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Large coercivity and exchange bias in Mn3O4 nanoparticles prepared by laser ablation method
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Large coercivity and exchange bias in Mn3O4 nanoparticles prepared by laser ablation method
T
Y.T. Yanga,⁎, P.Z. Sia, C.J. Choib, H.L. Gea,⁎ a b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China Powder & Ceramic Division, Korea Institute of Materials Science, Changwon 51508, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn3O4 Large coercivity Exchange bias Laser ablation Nanoparticles
A facile and easily reproducible laser ablation method has been developed to synthesize high quality Mn3O4 nanoparticles from Mn pieces in ambient air. By changing the laser ablation air pressure to ~20 Pa, a mixture of α-Mn and Mn3O4 composite nanoparticles were obtained. Most of the Mn3O4 nanoparticles are spherical in shape size ranging from 20 to 90 nm. Besides spherical shape, a considerable amount of α-Mn/Mn3O4 nanoparticles show irregular shape and aggregation. The oxide nanoparticles exhibit large coercivity up to 1.05 T, which is almost four times to that of the Mn3O4 single crystals, owing to size effect. The coercivity was further enhanced to 1.17 T in α-Mn/Mn3O4 nanoparticles, owing to the additional interfacial effect between α-Mn and Mn3O4 besides size effect. The presence of the antiferromagnetic/ferrimagnetic α-Mn/Mn3O4 interface results in an exchange bias field up to ~65 mT in the nanoparticles.
1. Introduction The hausmannite (Mn3O4) nanostructures have recently attracted broad research interests for its potential applications in gas-sensing [1], lithium/sodium-ion batteries [2], electrocatalysts [3], and electrochemical supercapacitors etc. [4] The magnetic properties of Mn3O4 have attracted great research interests for a long time because of its complex magnetic structures and size-dependent behaviors [5,6]. The magnetic properties of bulk Mn3O4, having spinel structure with Mn2+ at the tetrahedral sites and Mn3+ at the octahedral sites, were studied systematically in the middle of last century [5]. Usually, the physical properties of Mn3O4 can be greatly affected with the reduction of their particle size. A significantly enhanced coercivity and a reduced saturation magnetization were observed in antiferromagnetic Mn nanoparticles and oxide-coated Mn nanoparticles prepared by arc discharge method, owing to the presence of a small amount of ferrimagnetic Mn3O4 phase in the samples [7,8]. Nanoscale Mn3O4 has since attracted intensive research interests from both experimental and theoretical points of views in the past decade [9–21]. Most of these nanoparticles containing Mn3O4 phase were prepared by chemical solution method. Sicard et al prepared Mn3O4 nanocrystals by chemical method using manganese acetate tetrahydrate and diethylene glycol as precursors [11]. Regmi et al. synthesized Mn3O4 nanoparticles by a co-precipitation method using MnCl2, HCl, water, and
⁎
NH4OH [12]. Mn3O4/MnO shell/core structured nanoparticles were prepared by using chemical method from manganese (II) acetylacetonate, and thermal decomposition of manganese oleate, respectively. [13–15] Wang et al prepared Mn3O4 nanopatterns by using a typical reaction between KOH-C2H5OH and manganese acetate dissolved in ethanol [16]. Single-crystalline Mn3O4 nanowires were synthesized by the low-temperature solvothermal technique using MnCl2·4H2O dissolved in ethanol [17]. Mn/Mn3O4 nanocomposites were synthesized from the reaction of KMnO4 and glycerol solutions [18]. In this work, the laser ablation method was employed for one-step facile syntheses of Mn3O4 nanoparticles and α-Mn/Mn3O4 nanoparticles under varied air pressures, respectively. Different methods for synthesizing Mn3O4 nanostructures usually result in different size and morphology of the product, which will further result in different magnetic properties. A coercivity up to 0.82 T (5 K) has been observed in Mn3O4 nanocrystals prepared from manganese acetate tetrahydrate [11]. Single-crystalline Mn3O4 nanowires prepared from MnCl2·4H2O show a coercivity of 0.875 T at 5 K [17]. The inverted core-shell MnO/Mn3O4 nanoparticles prepared from manganese(II) acetylacetonate show a coercivity of ~0.8 T [19]. A coercivity up to 1.1 T has been reported in the oxide-coated manganese nanoparticles prepared by arc discharge and passivation process [7]. In this work, the coercivities up to 1.05 T and 1.17 T were found in Mn3O4 nanoparticles and α-Mn/Mn3O4 nanoparticles prepared by laser
Corresponding authors. China Jiliang University, Hangzhou 310018, China. E-mail addresses: [email protected] (Y.T. Yang), [email protected] (H.L. Ge).
https://doi.org/10.1016/j.jmmm.2019.165481 Received 13 May 2019; Received in revised form 18 June 2019; Accepted 20 June 2019 Available online 21 June 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Y.T. Yang, et al.
Fig. 1. The XRD patterns of the nanoparticles prepared by laser ablation of Mn under (a) ambient air pressure and (b) ~20 Pa could be indexed with (a) Mn3O4; and (b) α-Mn + Mn3O4, respectively.
Fig. 3. The normalized M−T curves of the Mn3O4 nanoparticles prepared by laser ablation of Mn in ambient air. The curves were measured by cooling the sample from room temperature under 0 T (black curve) and then heating the sample under 0.05 T (red curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ablation process.
deposited on the chamber wall were collected in air for subsequent characterizations. The distance from the ablation spot to the chamber wall was estimated to be in the range of 6–10 cm. The X-ray diffraction (XRD) patterns of the samples were collected at room temperature using a Rigaku D/Max 2500 automatic diffractometer operating at 40 kV and 100 mA with Cu-Kα radiation, a Ni filter, a scintillation counter and an angular step of 0.02°. The morphology and high-resolution images of the nanoparticles were observed by using a JEM2100 transmission electron microscopy (TEM). The magnetic properties
2. Experimental The commercial manganese (purity 99.95%) pieces were first loaded into a chamber with a quartz window, through which the laser beams (wavelength ~1064 nm) were focused on the surface of the Mn pieces. The laser ablation process was conducted in ambient pressure and air with a gas pressure of ~20 Pa, respectively. The 20 Pa pressure was generated by using a rotary vane vacuum pump and the pressure was measured by using a Pirani vacuum gauge. The nanoparticles
Fig. 2. The typical morphology and high-resolution images of the nanoparticles prepared by laser ablation of Mn in air with (a and b) ambient pressure and (c and d) 20 Pa. 2
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Y.T. Yang, et al.
exclude the presence of α-Mn phase in the Mn3O4 nanoparticles because of the limited sensitivity of XRD. Usually, it is difficult to detect phases of trace amount and/or of tiny crystalline size by XRD. In comparison with the complicated process of the arc discharge process and the chemical solution method as mentioned above, the laser ablation method is much more simplified for preparation of Mn3O4 nanoparticles. The broadening of the XRD peaks was ascribed to the tiny crystalline size of both Mn3O4 and α-Mn phases as confirmed by the TEM observations. Fig. 2 shows the morphology and high-resolution images of the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles prepared by the laser ablation method. Most of the Mn3O4 nanoparticles are spherical in shape with a narrow size distribution ranging from 20 nm to about 90 nm, as shown in Fig. 2(a). Fig. 2(b) shows the high-resolution image of a Mn3O4 nanoparticle, which is well-crystalized within the nanoparticle. However, we could also observe the disordered arrangement of the surface atoms. Both spherical and irregular shaped nanoparticles were observed in the α-Mn/Mn3O4 sample, as shown Fig. 2(c), in which aggregated arrays of the α-Mn/Mn3O4 nanoparticles are frequently observed. The more active surface atoms due to in-sufficient oxidation under low air pressures may result in aggregation of the nanoparticles to minimize the surface energy. The size of the α-Mn/ Mn3O4 nanoparticles is similar to that of the Mn3O4 nanoparticles. Both the surface and the core of the α-Mn/Mn3O4 nanoparticles show atomic disordered regions, as shown in Fig. 2(d). The high temperature laser beams may evaporate or sublimate the bulk Mn at a very high rate and the ejected Mn nanoparticles would react with oxygen in air spontaneously. The rough surface, irregular shape, and presence of secondary phase in the nanoparticles result in a large surface/interface area of the nanoparticles and possibly the presence of uncompensated surface/interface spins of the surface/interface atoms. The oxidation of metallic nanoparticles usually occurs from the surface of the nanoparticles. It is reasonable for us to assume that the α-Mn phase may present inside the particles. The temperature dependence of magnetization of the Mn3O4 nanoparticles is shown in Fig. 3. The M−T curves of the α-Mn/Mn3O4 nanoparticles are not shown here because it is almost the same to that of the Mn3O4 nanoparticles. The spontaneous magnetization of the Mn3O4 nanoparticles increases dramatically at temperatures below 42 K and reaches saturation at 30 K. The Curie temperature (Tc) of the samples was thus determined to be 42 K, which is the same to that of the bulk Mn3O4 [5]. Although the Tc of the Mn3O4 nanoparticles was reported to be size dependent in some reports [20,21], we did not observe obvious deviation of the Tc of the Mn3O4 nanoparticles from that of the bulk. One possible reason might be that the size (20–90 nm) of our Mn3O4 nanoparticles is still too large to affect the Tc. The zero-field cooling curve of the samples at 0.05 T, measured with increasing temperature, also shows a peak magnetization at 42 K. The increasing M with increasing T as seen in the ZFC curve was ascribed to superparamagnetic behaviors of Mn3O4 nanoparticles at temperatures below Tc. Superparamagnetism has been frequently observed in magnetic nanoparticles [8]. At low temperatures, the magnetization of each randomly oriented magnetic nanoparticle is blocked because of anisotropy. With increasing temperature, the thermal energy overcomes the energy barrier due to anisotropy of some small nanoparticles, which show superparamagnetic behaviors and thus the M of the sample increases with increasing T. A higher enough temperature (above Tc) would result in a paramagnetic state of the sample and thus a decrease of M with increasing T. The magnetic hysteresis loops of the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles measured in the temperature range of 5–35 K are shown in Fig. 4. The coercivity of the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles at 5 K reached up to 1.05 T and 1.17 T, respectively. For comparison, the coercivity of the single crystals of Mn3O4 at 4.2 K is 0.265 T while the typical coercivity of Mn3O4 nanoparticles prepared from manganese acetate tetrahydrate by
Fig. 4. The magnetic hysteresis loops of the nanoparticles prepared by laser ablation of Mn in air with (a) ambient pressure and (b) 20 Pa, respectively.
Table 1 The magnetic properties of the nanoparticle prepared by laser ablation of Mn in ambient air pressure and 20 Pa (parameters in brackets), respectively. T/K
Hc/T
5 15 25 35
1.05 0.65 0.49 0.32
(1.17) (0.77) (0.59) (0.35)
Ms/Am2kg−1
Mr/Am2kg−1
6.96 6.88 6.66 6.19
4.5 4.1 3.8 3.3
(18.5) (18) (17.2) (16)
(11.5) (10.5) (9.6) (8.5)
of the samples were measured by using a Quantum Design physical property measurement system equipped with a VSM. The magnetic moment of the system is calibrated by using a standard Ni sample. The weight of the sample is measured by using a high-precision balance. The M−T curves were measured in the temperature range of 5–100 K while the M−H curves of the samples were measured at 5 K, 15 K, 25 K, and 35 K, respectively. 3. Results and discussion As shown in Fig. 1(a), the XRD patterns of the nanoparticles prepared by laser ablation of Mn in ambient pressure can be indexed merely with Mn3O4. However, the XRD patterns of the nanoparticles prepared under 20 Pa air could be indexed with Mn3O4 as the major phase and α-Mn as the minor phase, indicating incomplete oxidation of Mn at lower air pressures. The α-Mn is the most stable polymorph of Mn below 1000 K, which has an exotic crystal structure containing 58 atoms in a cubic unit cell. It seems the ratio of α-Mn to Mn3O4 might be dependent on the air pressure. It should be noted that we could not 3
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Y.T. Yang, et al.
Appendix A. Supplementary data
chemical synthesis is ~0.82 T or less [11]. We ascribe the large coercivity of 1.05 T in the Mn3O4 nanoparticles to the small crystalline size of Mn3O4 phase. The further enhanced coercivity (1.17 T) in the α-Mn/ Mn3O4 nanoparticles was ascribed to the interfacial exchange coupling between antiferromagnetic α-Mn and ferrimagnetic Mn3O4 phase. And this has been proved by the shift of the magnetic hysteresis loops, which indicate the presence of exchange bias effect. An exchange bias field up to ~65 mT was observed in the α-Mn/Mn3O4 nanoparticles. The exchange bias effect was first observed by Meiklejohn and Bean in fine particles of ferromagnetic (FM) cobalt with antiferromagnetic (AFM) cobaltous oxide shell in 1956 [22]. In this work, the exchange bias effect may originate mainly from interfacial exchange coupling between antiferromagnetic α-Mn and ferrimagnetic Mn3O4. The α-Mn undergoes a paramagnetic to antiferromagnetic phase transition at its Néel temperature TN ~ 100 K. It seems the coercivity of the laser ablated Mn3O4 nanoparticles is usually larger than that reported in the Mn3O4 nanoparticles produced by chemical solution method. The laser ablation process is a high temperature process while the chemical solution method is room temperature or low temperature method. We speculate that the high rate temperature changes of the sample during laser ablation process may result in more defects and strains in the nanoparticles, and thus a higher coercivity. It is interesting that a similar exchange bias field was also observed in the Mn3O4 nanoparticles, even though the antiferromagnetic α-Mn phase in this sample was not detected by XRD. We speculate that α-Mn phase might also present in the Mn3O4 nanoparticles. As mentioned above, the small crystalline size and/or small fraction of the α-Mn phase make it difficult to be detected by XRD. The disordered/uncompensated surface spins of the nanoparticles may also induce an exchange bias effect. In fact, the exchange bias effect originating from disordered surface spins has been observed in small ferrite nanoparticles [23]. The Hc of both samples decreases with increasing temperature, as shown in Fig. 4 and Table 1. The saturation magnetization (Ms) of the Mn3O4 nanoparticles at 5 K and under an applied field of 5 T is 6.96 Am2/kg, which is approximately 15% of that of the Mn3O4 single crystals [5]. A magnetization reduction is frequently observed in most ferromagnetic nanoparticles because of the large surface area, resulting in a large fraction of surface atoms with uncompensated/disordered/canted spins. It is interesting that the α-Mn/Mn3O4 nanoparticles at 5 K and 5 T exhibits a Ms of 18.5 Am2/kg, which is higher than that of the Mn3O4 nanoparticles for unknown reason. Both Ms and remanent magnetization of the two samples decrease slightly with increasing temperature, as shown in Table 1.
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165481. References [1] S.X. Cao, T. Han, L.L. Peng, B.T. Liu, Hydrothermal preparation, formation mechanism and gas-sensing properties of novel Mn3O4 nano-octahedrons, Mater. Lett. 246 (2019) 210–213, https://doi.org/10.1016/j.matlet.2019.02.127. [2] B.B. Wang, F. Li, X.J. Wang, G. Wang, H. Wang, J.T. Bai, Mn3O4 nanotubes encapsulated by porous graphene sheets with enhanced electrochemical properties for lithium/sodium-ion batteries, Chem. Eng. J. 364 (2019) 57–69, https://doi.org/10. 1016/j.cej.2019.01.155. [3] M.P. Araujo, M. Nunes, I.M. Rocha, M.F.R. Pereira, C. Freire, Electrocatalytic activity of new Mn3O4@oxidized graphene flakes nanocomposites toward oxygen reduction reaction, J. Mater. Sci. 54 (2019) 8919–8940, https://doi.org/10.1007/ s10853-019-03508-6. [4] K.T. Kubra, A. Javaid, B. Patil, R. Sharif, A. Salman, S. Shahzadi, S. Siddique, S. Ghani, Synthesis and characterization of novel Pr6O11/Mn3O4 nanocomposites for electrochemical supercapacitors, Ceram. Int. 45 (2019) 6819–6827, https://doi. org/10.1016/j.ceramint.2018.12.175. [5] K. Dwight, N. Menyuk, Magnetic properties of Mn3O4 and canted spin problem, Phys. Rev. 119 (1960) 1470–1479, https://doi.org/10.1103/PhysRev.119.1470. [6] T. Tajiri, K. Sakai, H. Deguchi, M. Mito, A. Kohno, Size effects on magnetic property and crystal structure of Mn3O4 nanoparticles in mesoporous silica, IEEE Tran. Magn. 55 (2019) 2300204, https://doi.org/10.1109/TMAG.2018.2859950. [7] P.Z. Si, D. Li, J.W. Lee, C.J. Choi, Z.D. Zhang, D.Y. Geng, E. Brück, Unconventional exchange bias in oxide-coated manganese nanoparticles, Appl. Phys. Lett. 87 (2005) 133122, https://doi.org/10.1063/1.2072807. [8] P.Z. Si, E. Brück, Z.D. Zhang, O. Tegus, W.S. Zhang, K.H.J. Buschow, J.C.P. Klaasse, Structural and magnetic properties of Mn nanoparticles prepared by arc-discharge, Mater. Res. 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López-Ortega, D. Tobia, E. Winkler, I.V. Golosovsky, G. Salazar-Alvarez, Sónia Estradé, Size-dependent passivation shell and magnetic properties in Antiferromagnetic/Ferrimagnetic core/shell MnO nanoparticles, J. Am. Chem. Soc. 132 (2010) 9398–9407, https://doi.org/10.1021/ja1021798. [14] A. Omelianchik, G. Singh, B.H. McDonagh, V. Rodionova, D. Fiorani, D. Peddis, S. Laureti, From Mn3O4/MnO core-shell nanoparticles to hollow MnO: evolution of magnetic properties, Nanotechnology 29 (2018) 55703, https://doi.org/10.1088/ 1361-6528/aa9e59. [15] A.E. Berkowitz, G.F. Rodriguez, J.I. Hong, K. An, T. Hyeon, N. Agarwal, D.J. Smith, E.E. Fullerton, Antiferromagnetic MnO nanoparticles with ferrimagnetic Mn3O4 shells: doubly inverted core-shell system, Phys. Rev. B 77 (2008) 24403, https:// doi.org/10.1103/PhysRevB.77.024403. [16] N. Wang, L. Guo, L. He, X. Cao, C. Chen, R. Wang, S. Yang, Facile synthesis of monodisperse Mn3O4 tetragonal nanoparticles and their large-scale assembly into highly regular walls by a simple solution route, Small 3 (2007) 606–610, https:// doi.org/10.1002/smll.200600283. [17] S. Sambasivam, G.J. Li, J.H. Jeong, B.C. Choi, K.T. Lim, S.S. Kim, T.K. Song, Structural, optical, and magnetic properties of single-crystalline Mn3O4 nanowires, J. Nanopart. Res. 14 (2012) 1138, https://doi.org/10.1007/s11051-012-1138-4. [18] A.K.M. Atique Ullah, A. Hossain, M. Akter, M.F. Kabir, M.N.I. Khan, A.K.M. Fazle Kibria, S.H. Firoz, Room temperature ferromagnetic behavior of manganese/manganese oxides nanocomposites, Mater. Lett 238 (2019) 151–154, https://doi.org/ 10.1016/j.matlet.2018.12.003. [19] G. Salazar-Alvarez, J. Sort, S. Surińach, M.D. Baró, J. Nogués, Synthesis and sizedependent exchange bias in inverted core-shell MnO Mn3O4 nanoparticles, J. Am. Chem. Soc. 129 (2007) 9102–9108, https://doi.org/10.1021/ja0714282. [20] N. Zhao, W. Nie, X. Liu, S. Tian, Y. Zhang, X. Ji, Shape- and size-controlled synthesis and dependent magnetic properties of nearly monodisperse Mn3O4 nanocrystals, Small 4 (2008) 77–81, https://doi.org/10.1002/smll.200700698. [21] L. Narayani, V.J. Angadi, A. Sukhdev, M. Challa, S. Matteppanavar, P.R. Deepthi, P.M. Kumar, M. Pasha, Mechanism of high temperature induced phase transformation and magnetic properties of Mn3O4 crystallites, J. Magn. Magn. Mater. 476 (2019) 268–273, https://doi.org/10.1016/j.jmmm.2018.12.072. [22] W.P. Meiklejohn, C.P. Bean, New magnetic anisotropy, Phys. Rev. 102 (1956) 1413, https://doi.org/10.1103/PhysRev.102.1413. [23] M.H. Phan, J. Alonso, H. Khurshid, P. Lampen-Kelley, S. Chandra, K.S. Repa, Z. Nemati, R. Das, Ó. Iglesias, H. Srikanth, Exchange bias effects in iron oxide-based nanoparticle systems, Nanomaterials 6 (2016) 221, https://doi.org/10.3390/ nano6110221.
4. Conclusions The Mn3O4 nanoparticles and the α-Mn/Mn3O4 composite nanoparticles were prepared by laser ablation of Mn in air with a gas pressure of 1 atm and 20 Pa, respectively. The structure and magnetic properties of these nanoparticles were studied. The large coercivities up to 1.05 T and 1.17 T were observed in the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles, respectively. We ascribe the large coercivity to the tiny crystalline size of Mn3O4 and interfacial exchange coupling between α-Mn and Mn3O4. An exchange bias effect was also observed in the nanoparticles, owing to the presence of antiferromagnetic α-Mn and ferrimagnetic Mn3O4 phase. Acknowledgments We acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 51671177, 11074227), and the Future Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2016M3D1A1027835).
4
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Large coercivity and exchange bias in Mn3O4 nanoparticles prepared by laser ablation method
T
Y.T. Yanga,⁎, P.Z. Sia, C.J. Choib, H.L. Gea,⁎ a b
College of Materials Science and Engineering, China Jiliang University, Hangzhou 310018, China Powder & Ceramic Division, Korea Institute of Materials Science, Changwon 51508, Republic of Korea
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn3O4 Large coercivity Exchange bias Laser ablation Nanoparticles
A facile and easily reproducible laser ablation method has been developed to synthesize high quality Mn3O4 nanoparticles from Mn pieces in ambient air. By changing the laser ablation air pressure to ~20 Pa, a mixture of α-Mn and Mn3O4 composite nanoparticles were obtained. Most of the Mn3O4 nanoparticles are spherical in shape size ranging from 20 to 90 nm. Besides spherical shape, a considerable amount of α-Mn/Mn3O4 nanoparticles show irregular shape and aggregation. The oxide nanoparticles exhibit large coercivity up to 1.05 T, which is almost four times to that of the Mn3O4 single crystals, owing to size effect. The coercivity was further enhanced to 1.17 T in α-Mn/Mn3O4 nanoparticles, owing to the additional interfacial effect between α-Mn and Mn3O4 besides size effect. The presence of the antiferromagnetic/ferrimagnetic α-Mn/Mn3O4 interface results in an exchange bias field up to ~65 mT in the nanoparticles.
1. Introduction The hausmannite (Mn3O4) nanostructures have recently attracted broad research interests for its potential applications in gas-sensing [1], lithium/sodium-ion batteries [2], electrocatalysts [3], and electrochemical supercapacitors etc. [4] The magnetic properties of Mn3O4 have attracted great research interests for a long time because of its complex magnetic structures and size-dependent behaviors [5,6]. The magnetic properties of bulk Mn3O4, having spinel structure with Mn2+ at the tetrahedral sites and Mn3+ at the octahedral sites, were studied systematically in the middle of last century [5]. Usually, the physical properties of Mn3O4 can be greatly affected with the reduction of their particle size. A significantly enhanced coercivity and a reduced saturation magnetization were observed in antiferromagnetic Mn nanoparticles and oxide-coated Mn nanoparticles prepared by arc discharge method, owing to the presence of a small amount of ferrimagnetic Mn3O4 phase in the samples [7,8]. Nanoscale Mn3O4 has since attracted intensive research interests from both experimental and theoretical points of views in the past decade [9–21]. Most of these nanoparticles containing Mn3O4 phase were prepared by chemical solution method. Sicard et al prepared Mn3O4 nanocrystals by chemical method using manganese acetate tetrahydrate and diethylene glycol as precursors [11]. Regmi et al. synthesized Mn3O4 nanoparticles by a co-precipitation method using MnCl2, HCl, water, and
⁎
NH4OH [12]. Mn3O4/MnO shell/core structured nanoparticles were prepared by using chemical method from manganese (II) acetylacetonate, and thermal decomposition of manganese oleate, respectively. [13–15] Wang et al prepared Mn3O4 nanopatterns by using a typical reaction between KOH-C2H5OH and manganese acetate dissolved in ethanol [16]. Single-crystalline Mn3O4 nanowires were synthesized by the low-temperature solvothermal technique using MnCl2·4H2O dissolved in ethanol [17]. Mn/Mn3O4 nanocomposites were synthesized from the reaction of KMnO4 and glycerol solutions [18]. In this work, the laser ablation method was employed for one-step facile syntheses of Mn3O4 nanoparticles and α-Mn/Mn3O4 nanoparticles under varied air pressures, respectively. Different methods for synthesizing Mn3O4 nanostructures usually result in different size and morphology of the product, which will further result in different magnetic properties. A coercivity up to 0.82 T (5 K) has been observed in Mn3O4 nanocrystals prepared from manganese acetate tetrahydrate [11]. Single-crystalline Mn3O4 nanowires prepared from MnCl2·4H2O show a coercivity of 0.875 T at 5 K [17]. The inverted core-shell MnO/Mn3O4 nanoparticles prepared from manganese(II) acetylacetonate show a coercivity of ~0.8 T [19]. A coercivity up to 1.1 T has been reported in the oxide-coated manganese nanoparticles prepared by arc discharge and passivation process [7]. In this work, the coercivities up to 1.05 T and 1.17 T were found in Mn3O4 nanoparticles and α-Mn/Mn3O4 nanoparticles prepared by laser
Corresponding authors. China Jiliang University, Hangzhou 310018, China. E-mail addresses: [email protected] (Y.T. Yang), [email protected] (H.L. Ge).
https://doi.org/10.1016/j.jmmm.2019.165481 Received 13 May 2019; Received in revised form 18 June 2019; Accepted 20 June 2019 Available online 21 June 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Y.T. Yang, et al.
Fig. 1. The XRD patterns of the nanoparticles prepared by laser ablation of Mn under (a) ambient air pressure and (b) ~20 Pa could be indexed with (a) Mn3O4; and (b) α-Mn + Mn3O4, respectively.
Fig. 3. The normalized M−T curves of the Mn3O4 nanoparticles prepared by laser ablation of Mn in ambient air. The curves were measured by cooling the sample from room temperature under 0 T (black curve) and then heating the sample under 0.05 T (red curve). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
ablation process.
deposited on the chamber wall were collected in air for subsequent characterizations. The distance from the ablation spot to the chamber wall was estimated to be in the range of 6–10 cm. The X-ray diffraction (XRD) patterns of the samples were collected at room temperature using a Rigaku D/Max 2500 automatic diffractometer operating at 40 kV and 100 mA with Cu-Kα radiation, a Ni filter, a scintillation counter and an angular step of 0.02°. The morphology and high-resolution images of the nanoparticles were observed by using a JEM2100 transmission electron microscopy (TEM). The magnetic properties
2. Experimental The commercial manganese (purity 99.95%) pieces were first loaded into a chamber with a quartz window, through which the laser beams (wavelength ~1064 nm) were focused on the surface of the Mn pieces. The laser ablation process was conducted in ambient pressure and air with a gas pressure of ~20 Pa, respectively. The 20 Pa pressure was generated by using a rotary vane vacuum pump and the pressure was measured by using a Pirani vacuum gauge. The nanoparticles
Fig. 2. The typical morphology and high-resolution images of the nanoparticles prepared by laser ablation of Mn in air with (a and b) ambient pressure and (c and d) 20 Pa. 2
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Y.T. Yang, et al.
exclude the presence of α-Mn phase in the Mn3O4 nanoparticles because of the limited sensitivity of XRD. Usually, it is difficult to detect phases of trace amount and/or of tiny crystalline size by XRD. In comparison with the complicated process of the arc discharge process and the chemical solution method as mentioned above, the laser ablation method is much more simplified for preparation of Mn3O4 nanoparticles. The broadening of the XRD peaks was ascribed to the tiny crystalline size of both Mn3O4 and α-Mn phases as confirmed by the TEM observations. Fig. 2 shows the morphology and high-resolution images of the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles prepared by the laser ablation method. Most of the Mn3O4 nanoparticles are spherical in shape with a narrow size distribution ranging from 20 nm to about 90 nm, as shown in Fig. 2(a). Fig. 2(b) shows the high-resolution image of a Mn3O4 nanoparticle, which is well-crystalized within the nanoparticle. However, we could also observe the disordered arrangement of the surface atoms. Both spherical and irregular shaped nanoparticles were observed in the α-Mn/Mn3O4 sample, as shown Fig. 2(c), in which aggregated arrays of the α-Mn/Mn3O4 nanoparticles are frequently observed. The more active surface atoms due to in-sufficient oxidation under low air pressures may result in aggregation of the nanoparticles to minimize the surface energy. The size of the α-Mn/ Mn3O4 nanoparticles is similar to that of the Mn3O4 nanoparticles. Both the surface and the core of the α-Mn/Mn3O4 nanoparticles show atomic disordered regions, as shown in Fig. 2(d). The high temperature laser beams may evaporate or sublimate the bulk Mn at a very high rate and the ejected Mn nanoparticles would react with oxygen in air spontaneously. The rough surface, irregular shape, and presence of secondary phase in the nanoparticles result in a large surface/interface area of the nanoparticles and possibly the presence of uncompensated surface/interface spins of the surface/interface atoms. The oxidation of metallic nanoparticles usually occurs from the surface of the nanoparticles. It is reasonable for us to assume that the α-Mn phase may present inside the particles. The temperature dependence of magnetization of the Mn3O4 nanoparticles is shown in Fig. 3. The M−T curves of the α-Mn/Mn3O4 nanoparticles are not shown here because it is almost the same to that of the Mn3O4 nanoparticles. The spontaneous magnetization of the Mn3O4 nanoparticles increases dramatically at temperatures below 42 K and reaches saturation at 30 K. The Curie temperature (Tc) of the samples was thus determined to be 42 K, which is the same to that of the bulk Mn3O4 [5]. Although the Tc of the Mn3O4 nanoparticles was reported to be size dependent in some reports [20,21], we did not observe obvious deviation of the Tc of the Mn3O4 nanoparticles from that of the bulk. One possible reason might be that the size (20–90 nm) of our Mn3O4 nanoparticles is still too large to affect the Tc. The zero-field cooling curve of the samples at 0.05 T, measured with increasing temperature, also shows a peak magnetization at 42 K. The increasing M with increasing T as seen in the ZFC curve was ascribed to superparamagnetic behaviors of Mn3O4 nanoparticles at temperatures below Tc. Superparamagnetism has been frequently observed in magnetic nanoparticles [8]. At low temperatures, the magnetization of each randomly oriented magnetic nanoparticle is blocked because of anisotropy. With increasing temperature, the thermal energy overcomes the energy barrier due to anisotropy of some small nanoparticles, which show superparamagnetic behaviors and thus the M of the sample increases with increasing T. A higher enough temperature (above Tc) would result in a paramagnetic state of the sample and thus a decrease of M with increasing T. The magnetic hysteresis loops of the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles measured in the temperature range of 5–35 K are shown in Fig. 4. The coercivity of the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles at 5 K reached up to 1.05 T and 1.17 T, respectively. For comparison, the coercivity of the single crystals of Mn3O4 at 4.2 K is 0.265 T while the typical coercivity of Mn3O4 nanoparticles prepared from manganese acetate tetrahydrate by
Fig. 4. The magnetic hysteresis loops of the nanoparticles prepared by laser ablation of Mn in air with (a) ambient pressure and (b) 20 Pa, respectively.
Table 1 The magnetic properties of the nanoparticle prepared by laser ablation of Mn in ambient air pressure and 20 Pa (parameters in brackets), respectively. T/K
Hc/T
5 15 25 35
1.05 0.65 0.49 0.32
(1.17) (0.77) (0.59) (0.35)
Ms/Am2kg−1
Mr/Am2kg−1
6.96 6.88 6.66 6.19
4.5 4.1 3.8 3.3
(18.5) (18) (17.2) (16)
(11.5) (10.5) (9.6) (8.5)
of the samples were measured by using a Quantum Design physical property measurement system equipped with a VSM. The magnetic moment of the system is calibrated by using a standard Ni sample. The weight of the sample is measured by using a high-precision balance. The M−T curves were measured in the temperature range of 5–100 K while the M−H curves of the samples were measured at 5 K, 15 K, 25 K, and 35 K, respectively. 3. Results and discussion As shown in Fig. 1(a), the XRD patterns of the nanoparticles prepared by laser ablation of Mn in ambient pressure can be indexed merely with Mn3O4. However, the XRD patterns of the nanoparticles prepared under 20 Pa air could be indexed with Mn3O4 as the major phase and α-Mn as the minor phase, indicating incomplete oxidation of Mn at lower air pressures. The α-Mn is the most stable polymorph of Mn below 1000 K, which has an exotic crystal structure containing 58 atoms in a cubic unit cell. It seems the ratio of α-Mn to Mn3O4 might be dependent on the air pressure. It should be noted that we could not 3
Journal of Magnetism and Magnetic Materials 489 (2019) 165481
Y.T. Yang, et al.
Appendix A. Supplementary data
chemical synthesis is ~0.82 T or less [11]. We ascribe the large coercivity of 1.05 T in the Mn3O4 nanoparticles to the small crystalline size of Mn3O4 phase. The further enhanced coercivity (1.17 T) in the α-Mn/ Mn3O4 nanoparticles was ascribed to the interfacial exchange coupling between antiferromagnetic α-Mn and ferrimagnetic Mn3O4 phase. And this has been proved by the shift of the magnetic hysteresis loops, which indicate the presence of exchange bias effect. An exchange bias field up to ~65 mT was observed in the α-Mn/Mn3O4 nanoparticles. The exchange bias effect was first observed by Meiklejohn and Bean in fine particles of ferromagnetic (FM) cobalt with antiferromagnetic (AFM) cobaltous oxide shell in 1956 [22]. In this work, the exchange bias effect may originate mainly from interfacial exchange coupling between antiferromagnetic α-Mn and ferrimagnetic Mn3O4. The α-Mn undergoes a paramagnetic to antiferromagnetic phase transition at its Néel temperature TN ~ 100 K. It seems the coercivity of the laser ablated Mn3O4 nanoparticles is usually larger than that reported in the Mn3O4 nanoparticles produced by chemical solution method. The laser ablation process is a high temperature process while the chemical solution method is room temperature or low temperature method. We speculate that the high rate temperature changes of the sample during laser ablation process may result in more defects and strains in the nanoparticles, and thus a higher coercivity. It is interesting that a similar exchange bias field was also observed in the Mn3O4 nanoparticles, even though the antiferromagnetic α-Mn phase in this sample was not detected by XRD. We speculate that α-Mn phase might also present in the Mn3O4 nanoparticles. As mentioned above, the small crystalline size and/or small fraction of the α-Mn phase make it difficult to be detected by XRD. The disordered/uncompensated surface spins of the nanoparticles may also induce an exchange bias effect. In fact, the exchange bias effect originating from disordered surface spins has been observed in small ferrite nanoparticles [23]. The Hc of both samples decreases with increasing temperature, as shown in Fig. 4 and Table 1. The saturation magnetization (Ms) of the Mn3O4 nanoparticles at 5 K and under an applied field of 5 T is 6.96 Am2/kg, which is approximately 15% of that of the Mn3O4 single crystals [5]. A magnetization reduction is frequently observed in most ferromagnetic nanoparticles because of the large surface area, resulting in a large fraction of surface atoms with uncompensated/disordered/canted spins. It is interesting that the α-Mn/Mn3O4 nanoparticles at 5 K and 5 T exhibits a Ms of 18.5 Am2/kg, which is higher than that of the Mn3O4 nanoparticles for unknown reason. Both Ms and remanent magnetization of the two samples decrease slightly with increasing temperature, as shown in Table 1.
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4. Conclusions The Mn3O4 nanoparticles and the α-Mn/Mn3O4 composite nanoparticles were prepared by laser ablation of Mn in air with a gas pressure of 1 atm and 20 Pa, respectively. The structure and magnetic properties of these nanoparticles were studied. The large coercivities up to 1.05 T and 1.17 T were observed in the Mn3O4 nanoparticles and the α-Mn/Mn3O4 nanoparticles, respectively. We ascribe the large coercivity to the tiny crystalline size of Mn3O4 and interfacial exchange coupling between α-Mn and Mn3O4. An exchange bias effect was also observed in the nanoparticles, owing to the presence of antiferromagnetic α-Mn and ferrimagnetic Mn3O4 phase. Acknowledgments We acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 51671177, 11074227), and the Future Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2016M3D1A1027835).
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