ARTICLE IN PRESS

Journal of Magnetism and Magnetic Materials 304 (2006) e179–e181 www.elsevier.com/locate/jmmm

Ultrasoft magnetic properties in nanocrystalline alloy Finemet with Au substituted for Cu N. Chau, N.Q. Hoa, N.D. The, P.Q. Niem Center for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam Available online 6 March 2006

Abstract The amorphous ribbon Fe73.5Si13.5B9Nb3Au1 has been prepared by rapid cooling on a copper wheel. The ribbon is 16.8 mm thick and 7 mm wide. The DSC curves show the first peak at 547–579 1C (corresponds to the crystallization of a-Fe(Si) phase) depending on heating rate from 10 to 50 1C/min which is a little higher than that of pure Finemet (542–570 1C, respectively). From the Kissinger plot, the crystallization activation energy is determined and shown to be 2.8 eV for a-Fe(Si) phase, less than that of Finemet (E ¼ 3:25 eV). By annealing at 530 1C for 30, 60 and 90 min, the crystallization volume fraction of a-Fe(Si) phase increased from 73% to 78% and 84%, respectively. After appropriate annealing, the ultrasoft magnetic properties are achieved. The maximum magnetic entropy change, jDSmjmax, showed a giant value of 7.8 J/kg K which occurred at around Curie temperature of amorphous phase of the ribbon. r 2006 Published by Elsevier B.V. PACS: 75.50.Tt; 75.30.Sg; 71.55.Jv; 73.63.Bd Keywords: Nanocrystalline alloy; Soft magnetic amorphous system; Nanoparticle; Magnetocaloric effect

Excellent soft magnetic properties of nanocrystalline alloys are due to the refinement of the grain size [1,2] and well described in terms of the random anisotropy model [3]. It was shown that Cu and Nb play a very important role to produce the nanocrystalline structure. A small amount of Cu facilitates to form a-Fe(Si) phase as crystallization nucleation but Nb with high melting temperature is ascribed to hinder the grain growth. In the previous work, we have studied the crystallization in Finemet with Cu substituted by Ag [4]. In this report, we present our study on the influence of Au substituted for Cu in Finemet on the crystallization and properties of alloy Fe73.5Si13.5B9Nb3Au1. This alloy has been fabricated by rapid quenching technology on a single copper wheel. The ribbon is 16.8 mm thick (observed by SEM) and 7 mm wide. The X-ray diffraction (XRD) analysis showed that the as-cast ribbon is amorphous.

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E-mail address: [email protected] (N. Chau). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.01.225

The DSC measurements on as-cast samples were performed with different heating rates from 10 to 50 1C/ min (Fig. 1). From Fig. 1, two exothermal peaks are clearly exhibited. The first peak Tp1 (in the range of 547–579 1C) corresponds to the crystallization of a-Fe(Si) phase and the second one relates to the forming of boride phase. From the Kissinger’s linear dependence, the crystallization activation energy E1 of a-Fe(Si) phase and E2 of boride phase have been determined and shown to be E 1 ¼ 2:8 eV and E 2 ¼ 4:6 eV. While the crystallization temperature of a-Fe(Si) phase is a little higher than that of pure Finemet (542–570 1C), the value of E1 is less than that of pure Finemet (3.25 eV) [5]. It is well known that in Finemet, Cu forms the cluster prior to the primary crystallization reaction of a-Fe(Si) phase and Cu-enriched regions are observed at the grain boundary [6]. We suppose that the role of Au in studied sample is similar to that of Cu on the crystallization in Finemet but with high diffusion coefficient, Au facilitating the crystallization (decreasing Ea1). The crystallization feature of the studied ribbon could be observed by measurement of thermomagnetic curves

ARTICLE IN PRESS N. Chau et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e179–e181

579°C

714°C

574°C

710°C

Heat flow (a.u.)

568°C

705°C

60 α-Fe(Si)

50°C/min

Intensity (Cps)

e180

40°C/min

559°C 698°C

30°C/min

547°C 687°C

40

20 20°C/min

10°C/min 300

400

500

600

700

800

0

900

20

30

40

T(°C) Fig. 1. DSC curves of as-cast ribbons Fe73.5Si13.5B9Nb3Au1 measured with heating rate from 10 to 50 1C/min.

70

80

90

Fig. 3. X-ray diffraction pattern of annealed ribbon: T a ¼ 530 1C for 90 min.

12

120

as-cast

H = 50 Oe 100

8

80

4 B (kG)

60

annealed

0 2.5

(2)

40

2.0

-4 B (kG)

M (emu/g)

50 60 2 Theta (deg.)

(1)

1.5 1.0 0.5

-8

20

0.0 0.0

-12 -1.0

0 0

200

400

600

800

as-cast

-0.5

0.0 H (Oe)

0.3 0.6 H (Oe)

0.5

0.9

1.0

T (°C) Fig. 2. Thermomagnetic curves of as-cast ribbon (1): heating cycle, (2): cooling cycle.

(Fig. 2). It can see from Fig. 2 that the Curie temperature of amorphous state is similar to Tc of pure Finemet and also there is a single phase structure in the M(T) curve measured along cooling cycle whereas in the case of Ag and Zn substituted for Cu in Finemet, the multiphase structure occurred [4,7]. The ribbons have been annealed in vacuum at optimum temperature T a ¼ 530 1C for keeping time ta ¼ 30, 60 and 90 min, respectively. Fig. 3 shows the XRD pattern of sample annealed at T a ¼ 530 1C for 90 min. It is clearly seen that after annealing, the crystallization of a-Fe(Si) phase occurred. Using Scherrer expression, the grain size of a-Fe(Si) nanocrystallites is determined and shown to be in range of 10.8–11.6 nm for above keeping

Fig. 4. Hysteresis loops of as-cast and annealed samples (at 530 1C for 30 min).

time which is less than ferromagnetic exchange interaction length in Finemet (35 nm). Based on DSC measurements, and Leu and Chin expression [8], we derived the crystallization volume fraction of the a-Fe(Si) phase to be 73%, 78% and 84%, respectively, for the above annealing conditions. Fig. 4 shows the hysteresis loops of as-cast and annealed ribbons (T a ¼ 530 1C, ta ¼ 90 min) measured at low field. Obviously, different from pure Finemet, here hysteresis loop of as-cast sample has quite high rectangular coefficient of more than 90% by pinning of domain wall displacement (see B2H curve in inset of Fig. 4) due to high mechanical strain of Au atoms locating at grain boundaries. The magnetic parameters of as-cast and annealed

ARTICLE IN PRESS N. Chau et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e179–e181 Table 1 The magnetic characteristics of studied samples (as-cast sample and samples annealed at 530 1C for different time)

as-cast 30 min 60 min 90 min

mi

mmax

Hc (Oe)

Ms (emu/g)

1300 13,000 15,000 19,000

6900 50,000 62,000 99,000

0.144 0.036 0.024 0.022

127 144 151 167

9 7.8 J/kgK

e181

13.9 J/kg K [9]. Here, the magnetic entropy change jDSmj of studied sample has been determined depending on the temperature (Fig. 5) and shows to be a maximum value of 7.8 J/kg K. This value belongs to GMCE. We note that this GMCE has reached at quite low magnetic field variation of 13.5 kOe. In conclusion, the magnetic ribbon with Cu fully substituted by Au in Finemet is prepared with amorphous structure. The ribbon exhibits higher plasticity, higher solidity and more easy to bend in comparison with those of Finemet. The appropriate annealing leads to nanocomposite state in the sample with ultrasoft magnetic properties. The GMCE with j DS m jmax ¼ 7:8 J=kg K which occurred at T ¼ 342 1C has been discovered. The studied sample could be considered as a good magnetic refrigerant material working at high temperature.

|∆Sm| (J/kgK)

6

The authors would like to thank Vietnam National Fundamental Research Program for financial support (Project 811204). 3

References

0 280

300

320

340 T (°C)

360

380

400

Fig. 5. Magnetic entropy change jDSmj of the studied sample versus temperature.

[1] [2] [3] [4] [5] [6] [7]

ribbons are collected in Table 1. After annealing, ultrasoft magnetic properties are achieved (Fig. 4 and Table 1). From a series of isothermal magnetization curves measured at different temperatures, giant magnetocaloric effect (GMCE) was firstly discovered by us for amorphous phase of Finemet compound with very high jDSmjmax of

[8] [9]

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