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Nuclear magnetic resonance of 57Fe in Al-, Ga- and Ti-substituted magnetite above Verwey temperature
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 2555–2557 www.elsevier.com/locate/jmmm
Nuclear magnetic resonance of 57Fe in Al-, Ga- and Ti-substituted magnetite above Verwey temperature V. Chlana,, E. Gamaliya, H. Sˇteˇpa´nkova´a, K. Kourˇ ila, J. Englicha, J. Kohouta, V. Brabersb a
Faculty of Mathematics and Physics, Charles University, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic Department of Physics, Eindhoven University of Technology, POB 512, NL-5600 MB Eindhoven, Netherlands
b
Available online 17 November 2006
Abstract Magnetite single crystals with Al, Ga and Ti substitutions were measured by means of 57Fe nuclear magnetic resonance (NMR) technique. Satellite structure of NMR spectra well above the Verwey temperature was detected and analyzed to obtain information on the distribution of substituting cations on iron sublattices. It was confirmed that Al and Ti enter iron octahedral B-sites. Ga strongly prefers tetrahedral A-sites, nevertheless a low presence of Ga cations in B-sites was detected. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Gg; 76.60.k Keywords: Magnetite; NMR; Spin echo; Cation substitution; Hyperfine interaction; Magnetic oxide
Magnetite ðFe3 O4 Þ is the earliest discovered magnet and one of the best known magnetic compounds. The physics of magnetite and in particular the question of its electronic structure is still an open problem of the solid state physics. Apart from its magnetic properties magnetite is of special interest due to the Verwey phase transition which occurs depending on the sample purity at a temperature T V 1202125 K. The most pronounced effect at T V is an abrupt change of electric conductivity by two orders. Just above the Verwey temperature reorientation transition appears (at T SR 130 K) when easy axis of magnetization turns its direction from h 0 0 1 i (for ToT SR ) to h1 1 1i (for T4T SR ). Substitution of cations at iron sites lowers T V [1]. Despite a trivalent state of both gallium and aluminium ions the aluminium substitution is known to affect the Verwey temperature and the conductivity more pronouncedly and substitution of tetravalent titanium cations has a comparable effect as Al. An explanation of these features could be based on differences in ionic radii and occupation preferences inducing changes in electronic structure. Corresponding author. Tel.: +420 2 2191 2884; fax: +420 2 2191 2567.
E-mail address: [email protected] (V. Chlan). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.867
In this contribution we present that nuclear magnetic resonance (NMR) spectroscopy of 57Fe may serve as a helpful tool to examine the distribution of a substitution in octahedral (B) and tetrahedral (A) iron sites in magnetite and to determine its occupancy even quantitatively. Single crystal samples Fe3 O4 , Fe3x Alx O4 (x ¼ 0:005, 0.01, 0.02 and 0.03), Fe3x Gax O4 (x ¼ 0:05) and Fe3x Tix O4 (x ¼ 0:008) used in the present study were prepared by means of floating zone technique. NMR spectra were measured by the spin-echo method using the phase-coherent pulse spectrometer with an averaging technique and the Fourier transformation. The measurements were performed in zero external magnetic field at temperatures above T V . The signal-to-noise ratio was significantly improved by using the Carr–Purcell pulse sequence. NMR spectra measured well above T V exhibit relatively simple structure. In agreement with [2], spectra consist of one main spectral line originated from nuclei in A-sites and of two main lines (with integral intensity ratio 1:3) from B-sites for magnetization along h1 1 1i. For a pure magnetite at a temperature of 273 K these lines occur at NMR frequencies of 68.04 MHz (A-line, see Fig. 1), 63.11 MHz and 64.08 MHz (B-lines). Positions of main
ARTICLE IN PRESS 2556
V. Chlan et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2555–2557
Fig. 1. 57Fe NMR spectra of A-sites in pure and substituted magnetite. Horizontal brackets denote the satellite lines that are induced by a substituent occupying one of the nearest neighbour B-sites to the resonating iron in A-site.
lines were not significantly affected by substitution in studied samples. Cation substitution induces spectral line broadening making difficulties in analysis of some neighbouring satellites. Spectral lines of 57Fe in octahedral B-sites are much more broadened compared with those of tetrahedral A-sites. Therefore, we analyzed the influence of temperature and substitution on A-spectrum. In general, substitutional defect in a cation site modifies slightly hyperfine magnetic field on neighbouring iron nuclei. Resonant frequencies of the nuclei close to the defect may be shifted enough to give rise to a pattern of satellite lines in an NMR spectrum. The shift of a satellite is determined by the type of substituent, by a configuration of the substituent and the resonating nucleus site and also by the magnetization direction. As a rule, substitution in a site with stronger exchange interaction induces more pronounced satellite shifts. Furthermore, temperature dependences of satellite lines belonging to crystallographically equivalent configurations of resonating nucleus and substituent are identical. Numbers and relative intensities of satellite lines induced by a substituent in a given site can be predicted knowing the crystal structure, magnetization direction and concentration of substitution.
Each iron ion in A-site has 12 nearest neighbour B-sites at a distance of 0.3453 nm. Following the symmetry considerations and for magnetization direction along h1 1 1i, if a substituent is in a B-site, the nearest resonating nuclei in A-sites give three satellite lines with intensity ratio 1:1:2 in the A-spectrum. The sum of integral intensities of these satellites with respect to the intensity of the main line depends on a concentration cB of the substituent in B-sites according to formula 12cB =ð1 cB Þ, when random distribution of a substituent on B-sites is supposed. For each A-sited iron ion, there are four nearest neighbour A-sites at a distance of 0.3607 nm. Substitution in the nearest A-site gives two satellite lines with intensity ratio 1:3 for resonating nuclei in A-sites. The sum of satellite intensities with respect to main line intensity is given by relation 4cA =ð1 cA Þ, where cA is concentration of substituent in A-sites. Frequency shifts of these satellites are supposed to be smaller due to weaker exchange interactions and longer mutual distance of the sites. Examples of measured 57Fe NMR spectra of Al, Ga and Ti substituted samples are shown in Fig. 1. Satellite structure in the A-subspectrum of Al substituted samples displays three pronounced satellites (67.01, 67.36 and 67.67 MHz at 273 K) with relative intensities 1:1:2 in agreement with substitution of Al occupying B-sites. Intensity ratio of satellites and main A-line yields concentration cB ¼ 0:0085ð20Þ, which gives x ¼ 2cB ¼ 0:017ð4Þ. This corresponds to nominal value x ¼ 0:02 of the sample. A similar satellite pattern was observed in Ga substituted sample, although intensities of satellites were much weaker and yielded concentration of Ga in B-site cB t0:006 (and corresponding xt0:012, what is much less than the nominal value x ¼ 0:05). It is in agreement with preferential Ga occupation of A-sites and is supported by observed satellite structure close to main resonance A-line, which is more pronounced at higher temperatures. These satellites (at frequencies of 65.94, 66.04 and 66.13 MHz for 370 K—see insets in Fig. 1) can be assigned to Ga substitution in A-sites. Spectra of Al substituted samples display no similar structure. Spectra of A-sites for substitution of Ti have three satellite lines with intensities 1:1:2 (at 66.42, 66.48 and 66.64 MHz for 273 K, see Fig. 1). Their shifts are larger than for Al. These satellites correspond to substitution of Ti in the nearest B-sites of resonating nucleus in A-sites. Evaluated concentration cB ¼ 0:005ð1Þ and x ¼ 0:010ð2Þ correspond within a given precision to nominal value x ¼ 0:008. In conclusion, it was confirmed that Al and Ti enter iron octahedral B-sites. Ga strongly prefers tetrahedral A-sites, nevertheless, a low presence of Ga cations in B-sites was detected. From this analysis follows that preference of Ga for B-sites is lower than the ratio 1:2 given in Ref. [3].
ARTICLE IN PRESS V. Chlan et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2555–2557
We acknowledge the support of the Grant Agency of Charles University (Grant no. 202/2006 B FYZ) and of the Grant Agency of the Czech Republic (Grant no. 202/ 06/0051). This work is a part of the research plan MS0021620834 that is financed by the Ministry of Education of the Czech Republic.
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References [1] V. Brabers, F. Waltz, H. Kronmuller, Phys. Rev. B 58 (1998) 14163. [2] P. Nova´k, H. Sˇteˇpa´nkova´, J. Englich, J. Kohout, V. Brabers, Phys. Rev. B 61 (2000) 1256. [3] M. Rosenberg, P. Deppe, H.U. Janssen, V.A.M. Brabers, F.S. Li, S. Dey, J. Appl. Phys. 57 (1985) 3740.
Journal of Magnetism and Magnetic Materials 310 (2007) 2555–2557 www.elsevier.com/locate/jmmm
Nuclear magnetic resonance of 57Fe in Al-, Ga- and Ti-substituted magnetite above Verwey temperature V. Chlana,, E. Gamaliya, H. Sˇteˇpa´nkova´a, K. Kourˇ ila, J. Englicha, J. Kohouta, V. Brabersb a
Faculty of Mathematics and Physics, Charles University, V Holesˇovicˇka´ch 2, 180 00 Prague 8, Czech Republic Department of Physics, Eindhoven University of Technology, POB 512, NL-5600 MB Eindhoven, Netherlands
b
Available online 17 November 2006
Abstract Magnetite single crystals with Al, Ga and Ti substitutions were measured by means of 57Fe nuclear magnetic resonance (NMR) technique. Satellite structure of NMR spectra well above the Verwey temperature was detected and analyzed to obtain information on the distribution of substituting cations on iron sublattices. It was confirmed that Al and Ti enter iron octahedral B-sites. Ga strongly prefers tetrahedral A-sites, nevertheless a low presence of Ga cations in B-sites was detected. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Gg; 76.60.k Keywords: Magnetite; NMR; Spin echo; Cation substitution; Hyperfine interaction; Magnetic oxide
Magnetite ðFe3 O4 Þ is the earliest discovered magnet and one of the best known magnetic compounds. The physics of magnetite and in particular the question of its electronic structure is still an open problem of the solid state physics. Apart from its magnetic properties magnetite is of special interest due to the Verwey phase transition which occurs depending on the sample purity at a temperature T V 1202125 K. The most pronounced effect at T V is an abrupt change of electric conductivity by two orders. Just above the Verwey temperature reorientation transition appears (at T SR 130 K) when easy axis of magnetization turns its direction from h 0 0 1 i (for ToT SR ) to h1 1 1i (for T4T SR ). Substitution of cations at iron sites lowers T V [1]. Despite a trivalent state of both gallium and aluminium ions the aluminium substitution is known to affect the Verwey temperature and the conductivity more pronouncedly and substitution of tetravalent titanium cations has a comparable effect as Al. An explanation of these features could be based on differences in ionic radii and occupation preferences inducing changes in electronic structure. Corresponding author. Tel.: +420 2 2191 2884; fax: +420 2 2191 2567.
E-mail address: [email protected] (V. Chlan). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.867
In this contribution we present that nuclear magnetic resonance (NMR) spectroscopy of 57Fe may serve as a helpful tool to examine the distribution of a substitution in octahedral (B) and tetrahedral (A) iron sites in magnetite and to determine its occupancy even quantitatively. Single crystal samples Fe3 O4 , Fe3x Alx O4 (x ¼ 0:005, 0.01, 0.02 and 0.03), Fe3x Gax O4 (x ¼ 0:05) and Fe3x Tix O4 (x ¼ 0:008) used in the present study were prepared by means of floating zone technique. NMR spectra were measured by the spin-echo method using the phase-coherent pulse spectrometer with an averaging technique and the Fourier transformation. The measurements were performed in zero external magnetic field at temperatures above T V . The signal-to-noise ratio was significantly improved by using the Carr–Purcell pulse sequence. NMR spectra measured well above T V exhibit relatively simple structure. In agreement with [2], spectra consist of one main spectral line originated from nuclei in A-sites and of two main lines (with integral intensity ratio 1:3) from B-sites for magnetization along h1 1 1i. For a pure magnetite at a temperature of 273 K these lines occur at NMR frequencies of 68.04 MHz (A-line, see Fig. 1), 63.11 MHz and 64.08 MHz (B-lines). Positions of main
ARTICLE IN PRESS 2556
V. Chlan et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2555–2557
Fig. 1. 57Fe NMR spectra of A-sites in pure and substituted magnetite. Horizontal brackets denote the satellite lines that are induced by a substituent occupying one of the nearest neighbour B-sites to the resonating iron in A-site.
lines were not significantly affected by substitution in studied samples. Cation substitution induces spectral line broadening making difficulties in analysis of some neighbouring satellites. Spectral lines of 57Fe in octahedral B-sites are much more broadened compared with those of tetrahedral A-sites. Therefore, we analyzed the influence of temperature and substitution on A-spectrum. In general, substitutional defect in a cation site modifies slightly hyperfine magnetic field on neighbouring iron nuclei. Resonant frequencies of the nuclei close to the defect may be shifted enough to give rise to a pattern of satellite lines in an NMR spectrum. The shift of a satellite is determined by the type of substituent, by a configuration of the substituent and the resonating nucleus site and also by the magnetization direction. As a rule, substitution in a site with stronger exchange interaction induces more pronounced satellite shifts. Furthermore, temperature dependences of satellite lines belonging to crystallographically equivalent configurations of resonating nucleus and substituent are identical. Numbers and relative intensities of satellite lines induced by a substituent in a given site can be predicted knowing the crystal structure, magnetization direction and concentration of substitution.
Each iron ion in A-site has 12 nearest neighbour B-sites at a distance of 0.3453 nm. Following the symmetry considerations and for magnetization direction along h1 1 1i, if a substituent is in a B-site, the nearest resonating nuclei in A-sites give three satellite lines with intensity ratio 1:1:2 in the A-spectrum. The sum of integral intensities of these satellites with respect to the intensity of the main line depends on a concentration cB of the substituent in B-sites according to formula 12cB =ð1 cB Þ, when random distribution of a substituent on B-sites is supposed. For each A-sited iron ion, there are four nearest neighbour A-sites at a distance of 0.3607 nm. Substitution in the nearest A-site gives two satellite lines with intensity ratio 1:3 for resonating nuclei in A-sites. The sum of satellite intensities with respect to main line intensity is given by relation 4cA =ð1 cA Þ, where cA is concentration of substituent in A-sites. Frequency shifts of these satellites are supposed to be smaller due to weaker exchange interactions and longer mutual distance of the sites. Examples of measured 57Fe NMR spectra of Al, Ga and Ti substituted samples are shown in Fig. 1. Satellite structure in the A-subspectrum of Al substituted samples displays three pronounced satellites (67.01, 67.36 and 67.67 MHz at 273 K) with relative intensities 1:1:2 in agreement with substitution of Al occupying B-sites. Intensity ratio of satellites and main A-line yields concentration cB ¼ 0:0085ð20Þ, which gives x ¼ 2cB ¼ 0:017ð4Þ. This corresponds to nominal value x ¼ 0:02 of the sample. A similar satellite pattern was observed in Ga substituted sample, although intensities of satellites were much weaker and yielded concentration of Ga in B-site cB t0:006 (and corresponding xt0:012, what is much less than the nominal value x ¼ 0:05). It is in agreement with preferential Ga occupation of A-sites and is supported by observed satellite structure close to main resonance A-line, which is more pronounced at higher temperatures. These satellites (at frequencies of 65.94, 66.04 and 66.13 MHz for 370 K—see insets in Fig. 1) can be assigned to Ga substitution in A-sites. Spectra of Al substituted samples display no similar structure. Spectra of A-sites for substitution of Ti have three satellite lines with intensities 1:1:2 (at 66.42, 66.48 and 66.64 MHz for 273 K, see Fig. 1). Their shifts are larger than for Al. These satellites correspond to substitution of Ti in the nearest B-sites of resonating nucleus in A-sites. Evaluated concentration cB ¼ 0:005ð1Þ and x ¼ 0:010ð2Þ correspond within a given precision to nominal value x ¼ 0:008. In conclusion, it was confirmed that Al and Ti enter iron octahedral B-sites. Ga strongly prefers tetrahedral A-sites, nevertheless, a low presence of Ga cations in B-sites was detected. From this analysis follows that preference of Ga for B-sites is lower than the ratio 1:2 given in Ref. [3].
ARTICLE IN PRESS V. Chlan et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2555–2557
We acknowledge the support of the Grant Agency of Charles University (Grant no. 202/2006 B FYZ) and of the Grant Agency of the Czech Republic (Grant no. 202/ 06/0051). This work is a part of the research plan MS0021620834 that is financed by the Ministry of Education of the Czech Republic.
2557
References [1] V. Brabers, F. Waltz, H. Kronmuller, Phys. Rev. B 58 (1998) 14163. [2] P. Nova´k, H. Sˇteˇpa´nkova´, J. Englich, J. Kohout, V. Brabers, Phys. Rev. B 61 (2000) 1256. [3] M. Rosenberg, P. Deppe, H.U. Janssen, V.A.M. Brabers, F.S. Li, S. Dey, J. Appl. Phys. 57 (1985) 3740.