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Detailed study of the structural and magnetic transitions in Pr1−xSrxMnO3 single crystals (0.48⩽x⩽0.57)
Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
Detailed study of the structural and magnetic transitions in Pr1xSrxMnO3 single crystals (0:48pxp0:57) E. Pollerta,*, Z. Jira! ka, J. Hejtma! neka, A. Strejca, R. Ku$zelb, V. Hardyc a
Institute of Physics ASCR, Cukrovarnicka! 10, 162 53 Praha 6, Czech Republic b Faculty of Mathematics and Physics, 121 16 Prague 2, Czech Republic c Laboratoire CRISMAT, ISMRA, 14050 Caen, France Received 17 July 2001; received in revised form 4 December 2001
Abstract The X-ray diffraction, magnetic, electric and specific heat measurements have been performed on single crystals Pr1xSrxMnO3 close to boundary between the Pbnm and I4/mcm perovskite types where subtle changes of composition are found sufficient for considerable variation of magnetic ground states and character of carriers. The sample of x ¼ 0:48 undergoes a transition from the paramagnetic phase of orthorhombic Pbnm symmetry to the ferromagnetic metallic phase of tetragonal I4/mcm symmetry (TC ¼ 290 K). The sample of a slightly higher x ¼ 0:50 exhibits the I4/ mcm symmetry already in the paramagnetic state. The ferromagnetism below TC ¼ 265 K is followed at lower temperatures by a transition to the orbitally polarized state of the Fmmm symmetry associated with the A-type antiferromagnetic arrangement (TN ¼ 155 K). The end composition of x ¼ 0:57 undergoes the antiferromagnetic transition directly from the paramagnetic state at TN ¼ 217 K and the experiments confirm the metallic character of the A-type ground state. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.25.+z; 75.30.m Keywords: Manganites; Single crystals; Magnetic properties; Electric properties
1. Introduction The contemporary interest about the manganites R1xMxMnO3, clearly comprehensible in the recent period by their colossal magnetoresistance properties, led to the discovery of various structural and magnetic phase transitions involving orbital and charge ordering phenomena. They are strongly influenced by the variation of the chemical composition, substitution of trivalent *Corresponding author. Tel.: +4202-20318417; fax: +420233343184. E-mail address: [email protected] (E. Pollert).
rare earth for bivalent alkaline earth, eventually alkaline metal ions. Actually, an interplay of two effects exists: *
*
variation of the Mn3+/Mn4+ ratio effectuated by the controlled valency mechanism, variation of the mean size of cations placed into oxygen matrix in twelve-fold coordination and in octahedral holes with six-fold coordination.
It seems to be pertinent to mention various fundamental types of ordering of the occupied dz2 orbitals, namely for 1:0, 1:1 and 1:3 ratio of Mn3+ and Mn4+ ions [1,2]. While in the first case only
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 0 7 6 - 8
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
Mn3+ ions in the octahedral sites are present and the cooperative Jahn–Teller effect is the determining interaction, the arrangement of dz2 orbitals for two latter ratios is combined with a long range ordering of the Mn3+ and Mn4+ ions mediated by the Coulomb interactions. Such a type of ordering can strongly modify the crystal structure and magnetic interactions as well. With respect to that the Mn3+/Mn4+B1 ratio has a particular attraction. The following correlation effects should be distinguished: *
*
the ferromagnetic double exchange Mn3+-O2Mn4+, the orbital–lattice interaction of Mn3+ ions controlled by frequency of the electron hopping Mn3+Mn4+ which becomes frozen with the lowering of the temperature.
Both the correlation effects compete each with other and one can easily realize that the prevailing character will depend on the geometrical arrangement of the Mn-O-Mn bonds, i.e. on an eventual distortion of the ideal perovskite structure. The consequences of a structural deformation are evident, low transfer integral for eg electrons and a strong tendency to charge localization which causes a weakening of the ferromagnetic interactions and preference to the charge ordering. An important role in this context plays the steric effects, macroscopic cooperative distortions called usually as tilting or buckling and the local deformations of the MnO6 octahedra caused by
291
the size effects. Due to a persisting slight buckling a tendency to the charge ordering prevails in the systems Pr1xCaxMnO3 for 0:4oxo0:7; (TCO B250 K, TN B175 K), [2] while in the Pr1xSrxMnO3 series for x ¼ 0:5 the role of tilting is minor and the ferromagnetic double exchange is preferred (TC B265 K, TN B135 K) [3]. Thus, we have now focused our attention to a detail study of the system Pr1xSrxMnO3 carried out on the single crystals in a narrow range of 0:48pxp0:57:
2. Experimental The single crystals in the system Pr1xSrxMnO3 of x ¼ 0:48 and 0.57 were grown from high temperature solutions by a modified top seeded method [4], for some details see Table 1, while the crystal of x ¼ 0:5 grown by the floating zone technique was supplied from JRCAT Tsukuba, Japan [5]. A considerable attention was paid to the characterization of the studied samples. The EMA and SEM revealed negligible concentration fluctuations and no inclusions in the single crystals. The methods described elsewhere were employed for the measurement of the electric resistivity and DC magnetic susceptibility vs. temperature dependences, for some details see e.g. [6]. The X-ray experiments were performed using Cu Ka radiation on a diffractometer Siemens D500
Table 1 Conditions of the Pr1xSrxMnO3 single crystals growth from high-temperature solutions Starting composition
A (mol %)
B (mol %)
PrO11/6 MnCO3 SrCO3 SrF2 B2O3
7.3 17.3 30.42 13.72 31.26
2.3 17.3 35.42 13.72 31.26
Growth conditions Temperature interval of the growth Cooling rate Vertical temperature gradient in the crucible Mean composition of the crystals
1133–11171C 0.1 K/h 1.4 K/cm Pr0.52Sr0.48MnO3
1118–10551C 0.3 K/h 4 K/cm Pr0.43Sr0.57MnO3
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
The positions of the investigated crystals in the phase diagram constructed on a basis of the recent neutron diffraction data and previous results [3,7] can be seen in Fig. 1. The narrow range of 0:48pxp0:57 represents an area where a subtle shift of the composition, namely change of the Mn3+/Mn4+ ratio can easily provoke a change of the orbital and magnetic arrangement. Indeed the observed susceptibility and resistivity data confirm this presumption, see Figs. 2 and 3. Thus the sample of x ¼ 0:48 exhibits at TC ¼ 290 K a transition to the ferromagnetic state with metallic conductivity. The magnetoresistance under the field of 9 T is rather weak and limited to the vicinity of the TC : In contrast, the composition of x ¼ 0:57 undergoes at TN ¼ 217 K transition to the A-type antiferromagnetic ordering which is characterized with antiferromagnetic interactions between the MnO2 planes and the 2D metallic character of the ferromagnetic MnO2 planes, see Fig. 4. This antiferromagnetic state is stable up to very high fields and a weak magnetoresistance can be associated with the interlayer canting induced by paraprocess. An intermediate situation occurs for x ¼ 0:50: The compound first undergoes a ferromagnetic transition at TC ¼ 265 K. The second (antiferromagnetic) transition takes place at TN B155 K. The onset of antiferromagnetism can be suppressed totally by the field of 7 T as shown in
Temperature (K)
350
I4/ mcm
Pbnm
Pm3m
300 250
TN (C)
TC
200
TN (A)
150
C type
100
A type
50
Fmmm
0
0.5
I4/ mcm
0.6
0.7
0.8
x
Fig. 1. The magnetic phase diagram of the Pr1xSrxMnO3 series in the range of 0:45pxp0:85: The dashed lines delimit the existence region of the cubic, orthorhombic and tetragonal perovskite phases, doted areas correspond approximately to the two-phase regions.
12 x= 0.48
10
TC
8 6 4 -1
3. Results and discussion
400
2
-1
with a helium cooled chamber Oxford Instruments. The diffraction patterns (2Y ¼ 2021401) have been taken with an aid of the Brown position sensitive detector in a broad temperature range 70–360 K for x ¼ 0:48 and 20–300 K for x ¼ 0:57; respectively. The specific-heat measurements were carried out by means of a commercial setup (Quantum Design) using a 2t-relaxation method. The samples masses were about 25 mg. The background signal, including the effect of the known amount of Apiezon N used to paste the sample on the platform, was recorded in a first run and then subtracted from the total heat capacity.
magnetic susceptibility (emu mol Oe )
292
0
TC
8 x= 0.50
TN
6 4 2 0
x=0.57
TN
0.2 0.1 0.1 0.0
0
50
100
150
200
250
300
350
T (K) Fig. 2. The temperature dependence of magnetic susceptibility in the investigated Pr1xSrxMnO3 single crystals.
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
TC
x= 0.48 0T
1
9T Electrical resistivity (mΩcm)
0.1 x= 0.5 10
TC
TN 0T
1
7T
0.1 5 4
TN
x= 0.57
0T
3 2
9T
TN 1
0
50
100 150 200 250 300 350 T (K)
Fig. 3. The temperature dependence of electrical resistivity in zero field and in the fields of 7 and 9 T of the investigated Pr1xSrxMnO3 single crystals.
Fig. 3. Some significant data on the magnetism and electric transport, concerning the studied crystals are summarized in Table 2. The magnetic transitions in the range 0:48pxp0:57 are accompanied with structural changes. The sequence of the crystallographic phases and temperature dependence of the lattice parameters are shown in Fig. 5. The crystal of x ¼ 0:48 occurs at the boundary between two perovskite types of the different tilt patterns. The hightemperature phase is of the orthorhombic Pbnm type. Approximately with the onset of ferromagnetism, actually at Tt ¼ 270 K, it undergoes transition to the tetragonal I4/mcm structure, which is associated with a pronounced decrease of the unit cell volume, see Fig. 6. This situation is analogous to the behavior upon the ferromagnetic
293
transition of another manganite of a similar B50% Mn4+ content, Pr0.5Sr0.41Ca0.09MnO3. This compound, however, undergoes at lower temperatures second transition to the antiferromagnetic charge-ordered state and the Pbnm perovskite phase re-enters [8]. In contrast, the ferromagnetic I4/mcm phase in the present crystal Pr0.52Sr0.48MnO3 is stable down to the lowest temperatures. The Pr0.5Sr0.5MnO3 compound (x ¼ 0:50) exhibits the orthorhombic–tetragonal transition well above room temperature, i.e. in the paramagnetic state and the ferromagnetic transition at TC ¼ 265 K brings neither change of the I4/mcm crystal symmetry nor clear anomaly in the unit cell volume. The second transition to the A-type antiferromagnetic state is due to the onset of orbital polarization shown in Fig. 4, which is itself responsible for the change of symmetry from I4/ mcm to Fmmm [3]. While the tetragonal deformation c=aO2B1.02 of the I4/mcm phase in x ¼ 0:50 is temperature independent and can be attributed mainly to the effect of the MnO6 octahedral tilting, in the crystal of x ¼ 0:57 it increases with decreasing temperature, which manifests an increasing orbital polarization of the d3z2 r2 type, i.e. the eg electron density is extended along the c-axis (see the right part of Fig. 4). At the concomitant structural and antiferromagnetic transition at TN ¼ 217 K, the orbital polarization becomes of the dz2 y2 type (see the left part of Fig. 4). The eg electron density is then extended along two mutually perpendicular directionsFthe original c-axis of the I4/mcm hightemperature phase and one of the [1 1 0] diagonals (the bo - and co -axis in Fmmm cell). The transition is accompanied with an abrupt decrease of the unit cell volume. The specific heat data on crystals x ¼ 0:48; 0.50 and 0.57 are displayed in Fig. 7. The results in the upper panel show for x ¼ 0:48 a broadened l-like peak centered at 285 K, which well correlates with the middle of the ferromagnetic transition while no effect is observable at Tt ¼ 270 K. On the other hand, the coupled orbital and magnetic ordering in x ¼ 0:57 at TN ¼ 217 K is abrupt with strong d peak in the specific heat. Both kinds of the anomalies are present in x ¼ 0:50; but of much
294
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
Fig. 4. The eg orbital polarization in the A-type (0:48oxo0:60) and C-type (0:60oxo0:95) antiferromagnetic phases of Pr1xSrxMnO3.
weaker intensities. There is a rounded peak centered around 255 K, i.e. at temperature slightly lower than the measured TC ¼ 265 K and a sharp peak at TN ¼ 155 K. The low-temperature specific heat displayed in the lower panel of Fig. 7 shows nearly straight lines yielding by an extrapolation to zero of Cp =T vs. T 2 a reliable estimate of the linear in T contribution to Cp ; which in our case can be ascribed to the free charge carriers. The observed linear coefficient g ¼ 5:5 mJ K2 mol1 for the ferromagnetic 3D metallic ground state in x ¼ 0:48 is in a close agreement with the value 5.4 mJ K2 mol1 reported for the related ferromagnet Pr0.6Sr0.4MnO3 of the Pbnm symmetry [9] and comparable with the results published for other metallic manganites in the B30% Mn4+ range [10]. For the A-type antiferromagnetic
Table 2 Magnetic and electric properties of Pr1xSrxMnO3 single crystals x ¼ 0:48
x ¼ 0:50
x ¼ 0:57
Paramagnetic EA B14 meV Adiabatic hopping
Paramagnetic EA B17 meV Adiabatic hopping
Paramagnetic EA B39 meV Adiabatic hopping
Ferromagnetic TC ¼ 290 K r0 B55265 mO cm
Ferromagnetic TC ¼ 265 K r0 B130 mO cm
Antiferromagnetic A-type TN ¼ 217 K Quasi 2D metal r0 B1 mO cm
Antiferromagnetic A-type TN ¼ 155 K Quasi 2D metal r0 B1 mO cm
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
454
a t√2
x=0.57
x=0.48
3
ao
cell volume(Å )
7.5
Tt
I4/mcm
co
7.6
455
ct
bo
7.7
lattice parameters (Å)
TN
Fmmm
7.8
Fmmm
7.9
TN
I4/mcm
bo
7.8
ct
7.7
TC
co
7.6
a t√2
ao
453
452 TN x=0.50
x=0.50
TN
451 7.9
I4/mcm
7.8
Tt ao√ 2
7.7
bo√ 2 co
a t√2
0
50
x=0.57
Pbnm
x=0.48
ct
7.6
295
Tc 100
150 200 250
450 0
50
100 150 200 250 300 T (K)
300
T (K) Fig. 5. Temperature evolution of the Pr1xSrxMnO3 crystal structures. (The data for x ¼ 0:50 are taken from Ref. [8]).
ground state in x ¼ 0:57; which is essentially 2D conductor (see e.g. resistivity data on a single domain specimen of the related compound Nd0.46Sr0.54MnO3 [11]), also an appreciable value g ¼ 3:5 mJ K2 mol1 is observed. This is an evidence of a considerable density of states at Fermi level NðEÞ ¼ 0:95 eV1 per formula unit (1.7 1022 eV1 cm3) and gives an additional argument for the metallic (band-like) character of carriers in the A-type antiferromagnets. It is worth mentioning that the non-zero density at Fermi level was determined also in ab initio theoretical calculations using a simplified structural model [12]. The calculated value is, however, about twice smaller.
4. Conclusion The single crystals of the Pr1xSrxMnO3 manganites have been studied in a narrow
Fig. 6. Temperature dependence of the unit cell volume in Pr1xSrxMnO3.
compositional range 0:48pxp0:57 close to the boundary between the Pbnm and I4/mcm perovskite types where subtle changes of the steric distortion (t ¼ 0:984 for x ¼ 0:48 and t ¼ 0:993 for x ¼ 0:57) and Mn4+ content 48–57% are sufficient for considerable variation of the magnetic ground states and character of carriers. The end compositions x ¼ 0:48 and 0.57 are characterized as ideal examples of the 3D metallic ferromagnet and 2D metallic antiferromagnet in the compositionally close systems with the same MnO6 tilt pattern. A novelty represents the tetragonal I4/mcm symmetry of the ferromagnetic ground state in x ¼ 0:48 which exhibits different distribution of Mn-OMn bonds compared to more common orthoperovskites Ln1xCaxMnO3 (Ln=La or rare earths) or rhombohedral perovskites La1xSrxMnO3. Namely, in the latter two systems all three MnO-Mn bond angles are alike while for the present I4/mcm ferromagnet, as well as the Fmmm antiferromagnets, one Mn-O-Mn angle is 1801 and two others make about 1651 [3].
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
296
250 x=0.57 x=0.50 TN x=0.48
-1
cP (JK mol )
200
TC
-1
150 100
Tt
50 0
0
80
160 240 T (K)
TC 320
-1
15
cP /T ( mJK mol )
-2
25 20
Acknowledgements This work was supported by the grants 202/99/ 0413 of Grant Agency of the Czech Republic and A1010004/00 of Grant Agency of the Academy of Sciences of the Czech Republic.
x=0.57 x=0.50 x=0.48
10
References
5 0
conductivity in the metallic ferromagnetic planes. The character of the interlayer electronic transport remains an open question and represents a challenge for more detailed electronic structure calculations. Namely, it is of basic interest how the transport is influenced by the degree of spin polarization of carriers at Fermi level and their orbital polarization (which might be different from the average shown schematically in Fig. 4). An important role should play also the interlayer MnO-Mn bond angles, actually B1651.
0
20
40
60
80 100 120 140 2
2
T (K ) Fig. 7. Specific heat data showing the anomaly at TC for x ¼ 0:48 and 0.50 and a first order transition at TN ; observed for 0.50 and 0.57. For a clarity the dependences for x ¼ 0:5 and 0.57 are shifted of 25 and 50 J K1 mol1 upwards, respectively. The lower panel shows the Cp =T vs. T 2 graph which determines the g coefficient of the linear electronic term and the cubic lattice term of the low-temperature specific heat.
Another interesting issue is the occurrence of 2D metallic conductivity in the A-type antiferromagnet x ¼ 0:57: It should be noted that it was not possible to determine the anisotropy of the transport properties. The reason is the multidomain nature of the Fmmm phase, which appears to be a consequence of its I4/mcm parentage. The measured data thus reflect essentially the
[1] E. Pollert, S. Krupi$cka, E. Kuzmi$cov!a, J. Phys. Chem. Sol. 43 (1982) 1137. $ sa, M. Dlouh!a, S. Vratislav, [2] Z. Jir!ak, S. Krupi$cka, Z. Sim$ J. Magn. Magn. Mater. 53 (1985) 153. [3] F. Damay, C. Martin, M. Hervieu, A. Maignan, B. Raveau, G. Andr!e, F. Bour!ee, J. Magn. Magn. Mater. 184 (1998) 71. [4] E. Pollert, M. Nev$riva, S. Dur$cok, Prog. Cryst. Growth Characterization 22 (1991) 143. [5] Y. Tomioka, A. Asamitsu, Y. Morimoto, H. Kuwahara, Y. Tokura, Phys. Rev. Lett. 74 (1995) 5108. [6] S. Krupi$cka, M. Mary$sko, Z. Jir!ak, J. Hejtm!anek, J. Magn. Magn. Mater. 206 (1999) 45. [7] C. Martin, A. Maignan, M. Hervieu, B. Raveau, Phys. Rev. B 60 (1999) 12191. [8] F. Damay, Z. Jir!ak, M. Hervieu, C. Martin, A. Maignan, B. Raveau, G. Andr!e, F. Bour!ee, J. Magn. Magn. Mater. 190 (1998) 221. [9] M.R. Lees, O.A. Petrenko, G. Balakrishnan, D.McK. Paul, Phys. Rev. B 59 (1999) 1298. [10] B.F. Woodfield, M.L. Wilson, J.M. Byres, Phys. Rev. Lett. 78 (1997) 3201. [11] H. Kuwahara, T. Okuda, Y. Tomioka, A. Asamitsu, Y. Tokura, Phys. Rev. Lett. 82 (1999) 4316. [12] Z. Feng, I.V. Solovyev, K. Terakura, Phys. Rev. Lett. 84 (2000) 3169.
Detailed study of the structural and magnetic transitions in Pr1xSrxMnO3 single crystals (0:48pxp0:57) E. Pollerta,*, Z. Jira! ka, J. Hejtma! neka, A. Strejca, R. Ku$zelb, V. Hardyc a
Institute of Physics ASCR, Cukrovarnicka! 10, 162 53 Praha 6, Czech Republic b Faculty of Mathematics and Physics, 121 16 Prague 2, Czech Republic c Laboratoire CRISMAT, ISMRA, 14050 Caen, France Received 17 July 2001; received in revised form 4 December 2001
Abstract The X-ray diffraction, magnetic, electric and specific heat measurements have been performed on single crystals Pr1xSrxMnO3 close to boundary between the Pbnm and I4/mcm perovskite types where subtle changes of composition are found sufficient for considerable variation of magnetic ground states and character of carriers. The sample of x ¼ 0:48 undergoes a transition from the paramagnetic phase of orthorhombic Pbnm symmetry to the ferromagnetic metallic phase of tetragonal I4/mcm symmetry (TC ¼ 290 K). The sample of a slightly higher x ¼ 0:50 exhibits the I4/ mcm symmetry already in the paramagnetic state. The ferromagnetism below TC ¼ 265 K is followed at lower temperatures by a transition to the orbitally polarized state of the Fmmm symmetry associated with the A-type antiferromagnetic arrangement (TN ¼ 155 K). The end composition of x ¼ 0:57 undergoes the antiferromagnetic transition directly from the paramagnetic state at TN ¼ 217 K and the experiments confirm the metallic character of the A-type ground state. r 2002 Elsevier Science B.V. All rights reserved. PACS: 75.25.+z; 75.30.m Keywords: Manganites; Single crystals; Magnetic properties; Electric properties
1. Introduction The contemporary interest about the manganites R1xMxMnO3, clearly comprehensible in the recent period by their colossal magnetoresistance properties, led to the discovery of various structural and magnetic phase transitions involving orbital and charge ordering phenomena. They are strongly influenced by the variation of the chemical composition, substitution of trivalent *Corresponding author. Tel.: +4202-20318417; fax: +420233343184. E-mail address: [email protected] (E. Pollert).
rare earth for bivalent alkaline earth, eventually alkaline metal ions. Actually, an interplay of two effects exists: *
*
variation of the Mn3+/Mn4+ ratio effectuated by the controlled valency mechanism, variation of the mean size of cations placed into oxygen matrix in twelve-fold coordination and in octahedral holes with six-fold coordination.
It seems to be pertinent to mention various fundamental types of ordering of the occupied dz2 orbitals, namely for 1:0, 1:1 and 1:3 ratio of Mn3+ and Mn4+ ions [1,2]. While in the first case only
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 0 0 7 6 - 8
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
Mn3+ ions in the octahedral sites are present and the cooperative Jahn–Teller effect is the determining interaction, the arrangement of dz2 orbitals for two latter ratios is combined with a long range ordering of the Mn3+ and Mn4+ ions mediated by the Coulomb interactions. Such a type of ordering can strongly modify the crystal structure and magnetic interactions as well. With respect to that the Mn3+/Mn4+B1 ratio has a particular attraction. The following correlation effects should be distinguished: *
*
the ferromagnetic double exchange Mn3+-O2Mn4+, the orbital–lattice interaction of Mn3+ ions controlled by frequency of the electron hopping Mn3+Mn4+ which becomes frozen with the lowering of the temperature.
Both the correlation effects compete each with other and one can easily realize that the prevailing character will depend on the geometrical arrangement of the Mn-O-Mn bonds, i.e. on an eventual distortion of the ideal perovskite structure. The consequences of a structural deformation are evident, low transfer integral for eg electrons and a strong tendency to charge localization which causes a weakening of the ferromagnetic interactions and preference to the charge ordering. An important role in this context plays the steric effects, macroscopic cooperative distortions called usually as tilting or buckling and the local deformations of the MnO6 octahedra caused by
291
the size effects. Due to a persisting slight buckling a tendency to the charge ordering prevails in the systems Pr1xCaxMnO3 for 0:4oxo0:7; (TCO B250 K, TN B175 K), [2] while in the Pr1xSrxMnO3 series for x ¼ 0:5 the role of tilting is minor and the ferromagnetic double exchange is preferred (TC B265 K, TN B135 K) [3]. Thus, we have now focused our attention to a detail study of the system Pr1xSrxMnO3 carried out on the single crystals in a narrow range of 0:48pxp0:57:
2. Experimental The single crystals in the system Pr1xSrxMnO3 of x ¼ 0:48 and 0.57 were grown from high temperature solutions by a modified top seeded method [4], for some details see Table 1, while the crystal of x ¼ 0:5 grown by the floating zone technique was supplied from JRCAT Tsukuba, Japan [5]. A considerable attention was paid to the characterization of the studied samples. The EMA and SEM revealed negligible concentration fluctuations and no inclusions in the single crystals. The methods described elsewhere were employed for the measurement of the electric resistivity and DC magnetic susceptibility vs. temperature dependences, for some details see e.g. [6]. The X-ray experiments were performed using Cu Ka radiation on a diffractometer Siemens D500
Table 1 Conditions of the Pr1xSrxMnO3 single crystals growth from high-temperature solutions Starting composition
A (mol %)
B (mol %)
PrO11/6 MnCO3 SrCO3 SrF2 B2O3
7.3 17.3 30.42 13.72 31.26
2.3 17.3 35.42 13.72 31.26
Growth conditions Temperature interval of the growth Cooling rate Vertical temperature gradient in the crucible Mean composition of the crystals
1133–11171C 0.1 K/h 1.4 K/cm Pr0.52Sr0.48MnO3
1118–10551C 0.3 K/h 4 K/cm Pr0.43Sr0.57MnO3
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
The positions of the investigated crystals in the phase diagram constructed on a basis of the recent neutron diffraction data and previous results [3,7] can be seen in Fig. 1. The narrow range of 0:48pxp0:57 represents an area where a subtle shift of the composition, namely change of the Mn3+/Mn4+ ratio can easily provoke a change of the orbital and magnetic arrangement. Indeed the observed susceptibility and resistivity data confirm this presumption, see Figs. 2 and 3. Thus the sample of x ¼ 0:48 exhibits at TC ¼ 290 K a transition to the ferromagnetic state with metallic conductivity. The magnetoresistance under the field of 9 T is rather weak and limited to the vicinity of the TC : In contrast, the composition of x ¼ 0:57 undergoes at TN ¼ 217 K transition to the A-type antiferromagnetic ordering which is characterized with antiferromagnetic interactions between the MnO2 planes and the 2D metallic character of the ferromagnetic MnO2 planes, see Fig. 4. This antiferromagnetic state is stable up to very high fields and a weak magnetoresistance can be associated with the interlayer canting induced by paraprocess. An intermediate situation occurs for x ¼ 0:50: The compound first undergoes a ferromagnetic transition at TC ¼ 265 K. The second (antiferromagnetic) transition takes place at TN B155 K. The onset of antiferromagnetism can be suppressed totally by the field of 7 T as shown in
Temperature (K)
350
I4/ mcm
Pbnm
Pm3m
300 250
TN (C)
TC
200
TN (A)
150
C type
100
A type
50
Fmmm
0
0.5
I4/ mcm
0.6
0.7
0.8
x
Fig. 1. The magnetic phase diagram of the Pr1xSrxMnO3 series in the range of 0:45pxp0:85: The dashed lines delimit the existence region of the cubic, orthorhombic and tetragonal perovskite phases, doted areas correspond approximately to the two-phase regions.
12 x= 0.48
10
TC
8 6 4 -1
3. Results and discussion
400
2
-1
with a helium cooled chamber Oxford Instruments. The diffraction patterns (2Y ¼ 2021401) have been taken with an aid of the Brown position sensitive detector in a broad temperature range 70–360 K for x ¼ 0:48 and 20–300 K for x ¼ 0:57; respectively. The specific-heat measurements were carried out by means of a commercial setup (Quantum Design) using a 2t-relaxation method. The samples masses were about 25 mg. The background signal, including the effect of the known amount of Apiezon N used to paste the sample on the platform, was recorded in a first run and then subtracted from the total heat capacity.
magnetic susceptibility (emu mol Oe )
292
0
TC
8 x= 0.50
TN
6 4 2 0
x=0.57
TN
0.2 0.1 0.1 0.0
0
50
100
150
200
250
300
350
T (K) Fig. 2. The temperature dependence of magnetic susceptibility in the investigated Pr1xSrxMnO3 single crystals.
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
TC
x= 0.48 0T
1
9T Electrical resistivity (mΩcm)
0.1 x= 0.5 10
TC
TN 0T
1
7T
0.1 5 4
TN
x= 0.57
0T
3 2
9T
TN 1
0
50
100 150 200 250 300 350 T (K)
Fig. 3. The temperature dependence of electrical resistivity in zero field and in the fields of 7 and 9 T of the investigated Pr1xSrxMnO3 single crystals.
Fig. 3. Some significant data on the magnetism and electric transport, concerning the studied crystals are summarized in Table 2. The magnetic transitions in the range 0:48pxp0:57 are accompanied with structural changes. The sequence of the crystallographic phases and temperature dependence of the lattice parameters are shown in Fig. 5. The crystal of x ¼ 0:48 occurs at the boundary between two perovskite types of the different tilt patterns. The hightemperature phase is of the orthorhombic Pbnm type. Approximately with the onset of ferromagnetism, actually at Tt ¼ 270 K, it undergoes transition to the tetragonal I4/mcm structure, which is associated with a pronounced decrease of the unit cell volume, see Fig. 6. This situation is analogous to the behavior upon the ferromagnetic
293
transition of another manganite of a similar B50% Mn4+ content, Pr0.5Sr0.41Ca0.09MnO3. This compound, however, undergoes at lower temperatures second transition to the antiferromagnetic charge-ordered state and the Pbnm perovskite phase re-enters [8]. In contrast, the ferromagnetic I4/mcm phase in the present crystal Pr0.52Sr0.48MnO3 is stable down to the lowest temperatures. The Pr0.5Sr0.5MnO3 compound (x ¼ 0:50) exhibits the orthorhombic–tetragonal transition well above room temperature, i.e. in the paramagnetic state and the ferromagnetic transition at TC ¼ 265 K brings neither change of the I4/mcm crystal symmetry nor clear anomaly in the unit cell volume. The second transition to the A-type antiferromagnetic state is due to the onset of orbital polarization shown in Fig. 4, which is itself responsible for the change of symmetry from I4/ mcm to Fmmm [3]. While the tetragonal deformation c=aO2B1.02 of the I4/mcm phase in x ¼ 0:50 is temperature independent and can be attributed mainly to the effect of the MnO6 octahedral tilting, in the crystal of x ¼ 0:57 it increases with decreasing temperature, which manifests an increasing orbital polarization of the d3z2 r2 type, i.e. the eg electron density is extended along the c-axis (see the right part of Fig. 4). At the concomitant structural and antiferromagnetic transition at TN ¼ 217 K, the orbital polarization becomes of the dz2 y2 type (see the left part of Fig. 4). The eg electron density is then extended along two mutually perpendicular directionsFthe original c-axis of the I4/mcm hightemperature phase and one of the [1 1 0] diagonals (the bo - and co -axis in Fmmm cell). The transition is accompanied with an abrupt decrease of the unit cell volume. The specific heat data on crystals x ¼ 0:48; 0.50 and 0.57 are displayed in Fig. 7. The results in the upper panel show for x ¼ 0:48 a broadened l-like peak centered at 285 K, which well correlates with the middle of the ferromagnetic transition while no effect is observable at Tt ¼ 270 K. On the other hand, the coupled orbital and magnetic ordering in x ¼ 0:57 at TN ¼ 217 K is abrupt with strong d peak in the specific heat. Both kinds of the anomalies are present in x ¼ 0:50; but of much
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E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
Fig. 4. The eg orbital polarization in the A-type (0:48oxo0:60) and C-type (0:60oxo0:95) antiferromagnetic phases of Pr1xSrxMnO3.
weaker intensities. There is a rounded peak centered around 255 K, i.e. at temperature slightly lower than the measured TC ¼ 265 K and a sharp peak at TN ¼ 155 K. The low-temperature specific heat displayed in the lower panel of Fig. 7 shows nearly straight lines yielding by an extrapolation to zero of Cp =T vs. T 2 a reliable estimate of the linear in T contribution to Cp ; which in our case can be ascribed to the free charge carriers. The observed linear coefficient g ¼ 5:5 mJ K2 mol1 for the ferromagnetic 3D metallic ground state in x ¼ 0:48 is in a close agreement with the value 5.4 mJ K2 mol1 reported for the related ferromagnet Pr0.6Sr0.4MnO3 of the Pbnm symmetry [9] and comparable with the results published for other metallic manganites in the B30% Mn4+ range [10]. For the A-type antiferromagnetic
Table 2 Magnetic and electric properties of Pr1xSrxMnO3 single crystals x ¼ 0:48
x ¼ 0:50
x ¼ 0:57
Paramagnetic EA B14 meV Adiabatic hopping
Paramagnetic EA B17 meV Adiabatic hopping
Paramagnetic EA B39 meV Adiabatic hopping
Ferromagnetic TC ¼ 290 K r0 B55265 mO cm
Ferromagnetic TC ¼ 265 K r0 B130 mO cm
Antiferromagnetic A-type TN ¼ 217 K Quasi 2D metal r0 B1 mO cm
Antiferromagnetic A-type TN ¼ 155 K Quasi 2D metal r0 B1 mO cm
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
454
a t√2
x=0.57
x=0.48
3
ao
cell volume(Å )
7.5
Tt
I4/mcm
co
7.6
455
ct
bo
7.7
lattice parameters (Å)
TN
Fmmm
7.8
Fmmm
7.9
TN
I4/mcm
bo
7.8
ct
7.7
TC
co
7.6
a t√2
ao
453
452 TN x=0.50
x=0.50
TN
451 7.9
I4/mcm
7.8
Tt ao√ 2
7.7
bo√ 2 co
a t√2
0
50
x=0.57
Pbnm
x=0.48
ct
7.6
295
Tc 100
150 200 250
450 0
50
100 150 200 250 300 T (K)
300
T (K) Fig. 5. Temperature evolution of the Pr1xSrxMnO3 crystal structures. (The data for x ¼ 0:50 are taken from Ref. [8]).
ground state in x ¼ 0:57; which is essentially 2D conductor (see e.g. resistivity data on a single domain specimen of the related compound Nd0.46Sr0.54MnO3 [11]), also an appreciable value g ¼ 3:5 mJ K2 mol1 is observed. This is an evidence of a considerable density of states at Fermi level NðEÞ ¼ 0:95 eV1 per formula unit (1.7 1022 eV1 cm3) and gives an additional argument for the metallic (band-like) character of carriers in the A-type antiferromagnets. It is worth mentioning that the non-zero density at Fermi level was determined also in ab initio theoretical calculations using a simplified structural model [12]. The calculated value is, however, about twice smaller.
4. Conclusion The single crystals of the Pr1xSrxMnO3 manganites have been studied in a narrow
Fig. 6. Temperature dependence of the unit cell volume in Pr1xSrxMnO3.
compositional range 0:48pxp0:57 close to the boundary between the Pbnm and I4/mcm perovskite types where subtle changes of the steric distortion (t ¼ 0:984 for x ¼ 0:48 and t ¼ 0:993 for x ¼ 0:57) and Mn4+ content 48–57% are sufficient for considerable variation of the magnetic ground states and character of carriers. The end compositions x ¼ 0:48 and 0.57 are characterized as ideal examples of the 3D metallic ferromagnet and 2D metallic antiferromagnet in the compositionally close systems with the same MnO6 tilt pattern. A novelty represents the tetragonal I4/mcm symmetry of the ferromagnetic ground state in x ¼ 0:48 which exhibits different distribution of Mn-OMn bonds compared to more common orthoperovskites Ln1xCaxMnO3 (Ln=La or rare earths) or rhombohedral perovskites La1xSrxMnO3. Namely, in the latter two systems all three MnO-Mn bond angles are alike while for the present I4/mcm ferromagnet, as well as the Fmmm antiferromagnets, one Mn-O-Mn angle is 1801 and two others make about 1651 [3].
E. Pollert et al. / Journal of Magnetism and Magnetic Materials 246 (2002) 290–296
296
250 x=0.57 x=0.50 TN x=0.48
-1
cP (JK mol )
200
TC
-1
150 100
Tt
50 0
0
80
160 240 T (K)
TC 320
-1
15
cP /T ( mJK mol )
-2
25 20
Acknowledgements This work was supported by the grants 202/99/ 0413 of Grant Agency of the Czech Republic and A1010004/00 of Grant Agency of the Academy of Sciences of the Czech Republic.
x=0.57 x=0.50 x=0.48
10
References
5 0
conductivity in the metallic ferromagnetic planes. The character of the interlayer electronic transport remains an open question and represents a challenge for more detailed electronic structure calculations. Namely, it is of basic interest how the transport is influenced by the degree of spin polarization of carriers at Fermi level and their orbital polarization (which might be different from the average shown schematically in Fig. 4). An important role should play also the interlayer MnO-Mn bond angles, actually B1651.
0
20
40
60
80 100 120 140 2
2
T (K ) Fig. 7. Specific heat data showing the anomaly at TC for x ¼ 0:48 and 0.50 and a first order transition at TN ; observed for 0.50 and 0.57. For a clarity the dependences for x ¼ 0:5 and 0.57 are shifted of 25 and 50 J K1 mol1 upwards, respectively. The lower panel shows the Cp =T vs. T 2 graph which determines the g coefficient of the linear electronic term and the cubic lattice term of the low-temperature specific heat.
Another interesting issue is the occurrence of 2D metallic conductivity in the A-type antiferromagnet x ¼ 0:57: It should be noted that it was not possible to determine the anisotropy of the transport properties. The reason is the multidomain nature of the Fmmm phase, which appears to be a consequence of its I4/mcm parentage. The measured data thus reflect essentially the
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