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Magnetic properties of SmNiO3
jR
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 541 - 542
Journalof
magnetism and magnetic materials
Magnetic properties of SmNiO3 J. P6rez a, J. Blasco a'b'*, J. Garcia a, M. Castro a, J. Stankiewicz a, M.C. Sfinchez a, R.D. S/mchez c "Instituto de Ciencia de Materiales de Arag6n, Consejo Superior de Investigaciones Cientificas, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain b Servicio Nacional EXAFS, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain c Departamento de Fisiea Aplicada, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain
Abstract
SmNiO3 synthesized under high oxygen pressure has been studied from 1.8 to 650 K by means of both, DC magnetization and AC calorimetry. This compound shows a metal-insulator transition at 400 K and an antiferromagnetic ordering at lower temperatures. The DC magnetic susceptibility and the heat capacity clearly show the magnetic ordering at 220 K. In addition, a change in the slope of the M ( T ) curve is detected at the metal-insulator transition. The heat capacity measurements give an entropy content for the magnetic transition lower than the expected for a S = ½ magnetic ion. This result suggests strong antiferromagnetic correlations above TN in this material. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Antiferromagnetism; Perovskite; Metal-insulator transition; Orbital ordering
Mixed oxides are of great interest since they exhibit many interesting properties such as superconductivity, giant magnetoresistance, charge or spin ordering, etc. [1]. In particular, in RENiO3 (RE = rare earth) system exists a metal-insulator (MI) transition that strongly depends on the RE size [2]. LaNiO3 is metallic in the whole temperature range while PrNiO3 and NdNiO3 show the MI transition simultaneously with a complex antiferromagnetic ordering [3]. Finally, SmNiO3 and EuNiO3 develop the MI transition at temperatures higher than the magnetic ordering. Recently, some authors have argued that an orbital ordering induces electronic localization [4]. This is supported by the low-temperature magnetic ordering in these compounds that can only be explained by the existence of orbital superlattice [3,4]. We have prepared SmNiO3 in order to study both, the MI and the magnetic
* Corresponding address: Servicio Nacional EXAFS, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain. Fax: + 34-76-761229; e-mail: [email protected].
transitions, separately and tried to determine the role, if any, of the orbital ordering in the electronic localization which takes place above the magnetic transition. SmNiO3 was prepared following a sol-gel method described elsewhere [5]. The resulting powder was sintered at 1000°C for 12 h under oxygen pressure of 200 bar. X-ray diffraction (XRD) measurements were performed by using a Rigaku D-max system with a rotating anode. XRD patterns with high counting rates were collected. The sample was of single phase with an orthorhombic perovskite structure. The oxygen content was determined by thermogravimetric analysis in reducing flow. The material obtained showed a slight oxygen deficiency. The nominal composition was SmNiO2.96 ±0.02. A commercial quantum design (SQUID) magnetometer was used to measure DC magnetization at H = 1 T between 1.8 and 650 K. Calorimetric measurements were done with an AC calorimeter (ACCI-VL Sinku-Riko) from 5 to 250 K and with a DSC (Perkin-Elmer, DSC 7) up to 500 K. The DSC measurements were used to scale the relative heat-capacity values obtained from AC calorimetry. XRD patterns do not show any evidence
0304-8853/99/$ - see front matter @ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 8 4 - 1
542
J. P~rez et al.
Journal o f Magnetism atut Magnetic Materials 1 9 6 - 1 9 7 f 1999) 541 - 542
of superstructure peaks related to an orbital ordering of the Ni eg orbitals, in agreement with recent neutron diffraction experiments [4]. D S C measurements show a first-order transition at TM~ = 404 K that corresponds to the MI transition reported for this system [3]. The entropy content (AS) associated to this transition has been calculated to be approximately 0.2 R This value is similar to that obtained for N d N i O s in previous works [5-7]. However, we note that NdNiO3 develops simultaneously both, the MI and the magnetic transitions, so the magnetic entropy involved in the peak which determines the transition seems negligible with respect to the MI contribution. Heat capacity for SmNiO3 between 5 and 300 K is shown in Fig. 1. A clear anomaly is detected at 220 K corresponding to the magnetic ordering of the Ni 3 + ions. The non-magnetic heat-capacity contribution is plotted in the same figure as a solid line. It has been calculated by an interpolation method using an effective Debye temperature which changes smoothly with the temperature and allows an easy interpolation. The inset of Fig. 1 shows the heat capacity less than the non-magnetic contribution, ZXCp. ACp peaks at about 220 K. AS calculated between 180 and 230 K gives 0.03 R. This value is very low compared with the expected one for the magnetic ordering of Ni 3 + ions. Note that S = ½ for Ni 3 +, so AS = R ln2 = 0.69 R. A study of the magnetism associated with the Ni s* sublattice in the S m N i O s is difficult due to the simultaneous presence of Sm s +. The susceptibility of Sm s + ions, with a 6H5/2 ground state and 6H7/2 first excited one at low energy, does not follow a Curie-Weiss law. We have synthesized the isostructural SmScO3 c o m p o u n d in order to obtain the magnetic contribution of the Sm 3 ions. Fig. 2 shows the ZDC(ir) curves for SmNiO3 and SmScO3. The difference between them, shown in the inset of Fig. 2, is the Ni a + magnetic contribution. This curve peaks at 220 K, in agreement with the Neel temperature (TN) obtained for this c o m p o u n d by neutron diffraction measurements [4] and from AC calorimetry shown in Fig. 1. Below TN, the paramagnetic signal decreases down to 2 of the cusp value as is expected for a long-range antiferromagnetic ordering. Although above TN, the curve does not follow a Curie-Weiss law, a rough approximation of the effective magnetic moments (tt~rr) can be obtained from the slope of the 1/X vs. T curve. We obtain that #~rr increases with decrease in temperature from a value of ll~rr = 1.5 lab between 420 and 640 K (metallic phase) up to #~rr = 1.69 lab between 280 and 400 K (insulating phase). Both values are slightly lower than the expected ones for a S = ½ free paramagnetic ion (1.76 ~tB). Moreover, the Weiss constant is practically zero in the metallic phase while it is ~ - I l O K in the insulating phase. This is a hallmark of strong antiferromagnetic correlations when electronic localization is produced.
15
.,
r
10 ~ ,., i
5 ./
[
()
-J I()0
TIK)
200
~;11[
Fig. I. Cp vs. Yfor SmNiO3 (O)and the non-magnetic contribution (solid line). Inset: Excess in the heat capacity for SmNiO.~ after subtracting the non-magnetic contribution.
r-
"1' " ~ l u s ~ t qJ
]{lll T
IKI
Fig. 2. DC susceptibility vs. T for SmNiO3 and SmScO3 Z tSmScOsj.
(tt = 1 T ). Inset: Zsub VS. T, Zsub = 7, (SmNiOsl
In conclusion, we have studied SmNiO3 by DC magnetization and AC calorimetry. Our results indicatc strong antiferromagnetic correlations for this system above TN which appear simultaneously with the electronic localization. Moreover, the very small value of AS for the magnetic transition agrees with the existence of short-range antiferromagnetic correlations in the paramagnetic insulating phase. These results suggest a relationship between MI transition and short-range antiferromagnetic correlations. D G Y C I T financial support under MAT96-0491 and MAT97-0987 projects is acknowledged.
References [1] N. Tsuda et al., Electronic Conduction in Oxides, Springer, Berlin, 1991. [2] P. Lacorre et al., J. Solid State Chem. 91 (1991) 225. [3] J.L. Garcia-Mufioz et al., Europhys. Lett. 20 (1992) 241. E4] J. Rodriguez-Carvajal et al.. Phys. Rev B 57 (19981 456. [5] J. P6rez et al., J. Phys.: Condens. Matter 8 t1996) 10393. [6] J. Blasco et al., J. Phys.: Condens. Matter 6 (1994) 5875. E7] X. Granados et al., Phys. Rev. B 48 (1993) 11666.
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 541 - 542
Journalof
magnetism and magnetic materials
Magnetic properties of SmNiO3 J. P6rez a, J. Blasco a'b'*, J. Garcia a, M. Castro a, J. Stankiewicz a, M.C. Sfinchez a, R.D. S/mchez c "Instituto de Ciencia de Materiales de Arag6n, Consejo Superior de Investigaciones Cientificas, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain b Servicio Nacional EXAFS, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain c Departamento de Fisiea Aplicada, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain
Abstract
SmNiO3 synthesized under high oxygen pressure has been studied from 1.8 to 650 K by means of both, DC magnetization and AC calorimetry. This compound shows a metal-insulator transition at 400 K and an antiferromagnetic ordering at lower temperatures. The DC magnetic susceptibility and the heat capacity clearly show the magnetic ordering at 220 K. In addition, a change in the slope of the M ( T ) curve is detected at the metal-insulator transition. The heat capacity measurements give an entropy content for the magnetic transition lower than the expected for a S = ½ magnetic ion. This result suggests strong antiferromagnetic correlations above TN in this material. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Antiferromagnetism; Perovskite; Metal-insulator transition; Orbital ordering
Mixed oxides are of great interest since they exhibit many interesting properties such as superconductivity, giant magnetoresistance, charge or spin ordering, etc. [1]. In particular, in RENiO3 (RE = rare earth) system exists a metal-insulator (MI) transition that strongly depends on the RE size [2]. LaNiO3 is metallic in the whole temperature range while PrNiO3 and NdNiO3 show the MI transition simultaneously with a complex antiferromagnetic ordering [3]. Finally, SmNiO3 and EuNiO3 develop the MI transition at temperatures higher than the magnetic ordering. Recently, some authors have argued that an orbital ordering induces electronic localization [4]. This is supported by the low-temperature magnetic ordering in these compounds that can only be explained by the existence of orbital superlattice [3,4]. We have prepared SmNiO3 in order to study both, the MI and the magnetic
* Corresponding address: Servicio Nacional EXAFS, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain. Fax: + 34-76-761229; e-mail: [email protected].
transitions, separately and tried to determine the role, if any, of the orbital ordering in the electronic localization which takes place above the magnetic transition. SmNiO3 was prepared following a sol-gel method described elsewhere [5]. The resulting powder was sintered at 1000°C for 12 h under oxygen pressure of 200 bar. X-ray diffraction (XRD) measurements were performed by using a Rigaku D-max system with a rotating anode. XRD patterns with high counting rates were collected. The sample was of single phase with an orthorhombic perovskite structure. The oxygen content was determined by thermogravimetric analysis in reducing flow. The material obtained showed a slight oxygen deficiency. The nominal composition was SmNiO2.96 ±0.02. A commercial quantum design (SQUID) magnetometer was used to measure DC magnetization at H = 1 T between 1.8 and 650 K. Calorimetric measurements were done with an AC calorimeter (ACCI-VL Sinku-Riko) from 5 to 250 K and with a DSC (Perkin-Elmer, DSC 7) up to 500 K. The DSC measurements were used to scale the relative heat-capacity values obtained from AC calorimetry. XRD patterns do not show any evidence
0304-8853/99/$ - see front matter @ 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 8 8 4 - 1
542
J. P~rez et al.
Journal o f Magnetism atut Magnetic Materials 1 9 6 - 1 9 7 f 1999) 541 - 542
of superstructure peaks related to an orbital ordering of the Ni eg orbitals, in agreement with recent neutron diffraction experiments [4]. D S C measurements show a first-order transition at TM~ = 404 K that corresponds to the MI transition reported for this system [3]. The entropy content (AS) associated to this transition has been calculated to be approximately 0.2 R This value is similar to that obtained for N d N i O s in previous works [5-7]. However, we note that NdNiO3 develops simultaneously both, the MI and the magnetic transitions, so the magnetic entropy involved in the peak which determines the transition seems negligible with respect to the MI contribution. Heat capacity for SmNiO3 between 5 and 300 K is shown in Fig. 1. A clear anomaly is detected at 220 K corresponding to the magnetic ordering of the Ni 3 + ions. The non-magnetic heat-capacity contribution is plotted in the same figure as a solid line. It has been calculated by an interpolation method using an effective Debye temperature which changes smoothly with the temperature and allows an easy interpolation. The inset of Fig. 1 shows the heat capacity less than the non-magnetic contribution, ZXCp. ACp peaks at about 220 K. AS calculated between 180 and 230 K gives 0.03 R. This value is very low compared with the expected one for the magnetic ordering of Ni 3 + ions. Note that S = ½ for Ni 3 +, so AS = R ln2 = 0.69 R. A study of the magnetism associated with the Ni s* sublattice in the S m N i O s is difficult due to the simultaneous presence of Sm s +. The susceptibility of Sm s + ions, with a 6H5/2 ground state and 6H7/2 first excited one at low energy, does not follow a Curie-Weiss law. We have synthesized the isostructural SmScO3 c o m p o u n d in order to obtain the magnetic contribution of the Sm 3 ions. Fig. 2 shows the ZDC(ir) curves for SmNiO3 and SmScO3. The difference between them, shown in the inset of Fig. 2, is the Ni a + magnetic contribution. This curve peaks at 220 K, in agreement with the Neel temperature (TN) obtained for this c o m p o u n d by neutron diffraction measurements [4] and from AC calorimetry shown in Fig. 1. Below TN, the paramagnetic signal decreases down to 2 of the cusp value as is expected for a long-range antiferromagnetic ordering. Although above TN, the curve does not follow a Curie-Weiss law, a rough approximation of the effective magnetic moments (tt~rr) can be obtained from the slope of the 1/X vs. T curve. We obtain that #~rr increases with decrease in temperature from a value of ll~rr = 1.5 lab between 420 and 640 K (metallic phase) up to #~rr = 1.69 lab between 280 and 400 K (insulating phase). Both values are slightly lower than the expected ones for a S = ½ free paramagnetic ion (1.76 ~tB). Moreover, the Weiss constant is practically zero in the metallic phase while it is ~ - I l O K in the insulating phase. This is a hallmark of strong antiferromagnetic correlations when electronic localization is produced.
15
.,
r
10 ~ ,., i
5 ./
[
()
-J I()0
TIK)
200
~;11[
Fig. I. Cp vs. Yfor SmNiO3 (O)and the non-magnetic contribution (solid line). Inset: Excess in the heat capacity for SmNiO.~ after subtracting the non-magnetic contribution.
r-
"1' " ~ l u s ~ t qJ
]{lll T
IKI
Fig. 2. DC susceptibility vs. T for SmNiO3 and SmScO3 Z tSmScOsj.
(tt = 1 T ). Inset: Zsub VS. T, Zsub = 7, (SmNiOsl
In conclusion, we have studied SmNiO3 by DC magnetization and AC calorimetry. Our results indicatc strong antiferromagnetic correlations for this system above TN which appear simultaneously with the electronic localization. Moreover, the very small value of AS for the magnetic transition agrees with the existence of short-range antiferromagnetic correlations in the paramagnetic insulating phase. These results suggest a relationship between MI transition and short-range antiferromagnetic correlations. D G Y C I T financial support under MAT96-0491 and MAT97-0987 projects is acknowledged.
References [1] N. Tsuda et al., Electronic Conduction in Oxides, Springer, Berlin, 1991. [2] P. Lacorre et al., J. Solid State Chem. 91 (1991) 225. [3] J.L. Garcia-Mufioz et al., Europhys. Lett. 20 (1992) 241. E4] J. Rodriguez-Carvajal et al.. Phys. Rev B 57 (19981 456. [5] J. P6rez et al., J. Phys.: Condens. Matter 8 t1996) 10393. [6] J. Blasco et al., J. Phys.: Condens. Matter 6 (1994) 5875. E7] X. Granados et al., Phys. Rev. B 48 (1993) 11666.