Pt-Co MAGNETS PRODUCED BY DC TRIODE SPUTTERING

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Pt-Co MAGNETS PRODUCED BY DC TRIODE SPUTTERING

E. Hill, G. Hoffman, A. Mccullough

To cite this version:

E. Hill, G. Hoffman, A. Mccullough. Pt-Co MAGNETS PRODUCED BY DC TRIODE SPUTTER- ING. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-291-C6-294. �10.1051/jphyscol:1985651�.

�jpa-00224906�

J O U R N A L D E PHYSIQUE

Colloque C6, supplbment au n09, Tome 46, septembre 1985 page C6-291

P t - C o MAGNETS PRODUCED BY DC TRIODE SPUTTERING

E.W. H i l l , G . R . Hoffman and A.M. McCullough

Department of EZectricaZ Engineering, University of Manchester, Manchester MI3 9PL, U.K.

RBsum6 - Des f i l m s minces 2 magndtisme permanent de PtCo Bquiatomique o n t 6 t 6 p r o d u i t s p a r bombardement t r i o d i q u e en c o u r a n t c o n t i n u . Des f i l m s a y a n t j u s q u ' s 6ym d ' 6 p a i s s e u r o n t d t 6 ddpos6s s u r d e s s u b s t r a t s d e s a p h i r e t donnent d e s p r o d u i t s (BH) maximums de 7 2 k ~ m - ~ quand i l s s o n t convenablement t r a i t & thermiquement.

Abstract - Thin permanent magnet f i l m s of equiatomic PtCo have been produced by d.c. t r i o d e s p u t t e r i n g . Films up t o 6pa t h i c k have been deposited o n t o sapphire s u b s t r a t e s and g i v e (BH) max products of 72kLrmd3 when o p t i m a l l y h e a t t r e a t e d .

W e have produced t h i n f i l m magnetoresistive s e n s o r s /1/ which r e q u i r e d.c. bias f i e l d s from 400 t o 800~m-l. The need t o generate t h e s e bias f i e l d s without continuous power consumption has l e d u s t o i n v e s t i g a t e t h e p o s s i b i l i t y of making s p u t t e r e d permanent magnet f i l m s , which by p a t t e r n i n g with photolithographic methods can produce permanent magnets of a given geometry. Equiatomic platinum c o b a l t w a s chosen f o r i n v e s t i g a t i o n s i n c e it e x h i b i t s e x c e l l e n t bulk permanent magnet p r o p e r t i e s /2/ and h a s been e x t e n s i v e l y s t u d i e d /3/, /4/.

The m a t e r i a l h a s a low anisotropy disordered f c c phase which is stable above about 800°c. A t lower temperature the ordered t e t r a g o n a l phase is stable. This phase h a s a lower s a t u r a t i o n magnetisation b u t a higher anisotropy than t h e disordered f c c phase. Optimum permanent magnet p r o p e r t i e s are obtained by h e a t i n g t h e disordered phase t o about 600°C, which h a s been shown by m i c r o s t r u c t u r a l s t u d i e s /5/ t o p r e c i p i t a t e t h e ordered phase i n t h e form of t h i n p l a t e l e t s giving rise t o high c o e r c i v i t i e s i n t h e b u l k m a t e r i a l . Aboaf et. a l . have produced f i l m s o f PtCo up t o 300 tun t h i c k by r . f . s p u t t e r i n g /6/ and a f t e r a 6 0 0 ~ ~ anneal found c o e r c i v i t i e s t o o l a r g e f o r t h e i r instrumentation t o measure. This l a t t e r p o i n t c l e a r l y i n d i c a t e s t h a t PtCo has a high p o t e n t i a l a s a material f o r spubtered permanent magnets.

FILM DEPOSITION

For use as bias magnets f i l m s of up t o 5pn i n t h i c k n e s s were needed, r e q u i r i n g a high d e p o s i t i o n r a t e technique i n o r d e r t o minimiae d e p o s i t i o n time. A commercially a v a i l a b l e d . c . t r i o d e s p u t t e r i n g source with some magnetic enhancement ("Trimag"

manufactured by L.M. Sirnard) was used f o r t h e i n v e s t i g a t i o n . The source w a s mounted i n a conventional d i f f u s i o n pumped vacuum system with a base p r e s s u r e o f 3 x m b a r . P u r i f i e d argon was admitted i n t o t h e chamber a t a r a t e o f 80 sccm giving a background p r e s s u r e of 3 x loe2 mbar during d e p o s i t i o n . The t r i o d e source h a s a higher s p u t t e r i n g rate t h a n a convenkional diode system and a l s o allows independent adjustment of t h e plasma c u r r e n t and t a r g e t p o t e n t i a l . Since no magnetic f i e l d s t h r e a d t h e t a r g e t , t h i c k t a r g e t s of magnetic material Could be used, avoiding t h e bonding problems a s s o c i a t e d w i t h t h i n f o i l t a r g e t s .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985651

C6-292 JOURNAL DE PHYSIQUE

The target consisted of a 50mrm diameter 6 m t h i c k cobalt d i s c with segments of 5 0 0 p t h i c k platinum f o i l spark welded t o the surface. The a r e a l r a t i o of platinum t o cobalt determines t h e camposition of the deposited material. This r a t i o w a s v a r i e d and t h e composition of t h e f i l m s determined using an e l e c t r o n microprobe. A f t e r the t a r g e t had been c a l i b r a t e d i n t h i s way it was p o s s i b l e t o reproduce any given a m p o s i t i o n t o within two percent.

'Phe s u b s t r a t e s were prepared f o r deposition by a detergent r i n s e followed by u l t r a s o n i c a l l y cleaning i n deionised w a t e r . They were then placed i n copper s u b s t r a t e holders and could be heated t o 300°C using r a d i a n t h e a t d i r e c t e d onto the back o f the holder. The s u b s t r a t e s were baked i n t h e vacuum system f o r 30 min. at 300- p r i o r t o deposition.

The rate o f deposition o f the films was 100nms-~ and was limited by t h e maximum power which could be supplied t o t h e composite t a r g e t before heating o f t h e platinum f o i l became a problem. The films w e r e deposited through a s t e n c i l mask onto 2 2 m square s u b s t r a t e s t o give twelve 2 . 5 m by 4 m rectangular samples. A f t e r deposition the s u b s t r a t e s w e r e divided by s c r i b i n g and cracking t o give twelve samples which could be individually h e a t t r e a t e d . The f i l m thickness was determined p r i o r t o d i v i s i o n of the s u b s t r a t e using a Talystep and unless s t a t e d otherwise a l l f i l m s w e r e between 2.7 and 4pm thick.

FILM ADHESION

Aboaf et. al. ^/6/ found t h a t films of up t o 300m thickness adhered w e l l on a v a r i e t y o f s u b s t r a t e s but w e have established t h a t f i l m adhesion is a problem f o r thicknesses g r e a t e r than 1 ~W e . deposited f i l m s onto a v a r i e t y of s u b s t r a t e s at d i f f e p n t temperatures. The films were then annealed f o r 2 hours a t 600°c and subjected t o mild abrasion with a s c a l p e l blade. The r e s u l t s a r e summarised i n t a b l e 1. It was posible t o achieve good adhesion only on sapphire and polished alumina s u b s t r a t e s with a deposition temperature a t l e a s t 300°C.

Film-Substrate Adhesion Q u a l i t y

~ e p o s i i t i o n temp. OC 25 150 300

B o r o s i l i c a t e G l a s s '"=Y poor poor poor

Oxidised S i l i c o n Poor poor f a i r

Polished Alumina poor

(MRC Superstrate 997)

f a i r good

Sapphire [ l l z o ] poor f a i r good

Very poor adhesion

-

Poor adhesion

-

P a i r adhesion

-

-

Table 1. Adhesion of PtCo t o various s u b s t r a t e m a t e r i a l s .

HEAT TREATUENT

Pilms o f two compositions were considered, one having 45 at% cobalt and t h e o t h e r 49

at% c o b a l t . Samples of each composition were heat t r e a t e d i n a vacuum furnace at 6 0 0 ~ ~ f o r varying lengths of time. They were a l l raised t o 6 0 0 ~ ~ a t a ramp rate of 10°C per minute t o minimise t h e thermal shock t o t h e film. After dwell times ranging from zero t o six hours a t 600°c the samples' were cooled t o ambient temperature at t h e same ramp rate.

MAGNErIC MEASUREMENTS

Magnetisation curves were p l o t t e d using a V.S.M. i n f i e l d s up t o 1200kAK1 a t room

temperature. Figure 1 compares the magnetisation curves for films deposited onto polished alumina and sapphire substrates.

PT-CO SAPPHIRE PHRS AT 600 DEGREES

T(kA/m) n

PT-CO ALUMINA 2HRS AT 600 DEGREES

~ ( k ! / d

Figure 1. Comparison of magnetisation curves for films on sapphire and polished alumina.

It is clear that the film on the polished alumina substrate has inferior permanent magnet properties with a laver coercivity than the film on sapphire. The polished alumina substrate had a high incidence of surface pits which would give rise to the formation of spikes on the underside of the film. The large demagneti.sing field present at such spikes could give rise to reverse magnetic domain nc~cleation which would in turn reduce the coercivity and loop squareness. As the magnets deposited onto the sapphire substrates gave superior properties this material was chosen as the substrate material for all films subsequently made.

The saturation magnfkisation and coercivity for the two film compositions considered in this investigation are shown plotted against dwell time at 600°& in figure 2.

The film with the higher cobalt concentration exhibits a higher initial saturation magnetisation before heat treatment but gives lower coercivities for a given dwell time. The zero dwell time samples were heated to the annealing temperature and then allowed to cool immediately but this heat treatment produces a coercivity which is higher than for the as-deposited material indicating that some ordered phase is produced during the ramp up to and down from temperature.

O 49% COEALT

-.

'

E

U n=

W 0 U

0

2

4 0 0

-

1 2 3 4 5 6

DWELL TIME (HOURS) DWELL TIME (HOURS)

Figure 2. C~eYCivity and saturation magnetisation vs. dwell time at 600°C

JOURNAL DE PHYSIQUE

Figure 3 shows t h e (BH)max product p l o t t e d a g a i n s t dwell time. I t is clear t h a t it is p o s s i b l e t o optimise t h e (BH)max product by s e l e c t i n g a dwell time o f 2 hours f o r t h e 49% cobalt f i l m and 1 . 5 hours f o r t h e 45% c o b a l t f i l m .

The above r e s u l t s are i n broad agreement with t h o s e f o r bulk m a t e r i a l /7/ and i n d i c a t e t h a t t h e as-deposited material is mainly t h e disordered fcc phase. The

maximum BH product of 72k3tak3ta3 obtained i n t h i s work compares w e l l w i t h t h e maximum reported f o r bulk m a t e r i a l o f 96kJK3 /2/.

45% COBALT

O 49% COBALT

1 2 3 4 5 6

DWELL TIME (HOURS) Figure 3. ( ~ ~ ) m a x vs. dwell t i m e a t 600°C

(~emagnetisation factor estimated using uniform magnetisation approximation).

SUMMARY AND CONCLUSIONS

W e have produced permanent magnet f i l m s using d . c . t r i o d e s p u t t e r e d PtCo with energy products up t o For f i l m s more t h a n o f lpn t h i c k n e s s good adhesion and high (BH)max product are obtained on sapphire s u b s t r a t e s with a d e p o s i t i o n temperature o f 300°C. The opt%mum (BH)max product is obtained a f t e r a 600°C anneal f o r 2 hours and is dependent on t h e composition of t h e f i l m . It w a s a l s o found t h a t a substrate w i t h a rough o r p i t t e d s u r f a c e caused a d e t e r i o r a t i o n i n t h e permanent magnet p r o p e r t i e s of t h e film.

I f t h e r e s u l t s of t h e magnetic measurements a r e campared with t h o s e f o r bulk

material it apears t h a t t h e as-deposited material is mainly t h e disordered f c c phase s i n c e t h e s a t u r a t i o n magnetisation and c m r c i v i t y are i n agreement w i t h t h o s e f o r bulk materials s o l u t i o n t r e a t e d at 900°C t o produce t h e disordered phase. T h i s latter conclusion h a s y e t t o be confirmed by m i c r o s t r u c t u r a l observations on t h e deposited f i l m s

.

/1/ Hoffman G.R., H i l l W. and B i r t w i s t l e J . K . , IEEE Trans. Mag., -20, NO. 5, Sept. 1984.

/2/ Kaneko E., Earnma M., suzuki K., Trans. Japn. I n s t . Met., 9, 1968.

/3/ Darling A.S., Platinum M e t . Rev., 2, 1963.

/4/ Craik D.J., Platinum M e t . Rev., l6, 1972.

/5/ Penisson J . M . , Bourret A. and Eurin P., Acta M e t . , l9, 1971.

/6/ Aboaf J.A., Herd S.R. and Klokholm E., IEEE Trans. Mag., -19, No. 4, July 1983.

/ 7 / Martin D.L., Trans. AIME, 212, 1958.

/ 8 / McCurrie R.A. and Gaunt P., Int. Conf. Magn. Nottingham, Proc. Phys. Soc.

(1964) 780.