Hyperfine magnetic field of Zn in iron

HYPERFINE FIELDS

Hyperfine Interactions 75( 1992)301 - 306 301

H Y P E R F I N E M A G N E T I C FIELD OF Zn IN IRON

I. BERKES, M. DE JESUS, B. HLIMI*, M. MASSAQ and E.H. SAYOUTY**

lnstitut de Physique Nucl&ire de Lyon, IN2P3-CNRS et Universit~ Claude Bernard, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France

and the NICOLE Collaboration, CERN

Precise h y p e r f i n e f i e l d value of zinc Ln iron has been d e t e r m i n e d by nuclear m a g n e t i c resonance on o r i e n t e d nuclei ( N M R / O N ) : Bhf (ZnFe) = - 18.785 (35) T at

" - h - -

7 mK. The r e l a x a t i o n constant of Zn in tron ts establls ed CK 14(3) Ks. The new h y p e r f i n e f i e l d value of zinc in iron allows a m o r e precise reevaluaLLon of the m a g n e t i c m o m e n t s of 6 9 m z n and 7 1 m z n measured w i t h N M R / O N .

1. I N T R O D U C T I O N

The h y p e r f i n e m a g n e t i c field of Zn in iron has been d e t e r m i n e d by several m e t h o d s . W h i l e i n t e g r a l t y p e m e a s u r e m e n t s as n u c t e a r o r i e n t a t i o n (NO) on Zn and i n t e g r a l p e r t u r b e d a n g u l a r c o r r e t a t i o n ( I P A C ) y i e l d e d v a l u e s a r o u n d -10 T / 1 , 2 / , d i f f e r e n t i a l p e r t u r b e d a n g u l a r d i s t r i b u t i o n ( D P A 9 g a v e -18./4(3)T at r o o m t e m p e r a t u r e / 3 / .

In i m p e r f e c t , a l l o y s e v e r y i m p u r i t y n u c l e u s does n o t o c c u p y s u b s t i t u t i o n a l l a t t i c e s i t e s in t h e h o s t and t h e a v e r a g e h y p e r f i n e f i e l d d e r i v e d f r o m i n t e g r a l t y p e m e a s u r e m e n t s as N'O and I P A C d e p e n d s on t h e f i e l d d i s t r i b u t i o n . A n o t h e r d i f f i c u l t y is t h e e x t r a c t i o n of t h e h y p e r f i n e f i e l d f r o m t e m p e r a t u r e d e p e n d e n t NO m e a s u r e m e n t when the t h e r m a l r e l a x a t i o n t i m e is not short enough as compare d to the m e a s u r i n g t i m e . This m a y be t h e ease f o r Zn in i r o n : t h e r e l a x a t i o n t i m e s for 6 5 ' 6 9 m ' 7 1 m z n in iron are of the order of Q.5 - 1 hour in the range of 4-9 m K / 4 / .

As the h y p e r f i n e f i e l d enters d i r e c t l y into the N M R / O N d e t e r m i n a t i o n of m a g n e t i c m o m e n t s i t is n e c e s s a r y to m e a s u r e t h i s f i e l d d i r e c t l y at l o w t e m p e r a t u r e . N M R / O N on 65Zn in iron is well adapted for this m e a s u r e m e n t , as the m a g n e t i c m o m e n t of 65Zn is well known # = + 0.7690 (2) #N / 5 / .

2. E X P E R I M E N T A L P R O C E D U R E A N D RESULT

65Zn a c t i v i t y has been prepared by b o m b a r d i n g a copper foil w i t h - 25 MeV d e u t e r o n s in t h e i n t e r n a l b e a m of t h e s y n c h r o c y c l o t r o n of L y o n . The 65Zn a c t i v i t y has been separated c h e m i c a l l y f r o m the copper.

One of the 65Zn sources has been prepared by e l e c t r o p l a t i n g 65Zn on an iron f o i l ; t h e a c t i v i t y was d i f f u s e d u n d e r a r g o n a t m o s p h e r e i n t o t h e i r o n and t h e undiffused activit.y was e l i m i n a t e d f r o m the surface by etching, The depth of the d i f f u s i o n of 65Zn has not been c o n t r o l l e d ,

Permanent address : * U n i v e r s i t y IBN ZOHR, Fac. S c i e n c e s , Agadir, Morocco 9 * University Hassan II, Fac. Sciences-I, Casablanca, Morocco

9 Baltzer A.G., Scientific Publishing C o m p a n y

302 1. Berkes et al., Hyperfine magnetic field of Zn in iron

A second source has been p r e p a r e d by isotope s e p a r a t o r i m p l a n t a t i o n of 65Zn. In o r d e r to d i m i n i s h the density of i m p u r i t y a t o m s ( m a i n l y 65Cu) in iron, t w o i m p l a n t a t i o n e n e r g i e s h a v e b e e n used : 7.5 x 1014 a t o m s / c m 2 h a v e b e e n i m p l a n t e d w i t h 110 keV e n e r g y and 2.5 x 101/4 a t o m s / c m 2 w i t h ~0 keV e n e r g y , and t h e i s o t o p e s e p a r a t o r b e a m has b e e n s w e p t w i t h an e l e c t r o s t a t i c d e f l e c t o r to c o l l e c t on a t o t a l surface of 3.5 c m 2. The p r o f i l e of d e n s i t y r e p r e s e n t e d on fig.1 has been c a l c u l a t e d using the M o n t e C a r l o m e t h o d d e s c r i b e d in ref.6. As fig. 1 s h o w s , t h e d e n s i t y of i m p l a n t e d a t o m s d i d n ' t e x c e e d 0.2_5 a t . %. The i r o n has been annealed a f t e r i m p l a n t a t i o n at. 150~ for 1/4 hour : at this t e m p e r a t u r e the d i f f u s i o n of Zn was n e g l i g i b l e .

0 . 3

z

o

"< 0.2

i ' Y I,--

z ,.l (o

z

o 0 . 1 (.9 [.9 0 ~- 0.0 .<

0

i i i i i I i

10 20 50 40 50 60 70 80

DEPTH ( n m )

Fig. I. Density of implanted impurity atoms (mainly copper) in iron. Implantation energy:60-110 keV (see text)

The s a m p l e s h a v e b e e n s o f t - s o l d e r e d to t h e c o l d f i n g e r of a d i l u t i o n r e f r i g e r a t o r t o g e t h e r w i t h a 60CoFe__ or a 6 0 C o C o n u c l e a r t h e r m o m e t e r .

The pulse height s p e c t r a of i n t r i n s i c Ge d e t e c t o r s w e r e r e g i s t e r e d w i t h c o m p u t e r i - zed data a c q u i s i t i o n systems. The c o m p u t e r c o n t r o l l e d also the r.f. signal g e n e r a t o r for Nt,.4R/ON, the f r e q u e n c y of which was m o d u l a t e d in a continuous r a m p u p - d o w n mode.

F i g u r e 2 shows the i n t e n s i t y of the 1116 keV y - r a y versus inverse t e m p e r a t u r e in the

1.2 , ,

, ( 0 ) 1.1

1.0

0 . 9 I I

0 50 1 O0 150

, / T (K -~)

Fig. 2. Intensity of the 1116 keV gamma ray in the direction of

the magnetic field versus reciprocal temperature in the diffused

source. The continuous line corresponds to B2eff A 2 U2 = 394 T 2 .

1. Berkes et al., Hyperfine magnetic field of Zn in iron 303

diffused source. On account of the slow r e l a x a t i o n , points corresponding to the cooling- down o p e r a t i o n mode are o m i t t e d ; measurements at d i f f e r e n t t e m p e r a t u r e s were p e r f o r m e d by heating the m i x i n g chamber and w a i t i n g the s t a b i l i z a t i o n of the anisotropy, i.e. the t h e r m a l i z a t i o n of the 65Zn nucleus. The f i t t e d curve does not take into account the points corresponding to i m p o r t a n t t e m p e r a t u r e v a r i a t i o n s . We come back to the f i t t e d p a r a m e t e r s l a t e r .

Several a t t e m p s were p e r f o r m e d to destroy the o r i e n t a t i o n by nuclear m a g n e t i c resonance in the frequency range of 40 MHz to 66 MHz. No resonance has been found w i t h this source. As the d i f f u s i o n depth has not been c o n t r o l l e d , it could have been deeper than the r a d i o f r e q u e n c y skin depth.

N M R / O N e x p e r i m e n t s have been p e r f o r m e d w i t h the i m p l a n t e d source using d i f f e r e n t frequency sweep speeds. In the slow sweep mode several 2.4 M H z / h o u r increasing and decreasing f r e q u e n c y sweeps are summed up t o g e t h e r . The fit on one of these runs is presented on fig. 3. The summing up of equal i n t e n s i t y increasing and decreasing frequency measurements yields a r e l a x a t i o n - b r o a d e n e d s y m m e t r i c a l curve.

2.1 t,o

' 0

~--~ 2.05

0 CO v

2.C

1.95 39

6.4 u3

' 0

~ s

0 3 v

O

~ 6.3

6.25

tttttttttttttttt

41 43 45 47 49 51

Freq (MHz)

. I t ,

39 41 43 45 47

Freq (MHz,)

l

'1

49 51

Fig. 3. Lower part : NMR/ON resonance in 65ZnFe in one of the

runs. Increasing and decreasing frequency resonance curves of the

1116 keV gamma ray observed in the direction of the magnetic

field h a v e been summed up. I / T = 133 / K, Bex t = 0.096 T ,

modulation + 0.2 MHz, frequency sweep speed 2.4 MHz/h. Fitted

parameters : ~) = 43.77 ( I i ) MHz, FWHM = 0.9(i) MHz. Upper

part : Intensity o'f the 1116 keV gamma ray without frequency res

m o d u l a t i o n .

3 0 4 1. Berkes et al., Hyperfine magnetic field of Zn in iron

The w e i g h t e d c e n t e r of g r a v i t y o f the m e a s u r e m e n t s g i v e s ,ore s

4:~.82(8) M H z w h i c h , usinq the m a g n e t i c m o m e n t nf 65Zn / ' , / y i e l d s Rel I - 1s163 ~ (',L41 T. (Thr ~ !;iqn i~.

a d o p t e d From r e f . 3 . ) With B . = 0.096 (2) T we f i n d B h f ( Z n F e ) -- ~ ' e f f - B e x t = - 18.785(35) T at low t e m p e r a t u r e X L T h e q u o t e d e r r o r t a k e s i n t o a c c o u n t an e v e n t u a l ~mall K n i g h t s h i f t due [o the e x t e r n a l field, <K < 2 %).

C o m p a r i n g the m e a s u r e d a n i s o t r o p i e s of the g a m m a rays o f the d i f f u s e d s o u r c e and the i m p l a n t e d one, we f i n d t h a t only 39 (/4) % of n u c l e i are s u b i e c t Io the full s u b s t i t u t i o n a l h y p e r f i n e f i e l d ~n Ihis l a t t e r s o u r c e .

In o r d e r to conl.ro] thi.,; m e a s u r e m e n t , the f r e q u e n c y has been .';wept also w i t h a f a s t e r speed of B M H z / h o u r . G a u s s i a n line b r o a d e n i n g , n o n - z e r o m o d u l a t i o n w i d t h and a s i n g l e e x p o n e n t i a l r e l a x a t i o n a f t e r r e s o n a n c e h a v e been c o n s i d e r e d in the f i t o f these c u r v e s . I n c r e a s i n g and d e c r e a s i n g f r e q u e n c y c u r v e s h a v e been f i t t e d s i m u l t a n e o u s l y w i t h the same set of p a r a m e t e r s ( f i g /4/.

LD 3.15 _ 9 ' O

O

3,1 4-

3.05

a 3.0i

36

3.15 ~_____,_

~O ' O

o 3.1 i

+

C_..~3.05

b

3.0 36

i t I

[ ~ i~ Li

!tl'tl

, [

1

i

40 44 4 8

Freq ( M H z )

52

P

I

40 44 4 8 52

Freq ( M H z )

Fig. 4. Increasing (a) and decreasing (b) frequency resonance curves taken with a frequency sweep of 8 MHz/h. The gaussian curve is the fitted adiabatic resonance (see text), which could be measured with a total sweep time > > T'

I "

I. Berkes et al., Hyperfine magnetic field of Zn in iron 305

T h e f i t t e d v a l u e s are :

R e s o n a n c e f r e q u e n c y v : 143.91 (15) [VlHz

r e s

Width of the a d i a b a t i c r e s o n a n c e F W H M : 0.9 (3) M H z R e l a x a t i o n t i m e c o n s t a n t

at I/T : 133 K -1 T' : 2350 {550) s.

I D e s t r u c t i o n of o r i e n t a t i o n D=8/4 (7) %

It must be r e m a r k e d t h a t the v a l u e of the r e s o n a n c e f r e q u e n c y ~s p r a t t , t a l l y i n d e p e n d e n t of the c h o i c e of the o t h e r p a r a m e t e r s , w h i l e F W H M , T' 1 and D are s t r o n g l y c o r r e l a t e d in the f i t , w h i c h e x p l a i n s t h e i r l a r g e e r r o r s . The r e s o n a n c e f r e q u e n c y o b t a i - ned f r o m the f a s t sweep m e a s u r e m e n t s is in e x c e l l e n t a g r e e m e n t w i t h the p r e v i o u s d e t e r - m i n a t i o n . The c o m p a r i s o n of t h e f i t t e d a d i a b a t i c w i d t h w i t h t h a t o b t a i n e d in the slow sweep runs shows t h a t the r e l a x a t i o n b r o a d e n i n g was n e g l i g i b l e al Z.4 M H z / h o u r s w e e p speed and the m e a s u r e d r e s o n a n c e w i d t h . The p r o d u c t TT' 1 =18 (4) Ks a g r e e s w e l l w i t h t h a t o b t a i n e d by f a s t c o o l i n g by C h i l a s v i l e et ah / 1 / .

The 6 ( E 2 / M 1 ) m i x i n g r a t i o of the 5/2- -* 3/2 1116 keV t r a n s i t i o n is g i v e n as -0.1437(15) in NDS / 7 / but v a l u e s f r o m d i f f e r e n t d e t e r m i n a t i o n s a r e r a t h e r s p r e a d out o v e r a r a n g e f r o m -0.19 to - 1.9 . The r e d u c e d r a t e of the 5/2- -~ 5/2 e l e c t r o n c a p t u r e is log ft : 5.9. This log ft w o u l d suggest a JR : I E C transition, thus U 2 (5/2,5/2,1) : 0.&57.

The anisotropy of the 1116 k e V g a m m a " ray calculated with these p a r a m e t e r s is about 60 % Lower than the m e a s u r e d one for the diffused source. As in this m e a s u r e m e n t

?

g u N B / k T < < I, thus B 2 A 2 U 2 c~ ( g B / k T ) - A 2 U 2, the m e a s u r e m e n t does not allow to assign the deviation u n a m b i g u o u s l y to one parameter. T h o u g h a m e a s u r e m e n t at m u c h lower t e m p e r a t u r e s (e.g. by m e a n s of nuclear demagnetization) would allow the separation of the B~ and A), U;~ terms, in this case the spin lattice relaxation time w o u l d b e c o m e too long.

3. D I S C U S S I O N

Zn is s o l u b l e in iron up to a f e w p e r c e n t s at r o o m t e m p e r a t u r e / 8 / . Thus we can suppose t h a t the m a g n e t i c h y p e r f i n e f i e l d m e a s u r e d w i t h the r e s o n a n c e and t h a t of A g a r w a ] et al. / 3 / c o r r e s p o n d to the s u b s t i t u t i o n a l l a t t i c e s i t e of d i l u t e a l l o y s . The v a l u e lies on the s y s t e m a t i c t r e n d of h . f . f i e l d s of 3d4s e l e m e n t s in iron.

The r a t i o o f the h y p e r f i n e f i e l d s of 57Fe in iron at r o o m t e m p e r a t u r e and al q._' "' I/.

has been m e a s u r e d w i t h N 4 6 s s b a u e r - e f f e c t to a high p r e c i s , o n : Bhf(3OOIK)/Bhf{/4.ZK) : 0 9 7 4 6 ( 1 ) / 9 / . The high t e m p e r a t u r e d e p e n d e n c e of Bhf ( Z n F e ) shows t h a t it f o l l o w s a p p r o x i m a t i v e l y the m a g n e t i z a t i o n in iron / 3 / . So, we can suppose t h a t the h y p e r f i n e f i e l d o f zinc in i r o n shows s i m i l a r v a r i a t i o n b e t w e e n 300 K and 0 K as t h a t of i r o n , but a f f e c t i n g the c o r r e c t i o n f a c t o r w i t h a 1 % a b s o l u t e e r r o r : 0.975(10), g i v i n g Bhf ( Z n F e , 300K) : - 18.31(18) T, w h i c h is in good a g r e e m e n t w i t h t h a t m e a s u r e d w i t h D P A 9 by A g a r w a l et ah -18.4(3) T.

H e r z o g et al. e s t a b l i s h e d the r e l a x a t i o n t i m e f o r 6 9 m z n as T' : 7;600 (&O) s

1 5

/ 4 / . With the g i v e n l a t t i c e t e m p e r a t u r e s T we f i n d T T ' l ( 6 9 Z n F e ) / T T ' l (6 ZnFIe) : (65)/v (69) ]2 : 1.69 1.8(4). Tile s q u a r e of the r a t i o of the r e s o n a n c e f r e q u e n c i e s [ Vre s res

a g r e e s w , t h i n the s t a t i s t i c a l a c c u r a c y w i t h the i n v e r s e r a t i o of the TT' 1 p r o d u c t s in a g r e e m e n t w i t h the s y s t e m a t i c s of r e l a x a t i o n t i m e s in the high t e m p e r a t u r e l i m i t g B / k T

<< I.

The r e l a x a t i o n t i m e c o n s t a n t C. is d e r i v e d c o m p a r i n g the v a r i a t i o n of the

a n i s o t r o p y c a l c u l a t e d f r o m the t i m e e v o u t m n K m a s t e r e q u a t i o n s / 1 0 / w i t h the single

e x p o n e n t i a l v a r i a t i o n C K = 1/4 (3) Ks.

306 I. Berkes et at,, Hyperfine magnetic field of Zn in iron

Our m a g n e t i c f~eld d e t e r m m a D o n allows to Lmprove the prec~sLon of the m a g n e D c mornents of 6 9 m ' 7 ] m z n . H e r z o g et al. //4/ e s t a b l i s h e d the zero e x t e r n a l f i e l d N M R / O N

frequencLes as 36.81/4:3'-,J 14Hz for 6 9 m z n F e and 33./47 119) Mmz for 7 1 m z n F e , wlc,h y i e l d s ~1 ( 6 9 m z n l 1.1S7~'21 ~.l N and It ( 7 1 m z n ] 1.052 (6) UN, r e s p e c t i v e l y .

II ~s diffLcult to e x p l a i n the greal antsolrop~, observed in Ihe d i f f u s e d source. If we accept Ihal each r~ucleus Is subject to the h y p e r f l n e fLeld of - 18.8 T, the measured a n l s o t r o p l e s impose A.~ : A.> ':max) 1.1)R wLlh #5 (F2/M1 ; 111(, keV] - 0.7/4 and IJ~

~,/3, ',/2, 1, Ihus a pure JB () I-(7 Irans, l , o n whtc,h would be unusual w r t h respect to the log ft value. No i m p o r t a n t J13 : 0 c o n t m b u t l o n has been found n e i t h e r for o t h e r nuc,lel in Lh~s mass region for non 0+O a l l o w e d 13-translttons. H o w e v e r , ~f we a c c e p t the l i t e r a t u r e m+xlnc I r a t i o for lhe 1116 keV I r a n s l t l o n and J6 :: 1 for Ihe [ [7, Ihe a v e r a g e h y p e r f l n e h e l d would be 26 /4(9} T. A s h w o r t h et al. observed such a b e h a v t o u r ~n Cs +replanted ~nto ~ron and annealed al room t e m p e r a l u r e : Ihe resonant f i e l d was much l o w e r than the average e f f e c l t v e h y p e r f l n e h e l d / 1 1 / . l-he two s~tuatLons are, h o w e v e r , d~fferent. Cs ts not soluble ~n iron, c o n t r a r y tu Zn. On the o t h e r hand, the o b s e r v e d resonance w t d t h and the m t e g r a l destruc,tlon ~n our sample are sirnilar to those of the c ' o l d - l m p l a n t e d 6 9 m ' 7 1 m z n of H e r z o g et al. //4/ whlc,h shows Ihat the anneahng has not an ~mportant e f f e c t on the sites of Zn in tron.

A C K N O W L E D G E M E N T S

A u t h o r s are k~ndly i n d e p l e d to Dr. R. E d e r for helpful discussions and for hts help

~n the m e a s u r e m ~ n l s , to I . V~dal for chemlc,al p r e p a r a t i o n , to A. P l a n t l e r for tsolope s e p a r a t o r ~mplanlat~on, to R. Bou~mr for sync,hroc,yc,lotron i r r a d i a t i o n s and to 3.P.

HadioLJt for t e c h n i c a l c o n l r l b u t l o n . R [ FEI:(ENCES

/ l / O . A . (-hllasvLle, C..]. S a n c l u a r y and N.J. Stone,

Proc. 11-th Conf. on L o w Temp. Phys. (1968) ~North H o l l a n d PubI.Co) p.523 / 2/ P. [nia, Y . K . A g a r w a l and H.de Waard, Phys. Rev., 188 (1969) 605

/ ',/ Y.t4. A g a r w a l , B. B e r t s c h a t , m. Haas, F. P l e t t e r , E. R e c k n a g e l , E. Schlodder and 13. !}pellmeyer, Phys. L el.t., A17 (197/4) 161

/ 4/ P. H e r z o g , U. D ~ m m r l c h , 14.. F r e t t a g , C . D . H e r m a n n and !4. S c h l 6 s s e r , Z. Ph~.s., A332 (1989) 2b. 7

/ 5/ s l e d e r e r and V.S. Shtrley,

T a b l e of I s o t o p e s , 7 - t h E d + t l o n (ed. Yohn W~ley & Sons, N e w Y o r k , 1978) / 6/ Y.F. Z+egler, .}.P. Blersac,k and U. L i t t m a n ,

t h e stopp,ng p o w e r and range of ions in sohds. ( P e r g a m o n Press, N e w Y o r k (1985) / 7/ N..]. Ward and Y.K. TulL, N u c l e a r D a t a Sheets, 47 (1986) 135

/ U/ M. Hansen,

C o n s t , t u t l o n of B,nary A l l o y s (and Lts supplements), Mc ( } r a w mJIl, N e w Y o r k , (1958)

/ 9/ C. V i o l e t and D. P l p k o r n , a. Appl. Phys., 82 (1971) /4339 / 1 0 / E. I<leln,

In I ow T e m p e r a t u r e N u c l e a r (3r~enlat~on, eds. N.J. 51one and m. P o s t m a p.607 +Norlh H o l l a n d P u b l . , C o , A m s t e r d a m , 1986)

/ 1 1 / C..]. A s h w o r t h , P. Back, N.Y. Stone and .].P. Wh,te,

Idyp. lnl. 60 (1990) 929