- Email: [email protected]
Shape-induced anisotropy in epitaxial Fe3O4 nanopillars
JID:PLA AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.1 (1-4)
Physics Letters A ••• (••••) ••••••
1
Contents lists available at ScienceDirect
67 68
2 3
Physics Letters A
4
69 70 71
5
72
6
www.elsevier.com/locate/pla
7
73
8
74
9
75
10
76
11 12 13 14 15 16 17
Shape-induced anisotropy in epitaxial Fe3 O4 nanopillars M. Guan a , G. Dong a , Z. Hu a,∗ , B. Peng a , Y. He b , W.Z. Cui b , Z. Zhou a , M. Liu a a
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China b National Key Laboratory of Science and Technology on Space Microwave, China Academy of Space Technology (Xi’an), Xi’an 710100, China
a r t i c l e
i n f o
24 25 26
29 30 31
81 82 83
87
Article history: Received 3 September 2018 Received in revised form 9 July 2019 Accepted 19 July 2019 Available online xxxx Communicated by M. Wu
27 28
80
86
a b s t r a c t
21 23
79
85
19
22
78
84
18 20
77
Keywords: Fe3 O4 nanopillars AAO template Ferromagnetic resonance Shape anisotropy
Fe3 O4 nanopillars with remarkably high uniformity and crystallinity are grown on (100)-oriented magnesium aluminate substrates by pulsed laser deposition with a simple anodic-aluminum-oxide template method. Compared with Fe3 O4 full film, the nanopillars exhibit higher in-plane magnetic saturation field and lower out-of-plane saturation field. The angular-dependent ferromagnetic resonance, investigated by electron spin resonance spectroscopy, confirms the shape-induced anisotropy change in Fe3 O4 nanopillars. High Gilbert damping parameter of 0.23 is obtained by measuring the resonance field at different microwave frequencies. © 2019 Published by Elsevier B.V.
88 89 90 91 92 93 94 95 96 97
32
98
33
99 100
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
1. Introduction Magnetic particles with different morphologies have attracted a lot of interests for their unique magnetic properties and potential applications in data storage [1], medicine [2]. Magnetite (Fe3 O4 ) is one of the most important transition metal oxides. Fe3 O4 nanostructure with different morphologies have been synthesized in recent years, [3,4] which exhibit potential applications in many fields such as lithium ion batteries [5] and microwave absorption coating [6]. As the morphology of the Fe3 O4 is a key interfering factor for its performance, improving the homogeneity of the morphology with an easy-to-scale process becomes significant for practical applications. However, one-dimensional Fe3 O4 nanostructures have been mostly prepared by chemical reaction methods, which is difficult to maintain the uniformity. [7,8] Anodic aluminum oxide (AAO) template has been recently used as a mask during pulsed laser deposition (PLD) to form nanostructures with designed shape. [9] In this work, we deposit epitaxial Fe3 O4 nanopillars using AAO template, and investigate the magnetic properties of the Fe3 O4 nanopillars. 2. Experimental details Fe3 O4 nanopillars were deposited on the MgAl2 O4 (MAO) substrate using a PLD system. Before deposition, the AAO template
62 63 64 65 66
*
Corresponding author. E-mail addresses: [email protected] (Z. Hu), [email protected] (M. Liu). https://doi.org/10.1016/j.physleta.2019.125850 0375-9601/© 2019 Published by Elsevier B.V.
101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123
3. Results and discussion
124 125
3.1. Structure and morphology property
60 61
was first disperse in acetone to remove the polymethyl methacrylate (PMMA) back support, and then the template was transferred into deionized water for hydrophilic treatment to ensure that the AAO membrane was tightly connected with the MAO substrate. Then, the MAO substrate was dried at 80 ◦ C on a hot plate and then transferred to the PLD vacuum chamber. The Fe3 O4 was deposited subsequently with a base pressure of approximately 2 × 10−7 Torr at a temperature of 650 ◦ C by using KrF excimer laser (λ = 248 nm). After cooling down to room temperature, the sample was soaked in NaOH solution to remove the AAO template. We also prepared a Fe3 O4 film under the same condition as the contrast sample. The phase and surface morphology of the Fe3 O4 nanopillar and film were characterized by high-resolution Xray diffraction (HRXRD, PANalytical X’Pert MRD) and atomic force microscopy (AFM, Bruker nanoscope V). The M-H hysteresis loops of the Fe3 O4 nanopillars and film were investigated by vibrating sample magnetometer (VSM, Lake Shore 7404). The angular dependence electron spin-resonance (ESR) spectra measurements were performed by an X-band (∼9.2 GHz) ESR system (JEOL, JESFA200) operated at TE 011 mode. The frequency dependence of the resonance field was performed in a broad-band ferromagnetic resonance spectroscopy system.
126 127
The schematic of the Fe3 O4 nanopillars in the AAO template is shown in Fig. 1. Inside the template’s holes, the Fe3 O4 pillars
128 129 130 131 132
JID:PLA
2
AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.2 (1-4)
M. Guan et al. / Physics Letters A ••• (••••) ••••••
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Fig. 1. The schematic of the Fe3 O4 nanopillars deposited by an AAO template method.
17 18 19 20 21 22 23 24 25 26 27 28
are epitaxy on the MAO substrate directly. While the Fe3 O4 in the other place is blocked by the template. After vanishing of the AAO template, Fe3 O4 nanopillars are left on the MAO substrate. The structure of the pillars and the film are determined by the X-ray diffraction. Fig. 2a shows the (004) diffraction peaks of the as prepared Fe3 O4 pillars and film taken in the vicinity of MAO (004) Bragg reflection. Both the Fe3 O4 film and nanopillars curve showed a diffraction peak around 2θ = 42.5◦ , which correspond well with the magnetite (JCPD no. 19-0629). No other phase is observed in the XRD curve, indicated both the pillars and film are
epitaxy single crystal and that the AAO template had not induced any second phase during the PLD deposition. As However, we also noticed that the (004) peak of the nanopillars sample is broader than that of the film. As the mismatch between Fe3 O4 and MAO is very large (−3.77%), and the film and pillars grow on the same substrates with similar thickness of around 30 nm, the strain is relaxed in those samples [10]. The broaden in peak indicates smaller domain size in the nanopillars than that in the film. As the Verwey transition is the key hallmark of Fe3 O4 , [10–12] the ferromagnetic resonance fields (Hr ) of the Fe3 O4 nanorods at different temperature were recorded and presented in the inset of Fig. 2(a). It can be seen that obviously different tendency of Hr -T at temperature below and above ∼117 K, indicating that the Verwey transition occurs in the samples as reported in recent works. To observe the morphology difference between the Fe3 O4 nanopillars and film, we applied AFM to observed the surface of the samples. The film roughness, which can be deduced from the AFM pattern in Fig. 2(b), is 0.427 nm. This indicating that the Fe3 O4 film got a smooth and uniform surface. Since the MAO we used has been polished to a roughness less than 0.5 nm, the smooth surface of the Fe3 O4 also predicted that the pillars are uniform in height as they can be seen as a “part” of the film. Fig. 2(c) shows the AFM pattern of the Fe3 O4 nanopillars, the details of the nanopillars is shown in Fig. 2(d). It should be highlighted that the nanopillars are uniform in shape, the width of the pillars are around 90 nm. The distance between two neighbors is 125 nm, which is also the distance of the neighbor holes in AAO tem-
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94
29
95
30
96
31
97
32
98
33
99
34
100
35
101
36
102
37
103
38
104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 2. (a) XRD patterns of the Fe3 O4 nanopillars and film; (b) AFM image of the Fe3 O4 film; (c-d) AFM image of the Fe3 O4 nanopillars, showing high uniformity.
132
JID:PLA AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.3 (1-4)
M. Guan et al. / Physics Letters A ••• (••••) ••••••
1
HS =
2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 36
Fig. 3. Normalized magnetization as a function of the applied field of the Fe3 O4 filmand Fe3 O4 nanopillars for (a) in plane and (b) out-of-plane.
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
D2
,
(1)
where the MS is the saturation magnetization which is 1.74 × 104 A/m for Fe3 O4 , D is the average diameter of the nanopillars, which is 90 nm in this case, q is the geometrical factor (q = 1.8412 for a cylinder and 2.0816 for a sphere [17]), lex is the exchange length (27 for Fe3 O4 ). According to Eq. (1) HS is 2654 Oe for the Fe3 O4 nanorods, which fitted well with the experiment results. As we know, the magnetic moments of ferrite resonance at microwave frequency in magnetic field, which is the basic of microwave devices like filters, isolators and so on. The microwave properties of the Fe3 O4 samples are characterized using the ESR systems, and the results are shown in Fig. 4. Fig. 4a shows the both the in plane and out of plane ESR curve of the pillars and film. When we applied an in plane magnetic field to the samples, the resonance filed of the film and pillars, which dedicated at dP/dH = 0, are 922 Oe and 1833 Oe, respectively. For the out of plane set, the resonance field are 10087 Oe and 4839 Oe, respectively. We also rotated the sample to see the magnetic field direction-dependence of the resonance field. The result is shown in Fig. 4 (b), the in-plane set is shown as theta = 0◦ and 180◦ , while the out-of-plane is shown in theta = 90◦ and 270◦ . We observed that resonance field of the pillars increased from 1833 Oe to 4839 Oe with the magnetic field rotated from in plane direction to our of plane. For the Fe3 O4 film, the resonance field increased from 922 Oe in plane to 10087 Oe out of plane. This indicated that the in-plane demagnetization decreased in the Fe3 O4 nanopillars. We also tested the magnetic resonance field in different frequencies to fit the data and assumed the value of the effective anisotropy field. The frequency f dependence of resonance magnetic field H can be described as
4
35
2 2 4lex q MS
3
plate. This means that we can design the shape of Fe3 O4 by just choosing the appropriate AAO template. The well-defined pillar structure also shown that the AAO template method is an effective to achieve pillar structure. 3.2. Magnetic and microwave property To test the difference of the magnetic properties of the Fe3 O4 samples, VSM and ESR are applied to characterize the magnetism of Fe3 O4 film and nanopillars. Fig. 3 shows the magnetization loops of the Fe3 O4 nanopillars and film for the in plane (IP) and out of plane (OOP) at room temperature. The magnetization exhibits a shape anisotropy with both the easy axis of the magnetization located in the plane of the samples. For the Fe3 O4 film, the magnetization saturated at H = 1399 Oe in plane and at H = 6500 Oe out of plane. For the Fe3 O4 nanopillars, the saturation field H = 2200 Oe in plane and H = 3500 Oe at the out of plane condition. The hysteresis of the Fe3 O4 nanorods is also slimmer than that of the films, which means that rod-shape of the pillars can induce a shape anisotropy and will help in rotating the magnetism of the pillars from in plane to out of plane direction. The magnetism of the nanopillars can be explained by using a curling model and a chain-of-shape model [13–15]. In terms of the two theories, the in-plane saturation field of the 1-dimensional Fe3 O4 is bigger than that of the film. This behavior has been observed in lots of magnetic 1-Dimensional structures such as Fe nanowires [16]. The saturation field of the pillars should be determined by equation:
γ f = ( H + H a )( H + H a + 4π M s ) 2π
4πα
γ
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
(2)
where H a is the magnetic anisotropy field, M s the magnetization, γ the gyromagnetic ratio (2.8 MHz/Oe). By fitting the experiment data, we obtain an anisotropy field of 2000 Oe, which agrees well with the VSM results. We tested the linewidth at different frequency. The Gilbert damping parameter α can be obtained by fitting the dependence of FMR linewidth H (full width at half maximum) on frequency f as shown in Fig. 4 (d) and Eq. (3),
H =
67
101 102 103 104 105 106 107 108 109 110 111 112
f + H0
(3)
where H 0 is a zero-frequency linewidth broadening. The α parameter of the nanopillars is 0.23 which is about one order of magnitude larger than that of the film (0.015). This indicated that the Fe3 O4 nanopillars had a strong absorption to the microwave energy, thus made Fe3 O4 nanopillars a very good candidate for microwave absorption coating.
113 114 115 116 117 118 119 120 121 122
4. Conclusion
123 124
Fe3 O4 nanopillars are successfully deposited by using an AAOtemplate pulsed laser deposition method. The nanopillars are epitaxy on the MAO substrate. The in-plane saturation field increased to 2200 Oe while the out-of-plane saturation decrease to 3500 Oe. The Gilbert damping constant of the Fe3 O4 nanopillars is 0.23, about one order of magnitude higher than that of the Fe3 O4 full film. The changes in this magnetic anisotropy may attributed to the 1-dimensional-shape of the samples.
125 126 127 128 129 130 131 132
JID:PLA
AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.4 (1-4)
M. Guan et al. / Physics Letters A ••• (••••) ••••••
4
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92
27
93
28
94
29
95
30
96
31
97
32
98
33
99
34
100
35
101
36
102
37
103 104
38 39 40
Fig. 4. (a) In-plane and out-of-plane FMR curve of the Fe3 O4 nanopillars and film. (b) Angular-dependent ferromagnetic resonance fields for the Fe3 O4 nanopillars and Fe3 O4 film. (c-d) Frequency-dependent resonance field and FMR linewidth of the nanopillars and the film, in which the line is the fitting curve.
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
106 107
41 42
105
Acknowledgements The work is supported by the National Natural Science Foundation of China (Grant Nos. 51472199, 51602244 and 11534015), the key Research and Development plan of Shaanxi Province (Grant No. 2018GY-109), the National 111 Project of China (B14040), and the Fundamental Research Funds for the Central Universities. References [1] B.D. Terris, T. Thomson, Nanofabricated and self-assembled magnetic structures as data storage media, J. Phys. D, Appl. Phys. 38 (12) (2005) R199. [2] K. Niemirowicz, K.H. Markiewicz, A.Z. Wilczewska, H. Car, Magnetic nanoparticles as new diagnostic tools in medicine, Adv. Med. Sci. 57 (2) (2012) 196–207. [3] G. Zou, et al., Magnetic Fe3 O4 nanodisc synthesis on a large scale via a surfactant-assisted process, Nanotechnology 16 (16) (2005) 1584. [4] H. Khurshid, et al., Synthesis and magnetic properties of core/shell FeO/Fe3 O4 nano-octopods, J. Appl. Phys. 113 (17) (2013) 09E301. [5] Q. Zhou, et al., Low temperature plasma synthesis of mesoporous Fe3 O4 nanorods grafted on reduced graphene oxide for high performance lithium storage, Nanoscale 6 (4) (Feb 21 2014) 2286–2291. [6] G. Sun, B. Dong, M. Cao, B. Wei, C. Hu, Hierarchical dendrite-like magnetic materials of Fe3 O4 , γ -Fe2 O3 , and Fe with high performance of microwave absorption, Chem. Mater. 23 (6) (2011).
[7] D. Zhang, et al., Magnetite (Fe3 O4 ) core−shell nanowires: synthesis and magnetoresistance, Nano Lett. 4 (2004) 5. [8] B. Lei, S. Han, C. Li, D. Zhang, Z. Liu, C. Zhou, Synthesis and electronic properties of transition metal oxide core shell nanowires, Nanotechnology 18 (44019) (2007) 333–341. [9] Q.F. Zhan, et al., Applied field Mössbauer study of shape anisotropy in Fe nanowire arrays, Appl. Phys. Lett. 85 (20) (2004) 4690–4692. [10] X.H. Liu, W. Liu, Z.D. Zhang, Evolution of magnetic properties in the vicinity of the Verwey transition in Fe3 O4 thin films, Phys. Rev. B 96 (9) (2017). [11] X. Liu, C.F. Chang, A.D. Rata, A.C. Komarek, L.H. Tjeng, Fe3 O4 thin films: controlling and manipulating an elusive quantum material, Nature 1 (2016). [12] H.Y. Huang, et al., Jahn-Teller distortion driven magnetic polarons in magnetite, Nat. Commun. 8 (2017) 15929. [13] I.S. Jacobs, C.P. Bean, An approach to elongated fine-particle magnets, Phys. Rev. 100 (4) (1955) 1060–1067. [14] L. Zhang, Y. Zhang, Fabrication and magnetic properties of Fe3 O4 nanowire arrays in different diameters, J. Magn. Magn. Mater. 321 (5) (2009) L15–L20. [15] J.B. Wang, et al., Magnetic texture in iron nanowire arrays, Nanotechnology 15 (5) (2004) 485. [16] L. Sun, Y. Hao, C.L. Chien, P.C. Searson, Tuning the properties of magnetic nanowires, IBM J. Res. Dev. 49 (1) (2005) 79–102. [17] G. Tian, et al., BiFeO3 nanorings synthesized via AAO template-assisted pulsed laser deposition and ion beam etching, RSC Adv. 7 (65) (2017) 41210–41216.
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128
63
129
64
130
65
131
66
132
[m5G; v1.260; Prn:25/07/2019; 11:39] P.1 (1-4)
Physics Letters A ••• (••••) ••••••
1
Contents lists available at ScienceDirect
67 68
2 3
Physics Letters A
4
69 70 71
5
72
6
www.elsevier.com/locate/pla
7
73
8
74
9
75
10
76
11 12 13 14 15 16 17
Shape-induced anisotropy in epitaxial Fe3 O4 nanopillars M. Guan a , G. Dong a , Z. Hu a,∗ , B. Peng a , Y. He b , W.Z. Cui b , Z. Zhou a , M. Liu a a
Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education and International Center for Dielectric Research, School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, China b National Key Laboratory of Science and Technology on Space Microwave, China Academy of Space Technology (Xi’an), Xi’an 710100, China
a r t i c l e
i n f o
24 25 26
29 30 31
81 82 83
87
Article history: Received 3 September 2018 Received in revised form 9 July 2019 Accepted 19 July 2019 Available online xxxx Communicated by M. Wu
27 28
80
86
a b s t r a c t
21 23
79
85
19
22
78
84
18 20
77
Keywords: Fe3 O4 nanopillars AAO template Ferromagnetic resonance Shape anisotropy
Fe3 O4 nanopillars with remarkably high uniformity and crystallinity are grown on (100)-oriented magnesium aluminate substrates by pulsed laser deposition with a simple anodic-aluminum-oxide template method. Compared with Fe3 O4 full film, the nanopillars exhibit higher in-plane magnetic saturation field and lower out-of-plane saturation field. The angular-dependent ferromagnetic resonance, investigated by electron spin resonance spectroscopy, confirms the shape-induced anisotropy change in Fe3 O4 nanopillars. High Gilbert damping parameter of 0.23 is obtained by measuring the resonance field at different microwave frequencies. © 2019 Published by Elsevier B.V.
88 89 90 91 92 93 94 95 96 97
32
98
33
99 100
34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59
1. Introduction Magnetic particles with different morphologies have attracted a lot of interests for their unique magnetic properties and potential applications in data storage [1], medicine [2]. Magnetite (Fe3 O4 ) is one of the most important transition metal oxides. Fe3 O4 nanostructure with different morphologies have been synthesized in recent years, [3,4] which exhibit potential applications in many fields such as lithium ion batteries [5] and microwave absorption coating [6]. As the morphology of the Fe3 O4 is a key interfering factor for its performance, improving the homogeneity of the morphology with an easy-to-scale process becomes significant for practical applications. However, one-dimensional Fe3 O4 nanostructures have been mostly prepared by chemical reaction methods, which is difficult to maintain the uniformity. [7,8] Anodic aluminum oxide (AAO) template has been recently used as a mask during pulsed laser deposition (PLD) to form nanostructures with designed shape. [9] In this work, we deposit epitaxial Fe3 O4 nanopillars using AAO template, and investigate the magnetic properties of the Fe3 O4 nanopillars. 2. Experimental details Fe3 O4 nanopillars were deposited on the MgAl2 O4 (MAO) substrate using a PLD system. Before deposition, the AAO template
62 63 64 65 66
*
Corresponding author. E-mail addresses: [email protected] (Z. Hu), [email protected] (M. Liu). https://doi.org/10.1016/j.physleta.2019.125850 0375-9601/© 2019 Published by Elsevier B.V.
101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123
3. Results and discussion
124 125
3.1. Structure and morphology property
60 61
was first disperse in acetone to remove the polymethyl methacrylate (PMMA) back support, and then the template was transferred into deionized water for hydrophilic treatment to ensure that the AAO membrane was tightly connected with the MAO substrate. Then, the MAO substrate was dried at 80 ◦ C on a hot plate and then transferred to the PLD vacuum chamber. The Fe3 O4 was deposited subsequently with a base pressure of approximately 2 × 10−7 Torr at a temperature of 650 ◦ C by using KrF excimer laser (λ = 248 nm). After cooling down to room temperature, the sample was soaked in NaOH solution to remove the AAO template. We also prepared a Fe3 O4 film under the same condition as the contrast sample. The phase and surface morphology of the Fe3 O4 nanopillar and film were characterized by high-resolution Xray diffraction (HRXRD, PANalytical X’Pert MRD) and atomic force microscopy (AFM, Bruker nanoscope V). The M-H hysteresis loops of the Fe3 O4 nanopillars and film were investigated by vibrating sample magnetometer (VSM, Lake Shore 7404). The angular dependence electron spin-resonance (ESR) spectra measurements were performed by an X-band (∼9.2 GHz) ESR system (JEOL, JESFA200) operated at TE 011 mode. The frequency dependence of the resonance field was performed in a broad-band ferromagnetic resonance spectroscopy system.
126 127
The schematic of the Fe3 O4 nanopillars in the AAO template is shown in Fig. 1. Inside the template’s holes, the Fe3 O4 pillars
128 129 130 131 132
JID:PLA
2
AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.2 (1-4)
M. Guan et al. / Physics Letters A ••• (••••) ••••••
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Fig. 1. The schematic of the Fe3 O4 nanopillars deposited by an AAO template method.
17 18 19 20 21 22 23 24 25 26 27 28
are epitaxy on the MAO substrate directly. While the Fe3 O4 in the other place is blocked by the template. After vanishing of the AAO template, Fe3 O4 nanopillars are left on the MAO substrate. The structure of the pillars and the film are determined by the X-ray diffraction. Fig. 2a shows the (004) diffraction peaks of the as prepared Fe3 O4 pillars and film taken in the vicinity of MAO (004) Bragg reflection. Both the Fe3 O4 film and nanopillars curve showed a diffraction peak around 2θ = 42.5◦ , which correspond well with the magnetite (JCPD no. 19-0629). No other phase is observed in the XRD curve, indicated both the pillars and film are
epitaxy single crystal and that the AAO template had not induced any second phase during the PLD deposition. As However, we also noticed that the (004) peak of the nanopillars sample is broader than that of the film. As the mismatch between Fe3 O4 and MAO is very large (−3.77%), and the film and pillars grow on the same substrates with similar thickness of around 30 nm, the strain is relaxed in those samples [10]. The broaden in peak indicates smaller domain size in the nanopillars than that in the film. As the Verwey transition is the key hallmark of Fe3 O4 , [10–12] the ferromagnetic resonance fields (Hr ) of the Fe3 O4 nanorods at different temperature were recorded and presented in the inset of Fig. 2(a). It can be seen that obviously different tendency of Hr -T at temperature below and above ∼117 K, indicating that the Verwey transition occurs in the samples as reported in recent works. To observe the morphology difference between the Fe3 O4 nanopillars and film, we applied AFM to observed the surface of the samples. The film roughness, which can be deduced from the AFM pattern in Fig. 2(b), is 0.427 nm. This indicating that the Fe3 O4 film got a smooth and uniform surface. Since the MAO we used has been polished to a roughness less than 0.5 nm, the smooth surface of the Fe3 O4 also predicted that the pillars are uniform in height as they can be seen as a “part” of the film. Fig. 2(c) shows the AFM pattern of the Fe3 O4 nanopillars, the details of the nanopillars is shown in Fig. 2(d). It should be highlighted that the nanopillars are uniform in shape, the width of the pillars are around 90 nm. The distance between two neighbors is 125 nm, which is also the distance of the neighbor holes in AAO tem-
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94
29
95
30
96
31
97
32
98
33
99
34
100
35
101
36
102
37
103
38
104
39
105
40
106
41
107
42
108
43
109
44
110
45
111
46
112
47
113
48
114
49
115
50
116
51
117
52
118
53
119
54
120
55
121
56
122
57
123
58
124
59
125
60
126
61
127
62
128
63
129
64
130 131
65 66
Fig. 2. (a) XRD patterns of the Fe3 O4 nanopillars and film; (b) AFM image of the Fe3 O4 film; (c-d) AFM image of the Fe3 O4 nanopillars, showing high uniformity.
132
JID:PLA AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.3 (1-4)
M. Guan et al. / Physics Letters A ••• (••••) ••••••
1
HS =
2 3 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 36
Fig. 3. Normalized magnetization as a function of the applied field of the Fe3 O4 filmand Fe3 O4 nanopillars for (a) in plane and (b) out-of-plane.
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
D2
,
(1)
where the MS is the saturation magnetization which is 1.74 × 104 A/m for Fe3 O4 , D is the average diameter of the nanopillars, which is 90 nm in this case, q is the geometrical factor (q = 1.8412 for a cylinder and 2.0816 for a sphere [17]), lex is the exchange length (27 for Fe3 O4 ). According to Eq. (1) HS is 2654 Oe for the Fe3 O4 nanorods, which fitted well with the experiment results. As we know, the magnetic moments of ferrite resonance at microwave frequency in magnetic field, which is the basic of microwave devices like filters, isolators and so on. The microwave properties of the Fe3 O4 samples are characterized using the ESR systems, and the results are shown in Fig. 4. Fig. 4a shows the both the in plane and out of plane ESR curve of the pillars and film. When we applied an in plane magnetic field to the samples, the resonance filed of the film and pillars, which dedicated at dP/dH = 0, are 922 Oe and 1833 Oe, respectively. For the out of plane set, the resonance field are 10087 Oe and 4839 Oe, respectively. We also rotated the sample to see the magnetic field direction-dependence of the resonance field. The result is shown in Fig. 4 (b), the in-plane set is shown as theta = 0◦ and 180◦ , while the out-of-plane is shown in theta = 90◦ and 270◦ . We observed that resonance field of the pillars increased from 1833 Oe to 4839 Oe with the magnetic field rotated from in plane direction to our of plane. For the Fe3 O4 film, the resonance field increased from 922 Oe in plane to 10087 Oe out of plane. This indicated that the in-plane demagnetization decreased in the Fe3 O4 nanopillars. We also tested the magnetic resonance field in different frequencies to fit the data and assumed the value of the effective anisotropy field. The frequency f dependence of resonance magnetic field H can be described as
4
35
2 2 4lex q MS
3
plate. This means that we can design the shape of Fe3 O4 by just choosing the appropriate AAO template. The well-defined pillar structure also shown that the AAO template method is an effective to achieve pillar structure. 3.2. Magnetic and microwave property To test the difference of the magnetic properties of the Fe3 O4 samples, VSM and ESR are applied to characterize the magnetism of Fe3 O4 film and nanopillars. Fig. 3 shows the magnetization loops of the Fe3 O4 nanopillars and film for the in plane (IP) and out of plane (OOP) at room temperature. The magnetization exhibits a shape anisotropy with both the easy axis of the magnetization located in the plane of the samples. For the Fe3 O4 film, the magnetization saturated at H = 1399 Oe in plane and at H = 6500 Oe out of plane. For the Fe3 O4 nanopillars, the saturation field H = 2200 Oe in plane and H = 3500 Oe at the out of plane condition. The hysteresis of the Fe3 O4 nanorods is also slimmer than that of the films, which means that rod-shape of the pillars can induce a shape anisotropy and will help in rotating the magnetism of the pillars from in plane to out of plane direction. The magnetism of the nanopillars can be explained by using a curling model and a chain-of-shape model [13–15]. In terms of the two theories, the in-plane saturation field of the 1-dimensional Fe3 O4 is bigger than that of the film. This behavior has been observed in lots of magnetic 1-Dimensional structures such as Fe nanowires [16]. The saturation field of the pillars should be determined by equation:
γ f = ( H + H a )( H + H a + 4π M s ) 2π
4πα
γ
68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100
(2)
where H a is the magnetic anisotropy field, M s the magnetization, γ the gyromagnetic ratio (2.8 MHz/Oe). By fitting the experiment data, we obtain an anisotropy field of 2000 Oe, which agrees well with the VSM results. We tested the linewidth at different frequency. The Gilbert damping parameter α can be obtained by fitting the dependence of FMR linewidth H (full width at half maximum) on frequency f as shown in Fig. 4 (d) and Eq. (3),
H =
67
101 102 103 104 105 106 107 108 109 110 111 112
f + H0
(3)
where H 0 is a zero-frequency linewidth broadening. The α parameter of the nanopillars is 0.23 which is about one order of magnitude larger than that of the film (0.015). This indicated that the Fe3 O4 nanopillars had a strong absorption to the microwave energy, thus made Fe3 O4 nanopillars a very good candidate for microwave absorption coating.
113 114 115 116 117 118 119 120 121 122
4. Conclusion
123 124
Fe3 O4 nanopillars are successfully deposited by using an AAOtemplate pulsed laser deposition method. The nanopillars are epitaxy on the MAO substrate. The in-plane saturation field increased to 2200 Oe while the out-of-plane saturation decrease to 3500 Oe. The Gilbert damping constant of the Fe3 O4 nanopillars is 0.23, about one order of magnitude higher than that of the Fe3 O4 full film. The changes in this magnetic anisotropy may attributed to the 1-dimensional-shape of the samples.
125 126 127 128 129 130 131 132
JID:PLA
AID:125850 /SCO Doctopic: Nanoscience
[m5G; v1.260; Prn:25/07/2019; 11:39] P.4 (1-4)
M. Guan et al. / Physics Letters A ••• (••••) ••••••
4
1
67
2
68
3
69
4
70
5
71
6
72
7
73
8
74
9
75
10
76
11
77
12
78
13
79
14
80
15
81
16
82
17
83
18
84
19
85
20
86
21
87
22
88
23
89
24
90
25
91
26
92
27
93
28
94
29
95
30
96
31
97
32
98
33
99
34
100
35
101
36
102
37
103 104
38 39 40
Fig. 4. (a) In-plane and out-of-plane FMR curve of the Fe3 O4 nanopillars and film. (b) Angular-dependent ferromagnetic resonance fields for the Fe3 O4 nanopillars and Fe3 O4 film. (c-d) Frequency-dependent resonance field and FMR linewidth of the nanopillars and the film, in which the line is the fitting curve.
43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
106 107
41 42
105
Acknowledgements The work is supported by the National Natural Science Foundation of China (Grant Nos. 51472199, 51602244 and 11534015), the key Research and Development plan of Shaanxi Province (Grant No. 2018GY-109), the National 111 Project of China (B14040), and the Fundamental Research Funds for the Central Universities. References [1] B.D. Terris, T. Thomson, Nanofabricated and self-assembled magnetic structures as data storage media, J. Phys. D, Appl. Phys. 38 (12) (2005) R199. [2] K. Niemirowicz, K.H. Markiewicz, A.Z. Wilczewska, H. Car, Magnetic nanoparticles as new diagnostic tools in medicine, Adv. Med. Sci. 57 (2) (2012) 196–207. [3] G. Zou, et al., Magnetic Fe3 O4 nanodisc synthesis on a large scale via a surfactant-assisted process, Nanotechnology 16 (16) (2005) 1584. [4] H. Khurshid, et al., Synthesis and magnetic properties of core/shell FeO/Fe3 O4 nano-octopods, J. Appl. Phys. 113 (17) (2013) 09E301. [5] Q. Zhou, et al., Low temperature plasma synthesis of mesoporous Fe3 O4 nanorods grafted on reduced graphene oxide for high performance lithium storage, Nanoscale 6 (4) (Feb 21 2014) 2286–2291. [6] G. Sun, B. Dong, M. Cao, B. Wei, C. Hu, Hierarchical dendrite-like magnetic materials of Fe3 O4 , γ -Fe2 O3 , and Fe with high performance of microwave absorption, Chem. Mater. 23 (6) (2011).
[7] D. Zhang, et al., Magnetite (Fe3 O4 ) core−shell nanowires: synthesis and magnetoresistance, Nano Lett. 4 (2004) 5. [8] B. Lei, S. Han, C. Li, D. Zhang, Z. Liu, C. Zhou, Synthesis and electronic properties of transition metal oxide core shell nanowires, Nanotechnology 18 (44019) (2007) 333–341. [9] Q.F. Zhan, et al., Applied field Mössbauer study of shape anisotropy in Fe nanowire arrays, Appl. Phys. Lett. 85 (20) (2004) 4690–4692. [10] X.H. Liu, W. Liu, Z.D. Zhang, Evolution of magnetic properties in the vicinity of the Verwey transition in Fe3 O4 thin films, Phys. Rev. B 96 (9) (2017). [11] X. Liu, C.F. Chang, A.D. Rata, A.C. Komarek, L.H. Tjeng, Fe3 O4 thin films: controlling and manipulating an elusive quantum material, Nature 1 (2016). [12] H.Y. Huang, et al., Jahn-Teller distortion driven magnetic polarons in magnetite, Nat. Commun. 8 (2017) 15929. [13] I.S. Jacobs, C.P. Bean, An approach to elongated fine-particle magnets, Phys. Rev. 100 (4) (1955) 1060–1067. [14] L. Zhang, Y. Zhang, Fabrication and magnetic properties of Fe3 O4 nanowire arrays in different diameters, J. Magn. Magn. Mater. 321 (5) (2009) L15–L20. [15] J.B. Wang, et al., Magnetic texture in iron nanowire arrays, Nanotechnology 15 (5) (2004) 485. [16] L. Sun, Y. Hao, C.L. Chien, P.C. Searson, Tuning the properties of magnetic nanowires, IBM J. Res. Dev. 49 (1) (2005) 79–102. [17] G. Tian, et al., BiFeO3 nanorings synthesized via AAO template-assisted pulsed laser deposition and ion beam etching, RSC Adv. 7 (65) (2017) 41210–41216.
108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128
63
129
64
130
65
131
66
132