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Al2O3 multilayers determined by nuclear magnetic resonance
Journal of Magnetism and Magnetic Materials 242–245 (2002) 943–945
Local structure in CoFe/Al2O3 multilayers determined by nuclear magnetic resonance N.A. Lesnika, P. Panissodb, G.N. Kakazeia,c,d, Yu.G. Pogorelovc,*, J.B. Sousac, E. Snoecke, S. Cardosof, P.P. Freitasf, P.E. Wigend a
Institute of Magnetism NAS of Ukraine, 36 Vernadskogo str., 03142 Kiev, Ukraine b IPCMS, Unit!e Mixte CNRS-ULP, 23 Rue du Loess, 67037 Strasbourg, France c IFIMUP/CFP, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal d Department of Physics, The Ohio State University, Columbus, OH 43210, USA e CEMES-CNRS, BP 4347, F-31055 Toulouse Cedex 4, France f INESC, Rua Alves Redol 9-1, 1000 Lisbon, Portugal
Abstract We studied NMR spectra at 1.5 K in a series of [Co80Fe20(t)/Al2O3(3 nm)]10 multilayers. The spectra are interpreted by mixed contributions from Co atoms inside CoFe granules and at their interface with Al2O3. They display three characteristic shapes when t varies from 0.8 to 2 nm. These shapes have been interpreted as resulting from granular CoFe layers for the thinnest ones (to1:3 nm), and continuous CoFe layers for the thickest ones (t > 1:6 nm), with an intermediate regimeFpancake grains of different sizesFin between. These structural data correlate with three magnetic states deduced from the FMR and SQUID measurements in this system. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Nuclear magnetic resonance; Multilayer film; Alloys; Alumina
Granular magnetic materials are interesting both for their applications in magnetic storage and sensing, and for fundamental studies. An important class among them is presented by discontinuous metal–insulator multilayers (DMIM) [1,2], where a possibility for longrange magnetic order in structurally discontinuous magnetic subsystem was recently indicated. The relevant parameter for the transition from para- to ferromagnetic state is the nominal thickness t of magnetic layer. For a better understanding of this transition mechanism, apart from the global characteristics like DC susceptibility, local magnetic and structural properties are highly informative as was shown in another granular system, the CoAg alloys [3]. Using the NMR technique, the chemical short-range order in the vicinity of resonating atoms can be deduced. In single domain spheres, the *Corresponding author. Tel.: +351-22-608-26-08; fax: +351-22-608-26-22. E-mail address: [email protected] (Yu.G. Pogorelov).
resonance frequencies increase due to a contribution from the demagnetizing field [4]. However, in this 2D granular system the interaction between granules in the layer plane is strong enough to keep the frequencies unshifted. ( Multilayered ½Co80 Fe20 ðtÞ=Al2 O3 ð30 AÞ 10 films with t varied from 0.8 to 2.5 nm have been fabricated using the ion-beam deposition technique described in Ref. [2]. The structural characterization of the multilayers by the lowangle X-ray diffraction and high resolution transmission electron microscopy (HRTEM) confirmed the existence of a periodic multilayered structure, although the thinnest CoFe layers consist actually of spherical granules (Fig. 1). The diffraction analysis of HRTEM images permitted to identify the CoFe crystalline structure of these granules as FCC with a lattice parameter of about 0.34 nm. NMR spectra have been recorded in a zero external DC field at 1.5 K using a phase-coherent swept frequency spin-echo spectrometer. Fig. 2a–e shows the
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 3 0 6 - 3
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N.A. Lesnik et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 943–945
NMR spectra recorded in a series of ½Co80 Fe20 ðtÞ=Al2 O3 ð3 nmÞ10 films (a–c) and fragments of these spectra (d–e) in the central 190–235 MHz range.
Fig. 1. A cross-section HRTEM micrograph of t ¼ 1:3 nm DMI film and the diffraction picture from a single CoFe granule (dash-rounded) permitting to identify an FCC structure with a lattice parameter E0.34 nm.
These spectra display three characteristic thickness regions when t is varied from 0.8 to 2 nm: (1) to1:3 nm (Fig 1a); (2) 1.3 nm oto1:6 nm (Fig. 1b); and (3) t > 1:6 nm (Fig. 1c). Indeed, the spectra are similar for the samples from the same region and change rather abruptly when passing to the next one. They were interpreted in terms of contributions from Co atoms in bulk and interfaces of granules as described below. Let us start from the simplest NMR spectrum obtained in the film from region 3, with t ¼ 2:0 nm (Fig. 2c, curve 3 and Fig. 2d, open squares). Curve 3 represents the narrowest resonance line among all the spectra in Fig. 2a–c. It peaks at 220 MHz with a large tail towards high frequencies and a low plateau in the 160–180 MHz range. This broad spectrum and its highfrequency tail are typical of both FCC and BCC CoFe disordered alloys for 20% Fe content. Actually, a mixture of the two phases is expected from the bulk phase diagram. The signal in the low-frequency range is similar to that observed earlier in bulk CoFe ordered compounds (B2 phase) where it was attributed to Co atoms lying in the Fe sublattice of the B2 structure (antisites). However, the rest of the spectrum does not show the multiple narrow lines typical of the offstoichiometric-ordered B2 phase [5]. Therefore, this lowfrequency signal must be attributed to Co atoms located at the interfaces, where the hyperfine field is lowered due to the presence of non-magnetic neighbors. This will be confirmed by the evolution of the spectral shape in the thinner samples. The next thinner film (with t ¼ 1:8 nm) exhibits a similar spectrum, although slightly broader. The relative intensity at the range of 160–180 MHz is the lowest in these samples, indicating a minimum interfacial roughness. This is also confirmed by the transmission electron microscopy showing that the CoFe layers are continuous in this thickness range. The spectrum of the sample with t ¼ 1:6 nm is still similar to the previous
Fig. 2. NMR spectra in CoFe/Al2O3 multilayers at different thickness values of ferromagnetic layers.
N.A. Lesnik et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 943–945
ones as to its main part, but it already shows an increase of the signal in the interfacial frequency range. This indicates that the CoFe layers have started to break for this thickness, thus increasing the surface area of the interfaces. Therefore, the continuous–discontinuous film transition can be fixed near t ¼ 1:8 nm (the percolation thickness). The NMR spectra in the thinnest layers (region 1) are shown in Fig. 2a and e (open circles and squares). Curves 1 and 2 correspond to the films with t ¼ 1:1 and 1.2 nm, respectively. The signal intensity at the 160– 180 MHz range dominates in these spectra, indicating a very high surface/bulk ratio: the samples in this thickness range consist of granular CoFe layers with a large interface area. Also associated with the large interface area is the weak feature appearing in the spectra at B330 MHz, which is extremely high for any CoFe alloy (even diluted Co in Fe). Most probably, the signal comes from the enhanced Co magnetic moments near the interface due to a transition from band to localized d-electrons at the formation of CoO. A similar signal at the enhanced frequencies was observed in Co nanoclusters embedded into SiO2 matrix [6]. This resonance signal disappears in region 3 (Fig. 2c) where the relative granule surface area is less than in regions 1 and 2. The ‘‘bulk’’ part of the spectra shows resonance peaks of about equal intensity in the vicinity of 200 and 225 MHz and a hump at higher frequencies. Such a set of three lines at 200, 225 and 250 MHz corresponds fairly well to the spectra observed in thin BCC FeCo films, where the relative intensities of the lines vary with the composition [7]. They correspond to Co environments with dominant even numbers of Fe neighbors (0, 2 and 4), a kind of short-range order that is not observed in bulk-disordered and ordered (B2) CoFe alloys. Therefore, in the FeCo/Al2O3 multilayers we rather observe a mixture of unknown BCC phases as in thin CoFe films. Note that in regions 1 and 2 a signal at 217– 218 MHz corresponding to the FCC Co (arrow B) is also observed. This peak starts to resolve from the film of 1.2 nm (open squares) and it is shifted towards high frequencies in the film of 1.4 nm (solid triangles). However, in region 3, the peak is not resolved any more. The FCC structure of the granules in the film of 1.3 nm is confirmed by HRTM (see Fig. 1). The fact that the BCC phase had not been observed by HRTEM can be due to the reason that only a few most-visible and well-crystallized granules were studied. It suggests that the BCC phase is present only in more disordered and less spheroidal regions. In fact, granules have a large size distribution causing different formation conditions, hence a complex film structure is expected. The NMR spectra in the intermediate region 2 (Figs. 2b and d, solid triangles and diamonds) display an abrupt decrease of intensity from 160 to 180 and at B330 MHz in comparison with the spectra in region 1
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and a corresponding increase at 220–255 MHz. This, in addition to the above remark, excludes the B2 ‘‘antisite’’ origin of the low-frequency signal: indeed, in the case of off-stoichiometric B2 phase the intensity of this region should evolve like the intensity of the region f > 250 MHz, which is absolutely not the case. Moreover, the signal below 180 MHz decreases similar to the high-frequency peak C (due to oxide at the interface). Therefore, we have the confirmation that it arises purely from the interfaces. It must be stressed that the decrease rate of these interfacial regions (metallic f o180 and oxide f > 300 MHz) never obeys the 1=t law that would be valid for a continuous film: the surface/volume ratio evolves much faster and discontinuously. At last, it can be seen that globally the center of gravity of the spectra shifts towards high frequencies when the thickness increases. This is due, on the one hand, to the decrease of the relative contribution from the interface. On the other hand, the variation of the relative intensities of the main peaks at 200 and 225 MHz shows that the environment with 2 Fe neighbors (225 MHz) is favored at its expense with no Fe neighbors (200 MHz). This suggests that the metallic FeCo alloy gets richer in Fe as the thickness increases. In fact, iron is more easily oxidized than cobalt, hence the metallic layers contain less iron near the interfaces because of the chemical reaction between Fe and Al2O3. This is the same reaction that produces the region C ( f B330 MHz) but it is more important for Fe than for Co. In summary, this NMR study shows that the evolution of the structure of CoFe layers takes place in three stages as follows: at to1:3 nm the films are granular; at 1:3oto1:6 nm they present a mixture of granules and flat local regions; and at t > 1:6 nm the films are continuous. This evolution scheme agrees well with the FMR data (effective fields versus thickness) measured in the same film series [8]. We also found 3 thickness regions in the analysis of the magnetization curves obtained in these samples by SQUID. These results will be published elsewhere.
References [1] [2] [3] [4] [5]
B. Dieny, et al., J. Magn. Magn. Mater. 185 (1998) 283. G.N. Kakazei, et al., IEEE Trans. Magn. 35 (1999) 2895. Yu.G. Pogorelov, et al., Phys. Rev. B 60 (1999) 12200. A.C. Gossard, et al., Phys. Rev. B 138 (1965) 1415. J.Ph. Jay, M. Wojcik, P. Panissod, Z. Phys. B 101 (1996) 471. [6] T. Thomson, et al., J. Appl. Phys. 81 (1997) 5549. [7] M. Wojcik, et al., Z. Phys. B 103 (1997) 5. [8] G.N. Kakazei, et al., J. Magn. Magn. Mater. 226–230 (2001) 1865.
Local structure in CoFe/Al2O3 multilayers determined by nuclear magnetic resonance N.A. Lesnika, P. Panissodb, G.N. Kakazeia,c,d, Yu.G. Pogorelovc,*, J.B. Sousac, E. Snoecke, S. Cardosof, P.P. Freitasf, P.E. Wigend a
Institute of Magnetism NAS of Ukraine, 36 Vernadskogo str., 03142 Kiev, Ukraine b IPCMS, Unit!e Mixte CNRS-ULP, 23 Rue du Loess, 67037 Strasbourg, France c IFIMUP/CFP, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal d Department of Physics, The Ohio State University, Columbus, OH 43210, USA e CEMES-CNRS, BP 4347, F-31055 Toulouse Cedex 4, France f INESC, Rua Alves Redol 9-1, 1000 Lisbon, Portugal
Abstract We studied NMR spectra at 1.5 K in a series of [Co80Fe20(t)/Al2O3(3 nm)]10 multilayers. The spectra are interpreted by mixed contributions from Co atoms inside CoFe granules and at their interface with Al2O3. They display three characteristic shapes when t varies from 0.8 to 2 nm. These shapes have been interpreted as resulting from granular CoFe layers for the thinnest ones (to1:3 nm), and continuous CoFe layers for the thickest ones (t > 1:6 nm), with an intermediate regimeFpancake grains of different sizesFin between. These structural data correlate with three magnetic states deduced from the FMR and SQUID measurements in this system. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Nuclear magnetic resonance; Multilayer film; Alloys; Alumina
Granular magnetic materials are interesting both for their applications in magnetic storage and sensing, and for fundamental studies. An important class among them is presented by discontinuous metal–insulator multilayers (DMIM) [1,2], where a possibility for longrange magnetic order in structurally discontinuous magnetic subsystem was recently indicated. The relevant parameter for the transition from para- to ferromagnetic state is the nominal thickness t of magnetic layer. For a better understanding of this transition mechanism, apart from the global characteristics like DC susceptibility, local magnetic and structural properties are highly informative as was shown in another granular system, the CoAg alloys [3]. Using the NMR technique, the chemical short-range order in the vicinity of resonating atoms can be deduced. In single domain spheres, the *Corresponding author. Tel.: +351-22-608-26-08; fax: +351-22-608-26-22. E-mail address: [email protected] (Yu.G. Pogorelov).
resonance frequencies increase due to a contribution from the demagnetizing field [4]. However, in this 2D granular system the interaction between granules in the layer plane is strong enough to keep the frequencies unshifted. ( Multilayered ½Co80 Fe20 ðtÞ=Al2 O3 ð30 AÞ 10 films with t varied from 0.8 to 2.5 nm have been fabricated using the ion-beam deposition technique described in Ref. [2]. The structural characterization of the multilayers by the lowangle X-ray diffraction and high resolution transmission electron microscopy (HRTEM) confirmed the existence of a periodic multilayered structure, although the thinnest CoFe layers consist actually of spherical granules (Fig. 1). The diffraction analysis of HRTEM images permitted to identify the CoFe crystalline structure of these granules as FCC with a lattice parameter of about 0.34 nm. NMR spectra have been recorded in a zero external DC field at 1.5 K using a phase-coherent swept frequency spin-echo spectrometer. Fig. 2a–e shows the
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 1 3 0 6 - 3
944
N.A. Lesnik et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 943–945
NMR spectra recorded in a series of ½Co80 Fe20 ðtÞ=Al2 O3 ð3 nmÞ10 films (a–c) and fragments of these spectra (d–e) in the central 190–235 MHz range.
Fig. 1. A cross-section HRTEM micrograph of t ¼ 1:3 nm DMI film and the diffraction picture from a single CoFe granule (dash-rounded) permitting to identify an FCC structure with a lattice parameter E0.34 nm.
These spectra display three characteristic thickness regions when t is varied from 0.8 to 2 nm: (1) to1:3 nm (Fig 1a); (2) 1.3 nm oto1:6 nm (Fig. 1b); and (3) t > 1:6 nm (Fig. 1c). Indeed, the spectra are similar for the samples from the same region and change rather abruptly when passing to the next one. They were interpreted in terms of contributions from Co atoms in bulk and interfaces of granules as described below. Let us start from the simplest NMR spectrum obtained in the film from region 3, with t ¼ 2:0 nm (Fig. 2c, curve 3 and Fig. 2d, open squares). Curve 3 represents the narrowest resonance line among all the spectra in Fig. 2a–c. It peaks at 220 MHz with a large tail towards high frequencies and a low plateau in the 160–180 MHz range. This broad spectrum and its highfrequency tail are typical of both FCC and BCC CoFe disordered alloys for 20% Fe content. Actually, a mixture of the two phases is expected from the bulk phase diagram. The signal in the low-frequency range is similar to that observed earlier in bulk CoFe ordered compounds (B2 phase) where it was attributed to Co atoms lying in the Fe sublattice of the B2 structure (antisites). However, the rest of the spectrum does not show the multiple narrow lines typical of the offstoichiometric-ordered B2 phase [5]. Therefore, this lowfrequency signal must be attributed to Co atoms located at the interfaces, where the hyperfine field is lowered due to the presence of non-magnetic neighbors. This will be confirmed by the evolution of the spectral shape in the thinner samples. The next thinner film (with t ¼ 1:8 nm) exhibits a similar spectrum, although slightly broader. The relative intensity at the range of 160–180 MHz is the lowest in these samples, indicating a minimum interfacial roughness. This is also confirmed by the transmission electron microscopy showing that the CoFe layers are continuous in this thickness range. The spectrum of the sample with t ¼ 1:6 nm is still similar to the previous
Fig. 2. NMR spectra in CoFe/Al2O3 multilayers at different thickness values of ferromagnetic layers.
N.A. Lesnik et al. / Journal of Magnetism and Magnetic Materials 242–245 (2002) 943–945
ones as to its main part, but it already shows an increase of the signal in the interfacial frequency range. This indicates that the CoFe layers have started to break for this thickness, thus increasing the surface area of the interfaces. Therefore, the continuous–discontinuous film transition can be fixed near t ¼ 1:8 nm (the percolation thickness). The NMR spectra in the thinnest layers (region 1) are shown in Fig. 2a and e (open circles and squares). Curves 1 and 2 correspond to the films with t ¼ 1:1 and 1.2 nm, respectively. The signal intensity at the 160– 180 MHz range dominates in these spectra, indicating a very high surface/bulk ratio: the samples in this thickness range consist of granular CoFe layers with a large interface area. Also associated with the large interface area is the weak feature appearing in the spectra at B330 MHz, which is extremely high for any CoFe alloy (even diluted Co in Fe). Most probably, the signal comes from the enhanced Co magnetic moments near the interface due to a transition from band to localized d-electrons at the formation of CoO. A similar signal at the enhanced frequencies was observed in Co nanoclusters embedded into SiO2 matrix [6]. This resonance signal disappears in region 3 (Fig. 2c) where the relative granule surface area is less than in regions 1 and 2. The ‘‘bulk’’ part of the spectra shows resonance peaks of about equal intensity in the vicinity of 200 and 225 MHz and a hump at higher frequencies. Such a set of three lines at 200, 225 and 250 MHz corresponds fairly well to the spectra observed in thin BCC FeCo films, where the relative intensities of the lines vary with the composition [7]. They correspond to Co environments with dominant even numbers of Fe neighbors (0, 2 and 4), a kind of short-range order that is not observed in bulk-disordered and ordered (B2) CoFe alloys. Therefore, in the FeCo/Al2O3 multilayers we rather observe a mixture of unknown BCC phases as in thin CoFe films. Note that in regions 1 and 2 a signal at 217– 218 MHz corresponding to the FCC Co (arrow B) is also observed. This peak starts to resolve from the film of 1.2 nm (open squares) and it is shifted towards high frequencies in the film of 1.4 nm (solid triangles). However, in region 3, the peak is not resolved any more. The FCC structure of the granules in the film of 1.3 nm is confirmed by HRTM (see Fig. 1). The fact that the BCC phase had not been observed by HRTEM can be due to the reason that only a few most-visible and well-crystallized granules were studied. It suggests that the BCC phase is present only in more disordered and less spheroidal regions. In fact, granules have a large size distribution causing different formation conditions, hence a complex film structure is expected. The NMR spectra in the intermediate region 2 (Figs. 2b and d, solid triangles and diamonds) display an abrupt decrease of intensity from 160 to 180 and at B330 MHz in comparison with the spectra in region 1
945
and a corresponding increase at 220–255 MHz. This, in addition to the above remark, excludes the B2 ‘‘antisite’’ origin of the low-frequency signal: indeed, in the case of off-stoichiometric B2 phase the intensity of this region should evolve like the intensity of the region f > 250 MHz, which is absolutely not the case. Moreover, the signal below 180 MHz decreases similar to the high-frequency peak C (due to oxide at the interface). Therefore, we have the confirmation that it arises purely from the interfaces. It must be stressed that the decrease rate of these interfacial regions (metallic f o180 and oxide f > 300 MHz) never obeys the 1=t law that would be valid for a continuous film: the surface/volume ratio evolves much faster and discontinuously. At last, it can be seen that globally the center of gravity of the spectra shifts towards high frequencies when the thickness increases. This is due, on the one hand, to the decrease of the relative contribution from the interface. On the other hand, the variation of the relative intensities of the main peaks at 200 and 225 MHz shows that the environment with 2 Fe neighbors (225 MHz) is favored at its expense with no Fe neighbors (200 MHz). This suggests that the metallic FeCo alloy gets richer in Fe as the thickness increases. In fact, iron is more easily oxidized than cobalt, hence the metallic layers contain less iron near the interfaces because of the chemical reaction between Fe and Al2O3. This is the same reaction that produces the region C ( f B330 MHz) but it is more important for Fe than for Co. In summary, this NMR study shows that the evolution of the structure of CoFe layers takes place in three stages as follows: at to1:3 nm the films are granular; at 1:3oto1:6 nm they present a mixture of granules and flat local regions; and at t > 1:6 nm the films are continuous. This evolution scheme agrees well with the FMR data (effective fields versus thickness) measured in the same film series [8]. We also found 3 thickness regions in the analysis of the magnetization curves obtained in these samples by SQUID. These results will be published elsewhere.
References [1] [2] [3] [4] [5]
B. Dieny, et al., J. Magn. Magn. Mater. 185 (1998) 283. G.N. Kakazei, et al., IEEE Trans. Magn. 35 (1999) 2895. Yu.G. Pogorelov, et al., Phys. Rev. B 60 (1999) 12200. A.C. Gossard, et al., Phys. Rev. B 138 (1965) 1415. J.Ph. Jay, M. Wojcik, P. Panissod, Z. Phys. B 101 (1996) 471. [6] T. Thomson, et al., J. Appl. Phys. 81 (1997) 5549. [7] M. Wojcik, et al., Z. Phys. B 103 (1997) 5. [8] G.N. Kakazei, et al., J. Magn. Magn. Mater. 226–230 (2001) 1865.