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RELAXATION EFFECTS IN MAGNETITE (Fe3O4)
USING SELECTIVE EXCITATION / DOUBLE
MÖSSBAUER (SEDM) PROCEDURES
B. Balko, G. Hoy
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n° 12, Tome 37, Décembre 1976, page C6-89
RELAXATION EFFECTS IN MAGNETITE (Fe
30
4)
USING SELECTIVE EXCITATION
DOUBLE MOSSBAUER (SEDM) PROCEDURES (*)
B. BALKO (t) and G. R. HOY
Physics Department Boston University, Boston, Massachusetts, U. S. A.
Résumé. — Nous avons utilisé la technique SEDM (diffusion Môssbauer en présence d'excita-tion sélective) pour étudier le temps de saut des électrons dans les sites octaédriques B de la magné-tite. Nous avons trouvé qu'à la température ambiante et à 125 K ce temps de saut est, ou nul, ou inférieur à 10- 1 J s. A 112 K les résultats SEDM pour les sites octaédriques B sont nettement
diffé-rents de ceux obtenus à température ambiante et à 125 K. Il est possible qu'il y ait de la relaxation, mais nous ne pouvons évaluer le temps de saut vu l'absence de modèle théorique adéquat. Le résul-tat le plus surprenant de cette étude est l'observation d'une relaxation sur les sites A aux trois tem-pératures.
Abstract. — We have applied the SEDM technique to study the electron hopping time at the octahedral (B) sites in magnetite. We have found that this relaxation time is either zero or less than 10"11 s at room temperature and 125 K. At 112 K the SEDM results for the octahedral (B)
sites are quite different from those at room temperature and 125 K. Relaxation may be occurring, but in the absence of a theoretical model no time estimate can be made. The most surprising result was the observation of relaxation at the A sites at all three temperatures.
Although magnetite, F e304, has been studied for
quite some time, there are still some unresolved pro-blems. Magnetite crystallizes in a cubic inverse spinel structure above the Verwey transition (Tv — 119 K). In magnetite both ferrous and ferric ions are present at the octahedral (B) sites, while only ferric ions are on the tetrahedral (A) sites. At room temperature the electrical conductivity of F e304 is unusually high
(250 £ 1- 1 c m- 1) . The explanation of this fact is
thought to be a rapid electron exchange between the ferrous and ferric ions on the octahedral (B) sites. The electrical conductivity results suggest that the electron exchange relaxation time is about 10"1 1 s at
room temperature. However, previous Mossbauer absorption measurements [1] on magnetite at room temperature have found the electron exchange rela-xation time to be 10 ~9 s. The Mossbauer result was
obtained by examining the line broadening in the hyperfine pattern associated with the octahedral (B) sites. It was the discrepancy between the electrical conductivity, and Mossbauer results that prompted our interest in magnetite.
Conventional Mossbauer absorption spectroscopy has some disadvantages when applied to the study of relaxation problems. Thickness, inhomogeneous, and/ or small field effects will produce line broadening which can be confused with relaxation in certain cases. Recently, selective excitation double Mossbauer
(*) Supported by the National Science Foundation Grant No. DMR 73-07665A03.
(f) Present address, National Institutes of Health, Bethesda, Maryland.
(SEDM) procedures [2] have been developed which can detect relaxation processes in cases where it is difficult to observe otherwise. A complete, general theory which can be applied quantitatively to SEDM experiments has not yet been formulated, although calculations neglecting thickness considerations and Rayleigh scattering have recently been published [3, 4]. In the meantime, we can still perform experiments and compare the results with the rather well established SEDM theory in the absence of relaxation [2].
In figure 1 we show a schematic diagram of an
DRIVE FUNCTION ELECTRONICS 8ENERAT0R 1 I ' ' ' SYNCH PULSE DRIVE j 1 . • 2 M U L T I -C A D CHANNEL ANALYZER 57 I C o (SOURCE) I • I
i J h J T
D R I V E * I f ^ SCATTERER i — • CVO H - * - \ C " SCA ' • « w J » _ S l N 8 L E LINE ' 1 r^—l ANALYZER DRIVE , „ . „ . . • 1 ELECTRONICS DETECTOR ' ' AMPI
PREAMP. — — FUNCTION GENERATORFIG. 1. —-In this figure we show a schematic representation of the experimental configuration used in SEDM. CVD stands for constant velocity drive, and CAD for constant acceleration drive.
C6-90 B. BALK0 AND G. R. HOY
experimental SEDM configuration. In this technique a relative to each other for visual display purposes. single line source is driven at a pre-determined cons- One line is shown for 296 K, one for 125 K, and one for tant velocity. The source radiation impinges on the 112 K. On this figure we have plotted the CVD setting scatterer, which is made of the material under investi-
gation i. e. magnetite in this case. Consequently some of the nuclei in the scatterer become resonantly excited. The radiation subsequently emitted by the excited scatterer is analyzed using a resonant single line absor- ber moving at constant acceleration. In this way, the energy spectral distribution of the radiation coming from the scatterer can be determined.
Although, as mentioned above, no complete theory a
of SEDM including relaxation exists, we can make - Z
I-
several observations. When there is no relaxation I- W '0
occurring in the scatterer, the experimental SEDM peak 0 5
appears at the excitation energy [2] with perhaps some U
asymmetry. However, when relaxation is present, the experimental peak is shifted toward the resonance position and the asymmetry can be quite pronounced. In figure 2 we show a typical Mossbauer spectrum of magnetite at room temperature [ 5 ] . The dips labeled B
Velocity (crn/s)
FIG. 2. - This figure shows a typical Mossbauer absorption spectrum using magnetite at room temperature [ 5 ] . The dips labeled B arise from the contribution due to the octahedral sites,
and those labeled A are due to the tetrahedral sites.
correspond to the hyperfine pattern due to iron nuclei on the octahedral (B) sites, while those labeled A
correspond, similarly, to the tetrahedral (A) sites. We
have concentrated our SEDM studies on the region in the neighborhood of the 1 A and 1 B dips. To be more precise, at room temperature we excited the scatterer at five different CVD settings : on the 1 A resonance, on the 1 B resonance and at three other
positions off these resonances. At 125 K we excited the magnetite sample at seven different positions, and at 112 K five positions.
As mentioned above, when a non-relaxing resonance peak is excited on or off resonance, the resulting SEDM dip appears at the excitation energy. In figure 3 we show three straight lines which have been shifted
VELOCITY POSITION OF SCATTERED PEAK
FIG. 3. -The velocity position of the experimental SEDM dips is plotted versus the corresponding CVD setting. The filled triangles represent our results at room temperature. The open circles are from data at 125 K and the filled circles from results at 112 K. Data that fall on the straight lines indicate that the experimental SEDM dips have minima at the excitation positions.
97
I I 1 1 1 1 . 1 I I I I I
60 70 80 90 100 110 120 130 140 I50
VELOCITY (IN CHANNELS)
FIG. 4. - Our SEDM results for magnetite at room temperature are shown in this figure. The arrows labeled 1A and 1B and the dashed vertical lines locate the positions of those resonances (see Fig. 2). The small arrows locate the excitation energy (i. e.
CVD setting) for each of the cases. The dots are the data and the solid curves give the theoretical SEDM results assuming no
RELAXATION EFFECTS I N MAGNETITE USING SEDM C6-9 1
versus the position of the resulting experimental
SEDM dip. The meaning of figure 3 is that, if a plotted
point falls on the appropriate straight line, the SEDM
dip has appeared at the excitation energy. Thus from figure 3 we see that, for the most part, the various data points do fall on the appropriate straight lines. This indicates that relaxation may not be occurring at all except, perhaps, at the A sites. Notice that those data points obtained by exciting the 1 A resonance off
resonance deviate considerably from the straight line. We can obtain much more definitive information by looking at the actual SEDM experimental results and comparing them with non-relaxing SEDM calculations.
These results are shown in figures 4, 5, and 6. Consider first the room temperature results given in figure 4. We see that, except for bottom spectrum in the figure, the data are well represented by the calculations without including relaxation effects. It is important to point out that there is only one free parameter per figure. This parameter is simply a normalization factor
VELOCITY (IN CHANNELS)
FIG. 5. - Our SEDM results for magnetite at 125 K are shown in this figure. The arrows labeled 1A and 1B and the dashed vertical lines locate the positions of those resonances (see Fig. 2). The small arrows locate the excitation energy (i. e. CVD setting) for each of the cases. The dots are the data and the solid curves give the theoretical SEDM results assuming no relaxation.
T i :
50 I 60 70 80 90 100 110 I I I I ~ E 120 130 I I 140 I 150 I I VELOCITY (IN CHANNELS)
FIG. 6.
-
Our SEDM results for magnetite at 112 K are shown in this figure. The arrows labeled 1A and 1B and the dashed vertical lines locate the positions of those resonances (see Fig. 2).The small arrows locate the excitation energy (i. e. CVD setting) for each of the cases. The dots are the data and the solid curves give the theoretical SEDM results assuming no relaxation.
chosen to fit one spectrum in the figure. For the case of figure 4, the second spectrum from the top, labeled N, was selected for this purpose. The calculations for the other spectra in figure 4 have no free parameters. These calculations were made using a ratio of B to A sites equal to two and a sample thickness parameter
p
= 336.We see that figure 4 presents two interesting results. First, it looks as if there is no relaxation at the B sites, or the electron exchange relaxation time is so short that we obtain a motional narrowedresult. In the second interpretation the relaxation rate must be so rapid that the SEDM technique can not distinguish between that and a static effect. Our best estimate of the relaxation time according to the second interpretation is that it must be less than lo-'' s. It certainly is not l o v 9 s. The second result of figure 4 is even more surprising. It appears that there is relaxation at the A sites at room temperature. We have observed this result over a period of several months on the same sample.
C6-92 B. BALK0 AND G . R. HOY
relaxation, or very rapid relaxation is occurring at the B sites at 125 K. Next, notice the bottom spectrum of figure 5. As noted in figure 3, we see that the dip hasn't moved to the right very far, but the size of the dip is too small and the shape is quite distorted.
In figure 6 we see our results at 112 K i. e. below the Verwey transition. Previous Mossbauer measure- ments below the Verwey transition [6] have shown that the spectrum is composed of five components. It is of considerable interest to realize that complicated
component structure does not seriously hinder SEDM relaxation studies. Thus SEDM studies even below T., are very useful. From figure 6 we see that the 1 B SEDM experimental line shapes can not be adequately fitted using static SEDM theory. However, we do not know of a relaxation model for the B sites in the magne- tite at 112 K, which we could use in order to estimate any relaxation times. In conclusion, we also note in figure 6 that the A sites again show what appears to be relaxation effects.
References
[I] KUNDIG, W. and HARGROVE, R. S., Solid State Commun. 7 [4] HARTMANN-BOUTRON, F., J. Physique 37 (1976) 549. (1969) 223.
[2] BALKO, B. and HOY, G. R., Phys. Rev. B 10 (1974) 36. [5] Taken from VAN DER WOUDE, F., SAWATZKY, G . A. and [31 AFANASEV, A. M. and GOROBCHENKO, V. D., Zh. T. E. F. 67 MORRISH, A. H., Phys. Rev. 167 (1968) 533.
(1974) 2246, English translation Sov. .phys. JETP 40 [6] HARGROVE, R. S. and KUNDIG, W., Solid State Comrnnn. 8
