To present day, the maximum magnetocaloric effect (MCE) at room temperature for a magnetic field change of 5 T is 40 J/(kg K) for MnAs. In this Letter we present colossal MCE measurements on MnAs under pressure, reaching values up to 267 J/(kg K), far greater than the magnetic limit arising from the assumption of magnetic field independence of the lattice and electronic entropy contributions. The origin of the effect is the contribution to the entropy variation coming from the lattice through the magnetoelastic coupling.9323720

Carvalho, A Magnus G

Coelho, Adelino A

de Campos, Ariana

de Oliveira, Nilson A

Gama, Sergio

Gandra, Flávio C G

von Ranke, Pedro J

Physical Review Letters

English

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PRL 93, 237202 (2004) P H Y S I C A L R E V I E W L E T T E R S
week ending
3 DECEMBER 2004Pressure-Induced Colossal Magnetocaloric Effect in MnAs
Sergio Gama, Adelino A. Coelho, Ariana de Campos, A. Magnus G. Carvalho, and Flávio C. G. Gandra
Instituto de Fı́sica Gleb Wataghin, Universidade Estadual de Campinas-UNICAMP,
Caixa Postal 6165, 13083-970 Campinas, SP, Brazil
Pedro J. von Ranke and Nilson A. de Oliveira
Instituto de Fı́sica, Universidade do Estado do Rio de Janeiro-UERJ,
Rua São Francisco Xavier, 524, 20550-013 Rio de Janeiro, RJ, Brazil
(Received 22 February 2004; published 30 November 2004)0031-9007=To present day, the maximum magnetocaloric effect (MCE) at room temperature for a magnetic field
change of 5 T is 40 J=kg K for MnAs. In this Letter we present colossal MCE measurements on MnAs
under pressure, reaching values up to 267 J=kg K, far greater than the magnetic limit arising from the
assumption of magnetic field independence of the lattice and electronic entropy contributions. The
origin of the effect is the contribution to the entropy variation coming from the lattice through the
magnetoelastic coupling.
DOI: 10.1103/PhysRevLett.93.237202 PACS numbers: 75.30.Sg, 71.20.Lp, 81.05.Bx, 81.40.VwThe MCE is the basis for the magnetic refrigeration
technique [1–5]. Recently, several apparatuses for refrig-
eration around room temperature were disclosed, clearly
showing the plausibility of such technology [6,7]. The
importance of the magnetic refrigeration relies on the
use of environmentally friendly materials and on greater
energy efficiency. A limitation for this technology is the
need of high magnetic fields to obtain large enough
temperature variations. For this reason, one of the main
topics on the study of the MCE is the search for materials
with enhanced effects compared to the ones already
known.
The MCE is significant around the magnetic phase
transitions. In 1997 it was discovered that the compound
Gd5Ge1xSix4; x  0:5, presents a giant MCE (GMCE)
with TC’s tuned from 20 Kx  0 up to 276 Kx  0:5
[8–11]. This remarkable material presents simultaneously
a first order magnetic transition coupled to a crystallo-
graphic one. In 2001 it was shown that the compound
LaFe11:4Si1:6 also presents a GMCE [12], and later in 2003
it was announced that the compound LaFe0:88Si0:1213Hy
presents the GMCE with low values of the thermal hys-
teresis [13]. In 2001 two other materials were shown to
present the GMCE: MnAs, with TC  318 K, and the
series of compounds MnAs1xSbx; 0 < x< 0:3 [14–16].
MnAs also presents a first order magnetic transition
coupled to a crystallographic one, from hexagonal to
orthorhombic structure, with a large thermal hysteresis
and a GMCE that is twice the value first reported for
Gd5Ge2Si2 [8]. In 2002 it was disclosed that the com-
pound FeMnAs0:55P0:45 also presents a GMCE with a first
order magnetic transition at 300 K but with a low value
for the thermal hysteresis [17,18]. Among all these mate-
rials, for a field variation of 5 T, the highest reported S
is 40 J=kg K for MnAs [14] around room temperature.
The magnetocaloric effect corresponds to the tempera-
ture variation a magnetic body suffers when submitted to04=93(23)=237202(4)$22.50 237202a magnetic field change. It is described by the isothermal
entropy variation observed in these operations.
Integration of the Maxwell relation
@ST;H
@H

T


@MT;H
@T

H
(1)
allows one to obtain the entropy variation from isother-
mal magnetization measurements as a function of the
applied field. The Maxwell relation involves the total
entropy of the body, comprising the contributions from
the lattice, the electrons, and the magnetic system. For
materials with localized moments it is a good approxi-
mation to take the lattice and electronic contributions as
magnetic field independent, so that all the field induced
isothermal entropy variation can be ascribed to the mag-
netic contribution to the entropy, SM. When we are
dealing with itinerant magnetic materials, the assumption
of field independence of the lattice and electronic contri-
butions is only valid if the electron-phonon interaction is
small [1]. The same can be said for systems showing
magnetoelastic interactions. However, the field indepen-
dence of the electronic and lattice terms is usually
adopted in practice for all materials. Consequently, it is
also usually accepted that the upper limit for the molar
entropy variation is given by the magnetic contribution
SmaxM  R ln2J 1; (2)
even though this can be, in principle, surpassed when a
system undergoes a first order transition, once the entropy
variation of the system is dominated by the latent heat of
the transition [1,19,20]. However, to the best of our
knowledge, no GMCE material is reported as presenting
an entropic MCE that reaches this limit, as can be seen
from Table I.
Also, phenomenological models of the GMCE for sev-
eral of the materials in Table I at ambient pressure confirm
that the observed MCE’s are well described using this-1  2004 The American Physical Society
FIG. 1 (color online). Magnetization curves for as prepared
MnAs at a pressure of 1.80 kbar with increasing field and
temperature.
TABLE I. Curie temperatures, effective moments, and observed and expected SM for
materials presenting giant MCE for a field variation of 5 T around or near room temperature.
Material
Curie
Temperature (K) J or S
Observed
SmaxM
[J=kg K]
Expected
SmaxM
[J=kg K]
References
Gd5Si2Ge2 272 3.5 36.4 88.9 [29]
MnAs 318 2 40 103 [14]
FeMnAsP 305 1 18 117 [17]
MnAs0:9Sb0:1 283 2 30 103 [14–16]
LaFe0:88Si0:1213H1 274 1 23 127 [13]
PRL 93, 237202 (2004) P H Y S I C A L R E V I E W L E T T E R S
week ending
3 DECEMBER 2004approximation of magnetic field independence [21–23] of
the lattice and electronic contributions to the entropy.
In Table I, MnAs is the compound in which the first
order magnetic transition was first observed [24–27]. It
presents a crystallographic transition from hexagonal
(NiAs) to orthorhombic (MnP) structure, simultaneously
with the first order magnetic transition. Its peculiar mag-
netic properties motivated the development of the first
phenomenological model to explain the observed mag-
netic first order transition [25]. This motivated the study
of its magnetic properties under hydrostatic pressures
[27], showing a decrease of the Curie temperature as
pressure increases, together a widening of the thermal
hysteresis.
Recently we started the study of the magnetic proper-
ties and the magnetocaloric effect of MnAs under hydro-
static pressures. The MnAs sample was prepared from
99.99% pure Mn and 99.9999% pure As and is single
phase. The sample was fully magnetically characterized,
presenting a saturation magnetization (MS) at 4 K of
3:2 B=Mn, TC  318 K when increasing the tempera-
ture, and 311 K when decreasing the temperature, and a
maximum S of 47 J=kg K. These values are in ex-
cellent agreement with the literature [24–27].
For the pressure measurements we used a Cu-Be clamp
type cell, able to work up to 12 kbar at 300 K in our
SQUID magnetometer. Our pressure scale has been ob-
tained from the transition temperatures determined in-
creasing the temperature, and comparing these values
with the ones from Ref. [27]. For the MCE determination,
MH curves up to H  5 T at several temperatures
were taken with both field and temperature increasing.
Near TC a step of 1 K was adopted.
Figure 1 shows the magnetization isotherms used to
calculate the MCE for a pressure of 1.80 kbar, similar for
all the other measured pressures. Below TC, the behavior
is ferromagnetic, and above, we have a strong initial
response of the magnetization, followed by a plateau
and by a metamagnetic transition to the ferromagnetic
state at a critical field. As the temperature is increased,
this critical field shifts to higher values.
Figure 2 shows the measured MCE. As pressure in-
creases, TC decreases, following the behavior described237202in the literature [27]. Notably, the MCE increases very
fast with pressure, reaching a maximum value of
267 J=kg K for 2.23 kbar. Increasing the pressure be-
yond this value, the effect starts to diminish, and for
pressures above 2.64 kbar it shows very low values. The
disappearance of the MCE is associated with a strong
decrease of the saturation magnetization (MS) with pres-
sure, as well as the onset of a new magnetic ordering,
possibly antiferromagnetism, as also described in the
literature [27,28]. These maximum MCE values are far
above the values predicted by Eq. (2) based on the hy-
pothesis of magnetic field independence of the lattice
and electronic contributions to the entropy, as shown in
Fig. 3. From this figure we obtain that the maximum value
of the entropic MCE varies with pressure at a rate of
92:4 J=kg K kbar. We note that already for the pressure
of 0.38 kbar the measured MCE surpasses the limit
established by Eq. (2). The experimental values in Fig. 3
are far higher than the ones presented by MnAs at am-
bient pressure or presented by the other materials in
Table I. Figure 4 compares some of our values for the
MCE with the ones for the most representative materials
in Table I. These values are also far higher than the ones-2
FIG. 2 (color online). The MCE as a function of temperature
and pressure for as prepared MnAs.
PRL 93, 237202 (2004) P H Y S I C A L R E V I E W L E T T E R S
week ending
3 DECEMBER 2004expected from the latent heat of the first order transition,
1:79 cal=g [23:5 J=kg K] [25].
Examining closely Fig. 2, we observe that, as pressure
increases, the shape of the MCE curve changes. For
ambient and low pressures, it presents a peak followed
by a plateau. As the pressure increases, the width of the
plateau decreases and the peak increases very fast, until
the plateau disappears. This suggests some kind of area
conservation is happening here. In fact, the area of the
entropy variation curve of the MCE defines the refriger-
ating power of the material, and has an upper limit given,
even for materials presenting first order magnetic transi-
tions, by the expression [1]
Z 1
0
SdT  MSH (3)
In this equation, S is the total entropy and H is the field
variation. As far as MS does not change, it is to be
expected this area to be constant for all pressures. The
inset in Fig. 4 shows the value calculated from Eq. (3) as aFIG. 3. Experimental maximum S values (circles) for
MnAs under pressure, and comparison with the theoretical
magnetic upper limit value for Jeff  2 for the Mn atom.
237202dashed line and the experimental values of the areas
obtained by numerical integration of the experimental
curves. There is a small variation of the area as pressure
increases, at a rate of 32 J=kg kbar, but all values are
below the one calculated from Eq. (3), that represents an
upper limit for all the curves. We emphasize that these
data are in accordance with the equation that involves the
total entropy and an intrinsic property of the material,MS.
However, the maximum values of the MCE are far greater
than the ones expected from the measured latent heat of
the transition or from Eq. (2), obtained exclusively from
the magnetic contribution to the entropy, i.e., using ex-
plicitly the assumption of magnetic field independence of
the lattice and electronic contributions to the entropy.
As the maximum values for the MCE of MnAs under
pressure are so far greater than the ones observed for all
the other materials up to this day, we termed the effect
colossal magnetocaloric effect (CMCE). The maximum
value of the entropic CMCE reaches a peak at
267 J=kg K for T  281 K and p  2:23 kbar, 7.3 times
greater than the value for optimally prepared Gd5Si2Ge2
[36:4 J=kg K and TC  272 K [29]], establishing a re-
cord value for the entropic variation under 5 T field
variation. In molar terms, it results in a value of
34:7 J=mol K, 1.47 times greater than the theoretical
limit for holmium, 23:6 J=mol K, the rare earth with
the highest J value. Preliminary results from a phenome-
nological model, based on an extension of the Bean and
Rodbell model [25], applied to MnAs under pressure [30]
confirm that the lattice contribution can account for the
experimental CMCE values. The strong magnetoelastic
interaction in MnAs is the driving force for the structural
and the first order magnetic transitions [24–27], and
imposes a strong lattice contribution to the entropy, ex-FIG. 4 (color online). Comparison of the CMCE for MnAs
under pressure with MnAs [14], MnAs0:9Sb0:1 [14],
FeMnAs0:55P0:45 [17], and optimally prepared Gd5Ge2Si2 [29].
In the inset, experimental areas under the S vs T curves
(circles) compared with the calculated ones (dashed line) for
MnAs.
-3
PRL 93, 237202 (2004) P H Y S I C A L R E V I E W L E T T E R S
week ending
3 DECEMBER 2004plaining the CMCE. This means that other materials
presenting strong magnetoelastic interaction can also dis-
play the CMCE, as is the case of the compounds
Gd5Ge1xSix4 and FeMnAs1xPx.
It is tempting to think that to the colossal entropic
effect corresponds a colossal temperature MCE. This is
not the case, however, as demonstrated by our prelimi-
nary calculations of the total entropy curves for MnAs
under pressure [30]. The calculations show that the co-
lossal entropic effect shows up as a quasi discontinuity in
both total entropy curves, presenting a small temperature
separation around the transition of the order of only 8 K.
(This parallels the description of the GMCE in Ref. [19].)
This means that the temperature variation is relatively
small, but the refrigerating power of the material is fairly
great, but concentrated in a small temperature interval.
The discovery of the CMCE in MnAs is a clear ex-
perimental verification that the idea of field independence
of the lattice and electronic entropy contributions to the
total entropy under a field variation is not valid in general.
It also highlights the possibility of a fairly large contri-
bution from the lattice to the total entropy change of the
material under a magnetic field variation, and that the
available data on latent heat of the first order transition
can not account for the observed colossal values mea-
sured. Without violating the sum rule, MnAs under pres-
sure presents the highest S peak ever seen, in a narrow
temperature window, and this result may be exploited in
the design of new materials to work as refrigerants for
magnetic refrigerators.
The authors acknowledge financial support from
Fundação de Amparo à Pesquisa do Estado de São
Paulo-Fapesp, through Grant No. 01/05883-0, and from
Conselho Nacional de Desenvolvimento Cientı́fico e
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Pressure-induced Colossal Magnetocaloric Effect In Mnas.

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Pressure-induced Colossal Magnetocaloric Effect In Mnas.

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