In this paper, the copper composite wires of 75 μm in diameter with a sputtered layer of Ni80Fe20 permalloy were prepared, with a DC current applied to the basal Cu terminals during the fabrication process. The influence of the DC current on the magnetic configuration and Magneto-Impedance (MI) effect was studied. The results indicate that both the current amplitude and actuation duration have significant effect on the magnetic properties of the Ni80Fe20 layer. With appropriate current applied, the induced magnetic field leads to a circumferential magnetic domain structure and reduces significantly the equivalent anisotropy field of Ni80Fe20 layer. Then, the GMI ratio of the composite wires was significantly increased. A maximum GMI of 194.8% can be reached when the current was fixed at 100 mA and the Ni80Fe20 thickness is 780 nm. If the Ni80Fe20 thickness is above 780 nm, the coercivity of the coating layer increases while the GMI ratio of the composite wire reduces, since the magnetic anisotropy of the Ni80Fe20 layer varies from circumferential to longitudinal. The results were explained combining the thermal and magnetic effects of current.

When a soft ferromagnetic conductor is subject to a small alternating current (ac), a large change in the impedance of the conductor can be achieved upon applying a magnetic field, and this phenomenon is called giant magneto-impedance (GMI) effect.1,2 GMI effect was firstly discovered in 1992 in Co-based amorphous wires. Because of its high sensitivity, fast response and no hysteresis, magnetic sensors based on GMI effect is expected to be widely used in industrial and engineering, such as non-destructive testing and target recognition.3 The soft magnetic materials for GMI study have initially been focused on homogeneous materials such as Co-based or Fe-based amorphous/nanocrystalline wires, ribbons and films. However, recently heterogeneous GMI material structure has been developed such as sandwiched films and composite wires.4,5,19,24 One can observe enhanced GMI effect at much lower frequencies in heterogeneous materials, which makes them promising in practical applications, e.g. the pT resolution GMI sensors have been developed and applied in smart phones, security system and bio-magnetic measurements.6,7

Skin effect is the main origin of the GMI effect.8 The skin depth is:

(1)

where ω is the circular frequency, σ is the conductivity, and μϕ is the circumferential/transverse permeability of the wire/film samples. For strong GMI effect, a primary prerequisite is to improve the dynamic magnetic permeability of the ferromagnetic material.9 Except for varying the composition and geometric structure of the materials, many studies employed field or stress annealing to induce circumferential/transverse magnetic domain structure.10,11 The most commonly used fabrication methods for composite wires, such as RF sputtering,20 electroplating17,21 and chemical plating,22,23 cannot control well the magnetic structure. Recently, we imposed a DC current during the process of the chemical plating, and it can significantly improve the GMI effect of composite wire.12 A maximum GMI ratio of 868% was obtained for the sample deposited under 150 mA. Though the result is a little lower than 1100% in electroplated NiFe/Cu composite wire after suitable Joule annealing,20 it is only one step method. The role of DC current is equivalent in function to sample preparation optimization and field annealing simultaneously.

In this paper, a DC current has been imposed on the sample in a magnetron sputtering system. During the permalloy Ni80Fe20 sputtering, the substrate Cu wire was rotated to ensure the film uniformity. The influence of the current on microstructure and GMI effect of the composite Fe20Ni80/Cu wires was studied.

The Ni80Fe20 magnetic layer was deposited onto the copper wire of 75 μm in diameter by RF magnetron sputtering. The Cu wire was connected to a self designed rotating system in the vacuum chamber. During sputtering, the rotation rate was fixed at 120 r/min to obtain a geometrically uniform layer. The base pressure is 2.0 × 10−4 Pa, and the Argon working pressure was kept at 0.65 Pa. The sputtering power was set to be 190 W and the sputtering rate is 13 nm/min calibrated by SEM.

During the sputtering process, a DC current was connected with the Cu terminals, shown in Figure 1. Two series of samples have been prepared. Firstly, Ni80Fe20/Cu composite wires with Ni80Fe20 thickness of 520 nm have been prepared under different current amplitudes ranging from 25 to 150 mA. Secondly, the composite wires with different Ni80Fe20 thicknesses of 260–1300 nm have been prepared under fixed current amplitude of 100 mA.

FIG. 1.

Device diagram of magnetron sputtering with current applied on the Cu wire base.

FIG. 1.

Device diagram of magnetron sputtering with current applied on the Cu wire base.

Close modal

The surface morphology and Ni80Fe20 coating thickness of the wires were checked by SEM. The hysteresis loops of the wires were measured by vibrating sample magnetometer (VSM), with the magnetic field parallel to the wire axis. Magneto-impedance measurements were carried out using Agilent 4294A impedance analyzer controlled by a computer. The rms value of the ac driving current was kept constant at 20 mA, and its frequency was varied from 100 Hz to 100 MHz. The external field was provided by a pair of Helmholtz coils. The maximum external magnetic field used for the measurements (Hmax) was ±100 Oe. Magneto-impedance ratio was defined as

(2)

Where Z(Hex) and Z(Hmax) are the sample impedance values under external magnetic fields of Hex and Hmax, respectively.

Firstly, the first series of Ni80Fe20/Cu composite wires were sputtered under different current amplitude while the Ni80Fe20 coating thickness was fixed at 520 nm. Figure 2 shows the SEM photos of the composite wires. It can be seen from Fig. 2 that the sample surface is rough and has poor densification when the current is 0 mA. With the current imposed, the temperature of coating layer increases according to Joule's law Q = I2Rt, where R is the dc resistance of the wire and t is the actuation duration. The thermal effect may release the stress partly and enhance the diffusion capacity of atom, which results in particles epitaxial growth. Thereby it forms the uniformly dense membrane. With the current increasing, the density of the coating surface and the size of particle don't change significantly, indicating that the current amplitude makes little impact on the film growth.

FIG. 2.

SEM photos of the sputtered composite wires under various current: (a) 0 mA, (b) 50 mA, (c) 100 mA, (d) 150 mA.

FIG. 2.

SEM photos of the sputtered composite wires under various current: (a) 0 mA, (b) 50 mA, (c) 100 mA, (d) 150 mA.

Close modal

Figure 3 shows the GMI curves measured at 1 MHz for the series of samples. It can be seen from the figure that almost all of the samples show double-peak MI profile except for the sample under 150 mA, which is typical for wires owning circumferential magnetic structure. Initially, the MI ratio is only 10%, it increases with the current and reaches 85% when the current is 100 mA. Further increase of current brings down the MI ratio, which is only 12% for 150 mA current. Simultaneously, the MI curve turns into a single peak profile, which means that the magnetic anisotropy of the coating layer changes from circumferential to longitudinal. The value of anisotropy field of the coating layer, which generally corresponds to the applied field at peak MI ratio, reduces from 29.9 Oe to 4.0 Oe.

FIG. 3.

GMI response of the sputtered composite wires with various current amplitude.

FIG. 3.

GMI response of the sputtered composite wires with various current amplitude.

Close modal

Fig. 4 is the hysteresis loops of the selected samples with different imposed current. It can be inferred that the magnetic structure of the samples with imposed current tends to the circumferential direction. When the current is 150 mA, the loop shows complicated magnetic configuration. This will be discussed later by considering the effect of current actuation duration as well.

FIG. 4.

Hysteresis loops of various currents for wires with a 520 nm thick NiFe.

FIG. 4.

Hysteresis loops of various currents for wires with a 520 nm thick NiFe.

Close modal

There are at least two kinds of effects with the current imposed during the sputtering process. One is the magnetic field effect. The current will induce a magnetic field around the wire. The amplitude of the field can be calculated using Ampere's law.

(3)

Where r is the distance from the wire, I is the amplitude of the current. The direction of the field should be circumferential. Due to the presence of the filed, the deposition of Fe and Ni atoms tends to the magnetically circumferential direction. Another effect is the heat effect discussed above. The temperature rising can be estimated based on surface heat transfer equation and heat balance equation.13 The heat effect may release the internal stress of the magnetic layer partly. Therefore, the coating layer becomes softer with the imposed current. Hence, the effective anisotropy field reduces and the MI ratio increases. When the current increases to 150 mA, the temperature rising may reach 75 °C.13 Taking into account that the substrate temperature is near 200 °C in the vacuum chamber, the samples would be subject to an annealing process at about 275 °C with the current applied during deposition. Studies [14 ] have shown that magnetic anisotropy is governed by the spontaneous magnetization and interaction between the lattices, which is affected by the temperature. Finally it represents that the magnetic domain structure of the outer shell changes from the circumferential to longitudinal direction. The impedance curve decreases monotonically with the external magnetic field, and the MI value becomes lower at 150 mA.

For a given imposed current of 100 mA, we prepared the second series of samples with different coating thicknesses. The Ni80Fe20 coating thicknesses are 260 nm, 520 nm, 780 nm, 1040 nm and 1300 nm, respectively. Figures 5 and 6 respectively illustrate the GMI curves measured at 1 MHz and the MI ratio spectrum for the series of samples. It can be seen from Fig. 5 that the GMI curve has a double-peak profile when the coating thickness is below 520 nm. However, the single peak turns up when the thickness is above 780 nm, indicating that the magnetic anisotropy of the coating layer changes from circumferential to longitudinal. The MI ratio increase initially and decreases with further increase of the coating thickness. The maximum MI ratio of 194.8% is obtained for the composite wire with a coating layer of 780 nm. The result is consistent with the above discussion. With the increases of sputtering time, the enhanced thermal effect of the current continues to rise. Meanwhile, the magnetic field effect will be weakened as the distance from the wire r is increased, and accordingly the ability to induce circumferential magnetic structure reduces. Ultimately, the magnetic domain structure shifts from the circumferential to the longitudinal configuration.

FIG. 5.

Impedance ratio curves for composite wires with different NiFe coating layer thicknesses (I = 100 mA).

FIG. 5.

Impedance ratio curves for composite wires with different NiFe coating layer thicknesses (I = 100 mA).

Close modal
FIG. 6.

Frequency dependence of the maximum MI ratios of the composite wires with various coating thicknesses.

FIG. 6.

Frequency dependence of the maximum MI ratios of the composite wires with various coating thicknesses.

Close modal

It can be seen from Fig. 6 that all of the composites wire show obvious MI effect even at low frequency of 100 kHz. With the increase of the testing frequency, GMI ratio increases significantly. At about 1 MHz, the MI ratio reaches its maximum value. Theoretical study shows that the skin effect appears in ferromagnetic layer of composite materials at much lower frequency than homogeneous materials because of the electromagnetic interaction between the conductive core and the magnetic coating layer.15 When the frequency continues to increase, the loss caused by the eddy damp enhances.16,17 The magnetization caused by the magnetic domain motion gradually weakens until it completely disappears. It only has the magnetization caused by the rotation of the magnetic moment, so the magnetic permeability decreases, and the maximum impedance ratio reduces with frequency increasing.18 

It can be also seen from Fig. 6 that the characteristic frequency fmax, at which the MI ratio reaches the maximum value, increases firstly and then decreases with the coating thickness. For a composite wire, one cannot get a simple formula to describe skin depth. However, we can still regard fmax as a function of the variables related to skin depth of a homogeneous wire.

(4)

where μΦS is the dynamic circumferential permeability under a maximum field of 100 Oe. Then, we can draw the conclusion that the thicker the coating layer, the smaller the characteristic frequency. However, it does not agree well with results shown in Fig. 6. The geometric structure of the coating layer is not fully uniform when the thickness is 260 nm. Its anisotropy field is much larger than the sample with coating thickness of 520 nm, though both samples own circumferential magnetic structure. When the thickness is above 780 nm, the magnetic configuration of the coating layer becomes longitudinal. Their characteristic frequencies indeed decrease with thickness. The characteristic frequencies are even related to domain structure and thickness.

Figure 7 shows the hysteresis loops of composite wires with different coating thicknesses, the inset shows the relationship between the thickness and coercivity. It can be seen that as the domain structure shifts from circumferential to longitudinal direction, the coercive field firstly decreases and then increases with the coating thickness. It agrees well with the conclusion drawn from above mentioned GMI curves. GMI ratio is related to domain structure, softness and geometric size.

FIG. 7.

Hysteresis loops of composite wires with different NiFe thicknesses (I = 100 mA).

FIG. 7.

Hysteresis loops of composite wires with different NiFe thicknesses (I = 100 mA).

Close modal

Figure 8 illustrates the SEM photos of composite wires with different coating thicknesses. As seen from the figure, the particles of coating surface significantly grows and the density deteriorates with the thickness increases. It indicates that the deposition temperature of chamber raises after the long time sputtering and current imposing. It results in the enhancement of atomic diffusion. The phenomenon of particle aggregation and integration intensifies, so the particles of the outer shell surface become coarse, and ultimately the soft magnetic properties deteriorate.

FIG. 8.

SEM photos of composite wires with various coating thicknesses (I = 100 mA).

FIG. 8.

SEM photos of composite wires with various coating thicknesses (I = 100 mA).

Close modal

Ni80Fe20/Cu composite wires were prepared by magnetron sputtering, with a DC current on the Cu base wire. The current can improve the magnetic softness of the coating layer because of the magnetic field and heating effects. A maximum MI ratio of 194.8% is obtained when the Ni80Fe20 thickness is 780 nm and current is 100 mA. If the current amplitude is too high or the actuation duration is too long, the domain structure of the coating layer will shift from circumferential to longitudinal owing to the spontaneous magnetization and electromagnetic interaction between the lattices.

This work was supported by Shanghai Automotive Science and Technology Development Foundation (SAISTDF/12-06), East China Normal University Program (78210142, 78210183), large instruments Open Foundation of East China Normal University (2013-69), and National Natural Science Foundation of China (51302085).

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