Platinum Group Metals and Compounds

1

M a g n e t i c Properties o f the P l a t i n u m Metals a n d T h e i r A l l o y s H. J. ALBERT and L. R. RUBIN Downloaded by 117.244.18.115 on October 30, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0098.ch001

Engelhard Minerals and Chemicals Corp., Newark, N. J. 07105 Although they are paramagnetic, the platinum metals, especially platinum, palladium, and rhodium, are capable of interacting in alloys with other metals to form ferromagnetic or very nearly ferromagnetic materials. Dilute additions of elements of the iron group and its neighbors with platinum and palladium take on enhanced moments which are interpreted as arising from an interaction of the solute moment with the 4d or 5d host electrons. Ferromagnetic and anti-ferromagnetic structures may result from greater additions of the iron group to the platinum metals. Examples are FeRh, Pt Fe, Pd Fe, and PtCo. Compounds of the rare earths with the platinum metals also form magnetic structures with unusual properties. 3

3

*Tphe platinum metals—ruthenium, rhodium, and palladium in the 2nd long row of the periodic system and osmium, iridium, and platinum in the 3rd long row—are all paramagnetic. That is, none of these elements have a permanent magnetic moment associated with it. However, as shown in Table I, the mass susceptibility of these elements varies widely over two orders of magnitude. Further, the "nonmagnetic" platinum metals are the elements immediately beneath the "magnetic" series iron-cobalt-nickel and, of course, are all in the transition group. Platinum, palladium, and rhodium form a number of ferromagnetic alloys with the iron group metals and with manganese. The present paper is not an exhaustive review but presents some of the more interesting magnetic work on the platinum metals which has appeared in the last 10-20 years. Palladium has the highest magnetic susceptibility of the platinum group, and because it is so high it has been termed an "incipient ferromagnet." The implication is that palladium could be induced to become A

1 In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

2

P L A T I N U M

Table I.

GROUP

M E T A L S

A N D

C O M P O U N D S

Mass Susceptibility of the Platinum Metals (Χ

Ru(+0.427)»

10

- 6

cgs U n i t s )

Rh(+0.9903)

0

Pd(+5.231)

c

c

Os(+0.052) Ir(+0.133) Pt(+0.9712) ° Reprinted from Engelhard Industries Technical Bulletin. Determinations made at 25°C. Determinations made at 20°C. b

6

c

6

e

ferromagnetic.

I n a w e a k l y p a r a m a g n e t i c m a t e r i a l , t h e s u s c e p t i b i l i t y of

the m a t e r i a l is i n d e p e n d e n t of t e m p e r a t u r e .

F i g u r e 1 ( 3 9 ) shows this

to b e q u i t e true f o r r h o d i u m . P a l l a d i u m , o n t h e other h a n d , exhibits a Downloaded by 117.244.18.115 on October 30, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0098.ch001

s t r o n g t e m p e r a t u r e d e p e n d e n c e of s u s c e p t i b i l i t y w i t h a p e a k at about 7 0 ° K . T h i s has b e e n t a k e n i n the past as évidence f o r a n a n t i f e r r o m a g n e t i c t r a n s i t i o n . H o w e v e r , n e u t r o n d i f f r a c t i o n has s h o w n n o e v i d e n c e of a n t i f e r r o m a g n e t i s m , a n d i t n o w seems l i k e l y that this effect is c a u s e d b y the F e r m i surface of p a l l a d i u m b e i n g associated w i t h a n extremely sharp p e a k i n t h e density-of-states.

T h e s u s c e p t i b i l i t y vs. t e m p e r a t u r e

curve

f o r t h e p a l l a d i u m - 5 % r h o d i u m a l l o y i n d i c a t e s t h e s e n s i t i v i t y of p a l l a d i u m - b a s e d alloys to electron c o n c e n t r a t i o n a n d t h e density-of-states i n r e l a t i o n to the F e r m i l e v e l

(13).

I r o n i m p u r i t i e s g r e a t l y c o m p l i c a t e t h e s u s c e p t i b i l i t y measurements o n p a l l a d i u m . A s n o t e d a b o v e , p a l l a d i u m is close to b e i n g f e r r o m a g n e t i c ,

I200xl0"

6

1000

800

600 mol 400

200

0.

100

200

Temperature,

300

°K

Weiss, J . R. "Solid State Physics for Metallurgists," Pergamon f

Figure 1. The susceptibility (10~ ergs/gauss?/mole) for palladium, rhodium, and paUadium-5% rhodium as a function of temperature (39) e

In Platinum Group Metals and Compounds; Rao, U.; Advances in Chemistry; American Chemical Society: Washington, DC, 1971.

1.

A L B E R T

Magnetic

A N D R U B I N

50

' ' ΊΟο' '

3

Properties

'

Ι5θ' ' ' T(°K)

ZOO' ' ' 250'

Downloaded by 117.244.18.115 on October 30, 2015 | http://pubs.acs.org Publication Date: June 1, 1971 | doi: 10.1021/ba-1971-0098.ch001

Journal of Applied Physics

Figure 2. Susceptibility vs. temperature for several "pure" palladium samples (17) φ 3 ppm iron, Ο 2 ppm iron, Δ zone refined, solid line less than 1 ppm iron

TCK)

Journal of Applied

Physics

Figure 3. Susceptibility vs. temperature for phtinum with approximately 3 ppm of iron (17)

a n d s m a l l a d d i t i o n s of i r o n w i l l f o r m f e r r o m a g n e t i c alloys w i t h

Curie

temperatures i n t h e c r y o g e n i c range. T h u s , as s h o w n i n F i g u r e 2 ( 1 7 ) , e v e n parts p e r m i l l i o n of i r o n i n p a l l a d i u m m a y b e n o t i c e d r e a d i l y i n s u s c e p t i b i l i t y measurements

at l o w temperatures.

N e u t r o n scattering

measurements h a v e s h o w n that i r o n i m p u r i t i e s h a v e a n effect f a r b e y o n d the n e a r e s t - n e i g h b o r p a l l a d i u m atoms, e x t e n d i n g p e r h a p s to t h e nearest 100 p a l l a d i u m atoms, a c c o u n t i n g f o r the extreme s e n s i t i v i t y of p a l l a d i u m to i r o n i m p u r i t i e s

(28).

P l a t i n u m , F i g u r e 3 (17), shows b e h a v i o r s i m i l a r t o t h a t of p a l l a d i u m b u t its s u s c e p t i b i l i t y a n d the effect of i r o n i m p u r i t i e s are m u c h s m a l l e r . T h e s m a l l p e a k at 100°Κ is analogous to that f o r p a l l a d i u m .


4

P L A T I N U M

GROUP

M E T A L S

A N D

C O M P O U N D S

Local Moments C o n s i d e r a b l e research i n recent years has b e e n c a r r i e d o u t o n the m a g n e t i c properties of alloys of elements of the s e c o n d a n d t h i r d l o n g r o w s of the p e r i o d i c table w i t h s m a l l amounts of elements i n the

first

l o n g r o w . S o m e of the most i n t e r e s t i n g results of this w o r k are c e n t e r e d o n t h e p l a t i n u m metals. O n e aspect of the results is g i v e n i n F i g u r e 4

(11),

s h o w i n g the m a g n e t i c m o m e n t of a n i r o n a t o m d i s s o l v e d i n the s e c o n d r o w t r a n s i t i o n metals. A n e n h a n c e m e n t of the i r o n m o m e n t is a p p a r e n t for alloys w i t h electron concentrations of a b o u t 5.5 to 7 a n d 8.25 to 9. F o r electron concentrations f r o m 9 to 10.25 there is a v e r y l a r g e e n h a n c e m e n t , the m o m e n t e x c e e d i n g t h a t of i r o n i n its b u l k state ( a b o u t 2.2

μ ). Β


A l l o y s s h o w i n g this large e n h a n c e m e n t are r e g a r d e d as h a v i n g " g i a n t moments."

E n h a n c e m e n t effects of the solute m o m e n t h a v e b e e n f o u n d

i n m a n y a l l o y systems, i n c l u d i n g C r (2), C o (9, 10, 38)

M n (2),

F e (10,

11, 18),

and

i n p a l l a d i u m a n d alloys of p a l l a d i u m a n d r h o d i u m . T h e

e x p e r i m e n t a l t e c h n i q u e w i d e l y u s e d to s h o w the existence of s u c h l o c a l m o m e n t s is the m e a s u r e m e n t of s u s c e p t i b i l i t y . T h e s u s c e p t i b i l i t y is m e a s u r e d as a f u n c t i o n of t e m p e r a t u r e , u s u a l l y f r o m 1.4° Κ to r o o m t e m p e r a ture or higher.

T h e d a t a f r o m alloys w i t h

a

temperature-dependent

s u s c e p t i b i l i t y are fitted to a C u r i e - W e i s s t y p e of c u r v e w h i c h leads to 14

3

4

Y

Zr

5

6

Nb

7

Mo

Re

ELECTRON

8

9

10

11

Ru

Rh

Pd

Ag

Ν

CONCENTRATION

Physical Review

Figure 4. iron atom metals and and alloy)

Magnetic moment in Bohr magnetons of an dissolved in various second row transition alloys {one atomic per cent iron in each metal as a function of electron concentration (11)



1.

A L B E R T

A N D RUBIN

Os . I o.e 0

4

r

Magnetic

I

5

Properties

r

COMPOSITION

p

(ATOMIC

P E R

t

C E N T )

Journal of Applied

Physics

Figure 5. Susceptibility per mole for alloys containing one atomic per cent iron at 3 different temperatures (18) the i n t e r p r e t a t i o n that a l o c a l m o m e n t exists o n the solute a t o m a n d p e r mits the c a l c u l a t i o n of the m o m e n t associated w i t h the a l l o y . A l l o y s of i r i d i u m a n d p l a t i n u m w i t h s i m i l a r s m a l l amounts of i r o n also e x h i b i t e n h a n c e m e n t effects a n d a giant m o m e n t i n the case of i r o n i n p l a t i n u m a n d p l a t i n u m - r i c h alloys as s h o w n i n F i g u r e 5

(18).

It is o b v i o u s that, i n v i e w of the l a r g e m e a s u r e d m o m e n t s o n the solute atoms, there is m o r e i n v o l v e d t h a n s i m p l y the m a g n e t i c

moment

of the solute a t o m . T h e postulate most w i d e l y u s e d suggests a m o d e l i n w h i c h the m o m e n t o n the solute atoms interacts w i t h the m a g n e t i c m o ments o n the i t i n e r a n t 4d or 5d electrons of the host m e t a l . T h i s i n t e r a c t i o n p r o d u c e s a p o l a r i z a t i o n of the spins i n the 4d or 5d b a n d , at t h e same t i m e a l i g n i n g the l o c a l i z e d m o m e n t s on the solute atoms i n the same d i r e c t i o n as the p o l a r i z e d d-electrons.


6

P L A T I N U M

GROUP

M E T A L S

A N D

C O M P O U N D S

U s i n g this l o c a l m o m e n t m o d e l , a n d u s i n g b a n d t h e o r y or its v a r i a tions, a n u m b e r of w o r k e r s h a v e b e e n a b l e to f o r m u l a t e expressions w h i c h represent the m e a s u r e d m a g n e t i c d a t a r e a s o n a b l y w e l l , at least for the case w h e r e w e l l - l o c a l i z e d m o m e n t s are d e v e l o p e d o n the solute atoms (11,

18).

H o w e v e r , c o n s i d e r a b l y m o r e d a t a has b e c o m e a v a i l a b l e o n

o t h e r properties of d i l u t e a l l o y s , i n c l u d i n g d a t a o n resistivity a n d specific heat, n e u t r o n scattering, v a r i o u s m a g n e t i c resonance experiments, M o s s b a u e r measurements, K o n d o effect,

a n d the l i k e .

Measurements have

b e e n e x t e n d e d also to alloys of m a n y other systems besides those i n v o l v -


i n g the p l a t i n u m metals.

Journal of Applied Physics

Figure 6. ceptibility,

(a) Magnetization (at 5 KOe) and inverse initial sus(b) Electrical resistivity of ordered FeRh for increasing (%) and decreasing (O) temperature (25)


1.

A L B E R T

A N D

R U B I N

Magnetic

7

Properties

T h e scope of this r e v i e w p r e c l u d e s a d e s c r i p t i o n of these d a t a , b u t the results of some of these experiments h a v e s h o w n weaknesses i n t h e o r i g i n a l b a n d m o d e l a p p r o a c h (9, 20, 34). ments (21,

22, 26)

H o w e v e r , n e w e r t h e o r e t i c a l treat-

are p r o v i d i n g m o r e i n s i g h t i n t o the p r o b l e m .

The

subject of l o c a l i z e d m a g n e t i c states i n d i l u t e m a g n e t i c alloys is s t i l l i n a state of v e r y a c t i v e r e s e a r c h a n d t h e o r e t i c a l d e v e l o p m e n t , as s h o w n b y the p r o g r a m s of e v e n the most recent m a g n e t i c conferences, s u c h as the Fifteenth A n n u a l Conference

on Magnetism and Magnetic Materials,

P h i l a d e l p h i a , P a . , N o v e m b e r 1969.


Ordered Alloys I n a d d i t i o n to the d i l u t e alloys a l r e a d y discussed, there are a n u m b e r of alloys of the metals of the p l a t i n u m g r o u p w i t h manganese,

iron,

cobalt, a n d n i c k e l w h i c h have m a g n e t i c properties b a s e d o n t h e f o r m a t i o n of o r d e r e d structures at some p a r t i c u l a r c o m p o s i t i o n . C o n s i d e r i n g i r o n alloys first, the i r o n - r h o d i u m system is a n interesti n g e x a m p l e of m a g n e t i c o r d e r i n g . E a r l y m a g n e t i c measurements i n the system (16)

established that alloys of a b o u t 50 a t o m i c %

rhodium in-

creased i n m a g n e t i z a t i o n as t h e i r t e m p e r a t u r e was r a i s e d t h r o u g h a c r i t i c a l v a l u e . Since 1960, a n u m b e r of w o r k e r s (23, 25, 36) change is o w i n g to a

first-order

h a v e s h o w n t h a t this

a n t i f e r r o m a g n e t i c ( A F M ) to f e r r o m a g -

n e t i c ( F M ) t r a n s i t i o n w i t h i n c r e a s i n g t e m p e r a t u r e , the t r a n s i t i o n t e m p e r a t u r e for a 52 a t o m i c % Figure 6 (25).

r h o d i u m b e i n g a b o u t 350°K, as s h o w n i n

X - r a y d i f f r a c t i o n studies h a v e s h o w n that the c r y s t a l

structure a b o v e a n d b e l o w the t r a n s i t i o n t e m p e r a t u r e is a n o r d e r e d C s C l type, the t r a n s i t i o n b e i n g a u n i f o r m r a p i d v o l u m e e x p a n s i o n of about 1 % w i t h increasing temperature.

C h a n g e s i n l a t t i c e d i m e n s i o n s i n this t y p e

of t r a n s f o r m a t i o n r e s u l t f r o m the differences b e t w e e n the magnetoelastic expansion of the f e r r o m a g n e t i c a l l y o r d e r e d lattice a n d the c o n t r a c t i o n o w i n g to the a n t i f e r r o m a g n e t i c lattice. M e a s u r e m e n t s of the l o w - t e m p e r a t u r e specific heat of i r o n - r h o d i u m alloys a n d i r o n - r h o d i u m - p a l l a d i u m alloys a n d of e n t r o p y changes of the t r a n s i t i o n h a v e s h o w n that the difference i n t h e energies of the F M a n d A F M state is r e l a t i v e l y s m a l l . T h i s suggests a m o d e l i n w h i c h b e l o w the t r a n s i t i o n t e m p e r a t u r e i n the A F M state, r h o d i u m atoms, b y s y m m e t r y , h a v e no net field

exchange

exerted b y the i r o n atoms a n d the energy is d o m i n a t e d b y

iron-

i r o n interactions. A b o v e the t r a n s i t i o n t e m p e r a t u r e , i n the F M state, the r h o d i u m atoms are p o l a r i z e d b y a n exchange field w h i c h i n d u c e s a s i g nificant l o c a l m o m e n t o n the r h o d i u m atoms.

This introduces

another

e n e r g y t e r m w h i c h , i f the s u s c e p t i b i l i t y of the r h o d i u m atoms is l a r g e e n o u g h , w i l l a c c o u n t for the c h a n g e to the F M state (-23).



P L A T I N U M

300

GROUP

M E T A L S

A N D

C O M P O U N D S

400 Τ (°K) Journal of Applied

Figure

7.

Magnetization in a 12.5 KOe field of FeRh line is for the bulk alloys (27).

filings.

Physics

The dashed

A n i n t e r e s t i n g effect i n i r o n - r h o d i u m alloys is n o t e d i n c o l d - w o r k i n g the a l l o y . F i l i n g a n a l l o y to m a k e p o w d e r is a c o n v e n i e n t w a y of c o l d w o r k i n g . M a g n e t i c measurements o n filings are s h o w n i n F i g u r e 7

(27).

S t a r t i n g at p o i n t A , the a l l o y is c o o l e d o 7 0 ° K a n d t h e n h e a t e d to 500°K. l

I t is e v i d e n t t h a t i n the c o l d - w o r k e d a l l o y there is no trace of the A F M to F M t r a n s i t i o n i n this range. B e t w e e n 500° to 700 ° C , a n o r m a l C u r i e p o i n t b e h a v i o r emerges a n d , o n c o o l i n g , the reappears.

first-order

transformation

X - r a y d i f f r a c t i o n measurements o n the filings i n d i c a t e d t h a t

i n the as-filed c o n d i t i o n the filings h a v e a d i s o r d e r e d fee structure, b u t q u e n c h i n g f r o m temperatures as h i g h as 1 4 0 0 ° C d i d not result i n the a p p e a r a n c e of the fee structure. F i r s t - o r d e r transitions also exist for the compositions P t F e , b a s e d o n the C u A u - t y p e structure. 3

3

P d F e and 3

I n the o r d e r e d state, the

f o r m e r a l l o y is f e r r o m a g n e t i c a n d the latter is a n t i f e r r o m a g n e t i c .

Inter

m e d i a t e compositions b a s e d o n F e ( P d , P t ) h a v e s h o w n the existence of 3

a state at l o w temperatures i n w h i c h , s t i l l b a s e d o n the o r d e r e d C u A u 3

structure, the i r o n m o m e n t s m o v e i n t o a " c a n t e d " s t r u c t u r e as s h o w n i n F i g u r e 8 (24).

F o r the c o m p o s i t i o n F e P d i . P t i . , the t r a n s i t i o n f r o m a 6

4


1.

A L B E R T

Magnetic

A N D R U B I N

9

Properties

s i m p l e f e r r o m a g n e t i c state to this f e r r i m a g n e t i c state w i t h c a n t e d i r o n m o m e n t s occurs at a b o u t 140 °K. T h e a l l o y P t F e w i t h a d d e d i r o n is a n i n t e r e s t i n g case b y itself ( 1 ). 3

T h e o r d e r e d a l l o y is a n t i f e r r o m a g n e t i c w i t h f e r r o m a g n e t i c sheets of i r o n atoms a r r a n g e d o n ( 1 1 0 )

planes a n t i f e r r o m a g n e t i c a l l y a l o n g the

[001]

axis. A s the i r o n content is i n c r e a s e d over the s t o i c h i o m e t r i c 25 a t o m i c % , a different A F M structure appears w i t h the sheets a r r a n g e d o n ( 100 ) planes. A t 30 a t o m i c % i r o n , this structure p r e d o m i n a t e s . B e t w e e n these t w o extremes, b o t h types of s t r u c t u r e coexist.

F r o m neutron diffraction

e x p e r i m e n t s , i t is c o n c l u d e d t h a t phase coherence of the t w o structures occurs o v e r m a n y u n i t cells a n d t h a t there is a n i n t e r t w i n i n g of t h e t w o


structures r a t h e r t h a n s e p a r a t i o n i n t o d o m a i n s

of

one

or

the

other

structure. T h e platinum—cobalt system is e s p e c i a l l y i m p o r t a n t i n that i t has the o n l y p r e c i o u s m e t a l a l l o y ever to b e d e v e l o p e d f o r p r a c t i c a l use of its m a g n e t i c p r o p e r t i e s . stoichiometric P t C o .

T h i s a l l o y is b a s e d o n s t o i c h i o m e t r i c or n e a r -

It is d i s t i n g u i s h e d b y a v e r y h i g h - e n e r g y p r o d u c t ,

a r e l a t i v e l y l o w r e m a n e n c e , a n d h i g h c o e r c i v i t y . F i e l d s of 30,000 oersteds or h i g h e r are r e q u i r e d for s a t u r a t i o n a n d P t C o magnets are r e g u l a r l y p r o d u c e d w i t h e n e r g y p r o d u c t s greater t h a n 9 m i l l i o n

gauss-oersteds,

coercive forces of 4300 oersteds, a n d r e m a n e n c e near 6400 gauss.

•

Fe

°Pd,Pt Journal of Applied

Figure

8.

Low-temperature canted-ferrimagnetic ofFePd^Pt^ (24)

Physics

structure


FePt

10

P L A T I N U M

DISORDERED

CUBIC

GROUP

M E T A L S

ORDERED

{ll0}

c

A N D

C O M P O U N D S

TETRAGONAL

II ( I O I ) t


C

Figure

9.

Crystallographic relationships disordered and ordered PtCo

between

is i s o m o r p h o u s to P t C o a n d p r e s u m a b l y shares m a g n e t i c h a r d e n i n g m e c h anisms. It has never b e e n u s e d p r a c t i c a l l y because

its c o e r c i v i t y a n d

e n e r g y p r o d u c t s are l o w e r t h a n those of P t C o . P t C o is a n o r d e r i n g a l l o y , f o r m i n g a f a c e - c e n t e r e d - t e t r a g o n a l t y p e structure (c/a tice (31).

CuAu-

— 0.98) f r o m a d i s o r d e r e d face-centered c u b i c l a t -

M a x i m u m c o e r c i v e force a n d m a x i m u m energy p r o d u c t are

a c h i e v e d at less t h a n c o m p l e t e o r d e r , a n d at different stages i n the a p p r o a c h to c o m p l e t e order.

O r d e r i n g starts i n the d i s o r d e r e d a l l o y w i t h

the f o r m a t i o n of a system of o r d e r e d platelets, e a c h c o n t a i n i n g a tetrag o n a l c-axis p a r a l l e l to one of the o r i g i n a l o r t h o g o n a l c u b e axes. are (110)

These

platelets; that is, they are not p a r a l l e l to the " c u b e faces" i n

the d i s o r d e r e d m a t e r i a l . I n a n y g i v e n r e g i o n , p r i m a r i l y as a n a c c o m m o d a t i o n to s t r a i n , o n l y t w o of the three possible p l a t e l e t orientations o c c u r F i g u r e 9 s u m m a r i z e s the c r y s t a l l o g r a p h i c aspects of t h i s system.

(8).

E l e c t r o n m i c r o s c o p i c a n d field i o n m i c r o s c o p i c w o r k has s h o w n t h a t m a x i m u m c o e r c i v i t y is a c h i e v e d i n the t e t r a g o n a l phase w i t h p l a t e l e t w i d t h of 2 0 0 - 5 0 0 angstroms a n d w i t h p l a t e l e t thickness of a b o u t 20 angstroms (30, 33).

T h e d i r e c t i o n of easy m a g n e t i z a t i o n ( c - a x i s ) is n o t i n the p l a n e of

the platelet. T h e degree of t r a n s f o r m a t i o n is f a r f r o m c o m p l e t e at m a x i m u m p r o p e r t i e s , p e r h a p s as little as 5 0 % t r a n s f o r m a t i o n b y v o l u m e

(32).

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 of the d i s o r d e r e d a l l o y is 43.5 gauss c m / g m at 30,000 oersteds (14). 3

T h e m a x i m u m internal magnetization


1.

A L B E R T

320


Magnetic

A N D R U B I N

220

11

Properties

340

0

20

40

200

180

160

f+0

Engelhard Industries Technical Bulletin

Figure 10.

Magnetic anisotropy in a PtCo single crystal (37)

Journal

of Applied

Physics

Figure 11. Crystallite distribution in ordered PtCo (8)


12

P L A T I N U M

GROUP

M E T A L S

A N D

C O M P O U N D S

of the o r d e r e d phase has b e e n estimated as 3 5 - 4 0 % l o w e r t h a n t h a t of the d i s o r d e r e d phase (14).

T h e average m a g n e t i c m o m e n t o f d i s o r d e r e d

P t C o at r o o m t e m p e r a t u r e is 1.04 b o h r magnetons w i t h a s p h e r i c a l d i s t r i b u t i o n of m o m e n t ( 8 ) .

T h e easy m a g n e t i z a t i o n d i r e c t i o n i n the or-

d e r e d phase corresponds

to its c-axis a n d the a n i s o t r o p y constant is

a p p r o x i m a t e l y 50 m i l l i o n e r g / c m

3

(8).

S i n g l e - c r y s t a l w o r k b y W a l m e r (37)

(on a fully-ordered crystal)

s h o w e d a m a x i m u m of the coercive force i n the 111 d i r e c t i o n a n d m i n i m a i n the 110 a n d 100 d i r e c t i o n s , the 100 m i n i m u m b e i n g the l o w e r of t h e t w o (Figure 10).

W o r k b y Brissonneau a n d coworkers (8)

o n the d i s t r i b u -

t i o n of platelets i n c o m p l e t e l y - o r d e r e d P t C o has l e d to a m o d e l s h o w n


i n F i g u r e 11, w h e r e i n e a c h z o n e s h o w n i n the figure contains a f u l l y d e v e l o p e d n e t w o r k of ( 110) platelets o r i e n t e d i n t w o of the three p o s s i b l e orthogonal directions. T h e z o n e m o d e l p r o p o s e d b y B r i s s o n n e a u is consistent n o t o n l y w i t h his o w n field i o n m i c r o s c o p i c observations, b u t also w i t h l a r g e r scale a n d often p u z z l i n g p h e n o m e n a observed.

P t C o magnets, for instance, f o r m

B i t t e r patterns o n a scale q u i t e easily v i s i b l e u n d e r a l i g h t m i c r o s c o p e . B i t t e r patterns d o not fit the p i c t u r e of a " f i n e - p a r t i c l e " h a r d e n i n g m e c h a n i s m unless t h e y also d e s c r i b e a l a r g e r scale p h e n o m e n o n , s u c h as B r i s sonneau zone boundaries. B e c a u s e of the s e n s i t i v i t y of the platelets to strain energy, t h e i r n u c l e a t i o n s h o u l d be sensitive to the w o r k i n g h i s t o r y of the b i l l e t s f r o m w h i c h the a l l o y s p e c i m e n is m a d e , a n d , n o t s u r p r i s i n g l y , P t C o responds w e l l to c o l d w o r k i n g b e f o r e heat t r e a t i n g . W o r k b y S h i m i z u a n d H a s h i m o t o (35),

w h o t e m p e r e d P t C o u n d e r elastic stress f a r b e l o w its elastic

l i m i t , has s h o w n that the a l l o y develops

considerably lower coercivity

Journal of Applied

Physics

Figure 12. Effects of compressive and tensile stresses on PtCo hysteresis curves (35)


1.

A L B E R T

Δ


Magnetic

A N D RUBIN

MRu

13

Properties

2

RARE-EARTH

ELEMENT

I NM R U

2

, M 0S2t Μ ΐ Γ

2

Physical Review

Figure

13. Curie temperatures for MX compounds (M = rare earth, X = Ru, Os, or Ir) (7) 2

( a n d higher remanence)

i n the direction of the a p p l i e d

compressive

stress t h a n at r i g h t angles to i t ( F i g u r e 1 2 ) . T h e y also r e p o r t t h e c o n verse to b e true w h e n tensile stress is a p p l i e d .

Platinum Metal—Rare Earth Alloys T h e m a g n e t i c m o m e n t s o f rare e a r t h elements a r e c a u s e d b y t h e u n p a i r e d electrons i n t h e i r 4f shells. T h e s e shells" are s h i e l d e d b y t h e outer shells so t h a t c h e m i c a l b o n d i n g h a s r e l a t i v e l y l i t t l e effect o n t h e m a g n e t i c m o m e n t s o f these elements. L a v e s phases o f c o m p o s i t i o n M B

2

T h e rare earths f o r m a series o f

( M = rare e a r t h , Β == p r e c i o u s m e t a l )

w h i c h share t h e c h a r a c t e r i s t i c of f e r r o m a g n e t i c c o u p l i n g a t l o w t e m p e r a tures ( 7 ) . F i g u r e 13 shows t h e C u r i e t e m p e r a t u r e s o f a series o f corn-

Table II.

Θ, °K

Compound PrRu PrRh PrOs Prlr PrPt GdRh Gdlr GdPt

40 8.6 >35 18.5 7.9 >77 >77 >77

2 2

2

2

2

2

2

2

° Reprinted from

Ferromagnetic Curie Points

Acta

Crystallographica

a

Compound

Θ, °K

NdRu NdRh Ndlr NdPt

35 8.1 11.8 6.7

2 2

2

2

{12).


14

P L A T I N U M


-12

-8 -4 H, MAGNETIC

GROUP

0 F I E L D IN

M E T A L S

A N D

4 8 KILO-OERSTEDS

C O M P O U N D S

12

(a)

-8

-4

0

4

H, MAGNETIC FIELD IN

8

ΚILO - OERSTEDS

(b) Journal of Applied Physics

Figure 14. Hysteresis loops for 2 GdRu -CeRu ing (a) superconductivity and ferromagnetism, ductivity alone (4) 2

pounds where Β = MgCu

2

2

alloys show (b) supercon

R u , O s , a n d I r . T h e s e c o m p o u n d s f o r m i n the c u b i c

( C 1 5 ) o r the h e x a g o n a l M g Z n

2

( C 1 4 ) structures ( 7 ) .

The Curie

p o i n t is h i g h e s t w i t h c o m p o u n d s c o n t a i n i n g G d a n d decreases w i t h l a r g e r o r s m a l l e r a t o m i c n u m b e r s of the r a r e earths ( F i g u r e 1 3 ) . S i m i l a r results have been obtained w i t h Β =

R h or P t (12)

(Table II).

The variation

i n C u r i e temperatures results p r i n c i p a l l y f r o m the i n t e r a c t i o n b e t w e e n the spins of the 4f shells of the rare earths a n d the c o n d u c t i o n electrons. P s e u d o b i n a r y alloys of ( C e R u - G d R u ) (4), 2

(GdRu^ThRujî)

(6),

(GdOs -LaOs ) 2

2

2

(5),

(YOs -GdOs ) 2

2

(29),

a n d p e r h a p s some others


1.

A L B E R T

A N D

Magnetic

R U B I N

15

Properties

s h o w s i m u l t a n e o u s f e r r o m a g n e t i c a n d s u p e r c o n d u c t i n g p r o p e r t i e s over a l i m i t e d range of c o m p o s i t i o n .

T h e system G d R u

2

in CeRu

2

has

been

s t u d i e d extensively. T h i s system shows a s u p e r c o n d u c t i n g t r a n s i t i o n w h e n the c o n c e n t r a t i o n of G d R u more than 6 %

GdRu

2

2

is less t h a n 10 a t o m i c % .

c r e a s i n g w i t h c o n c e n t r a t i o n (4). 8 and 4 % G d R u

2

Alloys containing

are f e r r o m a g n e t i c w i t h C u r i e t e m p e r a t u r e s i n F i g u r e 14 shows hysteresis loops

for

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

being simultaneously superconducting and ferromagnetic.

Both

loops

s h o w the e x c l u s i o n of field c h a r a c t e r i s t i c of s u p e r c o n d u c t i n g m a t e r i a l s ,


Table III.

Curie Temperature and Coercive Force of R s P d Compounds (3) 2

Θ, °K

Compound Gd Pd 6

Hc(at

4.2°K)oe

335

2

Tb5.10Pd1.90

~30

12,800

Dy5.07Pd1.93

~25

9,700

H o . o P d i •96

~10

1,300

5

4

i.e., n e g a t i v e slope o n i n i t i a l m a g n e t i z a t i o n a n d n e g a t i v e s l o p e n e a r r e m a nence i n the first q u a d r a n t . T h e n o r m a l or n o n f e r r o m a g n e t i c s u p e r c o n d u c t o r exhibits a " r e m a n e n c e " a t t r i b u t e d to " f r o z e n - i n " flux. T h e m a g n e t i z a t i o n c u r v e for the f e r r o m a g n e t i c ( 8 % )

a l l o y lies w e l l a b o v e that

c a l c u l a t e d for a p a r a m a g n e t i c m a t e r i a l of the same G d content, a n d the r e m a n e n c e is also w e l l a b o v e that e x p e c t e d for a p a r a m a g n e t i c s u p e r conductor.

I t is q u e s t i o n a b l e w h e t h e r s u p e r c o n d u c t i v i t y a n d f e r r o m a g -

n e t i s m exist i n the same d o m a i n s i n a g i v e n s p e c i m e n .

T h e minor loop

P Q R S shows t h a t some s u p e r c o n d u c t i v i t y s t i l l exists i n parts of the a l l o y after s u p e r c o n d u c t i v i t y as a w h o l e has b e e n d e s t r o y e d . C o m p o u n d s of n o m i n a l c o m p o s i t i o n Μ Β , w h e r e M is a g a i n a r a r e δ

2

earth ( G d , T b , H o , D y ) a n d Β a precious m e t a l (Pt, P d ) , show m a g netizations close to those c a l c u l a t e d f r o m t h e m o m e n t s of the r a r e e a r t h atoms

(3).

Gd Pd 5

2

is a soft m a g n e t i c m a t e r i a l w i t h a c o e r c i v e

force

less t h a n 100 oersteds b u t w i t h a h i g h s a t u r a t i o n associated w i t h the h i g h m o m e n t of G d ( 3 ) .

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

erties of the p a l l a d i u m alloys. T h e p l a t i n u m alloys are i s o s t r u c t u r a l a n d h a v e t h e same C u r i e t e m p e r a t u r e s ( 19).

E n e r g y p r o d u c t s for T b P d a n d

D y P d at 4.2°K are 20 X

1 0 gauss-oersteds, r e s p e c t i v e l y

5

(3).

1 0 a n d 26 X 6

5

6

W h i l e these e n e r g y p r o d u c t s are h i g h e r t h a n p l a t i n u m - c o b a l t , the

l o w C u r i e t e m p e r a t u r e s p r e c l u d e use of these c o m p o u n d s

i n the u s u a l

platinum-cobalt applications.



16

PLATINUM GROUP METALS AND COMPOUNDS

Literature Cited (1) Bacon, G. E., Crangle, J., Proc. Roy. Soc. 1963, A272, 387. (2) Barton, E. E., Claus, H., private communication. (3) Berkowitz, A. E., Holtzberg, F., Methfessel, S.,J.Appl. Phys. 1964, 35, 1030. (4) Bozorth, R. M., Davis, D. D., J. Appl. Phys. 1960, 31, 321S. (5) Bozorth, R. M., Davis, D. D., Williams, A. J., Phys. Rev. 1960, 119, 1570. (6) Bozorth, R. M., Matthias, B. T., Davis, D. D., Proc. Intern. Conf. Low Temp. Phys., 7th, Toronto, Canada, 1960, 1961, p. 385. (7) Bozorth, R. M., Matthias, B. T., Suhl, H., Corenzwit, E., Davis, D. D., Phys. Rev. 1959, 115, 1595. (8) Brissonneau, P., Blanchard, Α., Schlenker, M., Laugier, J., J. Appl. Phys. 1969, 39, 1266. (9) Brog, K.C.,Jones, W. H., Booth, J. G.,J.Appl.Phys. 1967, 38, 1151. (10) Cape, J. Α., Hake, R. R., Phys. Rev. 1965, 139, A142. (11) Clogston, A. M.,etal.,Phys. Rev. 1962, 125, 541. (12) Compton, V. B., Matthias, B. T., Acta Cryst. 1959, 12, 651. (13) Doclo, R., Foner, S., Narath, Α.,J.Appl.Phys. 1969, 40, 1206. (14) Dunaev, F. N., Kalinin, V. M., Kryokov, I. P., Maisinovich, V. I., Fiz. Metal i Metaloved. 1965, 20, 460. (15) Engelhard Ind. Tech.Bull.VI, No. 3, December, 1965. (16) Fallot, M., Hocart, R., Rev. Sci. 1939, 77, 498. (17) Foner, S., Doclo, R., McNiff, E. J., Jr.,J.Appl.Phys. 1968, 39, 551. (18) Geballe, T. H.,etal.,J.Appl.Phys. 1965, 37, 1181. (19) Holtzberg, F., Methfessel, S. J., U. S. Patent 3,326,637 (1967). (20) Knapp, G. S.,J.Appl.Phys. 1967, 38, 1267. (21) Knapp, G. S., Phys. Letters 1967, 25A, 114. (22) Kondo, J., Phys. Rev. 1968, 169, 437. (23) Kouvel, J. S.,J.Appl.Phys. 1966, 37, 1257. (24) Kouvel, J. S., Forsyth, J. B.,J.Appl.Phys. 1969, 40, 1359. (25) Kouvel, J. S., Hartelius, C.C.,J.Appl.Phys. 1962, 33, 1343. (26) Lederer, P., Mills, D. L., Phys. Rev. 1968, 165, 837. (27) Lommel, J. M., Kouvel, J. S.,J.Appl. Phys. 1967, 38, 1263. (28) Low, G. G., Holden, T. M., Proc. Phys. Soc. 1966, 89, 119. (29) Matthias, B. T., U. S. Patent 2,970,961 (1961). (30) Mishin, D. D., Greshishkin, R. M., Phys. Status Solidi 1967, 19, K1-K3. (31) Newkirk, J. B., Geisler, A. H., Martin, D. L., Smoluchowski, R.,J.Metals 1950, 188, 1249. (32) Rabinkin, A. G., Tyapkin, Yu. D., Yamaleev, Κ. M., Fiz. Metal i Metaloved. 1965, 19, 360. (33) Ralph, B., Brandon, D. G., Proc. European Conf. Electron Microsco 3rd, Prague 1964 (A) 303. (34) Sarachik, M. P., Phys. Rev. 1968, 170, 679. (35) Shimizu, S., Hashimoto, E.,J.Appl. Phys. 1968, 39, 2369. (36) Tu, P.,etal.,J. Appl. Phys. 1969, 40, 1368. (37) Walmer, M. L., Engelhard Ind. Tech.Bull.1962, 2, 117. (38) Walstedt, R. E., Sherwood, R.C.,Wernicke, J. H., J. Appl. Phys. 1968, 39, 555. (39) Weiss, R. J., "Solid State Physics for Metallurgists," p. 323, Pergamon, New York, 1963. RECEIVED January 16, 1970.


Platinum Group Metals and Compounds

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