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The surface metal-insulator phase transition of MBE (1 0 0) magnetite thin film
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The surface metal-insulator phase transition of MBE (1 0 0) magnetite thin "lm W. Soszka *, N.-T.H. Kim-Ngan , D. Sitko , G. Jag"o , J. Korecki, B. Handke Institute of Physics, Pedagogical University, ul.Podchorazych 2, 30-084 Krako& w, Poland Department of Solid State Physics, Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, 30-059 Krako& w, Poland
Abstract The (1 0 0) surface of the magnetite thin "lm has been investigated by low-energy ion scattering in the small-angle geometry and in the temperature range of 85}300 K. Thin "lm of Fe O was grown on MgO (1 0 0) substrate by the molecular beam epitaxy (MBE) and characterized by low-energy electron di!raction, conversion electron MoK ssbauer spectroscopy and scanning tunneling microscopy. The scattered ion spectra have shown two maxima, in which the intensity of the maximum at low-energy side increases enormously with increasing bombarding energy values. The temperature dependence of scattering ion yield, R>(¹), for 5.0, 5.5 and 6.0 keV Ne> bombardments exhibit two minima, around 110 K and around 125 K, related to the metal-insulator phase transition of this material. The high-temperature minimum was found to disappear at 6.5 keV Ne> bombardment, indicating the ion velocity dependence of the character of the R>(¹) curve, which has been observed previously for the MBE (1 1 1) magnetite thin "lm. This phenomenon has been explained in the framework of the resonant and Auger neutralization. 2001 Elsevier Science Ltd. All rights reserved. PACS: 68.35.-p; 61.14.Hg; 61.18.Bn; 71.30.#h Keywords: LEIS; Magnetite; LEED epitaxial thin "lm; Metal-insulator phase transition
1. Introduction The low-energy ion scattering (LEIS) technique or ion scattering spectroscopy (ISS) has proved itself to be a very useful tool for studying solid surfaces [1,2]. The high surface sensitivity of this technique is attributed to the preferential neutralization of the ions scattered from atoms beneath the surface and to the small penetration depth of
* Corresponding author. Tel./Fax: #48-12-6372243. E-mail address: [email protected] (W. Soszka).
the ions due to the large di!erential scattering cross section in this energy range. As a consequence, the majority of the re#ected ions are scattered from the outermost surface layer of solids. Recently, attention has been focused on using this method for investigation of the metal-insulator phase transition (MIT) in magnetite, i.e. the Verwey transition [3}6]. Magnetite, Fe O , has a cubic inverse spinel structure with a lattice constant of 8.3967 As . The valence structure is [Fe>](Fe> Fe>)(O\) , where half of the Fe> ions occupy the tetrahedrally-coordinated sites (A-sites) and the other half
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 1 2 - 3
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together with the Fe> ions are located in the octahedrally-coordinated sites (B-sites). MIT in magnetite corresponds to the order/disorder transition in the distribution of Fe> and Fe> ions in the B-sites. Namely, the electron hopping between neighboring Fe> and Fe> on B-sites is the reason for the good electrical conductivity at room temperature. Upon cooling, magnetite undergoes the Verwey transition at a temperature in the range of 115}125 K, where the electron hopping is frozen and the crystal becomes insulating [7]. The ion scattering experiments have revealed a strong e!ect of MIT on the ion scattering from several di!erent surfaces of magnetite. Namely, a deep minimum around 120 K in the temperature-dependent curve of scattered ion yield, R>(T), has been observed [3}6]. Moreover, the ion-scattering investigations on a (1 1 1) surface of a molecular-beam-epitaxy (MBE) thin "lm of magnetite have revealed the ion velocity dependence of the character of the R>(T) curve [6]. In the present paper, we report our further investigations of the Verwey transition of MBE (1 0 0) thin "lm surface of magnetite and its e!ect on ion scattering.
2. Experimental details The ion-scattering experiments were performed using a standard ISS in the temperature range of 85}300 K and with a small-angle geometry [6]. This angle geometry, the incident angle relative to the surface target and the detection angle being in the order of 03}103, has been developed for ion-scattering experiments on a thin "lm surface. It is favorable for the observation of multiple scattering e!ects due to the atomic screening on the incoming and outgoing paths. Moreover, due to the large footprint at small angles, there was less damage caused by the ion beam to the thin "lm surface. The 200 As -thick magnetite thin "lm was grown on the MgO (0 0 1) substrate in the UHV MBE system. The "lm structure was controlled by a standard four-grid LEED-AES spectrometer. Before deposition, the MgO substrate was annealed for 20 min at 6003C. The Fe O layers were depos ited by Fe evaporation in O ambient pressure at the rate of 14 As /min, and at the substrate temper-
ature of 2503C. The Fe isotope has been used as a probe for conversion electron MoK ssbauer spectroscopy (CEMS). For further discussions, we address this thin "lm surface of magnetite as the Fe O (1 0 0)-"lm surface. The ion-scattering experiments, as well as the additional investigations for checking the in#uence of bombarding time to the "lm structure, etc have been carried out similarly to those on the Fe O (1 1 1)-"lm surface [6]. We have found that: E no disturbance related to the surface oxidization on the ion scattering investigations was observed for magnetite surface, E preferential sputtering has shown no visible in#uence on the shape of energy spectra, E the negative ion yields were negligible, E the time dependence was also negligible in the temperature-dependent investigation due to the fact that a very small change in the intensity of a chosen scattering peak was observed within 4 h (i.e. within the time limit of each temperature-dependent run). For the temperature-dependence investigations we have used He> and Ne> ions. The measurements have been carried out at di!erent energies between 5.0 and 6.5 keV with an energy step of 0.5 keV. We optimized a scattering peak similar to that for the (1 1 1)-"lm surface (at grazing angles) for the temperature-dependent investigations due to the fact that it was strong enough and that the bombarding-time e!ect to the thin "lm surface was expected to be the weakest for this position.
3. Results and discussions The in situ LEED image of this thin "lm has revealed a clean, well-ordered (1 0 0) surface, as shown in Fig. 1. The CEMS spectra were taken in situ. Their temperature dependence has shown that: (i) the room temperature spectrum could be "tted with two magnetic component of `2.5#a and `3#a with the intensity ratio of nearly 1 : 2 and, (ii) the drastic change of the spectra related to the Verwey transition has occurred at temperatures below 130 K. These results have re#ected the good stoichiometry of the thin "lm.
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Fig. 1. LEED patterns observed for the Fe O (1 0 0)-"lm sur face grown on the MgO (0 0 1) substrate at temperature of 2503C. The LEED image was taken at 95 eV.
The surface topology of such a (1 0 0)-"lm surface of magnetite has been investigated by scanning tunneling microscopy (STM) [8]. Our STM data were in good agreement with those reported by several groups [9,10]. There are two di!erent topographies corresponding to two (1 0 0) crystal planes; one built up by the octahedrally-coordinated iron and oxygen atoms and the other one by tetrahedrally-coordinated iron atoms. The layer spacing between those two planes is 1 As ("1/8 of the spinel lattice constant), as shown in Fig. 2. Along the crystallographic direction, the semi-channel surface on the magnetite surface was also found, as shown in Fig. 2b. The channel height is about 1 As . Such a semi-channel surface is favorable for the zigzag collisions with related large energy loss, as described earlier [5,6]. In Fig. 3, the energy spectra of ions scattered from the Fe O (1 0 0)-"lm surface under 5.0 keV of He> and Ne> bombardments at 87 K are shown. Very broad peaks around 4.3 keV, corresponding presumably to the ion/Fe scattering, were observed. The He>/Fe scattering peak was found to be much stronger than the Ne>/Fe one. Moreover, the He>/Fe scattering peak was much broader at the
Fig. 2. (a) The cubic inverse spinel structure of magnetite. For clarity, the atoms are shown only in the front half. Oxygen anions are shown as big open spheres, Fe atoms in octahedral sites as small shaded spheres and those in tetrahedral sites as small solid spheres. The layer spacing is 1 As . The crystallographic directions of magnetite are also shown. (b) The stacking sequence of di!erent (1 0 0) layers in magnetite. The surface semi-channel with the channel height of 1 As exists on magnetite surface.
low-energy side. Unlike the case of ion scattering from the cleavage surface and from the (1 1 1)-"lm surface of magnetite, no visible downwards-shift of the scattering peak with increasing ion mass was observed for the (1 0 0)-"lm surface. In Fig. 4, the energy spectra of ions scattered from the Fe O (1 0 0)-"lm surface under 5.0, 5.5, 6.0 and 6.5 keV Ne> bombardments at 87 K are shown. The intensity of the scattering peak was found to increase strongly with increasing primary energies. In general, the intensity of the scattering peak obtained for the (1 0 0)-"lm surface was much stronger than that for the (1 1 1)-"lm surface of magnetite. Two maxima were observed which became more visible with increasing bombarding energy. For energies higher than 5.0 keV, while no change in the inten-
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Fig. 3. Energy spectra of 5.0 keV He> (䊏) and Ne> (*) ions scattered from the Fe O (1 0 0)-"lm surface at grazing angles and at the target temperature of 87 K.
Fig. 4. Energy spectra of 5.0 (䊏), 5.5 (䉱), 6.0 (*) and 6.5 keV () Ne> ions scattered from the Fe O (1 0 0)-"lm surface at grazing angles and at the target temperature of 87 K. The vertical line shows the position of the minimum between two maxima, which is at the same relative energy loss ratio, E /E , of 0.865 for di!erent bombarding energies.
sity of the maximum at the high-energy side was observed, the maximum at the low-energy side (the low-energy maximum) increases enormously with increasing primary energies. We notice here that the position of the minimum between two maxima is always located at the same relative energy loss ratio, E /E , of 0.865, as shown by vertical line in Fig. 4. In the case of ion scattering from the cleaved surface and (1 1 1)-"lm surface, only broad scattering peaks had been typically observed for the mul-
Fig. 5. Temperature dependence of the scattered ion yield, R>(T), for 5.0 (䊏), 5.5 (䉱), 6.0 (*) and 6.5 keV () Ne> ions scattered from the Fe O (1 0 0)-"lm surface. Solid curves serve as guide for eyes.
tiple scattering. The broad scattering peak is always considered as a superposition of many scattering peaks, in which each is with di!erent scattering con"guration (sub-scattering peak) and at di!erent scattering angles. We assumed that, due to the e!ect of neutralization, some of sub-scattering peaks at certain scattering angles may disappear, resulting in the minimum observed in the scattering spectrum of the (1 0 0)-"lm surface. Moreover, this minimum became more visible with increasing the primary energy. Probably due to the shadow cone decreasing at larger bombarding energies, the ions can go deeper into the semi-channel and reach the bottom. As a consequence, the neutralization by the surface semi-channel became more distinct with increasing primary energies. Although beyond the scope of this article, it would be interesting to perform the measurements of the energy spectra of neutrals, by a time of #ight (TOF) spectrometer instance. The comparison of energy spectra of scattered neutral particles with ion scattering spectra would provide information on neutralization. The temperature dependence of the scattered ion yields (R>(¹)) from the Fe O (1 0 0)-"lm surface under the Ne> ion bombardments was shown in Fig. 5. Two minima, one located around 100 K and the other one at around 125 K, were observed in the R(¹) curves corresponding to the bombarding energy of 5.0, 5.5 and 6.0 keV. The scattering ion yields decrease by about 20% of magnitude at the
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minimum. Above 150 K, the scattered ion yields were almost constant with temperature. For the 6.5 keV Ne> ion bombardment, only one deep minimum has existed at around 100 K. Moreover, a bigger decrease in the scattering ion yield, by two times of magnitude, was observed at the transition point. Some di!erent behavior of the R>(¹) curves for Fe O (1 0 0)-"lm surface was observed with re spect to other surfaces: 1. appearance of the low-temperature minimum (i.e. the minimum at around 100 K) in the R>(¹) curves, besides the minimum around 125 K, 2. disappearance of the small maximum around 135 K, and 3. change of the character of the R>(¹) curve at higher primary energies. We notice here that for both (1 1 1)- and (1 0 0)"lm surfaces we have observed the change of the character of the R>(¹) curve at higher primary energies. The characterization of such a change, however, was di!erent. For the (1 1 1)-"lm surface, the broad minimum in the R(¹) curves around 125 K was contributed to the Verwey transition. However, under the 6.5 keV Ne> bombardment the minimum has disappeared and only a very fast decrease of the scattered ion yield around 100 K was observed. The character of the temperaturedependent scattered ion yield curve of the (1 1 1)"lm surface at the same bombarding energy di!ers from that of the (1 0 0)-"lm surface. Namely, for the latter one (i.e. for the (1 0 0)-"lm surface) two minima existed. In such case, the change of the character of the ion scattering variation in temperature with increasing primary energy was indicated by the disappearance of the high-temperature minimum. Namely, the minimum around 125 K disappeared under 6.5 keV Ne> ion bombardment of the (1 0 0)-"lm surface. The three following factors have been taken into account for explanation of the phase transition e!ect on ion scattering from other surfaces of magnetite: 1. change of the neutralization probability of incoming and outgoing ions,
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2. change of the crystal transparency, and 3. existence of the so-called ionizing trajectories, i.e. trajectories along which the particles become re-ionized (in other words, ion trajectories containing ionizing collisions). The last one was found to play the most important role for ion scattering from the cleavage (2 1 3) surface of magnetite in the phase transition region. A very limited choice of ionizing trajectories was found in case of the small angle geometry. Any small in#uence on such a group of ion trajectories can cause a large change in the scattered ion yield especially in the phase transition region. For the (1 1 1)-"lm surface of magnetite, the character of the scattered ion yield was found to depend on the velocity of the incoming ions, but not on the primary energy value. These results indicate that the neutralization from the resonant and Auger processes plays an important role. Such neutralization e!ect was considered to exist also in case of ion scattering from the (1 0 0)-thin surface, where the change of the character of the R>(¹) curve at 6.5 keV Ne> ion bombardment was also observed. The loss of ions from the ultimate scattered signal is exponential in 1/< , where < is , , the component of the ions velocity perpendicular to the surface [6]. An increase of the ion velocity implies an increase of the remaining-ion probability (i.e. the probability that the ion escapes neutralizations). Such an increase can cause the disappearance of the minimum at 125 K for both (1 1 1)- and (1 0 0)-"lm surfaces. In our experiments, i.e. for ion scattering with the small angle geometry, where the perpendicular component of the velocity < is small, the neutralization is , certainly very sensitive to the change of such component.
4. Conclusion For the (1 0 0)-"lm surface of magnetite the scattered ion spectra exhibit a double maxima under the Ne> bombardments. The minimum between two maxima is always located at the same relative energy loss ratio for di!erent bombarding energy values. Such a minimum was assumed to be a result
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of the lack of some scattering peaks due to an extra neutralization, which became dominant for some scattering con"guration. The large e!ect of the Verwey transition on ion scattering was also observed by two minima, one at around 100 K and the other one at 125 K, in the temperature dependent curve of the scattered ion yield, R>(¹). The high-temperature minimum was found to disappear under 6.5 keV Ne> ion bombardment, indicating the ion velocity dependence of the character of the scattered ion yield, which was also observed in the case of the (1 1 1)-"lm surface of magnetite. The velocity dependence of the character of the R>(¹) curve shows that the resonant and Auger neutralization involving the tunneling or the loss of electrons from the solid surface valence band can give dominant contributions a!ecting the ion scattering at the Verwey phase transition.
References [1] For a review see e.g. Niehus H, Heiland W, Taglauer E. Surface Sci Reports 1993;17:213. [2] Algra AJ, Luitjens SB, Suurmeijer EPThM, Boers AL. Nucl Instr and Meth 1982;203:515 and references therein. [3] Soszka W, Kim-Ngan N-TH, Sitko D, Jaglo G, Kozlowski A. Vacuum 1999;54:83. [4] Kim-Ngan N-TH, Soszka W. J Magn Magn Mater 1999;202:327. [5] Soszka W, Kim-Ngan N-TH. Surf Sci 1999;441:331. [6] Kim-Ngan N-TH, Soszka W, Sitko D, Jag"o G, Korecki J, Handke B. Nucl Instr and Meth 2000;B164}165:992. [7] Tsuda N, Nasu K, Yanase Y, Siratori K. In: Cardona M, Fulde P, von Klitzing K, Queisser H-J, editors. Electronic conductions in oxides, siries in solid-state sciences, vol. 94. Berlin: Springer, 1990. [8] Kim-Ngan N-TH, Soszka W, Hietschold M, Acta Physica Polonica 2001; A99. [9] Wiesendanger R, Shvets IV, Coey JMD. Vac Sci Technol 1994;B12:2118. and references therein. [10] Gaines JM, et al. Surf Sci 1997;373:85. and references therein.
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The surface metal-insulator phase transition of MBE (1 0 0) magnetite thin "lm W. Soszka *, N.-T.H. Kim-Ngan , D. Sitko , G. Jag"o , J. Korecki, B. Handke Institute of Physics, Pedagogical University, ul.Podchorazych 2, 30-084 Krako& w, Poland Department of Solid State Physics, Faculty of Physics and Nuclear Techniques, University of Mining and Metallurgy, 30-059 Krako& w, Poland
Abstract The (1 0 0) surface of the magnetite thin "lm has been investigated by low-energy ion scattering in the small-angle geometry and in the temperature range of 85}300 K. Thin "lm of Fe O was grown on MgO (1 0 0) substrate by the molecular beam epitaxy (MBE) and characterized by low-energy electron di!raction, conversion electron MoK ssbauer spectroscopy and scanning tunneling microscopy. The scattered ion spectra have shown two maxima, in which the intensity of the maximum at low-energy side increases enormously with increasing bombarding energy values. The temperature dependence of scattering ion yield, R>(¹), for 5.0, 5.5 and 6.0 keV Ne> bombardments exhibit two minima, around 110 K and around 125 K, related to the metal-insulator phase transition of this material. The high-temperature minimum was found to disappear at 6.5 keV Ne> bombardment, indicating the ion velocity dependence of the character of the R>(¹) curve, which has been observed previously for the MBE (1 1 1) magnetite thin "lm. This phenomenon has been explained in the framework of the resonant and Auger neutralization. 2001 Elsevier Science Ltd. All rights reserved. PACS: 68.35.-p; 61.14.Hg; 61.18.Bn; 71.30.#h Keywords: LEIS; Magnetite; LEED epitaxial thin "lm; Metal-insulator phase transition
1. Introduction The low-energy ion scattering (LEIS) technique or ion scattering spectroscopy (ISS) has proved itself to be a very useful tool for studying solid surfaces [1,2]. The high surface sensitivity of this technique is attributed to the preferential neutralization of the ions scattered from atoms beneath the surface and to the small penetration depth of
* Corresponding author. Tel./Fax: #48-12-6372243. E-mail address: [email protected] (W. Soszka).
the ions due to the large di!erential scattering cross section in this energy range. As a consequence, the majority of the re#ected ions are scattered from the outermost surface layer of solids. Recently, attention has been focused on using this method for investigation of the metal-insulator phase transition (MIT) in magnetite, i.e. the Verwey transition [3}6]. Magnetite, Fe O , has a cubic inverse spinel structure with a lattice constant of 8.3967 As . The valence structure is [Fe>](Fe> Fe>)(O\) , where half of the Fe> ions occupy the tetrahedrally-coordinated sites (A-sites) and the other half
0042-207X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 2 - 2 0 7 X ( 0 1 ) 0 0 2 1 2 - 3
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together with the Fe> ions are located in the octahedrally-coordinated sites (B-sites). MIT in magnetite corresponds to the order/disorder transition in the distribution of Fe> and Fe> ions in the B-sites. Namely, the electron hopping between neighboring Fe> and Fe> on B-sites is the reason for the good electrical conductivity at room temperature. Upon cooling, magnetite undergoes the Verwey transition at a temperature in the range of 115}125 K, where the electron hopping is frozen and the crystal becomes insulating [7]. The ion scattering experiments have revealed a strong e!ect of MIT on the ion scattering from several di!erent surfaces of magnetite. Namely, a deep minimum around 120 K in the temperature-dependent curve of scattered ion yield, R>(T), has been observed [3}6]. Moreover, the ion-scattering investigations on a (1 1 1) surface of a molecular-beam-epitaxy (MBE) thin "lm of magnetite have revealed the ion velocity dependence of the character of the R>(T) curve [6]. In the present paper, we report our further investigations of the Verwey transition of MBE (1 0 0) thin "lm surface of magnetite and its e!ect on ion scattering.
2. Experimental details The ion-scattering experiments were performed using a standard ISS in the temperature range of 85}300 K and with a small-angle geometry [6]. This angle geometry, the incident angle relative to the surface target and the detection angle being in the order of 03}103, has been developed for ion-scattering experiments on a thin "lm surface. It is favorable for the observation of multiple scattering e!ects due to the atomic screening on the incoming and outgoing paths. Moreover, due to the large footprint at small angles, there was less damage caused by the ion beam to the thin "lm surface. The 200 As -thick magnetite thin "lm was grown on the MgO (0 0 1) substrate in the UHV MBE system. The "lm structure was controlled by a standard four-grid LEED-AES spectrometer. Before deposition, the MgO substrate was annealed for 20 min at 6003C. The Fe O layers were depos ited by Fe evaporation in O ambient pressure at the rate of 14 As /min, and at the substrate temper-
ature of 2503C. The Fe isotope has been used as a probe for conversion electron MoK ssbauer spectroscopy (CEMS). For further discussions, we address this thin "lm surface of magnetite as the Fe O (1 0 0)-"lm surface. The ion-scattering experiments, as well as the additional investigations for checking the in#uence of bombarding time to the "lm structure, etc have been carried out similarly to those on the Fe O (1 1 1)-"lm surface [6]. We have found that: E no disturbance related to the surface oxidization on the ion scattering investigations was observed for magnetite surface, E preferential sputtering has shown no visible in#uence on the shape of energy spectra, E the negative ion yields were negligible, E the time dependence was also negligible in the temperature-dependent investigation due to the fact that a very small change in the intensity of a chosen scattering peak was observed within 4 h (i.e. within the time limit of each temperature-dependent run). For the temperature-dependence investigations we have used He> and Ne> ions. The measurements have been carried out at di!erent energies between 5.0 and 6.5 keV with an energy step of 0.5 keV. We optimized a scattering peak similar to that for the (1 1 1)-"lm surface (at grazing angles) for the temperature-dependent investigations due to the fact that it was strong enough and that the bombarding-time e!ect to the thin "lm surface was expected to be the weakest for this position.
3. Results and discussions The in situ LEED image of this thin "lm has revealed a clean, well-ordered (1 0 0) surface, as shown in Fig. 1. The CEMS spectra were taken in situ. Their temperature dependence has shown that: (i) the room temperature spectrum could be "tted with two magnetic component of `2.5#a and `3#a with the intensity ratio of nearly 1 : 2 and, (ii) the drastic change of the spectra related to the Verwey transition has occurred at temperatures below 130 K. These results have re#ected the good stoichiometry of the thin "lm.
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Fig. 1. LEED patterns observed for the Fe O (1 0 0)-"lm sur face grown on the MgO (0 0 1) substrate at temperature of 2503C. The LEED image was taken at 95 eV.
The surface topology of such a (1 0 0)-"lm surface of magnetite has been investigated by scanning tunneling microscopy (STM) [8]. Our STM data were in good agreement with those reported by several groups [9,10]. There are two di!erent topographies corresponding to two (1 0 0) crystal planes; one built up by the octahedrally-coordinated iron and oxygen atoms and the other one by tetrahedrally-coordinated iron atoms. The layer spacing between those two planes is 1 As ("1/8 of the spinel lattice constant), as shown in Fig. 2. Along the crystallographic direction, the semi-channel surface on the magnetite surface was also found, as shown in Fig. 2b. The channel height is about 1 As . Such a semi-channel surface is favorable for the zigzag collisions with related large energy loss, as described earlier [5,6]. In Fig. 3, the energy spectra of ions scattered from the Fe O (1 0 0)-"lm surface under 5.0 keV of He> and Ne> bombardments at 87 K are shown. Very broad peaks around 4.3 keV, corresponding presumably to the ion/Fe scattering, were observed. The He>/Fe scattering peak was found to be much stronger than the Ne>/Fe one. Moreover, the He>/Fe scattering peak was much broader at the
Fig. 2. (a) The cubic inverse spinel structure of magnetite. For clarity, the atoms are shown only in the front half. Oxygen anions are shown as big open spheres, Fe atoms in octahedral sites as small shaded spheres and those in tetrahedral sites as small solid spheres. The layer spacing is 1 As . The crystallographic directions of magnetite are also shown. (b) The stacking sequence of di!erent (1 0 0) layers in magnetite. The surface semi-channel with the channel height of 1 As exists on magnetite surface.
low-energy side. Unlike the case of ion scattering from the cleavage surface and from the (1 1 1)-"lm surface of magnetite, no visible downwards-shift of the scattering peak with increasing ion mass was observed for the (1 0 0)-"lm surface. In Fig. 4, the energy spectra of ions scattered from the Fe O (1 0 0)-"lm surface under 5.0, 5.5, 6.0 and 6.5 keV Ne> bombardments at 87 K are shown. The intensity of the scattering peak was found to increase strongly with increasing primary energies. In general, the intensity of the scattering peak obtained for the (1 0 0)-"lm surface was much stronger than that for the (1 1 1)-"lm surface of magnetite. Two maxima were observed which became more visible with increasing bombarding energy. For energies higher than 5.0 keV, while no change in the inten-
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Fig. 3. Energy spectra of 5.0 keV He> (䊏) and Ne> (*) ions scattered from the Fe O (1 0 0)-"lm surface at grazing angles and at the target temperature of 87 K.
Fig. 4. Energy spectra of 5.0 (䊏), 5.5 (䉱), 6.0 (*) and 6.5 keV () Ne> ions scattered from the Fe O (1 0 0)-"lm surface at grazing angles and at the target temperature of 87 K. The vertical line shows the position of the minimum between two maxima, which is at the same relative energy loss ratio, E /E , of 0.865 for di!erent bombarding energies.
sity of the maximum at the high-energy side was observed, the maximum at the low-energy side (the low-energy maximum) increases enormously with increasing primary energies. We notice here that the position of the minimum between two maxima is always located at the same relative energy loss ratio, E /E , of 0.865, as shown by vertical line in Fig. 4. In the case of ion scattering from the cleaved surface and (1 1 1)-"lm surface, only broad scattering peaks had been typically observed for the mul-
Fig. 5. Temperature dependence of the scattered ion yield, R>(T), for 5.0 (䊏), 5.5 (䉱), 6.0 (*) and 6.5 keV () Ne> ions scattered from the Fe O (1 0 0)-"lm surface. Solid curves serve as guide for eyes.
tiple scattering. The broad scattering peak is always considered as a superposition of many scattering peaks, in which each is with di!erent scattering con"guration (sub-scattering peak) and at di!erent scattering angles. We assumed that, due to the e!ect of neutralization, some of sub-scattering peaks at certain scattering angles may disappear, resulting in the minimum observed in the scattering spectrum of the (1 0 0)-"lm surface. Moreover, this minimum became more visible with increasing the primary energy. Probably due to the shadow cone decreasing at larger bombarding energies, the ions can go deeper into the semi-channel and reach the bottom. As a consequence, the neutralization by the surface semi-channel became more distinct with increasing primary energies. Although beyond the scope of this article, it would be interesting to perform the measurements of the energy spectra of neutrals, by a time of #ight (TOF) spectrometer instance. The comparison of energy spectra of scattered neutral particles with ion scattering spectra would provide information on neutralization. The temperature dependence of the scattered ion yields (R>(¹)) from the Fe O (1 0 0)-"lm surface under the Ne> ion bombardments was shown in Fig. 5. Two minima, one located around 100 K and the other one at around 125 K, were observed in the R(¹) curves corresponding to the bombarding energy of 5.0, 5.5 and 6.0 keV. The scattering ion yields decrease by about 20% of magnitude at the
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minimum. Above 150 K, the scattered ion yields were almost constant with temperature. For the 6.5 keV Ne> ion bombardment, only one deep minimum has existed at around 100 K. Moreover, a bigger decrease in the scattering ion yield, by two times of magnitude, was observed at the transition point. Some di!erent behavior of the R>(¹) curves for Fe O (1 0 0)-"lm surface was observed with re spect to other surfaces: 1. appearance of the low-temperature minimum (i.e. the minimum at around 100 K) in the R>(¹) curves, besides the minimum around 125 K, 2. disappearance of the small maximum around 135 K, and 3. change of the character of the R>(¹) curve at higher primary energies. We notice here that for both (1 1 1)- and (1 0 0)"lm surfaces we have observed the change of the character of the R>(¹) curve at higher primary energies. The characterization of such a change, however, was di!erent. For the (1 1 1)-"lm surface, the broad minimum in the R(¹) curves around 125 K was contributed to the Verwey transition. However, under the 6.5 keV Ne> bombardment the minimum has disappeared and only a very fast decrease of the scattered ion yield around 100 K was observed. The character of the temperaturedependent scattered ion yield curve of the (1 1 1)"lm surface at the same bombarding energy di!ers from that of the (1 0 0)-"lm surface. Namely, for the latter one (i.e. for the (1 0 0)-"lm surface) two minima existed. In such case, the change of the character of the ion scattering variation in temperature with increasing primary energy was indicated by the disappearance of the high-temperature minimum. Namely, the minimum around 125 K disappeared under 6.5 keV Ne> ion bombardment of the (1 0 0)-"lm surface. The three following factors have been taken into account for explanation of the phase transition e!ect on ion scattering from other surfaces of magnetite: 1. change of the neutralization probability of incoming and outgoing ions,
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2. change of the crystal transparency, and 3. existence of the so-called ionizing trajectories, i.e. trajectories along which the particles become re-ionized (in other words, ion trajectories containing ionizing collisions). The last one was found to play the most important role for ion scattering from the cleavage (2 1 3) surface of magnetite in the phase transition region. A very limited choice of ionizing trajectories was found in case of the small angle geometry. Any small in#uence on such a group of ion trajectories can cause a large change in the scattered ion yield especially in the phase transition region. For the (1 1 1)-"lm surface of magnetite, the character of the scattered ion yield was found to depend on the velocity of the incoming ions, but not on the primary energy value. These results indicate that the neutralization from the resonant and Auger processes plays an important role. Such neutralization e!ect was considered to exist also in case of ion scattering from the (1 0 0)-thin surface, where the change of the character of the R>(¹) curve at 6.5 keV Ne> ion bombardment was also observed. The loss of ions from the ultimate scattered signal is exponential in 1/< , where < is , , the component of the ions velocity perpendicular to the surface [6]. An increase of the ion velocity implies an increase of the remaining-ion probability (i.e. the probability that the ion escapes neutralizations). Such an increase can cause the disappearance of the minimum at 125 K for both (1 1 1)- and (1 0 0)-"lm surfaces. In our experiments, i.e. for ion scattering with the small angle geometry, where the perpendicular component of the velocity < is small, the neutralization is , certainly very sensitive to the change of such component.
4. Conclusion For the (1 0 0)-"lm surface of magnetite the scattered ion spectra exhibit a double maxima under the Ne> bombardments. The minimum between two maxima is always located at the same relative energy loss ratio for di!erent bombarding energy values. Such a minimum was assumed to be a result
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of the lack of some scattering peaks due to an extra neutralization, which became dominant for some scattering con"guration. The large e!ect of the Verwey transition on ion scattering was also observed by two minima, one at around 100 K and the other one at 125 K, in the temperature dependent curve of the scattered ion yield, R>(¹). The high-temperature minimum was found to disappear under 6.5 keV Ne> ion bombardment, indicating the ion velocity dependence of the character of the scattered ion yield, which was also observed in the case of the (1 1 1)-"lm surface of magnetite. The velocity dependence of the character of the R>(¹) curve shows that the resonant and Auger neutralization involving the tunneling or the loss of electrons from the solid surface valence band can give dominant contributions a!ecting the ion scattering at the Verwey phase transition.
References [1] For a review see e.g. Niehus H, Heiland W, Taglauer E. Surface Sci Reports 1993;17:213. [2] Algra AJ, Luitjens SB, Suurmeijer EPThM, Boers AL. Nucl Instr and Meth 1982;203:515 and references therein. [3] Soszka W, Kim-Ngan N-TH, Sitko D, Jaglo G, Kozlowski A. Vacuum 1999;54:83. [4] Kim-Ngan N-TH, Soszka W. J Magn Magn Mater 1999;202:327. [5] Soszka W, Kim-Ngan N-TH. Surf Sci 1999;441:331. [6] Kim-Ngan N-TH, Soszka W, Sitko D, Jag"o G, Korecki J, Handke B. Nucl Instr and Meth 2000;B164}165:992. [7] Tsuda N, Nasu K, Yanase Y, Siratori K. In: Cardona M, Fulde P, von Klitzing K, Queisser H-J, editors. Electronic conductions in oxides, siries in solid-state sciences, vol. 94. Berlin: Springer, 1990. [8] Kim-Ngan N-TH, Soszka W, Hietschold M, Acta Physica Polonica 2001; A99. [9] Wiesendanger R, Shvets IV, Coey JMD. Vac Sci Technol 1994;B12:2118. and references therein. [10] Gaines JM, et al. Surf Sci 1997;373:85. and references therein.
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