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The B-site Mössbauer linewidth in Fe3O4
Volume 57A, number 4
PHYSICS LETTERS
28 June 1976
THE B-SITE MOSSBAUER LLNEWIDTH IN Fe3 04 A.M. Van DIEPEN Philips Research Laboratories, Eindhoven, The Netherlands
Received 21 May 1976 It is found that the lmewidth of the B-site MOssbauer spectrum of an Fe304 single crystal at room temperature is strongly dependent on the direction of the externally applied magnetic field. The broadening of the line lB observed in polycrystalilne materials is concluded to be largely determined by magnetic dipolar and electric quadrupolar effects, and therefore cannot serve as proof for an electron hopping model. Magnetite (Fe304) is a good electrical conductor
above the Verwey transition (119 K) and an insulator below it. The conduction originates from the presence on the2~and (octahedral) B-sites of nominally equal amounts Fe3~that are supposed either to take part of Fe in a fast thermally activated electron exchange (hopping) or to produce a conduction band. A review of experimental data can be found in ref. [lj where it is concluded that a band description is most appropriate for the conduction mechanism above the Verwey transition. The strongest argument in favor of the electronhopping picture was the broadening of the B-site lines in the Mössbauer spectra and its temperature dependence, as compared to those from the (tetrahedral) Asites [2, 3J. In particular the Bl and Al lines, i.e., the outermost negative velocity peaks of the A and B sublattices, were studied this and respect, are well separated from each in other fromsince otherthey peaks. On the other hand, Iineshape analyses of spectra taken on doped magnetite as compared with undoped samples are used to provide evidence for spin and charge density oscillations [4]. Thus, fargoing conclusions about the conduction mechanism and electronic states in Fe3 04 are derived from the shape and width of the
B-site Mdssbauer lines. It is evident that a critical analysis of this spectrum is imperative and that, in particular, other possible mechanisms for line broadening have to be investigated. This is the purpose of the present study.
The octahedral sites in spinels have trigonal symmetry with the local axis along a [111] direction, which implies that nuclear quadrupole interactions and magnetic dipolar fields are nonzero. Both of these produce a splitting of the Mdssbauer spectrum that may be ob-
354
served as line broadening. In this study it will be shown that contributions from quadrupolar and magnetic dipolar effects can be separated from the B-site linewidth
by a relatively small magnetic field alongsample. differentapplying crystallographic directions in a single crystal It is found that the B-site linewidth in Fe 3 04 is basically not different from that of the A-site, so that it cannot be used as a proof for the electron hopping model. Secondly, it is concluded that an analysis of the lineshape based on an unperturbed Bl Lorentz line with a width much larger than 0.33 mm/sec may lead to unphysical results. Mossbauer spectra were taken at room temperature with a conventional constant-acceleration spectrometer operating in the sawtooth mode. The spectrometer was calibrated n-Fe foil and a-Fe203. The source 57Co in Pdversus matrix. A (110) single-crystal platelet of was Bridgeman-grown Fe 3 04, 85 pm thick, was used as absorber. It could be rotated in an external magnetic field H of 8 kG. Measurements were taken with Hparallel to the [001], [110], and [1111 directions. The field was always applied in the plane of the platelet while the direction of the ‘y rays was perpendicular to both
platelet and field. Laue diffraction was used to determine the main directions in the platelet. To first order the electric quadrupolar and magnetic dipolar shifts in Ml5ssbauer both proportional 2O 1), where 0 is spectra the anglearebetween the directo (3cos tion of the hyperfine field H~and the local symmetry axis. This notation if frequently used in Massbauer as well as in NMR spectroscopy to determine the direction of easy magnetization, when only polycrystalline samples are available. For the B-site in the spinel structure the symmetry axis is [111]. In fact, in Fe 304 this is also —
Volume 57A, number 4
PHYSICS LETTERS
28 June 1976
~ H//boll
~
~
~
~
~
•.;~‘
Al
:‘~~
~:
~ ~‘
,
‘•
B1 —10-8 Al
Bl ~,
Hll[llll
,~
.
6
8
10
the applied field in the [001] direction, showing that under well-chosen conditions the A and B sites give narrow lines of
~“
practically the same width.
Bi
-lb
—2 C 2 Velocity lmm/sec)
304 with
/ Al
-~
Fig. 2. Room temperature Mdssbauer spectrum of Fe
~
,
-~
external field along [001], i.e., with only one narrow B-site subspectrum. The first estimate for a quadrupole interaction was I
Velocity )mm/sec)
Fig. 1. Low-velocity parts of room temperature MOssbauer
made by Evans [5] ,who decomposed the B-site pattern
spectra of Fe3 04 with a field of 8 kG applied as indicated in the three crystal platelet. main crystallographic directions of a (110) single-
into two sub spectra with 3 : 1 intensity and 2qQ determined at the a value of 0.100mm/sec for the value of I e B-site. Later estimates by Nistor et al. [4,6] arrive at a splitting in the line Bi also of the order of 0.10 mm/sec, which lead them to estimate a broadening of approximately 0.03 mm/sec. In an earlier study on some spinel ferrites in the paramagnetic region we have found that in our sample Fe3+ on the B-site has a quadrupole splitting QS = 9 Ie2qQI varying between 0.30 and 0.54 mm/sec [7] , considerably larger than 0.05 mm! sec. For Fe2~on the B-site we found 2.2 mm/sec. It is noted here that the observation of a small quadrupole splitting in Fe 3 04 disagrees withFe3~as Fe being in a state 2~and required for intermediate between Fe the hopping model to apply. Magnetic dipolar fields on the octahedral sites in doped and undoped Y 3Fe5012, a similar compound, are of the order of 10—15 kG, also leading1 to line shifts of 0.3—0.5 mm/sec [8]. in the as compared to a splitting From r former of about 0.27 mm/sec is derived, which implies a splitting of 0.36 mm/sec in ~ This should be compared with the earlier estimates of 0.10 mm/sec [5,6]. Since ~ in the ground state has no interaction with an electric field gradient, the magnetic dipolar splitting is conveniently obtained in an NMR experiment. Data at 300 and 185K show a splitting of 1 MHz with the lowest internal field for the 0 = 0 line [9,10], corresponding to a magnetic dipolar splitting of 7.5 kG or 0.24 mm/sec for r~’1. The remaining .
the axis of easy magnetization. The anisotropy is low (K 3 at 300 K), so that an external 1 = of —18 Xl erg/cmis an order of magnitude higher field kG,o~ which than the anisotropy field and also large as compared to demagnetizing fields, easily pulls the magnetization, and thus the hyperfine field, to a different direction, Since there are four body diagonals in the cube, there are four different angles 0 for an arbitrary direction of the hyperfme field, and thus four subspectra for the B-site. If H~ along an [001] direction they coincide, so that onlyis one B-site spectrum is observed, For H~II[110] there are two subspectra of equal intensity, while with H~II[111] there are two subspectra with relative intensities 3:1. Fig. 1 gives the low-velo. city of the three spectra. It is immediately seen that parts the line-shape of the line BI reflects the (3 cos20 1) dependence: For HIl [001] only one line is observed of practically the same width as the line Al, for HIl [110] the line Bl is about twice as wide but still symmetric, and for HIl [111] it is asymmetrically broadened.The A-site is perfectly cubic, so that its Mossbauer spectrum is not affected by turning the magnetization, Effective line-widths at half height are: ~A1 = 0.28 mm/sec for all three directions, r~°’1 = 0.30 mm/sec, 0.57 mm/sec, and r}~”]= 0.45 mm/sec. Fig. 2 shows the Mossbauerspectrum of Fe 304 with the —
0.12 mm/sec result from nuclear quadrupole coupling 355
Volume 57A, number 4
PHYSICS LETTERS
alone, leading to I e2qQ I 0.09 mm/sec. The total broadening thus originates for two thirds from the magnetic dipolar field and for one third from the nuclear quadrupole interaction. In the present study it is demonstrated that the linebroadening observed in the B-site Mossbauer spectrum of Fe 3 04 can strongly be suppressed by turning the hyperfine field into the [0011 direction by means of an externally applied field. Thus B-site linebroadening is shown to be the result of electric quadrupolar 20 and—1 magnetic dipolar effects which arethe zeroB-site for 3linewidth, cos = 0. Any conclusions drawn from for instance regarding the electron hopping model, or from its shape starting form an unperturbed value broaderthan 0.30 mm/sec, are therefore unrealistic. Grateful acknowledgement is made to Jacques Verheijden for his valuable technical assistance, and to J.P.M. Damen for growing the crystal.
356
28 June 1976
References [1] B.J. Evans, AlP Conf. Proc. 24 (1975) 73. [2] W. Kundig and R.S. Hargrove, Solid State Comm. 7 (1969)
223. [3] G.A. Phys.Sawatzky, 40 (1969)J.M.D. 1402. Coey and A.H. Morrish, J. Appl. [4] C.I. Nistor, C. Boekema, F. Van der Woude and G.A. Sawatzky,
Proc. 5th Intern. Conf. on Mdssbauer Spectrometry, Bratislava 1973, p. 99.
[51 C.l. B.J. Evans, AlP Proc. 5 (1971) 296.867. [61 Rev.Conf. Roum. Phys. 18 (1973) [71F.K. Nistor, Lotgering and A.M. Van Diepen, J. Phys. Chem. Sob. 34 (1973) 1369.
[81 T.J.A. Popma,
A.M. Van Diepen, and P.F. Bongers, J. Phys.
Chem. Sob. 35 (1974) 201; T.J.A. Popma, A.M. Van Diepen, and J.M. Robertson,
Mat. Res. Bull. 9 (1974) 699. [9] M. Rubinstein, G.H. Strauss, F.J. Bruni, ALP Conf. Proc. 10 (1973) 1384. [10] N.M. Kovtun and A.A. Shamyakov, Solid State Comm. 13 (1973) 1345.
PHYSICS LETTERS
28 June 1976
THE B-SITE MOSSBAUER LLNEWIDTH IN Fe3 04 A.M. Van DIEPEN Philips Research Laboratories, Eindhoven, The Netherlands
Received 21 May 1976 It is found that the lmewidth of the B-site MOssbauer spectrum of an Fe304 single crystal at room temperature is strongly dependent on the direction of the externally applied magnetic field. The broadening of the line lB observed in polycrystalilne materials is concluded to be largely determined by magnetic dipolar and electric quadrupolar effects, and therefore cannot serve as proof for an electron hopping model. Magnetite (Fe304) is a good electrical conductor
above the Verwey transition (119 K) and an insulator below it. The conduction originates from the presence on the2~and (octahedral) B-sites of nominally equal amounts Fe3~that are supposed either to take part of Fe in a fast thermally activated electron exchange (hopping) or to produce a conduction band. A review of experimental data can be found in ref. [lj where it is concluded that a band description is most appropriate for the conduction mechanism above the Verwey transition. The strongest argument in favor of the electronhopping picture was the broadening of the B-site lines in the Mössbauer spectra and its temperature dependence, as compared to those from the (tetrahedral) Asites [2, 3J. In particular the Bl and Al lines, i.e., the outermost negative velocity peaks of the A and B sublattices, were studied this and respect, are well separated from each in other fromsince otherthey peaks. On the other hand, Iineshape analyses of spectra taken on doped magnetite as compared with undoped samples are used to provide evidence for spin and charge density oscillations [4]. Thus, fargoing conclusions about the conduction mechanism and electronic states in Fe3 04 are derived from the shape and width of the
B-site Mdssbauer lines. It is evident that a critical analysis of this spectrum is imperative and that, in particular, other possible mechanisms for line broadening have to be investigated. This is the purpose of the present study.
The octahedral sites in spinels have trigonal symmetry with the local axis along a [111] direction, which implies that nuclear quadrupole interactions and magnetic dipolar fields are nonzero. Both of these produce a splitting of the Mdssbauer spectrum that may be ob-
354
served as line broadening. In this study it will be shown that contributions from quadrupolar and magnetic dipolar effects can be separated from the B-site linewidth
by a relatively small magnetic field alongsample. differentapplying crystallographic directions in a single crystal It is found that the B-site linewidth in Fe 3 04 is basically not different from that of the A-site, so that it cannot be used as a proof for the electron hopping model. Secondly, it is concluded that an analysis of the lineshape based on an unperturbed Bl Lorentz line with a width much larger than 0.33 mm/sec may lead to unphysical results. Mossbauer spectra were taken at room temperature with a conventional constant-acceleration spectrometer operating in the sawtooth mode. The spectrometer was calibrated n-Fe foil and a-Fe203. The source 57Co in Pdversus matrix. A (110) single-crystal platelet of was Bridgeman-grown Fe 3 04, 85 pm thick, was used as absorber. It could be rotated in an external magnetic field H of 8 kG. Measurements were taken with Hparallel to the [001], [110], and [1111 directions. The field was always applied in the plane of the platelet while the direction of the ‘y rays was perpendicular to both
platelet and field. Laue diffraction was used to determine the main directions in the platelet. To first order the electric quadrupolar and magnetic dipolar shifts in Ml5ssbauer both proportional 2O 1), where 0 is spectra the anglearebetween the directo (3cos tion of the hyperfine field H~and the local symmetry axis. This notation if frequently used in Massbauer as well as in NMR spectroscopy to determine the direction of easy magnetization, when only polycrystalline samples are available. For the B-site in the spinel structure the symmetry axis is [111]. In fact, in Fe 304 this is also —
Volume 57A, number 4
PHYSICS LETTERS
28 June 1976
~ H//boll
~
~
~
~
~
•.;~‘
Al
:‘~~
~:
~ ~‘
,
‘•
B1 —10-8 Al
Bl ~,
Hll[llll
,~
.
6
8
10
the applied field in the [001] direction, showing that under well-chosen conditions the A and B sites give narrow lines of
~“
practically the same width.
Bi
-lb
—2 C 2 Velocity lmm/sec)
304 with
/ Al
-~
Fig. 2. Room temperature Mdssbauer spectrum of Fe
~
,
-~
external field along [001], i.e., with only one narrow B-site subspectrum. The first estimate for a quadrupole interaction was I
Velocity )mm/sec)
Fig. 1. Low-velocity parts of room temperature MOssbauer
made by Evans [5] ,who decomposed the B-site pattern
spectra of Fe3 04 with a field of 8 kG applied as indicated in the three crystal platelet. main crystallographic directions of a (110) single-
into two sub spectra with 3 : 1 intensity and 2qQ determined at the a value of 0.100mm/sec for the value of I e B-site. Later estimates by Nistor et al. [4,6] arrive at a splitting in the line Bi also of the order of 0.10 mm/sec, which lead them to estimate a broadening of approximately 0.03 mm/sec. In an earlier study on some spinel ferrites in the paramagnetic region we have found that in our sample Fe3+ on the B-site has a quadrupole splitting QS = 9 Ie2qQI varying between 0.30 and 0.54 mm/sec [7] , considerably larger than 0.05 mm! sec. For Fe2~on the B-site we found 2.2 mm/sec. It is noted here that the observation of a small quadrupole splitting in Fe 3 04 disagrees withFe3~as Fe being in a state 2~and required for intermediate between Fe the hopping model to apply. Magnetic dipolar fields on the octahedral sites in doped and undoped Y 3Fe5012, a similar compound, are of the order of 10—15 kG, also leading1 to line shifts of 0.3—0.5 mm/sec [8]. in the as compared to a splitting From r former of about 0.27 mm/sec is derived, which implies a splitting of 0.36 mm/sec in ~ This should be compared with the earlier estimates of 0.10 mm/sec [5,6]. Since ~ in the ground state has no interaction with an electric field gradient, the magnetic dipolar splitting is conveniently obtained in an NMR experiment. Data at 300 and 185K show a splitting of 1 MHz with the lowest internal field for the 0 = 0 line [9,10], corresponding to a magnetic dipolar splitting of 7.5 kG or 0.24 mm/sec for r~’1. The remaining .
the axis of easy magnetization. The anisotropy is low (K 3 at 300 K), so that an external 1 = of —18 Xl erg/cmis an order of magnitude higher field kG,o~ which than the anisotropy field and also large as compared to demagnetizing fields, easily pulls the magnetization, and thus the hyperfine field, to a different direction, Since there are four body diagonals in the cube, there are four different angles 0 for an arbitrary direction of the hyperfme field, and thus four subspectra for the B-site. If H~ along an [001] direction they coincide, so that onlyis one B-site spectrum is observed, For H~II[110] there are two subspectra of equal intensity, while with H~II[111] there are two subspectra with relative intensities 3:1. Fig. 1 gives the low-velo. city of the three spectra. It is immediately seen that parts the line-shape of the line BI reflects the (3 cos20 1) dependence: For HIl [001] only one line is observed of practically the same width as the line Al, for HIl [110] the line Bl is about twice as wide but still symmetric, and for HIl [111] it is asymmetrically broadened.The A-site is perfectly cubic, so that its Mossbauer spectrum is not affected by turning the magnetization, Effective line-widths at half height are: ~A1 = 0.28 mm/sec for all three directions, r~°’1 = 0.30 mm/sec, 0.57 mm/sec, and r}~”]= 0.45 mm/sec. Fig. 2 shows the Mossbauerspectrum of Fe 304 with the —
0.12 mm/sec result from nuclear quadrupole coupling 355
Volume 57A, number 4
PHYSICS LETTERS
alone, leading to I e2qQ I 0.09 mm/sec. The total broadening thus originates for two thirds from the magnetic dipolar field and for one third from the nuclear quadrupole interaction. In the present study it is demonstrated that the linebroadening observed in the B-site Mossbauer spectrum of Fe 3 04 can strongly be suppressed by turning the hyperfine field into the [0011 direction by means of an externally applied field. Thus B-site linebroadening is shown to be the result of electric quadrupolar 20 and—1 magnetic dipolar effects which arethe zeroB-site for 3linewidth, cos = 0. Any conclusions drawn from for instance regarding the electron hopping model, or from its shape starting form an unperturbed value broaderthan 0.30 mm/sec, are therefore unrealistic. Grateful acknowledgement is made to Jacques Verheijden for his valuable technical assistance, and to J.P.M. Damen for growing the crystal.
356
28 June 1976
References [1] B.J. Evans, AlP Conf. Proc. 24 (1975) 73. [2] W. Kundig and R.S. Hargrove, Solid State Comm. 7 (1969)
223. [3] G.A. Phys.Sawatzky, 40 (1969)J.M.D. 1402. Coey and A.H. Morrish, J. Appl. [4] C.I. Nistor, C. Boekema, F. Van der Woude and G.A. Sawatzky,
Proc. 5th Intern. Conf. on Mdssbauer Spectrometry, Bratislava 1973, p. 99.
[51 C.l. B.J. Evans, AlP Proc. 5 (1971) 296.867. [61 Rev.Conf. Roum. Phys. 18 (1973) [71F.K. Nistor, Lotgering and A.M. Van Diepen, J. Phys. Chem. Sob. 34 (1973) 1369.
[81 T.J.A. Popma,
A.M. Van Diepen, and P.F. Bongers, J. Phys.
Chem. Sob. 35 (1974) 201; T.J.A. Popma, A.M. Van Diepen, and J.M. Robertson,
Mat. Res. Bull. 9 (1974) 699. [9] M. Rubinstein, G.H. Strauss, F.J. Bruni, ALP Conf. Proc. 10 (1973) 1384. [10] N.M. Kovtun and A.A. Shamyakov, Solid State Comm. 13 (1973) 1345.