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Magnetic properties and Mössbauer studies of Bi2Sr3Fe2O9+δ, isostructural with Bi-2122 superconductors
Physica B 262 (1999) 410—414
Magnetic properties and Mo¨ssbauer studies of Bi Sr Fe O , >B isostructural with Bi-2122 superconductors D. Hechel*, I. Felner The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 29 June 1998
Abstract We have investigated Bi Sr Fe O materials by complementary experimental techniques. The materials are derived >B from the superconducting Bi Sr CaCu O (Bi-2122) by replacing all Cu by Fe and Ca atoms by Sr. The materials studied are not superconducting, but rather magnetically ordered. Mo¨ssbauer spectroscopy (MS) and magnetic-susceptibility studies reveal, that the magnetic ordering temperature (¹ ) is very sensitive to oxygen concentration. ¹ "100(5) K , , (determined by MS) for the oxygen-rich sample, is shifted towards 610 K when oxygen is depleted. Magnetic susceptibility data yield somewhat lower ¹ values (80 and 520 K, respectively). 1999 Published by Elsevier Science B.V. , All rights reserved. PACS: 74.10#V; 74.62Bf; 75.50Ee; 76.80#y Keywords: Magnetic properties; Mo¨ssbauer spectroscopy; Bi Sr Fe O d ; Superconductors >
1. Introduction The discovery of the high-¹ cuprates [1] ini tiated also the study of related compounds in order to understand hopefully more the interesting properties of these superconducting oxides. One of the striking features is the interplay between superconductivity (SC) and antiferromagnetic (AFM) order. Mo¨ssbauer spectroscopy (MS) of a Fe probe is a very powerful tool to determine the appearance of magnetism at Cu-sites. It was shown that in various cationic substitutions in oxygen-rich YBa Cu O * Corresponding author. Tel.: #972-2-6585752; fax: #9722-6586347; e-mail: [email protected].
(YBCO), whenever superconductivity disappears due to doping out of the CuO planes, the Cu(2) moments order antiferromagnetically. A similar phase diagram was established in the La—Sr—Cu—O system and in several others more [2]. In the 110 K superconductor Bi Sr CaCu O (Bi-2212), successive substitution of Ca by Y changes the electronic and magnetic properties, and a similar phase diagram of SC and AFM has been obtained, with ¹ "385 K in Bi Sr , YCu O [3]. In YBCO partial substitution of Cu >B by Fe decreases ¹ , where superconductivity disap pears for a Fe concentration above 13% at the Cu-sites. For slightly higher iron concentration, AFM was observed with ¹ "420 K [4]. The next , interesting step is the full substitution of the
0921-4526/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 5 5 - 7
D. Hechel, I. Felner / Physica B 262 (1999) 410—414
copper site by iron atoms. In the RBa Fe O (R"Y, Eu) compounds the iron moments order antiferromagnetically at the Ne´el temperature ¹ "700 K [5,10]. , Much attention has been attracted to a phase resembling the superconducting Bi Sr CaCu O (Bi-2122) having the composition Bi Sr Fe O . >B The Bi Sr Fe O structure evolves from the Bi >B 2122 structure by replacing all copper by iron and divalent Ca ions by Sr. Substitution of only Cu by Fe yields the composition Bi Sr CaFe O >B which has been studied by Sedykh et al [6], who measured also the MS at room temperature (RT). They show that the MS spectrum is composed of three paramagnetic doublets, and no magnetic hyperfine sextet is observed. On the other hand, in the polycrystalline Bi Sr Fe O , whose structure >B was determined by LePage et al. [7], the MS spectrum at RT contains a well defined singlet and two doublets [8]. The main purpose of this study, employing several experimental techniques, is to study in more details the magnetic properties of the Bi Sr Fe O system. Magnetic studies on oxygen-rich >B sample exhibit only one anomaly, at relatively low temperature ¹ "80 K which is attributed to mag, netic ordering of the Fe sublattices. ¹ is extremely , sensitive to oxygen concentration, and for oxygenreduced samples shifts to 520 K. MS reveal that the ¹ values are somewhat higher 90—100 and 610 K. , 2. Experimental details Ceramic samples with a nominal composition Bi Sr Fe O were prepared by solid state reaction techniques. Prescribed amounts of Bi O , SrCO , and Fe (metal) were mixed and pressed into pellets and preheated at 900°C for about 1 day. The products were cooled, reground (at each step) and sintered at 1100°C and 1080°C for 24 and 72 h at 1050°C. A part of the as-prepared sample were post annealed at 1050°C for 2 h and then quenched into liquid nitrogen. This is referred to as oxygen reduced “quenched” samples. The other half was cooled in a rate of 200°C/h from 1050 to 200°C, in flowing oxygen and denoted as the “annealed” sample. The oxygen content was not determined.
411
Powder X-ray diffraction (XRD) measurements indicate that both Bi Sr Fe O materials are near ly single phase (&95%) and have the orthorhombic structure. A few unidentified additional peaks were observed in the XRD pattern and our attempts to get completely rid of them were unsuccessful. The DC magnetic measurements on solid ceramic pieces in the range of 5—600 K were performed in a commercial (Quantum Design) superconducting quantum interference device magnetometer (SQUID). The magnetization was measured by two different procedures: (a) The sample was zero field cooled (ZFC) to 5 K, a field was applied and the magnetization was measured as a function of temperature; (b) The sample was field cooled (FC) from above 300 to 5 K and then the magnetization was measured. The Mo¨ssbauer spectroscopic studies were carried out using a conventional constant acceleration spectrometer. The Fe MS were measured with a 100 mCi Co : Rh source. The Fe isomer shifts (IS) are reported with respect to metallic Fe.
3. Experimental results 3.1. Lattice parameters and oxygen content XRD studies show that Bi Sr Fe O has an >B orthorhombic structure and the lattice parameters are listed in Table 1. Note the contraction of the c-axis of the annealed sample. The lattice parameters for this material are in excellent agreement with data published in Ref. [8]. Determination of the absolute oxygen content in these materials is difficult. 3.2. Magnetic measurements The ZFC and FC magnetic susceptibilities, measured at 100 Oe of the annealed Bi Sr Fe O >B sample are shown in Fig. 1. In both branches, only one distinct peak at ¹ "79 K is observed, and no , other anomalies were observed at higher temperatures. As will be shown below, the Fe sublattice is magnetically ordered, therefore, the magnetic anomaly in these curves is related to the Fe sites. ¹ (Fe) , is defined as the merging temperature of the ZFC
1.02 0.63 0.18 1.04 0.52 536 483 31.90 5.486 Quenched
5.550
5.461 Annealed
5.517
31.59
0.53 0.64 0.64 0.34 0.63
0.18 1.13 0.24 !0.01 !0.30
254
59 27 14 56 44
0.61 0.37 0.17 0.45 0.33
0.49 0.44
0.29 0.25 0.17 0.07 0.13 0.37
D""eqQ" (mm/s) $0.01 IS (mm/s) $0.01 a (As ) $0.005
b (As ) $0.005
c (As ) $0.04
Width (mm/s) $0.01
D""eqQ" (mm/s) $0.01
H (kOe) $5
Intensity IS (mm/s) (%) $0.01
Width (mm/s) $0.01
Hyperfine parameters above ¹ Hyperfine parameters at 90 K Lattice parameters Compound
Table 1 Lattice constants and hyperfine parameters deducted from Mo¨ssbauer measurements for the annealed and quenched Bi Sr Fe O samples
50 31 19 63 37
D. Hechel, I. Felner / Physica B 262 (1999) 410—414 Intensity (%)
412
Fig. 1. Temperature dependence of the magnetic moment of annealed Bi Sr Fe O . Insert shows isotherm magnetization M(H) curve at 5 K.
and FC branches. The irreversibility probably arises from the AFM alignment of the Fe sublattice. It is assumed that in the FC process the external field causes the spins to cant slightly out of their original direction. This canting abruptly aligns a component of the moments with the direction of the field, and the FC branch is obtained. Above ¹ , the susceptibility s(T ) of Bi Sr , Fe O compounds adheres closely to the >B Curie—Weiss law over the major portion of the measured temperatures. s(T ) can be well described by s(¹)"s #C/(¹!h), where s is the temper ature independent part of the susceptibility, C is the molar Curie constant and h is the Curie—Weiss temperature. The fit of s(T ) (in the range 100 K(T( 300 K) yields: s "2.48;10\ emu/mol Oe, C"1.27 emu K/mol Oe, and h"!19 K. Since the other cations (Bi> and Sr>) are non-magnetic, the whole paramagnetic moment is presumed to be due to the Fe ions. The effective magnetic moment derived from C, P "2.25k , is attributed to Fe>. The almost linear virgin magnetization curve at 5 K up to 50 kOe (Fig. 1, inset), and the negative value obtained for h, both indicate AFM ordering in the Fe sites. It appears that a slight deficiency of oxygen, shifts ¹ (Fe) to higher temperatures. The ,
D. Hechel, I. Felner / Physica B 262 (1999) 410—414
Fig. 2. The variation of the magnetic moment (in arbitrary units) of quenched Bi Sr Fe O measured at elevated temper atures.
maximum in the magnetization curve of the quenched sample is shifted to about 520 K (Fig. 2), and no anomaly is observed at lower temperatures. In that respect, the present system behaves in a manner similar to the well known YBCO [9]. The magnetic results, shown in Fig. 2, confirm our MS studies which are displayed and discussed in the next section. 3.3. Mo¨ ssbauer studies The Fe Mo¨ssbauer spectra of the annealed sample below and above ¹ are presented in Fig. 3, , and the hyperfine parameters obtained are listed in Table 1. At 4.2 K, the spectrum is splitted into two magnetic subspectra with a relative intensity ratio of 3 : 1. At 85 K the spectrum is a complicated one, because partly the sextet has already collapsed into a doublet. This result indicates a distribution of magnetic ordering temperatures. This is probably due to the inhomogenity of the oxygen content. At 300 K the spectrum is composed of three doublets with quadrupole splittings of eqQ" D"1.02, 0.63 and 0.18 mm/s. These values compare well with those obtained by Sedykh et al. [8]. However, in contrast to the small splitting of the third doublet in our sample, they observed a small singlet.
413
Fig. 3. Mo¨ssbauer spectra of annealed Bi Sr Fe O , at various temperatures. Note the extended scale at 300 K.
These parameters (Table 1) indicate that the Featoms are in the trivalent state. The different values of the two large doublets are a result of different local environment of the Fe atoms, probably due to inhomogenity of the oxygen distribution. Sedykh et al. concluded that their singlet is connected with an additional phase, since this singlet was absent in single crystals. In the next paragraph we will show that this third doublet does not appear in the quenched sample. Thus in our opinion, it is not clear whether this third subspectrum belongs to an additional phase or whether it represents another oxygen environment of Fe in Bi Sr Fe O in the annealed phase only. A different picture we find for the quenched sample. The Mo¨ssbauer spectra at various temperatures are presented in Fig. 4. The spectra at 4.2 (not shown) and 90 K are composed of two magnetic subspectra. Table 1 shows that at 90 K, the major sextet, which counts 56% of the spectral area, has an effective magnetic field H (1)" 536 kOe, whereas for the minor sextet H (2)" 483 kOe. As the temperatures are raised, both magnetic splittings decrease and disappear around 610 K. At 650 K, the paramagnetic spectrum is composed of two doublets (see Table 1). By comparing the intensities of the subspectra, we can assume,
414
D. Hechel, I. Felner / Physica B 262 (1999) 410—414
the Fe) does not appear in the quenched sample. Thus, in contrast to Sedykh et al., we conclude that this doublet belongs to Bi Sr Fe O . 4. Conclusions We performed susceptibility and Mo¨ssbauer studies of Bi Sr Fe O . The results show that the heat treatment is crucial for the magnetic behavior of the samples. The annealed sample is antiferromagnetically ordered at ¹ "100 K, whereas the , quenched sample has ¹ "610$10 K. These , ¹ values obtained by MS are somewhat higher , that the values obtained by DC magnetization measurements. The Mo¨ssbauer spectra of the annealed sample show three subspectra, and for the quenched sample we found two sites only.
Acknowledgements
Fig. 4. Mo¨ssbauer spectra of quenched Bi Sr Fe O at various temperatures.
that the major and the minor sextets collapse into the doublets with D"1.02 and 0.52 mm/s, respectively. Using the relation: D "D(3 cosh!1)/2, where D is the effective quadrupole interaction below ¹ , and h is the hyperfine field orientation , relative to the c-axis, we obtain for the quenched sample, h "55° for the first site, and h "90° for the second site. The D values of the quenched sample are quite similar to the parameters of main doublets of the annealed sample. However, the third doublet of the annealed sample with D"0.18 mm/s, (indicating a symmetric distribution of oxygen atoms around
The research was supported by the Israel Academy of Science and Technology and by the Klachky Foundation for Superconductivity.
References [1] J.G. Bednorz, K.A. Mu¨ller, Z. Physik B 64 (1986) 189. [2] I. Felner, I. Nowik, Supercond. Sci. Technol. 8 (1995) 121. [3] I. Nowik, I. Felner, E.R. Bauminger, Phys. Rev. B 45 (1992) 4912. [4] D. Hechel, I. Nowik, E.R. Bauminger, I. Felner, Phys. Rev. B 42 (1990) 2166. [5] I. Felner et al, Phys. Rev. B 48 (1993) 16040. [6] V. Sedykh et al., Appl. Phys. A 60 (1995) 71. [7] Y. LePage, W.R. McKinnon, J.M. Tarascon, P. Barboux, Phys. Rev. B 40 (1989) 6810. [8] V. Sedykh et al., Jpn. J. Appl. Phys. 34 (I) (1995) 4033. [9] J.M. Tranquada et al., Phys. Rev. B 38 (1988) 2477. [10] M. El Massalami et al., Physica C 183 (1991) 143.
Magnetic properties and Mo¨ssbauer studies of Bi Sr Fe O , >B isostructural with Bi-2122 superconductors D. Hechel*, I. Felner The Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 29 June 1998
Abstract We have investigated Bi Sr Fe O materials by complementary experimental techniques. The materials are derived >B from the superconducting Bi Sr CaCu O (Bi-2122) by replacing all Cu by Fe and Ca atoms by Sr. The materials studied are not superconducting, but rather magnetically ordered. Mo¨ssbauer spectroscopy (MS) and magnetic-susceptibility studies reveal, that the magnetic ordering temperature (¹ ) is very sensitive to oxygen concentration. ¹ "100(5) K , , (determined by MS) for the oxygen-rich sample, is shifted towards 610 K when oxygen is depleted. Magnetic susceptibility data yield somewhat lower ¹ values (80 and 520 K, respectively). 1999 Published by Elsevier Science B.V. , All rights reserved. PACS: 74.10#V; 74.62Bf; 75.50Ee; 76.80#y Keywords: Magnetic properties; Mo¨ssbauer spectroscopy; Bi Sr Fe O d ; Superconductors >
1. Introduction The discovery of the high-¹ cuprates [1] ini tiated also the study of related compounds in order to understand hopefully more the interesting properties of these superconducting oxides. One of the striking features is the interplay between superconductivity (SC) and antiferromagnetic (AFM) order. Mo¨ssbauer spectroscopy (MS) of a Fe probe is a very powerful tool to determine the appearance of magnetism at Cu-sites. It was shown that in various cationic substitutions in oxygen-rich YBa Cu O * Corresponding author. Tel.: #972-2-6585752; fax: #9722-6586347; e-mail: [email protected].
(YBCO), whenever superconductivity disappears due to doping out of the CuO planes, the Cu(2) moments order antiferromagnetically. A similar phase diagram was established in the La—Sr—Cu—O system and in several others more [2]. In the 110 K superconductor Bi Sr CaCu O (Bi-2212), successive substitution of Ca by Y changes the electronic and magnetic properties, and a similar phase diagram of SC and AFM has been obtained, with ¹ "385 K in Bi Sr , YCu O [3]. In YBCO partial substitution of Cu >B by Fe decreases ¹ , where superconductivity disap pears for a Fe concentration above 13% at the Cu-sites. For slightly higher iron concentration, AFM was observed with ¹ "420 K [4]. The next , interesting step is the full substitution of the
0921-4526/99/$ — see front matter 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 1 5 5 - 7
D. Hechel, I. Felner / Physica B 262 (1999) 410—414
copper site by iron atoms. In the RBa Fe O (R"Y, Eu) compounds the iron moments order antiferromagnetically at the Ne´el temperature ¹ "700 K [5,10]. , Much attention has been attracted to a phase resembling the superconducting Bi Sr CaCu O (Bi-2122) having the composition Bi Sr Fe O . >B The Bi Sr Fe O structure evolves from the Bi >B 2122 structure by replacing all copper by iron and divalent Ca ions by Sr. Substitution of only Cu by Fe yields the composition Bi Sr CaFe O >B which has been studied by Sedykh et al [6], who measured also the MS at room temperature (RT). They show that the MS spectrum is composed of three paramagnetic doublets, and no magnetic hyperfine sextet is observed. On the other hand, in the polycrystalline Bi Sr Fe O , whose structure >B was determined by LePage et al. [7], the MS spectrum at RT contains a well defined singlet and two doublets [8]. The main purpose of this study, employing several experimental techniques, is to study in more details the magnetic properties of the Bi Sr Fe O system. Magnetic studies on oxygen-rich >B sample exhibit only one anomaly, at relatively low temperature ¹ "80 K which is attributed to mag, netic ordering of the Fe sublattices. ¹ is extremely , sensitive to oxygen concentration, and for oxygenreduced samples shifts to 520 K. MS reveal that the ¹ values are somewhat higher 90—100 and 610 K. , 2. Experimental details Ceramic samples with a nominal composition Bi Sr Fe O were prepared by solid state reaction techniques. Prescribed amounts of Bi O , SrCO , and Fe (metal) were mixed and pressed into pellets and preheated at 900°C for about 1 day. The products were cooled, reground (at each step) and sintered at 1100°C and 1080°C for 24 and 72 h at 1050°C. A part of the as-prepared sample were post annealed at 1050°C for 2 h and then quenched into liquid nitrogen. This is referred to as oxygen reduced “quenched” samples. The other half was cooled in a rate of 200°C/h from 1050 to 200°C, in flowing oxygen and denoted as the “annealed” sample. The oxygen content was not determined.
411
Powder X-ray diffraction (XRD) measurements indicate that both Bi Sr Fe O materials are near ly single phase (&95%) and have the orthorhombic structure. A few unidentified additional peaks were observed in the XRD pattern and our attempts to get completely rid of them were unsuccessful. The DC magnetic measurements on solid ceramic pieces in the range of 5—600 K were performed in a commercial (Quantum Design) superconducting quantum interference device magnetometer (SQUID). The magnetization was measured by two different procedures: (a) The sample was zero field cooled (ZFC) to 5 K, a field was applied and the magnetization was measured as a function of temperature; (b) The sample was field cooled (FC) from above 300 to 5 K and then the magnetization was measured. The Mo¨ssbauer spectroscopic studies were carried out using a conventional constant acceleration spectrometer. The Fe MS were measured with a 100 mCi Co : Rh source. The Fe isomer shifts (IS) are reported with respect to metallic Fe.
3. Experimental results 3.1. Lattice parameters and oxygen content XRD studies show that Bi Sr Fe O has an >B orthorhombic structure and the lattice parameters are listed in Table 1. Note the contraction of the c-axis of the annealed sample. The lattice parameters for this material are in excellent agreement with data published in Ref. [8]. Determination of the absolute oxygen content in these materials is difficult. 3.2. Magnetic measurements The ZFC and FC magnetic susceptibilities, measured at 100 Oe of the annealed Bi Sr Fe O >B sample are shown in Fig. 1. In both branches, only one distinct peak at ¹ "79 K is observed, and no , other anomalies were observed at higher temperatures. As will be shown below, the Fe sublattice is magnetically ordered, therefore, the magnetic anomaly in these curves is related to the Fe sites. ¹ (Fe) , is defined as the merging temperature of the ZFC
1.02 0.63 0.18 1.04 0.52 536 483 31.90 5.486 Quenched
5.550
5.461 Annealed
5.517
31.59
0.53 0.64 0.64 0.34 0.63
0.18 1.13 0.24 !0.01 !0.30
254
59 27 14 56 44
0.61 0.37 0.17 0.45 0.33
0.49 0.44
0.29 0.25 0.17 0.07 0.13 0.37
D""eqQ" (mm/s) $0.01 IS (mm/s) $0.01 a (As ) $0.005
b (As ) $0.005
c (As ) $0.04
Width (mm/s) $0.01
D""eqQ" (mm/s) $0.01
H (kOe) $5
Intensity IS (mm/s) (%) $0.01
Width (mm/s) $0.01
Hyperfine parameters above ¹ Hyperfine parameters at 90 K Lattice parameters Compound
Table 1 Lattice constants and hyperfine parameters deducted from Mo¨ssbauer measurements for the annealed and quenched Bi Sr Fe O samples
50 31 19 63 37
D. Hechel, I. Felner / Physica B 262 (1999) 410—414 Intensity (%)
412
Fig. 1. Temperature dependence of the magnetic moment of annealed Bi Sr Fe O . Insert shows isotherm magnetization M(H) curve at 5 K.
and FC branches. The irreversibility probably arises from the AFM alignment of the Fe sublattice. It is assumed that in the FC process the external field causes the spins to cant slightly out of their original direction. This canting abruptly aligns a component of the moments with the direction of the field, and the FC branch is obtained. Above ¹ , the susceptibility s(T ) of Bi Sr , Fe O compounds adheres closely to the >B Curie—Weiss law over the major portion of the measured temperatures. s(T ) can be well described by s(¹)"s #C/(¹!h), where s is the temper ature independent part of the susceptibility, C is the molar Curie constant and h is the Curie—Weiss temperature. The fit of s(T ) (in the range 100 K(T( 300 K) yields: s "2.48;10\ emu/mol Oe, C"1.27 emu K/mol Oe, and h"!19 K. Since the other cations (Bi> and Sr>) are non-magnetic, the whole paramagnetic moment is presumed to be due to the Fe ions. The effective magnetic moment derived from C, P "2.25k , is attributed to Fe>. The almost linear virgin magnetization curve at 5 K up to 50 kOe (Fig. 1, inset), and the negative value obtained for h, both indicate AFM ordering in the Fe sites. It appears that a slight deficiency of oxygen, shifts ¹ (Fe) to higher temperatures. The ,
D. Hechel, I. Felner / Physica B 262 (1999) 410—414
Fig. 2. The variation of the magnetic moment (in arbitrary units) of quenched Bi Sr Fe O measured at elevated temper atures.
maximum in the magnetization curve of the quenched sample is shifted to about 520 K (Fig. 2), and no anomaly is observed at lower temperatures. In that respect, the present system behaves in a manner similar to the well known YBCO [9]. The magnetic results, shown in Fig. 2, confirm our MS studies which are displayed and discussed in the next section. 3.3. Mo¨ ssbauer studies The Fe Mo¨ssbauer spectra of the annealed sample below and above ¹ are presented in Fig. 3, , and the hyperfine parameters obtained are listed in Table 1. At 4.2 K, the spectrum is splitted into two magnetic subspectra with a relative intensity ratio of 3 : 1. At 85 K the spectrum is a complicated one, because partly the sextet has already collapsed into a doublet. This result indicates a distribution of magnetic ordering temperatures. This is probably due to the inhomogenity of the oxygen content. At 300 K the spectrum is composed of three doublets with quadrupole splittings of eqQ" D"1.02, 0.63 and 0.18 mm/s. These values compare well with those obtained by Sedykh et al. [8]. However, in contrast to the small splitting of the third doublet in our sample, they observed a small singlet.
413
Fig. 3. Mo¨ssbauer spectra of annealed Bi Sr Fe O , at various temperatures. Note the extended scale at 300 K.
These parameters (Table 1) indicate that the Featoms are in the trivalent state. The different values of the two large doublets are a result of different local environment of the Fe atoms, probably due to inhomogenity of the oxygen distribution. Sedykh et al. concluded that their singlet is connected with an additional phase, since this singlet was absent in single crystals. In the next paragraph we will show that this third doublet does not appear in the quenched sample. Thus in our opinion, it is not clear whether this third subspectrum belongs to an additional phase or whether it represents another oxygen environment of Fe in Bi Sr Fe O in the annealed phase only. A different picture we find for the quenched sample. The Mo¨ssbauer spectra at various temperatures are presented in Fig. 4. The spectra at 4.2 (not shown) and 90 K are composed of two magnetic subspectra. Table 1 shows that at 90 K, the major sextet, which counts 56% of the spectral area, has an effective magnetic field H (1)" 536 kOe, whereas for the minor sextet H (2)" 483 kOe. As the temperatures are raised, both magnetic splittings decrease and disappear around 610 K. At 650 K, the paramagnetic spectrum is composed of two doublets (see Table 1). By comparing the intensities of the subspectra, we can assume,
414
D. Hechel, I. Felner / Physica B 262 (1999) 410—414
the Fe) does not appear in the quenched sample. Thus, in contrast to Sedykh et al., we conclude that this doublet belongs to Bi Sr Fe O . 4. Conclusions We performed susceptibility and Mo¨ssbauer studies of Bi Sr Fe O . The results show that the heat treatment is crucial for the magnetic behavior of the samples. The annealed sample is antiferromagnetically ordered at ¹ "100 K, whereas the , quenched sample has ¹ "610$10 K. These , ¹ values obtained by MS are somewhat higher , that the values obtained by DC magnetization measurements. The Mo¨ssbauer spectra of the annealed sample show three subspectra, and for the quenched sample we found two sites only.
Acknowledgements
Fig. 4. Mo¨ssbauer spectra of quenched Bi Sr Fe O at various temperatures.
that the major and the minor sextets collapse into the doublets with D"1.02 and 0.52 mm/s, respectively. Using the relation: D "D(3 cosh!1)/2, where D is the effective quadrupole interaction below ¹ , and h is the hyperfine field orientation , relative to the c-axis, we obtain for the quenched sample, h "55° for the first site, and h "90° for the second site. The D values of the quenched sample are quite similar to the parameters of main doublets of the annealed sample. However, the third doublet of the annealed sample with D"0.18 mm/s, (indicating a symmetric distribution of oxygen atoms around
The research was supported by the Israel Academy of Science and Technology and by the Klachky Foundation for Superconductivity.
References [1] J.G. Bednorz, K.A. Mu¨ller, Z. Physik B 64 (1986) 189. [2] I. Felner, I. Nowik, Supercond. Sci. Technol. 8 (1995) 121. [3] I. Nowik, I. Felner, E.R. Bauminger, Phys. Rev. B 45 (1992) 4912. [4] D. Hechel, I. Nowik, E.R. Bauminger, I. Felner, Phys. Rev. B 42 (1990) 2166. [5] I. Felner et al, Phys. Rev. B 48 (1993) 16040. [6] V. Sedykh et al., Appl. Phys. A 60 (1995) 71. [7] Y. LePage, W.R. McKinnon, J.M. Tarascon, P. Barboux, Phys. Rev. B 40 (1989) 6810. [8] V. Sedykh et al., Jpn. J. Appl. Phys. 34 (I) (1995) 4033. [9] J.M. Tranquada et al., Phys. Rev. B 38 (1988) 2477. [10] M. El Massalami et al., Physica C 183 (1991) 143.