DSpace at VNU: Magnetotransport properties and magnetocaloric effect in La 0.67Ca 0.33Mn 1-xTM xO 3 (TM=Cu, Zn) perovskite manganites

Journal of Magnetism and Magnetic Materials 324 (2012) 2363–2367 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Magnetotransport properties and magnetocaloric effect in La0.67Ca0.33Mn1 À xTMxO3 (TM¼ Cu, Zn) perovskite manganites Quoc Thanh Phung a, Van Khai Vu b, An Bang Ngac b, Huy Sinh Nguyen b, Nam Nhat Hoang a,n a b 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 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 abstract Article history: Received September 2011 Received in revised form 22 February 2012 Available online 13 March 2012 The magnetic and transport properties of the perovskites La0.67Ca0.33Mn1-xTMxO3 were 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 À I, and TCMR were 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 kOe) of magnetoresistance ratio (CMR) were 27.8%, and 24.5% and of magnetic entropy change (À DSM) were 3.9 and 3.2 J/kg K for Cu and Zn-doped, respectively & 2012 Elsevier B.V All rights reserved Keywords: Perovksite Manganate Magnetotransport Magnetocaloric effect Introduction Recently, a large number of works have been devoted to the substitution of Mn-site by 3d-ferromagnetic transition metals in manganites La1À yCayMnO3 In this class of compounds many important effects, such as colossal magnetoresistance (CMR) and giant magnetocaloric effect (MCE), have been observed [1–6] By introduction of the rare earth or alkaline earth ions into the A-sites one might reasonably expect a change in Mn3 ỵ /Mn4 ỵ 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) interactions can be well controlled Previous studies showed that a good CMR could be achieved when the Mn3 ỵ /Mn4 ỵ ratio 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 La2/3Ca1/3MnO3 system An abrupt decrease of magnetizations near TC and an increase of magnetoresistance ratios ({R(0)À R(H)}/R(0)¼17% and 8.3% at H¼4 kOe, respectively) were observed These observations provided a support argument for n Corresponding author Tel.: ỵ84 98 300 6668; fax: ỵ84 768 2007 E-mail address: namnhat@gmail.com (N.N Hoang) 0304-8853/$ - see front matter & 2012 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.jmmm.2012.03.001 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 elsewhere, and for now in this paper we report a study of magnetic properties of Mn-based perovskites La0.67Ca0.33Mn1 À xTMxO3 (TM¼Cu,Zn; x ¼0.15), including the colossal magnetoresistance and magnetocaloric effects, when the divalent ferromagnetic 3dtransition metal cations Cu (3d9) and Zn (3d10) were substituted for Mn For a similar compound La0.7Ca0.3Mn0.9Zn0.1O3 we have reported in the previous publication À DSM ¼1.1 and 2.7 J/kg K in an applied field of 10 and 35 kOe respectively, at the vicinity of TC ¼206.75 K [8] The Cu-doped La0.7Ca0.3Mn0.95Cu0.05O3 was found to have TC ¼197.3 K [9] These values of DSM were 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 concentrations of Cu and Zn in La0.67Ca0.33MnO3 can lead to higher values of CMR and MCE in comparison with that of the low doped compounds For the critical behavior, the detailed analysis reported in Refs [8,9] revealed that both La0.7Ca0.3Mn0.9Zn0.1O3 and La0.7Ca0.3 Mn0.95Cu0.05O3 exhibit a second-order magnetic phase transition with the critical exponents b and g (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 2364 Phung Quoc Thanh et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2363–2367 results were reported for La0.7Ca0.3À xSrxMnO3 [15]: the Sr-doping concentration of 10% separates the first-order from the secondorder 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 TC from 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 Experimental The samples with nominal composition of La0.67Ca0.33Mn1À xTMxO3 (TM¼Cu, Zn; x¼0.15) were prepared by using the solidstate 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 Ka radiation at room temperature The magnetization measurements were carried out on a Digital Measurement System (DMS) vibrating samples magnetometer (VSM) DMS 880 in the magnetic field H up to 13.5 kOe The resistance curves at and 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 Results and discussion As shown in Fig 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 diffraction positions (vertical bars) and difference curve are also given in Fig The obtained refined lattice parameters (shown in Table 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 ỵ and 3d9 Cu2 ỵ cations versus 3d4 Mn3 ỵ and 3d3 Mn4 þ (0.74 and 0.73 A˚ ˚ respectively) The systematical evolution versus 0.66 and 0.60 A, 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 ỵ and Fig XDR patterns of undoped and 15% Cu and Zn-doped samples The Rietveld profile fitting was taken in the orthorhombic space group Pnma Table ˚ and the volumes of unit cells (A˚ 3) Lattice parameters of samples (A) Samples a b c V La0.67Ca0.33Mn0.85Zn0.15O3 La0.67Ca0.33Mn0.85Cu0.15O3 La0.67Ca0.33MnO3 5.466 5.469 5.459 7.734 7.732 7.728 5.464 5.466 5.465 231.0 231.1 230.5 Zn2 ỵ 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 ỵ /Mn4 ỵ ratios as a possible magnetic exchange between Cu2 þ and Mn3 þ and Mn4 þ cations might be expected Such an interaction should not occur in the case of the singlet 3d10 Zn2 ỵ cations; therefore, 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 shows 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 ỵ and Mn4 þ cations 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 ỵ agrees well with the larger initial magnetization of the Cu-doped sample in comparison with that of the Zn-doped one (containing the singlet 3d10 Zn2 ỵ cations) Second, the substitution should force the Mn3 ỵ /Mn4 ỵ ratio away from the optimal value of 7/3 by Phung Quoc Thanh et al / Journal of Magnetism and Magnetic Materials 324 (2012) 23632367 increasing the portion of Mn4 ỵ cations (as the charge compensation required when a divalent cation replaces the trivalent one) This consequently reduced the ferromagnetic interaction between Mn3 ỵ and Mn4 ỵ cations and led to the lowering of the temperature at which ferromagnetic ordering prevails, i.e the temperature of the ferromagnetic-to-paramagnetic phase Fig 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 TC according to content of substitution for Cu-doped case Fig 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 shows the case of undoped sample 2365 transition TC Furthermore, as the doped elements possess different spin states, the Cu-doped sample (doublet spin state) should show a lower TC in 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 dependences of spontaneous magnetization Ms and inverse initial susceptibility w0À 1; one might also estimate TC by differentiation of, or extrapolation from, ZFC curves given in Fig 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 (TC ¼240 K) Recall that the TC for 5% Cu-doped La0.7Ca0.3Mn0.95Cu0.05O3 was 197.3 K [9]; for our case of La0.67Ca0.33Mn0.95Cu0.05O3 we have obtained TC ¼200 K For 20% Cu-doped La0.67Ca0.33Mn0.8Cu0.2O3 we achieved TC ¼183 K So far, for all Cu-doped samples the development of TC according to Cu content was linear and no abrupt decrease of TC was 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% continuously lowered the TC However, the reduction of TC was not large, as 10% increase in Cu-doping concentration induced only Fig CMR ratio versus applied field for (a) 15% Cu-doped and (b) 15% Zn-doped samples 2366 Phung Quoc Thanh et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2363–2367 a 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 Zndoped on temperature, in the absence and presence of magnetic field, are shown in Fig For all investigated cases, i.e with Cudoping 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 in Fig 3, for H¼0, the maxima of the resistance curves happened at Tp ¼185 (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 Tp the resistance reduced about 27% of its magnitude at kOe for the Cu-doped case, 25% for the Zn-doped case and 17% for the undoped case (graph shown in the inset of Fig 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 Fig Field dependence of magnetization for (a) 15% Cu-doped and (b) 15% Zn-doped samples presented, we have calculated the magnetoresistance ratio (MR) using the formula MR%ị ẳ ẵR0ị-RHịị=R0ị 100% 1ị The magnetoresistance ratio achieved the highest value in the vicinity of TC In Fig we 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 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 ÀH curves) in the vicinity of ferromagnetic–paramagnetic 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] 9DSM 9T, DHị ẳ X i M i M i ỵ ịH DHi T i ỵ T i 2ị where Mi and Mi ỵ are the magnetization values measured in a field H at temperatures Ti and Ti ỵ 1, respectively The relative Fig Temperature dependences of magnetic-entropy change under applied magnetic fields of 10, 30 and 45 kOe for (a) Cu and (b) Zn-doped samples Phung Quoc Thanh et al / Journal of Magnetism and Magnetic Materials 324 (2012) 2363–2367 Table Curie temperature (TC) and maximum magnetic entropy change (9DSM9max) for different materials Materials TC (K) DH 9DSM9max ( Â 10 kOe) (J/kg K) RCP (J/kg) References La0.67Ca0.33MnO3 La0.7Ca0.3MnO3 La0.7Sr0.3Mn0.95Cu0.05O3 La0.7Sr0.3Mn0.9Cu0.1O3 La0.7Sr0.3Mn0.95Cu0.05O3 Nd0.5Sr0.5Mn0.9Cu0.1O3 La0.7Sr0.3Mn0.95Cu0.05O3 La0.7Sr0.3Mn0.9Cu0.1O3 La0.7Sr0.3Mn0.95Cu0.05O3 La0.7Sr0.3Mn0.9Cu0.1O3 La0.7Ca0.3Mn0.9Zn0.1O3 La0.7Ca0.3Mn0.9Zn0.1O3 La0.7Ca0.3Mn0.9Zn0.1O3 La0.67Ca0.33MnO3 La0.67Ca0.33Mn0.95Zn0.15O3 La0.67Ca0.33Mn0.95Zn0.15O3 La0.67Ca0.33Mn0.95Zn0.15O3 La0.67Ca0.33Mn0.95Cu0.15O3 La0.67Ca0.33Mn0.95Cu0.15O3 La0.67Ca0.33Mn0.95Cu0.15O3 260 227 350 350 346 260 345 347 346 348 207 207 207 240 202 202 202 194 194 194 1.0 1.0 1.35 1.35 1.5 1.35 1.0 1.0 1.5 1.5 1.0 2.0 3.5 4.5 1.5 3.0 4.5 1.5 3.0 4.5 – – 39 43 312 – – – – – – – – 151 57 123 210 85 162 214 [21] [22] [23] [23] [24] [25] [24] [24] [24] [24] [8] [8] [8] This This This This This This This 1.20 1.95 1.96 2.07 5.20 1.25 3.05 3.24 5.20 5.51 1.1 1.7 2.7 2.6 1.5 2.3 3.2 2.1 3.0 3.9 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 La0.7Ca0.3MnO3 in Ref [16] Acknowledgments work work work work work work work cooling power (RCP) can be calculated as [2] RCP ẳ DSM ị9max DT FWHM 2367 3ị where DTFWHM is the full-width at the half maximum of the magnetic entropy change curve Fig shows 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 in Fig 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 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 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 Conclusions The magnetocaloric and magnetoresistance effects of 15% Cu-doped and 15% Zn-doped La0.67Ca0.33MnO3 manganites were investigated The maximum values of CMR and MCE showed that these divalent ferromagnetic 3d-transition metal cations played 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 References [1] K.A Gschneidner, V.K Pecharsky, A.O Tsokol, Reports on Progress in Physics 68 (2005) 1479–1539 [2] M.H Phan, S.C Yu, Journal of Magnetism and Magnetic Materials 308 (2007) 325–340 [3] E.L Nagaev, Physics Reports 346 (2001) 387–531 [4] A.P Ramirez, Journal of Physics: Condensed Matter (1997) 8171–8199 [5] N Chau, H.N Nhat, N.H Luong, D.L Minh, N.D Tho, N.N Chau, Physica B: Condensed Matter 327 (2003) 270 [6] N.H Luong, D.T Hanh, N Chau, N.D Tho, T.D Hiep, Journal of Magnetism and Magnetic Materials 290–291 (2005) 690 [7] N.H Sinh, V.V Khai, N.T Thuong, Proceedings of the 5th National Conference on Solid State Physics, Vungtau, 12–14 Nov 2007 [8] T.L Phan, P.Q 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La0 .67Ca0 .33 Mn0.95Zn0.15O3 La0 .67Ca0 .33 Mn0.95Zn0.15O3 La0 .67Ca0 .33 Mn0.95Zn0.15O3 La0 .67Ca0 .33 Mn0.95Cu0.15O3 La0 .67Ca0 .33 Mn0.95Cu0.15O3 La0 .67Ca0 .33 Mn0.95Cu0.15O3 260 227 35 0 35 0 34 6 260 34 5 34 7 34 6 34 8... Nd0.5Sr0.5Mn0.9Cu0.1O3 La0 .7Sr0.3Mn0.95Cu0.05O3 La0 .7Sr0.3Mn0.9Cu0.1O3 La0 .7Sr0.3Mn0.95Cu0.05O3 La0 .7Sr0.3Mn0.9Cu0.1O3 La0 .7Ca0.3Mn0.9Zn0.1O3 La0 .7Ca0.3Mn0.9Zn0.1O3 La0 .7Ca0.3Mn0.9Zn0.1O3 La0 .67Ca0 .33 MnO3 La0 .67Ca0 .33 Mn0.95Zn0.15O3... (A˚ 3) Lattice parameters of samples (A) Samples a b c V La0 .67Ca0 .33 Mn0.85Zn0.15O3 La0 .67Ca0 .33 Mn0.85Cu0.15O3 La0 .67Ca0 .33 MnO3 5.466 5.469 5.459 7. 734 7. 732 7.728 5.464 5.466 5.465 231 .0 231 .1