- Email: [email protected]
3ZrxMn1−xO3 manganites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e658–e660 www.elsevier.com/locate/jmmm
Investigation of chemical and grain-boundary effects in the Zr-doped La2/3Sr1/3ZrxMn1xO3 manganites R. Teteana,, I.G. Deaca, E. Burzoa, A. Takacsb, M. Neumannb a
Babes-Bolyai University, Faculty of Physics, RO-400084 Cluj-Napoca, Romania b Osnabru¨ck University, Barbara Str.7, Osnabru¨ck 49069, Germany Available online 20 November 2006
Abstract We present a detailed study of the polycrystalline perovskite manganites La2/3Sr1/3ZrxMn1xO3 (xo0.3) at low temperatures and high magnetic fields, by means of electrical resistance, magnetization and X-ray photoelectron spectroscopy (XPS). All the samples show large negative magnetoresistance behavior. The data suggest that the magnetoresistance of the system is dominated by intergranular effects, but some intrinsic effects are also to be considered. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50y; 75.30.Kz; 72.80.Ga Keywords: Perovskites; Magnetoresistance; X-ray photoelectron spectroscopy; Electrical resistivity; Magnetization
The perovskite manganese oxides like La1xSrxMnO3 display a number of remarkable anomalously magnetic and transport properties, including a large negative magnetoresistance, the so-called ‘‘colossal’’ magnetoresistance [1,2]. The richness of the phase diagrams of these materials was considered to be determined by the competition of double exchange and super-exchange interactions, charge/orbital ordering instabilities, and strong coupling to the lattice deformations. This coupling has recently been shown to commonly result in electronic-phase separation between different magnetoelectronic states at low temperatures [3,4]. The electronic properties are nearly half metallic [1,5]. The compound with x ¼ 0.3 have a high transition temperature (Tc370 K) into ferromagnetic state that makes this compound of great importance for applications in spintronics. This compound shows a metal-like behavior of resistivity below Tc, and a maximum of magnetoresistance around Tc [1,2]. It is interesting to study the effect of the distances between Mn ions in the system as well as the Mn3+/ Mn4+ ratio and to introduce disorder in the Mn lattice. Corresponding author. Tel.: +4 026 459 4315; fax: +4 026 459 1906.
E-mail address: [email protected] (R. Tetean). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.912
Substitution of manganese with Zr4+ leads to the decrease of Mn4+/Mn3+ ratio and reduces the interaction between Mn4+ and Mn3+ ions. Polycrystalline samples with nominal composition La2/ 3Sr1/.3ZrxMn1xO3 (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3) were prepared by standard ceramic reaction [1]. The compounds were sintered in air at 1400 1C for 24 h. The samples were studied by resistivity measurements in magnetic fields up to 6 T in the temperature ranges 5–300 K, DC susceptibility measurement in the range 300–950 K and XPS by using a PHI 5600 ci Multitechnique system. The compounds are single phases, within the limit of experimental errors. All the compounds crystallize in a rhombohedral structure. The lattice parameters are nearly unchanged upon doping, due to relatively closed values of the ionic radii for Mn+4 and Zr4+. Both valence band and core level spectra were studied by XPS. The XPS valence band spectra for La0.67Sr0.33Mn1xZrxO3 are shown in Fig. 1. Near to the Fermi level, a strong hybridization can be observed between the Mn 3d states and La 4f, for the compound with a concentration of x ¼ 0. By doping with Zr, the electronic structure is changed near to the Fermi level, the Zr 4d states, also contributing to the hybridization.
ARTICLE IN PRESS R. Tetean et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e658–e660
e659
1.2 La0.7Sr0.3ZrxMn1-xO3 1 x=0.15 ρ(T)/r(300 K)
Intensity (arb.units)
x=0.3
x=0.1
0.8
0.6
x = 0, 0T
x=0
x = 0, 1T 0.4
15
10
5 Binding energy (eV)
x = 0.2, 0T x = 0.2, 1T
0 0.2 0.00
Fig. 1. Valence band spectra for La2/3Sr1/.3ZrxMn1xO3 compounds.
50.0
100
150 T (K)
200
250
300
Fig. 3. r(T)/r(0) curves for La2/3Sr1/.3ZrxMn1xO3 system taken in 1 T (open symbols) and in zero applied magnetic field (solid symbols).
Intensity (arb. units)
x=0.3
x=0.15 x=0.1
x=0
675
670
665
660 655 650 645 Binding energy (eV)
640
635
63
Fig. 2. Mn 2p XPS spectra for La0.67Sr0.33Mn1xZrxO3.
The strong hybridization of the Zr, Mn and La states leads to a shift of the valence-bands towards higher bindings energies as the Zr concentration increases. The Mn 2p XPS spectra for La0.67Sr0.33Mn1xZrxO3 are shown in Fig. 2. The observed spin-orbit splitting, which can be identified in the distance between the two peaks of the 2p3/2 and 2p1/2 states is Dsp ¼ 12 eV. The first observation of the magnetic exchange splitting in the core level Mn 2p3/2 photoelectron spectra (Dex ¼ 1.0–1.5 eV) was reported on Heusler alloys [6,7]. Both these states are accompanied by features at about 5 eV towards higher binding energies for the case of x ¼ 0.15. The Mn ground state is mainly the 3d6, one with a small admixture of 3d5 configuration, representing states of local moments in the Anderson’s model [8,9]. Upon creation of a 2p core hole, one may expect a well-screened state 2p53d6 and a low screened 2p53d5 final state occur. In this case, the 5 eV satellite located at 648 eV is assigned to a 2p5d5 final state configuration, whereas the main line at 643 eV was associated with a 2p53d6 final state configuration. For the Mn 2p spectra, with x ¼ 0.3, the main line (2p3/2) presents an asymmetry, which can be related to the exchange interactions between the core hole and the valence
electrons. The two binding energy components correspond to the majority and minority spin core-hole states, respectively, and the energy separation between them, Dex, corresponds to the exchange splitting of the Mn 2p core-holes states, giving direct evidence for the existence of local moments in the given compound. The resistivities increase with temperature and show a maximum at Tmax in the range 240–275 K, Fig. 3. In the lowtemperature region (To30 K), a shallow minimum appeared that seems to be indicative of the intergrain conductivity and of the associated intergrain magnetoresitivity [10]. The temperature dependence of magnetoresistance is not typical for a CMR effect, since the magnetoresistance does not show a maximum near Tmax and it seems to be an intergranular electrical conductivity effect [11]. The magnetoresistance rapidly increases in relatively low magnetic fields and then, in higher fields the changes are almost linear and were explained by tunneling between AFM-coupled grains [10]. The linear behavior of MR(H), in the region of high fields is a result of the suppression of spin fluctuations by the applied magnetic field [10]. The Mn4+ concentration, f4+ Mn determined from DC susceptibility measurements decreases with increasing Zr content from 36.65% for x ¼ 0 to 11.54% when x ¼ 0.3, suggesting that Zr replaces mainly the Mn4+ ions. The behavior of magnetoresistance in our samples seems to be dominated by transport across the grain boundaries, since it increases monotonically with decreasing temperature in the low-temperature region. The upturn trend of MR(T) curves near by 300 K, however, suggests that magnetoresistance has also an intrinsic component. References [1] For reviews see J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167 and the references therein. [2] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 41103.
ARTICLE IN PRESS e660
R. Tetean et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e658–e660
[3] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [4] I.G. Deac, J.F. Mitchell, P. Schiffer, Phys. Rev. B 63 (2001) 172408. [5] B. Nadgorny, I.I. Mazin, M. Osofsky, R.J. Soulen Jr., P. Broussard, R.M. Stroud, D.J. Singh, V.G. Harris, A. Arsenov, Y. Mukovskii, Phys. Rev. B. 63 (2001) 184433. [6] Zu. M. Zarmoshenko, M.I. Katsnelson, E.I. Shreder, E.Z. Kurmaev, A. Slebarski, S. Plogmann, T. Schlatho¨lter, J. Braun, Eur. Phys. J. B 2 (1998) 1.
[7] S. Plogmann, T. Schlatho¨lter, J. Braun, M. Neumann, Zu. M. Zarmoshenko, M.V. Zablonskikh, E.I. Shreder, E.Z. Kurmaev, Phys. Rev. B 60 (1999) 6428. [8] P.W. Anderson, Phys. Rev. 124 (1961) 41. [9] T. Moriya, J. Magn. Magn. Mater. 31–34 (1983) 10. [10] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [11] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041.
Journal of Magnetism and Magnetic Materials 310 (2007) e658–e660 www.elsevier.com/locate/jmmm
Investigation of chemical and grain-boundary effects in the Zr-doped La2/3Sr1/3ZrxMn1xO3 manganites R. Teteana,, I.G. Deaca, E. Burzoa, A. Takacsb, M. Neumannb a
Babes-Bolyai University, Faculty of Physics, RO-400084 Cluj-Napoca, Romania b Osnabru¨ck University, Barbara Str.7, Osnabru¨ck 49069, Germany Available online 20 November 2006
Abstract We present a detailed study of the polycrystalline perovskite manganites La2/3Sr1/3ZrxMn1xO3 (xo0.3) at low temperatures and high magnetic fields, by means of electrical resistance, magnetization and X-ray photoelectron spectroscopy (XPS). All the samples show large negative magnetoresistance behavior. The data suggest that the magnetoresistance of the system is dominated by intergranular effects, but some intrinsic effects are also to be considered. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50y; 75.30.Kz; 72.80.Ga Keywords: Perovskites; Magnetoresistance; X-ray photoelectron spectroscopy; Electrical resistivity; Magnetization
The perovskite manganese oxides like La1xSrxMnO3 display a number of remarkable anomalously magnetic and transport properties, including a large negative magnetoresistance, the so-called ‘‘colossal’’ magnetoresistance [1,2]. The richness of the phase diagrams of these materials was considered to be determined by the competition of double exchange and super-exchange interactions, charge/orbital ordering instabilities, and strong coupling to the lattice deformations. This coupling has recently been shown to commonly result in electronic-phase separation between different magnetoelectronic states at low temperatures [3,4]. The electronic properties are nearly half metallic [1,5]. The compound with x ¼ 0.3 have a high transition temperature (Tc370 K) into ferromagnetic state that makes this compound of great importance for applications in spintronics. This compound shows a metal-like behavior of resistivity below Tc, and a maximum of magnetoresistance around Tc [1,2]. It is interesting to study the effect of the distances between Mn ions in the system as well as the Mn3+/ Mn4+ ratio and to introduce disorder in the Mn lattice. Corresponding author. Tel.: +4 026 459 4315; fax: +4 026 459 1906.
E-mail address: [email protected] (R. Tetean). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.912
Substitution of manganese with Zr4+ leads to the decrease of Mn4+/Mn3+ ratio and reduces the interaction between Mn4+ and Mn3+ ions. Polycrystalline samples with nominal composition La2/ 3Sr1/.3ZrxMn1xO3 (x ¼ 0, 0.1, 0.15, 0.2, 0.25, 0.3) were prepared by standard ceramic reaction [1]. The compounds were sintered in air at 1400 1C for 24 h. The samples were studied by resistivity measurements in magnetic fields up to 6 T in the temperature ranges 5–300 K, DC susceptibility measurement in the range 300–950 K and XPS by using a PHI 5600 ci Multitechnique system. The compounds are single phases, within the limit of experimental errors. All the compounds crystallize in a rhombohedral structure. The lattice parameters are nearly unchanged upon doping, due to relatively closed values of the ionic radii for Mn+4 and Zr4+. Both valence band and core level spectra were studied by XPS. The XPS valence band spectra for La0.67Sr0.33Mn1xZrxO3 are shown in Fig. 1. Near to the Fermi level, a strong hybridization can be observed between the Mn 3d states and La 4f, for the compound with a concentration of x ¼ 0. By doping with Zr, the electronic structure is changed near to the Fermi level, the Zr 4d states, also contributing to the hybridization.
ARTICLE IN PRESS R. Tetean et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e658–e660
e659
1.2 La0.7Sr0.3ZrxMn1-xO3 1 x=0.15 ρ(T)/r(300 K)
Intensity (arb.units)
x=0.3
x=0.1
0.8
0.6
x = 0, 0T
x=0
x = 0, 1T 0.4
15
10
5 Binding energy (eV)
x = 0.2, 0T x = 0.2, 1T
0 0.2 0.00
Fig. 1. Valence band spectra for La2/3Sr1/.3ZrxMn1xO3 compounds.
50.0
100
150 T (K)
200
250
300
Fig. 3. r(T)/r(0) curves for La2/3Sr1/.3ZrxMn1xO3 system taken in 1 T (open symbols) and in zero applied magnetic field (solid symbols).
Intensity (arb. units)
x=0.3
x=0.15 x=0.1
x=0
675
670
665
660 655 650 645 Binding energy (eV)
640
635
63
Fig. 2. Mn 2p XPS spectra for La0.67Sr0.33Mn1xZrxO3.
The strong hybridization of the Zr, Mn and La states leads to a shift of the valence-bands towards higher bindings energies as the Zr concentration increases. The Mn 2p XPS spectra for La0.67Sr0.33Mn1xZrxO3 are shown in Fig. 2. The observed spin-orbit splitting, which can be identified in the distance between the two peaks of the 2p3/2 and 2p1/2 states is Dsp ¼ 12 eV. The first observation of the magnetic exchange splitting in the core level Mn 2p3/2 photoelectron spectra (Dex ¼ 1.0–1.5 eV) was reported on Heusler alloys [6,7]. Both these states are accompanied by features at about 5 eV towards higher binding energies for the case of x ¼ 0.15. The Mn ground state is mainly the 3d6, one with a small admixture of 3d5 configuration, representing states of local moments in the Anderson’s model [8,9]. Upon creation of a 2p core hole, one may expect a well-screened state 2p53d6 and a low screened 2p53d5 final state occur. In this case, the 5 eV satellite located at 648 eV is assigned to a 2p5d5 final state configuration, whereas the main line at 643 eV was associated with a 2p53d6 final state configuration. For the Mn 2p spectra, with x ¼ 0.3, the main line (2p3/2) presents an asymmetry, which can be related to the exchange interactions between the core hole and the valence
electrons. The two binding energy components correspond to the majority and minority spin core-hole states, respectively, and the energy separation between them, Dex, corresponds to the exchange splitting of the Mn 2p core-holes states, giving direct evidence for the existence of local moments in the given compound. The resistivities increase with temperature and show a maximum at Tmax in the range 240–275 K, Fig. 3. In the lowtemperature region (To30 K), a shallow minimum appeared that seems to be indicative of the intergrain conductivity and of the associated intergrain magnetoresitivity [10]. The temperature dependence of magnetoresistance is not typical for a CMR effect, since the magnetoresistance does not show a maximum near Tmax and it seems to be an intergranular electrical conductivity effect [11]. The magnetoresistance rapidly increases in relatively low magnetic fields and then, in higher fields the changes are almost linear and were explained by tunneling between AFM-coupled grains [10]. The linear behavior of MR(H), in the region of high fields is a result of the suppression of spin fluctuations by the applied magnetic field [10]. The Mn4+ concentration, f4+ Mn determined from DC susceptibility measurements decreases with increasing Zr content from 36.65% for x ¼ 0 to 11.54% when x ¼ 0.3, suggesting that Zr replaces mainly the Mn4+ ions. The behavior of magnetoresistance in our samples seems to be dominated by transport across the grain boundaries, since it increases monotonically with decreasing temperature in the low-temperature region. The upturn trend of MR(T) curves near by 300 K, however, suggests that magnetoresistance has also an intrinsic component. References [1] For reviews see J.M.D. Coey, M. Viret, S. von Molnar, Adv. Phys. 48 (1999) 167 and the references therein. [2] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 41103.
ARTICLE IN PRESS e660
R. Tetean et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e658–e660
[3] A. Moreo, S. Yunoki, E. Dagotto, Science 283 (1999) 2034. [4] I.G. Deac, J.F. Mitchell, P. Schiffer, Phys. Rev. B 63 (2001) 172408. [5] B. Nadgorny, I.I. Mazin, M. Osofsky, R.J. Soulen Jr., P. Broussard, R.M. Stroud, D.J. Singh, V.G. Harris, A. Arsenov, Y. Mukovskii, Phys. Rev. B. 63 (2001) 184433. [6] Zu. M. Zarmoshenko, M.I. Katsnelson, E.I. Shreder, E.Z. Kurmaev, A. Slebarski, S. Plogmann, T. Schlatho¨lter, J. Braun, Eur. Phys. J. B 2 (1998) 1.
[7] S. Plogmann, T. Schlatho¨lter, J. Braun, M. Neumann, Zu. M. Zarmoshenko, M.V. Zablonskikh, E.I. Shreder, E.Z. Kurmaev, Phys. Rev. B 60 (1999) 6428. [8] P.W. Anderson, Phys. Rev. 124 (1961) 41. [9] T. Moriya, J. Magn. Magn. Mater. 31–34 (1983) 10. [10] M. Ziese, Rep. Prog. Phys. 65 (2002) 143. [11] H.Y. Hwang, S.W. Cheong, N.P. Ong, B. Batlogg, Phys. Rev. Lett. 77 (1996) 2041.