DSpace at VNU: The crystallization and properties of alloys with Fe partly substituted by Cr and Cu fully substituted by Au in Finemet

Thoa aCenter for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam bDepartment of Physics, Chungbuk National University, Cheongju

Materials Science and Engineering A 449–451 (2007) 364–367

The crystallization and properties of alloys with Fe partly substituted

by Cr and Cu fully substituted by Au in Finemet N.Q Hoaa, N Chaua, S.-C Yub,∗, T.M Thanga, N.D Thea, N.D Thoa

aCenter for Materials Science, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam

bDepartment of Physics, Chungbuk National University, Cheongju 361-736, Republic of Korea

Received 23 August 2005; received in revised form 3 March 2006; accepted 20 March 2006

Abstract

The structure, crystallization and magnetic properties of ribbons obtained by first making amorphous ribbons and then objecting them to a crystallization annealing have been published elsewhere by us previously In the present work the soft magnetic ribbons Fe73.5−xCrxSi13.5B9Nb3Au1

(numbers indicate at.%, x = 1–5) are prepared by fast quenching on a single copper wheel X-ray diffraction patterns show that the as-cast samples

are amorphous Differential scanning calorimetry analysis indicates that the crystallization temperature of the␣-Fe(Si) phase is a little higher than that of pure Finemet With the same annealing conditions, the crystallization volume fraction decreases with increasing Cr content substituted for

Fe Hysteresis loops of as-cast samples measured by Permagraph show that domain walls are pinned After appropriate annealing, the ultrasoft magnetic properties of nanocomposite materials are established The magnetic entropy change,|Sm|, of studied samples has been determined, and a giant magnetocaloric effect is found Our materials could be considered as promising magnetic refrigerants working at high temperatures (several hundreds◦C)

© 2006 Elsevier B.V All rights reserved

Keywords: Nanocrystalline alloy; Soft magnetic amorphous system; Magnetocaloric effect

1 Introduction

Si13.5B9Nb3Cu1(numbers indicate at.%) are directly correlated

to its ultrafine structure composed of bcc-Fe rich crystals of

nanometer size surrounded by a residual amorphous matrix

[1,2] This magnetic softening is mainly ascribed to the

aver-aging out of magnetocrystalline anisotropy via ferromagnetic

interaction between two constituent magnetic phases, and is

reinforced by the negligible magneto elastic contribution due

to desirable reduction of both internal quenched stresses as well

as effective magnetostriction

Recent studies have been devoted to investigate the

substi-tution effect of P for B [3], Ag for Cu [4], Au for Cu [5] in

Finemet, Co for Fe[6,7], Cr for Fe[8,9]as well as of Mn for

Fe[10]on the structural, crystallization and magnetic properties

of these alloys When Cu substituted by Ag, there is very sharp

exothermal peak exhibited in differential scanning calorimetry

∗Corresponding author Tel.: +82 43 2612269; fax: +82 43 2756415.

E-mail address:scyu@chungbuk.ac.kr (S.-C Yu).

(DSC)[4], also the crystallization temperature of␣-Fe(Si) phase

as well as crystallization activation energy showed to be higher than those of pure Finemet If Au is substituted for Cu, the crys-tallization temperature of the bcc-Fe(Si) phase is a little higher but crystallization activation energy is found to be less than that

of pure Finemet If Co is substituted for Fe, saturation

magneti-zation, Ms, Curie temperature of amorphous state, TC, showed to

be higher than those of pure Finemet[6,7]but if Cr is substituted

for Fe, TCis drastically decreased[8,9] The aim of this work is to present our study of the inclusion effect of Cr and Au on crystallization and properties of Finemet-type alloys

2 Experiment

CrxSi13.5B9Nb3Au1 (x = 1–5) have been prepared by rapid

and 8 mm wide The structure of samples was examined by X-ray diffraction (XRD; D 5005, Bruker) with Cu K␣ radiation The evolution of crystallization process was studied on differ-ential scanning calorimetry (SDT 2900, TA Instruments) The

0921-5093/$ – see front matter © 2006 Elsevier B.V All rights reserved.

doi: 10.1016/j.msea.2006.03.139

N.Q Hoa et al / Materials Science and Engineering A 449–451 (2007) 364–367 365

ribbons were annealed in vacuum Thermomagnetic curves were

measured by a vibrating sample magnetometer (VSM-DMS

880, Digital Measurement Systems) Hysteresis loop

parame-ters were carried out using automatic magnetic hysteresis graph

(AMH-401A, Walker)

3 Results and discussion

The XRD patterns of as-cast samples showed one broad peak

centered at approximately 2θ = 45◦which appears to be typical

of an amorphous phase

DSC measurement on amorphous ribbons was performed in

Ar atmosphere andFig 1presents these results There are clearly

ascribed to the precipitation of bcc-Fe(Si) and boride phase,

respectively The first peak Tp1 is ranging from 564 to 579◦C

depending on the Cr content substituted for Fe, it is a little higher

than that of pure Finemet[7]and of Finemet with Cu substituted

by Au[5] FromFig 1we can see also that the second peak Tp2

exhibited with high sharpness relating to strong crystallization

of boride phase These results are fully agree with those reported

in[9].Fig 2presents the linear dependence of crystallization

Fig 1 DSC curves of as-cast ribbons Fe 73.5−xCrxSi 13.5 B 9 Nb 3 Au 1 (x = 1–5)

with heating rate of 20 K/min.

Fig 2 Dependence of crystallization activation energies at the first peak Ea1

and the second peak E on Cr concentration.

Fig 3 Thermomagnetic curves of the samples x = 1 and x = 4 measured in

magnetic field of 50 Oe.

activation energy of␣-Fe(Si) phase (Ea1) and of boride phase

(Ea2) on Cr content

Crystallization kinetics of samples could be studied by

the results for samples x = 1 and x = 4 We can see from this figure that at Curie temperature, TC, of amorphous phase, magnetiza-tion suddenly decreases then material is in superparamagnetic state up to temperature region starting to crystallize Magneti-zation curves measured along cooling cycle showing that the samples are of single phase in contrary with that of Ag substi-tuted for Cu[4] Insert of this figure is the magnetization curves

of studied samples The drastic decrease of TCand Mscould be explained by ferromagnetic dilution due to Cr substitution for

Fe as also observed in[8,9]

To promote the characteristic nanocrystalline structure, the samples were submitted to isothermal annealing in vacuum for

45 min The DSC apparatus has been used to estimate the crys-tallized volume fraction (χf) of␣-Fe(Si) phase[8]and the results showed that at the same annealing conditionsχfdecreases with increasing Cr content substituted for Fe which confirming that

Cr atoms enhances crystallization temperature as observed from

Fig 1 and it could be associated with atomic rearrangements within the amorphous and FeCr phase which give rise to the for-mation of bcc-Fe(Si) nanograins, which coexist with the FeCr nanograins as assumed in[11]

Fig 4shows the XRD patterns of studied samples annealed at

550◦C for t

a= 45 min The mean crystalline size (dg) of␣-Fe(Si) phase determined by XRD peak broadening analysis, using the Scherrer expression, is also indicated inFig 4 dgdecreased from

11.7 nm for sample x = 1 to 8.2 nm for sample x = 5 showing one

again that Cr hinders crystallization

Fig 5shows hysteresis loops of as-cast and annealed ribbon

x = 3 (Ta= 550◦C, t

a= 45 min) measured at low magnetic field Similar to that of Fe73.5Si13.5B9Nb3Ag1alloy[4](but different for Finemet) the as-cast sample exhibits high rectangular coef-ficient of hysteresis loop We suppose as in[8] that the FeCr nanograins, present in sample from as-cast state could act as wall pinning centers during magnetization process As we see from this figure, after annealing, the soft magnetic properties

366 N.Q Hoa et al / Materials Science and Engineering A 449–451 (2007) 364–367

Fig 4 X-ray diffraction patterns of studied samples annealed at 550 ◦C for

45 min.

of sample significantly improved The magnetic parameters of

as-cast and annealed samples are collected inTable 1

We can see fromTable 1that the ultrasoft magnetic properties

are established in the whole series of samples after the same

annealing condition

asso-ciated with the second-order phase transition were performed

using isothermal magnetization curves measured around the

respective Curie temperatures of amorphous-state samples and

Fig 6shows the|Sm|(T) curves of studied samples It seems

from Fig 6 that quite high values of |Sm|max have been

achieved in all samples and the studied ribbons could be

consid-ered as materials with giant magnetocaloric effect (GMCE) By

doping of Cr, we can shift the peak of|Sm| to lower temperature

similar to that of TCdepending on x.

In conclusion, the Fe73.5−xCrxSi13.5B9Nb3Au1 samples are

prepared with amorphous structure While crystallization

tem-perature of bcc-Fe(Si) phase increases with increasing of Cr

content, the crystallization volume fraction, particle size of

crystallites, Ms and TCdecrease with increasing of Cr content

substituted for Fe The doping of Cr leading to pinning of

dis-placement of domains walls in as-cast samples The samples

Fig 5 Hysteresis loops of as-cast and annealed ribbons of the sample x = 3.

Table 1 The magnetic characteristics (μi , μmax = initial and final permeability,

Hc = coercive field) of as-cast and Fe 73.5−xCrxSi 13.5 B 9 Nb 3 Cu 1 annealed samples

(Ta = 550 ◦C, t

a = 45 min)

x = 1

x = 2

x = 3

x = 4

x = 5

Fig 6 The|Sm|(T) curves of studied samples measured in 13.5 kOe magnetic

field change.

exhibited a GMCE and they could be considered as promising magnetic refrigerants working at high temperatures Ultrasoft magnetic properties of studied nanocomposite materials are established

Acknowledgements

The authors express their sincere thanks to the National Research Program in Natural Science for financial support of the Project 811204

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