The magnetic field induced resistivity and magnetic entropy change of these samples showed abrupt changes near TC194.2 and 201.5 K for Cu and Zn-doped case respectively and attained the h
Trang 1Magnetotransport properties and magnetocaloric effect in
Quoc Thanh Phunga, Van Khai Vub, An Bang Ngacb, Huy Sinh Nguyenb, Nam Nhat Hoanga,n
a
Faculty of Technical Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University, Ha Noi, 144 Xuan Thuy, Cau Giay, Ha Noi, Viet Nam
b Faculty of Physics, Ha Noi University of Science, Vietnam National University, Ha Noi, 334 Nguyen Trai, Thanh Xuan, Ha Noi, Viet Nam
a r t i c l e i n f o
Article history:
Received 9 September 2011
Received in revised form
22 February 2012
Available online 13 March 2012
Keywords:
Perovksite
Manganate
Magnetotransport
Magnetocaloric effect
a b s t r a c t
The magnetic and transport properties of the perovskites La0.67Ca0.33Mn1-xTMxO3were found to be sufficiently changed with the substitution of Mn-sites by other 3d transition-metal cations (TM ¼ Cu,Zn;
x ¼ 0.15) The values of TC, TM I, and TCMRwere surveyed when Mn was replaced by Cu and Zn The magnetic field induced resistivity and magnetic entropy change of these samples showed abrupt changes near TC(194.2 and 201.5 K for Cu and Zn-doped case respectively) and attained the highest values among the doped cases (up to 20% Cu) The maximum values (obtained at H¼ 4 kOe) of magnetoresistance ratio (CMR) were 27.8%, and 24.5% and of magnetic entropy change (DSM) were 3.9 and 3.2 J/kg K for Cu and Zn-doped, respectively
&2012 Elsevier B.V All rights reserved
1 Introduction
Recently, a large number of works have been devoted to the
substitution of Mn-site by 3d-ferromagnetic transition metals in
manganites La1 yCayMnO3 In this class of compounds many
important effects, such as colossal magnetoresistance (CMR) and
giant magnetocaloric effect (MCE), have been observed [1– ] By
introduction of the rare earth or alkaline earth ions into the A-sites
one might reasonably expect a change in Mn3 þ/Mn4 þ ratio and
distortion of original lattice which should be induced by the effect of
valence mismatch and difference of ionic radii As a consequence,
the magnetic and electrical properties of materials should be
modified Hence by a subsequent insertion of the proper transition
metals into the B-sites, the bonding geometry (length, angle) around
the Mn cations might be fine-tuned in a manner so that the
magnitude of super-exchange (SE) and double-exchange (DE)
inter-actions can be well controlled Previous studies showed that a good
CMR could be achieved when the Mn3 þ/Mn4 þratio was around 7/3
(that is when y¼0.3) since at this ratio the ferromagnetic exchange
should prevail In the previous paper [7], we have reported the
effects of Mn-site substitution by Ni and Co (3d-ferromagnetic
transition metals) in the La2/3Ca1/3MnO3system An abrupt decrease
of magnetizations near TC and an increase of magnetoresistance
ratios ({R(0) R(H)}/R(0)¼17% and 8.3% at H¼4 kOe, respectively)
were observed These observations provided a support argument for
the assumption that there was a coexistence of separated AFM–FM phases and a competition between SE and DE interactions in the lattice It was clear that the modifications of Ni–O–Mn and Co–O–Mn bond lengths and angles strongly influenced the MR and MCE effects We will consider the mentioned theoretical issues else-where, and for now in this paper we report a study of magnetic properties of Mn-based perovskites La0.67Ca0.33Mn1 xTMxO3 (TM¼Cu,Zn; x ¼0.15), including the colossal magnetoresistance and magnetocaloric effects, when the divalent ferromagnetic 3d-transition metal cations Cu (3d9) and Zn (3d10) were substituted for Mn For a similar compound La0.7Ca0.3Mn0.9Zn0.1O3we have reported in the previous publication DSM¼1.1 and 2.7 J/kg K in
an applied field of 10 and 35 kOe respectively, at the vicinity of
TC¼206.75 K[8] The Cu-doped La0.7Ca0.3Mn0.95Cu0.05O3was found
to have TC¼197.3 K[9] These values ofDSMwere still lower than 4.3 J/kg K (obtained at 15 kOe) for the undoped La0.67Ca0.33MnO3
[10]and 6.25 J/kg K (at 10 kOe) for La0.7Ca0.3MnO3[11,12] As the search for new materials with better CMR and MCE is still of continuous interests, we show here that higher doping concentra-tions of Cu and Zn in La0.67Ca0.33MnO3can lead to higher values of CMR and MCE in comparison with that of the low doped com-pounds For the critical behavior, the detailed analysis reported in Refs.[8,9] revealed that both La0.7Ca0.3Mn0.9Zn0.1O3and La0.7Ca0.3
Mn0.95Cu0.05O3exhibit a second-order magnetic phase transition with the critical exponentsbandg(deduced from the plots of M1/ b
versus (H/M)1/g [13,14]) being very close to the ones predicted
by the mean-field theory This result suggests that for the cases of higher doping content, the same second-order magnetic phase transitions should prevail It is worthwhile noting that similar
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n
Corresponding author Tel.: þ84 98 300 6668; fax: þ84 4 768 2007.
E-mail address: namnhat@gmail.com (N.N Hoang).
Trang 2results were reported for La0.7Ca0.3 xSrxMnO3[15]: the Sr-doping
concentration of 10% separates the first-order from the
second-order magnetic phase transition Therefore, we will not go deeper
into details of magnetic transition here The magnetic properties of
Cu-doped La0.7Ca0.3MnO3, for the concentration range up to 20%,
were reported by Wang et al.[16] The conclusions from this work,
however, posed questions regarding the purity of phase, nature of
phase transition and therefore the exactness of data presented At
least the following points are worth re-examining: the collapse of
unit-cell volume at larger doping concentration than 15%, the
multi-phase transition as seen in all documented resistance versus
temperature curves and the abrupt drop of TCfrom 244 to 143 K at
the doping concentration of 15% whereas the samples with Cu
content of 5% and less showed a negligible change of TC The
reported extreme by high MR ratio (of order 106% at H¼50 kOe) for
15% Cu-doped sample was obtained at a low temperature around
37 K, too far from TC and could hardly be associated with the
magnetic phase transition that happened around TC
2 Experimental
The samples with nominal composition of La0.67Ca0.33Mn1 x
-TMxO3(TM¼Cu, Zn; x¼0.15) were prepared by using the
solid-state reaction method with repeated grinding in methanol, then
heating up to 600 1C for several hours, pressing into the pellets and
sintering at 1100 1C for 24 h in open air The final sintering took
place at 1140 1C for 48 h in air The structure and phase purity of
prepared samples were checked by powder X-ray diffraction (XRD)
using a Brucker D5005 diffractometer with Cu Karadiation at room
temperature The magnetization measurements were carried out
on a Digital Measurement System (DMS) vibrating samples
mag-netometer (VSM) DMS 880 in the magnetic field H up to 13.5 kOe
The resistance curves at 0 and 4 kOe were measured by the
standard four-probe method The isothermal magnetization curves
were measured at 45 kOe using a superconducting quantum
interference device (MPMS Controller Model 1822-Quantum) For
the purpose of comparison with other results reported we have
also prepared the samples with x¼0 (undoped), 0.05 and 0.20
(for Cu-doped case) As mentioned, we have discussed the
critical behavior of a slightly modified 5% Cu-doped sample
(La0.7Ca0.3Mn0.95Cu0.05O3) in the previous paper[8], so this work
will be focused on the magnetic properties of 15% doped samples
3 Results and discussion
As shown inFig 1, the XRD patterns contain only peaks that
belong to a typical orthorhombic structure The Rietveld profile
refinement was taken on the space group Pnma and achieved the
final R (profile) less than 4% for all cases The calculated
diffrac-tion posidiffrac-tions (vertical bars) and difference curve are also given in
Fig 1 The obtained refined lattice parameters (shown inTable 1)
of the Cu-doped and Zn-doped samples were slightly larger in
comparison with those of the undoped one This was caused by a
small difference between the ionic radii of 3d10 Zn2 þ and 3d9
Cu2 þ cations versus 3d4 Mn3 þ and 3d3Mn4 þ (0.74 and 0.73 ˚A
versus 0.66 and 0.60 ˚A, respectively) The systematical evolution
of lattice constants obtained for x¼0, 0.05, 0.15 and 0.2 in
Cu-doped cases was monotonous and quite different from that
reported for the Cu-doped La0.7Ca0.3MnO3[16]where the collapse
of unit-cell volume appeared for the doping concentration of 15%
It is reasonable to expect that the increase in doping content of
larger cations should relax the lattice and increase the volume
Indeed, we have observed a continuous trend of lattice relaxation
when the content of Cu increased from 0% to 20% Since Cu2 þand
Zn2 þ possess nearly identical ionic radii, the evolutions of lattice relaxation in two cases under doping were quite similar However, despite this overall similarity, the substitution of Cu and Zn for Mn should introduce different changes in Mn3 þ/Mn4 þ
ratios as a possible magnetic exchange between Cu2 þand Mn3 þ
and Mn4 þcations might be expected Such an interaction should not occur in the case of the singlet 3d10 Zn2 þ cations; there-fore, besides the effect of static charge compensation caused by substitution of divalent cations (that should be equal for both Cu and Zn), the final image of ferro–antiferromagnetic exchange, of
DE and SE competition, may be quite different in the two cases Fig 2shows the temperature dependence of magnetization (Zero-Field-Cooling, ZFC) of the samples The ZFC curve for the undoped case and the development of TC according to the doping content for Cu-doped case are also shown in the insets
As seen, the ZFC curves demonstrate that magnetic phase transitions were abrupt, and the magnetizations strongly decreased near TC It is obvious that both initial magnetizations were relatively weak and varied almost identically The similar ionic radii and mix-valence effect seemed to induce the similar behaviors of magnetic property As seen in Table 1, the close lattice constants in both cases cause almost equal Mn–O bond lengths and angles, so the same local variation of ferromagnetic interaction between Mn3 þand Mn4 þcations is expected Thus the different magnetizations should be explained on only the basis of the difference of spin states of substituted cations First, the presence of an unpaired electron in Cu2 þ agrees well with the larger initial magnetization of the Cu-doped sample in comparison with that of the Zn-doped one (containing the singlet 3d10Zn2 þcations) Second, the substitution should force the Mn3 þ/Mn4 þ ratio away from the optimal value of 7/3 by
Fig 1 XDR patterns of undoped and 15% Cu and Zn-doped samples The Rietveld profile fitting was taken in the orthorhombic space group Pnma.
Table 1 Lattice parameters of samples ( ˚A) and the volumes of unit cells ( ˚A 3
).
La 0.67 Ca 0.33 Mn 0.85 Zn 0.15 O 3 5.466 7.734 5.464 231.0
La 0.67 Ca 0.33 Mn 0.85 Cu 0.15 O 3 5.469 7.732 5.466 231.1
La 0.67 Ca 0.33 MnO 3 5.459 7.728 5.465 230.5
Trang 3increasing the portion of Mn4 þcations (as the charge
compen-sation required when a divalent cation replaces the trivalent
one) This consequently reduced the ferromagnetic interaction
between Mn3 þand Mn4 þcations and led to the lowering of the
temperature at which ferromagnetic ordering prevails, i.e the
temperature of the ferromagnetic-to-paramagnetic phase
transition TC Furthermore, as the doped elements possess different spin states, the Cu-doped sample (doublet spin state) should show a lower TCin comparison with that of the Zn-doped sample (singlet spin state) due to a possible competition of the antiferromagnetic ordering mediated by Cu sites Indeed, the Curie temperature TC as determined by the equation of state in phase transition region (e.g by a procedure discussed in Ref [8]: TC could be obtained from the temperature depen-dences of spontaneous magnetization Ms and inverse initial susceptibilityw01; one might also estimate TCby differentiation
of, or extrapolation from, ZFC curves given inFig 2) was shown
to be 194.2 and 201.5 K for Cu and Zn respectively These values are different from the ones reported by the other authors [16–19], and both are lower than the TC of the undoped polycrystalline sample (TC¼240 K) Recall that the TC for 5% Cu-doped La0.7Ca0.3Mn0.95Cu0.05O3was 197.3 K[9]; for our case
of La0.67Ca0.33Mn0.95Cu0.05O3 we have obtained TC¼200 K For 20% Cu-doped La0.67Ca0.33Mn0.8Cu0.2O3 we achieved TC¼183 K
So far, for all Cu-doped samples the development of TC accord-ing to Cu content was linear and no abrupt decrease of TCwas observed at the concentration of 15% as reported in Ref.[16] TC
for the Cu-doped sample is lower than that for the Zn-doped one
by 7.3 K In comparison with the previously reported TC (206.75 K in 10% Zn-doped[8]and 197.3 K in 5% Cu-doped[9] samples), the increase in doping concentration to 15% continu-ously lowered the TC However, the reduction of TC was not large, as 10% increase in Cu-doping concentration induced only
Fig 2 Temperature dependence of magnetization for Cu and Zn-doped samples
(the undoped case is shown in the inset) The inset (upper) also shows the
development of T C according to content of substitution for Cu-doped case.
Fig 3 Temperature dependence of resistance for (a) 15% Cu-doped and (b) 15%
Zn-doped sample in the absence and the presence of applied magnetic field The inset Fig 4 CMR ratio versus applied field for (a) 15% Cu-doped and (b) 15% Zn-doped
samples.
Trang 4a small loss of 3.1 K, so the gain of a better CMR effect in this
case is worth considering
The dependences of resistance of samples with 15% Cu and
Zn-doped on temperature, in the absence and presence of magnetic
field, are shown inFig 3 For all investigated cases, i.e with
Cu-doping concentration of 0%, 5%, 15% and 20%, only one sharp
maximum was observed (near TC) for each case from room
temperature down to a region below 30 K Thus all samples
exhibit only one metal-to-semiconductor phase transition in the
whole temperature range examined This observation is totally
different from that reported in Ref [16], where two maxima
appeared at the Cu-doping content less than 15%; at 15% two
broad peaks far below TC arose and at 20% Cu no maximum
occurred As seen inFig 3, for H¼0, the maxima of the resistance
curves happened at Tp¼185 (Cu-doped) and 192 K (Zn-doped)
For H¼2 kOe, the maxima shifted to 190 (Cu-doped) and 196 K
(Zn-doped) At H¼4 kOe, the maxima appeared almost at the
same temperature as TC, i.e 195 and 200 K for Cu and Zn-doped
respectively Overall, Tp shifted to a higher region and the
resistance of samples decreased under applied field In the
vicinity of Tpthe resistance reduced about 27% of its magnitude
at 4 kOe for the Cu-doped case, 25% for the Zn-doped case and
17% for the undoped case (graph shown in the inset ofFig 3) The
increase of conductivity under the applied field is an interesting
observation in the Cu and Zn-doped polycrystalline samples and
may be attributed to the reduction of magnetic scattering upon
the conduction electrons hopping between dopant sites The same
decrease of resistance upon the applied field was also reported for
both single crystal and polycrystalline La0.7Ca0.3MnO3[8,9,20] and
Cu-doped polycrystalline La0.7Ca0.3MnO3 [16] Using the data
presented, we have calculated the magnetoresistance ratio (MR) using the formula
MRð%Þ ¼ ½ðRð0Þ-RðHÞÞ=Rð0Þ 100% ð1Þ The magnetoresistance ratio achieved the highest value in the vicinity of TC InFig 4we show the dependence of CMR on applied field at different temperatures The maximal values of CMR are 27.8% (at 195 K in Cu-doped sample) and 24.5% (at 200 K in Zn-doped one) at the magnetic field of 4 kOe These values are larger than the one obtained for the undoped sample La0.67Ca0.33
MnO3(CMR¼ 17.6% at 240 K, H¼4 kOe)
The applied magnetic field dependences of magnetization of samples (M H curves) in the vicinity of ferromagnetic–para-magnetic phase transition temperature (TC) were investigated by the MPMS equipment (SQUID) The magnetocaloric effect (MCE) can be evaluated by measuring the magnetization and interpreted using the thermodynamic theory With an indirect method, the magnetic entropy change as a function of temperature and field,
DSm(T,DH), can be approximated by[2]
9DSM9ðT,DHÞ ¼X
i
1
Ti þ 1Ti
ðMiMi þ 1ÞHD i ð2Þ
where Miand Mi þ 1are the magnetization values measured in a field H at temperatures Ti and Ti þ 1, respectively The relative
Fig 5 Field dependence of magnetization for (a) 15% Cu-doped and (b) 15%
Zn-doped samples.
Fig 6 Temperature dependences of magnetic-entropy change under applied
Trang 5cooling power (RCP) can be calculated as[2]
RCP ¼ ðDSMÞ9maxDTFWHM ð3Þ
where DTFWHM is the full-width at the half maximum of the
magnetic entropy change curve.Fig 5shows the M(H) curves of
samples for the temperature range from 174 to 214 K (Cu-doped
samples) and from 180 to 220 K (Zn-doped samples) as functions
of temperature and magnetic field The magnetic entropy changes
of these samples were calculated in the magnetic fields of 15, 30
and 45 kOe and the results are featured inFig 6 As seen, the
magnetic entropy changes of samples attained the maximum
values 1.5, 2.3 and 3.2 J/kg K around 200 K (Zn-doped samples)
and 2.1, 3.0 and 3.9 J/kg K around 195 K (Cu-doped samples) in
the magnetic fields of 15, 30 and 45 kOe respectively The
corresponding RCP values are 57, 123 and 210 J/kg (Zn-doped
samples) and 85, 162 and 214 J/kg (Cu-doped samples) To
compare, in the same magnetic field, values of both 9DSM9max
and corresponding RCP for the Cu-doped samples are higher than
those for the Zn-doped ones, but the maximum RCPs obtained at
4 kOe for both samples are close to each other According to the
literature, the large spin–lattice coupling in the magnetic ordering
process was responsible for large magnetic entropy change in
manganites[2,16,17] This effect appeared to be of the same order
when Cu and Zn were substituted for Mn Table 2 lists some
values for MCE as obtained for different Mn-based perovskites,
which were recently published in literature According to the
presented RCP, which was usually a good measure for comparing
various magnetocaloric materials, our doped materials can be
considered as the potential magnetic refrigerants
4 Conclusions
The magnetocaloric and magnetoresistance effects of 15%
Cu-doped and 15% Zn-doped La0.67Ca0.33MnO3manganites were
investigated The maximum values of CMR and MCE showed that
these divalent ferromagnetic 3d-transition metal cations played
the similar roles when they were substituted in B-site for Mn Although the obtained values were still below the values reported for the undoped La0.7Ca0.3MnO3 they were the highest values achieved until now for Cu and Zn-doped La0.67Ca0.33MnO3(up to 20% doping content) The results also showed that there was no abrupt change of electrical and magnetic properties of Cu-doped compounds at 15% doping as demonstrated for La0.7Ca0.3MnO3in Ref.[16]
Acknowledgments The authors would like to thank Prof K.W Lee (Korea Research Institute of Standards and Science) for technical supports in performing the magnetic measurements on SQUID Our thanks also go to Prof Seong-Cho Yu and Dr Phan The Long (Chungbuk National University, Korea) for fruitful discussions and comments This work has been supported by the National Foundation for Scientific and Technology Development (NAFOSTED), Code 103.02-2010.38
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Table 2
Curie temperature (T C ) and maximum magnetic entropy change (9DS M 9max) for
different materials.
(K)
D
( 10 kOe)
9DS M 9max (J/kg K)
RCP (J/kg) References
La 0.67 Ca 0.33 MnO 3 260 1.0 1.20 – [21]
La 0.7 Ca 0.3 MnO 3 227 1.0 1.95 – [22]
La 0.7 Sr 0.3 Mn 0.95 Cu 0.05 O 3 350 1.35 1.96 39 [23]
La 0.7 Sr 0.3 Mn 0.9 Cu 0.1 O 3 350 1.35 2.07 43 [23]
La 0.7 Sr 0.3 Mn 0.95 Cu 0.05 O 3 346 1.5 5.20 312 [24]
Nd 0.5 Sr 0.5 Mn 0.9 Cu 0.1 O 3 260 1.35 1.25 – [25]
La 0.7 Sr 0.3 Mn 0.95 Cu 0.05 O 3 345 1.0 3.05 – [24]
La 0.7 Sr 0.3 Mn 0.9 Cu 0.1 O 3 347 1.0 3.24 – [24]
La 0.7 Sr 0.3 Mn 0.95 Cu 0.05 O 3 346 1.5 5.20 – [24]
La 0.7 Sr 0.3 Mn 0.9 Cu 0.1 O 3 348 1.5 5.51 – [24]
La 0.7 Ca 0.3 Mn 0.9 Zn 0.1 O 3 207 1.0 1.1 – [8]
La 0.7 Ca 0.3 Mn 0.9 Zn 0.1 O 3 207 2.0 1.7 – [8]
La 0.7 Ca 0.3 Mn 0.9 Zn 0.1 O 3 207 3.5 2.7 – [8]
La 0.67 Ca 0.33 MnO 3 240 4.5 2.6 151 This work
La 0.67 Ca 0.33 Mn 0.95 Zn 0.15 O 3 202 1.5 1.5 57 This work
La 0.67 Ca 0.33 Mn 0.95 Zn 0.15 O 3 202 3.0 2.3 123 This work
La 0.67 Ca 0.33 Mn 0.95 Zn 0.15 O 3 202 4.5 3.2 210 This work
La 0.67 Ca 0.33 Mn 0.95 Cu 0.15 O 3 194 1.5 2.1 85 This work
La 0.67 Ca 0.33 Mn 0.95 Cu 0.15 O 3 194 3.0 3.0 162 This work
La 0.67 Ca 0.33 Mn 0.95 Cu 0.15 O 3 194 4.5 3.9 214 This work