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Influence of the metal–insulator phase transition on low-energy Ar+ ions scattering from an Fe3O4 (0 0 1) surface
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2205–2207
Influence of the metal–insulator phase transition on low-energy Ar+ ions scattering from an Fe3O4 (0 0 1) surface N.-T.H. Kim-Ngan*, W. Soszka, G. Jag"o, D. Goc-Jag"o ! 30-084, Poland Institute of Physics, Pedagogical University, Krakow
Abstract We investigated a single crystalline Fe3O4 (0 0 1) surface by using a temperature-dependent low-energy ion spectroscopy with an Ar+ ion beam. The energy spectra have revealed distinguished peaks corresponding to the Ar+– Fe single scattering, the recoil Fe and recoil O ions. Two step-like increases were revealed in the temperature-dependent curves of the positive-charged scattered ion yield in the metal–insulator phase transition region. A very weak phasetransition effect was observed on the oxygen emissions from the magnetite surface. r 2003 Elsevier B.V. All rights reserved. PACS: 68.35.p; 61.18.Bn Keywords: Fe3O4; Metal–insulator phase transition; Low-energy ion scattering (LEIS)
The large influence from the metal–insulator phase transition in magnetite (i.e. the Verwey transition related to a freezing of the electron hopping in the Feoctahedral interstices) on the bulk properties has been thoroughly investigated over many years (see the review [1]). Recently, we have investigated the ‘surface’ Verwey transition of several magnetite surfaces using an ion scattering spectroscopy (ISS or LEIS technique) [2,3]. In a small-angle geometry, distinct anomalies around 100– 130 K have been revealed in the temperature-dependent curve of ion scattering yield, R+(T). In this paper, we present the LEIS investigations on a single crystalline magnetite (0 0 1) surface. The experiments have been carried out in an ISS at a large-angle geometry described elsewhere [4] with which we focus on the influence of the Verwey transition on the single scattering process from such a surface. A surface cleanness was done in situ with annealing at temperatures 500–600 K and with 6 keV Ar+ sputtering prior to each LEIS experiment. *Corresponding author. Tel.: +48-12-6626314; fax: +48-126372243. E-mail address: [email protected] (N.-T.H. Kim-Ngan).
According to a binary elastic collision model [5], the energy of the scattered ions and recoil ions is, respectively, expressed as E1 ¼ E0 f½cos y1 þ ðm2 sin2 y1 Þ1=2 =ð1 þ mÞg2 ; E2 ¼ E0 ½4m=ð1 þ mÞ2 cos2 y2 ; where E0 is the primary energy, y1 ; y2 are the related scattering angles, m ¼ m2 =m1 is the mass ratio between the target and projectile atoms. Fig. 1 showed the LEIS spectra of ions scattered from the Fe3O4 (0 0 1) surface under 6 keV Ar+ ion bombardments. Two well-defined peaks on a high background were observed. The peak intensity, and the shape of the LEIS spectra were strongly dependent on the angle geometry and on the target temperatures. At 300 K a symmetric energy spectrum with similar peak-heights was obtained at C ¼ 34 and Y ¼ 68 : The energy position of the high-energy peak was in a good agreement with that for a single binary Ar+–Fe scattering: the E1(Fe)/E0 ratio was of 0.374 and 0.352, respectively, for Y ¼ 66 and 68 . A significant contribution from the quasi-single scattering ions was present showed by a large peak width. Detailed investigations of the characteristic of the low-energy peak as a function of the detection angle and of the azimuthal angle indicated that it was due to the recoil Fe ions, E2 ðFeÞ [4]. The peak intensity was much enhanced
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.920
ARTICLE IN PRESS 2206
N.-T.H. Kim-Ngan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2205–2207
Fig. 1. The LEIS spectra of Fe3O4 (0 0 1) surface under 6 keV Ar+ ion bombardment at different angle geometries. T=300 K (open markers) and 90 K (solid markers).
Fig. 2. Energy spectra of the recoil O1 ions emitted from Fe3O4 (0 0 1) surface under 6 keV Ar+ ion bombardment. C ¼ 34 ; Y ¼ 68 :
at 90 K. The largest enhancement was found for the Ar+-Fe scattering peak at C ¼ 34 and Y ¼ 68 : A similar angle and temperature dependence of the LEIS spectra was observed for different primary energies. The energy ratio between the two peaks, E1 ðFeÞ=E2 ðFeÞ; was of 2.3 and was independent on the primary energy. A huge peak was observed in the LEIS spectra of the emitted negative ions (Fig. 2). The energy position of the maximum was in a good agreement with that of the recoil O1 ions from a binary collision, E2(O). It indicated that a big amount of negative-charged oxygen ions was present on this surface and joined the binary collisions. A decrease of the peak intensity was found at 90 K, indicating an enhancement of the shadowing effect related to the screening action of iron ions.
Fig. 3. Temperature dependence of the normalized ion yield, R(T)/R300K, of positive- ( ) and negative-charged (m) ions scattered from Fe3O4 (0 0 1) surface. Solid curves serve to guide the eyes.
The temperature dependence of the normalized ion yield (RðTÞ=R300 K ) for 6 keV Ar+ ions scattered from Fe3O4 (0 0 1) surface was shown in Fig. 3. The ion yield curve of the positive-charged ion, Rþ ðTÞ; has revealed two-step increases in the Verwey phase transition region respectively at 138 and 104 K. A similar characteristic of the R+(T) curve was observed at different primary energies. However, increasing the primary energy implied a widening of the plateau between the two steps. The results indicated a dominant contribution from the Auger and resonant neutralization. An increase of the electron localization degree below the Verwey temperature implied a decrease of the neutralization degree and thus an enormous increase in the peak intensity and in the ion yield. However, the plateau was considered as an evidence for a ‘compensated’ contribution from the re-ionization of neutralized particles in a collision with atoms in the deeper layers than the surface layer. The re-ionization was largely influenced by a crystal distortion at the Verwey transition due to a large change in the crystal transparency and thus the ion depth penetration. A very smooth minimum was observed in the R ðTÞ curve indicated a weak influence from the Verwey transition on the negative-charged oxygen ion emission. In summary, distinguished peaks in the LEIS spectra were attributed to Ar+–Fe scattering, the Fe-recoil and recoil oxygen ions. A large influence from the metal– insulator phase transition on the peak intensity and on the (positive-charged) scattered ion yield was observed. The characteristic of the Rþ ðTÞ curves was explained as a result of a complex interplay between two mechanisms: the dominated Auger-neutralization and the re-ionization.
ARTICLE IN PRESS N.-T.H. Kim-Ngan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2205–2207
References [1] N. Tsuda, et al., in: M. Cardona, P. Fulde, K. von Klitzing, H.-J. Queisser (Eds.), Electronic conductions in Oxides, Solid-State Sciences, Vol. 94, Springer, Berlin, 1990. [2] N.-T.H. Kim-Ngan, W. Soszka, J. Magn. Magn. Mater. 202 (1999) 327.
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[3] N.-T.H. Kim-Ngan, W. Soszka, Physica B 319 (2002) 133. [4] N.-T.H. Kim-Ngan, W. Soszka, Surface Science 536 (2003) 24. [5] E.S. Parilis, et al., in: Atomic Collisions on Solid Surface, North-Holland, Elsevier Science Publishers B.V., Amsterdam, 1993.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2205–2207
Influence of the metal–insulator phase transition on low-energy Ar+ ions scattering from an Fe3O4 (0 0 1) surface N.-T.H. Kim-Ngan*, W. Soszka, G. Jag"o, D. Goc-Jag"o ! 30-084, Poland Institute of Physics, Pedagogical University, Krakow
Abstract We investigated a single crystalline Fe3O4 (0 0 1) surface by using a temperature-dependent low-energy ion spectroscopy with an Ar+ ion beam. The energy spectra have revealed distinguished peaks corresponding to the Ar+– Fe single scattering, the recoil Fe and recoil O ions. Two step-like increases were revealed in the temperature-dependent curves of the positive-charged scattered ion yield in the metal–insulator phase transition region. A very weak phasetransition effect was observed on the oxygen emissions from the magnetite surface. r 2003 Elsevier B.V. All rights reserved. PACS: 68.35.p; 61.18.Bn Keywords: Fe3O4; Metal–insulator phase transition; Low-energy ion scattering (LEIS)
The large influence from the metal–insulator phase transition in magnetite (i.e. the Verwey transition related to a freezing of the electron hopping in the Feoctahedral interstices) on the bulk properties has been thoroughly investigated over many years (see the review [1]). Recently, we have investigated the ‘surface’ Verwey transition of several magnetite surfaces using an ion scattering spectroscopy (ISS or LEIS technique) [2,3]. In a small-angle geometry, distinct anomalies around 100– 130 K have been revealed in the temperature-dependent curve of ion scattering yield, R+(T). In this paper, we present the LEIS investigations on a single crystalline magnetite (0 0 1) surface. The experiments have been carried out in an ISS at a large-angle geometry described elsewhere [4] with which we focus on the influence of the Verwey transition on the single scattering process from such a surface. A surface cleanness was done in situ with annealing at temperatures 500–600 K and with 6 keV Ar+ sputtering prior to each LEIS experiment. *Corresponding author. Tel.: +48-12-6626314; fax: +48-126372243. E-mail address: [email protected] (N.-T.H. Kim-Ngan).
According to a binary elastic collision model [5], the energy of the scattered ions and recoil ions is, respectively, expressed as E1 ¼ E0 f½cos y1 þ ðm2 sin2 y1 Þ1=2 =ð1 þ mÞg2 ; E2 ¼ E0 ½4m=ð1 þ mÞ2 cos2 y2 ; where E0 is the primary energy, y1 ; y2 are the related scattering angles, m ¼ m2 =m1 is the mass ratio between the target and projectile atoms. Fig. 1 showed the LEIS spectra of ions scattered from the Fe3O4 (0 0 1) surface under 6 keV Ar+ ion bombardments. Two well-defined peaks on a high background were observed. The peak intensity, and the shape of the LEIS spectra were strongly dependent on the angle geometry and on the target temperatures. At 300 K a symmetric energy spectrum with similar peak-heights was obtained at C ¼ 34 and Y ¼ 68 : The energy position of the high-energy peak was in a good agreement with that for a single binary Ar+–Fe scattering: the E1(Fe)/E0 ratio was of 0.374 and 0.352, respectively, for Y ¼ 66 and 68 . A significant contribution from the quasi-single scattering ions was present showed by a large peak width. Detailed investigations of the characteristic of the low-energy peak as a function of the detection angle and of the azimuthal angle indicated that it was due to the recoil Fe ions, E2 ðFeÞ [4]. The peak intensity was much enhanced
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.920
ARTICLE IN PRESS 2206
N.-T.H. Kim-Ngan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2205–2207
Fig. 1. The LEIS spectra of Fe3O4 (0 0 1) surface under 6 keV Ar+ ion bombardment at different angle geometries. T=300 K (open markers) and 90 K (solid markers).
Fig. 2. Energy spectra of the recoil O1 ions emitted from Fe3O4 (0 0 1) surface under 6 keV Ar+ ion bombardment. C ¼ 34 ; Y ¼ 68 :
at 90 K. The largest enhancement was found for the Ar+-Fe scattering peak at C ¼ 34 and Y ¼ 68 : A similar angle and temperature dependence of the LEIS spectra was observed for different primary energies. The energy ratio between the two peaks, E1 ðFeÞ=E2 ðFeÞ; was of 2.3 and was independent on the primary energy. A huge peak was observed in the LEIS spectra of the emitted negative ions (Fig. 2). The energy position of the maximum was in a good agreement with that of the recoil O1 ions from a binary collision, E2(O). It indicated that a big amount of negative-charged oxygen ions was present on this surface and joined the binary collisions. A decrease of the peak intensity was found at 90 K, indicating an enhancement of the shadowing effect related to the screening action of iron ions.
Fig. 3. Temperature dependence of the normalized ion yield, R(T)/R300K, of positive- ( ) and negative-charged (m) ions scattered from Fe3O4 (0 0 1) surface. Solid curves serve to guide the eyes.
The temperature dependence of the normalized ion yield (RðTÞ=R300 K ) for 6 keV Ar+ ions scattered from Fe3O4 (0 0 1) surface was shown in Fig. 3. The ion yield curve of the positive-charged ion, Rþ ðTÞ; has revealed two-step increases in the Verwey phase transition region respectively at 138 and 104 K. A similar characteristic of the R+(T) curve was observed at different primary energies. However, increasing the primary energy implied a widening of the plateau between the two steps. The results indicated a dominant contribution from the Auger and resonant neutralization. An increase of the electron localization degree below the Verwey temperature implied a decrease of the neutralization degree and thus an enormous increase in the peak intensity and in the ion yield. However, the plateau was considered as an evidence for a ‘compensated’ contribution from the re-ionization of neutralized particles in a collision with atoms in the deeper layers than the surface layer. The re-ionization was largely influenced by a crystal distortion at the Verwey transition due to a large change in the crystal transparency and thus the ion depth penetration. A very smooth minimum was observed in the R ðTÞ curve indicated a weak influence from the Verwey transition on the negative-charged oxygen ion emission. In summary, distinguished peaks in the LEIS spectra were attributed to Ar+–Fe scattering, the Fe-recoil and recoil oxygen ions. A large influence from the metal– insulator phase transition on the peak intensity and on the (positive-charged) scattered ion yield was observed. The characteristic of the Rþ ðTÞ curves was explained as a result of a complex interplay between two mechanisms: the dominated Auger-neutralization and the re-ionization.
ARTICLE IN PRESS N.-T.H. Kim-Ngan et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2205–2207
References [1] N. Tsuda, et al., in: M. Cardona, P. Fulde, K. von Klitzing, H.-J. Queisser (Eds.), Electronic conductions in Oxides, Solid-State Sciences, Vol. 94, Springer, Berlin, 1990. [2] N.-T.H. Kim-Ngan, W. Soszka, J. Magn. Magn. Mater. 202 (1999) 327.
2207
[3] N.-T.H. Kim-Ngan, W. Soszka, Physica B 319 (2002) 133. [4] N.-T.H. Kim-Ngan, W. Soszka, Surface Science 536 (2003) 24. [5] E.S. Parilis, et al., in: Atomic Collisions on Solid Surface, North-Holland, Elsevier Science Publishers B.V., Amsterdam, 1993.