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Electron spin resonance studies on single crystalline Fe3O4 films
Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
Electron spin resonance studies on single crystalline Fe3O4 films c . Satilmis- Budaka, Fikret Yildizb,*, Mustafa Ozdemir , Bekir Aktas-b a Fatih University, Faculty of Art and Science, Physics Department, 34900 Istanbul, Turkey Department of Physics, Gebze Institute of Technology, 41410 Cayirova-Gebze, Kocaeli, Turkey c Faculty of Art and Science, Physics Department, Marmara University, Goztepe, Istanbul, Turkey . b
Abstract The effects of surface coating (Co and/or CoxOy) on magnetic anisotropies in single crystalline Fe3O4 films with different thickness that were epitaxially grown on MgO (1 0 0) substrate have been studied by electron spin resonance (ESR) technique. The angular dependent ESR spectra were analysed by using well-known SWR theory and the magnetic anisotropy parameters were deduced. The magnetic anisotropies are deeply affected from both film thickness ( thick films, and surface coating conditions. Magneto-crystalline cubic anisotropy (30 and 60 Oe for 110 and 2771 A respectively) is much smaller compared to its value for the bulk sample of the same compound (Hk ¼ 2K1 =M ¼ 460 Oe). The effective magnetization is influenced by the surface coating as well. The spectra for surface-coated films broaden especially for Co3O4-coated samples. r 2002 Elsevier Science B.V. All rights reserved. Keywords: ESR; Magnetic anisotropy; Magneto-crystalline energy
1. Introduction The members of Fe–O system are FeO, Fe2O3 and Fe3O4, which are the most important, and most abundant ferrimagnetic transition metal oxides. They are extensively used for magnetic storage information [1,2]. Furthermore, epitaxial Fe3O4 films grown on MgO substrates show magneto-resistance (MR) even at room temperature, whereas single crystals do not [3]. The substrate–sample structure match, the synthesis conditions, such as molecular beam flux, oxygen pressure and temperature can strongly influence the film structure and composition [4]. The magnetic properties depend on the structural order and the stoichiometry as well [5]. We have encountered only two FMR studies on FexOy in literature [6,7]. Therefore in this study, we have investigated magnetic properties of the Fe3O4 film prepared in different thickness and some of them were *Corresponding author. Tel.: +90-262-653-8497; fax: +90262-653-8490. E-mail address: [email protected] (F. Yildiz).
coated by Co and/or CoxOy to manipulate the magnetic surface anisotropy and obtain different surface spin pinning parameters.
2. Experimental results Fe3O4 thin films were prepared by deposition on MgO (1 0 0) substrates using reactive DC sputtering technique. Two sets of Fe3O4 films prepared in identical growth conditions have been used for electron spin resonance (ESR) measurements. The film thickness for the first and ( respectively. The the second sets are 110 and 2771 A, first set consists of non-coated film and two films coated ( Co and 18 A ( Co3O4 layers, respectively. The by 10 A second set includes non-coated film and two films coated ( CoO and 18.8 A ( Co3O4 layers. Fig. 1 shows by 17 A some examples for ESR spectra taken from Fe3O4 films ( in thickness for both perpendicular with 110 and 2771 A (where the magnetic field is perpendicular to the film plane, >) and parallel geometries (field lays in the sample plane, 8 geometries) as indicated on related
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 0 8 3 - 1
424
S. Budak et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
Fig. 1. Variation of SWR spectra with thickness and surface conditions of Fe3O4 films as indicated on relevant spectrum. Dotted line and straight line show theoretical and experimental results respectively. Parallel (8) and perpendicular (>) geometries are indicated.
spectra at room temperature. The very narrow multiple peaks, centred at the field corresponding g ¼ 2; originate from the Mn+2 impurities. Although all of the films have been epitaxially grown on single crystalline MgO substrate in similar conditions, the ESR spectra seem to be quite different from each other especially for the thinner set (Fig. 1a). The spectra are varied with both film thickness and outer surface coating conditions. The separation of absorption peaks in parallel and perpendicular geometries are remarkably increased for Co and Co3O4-coated films with respect to uncoated one. The lines for coated sample are broadened as well. However, the effects of the surface coating are much more dramatic on Co3O4-coated Fe3O4 film. The lines are so much broader that the amplitude of field derivative-absorption line is too small to observe for low modulation field of 1 Oe as used for non-coated ( samples. 110 A ( thick films are The SWR spectra taken from 2771 A shown in Fig. 1b. The spectra in perpendicular geometry are split into multiple peaks while single peak is observed for parallel case for both non-coated and coated films. However, as the field is rotated away from the film normal, the peaks in this multiplet come close to each other and overlap giving a relatively broader single absorption line. The distance between the main modes of two geometries almost remains constant regardless the surface coating of these thicker films. That is, the thinner films are much influenced from the surface coating. Fig. 2 shows resonance field values as a function of angle measured with respect to the film plane for all films. The curves for the resonance field values are
Fig. 2. Out of plane angular variation of resonance field of various Fe3O4 films with different coating conditions.
perfectly symmetrical with respect to the film normal. The bell shaped curves becomes much narrower for thicker films. The Co3O4 coating further enhances this effect for both thinner and thicker films. The angular variations of the resonance fields for all films are given in Fig. 3 for in plane geometry. The angle of the applied magnetic field is measured from /1 0 0S direction. Fe3O4 films on MgO (1 0 0) exhibits four-fold symmetry with a period of 901 in the film plane suggesting a cubic crystalline structure for the film. However, the maximum amplitude of the resonance field variation with the angle depends on both film thickness and surface coating condition.
S. Budak et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
425
Table 1 Magnetic parameters for various Fe3O4 films
o=g (G) Keff M0 (G) K1 =M0 (G) Ka =M0 (G) 2K1s L=ðDM0 Þ 2K2s L=DM0 D (109) (erg/cm)
1
2
3
4
5
6
3330 1286 22 6
3105 1628 10 4
3320 1900 63 Undet.
3335 2230 52 7 6.5 11.0 5.5
3360 2062 48 4 4.0 7.0 5.5
3305 2092 46 11 12.4 12.6 5.5
( (2) 10 A Co-coated Fe3O4(110 A) ( (3) 18.8 A ( (1) Fe3O4 (110 A), ( (4) Fe3O4 (2771 A), ( (5) 17 CoO–Co3O4-coated Fe3O4 (110 A), ( (6) 18.8 A ( CoO–Co3O4-coated CoO-coated Fe3O4 (2771 A) ( Fe3O4 (2771 A).
Fig. 3. In plane angular variation of resonance field in sample plane of various Fe3O4 films with different coating conditions.
3. Theoretical results and discussions The theoretical resonance fields in Figs. 2 and 3 are calculated by using the following expression for the magnetic energy density E: E ¼ MH þ K1 ða21 a22 þ a21 a23 þ a22 a23 Þ S 2 þ Ka Cos2 ðjÞ þ Keff a23 K1;2 a3 þ Eex ;
where the first term represents Zeeman energy density, the second term accounts for cubic magneto-crystalline energy density with anisotropy parameter, K1 ; and ai’s are the direction cosines of M. The third term in this expression was added to account for any possible induced in-plane axial anisotropy due to any possible geometrical asymmetry in sample preparation apparatus. The fourth term (Keff ¼ K u 2pM 2 ) represents the effective uniaxial anisotropy parameter including both the shape anisotropy (2pM 2 ) and any perpendicular anisotropy K u arising from magneto-elastic coupling S 2 due to lattice mismatch [8]. The term K1;2 a3 corresponds to easy axis surface anisotropy, and the last term represents exchange energy (Dk2, where D is exchange stiffness parameter, and k is spin-wave vector, related to the film thickness L) [6]. Using the above expression, we obtained best fit to the experimental data with the parameters given in Table 1. From the angular variation of the FMR spectra at higher temperature, the peak at higher field side was identified as a surface mode, which appears for perpendicular direction of measurement field with respect to the plane of the thicker films. The deduced cubic anisotropy parameter, 2K1 =M; is more than one order of magnitude smaller than a value of 460 Oe [9] for the bulk sample of the same compound due to under-
saturation of magnetization [6]. The curve for resonance values mainly oscillate with a period of 901. There is a small component with a period of 1801 for in-plane geometry, as seen from Fig. 3. This small amplitude modulation is well accounted for in plane axial anisotropy energy term with values for Ka as seen in Table 1. The effective magnetization is given in the second row of the table. Obviously, this value is too small compared to the bulk value of effective magnetization. This means that there is significant contribution from uniaxial magneto-crystalline anisotropy. One of the possible origins for such a small value for deduced effective anisotropy may be mechanical stress due to lattice mismatch between the Fe3O4 and MgO substrate. It is obvious that the lattice mismatch is more effective on thinner films. The lattice mismatch relaxes to inner region of thicker films, so the spins at interior region see mainly demagnetizing field that is linearly proportional to the magnetization. The surface treatment further enhances axial components of the magnetic anisotropy energy. The coating affects the surface spin wave modes by modifying pinning condition of the surface spins as well. This effect is clearly seen in Fig. 1b. The linewidth is also changed by coating. Since all the interaction is antiferromagnetic, this kind of behaviour can be attributed to the spin disorder, which can be modulated due to the interaction between Fe and Co spins in Fe3O4, and CoO layers, respectively. From this analysis we have found that the hard direction for surface spins is the film normal making easy plane surface anisotropy for thicker and Co3O4-coated films. SWR mode in X-band regions cannot be exited due to their relatively higher energy for thinner films. Therefore, for thinner films the surface anisotropy and surface spin pinning influence all the spins at interior regions, allowing to take into account the surface anisotropy in terms of effective bulk anisotropy parameters.
426
S. Budak et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
Acknowledgements The author would like to thank Dr. David Margulies for his kindness to supply the characterized samples. This work has been supported by Scientific and Technical Research Council of Turkey and Islamic Development Bank. References [1] J. Ahdjoudj, C. Martinsky, C. Minot, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 443 (1999) 133. [2] Kazuto Tokumitsu, Toshio Nasu, Scr. Mater. 44 (2001) 1421.
[3] W. Eerenstein, T.T.M. Palstra, T. Hibma, Thin Solid Films 400 (2001) 90. [4] B. Handke, J. Haber, T. Slezak, M. Kubik, J. Korecki, Vacuum 63 (2001) 331. [5] F. Schedin, P. Morrall, V.N. Petrov, S. Case, M.F. Thomas, E. Dudzik, G. Van der Laan, G. Thornton, J. Magn. Magn. Mater. 211 (2000) 266. [6] B. Aktas, Thin Solid Films 307 (1997) 250. [7] P.A.A. van der Heijden, M.G. van Opstal, C.H.W. Swuste, . P.H.J. Blomen, J.M. Gaines, W.J.M. de Jonge, J. Magn. Magn. Mater. 182 (1998) 71. [8] D.T. Margulies, F.T. Parker, F.E. Spada, R.S. Goldman, J. Li, R. Sinclair, A.E. Berkowitz, Phys. Rev. B 53 (1996) 9175. [9] B.A. Calhoun, Phys. Rev. 94 (1954) 1577.
Electron spin resonance studies on single crystalline Fe3O4 films c . Satilmis- Budaka, Fikret Yildizb,*, Mustafa Ozdemir , Bekir Aktas-b a Fatih University, Faculty of Art and Science, Physics Department, 34900 Istanbul, Turkey Department of Physics, Gebze Institute of Technology, 41410 Cayirova-Gebze, Kocaeli, Turkey c Faculty of Art and Science, Physics Department, Marmara University, Goztepe, Istanbul, Turkey . b
Abstract The effects of surface coating (Co and/or CoxOy) on magnetic anisotropies in single crystalline Fe3O4 films with different thickness that were epitaxially grown on MgO (1 0 0) substrate have been studied by electron spin resonance (ESR) technique. The angular dependent ESR spectra were analysed by using well-known SWR theory and the magnetic anisotropy parameters were deduced. The magnetic anisotropies are deeply affected from both film thickness ( thick films, and surface coating conditions. Magneto-crystalline cubic anisotropy (30 and 60 Oe for 110 and 2771 A respectively) is much smaller compared to its value for the bulk sample of the same compound (Hk ¼ 2K1 =M ¼ 460 Oe). The effective magnetization is influenced by the surface coating as well. The spectra for surface-coated films broaden especially for Co3O4-coated samples. r 2002 Elsevier Science B.V. All rights reserved. Keywords: ESR; Magnetic anisotropy; Magneto-crystalline energy
1. Introduction The members of Fe–O system are FeO, Fe2O3 and Fe3O4, which are the most important, and most abundant ferrimagnetic transition metal oxides. They are extensively used for magnetic storage information [1,2]. Furthermore, epitaxial Fe3O4 films grown on MgO substrates show magneto-resistance (MR) even at room temperature, whereas single crystals do not [3]. The substrate–sample structure match, the synthesis conditions, such as molecular beam flux, oxygen pressure and temperature can strongly influence the film structure and composition [4]. The magnetic properties depend on the structural order and the stoichiometry as well [5]. We have encountered only two FMR studies on FexOy in literature [6,7]. Therefore in this study, we have investigated magnetic properties of the Fe3O4 film prepared in different thickness and some of them were *Corresponding author. Tel.: +90-262-653-8497; fax: +90262-653-8490. E-mail address: [email protected] (F. Yildiz).
coated by Co and/or CoxOy to manipulate the magnetic surface anisotropy and obtain different surface spin pinning parameters.
2. Experimental results Fe3O4 thin films were prepared by deposition on MgO (1 0 0) substrates using reactive DC sputtering technique. Two sets of Fe3O4 films prepared in identical growth conditions have been used for electron spin resonance (ESR) measurements. The film thickness for the first and ( respectively. The the second sets are 110 and 2771 A, first set consists of non-coated film and two films coated ( Co and 18 A ( Co3O4 layers, respectively. The by 10 A second set includes non-coated film and two films coated ( CoO and 18.8 A ( Co3O4 layers. Fig. 1 shows by 17 A some examples for ESR spectra taken from Fe3O4 films ( in thickness for both perpendicular with 110 and 2771 A (where the magnetic field is perpendicular to the film plane, >) and parallel geometries (field lays in the sample plane, 8 geometries) as indicated on related
0304-8853/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 2 ) 0 1 0 8 3 - 1
424
S. Budak et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
Fig. 1. Variation of SWR spectra with thickness and surface conditions of Fe3O4 films as indicated on relevant spectrum. Dotted line and straight line show theoretical and experimental results respectively. Parallel (8) and perpendicular (>) geometries are indicated.
spectra at room temperature. The very narrow multiple peaks, centred at the field corresponding g ¼ 2; originate from the Mn+2 impurities. Although all of the films have been epitaxially grown on single crystalline MgO substrate in similar conditions, the ESR spectra seem to be quite different from each other especially for the thinner set (Fig. 1a). The spectra are varied with both film thickness and outer surface coating conditions. The separation of absorption peaks in parallel and perpendicular geometries are remarkably increased for Co and Co3O4-coated films with respect to uncoated one. The lines for coated sample are broadened as well. However, the effects of the surface coating are much more dramatic on Co3O4-coated Fe3O4 film. The lines are so much broader that the amplitude of field derivative-absorption line is too small to observe for low modulation field of 1 Oe as used for non-coated ( samples. 110 A ( thick films are The SWR spectra taken from 2771 A shown in Fig. 1b. The spectra in perpendicular geometry are split into multiple peaks while single peak is observed for parallel case for both non-coated and coated films. However, as the field is rotated away from the film normal, the peaks in this multiplet come close to each other and overlap giving a relatively broader single absorption line. The distance between the main modes of two geometries almost remains constant regardless the surface coating of these thicker films. That is, the thinner films are much influenced from the surface coating. Fig. 2 shows resonance field values as a function of angle measured with respect to the film plane for all films. The curves for the resonance field values are
Fig. 2. Out of plane angular variation of resonance field of various Fe3O4 films with different coating conditions.
perfectly symmetrical with respect to the film normal. The bell shaped curves becomes much narrower for thicker films. The Co3O4 coating further enhances this effect for both thinner and thicker films. The angular variations of the resonance fields for all films are given in Fig. 3 for in plane geometry. The angle of the applied magnetic field is measured from /1 0 0S direction. Fe3O4 films on MgO (1 0 0) exhibits four-fold symmetry with a period of 901 in the film plane suggesting a cubic crystalline structure for the film. However, the maximum amplitude of the resonance field variation with the angle depends on both film thickness and surface coating condition.
S. Budak et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
425
Table 1 Magnetic parameters for various Fe3O4 films
o=g (G) Keff M0 (G) K1 =M0 (G) Ka =M0 (G) 2K1s L=ðDM0 Þ 2K2s L=DM0 D (109) (erg/cm)
1
2
3
4
5
6
3330 1286 22 6
3105 1628 10 4
3320 1900 63 Undet.
3335 2230 52 7 6.5 11.0 5.5
3360 2062 48 4 4.0 7.0 5.5
3305 2092 46 11 12.4 12.6 5.5
( (2) 10 A Co-coated Fe3O4(110 A) ( (3) 18.8 A ( (1) Fe3O4 (110 A), ( (4) Fe3O4 (2771 A), ( (5) 17 CoO–Co3O4-coated Fe3O4 (110 A), ( (6) 18.8 A ( CoO–Co3O4-coated CoO-coated Fe3O4 (2771 A) ( Fe3O4 (2771 A).
Fig. 3. In plane angular variation of resonance field in sample plane of various Fe3O4 films with different coating conditions.
3. Theoretical results and discussions The theoretical resonance fields in Figs. 2 and 3 are calculated by using the following expression for the magnetic energy density E: E ¼ MH þ K1 ða21 a22 þ a21 a23 þ a22 a23 Þ S 2 þ Ka Cos2 ðjÞ þ Keff a23 K1;2 a3 þ Eex ;
where the first term represents Zeeman energy density, the second term accounts for cubic magneto-crystalline energy density with anisotropy parameter, K1 ; and ai’s are the direction cosines of M. The third term in this expression was added to account for any possible induced in-plane axial anisotropy due to any possible geometrical asymmetry in sample preparation apparatus. The fourth term (Keff ¼ K u 2pM 2 ) represents the effective uniaxial anisotropy parameter including both the shape anisotropy (2pM 2 ) and any perpendicular anisotropy K u arising from magneto-elastic coupling S 2 due to lattice mismatch [8]. The term K1;2 a3 corresponds to easy axis surface anisotropy, and the last term represents exchange energy (Dk2, where D is exchange stiffness parameter, and k is spin-wave vector, related to the film thickness L) [6]. Using the above expression, we obtained best fit to the experimental data with the parameters given in Table 1. From the angular variation of the FMR spectra at higher temperature, the peak at higher field side was identified as a surface mode, which appears for perpendicular direction of measurement field with respect to the plane of the thicker films. The deduced cubic anisotropy parameter, 2K1 =M; is more than one order of magnitude smaller than a value of 460 Oe [9] for the bulk sample of the same compound due to under-
saturation of magnetization [6]. The curve for resonance values mainly oscillate with a period of 901. There is a small component with a period of 1801 for in-plane geometry, as seen from Fig. 3. This small amplitude modulation is well accounted for in plane axial anisotropy energy term with values for Ka as seen in Table 1. The effective magnetization is given in the second row of the table. Obviously, this value is too small compared to the bulk value of effective magnetization. This means that there is significant contribution from uniaxial magneto-crystalline anisotropy. One of the possible origins for such a small value for deduced effective anisotropy may be mechanical stress due to lattice mismatch between the Fe3O4 and MgO substrate. It is obvious that the lattice mismatch is more effective on thinner films. The lattice mismatch relaxes to inner region of thicker films, so the spins at interior region see mainly demagnetizing field that is linearly proportional to the magnetization. The surface treatment further enhances axial components of the magnetic anisotropy energy. The coating affects the surface spin wave modes by modifying pinning condition of the surface spins as well. This effect is clearly seen in Fig. 1b. The linewidth is also changed by coating. Since all the interaction is antiferromagnetic, this kind of behaviour can be attributed to the spin disorder, which can be modulated due to the interaction between Fe and Co spins in Fe3O4, and CoO layers, respectively. From this analysis we have found that the hard direction for surface spins is the film normal making easy plane surface anisotropy for thicker and Co3O4-coated films. SWR mode in X-band regions cannot be exited due to their relatively higher energy for thinner films. Therefore, for thinner films the surface anisotropy and surface spin pinning influence all the spins at interior regions, allowing to take into account the surface anisotropy in terms of effective bulk anisotropy parameters.
426
S. Budak et al. / Journal of Magnetism and Magnetic Materials 258–259 (2003) 423–426
Acknowledgements The author would like to thank Dr. David Margulies for his kindness to supply the characterized samples. This work has been supported by Scientific and Technical Research Council of Turkey and Islamic Development Bank. References [1] J. Ahdjoudj, C. Martinsky, C. Minot, M.A. Van Hove, G.A. Somorjai, Surf. Sci. 443 (1999) 133. [2] Kazuto Tokumitsu, Toshio Nasu, Scr. Mater. 44 (2001) 1421.
[3] W. Eerenstein, T.T.M. Palstra, T. Hibma, Thin Solid Films 400 (2001) 90. [4] B. Handke, J. Haber, T. Slezak, M. Kubik, J. Korecki, Vacuum 63 (2001) 331. [5] F. Schedin, P. Morrall, V.N. Petrov, S. Case, M.F. Thomas, E. Dudzik, G. Van der Laan, G. Thornton, J. Magn. Magn. Mater. 211 (2000) 266. [6] B. Aktas, Thin Solid Films 307 (1997) 250. [7] P.A.A. van der Heijden, M.G. van Opstal, C.H.W. Swuste, . P.H.J. Blomen, J.M. Gaines, W.J.M. de Jonge, J. Magn. Magn. Mater. 182 (1998) 71. [8] D.T. Margulies, F.T. Parker, F.E. Spada, R.S. Goldman, J. Li, R. Sinclair, A.E. Berkowitz, Phys. Rev. B 53 (1996) 9175. [9] B.A. Calhoun, Phys. Rev. 94 (1954) 1577.