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Crystal morphology and magnetic domain structure of the magnetically hard alloy Pt0.7Ni0.3Fe
1472
CRYSTAL MORPHOLOGY AND MAGNETIC D O M M N STRUCTURE OF THE MAGNETICALLY
H A R D A L L O Y Pto.TNio.3Fe
P. GAUNT Department of Physics, University of Manitoba, Winnipeg, Canada R3T 2N2 A magnetically hard ordered tetragonal alloy has been examined using a 1 MeV electron microscope. A {ll0} lamellar structure is found; the c axes of the tetragonal lamellae being at 90° to the c axis of the locally dominant tetragonal matrix. Lorentz microscopy reveals 180° domain walls parallel to the matrix c axis. The lamellae are decorated with pairs of 90° domain walls. It is suggested that 180° walls can be pinned where they meet junctions between {110} lamellae. Moving a wall from such a junction increases the 180° domain wall area. The pinning force from this interaction is shown to be of the appropriate order of magnitude to explain the coercive field.
1. Introduction After appropriate heat treatment the alloys P t x N i l _ x F e w i t h x = 0.2 a n d 0.3 b e c o m e m a g n e t i c a l l y h a r d [1]. In t h e s t a t e o f o p t i m u m h a r d n e s s t h e a l l o y s h a v e an o r d e r e d t e t r a g o n a l s t r u c t u r e w i t h P t / N i at [000] a n d [~0] a n d F e at [0~] a n d [½0½]. T h e v a l u e o f c/a is = 0 . 9 6 . T h e p r e s e n t i n v e s t i g a t i o n w a s u n d e r t a k e n to d e t e r mine the nature of the interaction responsible f o r t h e high m a g n e t i c h a r d n e s s o f t h e alloy. I f a s e g m e n t o f d o m a i n wall, o f a r e a A , i n t e r a c t s w i t h a p i n n i n g site, o f e n e r g y U(x), t h e n t h e a b s o l u t e z e r o c o e r c i v e f o r c e is g i v e n b y : H0 = (d U[dx)m~x[2MA,
P e n i s s o n et al. [3] o n P t C o a l l o y s . In t h e o p t i m u m s t a t e , a f t e r a g e i n g f o r 1.33 h, {110} l a m e l lar t r a c e s w e r e o b s e r v e d . T h e l a m e l l a e h a d t h e i r c a x e s at 90 ° to t h e c a x i s o f t h e s u r r o u n d i n g dominant tetragonal matrix. These conclusions w e r e c o n f i r m e d f o r six d i f f e r e n t c r y s t a l o r i e n t a t i o n s b y t r a c e a n a l y s i s a n d d a r k field micrographs from super-lattice reflections. Fig. 1 s h o w s a d e f o c u s s e d i m a g e f r o m a d i s c
(1)
w h e r e M is t h e s a t u r a t i o n m a g n e t i z a t i o n p e r u n i t v o l u m e [2].
2. Specimen preparation for electron microscopy T h e s p e c i m e n s w e r e 0.2 c m d i s c s o f t h i c k n e s s 0.05 cm. T h e y w e r e a g e d at 700°C to p r o d u c e m a g n e t i c h a r d e n i n g . J e t e l e c t r o - p o l i s h i n g in concentrated hydrochloric acid and perforation by a bath method produced some thinned areas. A n A E I E M 7 1 M e V e l e c t r o n m i c r o s c o p e , at the Department of Metallurgy and Materials Science Oxford, was used while the author was on s a b b a t i c a l l e a v e . U s e o f an a p p r o p r i a t e objective pole-piece and entry stage reduced the m a g n e t i c field at t h e s p e c i m e n to l e s s t h a n 50Oe. This made Lorentz microscopy of domain walls possible but reduced the optimum resolution of the instrument.
3. Results The crystal morphology of the alloys closely p a r a l l e l s t h e high t e m p e r a t u r e a g e i n g r e s u l t s o f
Physica 86-88B (1977) 1472-1474 (~ North-Holland
Fig. 1. Defocussed image of alloy in optimum state. The walls parallel to [001] are 180° domain walls. The (011) and (011) lameUae are decorated by pairs of 90° domain walls. The ringed area shows the reversal of lamellar contrast on crossing a 180° wall.
1473 aged f o r 1.33 h at 700°C to give a c o e r c i v e f o r c e , at r o o m t e m p e r a t u r e , o f 1380 Oe. T h e foil plane is (100) and traces o f (011) and (01D lameUae can be seen. The, c o m p a r a t i v e l y , long range b l a c k and white c o n t r a s t lines, parallel to [001], are identified with 180 ° d o m a i n walls. T h e y are d e t e r m i n e d b y the d o m i n a n t phase, w h o s e c axis, and h e n c e e a s y direction o f m a g n e t i z a t i o n is parallel to [001]. E a c h lamellar trace is dec o r a t e d b y a black and white d o m a i n c o n t r a s t effect and these are identified with 90 ° d o m a i n walls; since the e a s y direction o f m a g n e t i z a t i o n , (the c axis), r o t a t e s t h r o u g h 90 ° in c r o s s i n g f r o m matrix to lamella. T h e s e relationships w e r e confirmed f o r o t h e r foil orientations. W h e n a lamella trace c r o s s e s a 180 ° wall the black-white c o n t r a s t o f the lamella reverses. A n e x a m p l e o f this is ringed in fig. 1 and a s k e t c h o f the effect is s h o w n in fig. 2. T h e sample o f fig. 1 had not e x p e r i e n c e d a field g r e a t e r than 50 O e since q u e n c h i n g f r o m a b o v e its Curie t e m p e r a t u r e . A field o f 2000 O e was, t h e r e f o r e , applied to the s p e c i m e n outside the m i c r o s c o p e and parallel to [001]. T h e mic r o g r a p h o f fig. 3, f r o m the s a m e area as fig. 1, w a s then taken.
[Ol o]
Fig. 3. Defocussed image of same area as fig. 1 after application and removal of a 2000Oe field parallel to [001]. Ringed area shows 180° wall crossing a lamellar junction.
4. Discussion [OOl]
w
W
w
Fig. 2 s h o w s that if a 180 ° wall c r o s s e s a lamellar j u n c t i o n it is n o t n e c e s s a r y to h a v e a d o m a i n wall s e g m e n t or u n c o m p e n s a t e d poles within the lamellae. If, h o w e v e r , the wall m o v e s , an e n e r g y o f interaction arises in the f o r m o f an extra s e g m e n t of 180 ° wall and u n c o m p e n s a t e d poles within the lamellae. This is illustrated in fig. 4. F r o m the d a t a o f [1] the 180 ° wall e n e r g y is e s t i m a t e d as ~ 1 0 erg c m -2. [OLO1
Fig. 2. Change in lamellar 90~ wall contrast on crossing a 180" domain wall and 180° wall meeting iamellar junctions, b and w indicate lines of black and white contrast.
[OOll b
T h e 180 ° d o m a i n walls o f fig. 3 h a v e shifted and in the n e w r e m a n e n t state s e e m to s h o w a p r e f e r e n c e f o r c r o s s i n g lamellae at a 90 ° junction b e t w e e n (011) a n d (011) lamellae. It is suggested that this r e p r e s e n t s the pinning mec h a n i s m in these alloys a n d is d i s c u s s e d below.
~
'
~
b
Fig. 4. 180° wall leaving lamelar junctions.
1474 Taking the lamellar thickness, t, as 7 × 10 -7 cm, the spacing, d, as 10 -5 cm and the other lamellar dimension as l0 -5 cm gives a m a x i m u m lamellar contribution to wall energy of 10-1°erg per junction. The m a x i m u m demagnetizing energy is estimated as 0.5 M2t2d. Taking, M, the saturation magnetization, as 1 0 3 e m u c m -3 gives a value of 5 x 10-~2erg which is negligible compared to the wall energy. The interaction energy reaches its m a x i m u m value w h e n the matrix wall has m o v e d a distance t, giving (dUldX)m~x ~3 x 10-4erg cm -~. Eq. (1) then gives the coercivity, H0, as ~1500 Oe. This is the right order of magnitude but m o r e exact calculations of the interaction energy are required. The coercive
field is particularly sensitive evaluation of (dUldx)ma,,.
to the
proper
A c k n o w l e d g e m e n t s are due to P r o f e s s o r Sir P e t e r Hirsch and Dr. J.P. J a k u b o v i c s for the provision of laboratory facilities and unstinting help. The w o r k was supported by the National R e s e a r c h Council of Canada. References [1] G. Hadjipanayis and P. Gaunt, IEEE Transactions on Magnetics, 1976, MAGI2, 393-395. [2] P. Gaunt, J. Appl. Phys. 43 (1972) 637. [3] J.M. Penisson, A. Bourret and Ph. Eurin, Acta Met. 19 (1971) 1195.
CRYSTAL MORPHOLOGY AND MAGNETIC D O M M N STRUCTURE OF THE MAGNETICALLY
H A R D A L L O Y Pto.TNio.3Fe
P. GAUNT Department of Physics, University of Manitoba, Winnipeg, Canada R3T 2N2 A magnetically hard ordered tetragonal alloy has been examined using a 1 MeV electron microscope. A {ll0} lamellar structure is found; the c axes of the tetragonal lamellae being at 90° to the c axis of the locally dominant tetragonal matrix. Lorentz microscopy reveals 180° domain walls parallel to the matrix c axis. The lamellae are decorated with pairs of 90° domain walls. It is suggested that 180° walls can be pinned where they meet junctions between {110} lamellae. Moving a wall from such a junction increases the 180° domain wall area. The pinning force from this interaction is shown to be of the appropriate order of magnitude to explain the coercive field.
1. Introduction After appropriate heat treatment the alloys P t x N i l _ x F e w i t h x = 0.2 a n d 0.3 b e c o m e m a g n e t i c a l l y h a r d [1]. In t h e s t a t e o f o p t i m u m h a r d n e s s t h e a l l o y s h a v e an o r d e r e d t e t r a g o n a l s t r u c t u r e w i t h P t / N i at [000] a n d [~0] a n d F e at [0~] a n d [½0½]. T h e v a l u e o f c/a is = 0 . 9 6 . T h e p r e s e n t i n v e s t i g a t i o n w a s u n d e r t a k e n to d e t e r mine the nature of the interaction responsible f o r t h e high m a g n e t i c h a r d n e s s o f t h e alloy. I f a s e g m e n t o f d o m a i n wall, o f a r e a A , i n t e r a c t s w i t h a p i n n i n g site, o f e n e r g y U(x), t h e n t h e a b s o l u t e z e r o c o e r c i v e f o r c e is g i v e n b y : H0 = (d U[dx)m~x[2MA,
P e n i s s o n et al. [3] o n P t C o a l l o y s . In t h e o p t i m u m s t a t e , a f t e r a g e i n g f o r 1.33 h, {110} l a m e l lar t r a c e s w e r e o b s e r v e d . T h e l a m e l l a e h a d t h e i r c a x e s at 90 ° to t h e c a x i s o f t h e s u r r o u n d i n g dominant tetragonal matrix. These conclusions w e r e c o n f i r m e d f o r six d i f f e r e n t c r y s t a l o r i e n t a t i o n s b y t r a c e a n a l y s i s a n d d a r k field micrographs from super-lattice reflections. Fig. 1 s h o w s a d e f o c u s s e d i m a g e f r o m a d i s c
(1)
w h e r e M is t h e s a t u r a t i o n m a g n e t i z a t i o n p e r u n i t v o l u m e [2].
2. Specimen preparation for electron microscopy T h e s p e c i m e n s w e r e 0.2 c m d i s c s o f t h i c k n e s s 0.05 cm. T h e y w e r e a g e d at 700°C to p r o d u c e m a g n e t i c h a r d e n i n g . J e t e l e c t r o - p o l i s h i n g in concentrated hydrochloric acid and perforation by a bath method produced some thinned areas. A n A E I E M 7 1 M e V e l e c t r o n m i c r o s c o p e , at the Department of Metallurgy and Materials Science Oxford, was used while the author was on s a b b a t i c a l l e a v e . U s e o f an a p p r o p r i a t e objective pole-piece and entry stage reduced the m a g n e t i c field at t h e s p e c i m e n to l e s s t h a n 50Oe. This made Lorentz microscopy of domain walls possible but reduced the optimum resolution of the instrument.
3. Results The crystal morphology of the alloys closely p a r a l l e l s t h e high t e m p e r a t u r e a g e i n g r e s u l t s o f
Physica 86-88B (1977) 1472-1474 (~ North-Holland
Fig. 1. Defocussed image of alloy in optimum state. The walls parallel to [001] are 180° domain walls. The (011) and (011) lameUae are decorated by pairs of 90° domain walls. The ringed area shows the reversal of lamellar contrast on crossing a 180° wall.
1473 aged f o r 1.33 h at 700°C to give a c o e r c i v e f o r c e , at r o o m t e m p e r a t u r e , o f 1380 Oe. T h e foil plane is (100) and traces o f (011) and (01D lameUae can be seen. The, c o m p a r a t i v e l y , long range b l a c k and white c o n t r a s t lines, parallel to [001], are identified with 180 ° d o m a i n walls. T h e y are d e t e r m i n e d b y the d o m i n a n t phase, w h o s e c axis, and h e n c e e a s y direction o f m a g n e t i z a t i o n is parallel to [001]. E a c h lamellar trace is dec o r a t e d b y a black and white d o m a i n c o n t r a s t effect and these are identified with 90 ° d o m a i n walls; since the e a s y direction o f m a g n e t i z a t i o n , (the c axis), r o t a t e s t h r o u g h 90 ° in c r o s s i n g f r o m matrix to lamella. T h e s e relationships w e r e confirmed f o r o t h e r foil orientations. W h e n a lamella trace c r o s s e s a 180 ° wall the black-white c o n t r a s t o f the lamella reverses. A n e x a m p l e o f this is ringed in fig. 1 and a s k e t c h o f the effect is s h o w n in fig. 2. T h e sample o f fig. 1 had not e x p e r i e n c e d a field g r e a t e r than 50 O e since q u e n c h i n g f r o m a b o v e its Curie t e m p e r a t u r e . A field o f 2000 O e was, t h e r e f o r e , applied to the s p e c i m e n outside the m i c r o s c o p e and parallel to [001]. T h e mic r o g r a p h o f fig. 3, f r o m the s a m e area as fig. 1, w a s then taken.
[Ol o]
Fig. 3. Defocussed image of same area as fig. 1 after application and removal of a 2000Oe field parallel to [001]. Ringed area shows 180° wall crossing a lamellar junction.
4. Discussion [OOl]
w
W
w
Fig. 2 s h o w s that if a 180 ° wall c r o s s e s a lamellar j u n c t i o n it is n o t n e c e s s a r y to h a v e a d o m a i n wall s e g m e n t or u n c o m p e n s a t e d poles within the lamellae. If, h o w e v e r , the wall m o v e s , an e n e r g y o f interaction arises in the f o r m o f an extra s e g m e n t of 180 ° wall and u n c o m p e n s a t e d poles within the lamellae. This is illustrated in fig. 4. F r o m the d a t a o f [1] the 180 ° wall e n e r g y is e s t i m a t e d as ~ 1 0 erg c m -2. [OLO1
Fig. 2. Change in lamellar 90~ wall contrast on crossing a 180" domain wall and 180° wall meeting iamellar junctions, b and w indicate lines of black and white contrast.
[OOll b
T h e 180 ° d o m a i n walls o f fig. 3 h a v e shifted and in the n e w r e m a n e n t state s e e m to s h o w a p r e f e r e n c e f o r c r o s s i n g lamellae at a 90 ° junction b e t w e e n (011) a n d (011) lamellae. It is suggested that this r e p r e s e n t s the pinning mec h a n i s m in these alloys a n d is d i s c u s s e d below.
~
'
~
b
Fig. 4. 180° wall leaving lamelar junctions.
1474 Taking the lamellar thickness, t, as 7 × 10 -7 cm, the spacing, d, as 10 -5 cm and the other lamellar dimension as l0 -5 cm gives a m a x i m u m lamellar contribution to wall energy of 10-1°erg per junction. The m a x i m u m demagnetizing energy is estimated as 0.5 M2t2d. Taking, M, the saturation magnetization, as 1 0 3 e m u c m -3 gives a value of 5 x 10-~2erg which is negligible compared to the wall energy. The interaction energy reaches its m a x i m u m value w h e n the matrix wall has m o v e d a distance t, giving (dUldX)m~x ~3 x 10-4erg cm -~. Eq. (1) then gives the coercivity, H0, as ~1500 Oe. This is the right order of magnitude but m o r e exact calculations of the interaction energy are required. The coercive
field is particularly sensitive evaluation of (dUldx)ma,,.
to the
proper
A c k n o w l e d g e m e n t s are due to P r o f e s s o r Sir P e t e r Hirsch and Dr. J.P. J a k u b o v i c s for the provision of laboratory facilities and unstinting help. The w o r k was supported by the National R e s e a r c h Council of Canada. References [1] G. Hadjipanayis and P. Gaunt, IEEE Transactions on Magnetics, 1976, MAGI2, 393-395. [2] P. Gaunt, J. Appl. Phys. 43 (1972) 637. [3] J.M. Penisson, A. Bourret and Ph. Eurin, Acta Met. 19 (1971) 1195.