Metallic glasses in devices for energy conversion and conservation

Journal of Non-Crystalline Solids 61 & 62 (1984) 725-736 North.Holland, Amsterdam

725

METALLIC GLASSES IN DEVICES FOR ENERGYCONVERSION AND CONSERVATION Ryusuke Hasegawa Materials Laboratory, Corporate Technology, A l l i e d Corporation, Morristown, NJ 07960, USA

The combination of various physical properties of m e t a l l i c glasses leads to the p o s s i b i l i t y of a number of energy conversion/conservation devices with high e f f i c i e n c i e s . Pertinent physical properties include low magnetic loss, high magnetic permeability, high e l e c t r i c a l r e s i s t i v i t y , superconductivity with a high upper c r i t i c a l f i e l d , high corrosion resistance, low radiation damage, high mechanical strength and hardness with high d u c t i l i t y , and hydrogen sorption° Among these, magnetic properties have been receiving the most a t t e n t i o n and a number of devices have been considered or a c t u a l l y fabricated. To i l l u s t r a t e the property-device r e l a t i o n s h i p , we select several representat i v e magnetic applications including commercial frequency transformers, sensors, transducers, and magnetic switches. The wide v a r i e t y of magnetic a p p l i cations realized thus far or to be explored arises l a r g e l y from the fact that the magnetic properties of glassy alloys can be modified widely by postf a b r i c a t i o n treatments. Other energy-related applications of m e t a l l i c glasses are in superconducting devices and hydrogen storage systems. 1. INTRODUCTION As in many cases with new m a t e r i a l s , liquid-quenched m e t a l l i c glasses were created to s a t i s f y s c i e n t i f i c c u r i o s i t y .

Glass f o r m a b i l i t y and basic physical

properties were of prime i n t e r e s t to early investigators I ,

Even the e f f o r t s

to a t t a i n ferromagnetism in n o n - c r y s t a l l i n e solids were not i n i t i a t e d to meet any s p e c i f i c practical needs, although t h e i r magnetic softness was immediately recognized2. til

Glassy metals were not seriously considered for applications un-

they became available in wire or ribbon form 3.

cal properties including high d u c t i l i t y

Having excellent mechani-

and high strength/hardness, these

materials have been considered for use in mechanical a p p l i c a t i o n s .

It has

been found, however, that dynamic properties such as fatigue of these materials are rather poor 4.

There are for the moment no p r a c t i c a l applica-

tions to mention u t i l i z i n g the excellent s t a t i c mechanical properties of m e t a l l i c glasses. The magnetic softness mentioned e a r l i e r in glassy Fe-P-C 2 has become more important for the materials in ribbon form because of t h e i r significance in various magnetic applications5.

We select several representative examples

including d i s t r i b u t i o n transformers, sensors, tranducers and magnetic switches to i l l u s t r a t e the property-device r e l a t i o n s h i p in energy e f f i c i e n t devices. These magnetic applications are among the most active of a l l applied research 0022-3093/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

726

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

in metallic glasses.

The major portion of the present review is therefore

centered around this subject.

Other energy-related applications of metallic

glasses are in superconducting devices and hydrogen storage systems. Someof the significant recent developments in these areas are also summarized.

2. MAGNETIC APPLICATIONS 2.1. Technological Properties 2.1.1. Magnetization Process Of all the pertinent quantities associated with magnetic materials, magnetization, M, or induction, B, is undoubtedly the most important.

In hard

magnets, B at remanence (Br) is one of the two major factors which determine the effectiveness (or strength) of a permanentmagnet. In soft ferromagnets, the quantity B or permeability (~=B/H) should be as large as possible with a given exciting f i e l d H.

It is thus instructive to compare B-H behavior of t y p i

cal glassy and crystalline soft ferromagnets, which is shown in Fig. 1.

It is

Magnetizing Field (A/m)

18 16

-8

0

8

16 i

24

32

40

48 f

56

04

4. , . . . . . . . . . .

72

1.4

14 A "~

1.8

. . . . 8 1.6

12 --

10

METALLIC GLASS

1.2

'~

1.o

t, b.. m

FeTsB13SI9

30 ~m Thick

~

. . . . ARMCO ORIENTED M-4 0.28 mm Thick

6

0.8

._~

0.6

~

4

0,4

2

0.2

0

0

0.1

0!2

01.3 0!4

0!5

0!6

0!7

018 0!9

0

Magnetizing Field H (Oe)

Figure 1 B-H characteristics of an Fe-base metallic glass and a crystalline s i l i con steel (M-4) widely used in distribution transformer cores.

R. Hasegawa / Metallic glosses in devices ]'or energy conversion and conservation

727

the smallness of the hysteresis loss t h a t makes glassy a l l o y s a t t r a c t i v e as core m a t e r i a l s in magnetic devices.

Glassy a l l o y s which are economically f e a s i b l e

f o r large scale devices have room temperature s a t u r a t i o n i n d u c t i o n , Bs, of about 1.6 Tesla which is somewhat lower than t h a t (~ 2 T) o f conventional s i l i c o n steels.

Efforts to increase Bs without much increasing the cost of the material

have not been very f r u i t f u l 6.

However, t h i s does not seem t o be a drawback any

longer, because the operating induction l e v e l of d i s t r i b u t i o n transformers is decreasing every year t o lower the core loss and compensate the increasing cost of e l e c t r i c i t y

generation 7.

In many magnetic devices operated at high frequencies (f>lO kHz), u, B-H squareness (Br/Bs) and r e s i s t i v i t y performance of devices. extensively.

(p) become important factors determining the

In t h i s area, f e r r i t e s and permalloys have been used

M e t a l l i c glasses having r e l a t i v e l y low magnetostriction (L) values

e x h i b i t properties comparable or superior t o those of supermalloys.

In the f r e -

quency range of f ) 5 MHz, ~ of about 500 can be achieved in some m e t a l l i c glasses 8.

This value of ~ is in the range of those of t y p i c a l f e r r i t e s

(u=lO0-1000).

Furthermore, the lower m a g n e t o s t r i c t i v e glassy metals tend t o

have higher B-H squareness r a t i o s due t o lower strain-induced magnetic anisot r o p y which is usually not c o l i n e a r with the bulk a n i s o t r o p y .

For devices

r e q u i r i n g fast f l u x reversal time, the combination of a high B-H squareness r a t i o , a high r e s i s t i v i t y

of about 150 u~-cm t y p i c a l of m e t a l l i c glasses (which

should be compared with p~50 ~ - c m for permalloys) and a Bs value between about 0.7 and 1.2 T in one material is p a r t i c u l a r l y a t t r a c t i v e . 2 . 1 . 2 . Curie and C r y s t a l l i z a t i o n Temperatures I n t e r p l a y between e l e c t r o n i c and atomic s t r u c t u r e e s s e n t i a l l y determines the Curie temperature (@f) of a glassy metal; there i s , as y e t , no d e t a i l e d theoretical

understanding of t h i s r e l a t i o n s h i p .

E m p i r i c a l l y , however, i t has been es-

t a b l i s h e d t h a t c r y s t a l l i n e and glassy Co-base and some Ni-base a l l o y s are magn e t i c a l l y and s t r u c t u r a l l y s i m i l a r whereas Fe-base and some Ni-base a l l o y s are not 9-12.

Based on these data, of of a s p e c i f i c glassy a l l o y can be predicted

t o some e x t e n t , enabling us t o design an a l l o y for a s p e c i f i c a p p l i c a t i o n . many magnetic a p p l i c a t i o n s , a higher Curie temperature is d e s i r a b l e .

In

However,

the need to h e a t - t r e a t m e t a l l i c glasses, in many cases, near t h e i r o f ' s forces us to set these temperatures below t h e i r c r y s t a l l i z a t i o n temperatures (Tx) which are t y p i c a l l y

in the range 350-500°C.

Higher Tx'S are obviously d e s i r a b l e .

There are a number of studies on c r y s t a l l i z a t i o n

k i n e t i c s of m e t a l l i c glasses

which i n d i c a t e t h a t t e c h n o l o g i c a l l y important Fe-B-Si base a l l o y s have a c t i v a t i o n energies f o r c r y s t a l l i z a t i o n

ranging between 2 and 4 eV13.

I t is now well

accepted t h a t these m a t e r i a l s meet the requirement of thermal s t a b i l i t y

for more

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

728

than 25 years at the operating temperature of most electromagnetic devices 14. 2.1.3. Magnetic Anisotropy and Magnetostriction Lack of c r y s t a l structure leads t o e s s e n t i a l l y zero magnetocrystalline anisot r o p y in m e t a l l i c glasses.

~though macroscopically random, as evidenced for

example in x - r a y d i f f r a c t i o n , range order.

these m a t e r i a l s have some kind of atomic short-

I t is also conceivable t h a t a small portion of atoms (on the or-

der of less than 1%) form atomic pairs 6. o r i g i n of f i e l d - i n d u c e d a n i s o t r o p y .

Atomic p a i r ordering seems to be the

These d e v i a t i o n s from complete randomness

lead to magnetic a n i s o t r o p y , the d e t a i l e d nature of which, however, is not well understood.

Whatever i t s o r i g i n , magnetic anisotropy a f f e c t s the magnetic

c h a r a c t e r i s t i c s of a glassy a l l o y .

F i r s t of a l l ,

i t defines the magnetically

preferred a x i s , which in turn defines the general area of a p p l i c a t i o n s of the ma. terial.

The magnitude of the a n i s o t r o p y , measured as an energy K, determines

the ease of magnetization o f f the anisotropy a x i s .

For example, i n i t i a l

per-

m e a b i l i l t y is p r o p o r t i o n a l t o Bs/K and dc c o e r c i v i t y Hc is proportional t o K/Ms , Ms being the s a t u r a t i o n magnetization 15. One of the unique properties of magnetic m e t a l l i c glasses is the ease t o control both the d i r e c t i o n and the magnitude of the anisotropy induced by f i e l d , stress, etc. during p o s t - f a b r i c a t i o n heat treatments.

For the Fe-base glassy

metals, the optimal f i e l d induced u n i a x i a l anisotropy Ku ranges between 500 and i000 J/m 3, and for the Co-base low m a g n e t o s t r i c t i v e a l l o y s , Ku is on the order of I00 J/m 3 16.

Recently a new technique to control the d i r e c t i o n of Ku has

been established by c a r e f u l l y c o n t r o l l i n g c r y s t a l l i n e p r e c i p i t a t e s in glassy materials.

Using t h i s approach, improved magnetic properties at high f r e -

quencies have been achieved in Fe-base a l l o y s 17. Magnetostriction (X), although a small q u a n t i t y of the order of 10-7-10 -5 , a f f e c t s the behavior of a magnetic m a t e r i a l , and t h e r e f o r e is an important fact o r in designing a device using magnetic m a t e r i a l s .

The l e v e l of understanding

the o r i g i n of x in m e t a l l i c glasses, however, is about the same as t h a t f o r the magnetic anisotropy mentioned above, although some phenomenological account has been proposedl8. The q u a n t i t y X is a magnetomechanical coupling c o e f f i c i e n t in a sense t h a t it

introduces an a d d i t i o n a l local f i e l d of the order of x where is the

average i n t e r n a l stress. rapid s o l i d i f i c a t i o n

Glassy metals of present i n t e r e s t are synthesized by

and t h e i r quenched-in stresses are considered t o be l a r g e .

Thus i f L is l a r g e , the o v e r a l l magnetic anisotropy of a glassy metal can be significantly

a l t e r e d by i t .

The q u a n t i t y may be reduced or modified by

h e a t - t r e a t m e n t s , but can be considerably increased during device f a b r i c a t i o n .

R. Hasegawa / Metallic glasses in devices/Or energy conversion attd conservation

Thus in many applications, a small value of x is preferred.

729

However,in devices

intended to sense, stress, pressure, e t c . , large x is necessary to increase the s e n s i t i v i t i e s of the devices.

The value of x increases with the applied f i e l d

and saturates beyond a certain f i e l d .

The saturation magnetostriction Xs is

mainly a function of transition metal atom composition and varies, for example, from about 35x10-6 in Fe-rich alloys to about -4x10-6 in Co-rich alloys, passing through Xs=O near Co/(Co+Fe)=O.93 in the glassy Co-Fe-B system19. 2.2. Magnetic Devices 2.2.1. Distribution Transformers Of all the transformers operating at different frequencies, distribution transformers c o l l e c t i v e l y are the most energy-inefficient7.

The two most im-

portant factors determining the quality of a transformer core are i t s core (or iron) loss and exciting power. The l a t t e r quantity is the power required to maintain a given induction level.

The core loss and exciting power of glassy

ferromagnets, lower by a factor of 5 than those of conventional silicon steel as shown in Fig. 2, have thus a great impact in these devices 6. transformers have been actually b u i l t and tested.

10

l

i

•

,

,

i

•

i

.

A number of such

For example, data taken on a

,

/Pe TABLE I

A

i

ARMCOM-4/ > o o. c~

1.0

Comparison of 10 kVA (60 Hz, 6600V/ 210-105V) distribution transformers made of metallic glass and silicon steel cores (data taken from Ref. 20).

9

uJ PROPERTIES

3

4)

oo

GLASSY ALLOY (Fe78B13Sig)

Core Loss (W)

8.6

No Load Current (%)

0.ii

Si-Fe (G-SH) 40

0.1

.

f

Metglas2605S2(ge7eB13Sig)

0'.6 ' o'.8 ' ,io

' ¢.= ' ¢.4 ' 1'.8 ' 118

0.6

Load Loss (W)

173

170

Impedance Voltage (V)

181

151

Core Temperature (°C)

35

65

Noise (phon)

35.3

M a x i m u m Induction Bm(Tesla)

Figure 2 Core loss and exciting power as a function of maximum induction for the same materials referred to in Fig. I.

Weight (kg)

115

34.5 95

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

730

10 kVA transformer using glassy Fe78B13Si9 alloy are summarized in Table 120. These data indicate that d i s t r i b u t i o n transformers using glassy alloys are a l most identical to the conventional ones excepting that the core loss and hence the temperature rise of the former is considerably lower.

It should be noted

that a r e l a t i v e l y large value of the magnetostriction (Xs~30 ppm) of the glassy alloy does not introduce additional noise and increased core loss due to core fabrication.

Distribution transformers larger than 10 kVA have been fabricated

in Japan and the US and t h e i r long term performance is being tested. 2.2.2. Magnetic Switches The B-H nonlinearity i l l u s t r a t e d in Fig. I demands that many magnetic devices be operated below saturation.

However,i f one excites a core from below to be-

yond saturation, the voltage appearing in the exciting coil suddenly drops to a small value due to a negligibly small permeability at B toward saturation.

This

situation is i l l u s t r a t e d in Fig. 3.

When

the switch S is activated, the core of the saturable inductor (L) is well below i t s magnetic saturation and L, therefore, is a high impedance element.

The capaci-

tance (C) is then charged with a time i

T

°

c?

}.L

B

constant TI=RC while the exciting f i e l d on L increases and eventually saturates the core in time x 2.

Whenthis happens,

t

the impedance of L decreases to a small

1 ,H

I

~t

T~ ,,

ve I ~ ,

,

1

t

T2 T3

,t

?

Vl

I

~2

L2

Vn Ln

C,T c,= VI

V2

. V.

Figure 3 Schematic i l l u s t r a t i o n of a magetic switch, li~e core of the inductor L is assumed to have a B-H behavior shown above. VA and VB are the voltages appearing at points A and B respectively in the c i r c u i t .

Figure 4 An example of pulse compressor. For i l l u s t r a t i o n purpose, time scale is expandedtoward the r i g h t hand side in the voltage-time curve.

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

731

value, discharging the capacitance C and allowing a current flow in RL (load resistor) with a time constant ~3=RLC.

These phenomenacan be u t i l i z e d as a

switch, the performance of which depends mainly on the squareness of the B-H loop and the eddy current loss of the core material. readily met by using metallic glasses as cores.

These requirements can be

The devices u t i l i z i n g the

above-mentioned phenomenaare often called saturable reactors.

One of the

simplest, but very clever, applications of this principle is a device called "spike k i l l e r " 2 1 .

This element is nothing but a saturable reactor to be con-

nected in series with a c i r c u i t , whose main purpose is to block spike-like voltages in the c i r c u i t protecting other elements.

Another straightforward

extension of the simple device in Fig. 3 is a pulse compressor. This is basically a cascade operation of the principle shown in Fig.

3.

As i l l u s t r a t e d in

Fig. 4, the pulse is compressed at each stage i f Li-1 >>Li, assuring energy transfer is toward the right-hand side of the c i r c u i t .

Devices based on this

principle have been constructed and tested in particle accelerators22-24. 2.2.3. Sensors and Transducers High permeability, a wide range of magnetostriction, high r e s i s t i v i t y , small material thickness, c o n t r o l l a b i l i t y of the Curie temperature and high tensile strength, which are characteristics of metallic glasses, can be u t i l i z e d to realize various energy converting 7 A

4

10-" t 0

7

L Amorphous

Metal H e a d s

devices such as sensors and transducers useful in automobiles, robots, control equipment and the l i k e .

Since

- Track Head ~ 24p.m " ~"~'~'~--------~.~ -~ T r a c k Head

the output of these devices is elec-

- 27 rn " - ,

t r i c a l , i t can be d i r e c t l y connected

\

Mn Zn ", ~ \ FerriteHead ,~ \ \ MetalTape \ ~\ (Hc = 1450 Oe, ~ I'~

4

"gr=2~ kG)

2

[ 0.5

to computers. Taking advantage of the above-mentioned properties, a number of devices have been deve-

k

loped.

Speed 3.45m/s I 1

I 3

1 5

I 10

F r e q u e n c y (MHz)

The majority of them,are

based on the high magnetostriction and the low acoustic attenuation commonly achieved in Fe-base alloys. Some of the examples are devices sen-

Figure 5

sing pressure, frost, displacement, distance, stress, tension and acous-

Comparison of output voltages of f e r r i t e and amorphous metal heads for video recording (taken from Ref. 8).

t i c wave propagation25.

Recent de-

velopment extends to use of large Barkausen and Matteuci effects in an

732

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

attempt to realize devices sensing rotation and electric current26. mentioned devices are currently not used commercially.

The above-

The most well-known com-

mercial sensing or transducer devices using metallic glasses are phonocartridges 27 and magnetic recording heads8. Figure 5 depicts the superior performance of video recording heads made of glassy metal cores based on near-zero magnetostrictive Co-Fe base alloy 8.

It is expected that home video casette re-

corders w i l l be equipped with these heads in the very near future.

3. METALLIC GLASS SUPERCONDUCTORS Whatever the applications of superconductivity of a material are, i t is essential to have high values of c r i t i c a l temperature (Tc) , c r i t i c a l fields (Hcl and Hc2) and c r i t i c a l current density (Jc).

For amorphous superconductors, an

empirical relationship28 between Tc and transition metal compositions has been well established and an example for the 4d transition metals is shown in Fig. 6. From this figure, one can estimate the highest Tc value for a glassy superconductor to be about 10 K.

It should be noted that most of the transition metal

base amorphous superconductors are medium coupled, i . e . , the electron-phonon coupling constant is between 1 and 231. not expected by alloying.

Drastic increase in Tc i s , therefore,

Efforts to increase Tc beyond the projected l i m i t

have been successful in some cases, in which crystalline phases with higher Tc are imbeddedin a noncrystalline matrix31.

One of the differences between

crystalline and glassy superconductors arises from shorter electron mean free paths of the order of interatomic spacing of the l a t t e r , resulting in a reduced coherence length {.

~assy superconductors, therefore, have high Ginzburg-

Landau parameters of K)50 and are considered as type I I superconductors.

Shor-

t e r coherence lengths are responsible for the enhanced -dHc2/dT values ranging typically between 20 and 30 kOe/K comparedto less than about 10 kOe/K for crystalline superconductors. ~though ~ is short, i t is typically about 100 A which is much larger than the size of the short-range atomic order (~10 A). Thus the local atomic disorder is not effective in pinning flux, resulting in relatively small values of Jc for noncrystalline superconductors.

Introduction

of crystalline particles of sizes larger than { greatly improves the situation. For example, Jc(H:l T) at 4.2 K increases from about 102 A/cm2 for a single glassy phase (Moo.6Ruo.4)8oSiloBIo to about 104 A/cm2 for the same alloy having about 1015 spherical crystalline (o-phase) particles per cm2 of sizes between 100-200 A32. At present, one can achieve Tc~9 K and Hc2(4.2 K)~110 kOe in Mo-Re base glassy alloys which are close to those of a crystalline Nb-Ti alloy 33. Further improvement of the c r i t i c a l properties, however, is necessary before these ma-

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

t e r i a l s can be used as, for example, superconducting magnet wires.

733

One promis-

ing approach may be to uniformly nucleate fine p a r t i c l e s of c r y s t a l l i n e superconducting materials with higher Tc's by properly h e a t - t r e a t i n g glassy metals. One such example has been reported for (Zro.7Hfo.3)6oV40 showing Jc>lO 5 A/cm2 up to about H=I60 kOe at 4.2 K34. Although there are at )resent no commercial applications of glassy metal superconductors, improvements of t h e i r superconducting properties are expected in the future.

Then these materials with excellent mechanical d u c t i l i t y and l i t t l e

s u s c e p t i b i l i t y to radiation damagew i l l undoubtedly be used in future energy generation systems.

10C

° =~; z,~,=,, o_., Zr- mt Pd.Zr

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/

i

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---- Vapa- DepositedAmqdwus Films /.",..

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RU

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Figure 6 Superconducting transition temperature versus electron-per-atom ratio for crystalline and amorphous alloys having 4d elements (taken from Refs. 28-31).

011 012 0'3

0!4

o's 0!6

07

Hydrogen Concentration H/M

Figure 7 Hydrogen pressure versus concentration at T=433 K for glassy Ni63.7Zr36.3 alloy (taken from Ref. 36).

4. HYDROGENSORPTION One of the earlier studies of glassy crystalline Ti-Cu and Zr-Cu showed that, under similar conditioffs of temperature and pressure, the metallic glasses had larger hydrogen absorption capacities than their crystalline counterparts 35. Encouraged by this work, more glassy metals containing Zr and Ti have been ex-

R. Hasegawa / Metallic glasses in devices for energy conversion and conservation

734

amined in terms of hydrogen sorption.

One such work36 indicates that no plateau

exists in the pressure-compostion isotherms of glassy Zr-Ni alloys as i t does in the crystalline counterpart (see Fig. 7). exists with a saturated solution of hydrogen.

This suggests that no hydride coIt is well-known that hydride-

forming crystalline LaNi5 disintegrates into fine particles during hydrogensorption cycles.

The absence of hydrides and high yield stress and d u c t i l i t y

of metallic glasses, therefore, have been thought to result in less disintegra£ion.

This seems to be indeed the case36. Recent structural work indicates

that hydrogen atoms tend to form bonding states with Zr atoms and are preferent i a l l y located at the tetrahedral-like sites in the glassy Zr-Ni case37.

These

finding would enhance our understanding of the problem, helping to develop improved hydrogen storage systems.

5. CONCLUSIONS Unique physical properties of metallic glasses are identified and their relationships to actually fabricated or possible devices in the area of energy conversion and conservation are b r i e f l y discussed.

With increasing knowledgeof

the present materials and discovery of new glassy metals, possible applications of these materials w i l l undoubtedly extend to the areas other than those covered in this review.

ACKNOWLEDGEMENT The author wishes to thank A. Maeland and C. H. Smith for t h e i r helpful advice in preparing this manuscript. REFERENCES 1) P. Duwez, Trans. ASM 60 (1967) 607. 2)

P. Duwez and S. C. H. Lin, J. Appl. Phys. 38 (1967) 4096.

3)

H. S. Chen and D. E. Polk, US Patent No. 3,856,513.

4)

Y. T. Yeow, private communication (1980)

5)

T. Egami, P. J. Flanders and C. D. Graham, J r . , AIP Conf. Proc. No. 24 (1975) 697.

6)

R. Hasegawa, Glassy Metals: Magnetic, Chemical Structural Properties (CRC Press, Boca Raton, 1983), Chapt. 5 and the references therein

7)

F. E. Werner, Energy Efficient Electrical Steels (TMS-AIME, Warrendale, PA, 1981) p. 1.

8)

K. Matsuura, K. Oyamadaand T. Yazaki, Presented at the Intermag Conference (1983); also Sanyo Data Sheet (1982-10).

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ii)

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R. Hasegawa, J. Appl. Phys. 53 (1982) 7819.

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24)

B. Birx, E. Cook, S. Hawkins, S. Poor, L, Reginato, J. Schmidt and M. Smith, IEEE Trans. Nuc. Sci. NS-30 (1983) 2763.

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27) 28)

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736

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M. Tenhover, IEEE Trans. Mag. MAG-17 (1981) 1021.

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37)

See for example, K. Suzuki, J. Less CommonMet. 80 (1983) 183.