluded that Fe3O4 films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C, not only have good crystallization with preferred orientation but also have a relatively low density of APBs. This is a wider Ts range than the previously reported 150 °C −250 °C [20] and 500 °C −550 °C [19] for polycrystalline Fe3O4 film on Si (100), which might be due to the higher based pressure and stable atmospheric condition without considering the oxygen partial pressure during depositing films.
3.3. Resistivity and magnetoresistance The room temperature resistivity of the films prepared with Ts = 550 °C and 600 °C are 7.9 and 8.3 mΩ•cm, respectively, both are comparable to the value of 8 mΩ•cm for 25-nm-thick epitaxial Fe3O4 film [37], even though they are larger than the bulk resistivity of 4 mΩ•cm [38]. The enhanced resistivity compared to the bulk is due to the presence of antiphase domain boundaries in the film [37]. A larger resistivity of 10.6 mΩ•cm for samples F500 and F650 were obtained, but still reasonable comparing with the reported 10 mΩ•cm for 55-nmthick [39] and 12 mΩ•cm for 240-nm-thick Fe3O4 films [40]. For samples F350, F400, F450, and F700, an abrupt increase in the resistivity was observed (see Table 1), which might be due to the poor crystallization, the existence of other FeO phase, and a decrease in the grain size [39]. The dependence of resistivity on temperature (T) was measured to further confirm the film quantity by investigating the Verwey transition for samples F500, F550, F600, and F650. The resistivity vs. 1000/T is shown in Fig. 4, along with the linear fitting at higher temperature range. A Verwey transition temperature (TV) at about 115 K for these four samples was observed. The most significant increasing of resistivity (not a sharp jump) below TV occurred in
3.2. Magnetic properties The in-plane and out-of-plane magnetization hysteresis (M-H) loops of all the prepared films were measured by VSM at room temperature. Fig. 3(a) shows a typical M-H loops for sample F550, and similar shape and anisotropy were obtained for all the films, except the sample F350 which manifests noticeable SP properties as shown in the inset of Fig.3(b). SP properties corresponding to the small grain size in XRD results were also observed for samples F400, F450, and F700 as the M-H curves reach saturation gradually with an increasing magnetic field (H), but do not saturate completely at H = =900 mT. Smaller remanence ratios (the ratio of remanence to saturation magnetization, Mr/Ms) for these three samples were also observed, while Fe3O4 films prepared at Ts = 500 °C, 550 °C, 600 °C, and 650 °C have higher remanence ratio as shown in Fig. 3(b). The saturation magnetizations (Ms) shown as the solid symbols in Fig. 3(c) were obtained by extrapolation the M-H loops with the maximum H of 900 mT. It was found that the highest value of Ms is 4.70 × 105 A/m for film prepared with Ts = 550 °C (with the smallest density of APBs), which is comparable to the bulk value of 4.71 × 105 A/m [34]. The values of 4.60 × 105 A/m and 4.12 × 105 A/m for samples F600 and F650 are still relatively large, and close to other reported values of 4.50 × 105 A/m for epitaxial Fe3O4 film [35] and 4.12 × 105 A/m for the polycrystalline film on Si (100) [20], respectively. A smaller value of 3.36 × 105 A/m for sample F500 coincides with the weaker intensity of A1g mode in Raman spectra. Nevertheless, these reasonable values of Ms mentioned above arise from the relatively smaller density of APBs. On the other hand, the coercivity as shown in Fig. 3(c) is decreasing as the increasing of Ts, which could attribute to the fluctuation of nanograins size and crystalline with the increasing of Ts [25, 36]. For samples F400 and F450, the lowest intensity of structurally active A1g mode, the smaller grain size, the lower Ms, and the smallest Hc indicate the presence of paramagnetic FeO phase along with the Fe3O4 phase in these films. The smaller value of Ms was also
Fig. 4. (color on line) Experimental data (hollow symbols) of ln (ρ) vs 1000/T for Fe3O4 films prepared at Ts = 500 °C, 550 °C, 600 °C, and 650 °C. Lines are the linear fittings at higher temperature range to obtain the TV. 4
Thin Solid Films 693 (2020) 137698
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Fig. 5. (color on line) The MR data (symbols) measured at room temperature with the maximum magnetic field of 900 mT for films deposited with Ts in the range of 400 °C - 650 °C and the low field simulated results (solid lines fitted with MR=A(M/Ms)2 and dashed line with Eq. (2)). The inset of (f) shows the obtained Ai parameters fitted by Eq. (2).
The spin polarizations (P) listed in Table 1 were determined from MR=P2/1 + +P2 for the granular ferromagnets [48] of samples F500, F550, F600, and F650 with pure Fe3O4 phase, fewer APBs, and other desirable properties. The dependence of P on the Ts is opposite to the change in density of APBs, i.e., enhanced MR and spin polarization is observed in films with a lower density of APBs .The deduced highest value of P for sample F550 is comparable to the results as recently reported [25], in spite of the thinner thickness of our studied films, and MR is decreasing with the decreasing thickness of Fe3O4 film [39, 40]. However, it is much smaller than the theoretical value of 100% for half metallic Fe3O4, and the deterioration of spin polarization is partly due to the unsaturated MR effect and the spin scattering at APBs [47]. The lowest value of λ, highest value of Ms, significant increasing of ρ at Verwey transition, and relatively larger spin polarization confirm the high quality and reduced density of APBs in the film grown at Ts = 550 °C.
samples F550 with the smallest density of APBs implies better metalinsulator transition (Verwey transition), and a better Fe3O4 phase formation in the samples F550. Regardless of the increase or decrease of the Ts, the misalignment is becoming weaker in the ln (ρ) vs 1000/T curves around TV, only slight misalignment occurred for the samples F650 and F500. Magneto-transport measurements of Fe3O4 films were performed with a current in plane geometry. The room-temperature MR shown in Fig. 5 is defined as [ρ (H)-ρ (Hc)]/ρ (Hc) due to that the maximum resistivity occurs at the coercive field, where ρ (H) and ρ (Hc) are the resistivities under the applied magnetic field and at the coercive field, respectively. It is noteworthy that no discernible MR was observed for samples F350 and F700 for which only obvious strong peak at (311) in the XRD pattern was observed. The phenomenon is consistent with the result that [111] oriented films have higher degree of spin polarization (in proportional to MR as discussed later) [41]. The larger MR values of samples F400 and F450 accompanying a larger resistivity is attributed to the spin-dependent tunneling effect [42], which also coincides with the SP magnetization of these films [43]. For films prepared at higher Ts, the good crystallization, preferred orientation, and larger grain size result in a decrease of MR [44], and higher value of MR occurs in the films with a lower density of APBs, similar results can also be found in previous reports [25]. The butterfly shaped MR curves can be approximately described by the relation for a granular tunneling system as MR ≈ -A(M/Ms)2 [25, 44, 45], where A is the spin dependent scattering coefficient, and M is the global magnetization. At low field (H < 100 mT) the MR curves can be well fitted as illustrated by the solid curves in Fig.5, indicating a typical granular tunneling conduction mechanism. The MR is predominantly determined by the magnetic tunneling process between nanograins in the nanocrystalline Fe3O4 film at low fields, and the moment far away from the APBs primarily orient to the magnetic field. The further slow increase of MR at high field is attributed to the moments near the APBs which gradually aligned as increasing of the magnetic field, thus leads to the decreases in the resistivity [25, 40, 45, 46, 47]. Considering of the interaction granular systems, high-order terms (i ≥ 2) were also introduced to further discuss the moment contribution using the equation
MR ∝
∑ Ai (M /Ms )2i i
4. Conclusions A series of 27nm-thick Fe3O4 polycrystalline films with different Ts were prepared on Si (100) with a naturally oxidized SiO2 layer. The results of XRD and Raman spectra verified that the films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C have good crystallization and preferred orientation. Compared with the data for bulk Fe3O4 and the previously reported films, all these above samples (except the film prepared at Ts = 500 °C) have large values of Ms. Considering the most significant change of resistivity around the TV, Fe3O4 film prepared at Ts = 550 °C possesses the highest crystalline quality. Moreover, by analyzing the magneto-transport properties, we found that film prepared with Ts = 550 °C has the largest value of MR and spin polarization, which are all coincident with the lowest density of APBs deduced from the smaller value of λ for T2g (3) mode. The larger MR values are obtained for Fe3O4 films prepared at Ts = 400 °C and 450 °C, which are attributed to the spin-dependent tunneling effect and the accompanying SP magnetization. The demonstrated results for Fe3O4 films prepared with different Ts could be beneficial for the development of spintronic devices base on Fe3O4 film.
(2)
CRediT authorship contribution statement
The experimental data up to 300 mT can be well fitted by Eq. (2) (the dashed lines in Fig. 5), similar to the results of Ref [40]. The obtained Ai are plotted in the inset of Fig. 5(f) except for samples F400 and F450 without pure phase of Fe3O4 or high quality. The larger A3 for sample F550 indicates a relative larger contribution of ferromagnetic grains far away from the APBs to the magnetization at low fields as the size of antiphase domains increases [40].
Li Sun: Writing - original draft, Funding acquisition, Project administration. Dongmei Ban: Data curation. Er Liu: Writing - review & editing. Xiaoyan Li: Data curation, Software. Hongyan Peng: Resources, Funding acquisition. Zhongyu Yao: Data curation, Software. Zhaocong Huang: Writing - review & editing. Ya Zhai: Project administration. Hongru Zhai: Formal analysis. 5
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Declaration of Competing Interest [18]
It is believed that this paper may be of particular interest. Firstly, the antiphase boundaries (APBs) as key parameters to sensitively influence the properties of Fe3O4 films deserve to pay much more attention. Secondly, Fe3O4 film on Si(100) are free from APBs, and electron-phonon (s-p) coupling constant (λ) for T2g(3) Roman mode of Fe3O4 film are numerously used to study APBs. Finally, hardly any report has been done on systematically analyze the effect of APBs on substrate temperature (Ts) of film on Si(100), especially using s-p coupling constant, although reports about epitaxial Fe3O4 on substrate with small mismatch were studied.
[19]
[20]
[21]
[22]
Acknowledgements [23]
This work is supported in part by the Natural Science Foundation of Hainan Province of China [grant number 117109]; the National Natural Science Foundation of China [grant number 11364015 and 51601093]; and Key Laboratory of Laser Technology and Optoelectronic Functional Materials of Hainan Province.
[24]
[25]
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Effect of substrate temperature on antiphase boundaries and spin polarization of thin Fe3O4 film on Si (100)
T
⁎
Li Suna, , Dongmei Bana, Er Liub, Xiaoyan Lia, Hongyan Penga, Zhongyu Yaoa, Zhaocong Huangc, Ya Zhaic, Hongru Zhaid a
College of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China c College of Physics, Southeast University, Nanjing 210096, China d National Laboratory of Solid Microstructures, Nanjing University, Nanjing 210093, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetite Verwey transition Antiphase boundaries Magnetoresistance Spin polarization
Polycrystalline Fe3O4 films with the thickness of 27 nm were grown at different temperature (Ts) on Si (100) substrate by Laser-Molecular Beam Epitaxy to study the influence of density of antiphase boundaries (APBs) and the spin polarization of Fe3O4. Films prepared with Ts in the range of 500 °C - 650 °C were proved to be of good crystallization, preferred orientation, and desirable values of saturation magnetization (a little smaller one for film prepared at Ts = 500 °C). The lowest density of APBs was verified by the measured bulk like electronphonon (s-p) coupling constant for film prepared at Ts = 550 °C, along with the highest value of spin polarization and the best crystalline quality as the most obvious inflection in the curves of lnρ vs. 1000/T around Verwey transition. This optimum value of Ts can be interpreted by the APBs thermally activated migration process with an activation energy. Films prepared with Ts = 500 °C, 600 °C, and 650 °C still have a reasonable value of the s-p coupling constant compared to recently reported results, while larger magnetoresistance accompanied by the superparamagnetic behavior was observed for films prepared at lower Ts (Ts = 400 °C and 450 °C).
1. Introduction As an ancient and fascinating material, the intriguing structural, electrical and magnetic properties of Magnetite (Fe3O4) [1, 2], in particular, the 100% theoretical spin polarization and high Curie temperature (~860 K) make it a ideal candidate for spintronics application such as spin valves and spin tunnel junctions [3-7] recently pursued [813]. However, the antiphase boundaries (APBs) which can influence the spin polarization and degrade electrical and magnetic properties of Fe3O4 films [14, 15] due to the antiferromagnetic coupling between adjacent structure domains [17, 18], are easily formed in Fe3O4 during film growth owing to the uncontrollable formation of growth defects like stacking faults. Various preparation parameters, such as oxygen partial pressure [19], subtract temperature (Ts) [18, 20-22], and nature of the substrate [16, 23, 24], as well as other factors, like post processing [25], even new approach of in-situ electric field assisted method [26] were investigated to reduce the APBs. Thus, engineering the APBs is one of the essential topics for preparing Fe3O4 film with good quantity.
⁎
It has been reported that the density of APBs can be reduced by using high oxygen partial pressure during depositing of Fe3O4 films [19] or post annealing Fe3O4 films at temperature up to 900 °C [25]. However, the effect of Ts on the density of APBs was systematically studied only on epitaxial Fe3O4 films deposited on TiN buffered Si (100), in which the λ (electron-phonon (s-p) coupling constant) values for T2g (3) mode of Fe3O4 were decreased from 0.86 to 0.72 with increasing Ts from 400 °C to 600 °C, indicating the decreases of APBs densities [21]. Considering the conclusion that Fe3O4 films grown on Si substrates are free from the APBs [17], it is interesting to study the density of APBs for Fe3O4 film on Si by controlling the Ts. As a powerful method to evaluate the APBs for Fe3O4 films, the evolution of λ for T2g (3) mode in Fe3O4 films deposited on Si with different Ts due to the change of APBs is still an open issue, even though intensive studies have been performed on Fe3O4/Si films. For example, Singh et al. [19] claim that the optimum Ts is 500 °C for the Fe3O4 film on Si (100) by reactive pulsed DC magnetron sputtering, given the observed strongest T2g and A1g Raman mode. Using Ts in the range of 150 °C −250 °C, Kumar et al. [20] revealed the optimum Ts for preparing Fe3O4 films by reactive ion
Corresponding author at: College of Physics and Electronic Engineering, Hainan Normal University, Haikou 571158, China E-mail address: [email protected] (L. Sun).
https://doi.org/10.1016/j.tsf.2019.137698 Received 26 March 2019; Received in revised form 9 November 2019; Accepted 13 November 2019 Available online 15 November 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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beam sputtering with low antiferromagnetic competitions is 175 °C, as indicated by the signature feature of the earlier room temperature magnetization. In this paper, the effect of Ts on APBs of single phase Fe3O4 films grown on Si (100) was systematically investigated by studying λ for T2g (3) mode in Raman spectra. At the same time, the spin polarization, which is much lower than the theoretical value and critical for the application of Fe3O4 in spintronics was also discussed from the magnetoresistance measurements. Benefiting from the almost broadest range of Ts (in a range of 350 °C - 700 °C) studied up to now, some interesting results were also obtained in the magneto-transport measurements, which would also provide useful information for preparing Fe3O4 film with desirable properties. 2. Experimental details The Fe3O4 thin films with the thickness of 27 nm were deposited using a α-Fe2O3 target on Si (100) at different Ts with intervals of 50 °C from 350 °C to 700 °C by Laser-Molecular Beam Epitaxy, which will be referred henceforth as F350, F400, F450, F500, F550, F600, F650, and F700, respectively. Before deposition, the Si (100) substrate with the naturally oxidized SiO2 layer was chemically cleaned successively with acetone, ethanol, and distilled water in an ultrasonic bath. All the samples were deposited with a based pressure of 5.0 × 10−7 Pa and a deposition rate of 0.15 Å/s. During deposition, the laser under constant energy mode with a pulse energy of 210 mJ and pulse repetition rate of 2 Hz were utilized, and the distance between the sample and the target was 4.5 cm. The standard configuration of Rigaku Ultima IV (185 mm) X-ray diffraction (XRD) was employed for ex situ investigations of the structural quality using Cu radiation (= 0.15418 nm) operated at 40 kV and 40 mA in air. The goniometer holds the sample horizontally and the sample is scanned by the X-ray generator and detector mounted on the arm of the goniometer. The phase purity of the samples was checked using Renishaw inVia Raman Microscope with He-Ne laser (632.8 nm) as an excitation source. The magnetic and transport properties (standard four-probe configuration) were measured by the 7404 Lakeshore vibrating sample magnetometer (VSM) equipped with a magnetoresistance (MR) option. All the above measurements were performed at room temperature. The resistivity vs. temperature measurements were performed in a physical property measurement system by Quantum Design.
Fig.1. X-ray diffraction patterns of Fe3O4 films deposited on Si (100) with different Ts.
while only about 20 nm for samples prepared with lower and higher Ts, and the smallest grain size of 14 nm was obtained for the film prepared at Ts = 350 °C with an obvious signature of superparamagnetic (SP) behavior occurred as discussed in the following magnetic properties. The Raman measurement which is sensitive to the different phases of iron oxides [18] on account of their local symmetry was performed on all samples to confirm the presence of Fe3O4 pure phase, as shown in Fig. 2(a). It is notable that only the strongest A1g mode of Fe3O4 was observed for samples prepared with low (350 °C and 400 °C) and the highest (700 °C) Ts, while three modes of A1g, T2g (2), and T2g (3) match to those of pure Fe3O4 films [19] were observed for films prepared with Ts in the range of 450 °C to 650 °C, even though the peak of T2g (2) mode merged into the Si Raman peak which is too strong to neglect. None of the Raman spectra revealed any signature of the corresponding modes of γ-Fe2O3 phase for which the peaks are expected to be observed at 350 cm−1, 500 cm−1, and 700 cm−1 [19, 28]. Considering the smaller intensities of A1g mode, which is link to the structural properties of Fe3O4 film as its sensitivities to the oxygen stoichiometry and disorder, we argue that samples F350, F400, and F700 have the minimum formation of Fe3O4 and poor crystalline quality, which is consistent with the XRD results. It should be mention that the Raman active modes at 306 cm−1 for Fe3O4 was assigned as Eg by O. N. Shebanova et al. [29] as the incident and scattered beams are polarized in a parallel way. It is believe that the Eg mode will be present along with the T2g modes in the depolarized Raman spectrum, unless a single crystal is in a specific orientation. Therefore, as defined in many
3. Results and discussions 3.1. Structural characterization Fig. 1 shows the X-ray diffraction patterns of Fe3O4 thin films grown at different Ts. For sample F350, a peak corresponding to (311) crystalline orientation was observed, which indicates the polycrystalline nature of Fe3O4 thin films. By increasing Ts to 450 °C, the intensity of diffraction peak is enhanced, and another 3 peaks corresponding to (111), (222) and (333) appear, implying the good crystallization of Fe3O4 thin film with Ts of 450 °C. As further increasing Ts to 650 °C, only the peaks corresponding to (111), (222), and (333) can be observed, which indicate the sample have good crystallization and preferred orientation. For Fe3O4 film prepared at the highest Ts of 700 °C, the intensities of peaks (111), (222), and (333) are rapidly reduced along with the reappearance of the strong peak (311). The results suggest that samples prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C have good crystallization and preferred orientation, while the broadening peaks in films with lower Ts of 350 °C and 400 °C and higher Ts of 700 °C indicate the poor crystallinity and the possibility of partial existence of other Fe-O phase(s) [19, 27]. Using the Scherrer formula, the average crystallite/grain size of these films were calculated to be 25 nm for films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C, 2
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Fig. 2. (color on line) (a) Raman spectra of Fe3O4 films deposited with different Ts recorded in the range 100cm−1–800cm−1 at room temperature. The Lorentzian fitted curves (solid smooth lines) of the peaks due to (b) A1g and (c) T2g (3) modes.
published articles [17, 19, 21, 30], the peak at around 306 cm−1 was marked as T2g (3). To further explore the structural and electronic properties of films prepared with different Ts, the λ for A1g Raman mode and λ for T2g (3) were estimated using Allen's formula [30]. For an mth phonon, the full width of half maximum (FWHM) and frequency are related to
Γ/ ω2 = (2π / gm) λm N (EF )
which further confirm the crystallinity improvement or variation in the films. Moreover, the almost constant value of λ for A1g mode are indicates a similar structure in the Fe3O4 films with Ts in the range of 400 °C - 650 °C. The small fluctuation in the peak position of T2g (3) mode as listed in Table 1 may be due to the surface oxidation of the prepared Fe3O4 films during exposure in air or in the course of the Raman experiment [29]. At the same time, the weak signal due to the thin film structure also affects the accuracy of simulation. Except the very large λ of T2g (3) Raman mode for sample F450 which implies high density of APBs, λ are varying in the range of 0.59–0.89 for films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C. The smallest value of 0.59 for film prepared with Ts = 550 °C is comparable to the bulk value (λ=0.51) of Fe3O4 [32] and much smaller than the reported value of film grown on MgO or TiN [17, 21, 24], which indicates the effective reduction of APBs and confirms the conclusion that Fe3O4 grown on Si is almost free from APBs. It should be mention that the value of 0.59 is a little bit larger than the previously reported value 0.48, which can be ascribed to the thinner thickness of our films [17]. With the decreasing or increasing of the Ts from 550 °C, the coupling constant λ always increases (listed in Table 1), implying an increasing density of APBs, which can be understood in terms of APBs migration approach reported by Eerenstein et al. [33]. The APBs migration process is a thermally activated process with activation energy. And this is the case of the
(1)
In which, the value of shape parameters ω (peak position), Γ (FWHM) of A1g and T2g (3) modes listed in Table 1 were obtained by fitting Raman spectra using a sum of Lorentzian as shown in Fig. 2(b)(f), gm is the degeneracy of the mth mode, and N (EF) is the density of states at Fermi level. The value of N (EF) =3 states/eV per Fe [1] at room temperature was used for estimating λ (tabulated in Table 1), and the λ for T2g (3) arising from the symmetric and asymmetric bending of oxygen with respect to Fe ions is used to evaluate the present of APBs in films [31]. When an APB is created in the Fe3O4 films, antiferromagnetic coupling is formed in the films by the super-exchange interactions at cationic/ionic sites, which enhances the localization of electron at Fe site, thus giving rise to an increased s-p coupling constant [17, 26]. In Table 1, the peak position of A1g shifted towards the value of pure Fe3O4 phase (668 cm−1) [32] for Ts in the range of 400 °C - 650 °C,
Table 1 The resistivity at room temperature ρRT, the deduced parameters from Raman spectra using Allen's formula, and the spin polarization estimated from the value of MR for Fe3O4 films deposited with different Ts. Ts ( °C)
350 400 450 500 550 600 650 700
A1g ω (cm−1)
Γ (cm−1)
λ
T2g ω (cm−1)
Γ (cm−1)
λ
ρRT (mΩ cm)
662.1 667.5 666.4 666.8 666.8 666.5 665.6 663.8
59.73 40.25 39.02 36.97 38.26 38.77 37.94 36.85
0.072 0.048 0.047 0.044 0.046 0.046 0.045 0.044
– – 309.1 302.6 304.0 303.1 308.8 –
– – 180.97 41.90 34.53 42.69 53.38 –
– – 3.016 0.73 0.59 0.74 0.89 –
142.9 53.7 35.6 10.6 7.9 8.3 10.6 159.0
3
P (%) – – – 8.17 9.07 7.79 7.24
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Fig. 3. (color on line) (a) Representative in-plane (solid black lines) and out-of-plane (red dash-dotted lines) hysteresis loops for Fe3O4 films deposited with Ts = 550 °C. (b) The squareness (Mr/Ms), (c) saturation magnetization (Ms), and coercivity (Hc) for Fe3O4 films deposited with different Ts. Insert: In-plane hysteresis loops for Fe3O4 films deposited with Ts = 350 °C.
obtained for film prepared with the highest Ts of 700 °C, along with an abrupt increase of Hc which could be ascribed to the change in the texture of Fe3O4 as shown in the results of XRD [36].
optimum value of Ts. As the Ts increased, the APBs will increase via the thermally activated aggregation of the ionic defects at the grain-surfaces/inter-granular regions. And the corresponding relatively large s-p coupling constants for samples with Ts = 500 °C, 600 °C, and 650 °C are still comparable to the previous reported value 0.81 of polycrystal Fe3O4 film on Si [25], but smaller than 0.98 for epitaxial Fe3O4 films deposited on MgO [24]. It can be concluded that Fe3O4 films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C, not only have good crystallization with preferred orientation but also have a relatively low density of APBs. This is a wider Ts range than the previously reported 150 °C −250 °C [20] and 500 °C −550 °C [19] for polycrystalline Fe3O4 film on Si (100), which might be due to the higher based pressure and stable atmospheric condition without considering the oxygen partial pressure during depositing films.
3.3. Resistivity and magnetoresistance The room temperature resistivity of the films prepared with Ts = 550 °C and 600 °C are 7.9 and 8.3 mΩ•cm, respectively, both are comparable to the value of 8 mΩ•cm for 25-nm-thick epitaxial Fe3O4 film [37], even though they are larger than the bulk resistivity of 4 mΩ•cm [38]. The enhanced resistivity compared to the bulk is due to the presence of antiphase domain boundaries in the film [37]. A larger resistivity of 10.6 mΩ•cm for samples F500 and F650 were obtained, but still reasonable comparing with the reported 10 mΩ•cm for 55-nmthick [39] and 12 mΩ•cm for 240-nm-thick Fe3O4 films [40]. For samples F350, F400, F450, and F700, an abrupt increase in the resistivity was observed (see Table 1), which might be due to the poor crystallization, the existence of other FeO phase, and a decrease in the grain size [39]. The dependence of resistivity on temperature (T) was measured to further confirm the film quantity by investigating the Verwey transition for samples F500, F550, F600, and F650. The resistivity vs. 1000/T is shown in Fig. 4, along with the linear fitting at higher temperature range. A Verwey transition temperature (TV) at about 115 K for these four samples was observed. The most significant increasing of resistivity (not a sharp jump) below TV occurred in
3.2. Magnetic properties The in-plane and out-of-plane magnetization hysteresis (M-H) loops of all the prepared films were measured by VSM at room temperature. Fig. 3(a) shows a typical M-H loops for sample F550, and similar shape and anisotropy were obtained for all the films, except the sample F350 which manifests noticeable SP properties as shown in the inset of Fig.3(b). SP properties corresponding to the small grain size in XRD results were also observed for samples F400, F450, and F700 as the M-H curves reach saturation gradually with an increasing magnetic field (H), but do not saturate completely at H = =900 mT. Smaller remanence ratios (the ratio of remanence to saturation magnetization, Mr/Ms) for these three samples were also observed, while Fe3O4 films prepared at Ts = 500 °C, 550 °C, 600 °C, and 650 °C have higher remanence ratio as shown in Fig. 3(b). The saturation magnetizations (Ms) shown as the solid symbols in Fig. 3(c) were obtained by extrapolation the M-H loops with the maximum H of 900 mT. It was found that the highest value of Ms is 4.70 × 105 A/m for film prepared with Ts = 550 °C (with the smallest density of APBs), which is comparable to the bulk value of 4.71 × 105 A/m [34]. The values of 4.60 × 105 A/m and 4.12 × 105 A/m for samples F600 and F650 are still relatively large, and close to other reported values of 4.50 × 105 A/m for epitaxial Fe3O4 film [35] and 4.12 × 105 A/m for the polycrystalline film on Si (100) [20], respectively. A smaller value of 3.36 × 105 A/m for sample F500 coincides with the weaker intensity of A1g mode in Raman spectra. Nevertheless, these reasonable values of Ms mentioned above arise from the relatively smaller density of APBs. On the other hand, the coercivity as shown in Fig. 3(c) is decreasing as the increasing of Ts, which could attribute to the fluctuation of nanograins size and crystalline with the increasing of Ts [25, 36]. For samples F400 and F450, the lowest intensity of structurally active A1g mode, the smaller grain size, the lower Ms, and the smallest Hc indicate the presence of paramagnetic FeO phase along with the Fe3O4 phase in these films. The smaller value of Ms was also
Fig. 4. (color on line) Experimental data (hollow symbols) of ln (ρ) vs 1000/T for Fe3O4 films prepared at Ts = 500 °C, 550 °C, 600 °C, and 650 °C. Lines are the linear fittings at higher temperature range to obtain the TV. 4
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Fig. 5. (color on line) The MR data (symbols) measured at room temperature with the maximum magnetic field of 900 mT for films deposited with Ts in the range of 400 °C - 650 °C and the low field simulated results (solid lines fitted with MR=A(M/Ms)2 and dashed line with Eq. (2)). The inset of (f) shows the obtained Ai parameters fitted by Eq. (2).
The spin polarizations (P) listed in Table 1 were determined from MR=P2/1 + +P2 for the granular ferromagnets [48] of samples F500, F550, F600, and F650 with pure Fe3O4 phase, fewer APBs, and other desirable properties. The dependence of P on the Ts is opposite to the change in density of APBs, i.e., enhanced MR and spin polarization is observed in films with a lower density of APBs .The deduced highest value of P for sample F550 is comparable to the results as recently reported [25], in spite of the thinner thickness of our studied films, and MR is decreasing with the decreasing thickness of Fe3O4 film [39, 40]. However, it is much smaller than the theoretical value of 100% for half metallic Fe3O4, and the deterioration of spin polarization is partly due to the unsaturated MR effect and the spin scattering at APBs [47]. The lowest value of λ, highest value of Ms, significant increasing of ρ at Verwey transition, and relatively larger spin polarization confirm the high quality and reduced density of APBs in the film grown at Ts = 550 °C.
samples F550 with the smallest density of APBs implies better metalinsulator transition (Verwey transition), and a better Fe3O4 phase formation in the samples F550. Regardless of the increase or decrease of the Ts, the misalignment is becoming weaker in the ln (ρ) vs 1000/T curves around TV, only slight misalignment occurred for the samples F650 and F500. Magneto-transport measurements of Fe3O4 films were performed with a current in plane geometry. The room-temperature MR shown in Fig. 5 is defined as [ρ (H)-ρ (Hc)]/ρ (Hc) due to that the maximum resistivity occurs at the coercive field, where ρ (H) and ρ (Hc) are the resistivities under the applied magnetic field and at the coercive field, respectively. It is noteworthy that no discernible MR was observed for samples F350 and F700 for which only obvious strong peak at (311) in the XRD pattern was observed. The phenomenon is consistent with the result that [111] oriented films have higher degree of spin polarization (in proportional to MR as discussed later) [41]. The larger MR values of samples F400 and F450 accompanying a larger resistivity is attributed to the spin-dependent tunneling effect [42], which also coincides with the SP magnetization of these films [43]. For films prepared at higher Ts, the good crystallization, preferred orientation, and larger grain size result in a decrease of MR [44], and higher value of MR occurs in the films with a lower density of APBs, similar results can also be found in previous reports [25]. The butterfly shaped MR curves can be approximately described by the relation for a granular tunneling system as MR ≈ -A(M/Ms)2 [25, 44, 45], where A is the spin dependent scattering coefficient, and M is the global magnetization. At low field (H < 100 mT) the MR curves can be well fitted as illustrated by the solid curves in Fig.5, indicating a typical granular tunneling conduction mechanism. The MR is predominantly determined by the magnetic tunneling process between nanograins in the nanocrystalline Fe3O4 film at low fields, and the moment far away from the APBs primarily orient to the magnetic field. The further slow increase of MR at high field is attributed to the moments near the APBs which gradually aligned as increasing of the magnetic field, thus leads to the decreases in the resistivity [25, 40, 45, 46, 47]. Considering of the interaction granular systems, high-order terms (i ≥ 2) were also introduced to further discuss the moment contribution using the equation
MR ∝
∑ Ai (M /Ms )2i i
4. Conclusions A series of 27nm-thick Fe3O4 polycrystalline films with different Ts were prepared on Si (100) with a naturally oxidized SiO2 layer. The results of XRD and Raman spectra verified that the films prepared with Ts = 500 °C, 550 °C, 600 °C, and 650 °C have good crystallization and preferred orientation. Compared with the data for bulk Fe3O4 and the previously reported films, all these above samples (except the film prepared at Ts = 500 °C) have large values of Ms. Considering the most significant change of resistivity around the TV, Fe3O4 film prepared at Ts = 550 °C possesses the highest crystalline quality. Moreover, by analyzing the magneto-transport properties, we found that film prepared with Ts = 550 °C has the largest value of MR and spin polarization, which are all coincident with the lowest density of APBs deduced from the smaller value of λ for T2g (3) mode. The larger MR values are obtained for Fe3O4 films prepared at Ts = 400 °C and 450 °C, which are attributed to the spin-dependent tunneling effect and the accompanying SP magnetization. The demonstrated results for Fe3O4 films prepared with different Ts could be beneficial for the development of spintronic devices base on Fe3O4 film.
(2)
CRediT authorship contribution statement
The experimental data up to 300 mT can be well fitted by Eq. (2) (the dashed lines in Fig. 5), similar to the results of Ref [40]. The obtained Ai are plotted in the inset of Fig. 5(f) except for samples F400 and F450 without pure phase of Fe3O4 or high quality. The larger A3 for sample F550 indicates a relative larger contribution of ferromagnetic grains far away from the APBs to the magnetization at low fields as the size of antiphase domains increases [40].
Li Sun: Writing - original draft, Funding acquisition, Project administration. Dongmei Ban: Data curation. Er Liu: Writing - review & editing. Xiaoyan Li: Data curation, Software. Hongyan Peng: Resources, Funding acquisition. Zhongyu Yao: Data curation, Software. Zhaocong Huang: Writing - review & editing. Ya Zhai: Project administration. Hongru Zhai: Formal analysis. 5
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Declaration of Competing Interest [18]
It is believed that this paper may be of particular interest. Firstly, the antiphase boundaries (APBs) as key parameters to sensitively influence the properties of Fe3O4 films deserve to pay much more attention. Secondly, Fe3O4 film on Si(100) are free from APBs, and electron-phonon (s-p) coupling constant (λ) for T2g(3) Roman mode of Fe3O4 film are numerously used to study APBs. Finally, hardly any report has been done on systematically analyze the effect of APBs on substrate temperature (Ts) of film on Si(100), especially using s-p coupling constant, although reports about epitaxial Fe3O4 on substrate with small mismatch were studied.
[19]
[20]
[21]
[22]
Acknowledgements [23]
This work is supported in part by the Natural Science Foundation of Hainan Province of China [grant number 117109]; the National Natural Science Foundation of China [grant number 11364015 and 51601093]; and Key Laboratory of Laser Technology and Optoelectronic Functional Materials of Hainan Province.
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