Journal of Nanomaterials

Volume 2012, Article ID 186138, 6 pages

http://dx.doi.org/10.1155/2012/186138

## Effect of Low-Frequency Alternative-Current Magnetic Susceptibility in Thin Films

Department of Materials Science and Engineering, I-Shou University, Kaohsiung 840, Taiwan

Received 7 January 2012; Revised 6 February 2012; Accepted 6 February 2012

Academic Editor: Zhi Li Xiao

Copyright © 2012 Yuan-Tsung Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

X-ray diffraction (XRD) results indicate that the NiFe thin films had a face-centered cubic (FCC) structure. Post-annealing treatment increased the crystallinity of NiFe films over those at room temperature (RT), suggesting that NiFe crystallization yields FCC (111) texturing. Post-annealing treatments increase crystallinity over that obtained at RT. This paper focuses on the maximum alternative-current magnetic susceptibility value of NiFe thin films with resonance frequency at low frequencies from 10?Hz to 25000?Hz. These results demonstrate that the of NiFe thin films increased with post-annealing treatment and increasing thickness. The NiFe (111) texture suggests that the relationship between magneto-crystalline anisotropy and the maximum value with optimal resonance frequency increased spin sensitivity at optimal . The results obtained under the three conditions revealed that the maximum value and optimal of a 1000?Å-thick NiFe thin film are 3.45?Hz and 500?Hz, respectively, following postannealing at °C for 1?h. This suggests that a 1000?Å NiFe thin film post-annealed at °C is suitable for gauge sensor and transformer applications at low frequencies.

#### 1. Introduction

Researchers have recently focused on measuring alternative-current magnetic susceptibility () at low frequency. However, most studies on this topic have examined the magnetic susceptibility at high frequency. Interference of a magnetic field with signal is most difficult to deal with at low frequencies associated with alternating current (AC). When the applied magnetic field is an AC magnetic field and the AC frequency is not high, the magnetic dipole direction can vary with the field. As a result, the low-frequency alternative-current magnetic susceptibility () varies. NiFe film is an important soft magnetic material that has been extensively adopted in industry and the operation of read heads. The characteristics of such a film vary with Ni content [1]. A low coercive field is associated with approximately 80 at% Ni; high saturation magnetic induction is associated with around 50 at% Ni, and low susceptibility but high electrical resistivity is associated with about 35 at% Ni. Ni content also affects the stability of the resonance frequency and the initial susceptibility [2]. Generally, is used at high frequency [3]. However, few studies have measured at low frequency. The maximum and optimal resonance frequency () are worthy of further study. The maximum and optimal demonstrate that the spin sensitivity is maximal at . This can be exploited in low-frequency gauge sensor and transformer applications [4]. The benefits associated with low-frequency are low power loss, low input power, and similarity to the magnetic properties of rocks, which is useful in environmental magnetism studies [5]. However, most studies in the field have extensively considered the applications of NiFe thin films at high frequency [6, 7]. Zawilski’s theory provides a sounder theoretical basis for frequency of susceptibility application [7]. Some soft material experiments have been performed using Ni_{80}Fe_{20} or CoFeB film because it has a high saturation magnetization (), low coercivity (), and a highly anisotropic field () for use in magnetoresistance random access memory (MRAM) applications [8–11]. Some studies have discussed the benefits of alternative magnetic susceptibility measurement approaches, such as using a wide range of frequencies from 10?Hz to 25000?Hz in investigations of environmental magnetism even if to estimate the distribution of grain sizes of superparamagnetic particles and to determine the critical temperature () of ferromagnetic materials [12].

This work uses Ni_{80}Fe_{20} films of various thicknesses to measure alternative-current magnetic susceptibility () and resonance frequency () at various temperatures and various frequencies of the current. X-ray diffraction (XRD) reveals that the NiFe thin films have a face-centered cubic (FCC) (111) textured structure and postannealed NiFe films are more crystalline than those formed at RT. The magneto-crystalline anisotropy of the (111) texture can be reasonably concluded to indicate that the maximum value is associated with the optimal resonance frequency ().

#### 2. Experimental Details

NiFe thin films with a thickness () of between 300?Å and 1000?Å were deposited on a glass substrate by DC magnetron sputtering under three conditions: (a) sustained at RT; (b) , followed by post-annealing at °C for 1?h; (c) , followed by post-annealing at °C for 1?h. The typical base chamber pressure was less than ?Torr, and the Ar-working chamber pressure was ?Torr. The target composition of the NiFe alloy was 80 at % Ni and 20 at % Fe.

The structure of the NiFe thin film was elucidated by X-ray diffraction (XRD), employing a CuK_{a1} line (Philips X'pert). The in-plane low-frequency alternative-current magnetic susceptibility () was examined using an analyzer (XacQuan, MagQu) to remove the demagnetization factor. The driving frequency ranged from 10?Hz to 25??000?Hz. The same shape and size of each sample were measured.

#### 3. Results and Discussion

Figure 1 displays the X-ray diffraction patterns of NiFe films under three conditions. Figures 1(a) to 1(c) present the X-ray results for the films formed at RT, with postannealing at °C and at °C, respectively. They include significant crystalline peaks in the range of 20°–90°, including peaks associated with a highly crystalline (111) diffraction peak and two weak (200), (220) peaks. XRD results indicate that the resulting NiFe thin films have a face-centered cubic (FCC) structure. The apparent (111) peak of the NiFe thin films measuring 500?Å, 700?Å, or 1000?Å thick revealed a greater diffracted intensity than the (200) and (220) peaks. The intensity of the NiFe (111) peak is three times stronger than the (200) and (220) peaks. This implies the relationship between FCC (111) texturing and NiFe crystallization. Corresponding to other NiFe thickness, the XRD intensity of the films with a thickness of 300?Å formed under three conditions is very weak, but the peaks still exist. This suggests that the film growth model is imperfect. The diffraction peaks of the NiFe thin films become more crystalline as the thickness increases. The crystallization achieved by annealing treatment exceeds that achieved at RT because the thermal energy can drive grain growth.

Figure 2 plots the corresponding full width at half maximum (FWHM, ) of the NiFe (111) peak at three different thicknesses. The XRD diffraction result indicates that the intensity of the NiFe (111) peak is much larger than other (200) and (220) peaks. Figure 3 shows the half maximum (FWHM, ) of the NiFe (111) peak to estimate the grain size distribution under three conditions and different NiFe thicknesses. Figure 2 shows that a thinner NiFe film yields a larger FWHM, and the FWHM for films formed at RT is larger than that of films that have undergone post-annealing.

Scherrer’s formula enables estimating the mean crystallite grain size () from the measured width of the diffraction peak under crystalline quality of three conditions, as Figure 3 shows. Scherrer’s formula is [13]
where (0.89) is Scherrer’s constant, is the X-ray wavelength of the CuK_{a1} line, is the relative value of the full width at half maximum (FWHM) of the (111) peak, and is the half angle of the diffraction peak. The formula states that is proportional to , and, thus, a larger corresponds to smaller grains. For example, Scherrer’s formula yields grain sizes of 124?Å, 142?Å, and 158?Å, respectively, for 1000?Å-thick NiFe thin films (a) with a substrate temperature () maintained at RT, (b) after post-annealing at °C for 1?h, and (c) after post-annealing at heat annealing °C for 1?h. This distribution of grain sizes is consistent with XRD results.

Figures 4(a) to 4(c) plot the alternative-current magnetic susceptibility () as a function of thickness at low frequency for NiFe films under the three preparation conditions. Apparently, no 300?Å-thick NiFe thin film, formed under any condition, yields an apparent signal at 10?Hz to 25?000?Hz. This result is consistent with the weak XRD diffraction crystalline results. Figure 4(a) shows that, at room temperature (RT), ??increases with thickness at a frequency range of 10?Hz to 30?Hz, but decreases with increasing thickness at a frequency range of 30?Hz to 50?Hz. Additionally, of 1000?Å-thick NiFe film reaches the corresponding maximum value at 50?Hz to 100?Hz, and again decreases with increasing thickness between 100?Hz and 25?000?Hz. Figure 4(a) showed that the maximum value and optimal resonance frequency () for a 1000?Å-thick NiFe thin film following RT treatment are 1.36?Hz and 100?Hz, respectively. Figures 4(b) and 4(c) show the same trend of of the NiFe films as a function of low frequency. The value first increases from 10?Hz to 30?Hz, and then falls from 30?Hz to 50?Hz under conditions (b) and (c). The of post-annealed films reaches its value in the 50?Hz to 500?Hz frequency range before falling as the frequency rises from 500?Hz to 25?000?Hz. Figure 4(b) shows that the maximum value and optimal resonance frequency () of a 1000?Å-thick NiFe thin film that has undergone post-annealing at °C for 1?h are 2.66?Hz and 500?Hz, respectively. Figure 4(c) shows that the corresponding values of a film that has been post-annealed at °C for 1?h are 3.45?Hz and 500?Hz, respectively. Apparently, the maximum value achieved following post-annealing at °C is 2.54 times that of a film prepared at RT. The results in Figure 4 show that post-annealing treatment increases because the magneto-crystalline anisotropy of (111) texture effect induces the maximum value with optimal resonance frequency () [14]. The optimal resonance frequency () is that at which the spin sensitivity is highest, suggesting that a 1000?Å-thick NiFe thin film that has been post-annealed at °C is suitable for use in a gauge sensor and transformer applications at low frequency. However, a 300?Å-thick NiFe thin film under any one of the three conditions is not suitable because it yields no apparent signal. The alternative-current magnetic susceptibility is generally correlated with film thickness and post-annealing temperature.

Figure 5 shows the results obtained at three temperatures for films of various thicknesses, including the maximum alternative-current magnetic susceptibility under three preparation conditions. The figure indicates that the maximum ??increases with NiFe thickness. The 1000?Å-thick NiFe thin film yielded the highest signal and corresponding optimal under all conditions. Furthermore, the maximum values of the films post-annealed films at °C for 1?h and °C for 1?h were significantly higher than the film prepared at RT. The 1000?Å-thick NiFe thin film that was post-annealed at °C for 1?h had the highest . These results show that the low-frequency alternative-current magnetic susceptibility is closely related to temperature, thickness, and resonant frequency.

Figure 6 plots the optimal frequency () associated with the maximum value as a function of NiFe thickness under three preparation conditions. This figure demonstrates that the optimal increases with thickness at RT. Additionally, the optimal resonant frequency post-annealing treatment at °C for 1?h increases to saturation at 500?Hz. The optimal post-annealing treatment initially increases from 30?Hz to 250?Hz, whereas post-annealing at °C for 1?h causes saturation at 500?Hz. Figures 5 and 6 suggest that the maximum value and optimal resonance frequency () of a 1000?Å-thick NiFe thin film that has been post-annealed at °C for 1?h are 3.45?Hz and 500?Hz, respectively.

Table 1 shows this relationship. Clearly, increasing post-annealing temperature, thickness, and resonance frequency positively affect value. This result supports previous research showing that annealing has a positive effect on the magnetic property of the electroplated NiFe film [15]. The effect of thickness on the alternative-current magnetic susceptibility of Ni_{80}Fe_{20} films is related to the degree of crystallinity [16]. The degree of crystallinity in thin film strongly affected the magnetic anisotropic field, magnetic coercivity, and relative susceptibility, which in turn affected magnetic properties [17, 18].

#### 4. Conclusions

In conclusion, post-annealing treatment positively affects the alternative-current magnetic susceptibility, , because the NiFe (111) texture induces the magneto crystalline anisotropy. This in turn results in a high value and high spin sensitivity. The maximum value and optimal resonance frequency () of a 1000?Å-thick NiFe thin film that was post-annealed at °C for 1?h are 3.45?Hz and 500?Hz, respectively, under preparation conditions, suggesting that a 1000?Å-thick NiFe thin film post-annealed °C is suitable for use in gauge sensors and transformer applications at low frequency. Increasing the thickness of an NiFe film increases its crystallinity, strongly affecting the magnetic anisotropy field, magnetic coercivity, and relative magnetic susceptibility.

#### Acknowledgment

This paper was supported by the National Science Council, under Grant no. NSC100-2112-M214-001-MY3.

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