Magnetic and structural properties of Joule-heated Fe73Al5Ga2P11−xC5B4Six amorphous thick ribbons

ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 281 (2004) 364–371

Magnetic and structural properties of Joule-heated Fe73Al5Ga2P11
xC5B4Six amorphous thick ribbons a a . . R. Sato Turtellia,*, D. Triyonob, G. Badurekc, M. Schonhart , R. Grossinger , d a a C.D. Dewhurst , H. Sassik , C. Bormio-Nunes a

Institut fur TU Wien, Wiedner Hauptstrasse 8–10, A-1040 Vienna, Austria . Festkorperphysik, . b Department Fisika—FMIPA, Universitas Indonesia, Depok 16424, Indonesia c Atominstitut, TU-Wien, Wiedner HauptstraX e 8, A-1040 Vienna, Austria d Institute Laue-Langevin, 6 rue Jules Horowitz, F-38042 Grenoble, France Received 8 April 2004 Available online 2 June 2004

Abstract The effect of Joule heating on the magnetic properties and microstructure was investigated in Fe73Al5Ga2P11
xC5B4Six (x ¼ 1; 3) ribbons with a thickness of about 40 mm. It turns out that, in these alloys, the coercive field, the pinning field and the time dependence of the initial permeability decrease with increasing current density applied during a thermal treatment via Joule heating up to the occurrence of crystallization. At the as-cast state, the sample with x ¼ 3 exhibits better soft magnetic properties than those of the sample with x ¼ 1; however, after an optimum annealing, both alloys present similar magnetic properties. The temperature dependence of the resistivity shows clear two stages crystallization process for the Fe73Al5Ga2P8C5B4Si3 alloy. The microstructural and magnetic inhomogeneities of the annealed Fe73Al5Ga2P8C5B4Si3 alloy with lowest coercivity were investigated by means of a small-angle neutron scattering method. The improvement of the soft magnetic properties caused by Joule heating of materials is due to the internal stress relieve originated during the production of the ribbon and to the enlargement of the domain wall thickness, consequently the reduction of the anisotropy. r 2004 Elsevier B.V. All rights reserved. PACS: 61.12.Ex; 75.50.Kj; 75.60.Lr; 75.60.Ej Keywords: Amorphous soft magnetic materials; Small-angle neutron scattering; Joule heating; Coercive field; Pinning field; Disaccommodation

1. Introduction

*Corresponding author. Tel.: +4315880113150; fax: +4315880113199. E-mail address: [email protected] (R.S. Turtelli).

Fe-based amorphous alloys in the system Fe(Al,Ga)-(P,C,B) were found to exhibit a large glass-forming ability enabling the production of bulk materials up to a diameter of 2 mm by copper

0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.04.129

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371

mould casting, as well as of thick amorphous ribbons of more than 100 mm thickness by meltspinning [1]. Mizushima et al. [2] have reported that the replacement of P by 1–2 at% Si in Fe72Al5Ga2P11
xC6B4Six causes an enlargement of the supercooled liquid region, thereby increasing the thermal stability and leading to improved soft magnetic properties. They found that the softmagnetic properties at a ribbon thickness larger than 70 mm are improved as well by replacement of 1–2 at% Si, although the optimum magnetic properties occur for ribbons of a thickness of only around 40–50 mm. Heat treatment of amorphous ribbons of the Fe-(Al,Ga)-(P,C,B,Si) system leads to an obvious decrease of the coercive field, but these type of ribbons is characterized by some brittleness even in the as-cast state. It is well known that the heat treatment of ribbons by Joule heating causes less brittleness than the traditional method of annealing. Therefore, in a previous work [3] we have studied the modifications of the structure and the soft-magnetic properties of an Fe73Al5Ga2P11C5B4 ribbon, which are caused by Joule heating. We found a higher crystallization temperature than that reported by other authors [1,2], due to a higher heating rate. Additionally, a reduction of the coercive field by a factor of more than three was observed at the optimum current density applied during the Joule heating process. In this work, we investigate the effect of Joule heating on the magnetic and structural properties of Fe73Al5Ga2P11
xC5B4Six ribbons (x ¼ 1; 3) of 40 mm thickness. A ribbon thickness of 40 mm was chosen because of their optimum magnetic properties. As important quantities for their magnetic characterization, the coercive field, the pinning field and saturation magnetostriction were measured at room temperature. It is very well known that in Fe-based amorphous alloys with high saturation magnetostriction, ls ; exhibit an after-effect of the initial permeability that is proportional to l2s and to the free volume quenched within the sample during the production [4]. Therefore, additionally, measurements of the after-effect of the initial permeability (disaccommodation) were also performed in Fe73Al5Ga2P11
xC5B4Six ribbons (x ¼ 1; 3). Due to the mixture of many different elements with strongly

365

differing atom size in amorphous alloys with extended supercooled region, it is expected that the free volume should be significantly reduced. The thermal stability of the samples was investigated by monitoring the temperature dependence of their resistivity. A small-angle neutron scattering (SANS) experiment with neutrons was performed at the Institute Laue Langevin (ILL) Grenoble in order to investigate both the compositional and the magnetic inhomogeneities of the Fe73Al5Ga2P8C5B4Si3 ribbon with the lowest coercivity.

2. Experimental procedure Amorphous Fe73Al5Ga2P11
xC5B4Six (x ¼ 0; 1, 3) ribbons with a width of 4 mm and a thickness of 40 mm were prepared by a planar flow casting process in an argon atmosphere. The ribbons (length=0.3 m) were undergone a subsequent Joule heating process in vacuum, applying different electrical current densities, J; for 1 min. During this heat treatment, for all samples, the resistance variations were controlled monitoring the current and the voltage drop across a standard resistance. X-ray diffraction characterization of these alloys was performed using Co Ka radiation. At room temperature, the coercive force Hc was determined by measuring quasi-static hysteresis loops (0.05 Hz) up to 180 kA/m and the disaccommodation, DB ¼ Bðt2 Þ
Bðt1 Þ, was measured using an impulsive technique [5]. The effective disaccommodation, DA, is defined by: DA ¼ HDB=Bðt2 Þ ¼ H  ½Bðt2 Þ
Bðt1 Þ=Bðt2 Þ; where Bðt2 Þ and Bðt1 Þ are the magnetic inductions at times t1 = 70 ms and at t2 = 5 s after a magnetic pulse, respectively, and H is the intensity of the driving field where the maximum in the DB as function of B occurs. The frequency of H was taken at 10 kHz. The intensity and width of the magnetic pulse were 200 A/m and 10 ms, respectively. The pinning field, Hp ; (i.e. the critical field where a typical step in the permeability versus H curve occurs) was also measured at a frequency of 10 kHz. The thermal stability was examined by means of the temperature dependence of resistivity monitored at a heating rate of 1.66 K/ min using a standard DC-method with a 4-contact

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371

366

arrangement. The effective saturation magnetostriction, ls ; of as-cast samples was measured by means of the so-called SAMR technique [6]. The compositional and magnetic inhomogeneities of the ribbon exhibiting the lowest coercivity after Joule heating were investigated by means of SANS at the instrument D11 of ILL in Grenoble. Two-dimensional SANS patterns were recorded at room temperature at a neutron wavelength l ¼ 0:6 nm for sample-detector distances of 2, 8 and 28 m, respectively, corresponding to a range of momentum transfers of about 0.025>Q> 1.6 nm
1 and structures in real space between about 4 and 250 nm. The individual measurements were normalized by comparison with a reference scatterer (H2O, 1 mm) with known macroscopic scattering cross-section (SB1 cm
1). The ribbon was aligned horizontally along an external magnetic field B ¼ 1:25 T to allow for a clear separation of nuclear and magnetic scattering contributions by comparing the scattered intensities along and perpendicular to the field direction.

crystallization of the sample with 3% Si occurs at two or even more stages. This result is interesting because in the amorphous phase of the sample free of Si the crystallization take places in a single step. Similar result has also observed by Mizushima and Makino [2] on Fe72Al5Ga2P11
xC6B4Six alloys by means of differential scanning calorimetry measurements. The onset of the crystallization of the first stage is almost independent of the Si content. Obviously the replacement of P by Si atoms, which causes the two-stage crystallization, reduces the thermal stability. It seems that the occurrence of this twostage crystallization process, of course under conditions of optimal heat treatment, is a basic ingredient for the formation of nanocrystalline grains embedded in the amorphous matrix and improves the soft magnetic properties [7]. Therefore, the structural and magnetic inhomogeneitity investigation by SANS were made on the Jouleheated Fe73Al5Ga2P8C5B4Si3 ribbon with lowest Hc which is of importance for a correct interpretation and understanding of the crystallization process.

3. Results and discussion 3.2. Magnetic properties 3.1. Thermal stability Fig. 1 shows the temperature dependence of the resistivity of amorphous Fe73Al5Ga2P11
xC5B4Six (x ¼ 1;3) alloys. As can be seen clearly the 1.20

Resistivity (a.u.)

Si 0 1.15

Si 1 1.10

Si 3 1.05 Fe73Al5Ga2P11-xC5B4Six (x = 1, 3) 1.00 400

600

800

Temperature (K) Fig. 1. Temperature dependence of the resistivity of Fe73Al5Ga2P11
xC5B4Six (x ¼ 1; 3) amorphous alloys. For comparison, rðTÞ of the Si free sample is also plotted.

The values of the coercive field, the pinning field, the magnetization saturation, the saturation magnetostriction and the effective disaccommodation of the as-cast Fe73Al5Ga2P11
xC5B4Six (x ¼ 1;3) samples measured at room temperature are given in Table 1. In the as-cast samples, the soft magnetic properties improve with increasing Si content. Figs. 2(a) and (b) show the evolution of the disaccommodation DB as function of the driving field and magnetic induction, respectively, with increasing current density, e.g. for the alloy containing 1 at% Si. For the same sample, the evolution of the pinning field (susceptibility, w; as function of the applied field) with J is shown in Fig. 3. Similar behaviors of the curves DB  H; DB  B and w  H for the Fe73Al5Ga2P8C5B4Si3 ribbon were found. One can see that with increasing current density the pinning field decreases. Additionally, with increasing J, the maximum in the DB  H curves is shifted towards

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371

367

Table 1 Values of the coercivity Hc ; pinning field Hp ; saturation magnetization Ms ; saturation magnetostricition ls and the disaccommodation DA ¼ HDB=Bðt2 Þ obtained from the measurements performed on the as-cast samples of Fe73Al5Ga2P11
xC5B4Six (x ¼ 0; 1, 3) amorphous alloys, at room temperature Sample

Hc (A/m)

Hp (A/m)

Ms (T)

ls (10
6)

DA (A/m)

Fe73Al5Ga2P10C5B4Si1 Fe73Al5Ga2P8C5B4Si3

5.6 5.0

2.2 2.0

1.13 1.15

26.5 22.4

0.58 0.52

Fe73Al5Ga2P11C5B4Si1

2

5.95 A/mm

0 2

5.76 A/mm

2

0

2

0 2

∆B (µT)

5.0 A/mm

2

∆B (µT)

2

5.95 A/mm

0 5.76 A/mm

2

0 2

4.98 A/mm

2

3.72 A/mm

2

0

5.0 A/mm

2

2

0 2

4.98 A/mm

2 0

2

2

3.72 A/mm

2

0

0 as cast

2 0

Fe73Al5Ga2P11C5B4Si1

2

2

0

2

(a)

4 6 H (A/m)

8

as cast

2 0

10 (b)

0

20

40 B (µT)

60

80

Fig. 2. DB ¼ Bðt2 Þ
Bðt1 Þ at times t1 ¼ 70 ms and at t2 ¼ 5 s after a magnetic pulse, as function of driving field H (a) and induction B (b) measured at ambient temperature on the Fe73Al5Ga2P10C5B4Si1 ribbon submitted to the Joule heating at different current density.

0.020

Fe73Al5Ga2P11C5B4Si1 5.95 A/mm

2

Susceptibility (a. u.)

2

5.76 A/mm

0.015

5.0 A/mm

2

4.98 A/mm

0.010

2

as cast 3.72 A/mm

0.005

0

2

4 6 H (A/m)

8

2

10

Fig. 3. Initial susceptibility as function of applied field of the Fe73Al5Ga2P10C5B4Si1 ribbon heat-treated with different current density.

smaller value and in the DB  B curves is shifted towards larger value. Consequently, the effective disaccommodation decreases with increasing current density. It is well known that in rapidly quenched soft-magnetic amorphous ribbons the value of B Bm where the maximum of DB ¼ Bðt2 Þ
Bðt1 Þ in the DB versus B curve occurs, is proportional to the domain wall thickness [4,8]. In Fig. 2(b), the shift of the maximum towards higher values of B is due to the occurrence of an enlargement of the domain wall thickness, consequently the anisotropy decreases and the material becomes magnetically softer. The decrease of the coercive field, the pinning field and the effective disaccommodation during Joule heating are shown in Figs. 4(a)–(c), respectively. As results, DA; Hc

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371

368

and Hp reach minimum values, which lowest values reached due to Joule heating (around J ¼ 12 A/mm2) are similar for both alloys. As shown in Fig. 5 our X-ray diffraction measurements indicates that, in principle, the samples which were heat treated with current densities lower than 12 A/mm2 can be considered like amorphous materials. However, the formation of very small precipitates or a very small admixture of a crystalline phase cannot be discarded since they are below the sensitivity limit of X-ray diffraction. The origin of the magnetic softness of the alloys due to the Joule heating process is not completely understood, whether such a Joule heating process causes only a structural relaxation of the amorphous phase or whether it induces the formation of exchange coupled ferromagnetic nanoparticles, which are embedded in the amorphous matrix

Hp (A/m)

Hc (A/m)

25

Fe73Al5Ga2P11-xC5B4Six

5

2 1

DA (A/m)

0 0.6 0.4 S1 S3

0.2 0.0

0

4

8

12

J (A/mm2) Fig. 4. Coercivity (a), pinning field (b) and disaccommodation (c) as function of current density measured on Fe73Al5Ga2P11
xC5B4Six (x ¼ 1; 3) samples at ambient temperature.

remained an open question. In order to investigate whether or not nanocrystallites are present after such a thermal treatment, the ribbon containing 3 at% Si and exhibiting the lowest coercivity was analyzed with respect to its compositional and magnetic inhomogeneities by a dedicated SANS experiment. 3.3. Small-angle neutron scattering For unpolarized neutrons the differential scattering cross-section of a magnetically saturated but structurally disordered sample can be described by the sum of an isotropic nuclear and an anisotropic magnetic term
 ds ðQÞ ¼ ðDbnucl Þ2 Snucl ðQÞ do Bb0 þ ðDbmagn Þ2 sin2 aSmagn ðQÞ:

ð1Þ

There Dbnucl and Dbmagn are the nuclear and magnetic scattering lengths differences of the respective phases and its boundary (i.e., in our specific case, the amorphous matrix), which generate the compositional and magnetic contrasts; Snucl and Smagn are the scattering functions of the phases and a is the angle between the neutron scattering vector Q and the field B. Scattering in horizontal direction (i.e., sin2 a ¼ 0) is of purely nuclear origin whereas in vertical direction magnetic scatting fully contributes. For statistical reasons we have applied the so-called ‘‘sector analysis’’ procedure: angular averaging for Qvectors forming an angle jajo15 with the external field yields to a sufficient approximation of the nuclear scattering cross-section, whereas an analogous sector average perpendicular to the field (ja
90 jo15 ) provides the sum of structural and magnetic cross-sections. Fig. 6 shows the Q-dependence of the nuclear cross-section, which has been derived by this procedure. From the low-Q region in the inset, which allows to infer the mean radius of gyration of the compositional inhomogeneities as Rg ¼ 7973 nm, one finds that these precipitates must be approximately of similar size and shape. Since otherwise, due to the then inevitable smearing effects, it would not be possible to obtain

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371 a b

Fe 73 Al5Ga2P 11 C5B4

Intensity [arb. unit]

16.4 A/mm

a b c

2

15.9 A/mm

c a d d f eb e a

c

2

14 .2 A/mm

2

12.1 A/mm

2

a

369

a Fe3B b Fe2C c Fe3 C d Fe2B e FeC f FeP

as-cast

(a)

Fe73Al5Ga2P8C 5B4Si3

11.5 A/mm 2 40

60

80

(b)

100

2θ

Fig. 5. X-ray diffraction pattern for the (a) Fe73Al5Ga2P11C5B4 and (b) Fe73Al5Ga2P8C5B4Si3 ribbons after Joule heating for a range of different current densities, which are indicated on the figures.

dΣ /dΩnucl (cm sr )

100

-1

-1

dΣ/dΩnucl (cm sr )

-1

-1

100

80

Rg=79±3 nm

60

10

40 20

1

0

2

3

4

5

6 -2

7

8

-1

Q (×10 nm )

0.1

0.01

NUCLEAR SCATTERING

0.1

1

Q (nm-1) Fig. 6. Log–log plot of the macroscopic nuclear SANS crosssection of the Joule-heated Fe73Al5Ga2P8C5B4Si3 ribbon with minimum coercivity. The inset shows the low-Q region, from which a radius of gyration of Rg ¼ 7973 nm has been derived using the Guinier approximation. The plotted solid curve, which is intended as a guide to the eye corresponds to the scattering function of a sphere with Guinier radius Rg :

the observed pronounced intensity modulation. Taking into account the short time interval of precipitate formation, which is associated with the pulsed Joule heating process, this mono-dispersivity

behavior is not unexpected. However, the statistical precision of the data is not sufficient to allow for a unique determination of the actual shape of the precipitates. Although corresponding to a spherical particle shape, the curve plotted in the inset of Fig. 6 therefore is just intended as a mere guide to the eye. From a comparison of the absolute scattering intensities with those of nanocrystalline samples, which were investigated with identical instrumental parameters, it follows that in accordance with the given production conditions of this kind of ribbons, the precipitate concentration must definitely be very low (therefore the absence of the crystalline lines in the X-ray diffraction pattern) and interaction effects between them hence negligible. Consequently, one can consider that the magnetic softness of this alloy arises mainly from the structural relaxation of the amorphous matrix. From the difference between scattering in perpendicular and horizontal sectors the magnetic cross-section can be derived, whose Q-dependence is plotted in Fig. 7. The magnetic correlation length x associated with ferromagnetic inhomogeneities can be evaluated from the

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371

370 1000

MAGNETIC SCATTERING

dΣ/dΩmagn (cm-1sr-1)

100

growing of the grains, at the same time, inhibits the formation of boride compounds [7].

Correlation length: ξ = 207+26-45 nm

4. Conclusions

10 1 0.1 0.01 0.1

1

Q (nm-1) Fig. 7. Magnetic cross-section deduced from the difference of the intensities scattered in horizontal and vertical directions using the so-called ‘‘sector analysis’’ method (see text). The solid curve corresponds to a least-squares fit to exponentially decaying correlation with cross-over length x [10]. Rather large systematic uncertainties of x are caused by the lack of information at still lower values of momentum transfer Q:

macroscopic differential cross-section by assuming an exponentially decaying correlation, which corresponds to a scattering law of the form [9]
 dS ðQÞ pð1 þ x2 Q2 Þ
2 : ð2Þ dO magn The least-squares fit yields a value x ¼ 207þ 26245 nm, which reveals that the magnetic fluctuations are more or less identical to the size of the precipitates (i.e., for spherical shape: pffiffiffiffiffiffiffi ffi DE2 5=3Rg D204 nm). This means that the stray fields of the ferromagnetic precipitates are effectively short-circuited by the surrounding amorphous phase. The inset shows the low-Q region, from which a radius of gyration of Rg ¼ 7973 nm has been derived using soft-magnetic matrix. The rather larger uncertainty of x is essentially caused by the lack of scattering data at still lower momentum transfers. The average grain size after the onset of crystallization is relatively large, about 200 nm. This indicates that the precipitation of Fe–B and Fe–C compounds occurs in the first step [3] due to the absence of both nucleation atoms, like Cu, and atoms like Nb, Cr, Ta, and V, which inhibit the

The structure and the magnetic properties of amorphous Fe73Al5Ga2P11
xC5B4Six (x ¼ 0;1,3) ribbons were investigated by X-ray diffraction, resistivity measurements and SANS as function of the current density after a Joule heating treatment. The coercive field, pinning field and disaccommodation decrease with Si content and with increasing current density. The magnetic properties indicate a magnetic softening of the material due to structural relaxation caused by the heat treatment. It is interesting to note that at current density values of 10 A/mm2 the coercivity and the pinning field is at a minimum which coincides to small DA values. This indicates that due to the structural relaxation a reduction of the free volume happens. Additionally the dependence of DA on J has a different character than the corresponding curves as obtained on Fe85B15 [11]. Generally the DA values in the samples presented here are above that of amorphous Finemet or Fe85B15 (these materials have similar lS values), which is in contradiction with the expected correlation between DA and the reduced free volume in materials with extended supercooled region. A structural investigation using SANS shows the existence of surprising large grains of about 200 nm size, which seams to be ferromagnetic. However, the amount of these crystallites is negligible and consequently they are not detectable by X-ray diffraction.

References [1] A. Inoue, A. Takeuchi, T. Zhang, A. Murakami, IEEE Trans. Magn. 32 (1996) 4866. [2] T. Mizushima, A. Makino, A. Inoue, IEEE Trans. Magn. 33 (1997) 3784. . . [3] H. Sassik, M. Schonhart, R. Grossinger, R. Sato Turtelli, A. Kottar, J. Magn. Magn. Mater. 215–216 (2000) 260. [4] P. Allia, F. Vinai, Phys. Rev. B 33 (1986) 422. [5] J.P. Sinnecker, R. Sato Turtelli, M. Knobel, J.F. Saeger, IEEE Trans. Magn. 30 (1994) 1069.

ARTICLE IN PRESS R.S. Turtelli et al. / Journal of Magnetism and Magnetic Materials 281 (2004) 364–371 [6] K. Narita, J. Yamasaki, H. Fukunaga, IEEE Trans. Magn. 16 (1980) 435. [7] Herzer G, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol. 10, 1997, p. 416. . [8] R. Sato Turtelli, R. Grossinger, C. Kussbach, J.P. Sinnecker, J. Appl. Phys. 83 (1998) 1581.

371

[9] P. Debye, H.R. Anderson, H. Brumberger, J. Appl. Phys. 28 (1957) 679. [10] A. Guinier, Nature 142 (1938) 569. [11] C. Kussbach, R. Sato Turtelli, C. KuX, D. Holzer, . R. Grossinger, J. Magn. Magn. Mater. 160 (1996) 293.