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Exchange dominated surface modes in amorphous ferromagnetic YCo3
Journal of Magnetism and Magnetic Materials 0 North-Holland Publishing Company
7 (1978)
178-181
EXCHANGE DOMINATED SURFACE MODES IN AMORPHOUS FERROMAGNETIC G. SURAN, R. KRISHNAN, Laboratoire de Magnthne,
YCo3
J. SZTERN
C.N.R.S.
92190 Meudon Bellevue, France
H. JOUVE and R. MEYER LETI, C.E. N. G. 85-X 38041
Grenoble Cedex, France
The ferromagnetic resonance spectra, in the band Ku of sputter deposited YCo3 amorphous films, show the presence of a surface mode which is due to a thin surface layer which forms at the initial stages of the deposition process. A simple model of two exchange coupled films enables us to determine as a function of temperature the magnetic properties and the effect of thickness of this surface layer. Induced anisotropy Ku in the bulk of the film and the thin surface layer has opposite signs and a comparative study of Ku = f(r) and the relaxation mechanism suggests that the origin of the Ku in the two cases could be different.
and K, in the bulk of the film
1. Introduction
2. Experimental
In transition metal (TM)-rare earth (RE) amorphous films the existence of a growth induced uniaxial anisotropy (K,) is a general feature [ 1,2]. However, the magnitude and sign of K, is largely dependent upon the film composition, the deposition method (evaporation or sputtering), the deposition parameters and the film thickness. The composition dependence of K, is largely complicated by the preferential oxidation of the magnetic rare earth constituent [3]. Under these conditions, it should be much easier to understand the main source of K, in an alloy where the RE is non magnetic, whence our choice of the YCo system. In YCo3 films, we have made a systematic study of K, as a function of deposition parameters, thickness and substrate material and some results have been reported earlier [4]. Here we present our investigations of the thin surface layer which seems to form right at the beginning of the sputtering process. In this surface layer KL is negative (easy plane) while in the bulk of the film K, is positive (easy axis normal to the film plane). It was felt that measurements as a function of temperature could throw some light on the question whether the same mechanism gives rise to K, of opposite signs, and here we describe the results.
Films of nominal composition YCo, were deposited by D.C. sputtering using Ar as sputtering gas [5]. Saturation magnetisation 47rM, and anisotropy field were determined by the ferromagnetic resonance techniques in the Ku band (f= 17.3 GHz) in the range 4.2 to 300 K. MS and K, were also controlled by the inplane loop tracer and static measurements. 4nM, and the anisotropy field Ha could be deduced from the resonance fields of the “as deposited” and annealed samples. After annealing (under vacuum at 200°C) K, in the bulk of the film was suppressed as confirmed also by the hysteresis measurements. In samples of thickness e larger than 0.2 pm, K, was in the range 2 to 8.104 erg/cm3 [4]. K, increases as the bias voltage Vb increases and decreases slightly with increasing film thickness and when the deposition temperature is decreased. This result is in favour of in plane Co-Co pair formation [4].
3. Surface mode: results, model and discussion Only for the perpendicular orientation of the film and for e > 0.2 1-1the surface mode is observed on the high field side of the main resonance mode. This sur178
G. &ran et al. j &face mode in arnor~~~~s ITo3 _Elms
179
*Y = d(Hs-Huk0+b43)
4
20-
20-
./
/
(c) lo-
0.75 I it
i?
d) ii
0, EL ~1
400
( f)
Fig. 1. (a) Diagram explaining the model, (b)-(e) angular dependence of the volume and the surface modes at 300, 77, 20 and 4.2K The straight line is computed from Hs and Hu, and (f) KS versus T.
G. Swan et al. / Surface made in amarpltaus
1
0
1000
YCa3 films
I 2000
3000
4000
5 000 T’l[K
Fig. 2. T3j2 dependences
of 4n tiff
and the resonance
face mode is localised at the substrate film interface and is related to the formation of a thin layer at the outset of the deposition process. In D.C. sputtering the bias Vb at the substrate surface will appear only after a continuous layer of 200-300 A thick is formed. In this surface layer, the easy induced axis is in the film plane in contrary to the bulk of the sample where the easy axis is normal to the plane. Annealing experiments indicated the negative sign of K, in the surface Iayer. Indeed after annealing in vacuum the resonance field of the main mode increases, in the perpendicular orientation Hy (corresponding to the suppression of K,), whereas the field of the surface mode H”I decreases (diminution of ---KL). Also, several experiments reported in the literature show that K, is negative in the sputtered RE-Co amorphous films when I’, = 0. In the “as deposited” films of different thicknesses the field separation between the surface
fields for the volume
and surface
modes.
and the main modes is typically 1000 Oe and the intensity of the surface mode decreases with respect to that of the main one when the total thickness c of the film increases, and the resonance line width AH, of the surface mode is also independent of e. These results show that the thickness of the surface layer is constant and independent of the total thickness of the film. Finally the localisation at the substrate interface is confirmed by the fact that in very thin films (e = 400 to 500 A) only one voiume mode is observed at a field close to that of the surface mode of thick samples. Under this condition the structure of the film could be described by the model in fig. 1. It is composed of two exchange coupled layers a thin one of thickness e where (47rMS)zff = 4n Me + ]Hhl and a thicker one of thickness e-e where (4nMS)gff = 4n Me IHal. The validity of the model and the properties of the surface layer can be fully demonstrated by mea-
G. Suran et al. / Surface mode in amorphous YCo3 films
suring the angular dependence f(0) of the field for resonance of volume and surface modes, as was shown by Spalek et al. in their recent calculation [6]. This calculation is valid when e < e/10 and predicts the following: (a) the surface mode should be observed for 0 < @ < at(b) There exists a critical angle ac < n/4 for which only the uniform mode is observed. (c)y = d(Hs - H,) cos(@ - t?), (H, and H,, being surface and bulk mode fields) is a linear function of cos 2Q. (d) The thickness of the surface layer is given by E = c/cos 2Qc, where c =A [(&)zff - (&f&rr] /zK,. (e) The slope of y is given by fiK,/A where K, is the surface anisotropy. A set of experimental results at various temperatures obtained on a typical sample of 0.2 pm thick is given in fig. 1 with the as deduced value of E and K,. The validity of the model is supported by the excellent agreement between the computed straight line for y and the experimental points. The agreement of Q’cobtained from the computed line and that determined directly is within 0.2” at low temperature. The main result is an apparent decrease of the magnetic thickness of the surface layer and a slight increase of K, with decreasing temperature. A possible explanation of this result could be that the transition between the two layers is not a sharp one as predicted by the theoretical model. Measurements as a function of temperature showed that the surface layer presents some un-
181
expected low temperature anomalies as shown in figs. 2 and 3. Fig. 2 shows the evolution Hlf and @I and the temperature dependence of 4n Mzff corresponding to the bulk mode. 4n Kff obeys fairly well the T3j2 Bloch law, so the spin wave dispersion coefficient D and the exchange constant A were deduced from the slope of 4n MJ(T312). We found D = 270 meV A2 and A = ;DM, = 5.2 X lo-’ erg/cm at 4.2 K, values which were used to calculate K, and E. The comparison of Hy = f(T3i2) and PL = f(T3i2) is also interesting and it is remarked that the H”r has a stronger variation with T. This result indicates that Hi in the surface layer has a stronger temperature dependence than Ha in the bulk of the film. Fig. 3 shows the temperature dependence of AH, and AH, and the ratio of the intensities of the two modes Is/I, where the indices s and u refer to the surface and volume modes. AH, is temperature independent as is usually observed in good metallic films. However, AH, increases as the temperature decreases and shows a maximum near 40 K. This result is reminiscent of the behaviour of cobalt ions in ferrimagnetic insulators with longitudinal relaxation mechanism [7]. This would suggest that cobalt in the surface layer has an ionic character unlike in the bulk of the film where it has a covalent character. In conclusion, the model proposed by Spalek et al. can be well adapted to the study of surface mode in YCo3 amorphous films. Measurements as a function of temperature suggest that the origin of the induced anisotropy of opposite signs in the bulk and the thin layer is not the same. The technical assistance of Mr. Tessier is gratefully acknowledged. References I [ 1] N. Heinman, A. Onton, D: Keyser,, K. Lee and CR. Guar-
[2] [3] [4] [5] ’ b
0
100
200
[“I 306
Fig. 3. Peak to peak derivative line width of the surface and volume modes and the ratio of the intensity of the above two modes I,/ZUas a function of temperature.
[6] [7] [8]
nering, A.I.P. Conf. Proc. 24 (1974) 173; K. Lee, ), N. Heinman ibid p. 108. R.J. Gambino, P. Chadhari and J.J. Cuomo, A.I.P. C&if. Proc. 18 (1973) 578. A. Brusch and J. Schneider, J. Appl. Phys. 48 (1974) 108. G. Suran, R. Krishnan, H. Jouve and R. Meyer, IEEE Trans. Mag. MAG-13 (1977) 1332. R. Meyer, H. Jouve and J.P. RebouilIat, I.E.E.E. Trans. Mag. Mag. 11 (1975) 1335. J. Spalek andW. Schmidt, Sol. Stat. Comm. 16 (1975) 193. See for example, A.M. Clogston, Bell. Syst. Tech; J. 34 (1962) 739. R.J. Gambino, P. Chaudhari and J.J. Cuomo, A.I.P. Conf. Proc. 18 (1973) 578.
7 (1978)
178-181
EXCHANGE DOMINATED SURFACE MODES IN AMORPHOUS FERROMAGNETIC G. SURAN, R. KRISHNAN, Laboratoire de Magnthne,
YCo3
J. SZTERN
C.N.R.S.
92190 Meudon Bellevue, France
H. JOUVE and R. MEYER LETI, C.E. N. G. 85-X 38041
Grenoble Cedex, France
The ferromagnetic resonance spectra, in the band Ku of sputter deposited YCo3 amorphous films, show the presence of a surface mode which is due to a thin surface layer which forms at the initial stages of the deposition process. A simple model of two exchange coupled films enables us to determine as a function of temperature the magnetic properties and the effect of thickness of this surface layer. Induced anisotropy Ku in the bulk of the film and the thin surface layer has opposite signs and a comparative study of Ku = f(r) and the relaxation mechanism suggests that the origin of the Ku in the two cases could be different.
and K, in the bulk of the film
1. Introduction
2. Experimental
In transition metal (TM)-rare earth (RE) amorphous films the existence of a growth induced uniaxial anisotropy (K,) is a general feature [ 1,2]. However, the magnitude and sign of K, is largely dependent upon the film composition, the deposition method (evaporation or sputtering), the deposition parameters and the film thickness. The composition dependence of K, is largely complicated by the preferential oxidation of the magnetic rare earth constituent [3]. Under these conditions, it should be much easier to understand the main source of K, in an alloy where the RE is non magnetic, whence our choice of the YCo system. In YCo3 films, we have made a systematic study of K, as a function of deposition parameters, thickness and substrate material and some results have been reported earlier [4]. Here we present our investigations of the thin surface layer which seems to form right at the beginning of the sputtering process. In this surface layer KL is negative (easy plane) while in the bulk of the film K, is positive (easy axis normal to the film plane). It was felt that measurements as a function of temperature could throw some light on the question whether the same mechanism gives rise to K, of opposite signs, and here we describe the results.
Films of nominal composition YCo, were deposited by D.C. sputtering using Ar as sputtering gas [5]. Saturation magnetisation 47rM, and anisotropy field were determined by the ferromagnetic resonance techniques in the Ku band (f= 17.3 GHz) in the range 4.2 to 300 K. MS and K, were also controlled by the inplane loop tracer and static measurements. 4nM, and the anisotropy field Ha could be deduced from the resonance fields of the “as deposited” and annealed samples. After annealing (under vacuum at 200°C) K, in the bulk of the film was suppressed as confirmed also by the hysteresis measurements. In samples of thickness e larger than 0.2 pm, K, was in the range 2 to 8.104 erg/cm3 [4]. K, increases as the bias voltage Vb increases and decreases slightly with increasing film thickness and when the deposition temperature is decreased. This result is in favour of in plane Co-Co pair formation [4].
3. Surface mode: results, model and discussion Only for the perpendicular orientation of the film and for e > 0.2 1-1the surface mode is observed on the high field side of the main resonance mode. This sur178
G. &ran et al. j &face mode in arnor~~~~s ITo3 _Elms
179
*Y = d(Hs-Huk0+b43)
4
20-
20-
./
/
(c) lo-
0.75 I it
i?
d) ii
0, EL ~1
400
( f)
Fig. 1. (a) Diagram explaining the model, (b)-(e) angular dependence of the volume and the surface modes at 300, 77, 20 and 4.2K The straight line is computed from Hs and Hu, and (f) KS versus T.
G. Swan et al. / Surface made in amarpltaus
1
0
1000
YCa3 films
I 2000
3000
4000
5 000 T’l[K
Fig. 2. T3j2 dependences
of 4n tiff
and the resonance
face mode is localised at the substrate film interface and is related to the formation of a thin layer at the outset of the deposition process. In D.C. sputtering the bias Vb at the substrate surface will appear only after a continuous layer of 200-300 A thick is formed. In this surface layer, the easy induced axis is in the film plane in contrary to the bulk of the sample where the easy axis is normal to the plane. Annealing experiments indicated the negative sign of K, in the surface Iayer. Indeed after annealing in vacuum the resonance field of the main mode increases, in the perpendicular orientation Hy (corresponding to the suppression of K,), whereas the field of the surface mode H”I decreases (diminution of ---KL). Also, several experiments reported in the literature show that K, is negative in the sputtered RE-Co amorphous films when I’, = 0. In the “as deposited” films of different thicknesses the field separation between the surface
fields for the volume
and surface
modes.
and the main modes is typically 1000 Oe and the intensity of the surface mode decreases with respect to that of the main one when the total thickness c of the film increases, and the resonance line width AH, of the surface mode is also independent of e. These results show that the thickness of the surface layer is constant and independent of the total thickness of the film. Finally the localisation at the substrate interface is confirmed by the fact that in very thin films (e = 400 to 500 A) only one voiume mode is observed at a field close to that of the surface mode of thick samples. Under this condition the structure of the film could be described by the model in fig. 1. It is composed of two exchange coupled layers a thin one of thickness e where (47rMS)zff = 4n Me + ]Hhl and a thicker one of thickness e-e where (4nMS)gff = 4n Me IHal. The validity of the model and the properties of the surface layer can be fully demonstrated by mea-
G. Suran et al. / Surface mode in amorphous YCo3 films
suring the angular dependence f(0) of the field for resonance of volume and surface modes, as was shown by Spalek et al. in their recent calculation [6]. This calculation is valid when e < e/10 and predicts the following: (a) the surface mode should be observed for 0 < @ < at(b) There exists a critical angle ac < n/4 for which only the uniform mode is observed. (c)y = d(Hs - H,) cos(@ - t?), (H, and H,, being surface and bulk mode fields) is a linear function of cos 2Q. (d) The thickness of the surface layer is given by E = c/cos 2Qc, where c =A [(&)zff - (&f&rr] /zK,. (e) The slope of y is given by fiK,/A where K, is the surface anisotropy. A set of experimental results at various temperatures obtained on a typical sample of 0.2 pm thick is given in fig. 1 with the as deduced value of E and K,. The validity of the model is supported by the excellent agreement between the computed straight line for y and the experimental points. The agreement of Q’cobtained from the computed line and that determined directly is within 0.2” at low temperature. The main result is an apparent decrease of the magnetic thickness of the surface layer and a slight increase of K, with decreasing temperature. A possible explanation of this result could be that the transition between the two layers is not a sharp one as predicted by the theoretical model. Measurements as a function of temperature showed that the surface layer presents some un-
181
expected low temperature anomalies as shown in figs. 2 and 3. Fig. 2 shows the evolution Hlf and @I and the temperature dependence of 4n Mzff corresponding to the bulk mode. 4n Kff obeys fairly well the T3j2 Bloch law, so the spin wave dispersion coefficient D and the exchange constant A were deduced from the slope of 4n MJ(T312). We found D = 270 meV A2 and A = ;DM, = 5.2 X lo-’ erg/cm at 4.2 K, values which were used to calculate K, and E. The comparison of Hy = f(T3i2) and PL = f(T3i2) is also interesting and it is remarked that the H”r has a stronger variation with T. This result indicates that Hi in the surface layer has a stronger temperature dependence than Ha in the bulk of the film. Fig. 3 shows the temperature dependence of AH, and AH, and the ratio of the intensities of the two modes Is/I, where the indices s and u refer to the surface and volume modes. AH, is temperature independent as is usually observed in good metallic films. However, AH, increases as the temperature decreases and shows a maximum near 40 K. This result is reminiscent of the behaviour of cobalt ions in ferrimagnetic insulators with longitudinal relaxation mechanism [7]. This would suggest that cobalt in the surface layer has an ionic character unlike in the bulk of the film where it has a covalent character. In conclusion, the model proposed by Spalek et al. can be well adapted to the study of surface mode in YCo3 amorphous films. Measurements as a function of temperature suggest that the origin of the induced anisotropy of opposite signs in the bulk and the thin layer is not the same. The technical assistance of Mr. Tessier is gratefully acknowledged. References I [ 1] N. Heinman, A. Onton, D: Keyser,, K. Lee and CR. Guar-
[2] [3] [4] [5] ’ b
0
100
200
[“I 306
Fig. 3. Peak to peak derivative line width of the surface and volume modes and the ratio of the intensity of the above two modes I,/ZUas a function of temperature.
[6] [7] [8]
nering, A.I.P. Conf. Proc. 24 (1974) 173; K. Lee, ), N. Heinman ibid p. 108. R.J. Gambino, P. Chadhari and J.J. Cuomo, A.I.P. C&if. Proc. 18 (1973) 578. A. Brusch and J. Schneider, J. Appl. Phys. 48 (1974) 108. G. Suran, R. Krishnan, H. Jouve and R. Meyer, IEEE Trans. Mag. MAG-13 (1977) 1332. R. Meyer, H. Jouve and J.P. RebouilIat, I.E.E.E. Trans. Mag. Mag. 11 (1975) 1335. J. Spalek andW. Schmidt, Sol. Stat. Comm. 16 (1975) 193. See for example, A.M. Clogston, Bell. Syst. Tech; J. 34 (1962) 739. R.J. Gambino, P. Chaudhari and J.J. Cuomo, A.I.P. Conf. Proc. 18 (1973) 578.