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CONTRIBUTION TO Ku OF
STRUCTURE-RELATED AND PSEUDODIPOLAR
ANISOTROPY : AN EXPERIMENTAL
DISCRIMINATION IN AMORPHOUS CoZrM (M =
Nb, Ti, Pt) THIN FILMS
G. Suran, N. Naili, F. Machizaud
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C8, Supplbment au no 12, Tome 49, dkembre 1988
CONTRIBUTION TO
KU
OF STRUCTURE-RELATED AND PSEUDODIPOLAR
ANISOTROPY: AN EXPERIMENTAL DISCRIMINATION IN AMORPHOUS
CoZrM (M = Nb,
Ti,
Pt)THIN FILMS
G . Suran, N. Naili and F. Machizaud
Laboratoire de Magngtisme, C N R S 92195 Meudon Cedex, France
Abstract. -The uniaxial magnetic anisotropy of CoZrM (M = Nb, Ti, Pt) films present large variations as a function of the pressure of the s utter gas. A thermomagnetic treatment permitted to show that Ku is the sum of two contributions
K: +Kt where K! is related to the directional short range order and K: originate from the anisotropy of Co atoms located in trigonal type clusters. The possibility of their experimental separation is related to the composition of the deposit.
Recently we reported a study of induced uniaxial anisotropy K u in amorphous Co-metal (M) ferroma- gnetic thin films where M is a non-magnetic transi- tion metal like Zr, Nb or Ti [I, 21. Following the results when a d.c. magnetic -field- was applied pa- rallel to the film plane during the deposition process a uniaxial anisotropy field
Hk
= 2Ku/
M, developed with its easy axis parallel to the applied field. A study as a function of deposition parameters revealed that the value of the as formed Ku is essentially related to the pressure of the sputtering gas PA,. Ku followsa somewhat bel-shaped like variation with increasing
PAr and present a well defined maximum (K,),, for a critical pressure
(PA,),,
.
This result, first obtained on a - Col-,Ti, was confirmed on films of composi- tion a-
Col-,-,Zr,M, with M = Nb, Ti, Pt. The as observed variations of Ku with PA, and the excep- tionally high value of (K,),, could not be explained by the classical directional short range order mecha- nism which is generally put forward as the origin of Ku in this type of alloy. We proposed a new model based upon a structural investigation where it is as-sumed that the structure of these alloys is formed by a random continuous matrix of clusters with icosahe- dral, octahedral (or fcc like), and trigonal (or hcp like) symmetries. The variations of
K,
was explained by the evolution of the local structure as a function of PA,, via the variations of the amount and orientation of trigonal type clusters with PA, [I, 21.One way t o identify the physical origin of Ku is to study its evolution as a function of anneling in the pre- sence of applied d.c. magnetic field. Here we report the results obtained on a - Col-,-,Zr,M, thin films deposited a t various PA=. The variations observed sho- wed clearly that K u is the sum of two contributions
The a
-
Col-,-,Zr,M, films were prepared by r.f. sputtering. During the deposition process a d.c. ma- gnetic field was applied. The thickness of the filmswere typically 0.2 p to 0.4 pm. The detailed deposition conditions and concentration range obtained are given in reference [2]. The annealing were performed on as prepared samples with no preannealing and carried out under a vacuum of 2
x
Torr. A d.c. magnetic field of 600 Oe was applied along the hard axis and the variations of H k were investigated in isothermal experiments. Ku was determined from the B-H loop corresponding t o the hard axis and its value controlled on some samples by ferromagnetic resonance measu- rements. The annealing temperature used was in the range of 80' to 300 OC and the variations of K, were studied systematically as a function of the composition and PAr used during the deposition of the films.For a given composition the as observed variations were qualitatively the same for the whole concentra- tion range investigated. Here were report the result of isothermal annealing performed a t 100 OC which illustrate the variations of Ku in Col-,-,Zr,M, with M = Zr, Ti, Nb or Pt. The possible dependence upon the particular M could be determined by comparing samples with practically the same amount of x
+
y.The as reported curves (Figs. la-ld) are given for three different characteristic values of PA,. P A I = ~ X
Torr correspond to the lowest pressure used, for which on films of a given composition the smallest va- lue of Ku (K,),,, was obtained. PA, = 8
x
Torr is closed to (PA,),, for films containing Zr, Ti or Nb, while 15 x Torr was the highest pressure used. Remark that in films containing P t the highest value of Ku correspond to this PA, [2].On all films deposited at 2 x Torr Ku decreases continously and after a sufficiently long annealing time
t
-
where t depend upon the annealing temperature - the original hard direction became the easy axis. The kinetics of decrease of Ku in also characteristic. In a first step Ku decreases fairly rapidly, processes which exhibit a very small relaxation time, followed in a se- cond step by a much slower mechanism. This second step appears t o follow an exp (-t/ra) law. This re-C8 - 1732 JOURNAL DE PHYSIQUE
Fig. 1. - Ku as a function of annealing time in CoZr(a), CoZrNb(b), CoZrTi(c) and CoZrPt(d). The annealing tem- perature was 100 OC. -Ku correspond to anisotropy in- duced along the applied field (see text).
sult is similar t o that reported in reference [31. On films deposited a t higher pressures the variations of Ku depend upon the composition. Layers containing Nb and Ti exhibit similar behaviour: with annealing a decrease AK, of Ku is observed, but after a certain critical time Ku remains unchanged even for further annealing. Some samples were annealed at higher tem- perature (typically 150' to 200 OC) and Ku remained constant. The decrease AKu is independent of PA, and within experimental uncertainties had the same value as (Ku),in in films of same composition. Result in CoZr differs only in the value of AKu which was gene- rally higher of 10-30 % than (Ku)mi, of corresponding composition. In CoZrPt films and for the concentra- tion of P t studied (0.15
<
y<
0.21) Ku could be suppressed practically on all films prepared at PA=<Torr whatever the initial value of Ku.
Let us consider first CoZrTi and CoZrNb layers. The present experiments show clearly that Ku is the sum of two contributions K,~+K: where
~ , d
is the term which is suppressed and K: which remains unchanged after annealing. Concerning their physical origin we propose that~ , d
is related t o the directional short range order via the chemical short range order (CSRO) and K: can be attributed t o the single ion anisotropy of Co atoms located in the trigonal type clusters, so related to the topological short range order (TSRO).In favor of this hypothesis is the generally accep ted characteristic of K t , that it can be suppressed or reoriented reversibly in a d.c. magnetic field, result observed in the present case. One can remark also that the value of (K,),, is only 20-40 % higher than Ku obtained on a-Co-M films prepared without an ap- plied field. Finally a necessary condition for the expe- rimental separation of K,d and K: is that the diffu- sional relaxation time (DRT) corresponding t o CSRO should be much smaller than the structural relaxation time (SRT) corresponding t o TSRO. Experiences per- formed on various amorphous materials showed effec- tively that generally the DRT is one or two order of magnitude shorter than the SRT [4].
Following these hypothesis and a structural study of Col-,-,Zr,M, alloys the results can be explained as follow.
In CoZrTi and CoZrNb thin films deposited at PA,= 2 mTorr K,=K,~. For films deposited at higher pres- sure Ku is the sum of two contribution K,~+K; where K; is constant depend upon the amount z+y but inde- pendent of PA=, while K: vary with PA, in agreement with structural investigations. On CoZr the results can be explained in the same way, but the higher value of AKu could be related to the higher local disorder so a small part of K: could be also suppressed by an- nealing via for exemple a distorsion of a small part of trigonal clusters. This last mechanism is believed t o be at the origin of the results obtained in CoZrPt thin films. Observations by transmission electron micros- copy revealed that the local disorder increases with in- creasing annealing temperature, behaviour extremely unusual. As a result in this alloy the DRT could be of the same order of magnitude as the SRT, so an an- nealing of this alloy should favorize a rearrangement of the local structure toward a configuration which has a lower energy i.e. icosahedral clusters.
[l] Suran, G., Ounadjela, K. and Machizaud, F.,
Phys. Rev. Lett. 57 (1986) 3109.
[2] Suran, G., Naili, M. and Sztern, J., J. Appl. Phys.
63 (1988) 4318.
