ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 303 (2006) e335–e338 www.elsevier.com/locate/jmmm Spin glass-like behavior, giant magnetocaloric and giant magnetoresistance effect in PrPb manganites N ChauÃ, D.T Hanh, N.D Tho, N.H Luong Center for Materials Science, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Hanoi, Vietnam Available online 20 February 2006 Abstract The Pr1ÀxPbxMnO3 (x ¼ 0.1–0.5) perovskites have been fabricated by solid-state reaction The X-ray diffraction patterns show that the samples are of single phase with orthorhombic structure The field-cooled (FC) and zero-field-cooled (ZFC) thermomagnetic curves measured at low field and low temperatures exhibit the spin glass-like state The Curie temperature of samples increased with increase in Pb content The maximum magnetic entropy change 9DSm9max reaches the giant values of 3.91 and 3.68 J/kg K for quite low magnetic field change of 1.35 T for the samples x ¼ 0.1 and 0.4, respectively The resistance measurements show that there is insulator–metal phase transition on the R(T) curves for samples with xX0.3 The giant magnetoresistance effect is also observed for all samples studied r 2006 Elsevier B.V All rights reserved PACS: 75.47.Lx; 75.30.Sg Keywords: Manganites; Spin glass-like state; Magnetocaloric effect Introduction The colossal magnetoresistance (CMR) manganites with the nominal composition R1ÀxAxMnO3 (R ¼ La, Pr, Nd,y and A ¼ Ca, Sr, Ba, Pb) have attracted considerable interest due to their extraordinary magnetic and electrical properties and their promise for future technological applications [1,2] Although the origin of mechanism of CMR is still the subject of discussion, it seems clearly that the double-exchange (DE) interaction between Mn3+ and Mn4+ sites plays an important role [3], where Jahn–Teller distortion [4] should also be considered Apart from the CMR effect that has potential applications in information storage and magnetic sensors, these compounds have also been found to exhibit a large magnetic entropy change under a moderate applied field [5–7] Recently, Hwang et al [8] have systematically investigated (La1ÀxRx)0.7Ca0.3MnO3 by replacing some of La3+ ions with Pr3+ and Y3+ ions They found that the Curie temperature (TC) and conductivity decrease with increasing Pr3+ and Y3+ contents Horng et al [9] have observed that ÃCorresponding author Tel.: +84 5582216; fax: +84 8589496 E-mail address: chau@cms.edu.vn (N Chau) 0304-8853/$ - see front matter r 2006 Elsevier B.V All rights reserved doi:10.1016/j.jmmm.2006.01.094 doping of Pr in La0.7ÀxPrxPb0.3MnO3 compounds leads to decrease in TC and metal–insulator transition temperature, but enhances the magnetoresistance effect and saturation magnetization Ms In our previous paper, overall investigation of properties of La1ÀxPbxMnO3 (x ¼ 0.1–0.5) has been performed [10] The results show that the symmetry decreases from cubic (x ¼ 0.5) to rhombohedral (x ¼ 0.4) and triclinic (x ¼ 0.3, 0.2, 0.1) Moreover, the TC increases from 153 K for x ¼ 0.1 to 350 K for x ¼ 0.5 In the present paper, we report our study of structure, electric, magnetic, magnetocaloric and magnetoresistance properties of Pr1ÀxPbxMnO3 (x ¼ 0.1–0.5) manganites Experimental The polycrystalline samples Pr1ÀxPbxMnO3 (x ¼ 0.1–0.5) were prepared using standard ceramic method The mixed powders were pre-sintered at 800 1C for 15 h and then reground to fine particles, pressed into pellets and again heated at 900 1C for 15 h The second reground powders were pressed and sintered at 1000 1C for 15 h Note that sintering process was carried out at not high temperature, since Pb has a quite low melting temperature and easily evaporates ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e335–e338 e336 Structure of the samples was examined in a Bruker D5005 Xray diffractometer (XRD) The microstructure was studied in a 5410 LV Jeol scanning electron microscope (SEM) Magnetic measurements were performed using a vibrating sample magnetometer (VSM) DMS 880 in magnetic field up to 13.5 kOe Resistivity measurements were performed by the four-probe method Results and discussion The SEM analysis indicates that the grain sizes of the samples are quite homogeneous and increase from around 0.5 mm (x ¼ 0.1) to around mm (x ¼ 0.5) with Pb addition, which is likely due to the existence of local liquid phases Fig shows the SEM image of a selected sample of Pr1ÀxPbxMnO3 with x ¼ 0.1 The X-ray diffraction of the compounds has been measured The obtained results reveal that the Pr1ÀxPbxMnO3 samples are of single phase with orthorhombic structure Note that the La1ÀxPbxMnO3 compounds have been investigated in Refs [11,12] with hexagonal cell of space group R3c for all samples x ¼ 0.1–0.5 The lattice parameters driven from the XRD patterns are shown in Table As can be seen from Table 1, parameter a and unit cell volume increase with increasing Pb2+ content substituted for Pr3+ In this series of samples the average ionic radius of A-site is systematically increased from orA4 ¼ 1.19 A˚ for x ¼ 0.1 to orA4 ¼ 1.26 A˚ for x ¼ 0.5 due to the substitution of Pr3+ ions by the larger Pb2+ ions In order to study the spin order and magnetic behavior of the samples studied, FC and ZFC magnetization measurements were performed in magnetic fields of 20 Oe and 20 kOe Fig shows the thermomagnetic curves of Pr1ÀxPbxMnO3 (x ¼ 0.1–0.5) compounds measured in the field of 20 kOe The FC and ZFC curves of sample Pr0.9Pb0.1MnO3 (inset in Fig 2) depart from each other below the freezing temperature Tf, indicating the onset of blocking of the cluster The system settles into the frozen state below the temperature Tf This behavior is attributed to the magnetic frustration arising from the co-existence of competing antiferromagnetic and ferromagnetic interactions The separation of FC and ZFC curves at low temperatures could be considered that the samples exhibit the spin glass-like state TC values have been determined as the maximum of the dM/dT on the FC and ZFC curves and listed in Table As shown in Fig 2, when increasing Fig Thermomagnetic curves of system Pr1ÀxPbxMnO3 measured by magnetic field of 20 kOe Inset is the FC and ZFC curves of the sample x ¼ 0.1 measured in magnetic field of 20 Oe Fig SEM picture of sample Pr0.9Pb0.1MnO3 Table Lattice parameters, Tc, Ms and maximum magnetic entropy change for all studied samples Pr1ÀxPbxMnO3 Sample orA4(A˚) Ms (emu/g) a (A˚) b (A˚) c (A˚) V (A˚3) TC (K) |DSm|max(J/kg K)a x ¼ 0.1 x ¼ 0.2 x ¼ 0.3 x ¼ 0.4 x ¼ 0.5 1.188 1.206 1.224 1.242 1.260 80.04 88.26 78.30 76.12 71.59 5.345 5.373 5.401 5.422 5.437 5.479 5.473 5.472 5.468 5.467 7.747 7.747 7.747 7.749 7.748 226.9 227.8 229.0 229.7 230.3 152 172 223 256 254 3.91 2.58 2.74 3.68 3.34 a |DSm|max measured with DH ¼ 13.5 kOe ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e335–e338 e337 Fig Magnetic entropy change as a function of temperature for sample Pr0.6Pb0.4MnO3 Fig The M–H loop of sample x ¼ 0.2 measured at T ¼ K the applied field, the TC of the sample with x ¼ 0.1 increases from $152 K for H ¼ 20 Oe (the inset of Fig 2) to 180 K for H ¼ 20 kOe This is attributed to enhancement in the ferromagnetic interaction with higher applied field One can see from Table that the TC increases monotonically with increasing Pb content in the sample from x ¼ 0.1 (TC ¼ 152 K) to x ¼ 0.4 (TC ¼ 256 K) and after that TC becomes somewhat lower in the sample with x ¼ 0.5 This can be attributed to gradual increasing Mn–O–Mn angles with increasing orA4 [8] Obviously, higher the Pb content, higher is the amount of Mn4+ ions formed, and this leads to the strengthening of ferromagnetic interaction (DE) Hence ferromagnetic transition temperature raises Magnetic entropy change as a function of temperature for all samples has been investigated at DH ¼ 10, 13.5, 30 and 50 kOe Fig presents the DSm(T) curves for the sample Pr0.6Pb0.4MnO3 Magnetic entropy change reaches the values of 2.02, 4.68 and 5.54 J/kg K for field change of 10, 30 and 50 kOe, respectively The values of 9DSm9max for all samples are quite high and are listed in Table Among the studied samples, 9DSm9max measured at DH ¼ 13.5 kOe reaches the highest value of 3.91 J/kg K for sample Pr0.9Pb0.1MnO3 These values of DSm are obviously larger than those of several manganese oxides [13–15] Our materials could be considered as giant magnetocaloric materials and suitable for working in magnetic refrigeration in the temperature range 150–270 K Magnetic hysteresis loops at K with a À5–5 T applied field are measured and Fig presents the result for sample x ¼ 0.2 The polycrystalline samples have relatively square hysteresis loops with the soft ferromagnetic characteristics for all cases When Pr3+ ions are substituted by Pb2+ ions, Ms decreases gradually from 80.04 emu/g for x ¼ 0.1 to 71.59 emu/g for x ¼ 0.5 sample (see Table 1) Thus, it illustrates that the magnetic Pr3+ ions with unpaired f-shell electrons contribute an additional value to the total moment and this is the reason for decreasing of Ms on x Resistance versus temperature in zero applied field is measured for all samples As an example, Fig presents the R(T) and MR(H) curves for sample x ¼ 0.1 The results show that at low doping contents x ¼ 0.1 and 0.2, the R(T) curves exhibit semiconducting behavior in the whole measured temperature range However, with higher Pb content, conductivity changed from metallic at low temperatures to semiconducting behavior at higher temperatures From this behavior we can understand that with higher Pb2+ content substituted for (LaPr)3+ (xX0.3) the hopping mechanism gets significantly established due to DE and therefore leads to metallic conductivity Magnetoresistance (MR) curves have been measured for all samples The results indicate that the MR values are in the range 12–28% and the samples studied belong to CMR materials Note that here MR is measured at a relatively low applied field of 13.5 kOe Conclusions Single-phase Pr1ÀxPbxMnO3 (x ¼ 0.1–0.5) manganites were prepared with orthorhombic structure TC increases from 152 K (x ¼ 0.1) to 256 K (x ¼ 0.4) and then there is a little decrease with x ¼ 0.5 There is spin glass-like state in all samples investigated The doping effect also leads to the decrease of Ms and enhancement of magnetocaloric effect Our compounds studied could be considered as good magnetic refrigerant materials working at temperatures lower than room temperature The studied manganites reveal GMCE and CMR materials ARTICLE IN PRESS e338 N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e335–e338 Fig The R(T) as well as MR(H) measurements for sample x ¼ 0.1 Acknowledgment The authors acknowledge the financial support from the Vietnam National Fundamental Research Program (Project 421004) References [1] R von Helmolt, J Wecker, B Holzapfel, L Schultz, K Samwer, Phys Rev Lett 71 (1993) 2331 [2] P Schiffer, A.P Ramirez, W Bao, S.-W Cheong, Phys Rev Lett 75 (1995) 3336 [3] C Zener, Phys Rev 82 (1951) 403 [4] A.J Millis, P.M Littlewood, B.I Shraiman, Phys Rev Lett 77 (1996) 175 [5] A Szewczyk, H Szymczak, A Winiewski, K Piotrowski, R Kartaszynski, B Dabrowski, S Kolesnik, Z Bukowski, Appl Phys Lett 77 (2000) 1026 [6] M.H Phan, T.L Phan, S.C Yu, N.D Tho, N Chau, Phys Status Solidi B 241 (2004) 1744 [7] M.H Phan, N.D Tho, N Chau, S.C Yu, M Kurisu, J Appl Phys 97 (2005) 103901 [8] H.Y Hwang, S.-W Cheong, P.G Radaelli, M Mazerio, B Batlogg, Phys Rev Lett 75 (1995) 941 [9] L Horng, P.C Kang, S.L Yong, Jpn J Appl Phys 42 (2003) 2685 [10] N Chau, H.N Nhat, N.H Luong, D.L Minh, N.D Tho, N.N Chau, Physica B 327 (2003) 270 [11] S.L Young, Y.C Chen, L Horng, T.C Wu, H.Z Chen, J.B Shi, J Magn Magn Mater 289 (2000) 5576 [12] I.O Troyanchuk, D.D Khalyavin, H Szymczak, Mater Sci Bull 32 (1997) 1637 [13] Y Sun, W Tong, Y.H Zhang, J Magn Magn Mater 323 (2001) 205 [14] N.H Luong, D.T Hanh, N Chau, N.D Tho, T.D Hiep, J Magn Magn Mater 290–291 (2005) 690 [15] Md.A Choudhury, J.A Akhter, D.L Minh, N.D Tho, N Chau, J Magn Magn Mater 272–276 (2004) 1295 ... and ZFC curves at low temperatures could be considered that the samples exhibit the spin glass-like state TC values have been determined as the maximum of the dM/dT on the FC and ZFC curves and. .. [13–15] Our materials could be considered as giant magnetocaloric materials and suitable for working in magnetic refrigeration in the temperature range 150–270 K Magnetic hysteresis loops at K with... The FC and ZFC curves of sample Pr0.9Pb0.1MnO3 (inset in Fig 2) depart from each other below the freezing temperature Tf, indicating the onset of blocking of the cluster The system settles into