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FeCoB sandwiched films with Permalloy underlayer
ARTICLE IN PRESS
Vacuum 82 (2008) 491–494 www.elsevier.com/locate/vacuum
Magnetoimpedance effect in FeCoB/Cu/FeCoB sandwiched films with Permalloy underlayer Zhiyong Zhonga,, Huaiwu Zhanga, Yulan Jinga, Xiaoli Tanga, Li Zhangb, Shuang Liuc a
State Key Laboratory of Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China b Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China c School of Opto-electronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China Received 12 May 2007; received in revised form 17 July 2007; accepted 23 July 2007
Abstract We studied the magnetoimpedance (MI) effect of FeCoB(100 nm)/Cu(100 nm)/FeCoB(100 nm) sandwiched films with different thickness of Permalloy as underlayer for the FeCoB ferromagnetic layer. The maximum MI ratio of sandwiched film is 9.2% when the thickness of the Permalloy underlayer is 2–3 nm. The improvement of MI ratio of sandwiched films with Permalloy underlayer was explained by exchange induced ripple reduction mechanism. r 2007 Elsevier Ltd. All rights reserved. PACS: 75.70.I; 75.30.Gw; 75.47.Np Keywords: Magnetoimpedance (MI); Magnetic thin films; Amorphous magnetic materials
1. Introduction The magnetoimpedance (MI) effect is defined as the change of the impedance experienced by an AC current flowing through magnetic materials when an external DC magnetic field is applied. This effect is promising in the application of small field sensors with high sensitivity and quick response [1,2]. Most studies performed on MI have been carried out on amorphous or nanocrystalline soft magnetic wires or ribbons. The use of thin film technology for MI is preferable in many applications because of its compatibility with integrated circuit technology. The investigation of MI in thin film systems includes a number of sandwiched systems with different composition, magnetic structure, and size [3–7]. But compared to wires or ribbons, the single magnetic thin films typically exhibit a lower MI sensitivity because of a higher anisotropy field induced through the fabrication process and annealing [8]. A high sensitivity MI has been reported to occur in Corresponding author. Tel.: +86 28 83201440; fax: +86 28 83201810.
E-mail addresses: [email protected], [email protected] (Z. Zhong). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.07.057
ferromagnetic layer(F)/non-ferromagnetic metal layer(M)/ ferromagnetic layer(F) sandwiched films, in which the ratio of the impedance change is several times larger than that in a similar ferromagnetic single-layer film [4–7,9]. For example, the MI ratio of a CoSiB/Cu/CoSiB sandwiched film (7 mm thick) is 340% at a frequency of 10 MHz and DC magnetic field of 9 Oe [8]. In the sandwiched structure F/M/F, a very large change in the impedance which is related to the outer magnetic layers becomes larger than the resistance determined mainly by the inner conductor [10]. Due to this advantage, MI in sandwiched films could have a potential application in developing sensitive micromagnetic sensors and magnetic heads for highdensity magnetic recording [11]. The MI effects of sandwiched films made of a soft ferromagnetic alloy of composition have been extensively studied. Those films are Co-riched CoSiB, CoFeSiB, CoNbZr, and Permalloy, etc. However, all those films have relatively low saturation magnetization. If the film is magnetically soft and has a well-defined anisotropy axis and large saturation magnetization, it will help enhance the MI effect due to the increased interaction with the external magnetic field [12]. It is well known that FeCo alloy thin
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films exhibit very large saturation magnetization. However, FeCo thin films also have very large saturation magnetostriction [13]. Such large saturation magnetostriction causes the degradation of soft magnetic properties. It has been proved that adding the third element such as B, N to form FeCobased amorphous or nanocrystalline materials can exhibit excellent soft magnetic properties with a lower coercivity. Among those compounds, Fe67 Co18 B15 (at%) amorphous thin film has been reported to be fabricated on a glass substrate [14]. But the dispersion of the induced magnetic anisotropy was too high to obtain high sensitivity for MI elements [12]. Wang et al. [15] have demonstrated that single layer thin film of Fe–Co–X (X ¼ N, B, etc) can be made magnetically soft by sandwiching the Fe–Co–X layer between two Permalloy layers. It was found that the Permalloy underlayer, rather than the cap layer, is more important in achieving low coercivity field and relatively low dispersion of magnetic anisotropy in Fe–Co–X films [16]. In this work, we investigated the MI effect of F/M/F, where F is FeCoB with different thickness of the Permalloy underlayer systematically. Because thinner films are favorable for microelectronic integration, we studied the MI effect in thin films under 1 mm thick.
The base pressure of the sputtering chamber was 2 107 Torr. A constant magnetic field of about 200 Oe was applied in parallel to the film surface during deposition. In order to eliminate the effect of stress formed by sputtering, the films were post-annealed in a high vacuum of 5 106 Torr at under 300 1C and 300 Oe applied magnetic field along the induced easy axis formed by sputtering for one half hour. Magnetic properties and structure of films were characterized by vibrating sample magnetometer (VSM) and X-ray diffraction (XRD), respectively. MI measurements on the sandwiched films were carried out using an HP4291B impedance analyzer. The RMS value of the AC current was kept at constant 10 mA and its frequency was varied from 1 to 500 MHz. The DC magnetic field was generated by a solenoid. The maximum intensity of the external magnetic field H max ¼ 100 Oe. In order to avoid the effect of earth’s magnetic field, the magnetic field of the solenoid was perpendicular to the earth field direction. In order to avoid the negative value, the MI ratio is defined as ½ZðHÞ Zð0Þ=Zð0Þ, where ZðHÞ and Zð0Þ are the MIs with and without the applied DC magnetic field, respectively. 3. Results and discussion
2. Experiment A schematic diagram of the Si substrate/Permalloy/ FeCoB/Cu/Permalloy/FeCoB films is shown in Fig. 1, with the top view in (a) and cross-section in (b). The films are composed of an inner Cu layer with two extended electrodes. The thickness of the Cu layer tc is 100 nm, and its width wc is 0.1 mm. The tm is the thickness of each FeCoB/Permalloy layer. The thickness of FeCoB is fixed at 100 nm, while that of Permalloy ranges from 0 to 10 nm. The width of the ferromagnetic layer wm is 2 mm, and the length l m is 10 mm. The films were prepared by a magnetron sputtering method. Element shapes were formed by deposition on thermally oxidized Si wafers with slit metal sheet masking. The nominal composition of the magnetic layer target is Fe67Co18B15 (at%), and the underlayer Permalloy is Ni81Fe19 (wt%). The purity of both targets is 99.9%.
Fig. 1. Schematic diagram of sandwiched GMI element: (a) top view and (b) cross-section view.
We applied XRD to confirm the amorphous structure of the as-deposited and post-annealed FeCoBð100 nmÞ= NiFeðtÞ films. The range of the thickness t is from 0 to 10 nm. The DC magnetization loops of the as-deposited and post-annealed FeCoBð100 nmÞ=NiFeðtÞ films were carried out along different directions in the film plane. It is found that there exists a strong uniaxial anisotropy in the film plane, in which an easy axis is shown to be parallel to the direction of applied field during deposition, and a hard axis perpendicular to the easy axis. All the films exhibit excellent soft magnetic properties with low coercivity. The induced uniaxial anisotropy comes from the directional atomic ordering mechanism for those amorphous thin films [17]. However, the as-deposited single FeCoB film is found to have a coercivity along the easy axis of 3:5 Oe and shows a uniaxial anisotropy H k of 40 Oe in the film plane. But upon annealing at 300 1C for 0.5 h, the magnetic properties of films changes significantly. The coercivity along the easy axis of films decreases to a much smaller value of 1:8 Oe, and the anisotropy field decreases to 23 Oe. When the thickness of FeCoB films is fixed at 100 nm, and the thickness of Permalloy underlayer varies, we found that the coercivity along the easy axis drops from 1.8 Oe to about 0.6 Oe when a Permalloy underlayer of 1 nm thick is placed beneath the FeCoB layer, and then it stays in between 0.5 and 0.8 Oe when the Permalloy thickness is less than 4 nm. The difference of the induced anisotropy between single FeCoB film and FeCoB films with Permalloy underlayer is small when the thickness of Permalloy underlayer is less than 4 nm. However, when the thickness of the Permalloy underlayer exceeds 4 nm,
ARTICLE IN PRESS Z. Zhong et al. / Vacuum 82 (2008) 491–494
the induced anisotropy increases dramatically to 40 Oe (as shown in Fig. 2). The improvement of soft magnetic materials of the post-annealed films can attribute to the elimination of stress formed by sputtering process. The variation of the induced anisotropy of the FeCoB films with underlayer could be related with change in domain structure in films, and will discuss it in the following section. We measured the evolution of the impedance change as a function of frequency for F/M/F sandwich films without Permalloy underlayer. We found that the maximum change of the impedance appeared at 450 MHz. Therefore, the MI ratio of all those samples was measured at a fixed frequency of 450 MHz. Fig. 3 shows the relationship between the impedance change and the applied field at
493
450 MHz for the post-annealed sputtered FeCoB with different thicknesses of Permalloy underlayer. As observed from Fig. 3, the change in the MI ratio first increases with the applied field, reaches a maximum value at a certain field, and then gradually drops with continued increase of the field. This is a typical profile of the impedance characteristics of films with transverse anisotropy. For the case of the well-defined transverse anisotropy, the maximum MI ratio should appear at near H k [12]. As pointed in Fig. 2, the H k of the FeCoB single layer and the FeCoB/Permalloy underlayer (when the thickness less than 4 nm) measured by VSM are very close, about 20 Oe. But in Fig. 3, we can see that peak fields, which corresponding to the maximum MI ratio are quite different. This phenomenon can be explained by the dispersion of transverse anisotropy or transverse permeability. It has been pointed in [18] ! d 2d 1 Z ¼ Rm 1 2jmt 2 , (1) d1 where Rm is the resistance of the conductive lead, mt the transverse permeability, d 1 and d 2 the thickness of the conductive and magnetic layers, respectively, d1 is the skindepth in the inner conductor. Meanwhile, based on ripple theory, the transverse permeability mt can be described by [19] mt ¼
Fig. 2. Dependence of coercivity along easy axis and induced magnetic anisotropy field of after stress-eliminated annealing films on Permalloy underlayer.
4pM s 1=4
H K ðh0 þ B h0
,
(2)
Þ
where h0 ¼ ðH=H k Þ 1, B is the ripple stray field parameter, which is approximately proportional to the dispersion angle a of the local magnetic moments [20], and H is the applied magnetic field. The dispersion angle a is defined as the angle
Fig. 3. Dependence on MI effects of post-annealed FeCoB with different thickness of Permalloy underlayer.
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4. Conclusion The MI effect of FeCoB(100 nm)/Cu(100 nm)/ FeCoB(100 nm) sandwiched films with different thickness of Permalloy as underlayer for FeCoB ferromagnetic layer were studied. Through optimizing the thickness of the Permalloy underlayer, it can be obtained an optimum MI ratio in FeCoB/NiFe double layers, which is favorable for improving the MI effect of F/M/F sandwiched films. The improvement of MI ratio ascribes to the exchange induced ripple reduction mechanism between the intrinsically soft Permalloy underlayer and the FeCoB layer. This study shows that it is possible to enhance MI effect through changing the structure of films.
Fig. 4. Dispersion angle of FeCoB film on Permalloy underlayer, with error bar showing the range of data scattering.
between the hard axis direction and the applied field at which the remanence is 90% during magnetic reversal, and this parameter can be obtained from B–H loops [21]. Fig. 4 shows the dispersion angle a of films. The dispersion angle is about 2.01 for the single layer FeCoB, while for the Peramlloy underlayer thickness in the range of 2–3 nm, a shows a minimum value of about 0.81 for the FeCoB/Permalloy film. Because the ripple stray field parameter B is related to the dispersion angle, the smaller dispersion angle corresponds to a smaller B, according to Eq. (2), the film with a Permalloy underlayer thickness of 2–3 nm will have the highest transverse permeability, therefore will have the largest MI ratio according to Eq. (1). While increasing the dispersion angle, the ripple stray field parameter B will increase, then lead to the decrease of the transverse permeability and the increase of the blocking field where corresponds to exhibit the maximum transverse permeability [18]. From the inset of Fig. 3, one can observe that the maximum MI change for the sandwiched film in our experiments appear at 2–3 nm thickness of Permalloy underlayer, and the maximum of MI ratio is 9.2%. This is due to the improved alignment of the induced anisotropy pointed out by Atkinson and Squire [21]. The reason for the Permalloy underlayer improving the local moment alignment in the FeCoB films can be ascribed to exchange coupling effect between the FeCoB layer and the Permalloy underlayer, which is intrinsically soft and well aligned with a low dispersion angle [22]. Regarding to the 4 nm thick Permalloy underlayer, the MI drops to 4.7%. For thicker Permalloy underlayer, inplane and out-of-plane XRD studies [23] clarify that the lattice spacing of planes along the easy axis direction is expanded than that along the hard axis direction. The difference would result in a compressive stress along the hard axis direction of the film, which plays an important role to induce a high magnetic anisotropy field in the double layer. According to Eqs. (1) and (2), the high magnetic anisotropy causes the decrease of the MI ratio.
Acknowledgments The work is supported in part by the National Natural Science Foundation of China under Grant nos. 60490296 and 60671029, the Key Project of Ministry of Education of China (no. 105151), and the Youth Science & Technology Foundation of Sichuan (no. 04ZQ026-003).
References [1] Beach RS, Berkowitz AE. Appl Phys Lett 1994;64:3652. [2] Knobel M, Pirota KR. J Magn Magn Mater 2002;242–245:33. [3] Senda M, Ishii O, Koshimoto Y, Toshima T. IEEE Trans Magn 1994;30:3400. [4] Morikawa T, Nishibe Y, Yamadera H, et al. IEEE Trans Magn 1996;32:4965. [5] Nishibe Y, Yamadera H, Ohta N, et al. IEEE Trans Magn 2003; 39:571. [6] Giouroudi I, Hauser H, Musiejovsky L, et al. J Appl Phys 2005;97:10M109. [7] de Cos D, Panian LV, Fry N, et al. IEEE Trans Magn 2005;41:3697. [8] Uhiyama T, Mohri K, Panina LV, Furuno K. IEEE Trans Japan 1995;19(115-A):949. [9] Morikawa T, Nishibe Y, Yamadera H. IEEE Trans Magn 1995; 33:4367. [10] Hika K, Pania LV, Mohri K. IEEE Trans Magn 1995;33:4367. [11] Panina LV, Mohri K. Sensors Actuators 2000;81:71. [12] Tannous C, Gigeraltowski J. J Mater Sci 2004;15:125. [13] Bozorth RM, Ferromagnetism. New York: IEEE Press; 1993. p. 194. [14] Chen LH, Kiemmer TJ, Ellis KA, van Dover RB, Jin S. J Appl Phys 2000;87:5858. [15] Wang SX, Sun NX, Yamaguchi M, Yabukami S. Nature 2000; 407:6801. [16] Sun NX, Wang SX, Hung C-y, Chien CX, Tong HC. Mater Res Soc Symp Proc 2000;614:F9.2. [17] O’Hnadley RC, Modern magnetic materials—principles and applications. New York: Wiley; 2000. p. 505. [18] Makhnovsky DP, Panina LV. Sensors Actuators A Phys 2000;81:91. [19] Hoffman H. Thin Solid Films 2000;373:107. [20] Bayreuther G, Hoffmann H. Czech J Phys B 1971;21:541. [21] Atkinson D, Squire PT. IEEE Trans Magn 1997;33:3364. [22] Nakatani R, Kobayashi T, Ootomo S, Kumasaka N. Jpn J Appl Phys 1988;27:937. [23] Kong SH, Okamoto T, Nakagawa S. J Magn Magn Mater 2004; 272–276:2184.
Vacuum 82 (2008) 491–494 www.elsevier.com/locate/vacuum
Magnetoimpedance effect in FeCoB/Cu/FeCoB sandwiched films with Permalloy underlayer Zhiyong Zhonga,, Huaiwu Zhanga, Yulan Jinga, Xiaoli Tanga, Li Zhangb, Shuang Liuc a
State Key Laboratory of Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China b Department of Applied Physics, University of Electronic Science and Technology of China, Chengdu 610054, China c School of Opto-electronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China Received 12 May 2007; received in revised form 17 July 2007; accepted 23 July 2007
Abstract We studied the magnetoimpedance (MI) effect of FeCoB(100 nm)/Cu(100 nm)/FeCoB(100 nm) sandwiched films with different thickness of Permalloy as underlayer for the FeCoB ferromagnetic layer. The maximum MI ratio of sandwiched film is 9.2% when the thickness of the Permalloy underlayer is 2–3 nm. The improvement of MI ratio of sandwiched films with Permalloy underlayer was explained by exchange induced ripple reduction mechanism. r 2007 Elsevier Ltd. All rights reserved. PACS: 75.70.I; 75.30.Gw; 75.47.Np Keywords: Magnetoimpedance (MI); Magnetic thin films; Amorphous magnetic materials
1. Introduction The magnetoimpedance (MI) effect is defined as the change of the impedance experienced by an AC current flowing through magnetic materials when an external DC magnetic field is applied. This effect is promising in the application of small field sensors with high sensitivity and quick response [1,2]. Most studies performed on MI have been carried out on amorphous or nanocrystalline soft magnetic wires or ribbons. The use of thin film technology for MI is preferable in many applications because of its compatibility with integrated circuit technology. The investigation of MI in thin film systems includes a number of sandwiched systems with different composition, magnetic structure, and size [3–7]. But compared to wires or ribbons, the single magnetic thin films typically exhibit a lower MI sensitivity because of a higher anisotropy field induced through the fabrication process and annealing [8]. A high sensitivity MI has been reported to occur in Corresponding author. Tel.: +86 28 83201440; fax: +86 28 83201810.
E-mail addresses: [email protected], [email protected] (Z. Zhong). 0042-207X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2007.07.057
ferromagnetic layer(F)/non-ferromagnetic metal layer(M)/ ferromagnetic layer(F) sandwiched films, in which the ratio of the impedance change is several times larger than that in a similar ferromagnetic single-layer film [4–7,9]. For example, the MI ratio of a CoSiB/Cu/CoSiB sandwiched film (7 mm thick) is 340% at a frequency of 10 MHz and DC magnetic field of 9 Oe [8]. In the sandwiched structure F/M/F, a very large change in the impedance which is related to the outer magnetic layers becomes larger than the resistance determined mainly by the inner conductor [10]. Due to this advantage, MI in sandwiched films could have a potential application in developing sensitive micromagnetic sensors and magnetic heads for highdensity magnetic recording [11]. The MI effects of sandwiched films made of a soft ferromagnetic alloy of composition have been extensively studied. Those films are Co-riched CoSiB, CoFeSiB, CoNbZr, and Permalloy, etc. However, all those films have relatively low saturation magnetization. If the film is magnetically soft and has a well-defined anisotropy axis and large saturation magnetization, it will help enhance the MI effect due to the increased interaction with the external magnetic field [12]. It is well known that FeCo alloy thin
ARTICLE IN PRESS 492
Z. Zhong et al. / Vacuum 82 (2008) 491–494
films exhibit very large saturation magnetization. However, FeCo thin films also have very large saturation magnetostriction [13]. Such large saturation magnetostriction causes the degradation of soft magnetic properties. It has been proved that adding the third element such as B, N to form FeCobased amorphous or nanocrystalline materials can exhibit excellent soft magnetic properties with a lower coercivity. Among those compounds, Fe67 Co18 B15 (at%) amorphous thin film has been reported to be fabricated on a glass substrate [14]. But the dispersion of the induced magnetic anisotropy was too high to obtain high sensitivity for MI elements [12]. Wang et al. [15] have demonstrated that single layer thin film of Fe–Co–X (X ¼ N, B, etc) can be made magnetically soft by sandwiching the Fe–Co–X layer between two Permalloy layers. It was found that the Permalloy underlayer, rather than the cap layer, is more important in achieving low coercivity field and relatively low dispersion of magnetic anisotropy in Fe–Co–X films [16]. In this work, we investigated the MI effect of F/M/F, where F is FeCoB with different thickness of the Permalloy underlayer systematically. Because thinner films are favorable for microelectronic integration, we studied the MI effect in thin films under 1 mm thick.
The base pressure of the sputtering chamber was 2 107 Torr. A constant magnetic field of about 200 Oe was applied in parallel to the film surface during deposition. In order to eliminate the effect of stress formed by sputtering, the films were post-annealed in a high vacuum of 5 106 Torr at under 300 1C and 300 Oe applied magnetic field along the induced easy axis formed by sputtering for one half hour. Magnetic properties and structure of films were characterized by vibrating sample magnetometer (VSM) and X-ray diffraction (XRD), respectively. MI measurements on the sandwiched films were carried out using an HP4291B impedance analyzer. The RMS value of the AC current was kept at constant 10 mA and its frequency was varied from 1 to 500 MHz. The DC magnetic field was generated by a solenoid. The maximum intensity of the external magnetic field H max ¼ 100 Oe. In order to avoid the effect of earth’s magnetic field, the magnetic field of the solenoid was perpendicular to the earth field direction. In order to avoid the negative value, the MI ratio is defined as ½ZðHÞ Zð0Þ=Zð0Þ, where ZðHÞ and Zð0Þ are the MIs with and without the applied DC magnetic field, respectively. 3. Results and discussion
2. Experiment A schematic diagram of the Si substrate/Permalloy/ FeCoB/Cu/Permalloy/FeCoB films is shown in Fig. 1, with the top view in (a) and cross-section in (b). The films are composed of an inner Cu layer with two extended electrodes. The thickness of the Cu layer tc is 100 nm, and its width wc is 0.1 mm. The tm is the thickness of each FeCoB/Permalloy layer. The thickness of FeCoB is fixed at 100 nm, while that of Permalloy ranges from 0 to 10 nm. The width of the ferromagnetic layer wm is 2 mm, and the length l m is 10 mm. The films were prepared by a magnetron sputtering method. Element shapes were formed by deposition on thermally oxidized Si wafers with slit metal sheet masking. The nominal composition of the magnetic layer target is Fe67Co18B15 (at%), and the underlayer Permalloy is Ni81Fe19 (wt%). The purity of both targets is 99.9%.
Fig. 1. Schematic diagram of sandwiched GMI element: (a) top view and (b) cross-section view.
We applied XRD to confirm the amorphous structure of the as-deposited and post-annealed FeCoBð100 nmÞ= NiFeðtÞ films. The range of the thickness t is from 0 to 10 nm. The DC magnetization loops of the as-deposited and post-annealed FeCoBð100 nmÞ=NiFeðtÞ films were carried out along different directions in the film plane. It is found that there exists a strong uniaxial anisotropy in the film plane, in which an easy axis is shown to be parallel to the direction of applied field during deposition, and a hard axis perpendicular to the easy axis. All the films exhibit excellent soft magnetic properties with low coercivity. The induced uniaxial anisotropy comes from the directional atomic ordering mechanism for those amorphous thin films [17]. However, the as-deposited single FeCoB film is found to have a coercivity along the easy axis of 3:5 Oe and shows a uniaxial anisotropy H k of 40 Oe in the film plane. But upon annealing at 300 1C for 0.5 h, the magnetic properties of films changes significantly. The coercivity along the easy axis of films decreases to a much smaller value of 1:8 Oe, and the anisotropy field decreases to 23 Oe. When the thickness of FeCoB films is fixed at 100 nm, and the thickness of Permalloy underlayer varies, we found that the coercivity along the easy axis drops from 1.8 Oe to about 0.6 Oe when a Permalloy underlayer of 1 nm thick is placed beneath the FeCoB layer, and then it stays in between 0.5 and 0.8 Oe when the Permalloy thickness is less than 4 nm. The difference of the induced anisotropy between single FeCoB film and FeCoB films with Permalloy underlayer is small when the thickness of Permalloy underlayer is less than 4 nm. However, when the thickness of the Permalloy underlayer exceeds 4 nm,
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the induced anisotropy increases dramatically to 40 Oe (as shown in Fig. 2). The improvement of soft magnetic materials of the post-annealed films can attribute to the elimination of stress formed by sputtering process. The variation of the induced anisotropy of the FeCoB films with underlayer could be related with change in domain structure in films, and will discuss it in the following section. We measured the evolution of the impedance change as a function of frequency for F/M/F sandwich films without Permalloy underlayer. We found that the maximum change of the impedance appeared at 450 MHz. Therefore, the MI ratio of all those samples was measured at a fixed frequency of 450 MHz. Fig. 3 shows the relationship between the impedance change and the applied field at
493
450 MHz for the post-annealed sputtered FeCoB with different thicknesses of Permalloy underlayer. As observed from Fig. 3, the change in the MI ratio first increases with the applied field, reaches a maximum value at a certain field, and then gradually drops with continued increase of the field. This is a typical profile of the impedance characteristics of films with transverse anisotropy. For the case of the well-defined transverse anisotropy, the maximum MI ratio should appear at near H k [12]. As pointed in Fig. 2, the H k of the FeCoB single layer and the FeCoB/Permalloy underlayer (when the thickness less than 4 nm) measured by VSM are very close, about 20 Oe. But in Fig. 3, we can see that peak fields, which corresponding to the maximum MI ratio are quite different. This phenomenon can be explained by the dispersion of transverse anisotropy or transverse permeability. It has been pointed in [18] ! d 2d 1 Z ¼ Rm 1 2jmt 2 , (1) d1 where Rm is the resistance of the conductive lead, mt the transverse permeability, d 1 and d 2 the thickness of the conductive and magnetic layers, respectively, d1 is the skindepth in the inner conductor. Meanwhile, based on ripple theory, the transverse permeability mt can be described by [19] mt ¼
Fig. 2. Dependence of coercivity along easy axis and induced magnetic anisotropy field of after stress-eliminated annealing films on Permalloy underlayer.
4pM s 1=4
H K ðh0 þ B h0
,
(2)
Þ
where h0 ¼ ðH=H k Þ 1, B is the ripple stray field parameter, which is approximately proportional to the dispersion angle a of the local magnetic moments [20], and H is the applied magnetic field. The dispersion angle a is defined as the angle
Fig. 3. Dependence on MI effects of post-annealed FeCoB with different thickness of Permalloy underlayer.
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4. Conclusion The MI effect of FeCoB(100 nm)/Cu(100 nm)/ FeCoB(100 nm) sandwiched films with different thickness of Permalloy as underlayer for FeCoB ferromagnetic layer were studied. Through optimizing the thickness of the Permalloy underlayer, it can be obtained an optimum MI ratio in FeCoB/NiFe double layers, which is favorable for improving the MI effect of F/M/F sandwiched films. The improvement of MI ratio ascribes to the exchange induced ripple reduction mechanism between the intrinsically soft Permalloy underlayer and the FeCoB layer. This study shows that it is possible to enhance MI effect through changing the structure of films.
Fig. 4. Dispersion angle of FeCoB film on Permalloy underlayer, with error bar showing the range of data scattering.
between the hard axis direction and the applied field at which the remanence is 90% during magnetic reversal, and this parameter can be obtained from B–H loops [21]. Fig. 4 shows the dispersion angle a of films. The dispersion angle is about 2.01 for the single layer FeCoB, while for the Peramlloy underlayer thickness in the range of 2–3 nm, a shows a minimum value of about 0.81 for the FeCoB/Permalloy film. Because the ripple stray field parameter B is related to the dispersion angle, the smaller dispersion angle corresponds to a smaller B, according to Eq. (2), the film with a Permalloy underlayer thickness of 2–3 nm will have the highest transverse permeability, therefore will have the largest MI ratio according to Eq. (1). While increasing the dispersion angle, the ripple stray field parameter B will increase, then lead to the decrease of the transverse permeability and the increase of the blocking field where corresponds to exhibit the maximum transverse permeability [18]. From the inset of Fig. 3, one can observe that the maximum MI change for the sandwiched film in our experiments appear at 2–3 nm thickness of Permalloy underlayer, and the maximum of MI ratio is 9.2%. This is due to the improved alignment of the induced anisotropy pointed out by Atkinson and Squire [21]. The reason for the Permalloy underlayer improving the local moment alignment in the FeCoB films can be ascribed to exchange coupling effect between the FeCoB layer and the Permalloy underlayer, which is intrinsically soft and well aligned with a low dispersion angle [22]. Regarding to the 4 nm thick Permalloy underlayer, the MI drops to 4.7%. For thicker Permalloy underlayer, inplane and out-of-plane XRD studies [23] clarify that the lattice spacing of planes along the easy axis direction is expanded than that along the hard axis direction. The difference would result in a compressive stress along the hard axis direction of the film, which plays an important role to induce a high magnetic anisotropy field in the double layer. According to Eqs. (1) and (2), the high magnetic anisotropy causes the decrease of the MI ratio.
Acknowledgments The work is supported in part by the National Natural Science Foundation of China under Grant nos. 60490296 and 60671029, the Key Project of Ministry of Education of China (no. 105151), and the Youth Science & Technology Foundation of Sichuan (no. 04ZQ026-003).
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