Despite some previous efforts [7,16 e18] , a clear understanding of the magneto- caloric effect and its association with the magnetic phase transition and magnetic interactions character[r]
Trang 1Nguyen Thi Maia,b, Nguyen Hai Yenc, Pham Thi Thanhc, Tran Dang Thanhc,
a The College of Printing Industry, Phuc Dien, Bac Tu Liem, Ha Noi, Viet Nam
b Hanoi University of Science, Vietnam National University, 334 Nguyen Trai, Thanh Xuan, Ha Noi, Viet Nam
c Institute of Material Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet, Cau Giay, Ha Noi, Viet Nam
d Hanoi Pedagogical University, No 2, 32 Nguyen Van Linh, Phuc Yen, Vinh Phuc, Viet Nam
e Hong Duc University, 565 Quang Trung, Dong Ve, Thanh Hoa, Viet Nam
f Quang Ninh University of Industry, Yen Tho, Dong Trieu, Quang Ninh, Viet Nam
a r t i c l e i n f o
Article history:
Received 15 February 2017
Received in revised form
17 February 2017
Accepted 19 February 2017
Available online 27 February 2017
Keywords:
Magnetocaloric effect
Magnetic phase transition
Heusler alloy
Critical parameter
Melt-spinning method
a b s t r a c t
Ni50xCoxMn50yAly(x¼ 7 and 9; y ¼ 17, 18 and 19) alloy ribbons were prepared by melt-spinning with a tangential velocity of copper wheel of 40 m s1 X-ray diffraction patterns reveal multi-crystalline phase behavior in the fabricated ribbons The shape of thermomagnetization curves clearly depends on Co and
Al concentrations The Curie temperatures (TC) of the alloy ribbons strongly increase with increasing the
Co concentration and slightly decrease with increasing the Al concentration The martensitic-austenitic phase transition in the alloy ribbons can be manipulated by tuning Co and Al concentrations The maximum magnetic entropy changejDSmjmaxof about 0.75 J kg1K1for afield change of 12 kOe at
TCz 364 K was achieved for the Ni43Co7Mn32Al18ribbon Critical analysis using the Arrott-Noaks and KouveleFisher methods demonstrates the existence of a long-range ferromagnetic order in this ribbon
© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)
1 Introduction
The magnetocaloric effect (MCE) is defined as the heating or
cooling of a magnetic material when a magneticfield is applied The
MCE occurs in a magnetic solid as a result of the entropy variation
due to the coupling of the magnetic spin system with the magnetic
field Since the discovery of the MCE, it has been widely utilized in
magnetic materials to reach low temperatures Nowadays, there is a
great deal of interest in using the MCE as an alternative technology
for refrigeration The magnetic refrigeration offers the prospect of
an energy-efficient and environmentally friendly alternative to the
common vapor cycle refrigeration technology used today[1e5]
Among magnetocaloric materials, NieMn-based Heusler alloys
are emerging as a promising candidate[6] Recently, Ni-Mn-based
alloys have been reported to exhibit the large magnetocaloric ef-fects, including both the conventional and inverse magnetocaloric effects [7e9] Besides that, the shape memory effect and other interesting properties have also been observed[10e12] In the Ni-Mn-Al Heusler alloys, the Neel temperature TN z 300 K was found to be virtually independent of composition[13] The
Ni-Mn-Al alloys with their relatively low cost and high ductility are a potentially attractive candidate material as a magnetic refrigerant The partial substitution of Co for Ni in these alloys has been re-ported to have a strong effect on the martensitic-austenitic trans-formation which greatly enhanced the MCE[14e16] Despite some previous efforts[7,16e18], a clear understanding of the magneto-caloric effect and its association with the magnetic phase transition and magnetic interactions characterized by critical exponents in Ni-Co-Mn-Al alloys has not been reached
To address this, we have systematically investigated the mag-netic, magnetocaloric and critical properties of Ni50 xCoxMn50 yAly
(x¼ 7 and 9; y ¼ 17, 18 and 19) rapidly quenched ribbons
* Corresponding author.
E-mail address: dannh@ims.vast.ac.vn (N.H Dan).
Peer review under responsibility of Vietnam National University, Hanoi.
http://dx.doi.org/10.1016/j.jsamd.2017.02.007
2468-2179/© 2017 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license
Trang 22 Experimental
Six alloy ingots with nominal compositions of Ni50x
-CoxMn50 yAly(x¼ 7 and 9; y ¼ 17, 18 and 19) were prepared from
pure elements (99.9%) of Ni, Co, Mn and Al using the arc-melting
method in Argon gas The melt-spinning method was then used
to fabricate the alloy ribbons with a tangential velocity of copper
wheel of 40 m s1 The structure of the alloys was investigated by
powder X-ray diffraction (XRD) technique using CuKa radiation
with measuring step of 0.02at room temperature The magnetic
properties and magnetocaloric effects of the alloys were
charac-terized on a vibrating sample magnetometer with temperature
range of 77e500 K and maximum magnetic field of 12 kOe
3 Results and discussion
The XRD patterns taken at room temperature (Fig 1) show that
crystalline phases of L10(face centered cubic), B2 (body centered
cubic) and 10M (orthorhombic) are formed in the ribbons Most of
the samples mainly contain B2 and L10 phases The 10M phase
appears in the ribbons with high concentrations of Al and Co The
change of the structure would probably affect magnetic and
mag-netocaloric properties of the alloys
The magnetic properties of the samples were characterized by
magnetization versus temperature (M-T) measurements (Fig 2a)
The results show that the shape of M-T curves clearly depends on Co
and Al concentrations For examples, the magnetization of the
sample with x¼ 7 increased from ~0.7 emu/g for y ¼ 17 to ~7.7 emu/g
for y¼ 19 The Curie temperatures of the alloy ribbons strongly increased with increasing the Co concentration and slightly decreased with increasing the Al concentration By increasing the Co concentration from 7 at.% to 9 at.%, the Curie temperature (TC) strongly increased from ~364 K (for x¼ 7 and y ¼ 18) to ~ 394 K (for
x¼ 9 and y ¼ 18) The magnetization of Ni50xCoxMn33Al17ribbons are also increased considerably with the substitution of Co for Ni The martensitic-austenitic phase transition in the alloy ribbons can
be tuned by adjusting Co and Al concentrations
In this series of samples, the Ni43Co7Mn32Al18ribbon shows two strong magnetic phase transitions near a room temperature region Therefore, it was chosen as a representative one for analyzing magnetic and magnetocaloric properties The magnetocaloric effect
in the ribbon was assessed by the magnetic entropy change (DSm)
as functions of temperature and magnetic field using Maxwell relationship:
DSm¼
ZH 2
H 1
vM vT
H
From a series of experimental curves M(T) (Fig 2b), the corre-sponding M(H) curves can be deduced (Fig 3a) Fig 3b shows
DSm(T) curves of the Ni43Co7Mn32Al18ribbon for different magnetic field changes (D ¼ 1, 4, 8, and 12 kOe) ForD ¼ 12 kOe, the maximum magnetic entropy changes (jDSmjmax) are determined to
be about 0.43 and0.74 J kg1K1for the conventional (negative) and inverse (positive) magnetocaloric effects, respectively As
Fig 1 XRD patterns of Ni50xCo x Mn50yAl y ribbons with x ¼ 7 (a) and x ¼ 9 (b).
Fig 2 Thermomagnetization curves of Ni 50x Co x Mn 50y Al y (x ¼ 7 and 9; y ¼ 17, 18 and 19) ribbons measured in a magnetic field of 100 Oe (a) and thermomagnetization curves of
fields (b).
N.T Mai et al / Journal of Science: Advanced Materials and Devices 2 (2017) 123e127 124
Trang 3expected, thejDSmjmaxincreases with increasing the magneticfield
(the inset ofFig 3b)
The temperature dependence ofDSmfor different appliedfield
changes for the second-order phase transition of materials can be
described by the so-called “universal master” curves [19e21]
Based on the DSm(T) curves, the DSm=DSmax versus q plots are
constructed Whiteqvalue is determined by the formula:
q¼
ðT TCÞ=ðTr1 TCÞ; T TC
ðT TCÞ=ðTr2 TCÞ; T> TC (2)
where Tr1and Tr2are the temperatures of the two reference points
For the present study, they are selected as those corresponding to
DSmðTr1 ;2Þ ¼ k:DSmaxðk ¼ 0:5Þ This choice of k does not affect the
actual construction of the universal curve, as it implies only
pro-portionality constant.Fig 4a shows the universal master curve of
the Ni43Co7Mn32Al18ribbon AllDSm(T) data are well collapsed onto
a universal master curve, affirming the nature of second-order
magnetic transition of the material This is an interesting
prop-erty of second-order phase transition materials and is distinct from
first-order phase transition materials
It has been shown that the magnetic orders of materials
exhibiting in a second-order magnetic phase transition can be
assessed by the critical parameters using Arrott plots [22] The
Arrott plots, H/M versus M2(Fig 4b), were constructed from the
M(H) data It can be observed that the M2eH/M curves are
non-linear at low magnetic field and linear at high magnetic field
Values of the spontaneous magnetization (MS) and the inverse
initial susceptibility (c0 1) at different temperatures were derived
from Arrott plots The critical parameters ofb,gand TCrelate to the two above quantities via the following equations:
MSðTÞ ¼ M0ðεÞH ε < 0; (3)
c10 ðTÞ ¼H0
where M0, H0and D are the critical amplitudes andε ¼ ðT TCÞ=TC
is the reduced temperature
The linear extrapolation from highfield to the intercepts with the M2 and H/M axes gives the values of MS(T) and c0 1(T),
respectively The critical parameters TC,bandgwere obtained from fitting MS(T) and c01(T) data (Fig 5a) following the according
formulas (3) and (4), whiledwas calculated by using the Widom scaling relation, equationd¼ 1 þg=b(5)[23] The Ni43Co7Mn32Al18
ribbon possesses TC ¼ 364.21 ± 0.61 K, b ¼ 0.469 ± 0.048,
g¼ 0.951 ± 0.035 anddz 3.027 The critical parameters can be obtained more accurately by the KouveleFisher method[25] Like Arrott-Noakes method, MS(T) andc10 ðTÞ are determined by plot-ting M1/bversus (H/M)1/gcurves The critical parameters ofbandg
relate to the two above quantities by these equations:
MS½dMS=dT1¼ ðT TCÞ=b (6)
Fig 3 Magnetic field dependence of the magnetization at different temperatures (a) andDS m (D ¼ 1, 4, 8, and 12 kOe) versus temperature (inset shows the field dependence of
jDS m j max ) (b) of the Ni 43 Co 7 Mn 32 Al 18 ribbon.
Trang 4c10 ðTÞhdc10 ðTÞ.dTi1
The critical parameters TC,bandgobtained fromfitting MS(T)
andc0 1(T) data by using the accordingformulas (6) and (7).Fig 5b
shows the KouveleFisher curves of the Ni43Co7Mn32Al18 ribbon
withfitting results of TCz 364 K,bz 0.462 andgz 0.948 Based
on the Widom scaling relation, the dvalue was calculated to be
3.051 Clearly, the critical parameter values determined from the
KouveleFisher method are in good agreement with those obtained
by the Arrott-Noakesfittings These critical parameters are quite
close to those of the mean-field model (b¼ 0.5,g¼ 1 andd¼ 3)
characterizing materials with long-range ferromagnetic
in-teractions [24] In a previous study, it has been shown that
Ni50Mn50 xSnx(x¼ 13 and 14) alloy ribbons show a short-range
ferromagnetic order for x¼ 13 but a long-range ferromagnetic
or-der for x¼ 14 at temperatures just below Tc, indicating that that Sn
addition tends to drive the system, in the austenitic ferromagnetic
phase, from the short-range (x¼ 13) to long-range (x ¼ 14)
ferro-magnetic order[16] The long-range ferromagnetism has recently
been reported for Co50 xNixCr25Al25(x¼ 0 and 5) alloys[17] In the
present case, the presence of Co and Al seem to establish a
long-range ferromagnetic order in the Ni43Co7Mn32Al18ribbon, thus
fa-voring the conventional (negative) magnetocaloric effect rather
than the inverse (positive) magnetocaloric effect Nevertheless, a
systematic study on effects of various Co and Al contents on the
magnetic ordering and magnetocaloric effect in Ni50 xCoxMn33Al17
ribbons will be essential to fully understand their relationship
4 Conclusion
The rapidly quenched Ni50xCoxMn50yAly(x¼ 7 and 9; y ¼ 17,
18 and 19) ribbons exhibit multi-crystalline phases of the L10, B2
and 10M types The Curie temperature of the alloy ribbon strongly
increases with increasing the Co-concentration and slightly
de-creases with increasing the Al-concentration For afield change of
12 kOe, the maximum magnetic entropy change of the Ni43
C-o7Mn32Al18 ribbon is about 0.43 and 0.74 J kg1 K1 for the
negative and positive magnetocaloric effects, respectively This
sample exhibits a long-range ferromagnetic order at temperatures
just below the TC
Acknowledgments
This work was supported by the National Foundation for Science
and Technology Development (NAFOSTED) of Viet Nam under
Grant numbers of 103.02e2014.35 Part of the work was done in
Key Laboratory for Electronic Materials and Devices and Laboratory
of Magnetism and Superconductivity, IMS-VAST, Viet Nam
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