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Magnetic properties and triple valence fluctuations of iron in PrSr2Fe3O9−δ
Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
Magnetic properties and triple valence fluctuations of iron in PrSr Fe O \B I. Nowik *, I. Felner , V.P.S. Awana Racah Institute of Physics, The Hebrew University, Jerusalem 919404, Israel Instituto de Fisica, Unicamp, 13083-970, Campinas, Sp, Brazil Received 4 August 1998; received in revised form 23 September 1998
Abstract The compound PrSr Fe O (d%(0.1) has been studied by magnetization and Fe Mo¨ssbauer spectroscopy \B measurements. The magnetization studies show magnetic hysteresis phenomena at low temperatures and three distinct phase transitions; an antiferromagnetic transition at ¹ "190 K, a transition at ¹ "170 K associated with change in , iron valence and ¹ "270 K, the merging temperature of the zero field and field cooled magnetization curves. Mo¨ssbauer studies confirm the ¹ and ¹ values. Well below and above ¹ , the spectra are composed of two , , well-defined sub-spectra (ratio 2 : 1), though all Fe ions reside in one crystallographic site. At 4.2 K the subspectra differ in their isomer shift (IS) by 0.4 mm/s and in the magnetic hyperfine field (469 and 269 kOe) whereas above ¹ the , difference in the IS is only 0.24 mm/s. For 90 K(¹(¹ the spectra are complex and cannot be interpreted in terms of , well defined (2 or even 3) sub-spectra. All spectra, in the entire temperature range can be well explained and reproduced by a triple valence fluctuation model, assuming that Fe appear in three valence states. (a) 2Fe>#Fe> at low temperatures, (b) Fe> and 2Fe> above ¹ and (c) at 90 K(¹(¹ triple valence fluctuations occur: Fe>Fe>Fe>. An electron , , is transferred from Fe> to Fe> to form two nonmagnetic Fe> ions. This process dilutes the magnetic ions concentration to a point of collapse of the magnetic order. 1999 Elsevier Science B.V. All rights reserved. PACS: 73.30.Kz; 75.30.Mb; 75.60.Ej; 76.80.#y Keywords: Phase transitions; Valence fluctuations; Canted antiferromagnetism; Mo¨ssbauer spectroscopy; Irreversibility; Magnetic hysteresis
1. Introduction The compounds RSr Fe O (R"rare earth \B 0)d)1) have been studied extensively in recent
* Corresponding author. Tel.: 972-2-6584347; fax: 972-26586347; e-mail: [email protected].
years [1—5]. The interest in these compounds is twofold; for high d values the compounds exhibit high magnetic ordering temperatures [1,10], for low values of d, the antiferromagnetic phase transition coincides with an iron valence phase transition [2]. Thus in LaSr Fe O (d)0.06) \B below ¹ +200 K, the iron occupies two magnetic , sublattices, one of Fe> and the other of Fe>
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 3 8 3 - 7
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I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
(relative occupancy") [2—4]. Above ¹ it seemed , that only one kind of iron exists, of average valence 3.67 [2]. In compounds with 0.06)d)0.25 the valence averaging process starts already much below ¹ [2]. , In the present paper we report studies of PrSr Fe O by Mo¨ssbauer spectroscopy (MS) \B and magnetometry. According to the analysis of the Mo¨ssbauer spectra one concludes that in our sample the value of d is less than 0.1. The magnetometry measurements show that PrSr Fe O orders antiferromagnetically below \B ¹ "190 K. At low temperatures it exhibits mag, netic hysteresis. The zero field cooled and field cooled magnetization curves merge at ¹ " 270 K, well above ¹ . , The detailed analysis of the Fe Mo¨ssbauer spectra in terms of a model which takes into account triple valence fluctuations (Fe>_Fe>, Fe>_Fe>) shows that below ¹ all three val, ences are present, Fe> and Fe> are magnetic, while Fe> (zero at 4.2 K, fraction increases with temperature) is nonmagnetic. Above ¹ only Fe> , and Fe> are present.
2. Experimental details The sample of PrSr Fe O was synthesized \B through a solid state reaction route with ingredients of Pr O , SrCO and Fe O . The sample was calcined and reground several times at temperatures of 950, 1000, 1050, 1100, 1150, 1200, 1300, 1350 and 1400°C, for 24 h at each temperature, to ensure the formation of a single phase compound. The sample was further annealed at 1000°C for 48 h in flowing oxygen and was subsequently cooled to room temperature over a span of 10 h. Neutron diffraction studies show that the sample is of single phase, the diffraction patterns are well fitted within a hexagonal structure, space group R3 c [5]. The DC magnetic measurements on solid ceramic pieces in the range 2—300 K were performed in a commercial SQUID (quantum design) superconducting quantum interface device magnetometer. 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 magnetiz-
ation was measured as a function of temperature, (b) the sample was field cooled (FC) from above 300 to 5 K and the magnetization was remeasured. The Mo¨ssbauer studies were performed using a Co:Rh source (50 mCi) and a conventional constant acceleration Mo¨ssbauer drive. The spectra were analyzed and least square fitted by a computer program of a model which allows triple valence fluctuations. The information obtained included the relative intensities, the hyperfine interaction parameters of the three iron valences and relaxation rates (Fe>_Fe>, Fe>_Fe>).
3. Experimental results and discussion 3.1. Magnetization studies The temperature dependence of the ZFC and FC magnetization in various field strengths have been measured, low field (20 Oe) curves are shown in Fig. 1. The merging temperature, T , of the ZFC and FC magnetization curves is at 270 K, well above ¹ "190 K. The magnetization curves ex, hibit strong dependence on applied magnetic field. The rise in isothermal magnetization as a function of magnetic field, Fig. 2, can be described in terms of M(H)"sH#p , where sH is the Fe antifer romagnetic and Pr paramagnetic linear contribution to the magnetization and p corresponds to
Fig. 1. Magnetic susceptibility of PrSr Fe O in a magnetic \B field of 20 Oe.
I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
Fig. 2. Magnetic field dependence of the PrSr Fe O mag \B netic moment at 5 K. The decomposition of the magnetization to paramagnetic and ferromagnetic components is displayed.
69
symmetry in the tetragonal FeO octahedra, which leads to a canted antiferromagnetic structure. Alternatively it can be a result of the oxygen deficiency, d'0, which may create ferromagnetic clusters around oxygen vacancies. The fact that ¹ '¹ indicates the presence of , short range order above ¹ . It could be the result of , two-dimensional short-range order, if the coupling between layers is much weaker than within layers. Alternatively, this order could be due to the ferromagnetic clusters, mentioned before, responsible for the hysteresis loops at low temperatures. In the PrSr Fe O magnetization curves, Fig. \B 1 (inset), we identify three critical temperatures, ¹ "190 K, ¹ "270 K and a sharp transition at , ¹ "170 K. This last transition is associated with a partial valence change of Fe> to Fe>, at ¹ the , valence change Fe>#Fe>P2Fe> is completed. These conclusions come from the Mo¨ssbauer studies described below. 3.2. Mo¨ ssbauer studies
Fig. 3. Magnetic hysteresis of PrSr Fe O at 5 K. \B
a weak ferromagnetic component of the Fe sublattice. This ferromagnetic component is responsible to the hysteresis loops, as that shown in Fig. 3. The irreversibility in the magnetization curves, Fig. 1, the saturation moments in high magnetic fields (p is 0.03 l /Fe, Fig. 2) and the hysteresis loops (the remanent moment and coercive field at 5 K are 235 emu/mol and 2.5 kOe respectively, Fig. 3), all indicate the presence of a small ferromagnetic component. p is much smaller than the saturation moment of Fe in any magnetic valence state. This ferromagnetic component can be a result of antisymmetric exchange coupling, due to induced low
The Fe Mo¨ssbauer spectra of PrSr Fe O \B are shown in Fig. 4. The parameters of the analysis of the spectra are given in Table 1 and Fig. 5. Magnetic order disappears completely only above 190 K. This is in complete agreement with the magnetization studies described above. The PrSr Fe O Mo¨ssbauer spectra are very similar \B to those of LaSr Fe O [1—4,10] proving that \B the Pr ion is also trivalent. The analysis of the spectra of PrSr Fe O in comparison to those of \B LaSr Fe O indicates that d is about 0.1. One \B observes in Fig. 4 that at 4.2 K the spectrum is well fitted with two iron subspectra (2 : 1 in intensity) of parameters given in Table 1. The spectra above 90 K cannot be fitted with well-defined hyperfine fields. Dynamic phenomena are evident and the analysis of all spectra was done using a triple valence fluctuation model. 3.3. The theoretical model In fitting theoretical curves to the experimental spectra we assumed that below ¹ , the iron in , PrSr Fe O is composed of predominantly Fe> \B ( at 4.2 K), Fe> ( at 4.2 K) and a fraction of Fe>
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I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
Fig. 4. Mo¨ssbauer spectra of Fe in PrSr Fe O . The spectrum of 210 K is shown in two velocity scales. \B
Table 1 Hyperfine interaction parameters, relative intensities and relaxation rates of Fe ions in PrSr Fe O \B Temperature Isomer shift (K) (mm/s) Fe> 4.2 90 110 130 140 150 160 170 190 210 240 300 400
0.37(2) 0.37(2) 0.36(2) 0.34(2) 0.33(2) 0.33(2) 0.35(2) 0.37(2) 0.37(2) 0.37(2) 0.34(2) 0.29(2) 0.23(2)
Fe>
#0.11(1) #0.13(1) 0.10(1) #0.05(1) 0.00(1)
Hyperfine field (kOe)
Relative population
Relaxation rate (10 s\)
Fe>
Fe>
Fe>
Fe>
Fe>
Fe>
Fe>
!0.03(1) !0.04(1) !0.05(1) !0.04(1) !0.05(1) !0.06(1) !0.07(1) !0.02(1)
469(1) 442(2) 438(2) 424(2) 418(2) 409(3) 398(4) 385(5) 0
— 54(10) 50(10) 47(10) 44(10) 36(10) 29(10) — 0
269(2) 258(2) 256(2) 249(2) 247(2) 244(3) 237(4) 224(5) 0
0.68(1) 0.62(2) 0.62(2) 0.57(2) 0.55(2) 0.49(2) 0.41(2) 0.32(3) 0.33(2) 0.35(2) 0.37(2) 0.38(2) 0.38(2)
— 0.04(4)
0.32(1) 0.34(1) 0.34(1) 0.35(1) 0.34(1) 0.34(1) 0.34(2) 0.33(2) 0
Isomer shift relative to iron metal at room temperature.
0.67 0.65 0.63 0.62 0.62
— 8(2) 9(2) 8(2) 7(2) 7(2) 6(2) 5(2) 0.6(6) 0.6(6) 0.6(6) 0.6(6) 0.6(6)
I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
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[6,11]. S(u)"Real(pA\1)
(1)
where p is a row vector containing the relative intensities of the three valences (p , p , p ). 1 is a coloumb of ones and (2) A"!CI#i(X!uI)#p where I is a three-dimensional unit matrix, C is the Mo¨ssbauer linewidth, X is a diagonal matrix containing the three energies corresponding to three valences of the given nuclear transition, and p is the fluctuation rate matrix. It contains the following off diagonal elements; p(3#P4#)"r p , p(5#P4#)"r p , p(4#P3#)"r p , p(4#P5#)"r p ,
Fig. 5. Temperature dependence of hyperfine interactions parameters and relative intensities of iron ions in PrSr Fe O . \B
which increases with temperature. It was assumed that Fe> is nonmagnetic and the increase with temperature of the Fe> fraction, dilutes the magnetic ions and eventually (when the Fe> fraction becomes above a critical value) causes the disappearance of magnetic order. Thus below ¹ we , assume the presence of Fe> (highest isomer shift, large hyperfine field), Fe> (lowest isomer shift, small hyperfine field) and Fe> (intermediate isomer shift and very small hyperfine field). The 4.2 K experimental Fe> and Fe> hyperfine fields scale exactly as expected theoretically for these two valences (the core polarization field is proportional to the ionic spin). We assume that valence fluctuations occur only of the kind Fe>_Fe> and Fe>_Fe> (single electron processes). We also assume that in the valence change process, nuclear quantum numbers do not change. Thus the closed formula which one can use for each given nuclear transition (m Pm ) is the standard formula
p(3#_5#)"0 and n "! p . II IJ I$J Thus the fitting parameters are the three isomer shifts, three hyperfine fields, two occupation probabilities p and p (p "1!p !p ) and two fluc tuation rates r and r . The quadrupole interactions were zero for all valences. In the fitting procedures it turned out that the Fe> hyperfine field is close to zero as expected for a nonmagnetic ion, however a small transferred hyperfine field &40 kOe is present. In order to limit the number of free parameters we put the constraints on the relaxation rates r "r and on the isomer shift (IS) of Fe>, IS (Fe>)"0.5 (IS(Fe>)#IS(Fe>)). Since each iron ion in PrSrFe O sees a valence \B variety, randomly distributed, of iron neighbors, the local hyperfine field has a width. This effect was taken into account by allowing a line broadening proportional to the magnetic hyperfine interaction of each nuclear transition, namely C "C#brod"(g m !g m ) ) H ". (3) The parameter ‘brod’ was less than 0.1 (broadening the most extreme lines by a factor less than two) even at the elevated temperatures (150—170 K). Above ¹ we assume the presence of only two , valences and one can use standard formulas for the valence fluctuation spectrum, the most common is the Wickman formula [7].
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I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
4. Conclusions Observing the experimental results displayed in the magnetization curves, in the Mo¨ssbauer spectra and in the temperature dependence of the parameters shown in Fig. 5 we come to the following conclusions. The magnetic order in PrSr Fe O is pre \B dominantly antiferromagnetic but with a small ferromagnetic component leading to irreversibility in the susceptibility curves, and to magnetic hysteresis loops. Short-range order persists above the antiferromagnetic ordering temperature. The hyperfine interaction parameters, in particular the hyperfine fields show that at low temperatures, the three iron ions per formula unit, located in equivalent crystallographic sites are 2Fe># Fe>. The isomer shift obtained for Fe> in PrSr Fe O is considerably higher than that ex \B pected for Fe> in pure ionic compounds [8]. Thus one may argue that in the present compound the ions below ¹ are not exactly Fe> and 2Fe>, but intermediate valences Fe\V> and 2Fe>V>. This makes the occurrence of the valence phase transition less unexpected. Above ¹ the least square fits to the spectra , prove with very little doubt that the system contains 2Fe>#Fe> . Previous claims [2] that all three ions are of intermediate valence 3.67 (single line spectrum) are not consistent with the present experimental spectra. The fit with one line has a three times larger s than the fit with two lines with an intensity ratio of 2Fe>#Fe>. Also the isomer shift between the two lines (0.24(2) mm/s) is consistent with this assignment. Slow valence fluctuation phenomena exist at all temperatures, even above ¹ , this may predict , a semi-metallic behavior of these materials due to hopping of electrons between iron ions. As the temperature is raised from 90 K, Fe> starts to convert to Fe>. At ¹ "170 K half of the Fe> ions are converted to Fe>. Fe> stays stable up to 170 K and then at ¹ "190 K all Fe> ions ,
convert to Fe>, Fig. 5. The two transitions observed in Fig. 5 are also observed in the magnetization curves of Fig. 1. The system PrSr Fe O behaves very sim \B ilarly to that of LaSr Fe O . The non-Kramers \B Pr> ion does not order magnetically at low temperatures (down to 2 K). The magnetic hyperfine field temperature dependence, Fig. 5, indicates that the magnetic and valence transition at ¹ may be of first order. , In the region of the magnetic and valence transition, one observes a discontinuity in the Fe> isomer shift, Fig. 5. This phenomenon can be associated with the magnetic transition with magnetostrictive effects [9]. It can also be due to the valence transition itself which changes the valence of iron neighbors to the central Fe> ion. The change with temperature of the isomer shifts of the Fe> and Fe> absorption lines between 200 to 400 K is of a slope !7;10\ mm/s/deg, fully consistent with the standard thermal shift expected for Fe at high temperatures. References [1] P.D. Battle, T.C. Gibb, S. Nixon, J. Solid State Chem. 79 (1989) 75. [2] P.D. Battle, T.C. Gibb, S. Nixon, J. Solid State Chem. 77 (1988) 124. [3] J.T. Wang, C.L. Lin, T. Mihalisin, J. Appl. Phys. 79 (8) (1996) 6608. [4] P.D. Battle, T.C. Gibb, P. Lightfoot, J. Solid State Chem. 84 (1990) 271. [5] V.S.P. Awana, S.X. Dou, I. Felner, I. Nowik, S.K. Malik, A. Mehta, R. Singh, A. Narlikar, W.B. Yelon, J. Appl. Phys. 83 (1998) 7312. In this short conference publication, unfortunately, several errors, corresponding to the Mo¨ssbauer figure and data, are present. [6] B. Khurgin, I. Nowik, M. Rakavy, S. Ofer, J. Phys. Chem. Solids 31 (1970) 49. [7] I. Nowik, H.H. Wickmann, Phys. Rev. Lett. 17 (1966) 949. [8] G. Demazeau, B. Buffat, M. Pouchard, P. Hagenmuller, Z. Anorg. Allg. Chem. 491 (1982) 60. [9] D. Feder, I. Nowik, J. Magn. Magn. Mater. 12 (1979) 149. [10] P.D. Battle, T.C. Gibb, S. Nixon, 79 (1989) 86. [11] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, Oxford, 1961, eq. 61 on p. 449.
Magnetic properties and triple valence fluctuations of iron in PrSr Fe O \B I. Nowik *, I. Felner , V.P.S. Awana Racah Institute of Physics, The Hebrew University, Jerusalem 919404, Israel Instituto de Fisica, Unicamp, 13083-970, Campinas, Sp, Brazil Received 4 August 1998; received in revised form 23 September 1998
Abstract The compound PrSr Fe O (d%(0.1) has been studied by magnetization and Fe Mo¨ssbauer spectroscopy \B measurements. The magnetization studies show magnetic hysteresis phenomena at low temperatures and three distinct phase transitions; an antiferromagnetic transition at ¹ "190 K, a transition at ¹ "170 K associated with change in , iron valence and ¹ "270 K, the merging temperature of the zero field and field cooled magnetization curves. Mo¨ssbauer studies confirm the ¹ and ¹ values. Well below and above ¹ , the spectra are composed of two , , well-defined sub-spectra (ratio 2 : 1), though all Fe ions reside in one crystallographic site. At 4.2 K the subspectra differ in their isomer shift (IS) by 0.4 mm/s and in the magnetic hyperfine field (469 and 269 kOe) whereas above ¹ the , difference in the IS is only 0.24 mm/s. For 90 K(¹(¹ the spectra are complex and cannot be interpreted in terms of , well defined (2 or even 3) sub-spectra. All spectra, in the entire temperature range can be well explained and reproduced by a triple valence fluctuation model, assuming that Fe appear in three valence states. (a) 2Fe>#Fe> at low temperatures, (b) Fe> and 2Fe> above ¹ and (c) at 90 K(¹(¹ triple valence fluctuations occur: Fe>Fe>Fe>. An electron , , is transferred from Fe> to Fe> to form two nonmagnetic Fe> ions. This process dilutes the magnetic ions concentration to a point of collapse of the magnetic order. 1999 Elsevier Science B.V. All rights reserved. PACS: 73.30.Kz; 75.30.Mb; 75.60.Ej; 76.80.#y Keywords: Phase transitions; Valence fluctuations; Canted antiferromagnetism; Mo¨ssbauer spectroscopy; Irreversibility; Magnetic hysteresis
1. Introduction The compounds RSr Fe O (R"rare earth \B 0)d)1) have been studied extensively in recent
* Corresponding author. Tel.: 972-2-6584347; fax: 972-26586347; e-mail: [email protected].
years [1—5]. The interest in these compounds is twofold; for high d values the compounds exhibit high magnetic ordering temperatures [1,10], for low values of d, the antiferromagnetic phase transition coincides with an iron valence phase transition [2]. Thus in LaSr Fe O (d)0.06) \B below ¹ +200 K, the iron occupies two magnetic , sublattices, one of Fe> and the other of Fe>
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 3 8 3 - 7
68
I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
(relative occupancy") [2—4]. Above ¹ it seemed , that only one kind of iron exists, of average valence 3.67 [2]. In compounds with 0.06)d)0.25 the valence averaging process starts already much below ¹ [2]. , In the present paper we report studies of PrSr Fe O by Mo¨ssbauer spectroscopy (MS) \B and magnetometry. According to the analysis of the Mo¨ssbauer spectra one concludes that in our sample the value of d is less than 0.1. The magnetometry measurements show that PrSr Fe O orders antiferromagnetically below \B ¹ "190 K. At low temperatures it exhibits mag, netic hysteresis. The zero field cooled and field cooled magnetization curves merge at ¹ " 270 K, well above ¹ . , The detailed analysis of the Fe Mo¨ssbauer spectra in terms of a model which takes into account triple valence fluctuations (Fe>_Fe>, Fe>_Fe>) shows that below ¹ all three val, ences are present, Fe> and Fe> are magnetic, while Fe> (zero at 4.2 K, fraction increases with temperature) is nonmagnetic. Above ¹ only Fe> , and Fe> are present.
2. Experimental details The sample of PrSr Fe O was synthesized \B through a solid state reaction route with ingredients of Pr O , SrCO and Fe O . The sample was calcined and reground several times at temperatures of 950, 1000, 1050, 1100, 1150, 1200, 1300, 1350 and 1400°C, for 24 h at each temperature, to ensure the formation of a single phase compound. The sample was further annealed at 1000°C for 48 h in flowing oxygen and was subsequently cooled to room temperature over a span of 10 h. Neutron diffraction studies show that the sample is of single phase, the diffraction patterns are well fitted within a hexagonal structure, space group R3 c [5]. The DC magnetic measurements on solid ceramic pieces in the range 2—300 K were performed in a commercial SQUID (quantum design) superconducting quantum interface device magnetometer. 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 magnetiz-
ation was measured as a function of temperature, (b) the sample was field cooled (FC) from above 300 to 5 K and the magnetization was remeasured. The Mo¨ssbauer studies were performed using a Co:Rh source (50 mCi) and a conventional constant acceleration Mo¨ssbauer drive. The spectra were analyzed and least square fitted by a computer program of a model which allows triple valence fluctuations. The information obtained included the relative intensities, the hyperfine interaction parameters of the three iron valences and relaxation rates (Fe>_Fe>, Fe>_Fe>).
3. Experimental results and discussion 3.1. Magnetization studies The temperature dependence of the ZFC and FC magnetization in various field strengths have been measured, low field (20 Oe) curves are shown in Fig. 1. The merging temperature, T , of the ZFC and FC magnetization curves is at 270 K, well above ¹ "190 K. The magnetization curves ex, hibit strong dependence on applied magnetic field. The rise in isothermal magnetization as a function of magnetic field, Fig. 2, can be described in terms of M(H)"sH#p , where sH is the Fe antifer romagnetic and Pr paramagnetic linear contribution to the magnetization and p corresponds to
Fig. 1. Magnetic susceptibility of PrSr Fe O in a magnetic \B field of 20 Oe.
I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
Fig. 2. Magnetic field dependence of the PrSr Fe O mag \B netic moment at 5 K. The decomposition of the magnetization to paramagnetic and ferromagnetic components is displayed.
69
symmetry in the tetragonal FeO octahedra, which leads to a canted antiferromagnetic structure. Alternatively it can be a result of the oxygen deficiency, d'0, which may create ferromagnetic clusters around oxygen vacancies. The fact that ¹ '¹ indicates the presence of , short range order above ¹ . It could be the result of , two-dimensional short-range order, if the coupling between layers is much weaker than within layers. Alternatively, this order could be due to the ferromagnetic clusters, mentioned before, responsible for the hysteresis loops at low temperatures. In the PrSr Fe O magnetization curves, Fig. \B 1 (inset), we identify three critical temperatures, ¹ "190 K, ¹ "270 K and a sharp transition at , ¹ "170 K. This last transition is associated with a partial valence change of Fe> to Fe>, at ¹ the , valence change Fe>#Fe>P2Fe> is completed. These conclusions come from the Mo¨ssbauer studies described below. 3.2. Mo¨ ssbauer studies
Fig. 3. Magnetic hysteresis of PrSr Fe O at 5 K. \B
a weak ferromagnetic component of the Fe sublattice. This ferromagnetic component is responsible to the hysteresis loops, as that shown in Fig. 3. The irreversibility in the magnetization curves, Fig. 1, the saturation moments in high magnetic fields (p is 0.03 l /Fe, Fig. 2) and the hysteresis loops (the remanent moment and coercive field at 5 K are 235 emu/mol and 2.5 kOe respectively, Fig. 3), all indicate the presence of a small ferromagnetic component. p is much smaller than the saturation moment of Fe in any magnetic valence state. This ferromagnetic component can be a result of antisymmetric exchange coupling, due to induced low
The Fe Mo¨ssbauer spectra of PrSr Fe O \B are shown in Fig. 4. The parameters of the analysis of the spectra are given in Table 1 and Fig. 5. Magnetic order disappears completely only above 190 K. This is in complete agreement with the magnetization studies described above. The PrSr Fe O Mo¨ssbauer spectra are very similar \B to those of LaSr Fe O [1—4,10] proving that \B the Pr ion is also trivalent. The analysis of the spectra of PrSr Fe O in comparison to those of \B LaSr Fe O indicates that d is about 0.1. One \B observes in Fig. 4 that at 4.2 K the spectrum is well fitted with two iron subspectra (2 : 1 in intensity) of parameters given in Table 1. The spectra above 90 K cannot be fitted with well-defined hyperfine fields. Dynamic phenomena are evident and the analysis of all spectra was done using a triple valence fluctuation model. 3.3. The theoretical model In fitting theoretical curves to the experimental spectra we assumed that below ¹ , the iron in , PrSr Fe O is composed of predominantly Fe> \B ( at 4.2 K), Fe> ( at 4.2 K) and a fraction of Fe>
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I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
Fig. 4. Mo¨ssbauer spectra of Fe in PrSr Fe O . The spectrum of 210 K is shown in two velocity scales. \B
Table 1 Hyperfine interaction parameters, relative intensities and relaxation rates of Fe ions in PrSr Fe O \B Temperature Isomer shift (K) (mm/s) Fe> 4.2 90 110 130 140 150 160 170 190 210 240 300 400
0.37(2) 0.37(2) 0.36(2) 0.34(2) 0.33(2) 0.33(2) 0.35(2) 0.37(2) 0.37(2) 0.37(2) 0.34(2) 0.29(2) 0.23(2)
Fe>
#0.11(1) #0.13(1) 0.10(1) #0.05(1) 0.00(1)
Hyperfine field (kOe)
Relative population
Relaxation rate (10 s\)
Fe>
Fe>
Fe>
Fe>
Fe>
Fe>
Fe>
!0.03(1) !0.04(1) !0.05(1) !0.04(1) !0.05(1) !0.06(1) !0.07(1) !0.02(1)
469(1) 442(2) 438(2) 424(2) 418(2) 409(3) 398(4) 385(5) 0
— 54(10) 50(10) 47(10) 44(10) 36(10) 29(10) — 0
269(2) 258(2) 256(2) 249(2) 247(2) 244(3) 237(4) 224(5) 0
0.68(1) 0.62(2) 0.62(2) 0.57(2) 0.55(2) 0.49(2) 0.41(2) 0.32(3) 0.33(2) 0.35(2) 0.37(2) 0.38(2) 0.38(2)
— 0.04(4)
0.32(1) 0.34(1) 0.34(1) 0.35(1) 0.34(1) 0.34(1) 0.34(2) 0.33(2) 0
Isomer shift relative to iron metal at room temperature.
0.67 0.65 0.63 0.62 0.62
— 8(2) 9(2) 8(2) 7(2) 7(2) 6(2) 5(2) 0.6(6) 0.6(6) 0.6(6) 0.6(6) 0.6(6)
I. Nowik et al. / Journal of Magnetism and Magnetic Materials 192 (1999) 67—72
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[6,11]. S(u)"Real(pA\1)
(1)
where p is a row vector containing the relative intensities of the three valences (p , p , p ). 1 is a coloumb of ones and (2) A"!CI#i(X!uI)#p where I is a three-dimensional unit matrix, C is the Mo¨ssbauer linewidth, X is a diagonal matrix containing the three energies corresponding to three valences of the given nuclear transition, and p is the fluctuation rate matrix. It contains the following off diagonal elements; p(3#P4#)"r p , p(5#P4#)"r p , p(4#P3#)"r p , p(4#P5#)"r p ,
Fig. 5. Temperature dependence of hyperfine interactions parameters and relative intensities of iron ions in PrSr Fe O . \B
which increases with temperature. It was assumed that Fe> is nonmagnetic and the increase with temperature of the Fe> fraction, dilutes the magnetic ions and eventually (when the Fe> fraction becomes above a critical value) causes the disappearance of magnetic order. Thus below ¹ we , assume the presence of Fe> (highest isomer shift, large hyperfine field), Fe> (lowest isomer shift, small hyperfine field) and Fe> (intermediate isomer shift and very small hyperfine field). The 4.2 K experimental Fe> and Fe> hyperfine fields scale exactly as expected theoretically for these two valences (the core polarization field is proportional to the ionic spin). We assume that valence fluctuations occur only of the kind Fe>_Fe> and Fe>_Fe> (single electron processes). We also assume that in the valence change process, nuclear quantum numbers do not change. Thus the closed formula which one can use for each given nuclear transition (m Pm ) is the standard formula
p(3#_5#)"0 and n "! p . II IJ I$J Thus the fitting parameters are the three isomer shifts, three hyperfine fields, two occupation probabilities p and p (p "1!p !p ) and two fluc tuation rates r and r . The quadrupole interactions were zero for all valences. In the fitting procedures it turned out that the Fe> hyperfine field is close to zero as expected for a nonmagnetic ion, however a small transferred hyperfine field &40 kOe is present. In order to limit the number of free parameters we put the constraints on the relaxation rates r "r and on the isomer shift (IS) of Fe>, IS (Fe>)"0.5 (IS(Fe>)#IS(Fe>)). Since each iron ion in PrSrFe O sees a valence \B variety, randomly distributed, of iron neighbors, the local hyperfine field has a width. This effect was taken into account by allowing a line broadening proportional to the magnetic hyperfine interaction of each nuclear transition, namely C "C#brod"(g m !g m ) ) H ". (3) The parameter ‘brod’ was less than 0.1 (broadening the most extreme lines by a factor less than two) even at the elevated temperatures (150—170 K). Above ¹ we assume the presence of only two , valences and one can use standard formulas for the valence fluctuation spectrum, the most common is the Wickman formula [7].
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4. Conclusions Observing the experimental results displayed in the magnetization curves, in the Mo¨ssbauer spectra and in the temperature dependence of the parameters shown in Fig. 5 we come to the following conclusions. The magnetic order in PrSr Fe O is pre \B dominantly antiferromagnetic but with a small ferromagnetic component leading to irreversibility in the susceptibility curves, and to magnetic hysteresis loops. Short-range order persists above the antiferromagnetic ordering temperature. The hyperfine interaction parameters, in particular the hyperfine fields show that at low temperatures, the three iron ions per formula unit, located in equivalent crystallographic sites are 2Fe># Fe>. The isomer shift obtained for Fe> in PrSr Fe O is considerably higher than that ex \B pected for Fe> in pure ionic compounds [8]. Thus one may argue that in the present compound the ions below ¹ are not exactly Fe> and 2Fe>, but intermediate valences Fe\V> and 2Fe>V>. This makes the occurrence of the valence phase transition less unexpected. Above ¹ the least square fits to the spectra , prove with very little doubt that the system contains 2Fe>#Fe> . Previous claims [2] that all three ions are of intermediate valence 3.67 (single line spectrum) are not consistent with the present experimental spectra. The fit with one line has a three times larger s than the fit with two lines with an intensity ratio of 2Fe>#Fe>. Also the isomer shift between the two lines (0.24(2) mm/s) is consistent with this assignment. Slow valence fluctuation phenomena exist at all temperatures, even above ¹ , this may predict , a semi-metallic behavior of these materials due to hopping of electrons between iron ions. As the temperature is raised from 90 K, Fe> starts to convert to Fe>. At ¹ "170 K half of the Fe> ions are converted to Fe>. Fe> stays stable up to 170 K and then at ¹ "190 K all Fe> ions ,
convert to Fe>, Fig. 5. The two transitions observed in Fig. 5 are also observed in the magnetization curves of Fig. 1. The system PrSr Fe O behaves very sim \B ilarly to that of LaSr Fe O . The non-Kramers \B Pr> ion does not order magnetically at low temperatures (down to 2 K). The magnetic hyperfine field temperature dependence, Fig. 5, indicates that the magnetic and valence transition at ¹ may be of first order. , In the region of the magnetic and valence transition, one observes a discontinuity in the Fe> isomer shift, Fig. 5. This phenomenon can be associated with the magnetic transition with magnetostrictive effects [9]. It can also be due to the valence transition itself which changes the valence of iron neighbors to the central Fe> ion. The change with temperature of the isomer shifts of the Fe> and Fe> absorption lines between 200 to 400 K is of a slope !7;10\ mm/s/deg, fully consistent with the standard thermal shift expected for Fe at high temperatures. References [1] P.D. Battle, T.C. Gibb, S. Nixon, J. Solid State Chem. 79 (1989) 75. [2] P.D. Battle, T.C. Gibb, S. Nixon, J. Solid State Chem. 77 (1988) 124. [3] J.T. Wang, C.L. Lin, T. Mihalisin, J. Appl. Phys. 79 (8) (1996) 6608. [4] P.D. Battle, T.C. Gibb, P. Lightfoot, J. Solid State Chem. 84 (1990) 271. [5] V.S.P. Awana, S.X. Dou, I. Felner, I. Nowik, S.K. Malik, A. Mehta, R. Singh, A. Narlikar, W.B. Yelon, J. Appl. Phys. 83 (1998) 7312. In this short conference publication, unfortunately, several errors, corresponding to the Mo¨ssbauer figure and data, are present. [6] B. Khurgin, I. Nowik, M. Rakavy, S. Ofer, J. Phys. Chem. Solids 31 (1970) 49. [7] I. Nowik, H.H. Wickmann, Phys. Rev. Lett. 17 (1966) 949. [8] G. Demazeau, B. Buffat, M. Pouchard, P. Hagenmuller, Z. Anorg. Allg. Chem. 491 (1982) 60. [9] D. Feder, I. Nowik, J. Magn. Magn. Mater. 12 (1979) 149. [10] P.D. Battle, T.C. Gibb, S. Nixon, 79 (1989) 86. [11] A. Abragam, The Principles of Nuclear Magnetism, Oxford University Press, Oxford, 1961, eq. 61 on p. 449.