Carbon-doping effects on the metamagnetic transition and magnetocaloric effect in MnAsCx

Article abstract of DOI:10.1016/j.jmmm.2010.02.014

Carbon doping effects in MnAs alloys have been investigated. More carbon doping in MnAs alloys leads to lower Curie temperature, larger thermal hysteresis and sharper slope of the dependence of critical field on reduced temperature due to severe lattice distortion. The obtained maximum of magnetic entropy change for a field change of 5 T is about 12.8 J kg^-1 K^-1 near room temperature, and increases with more doping carbon content to about 22.4 J kg^-1 K^-1 in MnAsC_0.03 and 13.2 J kg^-1K^-1 in MnAsC_0.05.

Full text of DOI:10.1016/j.jmmm.2010.02.014

ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 322 (2010) 2223–2226
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Carbon-doping effects on the metamagnetic transition and magnetocaloric
effect in MnAsCx
n
W.B. Cui, W. Liu , Q. Zhang, B. Li, X.H. Liu, F. Yang, X.G. Zhao, Z.D. Zhang
Shenyang National Laboratory for Materials Science, Institute of Metal Research, International Center for Materials Physics, Chinese Academy of Sciences, Shenyang 110016,
People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon doping effects in MnAs alloys have been investigated. More carbon doping in MnAs alloys leads
to lower Curie temperature, larger thermal hysteresis and sharper slope of the dependence of critical
Received 2 March 2009
Received in revised form
field on reduced temperature due to severe lattice distortion. The obtained maximum of magnetic
1
7 December 2010
ꢀ1 ꢀ1
entropy change for a field change of 5 T is about 12.8 J kg
K
near room temperature, and increases
Available online 13 February 2010
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
with more doping carbon content to about 22.4 J kg
MnAsC0.05
K
in MnAsC0.03 and 13.2 J kg
K
in
Keywords:
.
Magneto-caloric effect
Metamagnetic phase transition
Carbon-doping effect
&
2010 Elsevier B.V. All rights reserved.
1
. Introduction
Recently, much interest has been focused on using the
been investigated. To date, nearly all investigations focused on
substitution effects on MCE in MnAs system. In this paper,
interstitial effects of carbon doping on the phase transition and
MCE in the MnAs system are investigated. It is found that carbon
doping can lead to lower Curie temperature, increased thermal
hysteresis and sharper slope of dependence of critical field on
reduced temperature. The maximum of magnetic entropy change
for the field change of 5 T decreases with increase in carbon
doping.
magnetocaloric effect (MCE) as an alternate technology for
refrigeration, in order to replace common gas-compression/
expansion technology, due to efficiency and environmental
concerns [1]. In particular, room-temperature magnetic refrigera-
tion materials have been of much concern [2–5]. In order to
realize this purpose, it is important to explore the compounds
with large entropy changes near room temperature. Conventional
MCE is observed for materials with second-order magnetic
transitions and the MCE would be much increased when the
transition is of first-order phase transition [6–8]. There is also a
first-order phase transition in MnAs under hydrostatic pressure
2. Experiments
x
Polycrystalline MnAsC compounds (x=0, 0.015, 0.03 and 0.5)
[
8]. When subjected to pressure, similar large MCE was found in
were synthesized by mechanical alloying, which was developed to
avoid the volatilization of As and time-exhausting solid-state
reaction. Powders of Mn, As and C with purity more than 99.9%
were mixed according to the nominal compositions. The mixtures
of 10 g were sealed in hardened steel vials with steel balls of
Gd Ge Si [9], La0.8Sr0.2MnO [10] and La(Fe1ꢀxSi [11]
assisted by magnetoelastic interactions. Specifically, bulk MnAs
undergoes a magnetic transition from a ferromagnetic (FM) phase
5
2
2
3
x 13 y
) H
c
to a paramagnetic (PM) phase at T =313 K, accompanied by a
structural transition from hexagonal NiAs-type to orthorhombic
MnP-type [12]. The phase transition is strongly sensitive to the
pressure caused by lattice distortion [8]. The substitution effect of
12 mm diameter in a high purity argon filled glove box.
Mechanical alloying was carried out for 5 h using a high-energy
ball mill that was rotated in two dimensions perpendicular to the
horizontal plane [13]. The rotation speed of the mill was chosen to
be 800 rpm. The weight ratio of ball-to-powder was 30:1. The
mechanically alloyed powders were annealed at 773 K for 2 h and
then elevated to 973 K for another 2 h in vacuum more than
3
d elements, which is considered to mimic the role of pressure,
has been extensively investigated [13–15]. However, introducing
interstitial atoms with small radius can also play the role of
pressure. Interstitial effects of carbon [16] and hydrogen [11,17]
in La–Fe–Si system and boron in Ni–Mn–Sn system [18] have
ꢀ
3
10 Pa. X-ray diffraction (XRD) analysis was conducted using
CuK radiation with a Rigaku D/Max- A diffractometer equipped
a
g
with a graphite crystal monochromator. The magnetic properties
were measured using a superconducting quantum interference
device (SQUID) with fields up to 5 T. The change of magnetic
n
Corresponding author. Fax: +86 24 23891320.
E-mail address: wliu@imr.ac.cn (W. Liu).
0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2010.02.014

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entropy was calculated from M–B plots close to their respective
Curie temperature (T ).
To make clear the influence of the external field on the
structural transition and MCE in MnAsC , isothermal magnetiza-
c
x
tion M(B) curves are recorded near their Curie temperature in a
heating process (see Fig. 2). It can be seen that magnetization
increases rapidly and shows a tendency to saturate at a low field
at 272 and 275 K, indicating an FM state with NiAs-type structure
in MnAsC0.015. With increase in temperature, an abrupt increment
is observed on the isothermal magnetization process, revealing a
3
. Results and discussions
All XRD patterns of MnAsC
show the MnP-type orthorhombic structure at room temperature.
Fig. 1 shows the thermo-magnetic M–T curves for MnAsC
x=0.015, 0.03 and 0.05) under 0.01 T on cooling and heating
x
(x=0.015, 0.03 and 0.05) alloys
field-induced metamagnetic transition, accompanied by
a
x
structural transition from orthorhombic MnP-type to hexagonal
NiAs-type [8]. Similar stepwise behaviors also appear on the M–B
curves at different temperature for other carbon-doped MnAs
alloys. With increase in temperature, the occurrence of
metamagnetic transition moves to higher field.
(
processes. A sharp decrement is observed on magnetization near
Curie temperatures, which are all below 295 K, meaning a phase
transition from a NiAs-type ferromagnetic (FM) state to MnP-type
paramagnetic (PM) state. More carbon addition leads to the
decrease of T
2
Fig. 3 gives the dependence of critical fields (Bcr) to induce the
metamagnetic transition on the reduced temperature (t) for
c
from 280 K in MnAsC0.015 to 277 K in MnAsC0.03 and
56 K in MnAsC0.05 for a heating process. However, the presence
x
MnAsC (x=0.015 and 0.03). It can be seen from Fig. 2(c) that the
of thermal hysteresis indicates the first-order nature of the phase
transition. It can be seen that the thermal hysteresis is extended
with more carbon addition, indicating that enlarged potential
barrier is necessary to induce phase transition. Because the phase
transitions in MnAs-based alloys are of first-order structural
transition, the lattice distortion caused by increase in carbon
content would influence the structural transition as reported in
Refs. [11,16–18] and be responsible for the extended thermal
hysteresis.
occurrence of metamagnetic transition in MnAsC0.05 is greatly
postponed to more than 5 T, the dependence of Bcr on the reduced
temperature in MnAsC0.05 is not shown in Fig. 3. Additionally, the
slope of Bcr–t curve seems sharper for MnAsC0.03 than that of
MnAsC0.015. More carbon doping content leads to severe lattice
distortion and larger Gibbs energy barrier between two phases,
which needs higher magnetic field to overcome Gibbs energy
barrier to induce the magnetic transitions accompanied with the
structural transition. Therefore, steeper slopes of Bcr–t curves are
observed.
The magnetic entropy change is usually calculated by the
R
B
0
Maxwell relationship
temperature dependences of
D
S
M
ðTÞ ¼
ð@M=@TÞ dB. The calculated
B
D
S
M
(T) for MnAsC
x
(x=0.015, 0.03
and 0.05) under a field change of 5 T are shown in Fig. 4. It is
argued that in the first-order phase transformation the magnetic
entropy change calculated from Maxwell relationship (MR) is
significantly different from the real value, especially in the region
of phase transformation, where an abnormal ‘‘spike’’ would be
observed. Such deviation is mainly because of incorrectly
neglecting the co-existence of paramagnetic phase and
ferromagnetic phase. Some improvements have been made on
the calculation of magnetic entropy change [19,20]. Generally, in
the first-order phase transition, the Clausius–Clapeyron (CC)
equation is more accurate in determining the magnetic entropy
change, especially in the region of phase transition [21]. Here,
because the magnetization is not fully saturated in our system,
the real magnetic entropy change is calculated by considering the
co-existence of paramagnetic and ferromagnetic phases [19],
as shown in Fig. 4, in comparison with the results calculated
directly by the Maxwell relationship. It can be seen that magnetic
ꢀ
1
ꢀ1
entropy change increases from 12.8 J kg
K
at x=0.015 to
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
22.4 J kg
K
at x=0.03 and then decreases to 13.2 J kg
K
at x=0.05, all of which are much lower than the corresponding
nominal magnetic entropy change calculated by the Maxwell
relationship. Since Wada and Tanabe [3] reported the large
magnetocaloric effect in MnAs system, much work has been done
on the substitution effects in MnAs systems [12–15,20,22,23].
However, the interstitial effects in MnAs system are investigated
in the present paper. So it would be interesting to compare the
substitutional and interstitial effect on the
M c
DS , T and thermal
hysteresis in MnAs based alloys, as listed in Table 1. From Table 1,
it can be seen that the thermal hysteresis in MnAs system can be
reduced to nearly zero only by Cr substitution for Mn and Si
substitution for As. The accompanied phase transition is more
like a second-order one, with obscure or even no inflexion
point on the isothermal magnetization curves. The magnetic
entropy changes in MnAs based compound in Table 1 range from
ꢀ
1
ꢀ1
2
0 to 30 J kg
K
, except in Mn1ꢀxCu
x M
As system where DS is
Fig. 1. Temperature dependence of magnetization curves for MnAsC
.03 and 0.05) under 0.01 T on cooling and heating processes.
x
(x=0.015,
0
directly obtained by Maxwell relationship. However, if we ignore

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2225
Fig. 2. Isothermal magnetization curves near their respective Curie temperature under magnetic field from 0 to 5 T for (a) MnAsC0.015, (b) MnAsC0.03 and (c) MnAsC0.05. The
measurements were carried out with increasing temperature.
Fig. 4. Magnetic entropy changes as the function of temperature and carbon
content under a magnetic field change of 5 T (the solid lines). The dashed line
Fig. 3. Dependence of critical field (Bcr) on the reduced temperature for MnAsC
x=0.015 and 0.03). The closed and open symbols represent real and estimated
critical field.
x
shows the estimated value of magnetic entropy change considering the co-
existence of paramagnetic and paramagnetic phases and the calculation of the
estimated values could be referred in Ref. [19].
(

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Table 1
Comparison of some main parameters in MnAs based systems.
ꢀ
1
ꢀ1
K )
D
S (J kg
T
c
(in heating process)
Thermal hysteresis
Reference
Ref. [3]
MnAs1ꢀxSb
Mn1ꢀxFe As
Mn1ꢀxCu
Mn1ꢀxCr
x
32 (x=0) (Bmax=5 T)
318 K (x=0)–230 K (x=0.25)
5 K (x=0)–0 K(xZ0.05)
x
26 (x=0.01) (Bmax=5 T) Ref. [20]
320 K (x=0)–280 K (x=0.015) Ref. [15] 10 K (x=0)–28 K (x=0.015) (B=0.02 T) Ref. [15]
x
As
45–175 (x=0.02, peak value) (Bmax=5 T) 317 K–323 K
12 K (x=0.003)–20 K (x=0.02) (B=0.02 T)
E0 K (x=0.006)–3 K (x=0.01) (B=0.01 T)
Ref. [15]
x
As
13 (x=0.01)–20 (x=0.006) (Bmax=5 T)
As E30 (x=0.1) (Bmax=2 T)
265 K (x=0.01)–290 K (x=0.006)
Ref. [13]
Mn1ꢀx(Ti0.5
MnAs1ꢀxSi
MnAsC
V
0.5
)
x
318 K (x=0)–266 K (x=0.1)
Ref. [22]
x
14 (x=0.03)–10.9 (x=0.09) (Bmax=5 T)
270 K (x=0.03)–297 K (x=0.09)
5 K (x=0.03)–E0 K (x=0.09) (B=0.01 T)
18 K (x=0.015)–35 K (x=0.05) (B=0.01 T)
Ref. [23]
x
12.8 (x=0.015)–22.4 (x=0.03) (Bmax=5 T) 280 K (x=0.015)–256 K (x=0.05)
Present work
the ‘‘spike’’ value in the phase transition region in Mn1ꢀxCu
x
As
[4] A. Fujita, S. Fujieda, K. Fukamichi, H. Mitamura, T. Goto, Phys. Rev. B 65 (2001)
14410.
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0
system, the magnetic entropy values in the plateau region in
ꢀ
1
ꢀ1
DS
M
–T curves still range from 20 to 30 J kg
K
, which may be
5
the reasonable value for MnAs based compound.
1
[
[
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4
. Conclusions
In conclusion, carbon-doping effects in MnAs alloys are
[9] A.M.G. Carvalho, C.S. Alves, A. de Campos, A.A. Coelho, S. Gama, F.C.G. Gandra,
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investigated. Curie temperature decreases from 280 K in
MnAsC0.015 to 256 K in MnAsC0.03. Larger thermal hysteresis and
sharper slope of Bcr–t with more carbon content are observed,
which may be due to the severe lattice distortion. For a field
change of 5 T, maximum of magnetic entropy change in
[12] J. Mira, F. Rivadulla, J. Rivas, A. Fondado, T. Guidi, R. Caciuffo, F. Carsughi, P.G.
Radaelli, J.B. Goodenough, Phys. Rev. Lett. 90 (2003) 097203.
[
13] N.K. Sun, W.B. Cui, D. Li, D.Y. Geng, F. Yang, Z.D. Zhang, Appl. Phys. Lett. 92
ꢀ
1
ꢀ1
MnAsC0.015 is about 12.8 J kg
K
near room temperature.
(2008) 072504.
ꢀ
1
ꢀ1
More doping carbon content leads to about 22.4 J kg
K
in
[14] A. de Campos, D.L. Rocco, A.M.G. Carvalho, L. Caron, A.A. Coelho, S. Gama, L. da
Silva, F.C.G. Gandra, A. O dos Santos, L.P. Cardoso, P.J. von Ranke, N.A. de
Oliveira, Nat. Mater. 5 (2006) 802.
ꢀ
1
ꢀ1
MnAsC0.03 and 13.2 J kg
K
in MnAsC0.05.
[
15] D.L. Rocco, A. de Campos, A.M.G. Carvalho, L. Caron, A.A. Coelho, S. Gama,
F.C.G. Gandra, A. O dos Santos, L.P. Cardoso, P.J. von Ranke, N.A. de Oliveira,
Appl. Phys. Lett. 90 (2007) 242507.
Acknowledgements
[
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17] K. Mandal, D. Pal, O. Gutfleisch, P. Kerschl, K.H. M u¨ ller, J. Appl. Phys. 102
(2007) 053906.
This work has been supported by the National Nature Science
Foundation of China under Projects 50831006 and 50971123 and
National Basic Research Program (No. 2010CB934603) of China,
the Ministry of Science and Technology of China.
18] H.C. Xuan, D.H. Wang, C.L. Zhang, Z.D. Han, B.X. Gu, Y.W. Du, Appl. Phys. Lett.
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