- Email: [email protected]
The surface metal–insulator transition in MBE magnetite thin films
Physica B 319 (2002) 133–140
The surface metal–insulator transition in MBE magnetite thin films N.-T.H. Kim-Ngan*, W. Soszka ! Poland Institute of Physics, Pedagogical University, Podchorazych 2, PL 30-084 Krakow, Received 29 January 2002; received in revised form 26 February 2002
Abstract The (1 0 0) and (1 1 1) MBE thin-film surfaces of magnetite have been investigated by low-energy ion scattering (LEIS) in the small-angle geometry and in the temperature range of 85–300 K. Distinct anomalies in the temperature dependence of scattering ion yield Rþ ðTÞ in the temperature range 100–125 K are attributed to the metal–insulator phase transition of this material. A strong dependence of the Rþ ðTÞ curve on the primary energy of incoming ions and on the geometrical structure of the surface semi-channels was observed. Three processes are taken into account: the Auger neutralization of ions related to the electron localization at the Verwey transition, the re-ionization of bombarding particles in the zigzag collisions from the topmost atomic rows, and the re-ionization in a collision cascade with the atomic rows lying inside the semi-channel. The latter one plays a more important role in the case of the (1 0 0) surface with a larger channel width. The change of the scattered-ion yield with increasing primary energy for both two surfaces is considered as indication of a dominant contribution from the Auger neutralization. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.35.p; 61.14.Hg; 61.18.Bn; 71.30.+h Keywords: Magnetite; MBE thin films; Metal–insulator phase transition (MIT); LEIS
1. Introduction Magnetite (Fe3O4) is a ferrimagnet with Nel temperature TN ¼ 860 K and has played an important role in the discovery of magnetism. Its high electrical conductivity at room temperature is attributed to electron hopping between Fe2+ and Fe3+ ions at the octahedral sites (B-sites) of the inverse spinel structure. Upon cooling, magnetite *Corresponding author. Tel.: +48-12-637-8286; fax: +4812-637-2243. E-mail address: [email protected] (N.-T.H. Kim-Ngan).
undergoes a metal–insulator phase transition (i.e. the so-called Verwey transition), at a temperature in the range of 115–125 K, where the electron hopping is frozen and the crystal becomes insulating [1]. Magnetite has been an attractive candidate for technological applications in many important fields of electronics and catalysis. Recently, increasing attention has been focused on the investigations of the surface structure and electromagnetic behavior of Fe3O4 thin films and Fe3O4based multilayer structures [2–6]. Solid-surface analysis by ion-scattering spectroscopy (ISS) or the low-energy ion scattering (LEIS) technique has been carried out with
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 1 1 6 - X
134
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
rare-gas ions having a primary energy in the range of 1–20 keV. The surface sensitivity of this technique is due to the fact that the majority of the reflected ions are scattered from the outermost surface layer of solids [7]. Recently, we have used this method to investigate the Verwey transition of several magnetite surfaces. A very deep minimum around 120 K and a small maximum around 135 K in the temperature-dependent curve of scatteredion yield Rþ ðTÞ have been observed for a single-crystalline magnetite surface [8,9]. The characterization of the Rþ ðTÞ curves around the Verwey temperature was found to be more complicated for the magnetite thin-film surfaces [10,11]. In this paper, we report our detailed investigations of the Verwey transition of MBE (1 0 0) and (1 1 1) magnetite thin-film surfaces. We focus on the influence of the geometrical structure of the surface semi-channels on the magnetite surfaces on the conditions for realizing ionizing trajectories during isolated collisions especially around the Verwey transition.
ing effects. Irrespective of the primary energy, such a small-angle geometry always results in multiple scattering due to the atomic screening on the incoming and outgoing paths. Moreover, this small-angle geometry is especially needed for ionscattering experiments on a thin-film surface. Due to the large footprint at small angles, there will be less damage caused by the ion beam to the film surface. He+, Ne+ and Ar+ ion beams at primary energies between 5.0 and 7.5 keV and with an energy step of 0.5 keV were used. A scattering peak at grazing angles was optimized for the temperature-dependent investigations due to the fact that the bombarding-time effect to the thin-film surface was expected to be the weakest for this position. The reproducibility of the phase-transition effect and the insignificant effect of bombarding time on the film structure in our experiments has been discussed earlier [10].
3. Results and discussion
2. Experimental details
3.1. The Verwey transition of the MBE thin-film magnetite surfaces
( The MBE 200-A-thick Fe3O4 thin films with a well-ordered (1 0 0)- and (1 1 1)-surface were the subject of our studies. The film structure was monitored by a standard four-grid LEED-AES spectrometer and its stoichiometry was checked by . in situ conversion electron Mossbauer spectroscopy (CEMS) spectra. Details of the thin-film growth were published elsewhere [10,11]. The temperature dependence of the CEMS spectra reflects a good stoichiometry of the thin film. Namely, the spectra at room temperature could be fitted with two magnetic components of ‘‘2.5+’’ and ‘‘3+’’ with the intensity ratio of nearly 1:2, and a drastic change of the spectra related to the Verwey transition occurs at temperatures below 130 K. The experiments were carried out in a LEIS system in the temperature range 85–300 K described elsewhere [9]. A small-angle geometry was used (the incident angle C relative to the plane and the detection angle Y are in the order of 0–101) favorable for the observation of multiple-scatter-
The energy spectra of ions scattered from the two Fe3O4 thin-film surfaces exhibited very broad peaks corresponding to the ion/Fe multiple scattering. The dependence of the energy spectra on the incoming ions and the primary energies have been discussed elsewhere [10,11]. No change in the position of the scattering peak with temperature was observed in all cases. Changing the target temperature caused only a change in the intensity of the scattering peak. A visible splitting of the scattering peak in the phase transition region was observed only under 6.5 keV He+ ion bombardment of the (1 0 0) surface: the scattering peak exhibited two well-separated maxima with an energy gap of 400 eV around 120 K. Integration of the peak area measured at different temperatures gives the temperature dependence of the scattered-ion yield Rþ ðTÞ: In all cases, distinct anomalies in the phase-transition region were observed. The Rþ ðTÞ curves under 5.5 keV ion bombardments of the (1 0 0) surface are shown in Fig. 1. Under the He+
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
135
Fig. 1. Temperature dependence of the normalized scattered+ ions (,), Ne+ ions ion yield, Rþ ðTÞ=Rþ 85 K ; for 5.5 keV He (K) and Ar+ ions (&) scattered from MBE Fe3O4 (1 0 0)-film surface. Solid curves serve to guide the eyes.
Fig. 2. Temperature dependence of the scattered-ion yield, Rþ ðTÞ; for Ne+ ions (5.0 keV (&), 5.5 keV (K), 6.0 keV (J), 6.5 keV (n)) scattered from the MBE Fe3O4 (1 0 0)-film surface. Solid curves serve to guide the eyes.
bombardments, a very wide minimum around 125 K was observed. At the minimum, a relative decrease of 40% in magnitude of the scattered-ion þ yield, i.e. of the ratio Rþ 120 K =R85 K ; is found. Above 160 K, the scattered-ion yield is almost constant with temperature. In the case of Ne+ bombardment, two minima are observed, one located around 100 K and the other one around 125 K. The Rþ ðTÞ curve under Ar+ ion bombardment exhibits only one minimum around 100 K. Moreþ over, the ratio Rþ 120 K =R85 K ; is found to decrease with increasing the ion mass. These anomalies are attributed to the metal–insulator phase transition of this material. There is no visible change of the Rþ ðTÞ curves upon changing the primary energy of the He+ bombardment for both two surfaces. Namely, under different primary energy values one broad minimum around 120 K in the Rþ ðTÞ curve is always observed. A strong dependence of the Rþ ðTÞ curves on the primary energy, however, is observed under the Ne+- and Ar+-ion (i.e. heavier ions) bombardments. As an example, the results for the (1 0 0) surface obtained under Ne+ bombardment are shown in Fig. 2. On increasing the primary energy up to 6.0 keV two minima always exist. At the energy of 6.5 keV, the hightemperature minimum disappears and only the low-temperature one is observed. Moreover, the
þ ratio Rþ 120 K =R85 K decreases distinctly by a factor of two at the transition temperature. The Rþ ðTÞ curves for the (1 0 0) surface under the Ar+ ion bombardment and those for the (1 1 1) surface reveal a different character (see Fig. 5 Ref. [10]). In all cases, increasing the primary energy leads to a visible character change of the ion scattering variation with temperature.
3.2. Influence of the geometrical structure of semichannel on ion scattering from magnetite surfaces A visible difference of the Rþ ðTÞ curves for the two different surfaces has been found. In Fig. 3, we compare the results obtained under the Ne+ ion bombardment at the same energy of 5.5 keV of two different magnetite thin-film surfaces. For the (1 0 0) surface, the effect of the phase-transition on the ion scattering is shown by the two wide minima in the Rþ ðTÞ curve, whereas only one wide minimum around 125 K is observed for the (1 1 1) surface. A distinct increase in the scattered-ion yield was found at low temperatures. The minimum for both two thin-film surfaces was broader and with a smaller relative decrease of the scattered-ion yields. Moreover, above the phasetransition temperature, the Rþ ðTÞ curve does not reveal the small maximum around 135 K previously observed for the single crystalline surface
136
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
ion scattering from these two different surfaces can be summarized as follows:
Fig. 3. Temperature dependence of the normalized scattered+ ions scattered from ion yield, Rþ ðTÞ=Rþ 85 K ; for 5.5 keV Ne MBE Fe3O4 (1 0 0)- (K) and (1 1 1)-film surface (&). The dashed line shows the results from the single crystalline (2 1 3) surface.
Fig. 4. Temperature dependence of the normalized scattered+ ions scattered from ion yield, Rþ ðTÞ=Rþ 85 K ; for 6.5 keV Ar MBE Fe3O4 (1 0 0)- (J) and (1 1 1)-film surface (.).
of magnetite [9] shown by the dashed line in Fig. 3. The results obtained under the 6.5 keV Ar+ ion bombardment are shown in Fig. 4. In both cases, the Rþ ðTÞ curves exhibit only one minimum and an increase with decreasing temperature. A much smaller relative decrease in the scattered-ion yield at the minimum was found for the (1 0 0) surface, whereas a change of 40% in magnitude was estimated for the (1 1 1) one. The difference in
1. In both cases the scattered-ion yield (under Ne+- and Ar+-ion bombardment) was found to depend on the primary energy values. For the (1 0 0) surface, a change in its temperaturedependence variation for both Ne+-ion and Ar+-ion bombardment was observed at the energy value of 6.5 keV. For the (1 1 1) surface, a change of the Rþ ðTÞ curve at the same energy value of 6.5 keV was found for Ne+-ion bombardment. Under Ar+ ion bombardments, however, the main trend of the ion scattering variation with temperature changes at a higher energy value of 7.5 keV. 2. The scattered-ion yields for Ne+-ion bombardment of the (1 0 0) surface exhibit different behavior: up to 6.0 eV, two minima exist and for 6.5 eV the high-temperature minimum in the Rþ ðTÞ curve disappears. In all other cases (i.e. Ne+-ion bombardment of the (1 1 1) surface and Ar+-ion bombardment of both two surfaces), only one minimum is observed, which is replaced by a fast increase of the scattered-ion yield with decreasing temperature. 3. A sharp minimum at the transition region is observed under Ar+-ion bombardment only for the (1 1 1) surface, which is similar to what has been found for single-crystalline (2 1 3) magnetite [9]. The results indicate a strong influence of the surface geometrical structure on the Rþ ðTÞ curves. For magnetite along the [1 0 0] direction, there are two different (1 0 0) planes in the bulk; one built up by tetrahedrally coordinated iron atoms (A-layer) and the other one by the octahedrally coordinated iron and oxygen atoms (B-layer), as shown in Fig. 5a. Along the [1 1 1] direction, Fe ions form an alternating sequence of two distinct layers. The first one contains only the Fe ions at octahedral sites, while the second one has both octahedral and tetrahedral sites. These two layers are separated by ( as oxygen layers with a layer spacing of 2.5 A, shown in Fig. 5b. Thus, the Fe-ion arrangements lead to ‘square’- and ‘triangle-shaped’ semi-channels, respectively, on the (1 0 0) and (1 1 1) surfaces,
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
137
Fig. 5. The stacking sequence of the different (1 0 0) layers (a) and (1 1 1) layers (b) in magnetite. Oxygen anions are shown as large open spheres, Fe atoms in octahedral sites as small shaded spheres and those in tetrahedral sites as small solid spheres. The crystallographic directions of magnetite are shown. The Fe-ion arrangements lead to a square-shaped semi-channel on the (1 0 0) surface, whereas a triangular-shaped semi-channel is found on the (1 1 1) surface illustrated in the right-hand side of the figure. The ( characteristic surface parameters are the atomic distance a; the row distance l and the channel height h. h is 1.05 and 0.61 A, respectively, for the (1 0 0) and the (1 1 1) magnetite surface. The scattering geometry in a collision cascade inside the semi-channel and in a zigzag collision on the surface is also illustrated (see text).
as illustrated on the right-hand side of Fig. 5. A ( and a channel width (the channel height h of 1.05 A distance between rows of Fe ions at octahedral ( was estimated for the (1 0 0) surface. sites) of 4.20 A Along the (1 1 1) direction, a channel height of ( was found. The possible value for the 0.61 A channel width is equal to an oxygen layer spacing ( Thus the (1 1 1) semi-channel is narrower. of 2.50 A. 3.3. Neutralization and ionization processes on magnetite surfaces The fine structures of the Rþ ðTÞ curves, observed close to the Verwey point, indicate the
involvement of many different processes. The scattered-ion yield Rþ for scattering by element i can be expressed as [12] Rþ i ¼ I p ni
dsi þ P c; dO i
where Ip is the primary ion-beam current, ni the number of surface atoms of element i per surface area, ds=dO the differential cross section, Pþ i the ion fraction of element i and c an instrumental factor. In magnetite, there is a large number of free electrons and the contribution from basic processes such as the resonant and Auger
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
138
neutralization can be dominant. The ion fraction for the outward path of the trajectory is exponentially proportional to the reciprocal of the perpendicular component of the velocity, V> [13]. Such a process plays an important role in the small-angle geometry, in which V> is very small. An increase of the primary energy implies an increase of the ion velocity and thus of Pþ : For low-energy ions, especially in the smallangle geometry, a big influence on the final ionization degree Pþ for a particle escaping the surface from the zigzag collisions has been discussed previously [9]. A schematic illustration of the double zigzag collisions between an incoming ion and two surface Fe ions on the surface semi-channels of magnetite is shown in Fig. 5. We have taken into account only the double zigzag collisions involving the final ionizing collision, i.e. the trajectories of the scattered particles leaving the surface in the re-ionized state when the inelastic energy loss De is comparable to the ionization energy Ii of the incoming ions. Inelastic energy loss to the target atoms or via the excitation of the electron in either the target atoms or the projectiles itself during two-body collision sequences can be estimated by means of the
Kishinevsky–Parilis formula [14]: bv V ðr0 Þ 1 0:68 ; DeðE0 ; pÞ ¼ Er ð1 þ r=aÞ3
ð2Þ
where v; Er are the velocity and energy of relative atomic motion, V ðr0 Þ is the so-called universal potential. The parameters p; b; a and r were calculated accordingly. In a detailed analysis, we have taken into account only the part kDe; where k ¼ 0:43 for the Ne+ and Ar+ ion. From the obtained elastic-energy-loss ratio E1 =E0 (E0 is the primary energy), the possible range for the scattering angles yi was evaluated. The distance of closest approach r0 ; and the related inelastic energy loss kDei ; were estimated for the symmetric double scattering (y1 ¼ y2 ) for different primary energies on different magnetite surfaces and were listed in Table 1. The results indicate that: 1. Unlike the case of Ar+-ion bombardments, no symmetric double scattering was found for both two surfaces for bombardments with Ne+ ion with energies below 6.0 keV. 2. The double scattering angles under the Ne+-ion bombardment are larger than in the case of Ar+-ion bombardment.
Table 1 The double zigzag collisions on the MBE magnetite thin films between incoming ion with two surface Fe ions involving the final ionizing collision. The scattering angles yi are estimated from the elastic-energy-loss ratio E1 =E0 (E0 is the primary energy). The distance of closest approach r0, and the inelastic-energy-loss kDei; were estimated for the symmetric double scattering y1 ¼ y2 E0 (keV) Ne+/(1 0 0) 5.0 5.5 6.0 6.5 Ne+/(1 1 1) 5.0 5.5 6.0 6.5 Ar+/(1 0 0) 5.5 6.5 Ar+/(1 1 1) 5.5 6.5
( r0 (A)
E1 =E0
y1 ¼ y2
0.878 0.872 0.847 0.832
— — 28.01 29.51
0.865 0.861 0.858 0.852
— — 26.51 27.01
0.338 0.336
21.01 21.01 19.01 19.01
0.850 0.850
kDe2 (eV)
yi
22.11 23.85
y2 X30:0o ; y1 p17:5o y2 X28:0o ; y1 p22:5o y2 X27:0o ; y1 p28:5o y2 X26:0o ; y1 p32:0o
23.81 25.29
21.68 22.93
y2 X31:5o ; y1 p18:0o y2 X28:5o ; y1 p23:5o y2 X26:0o ; y1 p27:5o y2 X25:0o ; y1 p29:5o
0.338 0.338
19.99 21.15
16.32 18.46
y2 X20:0o ; y1 p22:5o y2 X16:5o ; y1 p25:0o
0.351 0.351
17.85 19.90
15.97 17.87
y2 X18:5o ; y1 p20:0o y2 X15:0o ; y1 p22:5o
0.332 0.324
kDe1 (eV)
23.99 26.74
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
3. Increasing the primary energy implied a wider choice of the scattering angles, i.e. a wider choice of ion trajectories fulfilling the reionization criteria indicating an increase in probability of particles leaving the surface in an ionized state at small out-going angle. Generally, the scattering angle ranges for the (1 0 0) surface are wider than those for the (1 1 1) one. Moreover, ion scattering from the (1 0 0) surface shows a more complicated behavior at the phase-transition region. This indicates that the ionizing mechanism for ions interacting with deeplying close-packed atomic rows can become dominant for the square-shaped semi-channel. For a sufficiently large surface semi-channel, the incident ions can move closely to the bottom atomic rows realizing a large number of weak and very weak collisions due to focusing effects [14]. A continuous summation of inelastic-energy losses (De) can occur in a collision cascade containing a suitable number of collisions for the projectiles with the velocity uXo0 d (o0 the relaxation frequency for a chosen target material, d the atomic distance). Thus, the scattered particles will leave the surface in the re-ionized state if De is comparable to Ii : The observed minima in the Rþ ðTÞ curves at the phase-transition temperature are considered as a result of such a ‘‘competition’’ between the two mechanisms. On the one hand, below the Verwey temperature the electron hopping is frozen out and the degree of electron localization increases, yielding a decrease of the neutralization and thus an increase of the scattered-ion yield. On the other hand, the change in the ionization degree leads to a decrease of the scattered-ion yield. A small change of the atomic position of the target atoms (due to the crystal distortion at the Verwey temperature) could affect strongly the narrow choice of the ionizing trajectories governed by the zigzag collisions. Consequently, this leads to a strong suppression of the scattered-ion yield. Another effect of the Verwey transition is on the re-ionizing process of the ions interacting with the deep-lying atomic rows in the semi-channel. Namely, o0 and d change at the phase-transition temperature due to a crystal distortion and a change in the electron
139
localization degree. Thus, a destruction of the semi-channel may occur, leading to a worse focusing condition and a decrease in the number of ionizing collisions leading to a decrease of the scattered-ion yield. The (1 0 0) surface with a wide square-shaped semi-channel provides a much higher probability of the interaction of the incoming ions with the atoms lying at the bottom of the surface channel. Since the neutralization/ionization rates can be higher, a larger influence of ionizing mechanism to the ion-scattering yield can be found for the (1 0 0) surface. Moreover, the Ne+ ion has a smaller size than Ar+ ion and consequently has a larger probability of going deeper into the surface channel. Indeed, for the (1 0 0) surface two minima exist under Ne+-ion bombardment. For the (1 1 1) surface under Ar+-ion bombardment with E0 p6:5 keV, the minimum is very narrow indicating a much smaller contribution from such an ionizing mechanism to the ion-scattering yield for the case of heavier-ion bombardment along a triangle-shaped semi-channel with a narrower width. For both surfaces, an increase of the primary energies causes the disappearance of the minimum and a fast increase of Rþ ðTÞ below the Verwey point. We assume that in the case of small-angle geometry, the contribution from the Auger neutralization mechanism becomes dominant at high energies.
4. Conclusions Anomalies in the temperature dependence of the ion-scattering yield of MBE magnetite thin-film surfaces are attributed to the metal–insulator phase transition of this material. The fine structure of the Rþ ðTÞ curve near the Verwey point indicates the involvement of different processes: the Auger neutralization, the re-ionization related to the zigzag scattering trajectories containing the final ionizing collision and the re-ionization in a collision cascade inside the surface semi-channel. Increase of the primary energies leads to a dominant contribution from the Auger neutralization. Different characteristic Rþ ðTÞ curves were
140
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
observed indicating an influence of the surface geometrical structure on the re-ionization in a collision cascade.
Acknowledgements One of the authors (N.-T.H. Kim-Ngan) would like to thank Prof. J.J.M. Franse (Van der WaalsZeeman Instituut, Universiteit van Amsterdam) for his great guidance during working in his group and for the motivation to go on with the phase transitions subject.
References [1] N. Tsuda, K. Nasu, Y. Yanase, K. Siratori, Electronic conductions in oxides, in: M. Cardona, P. Fulde, K. von Klitzing, H.-J. Queisser (Eds.), Solid-State Sciences, Vol. 94, Springer, Berlin, 1990. [2] N.G. Condon, F.M. Leibsle, T. Parker, A.R. Lennie, D.J. Vaughan, G. Thornton, Phys. Rev. B 55 (1997) 15885.
[3] J.M. Gaines, et al., Surf. Sci. 373 (1997) 85 (and references therein). [4] F.C. Voogt, Ph.D. Thesis, Groningen, Holland, 1998. [5] C. Seoighe, J. Naumann, I.V. Shvets, Surf. Sci. 440 (1999) 116. [6] A.V. Mijiriskii, M.H. Langelaar, D.O. Boerma, J. Magn. Magn. Mater. 211 (2000) 278. [7] S. Speller, M. Schleberger, H. Franke, C. Muller, W. Heiland, Mod. Phys. Lett. B 8 (1994) 491. [8] N.-T.H. Kim-Ngan, W. Soszka, J. Magn. Magn. Mater. 202 (1999) 327. [9] W. Soszka, N.-T.H. Kim-Ngan, Surf. Sci. 441 (1999) 311. [10] N-T.H. Kim-Ngan, W. Soszka, D. Sitko, G. Jag"o, J. Korecki, B. Handke, Nucl. Instrum. Methods B 164–165 (2000) 992. [11] W. Soszka, N-T.H. Kim-Ngan, D. Sitko, G. Jag"o, J. Korecki, B. Handke, Vacuum 63 (2001) 349. [12] G.C. van Leerdam, K.-M.H. Lenssen, H.H. Brongersma, Nucl. Instrum. Methods B 45 (1990) 390. [13] D.P. Woodruff, Nucl. Instrum. Methods 194 (1982) 639. [14] E.S. Parilis, L.M. Kishinevsky, N.Yu. Turaev, B.E. Baklitzky, F.F. Umarov, V.Kh. Verleger, S.L. Nizhnaya, I.S. Bitensky, Atomic Collisions on Solid Surface, North-Holland, Amsterdam, London, New York, Tokyo, 1993.
The surface metal–insulator transition in MBE magnetite thin films N.-T.H. Kim-Ngan*, W. Soszka ! Poland Institute of Physics, Pedagogical University, Podchorazych 2, PL 30-084 Krakow, Received 29 January 2002; received in revised form 26 February 2002
Abstract The (1 0 0) and (1 1 1) MBE thin-film surfaces of magnetite have been investigated by low-energy ion scattering (LEIS) in the small-angle geometry and in the temperature range of 85–300 K. Distinct anomalies in the temperature dependence of scattering ion yield Rþ ðTÞ in the temperature range 100–125 K are attributed to the metal–insulator phase transition of this material. A strong dependence of the Rþ ðTÞ curve on the primary energy of incoming ions and on the geometrical structure of the surface semi-channels was observed. Three processes are taken into account: the Auger neutralization of ions related to the electron localization at the Verwey transition, the re-ionization of bombarding particles in the zigzag collisions from the topmost atomic rows, and the re-ionization in a collision cascade with the atomic rows lying inside the semi-channel. The latter one plays a more important role in the case of the (1 0 0) surface with a larger channel width. The change of the scattered-ion yield with increasing primary energy for both two surfaces is considered as indication of a dominant contribution from the Auger neutralization. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.35.p; 61.14.Hg; 61.18.Bn; 71.30.+h Keywords: Magnetite; MBE thin films; Metal–insulator phase transition (MIT); LEIS
1. Introduction Magnetite (Fe3O4) is a ferrimagnet with Nel temperature TN ¼ 860 K and has played an important role in the discovery of magnetism. Its high electrical conductivity at room temperature is attributed to electron hopping between Fe2+ and Fe3+ ions at the octahedral sites (B-sites) of the inverse spinel structure. Upon cooling, magnetite *Corresponding author. Tel.: +48-12-637-8286; fax: +4812-637-2243. E-mail address: [email protected] (N.-T.H. Kim-Ngan).
undergoes a metal–insulator phase transition (i.e. the so-called Verwey transition), at a temperature in the range of 115–125 K, where the electron hopping is frozen and the crystal becomes insulating [1]. Magnetite has been an attractive candidate for technological applications in many important fields of electronics and catalysis. Recently, increasing attention has been focused on the investigations of the surface structure and electromagnetic behavior of Fe3O4 thin films and Fe3O4based multilayer structures [2–6]. Solid-surface analysis by ion-scattering spectroscopy (ISS) or the low-energy ion scattering (LEIS) technique has been carried out with
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 1 1 6 - X
134
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
rare-gas ions having a primary energy in the range of 1–20 keV. The surface sensitivity of this technique is due to the fact that the majority of the reflected ions are scattered from the outermost surface layer of solids [7]. Recently, we have used this method to investigate the Verwey transition of several magnetite surfaces. A very deep minimum around 120 K and a small maximum around 135 K in the temperature-dependent curve of scatteredion yield Rþ ðTÞ have been observed for a single-crystalline magnetite surface [8,9]. The characterization of the Rþ ðTÞ curves around the Verwey temperature was found to be more complicated for the magnetite thin-film surfaces [10,11]. In this paper, we report our detailed investigations of the Verwey transition of MBE (1 0 0) and (1 1 1) magnetite thin-film surfaces. We focus on the influence of the geometrical structure of the surface semi-channels on the magnetite surfaces on the conditions for realizing ionizing trajectories during isolated collisions especially around the Verwey transition.
ing effects. Irrespective of the primary energy, such a small-angle geometry always results in multiple scattering due to the atomic screening on the incoming and outgoing paths. Moreover, this small-angle geometry is especially needed for ionscattering experiments on a thin-film surface. Due to the large footprint at small angles, there will be less damage caused by the ion beam to the film surface. He+, Ne+ and Ar+ ion beams at primary energies between 5.0 and 7.5 keV and with an energy step of 0.5 keV were used. A scattering peak at grazing angles was optimized for the temperature-dependent investigations due to the fact that the bombarding-time effect to the thin-film surface was expected to be the weakest for this position. The reproducibility of the phase-transition effect and the insignificant effect of bombarding time on the film structure in our experiments has been discussed earlier [10].
3. Results and discussion
2. Experimental details
3.1. The Verwey transition of the MBE thin-film magnetite surfaces
( The MBE 200-A-thick Fe3O4 thin films with a well-ordered (1 0 0)- and (1 1 1)-surface were the subject of our studies. The film structure was monitored by a standard four-grid LEED-AES spectrometer and its stoichiometry was checked by . in situ conversion electron Mossbauer spectroscopy (CEMS) spectra. Details of the thin-film growth were published elsewhere [10,11]. The temperature dependence of the CEMS spectra reflects a good stoichiometry of the thin film. Namely, the spectra at room temperature could be fitted with two magnetic components of ‘‘2.5+’’ and ‘‘3+’’ with the intensity ratio of nearly 1:2, and a drastic change of the spectra related to the Verwey transition occurs at temperatures below 130 K. The experiments were carried out in a LEIS system in the temperature range 85–300 K described elsewhere [9]. A small-angle geometry was used (the incident angle C relative to the plane and the detection angle Y are in the order of 0–101) favorable for the observation of multiple-scatter-
The energy spectra of ions scattered from the two Fe3O4 thin-film surfaces exhibited very broad peaks corresponding to the ion/Fe multiple scattering. The dependence of the energy spectra on the incoming ions and the primary energies have been discussed elsewhere [10,11]. No change in the position of the scattering peak with temperature was observed in all cases. Changing the target temperature caused only a change in the intensity of the scattering peak. A visible splitting of the scattering peak in the phase transition region was observed only under 6.5 keV He+ ion bombardment of the (1 0 0) surface: the scattering peak exhibited two well-separated maxima with an energy gap of 400 eV around 120 K. Integration of the peak area measured at different temperatures gives the temperature dependence of the scattered-ion yield Rþ ðTÞ: In all cases, distinct anomalies in the phase-transition region were observed. The Rþ ðTÞ curves under 5.5 keV ion bombardments of the (1 0 0) surface are shown in Fig. 1. Under the He+
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
135
Fig. 1. Temperature dependence of the normalized scattered+ ions (,), Ne+ ions ion yield, Rþ ðTÞ=Rþ 85 K ; for 5.5 keV He (K) and Ar+ ions (&) scattered from MBE Fe3O4 (1 0 0)-film surface. Solid curves serve to guide the eyes.
Fig. 2. Temperature dependence of the scattered-ion yield, Rþ ðTÞ; for Ne+ ions (5.0 keV (&), 5.5 keV (K), 6.0 keV (J), 6.5 keV (n)) scattered from the MBE Fe3O4 (1 0 0)-film surface. Solid curves serve to guide the eyes.
bombardments, a very wide minimum around 125 K was observed. At the minimum, a relative decrease of 40% in magnitude of the scattered-ion þ yield, i.e. of the ratio Rþ 120 K =R85 K ; is found. Above 160 K, the scattered-ion yield is almost constant with temperature. In the case of Ne+ bombardment, two minima are observed, one located around 100 K and the other one around 125 K. The Rþ ðTÞ curve under Ar+ ion bombardment exhibits only one minimum around 100 K. Moreþ over, the ratio Rþ 120 K =R85 K ; is found to decrease with increasing the ion mass. These anomalies are attributed to the metal–insulator phase transition of this material. There is no visible change of the Rþ ðTÞ curves upon changing the primary energy of the He+ bombardment for both two surfaces. Namely, under different primary energy values one broad minimum around 120 K in the Rþ ðTÞ curve is always observed. A strong dependence of the Rþ ðTÞ curves on the primary energy, however, is observed under the Ne+- and Ar+-ion (i.e. heavier ions) bombardments. As an example, the results for the (1 0 0) surface obtained under Ne+ bombardment are shown in Fig. 2. On increasing the primary energy up to 6.0 keV two minima always exist. At the energy of 6.5 keV, the hightemperature minimum disappears and only the low-temperature one is observed. Moreover, the
þ ratio Rþ 120 K =R85 K decreases distinctly by a factor of two at the transition temperature. The Rþ ðTÞ curves for the (1 0 0) surface under the Ar+ ion bombardment and those for the (1 1 1) surface reveal a different character (see Fig. 5 Ref. [10]). In all cases, increasing the primary energy leads to a visible character change of the ion scattering variation with temperature.
3.2. Influence of the geometrical structure of semichannel on ion scattering from magnetite surfaces A visible difference of the Rþ ðTÞ curves for the two different surfaces has been found. In Fig. 3, we compare the results obtained under the Ne+ ion bombardment at the same energy of 5.5 keV of two different magnetite thin-film surfaces. For the (1 0 0) surface, the effect of the phase-transition on the ion scattering is shown by the two wide minima in the Rþ ðTÞ curve, whereas only one wide minimum around 125 K is observed for the (1 1 1) surface. A distinct increase in the scattered-ion yield was found at low temperatures. The minimum for both two thin-film surfaces was broader and with a smaller relative decrease of the scattered-ion yields. Moreover, above the phasetransition temperature, the Rþ ðTÞ curve does not reveal the small maximum around 135 K previously observed for the single crystalline surface
136
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
ion scattering from these two different surfaces can be summarized as follows:
Fig. 3. Temperature dependence of the normalized scattered+ ions scattered from ion yield, Rþ ðTÞ=Rþ 85 K ; for 5.5 keV Ne MBE Fe3O4 (1 0 0)- (K) and (1 1 1)-film surface (&). The dashed line shows the results from the single crystalline (2 1 3) surface.
Fig. 4. Temperature dependence of the normalized scattered+ ions scattered from ion yield, Rþ ðTÞ=Rþ 85 K ; for 6.5 keV Ar MBE Fe3O4 (1 0 0)- (J) and (1 1 1)-film surface (.).
of magnetite [9] shown by the dashed line in Fig. 3. The results obtained under the 6.5 keV Ar+ ion bombardment are shown in Fig. 4. In both cases, the Rþ ðTÞ curves exhibit only one minimum and an increase with decreasing temperature. A much smaller relative decrease in the scattered-ion yield at the minimum was found for the (1 0 0) surface, whereas a change of 40% in magnitude was estimated for the (1 1 1) one. The difference in
1. In both cases the scattered-ion yield (under Ne+- and Ar+-ion bombardment) was found to depend on the primary energy values. For the (1 0 0) surface, a change in its temperaturedependence variation for both Ne+-ion and Ar+-ion bombardment was observed at the energy value of 6.5 keV. For the (1 1 1) surface, a change of the Rþ ðTÞ curve at the same energy value of 6.5 keV was found for Ne+-ion bombardment. Under Ar+ ion bombardments, however, the main trend of the ion scattering variation with temperature changes at a higher energy value of 7.5 keV. 2. The scattered-ion yields for Ne+-ion bombardment of the (1 0 0) surface exhibit different behavior: up to 6.0 eV, two minima exist and for 6.5 eV the high-temperature minimum in the Rþ ðTÞ curve disappears. In all other cases (i.e. Ne+-ion bombardment of the (1 1 1) surface and Ar+-ion bombardment of both two surfaces), only one minimum is observed, which is replaced by a fast increase of the scattered-ion yield with decreasing temperature. 3. A sharp minimum at the transition region is observed under Ar+-ion bombardment only for the (1 1 1) surface, which is similar to what has been found for single-crystalline (2 1 3) magnetite [9]. The results indicate a strong influence of the surface geometrical structure on the Rþ ðTÞ curves. For magnetite along the [1 0 0] direction, there are two different (1 0 0) planes in the bulk; one built up by tetrahedrally coordinated iron atoms (A-layer) and the other one by the octahedrally coordinated iron and oxygen atoms (B-layer), as shown in Fig. 5a. Along the [1 1 1] direction, Fe ions form an alternating sequence of two distinct layers. The first one contains only the Fe ions at octahedral sites, while the second one has both octahedral and tetrahedral sites. These two layers are separated by ( as oxygen layers with a layer spacing of 2.5 A, shown in Fig. 5b. Thus, the Fe-ion arrangements lead to ‘square’- and ‘triangle-shaped’ semi-channels, respectively, on the (1 0 0) and (1 1 1) surfaces,
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
137
Fig. 5. The stacking sequence of the different (1 0 0) layers (a) and (1 1 1) layers (b) in magnetite. Oxygen anions are shown as large open spheres, Fe atoms in octahedral sites as small shaded spheres and those in tetrahedral sites as small solid spheres. The crystallographic directions of magnetite are shown. The Fe-ion arrangements lead to a square-shaped semi-channel on the (1 0 0) surface, whereas a triangular-shaped semi-channel is found on the (1 1 1) surface illustrated in the right-hand side of the figure. The ( characteristic surface parameters are the atomic distance a; the row distance l and the channel height h. h is 1.05 and 0.61 A, respectively, for the (1 0 0) and the (1 1 1) magnetite surface. The scattering geometry in a collision cascade inside the semi-channel and in a zigzag collision on the surface is also illustrated (see text).
as illustrated on the right-hand side of Fig. 5. A ( and a channel width (the channel height h of 1.05 A distance between rows of Fe ions at octahedral ( was estimated for the (1 0 0) surface. sites) of 4.20 A Along the (1 1 1) direction, a channel height of ( was found. The possible value for the 0.61 A channel width is equal to an oxygen layer spacing ( Thus the (1 1 1) semi-channel is narrower. of 2.50 A. 3.3. Neutralization and ionization processes on magnetite surfaces The fine structures of the Rþ ðTÞ curves, observed close to the Verwey point, indicate the
involvement of many different processes. The scattered-ion yield Rþ for scattering by element i can be expressed as [12] Rþ i ¼ I p ni
dsi þ P c; dO i
where Ip is the primary ion-beam current, ni the number of surface atoms of element i per surface area, ds=dO the differential cross section, Pþ i the ion fraction of element i and c an instrumental factor. In magnetite, there is a large number of free electrons and the contribution from basic processes such as the resonant and Auger
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
138
neutralization can be dominant. The ion fraction for the outward path of the trajectory is exponentially proportional to the reciprocal of the perpendicular component of the velocity, V> [13]. Such a process plays an important role in the small-angle geometry, in which V> is very small. An increase of the primary energy implies an increase of the ion velocity and thus of Pþ : For low-energy ions, especially in the smallangle geometry, a big influence on the final ionization degree Pþ for a particle escaping the surface from the zigzag collisions has been discussed previously [9]. A schematic illustration of the double zigzag collisions between an incoming ion and two surface Fe ions on the surface semi-channels of magnetite is shown in Fig. 5. We have taken into account only the double zigzag collisions involving the final ionizing collision, i.e. the trajectories of the scattered particles leaving the surface in the re-ionized state when the inelastic energy loss De is comparable to the ionization energy Ii of the incoming ions. Inelastic energy loss to the target atoms or via the excitation of the electron in either the target atoms or the projectiles itself during two-body collision sequences can be estimated by means of the
Kishinevsky–Parilis formula [14]: bv V ðr0 Þ 1 0:68 ; DeðE0 ; pÞ ¼ Er ð1 þ r=aÞ3
ð2Þ
where v; Er are the velocity and energy of relative atomic motion, V ðr0 Þ is the so-called universal potential. The parameters p; b; a and r were calculated accordingly. In a detailed analysis, we have taken into account only the part kDe; where k ¼ 0:43 for the Ne+ and Ar+ ion. From the obtained elastic-energy-loss ratio E1 =E0 (E0 is the primary energy), the possible range for the scattering angles yi was evaluated. The distance of closest approach r0 ; and the related inelastic energy loss kDei ; were estimated for the symmetric double scattering (y1 ¼ y2 ) for different primary energies on different magnetite surfaces and were listed in Table 1. The results indicate that: 1. Unlike the case of Ar+-ion bombardments, no symmetric double scattering was found for both two surfaces for bombardments with Ne+ ion with energies below 6.0 keV. 2. The double scattering angles under the Ne+-ion bombardment are larger than in the case of Ar+-ion bombardment.
Table 1 The double zigzag collisions on the MBE magnetite thin films between incoming ion with two surface Fe ions involving the final ionizing collision. The scattering angles yi are estimated from the elastic-energy-loss ratio E1 =E0 (E0 is the primary energy). The distance of closest approach r0, and the inelastic-energy-loss kDei; were estimated for the symmetric double scattering y1 ¼ y2 E0 (keV) Ne+/(1 0 0) 5.0 5.5 6.0 6.5 Ne+/(1 1 1) 5.0 5.5 6.0 6.5 Ar+/(1 0 0) 5.5 6.5 Ar+/(1 1 1) 5.5 6.5
( r0 (A)
E1 =E0
y1 ¼ y2
0.878 0.872 0.847 0.832
— — 28.01 29.51
0.865 0.861 0.858 0.852
— — 26.51 27.01
0.338 0.336
21.01 21.01 19.01 19.01
0.850 0.850
kDe2 (eV)
yi
22.11 23.85
y2 X30:0o ; y1 p17:5o y2 X28:0o ; y1 p22:5o y2 X27:0o ; y1 p28:5o y2 X26:0o ; y1 p32:0o
23.81 25.29
21.68 22.93
y2 X31:5o ; y1 p18:0o y2 X28:5o ; y1 p23:5o y2 X26:0o ; y1 p27:5o y2 X25:0o ; y1 p29:5o
0.338 0.338
19.99 21.15
16.32 18.46
y2 X20:0o ; y1 p22:5o y2 X16:5o ; y1 p25:0o
0.351 0.351
17.85 19.90
15.97 17.87
y2 X18:5o ; y1 p20:0o y2 X15:0o ; y1 p22:5o
0.332 0.324
kDe1 (eV)
23.99 26.74
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
3. Increasing the primary energy implied a wider choice of the scattering angles, i.e. a wider choice of ion trajectories fulfilling the reionization criteria indicating an increase in probability of particles leaving the surface in an ionized state at small out-going angle. Generally, the scattering angle ranges for the (1 0 0) surface are wider than those for the (1 1 1) one. Moreover, ion scattering from the (1 0 0) surface shows a more complicated behavior at the phase-transition region. This indicates that the ionizing mechanism for ions interacting with deeplying close-packed atomic rows can become dominant for the square-shaped semi-channel. For a sufficiently large surface semi-channel, the incident ions can move closely to the bottom atomic rows realizing a large number of weak and very weak collisions due to focusing effects [14]. A continuous summation of inelastic-energy losses (De) can occur in a collision cascade containing a suitable number of collisions for the projectiles with the velocity uXo0 d (o0 the relaxation frequency for a chosen target material, d the atomic distance). Thus, the scattered particles will leave the surface in the re-ionized state if De is comparable to Ii : The observed minima in the Rþ ðTÞ curves at the phase-transition temperature are considered as a result of such a ‘‘competition’’ between the two mechanisms. On the one hand, below the Verwey temperature the electron hopping is frozen out and the degree of electron localization increases, yielding a decrease of the neutralization and thus an increase of the scattered-ion yield. On the other hand, the change in the ionization degree leads to a decrease of the scattered-ion yield. A small change of the atomic position of the target atoms (due to the crystal distortion at the Verwey temperature) could affect strongly the narrow choice of the ionizing trajectories governed by the zigzag collisions. Consequently, this leads to a strong suppression of the scattered-ion yield. Another effect of the Verwey transition is on the re-ionizing process of the ions interacting with the deep-lying atomic rows in the semi-channel. Namely, o0 and d change at the phase-transition temperature due to a crystal distortion and a change in the electron
139
localization degree. Thus, a destruction of the semi-channel may occur, leading to a worse focusing condition and a decrease in the number of ionizing collisions leading to a decrease of the scattered-ion yield. The (1 0 0) surface with a wide square-shaped semi-channel provides a much higher probability of the interaction of the incoming ions with the atoms lying at the bottom of the surface channel. Since the neutralization/ionization rates can be higher, a larger influence of ionizing mechanism to the ion-scattering yield can be found for the (1 0 0) surface. Moreover, the Ne+ ion has a smaller size than Ar+ ion and consequently has a larger probability of going deeper into the surface channel. Indeed, for the (1 0 0) surface two minima exist under Ne+-ion bombardment. For the (1 1 1) surface under Ar+-ion bombardment with E0 p6:5 keV, the minimum is very narrow indicating a much smaller contribution from such an ionizing mechanism to the ion-scattering yield for the case of heavier-ion bombardment along a triangle-shaped semi-channel with a narrower width. For both surfaces, an increase of the primary energies causes the disappearance of the minimum and a fast increase of Rþ ðTÞ below the Verwey point. We assume that in the case of small-angle geometry, the contribution from the Auger neutralization mechanism becomes dominant at high energies.
4. Conclusions Anomalies in the temperature dependence of the ion-scattering yield of MBE magnetite thin-film surfaces are attributed to the metal–insulator phase transition of this material. The fine structure of the Rþ ðTÞ curve near the Verwey point indicates the involvement of different processes: the Auger neutralization, the re-ionization related to the zigzag scattering trajectories containing the final ionizing collision and the re-ionization in a collision cascade inside the surface semi-channel. Increase of the primary energies leads to a dominant contribution from the Auger neutralization. Different characteristic Rþ ðTÞ curves were
140
N.-T.H. Kim-Ngan, W. Soszka / Physica B 319 (2002) 133–140
observed indicating an influence of the surface geometrical structure on the re-ionization in a collision cascade.
Acknowledgements One of the authors (N.-T.H. Kim-Ngan) would like to thank Prof. J.J.M. Franse (Van der WaalsZeeman Instituut, Universiteit van Amsterdam) for his great guidance during working in his group and for the motivation to go on with the phase transitions subject.
References [1] N. Tsuda, K. Nasu, Y. Yanase, K. Siratori, Electronic conductions in oxides, in: M. Cardona, P. Fulde, K. von Klitzing, H.-J. Queisser (Eds.), Solid-State Sciences, Vol. 94, Springer, Berlin, 1990. [2] N.G. Condon, F.M. Leibsle, T. Parker, A.R. Lennie, D.J. Vaughan, G. Thornton, Phys. Rev. B 55 (1997) 15885.
[3] J.M. Gaines, et al., Surf. Sci. 373 (1997) 85 (and references therein). [4] F.C. Voogt, Ph.D. Thesis, Groningen, Holland, 1998. [5] C. Seoighe, J. Naumann, I.V. Shvets, Surf. Sci. 440 (1999) 116. [6] A.V. Mijiriskii, M.H. Langelaar, D.O. Boerma, J. Magn. Magn. Mater. 211 (2000) 278. [7] S. Speller, M. Schleberger, H. Franke, C. Muller, W. Heiland, Mod. Phys. Lett. B 8 (1994) 491. [8] N.-T.H. Kim-Ngan, W. Soszka, J. Magn. Magn. Mater. 202 (1999) 327. [9] W. Soszka, N.-T.H. Kim-Ngan, Surf. Sci. 441 (1999) 311. [10] N-T.H. Kim-Ngan, W. Soszka, D. Sitko, G. Jag"o, J. Korecki, B. Handke, Nucl. Instrum. Methods B 164–165 (2000) 992. [11] W. Soszka, N-T.H. Kim-Ngan, D. Sitko, G. Jag"o, J. Korecki, B. Handke, Vacuum 63 (2001) 349. [12] G.C. van Leerdam, K.-M.H. Lenssen, H.H. Brongersma, Nucl. Instrum. Methods B 45 (1990) 390. [13] D.P. Woodruff, Nucl. Instrum. Methods 194 (1982) 639. [14] E.S. Parilis, L.M. Kishinevsky, N.Yu. Turaev, B.E. Baklitzky, F.F. Umarov, V.Kh. Verleger, S.L. Nizhnaya, I.S. Bitensky, Atomic Collisions on Solid Surface, North-Holland, Amsterdam, London, New York, Tokyo, 1993.