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Mössbauer and magnetic studies of PbFe12−xCrxO19 hexagonal ferrites
Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
Mo¨ssbauer and magnetic studies of PbFe Cr O 12~x x 19 hexagonal ferrites G. Albanese!, B.E. Watts", F. Leccabue",*, S. Dı´ az Castan8 o´n# ! Istituto Nazionale di Fisica della Materia, Physics Department, Parma University, V. le delle Scienze, I-43100 Parma, Italy " MASPEC/CNR Institute, Via Chiavari 18/a, I-43100 Parma, Italy # Physics Faculty, La Habana University, La Habana, Cuba Received 28 March 1997; received in revised form 24 November 1997
Abstract Mo¨ssbauer and magnetic measurements have been performed on Cr substituted Pb-hexaferrite of composition PbFe Cr O (0)x)6). The main hyperfine parameters for the Fe3` nuclei of the various sublattices have been 12~x x 19 determined in the temperature range of 78—800 K. The data obtained give no evidence of deviation of the Fe3` magnetic moments from the axial magnetic order. In addition, the Cr3` ions do not occupy the hexahedral lattice sites. The rapid fall of the saturation magnetisation with increasing x cannot be simply explained with the replacement of the magnetic moment of the Fe3` with that of Cr3`. The onset of a complex magnetic order induced by the presence of Cr3` ions is involved. ( 1998 Elsevier Science B.V. All rights reserved.
Keywords: Ferrites — hexagonal; Hyperfine parameters; Mo¨ssbauer spectroscopy; Substitution effects
1. Introduction The substitution of Fe3` ions by other trivalent cations may lead to drastic variations in the magnetic structure of hexaferrites. The case of the substitution of Fe3` by Cr3` in M-type hexaferrites is of particular interest. The rapid fall in the values of the Curie temperature and of the saturation
* Corresponding author. Fax: (#39-521) 269206; e-mail: [email protected].
magnetisation at 0 K with increasing degree of substitution, cannot be explained only on the basis of a long range collinear spin order, and considering that the chromium ion has a magnetic moment of 3k . Detailed studies by Mo¨ssbauer spectroscopy, B neutron diffraction and magnetic measurements, performed by Obradors et al. [1—3], on chromium substituted Sr hexaferrites have evidenced that in these compounds, spin-glass re-entrance phenomena occur at low temperature. Moreover, in a Mo¨ssbauer study of BaFe Cr O , Wartewig 12~x x 19 et al. [4] have shown that with an external magnetic field of 6 T, applied to samples with x"0.8
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 1 1 6 2 - 1
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G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
and 1.8, at 4.2 K the Fe3` magnetic moments are aligned along the c axis. A satisfactory explanation for the magnetic structure of the Cr containing ferrites is still a matter of debate. Therefore, as part of a general study of M-type Pb ferrites substituted with trivalent ions, it seemed interesting to investigate further the effects of replacing Fe3` by Cr3` in PbFe O . 12 19 In this paper the results of Mo¨ssbauer and saturation magnetisation measurements on polycrystalline samples of PbFe Cr O (0)x)6) are 12~x x 19 presented and discussed. The Mo¨ssbauer data obtained indicate that in the substituted compounds the Fe3` magnetic moments remain axially aligned. The strong lowering of the saturation magnetisation with increasing degree of substitution can be explained by admitting the onset of a complex magnetic order.
2. Experimental Polycrystalline samples of PbFe Cr O 12~x x 19 (0)x)6) were prepared by a chemical coprecipitation method. Pb(NO ) , Fe(NO ) ) 9H O and 32 33 2 Cr(NO ) ) 9H O were dissolved in deionised water 33 2 with the stoichiometric ratio Pb/(Fe,Cr)" 1 , and 12 this solution was added drop by drop to a NaOH/Na CO buffer solution, having a pH" 2 3 10.5. The precipitate obtained was repeatedly washed with deionised water till all the NO~ ions 3 were eliminated. Afterwards, it was dried at 60°C and finely powdered. The powders were heattreated in air at 900—950°C for 2 h in order to crystallise the hexaferrite phase.
The samples obtained were characterised by Xray diffraction (XRD) using Co K radiation and by a DC susceptibility measurements, performed at 100 Oe with an extraction magnetometer, in the range 300—800 K. The saturation magnetisation, in the range 4.2—300 K and the coercive field ( H ) at * # room temperature were measured by means of a vibrating sample magnetometer, working at an H of 16 kOe. .!9 Mo¨ssbauer spectra of the Fe57 14.4 keV c radiation were recorded using a 20 mCi Co57 source in a Rh matrix, with the spectrometer working at constant acceleration, and the absorber samples held at temperatures between 78 and 800 K. The spectra were analysed using a program with which it is possible to optimise the isomer shift (IS), the quadrupole splitting (QS), the hyperfine magnetic field (H ), and the intensity (I) of the subspectra )& corresponding to the five iron sublattices of the M-type structure (Table 1). The intensities were calculated as the area submitted by lines with Lorentzian shape.
3. Results and discussion The analysis of the samples by X-ray diffraction confirms the formation of the M-type hexagonal structure; no spurious, secondary phases were detected. The a and c lattice parameters fall with increasing x from a"5.88 and c"23.12 A_ for x"0, to a"5.85 and c"22.71 A_ for x"6. However, accurate Mo¨ssbauer measurements made above and below the Curie temperature (¹ ), C for samples with x"3 and 5, revealed traces of
Table 1 Number of ions per unit formula, coordination and spin orientation for the cationic sublattices of the M-type hexagonal ferrites. The coordination of the sublattice b is indicated as pseudo-hexahedral because the cations are displaced from the symmetry plane of the trigonal bipyramid Sublattice
Structural block
Coordination
Number of ions
Spin
k a b f VI f IV
R—S S R R S
Octahedral Octahedral Pseudo-hexahedral Octahedral Tetrahedral
6 1 1 2 2
Up Up Up Down Down
G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
submicron a-Fe O particles. The small grain size 2 3 of the spurious phase rendered it undetectable by XRD but their dimensions were above the Mo¨ssbauer superparamagnetic limit. In the Mo¨ssbauer spectra, measured above and below ¹ , the peaks C belonging to the spurious a-Fe O can be clearly 2 3 distinguished from those of the ferrite, because aFe O has hyperfine parameters and a magnetic 2 3 transition temperature very different from those of the hexaferrite. The Curie temperatures were determined by measuring the dependence on the sample temperature of the transmitted intensity of the Fe57 14.4 keV Mo¨ssbauer radiation with the source and absorber at rest. A distinct change in transmitted intensity is observed at the temperature at which the magnetic hyperfine field disappears. This critical temperature can be assumed to be the Curie temperature of the sample at zero external magnetic field seen through the characteristic Mo¨ssbauer time window (+10~7 s). In Fig. 1 the dependence of ¹ on x is reported and compared C with the values obtained by DC, low field susceptibility measurements, reported in a previous brief report [5]. The Curie temperatures obtained by Mo¨ssbauer measurements are higher and the difference between the two series of data increases slightly with x. A discrepancy may occur when the upper
Fig. 1. Curie temperatures for PbFe Cr O hexaferrites 12~x x 19 determined by Mo¨ssbauer (m) and susceptibility measurements (j).
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limit of the spin relaxation rates in the sample is above the characteristic Mo¨ssbauer time [6]. For all the samples the temperature dependence of the saturation magnetisation has been measured down to 4.2 K. The p (¹) curves do not show dis4 continuities, revealing anomalous changes in the spin order. The values obtained by extrapolating the p (¹) curves to 0 K are reported in Fig. 2. The 4 p (0) values fall rapidly with increasing x, from 20k 4 B at x"0 down to 2.58k at x"6. The decrease in B p seen in Fig. 2 is much steeper than that expected, 4 assuming that the spin order remains collinear and assigning a magnetic moment of 3k to Cr3`. Data B on the order of substitution of the Fe3` by Cr3` in the various cationic sublattices and on the magnetic order, obtained from the Mo¨ssbauer spectra, are reported below. The Mo¨ssbauer spectra of the substituted PbFe Cr O , both below and above ¹ , have 12~x x 19 C been interpreted starting from the spectra of PbFe O , which have already been presented in 12 19 a previous paper [7], and following their evolution with increasing x. The spectra below ¹ can be decomposed into C five Zeeman sextets, corresponding to the five iron sublattices of the M-type structure, and have a
Fig. 2. Saturation magnetisation in Bohr magnetons per unit formula for PbFe Cr O hexaferrites at ¹"0 K (j). The 12~x x 19 full line denotes the calculated values in the hypothesis of axial magnetic order and equiprobable distribution of Cr3`ions in the octahedral k, a and f sublattices. The dotted line represents VI distribution in the spin up octahedral sublattices k and a.
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similar pattern to the spectrum of the PbFe O . 12 19 Changes in the relative intensities of the sextets are due to the replacement of iron by chromium. The quadrupole splitting for the various sublattices does not change markedly below ¹ . There is no C evidence for splitting of the sextet of the predominant k sublattice, as observed in Ga, In and Sc substituted M-type ferrites. The room temperature spectrum for x"2 is shown in Fig. 3. The sextet of the k sublattice is easily distinguishable from the rest of the spectrum, so that its hyperfine parameters can be determined with high accuracy. In Table 2 the Mo¨ssbauer data for the k sublattice, measured at 295 K are listed. The k sextet is made up of symmetric lines with Lorentzian shape, whose width, at a given temperature, strongly increases with increasing x. The width is temperature dependent and increases with increasing temperature. It is worth noting that in the spectra at liquid N temperature the linewidth 2 of the k sextet is close to the experimental values of the apparatus, showing that a distribution of the H values or of the angle H between the axis of )& the electric field gradient at the Fe57 nuclei and the
Table 2 Isomer shift (IS), hyperfine magnetic field (H ), quadrupole )& splitting (QS) and linewidth (C) for the Fe57 nuclei in the k sublattices of PbFe Cr O compounds, at 295 K 12~x x 19 x
IS (mm/s)
H (kOe) )&
QS (mm/s)
C (mm/s)
0 1 2 3 4 5
0.243(3) 0.243(3) 0.247(3) 0.241(3) 0.240(3) 0.241(3)
413(2) 412(2) 407(2) 398(2) 388(2) 373(2)
0.42(1) 0.44(1) 0.47(1) 0.50(1) 0.51(1) 0.57(1)
0.16(1) 0.18(1) 0.20(1) 0.30(1) 0.37(1) 0.55(1)
H direction for the k sublattice can be excluded at )& low temperatures. This remarkable broadening is seen only for the peaks of the k sublattice. It is worth noting that this sublattice is characterised by a relatively weak magnetic coupling and the temperature dependence of its magnetisation does not follow a Brillouin curve, like the other sublattices, but it falls more rapidly with increasing temperature, as revealed by the temperature dependence
Fig. 3. Fe57-Mo¨ssbauer spectrum for PbFe Cr O (x"2) compound at ¹"295 K. Experimental points (L L L); spectrum 12~x x 19 fitted as superposition of five sextets (—). For better clarity only the subspectra due to sublattices k (—) and e ( ) ) ) ) are traced.
G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
Fig. 4. Temperature dependence of the hyperfine magnetic field at the Fe57 nuclei in the k (m), a (j), e (#), f (*) and f (]) VI IV sublattices of PbFe O hexaferrite. 12 19
of the hyperfine magnetic fields in M-type ferrites (Fig. 4). This line broadening is not seen in the spectra measured above ¹ so that it C cannot be ascribed to a distribution of the
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quadrupole splittings, due to local crystallographic disorder. The Mo¨ssbauer spectra measured above ¹ show the typical pattern of the M-type hexaferC rite in the paramagnetic state. The large quadrupole splitting of the b sublattice makes the corresponding doublet clearly distinguishable from the central part of the spectrum, to which the remaining sublattices contribute. The same procedure used for Al substituted Pb [7] and Ba [8] ferrites was adopted to fix the starting parameters required for a univocal fitting of the spectra. This leads to the determination of the IS, QS and intensities of the quadrupole doublets of the five iron sublattices. In Fig. 5 the spectrum measured at 724 K for a compound with x"2 in the paramagnetic state is reported. The quadrupole splitting above ¹ for the various iron sublattices as a funcC tion of x are shown in Fig. 6. A comparison of these data with those obtained below ¹ , for both subC stituted and unsubstituted ferrite, shows a jump in the quadrupole splitting at ¹ for the octahedral C
Fig. 5. Fe57-Mo¨ssbauer spectrum for PbFe Cr O (x"2) compound at ¹"724 K. Experimental points (L L L); fitted 12~x x 19 spectrum (—); subl. k (- - -) ; subl. a (###) ; subl. e (***); subl. f ( ) ) ) ) ; subl. f (- ) - ) -). VI IV
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G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
Fig. 6. Quadrupole splitting for Fe57 nuclei in the various sublattices of PbFe Cr O compounds at ¹'¹ . Subl. k (m); 12~x x 19 C subl. e (j); subl. f (h); subl f (*). Within the experimental VI IV uncertainty the QS for the a sublattice equals zero for all x.
sublattices. As discussed in Refs. [7,8], this discontinuity may be attributed to a nonzero value of H for these sublattices. The QS values of the k sublattice measured at 77 K, at 295 K and ¹'¹ , increase smoothly with C x (Fig. 7). The parallel behaviour of QS versus x above and below ¹ indicates that no variation in C the values of the angle H occurs with varying degrees of substitution. No significant differences in the dependence on x of the QS values for the other sublattices were observed at ¹'¹ and ¹(¹ . C C From the data above, one can conclude that there is no deviation from axial order for the Fe3` magnetic moments, within the experimental uncertainty and in the range of temperatures explored. As mentioned above, in the Mo¨ssbauer spectra above ¹ , the doublet of the b sublattice is well C separated from the remaining part of the spectrum so that its relative intensity can be determined with a high degree of accuracy. In Fig. 8 the ratio of the areas submitted by the b doublet and the total area of the spectrum are compared with calculated values for different orders of substitution, assuming that Debye—Waller factors are not affected by the substitution. The data clearly show that the b sublattice is not occupied by Cr3` ions. As far as the other sublattices are concerned, the data deduced from the spectra measured, above
Fig. 7. Quadrupole splitting for Fe57 nuclei in the k lattice sites of PbFe Cr O compounds at ¹"295 K (m), ¹"77 K 12~x x 19 (h) and ¹'¹ (j). C
Fig. 8. Dependence on x of the ratio between the area submitted by the doublet e and the total area of the Mo¨ssbauer spectrum of PbFe Cr O compounds at ¹'¹ (j). The lines denote 12~x x 19 C the expected values in the hypotheses of a statistical distribution of chromium in all the lattice sites (- ) - ) -), with the exclusion of the sublattice e (- - - -) and the tetrahedral sublattice f (—). The IV data are normalised to the value determined for x"0.
and below ¹ , indicate that the Cr3` substitutes C Fe3` preferentially on the octahedral sites, k, a and f . The rate of occupancy of the f sites is lower VI VI than those of the k and a sublattices.
G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
From the above data one can conclude that the steep fall in p (0) with increasing x (see Fig. 2) 4 cannot be interpreted as being due simply to the replacement of the magnetic moment of Fe3` (5k ) with that of Cr3` (3k ). This is true even B B for the extreme hypothesis that the Cr3`, which has a lower moment, only enters the spin-up sublattices, k and a. This discrepancy between experimental and calculated values could be resolved by assigning a zero value to the axial component of the magnetic moment of the Cr3` ions. Unfortunately, the experimental techniques used in this work are unable to test this hypothesis, and the question still remains open. However, it is known that in mixed oxides, where the Cr3` participates in the superexchange interactions, the intensity and the sign of the interaction strongly depend on the lengths and angle of the Cr3`v—O2—Me3` bonds, and on the trivalent cation Me3` [9]. Hence, in substituted ferrites, canting or reversal of the Cr3` magnetic moments cannot be excluded as frustrated states of the complex net of superexchange interactions. The situation seems to be similar to that described by Obradors et al. [1—3] for the chromium-substituted Sr hexaferrites. Although these compounds show paramagnetic—ferrimagnetic long range transitions, some of their magnetic properties are similar to those of re-entrant spin glasses [3]. These authors have interpreted the results by hypothesising the coexistence of long range and short range magnetic order in the compounds investigated. However, for a rigorous determination of the magnetic order in PbFe Cr O hexaferrites, 12~x x 19 further investigations, using other experimental techniques as low field and high field susceptibility measurements at low temperature and neutron diffraction experiments are needed.
343
4. Conclusions Mo¨ssbauer spectroscopy has been performed on PbFe Cr O (0)x)6) hexaferrite, between 12~x x 19 77 and 800 K. The main hyperfine parameters for the Fe57 nuclei in the five iron sublattices have been determined. The data indicate that Cr3` ions do not occupy the hexahedral lattice sites (b) and that the magnetic moments of the Fe3` ions remain axially aligned for all values of x. The steep fall in the saturation magnetisation at 0 K, with increasing x cannot be interpreted as being due the simple replacement of the Fe3` magnetic moment (5k ) B with that of Cr3` (3k ), but it is a consequence of B the onset of a complex magnetic order. The differences in the values of the Curie temperatures deduced from Mo¨ssbauer and static magnetic measurements and some features of the Mo¨ssbauer spectra seems to indicate that complex spin—spin relaxation phenomena occur. References [1] X. Obradors, A. Isalgue, A. Collomb, M. Pernet, J. Pannetier, J. Rodrigues, J. Tejada, J.C. Joubert, IEEE Trans. Magn. 20 (1984) 1636. [2] J.A. Pereda, A. Isalgue, J. Tejada, F.J. Litterst, X. Obradors, Hyperfine Interactions 28 (1986) 569. [3] X. Obradors, A. Labarta, J. Tejada, M. Pernet, J.L. Tholence, M. Saint-Paul, B. Barbara, J. Appl. Phys. 63 (1988) 4091. [4] P. Wartewig, K. Melzer, M. Krause, R. Tellgren, J. Magn. Magn. Mater. 140—144 (1995) 2101. [5] S. Dı´ az, J.L. Sa´nchez, F. Leccabue, B.E. Watts, G. Bocelli, G. Albanese, J. de Physique 7 (1997) C1-331. [6] I.A. Campbell, Hyperfine Interactions 27 (1986) 15. [7] G. Albanese, F. Leccabue, B.E. Watts, S. Dı´ az, in: I. Ortalli (Ed.), Proc. ICAME-95, SIF, Bologna, Italy, 1996, p. 279. [8] G. Albanese, J. Magn. Magn. Mater. 147 (1995) 421. [9] D.G. Wickham, J.B. Goodenough, Phys. Rev. 115 (1959) 1156.
Mo¨ssbauer and magnetic studies of PbFe Cr O 12~x x 19 hexagonal ferrites G. Albanese!, B.E. Watts", F. Leccabue",*, S. Dı´ az Castan8 o´n# ! Istituto Nazionale di Fisica della Materia, Physics Department, Parma University, V. le delle Scienze, I-43100 Parma, Italy " MASPEC/CNR Institute, Via Chiavari 18/a, I-43100 Parma, Italy # Physics Faculty, La Habana University, La Habana, Cuba Received 28 March 1997; received in revised form 24 November 1997
Abstract Mo¨ssbauer and magnetic measurements have been performed on Cr substituted Pb-hexaferrite of composition PbFe Cr O (0)x)6). The main hyperfine parameters for the Fe3` nuclei of the various sublattices have been 12~x x 19 determined in the temperature range of 78—800 K. The data obtained give no evidence of deviation of the Fe3` magnetic moments from the axial magnetic order. In addition, the Cr3` ions do not occupy the hexahedral lattice sites. The rapid fall of the saturation magnetisation with increasing x cannot be simply explained with the replacement of the magnetic moment of the Fe3` with that of Cr3`. The onset of a complex magnetic order induced by the presence of Cr3` ions is involved. ( 1998 Elsevier Science B.V. All rights reserved.
Keywords: Ferrites — hexagonal; Hyperfine parameters; Mo¨ssbauer spectroscopy; Substitution effects
1. Introduction The substitution of Fe3` ions by other trivalent cations may lead to drastic variations in the magnetic structure of hexaferrites. The case of the substitution of Fe3` by Cr3` in M-type hexaferrites is of particular interest. The rapid fall in the values of the Curie temperature and of the saturation
* Corresponding author. Fax: (#39-521) 269206; e-mail: [email protected].
magnetisation at 0 K with increasing degree of substitution, cannot be explained only on the basis of a long range collinear spin order, and considering that the chromium ion has a magnetic moment of 3k . Detailed studies by Mo¨ssbauer spectroscopy, B neutron diffraction and magnetic measurements, performed by Obradors et al. [1—3], on chromium substituted Sr hexaferrites have evidenced that in these compounds, spin-glass re-entrance phenomena occur at low temperature. Moreover, in a Mo¨ssbauer study of BaFe Cr O , Wartewig 12~x x 19 et al. [4] have shown that with an external magnetic field of 6 T, applied to samples with x"0.8
0304-8853/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 1 1 6 2 - 1
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G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
and 1.8, at 4.2 K the Fe3` magnetic moments are aligned along the c axis. A satisfactory explanation for the magnetic structure of the Cr containing ferrites is still a matter of debate. Therefore, as part of a general study of M-type Pb ferrites substituted with trivalent ions, it seemed interesting to investigate further the effects of replacing Fe3` by Cr3` in PbFe O . 12 19 In this paper the results of Mo¨ssbauer and saturation magnetisation measurements on polycrystalline samples of PbFe Cr O (0)x)6) are 12~x x 19 presented and discussed. The Mo¨ssbauer data obtained indicate that in the substituted compounds the Fe3` magnetic moments remain axially aligned. The strong lowering of the saturation magnetisation with increasing degree of substitution can be explained by admitting the onset of a complex magnetic order.
2. Experimental Polycrystalline samples of PbFe Cr O 12~x x 19 (0)x)6) were prepared by a chemical coprecipitation method. Pb(NO ) , Fe(NO ) ) 9H O and 32 33 2 Cr(NO ) ) 9H O were dissolved in deionised water 33 2 with the stoichiometric ratio Pb/(Fe,Cr)" 1 , and 12 this solution was added drop by drop to a NaOH/Na CO buffer solution, having a pH" 2 3 10.5. The precipitate obtained was repeatedly washed with deionised water till all the NO~ ions 3 were eliminated. Afterwards, it was dried at 60°C and finely powdered. The powders were heattreated in air at 900—950°C for 2 h in order to crystallise the hexaferrite phase.
The samples obtained were characterised by Xray diffraction (XRD) using Co K radiation and by a DC susceptibility measurements, performed at 100 Oe with an extraction magnetometer, in the range 300—800 K. The saturation magnetisation, in the range 4.2—300 K and the coercive field ( H ) at * # room temperature were measured by means of a vibrating sample magnetometer, working at an H of 16 kOe. .!9 Mo¨ssbauer spectra of the Fe57 14.4 keV c radiation were recorded using a 20 mCi Co57 source in a Rh matrix, with the spectrometer working at constant acceleration, and the absorber samples held at temperatures between 78 and 800 K. The spectra were analysed using a program with which it is possible to optimise the isomer shift (IS), the quadrupole splitting (QS), the hyperfine magnetic field (H ), and the intensity (I) of the subspectra )& corresponding to the five iron sublattices of the M-type structure (Table 1). The intensities were calculated as the area submitted by lines with Lorentzian shape.
3. Results and discussion The analysis of the samples by X-ray diffraction confirms the formation of the M-type hexagonal structure; no spurious, secondary phases were detected. The a and c lattice parameters fall with increasing x from a"5.88 and c"23.12 A_ for x"0, to a"5.85 and c"22.71 A_ for x"6. However, accurate Mo¨ssbauer measurements made above and below the Curie temperature (¹ ), C for samples with x"3 and 5, revealed traces of
Table 1 Number of ions per unit formula, coordination and spin orientation for the cationic sublattices of the M-type hexagonal ferrites. The coordination of the sublattice b is indicated as pseudo-hexahedral because the cations are displaced from the symmetry plane of the trigonal bipyramid Sublattice
Structural block
Coordination
Number of ions
Spin
k a b f VI f IV
R—S S R R S
Octahedral Octahedral Pseudo-hexahedral Octahedral Tetrahedral
6 1 1 2 2
Up Up Up Down Down
G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
submicron a-Fe O particles. The small grain size 2 3 of the spurious phase rendered it undetectable by XRD but their dimensions were above the Mo¨ssbauer superparamagnetic limit. In the Mo¨ssbauer spectra, measured above and below ¹ , the peaks C belonging to the spurious a-Fe O can be clearly 2 3 distinguished from those of the ferrite, because aFe O has hyperfine parameters and a magnetic 2 3 transition temperature very different from those of the hexaferrite. The Curie temperatures were determined by measuring the dependence on the sample temperature of the transmitted intensity of the Fe57 14.4 keV Mo¨ssbauer radiation with the source and absorber at rest. A distinct change in transmitted intensity is observed at the temperature at which the magnetic hyperfine field disappears. This critical temperature can be assumed to be the Curie temperature of the sample at zero external magnetic field seen through the characteristic Mo¨ssbauer time window (+10~7 s). In Fig. 1 the dependence of ¹ on x is reported and compared C with the values obtained by DC, low field susceptibility measurements, reported in a previous brief report [5]. The Curie temperatures obtained by Mo¨ssbauer measurements are higher and the difference between the two series of data increases slightly with x. A discrepancy may occur when the upper
Fig. 1. Curie temperatures for PbFe Cr O hexaferrites 12~x x 19 determined by Mo¨ssbauer (m) and susceptibility measurements (j).
339
limit of the spin relaxation rates in the sample is above the characteristic Mo¨ssbauer time [6]. For all the samples the temperature dependence of the saturation magnetisation has been measured down to 4.2 K. The p (¹) curves do not show dis4 continuities, revealing anomalous changes in the spin order. The values obtained by extrapolating the p (¹) curves to 0 K are reported in Fig. 2. The 4 p (0) values fall rapidly with increasing x, from 20k 4 B at x"0 down to 2.58k at x"6. The decrease in B p seen in Fig. 2 is much steeper than that expected, 4 assuming that the spin order remains collinear and assigning a magnetic moment of 3k to Cr3`. Data B on the order of substitution of the Fe3` by Cr3` in the various cationic sublattices and on the magnetic order, obtained from the Mo¨ssbauer spectra, are reported below. The Mo¨ssbauer spectra of the substituted PbFe Cr O , both below and above ¹ , have 12~x x 19 C been interpreted starting from the spectra of PbFe O , which have already been presented in 12 19 a previous paper [7], and following their evolution with increasing x. The spectra below ¹ can be decomposed into C five Zeeman sextets, corresponding to the five iron sublattices of the M-type structure, and have a
Fig. 2. Saturation magnetisation in Bohr magnetons per unit formula for PbFe Cr O hexaferrites at ¹"0 K (j). The 12~x x 19 full line denotes the calculated values in the hypothesis of axial magnetic order and equiprobable distribution of Cr3`ions in the octahedral k, a and f sublattices. The dotted line represents VI distribution in the spin up octahedral sublattices k and a.
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G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
similar pattern to the spectrum of the PbFe O . 12 19 Changes in the relative intensities of the sextets are due to the replacement of iron by chromium. The quadrupole splitting for the various sublattices does not change markedly below ¹ . There is no C evidence for splitting of the sextet of the predominant k sublattice, as observed in Ga, In and Sc substituted M-type ferrites. The room temperature spectrum for x"2 is shown in Fig. 3. The sextet of the k sublattice is easily distinguishable from the rest of the spectrum, so that its hyperfine parameters can be determined with high accuracy. In Table 2 the Mo¨ssbauer data for the k sublattice, measured at 295 K are listed. The k sextet is made up of symmetric lines with Lorentzian shape, whose width, at a given temperature, strongly increases with increasing x. The width is temperature dependent and increases with increasing temperature. It is worth noting that in the spectra at liquid N temperature the linewidth 2 of the k sextet is close to the experimental values of the apparatus, showing that a distribution of the H values or of the angle H between the axis of )& the electric field gradient at the Fe57 nuclei and the
Table 2 Isomer shift (IS), hyperfine magnetic field (H ), quadrupole )& splitting (QS) and linewidth (C) for the Fe57 nuclei in the k sublattices of PbFe Cr O compounds, at 295 K 12~x x 19 x
IS (mm/s)
H (kOe) )&
QS (mm/s)
C (mm/s)
0 1 2 3 4 5
0.243(3) 0.243(3) 0.247(3) 0.241(3) 0.240(3) 0.241(3)
413(2) 412(2) 407(2) 398(2) 388(2) 373(2)
0.42(1) 0.44(1) 0.47(1) 0.50(1) 0.51(1) 0.57(1)
0.16(1) 0.18(1) 0.20(1) 0.30(1) 0.37(1) 0.55(1)
H direction for the k sublattice can be excluded at )& low temperatures. This remarkable broadening is seen only for the peaks of the k sublattice. It is worth noting that this sublattice is characterised by a relatively weak magnetic coupling and the temperature dependence of its magnetisation does not follow a Brillouin curve, like the other sublattices, but it falls more rapidly with increasing temperature, as revealed by the temperature dependence
Fig. 3. Fe57-Mo¨ssbauer spectrum for PbFe Cr O (x"2) compound at ¹"295 K. Experimental points (L L L); spectrum 12~x x 19 fitted as superposition of five sextets (—). For better clarity only the subspectra due to sublattices k (—) and e ( ) ) ) ) are traced.
G. Albanese et al. / Journal of Magnetism and Magnetic Materials 184 (1998) 337—343
Fig. 4. Temperature dependence of the hyperfine magnetic field at the Fe57 nuclei in the k (m), a (j), e (#), f (*) and f (]) VI IV sublattices of PbFe O hexaferrite. 12 19
of the hyperfine magnetic fields in M-type ferrites (Fig. 4). This line broadening is not seen in the spectra measured above ¹ so that it C cannot be ascribed to a distribution of the
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quadrupole splittings, due to local crystallographic disorder. The Mo¨ssbauer spectra measured above ¹ show the typical pattern of the M-type hexaferC rite in the paramagnetic state. The large quadrupole splitting of the b sublattice makes the corresponding doublet clearly distinguishable from the central part of the spectrum, to which the remaining sublattices contribute. The same procedure used for Al substituted Pb [7] and Ba [8] ferrites was adopted to fix the starting parameters required for a univocal fitting of the spectra. This leads to the determination of the IS, QS and intensities of the quadrupole doublets of the five iron sublattices. In Fig. 5 the spectrum measured at 724 K for a compound with x"2 in the paramagnetic state is reported. The quadrupole splitting above ¹ for the various iron sublattices as a funcC tion of x are shown in Fig. 6. A comparison of these data with those obtained below ¹ , for both subC stituted and unsubstituted ferrite, shows a jump in the quadrupole splitting at ¹ for the octahedral C
Fig. 5. Fe57-Mo¨ssbauer spectrum for PbFe Cr O (x"2) compound at ¹"724 K. Experimental points (L L L); fitted 12~x x 19 spectrum (—); subl. k (- - -) ; subl. a (###) ; subl. e (***); subl. f ( ) ) ) ) ; subl. f (- ) - ) -). VI IV
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Fig. 6. Quadrupole splitting for Fe57 nuclei in the various sublattices of PbFe Cr O compounds at ¹'¹ . Subl. k (m); 12~x x 19 C subl. e (j); subl. f (h); subl f (*). Within the experimental VI IV uncertainty the QS for the a sublattice equals zero for all x.
sublattices. As discussed in Refs. [7,8], this discontinuity may be attributed to a nonzero value of H for these sublattices. The QS values of the k sublattice measured at 77 K, at 295 K and ¹'¹ , increase smoothly with C x (Fig. 7). The parallel behaviour of QS versus x above and below ¹ indicates that no variation in C the values of the angle H occurs with varying degrees of substitution. No significant differences in the dependence on x of the QS values for the other sublattices were observed at ¹'¹ and ¹(¹ . C C From the data above, one can conclude that there is no deviation from axial order for the Fe3` magnetic moments, within the experimental uncertainty and in the range of temperatures explored. As mentioned above, in the Mo¨ssbauer spectra above ¹ , the doublet of the b sublattice is well C separated from the remaining part of the spectrum so that its relative intensity can be determined with a high degree of accuracy. In Fig. 8 the ratio of the areas submitted by the b doublet and the total area of the spectrum are compared with calculated values for different orders of substitution, assuming that Debye—Waller factors are not affected by the substitution. The data clearly show that the b sublattice is not occupied by Cr3` ions. As far as the other sublattices are concerned, the data deduced from the spectra measured, above
Fig. 7. Quadrupole splitting for Fe57 nuclei in the k lattice sites of PbFe Cr O compounds at ¹"295 K (m), ¹"77 K 12~x x 19 (h) and ¹'¹ (j). C
Fig. 8. Dependence on x of the ratio between the area submitted by the doublet e and the total area of the Mo¨ssbauer spectrum of PbFe Cr O compounds at ¹'¹ (j). The lines denote 12~x x 19 C the expected values in the hypotheses of a statistical distribution of chromium in all the lattice sites (- ) - ) -), with the exclusion of the sublattice e (- - - -) and the tetrahedral sublattice f (—). The IV data are normalised to the value determined for x"0.
and below ¹ , indicate that the Cr3` substitutes C Fe3` preferentially on the octahedral sites, k, a and f . The rate of occupancy of the f sites is lower VI VI than those of the k and a sublattices.
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From the above data one can conclude that the steep fall in p (0) with increasing x (see Fig. 2) 4 cannot be interpreted as being due simply to the replacement of the magnetic moment of Fe3` (5k ) with that of Cr3` (3k ). This is true even B B for the extreme hypothesis that the Cr3`, which has a lower moment, only enters the spin-up sublattices, k and a. This discrepancy between experimental and calculated values could be resolved by assigning a zero value to the axial component of the magnetic moment of the Cr3` ions. Unfortunately, the experimental techniques used in this work are unable to test this hypothesis, and the question still remains open. However, it is known that in mixed oxides, where the Cr3` participates in the superexchange interactions, the intensity and the sign of the interaction strongly depend on the lengths and angle of the Cr3`v—O2—Me3` bonds, and on the trivalent cation Me3` [9]. Hence, in substituted ferrites, canting or reversal of the Cr3` magnetic moments cannot be excluded as frustrated states of the complex net of superexchange interactions. The situation seems to be similar to that described by Obradors et al. [1—3] for the chromium-substituted Sr hexaferrites. Although these compounds show paramagnetic—ferrimagnetic long range transitions, some of their magnetic properties are similar to those of re-entrant spin glasses [3]. These authors have interpreted the results by hypothesising the coexistence of long range and short range magnetic order in the compounds investigated. However, for a rigorous determination of the magnetic order in PbFe Cr O hexaferrites, 12~x x 19 further investigations, using other experimental techniques as low field and high field susceptibility measurements at low temperature and neutron diffraction experiments are needed.
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4. Conclusions Mo¨ssbauer spectroscopy has been performed on PbFe Cr O (0)x)6) hexaferrite, between 12~x x 19 77 and 800 K. The main hyperfine parameters for the Fe57 nuclei in the five iron sublattices have been determined. The data indicate that Cr3` ions do not occupy the hexahedral lattice sites (b) and that the magnetic moments of the Fe3` ions remain axially aligned for all values of x. The steep fall in the saturation magnetisation at 0 K, with increasing x cannot be interpreted as being due the simple replacement of the Fe3` magnetic moment (5k ) B with that of Cr3` (3k ), but it is a consequence of B the onset of a complex magnetic order. The differences in the values of the Curie temperatures deduced from Mo¨ssbauer and static magnetic measurements and some features of the Mo¨ssbauer spectra seems to indicate that complex spin—spin relaxation phenomena occur. References [1] X. Obradors, A. Isalgue, A. Collomb, M. Pernet, J. Pannetier, J. Rodrigues, J. Tejada, J.C. Joubert, IEEE Trans. Magn. 20 (1984) 1636. [2] J.A. Pereda, A. Isalgue, J. Tejada, F.J. Litterst, X. Obradors, Hyperfine Interactions 28 (1986) 569. [3] X. Obradors, A. Labarta, J. Tejada, M. Pernet, J.L. Tholence, M. Saint-Paul, B. Barbara, J. Appl. Phys. 63 (1988) 4091. [4] P. Wartewig, K. Melzer, M. Krause, R. Tellgren, J. Magn. Magn. Mater. 140—144 (1995) 2101. [5] S. Dı´ az, J.L. Sa´nchez, F. Leccabue, B.E. Watts, G. Bocelli, G. Albanese, J. de Physique 7 (1997) C1-331. [6] I.A. Campbell, Hyperfine Interactions 27 (1986) 15. [7] G. Albanese, F. Leccabue, B.E. Watts, S. Dı´ az, in: I. Ortalli (Ed.), Proc. ICAME-95, SIF, Bologna, Italy, 1996, p. 279. [8] G. Albanese, J. Magn. Magn. Mater. 147 (1995) 421. [9] D.G. Wickham, J.B. Goodenough, Phys. Rev. 115 (1959) 1156.