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Magnetic properties and electron-transport properties in Fe0.92Cr2S4
Journal of Magnetism and Magnetic Materials 226}230 (2001) 518}520
Magnetic properties and electron-transport properties in Fe Cr S
Sam Jin Kim , Woo Chul Kim , Bo Wha Lee, Jung Chul Sur, Chul Sung Kim * Department of Physics, Kookmin University, 861-1 Chong rungdong, Songbuk-gu Seoul 136-702, South Korea Department of Physics, Hankuk University of Foreign Studies, Yongin, Kyungki, 449-791, South Korea Department of Physics, Wonkwang University, Jeonbuk 570-749, South Korea
Abstract Sample of Fe Cr S has been studied with MoK ssbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), SQUID magnetometer, and magnetoresistance (MR). The crystal structure was cubic spinel with its lattice constant a "9.9925(2) As . The maximum MR ratio was observed at 183 K about 12% under 1.6 T. The MoK ssbauer spectra were recorded from 18 K to room temperature. Below the Curie temperature the asymmetric line broadening is observed and considered to be dynamic Jahn}Teller distortion. Isomer shift value of the sample at room temperature was 0.53 mm/s, which means that charge state of Fe ions is ferrous in character. The conduction mechanism in this sample is di!erent from the double exchange mechanism in a point that there were no mixed iron charge valences. 2001 Elsevier Science B.V. All rights reserved. Keywords: MoK ssbauer spectroscopy; Magnetoresistance; Jahn}Teller e!fect
Recently, the colossal magnetoresistance (CMR) e!ect in sulfur spinel has been reported, and it is suggested that the conducting mechanism in this materials may not be caused by the double exchange (DE) of the carriers because there is no valence variation in stoichiometry as in the Mn perovskite structure [1]. MoK ssbauer studies on FeCr S have been reported already by many previous workers [2,3]. But the MoK ssbauer spectra on FeCr S were found to be dependent on the sample, furthermore, some of the papers reported the line broadening originating from the excess of B site Fe>. These di$culties may be eliminated by starting with a small iron de"ciency [4]. Here we present the results of MoK ssbauer experiments and compare them with those of X-ray, magnetoresistance, SQIUD magnetometer, and X-ray photoelectron spectrum (XPS). Fe Cr S was prepared by the usual direct reaction of the high-purity elements Fe (99.999%), Cr (99.999%),
* Corresponding author. Tel. : #82-2-910-4728; fax: #82-2910-4752. E-mail address: [email protected] (C.S. Kim).
and S (99.9999%) in an evacuated quartz tube. The compositional stoichiometry of the samples was con"rmed by XPS analysis. The MoK ssbauer spectra were recorded using a conventional spectrometer with a Co source in a rhodium matrix. Magnetoresistance and magnetization were measured with the Van Der Pauw method and a SQUID magnetometer, respectively. The crystal structure of the sample at room temperature is determined to be the normal cubic spinel structure with its lattice constant a "9.9925(2) As . The "nal oxygen parameter u and disagreement factor R were 0.3695(1) and 0.1342(1), respectively. Fig. 1(a) shows the temperature dependence of zero"eld resistance, and resistance measured in a "eld of 1.6 T. It shows metallic behavior in the temperature range between 140 and 185 K, and semiconducting behavior at ¹'185 K and ¹(140 K. Fig. 1(b) shows the temperature dependence of magnetization for the sample under the external "eld 5 kOe. Also, the inset of Fig. 1(b) shows the enlarged shape between 4 and 20 K. This cusp-like anomaly at 10 K is explained by changing from static to dynamic Jahn}Teller stabilization [3]. As shown in Fig. 1(a), the maximum magnetoresistance temper-
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 9 9 4 - X
S.J. Kim et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 518}520
Fig. 1. (a) Temperature dependence of zero-"eld resistance R and magnetoresistance MR at 1.6 T for Fe Cr S , where & MR "(R(0)!R(1.6 ¹))/R(0). (b) Temperature dependence of & magnetization M( ) at 5 kOe external "eld. The inset shows the enlarged data between 4 and 20 K.
ature(¹ ) was at 183 K with its MR ratio being 12%. + Assuming a simple exponential temperature dependence of resistance in the form R(¹)"R exp(E /k ¹), a "t to M the data of Fig .1(a) above 185 K yields activation energy E "58 meV [1]. M In order to study the conduction mechanism and local properties of iron ions, MoK ssbauer spectra of the sample were measured at various absorber temperatures from 18 K to room temperature. Some of the representative spectra are shown in Fig. 2. MoK ssbauer spectra were analyzed using the eight Lorentzians "tting [5]. The results of computer analysis are shown in Fig. 3. The isomer shift at room temperature is 0.53 mm/s relative to Fe metal, which means that the valance state of Fe ions is ferrous in character. The magnetic hyper"ne "eld at 13 K is 183 kOe, this value corresponds to the typical ferrous ion. This an important di!erent point in comparison to Mn perovskite structure, in which the conduction mechanism is accompanied by the heterogenous Mn> and Mn> with ferromagnetic DE coupling. But in this sample magnetic structure is ferrimagnetic and there are no iron species other than Fe>. Also, we note that the quadrupole splitting E rapidly / decreases with increasing temperature in Fig. 3. In the tetragonal site, the ground state of Fe> is in a degenerate orbital doublet E . This situation is unstable and must be resolved via a Jahn}Teller distortion, which may
519
Fig. 2. MoK ssbauer spectra for Fe Cr S at various temper atures.
Fig. 3. Temperature dependence of the quadrupole splitting and magnetic hyper"ne "elds for Fe Cr S
explain the observed appearance of quadrupole splitting below ¹ . However, a low-temperature X-ray photo! graph for FeCr S showed that no static Jahn}Teller distortion occurs above 12 K [6]. Even though no static Jahn}Teller distortion is observed, dynamic Jahn}Teller distortion can cause a quadrupole shift if the distortion relaxes between a number of equivalent crystal axes [7]. This kind of relaxation is consistent with the asymmetrical line broadening apparent in Fig. 2. In summary, we have studied magnetic properties and conduction mechanism in Fe Cr S . The MoK ssbauer
520
S.J. Kim et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 518}520
spectra result is explained in terms of the dynamic Jahn}Teller e!ect. The evidence that iron ions are ferrous in character led to the conclusion that conduction mechanism in this sample is di!erent from Mn perovskite structure. The present study was supported by the Korea Science and Engineering Foundation (97-0702-0401-5), by the KISTEP (HANARO Reactor Program) and by the Brain Korea 21 Program.
References [1] A.P. Ramirez, R.J. Cava, J. Krajewski, Nature 386 (1997) 156.
[2] A.M. Van Diepen, R.P. Van stapele, Solid State Commun. 13 (1973) 1651. [3] M.R. Spender, A.H. Morrish, Can. J. Phys. 50 (1972) 1125. [4] F.K. Lotgering, A.M. van Diepen, J.F. Olijhohoek, Solid State Commun. 11 (1972) 1417. [5] C.S. Kim, I.B. Shim, M.Y. Ha, C.S. Kim, J.Y. Park, J. Appl. Phys. 73 (1993) 5707. [6] P. Gibart, A. Beguoen-Demeaux, C. R. Acad. Sci. (France) 268C (1969) 816. [7] J.A. Tjon, M. Blume, Phys. Rev. 165 (1968) 456.
Magnetic properties and electron-transport properties in Fe Cr S
Sam Jin Kim , Woo Chul Kim , Bo Wha Lee, Jung Chul Sur, Chul Sung Kim * Department of Physics, Kookmin University, 861-1 Chong rungdong, Songbuk-gu Seoul 136-702, South Korea Department of Physics, Hankuk University of Foreign Studies, Yongin, Kyungki, 449-791, South Korea Department of Physics, Wonkwang University, Jeonbuk 570-749, South Korea
Abstract Sample of Fe Cr S has been studied with MoK ssbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), SQUID magnetometer, and magnetoresistance (MR). The crystal structure was cubic spinel with its lattice constant a "9.9925(2) As . The maximum MR ratio was observed at 183 K about 12% under 1.6 T. The MoK ssbauer spectra were recorded from 18 K to room temperature. Below the Curie temperature the asymmetric line broadening is observed and considered to be dynamic Jahn}Teller distortion. Isomer shift value of the sample at room temperature was 0.53 mm/s, which means that charge state of Fe ions is ferrous in character. The conduction mechanism in this sample is di!erent from the double exchange mechanism in a point that there were no mixed iron charge valences. 2001 Elsevier Science B.V. All rights reserved. Keywords: MoK ssbauer spectroscopy; Magnetoresistance; Jahn}Teller e!fect
Recently, the colossal magnetoresistance (CMR) e!ect in sulfur spinel has been reported, and it is suggested that the conducting mechanism in this materials may not be caused by the double exchange (DE) of the carriers because there is no valence variation in stoichiometry as in the Mn perovskite structure [1]. MoK ssbauer studies on FeCr S have been reported already by many previous workers [2,3]. But the MoK ssbauer spectra on FeCr S were found to be dependent on the sample, furthermore, some of the papers reported the line broadening originating from the excess of B site Fe>. These di$culties may be eliminated by starting with a small iron de"ciency [4]. Here we present the results of MoK ssbauer experiments and compare them with those of X-ray, magnetoresistance, SQIUD magnetometer, and X-ray photoelectron spectrum (XPS). Fe Cr S was prepared by the usual direct reaction of the high-purity elements Fe (99.999%), Cr (99.999%),
* Corresponding author. Tel. : #82-2-910-4728; fax: #82-2910-4752. E-mail address: [email protected] (C.S. Kim).
and S (99.9999%) in an evacuated quartz tube. The compositional stoichiometry of the samples was con"rmed by XPS analysis. The MoK ssbauer spectra were recorded using a conventional spectrometer with a Co source in a rhodium matrix. Magnetoresistance and magnetization were measured with the Van Der Pauw method and a SQUID magnetometer, respectively. The crystal structure of the sample at room temperature is determined to be the normal cubic spinel structure with its lattice constant a "9.9925(2) As . The "nal oxygen parameter u and disagreement factor R were 0.3695(1) and 0.1342(1), respectively. Fig. 1(a) shows the temperature dependence of zero"eld resistance, and resistance measured in a "eld of 1.6 T. It shows metallic behavior in the temperature range between 140 and 185 K, and semiconducting behavior at ¹'185 K and ¹(140 K. Fig. 1(b) shows the temperature dependence of magnetization for the sample under the external "eld 5 kOe. Also, the inset of Fig. 1(b) shows the enlarged shape between 4 and 20 K. This cusp-like anomaly at 10 K is explained by changing from static to dynamic Jahn}Teller stabilization [3]. As shown in Fig. 1(a), the maximum magnetoresistance temper-
0304-8853/01/$ - see front matter 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 9 9 4 - X
S.J. Kim et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 518}520
Fig. 1. (a) Temperature dependence of zero-"eld resistance R and magnetoresistance MR at 1.6 T for Fe Cr S , where & MR "(R(0)!R(1.6 ¹))/R(0). (b) Temperature dependence of & magnetization M( ) at 5 kOe external "eld. The inset shows the enlarged data between 4 and 20 K.
ature(¹ ) was at 183 K with its MR ratio being 12%. + Assuming a simple exponential temperature dependence of resistance in the form R(¹)"R exp(E /k ¹), a "t to M the data of Fig .1(a) above 185 K yields activation energy E "58 meV [1]. M In order to study the conduction mechanism and local properties of iron ions, MoK ssbauer spectra of the sample were measured at various absorber temperatures from 18 K to room temperature. Some of the representative spectra are shown in Fig. 2. MoK ssbauer spectra were analyzed using the eight Lorentzians "tting [5]. The results of computer analysis are shown in Fig. 3. The isomer shift at room temperature is 0.53 mm/s relative to Fe metal, which means that the valance state of Fe ions is ferrous in character. The magnetic hyper"ne "eld at 13 K is 183 kOe, this value corresponds to the typical ferrous ion. This an important di!erent point in comparison to Mn perovskite structure, in which the conduction mechanism is accompanied by the heterogenous Mn> and Mn> with ferromagnetic DE coupling. But in this sample magnetic structure is ferrimagnetic and there are no iron species other than Fe>. Also, we note that the quadrupole splitting E rapidly / decreases with increasing temperature in Fig. 3. In the tetragonal site, the ground state of Fe> is in a degenerate orbital doublet E . This situation is unstable and must be resolved via a Jahn}Teller distortion, which may
519
Fig. 2. MoK ssbauer spectra for Fe Cr S at various temper atures.
Fig. 3. Temperature dependence of the quadrupole splitting and magnetic hyper"ne "elds for Fe Cr S
explain the observed appearance of quadrupole splitting below ¹ . However, a low-temperature X-ray photo! graph for FeCr S showed that no static Jahn}Teller distortion occurs above 12 K [6]. Even though no static Jahn}Teller distortion is observed, dynamic Jahn}Teller distortion can cause a quadrupole shift if the distortion relaxes between a number of equivalent crystal axes [7]. This kind of relaxation is consistent with the asymmetrical line broadening apparent in Fig. 2. In summary, we have studied magnetic properties and conduction mechanism in Fe Cr S . The MoK ssbauer
520
S.J. Kim et al. / Journal of Magnetism and Magnetic Materials 226}230 (2001) 518}520
spectra result is explained in terms of the dynamic Jahn}Teller e!ect. The evidence that iron ions are ferrous in character led to the conclusion that conduction mechanism in this sample is di!erent from Mn perovskite structure. The present study was supported by the Korea Science and Engineering Foundation (97-0702-0401-5), by the KISTEP (HANARO Reactor Program) and by the Brain Korea 21 Program.
References [1] A.P. Ramirez, R.J. Cava, J. Krajewski, Nature 386 (1997) 156.
[2] A.M. Van Diepen, R.P. Van stapele, Solid State Commun. 13 (1973) 1651. [3] M.R. Spender, A.H. Morrish, Can. J. Phys. 50 (1972) 1125. [4] F.K. Lotgering, A.M. van Diepen, J.F. Olijhohoek, Solid State Commun. 11 (1972) 1417. [5] C.S. Kim, I.B. Shim, M.Y. Ha, C.S. Kim, J.Y. Park, J. Appl. Phys. 73 (1993) 5707. [6] P. Gibart, A. Beguoen-Demeaux, C. R. Acad. Sci. (France) 268C (1969) 816. [7] J.A. Tjon, M. Blume, Phys. Rev. 165 (1968) 456.