# Effect of Low-Frequency Alternative-Current Magnetic Susceptibility inNi80Fe20 Thin Films

Authors: Yuan-Tsung Chen; S. H. Lin; Y. C. Lin
Journal: Journal of Nanomaterials
 (2012)

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## 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(χac) value of NiFe thin films with resonance frequency (fres) at low frequencies from 10 Hz to 25000 Hz. These results demonstrate that the χac
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 χac value with optimal resonance frequency (fres) increased spin sensitivity at optimal fres. The results obtained under the three conditions revealed that the maximum χac value and optimal fres of a 1000 Å-thick NiFe thin film are 3.45 Hz and 500 Hz, respectively, following postannealing at TA=250°C for 1 h. This suggests that a 1000 Å NiFe thin film post-annealed at TA=250°C is suitable for gauge sensor and transformer applications at low frequencies.


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## Body

## 1. Introduction

Researchers have recently focused on measuring alternative-current magnetic susceptibility (χac) 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 (χac) 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, χac is used at high frequency [3]. However, few studies have measured χac at low frequency. The maximum χac and optimal resonance frequency (fres) are worthy of further study. The maximum χac and optimal fres demonstrate that the spin sensitivity is maximal at fres. This can be exploited in low-frequency gauge sensor and transformer applications [4]. The benefits associated with low-frequency χac 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 Ni80Fe20 or CoFeB film because it has a high saturation magnetization (Ms), low coercivity (Hc), and a highly anisotropic field (Hk) 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 (Tc) of ferromagnetic materials [12].This work uses Ni80Fe20 films of various thicknesses to measure alternative-current magnetic susceptibility (χac) and resonance frequency (fres) 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 χac value is associated with the optimal resonance frequency (fres).


## 2. Experimental Details

NiFe thin films with a thickness (tf) of between 300 Å and 1000 Å were deposited on a glass substrate by DC magnetron sputtering under three conditions: (a) Ts sustained at RT; (b) Ts=RT, followed by post-annealing at TA=150°C for 1 h; (c) Ts=RT, followed by post-annealing at TA=250°C for 1 h. The typical base chamber pressure was less than 1×10-7 Torr, and the Ar-working chamber pressure was 5×10-3 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 CuKα1 line (Philips X′pert). The in-plane low-frequency alternative-current magnetic susceptibility (χac) was examined using an χac 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

Figure1 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 TA=150°C and at TA=250°C, respectively. They include significant crystalline peaks in the 2θ 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.X-ray diffraction patterns of NiFe films with thicknesses from 300 Å to 1000 Å under three preparation conditions: (a) deposited at RT only, (b) postannealed atTA=150°C for 1 h, and (c) postannealed at TA=250°C for 1 h.
(a)(b)(c)Figure2 plots the corresponding full width at half maximum (FWHM, β) 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, β) 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.Figure 2
FWHM (β) of NiFe (111) peak as a function of thickness of NiFe thin films prepared under three conditions.Figure 3
Grain size as a function of thickness of NiFe thin films prepared under three conditions.Scherrer’s formula enables estimating the mean crystallite grain size (D) from the measured width of the diffraction peak under crystalline quality of three conditions, as Figure 3 shows. Scherrer’s formula is [13](1)D=kλβcos⁡θ,
where k (0.89) is Scherrer’s constant, λ is the X-ray wavelength of the CuKα1 line, β is the relative value of the full width at half maximum (FWHM) of the (111) peak, and θ is the half angle of the diffraction peak. The formula states that D is proportional to 1/β, and, thus, a larger β 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 (Ts) maintained at RT, (b) after post-annealing at TA=150°C for 1 h, and (c) after post-annealing at heat annealing TA=250°C for 1 h. This distribution of grain sizes is consistent with XRD results.Figures4(a) to 4(c) plot the alternative-current magnetic susceptibility (χac) 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 χac 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), χac  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, χac of 1000 Å-thick NiFe film reaches the corresponding maximum χac value at 50 Hz to 100 Hz, and χac again decreases with increasing thickness between 100 Hz and 25 000 Hz. Figure 4(a) showed that the maximum χac value and optimal resonance frequency (fres) 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 χac of the NiFe films as a function of low frequency. The χac value first increases from 10 Hz to 30 Hz, and then falls from 30 Hz to 50 Hz under conditions (b) and (c). The χac 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 χac value and optimal resonance frequency (fres) of a 1000 Å-thick NiFe thin film that has undergone post-annealing at TA=150°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 TA=250°C for 1 h are 3.45 Hz and 500 Hz, respectively. Apparently, the maximum χac value achieved following post-annealing at TA=250°C is 2.54 times that of a film prepared at RT. The results in Figure 4 show that post-annealing treatment increases χac because the magneto-crystalline anisotropy of (111) texture effect induces the maximum χac value with optimal resonance frequency (fres) [14]. The optimal resonance frequency (fres) is that at which the spin sensitivity is highest, suggesting that a 1000 Å-thick NiFe thin film that has been post-annealed at TA=250°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 χac signal. The alternative-current magnetic susceptibility is generally correlated with film thickness and post-annealing temperature.Alternative-current magnetic susceptibility (χac) as a function of low frequency for NiFe films under three preparation conditions: (a) deposited at RT only, (b) postannealed at TA=150°C for 1 h, and (c) postannealed at TA=250°C for 1 h.
(a)(b)(c)Figure5 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 χac  increases with NiFe thickness. The 1000 Å-thick NiFe thin film yielded the highest χac signal and corresponding optimal fres under all conditions. Furthermore, the maximum χac values of the films post-annealed films at TA=150°C for 1 h and TA=250°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 TA=250°C for 1 h had the highest χac. These results show that the low-frequency alternative-current magnetic susceptibility is closely related to temperature, thickness, and resonant frequency.Figure 5
Maximumχac at three temperatures and film thicknesses.Figure6 plots the optimal frequency (fres) associated with the maximum χac value as a function of NiFe thickness under three preparation conditions. This figure demonstrates that the optimal fres increases with thickness at RT. Additionally, the optimal resonant frequency post-annealing treatment at TA=150°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 TA=250°C for 1 h causes saturation at 500 Hz. Figures 5 and 6 suggest that the maximum χac value and optimal resonance frequency (fres) of a 1000 Å-thick NiFe thin film that has been post-annealed at TA=250°C for 1 h are 3.45 Hz and 500 Hz, respectively.Figure 6
Maximumχac at corresponding resonance frequency fres.Table1 shows this relationship. Clearly, increasing post-annealing temperature, thickness, and resonance frequency positively affect χac 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 Ni80Fe20 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].Table 1
Maximum alternative-current magnetic susceptibility and optimal frequency at various thicknesses and temperatures.
NiFe thicknessRoom temperature of maximumχacPost-annealed 150°C-1 hr of maximumχacPost-annealed 250°C-1 hr of maximumχacHighestχac corresponding optimal resonance frequency fres (Hz)300 Å0.1722990.2122150.8517130 Hz500 Å0.6613072.1480862.456315250 Hz700 Å1.1949912.3445012.541269250 Hz1000 Å1.3602312.6687163.456921500 Hz


## 4. Conclusions

In conclusion, post-annealing treatment positively affects the alternative-current magnetic susceptibility,χac, because the NiFe (111) texture induces the magneto crystalline anisotropy. This in turn results in a high χac value and high spin sensitivity. The maximum χac value and optimal resonance frequency (fres) of a 1000 Å-thick NiFe thin film that was post-annealed at TA=250°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 TA=250°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.



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*Source: 186138-2012-05-15.xml*