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MAGNETIC AND STRUCTURAL ANISOTROPY IN
COMPOSITION-MODULATED Cu-Ni FILMS
N. Flevaris, M. Porte, R. Krishnan
To cite this version:
N. Flevaris, M. Porte, R. Krishnan.
MAGNETIC AND STRUCTURAL ANISOTROPY IN
JOURNAL DE PHYSIQUE
Colloque C8, Suppl6ment au no 12, Tome 49, decembre 1988
MAGNETIC AND STRUCTURAL ANISOTROPY IN COMPOSITION-MODULATED Cu-Ni FILMS
N. K. Flevaris ( I ) , Mt Porte (') and R. Kdshnan (')
( I ) Dept. of Physics, Aristotle Univ., 54006 Thessaloniki, Greece (') Laboratoire de Magnktisme, CNRS, 92185 Meudon Cedex, France
Abstract. - Short-period compositionally modulated Cu-Ni multilayers were studied by transmission electron microscopy and torque magnetometry. A high density of embeded twins and a columnar-growth morphology, with a planar anisotropy, were observed. The strong uniaxial magnetic anisotropy was found to have additional rotational features. A phenomenc- logical single-domain equilibrium analysis will be discussed.
1. Introduction
The study of the magnetic properties of artifi- cially produced compositionally modulated multilay- ered (CMM) thin films appears t o grow intensely in re- cent times. Several CMM systems have been produced and studied [I] by various techniques. The present contribution will report on structural and magnetic studies of CMM films of Cu-Ni; this system has been extensively studied (e.g., see Refs. [l-61).
The CMM films were prepared by vapor deposition onto heated mica in a dual-e-gun unit; details can be found in references [4, 51. Specimens of the as- deposited materials were thinned by ion-beam tech- niques for the transmission electron microscopy, in the conventional planar (CTEM) and cross-sectional (XTEM), studies. For these TEM observations a 120 kV voltage was used. For the static magnetic torque studies a conventional torque-nulling magne- tometer was used.
2. Structural defects a n d anisotropy
The XTEM micrograph seen in figure 1 depicts the morphological characterjstics of growth for a CMM
C~6.8
-
Ni7.0 sample; 6.8 and 7.0 denote the number of atomic planes in each modulation period, X--
2.9 nm. The observed structure consists of: (a) commonly tex- tured crystallites in the growth direction, [lll], (b) crystallites with their [liO] or [2ii] along the beam di- rection; these crystallites are having an orientational relationship of 30 degrees, (c) small number of colum- nar crystallites which are t&n-related with the (b) ones, (d) a variety of embedded twins and (e) mi- crotwins. An unusual feature of this structure is the"inclinationn of modulation-fringe sequences adjacent t o the high-angle boundaries.
These fringes were observed to remain unaffected when crossing an embedded-twin boundary while their periodicity may vary from that of the (111)'s. The analysis of this structure [6] showed that this "inclination"
,
as well as the surface facets, followed closely certain crystallographic orientations. For ex- ample, it was observed that such a boundary may be formed between two [ l i ~ ] and [2ii] crystallites to be the (113) or (Oil) plane, respectively, when the "inclinations" followed the (110) abd (102) planes. It is obvious that such a growth manner resulted in a structure with planar anisotropy in addition to the ob- served tooth-like surface morphology.It must be reminded that the lattice mismatch be- tween the constituent lattices is -2.5 %
.
Since no significant dislocation density is observed, the CMM lattice must be considerably strained in order to ac- commodate this misfit. However, the resulting biaxial elastic modulus is generating an anisotropy in the film plane. Thus, when dealing with the possible growth mechanisms as well as any anisotropic physical prop- erties, the influence of the structural anisotropies and the coherency strains must be included.3. Uniaxial magnetic anisotropy
Fig. 1. - Cross-sectional transmission electron micrograph The magnetic anisoropy of CMM films of Cu-Ni has for a
-
Ni7.0 multilayer with columnar structure, been studied before i2,5l
by resonance Or static mag- twins and i ~ i ~ ~ l i ~ ~ t i ~ ~ ~ > of modulation fringes and surface netometry. Nevertheless, because of the structural facets. pecuIiarities of the CMM systems, it is desirable toC8 - 1772 JOURNAL DE PHYSIQUE
have a reasonably good knowledge of the structure be- Ni and K = -3.5 x
lo5
erg/cm6 erg/cm3-
Ni. These fore modeling physical properties. In this contribu- values designate an easy-plane system.tion we selected to discuss the magnetic anisotropy
of the Cu6.8
-
Ni7.0 sample for which extensive struc- 4. Discussion. Summaryt u r d investigations were already done; earlier studies on Cu,J\Ti, [5] will also be compared.
In figure 2 the variation of the magnetic torque is plotted as a function of increasing (solid line) or de- creasing (dashed) angle (8) between applied field and film plane; a field of H = 12 kOe was used. From sep- arate variable-H experiments, with 0 = 30 and 45 de- grees, it is possible [5, 71 from the (torque/H) ws. H curves t o determine the saturation magnetization den- sity (M) and the first-order uniaxial anisotropy con- stant ( K ) . For such a thin film system, in a single- domain model, one sets the magnetic free energy to consist of three terms: (a) the self-energy part, (b) the demagnetizing (shape) term and (c) the anisotropy contribution.
Fig. 2. - The angular variation of the magnetic torque at
H = 12 kOe.
The angle (cp) between the magnetization vector and the film plane follows the equilibrium relation
,B sin (0
-
(p) = a sin 2 cp, (1)yhere a = K + 2 T M ~ and ,B = MH, which allows sim- ulation of experimental curves, for given M, K, H and 0. This phenomenological single-domain treatment de- termines the distinction between easy-plane and easy- axis uniaxial systems, in terms of a! and ,B, and it also
sets limits for saturation and metastability.
By using such a model, for thin films of CMM Cu,
-
Ni, studied earlier [5], the values of K obtained, for n = 5, 6, 7, and 8, were, respectively; -6.4,-1.2,+8.2 and +14.4 (lo4 erg per cm3 of Ni content). For the Cu6.8-Ni7.0 sample we obtained: M = 307emu/cm3-
From the data shown in figure 2 the main differences from earlier studies on Cum -Ni, svnples with various m and n are: (a) the very high K-value for the m = 6.8, n = 7.0 sample and (b) the rotational-anisotropy fea- tures observed. The location (8) of the torque ex- trema is the same for both directions, confirms the easy-plane character and suggests that saturation was nearly reached. However, there is a -12 % difference in the torque values of subsequent peaks while there is an apparent correspondence in their values when ro- tating clock or counter-clock wise.
The Cu6.8
-
Ni7.0 sample has exhibited that unusual columnar-grain structure in the upper part of its thick- ness; its total thickness was 300 nm while the films with n = 6, 7 or 8 were 1.5-2.5 pm thick. It will be interesting t o further examine this anisotropy in re- lation to the structure and, also, its behavior under field-cooled conditions for possible unidirectional and exchange anisotropy.Acknowledgments
One of us (N.K.F.) acknowledges partial support by the Greek Secretariat of Research and Technology and the EEC (project No ST2J-0289-C). Also, the collab- orations of Prof. Th. Karakostas and Prof. J. Stoe- menos in the XTEM work are gratefully appreciated.
[I] See, for example, the books: Synthetic Modulated Structures, Eds. L. L. Chang and B. C. Giessen (Academic, New York) 1985; Modulated Struc- ture Materials, Ed. T. Tsakalakos (Martinus Ni- jhoff, Dordrecht) 1984.
[2] Gyorgy, E. M., Dillon, J. F., Jr., McWhan, D. B., Rupp, L. W. Jr., Testardi, L. R. and Flanders, P. J., Phys. Rev. Lett. 45 (1980) 57;
Dillon, J. F., Jr., Gyorgy, E. M., Rupp, L. W., Jr., Yafet, Y. and Testardi, L. R., J. Appl. Phys. 52 (1981) 2256.
[3] Zheng, J. Q., Ketterson, J. B., Falco, C. M. and
Schuller, I. K., J. Appl. Phys. 53 (1982) 3150. [4] Flevaris, N. K., Baral, D., Hilliard, J. E. and Ket-
terson, 3. B., Appl. Phys. Lett. 38 (1981) 992. [5] Flevaris, N. K., Ketterson, J. B. and Hliard, J.
E., J. Appl. Phys. 53 (1982) 8046;
Flevaris, N. K.,'Ph. D. thesis, Northwestern Uni- versity, Evanston, U.S.A. (1983).
[6] Flevaris, N. K. and Karakostas, Th., J. Appl. Phys. 63 (1988) 1228.
