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Low-temperature magnetic ordering in the perovskites Pr1−xAxCoO3 (A=Ca, Sr)
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 322 (2010) 1185–1188
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Low-temperature magnetic ordering in the perovskites Pr1xAxCoO3 (A ¼ Ca, Sr) Iosif G. Deac , Romulus Tetean, Istvan Balasz, Emil Burzo Faculty of Physics, Babes-Bolyai University, Str. M. Kogalniceanu 1, Cluj-Napoca 400084, Romania
a r t i c l e in fo
abstract
Available online 17 June 2009
The magnetic and electrical properties of polycrystalline Pr1xAxCoO3 cobaltites with A ¼ Ca, Sr and 0rxr0.5 were studied in the temperature range 4 KrTr1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO3 is a paramagnetic insulator in the range from 4 to 1000 K. The Srdoped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cobaltite Spin-state transition Magnetic phase transition Magnetic cluster
1. Introduction Since the discovery of ‘‘colossal’’ magnetoresistance (MR) effect in perovskite manganites, search for next MR materials has been widely conducted in various transition-metal oxides. Among them, cobaltites with the perovskite-type structure are of potential interest for their magnetoresistive properties. Doped cobaltite perovskites Ln1xAxCoO3 (Ln ¼ rare earth and A ¼ Ca, Sr, Ba), have a unique feature among some other perovskites, which change the Co ions spin-state. In these compounds, there are various spin states for trivalent (low-spin LS: t62ge0g; intermediate-spin IS: t52ge1g ; high-spin HS: t42ge2g) and tetravalent cobalt ions (LS: t52ge0g; IS: t42ge1g ; HS: t32ge2g ). The spin states of undoped LnCoO3 exhibit a gradual crossover with increasing temperature from the low-spin (LS) state (t62ge0g) to the intermediate-spin (IS) state (t52ge1g ) or to the high-spin (HS) state (t42ge2g ). Upon doping A2+ ions into LnCoO3, some of trivalent Co ions become tetravalent, and these also contain a mixture of low and higher spin sates. The Co-ion spin states are determined by the two competing energies, namely, crystal field and Hund coupling, respectively. The properties of these materials are expected to be strongly dependent on the average radius of the A-site cation. Substitution of cations with different ionic radii /rAS, at the A-site, distorts the
Corresponding author. Tel.: +4 026 440 5300; fax: +4 026 459 1906.
E-mail address: [email protected] (I.G. Deac). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.06.034
structure, introduces disorder, enhances antiferromagnetic (AFM) superexchange interactions and change the spin state of Co ions ˚ is ferromagnetic and [1–3]. While La0.7Ca0.3CoO3 (/rAS ¼ 1.354 A) ˚ do not metallic, Nd0.7Ca0.3CoO3 with smaller /rAS ¼ 1.168 A, show a distinct ferromagnetic transition and it is not metallic [4]. One could expect a different behavior of cobaltites doped with ˚ and Sr or Ca because of the difference in ionic radii of Sr2+(1.44 A) ˚ ions. The replacement of Pr3+ (1.14 A) ˚ with Ca2+ and Ca2+ (1.35 A) Sr2+ will result in an expansion of the lattice parameters. In this paper we report the physical properties of Pr1xAxCoO3, A ¼ Ca, Sr to obtain further information how the properties of PrCoO3 are changed as a result of substitutions. We found that the magnetic and electrical properties of these cobaltite are very sensitive to the Ca or Sr doping. Also, that the magnetic transitions in these materials are not true, long-range ferromagnetic transitions, but they are associated with the presence of the magnetic clusters and, probably, a change in the nature of magnetic coupling between Co ions.
2. Experimental Ceramic samples Pr1xAxCoO3, A ¼ Ca, Sr with x ¼ 0, 0.3 and 0.5 were synthesized using the conventional solid state reaction method. The Sr-doped compounds were calcinated at 1000 1C and then sintered in air at 1050 1C, while the Ca-doped samples were treated in the same way at 1100 and 1400 1C for 24 h with intermediate grindings. The powder X-ray diffraction patterns
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were recorded by using a Brucker Advance D8 AXS diffractometer with CuKa radiation. The lattice parameters were obtained by Rietveld analysis, using the FULLPROF program. The magnetization and ac susceptibility measurements were done in a multipurpose Oxford Instruments MagLab System 2000 in the temperature range 5–400 K. The amplitude of the ac field was 1 Oe and the ac susceptibility was measured in the frequency range from 10 to 10,000 Hz. The static magnetic susceptibility was measured by using Faraday equipment up to 1000 K in 1 T magnetic field. The resistivities were determined in a cryogen free magnet cryostat CFM-7T (Cryogenic Ltd.) using the four-probe technique in the temperature range from 5 to 300 K and in magnetic fields up to 7 T.
nonlinear ac susceptibility, w0 3(T) was also observed at TC. For the Ca-doped samples we found only a single magnetic phase transition at 16 K (when x ¼ 0.3) or at 80 K (for x ¼ 0.5), as indicated by maxima in w0 (T). All these maxima are frequency dependent, suggesting the presence of magnetic clusters [9]. In Fig. 3 it is shown the frequency dependence of w0 (T) for the sample Pr0.7Ca0.3CoO3. The inset shows the temperature dependence of the nonlinear susceptibility w0 3(T) for the Pr0.5Sr0.5CoO3 sample, in the region of the transition. The above-mentioned behavior suggests a low temperature inhomogeneous magnetic state, so-called cluster glass state, that presumes some ferromagnetic order but with a possible coexistence of superparamagnetic clusters [10]. The cluster glass
1.6 0.23
3. Results and discussions
0.35
0.03
χ '(a.u.)
1.2
0.21
0.02
x = 0.3
500 Hz 1000 Hz 3000 Hz 5000 Hz 10000 Hz
0.2
0.19
0.8 60
90
0.6
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80
100
T(K)
120
x = 0.3 0.4 0.2
x = 0.5
0 50
100
150
200
250
300
350
T(K) Fig. 2. The temperature dependencies of the real part, w0 (T), of the complex susceptibility for Pr1xSrxCoO3 with x ¼ 0.3 and 0.5. In the insets, the frequency dependencies of w0 (T) in the region of low temperature phase transition are shown.
0.075
Pr
χ (emu/mole)
0.3 0.25
0.05
Ca CoO
0.075 500 Hz 1000 Hz 5000 Hz 10000 Hz
x = 0.3 x = 0.5
0.025
0.2 μ 0 H = 0.1 T
0 0 20 40 60 80 100120140
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T(K)
χ' (a.u.)
Pr0.7 Sr 0.3 CoO 3
M (μB)
x = 0.5 0.22
1
χ '(a.u.)
The X-ray diffraction patterns for all samples showed the presence of a single phase in the limit of experimental errors. The samples crystallize in orthorhombic Pbnm (PrCoO3 and the Cadoped samples) or monoclinic P21/m (Sr-doped samples) type structures having the unit cell parameters close to those previously reported [5–9]. The parent sample PrCoO3 is paramagnetic on the whole investigated temperature range, from 4 to 1000 K. The magnetization of the doped samples has history dependence with a bifurcation between zero field cooling (ZFC) and field cooling (FC) at a characteristic temperature. For the Ca-doped samples, the wT curves are rather complex [9], not typical for a true ferromagnetic behavior (Fig. 1). The hysteresis loops are very narrow, while for the Sr-doped samples the coercivities are about 0.15 T at 5 K. Two magnetic phase transitions were seen in ac susceptibility data for the Sr-doped samples (Fig. 2) namely, sharp peaks in the real part of the complex susceptibility w0 (T) at TCE240 K and broad maxima in w0 (T) at TAE100 K. The fingerprints of these transitions can also be seen in the temperature dependence of the FC magnetization (Fig. 1) that increases abruptly below 240 K and then it decreases and shows a minimum below 100 K. A negative sharp minimum in the
500 Hz 1000 Hz 3000 Hz 5000 Hz 10000 Hz
χ '(a.u.)
1.4
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Pr0.7 Ca 0.3 CoO 3
0 χ' (a.u.) 3
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0.1 FC
0.05
-5 10
-1.5 10
0 50
Sr CoO
-1 10
ZFC
0
Pr
100 150 200 250 300 350 400
T(K) Fig. 1. Temperature dependences of the ZFC and FC magnetizations for the sample Pr0.7Sr0.3CoO3 in a 0.1 T field. In inset the static magnetic susceptibilities for the samples Pr1xCaxCoO3 with x ¼ 0.3 and 0.5 is shown.
0
200
220
240
T(K)
5
10
15
20
25
30
35
40
T(K) Fig. 3. The frequency dependence of w0 (T) for the Pr0.7Ca0.3CoO3 sample. In inset the temperature dependence of w0 3(T) for the sample Pr0.5Sr0.5CoO3.
ARTICLE IN PRESS I.G. Deac et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 1185–1188
x = 0.5 1
ρ(mΩ.cm)
100
ρ/ρ
0
0.3 0.5
0.9 6K 10 K 20 K
0.8
50 K 0
1
2
3
4
5
6
7
8
μ H(T)
10
0
Pr1-x Ca x CoO 3
1
0
50
100
150
200
250
300
T(K) Fig. 4. Electrical resistivity for Pr0.7Ca0.3CoO3 and Pr0.5Ca0.5CoO3 samples in a field of 0 T. In inset: r(H)/r(0), at 6, 10, 20 and 50 K.
3.5 Pr
0.7
Sr
0.3
CoO
3
3
ρ(mΩ-cm)
behavior is a characteristic feature of magnetic phase separated systems [11,12]. Thus we expect the same behavior in our case. While the high temperature phase transition in the Sr-doped samples, at TC, can be understood as the development of ferromagnetic clusters, the low-temperature phase transition is still controversial [13] and it seems to be related with a change in the nature of magnetic coupling between Co ions. The possibility of a change of the easy magnetic axis associated with the orbital ordering at a temperature TA has been pointed out by using Lorentz electron microscopy [14], thermal expansion and magnetostriction measurements [15]. Such anomaly has been previously observed only for Pr0.5Sr0.5CoO3 composition [13–16]. In the high temperatures range, 300rTr1000 K, the static magnetic susceptibility, for all the investigated samples shows a Curie–Weiss behavior, i.e., the reciprocal susceptibility has a linear temperature dependence. A paramagnetic Curie temperature (yp) can be estimated from the linear part of the temperature dependence of the reciprocal susceptibility and the corresponding Curie constants, or the effective magnetic moments, respectively. For PrCoO3 and Ca-doped samples the paramagnetic Curie temperatures are negative having values between yp ¼ 165 K for x ¼ 0, yp ¼ 120 K for x ¼ 0.5 and yp ¼ 25 K for x ¼ 0.3. The negative yp values suggest that the antiferromagnetic interactions are dominant. The Curie constants ranged from 3.22 emu K/mole for x ¼ 0, 1.72 emu K/mole for x ¼ 0.3 and 1.45 emu K/mole for x ¼ 0.5. For the Sr-doped samples the paramagnetic Curie temperatures are positive: yp ¼ 150 K for x ¼ 0.3 and yp ¼ 265 K for x ¼ 0.5. The Curie constants were C ¼ 1.3 emu K/mole for x ¼ 0.3 and C ¼ 1.09 emu K/mole for x ¼ 0.5. Admitting that the effective Pr magnetic moment is the same as that of free Pr3+ ion (Peff ¼ 3.58 mB) according to the addition law of susceptibilities we determined the contribution of Co ions to the Curie constant [17]. Charge neutrality condition implies that a fraction of the order of x of cobalt ions to be in Co4+ valence state. Since the Curie constants of IS Co4+ and HS Co4+ are rather high, as compared to the experimentally determined CCo value, we expect that Co4+ to be in LS state. A very small number of Co4+ ions in IS or HS states may be present but this lead to unphysical behavior. The Curie constant is consistent with Co3+ in all three spin states. Since the LS Co3+ is not magnetic we can estimate only the ratio of IS Co3+ to HS Co3+. This ratio can change from 2.33 for x ¼ 0 to 4.5 for A ¼ Ca and x ¼ 0.3. For A ¼ Sr, we need to have all the Co+3 ions in IS and LS states. Such estimation is rather raw, and the states can be greatly changed due to the magnetic phase separation in the system, to temperature variation and due to magnetic field. Temperature dependences of electrical resistivities r(T) are shown in Figs. 4 [9] and 5. While the PrCoO3 is a paramagnetic insulator, the Ca-doped samples were found to have a semiconducting behavior in the whole temperature range, in spite of their low resistivities. We found no sign of magnetic changes in the r(T) curves. Probably, these changes can be masked by disorder and grain boundary effects. For x ¼ 0.3 we found a negative magnetoresistance of a few percents only, while for the x ¼ 0.5 the magnetoresistance can be higher than 20% at 6 K and 7 T. The Sr-doped samples have a very different behavior (Fig. 5). They show a metallic behavior in the high temperatures range and an intriguing upturn at low temperatures, below 100 K for x ¼ 0.3 and below 50 K for x ¼ 0.5. If not intrinsic, we attribute this lowtemperature behavior to the tunneling effect through the grain boundaries, as in the case of polycrystalline manganites [18] and some other transition-metal oxides [19]. If we can see a change of slope in the r(T) curves around TC, such a change is probably hidden by these extrinsic effects in the region of the low-temperature transition, around TA. No significant magnetoresistance was observed for these Sr-doped samples.
1187
2.5
1T 0T 1T
0.4
0T
Pr0.5 Sr0.5 CoO 3
0.3
0.2
0
50
100
150
200
250
300
T(K) Fig. 5. Temperature dependences of resistivities for Pr0.7Ca0.3CoO3 and Pr0.5Ca0.5CoO3 in 0 and 1 T.
4. Conclusions High-quality Pr1xAxCoO3 (A ¼ Ca, Sr and x ¼ 0, 0.3 and 0.5) samples were prepared by standard ceramic reaction and their electrical and magnetic properties were investigated. These are strongly dependent on the type of the doping ions. The doped samples have a cluster glass behavior at low temperatures. The Ca-doped samples show a low-temperatures transition into the cluster glass state; they have semiconducting behavior and they can have high negative magnetoresistence (for x ¼ 0.5). The Srdoped samples show a double magnetic transition. While the high temperature magnetic transition takes place from a paramagnetic phase into a ‘‘nearly’’ ferromagnetic one, the second transition occurs inside of this latter phase. The Sr-doped samples have metallic behavior at high temperatures, and the grain boundary
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effects dominate in the low temperature range. The magnetoresitive effect is negligible.
Acknowledgments This work was partially supported by the Grants CEEX 21 and 45/2006 of NARS and PNCDI2 71-045/2007 of NCPM, Romania. References [1] [2] [3] [4]
G.H. Jonker, J.H. van Santen, Physica 19 (1953) 120. M.A. Senaris-Rodriguez, J.B. Goodenough, J. Solid State Chem. 118 (1995) 323. M. Itoh, M. Sugahara, I. Natori, K. Morita, J. Phys. Soc. Jpn. 64 (1995) 3967. A.K. Kundu, K. Ramesha, R. Seshadri, C.N.R. Rao, J. Phys.: Condens. Matter 16 (2004) 7955. [5] H.W. Brinks, H. Fjellvag, A. Kjekshus, B.C. Hauback, J. Solid State Chem. 147 (1999) 464.
[6] A.K. Kundu, E.V. Sampathkumaran, K.V. Gopalakrishnan, C.N.R. Rao, J. Magn. Magn. Mater. 28 (2004) 261. [7] S. Tsubouchi, T. Kyoˆ men, M. Itoh, P. Ganguly, M. Oguni, Y. Shimojo, Y. Morii, Y. Ishii, Phys. Rev. B 66 (2002) 052418. [8] A. Chichev, J. Hejtmanek, Z. Jirak, K. Knızek, M. Marysko, M. Dlouha, S. Vratislav, J. Magn. Magn. Mater. 316 (2007) e728. [9] I. G. Deac, R. Tetean and E. Burzo, IEEE Trans. Magn. 44 (2008) 2292. [10] R. Caciuffo, D. Rinaldi, G. Barucca, J. Mira, J. Rivas, M.A. Senaris-Rodriguez, P.G. Radaelli, D. Fiorani, J.B. Goodenough, Phys. Rev. B 59 (1999) 1068. [11] E. Dagotto, T. Hotta, A. Moreo, Phys. Rep. 344 (2001) 1. [12] I.G. Deac, J.F. Mitchell, P. Schiffer, Phys. Rev. B 63 (2001) 172408. [13] R. Mahendiran, P. Schiffer, Phys. Rev. B 68 (2003) 024427. [14] M. Uchida, R. Mahendiran, Y. Tomioka, Y. Matsui, K. Ishizuka, Y. Tokura, Appl. Phys. Lett. 86 (2005) 131913. [15] S. Hirahara, Y. Nakai, K. Miyoshi, K. Fujiwara, J. Takeuchi, J. Magn. Magn. Mater. 310 (2007) 1866. [16] I.O. Troyanchuk, D.V. Karpinski, A.N. Chobot, D.G. Votsekhovich, V.M. Dobryanski, JETP Lett. 84 (2006) 151. [17] J. Blasco, C. Ritter, J. Garcia, J.M. de Teresa, J. Perez-Carlo, M.R. Ibarra, Phys. Rev. B 62 (2000) 5609. [18] J. Wu, J.W. Lynn, C.J. Glinka, J. Burley, H. Zheng, J.F. Mitchell, C. Leighton, Phys. Rev. Lett. 94 (2005) 037201. [19] M. Ziese, Rep. Prog. Phys. 65 (2002) 143.
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Low-temperature magnetic ordering in the perovskites Pr1xAxCoO3 (A ¼ Ca, Sr) Iosif G. Deac , Romulus Tetean, Istvan Balasz, Emil Burzo Faculty of Physics, Babes-Bolyai University, Str. M. Kogalniceanu 1, Cluj-Napoca 400084, Romania
a r t i c l e in fo
abstract
Available online 17 June 2009
The magnetic and electrical properties of polycrystalline Pr1xAxCoO3 cobaltites with A ¼ Ca, Sr and 0rxr0.5 were studied in the temperature range 4 KrTr1000 K and field up to 7 T. The X-ray analyses show the presence of only one phase having monoclinic or orthorhombic symmetry. The magnetic measurements indicate that the Ca-doped samples have at low temperatures, similar properties to the frustrated magnetic materials. PrCoO3 is a paramagnetic insulator in the range from 4 to 1000 K. The Srdoped cobaltites exhibit two phase transitions: a paramagnetic–ferromagnetic (or magnetic phase separated state) phase transition at about 240 K and a second one at about 100 K. The magnetic measurements suggest the presence of magnetic clusters and a change in the nature of magnetic coupling between Co ions at low temperatures. A semiconducting type behavior and high negative magnetoresistance was found for the Ca-doped samples, while the Sr-doped ones were metallic and with negligible magnetoresistance. The results are analyzed in the frame of a phase separation scenario in the presence of the spin-state transitions of Co ions. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cobaltite Spin-state transition Magnetic phase transition Magnetic cluster
1. Introduction Since the discovery of ‘‘colossal’’ magnetoresistance (MR) effect in perovskite manganites, search for next MR materials has been widely conducted in various transition-metal oxides. Among them, cobaltites with the perovskite-type structure are of potential interest for their magnetoresistive properties. Doped cobaltite perovskites Ln1xAxCoO3 (Ln ¼ rare earth and A ¼ Ca, Sr, Ba), have a unique feature among some other perovskites, which change the Co ions spin-state. In these compounds, there are various spin states for trivalent (low-spin LS: t62ge0g; intermediate-spin IS: t52ge1g ; high-spin HS: t42ge2g) and tetravalent cobalt ions (LS: t52ge0g; IS: t42ge1g ; HS: t32ge2g ). The spin states of undoped LnCoO3 exhibit a gradual crossover with increasing temperature from the low-spin (LS) state (t62ge0g) to the intermediate-spin (IS) state (t52ge1g ) or to the high-spin (HS) state (t42ge2g ). Upon doping A2+ ions into LnCoO3, some of trivalent Co ions become tetravalent, and these also contain a mixture of low and higher spin sates. The Co-ion spin states are determined by the two competing energies, namely, crystal field and Hund coupling, respectively. The properties of these materials are expected to be strongly dependent on the average radius of the A-site cation. Substitution of cations with different ionic radii /rAS, at the A-site, distorts the
Corresponding author. Tel.: +4 026 440 5300; fax: +4 026 459 1906.
E-mail address: [email protected] (I.G. Deac). 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.06.034
structure, introduces disorder, enhances antiferromagnetic (AFM) superexchange interactions and change the spin state of Co ions ˚ is ferromagnetic and [1–3]. While La0.7Ca0.3CoO3 (/rAS ¼ 1.354 A) ˚ do not metallic, Nd0.7Ca0.3CoO3 with smaller /rAS ¼ 1.168 A, show a distinct ferromagnetic transition and it is not metallic [4]. One could expect a different behavior of cobaltites doped with ˚ and Sr or Ca because of the difference in ionic radii of Sr2+(1.44 A) ˚ ions. The replacement of Pr3+ (1.14 A) ˚ with Ca2+ and Ca2+ (1.35 A) Sr2+ will result in an expansion of the lattice parameters. In this paper we report the physical properties of Pr1xAxCoO3, A ¼ Ca, Sr to obtain further information how the properties of PrCoO3 are changed as a result of substitutions. We found that the magnetic and electrical properties of these cobaltite are very sensitive to the Ca or Sr doping. Also, that the magnetic transitions in these materials are not true, long-range ferromagnetic transitions, but they are associated with the presence of the magnetic clusters and, probably, a change in the nature of magnetic coupling between Co ions.
2. Experimental Ceramic samples Pr1xAxCoO3, A ¼ Ca, Sr with x ¼ 0, 0.3 and 0.5 were synthesized using the conventional solid state reaction method. The Sr-doped compounds were calcinated at 1000 1C and then sintered in air at 1050 1C, while the Ca-doped samples were treated in the same way at 1100 and 1400 1C for 24 h with intermediate grindings. The powder X-ray diffraction patterns
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were recorded by using a Brucker Advance D8 AXS diffractometer with CuKa radiation. The lattice parameters were obtained by Rietveld analysis, using the FULLPROF program. The magnetization and ac susceptibility measurements were done in a multipurpose Oxford Instruments MagLab System 2000 in the temperature range 5–400 K. The amplitude of the ac field was 1 Oe and the ac susceptibility was measured in the frequency range from 10 to 10,000 Hz. The static magnetic susceptibility was measured by using Faraday equipment up to 1000 K in 1 T magnetic field. The resistivities were determined in a cryogen free magnet cryostat CFM-7T (Cryogenic Ltd.) using the four-probe technique in the temperature range from 5 to 300 K and in magnetic fields up to 7 T.
nonlinear ac susceptibility, w0 3(T) was also observed at TC. For the Ca-doped samples we found only a single magnetic phase transition at 16 K (when x ¼ 0.3) or at 80 K (for x ¼ 0.5), as indicated by maxima in w0 (T). All these maxima are frequency dependent, suggesting the presence of magnetic clusters [9]. In Fig. 3 it is shown the frequency dependence of w0 (T) for the sample Pr0.7Ca0.3CoO3. The inset shows the temperature dependence of the nonlinear susceptibility w0 3(T) for the Pr0.5Sr0.5CoO3 sample, in the region of the transition. The above-mentioned behavior suggests a low temperature inhomogeneous magnetic state, so-called cluster glass state, that presumes some ferromagnetic order but with a possible coexistence of superparamagnetic clusters [10]. The cluster glass
1.6 0.23
3. Results and discussions
0.35
0.03
χ '(a.u.)
1.2
0.21
0.02
x = 0.3
500 Hz 1000 Hz 3000 Hz 5000 Hz 10000 Hz
0.2
0.19
0.8 60
90
0.6
T(K)
120
0.18
150
80
100
T(K)
120
x = 0.3 0.4 0.2
x = 0.5
0 50
100
150
200
250
300
350
T(K) Fig. 2. The temperature dependencies of the real part, w0 (T), of the complex susceptibility for Pr1xSrxCoO3 with x ¼ 0.3 and 0.5. In the insets, the frequency dependencies of w0 (T) in the region of low temperature phase transition are shown.
0.075
Pr
χ (emu/mole)
0.3 0.25
0.05
Ca CoO
0.075 500 Hz 1000 Hz 5000 Hz 10000 Hz
x = 0.3 x = 0.5
0.025
0.2 μ 0 H = 0.1 T
0 0 20 40 60 80 100120140
0.15
T(K)
χ' (a.u.)
Pr0.7 Sr 0.3 CoO 3
M (μB)
x = 0.5 0.22
1
χ '(a.u.)
The X-ray diffraction patterns for all samples showed the presence of a single phase in the limit of experimental errors. The samples crystallize in orthorhombic Pbnm (PrCoO3 and the Cadoped samples) or monoclinic P21/m (Sr-doped samples) type structures having the unit cell parameters close to those previously reported [5–9]. The parent sample PrCoO3 is paramagnetic on the whole investigated temperature range, from 4 to 1000 K. The magnetization of the doped samples has history dependence with a bifurcation between zero field cooling (ZFC) and field cooling (FC) at a characteristic temperature. For the Ca-doped samples, the wT curves are rather complex [9], not typical for a true ferromagnetic behavior (Fig. 1). The hysteresis loops are very narrow, while for the Sr-doped samples the coercivities are about 0.15 T at 5 K. Two magnetic phase transitions were seen in ac susceptibility data for the Sr-doped samples (Fig. 2) namely, sharp peaks in the real part of the complex susceptibility w0 (T) at TCE240 K and broad maxima in w0 (T) at TAE100 K. The fingerprints of these transitions can also be seen in the temperature dependence of the FC magnetization (Fig. 1) that increases abruptly below 240 K and then it decreases and shows a minimum below 100 K. A negative sharp minimum in the
500 Hz 1000 Hz 3000 Hz 5000 Hz 10000 Hz
χ '(a.u.)
1.4
0.05
Pr0.7 Ca 0.3 CoO 3
0 χ' (a.u.) 3
0.025
0.1 FC
0.05
-5 10
-1.5 10
0 50
Sr CoO
-1 10
ZFC
0
Pr
100 150 200 250 300 350 400
T(K) Fig. 1. Temperature dependences of the ZFC and FC magnetizations for the sample Pr0.7Sr0.3CoO3 in a 0.1 T field. In inset the static magnetic susceptibilities for the samples Pr1xCaxCoO3 with x ¼ 0.3 and 0.5 is shown.
0
200
220
240
T(K)
5
10
15
20
25
30
35
40
T(K) Fig. 3. The frequency dependence of w0 (T) for the Pr0.7Ca0.3CoO3 sample. In inset the temperature dependence of w0 3(T) for the sample Pr0.5Sr0.5CoO3.
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x = 0.5 1
ρ(mΩ.cm)
100
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0
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3
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T(K) Fig. 4. Electrical resistivity for Pr0.7Ca0.3CoO3 and Pr0.5Ca0.5CoO3 samples in a field of 0 T. In inset: r(H)/r(0), at 6, 10, 20 and 50 K.
3.5 Pr
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3
ρ(mΩ-cm)
behavior is a characteristic feature of magnetic phase separated systems [11,12]. Thus we expect the same behavior in our case. While the high temperature phase transition in the Sr-doped samples, at TC, can be understood as the development of ferromagnetic clusters, the low-temperature phase transition is still controversial [13] and it seems to be related with a change in the nature of magnetic coupling between Co ions. The possibility of a change of the easy magnetic axis associated with the orbital ordering at a temperature TA has been pointed out by using Lorentz electron microscopy [14], thermal expansion and magnetostriction measurements [15]. Such anomaly has been previously observed only for Pr0.5Sr0.5CoO3 composition [13–16]. In the high temperatures range, 300rTr1000 K, the static magnetic susceptibility, for all the investigated samples shows a Curie–Weiss behavior, i.e., the reciprocal susceptibility has a linear temperature dependence. A paramagnetic Curie temperature (yp) can be estimated from the linear part of the temperature dependence of the reciprocal susceptibility and the corresponding Curie constants, or the effective magnetic moments, respectively. For PrCoO3 and Ca-doped samples the paramagnetic Curie temperatures are negative having values between yp ¼ 165 K for x ¼ 0, yp ¼ 120 K for x ¼ 0.5 and yp ¼ 25 K for x ¼ 0.3. The negative yp values suggest that the antiferromagnetic interactions are dominant. The Curie constants ranged from 3.22 emu K/mole for x ¼ 0, 1.72 emu K/mole for x ¼ 0.3 and 1.45 emu K/mole for x ¼ 0.5. For the Sr-doped samples the paramagnetic Curie temperatures are positive: yp ¼ 150 K for x ¼ 0.3 and yp ¼ 265 K for x ¼ 0.5. The Curie constants were C ¼ 1.3 emu K/mole for x ¼ 0.3 and C ¼ 1.09 emu K/mole for x ¼ 0.5. Admitting that the effective Pr magnetic moment is the same as that of free Pr3+ ion (Peff ¼ 3.58 mB) according to the addition law of susceptibilities we determined the contribution of Co ions to the Curie constant [17]. Charge neutrality condition implies that a fraction of the order of x of cobalt ions to be in Co4+ valence state. Since the Curie constants of IS Co4+ and HS Co4+ are rather high, as compared to the experimentally determined CCo value, we expect that Co4+ to be in LS state. A very small number of Co4+ ions in IS or HS states may be present but this lead to unphysical behavior. The Curie constant is consistent with Co3+ in all three spin states. Since the LS Co3+ is not magnetic we can estimate only the ratio of IS Co3+ to HS Co3+. This ratio can change from 2.33 for x ¼ 0 to 4.5 for A ¼ Ca and x ¼ 0.3. For A ¼ Sr, we need to have all the Co+3 ions in IS and LS states. Such estimation is rather raw, and the states can be greatly changed due to the magnetic phase separation in the system, to temperature variation and due to magnetic field. Temperature dependences of electrical resistivities r(T) are shown in Figs. 4 [9] and 5. While the PrCoO3 is a paramagnetic insulator, the Ca-doped samples were found to have a semiconducting behavior in the whole temperature range, in spite of their low resistivities. We found no sign of magnetic changes in the r(T) curves. Probably, these changes can be masked by disorder and grain boundary effects. For x ¼ 0.3 we found a negative magnetoresistance of a few percents only, while for the x ¼ 0.5 the magnetoresistance can be higher than 20% at 6 K and 7 T. The Sr-doped samples have a very different behavior (Fig. 5). They show a metallic behavior in the high temperatures range and an intriguing upturn at low temperatures, below 100 K for x ¼ 0.3 and below 50 K for x ¼ 0.5. If not intrinsic, we attribute this lowtemperature behavior to the tunneling effect through the grain boundaries, as in the case of polycrystalline manganites [18] and some other transition-metal oxides [19]. If we can see a change of slope in the r(T) curves around TC, such a change is probably hidden by these extrinsic effects in the region of the low-temperature transition, around TA. No significant magnetoresistance was observed for these Sr-doped samples.
1187
2.5
1T 0T 1T
0.4
0T
Pr0.5 Sr0.5 CoO 3
0.3
0.2
0
50
100
150
200
250
300
T(K) Fig. 5. Temperature dependences of resistivities for Pr0.7Ca0.3CoO3 and Pr0.5Ca0.5CoO3 in 0 and 1 T.
4. Conclusions High-quality Pr1xAxCoO3 (A ¼ Ca, Sr and x ¼ 0, 0.3 and 0.5) samples were prepared by standard ceramic reaction and their electrical and magnetic properties were investigated. These are strongly dependent on the type of the doping ions. The doped samples have a cluster glass behavior at low temperatures. The Ca-doped samples show a low-temperatures transition into the cluster glass state; they have semiconducting behavior and they can have high negative magnetoresistence (for x ¼ 0.5). The Srdoped samples show a double magnetic transition. While the high temperature magnetic transition takes place from a paramagnetic phase into a ‘‘nearly’’ ferromagnetic one, the second transition occurs inside of this latter phase. The Sr-doped samples have metallic behavior at high temperatures, and the grain boundary
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effects dominate in the low temperature range. The magnetoresitive effect is negligible.
Acknowledgments This work was partially supported by the Grants CEEX 21 and 45/2006 of NARS and PNCDI2 71-045/2007 of NCPM, Romania. References [1] [2] [3] [4]
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