- Email: [email protected]
Tb multilayers
Journal of Magnetism and Magnetic Materials 156 (1996) 181 - 183
A~
~FBFi Jeurnal of
- ~ . " magnetism and magnetic malerlals
ELSEVIER
Magnetic anisotropy in amorphous Fe/Tb multilayers F. R i c h o m m e a j. T e i l l e t a,* A. F n i d i k i a j . p . L e b e r t o i s a p. A u r i c b p, V e i l l e t c LMA, URA CNRS 808, FacultF des Sciences de Rouen, 76821 Mont-Saint-Aignan CFdex, France 6 D R F M C / P h y s i q u e / M D T H , CENG. BP 85X, 38041 Grenoble CFdex, France c hzstitut d'Electronique Fondamentale, 91405 Orsay C('dex, France
Abstract F e / T b multilayers with fixed Tb thickness (1.0 nm) and various Fe thicknesses (0.7-2.4 nm) in the domain where both Fe and Tb layers are amorphous, have been investigated by CEMS and magnetic measurements in the 4-300 K range. The results on magnetic anisotropy are interpreted taking into account the composition modulation in the multilayers and the dominant magnetic subnetwork.
1. Introduction R E / T M magnetic multilayers are extensively studied because of their potential applications for perpendicular magneto-optic recording technology. Previously, from the determination of Curie temperatures [1] and irradiation experiments [2], we showed the composition modulation in the amorphous sputtered F e / T b multilayers with the appearance of pure amorphous iron at the centre of iron layer from a Fe thickness (eFe) threshold. Here, we present a study of the magnetic anisotropy taking into account this composition modulation in amorphous multilayers.
3. Results and discussion
2. Experimental procedures T b / F e multilayers were evaporated at room temperature on Si (111) substrates using a reactive diode rfsputtering system according to a procedure previously published [3]. To avoid the oxidation and corrosion of the samples, 10 nm Si3N 4 was deposited both before and after the deposition of the T b / F e stack. The layer thicknesses and the global structure of the films were determined by grazing X-ray reflectometry [4] and high angle X-ray diffraction. The number of periods was between 60 and 102. The magnetic properties of the multilayers were studied by temperature dependent CEMS and magnetization measurements in the 4 - 3 0 0 K range. We will focus here on a series of amorphous samples with fixed terbium layers ( e T b = 1.0 nm) and varying iron thicknesses (0.7 < eF~ < 2.4 nm).
Corresponding author. jacques.teillet @univ-rouen.fr.
Fax:
The magnetic measurements were performed with an applied field perpendicular to the layers. The magnetization M was obtained with a magnetic field of 1.0 T. At room temperature, the coercive fields H~ are deduced from hysteresis loops. Because of the various environments of Fe atoms in amorphous compounds, the Mfssbauer spectra were fitted using a distribution of hyperfine fields. The MiSssbauer angle /3 is deduced from the relative intensities of the 2nd and the 5th Mfssbauer lines of each elementary sextuplet and measures the mean angle between the iron magnetic moments and the normal to the film plane.
+ 33-3514-6652;
email:
A - The mean Tb atomic fraction X dependence of M, H~ and 13 at room temperature are reported in Fig. 1. The curves exhibit a compensation point ( Xc,,mp = 19.8 + 0.9%, eft = 1.5 ___0.1 nm) where M = 0 and H c is a maximum indicating the change of the dominant sublattice at room temperature. At this point, the demagnetizing field, proportional to the magnetization, vanishes, then /3 tends towards zero indicating a perfect perpendicular magnetic anisotropy (PMA) related to the interfaces anisotropy and the T b - F e ferrimagnetic exchange coupling. For comparison, F e - T b amorphous alloys do not exhibit a perfect PMA at the compensation point but only a minimum value (/3 ~ 20 °) [5]. For X higher than Xcomp, i.e. when ev:c decreases, /3 seems to follow the same increase as M. So, the increasing demagnetizing field produces a continuous departure of iron moments from the perpendicular direction. For eft = 0.7 nm (X = 34.5%), /3 (43 °) is very close to the value of the corresponding F e - T b amorphous alloys [5]. The composition modulation becomes smooth and the effect of interfaces is reduced. Down to Xcomp, the iron sublattice dominates magnetically, eF~ increases and the
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 2 5 - X
F. Richomme et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 181-183
182
350
90
F"\
2500 o
80
300
\
M /
70
50
/
250
,~ 6o ~
2000
,J /
I I /
•E2O0
1500
I
A
O
150
1000
I
~ 3o 2O
100500
_
~
~
~
~
"
0500
10 0
10
20
30 X (at % Tb)
40
Fig. I. M ( ,x ), Hc ( • ) and fl (°) versus the mean Tb atomic fraction X at room temperature.
pure amorphous iron layer enlarges. The demagnetizing field is still present. When the pure iron layer thickness is large enough (eFe > 1.7 nm, X < 17.7%), due to a strong decrease of Tc, the magnetization decreases. Nevertheless, /3 continues to increase up to 90 ° which is a much larger value than in amorphous alloys at the same composition ( X = 14%, /3 = 23 ° [5]) where the Fe sublattice is also magnetically dominant. This could be explained by the reduction of magnetic interactions between the F e - T b
interfaces related to the non-magnetic layer of pure amorphous iron (Tc = 200 K [6]). B - In Fig. 2, we report the temperature dependence of /3 for some samples of the series. At 20 K and for eFe < 1.7 nm, fl is close to 30 °. For these multilayers, the Tb sublattice magnetically dominates at low temperature (Fig. 3) and enforces its distributed spin structure, due to the structural disorder at the interfaces, via the F e - T b ferrimagnetic coupling. Increasing the temperature, the Fe
500
90
/10 K 80
400
7O 6O ca
=
300
50 O
_,3 4o
E
m
,~ 30
i . d 300 K
J
~E 200
k--II
i/li// / /~rl l
20
100
10
0
I 50
I 100
I I 150 200 T (K)
I 250
-t~ 300
\ / ', / // ',,'
l 350
Fig. 2. Temperature dependence of /3 for • 0.9 nm Fe (29.0%), [] 1.3 nm Fe (22.1%), • 1.7 nm Fe (17.7%), ~ 2.2 nm Fe (14.0%).
o
- - 4
10
J
~'
20
~-
I
-
,
30 X (at % Tb)
Fig. 3. M as a function of X at 300 K and l 0 K.
4-
40
F. Richomme et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 181 - 183
magnetic moments progressively rotate towards the orientation at 300 K, according to the decrease of Tb moments. For eFe = 2.2 nm, pure amorphous iron part in the center of Fe layer is thick and the Fe subnetwork dominates at every temperature (Fig. 3). At low temperature, /3 is larger than for thinner eFe because the planar anisotropy of pure amorphous iron which is magnetic and dominant is added to the random anisotropy of Tb at the interfaces (30°). When the temperature increases, /3 remains constant up to 200 K and then increases sharply towards the value at 300 K. This could be attributed to the decreasing of interlayer coupling and of the random anisotropy of Tb at the interfaces due to the decreasing of iron and terbium magnetic moments. 4. Conclusion A strong PMA related to the interface anisotropy has been evidenced at room temperature at the compensation point where the demagnetizing field has no effect. The role
183
and the temperature dependence of the interlayer coupling, shape anisotropy of individual layers and the strong local anisotropy of Tb at the interfaces were pointed out taking into account the composition modulation. A c k n o w l e d g e m e n t : This work was supported by the French Ministry of Research under the MRES Contract No. 92S0151. References [1] F. Richomme, A. Fnidiki, P. Auric, J. Teillet, P. Boher and Ph. Houdy, Hyperfine Interactions 92 (1994) 1243. [2] F. Richomme, J. Teillet, P. Auric, P. Veillet, A. Fnidiki, Ph. Houdy and P. Boher, J. Magn. Magn. Mater. 140-144 (1995) 627. [3] J. Teillet, A. Fnidiki, F. Richomme, P. Boher and Ph. Houdy, J. Magn. Magn. Mater. 123 (1993) 359, [4] F. Pierre, P. Boher, H. Kergoat, Ph. Houdy, J. Ferr~ and G. Penissard, J. Appl. Phys. 69 (1991)4565. [5] V.S. Rusakov, B.S. Vvedensky, E.T. Voropaeva and E.N. Nikolaev, IEEE Trans. Magn. 28 (1992) 2524. [6] J.J. Rhyne, IEEE Trans. Magn. 2t (1985) 1990.
A~
~FBFi Jeurnal of
- ~ . " magnetism and magnetic malerlals
ELSEVIER
Magnetic anisotropy in amorphous Fe/Tb multilayers F. R i c h o m m e a j. T e i l l e t a,* A. F n i d i k i a j . p . L e b e r t o i s a p. A u r i c b p, V e i l l e t c LMA, URA CNRS 808, FacultF des Sciences de Rouen, 76821 Mont-Saint-Aignan CFdex, France 6 D R F M C / P h y s i q u e / M D T H , CENG. BP 85X, 38041 Grenoble CFdex, France c hzstitut d'Electronique Fondamentale, 91405 Orsay C('dex, France
Abstract F e / T b multilayers with fixed Tb thickness (1.0 nm) and various Fe thicknesses (0.7-2.4 nm) in the domain where both Fe and Tb layers are amorphous, have been investigated by CEMS and magnetic measurements in the 4-300 K range. The results on magnetic anisotropy are interpreted taking into account the composition modulation in the multilayers and the dominant magnetic subnetwork.
1. Introduction R E / T M magnetic multilayers are extensively studied because of their potential applications for perpendicular magneto-optic recording technology. Previously, from the determination of Curie temperatures [1] and irradiation experiments [2], we showed the composition modulation in the amorphous sputtered F e / T b multilayers with the appearance of pure amorphous iron at the centre of iron layer from a Fe thickness (eFe) threshold. Here, we present a study of the magnetic anisotropy taking into account this composition modulation in amorphous multilayers.
3. Results and discussion
2. Experimental procedures T b / F e multilayers were evaporated at room temperature on Si (111) substrates using a reactive diode rfsputtering system according to a procedure previously published [3]. To avoid the oxidation and corrosion of the samples, 10 nm Si3N 4 was deposited both before and after the deposition of the T b / F e stack. The layer thicknesses and the global structure of the films were determined by grazing X-ray reflectometry [4] and high angle X-ray diffraction. The number of periods was between 60 and 102. The magnetic properties of the multilayers were studied by temperature dependent CEMS and magnetization measurements in the 4 - 3 0 0 K range. We will focus here on a series of amorphous samples with fixed terbium layers ( e T b = 1.0 nm) and varying iron thicknesses (0.7 < eF~ < 2.4 nm).
Corresponding author. jacques.teillet @univ-rouen.fr.
Fax:
The magnetic measurements were performed with an applied field perpendicular to the layers. The magnetization M was obtained with a magnetic field of 1.0 T. At room temperature, the coercive fields H~ are deduced from hysteresis loops. Because of the various environments of Fe atoms in amorphous compounds, the Mfssbauer spectra were fitted using a distribution of hyperfine fields. The MiSssbauer angle /3 is deduced from the relative intensities of the 2nd and the 5th Mfssbauer lines of each elementary sextuplet and measures the mean angle between the iron magnetic moments and the normal to the film plane.
+ 33-3514-6652;
email:
A - The mean Tb atomic fraction X dependence of M, H~ and 13 at room temperature are reported in Fig. 1. The curves exhibit a compensation point ( Xc,,mp = 19.8 + 0.9%, eft = 1.5 ___0.1 nm) where M = 0 and H c is a maximum indicating the change of the dominant sublattice at room temperature. At this point, the demagnetizing field, proportional to the magnetization, vanishes, then /3 tends towards zero indicating a perfect perpendicular magnetic anisotropy (PMA) related to the interfaces anisotropy and the T b - F e ferrimagnetic exchange coupling. For comparison, F e - T b amorphous alloys do not exhibit a perfect PMA at the compensation point but only a minimum value (/3 ~ 20 °) [5]. For X higher than Xcomp, i.e. when ev:c decreases, /3 seems to follow the same increase as M. So, the increasing demagnetizing field produces a continuous departure of iron moments from the perpendicular direction. For eft = 0.7 nm (X = 34.5%), /3 (43 °) is very close to the value of the corresponding F e - T b amorphous alloys [5]. The composition modulation becomes smooth and the effect of interfaces is reduced. Down to Xcomp, the iron sublattice dominates magnetically, eF~ increases and the
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 8 2 5 - X
F. Richomme et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 181-183
182
350
90
F"\
2500 o
80
300
\
M /
70
50
/
250
,~ 6o ~
2000
,J /
I I /
•E2O0
1500
I
A
O
150
1000
I
~ 3o 2O
100500
_
~
~
~
~
"
0500
10 0
10
20
30 X (at % Tb)
40
Fig. I. M ( ,x ), Hc ( • ) and fl (°) versus the mean Tb atomic fraction X at room temperature.
pure amorphous iron layer enlarges. The demagnetizing field is still present. When the pure iron layer thickness is large enough (eFe > 1.7 nm, X < 17.7%), due to a strong decrease of Tc, the magnetization decreases. Nevertheless, /3 continues to increase up to 90 ° which is a much larger value than in amorphous alloys at the same composition ( X = 14%, /3 = 23 ° [5]) where the Fe sublattice is also magnetically dominant. This could be explained by the reduction of magnetic interactions between the F e - T b
interfaces related to the non-magnetic layer of pure amorphous iron (Tc = 200 K [6]). B - In Fig. 2, we report the temperature dependence of /3 for some samples of the series. At 20 K and for eFe < 1.7 nm, fl is close to 30 °. For these multilayers, the Tb sublattice magnetically dominates at low temperature (Fig. 3) and enforces its distributed spin structure, due to the structural disorder at the interfaces, via the F e - T b ferrimagnetic coupling. Increasing the temperature, the Fe
500
90
/10 K 80
400
7O 6O ca
=
300
50 O
_,3 4o
E
m
,~ 30
i . d 300 K
J
~E 200
k--II
i/li// / /~rl l
20
100
10
0
I 50
I 100
I I 150 200 T (K)
I 250
-t~ 300
\ / ', / // ',,'
l 350
Fig. 2. Temperature dependence of /3 for • 0.9 nm Fe (29.0%), [] 1.3 nm Fe (22.1%), • 1.7 nm Fe (17.7%), ~ 2.2 nm Fe (14.0%).
o
- - 4
10
J
~'
20
~-
I
-
,
30 X (at % Tb)
Fig. 3. M as a function of X at 300 K and l 0 K.
4-
40
F. Richomme et al. / Journal of Magnetism and Magnetic Materials 156 (1996) 181 - 183
magnetic moments progressively rotate towards the orientation at 300 K, according to the decrease of Tb moments. For eFe = 2.2 nm, pure amorphous iron part in the center of Fe layer is thick and the Fe subnetwork dominates at every temperature (Fig. 3). At low temperature, /3 is larger than for thinner eFe because the planar anisotropy of pure amorphous iron which is magnetic and dominant is added to the random anisotropy of Tb at the interfaces (30°). When the temperature increases, /3 remains constant up to 200 K and then increases sharply towards the value at 300 K. This could be attributed to the decreasing of interlayer coupling and of the random anisotropy of Tb at the interfaces due to the decreasing of iron and terbium magnetic moments. 4. Conclusion A strong PMA related to the interface anisotropy has been evidenced at room temperature at the compensation point where the demagnetizing field has no effect. The role
183
and the temperature dependence of the interlayer coupling, shape anisotropy of individual layers and the strong local anisotropy of Tb at the interfaces were pointed out taking into account the composition modulation. A c k n o w l e d g e m e n t : This work was supported by the French Ministry of Research under the MRES Contract No. 92S0151. References [1] F. Richomme, A. Fnidiki, P. Auric, J. Teillet, P. Boher and Ph. Houdy, Hyperfine Interactions 92 (1994) 1243. [2] F. Richomme, J. Teillet, P. Auric, P. Veillet, A. Fnidiki, Ph. Houdy and P. Boher, J. Magn. Magn. Mater. 140-144 (1995) 627. [3] J. Teillet, A. Fnidiki, F. Richomme, P. Boher and Ph. Houdy, J. Magn. Magn. Mater. 123 (1993) 359, [4] F. Pierre, P. Boher, H. Kergoat, Ph. Houdy, J. Ferr~ and G. Penissard, J. Appl. Phys. 69 (1991)4565. [5] V.S. Rusakov, B.S. Vvedensky, E.T. Voropaeva and E.N. Nikolaev, IEEE Trans. Magn. 28 (1992) 2524. [6] J.J. Rhyne, IEEE Trans. Magn. 2t (1985) 1990.