MAGNETIC SOFTENING OF METALLIC GLASSESBY CURRENT ANNEALING TECHNIQUE

N. MITROVICa, S. ROTHb, S. DJUKICa, and J. ECKERTb

aJoint Laboratory for Advanced Materials of Serbian Academy of Science and ArtsSection for Amorphous SystemsTechnical Faculty a ak, Svetog Save 65, 32000 a ak, SerbiabLeibniz-Institute of Solid State and Materials Research Dresden Institute of Metallic MaterialsP.O. Box 270016, D-01171 Dresden, Germany

Corresponding author: M. Mitrovic, e-mail: drnm1nes@yahoo.com

Abstract: The current annealing (CA) method used for magnetic softening of amorphous alloys is observed by estimation of annealing temperature based on on-line infrared radiation and electrical resistivity measurements. Multi-step CA treatments were performed on FINEMET-type alloys as well as on novel Fe-based amorphous alloys with a large supercooled liquid region in order to attain different degrees of relaxation or nanocrystallization. Recent results on improvement of magneto-resistance and magnetoimpedance effects by applying this technique in ribbon and wire shaped samples are reviewed.

1. INTRODUCTION

In the last decade alternative annealing techniques have been a subject of increasing attention due to exploring their usefulness for improvement of the magnetic properties of ferromagnetic metallic glasses. The very high sensitivity of excellent soft magnetic behaviour on grain size of nanocrystalline alloys obtained from amorphous precursors [1] underlines the importance of optimization of the annealing parameters. The current annealing (CA) technique is especially interesting as thermo-magnetic sample annealing of two the simultaneous effects: Joule heating and a magnetic field induced by the current [2]. Moreover, FINEMET-type alloy samples are characterized by some brittleness even in the as-cast state, thus limiting their practical application. Some attempts to overcome this problem were based on the precipitation of nanocrystalline phases by dc Joule heating of amorphous ribbons [3].

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B. Idzikowski et al. (eds.), Properties and Applications of Nanocrystalline Alloys from Amorphous Precursors, 331–344.© 2005 Kluwer Academic Publishers. Printed in the Netherlands.

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Nowadays, there is a variety of methods to apply this technique: (i) single treatment with different current intensities and pulse duration [3, 4]; (ii) multi-step treatments with successive increase in current intensity with [5, 6] or

without cooling [7] between the steps;(iii) application of high frequency currents [8]. However, apart from the method applied, the main problem in practice is the difficulty

in estimating of the sample temperature [9, 10] as well as its (in)homogeneity throughout the specimen [11]. In this study we present our recent results on the magnetic softening of FINEMET-type alloys as well as novel Fe-based amorphous alloys with a large supercooled liquid region. Multi-step CA treatments were performed on ribbon and wire shaped samples in order to attain different degrees of relaxation and nanocrystallization [5, 12, 13]. An experimental method for determining the temperature of CA ribbons characterized by measuring the spectral response of a PbS detector on infrared radiation emitted during the treatment is compared with a method based on the on-line following of the electrical resistivity [9]. The evolution of soft magnetic properties obtained by controlled crystallization is investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), hysteresis, magnetization, magnetoresistance and magnetoimpedance measurement.

2. ESTIMATION OF THE SAMPLE TEMPERATURE

In experimental work, sample temperatures are estimated by monitoring the temperature dependence of chosen physical properties (usually magnetization M(T) [14] or electrical resistance R(T) [15]) and changes of these properties during the flow of different currents through the sample (M(I) or R(I)). The dependence T(I) is derived by comparing the values of these physical properties. In Fig. 1a the furnace annealed (FA) R(T)/R(300 K) curve of Fe73.5CuxNb3Si14.5 xB9 (x = 1.5) and the M(T)/M(300 K) curve for the same system (x = 1) presented by Kataoka et al. [16] are given. The different crystallization steps can be clearly seen for both curves. Good agreement between these two curves can be seen for the region of Curie temperature TC (I) and the region of the onset of the crystallization process TX1 (II).

Figure 1. a) Temperature dependence of M(T)/M(300 K) (x = 1) [15] and R(T)/R(300 K) (x = 1.5) (FA) for Fe73.5CuxNb3Si14.5–xB9 amorphous ribbons, and b) comparison of estimated sample tem-perature vs. annealing current made by the first two methods (light symbols resistance data, black symbols theoretical model) for Fe73.5Cu1.5Nb3Si13B9 and Fe72Cu1V5Si14B8 alloys [5]

Magnetic Softening of Metallic Glasses 333

In our experiments, the final temperature during the Joule heating was determined by three methods:(i) Using the temperature dependence of the relative electrical resistance R(T)/R(300 K),

thus avoiding extrapolation for T > TC, which would be required if the magnetization-temperature M(T) dependence were used.

(ii) Solving the transcendent equation in the theoretical model proposed by Barandiaran et al. [10], for details see [5]. The disagreement of the values obtained by resistance data and theoretical model is about 10 K (Fig. 1b).

(iii) Measuring the spectral response of a PbS detector on infrared radiation (IR) of the sample emitted during dc CA. A spectroradiometer SR-5000 was used. The princi-pal scheme is given in Fig. 2. The calibration source of infra-red radiation (black body) was placed at a distance of lSC = 3 m (Fig. 2a); the optical system was focused on the central region of the sample in order to avoid the region of inhomogeneous temperature distribution (about a tenth of the length taken from both ends of the sample [17]) as it is shown in Fig. 2b.

Figure 2. a) Calibration of the spectroradiometer, and b) measuring the spectral response of the radiation of a source at an unknown temperature (ribbon sample) [9]

The unknown temperature of the CA sample TAN can be determined from the equation:

2cmsr

W,

)(

)(,,

2

1

2

1

dTPS

S

U

UdTPT SC

AN

SC

SC

ANANAN (1)

where: ( ,TAN) is the spectral emissivity (we used 1), P( , TSC) is Planck’s function, USC( ) is the spectral response of the detector to a black body, UAN ( ) is the spectral response of the detector to the ribbon during annealing, SAN is the active area of the sample, and SSC is the active area of the black body. Figure 3 shows the functions

334 N. Mitrovic et al.

USC( ) and UAN( ) during the experiments on Fe75Ni2Si8B13C2 amorphous ribbons. The value on the right side of the Eq. 1 ( -radiance, i.e. radiation flux per unit solid angle per unit area) can be determined basing on the experimental data and using numerical methods for calculating a definite integral [18]. Finally, the sample temperature is calculated using an iterative procedure, where the iteration step TAN changes until the required accuracy 5 K is reached.

Figure 3. Spectral responses of the detector to a black body (USC) and to a ribbon sample (UAN) at the start of the annealing procedure [9]

Figure 4. Ia) Radiance during annealing of a sample of amorphous ribbon, and Ib) change of the relative electrical resistivity of a Fe75Ni2Si8B13C2 amorphous ribbon during CA(34 A/mm2). II) Temperatures of the sample during CA determined by: a) spectroradiometric measurements, and b) using curves R(t)/R(0) and R(T)/R(300 K) [9]

This method was combined with the comparative method based on the on-line following of the electrical resistivity R(t) of the amorphous ribbon during CA. As it can be seen from Fig. 4I excellent agreement between the radiance and the electrical resistivity methods was obtained. The maximum deviation of temperatures estimated by these methods is about 20 K (Fig. 4II).

Magnetic Softening of Metallic Glasses 335

3. ELECTRICAL AND MAGNETIC PROPERTIES OF CURRENT ANNEALED AMORPHOUS ALLOYS

3.1. Electrical resistivity followed during current annealing treatment

For the interpretation of the structural transformations during CA it is necessary to follow the electrical resistivity of amorphous alloys before, during and after annealing. In Fig. 5 selected curves of the relative electrical resistance during 60 s of dc CA for Fe72Cu1V5Si14B8 amorphous ribbon are given. The initial steep increase in resistivity is due to the increase of the sample temperature. For low current intensity (curves a and b) a plateau value exists, which is determined by the heat balance (the supplied power is equal to the speed of dissipation of heat by conduction, natural convection and radia-tion).

Figure 5. Relative electrical resistance during the first run of 60 s dc CA for amorphous Fe72Cu1V5Si14B8 ribbons [5]

Figure 6. Changes of relative electrical resistance for the sample (b – 1.9 A) of alloy Fe72Cu1V5Si14B8 during the second (b’ – 2.3 A) and third (b” – 2.5 A) heating run, and XRD patterns for the sample annealed with 2A during only 5 s and for the sample b” [5]

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The slight decrease observed with increasing annealing time may be due to the relaxation processes in the amorphous structure. Currents higher than 2 A (c-f), cause a resistance bump, shifting toward lower times with increasing current intensity. The maximum after this bump results from the crystallization energy, which is released during the exothermic phase transformation and raises the sample temperature.

In order to confirm the state of the sample b (I = 9 A) after cooling, we performed a second and third heating run. The second heating with increasing current intensity (curve b’– I’ = 2.3 A in Fig. 6) leads to crystallization, i.e. a peak is recorded before the isothermal region. Curve b’’– I” = 2.5 A for the third run shows a plateau, indicating that the crystallization process is almost completed during the second heating run. The XRD pattern (b”) in Fig. 6 is for this sample after the third heating run. Besides strong peaks of a bcc -Fe(Si) phase and very weak peak of the Fe2Si0.4B0.6, no Fe-borides (Fe2B and Fe3B) that precipitate after FA for 1 hour at 813 K [19] were noticed. The other XRD pattern in Fig. 6 for the sample heated with the current of 2 A for only 5 s confirms crystallization in the remaining amorphous matrix.

3.2. CA crystallization effect on longitudinal magnetoresistance

During the last decade considerable experimental research on magnetoresistance and magnetoimpedance effect of amorphous and nanocrystalline alloys has been conducted, aiming at the optimization of the microstructure for sensing applications. Measurements of the longitudinal magnetoresistance give positive values for all three groups of amorphous alloys (Fe-based [20], Ni/Fe-based [21] and Co-based [22]). However, a model of magnetoresistance proposed by Balberg and Helman assumed it to be always negative for crystalline ferromagnets [23]. Kuzminski et al. [24] have examined the behaviour of the Fe-Cu-Nb-Si-B alloys, showing that a negative magnetoresistance in the nanocrystalline state can be obtained by conventional (furnace) annealing.

Figure 7. Field dependence of the longitudi-nal magnetoresistance in Fe72Cu1V3Si16B8:a – as-cast state and after successive heatingruns (b – first 0.47 A; c – second 0.48 A;d – third 0.51 A ; e – fourth 0.52 A) [12]

In order to explore the longitudinal magnetoresistance of Fe72Cu1V3Si16B8 alloys,successive steps of dc CA were used to obtain a gradual transformation from the amor-

Magnetic Softening of Metallic Glasses 337

phous to the nanocrystalline state. The field dependence of the longitudinal magneto-resistance ( II) of the sample normalized to the resistivity in zero magnetic field ( 0)is shown in Fig. 7. With increasing magnetic field the absolute values of magneto-resistance increase. As the field is increased further all curves tend to saturate. The as-cast saturated value (Fig 7a, 0.096%) is close to that of amorphous Fe-based Metglas 2605 and 2605A alloys with positive longitudinal magnetoresistance (0.128% and 0.083%, respectively [22]).

After each heating run the magnetoresistance decreases. The decrease can be attributed to an increase of the content of nanocrystals in the residual amorphous matrix. The increase of the volumetric fraction of the crystallites was suggested by an increase of the relative electrical resistance during successive heating runs (for instance comparing curves c and d on Fig. 8I), or confirmed by the decrease of a ratio Ra/Rb

between electrical resistance after every heating run (Ra) and before the first heating (Rb

i.e. in amorphous state), values are listed in Table I.

Table I. Estimated final temperatures (Tf) during CA and the changes of Ra/Rb ratio after successive heating runs for the Fe72V3Cu1Si16B8 (Ra is electrical resistance after every heating run and Rb is electrical resistance in amorphous state)

First heating Second heating Third heating Fourth heating

I [A] 0.47 0.48 0.51 0.52

Tf [K] 710 716 761 772

Ra/Rb 0.975 0.943 0.868 0.854

Figure 8. I) Changes of relative electrical resistance for the Fe72V3Cu1Si16B8 at the beginning of the: (a) first 0.47 A, (b) second 0.48 A, (c) third 0.51 A, and (d) fourth 0.52 A successive heating run. II) XRD patterns for the specimens in the: (a) as-cast state and current annealed by (b) 0.47 A and 0.48 A; (c) 0.47 A and 0.52 A; (d) successively 0.47 A / 0.52 A / 0.57 A and 0.61 A [12]

The X-ray diffraction patterns for the as-cast (curve a) and annealed samples (b, c and d) are given in Fig. 8II. Only broad diffuse diffraction maximum of an amorphous phase can be observed for the non-treated samples. The first crystallization event on

338 N. Mitrovic et al.

current annealing is the formation of -Fe(Si) phase (curve b, D 15 nm; curve c, D 25 nm).

With increasing annealing current (curve d) diffraction peaks of -Fe(Si)(D 50 nm) and Fe2Si0.4B0.6 (D 45 nm) emerge from the amorphous maximum. A strong decrease in the lattice parameter of the annealed samples was observed. In accordance with the literature data (shift 17 10 5nm/1at.% Si [25]), the Si content in the precipitated -Fe(Si) was estimated from the position of the (110) peaks. The in-crease of the Si content observed after the first two annealing runs (17.1 at.%, curve b) is followed by a saturation after the next two (higher current) treatments (18.4 at.%, curve d).

Electrical resistivity measurement performed at room temperature and DSC data were used to study the increase of the volume fraction of the crystallites (p). Assuming that the total resistivity ( tot) can be obtained as the sum of the resistivity of the crystallites ( cr) and the resistivity of the amorphous matrix ( am ) [24], we have:

amcr

amtot

cram

totamp/1

/1 (2)

From successive CA up to 850 K (XRD pattern on Fig. 8IId), we find that the resistivity of the almost completely crystallized sample (p 1) normalized to the resistivity of the as-cast state ( cr / am) is around 0.8. Values of p calculated from Eq. (2) are given in Fig. 9Ia.

The increase of the volume fraction of the -Fe(Si) nanocrystals can also be analysed by DSC data. The area of the exothermic peak is a measure for the enthalpy of crystallization and is expected to be proportional to the volume fraction of amorphous phase transformed into the nanocrystalline -Fe(Si) during CA. This means that the vol-ume fraction of the nanocrystalline phase (p) may be calculated from the equation:

21

2211 ''

HH

HHHHp (3)

where H1 and H2 are the enthalpies for the first and second stage of crystallization of the amorphous samples; H1’ and H2’ are the enthalpies for the CA samples [12] (see Fig. 9II).

Figure 9Ia shows that there is a good agreement between the calculated values of the volume fraction of nanocrystals p from electrical resistivity (Eq. 2) and DSC (Eq. 3) measurements. The volume fraction of the -Fe(Si) and Fe2Si0.4B0.6 crystals precipitated after the highest current intensity annealing is about 90%. The observed dramatic in-crease of the Si content in -Fe(Si) nanocrystals after CA with low current intensity (Fig. 9Ib), was also detected by He et al. [26]. The saturation in the concentration of silicon in -Fe crystallites to value of 19 at.% Si has been observed. As a consequence, there is a significant change in the composition of the residual amorphous phase. A cal-culation shows that there is only 11 weight % with the composition of Fe24Cu6V19B51.This effect of the composition changes of the residual amorphous matrix with the precipitation of -Fe(Si) nanograins also has been observed by magnetostriction measurements [27].

Magnetic Softening of Metallic Glasses 339

Figure 9. I) a – Volume fraction of the -Fe(Si) nanocrystals p vs. normalized electrical resistivity tot/ am calculated from Eq. 2 ( ) and Eq. 3 ( ) and b – changes of the concentration of silicon in -Fe crystallites for the Fe72V3Cu1Si16B8, II) DSC traces for Fe72V3Cu1Si16B8 in the: (a) as-cast state and current annealed (b) 0.47 A and 0.48 A and (c) – successively 0.47 A/0.48 A/0.51 A and 0.52 A [12]

It was pointed out by Kuzminski et al. [24] that the anisotropy of magnetoresistivity i.e. ( II )/ 0 in Fe-based nanocrystalline alloys can be approximated by a linear function of p. Thus, it can be presumed that both longitudinal ( II) and transverse ( )magnetoresistivity are also proportional to p. The dependence of = ( II 0)/ 0

(saturated values) on the volume fraction of the crystalline phase p is shown in Fig. 10. A linear decrease of the longitudinal magnetoresistance with increase of the volume fraction of the crystallites is satisfied in the central region. The crossing point with the magnetoresistance axis was calculated to be 0.061%. This value is lower than the one measured for the as-cast state 0.096%. It can be attributed to the observed dramatic increase of the Si content in the -Fe(Si) nanocrystals after CA with low current intensity (Fig. 9Ib).

Figure 10. Saturated values of the magneto-resistance for Fe72V3Cu1Si16B8 as a function of the volume fraction of the crystallites (p)[12]

As a consequence, the significant changes in the composition of the residual amorphous phase lead to changes in the magnetoresistance contributions originating from amorphous matrix and nanocrystals. The magnetoresistance becomes zero for a fraction of nanocrystals of about 38%. Combining this figure with the data for a com-mercial nanocrystalline material (p 0.6-0.8 [28]) it can be deduced that the negative value of 0.05% corresponding to the preferable crystalline content. Thus,

340 N. Mitrovic et al.

magnetoresistance measurements may be used to estimate the crystalline fraction and provides an easy control of annealing procedures.

4. EFFECT OF CA ON MAGNETIC PROPERTIES

Bearing in mind that the nanocrystallization of FINEMET type alloys proceeds at temperatures of about 750-800 K (first crystallization step), it is necessary to apply a relatively high current intensity I (i.e. current density j, i.e. heating power per square area PS [6, 13]). The flow of current causes a transverse magnetic field during each CAtreatment (Fig. 11, right). The current of sufficiently high density, leads to induced of transverse magnetic anisotropy [2], aligning the magnetization transverse to the current and therefore to a decrease of remanence (Fig. 11, left).

-50 0 50 100 1500,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

Long. field

4.5 kA/m

F.A. 803K

30 min.

F.A. no field,

803K, 30 min.

as-cast

0.5

0.5

0.5

0.5

0.0

0.0

0.0

0.0

Mag

net

ic I

nd

uct

ion

, B

(T

)

AC Driving Field, H (A/m)

-50 0 50 100 150

-50 0 50 100 150

0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

C.A. 1.95 A

(estimated 770K)

5 sec.

Figure 11. Hysteresis curves of Fe73.5Cu1.5Nb3Si13B9 after different thermal treatments (left) and resultant domain structure under influence of the induced transverse magnetic field (right)

Figure 12. Bamboo-like domain structure in Fe-based as-cast wires and dependence of the MI-ratio in Fe73.5Cu1Nb3Si13.5B9 wires in as-cast (amorphous) and annealed (nano-crystalline) state for FA and CA samples on: I) frequency (Hmax1 = 1.35 kA/m), and II) on ex-ternal magnetic field Hex (driving current frequency as a parameter, Hmax2 = 20.3 kA/m) [35]

Magnetic Softening of Metallic Glasses 341

The one of the arising application of soft magnetic materials are the sensing devices. Alloys with very low coercivity and nearly zero magnetostriction such as amorphous Co-based and nanocrystalline Fe-based wires are most promising on materials for preparation of magneto-impedance (MI) elements [29, 30] used for magnetic field sensors. Figure 12 shows the MI ratio in Fe73.5Cu1Nb3Si13.5B9 wires after nano-crystallization performed by standard FA and dc CA multistep pulse thermal treatments [5]. The maximum change of 31% (FA at 470 C) and 12% (CA with Ian-max = 0.75 A) was obtained at a drive current frequency of 300 kHz, (Hmax1 = 1.35 kA/m). The FAsample has higher sensitivity (about 18%/kA/m) and a somewhat lower range of linearity (about 2.5 kA/m) compared with the CA sample (8%/kA/m and 4 kA/m respectively, Fig. 12II). The slightly lower sensitivity of the CA sample is due to the influence of the induced anisotropy, which leads to a bamboo-like magnetic domain structure (Fig. 12I, up).

Figure 13. I) DSC trace for Fe72Al5Ga2P11C6B4 amorphous ribbon measured at 40 K/min (the inset shows the determination of Curie temperature TC = 574 K) [33], and II) XRD patterns for as-cast, optimum CA and fully crystallized FA samples [13]

The development of multicomponent soft magnetic Fe-based amorphous alloys with a large supercooled liquid region has been a subject of an increasing attention of the scientific community in the last few years because of good soft magnetic proper-ties combined with the possibility to prepare these alloys by direct casting in final forms for application [31, 32]. These metallic glass forming systems have an extended supercooled liquid region before crystallization defined by the temperature span between the onset temperature of glass transition (Tg) and the onset temperature of crystallization (TX), ( TX = TX -Tg e.g. TX = 65 K for Fe72Al5Ga2P11C6B4, Fig. 13I [33]). The alloys of the Fe-(Al, Ga)-(P, C, B, Si) amorphous systems have very promising magnetic properties for various technical purposes [31]. The existence of a large TX

(50 to 70 K) gives the opportunity to perform annealing above Tg in the “liquid” state with efficient stress relief, i.e. an improvement of the magnetic permeability may be attained. It is therefore expected to have a decrease of the ratio ( / ) leading to a de-crease of the penetration depth ( m) and finally to an increase of the MI ratio.

342 N. Mitrovic et al.

For ribbon geometry the changes of the transverse magnetic permeability ( T) are crucial for MI-effect [34]; ( T) can be modified by thermal treatments, frequency and an external static magnetic fields, i.e. T = T (PS, f, Hex). The resistivity ( ) depends only on the annealing conditions, i.e. = (PS). Figure 14a shows the improvement of the MI ratio as a result of CA thermal treatments. The maximum variation occurs for PS 5 W/cm2. In order to explore the origin of the MI increase (Fig. 14b), we analysed the changes of resistivity vs. applied heating power. The decrease in the resistivity of the sample after each treatment is rather small in comparison with other Fe-based alloys [12, 35, 36] that exhibit a giant MI effect (only 1.5% for the highest PS, see Fig.14c). Contrary to the XRD pattern of a FA-sample (773 K/20 min) which is characterized by several iron-metalloid compounds (Fig. 13IIc) no evidence of crystals was found after applying a very high PS (5.5 W/cm2, Fig. 13IIb). Therefore, it can be concluded that

Figure 14. Dependence of the MI ratio in Fe72Al5Ga2P11C6B4 ribbons in as-cast and CA state (Hmax = 20.6 kA/m): a) on frequency, b) on PS at optimum driving frequencies, and (c) varia-tion of the resistivity of CA samples ( CA) normalized to the resistivity in the as-cast state ( O)

the highest MI response is associated with the magnetically softest state, i.e. after optimum relaxation of the amorphous structure as proved by coercivity/XRD/TEM analysis [6].

5. CONCLUSIONS

Current annealing technique is a widely applied non-conventional thermal treatment for improvement of technical properties of amorphous soft-magnetic materials. Multi-step current annealing with successive increase in heating power can be successfully used to attain different degrees of structural relaxation or nanocrystallization in order to optimize magnetic and mechanical properties.

Magnetic Softening of Metallic Glasses 343

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