The existence of giant magnetocaloric effect and laminar structure in Fe73.5 xCrxSi13.5B9Nb3Cu1

The substitution of Cr for Fe in Finemet leads to several interesting behaviors such as the increase of crystallization temperature and decrease of crystallization volume fraction and pa[r]

Journal of Magnetism and Magnetic Materials 304 (2006) 36–40

The existence of giant magnetocaloric effect and laminar

N Chaua, , P.Q Thanhb, N.Q Hoaa, N.D Thea

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

b Department of Physics, College of Science, Vietnam National University, Hanoi, 334 Nguyen Trai, Hanoi, Vietnam

Available online 3 March 2006

Abstract

Amorphous soft magnetic ribbons Fe73.5
xCrxSi13.5B9Nb3Cu1(x ¼ 1–5) have been fabricated by rapid quenching on a single copper wheel The differential scanning calorimetry (DSC) patterns showed that the crystallization temperature of a-Fe(Si) phase is ranging from 542 to 569 1C, a little higher than that of pure Finemet (x ¼ 0) With the same annealing regime, the crystallization volume fraction

as well as the particle size of a-Fe(Si) crystallites decreased with increasing Cr amount substituted for Fe in studied samples Especially, the interesting fact is that the laminar structure of heat-treated ribbons on the surface contacted to copper wheel in the fabricating process has been firstly discovered and explained to be related to the existence of Cr in studied samples The hysteresis loop measurement indicated that there is the pinning of displacement of domain walls The giant magnetocaloric effect (GMCE) has been found in amorphous state of the samples After annealing, the soft magnetic properties of investigated nanocomposite materials are desirably improved

r2006 Elsevier B.V All rights reserved

PACS: 75.50.Tt; 75.30.Sg; 71.55.Jv; 73.63.Bd

Keywords: Soft magnetic amorphous system; Nanocrystalline materials; Magnetocaloric effect; Magnetic properties of nanostructures

1 Introduction

A great interest has been paid to nanocrystalline Finemet

Fe73.5Si13.5B9Nb3Cu1 [1] With two existing ordered

mag-netic phases, the material almost vanishes effective

magnetostriction and magnetocrystalline anisotropy The

local anisotropy is averaged out over several grains, which

reduces effective anisotropy significantly owing to the grain

size smaller than the ferromagnetic exchange interaction

lengths[2], Finemet has excellent soft magnetic properties

In the previous papers, we have examined the influence

of P substituted for B[3], Ag for Cu[4]and Co for Fe[5]in

Finemet on the crystallization and properties of materials

The partial substitution of Fe by Al has been reported

[6–8] The effect of inclusion of Cr in Finemet has been

investigated [9] and the authors showed that there is

the enrichment in Cr of residual amorphous matrix in the

FeCr samples Gomez-Polo et al studied Fe73.5
xCrx

Si13.5B9Nb3Cu1[10]and reported that the inclusion of Cr atoms promotes a magnetic hardening at room tempera-ture for the highest Cr concentration x ¼ 10 associated to a drastic decrease of the Curie point of residual amorphous phase in alloy Recently, magnetic properties of Mn-doped Finemet nanocrystalline alloy have been examined[11]

In this paper, we present our study of influence of partial substitution of Fe by Cr on the structure and properties of Finemet-type alloys and for the first time we also show that there is giant magnetocaloric effect (GMCE) as well as laminar structure occurring in these alloys

2 Experiment Alloy ribbons (6–8 mm wide, 20 mm thick) with nominal compositions Fe73.5
xCrxSi13.5B9Nb3Cu1 (x ¼ 1–5) were prepared by rapid quenching on a single copper wheel Structure of the ribbons was studied using a X-ray diffractometer Bruker D5005 with Cu–Ka radiation The

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doi:10.1016/j.jmmm.2006.02.006

Corresponding author Tel.: +84 4 5582216; fax: +84 4 8589496.

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

evolution of crystallization process was monitored by

differential scanning calorimetry (DSC) SDT 2960 TA

Instruments Microstructure of annealed samples was

studied in a 5410 LV Jeol scanning electron microscope

(SEM) The magnetic properties of ribbons were measured

using a vibrating sample magnetometer (VSM) DMS 880

and Permagraph AMH 401 A Walker

3 Results and discussion

The XRD patterns of as-cast samples exhibit only one

broad peak around 2y ¼ 451, showing that the as-cast

ribbons are amorphous

DSC measurements on amorphous ribbons were

per-formed with heating rate of 20 1C/min in Ar atmosphere

and Fig 1 presents the results There are clearly two

separated exothermal peaks Tp1 and Tp2 on these curves,

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

phase, respectively Tp1 is ranging from 542 to 569 1C

(depending on Cr content), a little higher than that of pure

Finemet (x ¼ 0) [5] We suppose that it is related to the

higher melting temperature of Cr (1857 1C) compared with

that of Fe (1536 1C)

The DSC measurements for all compositions were also

examined with heating rate from 10 to 50 1C/min andFig 2

shows an example for ribbon x ¼ 3 As heating rate

increased, the primary crystallization peak shifts to higher

temperatures (Fig 2a) due to thermal activation necessary

for crystallization Kissinger technique has been used to

find activation energy and the plot inFig 2(b)shows this

value to be Ea1¼3.06 eV The crystallization activation

energy for bcc-Fe(Si) phase (Ea1) and boride phase (Ea2) of

the rest samples are also determined and shown inTable 1

The formation of ferromagnetic phase during primary crystallization is critical for obtained excellent soft magnetic properties since other phases might act as pinning sites for domain walls Another important feature forming the desired properties is proper nanocrystallization, namely the particle size as well as crystallization volume fraction DSC apparatus has been used to estimate the crystal-lization volume fraction[12]:

wf ¼DHa
DHt

x = 5

x = 4

x = 3

x = 2

705 °C

732 °C

709 °C

732 °C

562 °C

569 °C

561 °C

560 °C

542 °C

690 °C

T ( °C)

x = 1

Fig 1 DSC curves of as-cast ribbons Fe 73.5
x Cr x Si 13.5 B 9 Nb 3 Cu 1

(x ¼ 1–5) with heating rate of 20 1C/min.

Table 1 Crystallization activation energies of first peak, E a1 , and of second peak,

E a2 , on DSC curves

Tp

1/Tp (*10-3 K-1)

T ( °C)

-9.4 -9.6 -9.8 -10.0 -10.2 -10.4 -10.6 -10.8 -11.0 -11.2 1.162

587 °C

579 °C

576 °C

562 °C

557 °C

721 °C

732 °C

739 °C

723 °C

750 °C

50 °C/min

40 °C/min

30 °C/min

20 °C/min

10 °C/min

(a)

(b)

Fig 2 DSC curves with different heating rates (a) and Kissinger plot (b)

of sample x ¼ 3.

where DHaand DHt are crystallization enthalpy of as-cast

and annealed alloy for time t, respectively

Fig 3 shows the DSC curves for ribbon x ¼ 1 (as-cast

and annealed at 540 1C for 15 min) Using expression (1) we

derived the crystallization volume fraction of a-Fe(Si)

phase at the peak Tp1 to be wf¼84% for sample x ¼ 1

This value decreased consequently to 60, 58 and 32% for

samples x ¼ 2, 3, and 4, respectively At this annealing

condition, in the sample x ¼ 5 the crystallization only

partly occurred Increasing keeping time to ta¼30 min, wf

showed to be 88%, 86%, 83%, 77% and 69% for samples

x ¼ 1–5, respectively We note that the higher Cr content

substituted for Fe in the Finemet, the lower crystallization

ability at the same annealing condition occurred due to the

same reason as the increasing Tp1with x (seeFig 1)

Fig 4 shows the X-ray diffraction patterns of annealed

ribbons and insert part in this figure is the average value of

particle size determined according to Scherrer expression

Nanocrystalline alloys, because of their small grain size,

exhibit significant broadening in the Bragg diffraction

peaks The calculations show that the particle size

decreases with increasing Cr content in studied samples,

perhaps originating from high melting temperature as well

as high hardness of Cr

Both the surfaces of annealed ribbons (wheel-contacted

and air-contacted surfaces in fabricating process) are

observed by SEM One very interesting feature is that for

the first time the clear laminar (or cellular) structure of

grains with thickness of less that 25 nm and average length

of around 300 nm has been observed.Fig 5presents as an

example for sample x ¼ 1 It is also surprising that plans of

laminas are oriented almost perpendicularly to the surface

of ribbons The air-contacted surface (with lower cooling

rate) of this ribbon exhibits only granular structure with average grain size of 25 nm

We suppose that a small amount of Cr, which is forming FeCr nanograins is the reason for laminar structure on the annealed ribbon surface which has higher cooling rate in rapid quenching technology The investigation of a FeCrCuNbSiB nanocomposite alloy showed an atomic segregation during crystallization and the absence of chromium in crystalline phase [13] As in Ref [9], we suppose that beside residual amorphous phase, there is co-existence of bcc-Fe(Si) nanograins and small amount of FeCr nanograins

annealed ribbon

as-cast ribbon

542 °C

598 °C

T ( °C)

∆H a

∆H t

Fig 3 DSC curves of as-cast and annealed sample (x ¼ 1) for estimating

crystallization volume fraction.

x = 5

x = 4

x = 3

x = 2

x = 1

2 Theta (deg.)

x =1, dg = 9 nm

x =2, dg = 8.6 nm

x =3, dg = 8 nm

x =4, dg = 7 nm

x =5, dg = 6.5 nm

Fig 4 X-ray diffraction patterns of annealed ribbons Fe 73.5
x Cr

x-Si 13.5 B 9 Nb 3 Cu 1 (T a ¼ 540 1C–30 min).

Fig 5 SEM micrograph of annealed sample x ¼ 1 at the wheel-contacted surface (T a ¼ 540 1C–30 min).

The thermomagnetic curves of studied samples are

measured in applied field of 200 Oe andFig 6 shows the

M(T) curves of as-cast ribbons x ¼ 1 and 5 One can see

that magnetization is fastly reduced at TC of amorphous

phase, after that materials are in superparamagnetic state,

then magnetization starts to develop corresponding to the

crystallization of a-Fe(Si) phase The M(T) measurement

along cooling cycle exhibits clearly a two-step curve

relating to multiphase structure of the ribbon It is

suggested that in this cycle, beside a-Fe(Si) phase and

remaining amorphous phase, a little amount of boride

phase also co-exists Experiments showed that the Curie

temperature of amorphous state significantly decreased

with increasing Cr content (seeFig 6andTable 3), in good

agreement with results obtained in Ref.[9]

The hysteresis loops of as-cast and annealed ribbons

have been measured and Fig 7 presents the results for

compositions x ¼ 1 and 5 As seen from this figure,

different from pure Finemet, here hysteresis loops of

as-cast ribbons with Cr doping have quite high rectangular

coefficient of more than 90% showing the pinning of

displacement of domain walls It could be related to the

existence of high gradient of mechanical strain by Cr

doping after rapid quenching After annealing, the soft

magnetic properties of the studied nanocomposite samples

are desirably improved (seeFig 7andTable 2)

From a series of isotherms M(H) measured at different

temperatures around TC of respective amorphous phase,

the magnetic entropy change, |DSm|, of studied samples has

been determined Fig 8andTable 3 show the results and

clearly that we can control the maximum value and the

peak of |DSm| by Cr doping content These |DSm|max

indicate that the studied samples have GMCE and

according to our knowledge, this effect was firstly

discovered by us for Finemet[14]

0

40

80

120

160

TC,c TC,c

x = 1

x = 5

TC,a

TC,a

600

T ( °C)

Fig 6 Thermomagnetic curves of as-cast samples with x ¼ 1 and 5 in

applied magnetic field of 200 Oe.

1.0 -12

-8 -4 0 4 8

12

x=1 (annealed)

x=5 (annealed) x=5 (as-cast)

H (Oe)

x=1 (as-cast)

Fig 7 Hysteresis loops of studied samples x ¼ 1 and 5 (as-cast and annealed at 540 1C–30 min).

Table 2 Magnetic characteristics of Fe 73.5
x Cr x Si 13.5 B 9 Nb 3 Cu 1 annealed ribbons (measured in maximal magnetic field of 1 Oe)

0 3 6 9 12

x = 2

Sm

T ( °C)

x = 1

x = 3

Fig 8 Magnetic entropy change, |DS m |, as a function of temperature of amorphous samples Fe Cr Si B Nb Cu

4 Conclusions

The substitution of Cr for Fe in Finemet leads to several

interesting behaviors such as the increase of crystallization

temperature and decrease of crystallization volume fraction

and particle size of a-Fe(Si) phase, significantly reducing

Curie temperature of amorphous state, the existence of

pinning centers, and furthermore the laminar (or cellular)

structure and GMCE are discovered These materials have

been combined almost the advantages of well-known MCE

materials because of high magnetization (high Fe content),

sharp FM-PM transition (high atomic homogeneity), high

working temperature as well as low heat capacity (metallic

nature) The studied samples could be considered as

promising candidates for magnetic refrigerant materials

working at high temperature

Acknowledgement

The authors are grateful to the Vietnam National

Fundamental Research Program for financial support of

the Project 811204

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Table 3

Curie temperature, T C , and maximum magnetic entropy change, |DS m | max ,

of Fe 73.5
x Cr x Si 13.5 B 9 Nb 3 Cu 1 samples

|DS m | max (J/kg K) 9.8 7.7 5.9 5.6 5.1