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Magnetic properties of field-annealed FeCo thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742 www.elsevier.com/locate/jmmm
Magnetic properties of field-annealed FeCo thin films M. Coı¨ sson, F. Celegato, P. Tiberto, F. Vinai INRIM, strada delle Cacce 91, I-10135 Torino, Italy Available online 10 April 2008
Abstract Fe50Co50 thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 1C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films. r 2008 Elsevier B.V. All rights reserved. PACS: 75.70.Ak; 75.70.Kw; 75.60.Nt; 75.50.Bb Keywords: FeCo; Thin film; Field annealing
1. Introduction FeCo alloys have been the subject of a wide scientific interest because of their large saturation magnetization and the wide dependence on composition of physical quantities such as magnetostriction and anisotropy constants. Suitable compositions and thermal treatments can be employed to develop materials that can display good soft magnetic properties and large saturation and magnetostriction [1–4]. FeCo alloys recently gained interest as magnetic elements for tunnel magneto-resistance and in magnetic tunnel junctions [5–7], especially in thin films form with thickness from a few nanometers to a few tens of nanometers. In this paper, magnetic properties of Fe50Co50 thin films have been studied by means of atomic force microscopy (AFM) and magnetic force microscopy (MFM) and DC hysteresis loops performed on as-prepared and field-annealed samples, in order to study how their magnetic structure evolves with annealing and how its Corresponding author. Tel.: +39 11 3919 855; fax: +39 11 391 834.
E-mail address: [email protected] (M. Coı¨ sson). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.040
relationship with magnetic anisotropy is induced by means of annealing. 2. Experimental FeCo thin films have been produced by rf sputtering on glass substrates from a target made of a Fe50Co50 alloy. Chamber base pressure was E2.2 10 7 mbar, while Ar pressure was E1 10 2 mbar, and the rf power was 25 W. Two families of samples have been prepared, with thickness of 30 and 4 nm. Energy dispersive spectroscopy (SEM-EDS) analysis confirms that the produced thin films, which are in the crystalline state, substantially maintain the same composition as the target. All studied samples have been observed with AFM and MFM, using commercial Co-Cr Mesp tips (coercivity E400 Oe) for tapping-lift mode. Approximately 3 3 mm2 samples of both families have been cut from a uniformly sputtered substrate and then furnace annealed in vacuum (pressure E2 10 4 mbar), at temperatures of 300 and 350 1C for times ranging in the interval 20–120 min. A static magnetic field has been applied during annealing by means of a solenoid along one of the geometrical
ARTICLE IN PRESS e740
M. Coı¨sson et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742
directions of the samples (easy axis); the field, with an intensity of 100 Oe, was switched on immediately before the beginning and switched off 10 min after the end of annealing, to ensure that the field was applied during cooling down. DC hysteresis loops have been measured on all asprepared and annealed samples by means of an alternating gradient force magnetometer (AGFM), under applied fields up to 5 kOe applied in the films’ plane. The samples were positioned on the AGFM probe in such a way to have the magnetic field applied along the easy axis (direction of the field applied during annealing) or the hard axis (901 off the easy axis). 3. Results and discussion MFM images of selected 30 nm thick samples are reported in Fig. 1 over scan sizes of 10 10 mm2. The MFM technique, which is sensitive to magnetic field gradients in the direction perpendicular to the film plane, reveals a tilt of the magnetization off the film plane, resulting in submicron magnetic structures whose magnetization has a non-negligible component perpendicular to the film plane. This partial out-of-plane orientation of the magnetization is in agreement with previous results reported in ref. [2], where Fe50Co50 alloys sputtered on glass-like substrates grow with a columnar structure that favours perpendicular magnetization. A columnar structure has also been observed in the present samples through FESEM images. These compositions are characterized by a large positive magnetostriction value ls (E102 ppm) and a relatively large and negative first-order anisotropy constant K1 (E 1.5 10 5 erg/cm3) when in the disordered phase, and a smaller magnetostriction and vanishing first-order anisotropy constant when in the ordered phase [1,3,8].
Fig. 1. MFM images of 30 nm thick samples (as-prepared and annealed at 350 1C for 40, 80 and 100 min).
Fig. 2. Hysteresis loops measured along the easy axis of 30 nm thick samples annealed at 350 1C under an applied field of 100 Oe for different times.
In Fig. 2 selected DC hysteresis loops measured along the easy axis are reported for the 30 nm thick samples annealed at 350 1C for different times t. A moderate reduction of the coercive field is observed with respect to the as-prepared sample (from E100 to E50 Oe) for t p60 min, probably due to a small ordering of the crystalline phase that leads to reduced ls and K1 values; longer annealing times, on the contrary, are responsible for the development of a harder magnetic phase (with a coercive field of E300 Oe), that is superimposed to the softer phase and grows to its expense, giving rise to the characteristic loop shape visible in Fig. 2 for the samples annealed for 80 and 100 min. On these specimens, the magnetic remanence is reduced with respect to the as-prepared film, and the harder phase increases in volume with increasing annealing time. This phase may be responsible for an increased orientation of the magnetization off the film plane, in agreement with MFM images reported in Fig. 1, where the magnetic domain structure evolves towards smaller and more fragmented domains with a sharper contrast between clear and dark regions of the images. This behavior after annealing is reversed with what is observed in ref. [2], where annealing was performed in an Ar/H atmosphere and not in vacuum and led to an improved in-plane magnetization. An annealing temperature of 300 1C is seen to induce a very similar behavior in 30 nm thick samples. Slightly longer annealing times are required to achieve a similar development of the harder magnetic phase. On the contrary, 4 nm thick samples display very different magnetic properties, as reported in Fig. 3, where selected DC hysteresis loops measured along the easy axis for samples annealed at 350 1C at different times are
ARTICLE IN PRESS M. Coı¨sson et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742
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Fig. 3. Hysteresis loops measured along the easy axis of 4 nm thick samples annealed at 350 1C under an applied field of 100 Oe for different times.
shown. The observed loop shapes have been confirmed by multiple measurements performed on different, 4 nm thick samples. In the as-prepared state, 4 nm thick samples have a lower coercive field with respect to the 30 nm thick ones. In thin films, the dependence of the coercive field with sample thickness is usually hard to predict, and is influenced by a number of factors. An increase of the coercive field on increasing film thickness has been attributed to stresses induced during deposition in highly magnetostrictive materials [9], a condition fulfilled also by our samples. In addition, their hysteresis loops are less squared, and display a larger field at which the two loop branches close (i.e. where irreversible magnetization processes disappear and only magnetization rotations persist). Short-time annealing causes a slight reduction of the coercive field with respect to the as-prepared (from E30 to E10 Oe) specimen. Longer annealing times result in larger hysteresis loops (coercive field of E80 Oe), with a reduced slope; increasing annealing time beyond 40 min, however, does not significantly affect the loop shape. The effect of the magnetic field annealing is shown in Fig. 4 for selected 30 nm thick samples annealed at 350 1C. The as-prepared sample is clearly isotropic, with no detectable difference between the two loops measured along two orthogonal directions in the film plane (since the as-prepared sample has not been annealed, the definition of what direction is ‘‘easy’’ and what is ‘‘hard’’ is arbitrary). The sample annealed for 20 min shows the already discussed reduction of the coercive field, but still no significant difference between the two directions is observed. Longer annealing times (only 60 min annealing time is reported in Fig. 4a for clarity) continue to decrease the coercive field, and a slight anisotropy develops: the loop
Fig. 4. Hysteresis loops measured along the easy and hard axes of selected 30 nm thick samples annealed at 350 1C under an applied field of 100 Oe for different times.
measured along the easy axis (i.e. the direction along which the magnetic field was applied during annealing) is moderately larger and with a slightly larger slope near the coercive field with respect to the loop measured along the hard axis. It has to be remarked that the field applied during annealing, equal to 100 Oe, is sufficiently larger than the coercive field, and is thus adequate for trying to induce a magnetic anisotropy. However, the induced magnetic anisotropy is not to be expected to be large in any case, because of the large magnetoelastic and, possibly, magnetocrystalline contributions to the total anisotropy of these Fe50Co50 alloys. Longer annealing times (see an example in
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Fig. 4b) induce a harder magnetic phase that has a coercive field too large to be affected by the field applied during annealing; thus, a slight magnetic anisotropy is still observed in the softer magnetic phase, which disappears when the hard portion of the hysteresis loop is explored. The results of annealing 30 nm thick samples at 300 1C under a field of 100 Oe are similar to those discussed so far, only requiring longer annealing times to achieve comparable reduction of the coercive field and development of magnetic anisotropy. Contrary to 30 nm thick samples, 4 nm thick ones do not seem to be affected by the magnetic field during annealing: their hysteresis loops are isotropic both in the as-prepared and annealed states, even at 350 1C for the longest annealing time (120 min).
irreversible effects disappearing only at larger fields with respect to 30 nm thick samples), which is affected by annealing time (increase of the coercive field and decrease of the slope near the coercive field) but not by the field applied during annealing (magnetic response of 4 nm thick films is always isotropic). Acknowledgments This work has been partially supported by Regione Piemonte with the project ‘‘Sviluppo di un dimostratore di transistor ad effetto tunnel magnetico basato su nanostrutture ibride ferromagnete-semiconduttore’’. The authors would like to thank Dr. A. Chiodoni for helping with the FESEM images.
4. Conclusions References Fe50Co50 thin films have been produced on glass substrates by rf sputtering. Their columnar structure is responsible for a non-null component of the magnetization perpendicular to the film plane, as detected by magnetic force microscopy. Samples with a thickness of 30 nm are affected by annealing at 300 and 350 1C in vacuum, which promotes, for annealing times longer than 60 min, the development of a harder magnetic phase, more oriented off the film plane. Annealing under a magnetic field of 100 Oe induces a slight magnetic anisotropy when annealing is performed for at least 40 min. When the harder magnetic phase is developed, it is not affected by the field applied during annealing. On the contrary, 4 nm thick samples are characterized by a very different loop shape (less squared, and with
[1] R.C. Hall, J. Appl. Phys. 31 (5) (1960) 157S. [2] M.D. Cooke, L.-C. Wang, R. Watts, R. Zuberek, G. Heydon, W.M. Rainforth, G.A. Gehring, J. Phys. D: Appl. Phys. 33 (2000) 1450. [3] G.Y. Chin, J.H. Wernick, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 2, North Holland, Amsterdam, 1980. [4] M. Sorescu, A. Grabias, Intermetallics 10 (2002) 317. [5] S.S.P. Parkin, E. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yong, Nat. Mater. 3 (2004) 862. [6] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (16) (1995) 3273. [7] S. Mitani, T. Moriyama, K. Takanashi, J. Appl. Phys. 93 (10) (2003) 8041. [8] M.D. Cooke, M.R.J. Gibbs, R.F. Pettifer, J. Magn. Magn. Mater. 237 (2001) 175–180. [9] J.M. Barandiara´n, P. Minguez, G.V. Krlyandskaya, J. Non-Cryst. Solids 329 (2003) 8.
Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742 www.elsevier.com/locate/jmmm
Magnetic properties of field-annealed FeCo thin films M. Coı¨ sson, F. Celegato, P. Tiberto, F. Vinai INRIM, strada delle Cacce 91, I-10135 Torino, Italy Available online 10 April 2008
Abstract Fe50Co50 thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 1C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films. r 2008 Elsevier B.V. All rights reserved. PACS: 75.70.Ak; 75.70.Kw; 75.60.Nt; 75.50.Bb Keywords: FeCo; Thin film; Field annealing
1. Introduction FeCo alloys have been the subject of a wide scientific interest because of their large saturation magnetization and the wide dependence on composition of physical quantities such as magnetostriction and anisotropy constants. Suitable compositions and thermal treatments can be employed to develop materials that can display good soft magnetic properties and large saturation and magnetostriction [1–4]. FeCo alloys recently gained interest as magnetic elements for tunnel magneto-resistance and in magnetic tunnel junctions [5–7], especially in thin films form with thickness from a few nanometers to a few tens of nanometers. In this paper, magnetic properties of Fe50Co50 thin films have been studied by means of atomic force microscopy (AFM) and magnetic force microscopy (MFM) and DC hysteresis loops performed on as-prepared and field-annealed samples, in order to study how their magnetic structure evolves with annealing and how its Corresponding author. Tel.: +39 11 3919 855; fax: +39 11 391 834.
E-mail address: [email protected] (M. Coı¨ sson). 0304-8853/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.04.040
relationship with magnetic anisotropy is induced by means of annealing. 2. Experimental FeCo thin films have been produced by rf sputtering on glass substrates from a target made of a Fe50Co50 alloy. Chamber base pressure was E2.2 10 7 mbar, while Ar pressure was E1 10 2 mbar, and the rf power was 25 W. Two families of samples have been prepared, with thickness of 30 and 4 nm. Energy dispersive spectroscopy (SEM-EDS) analysis confirms that the produced thin films, which are in the crystalline state, substantially maintain the same composition as the target. All studied samples have been observed with AFM and MFM, using commercial Co-Cr Mesp tips (coercivity E400 Oe) for tapping-lift mode. Approximately 3 3 mm2 samples of both families have been cut from a uniformly sputtered substrate and then furnace annealed in vacuum (pressure E2 10 4 mbar), at temperatures of 300 and 350 1C for times ranging in the interval 20–120 min. A static magnetic field has been applied during annealing by means of a solenoid along one of the geometrical
ARTICLE IN PRESS e740
M. Coı¨sson et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742
directions of the samples (easy axis); the field, with an intensity of 100 Oe, was switched on immediately before the beginning and switched off 10 min after the end of annealing, to ensure that the field was applied during cooling down. DC hysteresis loops have been measured on all asprepared and annealed samples by means of an alternating gradient force magnetometer (AGFM), under applied fields up to 5 kOe applied in the films’ plane. The samples were positioned on the AGFM probe in such a way to have the magnetic field applied along the easy axis (direction of the field applied during annealing) or the hard axis (901 off the easy axis). 3. Results and discussion MFM images of selected 30 nm thick samples are reported in Fig. 1 over scan sizes of 10 10 mm2. The MFM technique, which is sensitive to magnetic field gradients in the direction perpendicular to the film plane, reveals a tilt of the magnetization off the film plane, resulting in submicron magnetic structures whose magnetization has a non-negligible component perpendicular to the film plane. This partial out-of-plane orientation of the magnetization is in agreement with previous results reported in ref. [2], where Fe50Co50 alloys sputtered on glass-like substrates grow with a columnar structure that favours perpendicular magnetization. A columnar structure has also been observed in the present samples through FESEM images. These compositions are characterized by a large positive magnetostriction value ls (E102 ppm) and a relatively large and negative first-order anisotropy constant K1 (E 1.5 10 5 erg/cm3) when in the disordered phase, and a smaller magnetostriction and vanishing first-order anisotropy constant when in the ordered phase [1,3,8].
Fig. 1. MFM images of 30 nm thick samples (as-prepared and annealed at 350 1C for 40, 80 and 100 min).
Fig. 2. Hysteresis loops measured along the easy axis of 30 nm thick samples annealed at 350 1C under an applied field of 100 Oe for different times.
In Fig. 2 selected DC hysteresis loops measured along the easy axis are reported for the 30 nm thick samples annealed at 350 1C for different times t. A moderate reduction of the coercive field is observed with respect to the as-prepared sample (from E100 to E50 Oe) for t p60 min, probably due to a small ordering of the crystalline phase that leads to reduced ls and K1 values; longer annealing times, on the contrary, are responsible for the development of a harder magnetic phase (with a coercive field of E300 Oe), that is superimposed to the softer phase and grows to its expense, giving rise to the characteristic loop shape visible in Fig. 2 for the samples annealed for 80 and 100 min. On these specimens, the magnetic remanence is reduced with respect to the as-prepared film, and the harder phase increases in volume with increasing annealing time. This phase may be responsible for an increased orientation of the magnetization off the film plane, in agreement with MFM images reported in Fig. 1, where the magnetic domain structure evolves towards smaller and more fragmented domains with a sharper contrast between clear and dark regions of the images. This behavior after annealing is reversed with what is observed in ref. [2], where annealing was performed in an Ar/H atmosphere and not in vacuum and led to an improved in-plane magnetization. An annealing temperature of 300 1C is seen to induce a very similar behavior in 30 nm thick samples. Slightly longer annealing times are required to achieve a similar development of the harder magnetic phase. On the contrary, 4 nm thick samples display very different magnetic properties, as reported in Fig. 3, where selected DC hysteresis loops measured along the easy axis for samples annealed at 350 1C at different times are
ARTICLE IN PRESS M. Coı¨sson et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742
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Fig. 3. Hysteresis loops measured along the easy axis of 4 nm thick samples annealed at 350 1C under an applied field of 100 Oe for different times.
shown. The observed loop shapes have been confirmed by multiple measurements performed on different, 4 nm thick samples. In the as-prepared state, 4 nm thick samples have a lower coercive field with respect to the 30 nm thick ones. In thin films, the dependence of the coercive field with sample thickness is usually hard to predict, and is influenced by a number of factors. An increase of the coercive field on increasing film thickness has been attributed to stresses induced during deposition in highly magnetostrictive materials [9], a condition fulfilled also by our samples. In addition, their hysteresis loops are less squared, and display a larger field at which the two loop branches close (i.e. where irreversible magnetization processes disappear and only magnetization rotations persist). Short-time annealing causes a slight reduction of the coercive field with respect to the as-prepared (from E30 to E10 Oe) specimen. Longer annealing times result in larger hysteresis loops (coercive field of E80 Oe), with a reduced slope; increasing annealing time beyond 40 min, however, does not significantly affect the loop shape. The effect of the magnetic field annealing is shown in Fig. 4 for selected 30 nm thick samples annealed at 350 1C. The as-prepared sample is clearly isotropic, with no detectable difference between the two loops measured along two orthogonal directions in the film plane (since the as-prepared sample has not been annealed, the definition of what direction is ‘‘easy’’ and what is ‘‘hard’’ is arbitrary). The sample annealed for 20 min shows the already discussed reduction of the coercive field, but still no significant difference between the two directions is observed. Longer annealing times (only 60 min annealing time is reported in Fig. 4a for clarity) continue to decrease the coercive field, and a slight anisotropy develops: the loop
Fig. 4. Hysteresis loops measured along the easy and hard axes of selected 30 nm thick samples annealed at 350 1C under an applied field of 100 Oe for different times.
measured along the easy axis (i.e. the direction along which the magnetic field was applied during annealing) is moderately larger and with a slightly larger slope near the coercive field with respect to the loop measured along the hard axis. It has to be remarked that the field applied during annealing, equal to 100 Oe, is sufficiently larger than the coercive field, and is thus adequate for trying to induce a magnetic anisotropy. However, the induced magnetic anisotropy is not to be expected to be large in any case, because of the large magnetoelastic and, possibly, magnetocrystalline contributions to the total anisotropy of these Fe50Co50 alloys. Longer annealing times (see an example in
ARTICLE IN PRESS e742
M. Coı¨sson et al. / Journal of Magnetism and Magnetic Materials 320 (2008) e739–e742
Fig. 4b) induce a harder magnetic phase that has a coercive field too large to be affected by the field applied during annealing; thus, a slight magnetic anisotropy is still observed in the softer magnetic phase, which disappears when the hard portion of the hysteresis loop is explored. The results of annealing 30 nm thick samples at 300 1C under a field of 100 Oe are similar to those discussed so far, only requiring longer annealing times to achieve comparable reduction of the coercive field and development of magnetic anisotropy. Contrary to 30 nm thick samples, 4 nm thick ones do not seem to be affected by the magnetic field during annealing: their hysteresis loops are isotropic both in the as-prepared and annealed states, even at 350 1C for the longest annealing time (120 min).
irreversible effects disappearing only at larger fields with respect to 30 nm thick samples), which is affected by annealing time (increase of the coercive field and decrease of the slope near the coercive field) but not by the field applied during annealing (magnetic response of 4 nm thick films is always isotropic). Acknowledgments This work has been partially supported by Regione Piemonte with the project ‘‘Sviluppo di un dimostratore di transistor ad effetto tunnel magnetico basato su nanostrutture ibride ferromagnete-semiconduttore’’. The authors would like to thank Dr. A. Chiodoni for helping with the FESEM images.
4. Conclusions References Fe50Co50 thin films have been produced on glass substrates by rf sputtering. Their columnar structure is responsible for a non-null component of the magnetization perpendicular to the film plane, as detected by magnetic force microscopy. Samples with a thickness of 30 nm are affected by annealing at 300 and 350 1C in vacuum, which promotes, for annealing times longer than 60 min, the development of a harder magnetic phase, more oriented off the film plane. Annealing under a magnetic field of 100 Oe induces a slight magnetic anisotropy when annealing is performed for at least 40 min. When the harder magnetic phase is developed, it is not affected by the field applied during annealing. On the contrary, 4 nm thick samples are characterized by a very different loop shape (less squared, and with
[1] R.C. Hall, J. Appl. Phys. 31 (5) (1960) 157S. [2] M.D. Cooke, L.-C. Wang, R. Watts, R. Zuberek, G. Heydon, W.M. Rainforth, G.A. Gehring, J. Phys. D: Appl. Phys. 33 (2000) 1450. [3] G.Y. Chin, J.H. Wernick, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, vol. 2, North Holland, Amsterdam, 1980. [4] M. Sorescu, A. Grabias, Intermetallics 10 (2002) 317. [5] S.S.P. Parkin, E. Kaiser, A. Panchula, P.M. Rice, B. Hughes, M. Samant, S.-H. Yong, Nat. Mater. 3 (2004) 862. [6] J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (16) (1995) 3273. [7] S. Mitani, T. Moriyama, K. Takanashi, J. Appl. Phys. 93 (10) (2003) 8041. [8] M.D. Cooke, M.R.J. Gibbs, R.F. Pettifer, J. Magn. Magn. Mater. 237 (2001) 175–180. [9] J.M. Barandiara´n, P. Minguez, G.V. Krlyandskaya, J. Non-Cryst. Solids 329 (2003) 8.