- Email: [email protected]
Light-modulated ferromagnetism of strained NiFe2O4 nanocrystals
Ceramics International xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Light-modulated ferromagnetism of strained NiFe2O4 nanocrystals Hang Zhou, Zhiwei An, Cailei Yuan∗, Xingfang Luo∗∗ Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Communication and Electronics, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang, 330022, Jiangxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocrystals Strain Ferromagnetism Light irradiation
Light provides a potential approach to modulate magnetism due to the low energy consumption, which could be used for the development of optical-magnetic coupling devices, data storage technology and quantum computation technology. In this work, NiFe2O4 nanocrystals with inverse spinel structure confined in non-magnetic Al2O3 film are synthesized. Based on theoretical calculation and experimental validation, it is revealed that the NiFe2O4 nanocrystals experience compressive strain from Al2O3 film. The compressive strain can be partially relaxed, leading to the formation of disorder layers at the surfaces of strained NiFe2O4 nanocrystals, which provide an ideal platform to modulate magnetism optically. A remarkable light-modulated ferromagnetism in the strained NiFe2O4 nanocrystals is observed. This work demonstrates a novel approach to optically engineer the ferromagnetic properties for applications in next generation of magnetic nano-devices.
1. Introduction Ferromagnetic NiFe2O4 materials have been widely used in many fields, including magnetic high-density data storage [1], biomedical applications [2], magnetic drug delivery [3], catalysts [4], etc. In most of these applications, reduction to nanoscale-size can optimize properties of materials for certain technical purposes [5]. The magnetic properties of NiFe2O4 nanocrystals are extremely sensitive to their size, purity and magnetic stability [6,7]. In recent years, the synthesis of high quality NiFe2O4 nanocrystals to obtain the best magnetic performance has attracted great interest. A common problem in the preparation of magnetic NiFe2O4 nanocrystals is agglomeration, which is caused by the magnetic attraction between NiFe2O4 nanocrystals. Incorporation of NiFe2O4 nanocrystals into solid-state film is an ideal approach for practical magnetic nano-devices applications, which can produce non-agglomerated NiFe2O4 nanocrystals with superior magnetic behaviors. However, the introduction of surface coatings will induce the formation of disordered layers at the surfaces of the NiFe2O4 nanocrystals during growth process because of the thermal expansion mismatch between nanocrystals and surrounding coating materials [8,9]. The disordered layers are mainly contributed by the broken symmetries and exchange bonds at the surfaces of NiFe2O4 nanocrystals [10], which result in the suppression of the ferromagnetic performance of nanocrystals with respect to bulk behavior [11]. Therefore, the device applications of NiFe2O4 nanocrystals are greatly hindered.
∗
Moreover, for the application in modern information technology, it is essential to modulate the ferromagnetism with non-magnetic filed approaches. Apart from electric field and current [12,13], light provides a potential approach to modulate the magnetism due to the low energy consumption, which could be used for the development of optical-magnetic coupling devices, data storage technology and quantum computation technology. Fortunately, the surface states of strained NiFe2O4 nanocrystals may provide an ideal platform to modulate magnetism optically. Therefore, in this work, pure-phase NiFe2O4/ Al2O3 nanocrystals are synthesized. In order to achieve superior magnetic performance, the modulation of magnetic properties of NiFe2O4/ Al2O3 nanocrystals by light irradiation are proposed. A remarkable light-modulated ferromagnetism in the strained NiFe2O4 nanocrystals is observed. This work demonstrates a novel approach to optically engineer the ferromagnetic properties for applications in next generation of magnetic nano-devices. 2. Experimental NiFe2O4 nanocrystals were synthesized in Al2O3 film by pulsed laser ablation (PLA) and rapid thermal annealing (RTA). Briefly, a target was ablated by KrF pulsed laser (248 nm) in an ultra-high vacuum chamber with base pressure of 5 × 10−8 torr. The laser ablation target was prepared by using a round Al2O3 (99.99% purity) target with a diameter of 4 cm and a small NiFe2O4 (99.99% purity) square plate with a
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Yuan), xfl[email protected] (X. Luo).
∗∗
https://doi.org/10.1016/j.ceramint.2019.04.022 Received 24 February 2019; Received in revised form 25 March 2019; Accepted 2 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hang Zhou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.04.022
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
As shown in Fig. 1 (d), the electron diffraction patterns of NiFe2O4 nanocrystals match well with the simulated NiFe2O4 electron diffraction patterns. It can be inferred that the crystal structure of NiFe2O4 nanocrystals is inverse spinel structure. Fig. 1 (e) also presents the schematic diagram of the NiFe2O4 nanocrystals unit cell, where the tetrahedral positions are completely occupied by Fe3+ and the octahedral positions are occupied by Ni2+ and Fe3+ ions [15]. Obviously, the NiFe2O4 nanocrystals grown in Al2O3 film will exhibit ferromagnetic behaviors because of the antiparallel spin between Fe3+ at tetrahedral site and Ni2+ at octahedral site [16]. More interesting, it should be noted that the lattice constants of the NiFe2O4 nanocrystals are about a = b = c = 8.1 Å, which are less than lattice constants of the NiFe2O4 bulk (a = b = c = 8.3 Å) [17]. The reduction of lattice constants of the NiFe2O4 nanocrystals can be attributed to the compressive strain existing in the NiFe2O4 nanocrystals grown in Al2O3 film [18]. As results, significant surface effect will be activated by the substantial compressive strain existing in NiFe2O4 nanocrystals. In order to fully understand the changes in microstructure of the NiFe2O4 nanocrystals, a high-resolution TEM image of single of NiFe2O4 nanocrystal is presented in Fig. 1 (f). Although this NiFe2O4 nanocrystal is single crystal, there is a very thin disorder layer formed at its surface region. The appearance disorder layer is mainly contributed by the broken symmetries and exchange bonds between the oxyanions and metallic atoms at the surface region of the NiFe2O4 nanocrystal [10], which may result in the suppression of ferromagnetic performance of NiFe2O4 nanocrystals with respect to NiFe2O4 bulk, and thus hinder the device applications of NiFe2O4 nanocrystals. The chemical composition and stoichiometry of NiFe2O4 nanocrystals in Al2O3 matrix are studied by XPS. Fig. 2 presents the high-resolution XPS and deconvolution spectra of NiFe2O4 nanocrystals grown in Al2O3 film, including Ni 2p, Fe 2p, Al 2p and O 1s peaks. The deconvolution peaks help to identify different cationic states. The Ni 2p high-resolution XPS spectrum of NiFe2O4 nanocrystals is shown in
length of 0.8 cm. The NiFe2O4 square plate was glued using chemically nonreactive adhesive material onto the surface of the Al2O3 target, creating an assembly of two composite materials, which were in physical, but not chemical contact. The deposited sample with thickness of about 300 nm was treated by RTA at 600 °C for 5 min in oxygen atmosphere. The structure of the sample was studied by JEOL 2010 transmission electron microscopy (TEM) operating at 200 kV. Kratos Axis Ultra DLD spectrometer was used to measure high-resolution X-ray photoelectron spectroscopy (XPS) with Al Ka radiation. The binding energies were analyzed with reference to C1s peak. Raman spectra were measured using HORIBA Scientific LabRAM HR Evolution system (excitation laser wavelength 514 nm). The magnetic properties of the sample were studied by using physical property measurement system (PPMS, Quantum Design) with the option of vibrating sample magnetometer. Using zero field cooling (ZFC) and field cooling (FC) modes, the temperature-dependent magnetization (M-T) of the sample was measured from 10 K to 300 K. The magnetic-field dependent magnetization (M − H) of the sample was measured at 10 K and 300 K, respectively.
3. Results and discussion Fig. 1 (a) shows the TEM image of the as-deposited film at lowmagnification. The corresponding electron diffraction patterns are presented in Fig. 1 (b). It clearly presents the voids of nanocrystals, which can be attributed that NiFe2O4 is not consumed or nucleated and the atomic dispersion of NiFe2O4 exists in amorphous Al2O3 film at room temperature. Fig. 1 (c) presents the low-magnification TEM image of NiFe2O4 nanocrystals grown in Al2O3 film after annealing at 600 °C. It can be seen that there are lots of NiFe2O4 nanocrystals with average size of about 10 nm formed in amorphous Al2O3 film. The simulated electron diffraction patterns of NiFe2O4 are generated by using the TEM simulator JAVA Electron Microscope Simulation (JEMS) software [14].
Fig. 1. (a) Low-magnification TEM image of as-deposited film; (b) electron diffraction pattern of asdeposited film; (c) low-magnification TEM image of NiFe2O4 nanocrystals grown in Al2O3 film; (d) electron diffraction pattern of NiFe2O4 nanocrystals grown in Al2O3 film; (e) a schematic diagram of NiFe2O4 nanocrystal unit cell; (f) high-resolution TEM image of a single NiFe2O4 nanocrystal grown in Al2O3 film.
2
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
Fig. 2. High-resolution XPS spectra of Ni 2p (a), Fe 2p (a), Al 2p (c) and O 1s (d) electrons from NiFe2O4 nanocrystals confined in Al2O3 film.
results are also consistent with the TEM observations in Fig. 1. In order to experimentally evaluate the compressive strain in NiFe2O4 nanocrystals, the Raman spectra of NiFe2O4 bulk and NiFe2O4/ Al2O3 film excited at a wavelength of 514 nm are shown in Fig. 4 for comparison. A Si peak at 520 cm−1 is used as a reference for calibration. The right inset is the expanded view of the same Raman spectrum of NiFe2O4 bulk in a range from 100 cm−1 to 900 cm−1. The Raman spectrum collected for bulk NiFe2O4 show five typical Raman bands of the inverse spinel structure of NiFe2O4, which matches well with those reported earlier [23,24]. In contrast, for the NiFe2O4 nanocrystals with spinel structure, only T2g (3) and A1g vibrational modes centered at about 574 cm−1 and 695 cm−1 are intense enough to be distinguished from the background noise, respectively. In comparison with the Raman peaks of bulk NiFe2O4, a distinguishable wavenumber shift (Δω = 12 cm−1) is observed in the NiFe2O4 nanocrystals. Usually, the wavenumber shift of Raman peak can be induced by the strain effect or/ and confinement effect. The compressive strain effect can lead to positive wavenumber shift, while the phonon confinement effect will result in negative wavenumber shift [25]. Thus, the positive value of Δω means that the compressive strain effect dominates the phonon confinement effect in these NiFe2O4 nanocrystals. The results furtherly demonstrate the TEM observations and FE calculations. The left inset of Fig. 4 displays the Raman intensity mapping image of A1g mode of the sample with confined NiFe2O4 nanocrystals. The image shows that the intensity distribution of the A1g peak is uniform. Since the peak position is significantly influenced by strain and confinement effects, the result furtherly demonstrates that NiFe2O4 nanocrystals with uniform size and strain distribution have been synthesized in Al2O3 film. To investigate the ferromagnetic properties of NiFe2O4 nanocrystals, the M-T curves were measured in ZFC and FC modes from 10 to 300 K, as shown in Fig. 5 (a). The inset is expanded view of the same M-T curves at low temperature range from 10 K to 40 K. The results show the overlapped ZFC and FC curves are separated at low temperature. It should be noted that the FC magnetization decreases with temperature increasing, while, the ZFC magnetization increases at first and then decreases with the temperature decreasing. The maximum value of ZFC curve (at about 17 K) is determined to be the blocking temperature (TB). The presence of superparamagnetism is therefore confirmed by the shape of FC and ZFC magnetization curves. At temperatures lower than TB, the magnetic moments of NiFe2O4 nanocrystals are confined along the local magnetic anisotropy axes, while being prone to the thermal
Fig. 2 (a). A Ni 2p3/2 signal appears at 856.0 eV with a satellite peak at 861.9 eV, and the peak value of 873.6 eV is attributed to the Ni 2p1/2 core level. Moreover, from Fig. 2 (a), an Auger electron peak of Ni (∼277.1 eV) can be found, which furtherly clarify that nickel is in the +2 oxidation state [19]. The high-resolution XPS spectrum of Fe 2p in NiFe2O4 nanocrystals is shown in Fig. 2 (b). The XPS peaks centered at ∼710.4 eV and ∼724.2 eV can be classified into Fe 2p3/2 and Fe 2p1/2 core levels, respectively. The main peaks of Fe 2p3/2 and Fe 2p1/2 are obviously accompanied by satellite structures on the side of higher binding energy (∼8 eV). From the overall spectral shape and binding energy positions of the Fe 2p core levels, it can be concluded that only Fe3+ species exist in the lattice, which matches well with those reported in the literature [20]. Fig. 2(c) and (d) show the Al 2p and O 1s spectra of NiFe2O4 nanocrystals confined in Al2O3 matrix, respectively. The binding energy of Al 2p is ∼74.3 eV, which corresponds to the chemical state of Al3+. The binding energy of O 1s centered at ∼531.4 eV is oxygen-metal bond [21]. These results indicate the formation of pure-phase NiFe2O4 nanocrystals in Al2O3 film, which are in good agreement with the TEM observations. Because of the difference in thermal expansion between NiFe2O4 and Al2O3, external strain will be introduced during the formation and growth of NiFe2O4 nanocrystals in Al2O3 film through RTA process. The strain distribution of NiFe2O4/Al2O3 system is qualitatively simulated by using ANSYS commercial software package [18]. The Young's modulus of NiFe2O4 and Al2O3 are 175 GPa and 360 GPa, respectively, and the Poisson's ratios of NiFe2O4 and Al2O3 are 0.34 and 0.24, respectively. Fig. 3 (a) presents the simulation results of cross-sectional strain distribution of a NiFe2O4 nanocrystal grown in Al2O3 film. The XeY plane strain profile of a NiFe2O4 nanocrystal grown in Al2O3 film is shown in Fig. 3 (b). It can be found that the NiFe2O4 nanocrystal is subjected to compressive strain in Al2O3 film. This is in good agreement with the smaller lattice constants of NiFe2O4 nanocrystals obtained by TEM observations. In addition, it should be noted that the compressive strain on the surface of NiFe2O4 nanocrystal is much larger than that in the center of NiFe2O4 nanocrystal. Thus, the microstructure and morphology of the confined NiFe2O4 nanocrystal may be significantly influenced by the net deviatoric strain [18]. By producing disordered layers at the surfaces of NiFe2O4 nanocrystals, the compressive strain can be partially relaxed, leading to the enhanced surface effect with the formation of a large number of breaking exchange bonds between the oxyanions and the metallic atoms in the disordered layers [22]. The 3
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
Fig. 3. (a) Cross sectional strain distribution of a NiFe2O4 nanocrystal confined in Al2O3 film; (b) XeY plane strain profiles of a NiFe2O4 nanocrystal confined in Al2O3 film.
agitation at higher temperatures, which leads to the separation between ZFC curve and FC curve [26]. Fig. 5 (b) shows the M − H curves at 10 K and 300 K, respectively. The inset is the expanded view of the same M − H curves at 10 K in low magnetic field from −0.2 T to 0.2 T. With temperature increasing, the saturation magnetization decreases. The distinguished coercivity and the magnetic hysteresis loop at 10 K exhibit ferromagnetic behavior, while a s-like behavior but without hysteresis can be observed at 300 K. These results indicate that the NiFe2O4 nanocrystals have superparamagnetism above the TB. It is noteworthy that the hysteresis loop is distorted at 10 K. As discussed previously, during the growth process, the Al2O3 matrix exerts compressive strain on the NiFe2O4 nanocrystals. This compressive strain can be partially relaxed with the formation of disordered layers at the surfaces of the NiFe2O4 nanocrystals. The disordered layers at the surfaces of the NiFe2O4 nanocrystals are contributed by the breakage of exchange bonds between oxyanions and the metallic atoms, resulting in the reduction of atomic coordination at the surfaces. The spins of electrons at the surfaces of the strained NiFe2O4 nanocrystals are canted (or disordered) because of the reduced coordination number and spin-spin exchange at the surfaces [10]. Therefore, these anomalies in magnetic behavior can be explained by surface phenomena in the NiFe2O4/Al2O3 system [27]. Fortunately, the surface states of strained NiFe2O4 nanocrystals can provide an ideal platform for modulating magnetism optically. The PPMS magnetometer equipped with an optical fiber and light pipe was utilized to investigate the manipulated magnetic properties of strained NiFe2O4 nanocrystals under light irradiation with wavelength of 436 nm (∼0.8 mW/cm2). The magnetization curves under light irradiation are also shown in Fig. 5. After irradiation, it can be found that the temperature dependent magnetization of strained NiFe2O4 nanocrystals is much stronger. The TB increases to around 22 K. During the measurements, we also observed that the magnetization of NiFe2O4 nanocrystals dropped to its dark value when the light was turned off. Therefore, the experimental results clearly show that the change in magnetization is purely electronic and does not accompany any structural or chemical changes caused by light and/or heat. The enhancement of the ferromagnetism and TB is possibly correlated with the photo-generated electrons. Because light excitation can generate electrons, the lower coordination of surface atoms is improved, thus the surface spin disorder is partly overcome. Based on the photo-generated electrons, the degree of ferromagnetic spin-spin exchange at the surfaces of strained NiFe2O4 nanocrystals can be modulated under light irradiation.
Fig. 4. Raman spectra of NiFe2O4 bulk and NiFe2O4/Al2O3 film. The left inset is Raman intensity mapping image of A1g mode of the NiFe2O4/Al2O3 film. The right inset is the expanded view of the same Raman spectra of NiFe2O4 bulk in a range from 100 cm−1 to 900 cm−1.
Fig. 5. M–T curves in ZFC and FC modes (a) and M − H curves at 10 K and 300 K (b) for NiFe2O4 nanocrystals confined in Al2O3 film with and without light irradiation.
4. Conclusions In summary, in order to achieve superior magnetic performance, the modulation of magnetic properties of strained NiFe2O4 nanocrystals by 4
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
light irradiation are proposed. NiFe2O4 nanocrystals with inverse spinel structure confined in non-magnetic Al2O3 film are synthesized. Based on theoretical calculation and experimental validation, it is revealed that the NiFe2O4 nanocrystals experience compressive strain from Al2O3 film. The compressive strain can be partially relaxed, leading to the formation of disordered layers at the surfaces of the NiFe2O4 nanocrystals which provide an ideal platform to modulate magnetism optically. A remarkable light-modulated ferromagnetism in the strained NiFe2O4 nanocrystals is observed. This work demonstrates an effective approach to engineer the ferromagnetic properties optically for applications in next generation of multifunctional devices.
[10] R.H. Kodama, A.E. Berkowitz, E.J. McNiff, S. Foner, Surface spin disorder in NiFe2O4 nanoparticles, Phys. Rev. Lett. 77 (1996) 394–397. [11] M. Atif, M. Nadeem, M. Siddique, Cation distribution and enhanced surface effects on the temperature-dependent magnetization of as-prepared NiFe2O4 nanoparticles, Appl. Phys. A 120 (2015) 571–578. [12] K.M. Cai, M.Y. Yang, H.L. Ju, S.M. Wang, Y. Ji, B.H. Li, K.W. Edmonds, Y. Sheng, B. Zhang, N. Zhang, S. Liu, H.Z. Zheng, K.Y. Wang, Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ ferroelectric structure, Nat. Mater. 16 (2017) 712–716. [13] E.B. Myers, D.C. Ralph, J.A. Katine, R.N. Louie, R.A. Buhrman, Current-induced switching of domains in magnetic multilayer devices, Science 285 (1999) 867–870. [14] Y.H. Peng, G. Wang, C.L. Yuan, J. He, S.L. Ye, X.F. Luo, Influences of oxygen vacancies on the enhanced nonlinear optical properties of confined ZnO quantum dots, J. Alloy. Comp. 739 (2018) 345–352. [15] J. Mantilla, L. eÓn Félix, M.A. Rodriguez, F.H. Aragon, P.C. Morais, J.A.H. Coaquira, E. Kuzmann, A.C. de Oliveira, I. Gonzalez, W.A.A. Macedo, V.K. Garg, Washing effect on the structural and magnetic properties of NiFe2O4 nanoparticles synthesized by chemical sol-gel method, Mater. Chem. Phys. 213 (2018) 295–304. [16] K.C. Verma, V.P. Singh, M. Ram, J. Shah, R.K. Kotnala, Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films, J. Magn. Magn. Mater. 323 (2011) 3271–3275. [17] J.M. Hastings, L.M. Corliss, Neutron diffraction studies of zinc ferrite and nickel ferrite, Rev. Mod. Phys. 25 (1953) 114–121. [18] C.L. Yuan, Z.X. Jiang, S.L. Ye, Strain-induced matrix-dependent deformation of GaAs nanoparticles, Nanoscale 6 (2014) 1119–1123. [19] B.Y. Yu, S.Y. Kwak, Self-assembled mesoporous Co and Ni-ferrite spherical clusters consisting of spinel nanocrystals prepared using a template-free approach, Dalton Trans. 40 (2011) 9989–9998. [20] X. Zhang, Y.G. Niu, X.D. Meng, Y. Li, J.P. Zhao, Structural evolution and characteristics of the phase transformations between α-Fe2O3, Fe3O4 and γ-Fe2O3 nanoparticles under reducing and oxidizing atmospheres, CrystEngComm 15 (2013) 81663-88172. [21] I. Iatsunskyi, M. Kempinski, M. Jancelewicz, K. Zaleski, S. Jurga, V. Smyntyna, Structural and XPS characterization of ALD Al2O3 coated porous silicon, Vacuum 113 (2015) 52–58. [22] C.L. Yuan, Q. Liu, B. Xu, Strain-induced structural phase transition of Si nanoparticles, J. Phys. Chem. C 115 (2011) 16374–16377. [23] Y. Wang, L.P. Li, Y.L. Zhang, X.Q. Chen, S.F. Fang, G.S. Li, Growth kinetics, cation occupancy, and magnetic properties of multimetal oxide nanoparticles: a case study on spinel NiFe2O4, J. Phys. Chem. C 121 (2017) 19467–19477. [24] A. Ahlawat, V.G. Sathe, Raman study of NiFe2O4 nanoparticles, bulk and films: effect of laser power, J. Raman Spectrosc. 42 (2011) 1087–1094. [25] Y.A. Pusep, G. Zanelatto, S.W. da Silva, J.C. Galzerani, P.P. Gonzalez-Borrero, A.I. Toropov, P. Basmaji, Raman study of interface modes subjected to strain in InAs/GaAs self-assembled quantum dots, Phys. Rev. B 58 (1998) R1770–R1773. [26] W. Liu, J. Zuo, M. Yue, Z. Cui, D. Zhang, J. Zhang, P. Zhang, H. Ge, Z. Guo, W. Li, Structure and magnetic properties of bulk anisotropic SmCo5/alpha-Fe nanocomposite permanent magnets with different alpha-Fe content, J. Appl. Phys. 109 (2011) 07A741. [27] N.K. Sahu, D. Bahadura, Influence of excess Fe accumulation over the surface of FePt nanocrystals: structural and magnetic properties, Appl. Phys. Lett. 113 (2013) 134303.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51661012, 51761017, 51871115 and 51561012), the Excellent Youth Science Foundation of Jiangxi Province of China (Grant No. 20171BCB23033) and the Natural Science Foundation of Jiangxi Province of China (Grant No. 20181BAB206001). References [1] P.X. Liu, Y.M. Ren, W.J. Ma, J. Ma, Y.C. Du, Degradation of shale gas produced water by Magnetic porous MFe2O4 (M = Cu, Ni, Co and Zn) heterogeneous catalyzed ozone, Chem. Eng. J. 345 (2018) 98–106. [2] S.V. Bhosale, P.S. Ekambe, S.V. Bhoraskar, V.L. Mathe, Effect of surface properties of NiFe2O4 nanoparticles synthesized by dc thermal plasma route on anti microbial activity, Appl. Surf. Sci. 441 (2018) 724–733. [3] A. Tomitaka, T. Koshi, S. Hatsugai, T. Yamada, Y. Takemura, Magnetic characterization of surface-coated magnetic nanoparticles for biomedical application, J. Magn. Magn. Mater. 323 (2011) 1398–1403. [4] B. Bhuyan, B. Paul, A. Paul, S.S. Dhar, Paederia foetida Linn. Promoted synthesis of CoFe2O4 and NiFe2O4 nanostructures and their photocatalytic efficiency, IET Nanobiotechnol. 12 (2017) 235–240. [5] E.R. Kumar, P.S.P. Reddy, G.S. Devi, S. Sathiyaraj, Structural, dielectric and gas sensing behavior of Mn subsitituted spinel MFe2O4 (M=Zn, Cu, Ni, and Co) ferrite nanoparticles, J. Magn. Magn. Mater. 398 (2016) 281–288. [6] T. Ahmad, H. Bae, Y. Iqbal, I. Rhee, S. Hong, Y. Chang, D. Sohn, Chitosan-coated nickel-ferrite nanoparticles as contrast agents in magnetic resonance imaging, J. Magn. Magn. Mater. 381 (2015) 151–157. [7] P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy, C. Muthamizhchelvan, Synthesis and characterization of NiFe2O4 nanoparticles and nanorods, J. Alloy. Comp. 563 (2013) 6–11. [8] S. Larumbe, J.I. Pérez-Landazábal, J.M. Pastor, C. Gómez-Polo, Sol-gel NiFe2O4 nanoparticles: effect of the silica coating, J. Appl. Phys. Lett. 111 (2012) 103911. [9] C.L. Yuan, Y.X. Mei, T. Yu, Y. Yang, Q.L. li, A.J. Hong, K. Xu, X.F. Luo, J. He, W. Lei, Tuning strain and photoluminescence of confined Au nanoparticles by hydrogen passivation, RSC Adv. 7 (2017) 6875–6879.
5
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Light-modulated ferromagnetism of strained NiFe2O4 nanocrystals Hang Zhou, Zhiwei An, Cailei Yuan∗, Xingfang Luo∗∗ Jiangxi Key Laboratory of Nanomaterials and Sensors, School of Physics, Communication and Electronics, Jiangxi Normal University, 99 Ziyang Avenue, Nanchang, 330022, Jiangxi, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nanocrystals Strain Ferromagnetism Light irradiation
Light provides a potential approach to modulate magnetism due to the low energy consumption, which could be used for the development of optical-magnetic coupling devices, data storage technology and quantum computation technology. In this work, NiFe2O4 nanocrystals with inverse spinel structure confined in non-magnetic Al2O3 film are synthesized. Based on theoretical calculation and experimental validation, it is revealed that the NiFe2O4 nanocrystals experience compressive strain from Al2O3 film. The compressive strain can be partially relaxed, leading to the formation of disorder layers at the surfaces of strained NiFe2O4 nanocrystals, which provide an ideal platform to modulate magnetism optically. A remarkable light-modulated ferromagnetism in the strained NiFe2O4 nanocrystals is observed. This work demonstrates a novel approach to optically engineer the ferromagnetic properties for applications in next generation of magnetic nano-devices.
1. Introduction Ferromagnetic NiFe2O4 materials have been widely used in many fields, including magnetic high-density data storage [1], biomedical applications [2], magnetic drug delivery [3], catalysts [4], etc. In most of these applications, reduction to nanoscale-size can optimize properties of materials for certain technical purposes [5]. The magnetic properties of NiFe2O4 nanocrystals are extremely sensitive to their size, purity and magnetic stability [6,7]. In recent years, the synthesis of high quality NiFe2O4 nanocrystals to obtain the best magnetic performance has attracted great interest. A common problem in the preparation of magnetic NiFe2O4 nanocrystals is agglomeration, which is caused by the magnetic attraction between NiFe2O4 nanocrystals. Incorporation of NiFe2O4 nanocrystals into solid-state film is an ideal approach for practical magnetic nano-devices applications, which can produce non-agglomerated NiFe2O4 nanocrystals with superior magnetic behaviors. However, the introduction of surface coatings will induce the formation of disordered layers at the surfaces of the NiFe2O4 nanocrystals during growth process because of the thermal expansion mismatch between nanocrystals and surrounding coating materials [8,9]. The disordered layers are mainly contributed by the broken symmetries and exchange bonds at the surfaces of NiFe2O4 nanocrystals [10], which result in the suppression of the ferromagnetic performance of nanocrystals with respect to bulk behavior [11]. Therefore, the device applications of NiFe2O4 nanocrystals are greatly hindered.
∗
Moreover, for the application in modern information technology, it is essential to modulate the ferromagnetism with non-magnetic filed approaches. Apart from electric field and current [12,13], light provides a potential approach to modulate the magnetism due to the low energy consumption, which could be used for the development of optical-magnetic coupling devices, data storage technology and quantum computation technology. Fortunately, the surface states of strained NiFe2O4 nanocrystals may provide an ideal platform to modulate magnetism optically. Therefore, in this work, pure-phase NiFe2O4/ Al2O3 nanocrystals are synthesized. In order to achieve superior magnetic performance, the modulation of magnetic properties of NiFe2O4/ Al2O3 nanocrystals by light irradiation are proposed. A remarkable light-modulated ferromagnetism in the strained NiFe2O4 nanocrystals is observed. This work demonstrates a novel approach to optically engineer the ferromagnetic properties for applications in next generation of magnetic nano-devices. 2. Experimental NiFe2O4 nanocrystals were synthesized in Al2O3 film by pulsed laser ablation (PLA) and rapid thermal annealing (RTA). Briefly, a target was ablated by KrF pulsed laser (248 nm) in an ultra-high vacuum chamber with base pressure of 5 × 10−8 torr. The laser ablation target was prepared by using a round Al2O3 (99.99% purity) target with a diameter of 4 cm and a small NiFe2O4 (99.99% purity) square plate with a
Corresponding author. Corresponding author. E-mail addresses: [email protected] (C. Yuan), xfl[email protected] (X. Luo).
∗∗
https://doi.org/10.1016/j.ceramint.2019.04.022 Received 24 February 2019; Received in revised form 25 March 2019; Accepted 2 April 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Hang Zhou, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.04.022
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
As shown in Fig. 1 (d), the electron diffraction patterns of NiFe2O4 nanocrystals match well with the simulated NiFe2O4 electron diffraction patterns. It can be inferred that the crystal structure of NiFe2O4 nanocrystals is inverse spinel structure. Fig. 1 (e) also presents the schematic diagram of the NiFe2O4 nanocrystals unit cell, where the tetrahedral positions are completely occupied by Fe3+ and the octahedral positions are occupied by Ni2+ and Fe3+ ions [15]. Obviously, the NiFe2O4 nanocrystals grown in Al2O3 film will exhibit ferromagnetic behaviors because of the antiparallel spin between Fe3+ at tetrahedral site and Ni2+ at octahedral site [16]. More interesting, it should be noted that the lattice constants of the NiFe2O4 nanocrystals are about a = b = c = 8.1 Å, which are less than lattice constants of the NiFe2O4 bulk (a = b = c = 8.3 Å) [17]. The reduction of lattice constants of the NiFe2O4 nanocrystals can be attributed to the compressive strain existing in the NiFe2O4 nanocrystals grown in Al2O3 film [18]. As results, significant surface effect will be activated by the substantial compressive strain existing in NiFe2O4 nanocrystals. In order to fully understand the changes in microstructure of the NiFe2O4 nanocrystals, a high-resolution TEM image of single of NiFe2O4 nanocrystal is presented in Fig. 1 (f). Although this NiFe2O4 nanocrystal is single crystal, there is a very thin disorder layer formed at its surface region. The appearance disorder layer is mainly contributed by the broken symmetries and exchange bonds between the oxyanions and metallic atoms at the surface region of the NiFe2O4 nanocrystal [10], which may result in the suppression of ferromagnetic performance of NiFe2O4 nanocrystals with respect to NiFe2O4 bulk, and thus hinder the device applications of NiFe2O4 nanocrystals. The chemical composition and stoichiometry of NiFe2O4 nanocrystals in Al2O3 matrix are studied by XPS. Fig. 2 presents the high-resolution XPS and deconvolution spectra of NiFe2O4 nanocrystals grown in Al2O3 film, including Ni 2p, Fe 2p, Al 2p and O 1s peaks. The deconvolution peaks help to identify different cationic states. The Ni 2p high-resolution XPS spectrum of NiFe2O4 nanocrystals is shown in
length of 0.8 cm. The NiFe2O4 square plate was glued using chemically nonreactive adhesive material onto the surface of the Al2O3 target, creating an assembly of two composite materials, which were in physical, but not chemical contact. The deposited sample with thickness of about 300 nm was treated by RTA at 600 °C for 5 min in oxygen atmosphere. The structure of the sample was studied by JEOL 2010 transmission electron microscopy (TEM) operating at 200 kV. Kratos Axis Ultra DLD spectrometer was used to measure high-resolution X-ray photoelectron spectroscopy (XPS) with Al Ka radiation. The binding energies were analyzed with reference to C1s peak. Raman spectra were measured using HORIBA Scientific LabRAM HR Evolution system (excitation laser wavelength 514 nm). The magnetic properties of the sample were studied by using physical property measurement system (PPMS, Quantum Design) with the option of vibrating sample magnetometer. Using zero field cooling (ZFC) and field cooling (FC) modes, the temperature-dependent magnetization (M-T) of the sample was measured from 10 K to 300 K. The magnetic-field dependent magnetization (M − H) of the sample was measured at 10 K and 300 K, respectively.
3. Results and discussion Fig. 1 (a) shows the TEM image of the as-deposited film at lowmagnification. The corresponding electron diffraction patterns are presented in Fig. 1 (b). It clearly presents the voids of nanocrystals, which can be attributed that NiFe2O4 is not consumed or nucleated and the atomic dispersion of NiFe2O4 exists in amorphous Al2O3 film at room temperature. Fig. 1 (c) presents the low-magnification TEM image of NiFe2O4 nanocrystals grown in Al2O3 film after annealing at 600 °C. It can be seen that there are lots of NiFe2O4 nanocrystals with average size of about 10 nm formed in amorphous Al2O3 film. The simulated electron diffraction patterns of NiFe2O4 are generated by using the TEM simulator JAVA Electron Microscope Simulation (JEMS) software [14].
Fig. 1. (a) Low-magnification TEM image of as-deposited film; (b) electron diffraction pattern of asdeposited film; (c) low-magnification TEM image of NiFe2O4 nanocrystals grown in Al2O3 film; (d) electron diffraction pattern of NiFe2O4 nanocrystals grown in Al2O3 film; (e) a schematic diagram of NiFe2O4 nanocrystal unit cell; (f) high-resolution TEM image of a single NiFe2O4 nanocrystal grown in Al2O3 film.
2
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
Fig. 2. High-resolution XPS spectra of Ni 2p (a), Fe 2p (a), Al 2p (c) and O 1s (d) electrons from NiFe2O4 nanocrystals confined in Al2O3 film.
results are also consistent with the TEM observations in Fig. 1. In order to experimentally evaluate the compressive strain in NiFe2O4 nanocrystals, the Raman spectra of NiFe2O4 bulk and NiFe2O4/ Al2O3 film excited at a wavelength of 514 nm are shown in Fig. 4 for comparison. A Si peak at 520 cm−1 is used as a reference for calibration. The right inset is the expanded view of the same Raman spectrum of NiFe2O4 bulk in a range from 100 cm−1 to 900 cm−1. The Raman spectrum collected for bulk NiFe2O4 show five typical Raman bands of the inverse spinel structure of NiFe2O4, which matches well with those reported earlier [23,24]. In contrast, for the NiFe2O4 nanocrystals with spinel structure, only T2g (3) and A1g vibrational modes centered at about 574 cm−1 and 695 cm−1 are intense enough to be distinguished from the background noise, respectively. In comparison with the Raman peaks of bulk NiFe2O4, a distinguishable wavenumber shift (Δω = 12 cm−1) is observed in the NiFe2O4 nanocrystals. Usually, the wavenumber shift of Raman peak can be induced by the strain effect or/ and confinement effect. The compressive strain effect can lead to positive wavenumber shift, while the phonon confinement effect will result in negative wavenumber shift [25]. Thus, the positive value of Δω means that the compressive strain effect dominates the phonon confinement effect in these NiFe2O4 nanocrystals. The results furtherly demonstrate the TEM observations and FE calculations. The left inset of Fig. 4 displays the Raman intensity mapping image of A1g mode of the sample with confined NiFe2O4 nanocrystals. The image shows that the intensity distribution of the A1g peak is uniform. Since the peak position is significantly influenced by strain and confinement effects, the result furtherly demonstrates that NiFe2O4 nanocrystals with uniform size and strain distribution have been synthesized in Al2O3 film. To investigate the ferromagnetic properties of NiFe2O4 nanocrystals, the M-T curves were measured in ZFC and FC modes from 10 to 300 K, as shown in Fig. 5 (a). The inset is expanded view of the same M-T curves at low temperature range from 10 K to 40 K. The results show the overlapped ZFC and FC curves are separated at low temperature. It should be noted that the FC magnetization decreases with temperature increasing, while, the ZFC magnetization increases at first and then decreases with the temperature decreasing. The maximum value of ZFC curve (at about 17 K) is determined to be the blocking temperature (TB). The presence of superparamagnetism is therefore confirmed by the shape of FC and ZFC magnetization curves. At temperatures lower than TB, the magnetic moments of NiFe2O4 nanocrystals are confined along the local magnetic anisotropy axes, while being prone to the thermal
Fig. 2 (a). A Ni 2p3/2 signal appears at 856.0 eV with a satellite peak at 861.9 eV, and the peak value of 873.6 eV is attributed to the Ni 2p1/2 core level. Moreover, from Fig. 2 (a), an Auger electron peak of Ni (∼277.1 eV) can be found, which furtherly clarify that nickel is in the +2 oxidation state [19]. The high-resolution XPS spectrum of Fe 2p in NiFe2O4 nanocrystals is shown in Fig. 2 (b). The XPS peaks centered at ∼710.4 eV and ∼724.2 eV can be classified into Fe 2p3/2 and Fe 2p1/2 core levels, respectively. The main peaks of Fe 2p3/2 and Fe 2p1/2 are obviously accompanied by satellite structures on the side of higher binding energy (∼8 eV). From the overall spectral shape and binding energy positions of the Fe 2p core levels, it can be concluded that only Fe3+ species exist in the lattice, which matches well with those reported in the literature [20]. Fig. 2(c) and (d) show the Al 2p and O 1s spectra of NiFe2O4 nanocrystals confined in Al2O3 matrix, respectively. The binding energy of Al 2p is ∼74.3 eV, which corresponds to the chemical state of Al3+. The binding energy of O 1s centered at ∼531.4 eV is oxygen-metal bond [21]. These results indicate the formation of pure-phase NiFe2O4 nanocrystals in Al2O3 film, which are in good agreement with the TEM observations. Because of the difference in thermal expansion between NiFe2O4 and Al2O3, external strain will be introduced during the formation and growth of NiFe2O4 nanocrystals in Al2O3 film through RTA process. The strain distribution of NiFe2O4/Al2O3 system is qualitatively simulated by using ANSYS commercial software package [18]. The Young's modulus of NiFe2O4 and Al2O3 are 175 GPa and 360 GPa, respectively, and the Poisson's ratios of NiFe2O4 and Al2O3 are 0.34 and 0.24, respectively. Fig. 3 (a) presents the simulation results of cross-sectional strain distribution of a NiFe2O4 nanocrystal grown in Al2O3 film. The XeY plane strain profile of a NiFe2O4 nanocrystal grown in Al2O3 film is shown in Fig. 3 (b). It can be found that the NiFe2O4 nanocrystal is subjected to compressive strain in Al2O3 film. This is in good agreement with the smaller lattice constants of NiFe2O4 nanocrystals obtained by TEM observations. In addition, it should be noted that the compressive strain on the surface of NiFe2O4 nanocrystal is much larger than that in the center of NiFe2O4 nanocrystal. Thus, the microstructure and morphology of the confined NiFe2O4 nanocrystal may be significantly influenced by the net deviatoric strain [18]. By producing disordered layers at the surfaces of NiFe2O4 nanocrystals, the compressive strain can be partially relaxed, leading to the enhanced surface effect with the formation of a large number of breaking exchange bonds between the oxyanions and the metallic atoms in the disordered layers [22]. The 3
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
Fig. 3. (a) Cross sectional strain distribution of a NiFe2O4 nanocrystal confined in Al2O3 film; (b) XeY plane strain profiles of a NiFe2O4 nanocrystal confined in Al2O3 film.
agitation at higher temperatures, which leads to the separation between ZFC curve and FC curve [26]. Fig. 5 (b) shows the M − H curves at 10 K and 300 K, respectively. The inset is the expanded view of the same M − H curves at 10 K in low magnetic field from −0.2 T to 0.2 T. With temperature increasing, the saturation magnetization decreases. The distinguished coercivity and the magnetic hysteresis loop at 10 K exhibit ferromagnetic behavior, while a s-like behavior but without hysteresis can be observed at 300 K. These results indicate that the NiFe2O4 nanocrystals have superparamagnetism above the TB. It is noteworthy that the hysteresis loop is distorted at 10 K. As discussed previously, during the growth process, the Al2O3 matrix exerts compressive strain on the NiFe2O4 nanocrystals. This compressive strain can be partially relaxed with the formation of disordered layers at the surfaces of the NiFe2O4 nanocrystals. The disordered layers at the surfaces of the NiFe2O4 nanocrystals are contributed by the breakage of exchange bonds between oxyanions and the metallic atoms, resulting in the reduction of atomic coordination at the surfaces. The spins of electrons at the surfaces of the strained NiFe2O4 nanocrystals are canted (or disordered) because of the reduced coordination number and spin-spin exchange at the surfaces [10]. Therefore, these anomalies in magnetic behavior can be explained by surface phenomena in the NiFe2O4/Al2O3 system [27]. Fortunately, the surface states of strained NiFe2O4 nanocrystals can provide an ideal platform for modulating magnetism optically. The PPMS magnetometer equipped with an optical fiber and light pipe was utilized to investigate the manipulated magnetic properties of strained NiFe2O4 nanocrystals under light irradiation with wavelength of 436 nm (∼0.8 mW/cm2). The magnetization curves under light irradiation are also shown in Fig. 5. After irradiation, it can be found that the temperature dependent magnetization of strained NiFe2O4 nanocrystals is much stronger. The TB increases to around 22 K. During the measurements, we also observed that the magnetization of NiFe2O4 nanocrystals dropped to its dark value when the light was turned off. Therefore, the experimental results clearly show that the change in magnetization is purely electronic and does not accompany any structural or chemical changes caused by light and/or heat. The enhancement of the ferromagnetism and TB is possibly correlated with the photo-generated electrons. Because light excitation can generate electrons, the lower coordination of surface atoms is improved, thus the surface spin disorder is partly overcome. Based on the photo-generated electrons, the degree of ferromagnetic spin-spin exchange at the surfaces of strained NiFe2O4 nanocrystals can be modulated under light irradiation.
Fig. 4. Raman spectra of NiFe2O4 bulk and NiFe2O4/Al2O3 film. The left inset is Raman intensity mapping image of A1g mode of the NiFe2O4/Al2O3 film. The right inset is the expanded view of the same Raman spectra of NiFe2O4 bulk in a range from 100 cm−1 to 900 cm−1.
Fig. 5. M–T curves in ZFC and FC modes (a) and M − H curves at 10 K and 300 K (b) for NiFe2O4 nanocrystals confined in Al2O3 film with and without light irradiation.
4. Conclusions In summary, in order to achieve superior magnetic performance, the modulation of magnetic properties of strained NiFe2O4 nanocrystals by 4
Ceramics International xxx (xxxx) xxx–xxx
H. Zhou, et al.
light irradiation are proposed. NiFe2O4 nanocrystals with inverse spinel structure confined in non-magnetic Al2O3 film are synthesized. Based on theoretical calculation and experimental validation, it is revealed that the NiFe2O4 nanocrystals experience compressive strain from Al2O3 film. The compressive strain can be partially relaxed, leading to the formation of disordered layers at the surfaces of the NiFe2O4 nanocrystals which provide an ideal platform to modulate magnetism optically. A remarkable light-modulated ferromagnetism in the strained NiFe2O4 nanocrystals is observed. This work demonstrates an effective approach to engineer the ferromagnetic properties optically for applications in next generation of multifunctional devices.
[10] R.H. Kodama, A.E. Berkowitz, E.J. McNiff, S. Foner, Surface spin disorder in NiFe2O4 nanoparticles, Phys. Rev. Lett. 77 (1996) 394–397. [11] M. Atif, M. Nadeem, M. Siddique, Cation distribution and enhanced surface effects on the temperature-dependent magnetization of as-prepared NiFe2O4 nanoparticles, Appl. Phys. A 120 (2015) 571–578. [12] K.M. Cai, M.Y. Yang, H.L. Ju, S.M. Wang, Y. Ji, B.H. Li, K.W. Edmonds, Y. Sheng, B. Zhang, N. Zhang, S. Liu, H.Z. Zheng, K.Y. Wang, Electric field control of deterministic current-induced magnetization switching in a hybrid ferromagnetic/ ferroelectric structure, Nat. Mater. 16 (2017) 712–716. [13] E.B. Myers, D.C. Ralph, J.A. Katine, R.N. Louie, R.A. Buhrman, Current-induced switching of domains in magnetic multilayer devices, Science 285 (1999) 867–870. [14] Y.H. Peng, G. Wang, C.L. Yuan, J. He, S.L. Ye, X.F. Luo, Influences of oxygen vacancies on the enhanced nonlinear optical properties of confined ZnO quantum dots, J. Alloy. Comp. 739 (2018) 345–352. [15] J. Mantilla, L. eÓn Félix, M.A. Rodriguez, F.H. Aragon, P.C. Morais, J.A.H. Coaquira, E. Kuzmann, A.C. de Oliveira, I. Gonzalez, W.A.A. Macedo, V.K. Garg, Washing effect on the structural and magnetic properties of NiFe2O4 nanoparticles synthesized by chemical sol-gel method, Mater. Chem. Phys. 213 (2018) 295–304. [16] K.C. Verma, V.P. Singh, M. Ram, J. Shah, R.K. Kotnala, Structural, microstructural and magnetic properties of NiFe2O4, CoFe2O4 and MnFe2O4 nanoferrite thin films, J. Magn. Magn. Mater. 323 (2011) 3271–3275. [17] J.M. Hastings, L.M. Corliss, Neutron diffraction studies of zinc ferrite and nickel ferrite, Rev. Mod. Phys. 25 (1953) 114–121. [18] C.L. Yuan, Z.X. Jiang, S.L. Ye, Strain-induced matrix-dependent deformation of GaAs nanoparticles, Nanoscale 6 (2014) 1119–1123. [19] B.Y. Yu, S.Y. Kwak, Self-assembled mesoporous Co and Ni-ferrite spherical clusters consisting of spinel nanocrystals prepared using a template-free approach, Dalton Trans. 40 (2011) 9989–9998. [20] X. Zhang, Y.G. Niu, X.D. Meng, Y. Li, J.P. Zhao, Structural evolution and characteristics of the phase transformations between α-Fe2O3, Fe3O4 and γ-Fe2O3 nanoparticles under reducing and oxidizing atmospheres, CrystEngComm 15 (2013) 81663-88172. [21] I. Iatsunskyi, M. Kempinski, M. Jancelewicz, K. Zaleski, S. Jurga, V. Smyntyna, Structural and XPS characterization of ALD Al2O3 coated porous silicon, Vacuum 113 (2015) 52–58. [22] C.L. Yuan, Q. Liu, B. Xu, Strain-induced structural phase transition of Si nanoparticles, J. Phys. Chem. C 115 (2011) 16374–16377. [23] Y. Wang, L.P. Li, Y.L. Zhang, X.Q. Chen, S.F. Fang, G.S. Li, Growth kinetics, cation occupancy, and magnetic properties of multimetal oxide nanoparticles: a case study on spinel NiFe2O4, J. Phys. Chem. C 121 (2017) 19467–19477. [24] A. Ahlawat, V.G. Sathe, Raman study of NiFe2O4 nanoparticles, bulk and films: effect of laser power, J. Raman Spectrosc. 42 (2011) 1087–1094. [25] Y.A. Pusep, G. Zanelatto, S.W. da Silva, J.C. Galzerani, P.P. Gonzalez-Borrero, A.I. Toropov, P. Basmaji, Raman study of interface modes subjected to strain in InAs/GaAs self-assembled quantum dots, Phys. Rev. B 58 (1998) R1770–R1773. [26] W. Liu, J. Zuo, M. Yue, Z. Cui, D. Zhang, J. Zhang, P. Zhang, H. Ge, Z. Guo, W. Li, Structure and magnetic properties of bulk anisotropic SmCo5/alpha-Fe nanocomposite permanent magnets with different alpha-Fe content, J. Appl. Phys. 109 (2011) 07A741. [27] N.K. Sahu, D. Bahadura, Influence of excess Fe accumulation over the surface of FePt nanocrystals: structural and magnetic properties, Appl. Phys. Lett. 113 (2013) 134303.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51661012, 51761017, 51871115 and 51561012), the Excellent Youth Science Foundation of Jiangxi Province of China (Grant No. 20171BCB23033) and the Natural Science Foundation of Jiangxi Province of China (Grant No. 20181BAB206001). References [1] P.X. Liu, Y.M. Ren, W.J. Ma, J. Ma, Y.C. Du, Degradation of shale gas produced water by Magnetic porous MFe2O4 (M = Cu, Ni, Co and Zn) heterogeneous catalyzed ozone, Chem. Eng. J. 345 (2018) 98–106. [2] S.V. Bhosale, P.S. Ekambe, S.V. Bhoraskar, V.L. Mathe, Effect of surface properties of NiFe2O4 nanoparticles synthesized by dc thermal plasma route on anti microbial activity, Appl. Surf. Sci. 441 (2018) 724–733. [3] A. Tomitaka, T. Koshi, S. Hatsugai, T. Yamada, Y. Takemura, Magnetic characterization of surface-coated magnetic nanoparticles for biomedical application, J. Magn. Magn. Mater. 323 (2011) 1398–1403. [4] B. Bhuyan, B. Paul, A. Paul, S.S. Dhar, Paederia foetida Linn. Promoted synthesis of CoFe2O4 and NiFe2O4 nanostructures and their photocatalytic efficiency, IET Nanobiotechnol. 12 (2017) 235–240. [5] E.R. Kumar, P.S.P. Reddy, G.S. Devi, S. Sathiyaraj, Structural, dielectric and gas sensing behavior of Mn subsitituted spinel MFe2O4 (M=Zn, Cu, Ni, and Co) ferrite nanoparticles, J. Magn. Magn. Mater. 398 (2016) 281–288. [6] T. Ahmad, H. Bae, Y. Iqbal, I. Rhee, S. Hong, Y. Chang, D. Sohn, Chitosan-coated nickel-ferrite nanoparticles as contrast agents in magnetic resonance imaging, J. Magn. Magn. Mater. 381 (2015) 151–157. [7] P. Sivakumar, R. Ramesh, A. Ramanand, S. Ponnusamy, C. Muthamizhchelvan, Synthesis and characterization of NiFe2O4 nanoparticles and nanorods, J. Alloy. Comp. 563 (2013) 6–11. [8] S. Larumbe, J.I. Pérez-Landazábal, J.M. Pastor, C. Gómez-Polo, Sol-gel NiFe2O4 nanoparticles: effect of the silica coating, J. Appl. Phys. Lett. 111 (2012) 103911. [9] C.L. Yuan, Y.X. Mei, T. Yu, Y. Yang, Q.L. li, A.J. Hong, K. Xu, X.F. Luo, J. He, W. Lei, Tuning strain and photoluminescence of confined Au nanoparticles by hydrogen passivation, RSC Adv. 7 (2017) 6875–6879.
5