MAGNETIC PROPERTIES OF MELT-SPUN AND SINTERED Fe-Nd-B MAGNETS AT ELEVATED TEMPERATURES

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MAGNETIC PROPERTIES OF MELT-SPUN AND SINTERED Fe-Nd-B MAGNETS AT ELEVATED

TEMPERATURES

G. Hadjipanayis, Y. Tao

To cite this version:

G. Hadjipanayis, Y. Tao. MAGNETIC PROPERTIES OF MELT-SPUN AND SINTERED Fe-Nd- B MAGNETS AT ELEVATED TEMPERATURES. Journal de Physique Colloques, 1985, 46 (C6), pp.C6-237-C6-241. �10.1051/jphyscol:1985641�. �jpa-00224894�

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JOURNAL DE PHYSIQUE

Colloque C6, supplément au n°9, Tome 46, septembre 1985 page C6-237

MAGNETIC PROPERTIES OF MELT-SPUN AND SINTERED Fe-Nd-B MAGNETS AT ELEVATED TEMPERATURES

G.C. Hadjipanayis and Y.F. Tao

Department of Physios, Kansas State University, Manhattan, Kansas 66506, U.S.A.

Résumé - Les propriétés magnétiques d'aimants Nd-Fe-B préparés par trempe sur rouleau et frittes ont été mesurés entre 20 et 350°C. La coercitivité de tous les échantillons décroit très rapidement lorsque la température augmente et devient extrêmement faible à des températures bien inférieures à la température de Curie qui est proche de 300°C. La variation thermique de l'aimantation M sous champ, en températures croissantes et décroissantes, a été étudiée. Elle correspond au comportement de la courbe de première aimantation ainsi qu'à la dépendance en champ de coercitivité.

Abstract - The magnetic properties of melt-spun and sintered Fe-Nd-B magnets have been measured in the temperature range of 20 - 350°C. The coercivity of all samples decreases drastically with increasing temperature and is very small at temperatures considerably lower than the Curie temperature which is around 300 C. The effects of temperature on magnetization during the heating and cooling cycles of the ii vs T experiment have been studied an'd found to be consistent with the initial magnetization behavior and field dependence of coercivity.

I ~,IiiTR0DUCTI0i;

The recent discovery of non-cobalt containing Fe-R-D alloys /1-4/ has generated a new revolution in permanent magnets. Coercive fields greater than 10 kOe and energy products up to 45 liGOe have been obtained in both melt-spun /1-3/ and sintered Fe- Nd-B magnets /4-5/. The hard magnetic properties of these materials are attributed to the presence of the highly anisotropic tetragonal phase Fe,,R„3 /6-7/. This phase has a relatively low Curie temperature around 300 C that causes large temperature coefficients of coercivity leading to small values of coercivity around 150 C thus limiting their use for electromechanical applications /l/.

In the present study we investigate the temperature dependence of the magnetic properties in more detail to check for any irreversible changes that might occur in the magnets upon heating to higher temperatures and more importantly to gain more insight on the magnetization reversal mechanism that occurs in this system.

II - EXPERIMENTAL METHODS

Commercial Fe-ild-B permanent magnets were obtained from Sumitomo Special iletals Co.

IZI (Neo 35, Neo 30 II) and Colt Crucible Industries /9/ (Cruraax 40). Neo 35 and Crumax 40 are ternary Fe-Nd-B magnets, while Ueo 30 II contains a small amount of Dy.

Melt spun ribbons were annealed at around 700 C for 10 - 15 min for optimum hard magnetic properties. The magnetic properties were measured with a vibrating sample magnetometer in fields up to 17 kOe and in the temperature range of 20 to 350 C.

Thermoraagnetic data (ii vs T) were obtained in the same temperature range by measuring the magnetization in a constant magnetic field as a function of temperature.

The magnetization was monitored continuously while the samples were heated to diffe- rent temperatures in various magnetic fields and then cooled to room temperature in the same measuring field.

(1) Work supported by the Office of Naval Research and U.S. Army Research.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985641

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C6-238 JOURNAL DE PHYSIQUE

111 - RCSiTLTS

Tie i n i t i a l i n a g n e t i z a t i o n c u r v e s and h y s t e r e s i s l o o p s f o r b o t h n e l t - s p i n and s i n - t e r e d le-;!d-3 s a a p l e s a r e shoi?m i n 2 i g . 1. The i n i t i a l r . l a g n e t i z a t i o n o f s i n t e r e d m a g n e t s i n c r e a s e s r a t h e r d r a s t i c a l l y w i t h a p p l i e d f i e l d a n d i t s a t u r a t e s a t t i i e raaxiinunl a v a i l a b l e f i e l d o f 1 7 W e . i n c o n t r a s t , t h e i n i t i a l i L i a g n e t i z a t i o n o f r i b - b o n s i n c r e a s e s g r a d u a l l y b u t s l o t r l y w i t h f i e l d a n d i t i s f a r froin s a t u r a t i o n a t t h e f i e l d o f 17 kOe. The c o e r c i v i t y i s a l s o found t o be d e p e n d e n t on t h e a p p l i e d f i e l d ( F i g . 2 ) i n d i c a t i n g t h a t t h e h y s t e r e s i s 100i; o b t a i n e d f o r r i b b o n s i s a iainor loo;, ( g . l ) T h e c o e r c i v i t y b e l l a v i o r shown i n f i g . 2 i n d i c a t e s t h e p r e s e n c e o f two doi~rain w a l l p i n n i n g mec:lanisn~s i i i t h e s e n a g n e t s / I D / .

The h y s t e r e s i s l o o p s o b t a i n e d a t e l e v a t e d t e f i i p r a t u r e s i n iteo 3 0 1: are shown i n Z i g . S. The s m a l l s i l o u l d e r o b s e r v e d o n t h e d e l n a g n e t i z a t i o n c u r v e d e c r e a s e s g r a d u a l l y a t h i g h e r t e i a p e r a t u r e s a n d i t d i s a p p e a r s a b o v e 2 9 0 ' ~ . T h i s i s p r o b a b l y due t o l a c k o f s a t u r a t i o n i n t h e l o w e r t e m p e r a t u r e r a n g e b e c a u s e o f t h e v e r y l a r g e v a l u e s o f c o e r - c i v i t y . Tine t e n p e r a t u r e d e p e n d e n c e o f m a g n e t i c p r o p e r t i e s c a n b e s e e n b e t t e r i i l

7 . k ~ g . 4. ifeo 30 B h a s much h i g h e r c o e r c i v i t i e s w i t h s l i g i l t l y l a r g e r t e m p e r a t u r e c o e f f i c i e n t s of c o e r c i v i t y , b u t a r o u n d 1 5 0 ' ~ i t s c o e r c i v i t y i s l a r g e r t h a n 5 kOe, a v a l u e w h i c h i s c o n s i C e r e d t o b e s a t i s f e c t o r y f o r e l e c t r o i n e c h a n i c a l s p p l i c a t i o n s . The i n a g n e t i c p r o p e r t i e s o f t h e s i n t e r e d m a g n e t s a n n e a l e d a t h i g h e r t e m p e r a t u r e s d o n o t c h a n g e when m e a s u r e d a g a i n a t rooili t e m p e r a t u r e . I n r i b b o n s , however, t h e c o e r - c i v i t y was f o u n d t o d e c r e a s e f r o m G kOe t o G l:0e when t h e s a i a p l e was h e a t e d t o 3 0 3 ~ ~ a n d t h e n cooleci t o roor!l t a i i p e r a t u r e . Y n i s i s p o s s i b l y d u e t o s o a e -Fe p r e c i p i t a - t i o n which o c c u r s d u r i n g a n n e a l i n g a t t h e s e t e n p e r a t u r e s .

Tile thermoiilagnetic d a t a a t d i f r ' e r e n t f i e l d s a n d t e m p e r a t u r e s are shown i n F i g . 5 -

F i g . 7. I n t h e c o n l i i e r c i a l m a g n e t s t i l e r i a g n e t i z a t i o n i n siilall a p p l i e d f i e l d s i n - c r e a s e s s i g n i f i c a n t l y iiitI1 t e i i i p e r a t u r e , a n d s u b s t a n t i a l l y when c o o l e d i n t h e n e a s u - r i n g f i e l d ( P i g . 5 a ) . T h i s b e h a v i o r i s d i i ~ i n i s i l i n a a t h i g h e r f i e l d s ( F i g . 5 b ) a n d a t a E i e l d o f 5 kOe o n l y a slnall c h a n g e h a s been o b s e r v e d d u r i n g t h e h e a t i n g a n d c o o l i n g c y c l e s . I n r i b b o n s a s l i g ! l t l y d i f f e r e n t b e i i a v i o r h a s b e e n o b s e r v e d . A t v e r y small f i e l d s n o c h a n g e h a s b e e n o b s e r v e d i n t h e l i l a g n e t i z a t i o l i d u r i n g t h e i.i v s T e x p e r i i n e n t s ( F i g . 6 a ) . I:owever, a t h i g h e r i ' i e l d s t h e magnetization c h a n g e s d r a s t i - c a l l y d u r i n g t i i e h e a t i n g a n d c o o l i n g c y c l e s ( F i g . Gb). I: s i L l i l a r b e h a v i o r was a l s o o b s e r v e d i n s i n t e r e d m a g n e t s w h i c h t i e r e f i r s t r j i a g n e t i z e d a n d t h e n d e m a g n e t i z e d f o r t h e L.1 v s T e x p e r i m e n t ( P i g . 7 ) .

The s u b s t a n t i a l d r o p i n c o e r c i v i t y a t h i g h e r t e m p e r a t u r e s i s r e l a t e d t o b o t h t h e r m a l a c t i v a t i o n a n d t l i e d e c r e a s e or' a n i s o t r o p y f i e l d v h i c h r e a c h e s v e r y s m a l l v a l u e s a t a r o u n d 3 0 0 ~ ~ /11/. Tlie h i g h e r c o e r c i v i t y o b t a i n e d i n :ieo 3 0 I1 is p r o b a b l y d u e t o t h e h i g h e r a r i s o t r o p y f i e l d o f t h i s nagn net c a u s e d by t h e s n a l l a d d i t i o n o f 3 y /12/.

Tile t h e r s o m a g n e t i c d a t a c a n b e u n d e r s t o o d by c o n s i d e r i n g b o t h t h e i n i t i a l magne- t i z a t i o n c u r v e s a n d t h e f i e l d d e p e n d e n c e o f c o e r c i v i t y as shoiin e a r l i e r . I n s i n - t e r c d m a g n e t s t i l e c o e r c i v i t y i s siilall i n sclall a p p l i e d f i e l d s a n d d o e s n o t c h a n g e much i n f i e l d s u p t o 3 kOe. T h e a p p l i e d f i e l d s u s e d f o r t h e i i v s T e x p e r i m e n t are l a r g e r t h a n t h e c o e r c i v i t y s o t h e sa:nple i s l n a g n e t i z e d somewhere a t t h e s t e e p p a r t 0 5 t h e i n a g n e t i z a t i o n c u r v e . i,Jhen t h e s a m p l e i s h e a t e d t o h i g h e r t e n p e r a t u r e s t h e i i r a g n e t i z a t i o n i s i n c r e a s e d b e c a u s e t i i e a n i s o t r o p y i i e l d i s r e d u c e d . A t h i g h e r a p - p l i e d f i e l d s , t h e sarople i s c l o s e r t o s a t u r a t i o n and t h e e f f e c t o f t e i i l p e r a t u r e i s d e c r e a s e d s u b s t a n t i a l l y . The d i f f e r e n t b e h a v i o r o b s e r v e d i n t h e f i e l d - d e m a g n e t i z e d s i n t e r e d s a m p l e s is d u e t o t h e d i f f e r e n t s h a p e o f t h e i r i n i t i a l i i r a g i l e t i z a t i o n c u r v e . I n t h e s e sarirples, t i l e i n i t i a l i : l a g n e t i z a t i o ~ l f i r s t i n c r e a s e s r a t h e r s l o r ~ r l y a n d t h e n more r a p i d l y w i t h a p p l i e d f i e l d /13/ a n d t h e sample n e e d s h i g h e r f i e l d s t o b e i m g n e t i z e d t h a n i n t h e t i l e r i r a l l y d e ~ . i a g n e t i z e d s t a t e . T h i s e x p l a i n s t h e l a r g e i n - c r e a s e s i n t h e n a g n e t i z a t i o 1 1 o b s e r v e d d u r i n g t h e i'i v s 'I' e x p e r i m e n t a t h i g h e r a p p l i e d L ' i e l d s , s i n c e t i l e sai!iple i s f a r froill s a t u r a t i o n i n t h e a p p l i e d f i e l d s u s e d . Iil r i b b o n s , a'c small a p p l i e d f i e l d s t h e c o e r c i v i t y or' t h e s a m p l e s i s a u c h h i g h e r t h a n t h e f i e l d u s e d s o t h a t t h e e f r e c t o f a p p l i e d f i e l d i n m a g n e t i z i n g t h e s a m p l e s i s

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JOURNAL DE PHYSIQUE

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r i b b o n s .

P i g . 7. Thermomagnetic d a t a i n f i e l d - d e c i a a n e t i z e d s i n t e r e d Fe-;Id-2 x a g n e t s .

minimum. At h i g h e r t e m p e r a t u r e s t h e c o e r c i v i t y i s d e c r e a s e d b u t i t i s s t i l l l a r g e r t h a n t h e a p p l i e d f i e l d a n d n o c h a n g e i s o b s e r v e d i n t h e m a g n e t i z a t i o n d u r i n g t h e :i v s T e x p e r i m e n t . Xowever, a t a r o u n d 2 5 0 ' ~ t h e a n i s o t r o p y i ' i e l d i s d e c r e a s e d d r a s - t i c a l l y a n d t h i s r e s u l t s t o a peal: i n ~ n a g n e t i z a t i o n i n t h e h e a t i n g c y c l e . A t l a r g e r a p p l i e d f i e l d s t h e c o e r c i v i t y i s s m a l l e r t h a n t h e a p p l i e d f i e l d , t h e sample i s n a g n e t i z e d t o a v a l u e below s ~ t u r a t i o n 2nd l a r g e i n c r e a s e s are o b s e r v e d i n t h e n a g n e t i z a t i o n a s i n t h e s i n t e r e d s a n p l e s .

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