Original Paper
phys. stat. sol. (a) 204, No. 12, 4125 – 4128 (2007) / DOI 10.1002/pssa.200777173
Ultrasoft magnetic properties of Co–Fe–Hf–O nanocomposite films N. D. Ha1, M. H. Phan2*, L. A. Tuan3, T. L. Phan4 C. G. Kim5, C. O. Kim5, and S. C. Yu3* 1 2 3 4 5
The Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands Advanced Composites Centre for Innovation and Science, University of Bristol, Queen’s Building, Bristol BS8 1TR, England Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea Microstructures Group, Department of Physics, University of Bristol, Bristol BS8 1TR, England Research Center for Advanced Magnetic Materials, Chungnam National University, Daejeon 305-764, South Korea
Received 8 May 2007, revised 1 November 2007, accepted 1 November 2007 Published online 10 December 2007 PACS 75.70.Ak, 85.70.Kh This paper reports on the excellent high-frequency magnetic performance of Co–Fe–Hf–O thin films, which were deposited on Si(100) substrates by the oxygen reactive rf-sputtering method. It was shown that the films possessed not only high electrical resistivity but also large saturation magnetization and hard-axis anisotropy field. Among the compositions investigated, Co19.35Fe53.28Hf7.92O19.35 exhibited the ultrasoft magnetic properties of high saturation magnetization, 4πMs∼19.86 kG, and low coercivity, Hc∼1.5 Oe. The magnetic permeability remained almost constant up to 3 GHz and reached a maximum at the ferromagnetic resonant frequency of 4.024 GHz. These properties of this film together with a high electrical resistivity of 3569 µΩcm make it ideal for producing micromagnetic devices for high-frequency applications. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1
Introduction
Co–Fe–Hf–O ferromagnetic oxide films are attractive candidate materials for high-frequency magnetic applications, owing to their combined properties of high electrical resistivity (ρ), large saturation magnetization (4πMs) and hard-axis anisotropy field (Hk) [1–4]. Here, the addition of a suitable amount of Co to a FeHfO film has been found to increase both the electrical resistivity and hard-axis anisotropy field, thereby improving the high-frequency characteristics [1,4]. The microstructure of a Co–Fe–Hf–O film comprised two disordered phases, including a nanocrystalline Co(Fe)-rich phase and an amorphous HfOrich phase, and the microstructural change could cause considerable variations in the film properties [4]. Therefore, it is possible to produce Co–Fe–Hf–O films with the desired properties through tailoring their microstructures. The present research aims to improve the high-frequency magnetic performance of Co–Fe–Hf–O films through fine-tuning the alloys composition with varying oxygen concentration by using the oxygen reactive rf-sputtering method. Our results reveal that the prepared Co–Fe–Hf–O films are ideal for high frequency applications of micromagnetic devices.
*
Corresponding authors: e-mail:
[email protected],
[email protected]
© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. D. Ha et al.: Ultrasoft magnetic properties of Co–Fe–Hf–O nanocomposite films
2
Experimental
Co–Fe–Hf–O films, including Co38.88Fe50.02Hf10.42O0.68 (sample No1), Co22.05Fe55.71Hf8.82O13.42 (sample No2), Co19.35Fe53.28Hf7.92O19.35 (sample No3), and Co10.78 Fe60.59 Hf7.32 O21.3 (sample No4), were deposited onto Si(100) substrates at ambient temperature by reactive rf-sputtering using an Ar+O2 atmosphere with a base pressure of less than 2.0 × 10–7 Torr and in a dc magnetic field of 100 Oe to induce an in-plane uniaxial anisotropy. The film composition was controllably modified by varying the aerial fraction of Hf chip on a Co0.3Fe0.7 target and the partial pressure of oxygen, and was analyzed by Auger electron spectroscopy (AES). The microstructures of the prepared films were examined by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (TEM). Magnetic hysteresis loops were measured using a vibrating sample magnetometer (VSM), along both the easy and hard axes of magnetization. Electrical resistivity was measured by a standard four-probe method. Hard-axis magnetic permeability was measured using a permeameter in the frequency range of 1–10 GHz.
3
Results and discussion
Figure 1 shows the magnetic hysteresis loops along the easy and hard axes of sample No3. The extracted magnetic parameters for all samples investigated are summarized in Table 1. As one can see from Fig. 1, the easy-axis hysteresis loop exhibited high coercive squareness, indicating that the magnetization reversal resulted mainly from irreversible domain wall motion across the film, whereas the magnetization reversal along the hard axis was caused by rotation of domain magnetization [5]. 1.2
M/Ms
0.4
300
Permeability
0.8
HardAxis EasyAxis
100 0
-100 -200
0.0
fFMR= 4.024 GHz
200
µ' µ''
2000 4000 6000 8000
Frequency (MHz)
-0.4 -0.8 -1.2 -200
-100
0
H (Oe)
HcE = 1.5 Oe HcH = 0.18 Oe HkH = 84 Oe 4πMs = 19.86 kG ρ = 3569 µΩcm fFMR = 4.024 GHz
100
Fig. 1 Hysteresis loops measured along the easy and hard directions for a Co19.35Fe53.28Hf7.92O19.35 film (sample No3). The inset shows the frequency dependences of the real and imaginary components of relative permeability.
200
It is interesting to note from Table 1 that the present film samples exhibited the superior properties over other films reported in the literatures. Among the samples investigated, Co19.35Fe53.28Hf7.92O19.35 (sample No3) exhibited the softest magnetic properties (4πMs = 19.86 kG and Hc = 1.5 Oe) with a high electrical resistivity (ρ = 3569 µΩcm). In addition, this film had the largest hard-axis anisotropy field, HkH = 84 Oe. These superior properties yielded the excellent high-frequency performance as shown in the inset of Fig. 1. It can be seen that the cut-off frequency of the real component of permeability (µ') reached a value as high as ~3.7 GHz, which is much higher than those of Fe–Hf–O and Co–Ta–Hf films (~90 MHz) reported in Ref. [1]. Interestingly, the imaginary component of permeability (µ'') remained almost unchanged up to f = 3 GHz and reached a maximum at the ferromagnetic resonant frequency of fFMR = 4.024 GHz. These features are beneficial for magnetic devices operating in the high frequency regime [1, 3]. In connection with the electrical resistivity and magnetization data, the excellent high-frequency performance of sample No3 can be attributed to the high electrical resistivity and to the large product of saturation magnetization and hard-axis anisotropy field. The superior magnetic properties of the Co–Fe–Hf–O films can be attributed to their microstrcuture. It should be noted that when the sputtering was carried out in Ar atmosphere, the atmosphere preferentially reacted with Co and Hf forming clusters of Fe-Fe(Co) and Co2Hf, due to chemical phase separation
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Original Paper
phys. stat. sol. (a) 204, No. 12 (2007)
4127
[4]. This was verified by the XRD pattern [Fig. 2, for sample No1], the TEM image [Fig. 3(a), for sample No1] and the selected area diffraction (SAD) pattern [the inset of Fig. 3(a)]. Therefore, the single peak at 430 is α-Fe(Co) (110) in a mixture of Co2Hf and Fe-Co amorphous matrix [Fig. 1, for sample No1]. Table 1 The magnetic and electrical parameters including the saturation magnetization, 4πMs, the hard-axis anisotropy field, HkH, and the electrical resistivity, ρ, for several magnetic thin films.
Composition
4πMs (kG)
HkH (Oe)
ρ (µΩcm)
Reference
Co85B21
12.00
16.0
1.20
[6]
Co83Ta6Hf11
8.10
10.3
170
[3]
Co85Nb12Zr3
8.50
6.8
120
[7]
Fe68Co18B13S1
18.00
35.0
1.3
[6]
Fe61Hf13O26
13.00
7.5
500
[1]
Co44Fe19Hf15O22
10.90
60.3
1700
[3]
Co38.88Fe50.02Hf10.42O0.68 (No1)
18.20
81.2
1401
present
Co22.05Fe55.71Hf8.82O13.42 (No2)
19.60
57.0
1160
present
Co19.35Fe53.28Hf7.92O19.35 (No3)
19.86
84.0
3569
present
Co10.78 Fe60.59 Hf7.32 O21.3 (No4)
13.75
58.5
5979
present
In contrast, when oxygen was present during sputtering, O preferentially combined with Hf, at sufficiently high concentration, forming a HfO2-rich amorphous phase, due to acquiring similar binding energies for metallic Hf and HfO2. Therefore, the co-existence of the two different phases [i.e. the α-Fe(Co) (110) and HfO2 (111) phases] arises mainly from chemical phase separation occurring during the film deposition, due to the difference in oxygen affinities of Hf and Fe(Co) [1–5].
Intensity (arb. units)
350
- CoFe(110) & Co(Fe)2Hf
- α-CoFe(110) - HfO2 - α-CoFe(211) - CoO
280 210 140 70
(a)
(b)
No4 No3 No2 No1
(d) (c)
0 20
30
40
50
60
70
80
90
2θ (deg.)
Fig. 2 The XRD patterns of the film samples. Fig. 3 Cross-sectional TEM images and SAD patterns of sample No1 (a), sample No2 (b) and sample No4 (d), respectively. In-plane TEM image of sample No3 (c) with arrows pointing to a preferentially anisotropic direction.
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© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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N. D. Ha et al.: Ultrasoft magnetic properties of Co–Fe–Hf–O nanocomposite films
It is a competitive coexistence of the two different phases, i.e. a nanocrystalline Fe(Co)-rich phase and an amorphous HfO2-rich phase, that governed the magnetic nature of Co–Fe–Hf–O films. The nanostructures were obtained in samples No2 and 3 [see Fig. 3(b) and (c)]. This resulted in the large values of 4πMs. In the present study, the largest value of 4πMs for sample No3 can be attributed to the strongest enhanced atomic pair correlation of Fe-Fe(Co) due to oxygen-induced chemical phase separation. It is noted that, for samples No. 3 and 4, the oxygen affinity went beyond Hf to combine with Co and converted part of it as magnetic CoO [see Fig. 3(c) and (d)]. Consequently, this sample contained high-moment metallic nanograins of α-Fe(Co) surrounded partially by antiferromagnetic CoO phases within an amorphous matrix dominated by HfO2. The presence of large CoO could weaken the exchange coupling of Fe-Fe(Co), thereby leading to a reduction in 4πMs. It should be noted, the CoO magnetic oxide also formed the basis for exchange coupling as well as provided a source for a preferential axis in the form of Co2+ ions. Since Co2+ ions possess large single-ion anisotropy, a small preferential occupation of these ions in octahedral sites with direction could be a source for large anisotropy and hence for an increase in HkH for sample No3 [8]. However, it alone cannot account for the large value of HkH for sample No3. It should be noted herein that unlike the structure of sample No1, 2 and 4, sample No3 had a lamilate nanostructure, which consisted of nanocrystalline α-Fe(Co)-rich layers separated by amorphous HfO2-rich layers. This structure can be considered as a [Fe(Co)-rich/HfO2-rich]n multilayer, where a exchange coupling of Fe–Fe(Co) takes place between two neighboring ferromagnetic layers through an insulating amorphous HfO2-rich layer. The Fe(Co) nanograins were distributed along a preferential direction in which the dc magnetic field of 100 Oe was applied during the fabrication process [see Fig. 3(c) and its insets] and this may also give rise to a preferentially uniaxial anisotropy. It is therefore concluded that the laminate nanostructure resulted in the superior properties of sample No3. The ferromagnetism of sample No4 was signifcantly modified as a direct consequence of the strong development of the antiferromagnetic CoO phase [see Fig. 2 and 3(d)] and the reduction of the ferromagnetic nanocrystalline Fe(Co)-rich phase.
4
Conclusions
The magnetic properties of the Co–Fe–Hf–O thin films were thoroughly investigated. The Co19.35Fe53.28Hf7.92O19.35 film exhibited the softest magnetic property, such as high saturation magnetization, 4πMs ∼ 19.86 kG, low coercivity, Hc ∼ 1.5 Oe and high electrical resistivity, ρ = 3569 µΩcm. In particular, the magnetic permeability remained almost constant up to 3 GHz and reached a maximum at the ferromagnetic resonant frequency of 4.024 GHz. The excellent magnetic properties of this film can be attributed to the formation of a novel laminate nanostructure. The Co19.35Fe53.28Hf7.92O19.35 films are excellent candidate materials for high-frequency micromagnetic device applications, such as magnetic thin film inductors, transformers, and thin film flux gate sensors. Acknowledgements The support by the BK 21 program at Chungbuk National University and the Research Center for Advanced Magnetic Materials (ReCAMM) at Chungnam National University is gratefully acknowledged.
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