IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 4, APRIL 2014 4003904

Stress-Induced Anisotropy in Electroplated FeNiRacetrack Fluxgate Cores

Mattia Butta1 and Ludek Kraus2

1Department of Measurement, Czech Technical University, Prague 16627, Czech Republic2Institute of Physics, Academy of Sciences of the Czech Republic, Prague 18221, Czech Republic

Magnetic anisotropy plays an important role in the behavior of a fluxgate core. It is desired to have an anisotropy orthogonalto the direction of the excitation field to move from one saturated state to the opposite saturated state without abrupt changeof magnetization. In this paper, we present a method for electroplating a Permalloy film in the shape of a racetrack core withbuilt-in anisotropy. The core is electroplated under bending and later released so that the resulting stress on the film generatesthe anisotropy. We show how the BH loop of the film changes as we increase the bending radius. Moreover, we demonstrate thatelectroplating under bending extends the frequency range before the inductance drops. Therefore, the fluxgates based on such corescan be used for higher frequencies (at the expense of lower sensitivity given by lower permeability).

Index Terms— Electroplating, fluxgate, magnetic anisotropy, magnetostriction, Permalloy, stress.

I. INTRODUCTION

ONE of the most important parameters for a fluxgatecore is the magnetic anisotropy, and it is essential that

it be controlled [1]–[4]. It is typically desired that magneticanisotropy be perpendicular to the direction of the excitationfield. This facilitates a smooth transition of the magnetizationfrom one saturation state to the opposite saturation state, thusreducing the noise of the sensor [5].

Anisotropy is often modified by annealing the core undermagnetic field in the direction of the desired anisotropy [6];however, this is not always possible, if the substrate can-not tolerate the high temperature required for annealing, forexample. Magnetic anisotropy of electroplated NiFe films canalso be controlled by electrodeposition on elastically strainedsubstrates.

In this paper, we present a method for controlling theanisotropy direction and magnitude on electroplated Permalloyracetrack core for fluxgates. The magnetic film is electroplatedunder bending and the back stress on the sample causes thechange of anisotropy when the strain is released [7].

II. ELECTROPLATING SETUP

The racetrack cores have been electroplated on a copper filmon a fiberglass-printed circuit board with 0.3 mm thickness.The copper film, with initial thickness of 18 μm, has beenelectropolished using orthophosphoric acid for 2 h to reduce itsthickness to 9 μm and decrease its roughness. The dimensionsof the racetrack core are shown in Fig. 1 together with a sketchshowing the longitudinal bending.

Two racetrack cores were orthogonally located on the samefiberglass substrate, so that one of them was subjected tobending longitudinally while the opposite was bent transver-sally. In this way, we could study the effect of bending duringelectroplating in the opposite direction while the samples wereplated under the same conditions.

Manuscript received August 7, 2013; revised October 21, 2013; acceptedOctober 22, 2013. Date of current version April 4, 2014. Correspondingauthor: M. Butta (e-mail: mattia@butta.org).

Digital Object Identifier 10.1109/TMAG.2013.2288179

Fig. 1. Upper part: dimensions of the racetrack core in millimeter (y-axis isthe longitudinal direction of bending, while x-axis is the transversal directionof bending). Lower part: sketch of the core bent in a longitudinal direction.The radius r is the radius of the equivalent circumference upon, which thecore is bent.

The composition of the bath was FeSO4.7H2O (8 g/l),H3BO3 (40 g/l), NiSO4.6H2O (125 g/l), NiCl2.6H2O(20 g/l), saccharine (6 g/l), and deionized water. The pH wasincreased to 2.8 by adding KOH; the temperature of the bathwas 55 °C [8].

The current density was 12 mA/cm2 and set as pulsing at0.5 Hz with 50% duty cycle (during the ON state the currentwas controlled in constant current mode by Keithley 2400).The anode was composed of a grid of platinum wire on apolytetrafluoroethylene disc placed above the sample to beelectroplated. The thickness of the resulting electroplated layerwas 6 μm.

We electroplated three types of core: flat, and with a bendingradius of 8.3 and 6.1 cm. As for electroplating under bending,the substrate was firmly joined to a plastic support so that thecurvature was uniform over the surface of the substrate. Afterthe electroplating process was completed, the substrate wasdetached from the bending support and the original flatnessrestored.

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4003904 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 4, APRIL 2014

Fig. 2. B/BS–H loops of racetrack cores electroplated with (A) no bending,(B) 8.3 cm longitudinal bending, and (C) 6.1 cm longitudinal bending.

III. INDUCED ANISOTROPY

To evaluate the anisotropy of the electroplated samples,we measured their B/BS–H loop by conventional inductionmethod. The H field was generated by a 300 turn toroidalcoils uniformly wound around the racetrack of the core.Induced voltage was picked up using 30 turn coils, 3 mmlong, placed in the middle on the straight part of the racetrackcore. The voltage was then digitized and numerically inte-grated to obtain the flux. Numerical correction was appliedto delete the flux in the air portion of the pick-up coil crosssection.

Figs. 2 and 3 show the B/BS–H loops of the cores electro-plated without bending, and with bending of 8.3 and 6.1 cm inlongitudinal and transversal directions, respectively. The loopsbecame shared in the longitudinally bent samples. Their slopesand coercivity fields decreased with increased bending strain(Fig. 2). This shows that the anisotropy is perpendicular toexcitation direction as desired for fluxgates. The larger thebend is, the harder the anisotropy axis becomes parallel to thebending direction.

In transversally bent samples, on the other hand, the loopsbecome rectangular and the coercivity slightly increases withthe bending strain (Fig. 3). In this case, the easy axisanisotropy is induced in the straight parts of the racetrackcore.

The anisotropy created in longitudinally bent racetracksis too high for practical sensors, because with such largeanisotropy, the core requires too large of a current to befully saturated (a necessary condition for proper workingmode of the fluxgate). Nevertheless, the resulting B/BS–Hloop shows how the anisotropy increases with decreasingbending radius (or increasing bending strain). Accordingto the desired anisotropy and saturation current, one canchoose the proper bending radius to achieve the best com-promise between the large anisotropy and the low saturationcurrent.

The induced anisotropy of the electroplated Permalloy filmis made by magnetoelastic coupling with the internal backstress appearing when the bending is released.

Fig. 3. B/BS–H loops of racetrack cores electroplated with (A) no bending,(B) 8.3 cm transversal bending, and (C) 6.1 cm transversal bending.

The anisotropy constant K is shown as

K = 3

2λsσ (1)

where λs is the saturation magnetostriction constant and σthe back stress. This is confirmed because when the racetrackcore is bent to the original bending ratio it was electroplatedwith, the B/BS–H loop is then restored to the nearly squareshape typically obtained when electroplated without bending.The sign of anisotropy constant K depends on the signs ofmagnetostriction constant λs and the back stress σ . Becausethe release of bending produces compresive back stress(σ < 0) along the bending direction the easy axis is parallelto bending direction for λs < 0 and perpendicular to it forλs > 0.

The measurement of saturation magnetization Js usingferromagnetic resonance at 49.1 GHz shows that Js is∼1.15–1.25 T for all the samples. This shows that the filmscontain more Fe than the zero magnetostrictive Permalloy withthe ratio 80:20. This means that the electroplated films aremagnetostrictive with the positive magnetostriction constant.Then, in longitudinaly bent race tracks, the easy axis isperpendicular to excitation direction and parallel to it intransversally bent ones. This well explains the behavior ofB/BS–H loops shown in Figs. 2 and 3.

IV. FREQUENCY DEPENDENCE OF THE INDUCTANCE

One of the most important parameters for fluxgate sensors isthe frequency with which they can be excited. Indeed, there isa large demand for fluxgates operated at large frequency, sincethis makes it possible to extend the bandwidth of the sensor.Unfortunately, this is not always possible. Many materials usedfor fluxgate cores show an inductance that rapidly disappearswith a frequency higher than some tens of kilohertz.

For this purpose, we measured the inductance of the coreselectroplated under both longitudinal and perpendicular bend-ing from 100 Hz to 1 MHz. The measurement was performedusing a HP 4284A RLC meter and the same toroidal excitationcoil used for the B/BS–H loop measurement to include thewhole track of the core in the inductance measurement. The

BUTTA AND KRAUS: STRESS-INDUCED ANISOTROPY 4003904

Fig. 4. Dependence of the inductance of cores electroplated under transversebending of 8.3 cm (circles) and 6.1 cm (diamonds).

RLC meter was set in an operative mode, which keeps thecurrent amplitude constant (10 mA) whatever is the frequencyselected. This is particularly important for measurement of theinductance of these types of cores; the permeability rapidlydiffers if the working point moves out of the middle of theBH loop. Therefore, we need to keep the test current at thevery same amplitude to be sure to test the core on the samerange of H field and have a fair comparison.

In Fig. 4, we can observe the dependence of the inductanceon the frequency for the samples electroplated under transver-sal bending, thus resulting with an increased anisotropy par-allel to excitation direction. The inductance is rather large atlow frequency (>70 μH at f < 1 kHz, for the sample plated<8.3 cm radius bending), but it quickly drops when the fre-quency is increased above ∼6 kHz. For f > 300 kHz, the coredoes not show any significant magnetic behavior anymore.If we consider the sample plated <6.1 cm radius, we observethat the sensitivity starts dropping even at lower frequencies.We can only observe the part of the curve already dropping.This shows that if we decrease the radius of transversalbending during electroplating, then the inductance of the coredrops faster at lower frequencies.

The opposite behavior is observed for samples electroplatedunder longitudinal bending, thus developing anisotropy orthog-onal to excitation direction, as shown in Fig. 5.

First, we observe that at low frequency, the value of theinductance is substantially lower than the inductance of theprevious samples electroplated under transverse bending: at1 kHz, we have ∼10 and 14 μH versus 23 and 72 μH. Thisis clearly due to lower permeability of the cores as observedfrom the measured BH loop.

However, the inductance keeps more constant when weincrease the frequency. The sample electroplated with 8.3 cmradius bending shows an inductance, which starts slightlydecreasing only at f > 100 kHz. The sample electroplated<6.1 cm radius bending does not even show any decrement ofits inductance, despite we increased the frequency up to 1 MHz(the maximum allowable frequency for the RLC meter).

Fig. 5. Dependence of the inductance of cores electroplated under longitu-dinal bending of 8.3 cm (squares) and 6.1 cm (triangles).

These results show that increasing the anisotropy orthogonalto excitation direction from one hand decreases the perme-ability (and thus the sensor’s sensitivity) but increases thefrequency range of core applicability. It is the consequenceof the fact that in many soft magnetic materials the cutofffrequency increases with decreasing dc permeability. Thisbehavior is sometimes referred as the Snoek’s law [9], [10].

The explanation for the observed behavior is that themagnetization reversal occurs predominantly by magnetizationrotations and low power losses in the part of the racetrackwhere bending is parallel to the excitation direction. In otherparts, mainly domain wall motion with high anomalous lossesoccurs.

To test the effect on these magnetic properties on the coreof the fluxgates, we employed the electroplated samples ascores for fluxgates. The excitation coil is the same toroidal coilused both for measurement of B/BS–H loops and inductance.The pick-up coil was instead wound around the whole sensor(limited to the straight section of the core). The pick-up coilwas wound on four layers for a total 300 turns. The excitationcurrent was 2.3 App , large enough to fully saturate the cores.An external capacitor tuned the pick-up coil to obtain themaximum sensitivity. The value of the tuning capacitor waschanged with every frequency value to achieve the best tuningcondition of the pick-up coil.

Fig. 6 shows the dependence of the sensitivity versus theoperative frequency for fluxgates with three different cores.The first core was electroplated with 8.3 cm radius bendingin transverse direction. As we saw before the permeability islarge, thus the resulting sensitivity of the sensor is also high.However, as the sensitivity increases, we rapidly reach a pointwhen the sensor loses linearity and its characteristic becomesbistable. Therefore, the sensor can be used with a frequencyof 20 kHz.

The second core was also electroplated <8.3 cm radiusbending but in longitudinal direction. As we have previ-ously observed, this increases the transverse anisotropy andso decreases the permeability. Therefore, we can observe in

4003904 IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 4, APRIL 2014

Fig. 6. Frequency dependence of the sensitivity of the fluxgate based on threeracetrack cores electroplated with transverse 8.3 cm radius bending (circles),longitudinal 8.3 cm radius bending (diamonds), and longitudinal 6.1 cm radiusbending (squares).

Fig. 6 that the sensitivity is much lower. On the other hand,we observed that the inductance was more stable when weincreased the frequency. This suggests that such a samplecould be used as a core for fluxgate at higher frequencies. Thisis indeed what we obtained: despite the fact that the sensitivityis lower, the sensor keeps stable up to 100 kHz, returninga linear characteristic for frequencies where the sample withopposite anisotropy was already unstable.

This phenomenon is confirmed if we consider the depen-dence of the sensitivity on the frequency of the sampleselectroplated <6.1 cm radius of longitudinal bending. As weobserve in Fig. 2, the permeability is lower, and therefore weexpect a lower sensitivity when the sample is used as a corefor a fluxgate, as we observe in Fig. 6. In addition, we noticethat the sensor can be used up to 140 kHz, since the frequencyrange is extended where it returns a linear characteristic.

This confirms that electroplating under longitudinal bendingnot only increases the anisotropy orthogonal to the excitationdirection, but also extends the bandwidth of the fluxgates(at the expenses of a lower sensitivity).

V. UNIFORMITY OF THE SAMPLE

A drawback of the system presented here is found at theterminations of the core. The endings of the racetrack aresemicircular sections that connect the two straight tracks.These endings are not exposed to longitudinal bending asthe straight tracks; on the contrary, the bending is rotatesrelative to the direction of the track, up to the most unfavorabledirection of π /2 in the middle of the ending. These parts ofthe core are electroplated with transverse bending, which hasbeen proven to move the anisotropy parallel to the excitationdirection, the opposite of what we desire for fluxgate. This isconfirmed by measuring the B/BS–H loop with pick-up coilon the terminations and simultaneously with another pick-upcoil on the straight track.

Therefore, despite the fact that we can stress induce thedesired anisotropy in the straight parts of the core, we willalways have the terminations that will have opposite behavior.

VI. CONCLUSION

In this paper, we have presented an effective method tomodify the anisotropy of a NiFe film in a racetrack shape,to be used as core of the fluxgate. We have shown how theanisotropy moves toward the direction orthogonal to excitationfield, if we electroplate the sample under longitudinal bending.The larger the bending, the larger is the achieved anisotropy.An alternative approach consists of electroplating the sampleunder a constant bending ratio and then tuning the anisotropychanging the magnetostriction (for example, this can beobtained modifying the current density of electroplating).

Unfortunately, this method does not prove to be useful forinducing proper anisotropy in the endings of the racetrackcore: indeed, while the straight parts of the core are bentin the correct direction, the endings are necessarily exposedto bending in opposite direction. Therefore, if we achieveanisotropy orthogonal to excitation field in the straight partsof the core, we inevitably create parallel anisotropy in theendings. Further development of this process will include amethod to minimize the effect of the ending with oppositeanisotropy.

ACKNOWLEDGMENT

This work was supported by the Grant Agency of the CzechRepublic under Grant P102/12/2177.

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