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THE DEPENDENCE OF THE STRUCTURE
CONSTANT OF UNIAXIAL PERMALLOY FILMS ON THE EVAPORATION TEMPERATURE AND THE
ALLOY COMPOSITION
K. Kempter, H. Hoffmann
To cite this version:
K. Kempter, H. Hoffmann. THE DEPENDENCE OF THE STRUCTURE CONSTANT OF UNIAXIAL PERMALLOY FILMS ON THE EVAPORATION TEMPERATURE AND THE ALLOY COMPOSITION. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-396-C1-398.
�10.1051/jphyscol:19711138�. �jpa-00213956�
LAMES MINCES (2 ème partie) ; DOMAINES, DISPERSION DE L'AIMANTATION
THE DEPENDENCE OF THE STRUCTURE CONSTANT OF UNIAXIAL PERMALLOY FILMS ON THE EVAPORATION TEMPERATURE
AND THE ALLOY COMPOSITION
by K. KEMPTER (*) and H. HOFFMANN (**) Sektion Physik der Universitat Munchen, Germany
Résumé. — Le facteur de structure S est l'une des constantes fondamentales, qui permettent de déterminer les pro- priétés de films ferromagnétiques polycristallins. Les variations de S en fonction de la température d'évaporatoin et de la composition de l'alliage ont été déterminés par des mesures de susceptibilité. Des lames minces de composition 79 % Mi 21 % Fe, obtenues sur un support en verre à 290 °C, montrent un monimum pour S.
Abstract. — The structure constant S is one of the fundamental constants, which determine the magnetic properties of polycrystalline ferromagnetic films. By means of susceptibility measurements the dependence of S on the evaporation temperature and alloy composition was determined. Films of the alloy composition 79 % Ni 21 % Fe show a minimum value of S when they were evaporated at 290 °C onto glass substrates.
I. Introduction. — In former papers by Hoffmann fl] and Kempter [2] it had been shown, that the struc- ture of thin polycrystalline ferromagnetic films can be investigated by the measurement of the transverse biased susceptibility. From the theory a structure constant £ had been derived, which is responsible for many properties of thin ferromagnetic films. An ideal uniaxial film is characterized by S = 0. So S can be used to describe the deviations of real (poly- crystalline) films from the expected behaviour of ideal uniaxial films. The structure constant
is given by a constant, K
s, of the local anisotropy, the mean diameter, D, of the crystallites and the number, n = d/D, of the crystallites through the film thickness d. The source of the local anisotropy is open for discussion. A first attempt of such an investigation was done by Leaver et al. [3]. At that time the analysis method of the susceptibility measu- rements had not been sufficiently developed.
The structure constant S can be used as a pheno- menological constant, which has to be determined by measurements, just like the other constants of ferromagnetics. It is the purpose of this paper to show the dependence of S on the alloy composition and the evaporation temperature.
II. The Transverse Biased Susceptibility. — The theoretical base of these investigations has been given in a former paper [1]. In the present investi- gation an a. c. tickle field was applied along the easy axis. The change of the magnetization component along the easy axis was observed, dependend on a d. c. field along the hard axis. For this special case the related susceptibility is given by
x
= Ml I . (2) H
k(h - 1) + B(h - I )
- 1 7 4+ E(h - 1 )
_ 1(*) Present address : Forschungslaboratorium der Siemens AG Munchen, Germany.
(**) Now at Fachbereich Physik der Universitat Regensburg, Germany.
The normalized applied d. c. field (in units of H^) along the hard axis is h. The constants B and E have to be determined by the measurement of the field dependence of %{K). They are related to the structure of the film by
B = =
_J__
s*jWfL
5 (3)
and
E = 3 < 6
2> . (4)
The first relation (3) determines the intrinsic dema- gnetizing field, caused by the ripple. The second term (4) determines the effective field due to the skew. The meaning of the constants is as follows : M
ssaturation magnetization, d film thickness, A exchange constant, K
uconstant of uniaxial anisotropy, < 9
Z>
1 / 2r. m. s.
skew angle.
III. Experiments. — In a first run films of the constant alloy composition 79 % Ni 21 % Fe, were evaporated at various temperatures Xj,, to determine 5(r
a) for this special alloy composition. In a second set films of different alloy composition were evapo- rated at a constant temperature (T
a= 295 °C).
The susceptibility was measured by the means of the magneto-optic Kerr effect. The diameter of the light spot on the film was about 0.7 mm. By experience it was found that the skew wavelength is larger than this distance and thereby its influence vanishes. This simplifies the analysis because of E = 0 in the eq. (2).
The anisotropy field was determined by an improved Kobelev's method [4]. The measured susceptibility (or the related voltage signal) in its dependence of the biasing field had to be fitted to the theoretical relation (2) by a suitable B. The figure 1 gives an exam- ple of the calculated curves and the measured suscepti- bilities for three different films. The theory is valid as long as the films are in a free ripple state, which is given for fields larger than roughly the peak-field.
The deviations at smaller fields are outside of the limits of the theory (eq. 2).
IV. The Structure Constant S. — From the above evaluated constant B the structure constant S can
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711138
THE DEPENDENCE OF THE STRUCTURE CONSTANT OF UNIAXIAL PERMALLOY FILMS C 1 -
397Applied Field h(--)- H Hk
FIG.
1.- The transverse biased susceptibility vs the bias field in the hard axis. Three films of the alloy composition 79 % Ni 21 % Fe evaporated at different temperatures
Ta.Film 822 A
Ta =200 "C. Film 24 1
T a =335 OC. Film TR 10
Ta =350 OC.
be determined as given in eq. (3). In figure 2 the dots give the measured values of S for 79 % Ni 21 % Fe- films. plotted vs the evaporation temperature T,.
To analyze this temperature dependence of S we compared the measured structure constant with a theoretical relation, approximated by Doyle and Finnegan [5] by averaging the magnetocrystalline and magnetoelastic energy of the polycrystalline sample. For the case of planar isotropic stresses their formula gives
FIG. 2.
-The dependence of the structure constant on the evaporation temperature for films of the alloy composition
79 % Ni 21 % Fe.
The magnetocrystalline energy is K,, the homogeneous planar stresses are given by oi, the magnetostriction constants are ill,,, All,. I n independent measurements
the mean diameter, D, of the crystallites was deter- 06~e/e 70ele 8O0/* 9 0 ' 1 ~ ~ i - mined by Reinbold 161 and the isotropic stresses were measured by Schmitt [7]. The values of K, and l i k l Composition -
were taken the literature for the FIG. 3. -The dependence of the structure constant on the
case. alloy composition for films evaporated at 295 OC.
C 1
-
398 K. KEMPTER AND H. HOFFMANNThe full curve of figure 2 shows the plot S
vsT,
as calculated from eq. (5) by using the experimentally determined values of
oiand D, which depend on the evaporation temperature. Very surprisingly not only the qualitative behaviour of S, determined in those two absolute different ways, but even the quantitative values agree. From this we may conclude that for the investigated 79 % Ni 21 % Fe films magneto- elastic and magentocrystalline effects of the randomly oriented crystallites can explain the above mentioned local anisotropy.
Figure 3 shows S versus the alloy composition for films evaporated at 295 OC. Dots and full curves have the same meaning as in figure 2. In this series the effect of the variation of the material constants is apparent. Here we observe deviations especially below 70 % Ni content. In this alloy range we expect accor- ding to (5) a decrease of S because A,,, - A,,, is
zero at 85 % Ni. The validity of ,the expression (5) presumes planar isotropic stress within the crystallites.
This last assumption has not yet been verified expe- rimentally.
We now look at S as a phenomenological constant the value of which is given by susceptibility measu- rement. In order to show its general importance several ripple induced properties of the films have been investigated and found in quantitative dependence of S. As examples we may quote the measurements on the fall back angle, a,,, done by Fujii et al.
[9], furthermore the r.m.s. ripple angle, the blocking process and the wall coercivity, which were deter- mined in our laboratories.
Aknowledgement. - The authors are greatly indeb- ted to the deutsche Forschungsgemeinschaft for the extensive support of this work.
References
[l] HOFFMANN (H.),
Phys.Stat. Sol., 1969, 33, 175. [6] REINBOLD (M.) and HOFFMANN (H.),
Phys.Stat. Sol.
[2] KEMPTER (K.) and HOFFMANN (H.),
Phys.Stat. Sol., (a), 1970,
2,K69.
1969, 34, 237. [7] SCHMITI (H.), Diplomarbeit, Universitat Munchen, [3] LEAVER (K. D.), PRUTTON (M.) and WEST (F. G.), 1969.
Phys.
