- Email: [email protected]
The role of local anisotropy in the origin of in-plane uniaxial anisotropy in amorphous CoZrNd thin films
~14 Journalof magnetism ,~
ELSEVIER
Journal of Magnetismand Magnetic Materials 152 (1996) 17-21
and
magnetic ~ i ~ materials
The role of local anisotropy in the origin of in-plane uniaxial anisotropy in amorphous CoZrNd thin films J.F. Calleja a, M.C. Contreras a.., M. Rivas a, J.A. Corrales a, G. Suran b, H. Ouahmane b a Departamento de Fisica de la Universidad de Oviedo, E-33007 Oviedo, Spain b Laboratoire de Magn£tisme et Mat~riaux Magn~tiques CNRS 92195 Meudon CEDEX, France
Received 4 January 1995; revised 14 April 1995
Abstract The magnetic properties of if-sputtered amorphous (Co93Zr7)lOo_xNd x thin films with 0 < x < 3.5 were investigated by transverse biased initial susceptibility (TBIS) measurements at both film/air and glass/film interfaces. The films exhibit a very well defined in-plane uniaxial anisotropy with negligible long-range fluctuations. A clear relationship could be established between Ku and Klo¢, data which is discussed in terms of the single-ion anisotropy of the Nd ions. Although the value of H k is the same at both interfaces, the as-determined ripple constants were found to be slightly different. This last result is believed to be related to some local defect, the origin of which is also advanced.
1. Introduction Thin films of amorphous CoZrRE have been the subject of recent papers [1-3]. These films possess magnetic properties that make them promising candidates to be employed in magnetic recording, particularly in magnetic heads: good frequency response of the permeability [4], high saturation magnetization, low coercivity, low magnetostriction [5], good mechanical properties [6] and a very well defined uniaxial anisotropy [1,2]. On the other hand, the films under study in the present paper are very interesting for studying the physical origin of in-plane uniaxial anisotropy in thin films. Thin amorphous T M - R E films have been the subject of many investigations. Such films obtained
* Corresponding author. Email: [email protected]; fax: +34 85 103324.
generally exhibit perpendicular anisotropy, the origin of which is partly related to macroscopic contributions induced by stress, columnar growth, etc. However, the uniaxial in-plane anisotropy K u is essentially related to the local structure; the local anisotropy in T M - R E amorphous alloys caused by spin-orbit coupling has been studied by several authors [7]. Although many investigations have been undertaken to determine the physical origin of K u, it is still poorly understood. It would be desirable to find a relationship between the uniaxial and local anisotropies. However, in most cases, the samples investigated exhibit defects related to the fabrication processes that mask the intrinsic perturbations due to the local anisotropy. Consequently, the deposition parameters must be chosen carefully. The films investigated in this work are a series of (Co93Zr7)10o_xNd x with 0 < x < 3.4. From our study we deduce the dependence of Ku on the local anisotropy.
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00440-8
18
J.F. Calleja et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 17-21
2. Experimental procedure The films were deposited by rf co-sputtering. The deposition was performed in a dc magnetic field of 700 Oe parallel to the film surface and we used a magnetic shield that constrains the field in the substrate region and does not affect the plasma. With this configuration, a uniaxial in-plane anisotropy develops. The magnitude of K u and the homogeneity of the amorphous structure depend on the sputter gas pressure used: PAr" The results reported in this paper correspond to PAr = 2 mTorr; for this value the amorphous structure appears to be the most homogeneous [8]. The other deposition parameters were a background pressure of 10 -7 Torr and a rf input power of 300 W, resulting in a deposition rate of 100 fk/min. The saturation magnetization M s was measured by VSM and the exchange stiffness constant by FMR. The uniaxial in-plane anisotropy and the micromagnetic properties were determined using the magneto-optical Kerr effect (MOKE) to perform transverse biased initial susceptibility (TBIS) measurements. The same MOKE system was used to obtain the in-plane dc hysteresis loops of the samples which allowed us to measure the coercive field H c (dc).
3. Theoretical considerations The existence of K u allows us to analyze the experimental results obtained in TBIS measurements by Hoffmann's ripple theory [9]. In this theory, as a consequence of taking into account all the components contributing to the local energy (external field, uniaxial anisotropy, exchange, local anisotropy and magnetostatic energy) the magnetization cannot suffer abrupt changes in direction, but it is correlated in the so-called coupling volumes. Following Hoffmann, the inverse of the transverse initial susceptibility may be expressed as [10]
1
s2Msv
b = 4"rrx/-2 ( a K u ) 5/4'
(2)
where the local anisotropy can thus be estimated according to [10] 3Kloed 3/2
S
1~
'
(3)
where Kioc is the local anisotropy constant, t is the thickness and d is the correlation length of local anisotropies, which is typically 10 ,~ in an amorphous solid. A careful analysis of the TBIS curves allows us to determine very precisely the three parameters Hk, b and c [1]. To get the value of the anisotropy field H k from the experimental data we used the procedure proposed by Feldtkeller [11]. The contribution of the skew was negligible in all the samples; effectively, the variations of the experimental X-1(/3)(h + 1) 1/4 versus (h + 1) 5/4 follows a linear law, as shown in Fig. 1 for the case x = 1.1. In this case the ripple parameter b corresponds to the extrapolated value of X - I ( / 3 X h -t- 1) 1/4 for (h 5: 1) 5/4 = 0.
4. Results and discussion
x;1(/3) ~--~k{(h + 1 ) + b ( h ±
where the plus sign corresponds to /3 = 0 (dc bias field applied along the easy axis), and the minus sign to /3 = xr/2 (dc bias field applied along the hard axis). M s is the saturation magnetization, H k = 2 K , / M s the uniaxial macroscopic anisotropy field, and h = H d c / H k, where Hdc is the applied field. The first term in Eq. (1) corresponds to the coherent rotation process (Stoner-Wohlfarth model); the second term describes the contribution of the ripple related to short-range fluctuations of the magnetization; and the third term is the 'skew' term caused by long-range fluctuations of the induced magnetic anisotropy. Information on the micromagnetic parameters is obtained from the ripple parameter b, related to the structure constant S by
1) - ' / 4 + c ( h s :
1)-1},
(1)
We have performed TBIS measurements at both film/air ( f / a ) and glass/film ( g / f ) interfaces. Effectively, as we shall see, TBIS measurements are sensitive enough to appreciate slight differences be-
J.F. Calleja et al./ Journal of Magnetism and Magnetic Materials 152 (1996) 17-21
19
3,000
°..
2.500
3 ~r'"
"~
e~
A. A
2.000 £x. ~ "
2
I ~00
"t
+~ . z s ~s " r ' "
"7 - 1.000
50o
I 2
I 4
I 6 (h+l)
I 8
I 10
I I 12
tween the two interfaces. We first consider the trends which become apparent from a study of the macroscopic magnetic parameters reported in Table 1. The coercive force is fairly low and varies in the range 0 . 5 - 2 Oe. H~ and thus K u have nearly the same values at the two interfaces. The data obtained here and those determined by VSM a n d / o r FMR [12] are in fairly good agreement. This confLrrns the high overall chemical uniformity of the films, as TBIS measurements refer to the surface and VSM and FMR to the bulk. The variations of K~ as a function of x for the two interfaces are shown in Fig. 2. The experimental results show that K~ varies linearly as a function of x. The ripple parameter b varies randomly with Nd content, a result also found for Tb [1]. For a given sample the value of b at the g / f interface is systematicaily lower than that at the f / a interface. From b the structure constant S, and finally Klocd 3/2 can be deduced. K~o¢ d 3/2 increases with Nd content at both
I 3
X
5/4
Fig. 1. Experimental curves of X-l(/3Xh+l) 1/4 versus (h+ 1)5/4 and linear extrapolations for (C093Zr7)98.9Ndl.i. Triangles represent /3 = O; squares fl = ~r/2.
[ 2
Fig. 2. Nd content dependence of the induced uniaxial anisotropy. Squares and the solid line refer to the f/a interface; triangles and the dashed line to the g/f interface.
interfaces, but it attains greater values at the f / a interface. The overall data indicate that the dominant contribution to the uniaxial anisotropy is related to the single-ion anisotropy of Nd via spin-orbit coupling. The linear increase in K u with increasing Nd concentration can be explained by a theoretical computation using the point charge approximation and based upon the hypothesis that K u is related to the singleion anisotropy of Nd via spin-orbit coupling [13]. The relationship between Ku and Klo¢ is shown in Fig. 3. The proportionality found between K u and Kloc is in accordance with the theoretical model proposed for the formation of the in-plane uniaxial anisotropy [13]; K~ results from a directional deformation along the applied field during deposition of the local sites involving Nd ions in order to minimize the full magnetic energy acting on the sample. Finally, we consider the somewhat surprising outcome that although the value of H~ is practically the
Table 1 Magnetic properties of amorphous (Co93Zr7)lOo_xNdx thin films. When two values are given they axe in the order (f/a)/(g/f) x t 41rM s Hk K a X 10 -4 b flocd3/2 X 104 0 0.2 0.5 1.1 2.2 3.4
(,~,)
(G)
(Oe)
(erg/cm 3)
2000 1500 1600 1950 1950 1800
12 947 13 110 12524 12180 11751 11350
27.7/30.1 30.6/32.3 35.2/34.9 42.7/42.7 55.4/53.2 66.1/63.1
1.42/ 1.55 1.59/ 1.68 1.75/1.74 2.07/2.07 2.59/2.49 2.98/2.85
(erg/cm 3/2)
0.14/0.06 0.18/0.08 0.13/0.11 0.16/0.09 0.14/0.06 0.29/0.08
3.51/2.42 3.95/2.72 3.70/3.39 4.85/3.64 5.31/3.39 8.70/4.44
20
J.F. Calleja et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 17-21
content in the film. During the sputtering process some increase in the effective deposition temperature is practically unavoidable. Consequently the major part of the difference between the values of K~o~ d 3/2 measured at the f / a and g / f interfaces could be related to the fact that the value of d at the f / a interface is higher than at the g / f interface, and this difference increases for higher Nd contents.
,,'"
o
zx ,""
o
,'"
o
I o
I
i 3
I 4
i 5
i 6
I 7
I 8
i 9
10
Kloc d 3/2 X 1 0 4 (erg/cm~ ) Fig. 3. Local anisotropy dependence of the induced uniaxial anisotropy. Squares and solid line refer to the f / a interface; triangles and the dashed line refer to the g / f interface.
same at the two interfaces, the dispersion of the magnetization which is characterized by the ripple parameter b is slightly but systematically higher at the f / a interface than at the g / f interface. Actually, we do not have a definite interpretation of these data but several hypotheses can be advanced. One possibility is that some contribution to the local anisotropy Klo~ occurs without affecting Ku. This could be related to the formation of some microstructural defects that are random from site to site, and so modify Klo¢, but do not affect the magnitude of K~. In favour of this mechanism is the result that the dispersion of H k is slightly bigger at the f / a interface. The defects could be related, for example, to an oxidation process which occurs on the surface when the films are exposed to air after deposition [14]. Previous investigations [15] have shown that as a result of such oxygen contamination occurring at room temperature a certain amount of metallic atoms will be transformed to an ionic state, but will remain exchange coupled with the metallic part of the films, so essentially K~o~ is modified. An alternative interpretation that can not be excluded at the present stage of the investigations, is one that takes into account the growth mechanism of the films. Experiments performed on CoZrDy films [16] showed that the orientational correlation length of the local anisotropy, so that the value of d increases with increasing deposition temperature; the larger this variation, the higher is the rare earth
5. Conclusions The magnetic properties of amorphous CoZrNd thin films were investigated by transverse biased initial susceptibility (TBIS) measurements. The measurements were carried out at both glass/film and film/air interfaces. The films exhibit a very well defined in-plane uniaxial anisotropy with negligible long-range fluctuations. This anisotropy is the same at both interfaces and its origin is related to the single-ion anisotropy of Nd ions through spin-orbit coupling. The local anisotropy was determined and different values are obtained at each interface. A clear relationship between K u and K~oc has been established that allowed us to discuss the origin of KIoc; it results from a combination of single-ion anisotropy of sites involving Nd ions and other contributions related to defects in the structure a n d / o r composition. The latter contributions do not contribute to the magnitude of the uniaxial anisotropy. An alternative explanation is based upon the hypothesis that the value of d is different at the f / a and g / f interfaces.
Acknowledgements This work is partially supported by the Comisi6n Interministerial de Ciencia y Tecnologla (CICYT) MAT92-0778 and MAT93-1431.
References [1] G. Suran, H. Ouahmane, I. Iglesias, M. Rivas, J.A. Corrales and M.C. Contreras, J. Appl. Phys. 76 (1994) 1749. [2] G. Suran, H. Ouahmane, M. Rivoire and J. Sztern, J. Appl. Phys. 73 (1993) 5721.
J.F. Calleja et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 17-21 [3] M. Rivoire, H. Ouahmane, G. Suran, M. Rivas, J.A. Corrales and M.C. Contreras, IEEE Trans. Magn. 29 (1993) 3482. [4] J. Russat, G. Suran, H. Ouahmane, M. Rivoire and J. Sztern, J. Appl. Phys. 73 (1993) 1386. [5] H. Warlimont, Prog. Rare Earth Sci. Technol. II (1968) 1. [6] A. Tago, C. Nishimura and K. Yanagisawa, IEEE Trans. Magn. 21 (1985) 2032. [7] C. Els~isser, M. F~mle, E.H. Brandt and M.C. B~hm, J. Phys. F: Metal Phys. 18 (1988) 2463. [8] G. Suran, K. Ounadjela and F. Machizaud, Phys. Rev. Lett. 57 (1986) 3109. [9] H. Hoffmann, IEEE Trans. Magn. 4 (1968) 32.
21
[10] H. Hoffmann, Phys. Stat. Solidi 33 (1969) 175. [11] E. Feldtkeller, Z. Phys. 176 (1963) 510. [12] G. Suran, H. Ouahmane and J. Sztem, J. Magn. Magn. Mater. 120 (1993) 232. [13] Y. Suzuki, S. Takayama, F. Kirino and N. Ohta, IEEE Trans. Magn. 23 (1987) 2275. [14] M.C. Contreras, I. Iglesias, J.M. Alameda and M. Fernandez, IEEE Trans. Magn. 28 (1992) 3243. [15] G. Suran, B. Prasad, H. Jouve and R. Meyer, J. Appl. Phys. 50 (1979) 1617. [16] G. Suran, K. Roky, J. Sztem, F. Machizaud and J.M. Mackowski, IEEE Trans. Magn. 30 (1994) 4770.
ELSEVIER
Journal of Magnetismand Magnetic Materials 152 (1996) 17-21
and
magnetic ~ i ~ materials
The role of local anisotropy in the origin of in-plane uniaxial anisotropy in amorphous CoZrNd thin films J.F. Calleja a, M.C. Contreras a.., M. Rivas a, J.A. Corrales a, G. Suran b, H. Ouahmane b a Departamento de Fisica de la Universidad de Oviedo, E-33007 Oviedo, Spain b Laboratoire de Magn£tisme et Mat~riaux Magn~tiques CNRS 92195 Meudon CEDEX, France
Received 4 January 1995; revised 14 April 1995
Abstract The magnetic properties of if-sputtered amorphous (Co93Zr7)lOo_xNd x thin films with 0 < x < 3.5 were investigated by transverse biased initial susceptibility (TBIS) measurements at both film/air and glass/film interfaces. The films exhibit a very well defined in-plane uniaxial anisotropy with negligible long-range fluctuations. A clear relationship could be established between Ku and Klo¢, data which is discussed in terms of the single-ion anisotropy of the Nd ions. Although the value of H k is the same at both interfaces, the as-determined ripple constants were found to be slightly different. This last result is believed to be related to some local defect, the origin of which is also advanced.
1. Introduction Thin films of amorphous CoZrRE have been the subject of recent papers [1-3]. These films possess magnetic properties that make them promising candidates to be employed in magnetic recording, particularly in magnetic heads: good frequency response of the permeability [4], high saturation magnetization, low coercivity, low magnetostriction [5], good mechanical properties [6] and a very well defined uniaxial anisotropy [1,2]. On the other hand, the films under study in the present paper are very interesting for studying the physical origin of in-plane uniaxial anisotropy in thin films. Thin amorphous T M - R E films have been the subject of many investigations. Such films obtained
* Corresponding author. Email: [email protected]; fax: +34 85 103324.
generally exhibit perpendicular anisotropy, the origin of which is partly related to macroscopic contributions induced by stress, columnar growth, etc. However, the uniaxial in-plane anisotropy K u is essentially related to the local structure; the local anisotropy in T M - R E amorphous alloys caused by spin-orbit coupling has been studied by several authors [7]. Although many investigations have been undertaken to determine the physical origin of K u, it is still poorly understood. It would be desirable to find a relationship between the uniaxial and local anisotropies. However, in most cases, the samples investigated exhibit defects related to the fabrication processes that mask the intrinsic perturbations due to the local anisotropy. Consequently, the deposition parameters must be chosen carefully. The films investigated in this work are a series of (Co93Zr7)10o_xNd x with 0 < x < 3.4. From our study we deduce the dependence of Ku on the local anisotropy.
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00440-8
18
J.F. Calleja et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 17-21
2. Experimental procedure The films were deposited by rf co-sputtering. The deposition was performed in a dc magnetic field of 700 Oe parallel to the film surface and we used a magnetic shield that constrains the field in the substrate region and does not affect the plasma. With this configuration, a uniaxial in-plane anisotropy develops. The magnitude of K u and the homogeneity of the amorphous structure depend on the sputter gas pressure used: PAr" The results reported in this paper correspond to PAr = 2 mTorr; for this value the amorphous structure appears to be the most homogeneous [8]. The other deposition parameters were a background pressure of 10 -7 Torr and a rf input power of 300 W, resulting in a deposition rate of 100 fk/min. The saturation magnetization M s was measured by VSM and the exchange stiffness constant by FMR. The uniaxial in-plane anisotropy and the micromagnetic properties were determined using the magneto-optical Kerr effect (MOKE) to perform transverse biased initial susceptibility (TBIS) measurements. The same MOKE system was used to obtain the in-plane dc hysteresis loops of the samples which allowed us to measure the coercive field H c (dc).
3. Theoretical considerations The existence of K u allows us to analyze the experimental results obtained in TBIS measurements by Hoffmann's ripple theory [9]. In this theory, as a consequence of taking into account all the components contributing to the local energy (external field, uniaxial anisotropy, exchange, local anisotropy and magnetostatic energy) the magnetization cannot suffer abrupt changes in direction, but it is correlated in the so-called coupling volumes. Following Hoffmann, the inverse of the transverse initial susceptibility may be expressed as [10]
1
s2Msv
b = 4"rrx/-2 ( a K u ) 5/4'
(2)
where the local anisotropy can thus be estimated according to [10] 3Kloed 3/2
S
1~
'
(3)
where Kioc is the local anisotropy constant, t is the thickness and d is the correlation length of local anisotropies, which is typically 10 ,~ in an amorphous solid. A careful analysis of the TBIS curves allows us to determine very precisely the three parameters Hk, b and c [1]. To get the value of the anisotropy field H k from the experimental data we used the procedure proposed by Feldtkeller [11]. The contribution of the skew was negligible in all the samples; effectively, the variations of the experimental X-1(/3)(h + 1) 1/4 versus (h + 1) 5/4 follows a linear law, as shown in Fig. 1 for the case x = 1.1. In this case the ripple parameter b corresponds to the extrapolated value of X - I ( / 3 X h -t- 1) 1/4 for (h 5: 1) 5/4 = 0.
4. Results and discussion
x;1(/3) ~--~k{(h + 1 ) + b ( h ±
where the plus sign corresponds to /3 = 0 (dc bias field applied along the easy axis), and the minus sign to /3 = xr/2 (dc bias field applied along the hard axis). M s is the saturation magnetization, H k = 2 K , / M s the uniaxial macroscopic anisotropy field, and h = H d c / H k, where Hdc is the applied field. The first term in Eq. (1) corresponds to the coherent rotation process (Stoner-Wohlfarth model); the second term describes the contribution of the ripple related to short-range fluctuations of the magnetization; and the third term is the 'skew' term caused by long-range fluctuations of the induced magnetic anisotropy. Information on the micromagnetic parameters is obtained from the ripple parameter b, related to the structure constant S by
1) - ' / 4 + c ( h s :
1)-1},
(1)
We have performed TBIS measurements at both film/air ( f / a ) and glass/film ( g / f ) interfaces. Effectively, as we shall see, TBIS measurements are sensitive enough to appreciate slight differences be-
J.F. Calleja et al./ Journal of Magnetism and Magnetic Materials 152 (1996) 17-21
19
3,000
°..
2.500
3 ~r'"
"~
e~
A. A
2.000 £x. ~ "
2
I ~00
"t
+~ . z s ~s " r ' "
"7 - 1.000
50o
I 2
I 4
I 6 (h+l)
I 8
I 10
I I 12
tween the two interfaces. We first consider the trends which become apparent from a study of the macroscopic magnetic parameters reported in Table 1. The coercive force is fairly low and varies in the range 0 . 5 - 2 Oe. H~ and thus K u have nearly the same values at the two interfaces. The data obtained here and those determined by VSM a n d / o r FMR [12] are in fairly good agreement. This confLrrns the high overall chemical uniformity of the films, as TBIS measurements refer to the surface and VSM and FMR to the bulk. The variations of K~ as a function of x for the two interfaces are shown in Fig. 2. The experimental results show that K~ varies linearly as a function of x. The ripple parameter b varies randomly with Nd content, a result also found for Tb [1]. For a given sample the value of b at the g / f interface is systematicaily lower than that at the f / a interface. From b the structure constant S, and finally Klocd 3/2 can be deduced. K~o¢ d 3/2 increases with Nd content at both
I 3
X
5/4
Fig. 1. Experimental curves of X-l(/3Xh+l) 1/4 versus (h+ 1)5/4 and linear extrapolations for (C093Zr7)98.9Ndl.i. Triangles represent /3 = O; squares fl = ~r/2.
[ 2
Fig. 2. Nd content dependence of the induced uniaxial anisotropy. Squares and the solid line refer to the f/a interface; triangles and the dashed line to the g/f interface.
interfaces, but it attains greater values at the f / a interface. The overall data indicate that the dominant contribution to the uniaxial anisotropy is related to the single-ion anisotropy of Nd via spin-orbit coupling. The linear increase in K u with increasing Nd concentration can be explained by a theoretical computation using the point charge approximation and based upon the hypothesis that K u is related to the singleion anisotropy of Nd via spin-orbit coupling [13]. The relationship between Ku and Klo¢ is shown in Fig. 3. The proportionality found between K u and Kloc is in accordance with the theoretical model proposed for the formation of the in-plane uniaxial anisotropy [13]; K~ results from a directional deformation along the applied field during deposition of the local sites involving Nd ions in order to minimize the full magnetic energy acting on the sample. Finally, we consider the somewhat surprising outcome that although the value of H~ is practically the
Table 1 Magnetic properties of amorphous (Co93Zr7)lOo_xNdx thin films. When two values are given they axe in the order (f/a)/(g/f) x t 41rM s Hk K a X 10 -4 b flocd3/2 X 104 0 0.2 0.5 1.1 2.2 3.4
(,~,)
(G)
(Oe)
(erg/cm 3)
2000 1500 1600 1950 1950 1800
12 947 13 110 12524 12180 11751 11350
27.7/30.1 30.6/32.3 35.2/34.9 42.7/42.7 55.4/53.2 66.1/63.1
1.42/ 1.55 1.59/ 1.68 1.75/1.74 2.07/2.07 2.59/2.49 2.98/2.85
(erg/cm 3/2)
0.14/0.06 0.18/0.08 0.13/0.11 0.16/0.09 0.14/0.06 0.29/0.08
3.51/2.42 3.95/2.72 3.70/3.39 4.85/3.64 5.31/3.39 8.70/4.44
20
J.F. Calleja et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 17-21
content in the film. During the sputtering process some increase in the effective deposition temperature is practically unavoidable. Consequently the major part of the difference between the values of K~o~ d 3/2 measured at the f / a and g / f interfaces could be related to the fact that the value of d at the f / a interface is higher than at the g / f interface, and this difference increases for higher Nd contents.
,,'"
o
zx ,""
o
,'"
o
I o
I
i 3
I 4
i 5
i 6
I 7
I 8
i 9
10
Kloc d 3/2 X 1 0 4 (erg/cm~ ) Fig. 3. Local anisotropy dependence of the induced uniaxial anisotropy. Squares and solid line refer to the f / a interface; triangles and the dashed line refer to the g / f interface.
same at the two interfaces, the dispersion of the magnetization which is characterized by the ripple parameter b is slightly but systematically higher at the f / a interface than at the g / f interface. Actually, we do not have a definite interpretation of these data but several hypotheses can be advanced. One possibility is that some contribution to the local anisotropy Klo~ occurs without affecting Ku. This could be related to the formation of some microstructural defects that are random from site to site, and so modify Klo¢, but do not affect the magnitude of K~. In favour of this mechanism is the result that the dispersion of H k is slightly bigger at the f / a interface. The defects could be related, for example, to an oxidation process which occurs on the surface when the films are exposed to air after deposition [14]. Previous investigations [15] have shown that as a result of such oxygen contamination occurring at room temperature a certain amount of metallic atoms will be transformed to an ionic state, but will remain exchange coupled with the metallic part of the films, so essentially K~o~ is modified. An alternative interpretation that can not be excluded at the present stage of the investigations, is one that takes into account the growth mechanism of the films. Experiments performed on CoZrDy films [16] showed that the orientational correlation length of the local anisotropy, so that the value of d increases with increasing deposition temperature; the larger this variation, the higher is the rare earth
5. Conclusions The magnetic properties of amorphous CoZrNd thin films were investigated by transverse biased initial susceptibility (TBIS) measurements. The measurements were carried out at both glass/film and film/air interfaces. The films exhibit a very well defined in-plane uniaxial anisotropy with negligible long-range fluctuations. This anisotropy is the same at both interfaces and its origin is related to the single-ion anisotropy of Nd ions through spin-orbit coupling. The local anisotropy was determined and different values are obtained at each interface. A clear relationship between K u and K~oc has been established that allowed us to discuss the origin of KIoc; it results from a combination of single-ion anisotropy of sites involving Nd ions and other contributions related to defects in the structure a n d / o r composition. The latter contributions do not contribute to the magnitude of the uniaxial anisotropy. An alternative explanation is based upon the hypothesis that the value of d is different at the f / a and g / f interfaces.
Acknowledgements This work is partially supported by the Comisi6n Interministerial de Ciencia y Tecnologla (CICYT) MAT92-0778 and MAT93-1431.
References [1] G. Suran, H. Ouahmane, I. Iglesias, M. Rivas, J.A. Corrales and M.C. Contreras, J. Appl. Phys. 76 (1994) 1749. [2] G. Suran, H. Ouahmane, M. Rivoire and J. Sztern, J. Appl. Phys. 73 (1993) 5721.
J.F. Calleja et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 17-21 [3] M. Rivoire, H. Ouahmane, G. Suran, M. Rivas, J.A. Corrales and M.C. Contreras, IEEE Trans. Magn. 29 (1993) 3482. [4] J. Russat, G. Suran, H. Ouahmane, M. Rivoire and J. Sztern, J. Appl. Phys. 73 (1993) 1386. [5] H. Warlimont, Prog. Rare Earth Sci. Technol. II (1968) 1. [6] A. Tago, C. Nishimura and K. Yanagisawa, IEEE Trans. Magn. 21 (1985) 2032. [7] C. Els~isser, M. F~mle, E.H. Brandt and M.C. B~hm, J. Phys. F: Metal Phys. 18 (1988) 2463. [8] G. Suran, K. Ounadjela and F. Machizaud, Phys. Rev. Lett. 57 (1986) 3109. [9] H. Hoffmann, IEEE Trans. Magn. 4 (1968) 32.
21
[10] H. Hoffmann, Phys. Stat. Solidi 33 (1969) 175. [11] E. Feldtkeller, Z. Phys. 176 (1963) 510. [12] G. Suran, H. Ouahmane and J. Sztem, J. Magn. Magn. Mater. 120 (1993) 232. [13] Y. Suzuki, S. Takayama, F. Kirino and N. Ohta, IEEE Trans. Magn. 23 (1987) 2275. [14] M.C. Contreras, I. Iglesias, J.M. Alameda and M. Fernandez, IEEE Trans. Magn. 28 (1992) 3243. [15] G. Suran, B. Prasad, H. Jouve and R. Meyer, J. Appl. Phys. 50 (1979) 1617. [16] G. Suran, K. Roky, J. Sztem, F. Machizaud and J.M. Mackowski, IEEE Trans. Magn. 30 (1994) 4770.