Rare Earth Intermetallics for Magnetostrictive Devices - Advances in

15 Rare Earth Intermetallics for Magnetostrictive

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Devices J O S E P H B.

MILSTEIN

1

N a v a l Research Laboratory, Washington, D.C. 20375

A discussion

of methods

of preparation

cubic

Laves phase compounds,

tions

is given.

studied

might be utilized

of the unusual

these physical

properties

R E F e 2 crystals,

confirmation

properties

of

the

methods by

materials.

in

measuring of the

inter-

and iron atoms in

single-ion

elucidation

of solu-

prepared

of the

obtained

anisotropy,

for and preferred

in both bulk

preparative

of rare-earth

to these systems,

crystals

the materials

lead to the explanation

interactions

the magnetocrystalline

of single

and their solid

devices are provided

results

atomic magnetic

the potential

the

physical

experimental

materials

for

of how

in magnetostrictive

Fundamental

appropriate

2

Motivation

and an explanation

a review

REFe ,

of the

model

as

nature

of

and a demonstration

mode of application

and surface

wave

of

of the

magnetoacoustic

devices.

ATagnetostrictive

materials have

the

property

that their physical

d i m e n s i o n s are c h a n g e d d u r i n g the process of m a g n e t i z a t i o n . T h e m a g n e t o s t r i c t i v e effect is u s e d , f o r e x a m p l e , i n c e r t a i n sonar devices s u c h as t h e free-flooded n i c k e l s c r o l l m a g n e t o a c o u s t i c t r a n s d u c e r . T h e m a g n e t o s t r i c t i v e s t r a i n a v a i l a b l e i n c u b e - t e x t u r e d o x i d e - a n n e a l e d n i c k e l is o n l y 35 p a r t s p e r m i l l i o n ( p p m ) , h o w e v e r .

L a r g e r strains, a v a i l a b l e at

r o o m t e m p e r a t u r e a n d s m a l l m a g n e t i c fields, w o u l d b e h i g h l y d e s i r a b l e for device applications. T h e l a n t h a n i d e r a r e e a r t h metals e x h i b i t l a r g e m a g n e t o s t r i c t i o n , u p to 10,000 p p m , b u t at c r y o g e n i c t e m p e r a t u r e s . T h e m e t a l s i r o n , c o b a l t , C u r r e n t a d d r e s s : P h o t o v o l t a i c A d v a n c e d S i l i c o n B r a n c h , Solar E n e r g y R e s e a r c h Institute, 1617 C o l e B o u l e v a r d , G o l d e n , C O 80401. 1

T h i s c h a p t e r n o t subject to U . S . c o p y r i g h t . P u b l i s h e d 1980 A m e r i c a n C h e m i c a l Society

Holt et al.; Solid State Chemistry: A Contemporary Overview Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

292

CHEMISTRY:

A

CONTEMPORARY

OVERVIEW


SOLID S T A T E

H(kOt) American Institute of Physics

Figure 1.

Room-temperature magnetostrictive strain and strain polarity as a function of magnetic field (5)


15. and

Intermetallics

MILSTEIN

for Magnetostrictive

293

Devices

n i c k e l are g o o d ferromagnets at r o o m t e m p e r a t u r e .

One

therefore

m i g h t expect that c o m b i n a t i o n s of these m e t a l s , as alloys or i n t e r m e t a l l i c c o m p o u n d s , c o u l d e x h i b i t l a r g e m a g n e t o s t r i c t i v e strains at r o o m t e m p e r a ture. T h e d i s c o v e r y of l a r g e , r o o m - t e m p e r a t u r e m a g n e t o s t r i c t i v e strains i n t h e class of c u b i c L a v e s - p h a s e ( I )

intermetallic compounds,

REFe , 2

w h e r e R E represents a l a n t h a n i d e rare e a r t h m e t a l , w a s m a d e at t h e N a v a l Research Laboratory ( N R L ) (2) a n d the N a v a l Surface Weapons Center ( N S W C ) (3).

E x a m p l e s of the s t r a i n as a f u n c t i o n of field r e l a


t i o n s h i p are s h o w n i n F i g u r e 1 f o r a n u m b e r of Pure binary R E F e however.

2

compositions.

c o m p o u n d s suffer f r o m a n u m b e r of

L a r g e magnetic

fields,

of

the order

of

r e q u i r e d to p r o d u c e the l a r g e magnetostrictions. from a device standpoint.

problems,

several tesla,

are

T h i s is i m p r a c t i c a l

I n c e r t a i n cases m a g n e t i c s a t u r a t i o n is n o t

o b t a i n e d e v e n at 12T. T h e m a g n e t o s t r i c t i v e strains v a r y as a f u n c t i o n of r a r e - e a r t h elements, t e m p e r a t u r e , a n d c r y s t a l l o g r a p h i c o r i e n t a t i o n . m a t e r i a l s are b r i t t l e i n p o l y c r y s t a l l i n e f o r m .

The

T h e r a r e e a r t h metals are

e x t r e m e l y reactive at e l e v a t e d t e m p e r a t u r e s , w h i c h poses serious f a b r i c a tion problems.

L a s t l y , the r a r e e a r t h metals are expensive at this t i m e .

T h e s e p r o b l e m s w i l l b e addressed i n the f o l l o w i n g sections.

The

Physics of Magnetostriction

in REFe

2

Compounds

T h e structure of the c u b i c L a v e s - p h a s e R E F e

2

c o m p o u n d s is s h o w n

i n F i g u r e 2. T h e l a t t i c e p a r a m e t e r for t h e T b , D y , H o , a n d E r b i n a r y c o m p o u n d s are a p p r o x i m a t e l y 7.34, 7.32, 7.30, a n d 7.28 A , r e s p e c t i v e l y (4).

F r o m a n e x a m i n a t i o n of t h e structure, a n d f r o m the l a t t i c e p a r a m

eter, one sees t h a t the shortest r a r e e a r t h - r a r e e a r t h d i s t a n c e is a p p r o x i m a t e l y 3.2 A , r o u g h l y the same or s l i g h t l y shorter t h a n t h a t i n p u r e r a r e - e a r t h metals. T h i s distance w i l l be d i s c u s s e d i n a l a t e r section. How, strictive

t h e n , does one strain

at l o w

o b t a i n a m a t e r i a l t h a t has l a r g e

fields?

The

pure

magneto

binary compositions

d e p e n d i n g o n the r a r e - e a r t h m e t a l , either p o s i t i v e or n e g a t i v e

have,

magneto

s t r i c t i o n ( A ) a n d either p o s i t i v e or negative a n i s o t r o p y constants ( K i , K ) . 2

T h e s e are p r e s e n t e d i n T a b l e I a n d are b a s e d o n o b s e r v e d values a n d t h e o r e t i c a l estimates b y C l a r k ( 5 ) .

T h e a n i s o t r o p y of the R E F e

2

phases

is d o m i n a t e d b y t h e r a r e - e a r t h m e t a l . T o h a v e the best m a g n e t o s t r i c t i v e response a m a t e r i a l s h o u l d h a v e as l a r g e a m a g n e t o s t r i c t i o n ( A ) a n d as s m a l l a t o t a l a n i s o t r o p y ( K )

as

p o s s i b l e , as the figure of m e r i t u s e d , w h i c h is p r o p o r t i o n a l to A / K , w i l l 2

b e m a x i m i z e d . E x a m i n i n g the t a b l e entries, one sees t h a t some c o m b i n a t i o n of T b , D y , H o , a n d F e w i l l y i e l d p o s i t i v e m a g n e t o s t r i c t i o n a n d , i n p r i n c i p l e , zero K

x

a n d K , a s s u m i n g that the parameters A, K i , a n d K 2

m a y be c o m b i n e d i n a l i n e a r m a n n e r .


2


294

SOLID

o

STATE CHEMISTRY:

O

A ATOM

(

A

) B

CONTEMPORARY

OVERVIEW

ATOM

Figure 2. Crystallographic structure of the cubic (C-15) Laves phase. atoms are large (RE) atoms; B atoms are small (Fe) atoms.

T h e s t u d y of the m a g n e t o s t r i c t i o n a n d a n i s o t r o p y constants

A

as

a

f u n c t i o n of c o m p o s i t i o n is c a r r i e d out best o n h o m o g e n e o u s s i n g l e - c r y s t a l specimens i n o r d e r to o b t a i n the c o m p o s i t i o n a n d c r y s t a l l o g r a p h i c o r i e n t a t i o n d e p e n d e n c e s as a c c u r a t e l y as possible. M a g n e t i c a n i s o t r o p y m e a s u r e ments on R E F e t o r q u e (6)

2

m a t e r i a l s h a v e b e e n c a r r i e d out b y u s i n g t h e m a g n e t i c

and magnetization (7)

techniques.

I n the f o r m e r

technique

one measures d i r e c t l y the energy r e q u i r e d to rotate t h e m a g n e t i z a t i o n f r o m one c r y s t a l l o g r a p h i c d i r e c t i o n to another.

I n the latter

o n e m u s t c a l c u l a t e the a n i s o t r o p y b a s e d o n the m a g n e t i z a t i o n

method behavior

i n selected c r y s t a l l o g r a p h i c d i r e c t i o n s .

Polarity of A , K i , and K2

Table I.

TbFe

2

DyFe

2

HoFe

2

A

+

+

+

#1

-

+

+

K

+

-

+

2


15.

Intermetallics

MILSTEIN


295

Devices

If s u i t a b l e single crystals are a v a i l a b l e , one

m a y w i s h to

study

other p h y s i c a l p r o p e r t i e s as w e l l .

Crystal

Growth crystals.

These

are t h e T r i a r c C z o c h r a l s k i m e t h o d ( 8 , 9 ) , t h e r a d i o f r e q u e n c y ( r f )

T h r e e m e t h o d s h a v e b e e n u s e d for g r o w i n g R E F e

levita-

t i o n C z o c h r a l s k i (10) Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch015

m e t h o d (11).

2

m e t h o d , a n d the r f l e v i t a t i o n h o r i z o n t a l - z o n i n g

I n a l l cases one m u s t observe c e r t a i n p r e c a u t i o n s because

of the extreme r e a c t i v i t y of r a r e - e a r t h metals.

Titanium-gettered argon

c a n b e u s e d as a p r o t e c t i v e a t m o s p h e r e to p r e v e n t the c o n t a m i n a t i o n of the m e l t b y o x y g e n , i n p a r t i c u l a r . O n e s h o u l d e x c l u d e f o r e i g n m a t e r i a l s to the greatest extent p o s s i b l e , p a r t i c u l a r l y f r o m objects that contact the m e l t , s u c h as seed w i r e s a n d crucibles.

O n e s h o u l d use t h e most r a p i d g r o w t h m e t h o d a v a i l a b l e i n

o r d e r to r e d u c e the t i m e d u r i n g w h i c h c o n t a m i n a t i o n m a y occur.

Addi

t i o n a l l y , one m a y w i s h to h a v e t h e p o s s i b i l i t y of u s i n g a seed i n o r d e r to p r e p a r e

crystals of

materials o f

selected

orientation.

One

s h o u l d use s t a r t i n g

t h e best p u r i t y a v a i l a b l e , e s p e c i a l l y w h e r e h i g h - q u a l i t y

crystals are d e s i r e d .

F o r c o m m e r c i a l p r o d u c t i o n of

REFe

materials,

2

h o w e v e r , the cost of s t a r t i n g m a t e r i a l s m a y l i m i t the p u r i t y of t h e r a r e e a r t h that c a n b e u s e d p r o f i t a b l y . T h e s t a r t i n g c o m p o s i t i o n that s h o u l d be u s e d to p r e p a r e a

given

c o m p o u n d frequently can be determined by consulting a suitable phase diagram.

U n f o r t u n a t e l y , for most rare e a r t h i n t e r m e t a l l i c s , s u c h p h a s e

d i a g r a m s d o n o t exist or are of q u e s t i o n a b l e a c c u r a c y .

Some

systems,

s u c h as E r - F e , h a v e b e e n i n v e s t i g a t e d b y a n u m b e r of w o r k e r s , w i t h v a r y i n g results

(12,13,14).

One

then must work

empirically, using

v a r i o u s s t a r t i n g c o m p o s i t i o n s ; b y c o r r e l a t i n g t h e results o b t a i n e d u n d e r the a p p r o p r i a t e t h e r m o d y n a m i c r e g i m e , o n e c a n d e d u c e c e r t a i n features of the p h a s e d i a g r a m of interest. I n this w a y H o F e a n d E r F e 2

s h o w n to b e h a v e as c o n g r u e n t l y m e l t i n g m a t e r i a l s ( 9 ) .

have been

2

F o r these m a t e

rials, as w e l l as ternaries a n d q u a t e r n a r i e s b a s e d o n H o F e , i t is u s e f u l to 2

start w i t h a 1 % r a r e e a r t h - r i c h c o m p o s i t i o n , t h a t is, R E F e i . , i n o r d e r t o 9 8

p r e v e n t the f o r m a t i o n of second-phase m a t e r i a l . T h e s t a r t i n g c o m p o s i t i o n c h o s e n i m p l i e s t h a t for a c o n g r u e n t l y m e l t i n g m a t e r i a l t h e r a r e e a r t h s h o u l d c o n t a i n n o m o r e t h a n 1 % a t o m i c o x y g e n , or r o u g h l y 1 0 p p m oxy 3

g e n b y w e i g h t . I f the o x y g e n c o n c e n t r a t i o n is l a r g e r t h a n this, one r u n s the risk of g r o w i n g a c r y s t a l c o n t a m i n a t e d w i t h ever i n c r e a s i n g q u a n t i t i e s of R E F e

3

p h a s e m a t e r i a l . F o r p e r i t e c t i c m a t e r i a l s , s u c h as D y F e

T b F e , the rare-earth concentration 2

must exceed 3 3 %

atomic b y

2

or an

a m o u n sufficient to c o m p e n s a t e f o r b o t h o x y g e n c o n c e n t r a t i o n a n d t h e r a r e - e a r t h excess r e q u i r e d b y

thermodynamic

considerations.


It

may

296

SOLID S T A T E

CHEMISTRY: A

CONTEMPORARY OVERVIEW

p r o v e necessary to t r y s e v e r a l s t a r t i n g c o m p o s i t i o n s i n o r d e r to g r o w a desired material, because the oxygen concentration b y weight i n pure c o m m e r c i a l r a r e earths is s o m e w h a t v a r i a b l e a n d m a y r e a c h 1 0 a l t h o u g h 1 0 p p m m a y b e f o u n d m o r e c o m m o n l y i n the m o r e 3

4

ppm,

expensive

d i s t i l l e d grades of the h e a v y r a r e - e a r t h metals. A

convenient

and straightforward method

for preparing starting

melts is the r f l e v i t a t i o n m e l t i n g of ingots of the metals i n t h e d e s i r e d proportions.

R e a c t i o n takes p l a c e

quickly, p r o d u c i n g a homogeneous


melt i n minutes. T h e a p p l i c a t i o n of t h e C z o c h r a l s k i m e t h o d r e q u i r e s a seed, w h i c h is d i p p e d i n t o the m o l t e n s t a r t i n g c h a r g e a n d w i t h d r a w n at a selected r a t e u n d e r c o n t r o l l e d t h e r m a l c o n d i t i o n s , l e a d i n g to the g r o w t h of boule.

a

I n the absence of a single c r y s t a l f r o m w h i c h a n o r i e n t e d seed

m a y b e o b t a i n e d , o n e m a y c u t a s e c t i o n of a p o l y c r y s t a l l i n e i n g o t s u c h that a s m a l l n u m b e r of grains, i d e a l l y o n l y one, w i l l contact t h e m e l t u p o n seeding.

A f t e r g r o w i n g a b o u l e , one m a y r e p e a t the process u n t i l

a seed of t h e d e s i r e d o r i e n t a t i o n is o b t a i n e d . T h e q u a l i t y of the g r o w n c r y s t a l w i l l b e d e t e r m i n e d i n p a r t b y t h e p r e s e n c e o r absence of contact b y f o r e i g n solids w i t h the c r y s t a l or the c r y s t a l - m e l t interface as g r o w t h p r o c e e d s .

A

s o l i d t h a t contacts

the

c r y s t a l m a y i n t r o d u c e defects b y a m e c h a n i c a l or a t h e r m a l stress m e c h a n i s m as the c r y s t a l cools f r o m the m e l t i n g p o i n t to r o o m

temperature.

T h e presence of a s o l i d at the c r y s t a l - m e l t i n t e r f a c e c a n result i n the s p u r i o u s n u c l e a t i o n of s e c o n d grains o r other defects.

Such a solid can

b e either a c o n t a i n e r for the m e l t , as i n the B r i d g m a n m e t h o d , or p a r t i c l e s of dross o r d i r t t h a t h e o n the surface of the m e l t .

Dark, apparently

o x i d i c drosses are p r o d u c e d f r e q u e n t l y u p o n a l l o y i n g r a r e earths w i t h t r a n s i t i o n metals. Arc-Powered

Czochralski

Crystal

Growth.

No

single-crystal

g r o w t h a p p a r a t u s or t e c h n i q u e is the best f o r the g r o w t h of a l l crystals. T h e a p p l i c a t i o n of a g i v e n t e c h n i q u e u s i n g c e r t a i n a p p a r a t u s to a selected c r y s t a l g r o w t h p r o b l e m m u s t b e m a d e o n the basis of the p r o p e r t i e s of the d e s i r e d m a t e r i a l a n d the c a p a b i l i t i e s o f the t e c h n i q u e a n d a p p a r a t u s . T h e use b y t h e a u t h o r of a n e l e c t r i c a r c - p o w e r e d C z o c h r a l s k i m e t h o d ( 8 ) f o r t h e g r o w t h of crystals of r a r e e a r t h i n t e r m e t a l l i c c o m p o u n d s b a s e d o n several c r i t e r i a .

T h e compounds

was

are a r c c o m p a t i b l e a n d are

p r e p a r e d frequently b y arc m e l t i n g on a water-cooled

copper hearth.

P r e l i m i n a r y d a t a o n a r c - m e l t e d m a t e r i a l s suggested t h a t t h e d e s i r e d c u b i c compounds

c r y s t a l l i z e d r e a d i l y , m a k i n g the C z o c h r a l s k i t e c h n i q u e

a

n a t u r a l c h o i c e f o r r a p i d c r y s t a l g r o w t h . T h e h i g h r e a c t i v i t y of r a r e e a r t h metals suggested f u r t h e r t h a t c r u c i b l e s b e a v o i d e d i f at a l l possible. A p p r o x i m a t e l y 25 g of p o l y c r y s t a l l i n e m a t e r i a l is t r a n s f e r r e d to the crystal growth furnace.

T h e m a t e r i a l is m e l t e d o n a r o t a t a b l e , c o o l e d


15.

MILSTEIN

Intermetallics


Devices

297

Figure 3. A crystal of Ho Tb Fe grown by the triarc Czochralski method, which was studied by inelastic neutron diffraction methods (20) 088

0

12


2

c o p p e r h e a r t h b y the a c t i o n of three electric arcs d i r e c t e d at i t f r o m t h o r i a t e d t u n g s t e n electrodes.

G e t t e r e d a r g o n at a n overpressure

several p o u n d s p e r square i n c h is u s e d as p r o t e c t i v e atmosphere.

of

Once

a p o o l of l i q u i d has b e e n e s t a b l i s h e d , a seed c r y s t a l is d i p p e d i n t o t h e melt and Czochralski growth commences. I n o r d e r to m i n i m i z e r a d i a l t h e r m a l gradients i n t h e system, the m e l t a n d g r o w i n g c r y s t a l are r o t a t e d .

R o t a t i o n of t h e m e l t

effectively

causes t h e l o c a l i z e d arcs to s w e e p the surface o f the m e l t , p r o v i d i n g m o r e u n i f o r m h e a t i n g , e l e c t r o m a g n e t i c s t i r r i n g , a n d the efficient r e m o v a l of dross f r o m the g r o w t h r e g i o n . A c r y s t a l of H o . 8 8 T b . i 2 F e 0

0

2

g r o w n b y this method a n d examined

b y n e u t r o n d i f f r a c t i o n b y N i c k l o w et a l . (15)

is s h o w n i n F i g u r e 3. Crystals

Radiofrequency Levitation Czochralski Crystal G r o w t h .

also h a v e b e e n g r o w n f r o m a w a t e r - c o o l e d H u k i n c r u c i b l e (16),

using

rf i n d u c t i o n m e l t i n g w i t h e l e c t r o m a g n e t i c l e v i t a t i o n of the m e l t . A p p r o x i m a t e l y 150 g of c h a r g e m a t e r i a l is p r e p a r e d . appears u p o n c o m p o u n d i n g

m a y be

removed

Dross that

by etching i n

aqueous

H N 0 , followed by repeated washing w i t h anhydrous methanol.

The

3

c h a r g e is m e l t e d i n the c r u c i b l e , w h i c h is h o u s e d i n a n A . D . m o d e l M P c r y s t a l - g r o w i n g f u r n a c e u n d e r a n overpressure argon.

of

Little

gettered

A seed c r y s t a l is d i p p e d i n t o the m e l t a n d C z o c h r a l s k i g r o w t h

commences. I n t h i s system the t o p p o r t i o n of the m e l t is a p p a r e n t l y the coolest r e g i o n . W h a t e v e r dross is s t i l l present f r e q u e n t l y appears as a t h i n s k i n o r p a t c h o n t h e u p p e r surface of the m e l t . m e c h a n i s m f o r its r e m o v a l .

T h e r e appears to b e

that has p a r t i c u l a t e m a t t e r at the c r y s t a l - m e l t interface.

If

extremely

p u r e s t a r t i n g m a t e r i a l s w e r e u s e d , the dross m i g h t b e absent. author's experience,

no

T h e c r y s t a l therefore grows f r o m a system

u s i n g c o m m e r c i a l r a r e - e a r t h metals, a

I n the

completely

dross-free m e l t surface has n o t b e e n a c h i e v e d .


SOLID S T A T E C H E M I S T R Y :

A

CONTEMPORARY


298


OVERVIEW

15.

Intermetallics

MILSTEIN


A c r y s t a l of H o . 7 7 T b o . 3 F e 0

[lll]-oriented

2

cylinder was

2

299

Devices

g r o w n b y this method, from w h i c h a

prepared

for

magnetoacoustic

study

by

T i m m e a n d M e e k s ( 1 7 ) , is s h o w n i n F i g u r e 4. In a

Radiofrequency Horizontal Levitation Zone Crystal G r o w t h . recent p a p e r M c M a s t e r s et a l . (11) REFe

2

r e p o r t e d t h e g r o w t h of a n u m b e r of

compositions b y using an rf induction-heated horizontal levitation

zone melting method.

A s is w e l l k n o w n , z o n e m e l t i n g is a t e c h n i q u e

v e r y w e l l s u i t e d t o m a t e r i a l s that d o not m e l t c o n g r u e n t l y


as T b F e

2

such

(18),

and D y F e . 2

F r o m the d e s c r i p t i o n g i v e n i t w o u l d a p p e a r that this m e t h o d presents t w o severe p r o b l e m s f r o m t h e c r y s t a l g r o w e r s p o i n t of v i e w . extremely large temperature

First, an

g r a d i e n t m u s t exist v e r t i c a l l y a l o n g

c r y s t a l - m e l t i n t e r f a c e , since t h e top

of

the c h a r g e ,

the

w h i c h is c o o l e d

p r i n c i p a l l y b y r a d i a t i o n , is v e r y n e a r the m e l t i n g p o i n t ( o f t h e o r d e r of 1 0 0 0 ° C ) , w h i l e the l o w e r surface of the s o l i d i f i e d b o u l e , r o u g h l y 1 c m a w a y , rests o n a surface m a i n t a i n e d at a p p r o x i m a t e l y r o o m t e m p e r a t u r e b y w a t e r c o o l i n g . S e c o n d , a s o l i d surface, t h a t of the c r u c i b l e , is present at the g r o w t h interface.

These

circumstances

m a y c o n t r i b u t e to

the

g e n e r a t i o n of defects b y t h e r m a l stress or b y the n u c l e a t i o n of s e c o n d grains, o r to t h e r m a l l y i n d u c e d c o m p o s i t i o n a l v a r i a t i o n s across the c r y s t a l diameter

i n the

case

of

ternary

or

higher-order

systems.

i n the v e r t i c a l d i r e c t i o n , as has b e e n a p p l i e d to R E C o i 2

M i l l e r a n d D ' S i l v a (19),

7

Zoning

systems

by

w o u l d e l i m i n a t e b o t h of these difficulties b u t

w o u l d b e m o r e difficult to p e r f o r m . A c o m p a r i s o n of the results o b t a i n e d b y the three m e t h o d s

appears

in Table II. A l t h o u g h t h e C z o c h r a l s k i m e t h o d s d e s c r i b e d h a v e p r o d u c e d t h e largesta n d the highest- (crystallographic)

quality R E F e

2

crystals to

date,'it

s h o u l d b e n o t e d i n fairness that t h e m a t e r i a l s s t u d i e d are c o n g r u e n t l y melting. Table II.

Properties of Crystals G r o w n by Various Arc Czochralski

Size ( t y p i c a l )

~ 1-cm d i a m e t e r X 1-2-cm length ( ~ 1.5 c m ) 3

Homogeneity

Homogeneous b y microprobe and magnetic torque measurements

Perfection

M o s a i c spread of a b o u t 0.1° ( b u l k , neutron-rocking curve)

Methods

RF Levitation Czochralski

RF Levitation Horizontal

~ 1-cm d i a m e t e r X ~ 9-cm length (-7 cm )

— 1-cm d i a m e t e r X 1-2-cm length (~ 1 cm )

3

M o s a i c spread a b o u t 1° to 3 ° (bulk, neutronr o c k i n g curve)


3

300

SOLID S T A T E C H E M I S T R Y : A

Physical


Measurements

S i n g l e crystals of c o m p o s i t i o n s

i n the H o - T b - D y - F e

system p r o

2

d u c e d by the C z o c h r a l s k i methods described above have been examined b y a v a r i e t y of p h y s i c a l t e c h n i q u e s , i n c l u d i n g m e a s u r e m e n t s

of

elastic

and inelastic magnetostriction, magnetic anisotropy, neutron diffraction, surface acoustic w a v e ( S A W ) v e l o c i t y , a n d m a g n e t o a c o u s t i c t r a n s d u c t i o n . A b r i e f d e s c r i p t i o n of these measurements

is g i v e n b e l o w ;

for a m o r e

c o m p l e t e d i s c u s s i o n , the o r i g i n a l references s h o u l d b e c o n s u l t e d . Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1970-0186.ch015

I n the H o - T b - D y - F e

2

system the easy d i r e c t i o n of m a g n e t i z a t i o n is

o b s e r v e d to v a r y as a f u n c t i o n of t e m p e r a t u r e a n d c o m p o s i t i o n .

A t room

t e m p e r a t u r e t h e H o - T b s y s t e m exhibits [111] -> [110] -> [100] as easy d i r e c t i o n s as the H o c o n t e n t increases, w h i l e the D y - T b system exhibits [111] - » [100] r e o r i e n t a t i o n o n l y . T h e s p i n r e o r i e n t a t i o n d i a g r a m s

(20)

f o r these systems are s h o w n i n F i g u r e 5. T h e s o l i d lines, w h i c h delineate t h e easy d i r e c t i o n regions, are o b t a i n e d b y t h e o r e t i c a l c a l c u l a t i o n s . h a t c h e d areas c o r r e s p o n d to easy directions o f m a g n e t i z a t i o n i n t h e

The (100)

p l a n e b e t w e e n [110] a n d [100]. T h e circles a n d squares are d a t a p o i n t s . T h i s b e h a v i o r is consistent w i t h the a s s u m p t i o n of K

2

system a n d K

2

>

0 i n the neighbor

T h u s a c o m b i n a t i o n of T b , D y , a n d H o i n

t h e p r o p e r a m o u n t s s h o u l d l e a d to K striction w i l l be small.

> 0 for t h e H o - T b

field

However,

=

K

2

necessary

=

0 at r o o m t e m p e r a t u r e .

to saturate t h e

magneto

since the m a g n e t o s t r i c t i o n

is q u i t e

Aioo, as s h o w n i n T a b l e I I I ( 2 1 ) , i t w i l l

be

necessary to h a v e a t e x t u r e d p o l y c r y s t a l l i n e m a t e r i a l or, i f p o s s i b l e ,

a

s i n g l e - c r y s t a l m a t e r i a l to o b t a i n the best efficiency.

30C-

Journal of Applied Physics

Figure 5a. Spin reorientation diagram for the Ho-Tb-Fe system as a function of rare-earth content and temperature (18) 2

70

80 at%Ho

90


100


15.

Intermetallics

MILSTEIN

50


60 ot.%Dy

301

Devices

70

80

Journal of Applied Physics

Figure 5b. Spin reorientation diagram for the Dy-Tb-Fe system as a function of rare-earth content and temperature (18J 2

N e u t r o n d i f f r a c t i o n m e t h o d s h a v e p r o v i d e d several i n t e r e s t i n g d a t a . The

crystallographic

determined

q u a l i t y of

Czochralski-grown

crystals

T r i a r c - g r o w n crystals p r o d u c e r o c k i n g

(10).

has

been

curve

peak

w i d t h s one-tenth that of l e v i t a t i o n - g r o w n crystals. T h e s e d a t a are p r e sented i n F i g u r e s 6 ( a )

and 6(b).

O n e sees c o n c l u s i v e l y the

appreciable

difference i n q u a l i t y , the t r i a r c - g r o w n crystals b e i n g s y s t e m a t i c a l l y m o r e perfect. T h e difference i n q u a l i t y is a s c r i b e d to the absence of dross at the g r o w t h interface i n the a r c - p o w e r e d dross i n the l e v i t a t i o n m e t h o d .

m e t h o d a n d to the presence of

T r i a r c - g r o w n crystals thus are

better for m o r e d e t a i l e d a n d precise measurements grown

specimens.

N e u t r o n inelastic scattering measurements Ho .88Tbo.i2Fe , 0

suited

t h a n are l e v i t a t i o n -

2

ErFe , 2

and

HoFe

2

have been performed

(15,22,23).

A l t h o u g h " there

v a r i a t i o n s i n the d e t a i l e d results o b t a i n e d f r o m these specimens, features are q u i t e systematic.

on are

several

T h e exchange interactions o c c u r as

F e - F e ^ tens of m i l l i e l e c t r o n v o l t s RE-Fe » RE-RE =

1 meV 0.00 ± 0.01 m e V

T h u s the F e - F e i n t e r a c t i o n is r o u g h l y e q u i v a l e n t to t h a t i n p u r e i r o n metal.


302

SOLID S T A T E

Table III.

CHEMISTRY:


Room-Temperature Magnetostrictive Constants TbFe

DyFe

2

A 111 ( X 10" ) A100 (X10" )

2460 300 ±

6

6


A

77.9°

HoFe

2

2

1060 ± 150 0 ± 4

100

78.9°

185 ± 2 0 -59 ± 6

79.9° Plenum Publishing Corporation

Figure grown

6a. Neutron diffraction rocking curves for triarc-CzochralskiREFe crystals. The angular values are arbitrary and merely serve to indicate the rocking-curve width (10). 2

t

J 61.0°

63.0°

65.0°

47.6°

49.6°

51.6° Plenum Publishing Company

Figure 6b. ralski-grown

Neutron diffraction rocking curves for rf levitation CzochREFe crystals. The angular values are arbitrary and merely serve to indicate the rocking-curve width (10). 2


15.

Intermetallics

MILSTEIN


303

Devices

T h e rare earths order b y c o u p l i n g v i a t h e i r o n s u b l a t t i c e . T h e rare earth's nearest n e i g h b o r s , a l t h o u g h at a b o u t the same d i s t a n c e as i n p u r e r a r e - e a r t h metals, d o n o t i n t e r a c t i n a n y m e a s u r a b l e w a y .

These

facts

are r a t h e r firm c o n f i r m a t i o n o f t h e s i n g l e - i o n m o d e l , w h i c h h a s b e e n involved i n calculations of various properties inelastic n e u t r o n d i f f r a c t i o n results f o r E r F e

2

o f these

systems. T h e

at r o o m t e m p e r a t u r e a r e

s h o w n i n F i g u r e 7 a n d a r e t y p i c a l o f t h e results f o u n d f o r a l l o f t h e REFe

2

systems s t u d i e d to date.


S A W studies (24) h a v e b e e n c a r r i e d o u t b y u s i n g several single crystals i n t h e H o - T b - D y - F e

2

system.

T h e wave

velocity could b e

a l t e r e d f r o m a b o u t 1.75 t o 2.15 X 1 0 c m • sec" , f o r p r o p a g a t i o n o f 5

1

waves h a v i n g f ^ 85 M H z , a l o n g t h e [001] d i r e c t i o n o f a [110]-cut

0.8

0.4

REDUCED

WAVE

0

0.2 0.4 VECTOR (X comp.)

Figure 7. Room-temperature inelastic neutron diffraction data for ErFe with fit curves and fitting parameters (21): ErFe , 295 K; — 3.6; 2

2

Fe

= 0.66;

d -e Fe

F

= 30 meV; d .

Experimental,

Fe

Er

Er

= -0.32 meV; d .

(O); model, (

Er

Er

).


= 0.

304

SOLID S T A T E

0.8

CHEMISTRY: A CONTEMPORARY OVERVIEW

0.8

r

r

T b

23

H (

?77

F e

2[ ] m

, 3 y.7 2


T b

0

20

40

60

80

100

0

Magnetic Bias Field (kA/m)

200

400

600

D

800

F e

1000

Bias Flux Density (mT)

Figure 8. Room-temperature coupling coefficient K for several REFe materials in polycrystalline and oriented form as a function of magnetic field (27) ss

2

c r y s t a l , w i t h a m a g n e t i c field of 8.6 K O e w h o s e d i r e c t i o n w a s v a r i e d f r o m [001] to [110]. T h i s v e l o c i t y c h a n g e of a p p r o x i m a t e l y 2 0 % is a p p a r e n t l y t h e largest c o n t i n u o u s l y v a r i a b l e surface v e l o c i t y c h a n g e r e p o r t e d t o date. E x a m i n a t i o n of R E F e at N R L a n d N S W C

polycrystalline a n d single-crystal materials

2

f o r use i n m a g n e t o a c o u s t i c

transducers has b e e n c a r r i e d o u t b y groups D a t a o b t a i n e d o n t h e same m a t e r i a l s

(17,25,26).

b y different t e c h n i q u e s a r e i n g o o d agreement. T h e p a r a m e t e r t h a t is most c o m m o n l y t a k e n as a m e a s u r e of t h e quality of a magnetoacoustic

t r a n s d u c e r element is K

m e c h a n i c a l c o u p l i n g coefficient.

3 3

, the magneto-

T h i s q u a n t i t y is a measure

of the

efficiency w i t h w h i c h m a g n e t i c e n e r g y s u p p l i e d to t h e t r a n s d u c e r e l e m e n t is c o n v e r t e d t o m e c h a n i c a l

( o r acoustic)

energy.

Another important

p a r a m e t e r is t h e strain d e v e l o p e d i n t h e t r a n s d u c e r element.

T h e larger

the a v a i l a b l e s t r a i n , t h e h i g h e r is t h e a t t a i n a b l e s o u n d pressure f o r a given geometrical configuration of the transducer. efficiency

S o u n d pressure a n d

a r e p a r a m e t e r s of some c o n s e q u e n c e to designers

o f sonar

equipment.


15.

Intermetallics

MILSTEIN


F i g u r e 8 d e p i c t s the r e l a t i o n s h i p b e t w e e n field

for

polycrystalline and oriented

i n c l u d e d as a reference v a l u e (27). oriented

Tb .23Ho .77Fe 0

0

REFe

K

and magnetic

w h i c h is

greater t h a n the values for the p o l y c r y s t a l l i n e specimens.

bias

with nickel

T h e c o u p l i n g coefficient for

is a p p r o x i m a t e l y 0.75,

2

3 3

specimens,

2

305

Devices

[111]-

appreciably T h i s v a l u e is

a m o n g t h e largest c o u p l i n g coefficients ever r e p o r t e d . T y p i c a l values f o r p i e z o e l e c t r i c c e r a m i c t r a n s d u c e r elements, t h e most w i d e l y u s e d

type,

are a b o u t 0.6.


T h e strain as a f u n c t i o n of bias field (27) s h o w n i n F i g u r e 9. [lll]-oriented

Tb . 3Ho .77Fe 0

for t h e same m a t e r i a l s is

T w o features are n o t i c e d r e a d i l y . T h e first is t h a t 2

0

saturates, w h i l e p o l y c r y s t a l l i n e T b . 5 -

2

0

2

H o . 7 5 F e does not saturate i n the l o w fields a p p l i e d . T h e second is t h a t 0

2

s a t u r a t i o n occurs r a p i d l y i n t h e o r i e n t e d s p e c i m e n .

T h u s the oriented

s p e c i m e n y i e l d s f a r h i g h e r strain w i t h a h i g h rate of c h a n g e

i n the

l o w - f i e l d r e g i o n . T h e i m p l i c a t i o n s of these p r o p e r t i e s is t h a t the use of

0

20

40

60

80

100 120 140

Magnetic Bias Field

160

180

(kA/m)

Figure 9. Room-temperature magnetostrictive strain as a function of magnetic field for several REFe materials in polycrystalline and oriented form (25) 2


306

SOLID S T A T E

oriented

CHEMISTRY: A


m a t e r i a l results i n a s u p e r i o r m a g n e t o a c o u s t i c

device.

The

d e s i g n e r w o u l d l i k e to use o r i e n t e d m a t e r i a l , w h i c h places a c o n s t r a i n t o n the f a b r i c a t o r of the m a g n e t o s t r i c t i v e elements. b e o p e r a t e d b y m a g n e t i c a l l y b i a s i n g t h e elements

T h e device w o u l d to a p o i n t

roughly

h a l f w a y u p t h e steep rise i n strain a n d b y a p p l y i n g a n o s c i l l a t i n g m a g n e t i c field to d r i v e the elements o v e r the f u l l d y n a m i c strain r a n g e .

This

m o d e of o p e r a t i o n w o u l d a l l o w the greatest e c o n o m y i n r e q u i r e d p o w e r s u p p l i e s , coils, v o l u m e , a n d mass, a l l of w h i c h are factors i n the p o t e n t i a l


a p p l i c a t i o n of s u c h systems. Conclusions T h e efforts of s e v e r a l w o r k e r s h a v e l e d to t h e a b i l i t y t o

prepare

materials that e x h i b i t i m p r o v e d m a g n e t o s t r i c t i v e b e h a v i o r at r o o m t e m perature.

M a j o r advances h a v e b e e n m a d e i n o u r u n d e r s t a n d i n g of t h e

basic m a g n e t i c interactions i n the R E F e Areas

that require

2

a d d i t i o n a l effort

intermetallic compounds. include

the

examination

of

a d d i t i o n a l i n t e r m e t a l l i c s y s t e m s — f o r e x a m p l e , those b a s e d o n S m r a t h e r t h a n T b — a n d the d e s i g n of t r a n s d u c e r devices that u t i l i z e the u n u s u a l c h a r a c t e r i s t i c s of these m a t e r i a l s .

Literature Cited 1. Nassau, K.; Cherry, L. V.; Wallace, W. E. J. Phys. Chem. Solids 1960, 16, 131. 2. Koon, N . C.; Schindler, A. I.; Carter, F. L. Phys. Lett. A 1971, 37, 413. 3. Clark, A. E. Belson, H. S. Phys. Rev. B 1972, 5, 3642. 4. Ray, A. E. Proc. Rare Earth Res. Conf. 1968, 2, 473. 5. Clark, A. E. AIP Conf. Proc. 1974, 18, 1015. 6. Williams, C. M . Koon, N . C.; Milstein, J. B. AIP Conf. Proc. 1976, 29, 191. 7. Clark, A. E.; Belson, H. S.; Tamagawa, N. Phys. Lett. A 1972, 42, 160. 8. Milstein, J. B.; Koon, N . C.; Johnson, L. R.; Williams, C. M. Mater. Res. Bull. 1974, 9, 1617. 9. Milstein, J. B. AIP Conf. Proc. 1976, 29, 592. 10. Milstein, J. B. "Crystallographic Quality of RFe Crystals Containing Ho," In "Rare Earths in Modern Science and Technology"; McCarthy, G. J.; Rhyne, J. J.; Eds.; Plenum: New York, 1978; p. 315. 11. McMasters, O. D.; Holland, G. E.; Gschneider, K. A., Jr. J. Cryst. Growth 1978, 43, 577. 12. Buschow, K. H. J.; van der Goot, A. S. Phys. Stat. Sol. 1969, 35, 515. 13. Meyer, A. J. Less-Common Metals 1969, 18, 41. 14. Kolesnikov, V. E.; Trekhova, V. F.; Savitskii, E. M . Izv. Akad. Nauk SSSR, Neorg. Mat. 1971, 7, 495. 15. Nicklow, R. M.; Koon, N. C.; Williams, C. M.; Milstein, J. B. Phys. Rev. Lett. 1976,36, 532. 16. Hukin, D. A.; Jones, D. W. Proc. Rare Earth Res. Conf. 1976, 891. 17. Meeks, S. W.; Timme, R. W. J. Accoust. Soc. Am. 1977, 62, 1158. 18. Pfann, W. G. "Zone Melting"; Wiley: New York, 1966. 19. Miller, A. E.; D'Silva, T.; Rodrigues, H . IEEE Trans. Mag. 1976, MAG12, 1006. ;

;

2


15.

MILSTEIN

Intermetallics


Devices

307


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