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A cems and absorption Mössbauer spectroscopy study of iron arachidate Langmuir-Blodgett films
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Solid State Communications, Vol. 86, No. 4, PP. 243-247, 1993. Prinied in Great Britain.
0038-1098/93 $6.00+. 00 Pergamon Press Ltd
A C E M S A N D A B S O R P T I O N M O S S B A U E R S P E C T R O S C O P Y STUDY O F I R O N A R A C H I D A T E L A N G M U I R - B L O D G E T T FILMS
E. Giesse, J. Dengler, G. Ritter, W. Wagner, D. Brandl, H. Voit, G. Saemann-Isehenko Physikalisches Institut der Universitat Erlangen-N0rnberg, Erwin-Rommel-Str. 1, W-8520 Edangen, FRG (Received 4 December 1992 by G. Giintherodt) Langmuir-Blodgett multilayers of ferric arachidate on silicon wafers have been investigated by means of conversion electron M6ssbauer spectroscopy (CEMS) and absorption MOssbauer spectroscopy at different temperatures between room temperature and 4.2 K without and with an external magnetic field of 5 T. The Mrssbauer spectra show Fe 3+. A change of the relative line intensities in the measured spectra dependent on the observation direction is visible. It was found that the lines in the spectra are significantly broadened, which is due to a distribution of electric field gradients and isomer shifts. The reason for this is a slightly disordered iron environment. The quadrupole splitting is linearly correlated to the isomer shift. At low temperatures antiferromagnetic ordering has been observed and the orientations of the internal magnetic field and the electric field gradient in the crystal system have been determined. Due to its chemical structure this substance is an example of a quasi two dimensional system•
solution of 4.10-5M FeCI3 (Fe enriched in 57Fe about 95%) in highly purified water. The pH of the solution was 3.9. A drop of arachidic acid (C19H39COOI--I) dissolved in chloroform was spread on this subphase and afterwards densified with a moving barrier to a surface pressure of 25.10-3N/m which was held constant during the preparation of the films. The substrates used for the preparation were carefully polished and cleaned silicon wafers. Films were produced by means of the dipping technique. An analysis of the films with heavy ion induced desorption mass spectrometry (I-IIID mass spectrometry, see for example2) was made to check the amount of unreacted arachidic acid in the films. As no peaks of the free arachidic acid have been observed this is a proof that the fatty acid has completely reacted with the Fe 3+ ions. With a length of about 2.7 nm for one chain of the fatty acid the films used for CEMS have a maximum thickness of about 13.5 nm for the 5 layer and 100 nm for the 37 layer film. The cross section of one fatty acid is 0.2 nm2, so the resulting 57Fe concentration is 8.3-1014/cm 2 and 5.9.1015/cm2, respectively. X-ray diffi'action patterns of the investigated iron conraining LB films show a bilayer spacing of 5.14 nm. The layer spacing in the multilayer film is always somewhat smaller than the length of two solitary fatty acids (see for example3,4). Conversion Electron Mrssbauer Spectroscopy (CEMS) measurements using the 7.3 keV K-shell conversion electrons were performed in a He/CH4-flow proportional backscatterer counter designed by us. This detector is supplied with three anode wires in order to obtain a small
In the last years great interest in the Langmuir-Blodgett technique for preparing well defined thin layers rose up because of the variety of possible scientific, biological and technological applications of such layered systems 1. These layers consist of uniformly oriented long chains of hydrocarbons which can contain for example metal ions such as Fe 3+ on the polar headgroups. Mrssbauer spectroscopy is a powerful method to elucidate electric and magnetic properties of such iron containing Langmuir-Blodgett (LB) films as it gives information about the local iron environment and possible distortions as well as defects in the structure of the films. Absorption Mrssbauer spectroscopy (AMS) measurements can be done at low temperatures in high external magnetic fields to study the magnetic behavior of the films. This is of special interest because, due to the structure of the films, the magnetic ions form quasi two dimensional •planes which are well separated from each other. So these films give the opportunity to study low dimensional magnetism. To study very thin films we used conversion electron Mrssbauer spectroscopy (CEMS) because the efficiency of CEMS is much higher than that of AMS. But with CEMS no measurements in external fields are possible. In this paper we report on conversion electron M6ssbauer spectroscopy (CEMS) and absorption Mrssbauer spectroscopy (AMS) measurements with iron arachidate Langmuir-Blodgett films. The LB films used for the measurements had 5 and 37 monolayers of iron arachidate (FeA) respectively. The films were prepared in a conventional Langmuir-Blodgett film trough made of teflon (KSV 3000). The subphase was a 243
244
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
variation of the electric field across the sample surface and therefore a high efficiency. The distance between the sample and the wires can be varied. In our measurements the distance was 2 ram. The proportional counter with the sample is placed in a liquid nitrogen bath cryostat and the sample holder is provided with two heating coils and a thermocouple in order to perform low temperature measurements between 90 K and room temperature. The temperature is controlled and held stable with an error of a:0.5 K during the whole measurement. Spectra were taken at different temperatures between 106 K and 300 K. Absorption MOssbauer spectroscopy measurements at 4.2 K in external magnetic fields up to 5 T were performed in a He-bath cryostat. The films used for these absorption measurements are prepared on Si wafers of 160 ~tm thickness which absorb only 38% of the 14.4 keV gammarays used for absorption Mrssbauer spectroscopy. As both sides of the Si wafer were covered with 37 layers of FeA the sample had a total 57Fe concentration of 1.2.1016/cm2. The source for all Mrssbauer measurements was 57Co in Rh at room temperature and all values for isomer shifts are given relative to ct-Fe. From the preparation conditions we expect two distinctive directions in the film, namely the normal to the surface of the film and the dipping direction during the film preparation which lies in the plane of the film. Therefore we describe the observation direction (gamma-ray direction) with the two angles ~ (polar angle relative to the
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Vol. 86, No. 4
normal of the film) and q~ (azimuth angle relative to the dipping direction). These angles have been varied. In Fig. 1 CEMS spectra of two LB films with 5 and 37 layers respectively, recorded at different temperatures and at different angles `9 between the direction normal to the surface and the gamma-ray direction are shown. At room temperature the spectra consist of a strikingly broadened and asymmetric quadrupole doublet. The energy splitting of the two lines is given by: AEQ = 1/2.e.Q-Vzz.[1/3.(3+~2)] 1/2 where Q is the nuclear quadrupole moment of the iron atom. Vzz is the largest component and rl is the asymmetry parameter of the electric field gradient (EFG) tensor, TI is defined as rl = (V~x-Vyy)/Vzz. As the measured intensity ratio of the two lines in the doublet is not unity at an angle `9 = 0 °, we can conclude that the orientation of the EFG is not randomly distributed in the system. At all angles ,q. the variation of the angle had no effect on the intensity ratio of the two lines. Therefore we conclude that concerning the EFG there is no distinctive direction in the plane of the film and that the intensities of the two lines in the quadrupole doublet have to be equal at an angle of,~ = 54.7 °. The broadened quadrupole doublet measured at an angle of`99.= 54.7 ° has been evaluated with a distribution of electric field gradients-and isomer shifts. We found a negative linear correlation between quadrupole splittings and isomer shifts and a distribution which can be described by a logarithmic normal distribution (see Fig. 2). The parameters of this distribution function remain unchanged for all the spectra recorded at different angles `9. The values for the quadrupole splitting AEQ and the isomer shift 8 at the center of the distribution are: AEQ = (0.58 ± 0.02) mm/s 6 = (0.352 ± 0.002) mm/s The isomer shift corresponds to values given by Meisel et al. 5 for ferric stearate LB films but the quadrupole splitting is different and also the asymmetry of the doublet has not been observed before. From the values of AEQ and
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Fig. 1. CEMS spectra of LB films layers of FeA at ,~ = 0 ° and T = RT, FeA at ,.q = 0 ° and T = RT, c) with ,9 = 54.7 ° and T =RT, and d) with `9=0 ° a n d T = 1 3 3 K .
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measured a) with 5 b) with 37 layers of 37 layers of FeA at 37 layers of FeA at
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Fig. 2. Calculated distribution of the quadrupole splittings and measured values from the spectra recorded at an angle `9 = 54.7 and at different azimuth angles q~.
245
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
Vol. 86, No. 4
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2
1_+=4. I+~-+0.5-(3c0s2 61-i)(3cos2fl-l+r/.sin 2,B.cOs2y) 1_
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where 13 and y are the polar and azimuth angle of the normal of the film in the principal axis system of the EFG and Vzz > 0, we can restrict the possible directions of the main component Vzz and the asymmetry parameter 11 of the macroscopic electric field gradient. The number of possible solutions for rl, 13, and ? that fit the measured angular dependence are shown in Fig. 4. One possible solution is 11 = 0.82, Vzz and Vyy randomly distributed in the plane of the film and Vxx parallel to the normal of the film. Spectra of the sample with 5 layers show no differences to the other measurements. This indicates that surface effects, if they can be observed with MOssbauer spectroscopy, can only be detected with measurements on samples with one or two layers. Then it may also be possible to observe the influence of the substrate on the structure of the first layers. Decreasing the temperature to 133 K leads to a slight broadening of the lines but no magnetically ordered phase can be observed at this temperature. This is in accordance with measurements made by Pomerantz et al. on iron stearate 7, who found a magnetic phase transition at 60 K. Absorption MOssbauer spectra measured without and with an external magnetic field of 5 T at 4.2 K at two angles ,9 = 0 ° and ,g = 35 ° between the direction normal to the surface of the film and the gamma-ray direction are plotted in Fig. 5. They show two different phases.
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246
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
About 23% of the total intensity is a paramagnetic quadrupole doublet with a quadrupole splitting AEQ = (0.71 + 0.01) mm/s, an isomer shift 8 = (0.55 + 0.01) mm/s, and a line width F = (0.43 + 0.01) mm/s. The intensity ratio 1./1+ of the left line to the right line is 0.78 _+ 0.03 for `5 = 0 ° and 0.88 + 0.03 for `5 = 35 °. We assume that this phase reflects the disordered parts of the film and is dependend on the preparation. It should decrease with a higher quality of the film but this has to be proved by further measurements on different samples. The main part of the spectrum is a magnetically split sextet with an internal magnetic field of 49.8T. The magnetic splitting does not change significantly when an external magnetic field is applied. Therefore we assume antiferromagnetic ordering. Earlier measurements7, 8 on manganese stearate and ferric stearate bulk material have also detected antiferromagnetic ordering. In ferric stearate bulk material the magnetic phase transition was found below 60 K 7 but there only a small amount of the total intensity shows magnetic ordering whereas in our spectra the main part is magnetically split. This further supports our suggestion that the non-magnetic phase is due to a disorder in the structure, as disorder occurs more frequently in bulk material. The isomer shift of the sextet is (0.48 4- 0.01)mm/s. Taking into account the second order doppler shift this value fits to the isomer shift of the quadrupole doublet measured at higher temperatures. All the six lines are broadened (see Table 1), which is again due to a distribution of the hyperfine fields, as can be concluded from the room temperature spectra. The quadrupole splitting of the sextett is between 0.06 mm/s and 0.09 mm/s (see Table 1). Assuming that the EFG does not change below the ordering temperature this means that the angle between the direction of the internal magnetic field and the main component of the EFG system at the nucleus is about 50 °. The relative line intensities of a magnetically split sextet depend on the angle between the effective magnetic hyper-
Vol. 86, No. 4
fine field and the observation direction6. For a polycrystalline sample an averaging over all observation directions yields an intensity ratio I1(6):I2(5):I3(4) = 3:2:1 where I n are the intensities, and the resonance lines are numbered from the lowest to the highest energy. From the values for the relative line intensities 12(5)/I1(6) measured at two different angles ~ and in an external magnetic field of 5 T (see Table 1) we conclude that the direction of the internal magnetic field has a preferred orientation at an angle 13= 58 ° relative to the surface normal. This angle has been calculated from the common angular dependence of the relative line intensities with a random distribution of orientations in the plane of the film:
12(5 )
4 . ( 1 - cos 2 fl. cos 2 ,9- 1~2-sin2 fl. sin 2 61)
11(6)
3.(1+ cos 2 fl. cos 2 ,9+ 1/2- sin2 fl. sin 2 ,9)
where 13 is the angle between the internal magnetic field and the direction normal to the surface of the film, and ,5 is the angle between the observation direction and the direction normal to the surface & t h e film. The relative line intensities calculated from this formula are also shown in Table 1. From these measurements at T < 60 K we conclude that such films are good examples of a quasi two dimensional antiferromagnetic system because the distance between two iron containing layers is about 5 nm. Direct interaction between these layers can be ruled out, whereas the distance of the iron atoms in one layer is only about 0.4 - 0.8 nm. But the spectra cannot be described with the model of a simple uniaxial antiferromagnet, the magnetic structure is more complicated. Therefore further measurements in high external fields at different orientations are in progress to get more information about the nature of the magnetism. Also measurements at higher temperature will be done to determine the ordering temperature of the system.
Table 1" Values for the quadrupole splitting (AEQ), the internal magnetic field (Hint) , the line widths (F) and the relative line intensities (I) for the absorption Mfssbauer spectra measured at 4.2 K at different angles `5 between the normal of the film and the gamma-ray direction and in different external magnetic fields (Hext). Hext is parallel to the gamma-ray direction
AISo (ram/s)
Hint (T)
FIr6) (ram/s)
F2(5~ (ram/s)
12f5)/11f6) measured
12fS~tI1f6) calculated
`5=0°
Hext=0T
0.06+0.01
49.774-0.05
0.884-0.04
0.654-0.03
0.754-0.04
0.749
8,=0 °
Hext=5T
0.094-0.01
49.444-0.10
1.304-0.06
0.854-0.04
0.754-0.04
0.750
`5=35 ° Hext=0T
0.074-0.01
49.864-0.05
0.90-4-0.03
0.67+0.03
0.694-0.03
0.706
References 1. G.G.Roberts, Advances in Physics 34, 475 (1985). 2. R.Schmidt, Ch.Schoppmann, D.Brandl, A.Ostrowski, H.Voit, D. Johannsmann, W.Knoll, Phys. Rev. B 44,560 (1991). 3. J.M.Bloch, W.Yun, K.M.Mohanty, Phys. Rev. B 40, 6529 (1989).
4. P.Tippmann-Krayer, g.M.Kenn, H.M0hwald, Thin Solid Films 210/211,577 (1992). 5. W.Meisel, P.Tippmann-Krayer, H.M0hwald, P.Gtitlich, Fresenius J. Anal. Chem 341,289 (1991). 6. J.K.Srivastava, S.C.Bhargava, P.K.Iyengar, B.V.Thosar in: Advances in MOssbauer Spectroscopy, (El sevier
Vol. 86, No. 4
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
Scientific Publishing Company, Amsterdam 1983), p. 19 and p.39. 7. M.Pomerantz, A.Aviram, AR.Tarenko,
J.Appl. Phys. 53(11), 7960 (1982) 8. A.Asaolu, BH.BIott, W.IKhan, D.Melville, Thin Solid Films 99, 263 (1983)
247
Solid State Communications, Vol. 86, No. 4, PP. 243-247, 1993. Prinied in Great Britain.
0038-1098/93 $6.00+. 00 Pergamon Press Ltd
A C E M S A N D A B S O R P T I O N M O S S B A U E R S P E C T R O S C O P Y STUDY O F I R O N A R A C H I D A T E L A N G M U I R - B L O D G E T T FILMS
E. Giesse, J. Dengler, G. Ritter, W. Wagner, D. Brandl, H. Voit, G. Saemann-Isehenko Physikalisches Institut der Universitat Erlangen-N0rnberg, Erwin-Rommel-Str. 1, W-8520 Edangen, FRG (Received 4 December 1992 by G. Giintherodt) Langmuir-Blodgett multilayers of ferric arachidate on silicon wafers have been investigated by means of conversion electron M6ssbauer spectroscopy (CEMS) and absorption MOssbauer spectroscopy at different temperatures between room temperature and 4.2 K without and with an external magnetic field of 5 T. The Mrssbauer spectra show Fe 3+. A change of the relative line intensities in the measured spectra dependent on the observation direction is visible. It was found that the lines in the spectra are significantly broadened, which is due to a distribution of electric field gradients and isomer shifts. The reason for this is a slightly disordered iron environment. The quadrupole splitting is linearly correlated to the isomer shift. At low temperatures antiferromagnetic ordering has been observed and the orientations of the internal magnetic field and the electric field gradient in the crystal system have been determined. Due to its chemical structure this substance is an example of a quasi two dimensional system•
solution of 4.10-5M FeCI3 (Fe enriched in 57Fe about 95%) in highly purified water. The pH of the solution was 3.9. A drop of arachidic acid (C19H39COOI--I) dissolved in chloroform was spread on this subphase and afterwards densified with a moving barrier to a surface pressure of 25.10-3N/m which was held constant during the preparation of the films. The substrates used for the preparation were carefully polished and cleaned silicon wafers. Films were produced by means of the dipping technique. An analysis of the films with heavy ion induced desorption mass spectrometry (I-IIID mass spectrometry, see for example2) was made to check the amount of unreacted arachidic acid in the films. As no peaks of the free arachidic acid have been observed this is a proof that the fatty acid has completely reacted with the Fe 3+ ions. With a length of about 2.7 nm for one chain of the fatty acid the films used for CEMS have a maximum thickness of about 13.5 nm for the 5 layer and 100 nm for the 37 layer film. The cross section of one fatty acid is 0.2 nm2, so the resulting 57Fe concentration is 8.3-1014/cm 2 and 5.9.1015/cm2, respectively. X-ray diffi'action patterns of the investigated iron conraining LB films show a bilayer spacing of 5.14 nm. The layer spacing in the multilayer film is always somewhat smaller than the length of two solitary fatty acids (see for example3,4). Conversion Electron Mrssbauer Spectroscopy (CEMS) measurements using the 7.3 keV K-shell conversion electrons were performed in a He/CH4-flow proportional backscatterer counter designed by us. This detector is supplied with three anode wires in order to obtain a small
In the last years great interest in the Langmuir-Blodgett technique for preparing well defined thin layers rose up because of the variety of possible scientific, biological and technological applications of such layered systems 1. These layers consist of uniformly oriented long chains of hydrocarbons which can contain for example metal ions such as Fe 3+ on the polar headgroups. Mrssbauer spectroscopy is a powerful method to elucidate electric and magnetic properties of such iron containing Langmuir-Blodgett (LB) films as it gives information about the local iron environment and possible distortions as well as defects in the structure of the films. Absorption Mrssbauer spectroscopy (AMS) measurements can be done at low temperatures in high external magnetic fields to study the magnetic behavior of the films. This is of special interest because, due to the structure of the films, the magnetic ions form quasi two dimensional •planes which are well separated from each other. So these films give the opportunity to study low dimensional magnetism. To study very thin films we used conversion electron Mrssbauer spectroscopy (CEMS) because the efficiency of CEMS is much higher than that of AMS. But with CEMS no measurements in external fields are possible. In this paper we report on conversion electron M6ssbauer spectroscopy (CEMS) and absorption Mrssbauer spectroscopy (AMS) measurements with iron arachidate Langmuir-Blodgett films. The LB films used for the measurements had 5 and 37 monolayers of iron arachidate (FeA) respectively. The films were prepared in a conventional Langmuir-Blodgett film trough made of teflon (KSV 3000). The subphase was a 243
244
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
variation of the electric field across the sample surface and therefore a high efficiency. The distance between the sample and the wires can be varied. In our measurements the distance was 2 ram. The proportional counter with the sample is placed in a liquid nitrogen bath cryostat and the sample holder is provided with two heating coils and a thermocouple in order to perform low temperature measurements between 90 K and room temperature. The temperature is controlled and held stable with an error of a:0.5 K during the whole measurement. Spectra were taken at different temperatures between 106 K and 300 K. Absorption MOssbauer spectroscopy measurements at 4.2 K in external magnetic fields up to 5 T were performed in a He-bath cryostat. The films used for these absorption measurements are prepared on Si wafers of 160 ~tm thickness which absorb only 38% of the 14.4 keV gammarays used for absorption Mrssbauer spectroscopy. As both sides of the Si wafer were covered with 37 layers of FeA the sample had a total 57Fe concentration of 1.2.1016/cm2. The source for all Mrssbauer measurements was 57Co in Rh at room temperature and all values for isomer shifts are given relative to ct-Fe. From the preparation conditions we expect two distinctive directions in the film, namely the normal to the surface of the film and the dipping direction during the film preparation which lies in the plane of the film. Therefore we describe the observation direction (gamma-ray direction) with the two angles ~ (polar angle relative to the
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Vol. 86, No. 4
normal of the film) and q~ (azimuth angle relative to the dipping direction). These angles have been varied. In Fig. 1 CEMS spectra of two LB films with 5 and 37 layers respectively, recorded at different temperatures and at different angles `9 between the direction normal to the surface and the gamma-ray direction are shown. At room temperature the spectra consist of a strikingly broadened and asymmetric quadrupole doublet. The energy splitting of the two lines is given by: AEQ = 1/2.e.Q-Vzz.[1/3.(3+~2)] 1/2 where Q is the nuclear quadrupole moment of the iron atom. Vzz is the largest component and rl is the asymmetry parameter of the electric field gradient (EFG) tensor, TI is defined as rl = (V~x-Vyy)/Vzz. As the measured intensity ratio of the two lines in the doublet is not unity at an angle `9 = 0 °, we can conclude that the orientation of the EFG is not randomly distributed in the system. At all angles ,q. the variation of the angle had no effect on the intensity ratio of the two lines. Therefore we conclude that concerning the EFG there is no distinctive direction in the plane of the film and that the intensities of the two lines in the quadrupole doublet have to be equal at an angle of,~ = 54.7 °. The broadened quadrupole doublet measured at an angle of`99.= 54.7 ° has been evaluated with a distribution of electric field gradients-and isomer shifts. We found a negative linear correlation between quadrupole splittings and isomer shifts and a distribution which can be described by a logarithmic normal distribution (see Fig. 2). The parameters of this distribution function remain unchanged for all the spectra recorded at different angles `9. The values for the quadrupole splitting AEQ and the isomer shift 8 at the center of the distribution are: AEQ = (0.58 ± 0.02) mm/s 6 = (0.352 ± 0.002) mm/s The isomer shift corresponds to values given by Meisel et al. 5 for ferric stearate LB films but the quadrupole splitting is different and also the asymmetry of the doublet has not been observed before. From the values of AEQ and
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Fig. 1. CEMS spectra of LB films layers of FeA at ,~ = 0 ° and T = RT, FeA at ,.q = 0 ° and T = RT, c) with ,9 = 54.7 ° and T =RT, and d) with `9=0 ° a n d T = 1 3 3 K .
0.3
measured a) with 5 b) with 37 layers of 37 layers of FeA at 37 layers of FeA at
0.0 0.0
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0.8
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t.6 AEa
Fig. 2. Calculated distribution of the quadrupole splittings and measured values from the spectra recorded at an angle `9 = 54.7 and at different azimuth angles q~.
245
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
Vol. 86, No. 4
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2
1_+=4. I+~-+0.5-(3c0s2 61-i)(3cos2fl-l+r/.sin 2,B.cOs2y) 1_
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where 13 and y are the polar and azimuth angle of the normal of the film in the principal axis system of the EFG and Vzz > 0, we can restrict the possible directions of the main component Vzz and the asymmetry parameter 11 of the macroscopic electric field gradient. The number of possible solutions for rl, 13, and ? that fit the measured angular dependence are shown in Fig. 4. One possible solution is 11 = 0.82, Vzz and Vyy randomly distributed in the plane of the film and Vxx parallel to the normal of the film. Spectra of the sample with 5 layers show no differences to the other measurements. This indicates that surface effects, if they can be observed with MOssbauer spectroscopy, can only be detected with measurements on samples with one or two layers. Then it may also be possible to observe the influence of the substrate on the structure of the first layers. Decreasing the temperature to 133 K leads to a slight broadening of the lines but no magnetically ordered phase can be observed at this temperature. This is in accordance with measurements made by Pomerantz et al. on iron stearate 7, who found a magnetic phase transition at 60 K. Absorption MOssbauer spectra measured without and with an external magnetic field of 5 T at 4.2 K at two angles ,9 = 0 ° and ,g = 35 ° between the direction normal to the surface of the film and the gamma-ray direction are plotted in Fig. 5. They show two different phases.
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.it a
Fig. 3. Measured and fitted relative line intensities I l l + for spectra recorded at different angles 9. from 0 ° to 54.7 °.
=
0.0
0.6
0.8
t.0
0
40
60
%
80
e ['1
Fig. 4. Possible solutions for the asymmetry parameter r I of the EFG, for the polar angle 13, and for the azimuth angle y of the normal of the film in the EFG system calculated from the measured angular dependence of the relative line intensities.
.
I
.
.
.
•
I
• •
"
,'.,"
$
4,
•
.
.
. ,,
.
.
! .
,,..
"
•
•
IL
>l--
Z tel l--
N~ ~
Z i-.4
•- : %
~.-_ ,._ " "'" z'~ >~ ." ,_:..~_ .-,~_.-". ' - "
l.-q I--
-. .'.-.,
•
._I IJJ rr
c)
-iO
'
0 VELOCITY
iO (mm/s)
Fig. 5. Absorption Mrssbauer spectra of a 37 layer FeA film measured a) at ,9 = 0 ° and Flext = 0T, b) at ,g = 0 ° and Hext = 5T, and c) at ,~ = 35 ° and Itext = 0T.
246
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
About 23% of the total intensity is a paramagnetic quadrupole doublet with a quadrupole splitting AEQ = (0.71 + 0.01) mm/s, an isomer shift 8 = (0.55 + 0.01) mm/s, and a line width F = (0.43 + 0.01) mm/s. The intensity ratio 1./1+ of the left line to the right line is 0.78 _+ 0.03 for `5 = 0 ° and 0.88 + 0.03 for `5 = 35 °. We assume that this phase reflects the disordered parts of the film and is dependend on the preparation. It should decrease with a higher quality of the film but this has to be proved by further measurements on different samples. The main part of the spectrum is a magnetically split sextet with an internal magnetic field of 49.8T. The magnetic splitting does not change significantly when an external magnetic field is applied. Therefore we assume antiferromagnetic ordering. Earlier measurements7, 8 on manganese stearate and ferric stearate bulk material have also detected antiferromagnetic ordering. In ferric stearate bulk material the magnetic phase transition was found below 60 K 7 but there only a small amount of the total intensity shows magnetic ordering whereas in our spectra the main part is magnetically split. This further supports our suggestion that the non-magnetic phase is due to a disorder in the structure, as disorder occurs more frequently in bulk material. The isomer shift of the sextet is (0.48 4- 0.01)mm/s. Taking into account the second order doppler shift this value fits to the isomer shift of the quadrupole doublet measured at higher temperatures. All the six lines are broadened (see Table 1), which is again due to a distribution of the hyperfine fields, as can be concluded from the room temperature spectra. The quadrupole splitting of the sextett is between 0.06 mm/s and 0.09 mm/s (see Table 1). Assuming that the EFG does not change below the ordering temperature this means that the angle between the direction of the internal magnetic field and the main component of the EFG system at the nucleus is about 50 °. The relative line intensities of a magnetically split sextet depend on the angle between the effective magnetic hyper-
Vol. 86, No. 4
fine field and the observation direction6. For a polycrystalline sample an averaging over all observation directions yields an intensity ratio I1(6):I2(5):I3(4) = 3:2:1 where I n are the intensities, and the resonance lines are numbered from the lowest to the highest energy. From the values for the relative line intensities 12(5)/I1(6) measured at two different angles ~ and in an external magnetic field of 5 T (see Table 1) we conclude that the direction of the internal magnetic field has a preferred orientation at an angle 13= 58 ° relative to the surface normal. This angle has been calculated from the common angular dependence of the relative line intensities with a random distribution of orientations in the plane of the film:
12(5 )
4 . ( 1 - cos 2 fl. cos 2 ,9- 1~2-sin2 fl. sin 2 61)
11(6)
3.(1+ cos 2 fl. cos 2 ,9+ 1/2- sin2 fl. sin 2 ,9)
where 13 is the angle between the internal magnetic field and the direction normal to the surface of the film, and ,5 is the angle between the observation direction and the direction normal to the surface & t h e film. The relative line intensities calculated from this formula are also shown in Table 1. From these measurements at T < 60 K we conclude that such films are good examples of a quasi two dimensional antiferromagnetic system because the distance between two iron containing layers is about 5 nm. Direct interaction between these layers can be ruled out, whereas the distance of the iron atoms in one layer is only about 0.4 - 0.8 nm. But the spectra cannot be described with the model of a simple uniaxial antiferromagnet, the magnetic structure is more complicated. Therefore further measurements in high external fields at different orientations are in progress to get more information about the nature of the magnetism. Also measurements at higher temperature will be done to determine the ordering temperature of the system.
Table 1" Values for the quadrupole splitting (AEQ), the internal magnetic field (Hint) , the line widths (F) and the relative line intensities (I) for the absorption Mfssbauer spectra measured at 4.2 K at different angles `5 between the normal of the film and the gamma-ray direction and in different external magnetic fields (Hext). Hext is parallel to the gamma-ray direction
AISo (ram/s)
Hint (T)
FIr6) (ram/s)
F2(5~ (ram/s)
12f5)/11f6) measured
12fS~tI1f6) calculated
`5=0°
Hext=0T
0.06+0.01
49.774-0.05
0.884-0.04
0.654-0.03
0.754-0.04
0.749
8,=0 °
Hext=5T
0.094-0.01
49.444-0.10
1.304-0.06
0.854-0.04
0.754-0.04
0.750
`5=35 ° Hext=0T
0.074-0.01
49.864-0.05
0.90-4-0.03
0.67+0.03
0.694-0.03
0.706
References 1. G.G.Roberts, Advances in Physics 34, 475 (1985). 2. R.Schmidt, Ch.Schoppmann, D.Brandl, A.Ostrowski, H.Voit, D. Johannsmann, W.Knoll, Phys. Rev. B 44,560 (1991). 3. J.M.Bloch, W.Yun, K.M.Mohanty, Phys. Rev. B 40, 6529 (1989).
4. P.Tippmann-Krayer, g.M.Kenn, H.M0hwald, Thin Solid Films 210/211,577 (1992). 5. W.Meisel, P.Tippmann-Krayer, H.M0hwald, P.Gtitlich, Fresenius J. Anal. Chem 341,289 (1991). 6. J.K.Srivastava, S.C.Bhargava, P.K.Iyengar, B.V.Thosar in: Advances in MOssbauer Spectroscopy, (El sevier
Vol. 86, No. 4
IRON ARACHIDATE LANGMUIR-BLODGETT FILMS
Scientific Publishing Company, Amsterdam 1983), p. 19 and p.39. 7. M.Pomerantz, A.Aviram, AR.Tarenko,
J.Appl. Phys. 53(11), 7960 (1982) 8. A.Asaolu, BH.BIott, W.IKhan, D.Melville, Thin Solid Films 99, 263 (1983)
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