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Super-transferred hyperfine interactions at diamagnetic ions in ferrimagnetic insulators
Solid State Communications, Vol. 6, pp. 277-280, 1968.
Pergamon Press.
Printed in Great Britain
SUPER-TRANSFERRED HYPERFINE INTERACTIONS AT DIAMAGNETIC IONS IN FERRIMAGNETIC INSULATORS* S.L. Ruby Argonne National Laboratory, Argonne, fllinois, U. S. A. and B.J. Evans and S.S. Hafner University of Chicago, Chicago, Illinois, U. S. A. (Received 1 January 1968 by G. Busch) Large magnetic fields at antimony nuclei octahedrally surrounded by oxygen in ferrimagnetic spinels have been measured. These results are similar to those found earlier for tin in garnets. In order to explain these fields as resulting mainly from spin polarization of the outer 5s electrons of these ions, the magnitude of this polarization must be rather large.
THERE is at present only a qualitative understanding of the relative magnitudes of those effects thought to be responsible for the transferred hyperfine structure in either metals or insulators. 1 The difficulties are mainly due to the fact that several mechanisms usually are simultaneously operative. Through the substitution of diamagnetic ions into magnetic systems, some progress has various been made in assessing the relative importance of the various mechanisms.2 The recent results on 119Sn inthe 4’ garnet~’ (Gd (Y3~Ca,,)[Fe2_,Sn, (Fe3 )012 and 3 Car) r Fe2 Sn,: (Fe3 )012 prompted the present M~ssbauerstudy on the isoelectronic Sb~ion in the spinels Ni1+2,Fe23,Sb,04. It was hoped that comparing the results from these two systems would enable us to make some assessment of the relative importance of the mechanisms responsible for super-transferred hyperfine interaction in insulators. There is also the possibility that the origin of the complexity of the ~‘Fe Mössbauer 9hyperfine can also structure be resolved in certain by a careful spinel compounds consideration of both the ‘~‘Sband Fe hyperfine i.nteractions. *Work performed under the auspices of the U.S. Atomic Energy Commission and National Science Foundation (under Grant GA-1134). -.
~“
277
The spinel system provides two crystallographically nonequivalent sets of sites for the cations: one set of tetrahedral (A) sites with a multiplicity of 8 in the face-centered unit cell and one set of octahedral rB sites with a multiplicity of 16. Studies of diffraction intensities have shown~ that Sb~occurs (almost) exclu3~ and Ni2~are sively at the [B] sites, but Fe distributed over the (A) as well as the [B: sites. The cation distribution over the sites is given by the formula (Ni Approximate values of y [Ni17+2,Fe1+7~,Sb,]O4. have been deduced from 1Fe1_7) saturation-magnetization measurements by use of the Néel model. The samples were prepared according to the technique of Blass&c and, by use of X-ray powder diffraction were determined to be pure spinel phases. Standard M~ssbauertransmission experiments were performed with both the CaSn(Sb)03 source and the absorber at 80°K. from The Curie 858° points to 590°K. of the various samples ranged The 18-line hyperfine structure of ~1Sb Mössbauer spectra was described previously. ‘~ A pure magnetic spectrum appears as an eightline pattern; the lines are equally spaced and their relative Intensities are 1, 9, 37, 37, 37, 9,1.
278
SUPER-TRANSFERRED HYPERFINE INTERACTIONS I
Vol. 6, No. 5
magnetic fields, a single isomeric shift, and a
I
zero quadrupole interaction. Further, the relative frequencies P(n,y) and magnitudes H~of the several magnetic fields were correlated with the parameter y in accordance with the expressions, P(n,y)
=
(~)y~(1
~y)E~~
H~= H, (,!_~i1,
2~ the six A the Both site fraction neighbors oflimited all of(A) athe available given sites Sb~ occupied andamong ion verification the and by large Ni2 y numis ~ ber where ofthe fields n resolution is the number ofquantitative Ni
________________________________ ~
~~fN\x~33~.o33
-16.0
-8.0 0 8.0 VELOCITY (mm/eec)
Fig. of the1definitely details have been of confirmed. this calculated result, by butuse the ofmain the above features were The solid lines in relation between the frequencies and magnitudes of the various fields. It should be noted that, even as x — 0, the six neighboring rB sites of each antimony ion are occupied by iron or nickel ions with equal probability. Thus, the spectrum for x = 0. 05 is a clear demonstration of the insignificance of the B-B interaction.
6.0
FIG. I Experimental and calculated “Sb spectra for Fe-Ni-Sb spinels of various concentrations X.
This is not the case for our ferrite spectra (Fig. 1). The observation that lines 4 and 5 are more intense than 3 and 6 suggests that the spectra consist of several superposed eight-line patterns resulting from several magnetically nonequivalent Sb~ positions in the structure. The isomeric shifts exclude any Sb’~ contributions, The symmetry in the spectra indicates that the electric-quadrupole interaction of ‘Sb is small, This is supported by our observation 2qQ
The probabilistic model used above is necessary to explain the numerous magnetic fields seen at the antimony nucleus. A similar model had been invoked earlier” to explain some features of the bulk-magnetization data. Again, two recent papers have used such considerations to explain some of the finer details of the E’~Fe NGR spectra for the cobalt ferrite~ CoFe,0 4 0 CuFe and the copper ferrite 2O4. From the above results, it may be concluded that H,.. at an Sb’ nucleus is due primarily to its (A)-site neighbors. It is also clear that each Fe’~ and Ni’’ contributes independently to H,... The maximum hyperfine field H:, that corresponds to an Sb” ion with six Fe’ neighbors, is 315 ~ 10 koe. A plausible mechanism for the origin of such a field can be obtained by extending the covalent mixing model used to explain the magnetic as compounds’ field KMnF, seen atto Finclude nuclei theinoxygen such bridge. This means a joining of work 4 with that together of transferred hyperfine interactions. Crudely, this means a on super-exchange’ LCAO mixing of 3d1 electrons at the Fe~’ion, 2p electrons at the oxygen, and the 5s electrons at the Sb~ion. One result will be a spin polarization of the Sb~’ 5s charge density. The nuclear
Vol. 6, No. 5
SUPER-TRANSFERRED HYPERFINE INTERACTIONS
magnetic field resulting directly from this will probably inner s shells. be augmented by polarization of the The isomer shift of the Sb~ ion in the [BJ site is + 8. 0 mm/sec relative to InSb. In particular, this value does not change between the diamagnetic and magnetic spinels. On the basis of a previous calibration of the isomer shifts, ‘~ 3 the corresponding electron charge density t (0)/a 0 is approximately six. If this were completely spin polarized, the resulting H1~,. would be about 3000 koe. It is, therefore, possible to explain the observed field if such a model can be shown to give about 10 per cent spin polarization. The NGR results obtained with “9Sn in the (Y. Car) Fe 2,,Sn, [(Fe3 )O1; and other garnet systems’’ are reasonably consistent with those obtained from the spinel data. Here the sublattices are designated c, d, and a, respectively, It is to be emphasized that the local environments for [B: sites in spinels is similar to [a sites in garnets; most importantly, each diamagnetic Ion is octahedrally bonded to oxygens that are also joined to the tetrahedral cations. An a appa-
279
The Isomer shift (relative to an Mg2 Sn 7) for the “ atom In the [aJ site was sourcemm/sec and is nearly Identical to that of -1.85 Sn’4 In diamagnetic Sn0 2. With the previously mentioned calibration, 3A this corresponds to a charge density of about 2 electrons per atomic volume. If 100 per cent polarized, this would produce a contact field of —~1000 koe. To explain the observed field of 210 koe, a 5s spin polarization approaching 20 per cent is required. Also, evidence on the direction of the hyperfine field is available. ~ The observation that the absorption lines increased In their splitting in the presence of an applied field means that H,,~ at the ‘~ nucleus in the octahedral 34 [a] ions site at is parallel the tetrahedral to the(d)moment site. The of the above Fe covalentmixing model is consistent with this observed
-,
direction. In summary: (a) The octahedral-oxygentetrahedral Interaction important for magnetic Ions is still dominant when the octahedral ion is replaced by Sb or Sn. (b) The electronic charge density, estimated from the isomer shift, at the diamagnetic nucleus does not differ noticeably between the paramagnetic and the ferrimagnetic
rent difference between the systems is the multi-
compounds studied.
plicity of fields needed to explain the spinel data, while only a single field is needed for the garnets. This problem disappears if the above-described (A) B interaction, which is equivalent to the (d) [a] interaction in garnets, is dominant. This indeed is one of the major conclusions, Only one field is observed since only Fe’ ions occupy the [a] sites, and thus each Sn’ ion experiences the same full six-fold (d) TaT interaction.
tions (about 10—20 per cent are required to explain the observed hyperfine fields by spin polarization of the 5s electrons.
(c) Rather large polariza-
—
Acknowledgments We wish to thank A.J. Freeman for several helpful discussions, R. Preston for improving our computer techniques to make possible the elaborate minimizations needed here, and B. Zabransky for clever and careful help in the laboratory.
—
-
—
References 1.
WATSON R.E. and FREEMAN A.J., Hyperfine Interactions, (eds. FREEMAN A.J. and FRANXEL R.B.) Academic Press, p. 53, New York.
2.
SHIRLEY D.A.
3.
BELOV K.P. and LYUBUTIN I.S.,
4.
GOLDANSKII V.1., TRUIGiTANOV V.A., 1, 19 (1965).
5.
GOLDANSKII V. I., V.A., ITETP Left.
6.
LYUBUTIN I.S.,
7.
BELOV K.P. and LYUBTJTIN LS., Soviet Phys. JETP 22, 518 (1966).
,
Phys. Lett. 25A, 129 (1967). JETP Lett.
DEVISHEVA M. N., 4, 42 (1966).
1, 16 (1965).
DEVISHEVA M.N. and VELOV V. F., MAKAROV E. F.,
JETP Lett.
NOVIKOV G. V. and TRUKHTANOV
Soviet Phys. solid St. 8, 519 (1966).
280
SUPER-TRANSFERRED HYPERFINE INTERACTIONS
Vol. 6, No. 5
8.
SAWATZKY G.A., van der WOUDE F. and MORRISH A.H., J. appi. Phys. bit. Cong. on Magnetism , to be published.
9.
EVANS B. J., HAFNER S., J. appi. Phys. Tnt. Cong. on Magnetism, to be published.
10. BLASSE G., PhUips Res. Rep. 3, Supple. 1, (1964). 11. RUBY S.L. and KALVIUS G.M., Phys. Rev.
155, 353 (1967).
12. GELLER S., WILLL&MS H. J., ESPINOSA G. P. and SHERWOOD R. C., Bell Syst. Tech. J. 43, 565 (1964). 13. NOWIK I. and OFER S., Phys. Rev. 153, 409 (1967). 14. HALPERN V., 15. RUBY S.L.,
Proc. Roy. Soc. Lond. 291A, 113 (1966). KALVIIJS G.M., BEARD G.B. and SNYDER R.E.,
Phys. Rev. 159, 239 (1967).
En étudiant des spinelles ferrimagnétiques, des forts champs magnétiques ont èté mesurés sur des noyaux d’antimoine entourés d’oxygenes en positions octahédriques. Ces résultats sont analogues a ceux trouvés antérleurenient pour l’étain dans les grenats. Dans l’hypothese o~ces champs seralent düs principalement a une pola.risation du spin des electrons 5s des couches extérieures, cette polarisation serait assez grande.
Pergamon Press.
Printed in Great Britain
SUPER-TRANSFERRED HYPERFINE INTERACTIONS AT DIAMAGNETIC IONS IN FERRIMAGNETIC INSULATORS* S.L. Ruby Argonne National Laboratory, Argonne, fllinois, U. S. A. and B.J. Evans and S.S. Hafner University of Chicago, Chicago, Illinois, U. S. A. (Received 1 January 1968 by G. Busch) Large magnetic fields at antimony nuclei octahedrally surrounded by oxygen in ferrimagnetic spinels have been measured. These results are similar to those found earlier for tin in garnets. In order to explain these fields as resulting mainly from spin polarization of the outer 5s electrons of these ions, the magnitude of this polarization must be rather large.
THERE is at present only a qualitative understanding of the relative magnitudes of those effects thought to be responsible for the transferred hyperfine structure in either metals or insulators. 1 The difficulties are mainly due to the fact that several mechanisms usually are simultaneously operative. Through the substitution of diamagnetic ions into magnetic systems, some progress has various been made in assessing the relative importance of the various mechanisms.2 The recent results on 119Sn inthe 4’ garnet~’ (Gd (Y3~Ca,,)[Fe2_,Sn, (Fe3 )012 and 3 Car) r Fe2 Sn,: (Fe3 )012 prompted the present M~ssbauerstudy on the isoelectronic Sb~ion in the spinels Ni1+2,Fe23,Sb,04. It was hoped that comparing the results from these two systems would enable us to make some assessment of the relative importance of the mechanisms responsible for super-transferred hyperfine interaction in insulators. There is also the possibility that the origin of the complexity of the ~‘Fe Mössbauer 9hyperfine can also structure be resolved in certain by a careful spinel compounds consideration of both the ‘~‘Sband Fe hyperfine i.nteractions. *Work performed under the auspices of the U.S. Atomic Energy Commission and National Science Foundation (under Grant GA-1134). -.
~“
277
The spinel system provides two crystallographically nonequivalent sets of sites for the cations: one set of tetrahedral (A) sites with a multiplicity of 8 in the face-centered unit cell and one set of octahedral rB sites with a multiplicity of 16. Studies of diffraction intensities have shown~ that Sb~occurs (almost) exclu3~ and Ni2~are sively at the [B] sites, but Fe distributed over the (A) as well as the [B: sites. The cation distribution over the sites is given by the formula (Ni Approximate values of y [Ni17+2,Fe1+7~,Sb,]O4. have been deduced from 1Fe1_7) saturation-magnetization measurements by use of the Néel model. The samples were prepared according to the technique of Blass&c and, by use of X-ray powder diffraction were determined to be pure spinel phases. Standard M~ssbauertransmission experiments were performed with both the CaSn(Sb)03 source and the absorber at 80°K. from The Curie 858° points to 590°K. of the various samples ranged The 18-line hyperfine structure of ~1Sb Mössbauer spectra was described previously. ‘~ A pure magnetic spectrum appears as an eightline pattern; the lines are equally spaced and their relative Intensities are 1, 9, 37, 37, 37, 9,1.
278
SUPER-TRANSFERRED HYPERFINE INTERACTIONS I
Vol. 6, No. 5
magnetic fields, a single isomeric shift, and a
I
zero quadrupole interaction. Further, the relative frequencies P(n,y) and magnitudes H~of the several magnetic fields were correlated with the parameter y in accordance with the expressions, P(n,y)
=
(~)y~(1
~y)E~~
H~= H, (,!_~i1,
2~ the six A the Both site fraction neighbors oflimited all of(A) athe available given sites Sb~ occupied andamong ion verification the and by large Ni2 y numis ~ ber where ofthe fields n resolution is the number ofquantitative Ni
________________________________ ~
~~fN\x~33~.o33
-16.0
-8.0 0 8.0 VELOCITY (mm/eec)
Fig. of the1definitely details have been of confirmed. this calculated result, by butuse the ofmain the above features were The solid lines in relation between the frequencies and magnitudes of the various fields. It should be noted that, even as x — 0, the six neighboring rB sites of each antimony ion are occupied by iron or nickel ions with equal probability. Thus, the spectrum for x = 0. 05 is a clear demonstration of the insignificance of the B-B interaction.
6.0
FIG. I Experimental and calculated “Sb spectra for Fe-Ni-Sb spinels of various concentrations X.
This is not the case for our ferrite spectra (Fig. 1). The observation that lines 4 and 5 are more intense than 3 and 6 suggests that the spectra consist of several superposed eight-line patterns resulting from several magnetically nonequivalent Sb~ positions in the structure. The isomeric shifts exclude any Sb’~ contributions, The symmetry in the spectra indicates that the electric-quadrupole interaction of ‘Sb is small, This is supported by our observation 2qQ
The probabilistic model used above is necessary to explain the numerous magnetic fields seen at the antimony nucleus. A similar model had been invoked earlier” to explain some features of the bulk-magnetization data. Again, two recent papers have used such considerations to explain some of the finer details of the E’~Fe NGR spectra for the cobalt ferrite~ CoFe,0 4 0 CuFe and the copper ferrite 2O4. From the above results, it may be concluded that H,.. at an Sb’ nucleus is due primarily to its (A)-site neighbors. It is also clear that each Fe’~ and Ni’’ contributes independently to H,... The maximum hyperfine field H:, that corresponds to an Sb” ion with six Fe’ neighbors, is 315 ~ 10 koe. A plausible mechanism for the origin of such a field can be obtained by extending the covalent mixing model used to explain the magnetic as compounds’ field KMnF, seen atto Finclude nuclei theinoxygen such bridge. This means a joining of work 4 with that together of transferred hyperfine interactions. Crudely, this means a on super-exchange’ LCAO mixing of 3d1 electrons at the Fe~’ion, 2p electrons at the oxygen, and the 5s electrons at the Sb~ion. One result will be a spin polarization of the Sb~’ 5s charge density. The nuclear
Vol. 6, No. 5
SUPER-TRANSFERRED HYPERFINE INTERACTIONS
magnetic field resulting directly from this will probably inner s shells. be augmented by polarization of the The isomer shift of the Sb~ ion in the [BJ site is + 8. 0 mm/sec relative to InSb. In particular, this value does not change between the diamagnetic and magnetic spinels. On the basis of a previous calibration of the isomer shifts, ‘~ 3 the corresponding electron charge density t (0)/a 0 is approximately six. If this were completely spin polarized, the resulting H1~,. would be about 3000 koe. It is, therefore, possible to explain the observed field if such a model can be shown to give about 10 per cent spin polarization. The NGR results obtained with “9Sn in the (Y. Car) Fe 2,,Sn, [(Fe3 )O1; and other garnet systems’’ are reasonably consistent with those obtained from the spinel data. Here the sublattices are designated c, d, and a, respectively, It is to be emphasized that the local environments for [B: sites in spinels is similar to [a sites in garnets; most importantly, each diamagnetic Ion is octahedrally bonded to oxygens that are also joined to the tetrahedral cations. An a appa-
279
The Isomer shift (relative to an Mg2 Sn 7) for the “ atom In the [aJ site was sourcemm/sec and is nearly Identical to that of -1.85 Sn’4 In diamagnetic Sn0 2. With the previously mentioned calibration, 3A this corresponds to a charge density of about 2 electrons per atomic volume. If 100 per cent polarized, this would produce a contact field of —~1000 koe. To explain the observed field of 210 koe, a 5s spin polarization approaching 20 per cent is required. Also, evidence on the direction of the hyperfine field is available. ~ The observation that the absorption lines increased In their splitting in the presence of an applied field means that H,,~ at the ‘~ nucleus in the octahedral 34 [a] ions site at is parallel the tetrahedral to the(d)moment site. The of the above Fe covalentmixing model is consistent with this observed
-,
direction. In summary: (a) The octahedral-oxygentetrahedral Interaction important for magnetic Ions is still dominant when the octahedral ion is replaced by Sb or Sn. (b) The electronic charge density, estimated from the isomer shift, at the diamagnetic nucleus does not differ noticeably between the paramagnetic and the ferrimagnetic
rent difference between the systems is the multi-
compounds studied.
plicity of fields needed to explain the spinel data, while only a single field is needed for the garnets. This problem disappears if the above-described (A) B interaction, which is equivalent to the (d) [a] interaction in garnets, is dominant. This indeed is one of the major conclusions, Only one field is observed since only Fe’ ions occupy the [a] sites, and thus each Sn’ ion experiences the same full six-fold (d) TaT interaction.
tions (about 10—20 per cent are required to explain the observed hyperfine fields by spin polarization of the 5s electrons.
(c) Rather large polariza-
—
Acknowledgments We wish to thank A.J. Freeman for several helpful discussions, R. Preston for improving our computer techniques to make possible the elaborate minimizations needed here, and B. Zabransky for clever and careful help in the laboratory.
—
-
—
References 1.
WATSON R.E. and FREEMAN A.J., Hyperfine Interactions, (eds. FREEMAN A.J. and FRANXEL R.B.) Academic Press, p. 53, New York.
2.
SHIRLEY D.A.
3.
BELOV K.P. and LYUBUTIN I.S.,
4.
GOLDANSKII V.1., TRUIGiTANOV V.A., 1, 19 (1965).
5.
GOLDANSKII V. I., V.A., ITETP Left.
6.
LYUBUTIN I.S.,
7.
BELOV K.P. and LYUBTJTIN LS., Soviet Phys. JETP 22, 518 (1966).
,
Phys. Lett. 25A, 129 (1967). JETP Lett.
DEVISHEVA M. N., 4, 42 (1966).
1, 16 (1965).
DEVISHEVA M.N. and VELOV V. F., MAKAROV E. F.,
JETP Lett.
NOVIKOV G. V. and TRUKHTANOV
Soviet Phys. solid St. 8, 519 (1966).
280
SUPER-TRANSFERRED HYPERFINE INTERACTIONS
Vol. 6, No. 5
8.
SAWATZKY G.A., van der WOUDE F. and MORRISH A.H., J. appi. Phys. bit. Cong. on Magnetism , to be published.
9.
EVANS B. J., HAFNER S., J. appi. Phys. Tnt. Cong. on Magnetism, to be published.
10. BLASSE G., PhUips Res. Rep. 3, Supple. 1, (1964). 11. RUBY S.L. and KALVIUS G.M., Phys. Rev.
155, 353 (1967).
12. GELLER S., WILLL&MS H. J., ESPINOSA G. P. and SHERWOOD R. C., Bell Syst. Tech. J. 43, 565 (1964). 13. NOWIK I. and OFER S., Phys. Rev. 153, 409 (1967). 14. HALPERN V., 15. RUBY S.L.,
Proc. Roy. Soc. Lond. 291A, 113 (1966). KALVIIJS G.M., BEARD G.B. and SNYDER R.E.,
Phys. Rev. 159, 239 (1967).
En étudiant des spinelles ferrimagnétiques, des forts champs magnétiques ont èté mesurés sur des noyaux d’antimoine entourés d’oxygenes en positions octahédriques. Ces résultats sont analogues a ceux trouvés antérleurenient pour l’étain dans les grenats. Dans l’hypothese o~ces champs seralent düs principalement a une pola.risation du spin des electrons 5s des couches extérieures, cette polarisation serait assez grande.