EFFECT OF IMPURITIES ON THE NÉEL TEMPERATURE AND SPIN-FLOP FIELD IN THE QUASI ID ANTIFERROMAGNET CsMnCl3·2H2O

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EFFECT OF IMPURITIES ON THE NÉEL

TEMPERATURE AND SPIN-FLOP FIELD IN THE

QUASI ID ANTIFERROMAGNET CsMnCl3

×2H2O

E. Velu, J. Renard, G. Corbel

To cite this version:

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JOURNAL DE PHYSIQUE Colloque

C6,

supplPmen/ au no

8,

Tome

39,

aolit

1978,

page

C6-7 17

EFFECT

OF

IMPURITIES

ON

T H E

N ~ E L

T E M P E R A T U R E

AND SPIN-FLOP

FIELD IN THE

QUASI

ID

A N T I F E R R O M A G N E T

C S M ~ C ~ ~ ~ ~ H ~ O

E. Velu, J.P. Renard and G. Corbel

I n s t i t u t d'Electronique Fondamentale, Laboratoire Associd au CNRS, BEZtirnent

220,

Universi

t d

Paris-Sud,

91

405 Orsay Ceder, France

Rdsum6.- Nous avons mesurd en fonction du champ et au-dessus de 1,2

K

la susceptibilit6 magn6tique parallsle de lrantiferromagn6tique en chafne CsMnC13,2H20(CMC) dope ar des

g

impuretds de Cobalt et Cuivre en concentrations atomiques respectives & 3,4x10- et 1,7x10-~ 3,4x10-~. Nous observons un net abaissement de la tempdrature ae NQel T di3 a m impure- tCs. De

T

et de la variation thermique de la susceptibilit6 au-dessus fle 30

K,

nous

N

dBduisons la valeur absolue de l'interaction d'dchange Cu-Mn dans CMC dopd au Cuivre. Nous avons dgalement determind la ligne de spin-flop de CMC avec impuret6s. On constate un abaissement du champ de spin-flop qui n'est pas compatible avec les theories de champ moyen.

Abstract.- The magnetic susceptibility of the quasi-ID antiferromagnet CsMnC13.2H20 (CMC) doped by cobalt and copper impurities in respective atomic concentrations 3.4x10-~ and 1.7~10 2, 3.4x10-~ has been measured versus field and temperature down to 1.2

K.

A net decrease of the NEells temperature TN due to impurities was observed. From T and

N temperature dependence of susceptibility above 30 K in Cu doped CMC, the absolute value of the Cu-Mn superexchange interaction was determined. In addition the spin-flop field is strongly depressed by impurities. The observed reduction is not explained by mean- field theory.

Several studies on CsMnC13.2H20 (CMC) /I, 2/ have shown that this salt becomes antiferroma- gnetic below Neel's temperature

T

= 4.89 K. With

N

temperatures above TN, CsMnC13.2H20 is well repre- sented by I-D Heisenberg model. The exchange inte-

++

gral between neighbor Mn belonging to the same chain is about

-

3 K, the interchain exchange about

-

0.03 K. Such a system is very sensitive to the presence of impurities. In effect, the latter cut the chains, shorten the correlation length and therefore modify Ndel's temperature.The effect of the impurities on the magnetic susceptibility has been theoretically studied by Richards /3/ for a system of classical spins with spin 1/2 impurities.

++

The theory can be applied to CMC (Mn

,

S

= 5/2)

++

which contain copper impurities (Cu

,

S' = 1/21. We prepared 3 monocrystals of CMC with impurities using the formula CsMn

X

C13.2H20. The concen-

l-X

X

tration X was measured by microanalysJs (table

I).

1) MAGNETIC SUSCEPTIBILITY VARIATION OF TN.- The magnetic susceptibility is measured using a mutual

inductance bridge functionning at 70

Hz,

at temperatures varying between 1.2 and 77 K. The temperature is measured with a carbon resistance, cali- brated using a CrK alum and recalibrated at each experiment at the fixed points 4.2 and 7.18

K

temperature of the superconductive transition of lead.

We have thus studied the molar parallel s u s c e p t i b i l i t y ~ ~ along the easy axis b of pure CMC and of the sample 3 between 4.2 and 77

K

(figure 1). An increase o f x b due to the impurities can be noticed.

Fig. 1 : Molar parallel susceptibility Xb versus T of pure CMC (black circ1es)and of the sample 3

(white circles). The crosses are the experimental data of the reference 2.The heavy line is calculated with J/k=-3.17K/3/ and x=O. The dotted lines are calculated for X ~0.034,

J/k

=

-

3.17 K ~ ' / k

= -1.85 K(1ong dots)and J1/k(short dots)=+ 1.85

K.

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The comparison o f

xb

o f p u r e CMC t o R i c h a r d ' s theo- r e t i c a l c u r v e a l l o w s u s t o d e t e r m i n e t h e i n t r a c h a i n

++

exchange between

Mn

J / k =

-

(3.17

2

0.03)K. T h i s v a l u e i s i n a c c o r d a n c e w i t h p r e v i o u s r e s u l t s . The d i f f e r e n c e between t h e t h e o r y and t h e e x p e r i m e n t below 25 K i s e x p l a i n e d by t h e f a c t t h a t , f o r T < 25 K t h e a n i s o t r o p y b e g i n s t o be f e l t w h i l e t h e t h e o r y d o e s n o t t a k e t h i s i n t o c o n s i d e r a t i o n .

The measure o f

xb

between 4.2 and 7 K a l l o w e d u s t o d e t e r m i n e N l e l ' s t e m p e r a t u r e o f t h e 3 s a m p l e s and o f t h e p u r e CMC ( t a b l e I ) . I n e f f e c t , i f we p l o t

xb

v e r s u s T we o b s e r v e a r u p t u r e i n t h e s l o p e a t T

N.

2 ) SPIN-FLOP LINE.- At T < TN, t h e s p i n - f l o p f i e l d HSF i s o b t a i n e d b y m e a s u r i n g

xb

v e r s u s a s t a t i c magnetic f i e l d , p a r a l l e l t o t h e e a s y a x i s b.xb p r e s e n t s , a t t h e s p i n - f l o p f i e l d a n e t peak. The peak i s narrow f o r t h e p u r e sample e l l i p s o i d and much l a r g e r f o r t h e impure samples ( p a r a l l e l e p i - p e d s ) . F o r t h e p u r e s a l t HSF

i s

measured a t

_+

10 G

w h i l e t h e u n c e r t a i n t y of HSF r e a c h e s

2

100 G f o r t h e sample 3 a t a b o u t 3 K ( F i g u r e 2 ) . The sample 2 p r e s e n t s two s u s c e p t i b i l i t y p e a k s which a r e a b o u t 250 G a p a r t . The s p l i t t i n g between t h e s e two peaks

i s r e p r e s e n t e d i n t h e F i g u r e 2 b y t h e u n c e r t a i n t y b a r s .

T a b l e I

We u s e T h o r p e ' s c a l c u l a t i o n s / 4 / t o de- duce, from t h e measured TN, t h e v a l u e o f t h e in- t r a c h a i n exchange Cu-Mn. J ' / k . The g e n e r a l formula o f t h e c o r r e l a t i o n l e n g t h f o r c l a s s i c a l s p i n s i s :

T)

E ( l

-

X) / U ( + 2x(1

-

X) u t 2 I

-

( 1

-

X)

[ " l

-

x ( l

-

x ) u ' 2 25 S ( S + I )

-

w i t h u = c o t h kT kT 25 S ( S + l )

'

2 J ' S S ' kT U ' = c o t h

-

kT

W'SS'

S(x, T N ( x ) ) TN(x) The r e l a t i o n = - E(0, T N ( 0 ) ) TN(0) ( 5 ) p e r m i t s u s t o c a l c u l a t e [ ~ ' / k ( i f we know J / k ( t a b l e I ) . T h e r e f o r e we c o n c l u d e I J f / k l = ( 1 . 8 5

2 +

0.10) K.

The TN of t h e samples 2 and 3 , c a l c u l a t e d on t h e b a s e of t h i s v a l u e o f ( J ' / k l a r e r e s p e c t i - v e l y 4.60 and 4.34 K , which a r e c o n s i s t e n t w i t h t h e measured v a l u e s . The comparison of

xb

v e r s u s T t o t h e t h e o r e t i c a l c u r v e s /3/ c a l c u l a t e d f o r J ' / k = 1.85K and

-

1.85 K s h o u l d a l l o w u s t o d e c i d e i f t h e exchange Cu-Mn i s f e r r o - o r a n t i f e r t o m a g n e t i c (£ i g u r e I ) . I n p r a c t i c e t h i s comparison h o l d s t r u e o n l y when T > 3 0 K and t h e n t h e d i f f e r e n c e between t h e two

2 3

TrKl

F i g . 2 : S p i n - f l o p l i n e s o f t h e f o u r s t u d i e d samples. Black c i r c l e s : p u r e CMC, b a r s : Sample 1 , w h i t e c i r c l e s : sample 2, t r i a n g l e s : sample 3. The heavy l i n e s r e p r e s e n t t h e l a w s H (T) o f t a b l e I . The d o t t e d l i n e r e p r e s e n t s t h e sp%-flop l i n e of p u r e CMC i n r e f e r e n c e / l / . G e n e r a l l y , t h e t e m p e r a t u r e dependence o f HSF i s w e l l f i t t e d by a l i n e a r f u n c t i o n , e x c e p t p e r h a p s i n t h e v i c i n i t y o f t h e b i c r i t i c a l p o i n t (sample 3 ) . Our s p i n - f l o p l i n e o f p u r e CMC i s s l i g h t l y d i f f e r e n t from t h e one i n r e f e r e n c e / I / . We o b s e r v e a c o n s i d e r a b l e d e c r e a s e o f HSF due t o t h e i m p u r i t i e s . Recent t h e o r e t i c a l work / 6 / sug- g e s t t h a t t h e e f f e c t of d i a m a g n e t i c i m p u r i t i e s i n c o n c e n t r a t i o n X on t h e p h a s e d i a g r a m i s a l i n e a r s c a l i n g of r a t i o I-X. T h i s r e s u l t i s n o t v e r i f i e d i n t h e p r e s e n t c a s e which we c a n a t t r i b u t e t o two r e a s o n s : t h e q u a s i I-D c h a r a c t e r of t h e s y s t e m and t h e paramagnetism o f t h e i m p u r i t i e s which c o u l d modify t h e m a g n e t i c a n i s o t r o p y .

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References

/ l / Butterworth, G.J. and Woollam, J.A., Phys. Lett.

a

(1969) 259.

/2/

Kobayashi,

H.

and Tsujikawa, J., J. of Low.Temp. Phys.

10

(1973) 6 2 1 .

/3/ Richards, P.M., Phys. Rev. B

_-

14 (1976) 1239. / 4 / Thorpe,

M.F.,

J. Physique

36

(1975) 1177.

/ S / Dupas, C. and Renard, J.P., Phys. Rev.

B,

to be published

( 1 9 7 8 ) .