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The adjustable anisotropy field in FeCoTiO/SiO2/FeCoTiO trilayer films by oblique sputtering and stripe patterning Yicheng Wang, Huaiwu Zhang, Luo Wang, and Feiming Bai Citation: AIP Advances 6, 055912 (2016); doi: 10.1063/1.4943243 View online: http://dx.doi.org/10.1063/1.4943243 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/6/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Controlling the microwave characteristic of FeCoB-ZnO granular thin films deposited by obliquely sputtering J. Appl. Phys. 117, 103902 (2015); 10.1063/1.4914033 Ferromagnetic resonance study of the misalignment between anisotropy axes in exchange-biased NiFe/FeMn/Co trilayers Appl. Phys. Lett. 104, 202403 (2014); 10.1063/1.4875929 Stress competition and vortex magnetic anisotropy in FeCoAlO high-frequency soft magnetic films with gradient Al-O contents J. Appl. Phys. 113, 17A332 (2013); 10.1063/1.4799480 Multilayer nanogranular films (Co40Fe40B20)50(SiO2)50/α-Si:H and (Co40Fe40B20)50(SiO2)50/SiO2: Magnetic properties J. Appl. Phys. 113, 17C105 (2013); 10.1063/1.4794361 FeCoSiN film with ordered FeCo nanoparticles embedded in a Si-rich matrix Appl. Phys. Lett. 90, 112506 (2007); 10.1063/1.2714280

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AIP ADVANCES 6, 055912 (2016)

The adjustable anisotropy field in FeCoTiO/ SiO2/ FeCoTiO trilayer films by oblique sputtering and stripe patterning Yicheng Wang, Huaiwu Zhang, Luo Wang, and Feiming Baia State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology, Chengdu, 610054, China

(Presented 12 January 2016; received 6 November 2015; accepted 15 December 2015; published online 1 March 2016)

A series of FeCoTiO thin films were deposited on Si (100) substrates using oblique sputtering and stripe patterning at the same time and the static and high frequency magnetic properties were studied in details. For the single-layered films, if the anisotropy fields induced by the two methods are in the same direction, the effective anisotropy field will be greatly enhanced, closed to 300 Oe. But if the two anisotropy fields are perpendicular to each other, there will be an opposite result. In the FM/NM/FM sandwich structures, the influence of shape anisotropy will be suppressed by the exchange coupling effect between the two FM layers. The resonance frequency and permeability are still above 3.5 GHz and 75 even the width of stripes change from 40 µm to 10 µm. C 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/1.4943243] INTRODUCTION

Soft magnetic thin films are being widely used for electromagnetic devices as high frequency components.1,2 With the increasing of operational frequency and degree of integration, the requirements to materials become increasingly high and a great deal of research effort about the soft magnetic thin films has been devoted to solve this problem.3–6 The high frequency properties of soft magnetic thin films are determined by the in-plane anisotropy field. Oblique sputtering and stripe patterning are the two of the most effective method to adjust the anisotropy field, so that the ferromagnetic resonance frequency and permeability.7,8 It will be very interesting if we use the two methods at the same time. On the other hand, in practical applications such as on-chip solenoid inductors,2 the size of patterned rectangular magnetic cores will be reduced to micron meter, and the long axis is required to be the hard axis of magnetic cores. So it is important to find a preparation method of thin films which can avoid the influence of shape anisotropy causing by the demagnetizing field. In current work, metal-insulator nanogranular films have been selected because of their excellent soft magnetic properties and high electrical resistivity. The TiO2 has also been chosen as nonmagnetic material to improve the soft magnetic properties and high frequency performance.9 Oblique sputtering and stripe patterning were both used to adjust the anisotropy field. A ferromagnetic/nonmagnetic/ferromagnetic (FM/NM/FM) sandwich structures were also fabricated to reduce the influence of demagnetizing field.

EXPERIMENTS

A series of FeCoTiO thin films were deposited on Si (100) substrates by RF magnetron sputtering using a Fe65Co35 target with TiO2 chips symmetrically placed on the erosion race-track. The films were deposited with an oblique angle of 25 degree, as shown in Figure 1(a). The films were

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FIG. 1. The schematic drawing of the sputtering device (a); the schematic drawing of the stripe line which it is parallel (b) and perpendicular (c) with x axis; the schematic drawing of FeCoTiO/SiO2/FeCoTiO sandwich structure, the stripe line is perpendicular with x axis.

also etched into stripes by conventional optical lithography and lift-off method. The designed widths of stripes were varied from 10 to 40 µm. The designed lengths of stripes were fixed as 2 µm. The stripe line was parallel and perpendicular to x axis to adjust the anisotropic field, shown in figure 1(b) and 1(c). The thickness was fixed at 300 nm for all the single layer films. In addition, a FeCoTiO/SiO2/FeCoTiO trilayer films were prepared to produce the easy-axis state in the element. The structure was shown in figure 1(d), the stripe axis was perpendicular to x axis, and the top layer was rotated 180 degree related to the bottom layer. The thickness of each magnetic layer was set as 300 nm, and the thickness of SiO2 was set as 10 nm. The base pressure was better than 2.0*10−4 Pa. The pressure of Ar gas and the sputtering power were optimized to 0.25 Pa and 250 W, respectively. No magnetic field was applied in the plane of the films during deposition. The phase structures of magnetic films were characterized by X-ray diffraction (XRD, Bede TM 2000, Dandong). The local microcompositions of the films were analyzed by energy dispersive x-ray spectroscopy (EDS, GENESIS-2000). The results justify that the films contain bcc FeCo nanograins embedded in a TiO2 amorphous matrix.9 The thickness of films was measured by a step profiler. The magnetic hysteresis loops were measured at room temperature by a vibrating sample magnetometer (VSM, BHV-525, Japan). The complex permeability spectra were obtained with an Agilent network analyzer (N5230A) using a shorted microstrip transmission-line perturbation method without external magnetic field.10

RESULTS AND DISCUSSION

Figure 2 shows the M-H loops and permeability spectra of a whole continuous film which was deposited with an oblique angle of 25 degree. It is found that the easy axis is parallel to x axis. The static saturation magnetization is keeping at 17.6 kGs. The anisotropy field (Hk ) and ferromagnetic resonance ( f r ) are 105 Oe and 3.8 GHz, respectively. It is well known that oblique deposition can induce the in-plane anisotropy field for soft magnetic films, and oblique angles also affect the intensity of anisotropy field.11–13 Figure 3 shows the M-H loops of the FeCo-TiO2 single layer films deposited by oblique sputtering and stripe patterning. The stripe lengthwise direction is parallel (a, b) and perpendicular (c, d) to the easy axis (induced by oblique sputtering, axis x). The size of stripe is set as 2000*30 µm2 (a, c) and 2000*10 µm2 (b, d) respectively. As can be seen, when the stripe lengthwise direction is parallel to the x axis, that is, the shape anisotropy and oblique sputtering anisotropy are in the same direction, the final anisotropy fields of the films will have a great enhancement. When the size of stripe is 2000*10 µm2, the anisotropy field closes to 300 Oe. On the other hand, when the stripe lengthwise direction is perpendicular to the x axis, two anisotropy fields will compete


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FIG. 2. The M-H loops (a) and permeability spectra (b) of FeCoTiO single layer film deposited by oblique sputtering.

with each other. As a result, the effective anisotropy decreases from 150 Oe to about 80 Oe for the 2000*30 µm2 pattered stripes. However, for the 2000*10 µm2 pattered stripes with a larger aspect ratio and a higher demagnetization factor, the effective anisotropic field drops from 300 Oe to less than 100 Oe. In addition, the coecivity field also increases, probably due to the formation of multiple flux closure domains. The same results are also shown in the permeability spectra. Figure 4 shows the permeability spectra of the FeCo-TiO2 single layer films deposited by oblique sputtering and stripe patterning. When the stripe lengthwise direction is parallel to the x axis, the ferromagnetic resonance frequencies are 4.7 and 6.4 GHz for the films with the width of stripe are 30 and 10 µm respectively. When the stripe lengthwise direction is perpendicular to the x axis, for the films with 30 µm width, a FMR frequency of 3.6 GHz is shown in the permeability spectra. However, when the width decreases to 10 µm, there is no peak related to the FMR and the relative permeability almost decreases to zero. This means there is no in-plane anisotropy in the films. The above results show that, for

FIG. 3. The M-H loops of FeCoTiO single layer films deposited by oblique sputtering and stripe patterning. The easy axis (induced by oblique sputtering) is parallel (a, b) and perpendicular (c, d) to the stripe lengthwise direction. The size of stripe is set as 2000*30 µm2 (a, c) and 2000*10 µm2 (b, d).


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FIG. 4. The permeability spectra of FeCoTiO single layer films deposited by oblique sputtering and stripe patterning. The easy axis (induced by oblique sputtering) is parallel (a, b) and perpendicular (c, d) to the stripe lengthwise direction. The size of stripe is set as 2000*30 µm2 (a, c) and 2000*10 µm2 (b, d).

the traditional preparation methods, the shape and size of films will have huge impact on film anisotropy. Figure 5 shows the M-H of FeCoTiO/SiO2/FeCoTiO trilayer patterned films with different stripe widths. Very interesting, the hard axis loops, which are very similar to the bulk continuous films, almost keep the same shape and static anisotropic field even with the decrease of stripe width.

FIG. 5. The M-H loops of FeCoTiO/SiO2/FeCoTiO trilayer films. The size of stripe is set as 2000*40 µm2 (a), 2000*30 µm2 (b), 2000*20 µm2 (c), 2000*10 µm2 (d).


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FIG. 6. The permeability spectra of FeCoTiO/SiO2/FeCoTiO trilayer films. The size of stripe is set as 2000*40 µm2 (a), 2000*30 µm2 (b), 2000*20 µm2 (c), 2000*10 µm2 (d).

On the other hand, there have a specific sharp at the easy axis, with an obvious step at low applied filed, centered at zero. This feature is shown in all the simples with different stripe widths, and with the decrease with the width, the plateau is much more accentuated. The classic work of Slonczewski has examined the domain configurations in infinitely long FM/NM/FM stripes thoroughly.14,15 In FM/NM/FM sandwich structures, the magnetization is aligned along easy axis due to the interlayer exchange coupling, and makes the magnetization anti-parallel alignment in the top and bottom layer. Therefore, the nearly anti-parallel alignment of magnetization manifests itself in a plateau in the magnetization curve around zero fields. In this work, we are more concerned with influence of the demagnetizing field to the high frequency properties in small sized films. Figure 6 shows the permeability of FeCoTiO/SiO2/FeCoTiO trilayer films with different stripe widths. All the films almost have the same results. Contrast with the continuous films, there is little changed in the resonance frequency and permeability. This can be explained by the anti-parallel magnetization in the top and bottom layer. And according to the Landau–Lifshitz–Gilbert (LLG) equation,16,17 the resonance frequency depends on the effective anisotropy field Heff . For the continuous films, there only have one induced anisotropy field Hk ; and for the patterning single layer flims, the demagnetizing field Hdemag was added; but for the FM/NM/FM sandwich structures films, as described previously, the interlayer exchange coupling introduce a domain originating contribution filed Hdomain or exchange filed, leading to Heff = Hk − Hdemag + Hdomain. As a result, the contribution of demagnetizing and domain offset with each other, make the performance of the films consistent with the continuous films. Such behavior make the films can get very good application in the integrated devices. On the other hand, we have also noticed the broad imaginary permeability peaks compare to that of the bulk continuous film, indicating the increased damping factor, and the narrower the stripe width, the broader the imaginary permeability or the higher FMR loss. Therefore, proper stripe width limit has to be set to balance the operational frequency and the magnetic loss in a real application.

CONCLUSION

In this paper, we have used the oblique sputtering and stripe patterning at the same time to adjust the anisotropy fields. The results show that if the anisotropy fields induced by the two


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methods are in the same direction, the effective anisotropy field will be greatly enhanced. But if the direction of two anisotropy fields is perpendicular, two anisotropy fields will conflict with each other, as a result, the films change to isotropy. By using the FM/NM/FM sandwich structures, we were able to introduce interlayer exchange coupling to compensate the shape anisotropy. The resonance frequency keeps between 3.5 GHz to 4 GHz, and the permeability keeps above 75. Such an excellent electromagnetic property makes the trilayer patterned films having a great potential in integrated device applications.

ACKNOWLEDGMENTS

The authors would like to acknowledge financial support from the National Basic Research Program of China under Grant 2012CB933104, the National Science Fund of China under Grant 61271031 and Sichuan Youth Science & Technology Fund under grant No.2014JQ0010. 1

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