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Influence of P substitution for B on the structure andNguyen Chaua,*, Nguyen Hoang Luonga, Nguyen Xuan Chiena, Phung Quoc Thanhb, Le Van Vub a Center for Materials Science, National Unive

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Influence of P substitution for B on the structure and

Nguyen Chaua,*, Nguyen Hoang Luonga, Nguyen Xuan Chiena,

Phung Quoc Thanhb, Le Van Vub

a Center for Materials Science, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam

b Department of Solid State Physics, National University of Hanoi, 334 Nguyen Trai, Hanoi, Viet Nam

Abstract

Amorphous ribbons of Fe73.5Si15.5Nb3Cu1B7xPx(x ¼ 0; 1; 2; 3; and 4) have been prepared by rapid cooling on a single copper wheel.The crystallization of a-Fe(Si) phase is independent of the P content in the alloys.Based on Kissinger plots, the activation crystallization energies are determined.The size of the nanoparticles crystallized on an amorphous matrix in heat-treated ribbons is found to be 10–12 nm.The crystallization fraction is determined by using thermal-analysis equipment and we show that after 30 min annealing, this fraction is over 80%.The thermomagnetic curves measured between room temperature and 1000 K revealed clearly two magnetic phases: an amorphous phase at low temperatures and a crystalline one at high temperatures

r2002 Elsevier Science B.V All rights reserved

Keywords: Crystallization kinetics; Grain size; Soft ferromagnetism; Nanocrystalline materials

Great interest has been paid to nanocrystalline

ferromagnets (FINEMET) since their invention by

Yoshizawa and co-workers in 1988 [1].This is an

attractive subject for scientists in both applied and

fundamental research.With two coexisting

or-dered magnetic phases, almost vanishing effective

magnetostriction and very low magnetocrystalline

anisotropy occur in the nanocrystalline state.After

an appropriate heat treatment on the as-cast

amorphous ribbon, nanosize grains are embedded

in the remaining amorphous matrix.Owing to the

grain size smaller than the ferromagnetic exchange

lengths, the local magnetocrystalline anisotropy is

averaged out over several grains, which reduces

the effective anisotropy significantly [2].It was

shown that Cu and Nb play an important role to produce the nanocrystalline structure.Moreover,

Nb is ascribed to hinder the grain growth, which is one of the decisive factors to achieve the excellent soft magnetic properties

In our previous work [3], we have studied the influence of Co on the structure and the properties

of the alloys based on FINEMET Fe73.5xCox

-Si13.5B9Nb3Cu1.This work is performed to in-vestigate the influence of P substituted for B on structure and properties of Vitroperm[4]

The soft-magnetic ribbons Fe73.5Si15.5Nb3

-Cu1B7xPx (x ¼ 0; 1; 2; 3 and 4) have been pre-pared by rapid cooling on a single copper wheel The ribbons are 20 mm thick and 10 mm wide The structure of the as-cast ribbons and the annealed ones was examined by X-ray diffraction (D 5005, Brucker).The thermal analysis was

*Corresponding author.Tel./fax: +84-4-858-9496.

E-mail address: chau@cms.edu.vn (N Chau).

0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 4 1 - 6

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performed on a differential scanning calorimeter

(SDT 2960-TA Instruments).The ribbons were

annealed in vacuum.Thermomagnetic curves of

the ribbons were measured by a vibrating-sample

magnetometer (VSM-DMS 880, Digital

Measure-ment Systems).Hysteresis loop parameters were

determined by automatic magnetic hysteresis

graph (AMH-401A, Walker)

Fig.1 shows the X-ray diffraction patterns of

as-cast Fe73.5Si15.5Nb3Cu1B7xPx ribbons.One

can see that these patterns exhibit only one broad

peak around 2y ¼ 451; showing that the ribbons of

all these compositions are amorphous

DSC measurements on as-cast amorphous

ribbons were performed with increasing

tempera-ture at a rate of 201C/min in Ar atmosphere.Fig.2

presents the DSC curves of our ribbons.One can

see clearly that the first exothermic peak TP1

(corresponding to crystallization of a-Fe(Si)

phase) does hardly depend on the P content

substituted for B and occurs in the temperature

interval 550–5571C.With increasing P content in

the ribbons, the second exothermic peak at 7261C

for the sample with x ¼ 0 (considered to

corre-spond to the crystallization of Fe–B phase) is split

into two peaks and shifted to lower temperatures

with increasing x: This may suggest that if the

ribbons contain P, the crystallization of the Fe2,3B

and Fe2,3P occurs more easily.Based on Kissinger

plots we have evaluated the crystallization

activa-tion energy E1 of the a-Fe(Si) phase

(correspond-ing to TP1) to be about 3.1–3.5 eV The value for

E1 weakly depends on P content.Calculations

performed for the second and third peak showed

that the crystallization activation energy decreases

with increasing x:

For the first time Leu and Chin[5]have pointed out the use of the DTA apparatus for estimating the crystallization fraction wf:

wf ¼DHa DHt

where DHa and DHt are the crystallization enthalpy of the as-cast alloy and of the alloy annealed for a time t; respectively

Fig.3 shows the DSC curves for Vitroperm ribbons (x ¼ 0) both as-cast and annealed in vacuum at 5401C for 30 min.Using expression (1) we derived a crystallization fraction of the a-Fe(Si) phase at peak TP1 to be wf=82%

Apparently the crystallization fraction deter-mines the magnetostriction of the ribbon while the grain size after annealing determines the magni-tude of the effective magnetic anisotropy.Both these factors play a decisive role in the soft-magnetic properties of nanocomposite soft-magnetic materials

All compositions have been annealed at 5401C for 30 min and subsequently examined by X-ray diffraction measurements.From X-ray analysis,

we can determine the average grain size of a-Fe(Si) formed after annealing according to the Scherrer expression[6].The size of nanograins crystallized

on an amorphous matrix shows to be 10–12 nm and is not dependent on x:

The crystallization kinetics of the ribbons can be observed by measurements of thermomagnetic curve Fig.4 shows the MðTÞ curves of ribbon

Fig.1 X-ray diffraction patterns of as-cast Fe 73.5 Si 15.5 Nb 3

-Cu B P ribbons.

x = 0

x = 1

x = 2

x = 3

x = 4

55 1 o C

726 o C

551 o C

550 o C

550 o C

557 o C

622 o C

643 o C

651 o C

690 o C

Fig.2 DSC curves of Fe 73.5 Si 15.5 Nb 3 Cu 1 B 7x P x ribbons.

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with x ¼ 0 measured in magnetic field of 100 Oe.

The other samples display similar MðT Þ behavior

It can be seen fromFig.4again that the ribbon is

amorphous at room temperature.When the

temperature increases, the magnetization abruptly

is reduced marking the Tc of the amorphous

phase.With further increasing temperature, the

magnetization is small and constant over a large

temperature interval up to a region where

crystal-lization starts the crystallites of a-Fe(Si) lead to an

increase of the magnetization.On returning from

high temperature a large amount of a-Fe(Si) grains

are crystallized in the sample and this leads to a

fast increase of the magnetization below Tc of

a-Fe(Si) (curve 2 ofFig.4)

From measurements performed for the other compositions we observed that the Curie tempera-ture of the amorphous phase varies around 3007101C

Tables 1 and 2 show the characteristics of hysteresis loops of as-cast and annealed ribbons From these tables we can see that the magnetic characteristics of the hysteresis loops were evi-dently improved after annealing and the materials became ultrasoft magnetic nanocomposites

Acknowledgements The authors are grateful to the Vietnam National Program for Natural Sciences for finan-cial support

References [1] Y.Yoshizawa, S.Oguma, K.Yamauchi, J.Appl.Phys.64 (1988) 6044.

[2] G.Herzer, J.Magn.Magn.Mater.112 (1992) 258 [3] N Chau, P.Q Thanh, N.H Luong, N.H Nghi, Invited paper presented at the Sixth Asean Science and Technology Week, Brunei 9/2001, to be published.

[4] J.Petzold, J.Magn, Magn.Mater.242–245 (2002) 84 [5] M.S Leu, T.S Chin, MRS Symp Proc 557 (1999) 557 [6] B.D Cullity, Element of X-ray Diffraction, 2nd Edition,

1978, Addison-Wesley, Reading, MA, p.102.

0

5

10

15

20

25

30

35

40

100 200 300 400 500 600 700

T ( o C) Fig.4 Thermomagnetic curves of Fe 73.5 Si 15.5 Nb 3 Cu 1 B 7

rib-bon.1: heating cycle, 2: cooling cycle.

T (oC)

55 1 8 o C

72 5.6 o C

59 8.1 o C

72 6.7 o C

as-cast ribbon

after annealing

Fig.3 DSC curves for Fe 73.5 Si 15.5 Nb 3 Cu 1 B 7 ribbons (as-cast

and annealed at 5401C for 30 min).

Table 1 Magnetic parameters of as-cast Fe 73.5 Si 15.5 Nb 3 Cu 1 B 7x P x

ribbons Composition x ¼ 0 x ¼ 1 x ¼ 2 x ¼ 3 x ¼ 4

mm 16,000 13,000 15,000 10,000 12,000

HC(Oe) 0.12 0.15 0.196 0.32 0.235

Bm(G) 6800 6100 4800 7100 3300

Table 2 Magnetic parameters of annealed Fe 73.5 Si 15.5 Nb 3 Cu 1 B 7x P x

ribbons Composition x ¼ 0 x ¼ 1 x ¼ 2 x ¼ 3 x ¼ 4

mi 56,000 41,000 39,000 33,000 40,000

mm 70,000 53,000 42,000 37,000 43.000

H C (Oe) 0.02 0.04 0.038 0.035 0.08

B m (G) 10,100 9300 7800 6700 6900

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