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Magnetization and magnetoresistance of magnetically soft manganite La0.67Sn0.33MnO3
Journal of Magnetism and Magnetic Materials 215}216 (2000) 204}206
Magnetization and magnetoresistance of magnetically soft manganite La Sn MnO B. Revzin , E. Rozenberg *, G. Gorodetsky, J. Pelleg , I. Felner Department of Materials Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Department of Physics, Faculty of Natural Science, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
Abstract Magnetic and magnetoresistive properties of polycrystalline ceramic La Sn MnO (LSMO) were investigated in the temperature range of 5}300 K and at magnetic "elds up to 5 T. According to X-ray di!raction, the compound was found to be composed of two phases: 73 at% perovskite-like and 27 at% a pyrochlore-like. It was found that the magnetic and magnetoresistive behavior of the LSMO samples could be attributed to the perovskite phase. 2000 Elsevier Science B.V. All rights reserved. Keywords: Doped manganites; Magnetic properties; Magnetoresistance
The compounds La Sn MnO (LSMO) [1,2] \V V belong to a family of doped ferromagnetic manganites that exhibit colossal magnetoresistance (CMR) [3,4]. The main di!erence between Sn and the traditional dopants (Ca, Sr, Ba, Pb, etc.) used in CMR compounds is its unstable valency during the ceramic sintering process. As indicated in Refs. [2,5] such instability leads to the existence of two phases in LSMO: a perovskiteike (LaSn>)MnO and a pyrochlore-like (LaSn>) Mn O . The coexistence of these phases was found for a doping level up to about x"0.5. In this paper we present measurements of magnetization (M) and magnetoresistance (MR) of La Sn MnO , versus temperature ¹ and magnetic "eld H. The measurement of the magnetic AC susceptibility versus temperature is also presented. Polycrystalline samples of LSMO were prepared by the standard ceramic sintering technique described in Ref. [5]. The analysis of the X-ray di!raction, given in Fig. 1, by the Rietveld method indicates the presence of two phases: a cubic phase (pyrochlore) having a lattice parameter a"1.067$0.001 nm and a distorted perov-
* Corresponding author. Fax: #972-7-6472903. E-mail address: [email protected] (E. Rozenberg).
skite phase with rhombohedral symmetry. The rhombohedral unit cell is described by a"0.5482$ 0.0002 nm and a"60.563 and it is very similar to that found in self-doped La Mn O [6]. A best "t to the experimental X-ray di!raction was obtained for 73 at% perovskite (formula ABO ) and 27 at% of a pyro chlore (formula A B O ). Topographic SEM images of the polycrystalline samples are typical of ceramics with an average grain size d)1 lm. The magnetic AC-susceptibility measured at a frequency of 1.4 kHz is given in Fig. 2. According to the results obtained, a ferro}paramagnetic transition occurs at ¹ "250$5 K and is found to be in agreement with previous measurements of Dai et al. [1]. It appears that the susceptibility s above T'260 K is described by s"C/(¹!h ) with h "240$2 K. The magnetization measurements, shown in Fig. 3, were carried out by a SQUID magnetometer at temperatures 5(¹(300 K and in magnetic "elds up to 5 T. As can be seen from Fig. 3, the magnetization at T"270 K, exhibits a
One should note an error in Ref. [5] regarding the proportion of the pyrochlore and the perovskite phases. It should read 73 and 27 at%, respectively, for the perovskite and the pyrochlore phases.
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 1 1 7 - 7
B. Revzin et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 204}206
205
Fig. 3. Magnetization curves of LSMO measured at di!erent temperatures. Fig. 1. (a) X-ray di!ractogram of La Sn MnO . (#) Experimental data, (**) best "t to a mixture of pyrochlore and perovskite phases; arrows mark the perovskite peaks. (b) Di!erence between experimental data and the simulated curve.
Fig. 4. Field-cooled (FC) and zero-"eld-cooled (ZFC) magnetization of LSMO measured at low magnetic "elds.
Fig. 2. (a) AC-susceptibility s of LSMO vs. temperature. (b) Inverse susceptibility vs. temperature in the vicinity of the Curie point.
deviation from a paramagnetic behavior. Field-cooled (FC) and zero-"eld-cooled (ZFC) magnetization vs. ¹ measured at relatively low magnetic "elds are presented in Fig. 4. The magnetization obtained at the lower "eld
(&2 Oe) is, in fact, the DC susceptibility. The di!erence between the FC and ZFC magnetization could be attributed to magnetic domains or ferromagnetic clusters. MR measurements were performed using a standard DC four-point method, in the temperature range 10(¹(300 K and at H up to 1.5 T. The results are given in Figs. 5 and 6. It should be noted that according to MR curves (Fig. 6) the coercive "eld H at ¹"10 K is about 80 Oe. It decreases to H (10 Oe at 200(¹(250 K. The resistivity (o) measured at H"0 shows a maximum at ¹ "225$1 K. It shifts to ¹"229$1 K
under an applied external "eld H"1.5 T as seen in Fig. 5. This temperature shift is a characteristic feature of CMR manganites having a perovskite structure [1,3,4]. The MR observed in the region 10(¹(¹ is about
* 20% and only slightly depends on temperature. Upon approaching ¹ a pronounced decrease in MR occurs.
206
B. Revzin et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 204}206
Fig. 5. Resistivity of LSMO measured as a function of temperature at H"0 and 1.5 T.
chlore-like phase in Sn-doped manganite. In this work, no signs of such transitions were observed. The results obtained for 1/s versus ¹ (Fig. 2b) are similar to those measured previously in perovskite manganites [8]. Upon approaching ¹ from above, one no tices various slopes in 1/s which may be attributed to various clustering of the Mn ions [8]. Independent experiments of ESR [5,9] provide evidence for such clustering. Two points regarding the transport properties in LSMO may be noted: (a) relatively high values of resistivity of the order of 10 ) cm and (b) a notable di!erence between ¹ and ¹ (see Fig. 5). The high resistivity of
LSMO may be associated with the presence of pyrochlore-like phase, which is characterized by small density of carriers compared to doped Mn-perovskites [10]. In the latter the double exchange Mn>}Mn> governs the mobility of the charge carriers. The temperature di!erence between ¹ and ¹ as well as the slight temperature
dependence of MR at 10 K(¹(¹ are characteristic
of the perovskite structure. The above e!ects are attributed to the grain boundaries (GB) in polycrystalline samples [4,11]. As to the MR versus H measurements (Fig. 6), the steep change in MR occurring at low "elds was attributed to the alignment of the magnetization within the grains. The high}"eld MR is due to the GB regions (see Chapters 1 and 5 in Ref. [12]). This research was supported by the Israeli Science Foundation administered by the Israel Academy of Sciences and Humanities. The authors thank Dr. D. Mogilyanski for the help in XRD analysis.
Fig. 6. Magnetoresistance of LSMO measured at 10 and 210 K. The inset presents the magnetoresistance at an extended H scale.
The curves of MR versus H (see Fig. 6) exhibit a steep change at relatively low magnetic "elds followed by a gradual decrease at "elds H'500 Oe. The value obtained for the saturation magnetization M at ¹"5 K is 38 emu/g. This value is lower than that expected for samples containing &73% of the perovskite-like phase (M &95 emu/g for manganites with &33% doping of Ca, Sr etc. [3,4,7]). It was noted in Ref. [2], that a magnetic transition of unknown nature in the interval of 50}65 K could be associated with the pyro-
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
S. Dai et al., Phys. Rev. B55 (1997) 14 125. Z.W. Li et al., J. Appl. Phys. 83 (1998) 7198. E.L. Nagaev, Phys. Usp. 39 (1996) 781. A.P. Ramirez, J. Phys.: Condens Matter 9 (1997) 8171. A.I. Shames et al., J. Magn. Magn. Mater. 203 (1999) 259. W.H. McCarroll et al., J. Solid State Chem. 136 (1998) 322. H.L. Ju, H. Sohn, J. Magn. Magn. Mater 167 (1997) 200. V.S. Amaral et al., J. Appl. Phys. 83 (1998) 7154. S.B. Osero! et al., Phys. Rev. B53 (1996) 6521. A.P. Ramirez, M.A. Subramanian, Science 277 (1997) 546. N. Zhang et al., Solid State Comm. 107 (1998) 417. C.N.R. Rao, B. Raveau, (Eds.), Colossal Magnetoresistance Charge Ordering and Related Properties of Manganese Oxides, World Scienti"c, Singapore, 1998, 345 p.
Magnetization and magnetoresistance of magnetically soft manganite La Sn MnO B. Revzin , E. Rozenberg *, G. Gorodetsky, J. Pelleg , I. Felner Department of Materials Engineering, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Department of Physics, Faculty of Natural Science, Ben-Gurion University of the Negev, P.O. Box 653, Beer-Sheva 84105, Israel Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel
Abstract Magnetic and magnetoresistive properties of polycrystalline ceramic La Sn MnO (LSMO) were investigated in the temperature range of 5}300 K and at magnetic "elds up to 5 T. According to X-ray di!raction, the compound was found to be composed of two phases: 73 at% perovskite-like and 27 at% a pyrochlore-like. It was found that the magnetic and magnetoresistive behavior of the LSMO samples could be attributed to the perovskite phase. 2000 Elsevier Science B.V. All rights reserved. Keywords: Doped manganites; Magnetic properties; Magnetoresistance
The compounds La Sn MnO (LSMO) [1,2] \V V belong to a family of doped ferromagnetic manganites that exhibit colossal magnetoresistance (CMR) [3,4]. The main di!erence between Sn and the traditional dopants (Ca, Sr, Ba, Pb, etc.) used in CMR compounds is its unstable valency during the ceramic sintering process. As indicated in Refs. [2,5] such instability leads to the existence of two phases in LSMO: a perovskiteike (LaSn>)MnO and a pyrochlore-like (LaSn>) Mn O . The coexistence of these phases was found for a doping level up to about x"0.5. In this paper we present measurements of magnetization (M) and magnetoresistance (MR) of La Sn MnO , versus temperature ¹ and magnetic "eld H. The measurement of the magnetic AC susceptibility versus temperature is also presented. Polycrystalline samples of LSMO were prepared by the standard ceramic sintering technique described in Ref. [5]. The analysis of the X-ray di!raction, given in Fig. 1, by the Rietveld method indicates the presence of two phases: a cubic phase (pyrochlore) having a lattice parameter a"1.067$0.001 nm and a distorted perov-
* Corresponding author. Fax: #972-7-6472903. E-mail address: [email protected] (E. Rozenberg).
skite phase with rhombohedral symmetry. The rhombohedral unit cell is described by a"0.5482$ 0.0002 nm and a"60.563 and it is very similar to that found in self-doped La Mn O [6]. A best "t to the experimental X-ray di!raction was obtained for 73 at% perovskite (formula ABO ) and 27 at% of a pyro chlore (formula A B O ). Topographic SEM images of the polycrystalline samples are typical of ceramics with an average grain size d)1 lm. The magnetic AC-susceptibility measured at a frequency of 1.4 kHz is given in Fig. 2. According to the results obtained, a ferro}paramagnetic transition occurs at ¹ "250$5 K and is found to be in agreement with previous measurements of Dai et al. [1]. It appears that the susceptibility s above T'260 K is described by s"C/(¹!h ) with h "240$2 K. The magnetization measurements, shown in Fig. 3, were carried out by a SQUID magnetometer at temperatures 5(¹(300 K and in magnetic "elds up to 5 T. As can be seen from Fig. 3, the magnetization at T"270 K, exhibits a
One should note an error in Ref. [5] regarding the proportion of the pyrochlore and the perovskite phases. It should read 73 and 27 at%, respectively, for the perovskite and the pyrochlore phases.
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 1 1 7 - 7
B. Revzin et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 204}206
205
Fig. 3. Magnetization curves of LSMO measured at di!erent temperatures. Fig. 1. (a) X-ray di!ractogram of La Sn MnO . (#) Experimental data, (**) best "t to a mixture of pyrochlore and perovskite phases; arrows mark the perovskite peaks. (b) Di!erence between experimental data and the simulated curve.
Fig. 4. Field-cooled (FC) and zero-"eld-cooled (ZFC) magnetization of LSMO measured at low magnetic "elds.
Fig. 2. (a) AC-susceptibility s of LSMO vs. temperature. (b) Inverse susceptibility vs. temperature in the vicinity of the Curie point.
deviation from a paramagnetic behavior. Field-cooled (FC) and zero-"eld-cooled (ZFC) magnetization vs. ¹ measured at relatively low magnetic "elds are presented in Fig. 4. The magnetization obtained at the lower "eld
(&2 Oe) is, in fact, the DC susceptibility. The di!erence between the FC and ZFC magnetization could be attributed to magnetic domains or ferromagnetic clusters. MR measurements were performed using a standard DC four-point method, in the temperature range 10(¹(300 K and at H up to 1.5 T. The results are given in Figs. 5 and 6. It should be noted that according to MR curves (Fig. 6) the coercive "eld H at ¹"10 K is about 80 Oe. It decreases to H (10 Oe at 200(¹(250 K. The resistivity (o) measured at H"0 shows a maximum at ¹ "225$1 K. It shifts to ¹"229$1 K
under an applied external "eld H"1.5 T as seen in Fig. 5. This temperature shift is a characteristic feature of CMR manganites having a perovskite structure [1,3,4]. The MR observed in the region 10(¹(¹ is about
* 20% and only slightly depends on temperature. Upon approaching ¹ a pronounced decrease in MR occurs.
206
B. Revzin et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 204}206
Fig. 5. Resistivity of LSMO measured as a function of temperature at H"0 and 1.5 T.
chlore-like phase in Sn-doped manganite. In this work, no signs of such transitions were observed. The results obtained for 1/s versus ¹ (Fig. 2b) are similar to those measured previously in perovskite manganites [8]. Upon approaching ¹ from above, one no tices various slopes in 1/s which may be attributed to various clustering of the Mn ions [8]. Independent experiments of ESR [5,9] provide evidence for such clustering. Two points regarding the transport properties in LSMO may be noted: (a) relatively high values of resistivity of the order of 10 ) cm and (b) a notable di!erence between ¹ and ¹ (see Fig. 5). The high resistivity of
LSMO may be associated with the presence of pyrochlore-like phase, which is characterized by small density of carriers compared to doped Mn-perovskites [10]. In the latter the double exchange Mn>}Mn> governs the mobility of the charge carriers. The temperature di!erence between ¹ and ¹ as well as the slight temperature
dependence of MR at 10 K(¹(¹ are characteristic
of the perovskite structure. The above e!ects are attributed to the grain boundaries (GB) in polycrystalline samples [4,11]. As to the MR versus H measurements (Fig. 6), the steep change in MR occurring at low "elds was attributed to the alignment of the magnetization within the grains. The high}"eld MR is due to the GB regions (see Chapters 1 and 5 in Ref. [12]). This research was supported by the Israeli Science Foundation administered by the Israel Academy of Sciences and Humanities. The authors thank Dr. D. Mogilyanski for the help in XRD analysis.
Fig. 6. Magnetoresistance of LSMO measured at 10 and 210 K. The inset presents the magnetoresistance at an extended H scale.
The curves of MR versus H (see Fig. 6) exhibit a steep change at relatively low magnetic "elds followed by a gradual decrease at "elds H'500 Oe. The value obtained for the saturation magnetization M at ¹"5 K is 38 emu/g. This value is lower than that expected for samples containing &73% of the perovskite-like phase (M &95 emu/g for manganites with &33% doping of Ca, Sr etc. [3,4,7]). It was noted in Ref. [2], that a magnetic transition of unknown nature in the interval of 50}65 K could be associated with the pyro-
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
S. Dai et al., Phys. Rev. B55 (1997) 14 125. Z.W. Li et al., J. Appl. Phys. 83 (1998) 7198. E.L. Nagaev, Phys. Usp. 39 (1996) 781. A.P. Ramirez, J. Phys.: Condens Matter 9 (1997) 8171. A.I. Shames et al., J. Magn. Magn. Mater. 203 (1999) 259. W.H. McCarroll et al., J. Solid State Chem. 136 (1998) 322. H.L. Ju, H. Sohn, J. Magn. Magn. Mater 167 (1997) 200. V.S. Amaral et al., J. Appl. Phys. 83 (1998) 7154. S.B. Osero! et al., Phys. Rev. B53 (1996) 6521. A.P. Ramirez, M.A. Subramanian, Science 277 (1997) 546. N. Zhang et al., Solid State Comm. 107 (1998) 417. C.N.R. Rao, B. Raveau, (Eds.), Colossal Magnetoresistance Charge Ordering and Related Properties of Manganese Oxides, World Scienti"c, Singapore, 1998, 345 p.