Their Curie temperature TC slightly increases with increasing Nd up to y ¼ 0:3: The most striking effect of the Nd substitution, however, is in their itinerant-electron metamagnetic beha
Trang 1Magnetism and magnetocaloric effect in
D.T Kim Anha, N.P Thuya,b, N.H Duca,*, T.T Nhiena, N.V Nongb a
Cryogenic Laboratory, Faculty of Physics, College of Natural Science, Vietnam National University of Hanoi, Nguyen Trai, Thanh
Xuan, Hanoi 334, Viet Nam
b
International Training Institute for Materials Science (ITIMS), Hanoi, Viet Nam
Abstract
Structural and magnetic properties of La1yNdy(Fe0.88Si0.12)13compounds have been investigated by means of X-ray diffraction and magnetization measurements The single-phase NaZn13-type cubic structure is stabilized for the compounds with y ¼ 0; 0.1, 0.3 and 0.4 All the synthesized compounds are ferromagnetic Their Curie temperature TC
slightly increases with increasing Nd up to y ¼ 0:3: The most striking effect of the Nd substitution, however, is in their itinerant-electron metamagnetic behavior and the magnetocaloric effect in the vicinity of TC: The maximum entropy change decreases somewhat, but the relative cooling power increases with increasing Nd content (i.e for y ¼ 0:3)
r2003 Elsevier Science B.V All rights reserved
PACS: 75.10.Lp; 75.30.Sg
Keywords: Rare earth—transition metal intermetallic compounds; Itinerant-electron metamagnetism; Magnetocaloric effect
Recently, the magnetic LaT13 (T=Fe and Co)
compounds of the NaZn13-type cubic structure
have been intensively studied It is, on the one
hand, due to the fact that these compounds have
the largest amount of transition metal in the
crystalline formula unit among the rare-earth
transition intermetallics [1,2] On the other hand,
this is a novel composition that exhibits
Fe-metamagnetism The cubic NaZn13-type structure
is easily stabilized in the binary La–Co compound
For the La–Fe, however, this structure can be
formed only in pseudo-binary La(FexM1x)13
(M=Al, Si) compounds[3] The magnetic
proper-ties of these compounds are sensitive to the kind
and the concentration of the M element The magnetic state in La(FexAl1x)13 compounds is ferromagnetic for 0:62oxp0:86; and antiferro-magnetic for 0:86oxp0:92 [4] La(FexSi1x)13
compounds are ferromagnetic in the region 0:62oxp0:86: However, their Curie temperature
TC decreases with increasing Fe concentration, whereas the saturation magnetic moment increases
[1] For these La(FexSi1x)13 compounds, it was reported that in the high Fe concentration region,
an itinerant-electron metamagnetic (IEMM) tran-sition, i.e a field-induced first-order paramagnetic-ferromagnetic transition, accompanied by a large negative lattice expansion, appeared just above the Curie temperature Finally, it is interesting to mention that the pseudo-binary La(FexM1x)13
compounds with M=Si and Al exhibit a giant
*Corresponding author.
E-mail address: duc@netnam.org.vn (N.H Duc).
0304-8853/03/$ - see front matter r 2003 Elsevier Science B.V All rights reserved.
doi:10.1016/S0304-8853(03)00073-8
Trang 2magnetostriction effect, which is rather promising
for applications [5,6]
In order to enhance the ordering temperature
and reduce the critical field of the
3d-metamag-netic transition, La was replaced partly by other
magnetic rare-earth elements such as R=Pr, Nd
and Gd La1yRy(Fe0.88Si0.12)13 compounds [5]
For these three rare-earth elements, the
substitu-tion is, however, limited up to y ¼ 0:2 only In
addition, the influence of the 4f(R)–3d(Fe)
exchange interactions on the TC is not so
helpful for yp0:2 [5] In this paper, we report a
study of the magnetocaloric effect (MCE) in
La1yNdy(Fe0.88Si0.12)13 (y ¼ 0; 0.1, 0.3 and 0.4)
pseudo-binary compounds
The La1yNdy(Fe0.88Si0.12)13 samples with y ¼
0; 0.1, 0.3, 0.4 were prepared by arc-melting the
appropriate amount of high purity (99.9% or
better) constituent elements in an inert purified Ar
atmosphere The arc-melted alloy buttons were
turned over and re-melted several times to ensure
homogeneity Subsequently, the samples were
annealed for 6 days at 1020C in evacuated quartz
tubes, followed by quenching in ice water X-ray
powder diffraction showed that the samples well
crystallize in the NaZn13-type structure However,
an amount of a-Fe exists as a secondary phase
The concentration dependence of the lattice
parameter a of the NaZn13-type structure in the
La1yNdy(Fe0.88Si0.12)13 (y ¼ 0:0; 0.1, 0.3 and 0.4)
compounds is shown inFig 1 A clear reduction of
the lattice parameter with increasing Nd
concen-tration is observed This is a good indication of the
introduction of Nd atoms into the La–Fe phase
The thermomagnetic curves MðT Þ were
mea-sured in an applied field of 0.1 T using a vibrating
sample magnetometer (VSM) The results are
illustrated in Fig 2 for samples with y ¼ 0:0 and
0.3 Note that both investigated samples are
ferromagnetic Their Curie temperature TCslightly
increases with increasing y (see Fig 3) The
observed enhancement of the ordering
tempera-ture may be attributed to the Nd–Nd
intrasublat-tice and the Nd–Fe intersublatintrasublat-tice exchange
interactions
Fig 4a displays the magnetization isotherms
MðHÞ measured in pulsed fields up to 5 T in a
wide temperature range around T for the sample
with y ¼ 0:0: Note that well below the TC; the magnetization curves are saturated In the vicinity
of TC; however, we observed only a tendency toward saturation A similar result is also obtained for the sample with y ¼ 0:3 (Fig 4b) This behavior can be associated with an IEMM
11.400 11.425 11.450 11.475
y
Fig 1 The concentration dependence of the lattice parameter a
in the La 1y Nd y (Fe 0.88 Si 0.12 ) 13 The dotted line is a guide to the eye.
5 10 15 20 25 30 35 40 45 50 55
T (K)
y = 0.3
y = 0.0
Fig 2 Thermal magnetization curves of La 1y Nd y (Fe 0.88 Si 0.12 ) 13
with y ¼ 0:0 and 0.3 at a field of 0.1 T.
Trang 3transition as already evidenced by means of the
magnetostriction measurements[5]
Based on general thermodynamics principles[7],
the magnetic entropy change for an
isobaric-isothermal process can be related to the
tempera-ture dependence of the magnetization, with one of
the Maxwell relation, by
qS
qH
T;p
¼ qM
qT
H;p
:
From a collection of magnetization isotherms,
the magnetic entropy change—DSM or the MCE
can be calculated approximately by[8],
DSM ¼X
i
1
Tiþ1 TiðMi Miþ1Þ DHi;
where Mi and Miþ1 are the magnetization values
obtained at temperature Ti and Tiþ1 in a field H;
respectively The plots of DSM as a function of
temperature in different magnetic fields are
pre-sented in Fig 5 for La(Fe0.88Si0.12)13 and
La0.7Nd0.3(Fe0.88Si0.12)13 In an applied field of
5 T, the maximal DSM(max) values of 12.6 and
9.7 J/kg K are found for La(Fe0.88Si0.12)13 and
La0.7Nd0.3(Fe0.88Si0.12)13, respectively The
magni-tude of the magnetic entropy change of
La0.7Nd0.3(Fe0.88Si0.12)13 is smaller than that of
La(Fe Si ) for the same field change
(DH ¼ 5 T), although it is comparable to that of
Gd metal (DSM(max)E10 J/kg K)[9] This result
is in good agreement with a previous report[10] For applications, however, it is interesting to consider the relative cooling power (RCP) based
on the magnetic entropy change [10], i.e the
180
190
200
210
220
230
240
250
y
T C
Fig 3 The concentration dependence of the Curie temperature
in the La 1y Nd y (Fe 0.88 Si 0.12 ) 13 (a)
0 2 4 6
H (T) 0
50 100 150
2 /kg)
180 K
195 K
200 K
205 K
210 K
220 K
150 K
170 K
(b)
0 2 4 6 0
50 100
150
180 K
195 K
205 K
215 K
220 K
225 K
230 K
240 K
255 K
275 K
2 /kg)
H (T)
Fig 4 The magnetization isotherms of La 1y Nd y -(Fe 0.88 Si 0.12 ) 13 compounds: y ¼ 0 (upper) and y ¼ 0:3 (lower).
Trang 4product of DSM(max) dTFWHM; where
dTFWHM¼ T2 T1 is the full-width at
half-max-imum (FWHM) [11] As shown in Fig 6, the
RCP equals 388 and 328 J/kg for La0.7Nd0.3
-(Fe0.88Si0.12)13and La(Fe0.88Si0.12)13, respectively
In summary, the Nd substitution in the
La Nd(Fe Si ) decreases the lattice
parameter, whereas the Curie temperature TC
slightly increases from 210 to 215 K In addition, the RCP increased in the La0.7Nd0.3(Fe0.88Si0.12)13
compounds Therefore, this alloy can be a candidate for magnetic refrigerant at the corre-sponding temperature range
This work was supported by the College of Natural Science, VNU, Hanoi under the Research Grants No TN-02.06 The authors would like to thank Mr N.N Phuoc for his help
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