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The influence of Al on the magnetic properties of synthetic goethite
ELSEVIER
Physica B 234-236 (1997) 620-621
The influence of A1 on the magnetic properties of synthetic goethite S.H. K i l c o y n e a'*, C. R i t t e r b aSchool of Physics and Astronomy, University o f St Andrews, St Andrews, KYI6 9SS, Scotland, UK blnstitut Laue-Langevin, BP156, 38042 Grenoble Cedex 9, France
Abstract
Neutron powder diffraction has been used to study the structural and magnetic properties of synthetic ~-(AIFe)OOH from 4.2 to 420 K. The lattice parameters vary little with composition, and the temperature dependence of the sublattice magnetisation closely follows an S = ~ Brillouin function at all concentrations.
Keywords: ~-(FeA1)OOH; Powder diffraction
Antiferromagnetic ~-FeOOH (goethite) is ubiquitous, occurring, for example, as a mineral, as a corrosion product and in modified form in the core of the iron storage protein, ferritin. Many such natural sources of goethite exhibit an isomorphous substitution of other cations for Fe, and of these substituents A1 is perhaps the most common. In M/Sssbauer effect studies of the magnetic properties of A1 substituted goethite, (see Ref. [1] and references therein), many of the results have been interpreted as arising from superparamagnetic behaviour. However, such interpretation is not always without ambiguity; with M6ssbauer spectroscopy alone, accurate magnetic characterisation of fine grain samples is not simple. In order to shed some light on these problems we have chosen to characterise the magnetic properties of synthetic Al-doped goethite using neutron powder diffraction. Five samples of (FeAI)OOH were prepared using the method described in Ref. [2] but replacing the * Corresponding author.
appropriate molar fraction of Fe(NOa)a •9H20 by Al(NO3)3 • 9H20. Final A1 concentrations of 0, 7.7, 13.6, 19.7 and 24.7 mol% were determined from EDAX measurements. Powder neutron diffraction spectra were collected on D1B at the ILL (Grenoble) in 3 K steps from 4.2 K to 450 K. The spectra obtained from all samples at T ~>TN are fully consistent with the expected orthorhombic structure (space group Pbnm) described in Ref. [3]. In none of the spectra was there evidence (e.g. nuclear superlattice reflections) of preferential partial spatial ordering of Al atoms at any of the four iron sites. The lattice parameters and cell volume were determined using a Rietveld-based refinement program. As expected, the substitution of the smaller A1a + ion for Fe 3 + leads to a decrease of the cell volume with increasing Al concentration of o3 0.063 A /mol% at 4.2 K. The antiferromagnetic structure, determined from diffraction patterns collected at 4.2 K, agrees fully with that of FeOOH [3]. The average magnetic moments per cation determined from the magnetic Bragg intensities at 4.2 K are as expected
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S092 1 - 4 5 2 6 ( 9 6 ) 0 10 6 2 - 9
621
S.H. Kilcoyne, C. Ritter / Physica B 234-236 (1997) 620-621
for a simple dilution of Fe by non-magnetic A1, decreasing from (5.0 _+ 0.1)~tB per cation for the parent compound. The N6el point of each sample was determined from the temperature dependence of the Bragg peak intensities. The values of TN decrease smoothly as A1 increased, from (377 +_ 1) K for the pure goethite sample to (320 _+ 5)K for the 24.7mo1% A1 sample. While TN for the pure goethite sample was relatively easy to ascertain, the task became increasingly difficult with increasing AI concentration due to the development of residual short range magnetic order close to TN. The temperature dependence of the magnetic Bragg intensities for all compounds is summarised as a universal curve in Fig. 1. Also shown is the calculated 5 squared Brillouin function for a spin of S = 5. The agreement between the data and calculation is excellent over the whole of the reduced temperature range. This feature is particularly important as there is considerable debate regarding the temperature dependence of the sublattice magnetisation in F e O O H . For example recent M6ssbauer effect measurements [-5] are interpreted as suggesting that the Brillouin function does not provide an adequate description of the temperature dependence of the hyperfine field in goethite. Moreover, the Neel temperatures previously determined from the T-dependence of the hyperfine fields in M6ssbauer studies [6] have suggested that addition of A1 to the goethite matrix reduces TN at a much faster rate than that observed here, with samples containing more than 12 mol% A1 showing only a paramagnetic doublet above room temperature. Although the discrepancies in the T-dependence of the magnetisation, and in TN itself, may reflect characteristics dependent upon material preparation, it is possible that they may indicate fundamental differences in the way that the magnetic order is sampled by neutrons and by the M6ssbauer effect. This observation leads to a cautionary note: the hyperfine fields of Fe-based minerals are often used as identifiers in analysis of mixtures of compounds. It is possible that the measured
I
~1.0 v ~_0..8
I
~
1
I
I
~0.6
00/oAI
-~0.4
,t •
~0.2
"" 19.7°/oAI • 24.7% AI
0.0
0.0
,I~
7.7% AI
z~ ~'~e
I
I
I
I
0.2
0.4
0.6
0.8
•
~
1.0
1.2
Reduced Temperature T/TN Fig. 1. Normalisedmagneticintensitiesfor all samples. The solid line is the squared Brillouinfunction for s = -~.The short range magnetic order appearing at TN as x increases can be seen.
hyperfine fields could differ substantially from those of the bulk, not because of significant differences in the structure of the compound, but because of the granularity and morphology of the material and the effects of these parameters on the superparamagnetic relaxation of the magnetisation of the grains. The authors wish to thank Prof. R. Cywinski for his help in this work.
References El] E. Murad and L.H. Bowen,Am. Miner. 72 (1987) 194. [2] A.M. van der Kraan and J. Medema,Inorg. Nucl. Chem. 31 (1969) 2039. ['3] J.B. Forsyth, I.G. Hedley and C.E. Johnson, J. Phys. C 1 (1968) 179. I-4] D.G. SchulzeClay Clay Min. 32 (1984) 36. [51 S. Bocquet,R.J. Pollard and J.D. Cashion, Phys. Re•. B 46 (1992) 11 657. [6] J. Fleish, R. Grimm, J. Griibler and P. Gutlich, J. de Phys. 41 (1980)C1-C169.
Physica B 234-236 (1997) 620-621
The influence of A1 on the magnetic properties of synthetic goethite S.H. K i l c o y n e a'*, C. R i t t e r b aSchool of Physics and Astronomy, University o f St Andrews, St Andrews, KYI6 9SS, Scotland, UK blnstitut Laue-Langevin, BP156, 38042 Grenoble Cedex 9, France
Abstract
Neutron powder diffraction has been used to study the structural and magnetic properties of synthetic ~-(AIFe)OOH from 4.2 to 420 K. The lattice parameters vary little with composition, and the temperature dependence of the sublattice magnetisation closely follows an S = ~ Brillouin function at all concentrations.
Keywords: ~-(FeA1)OOH; Powder diffraction
Antiferromagnetic ~-FeOOH (goethite) is ubiquitous, occurring, for example, as a mineral, as a corrosion product and in modified form in the core of the iron storage protein, ferritin. Many such natural sources of goethite exhibit an isomorphous substitution of other cations for Fe, and of these substituents A1 is perhaps the most common. In M/Sssbauer effect studies of the magnetic properties of A1 substituted goethite, (see Ref. [1] and references therein), many of the results have been interpreted as arising from superparamagnetic behaviour. However, such interpretation is not always without ambiguity; with M6ssbauer spectroscopy alone, accurate magnetic characterisation of fine grain samples is not simple. In order to shed some light on these problems we have chosen to characterise the magnetic properties of synthetic Al-doped goethite using neutron powder diffraction. Five samples of (FeAI)OOH were prepared using the method described in Ref. [2] but replacing the * Corresponding author.
appropriate molar fraction of Fe(NOa)a •9H20 by Al(NO3)3 • 9H20. Final A1 concentrations of 0, 7.7, 13.6, 19.7 and 24.7 mol% were determined from EDAX measurements. Powder neutron diffraction spectra were collected on D1B at the ILL (Grenoble) in 3 K steps from 4.2 K to 450 K. The spectra obtained from all samples at T ~>TN are fully consistent with the expected orthorhombic structure (space group Pbnm) described in Ref. [3]. In none of the spectra was there evidence (e.g. nuclear superlattice reflections) of preferential partial spatial ordering of Al atoms at any of the four iron sites. The lattice parameters and cell volume were determined using a Rietveld-based refinement program. As expected, the substitution of the smaller A1a + ion for Fe 3 + leads to a decrease of the cell volume with increasing Al concentration of o3 0.063 A /mol% at 4.2 K. The antiferromagnetic structure, determined from diffraction patterns collected at 4.2 K, agrees fully with that of FeOOH [3]. The average magnetic moments per cation determined from the magnetic Bragg intensities at 4.2 K are as expected
0921-4526/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved Pll S092 1 - 4 5 2 6 ( 9 6 ) 0 10 6 2 - 9
621
S.H. Kilcoyne, C. Ritter / Physica B 234-236 (1997) 620-621
for a simple dilution of Fe by non-magnetic A1, decreasing from (5.0 _+ 0.1)~tB per cation for the parent compound. The N6el point of each sample was determined from the temperature dependence of the Bragg peak intensities. The values of TN decrease smoothly as A1 increased, from (377 +_ 1) K for the pure goethite sample to (320 _+ 5)K for the 24.7mo1% A1 sample. While TN for the pure goethite sample was relatively easy to ascertain, the task became increasingly difficult with increasing AI concentration due to the development of residual short range magnetic order close to TN. The temperature dependence of the magnetic Bragg intensities for all compounds is summarised as a universal curve in Fig. 1. Also shown is the calculated 5 squared Brillouin function for a spin of S = 5. The agreement between the data and calculation is excellent over the whole of the reduced temperature range. This feature is particularly important as there is considerable debate regarding the temperature dependence of the sublattice magnetisation in F e O O H . For example recent M6ssbauer effect measurements [-5] are interpreted as suggesting that the Brillouin function does not provide an adequate description of the temperature dependence of the hyperfine field in goethite. Moreover, the Neel temperatures previously determined from the T-dependence of the hyperfine fields in M6ssbauer studies [6] have suggested that addition of A1 to the goethite matrix reduces TN at a much faster rate than that observed here, with samples containing more than 12 mol% A1 showing only a paramagnetic doublet above room temperature. Although the discrepancies in the T-dependence of the magnetisation, and in TN itself, may reflect characteristics dependent upon material preparation, it is possible that they may indicate fundamental differences in the way that the magnetic order is sampled by neutrons and by the M6ssbauer effect. This observation leads to a cautionary note: the hyperfine fields of Fe-based minerals are often used as identifiers in analysis of mixtures of compounds. It is possible that the measured
I
~1.0 v ~_0..8
I
~
1
I
I
~0.6
00/oAI
-~0.4
,t •
~0.2
"" 19.7°/oAI • 24.7% AI
0.0
0.0
,I~
7.7% AI
z~ ~'~e
I
I
I
I
0.2
0.4
0.6
0.8
•
~
1.0
1.2
Reduced Temperature T/TN Fig. 1. Normalisedmagneticintensitiesfor all samples. The solid line is the squared Brillouinfunction for s = -~.The short range magnetic order appearing at TN as x increases can be seen.
hyperfine fields could differ substantially from those of the bulk, not because of significant differences in the structure of the compound, but because of the granularity and morphology of the material and the effects of these parameters on the superparamagnetic relaxation of the magnetisation of the grains. The authors wish to thank Prof. R. Cywinski for his help in this work.
References El] E. Murad and L.H. Bowen,Am. Miner. 72 (1987) 194. [2] A.M. van der Kraan and J. Medema,Inorg. Nucl. Chem. 31 (1969) 2039. ['3] J.B. Forsyth, I.G. Hedley and C.E. Johnson, J. Phys. C 1 (1968) 179. I-4] D.G. SchulzeClay Clay Min. 32 (1984) 36. [51 S. Bocquet,R.J. Pollard and J.D. Cashion, Phys. Re•. B 46 (1992) 11 657. [6] J. Fleish, R. Grimm, J. Griibler and P. Gutlich, J. de Phys. 41 (1980)C1-C169.