MAGNETIC PROPERTIES OF τ PHASE AND κ PHASE FILMS IN TERNARY ALLOY SYSTEM

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MAGNETIC PROPERTIES OF τ PHASE AND κ

PHASE FILMS IN TERNARY ALLOY SYSTEM

A. Morisako, M. Matsumoto, M. Naoe

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C8, SupplBment au no 12, Tome 49, decembre 1988

MAGNETIC PROPERTIES OF

r

PHASE AND

rc

PHASE FILMS IN TERNARY

ALLOY SYSTEM

A. Morisako (I), M. Matsumoto (I) and M. Naoe (')

(I) Faculty of Engng., Shinshu University, 500 Wakasato Nagano, 380 Japan

(') Faculty of Engng., Tokyo Institute of Technology, 8-1-12 Oh-okayama, Megum-ku, Tokyo, 152 Japan

Abstract. - Magnetic properties of r and n phases in M ~ ~ ~ - , A I ~ ~ C ~ , ' films prepared by sputtering have been inves- tigated. The phases synthesized in the films changed from r phase to n phase through the mixture phase of them with increase of x. Saturation magnetization of the films for each single phase increased with inmase of a: until mixture phase influenced and that of n phase depends strongly on the lattice constant.

1. Introduction 3. Results a n d discussion

Three ferromagnetic regions are known in Mn-A1- Cu system. These are T phase (CuAu-superstructure)

[I], K phase (CsC1-type superstructure) [2] and Heusler

alloy (CuaMnAl) [3]. The authors have reported that

T phase in MnAl binary alloy films was syntehsized

a t Mn composition of about 60 at. % [4], while that in bulk alloy was synthesized at about 54 at. % [I]. In the case of sputtered films, excess Mn atoms occupy A1 site in tetragonal superstructure. This led to increase of anti-ferromagnetic coupling between neighboring pairs of Mn atoms and resulted in decrease of saturation magnetization, M,, of T phase.

In order t o clarify the effect of substitution by non- magnetic such as Cu for excess Mn atoms on M, of deposited films, Mn60-xA140Cu, films &re deposited by means of sputtering and their magnetic properties have been investigated.

2. Experimental procedure

Films were prepared by means of dc magnetron sput- tering system. A composite type of target, where Mn (99.9 % in purity) and Cu(99.9 % in purity) chips were placed on 100

4

A1 (99.99 % in purity) disk, was used in this study. The composition of the deposited fdm was controlled by alternating the number of Mn and Cu chips. Slide glasses were used as the substrate. Sub- strate temperature was measured by a thermocouple in contact with the substrate surface and was kept at 150 OC [4]. Argon (99.9995 % in purity) gas pressure was kept constant a t 1.5 m Torr and film thickness was about 7000

A.

Crystal structure and composition of the deposited films were determined by X-ray diffractometry and electron microprobe analysis, respectively. Magne- tic properties of the films were measured with vibra- ting sample magnetometer at room temperature in the maximum magnetic field of 10 kOe.

Crystallographic characteristics of Mn6o-,Al4oCu, films have been reported elsewhere [5] and only brief description is included here for summarization. When x is below 10, single T phase is synthesized in the film

with a preferred orientation o f , ( l l l ) plane. Both T and K phases are synthesized simultaneously in the film at

x

of around 11 and single K phase is synthesized a t x in the range of 13 N 25. When x is in the range from 26 to 40, non-magnetic Cu9A4 phase is synthesized. T phase has a tetragonal CuAu-type superstructure and rc phase has a body centered cubic CsC1-type super- structure. The tetragonality for T phase, which is de-

fined by c / a

-

1 (where c and a are lattice constant), decreases from 0.3 t o 0.24 with increase of x up to 12.5 [5]. This decrease of tetragonality results in the decrease of magnetocrystalline anisotropy of 7 phase.

As the authors have already reported, coercivity of T

phase films decreases drastically from 2.0 t o 1.0 kOe with an increase of the substitution of Cu for Mn.

Maximum values of M, for MnAl binary alloy films obtained in our previous study was 120 emu/cc at most, where Mn composition was about 60 at. % [4], while that for bulk alloy with a Mn composition of ap- proximately 54 at. % was 490 emu/cc [6]. Excess Mn atoms in the films are considered to occupy an A1 site in tetragonal Cu-Au-type superstructure. This leads to the increase of anti-ferromagnetic coupling be:ween neighboring pairs of Mn atoms and results in a de- crease of M, for 7 phase films.

Figure 1 shows the dependence of M, on Cu content x in the films. As shown in this figure, M, increases

from 120 to 220 emu/cc with increasing Cu content up to 10 at. ?6

.

Single T phase was synthesized in this

Cu compositional range, as mentioned above. l?&her increase of x up to 16 leads to a decrease of M, and M, reaches the minimum value of 130 emu/cc at

x

of 16. Both T and rc phase were synthesized at z in the range from 11 t o 12.5 and single n phase was synthesized at

x

above 13. In X-ray diffraction diagrams for the films

JOURNAL DE PHYSIQUE

X ( C U content I n a t . % )

Fig. 1. - Dependence of saturation magnetization, Cu content in films.

L a t t l c e constant a ( 8. )

Ms, on Fig. 2. - Relation between saturation magnetization, and lattice constant, a, for K. phase.

with Cu content in the range from 11 to 16, the full width a t half maximum of diffraction line correspon- ding t o each phase became significantly broad [5].

The increase of M, in single r phase films can be explained that Mn atoms on A1 site are substituted by non-magnetic Cu atoms and this leads t o decrease of anti-ferromagnetic coupling between the nearest neigh- bors of Mn atoms pairs. The decrease of M, of the films with Cu content in the range from 11 to 16 is consi- dered that incompleteness of superstructure in each phase increases and leads t o weaken a ferromagnetic coupling in Mn pairs.

Further increase of x from 16 to 23, where single rc phase was synthesized, leads to increase of Ms. The maximum value of M, is about 300 emu/cc. In this re- gion, the full width a t half maximum of diffraction line became vary narrow and average grain size (D) which was estimated from diffraction line increased from 200 (at x of 16) t o 400

a

(at x of 23) [5]. The increase of Ma is due to the improvement of degree of atomic ordering in superstructure (CsC1-type) for rc phase.

When

x

is above 30, M, decreases rapidly and reaches zero. This is due t o the formation of non- magnetic CusAlr phase in the films.

From these results, the authors can conclude that the increase of M, in T phase (x

5

10) is caused by de-

creasing the anti-ferromagnetic couplings between Mn atoms pairs by substituting nonmagnetic Cu atoms. Furthermore, coexistence of different type of super- structure leads to the increase of incompleteness of superstructure in each phase.

Figure 2 shows the relation between a latice constant, a, for rc phase and M, of the films prepa-

red in this study. The composition of these specimen

films are Mnso-xAl~o-,Cux+, (where y is in the range from -1 to 5 and x is in the range from 13 t o 25) and these films consist of single rc phase. Similar tendency is also found for other phases films prepared in our pre- vious studies [4, 71. The closer lattice constant of the films to that of bulk alloy revealed the higher Ms. 4. Conclusion

It was found that the substitution for Mn atoms by non-magnetic Cu atoms resulted in the increase of Ms in T phase. Coexistence of T and rc phase, however, resulted in decrease of Ms.

Ms of the films for K phase depended strongly on its

lattice constant and higher M, was obtained at the closer lattice constant to that of bulk alloy.

[I] Kono, H., J. Phys. J p n 13 (1958) 1444.

[2] Tsuboya, I. and Suguhara, M., ibid., 16 (1961) 571.

[3] Bozorth, R. M., Ferromagnetism (D. Van Nos- trand N.Y.) 1951, p. 329.

[4] Morisako, A., Matsumoto, M. and Naoe, M., J. Appl. Phys. 61 (1987) 4281.

[5] Matsumoto, M., Morisako, A. and Naoe, M., Sup. to Truns. JIM (Proc. of JIMIS-5, Kyoto Japan) 29 (1988) 269.

[6] Koch, A. J. J., Hokkeling, P., Steeg, M. G. V. D. and de Vos, K. J., J. Appl. Phys. 37 (1960) 75s. [7] Morisako, A., Matsumoto, M. and Naoe, M., Phy-.