DSpace at VNU: Magnetocaloric effects in RCo2 compounds

Journal of Magnetism and Magnetic Materials 242–245 (2002) 873–875

Magnetocaloric effects in RCo2 compounds
N.H. Duc*, D.T. Kim Anh
Faculty of Physics, Cryogenic Laboratory, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam

Abstract
Magnetisation isotherms were measured for a number of (R; R0 )Co2 and (R, Y)Co2 (R, R0 =rare earths) compounds.
A metamagnetic transition is observed just above the Curie temperature (TC ) of compounds having a first-order phase
transition, i.e. ErCo2, HoCo2 and (Dy, Y)Co2. The magnetic entropy change DSm shows a largest value of À11.8 J/
mol K at 35 K for ErCo2 and it decreases exponentially with increasing temperature. The obtained thermal variation of
DSm is compared to that of RAl2 and other intermetallic compounds. Giant magnetocaloric effects observed in RCo2based compounds are discussed in terms of the 4f(R)-localised spin, 3d(Co)-spin fluctuations as well as nature of the
phase transition. r 2002 Elsevier Science B.V. All rights reserved.
Keywords: Rare earth–transition metal compounds; Magnetocaloric effects; Metamagnetic transition

The best materials for magnetic refrigeration applications are compounds, which display first order or
combined transitions [1]. In this context, rare earth (R)
intermetallics are one of the promising candidates
because many of them (RCo2, RCo3, RCo5y) exhibit
a metamagnetic transition (MMT) in an applied
magnetic field and/or with a temperature variation [2,3
and Refs. therein]. The RCo2 compounds with R=Er,
Ho, Dy show the first-order transition (FOT) at low
temperatures (TC o178 K), whereas the RCo2 with
higher TC (i.e. TbCo2 and GdCo2) exhibit the secondorder transition (SOT). The formation of the 3d
magnetic moments is combined with the volume
expansion and the quenching of the spin fluctuations.
The size of the magnetocaloric effect (MCE) in these
compounds, thus, depends not only on the number of
(localised) 4f-spin, the nature of the transition, but also
on the contribution of the 3d-itinerant electrons.

The aim of this paper is to investigate MMT
and MCE in the vicinity of TC in (R,R0 )Co2 and
(R,Lu(Y))Co2 compounds (R=Er, Ho, Dy, Tb, Gd).
The (R0 ,R)Co2 and (R,Y)Co2 compounds were prepared by arc-melting the stoichiometric mixtures of rare
earth (3N) and cobalt (4N8) in an argon atmosphere in a

*Corresponding author.
E-mail address: (N.H. Duc).

water-cooled copper container. The samples were
annealed in an evacuated quartz tube at 9001C for
48 h. Magnetisation were measured using the induction
method in fields up to 10 T. Magnetisation isotherms are
illustrated in Fig. 1(a–d) for RCo2 compounds (R=Er,
Ho, Dy, Tb). MMT is observed in ErCo2, HoCo2 and
DyCo2. This MMT exists only in a small range of
temperature (DTE20 K) above the FOT. In addition,
the MMT is characterised by (i) the large hysteresis of
magnetisation, (ii) the increase of the critical fields (Bc )
and (iii) the decrease of the magnetisation jump with
increasing temperature. The disappearance of the MMT
is characterised by the disappearance of not only the
magnetisation jump but also of the hysteresis. For
DyCo2, the MMT is weakly evident in the hysteresis of
the magnetisation curves above TC : This behaviour has
completely disappeared in TbCo2.
The MMT is usually attributed to the 3d(Co)
sublattice. However, as can be seen from the magnetisation isotherms (e.g. Fig. 1a and b), the magnetisation
jump at the MMT reaches a magnitude of DM >
2 mB =f:u:; which is already over the contribution of 3d

itinerant electrons. This implies that, at the critical field
of the MMT, not only the magnetic moment is formed
at the Co sites, but also the rare earth moment becomes
ordered suddenly. In this magnetisation process, the
total magnetisation of both 4f and 3d moments must be
described as a proper thermodynamic variable.

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 3 2 8 - 2


N.H. Duc, D.T.K. Anh / Journal of Magnetism and Magnetic Materials 242–245 (2002) 873–875
7

6

36 K 40 K
6 T = 34 K

5

5
48 K

4
45 K

3

4

M (µB/f.u.)

M (µB /f.u.)

874

0

2

6

4

8

0

10

B(T)

(a)

85 K

1

ErCo2

0

90 K

2

2
1

100 K
95 K

3

HoCo2

T = 80 K
0

4

2

6

8

10

B(T)

(b)
4
230 K
160 K

155 K

4

150 K

3

2

270 K
260 K
250 K

1

DyCo2

1

(c)

T=210K

145 K

2

0

220 K

3

5

M (µB /f.u.)

M (µB /f.u.)

T= 140 K

0

2

6

4

240 K
0

8

10

B(T)

(d)

0

2

4

TbCo2
6

8

10

B(T)

Fig. 1. Magnetisation isotherms of ErCo2, HoCo2, DyCo2 and TbCo2.

The magnetic entropy change (DSm ) was calculated
from magnetisation data MT ðBÞ by the Maxwell relation
[4,5]:

Z B
qMðBÞTav

dB
DSm ðTav ÞH ¼
qT
0
H
Z B
1
½MðTiþ1 ; BÞ À MðTi ; Bފ dB:
ð1Þ
E
DT 0
Here, Tav ¼ ðTiþ1 þ Ti Þ=2 is an average temperature and
DT ¼ Tiþ1 À Ti is the temperature difference between
the two magnetisation isotherms measured at Tiþ1 and
Ti with the magnetic field changing from 0 to B: The
obtained results of DSm are illustrated in Fig. 2a and b
for ErCo2 and TbCo2. Note that, DSm always shows its
max
maximum ðDSm
Þ at TC :
max
For TbCo2, DSm
¼ À2:3 J/mol K in DB ¼ 5 T. In
addition, DSm is almost symmetric with respect to TC :
This is a general behaviour of the SOT. For ErCo2, at
max
TC DSm
reaches a huge value of À11.1 J/mol K in
DB ¼ 4 T. DSm falls abruptly just below TC ; but it still
has a rather large value in a small temperature range

above TC ; where the MMT occurrs.
As regard applications of MCE, three samples of
Gd0.4Tb0.6Co2, Gd0.7Lu0.3Co2 and Gd0.7Y0.3Co2 having
the SOT around room temperature are prepared and
investigated. These intermetallic compounds show a
max
value of DSm
E À 1:8 J/mol K in DB ¼ 6 T. Their

entropy change is comparable with that of Gd metal
(of À1.6 J/mol K), which is used as one of the working
materials nowadays [4].
max
DSm
is determined and collected in Fig. 3 for several
(R,R0 )Co2 and (R,Y)Co2 compounds. The data are in
good agreement with those reported for RCo2 [4 and
Refs. therein]. As the temperature decreases, an exmax
ponential tendency to increase of DSm
is found. This
temperature dependence of the entropy change was
previously reported for several intermetallic systems
such as RAl2, RNi2 [4]. For a more detailed comparison,
max
those data of DSm
are included in the same figure.
However, a similar variation is observed in the
temperature range T > 200 K only. At low temperatures,
max
DSm

(RAl2) is smaller than that of the corresponding
max
max
DSm
(RCo2),
e.g.
at
TC ¼ 35 K,
[DSm
max
(RCo2)ÀDSm (RAl2)]EÀ6 J/mol K. This large difference may be related to (i) the nature of the FOT and (ii)
the quenching of spin fluctuations observed in RCo2. It
was indicated that the quenching of spin fluctuations
reduced the electronic entropy DSe ¼ DgT; where g is the
electronic specific heat constant. From data of the
change of the electronic specific heat Dg at the MMT,
which was collected in Ref. [3] for several Y(Lu)Co2
related compounds, it turns out that the electronic
contribution to the entropy change is oÀ1 J/mol K.
Such a contribution is rather small with respect to the
above mentioned entropy difference in RCo2 and other


N.H. Duc, D.T.K. Anh / Journal of Magnetism and Magnetic Materials 242–245 (2002) 873–875

14

12.0
- ∆ Sm (J/mol.K)

ErCo 2

10.0
-∆ S (J/mol.K)

875

B=2T
B=4T

8.0
6.0

12

RRCo
C o2 2

10

RRAl
A l2 2
GGd
d3Al2
2Al3

8

GGd
d

6

∆B = 7 T

4
2

4.0

0
0

2.0

100

200

300

400

TC (K)

0.0
20

40
T (K)

(a)

60

max
Fig. 3. DSm
vs. TC of RCo2 and other intermetallics.

lower cost. Thus, they are rather promising for magnetic
refrigeration applications.

2.5
TbCo2

-∆ S (J/mol.K)

2.0

This work is supported by the National University of
Hanoi, within project QG.TD.00.01 and by the project
420.301 of the Fundamental Research Program of
Vietnam.

B=1T
B=3T
B=5T

1.5
1.0

0.5

References

0.0
200
(b)

220

240

260

280

300

T (K)
Fig. 2. DSm vs. T of ErCo2 (a) and TbCo2 (b).

rare earth intermetallics. Therefore, MCE observed in
RCo2 with low ordering temperature can be attributed
to the FOT.
In conclusion, MCE in RCo2 is mainly governed by
the nature of the type of the magnetic phase transition.
The size of the effect observed in (R, R0 )Co2 compounds
with the SOT at room temperature is comparable with
that of pure Gd metal. These materials, however, are of

[1] M. Foldeaki, A. Giguere, R. Chahine, T.K. Bose, Adv.
Cryogenic Eng. 43 (1998) 1533.
[2] N.H. Duc, T. Goto, in: K.A. Gschneirdner Jr., L. Eyring
(Eds.), Handbook on Physics and Chemistry of the Rare
Earths, North-Holland, Amsterdam, 1999, Vol. 26, Chapter
171, p. 301.
[3] N.H. Duc, P.E. Brommer, in: K.H.J. Buschow (Ed.),
Handbook on Magnetic Materials, North-Holland, Amsterdam, 1999, Vol. 12, Chapter 3, p. 259.
[4] M.A. Tishin, in: K.H.J. Buschow (Ed.), Handbook on
Magnetic Materials, North-Holland, Amsterdam, 1999,
Vol. 12, Chapter 4, p. 395.
[5] V.K. Pecharsky, K.A. Gschneidner Jr., J. Appl. Phys. 86
(1999) 565.