DSpace at VNU: The discovery of the colossal magnetocaloric effect in a series of amorphous ribbons based on Finemet

Materials Science and Engineering A 449–451 (2007) 360–363

The discovery of the colossal magnetocaloric effect in a series
of amorphous ribbons based on Finemet
N. Chau a , N.D. The a,b , N.Q. Hoa a,c , C.X. Huu a , N.D. Tho a , S.-C. Yu c,∗
a

Center for Materials Science, College of Science, Vietnam National University, 334 Nguyen Trai, Hanoi, Vietnam
b Department of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom
c Department of Physics, Chungbuk National University, Cheongju 361-763, Republic of Korea
Received 23 August 2005; received in revised form 10 February 2006; accepted 24 February 2006

Abstract
A large number of amorphous ribbons based on Finemet have been prepared by rapid quenching on a single copper wheel with linear
speed of v = 25–30 m/s. The ribbons are 20–25 ␮m thick and 6–8 mm wide. All as-cast samples are amorphous. Two criteria producing the
colossal magnetocaloric effect (CMCE) in magnetic materials working as magnetic refrigerants are high saturation magnetization and sharp
ferromagnetic–paramagnetic phase transition. The Fe-based amorphous ribbons fit these cretia. Thermomagnetic curves as well as isothermal
magnetization curves around the Curie temperature of all the studied samples have been determined. The results show that the magnetic entropy
change, | Sm |, belongs to a class of materials with CMCE and the | Sm |max values have been obtained at a moderately low magnetic field change
of 1.35 T, moreover | Sm |max occurred at quite high temperature.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Amorphous magnetic materials; Magnetocaloric effect; Nanocrystalline materials

1. Introduction
When an external magnetic field is applied to a material,
magnetic moments in material attempt to align with the magnetic field, thereby reducing the magnetic entropy of the spin
system. If this process is performed adiabatically, the reduction
in spin entropy is offset by an increase in lattice entropy, and
temperature of the sample will rise. When an applied magnetic
field is removed, the temperature of specimen will drop. This
magnetocaloric effect (MCE), or adiabatic temperature change,

which is detected as the heating or cooling of magnetic materials,
is due to the varying magnetic field. Magnetic refrigeration provides an alternative method for cooling. Recently, there has been
interest in extending the magnetic refrigeration technique to near
and higher than room temperature region because of the desire
to eliminate chlorofluoro-carbons present in high-temperature
gas-cycle systems and to save energy [1].
As we well known, the adiabatic magnetic entropy change,
Sm , and temperature change, Tad , are correlated with magnetization, magnetic field change, heat capacity and absolute
∗

Corresponding author. Tel.: +82 43 2612269; fax: +82 43 2756415.
E-mail address: (S.-C. Yu).

0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2006.02.354

temperature by Maxwell’s fundamental relations [2]:
Hmax

Sm (T, H) =
0

Hmax

Tad (T, H) = −
0

∂M(T, H)
∂T
T

C(T, H)H

dH

(1)

H

∂M(T, H)
∂T

dH

(2)

H

where Hmax is the final applied magnetic field.
From Eq. (1), we see that two criteria forming large
MCE in magnetic materials are a high saturation magnetization and a sharp change in magnetization at the
ferromagnetic–paramagnetic (FM–PM) phase transition.
The prototype material for room temperature range is lanthanide metal Gd which orders ferromagnetically at 294 K [3].
A series of Gd5 (Ge1−x Six )4 alloys was reported [4,5] to display a Sm at least two times larger than that of Gd near
room-temperature. The compound La(Fe,Co)11.83 Al1.17 has also
showed a considerable MCE near room temperature [6].
Recently, a new class of magnetic refrigerant materials
MnFe(P, As) and related compounds for room-temperature
applications have been discovered [7,8], also, interstitial modifications of compounds La(Fe, Si)13 with hydrogen, carbon
and nitrogen [9,10] have attracted much attention. These new

N. Chau et al. / Materials Science and Engineering A 449–451 (2007) 360–363

361

Fig. 1. X-ray diffraction patterns of as-cast samples Fe73.5−x Crx Si13.5 B9
Nb3 Au1 .

materials have important advantages over existing magnetic
coolants: they exhibit a large MCE and the operating temperature can be ranged from below 200 to about 400 K by adjusting
the chemical composition or the content of interstitial atoms.
Another class of materials also displaying a large MCE is
based on perovskite [11,12], where we examined the positive
entropy change in manganite with charge-ordering [13].
In this report we present our discovery of colossal magnetocaloric effect (CMCE) in a series of amorphous ribbons based
on Finemet.

Fig. 3. Magnetization as a function of applied field of the sample
Fe73.5 Si13.5 B9 Nb3 Au1 at different temperatures.

The structure of the ribbons was examined by X-ray diffrac´˚
tometery (D5005 Bruker) with Cu K␣ radiation (λ = 1.54056 A).
Isothermal magnetization curves and thermomagnetic curves
were measured by vibrating sample magnetometer (VSMDMS 880, Digital Measurement Systems) with maximal

2. Experiments
Amorphous ribbons with nominal compositions (number
indicate at.%)
No. 1: Fe73.5 Si13.5 B9 Nb3 Au1 ;
No. 2: Fe73.5−x Crx Si13.5 B9 Nb3 Cu1 (x = 1–9);

No. 3: Fe73.5−x Crx Si13.5 B9 Nb3 Au1 (x = 1–5);
No. 4: Fe73.5−x Mnx Si13.5 B9 Nb3 Cu1 (x = 1, 3 and 5).
have been prepared by rapid quenching melting alloys (using
elements of 99.9% purity) on a copper wheel with wheel speeds
v = 25–30 m/s. The ribbons are 20–25 ␮m thick and 6–8 mm
wide.

Fig. 2. Thermomagnetic curves of as-cast ribbon Fe73.5 Si13.5 B9 Nb3 Au1 : (1)
heating cycle and (2) cooling cycle.

Fig. 4. Thermomagnetic curves of as-cast ribbons Fe73.5−x Crx Si13.5 B9 Nb3 Cu1
(a) and magnetic entropy change, | Sm |, vs. temperature (b).


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N. Chau et al. / Materials Science and Engineering A 449–451 (2007) 360–363

applied 13.5 kOe. The heat capacity measurements
studied samples were performed by DSC TA 2960
ments and the results showed that these values
order of that of pure Fe (the maximum values of
ity reached at respective Curie temperatures, C were
400 J/kg K).

of the
Instruare in
capacaround

magnetic field up to 13.5 kOe. Fig. 3 shows the results for sample

No. 1.
When magnetization is measured at a small discrete field and
temperature interval, Sm could be determined from Eq. (1) by
formula:
Sm =

3. Results and discussion
Fig. 1 shows the XRD patterns of as-cast ribbons No. 3. These
patterns exhibit only one broad peak around 2θ = 45◦ , showing
that the samples are amorphous. The same behavior is observed
for all ribbons studied.
The thermomagnetic curves of all samples have been
measured at a low applied magnetic field of 50 Oe. Fig. 2
presents the M(T) curves for as-cast ribbon No. 1 as an example.
It is clear that at the Curie temperature, TC , of amorphous
state, magnetization suddenly decreases, after that the sample
is in a (super)paramagnetic state to above 550 ◦ C, then starts
to increase due to crystallization. To study the magnetocaloric
effect of the samples, a series of isothermal magnetization
curves around their respective TC have been measured in a

Mi − Mi+1
Hi
Ti − Ti+1

(3)

where Mi and Mi+1 are the experimental values of magnetization
at Ti and Ti+1 , respectively, under an applied magnetic field of Hi .
The magnetic entropy change, | Sm |, of sample No. 1 has been

calculated and has a maximum value of 7.8 J/kg K. This value
indicated that the mentioned sample has colossal magnetocaloric
effect (CMCE). We note that this CMCE was reached at a quite
low magnetic field variation of 13.5 kOe.
Figs. 4–5 display the thermomagnetic curves of as-cast ribbons as well as | Sm | versus temperature of the other samples.
These figures show that the doping of Cr and Mn in Finemettype alloys significantly decreased the Curie temperature of
amorphous state of respective compositions. In the systems with
Cr doping this could be explained by ferromagnetic dilution
as well as by the existence of FeCr at the grain boundary
[14]. In the case of Mn substituted for Fe in Finemet, the
authors of [15] reported that the migration of Mn atoms to
the grain boundary region would promote a reduction of the
magnetic coupling in the system. We also see that all studied
samples exhibit a large MCE and the temperature at which
| Sm | reached a maximum (close to the respective TC of
amorphous phase) could be controlled by adjusting the doping
content.
According to our knowledge, CMCE was first discovered
by us for amorphous phase of Finemet compound with very
high | Sm |max = 13.9 J/kg K [16]. The studied samples in the
present work belong to ultrasoft nanocomposite materials after
appropriate annealing similar to that obtained in the ribbons
Fe73.5−x Cox Si13.5 B9 Nb3 Cu1 [17] and in Finemet with Cu substituted by Ag [18].
4. Conclusions
The amorphous magnetic alloys based on Finemet have
essential advantages: high saturation magnetization Ms , sharp
change of Ms at FM–PM phase transition of the amorphous
state, high working temperature (∼TC ) and low heat capacity
(400 J/kg K). Therefore they are very well adapted for magnetocaloric materials and:

Fig. 5. Temperature dependence of magnetic entropy change, | Sm |, of ribbons
Fe73.5−x Crx Si13.5 B9 Nb3 Au1 : (a) and Fe73.5−x Mnx Si13.5 B9 Nb3 Cu1 (b).

(i) colossal magnetic entropy change, | Sm |, is discovered in
a large number of amorphous ribbons;
(ii) | Sm |max occurred at quite high temperature, which could
be controlled by substitution effect;
(iii) the | Sm |max value has been obtained at a moderately low
magnetic field change of 1.35 T and as a consequence, the
studied samples could be considered as promising magnetic refrigerant materials working in the high temperature
region.


N. Chau et al. / Materials Science and Engineering A 449–451 (2007) 360–363

Acknowledgements
The authors express sincere thanks to Vietnam National Fundamental Research Program for financial support and this work
was supported by Korea Science and Engineering Foundation
through the Research Center for Advanced Magnetic Materials
at Chungnam National University.
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