DSpace at VNU: Spin glass-like state, charge ordering, phase diagram and positive entropy change in Nd0.5-xPrxSr0.5MnO3 perovskites

ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405 www.elsevier.com/locate/jmmm Spin glass-like state, charge ordering, phase diagram and positive entropy change in Nd0.5ÀxPrxSr0.5MnO3 perovskites N Chaua,Ã, N.D Thoa, N.H Luonga, B.H Giangb, B.T Congb a Center for Materials Science, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Hanoi, Vietnam b Department of Physics, University of Science, Vietnam National University, Hanoi, 334 Nguyen Trai Street, Hanoi, Vietnam Available online 17 February 2006 Abstract The mixed rare earth manganites Nd0.5ÀxPrxSr0.5MnO3 (x ¼ 0.1–0.5) have been prepared using solid state reaction technique All samples are of single phase with orthorhombic structure The microstructure of the samples was determined by SEM The field-cooled (FC) and zero-field-cooled (ZFC) curves showed that samples with xX0.25 exhibit the spin glass-like state at low field and low temperatures, whereas, in the samples with xo0.25, there is the charge ordering (coexisting with FM–AFM transition) established at low temperatures The Curie temperature of the samples increases with increasing Pr content due to increase of orA4 Interesting feature is that at the FM–AFM transition region, the magnetic entropy change has positive value, in contrary to that at FM–PM transition region The electrical property of the samples from 10 K to room temperature is examined in detail r 2006 Elsevier B.V All rights reserved PACS: 75.47.Lx; 75.30.Kz; 75.30.Sg Keywords: Manganites; Charge ordering; Magnetocaloric effect The study of Ln0:5 A00:5 MnO3 manganites (Ln ¼ rare earth, A0 ¼ alkaline element) has brought out a novel effect: charge-ordering (CO) effect [1,2] There is the ferromagnetic (FM) interaction due to the double exchange (DE) interaction among the carriers and the antiferromagnetic (AFM) interaction caused by the super exchange (SE) interaction which depends on combination of Ln3+ and A0 2+ cations Previous works have pointed out that the CO state associated with insulating and AFM behaviors is strongly affected by the average radius of cations Ln3+ and A0 2+ or of A site, orA4 [3–5] In compound Nd0.5Sr0.5MnO3 with a middle orA4( ¼ 1.236 A˚), the ferromagnetic metallic (FMM) state (TC ¼ 250 K) transforms to the AFM CO state on cooling to 150 K Manganite Pr0.5Ca0.5MnO3 with small orA4 (p1.17 A˚) does not exhibit the FMM state at any temperature and CO occurs in the paramagnetic (PM) state Two types of charge ordering can be distinguished in manganites based on the dependence on magnetic field of the CO ÃCorresponding author Tel./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.2006.01.062 state [6,7] We have reported for the first time on the large positive magnetic entropy change in several CO perovskites Nd0.5Sr0.5Mn1ÀxCuxO3 (x ¼ 0:00, 0.02) and Nd0.25Pr0.25Sr0.5MnO3 [8,9] In this work, we report our study on spin glass-like state, charge ordering, phase diagram and positive entropy change in Nd0.5ÀxPrxSr0.5MnO3 (x ¼ 0:1, 0.2, 0.3, 0.4 and 0.5) perovskites The five compositions above were prepared by the solid state reaction technique The microstructure was studied in 5410 LV Jeol scanning electron microscope (SEM) The SEM pictures showed that the samples are homogeneous The grain size decreases from nearly 0.5 mm (x ¼ 0:1 — Fig 1a) to around 0.25 mm (x ¼ 0:2 — Fig 1b) and around 0.15 mm in sample with x ¼ 0:5 Substitution of Pr for Nd leads to refinement of particles To classify the structure symmetry in perovskites we use a geometrical index dened p as t ẳ rA ỵ rO ị= 2rMn þ rO Þ (where rA, rO and rMn are the ionic radius at A, O, and Mn site, respectively) Table presents the value of orA4 and tolerant factor t of the studied samples Obviously while Nd is partly substituted by Pr, orMn4 is constant, orA4 and t increase due to ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405 e403 Fig FC and ZFC thermomagnetic curves of sample with x ¼ 0:5 Fig SEM pictures of samples: (a) x ¼ 0; 1, (b) x ¼ 0:2 Table Some parameters of the studied samples Nd0.5ÀxPrxSr0.5MnO3 Sample orA4 (A˚) t TC (K) TCO (K) x ¼ 0:1 x ¼ 0:2 x ¼ 0:3 x ¼ 0:4 x ¼ 0:5 1.238 1.240 1.242 1.243 1.245 0.954 0.955 0.956 0.956 0.957 270 280 285 305 310 150 145 — — — orA4: average ionic radius at site A; t: tolerant factor, TC: Curie temperature, TCO: Charge-ordering transition temperature larger ionic radius of Pr3+ ion The value of t which is in the range of 0.954–0.957 corresponds to the stable perovskite structure [10] The structure of the samples was examined by Bruker X-ray Diffractometer D5005 and showed that all samples are of single phase with orthorhombic structure The lattice parameter a is slightly decreased with increasing x, whereas the parameters b and c as well as the volume of unit cell are continuously enhanced with increasing x When Nd is partly substituted by Pr, as mentioned above, orA4 increases due to larger ionic radius of Pr3+, leading to increase of internal pressure or volume of unit cell The field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements were carried out in the applied field of 20 Oe by using vibrating sample magnetometer (VSM) DMS 880 From Fig we can see that in sample with x ¼ 0:5, FC and ZFC curves separate each other at low temperatures The temperature at which FC and ZFC curves begin to split is called irreversibility temperature, Tr (TroTC) The low field ZFC curve clearly shows a cusp at a so-called freezing (or spin-glass transition) temperature, Tg These phenomena are the typical features, which belong to the spin glass-like state behavior [11] The samples with x ¼ 0:3 and 0.4 exhibit the same behavior With low doping content of Pr (x ¼ 0:1 and 0.2), there are two magnetic transitions from FC and ZFC curves: the PM to FM transition at TC and FM to AFM transition at Ne´el temperature (TN—in this case coincides with TCO) (Fig 3) The TC of sample with x ¼ 0:1 is less than that of sample with x ¼ 0:2, however TCO of sample with x ¼ 0:2 is less than that of sample with x ¼ 0:1 The values of TC and TCO depend on the average radius of A cations TC enhances with larger orA4 while TCO decreases (see Table 1) Based on the present results as well as of the previous work [9], phase diagram is composed and displayed in Fig TC of samples increases continuously with increasing Pr content due to increase in orA4 We suppose CO and AFM states exist in samples with x ¼ 0:020:25 When Pr is substituted for Nd at higher amount (x40:25), the CO state at low temperatures is vanished by competition between DE and SE interaction This result suggests that there is considerable mismatch effect located at A-site cations [12,13] Accordingly, for Ln0.5A0.5MnO3 ARTICLE IN PRESS e404 N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405 Fig The magnetic entropy change as a function of temperature for samples with x ¼ 0:2 Fig The existence of two magnetic transitions in FC and ZFC thermomagnetic curves for sample with x ¼ 0:2 Fig The magnetic entropy change as a function of temperature for sample with x ¼ 0:3 Fig The phase diagram of system Nd0.5ÀxPrxSr0.5MnO3 perovskites, the drastic changes in properties are seen over the small range of orA4, from 1.13 to 1.24 A˚ We can conclude that the value of orA41.241 A˚ is defines a limit at which magnetic and conducting properties drastically change in our studied samples Fig shows that TC increases quite sharply when orA4 is greater than 1.241 A˚ This feature is in full agreement with the remark in Ref [6] for Ln0.5A0 0,.5MnO3 perovskites For all studied samples the magnetic entropy change as a function of temperature, DSm(T), was evaluated and Fig displays DSm(T) for the sample with x ¼ 0:2 It is clear that besides the negative peak around TC we can see a sharp positive peak at TCO The existence of the positive peak of DSm originates from the increase of magnetic entropy by applied magnetic field when material makes a transition from AFM to FM state in heating ðqM=qT40Þ We could observe the interesting behavior that the material should be cooled by magnetizing at this AFM–FM transition Note that CO transition is the first-order transition while FM–PM transition is the second-order one For the rest samples with x ¼ 0:320:5, there is only a sharp peak of DSm(T) around TC Fig shows that behavior in sample with x ¼ 0:3, for instance The magnetic entropy change at TCO has been also studied by other groups Sande et al [14] have reported that there is a large magnetocaloric effect in manganites with CO transition They found that the magnitude of DSm(T) at the first-order transition is around three times larger than that obtained at the second-order transition for sample Nd0.5Sr0.5MnO3 Szewczyk et al [15] have measured the giant magnetocaloric effect in manganites La1ÀxSrxMnO3 (x ¼ 0:13, 0.16) with CO transition but they did not study the effect at TCO ARTICLE IN PRESS N Chau et al / Journal of Magnetism and Magnetic Materials 303 (2006) e402–e405 In conclusion, perovskites Nd0.5ÀxPrxSr0.5MnO3 (x ¼ 0:120:5) were prepared with single-phase and orthorhombic structure Two samples with x ¼ 0:1 and 0.2 exhibited CO transition (coincides with FM–AFM transition) at low temperatures Whereas, in the rest samples, there is spin glass-like state at low temperatures and low field The magnetic entropy change around CO transition has positive value, contrary to that around FM–PM transition Acknowledgments The authors acknowledge the financial support from the Vietnam National Fundamental Research Program (Project 421004) References [1] H Kuwahara, Y Tomioka, A Asamitsu, Y Moritomo, Y Tokura, Science 270 (1995) 961 [2] J Wolfman, C Simon, M Hervieu, A Maignan, B Raveau, J Solid State Chem 123 (1996) 413 [3] N Kumar, C.N.R Rao, J Solid State Chem 129 (1997) 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Wisniewski, K Piotrowski, R Kartaszynski, B Dabrowski, S Kolesnik, Z Bukowski, Appl Phys Lett 77 (2000) 1026  ... work [9], phase diagram is composed and displayed in Fig TC of samples increases continuously with increasing Pr content due to increase in orA4 We suppose CO and AFM states exist in samples... transition) at low temperatures Whereas, in the rest samples, there is spin glass-like state at low temperatures and low field The magnetic entropy change around CO transition has positive value,... originates from the increase of magnetic entropy by applied magnetic field when material makes a transition from AFM to FM state in heating ðqM=qT40Þ We could observe the interesting behavior that