BOND IMPURITY EFFECT IN THE LOW-DIMENSIONAL MAGNETIC SYSTEM (CH3)3 NHCoCl3.2H2O

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BOND IMPURITY EFFECT IN THE

LOW-DIMENSIONAL MAGNETIC SYSTEM (CH3)3

NHCoCl3.2H2O

J. Schouten, K. Takeda, K. Kopinga

To cite this version:

JOURNAL DE PHYSIQUE _{Colloque C6, supplhment au} no 8 , Tome 39, aoit 1978, page C6-723

BOND

EFFECT

THE

C SYSTEM

(cH~)

J.C. Schouten, K. ~akeda' and K. Kopinga

Department o f Physics, Eindhoven U n i v e r s i t y o f TeclmoZogy, Eindhoven, The NetherZmtds

R6sumd.- La chaleur specifique du syst2me unidimensionnel ( C H ~ ) ~ N H C O ~ - ~ C U ~ C ~ ~ . ~ H ~ O a 6tG mesurse pour un large eventail de concentrations X. Le comportement de TN(x) diffsre considdrablement des resultats publies 3 propos d'autres systgmes pseudo unidimensionnels en ce sens que TN est presque indgpendant de X jusqu'2 X P 0,lO.

Abstract.- The heat capacity of the pseudo one-dimensional system (CH3) 3NHCol-,CuXC13 .2H20 is measu- red for a wide range of concentrations X. The behaviour of TN(x) differs significantly from the re-

sults reported on other pseudo l-d systems in that respect that TN is almost independent of X up till

X = 0.10.

A onedirnensional (I-d) magnetic system shows no long range order for T > 0 K, although the intra- chain correlation length Eld(T) grows rapidly as T approaches zero. In a pseudo I-d system a transition to a 3-d ordered state will be triggered by the in- terplay of the highly correlated chain and the weak interchain interaction J'. Subsequently the ordering temperature of such systems is drastically affected by limitations set to the correlation length by ma- gnetic or nonmagnetic impurities/l,2/. In this paper we will report on TN(x) of (CH3)3NHCo Cu C13.2H20.

I-X X

The host system (CH3)3NHCoC1,.2H20 is composed of ferromagnetic Ising chains, J/kg = 15.4 K, with a weak interchain coupling J'/J = 0.01/3/. The 3-d transition occurs at TN = 4.135 K and is accompanied by a sharp peak in the heat capacity. (CH3)3NHCuC13

.2H20 is isomorphous to the cobalt salt and orders at T = 0.157 K with J/% = 0.28 K/4/. The crystals

N

used in the experiments were grown from an aqueous solution of CoC12.6H20, CuC12.2H20 and (CH3) 3NHCoCl. The Cu-concentration in the crystals was measured by comnercial chemical analysis, and was found to be systematically twice the solution concentration.

We measured the specific heat between 1.5 and

5 K on small single crystals with an average mass between 0.4 and 0.8 grams. The values of TN which were deduced from the maximum in the specific heat are plotted in figure 1 as a function of X. Inspec- tion of this figure reveals that in contrast to the behaviour reported for nonmagnetic impurities/2/, the decrease of the ordering temperature for x<<l is not proportional to X, but has zero slope for

M. Hone et a1 /l/ showed that for magnetic impuri-

+

Present address : Dept. of Physics, Faculty of Te- chnical Science, Osaka Univ. Toyonaka, Osaka,Japan.

Fig. 1 : TN(x)/TN(0) of (CH3) ~ N H C O ~ - ~ C U ~ C ~ ~ .2H20 as a function of X. The solid lines represent the theo-

ry for a pseudo one-dimensional Ising host system. a : JIH ' JII = 0 ; b : JIH ' Jm/2, JII = 0.02JHH ;

c : JIH = JHH, JII = 0.02JHH.

ties the interaction between host and impurity ions JIH and the interaction between the impurity ions JII may have a rather drastic influence. This is demonstrated by the curves a (JIH = JII = 0)

- - - - -

b(JIH =

1

a corresponds to non magnetic impurities ; in case b and c we used the value J =

I1 Jcu-cu derived by Stirrat141 from measurements on pure (CH3)3NHCuC13

.2H20. Apart from the fact that for low values of

X our data seem to indicate a small raise of T the N theoretical predictions are systematically too low. Qualitatively, however, the experimental behaviour seems to be rather well reproduced for J

IH = JHH- This value of JIH obviously disagrees with the usual approximation J IH -

-

mi

Id2.

The deviation of the theoretical prediction from the experimental data may be due to several effects. First of all the assumption J =

I I Jcu-cu

may be incorrect, since the local environment of the Cu-ions in the cobalt salt differs slightly from that in the pure copper salt. As already indicated above the theoretical predictions are very sensi ti- ve to this value. Secondly some clustering of the copper ions may occur, which will cause the actual number of copper clusters to be less than the num- ber based on a statistical distribution. However such a clustering effect is not very likely in view of isomorphy between the copper and cobalt salt. Mo- reover, the meanfield treatment of the interchain interaction used in the theory of Hone may be less appropriate to (CH3)3NHCoC13.2H20, since this salt has been shown to display rather pronounced two- dimensional characteristics. Currently ESR experi- ments in the Cu-doped ( C H ~ ) ~ N H C O C ~ ~ . ~ H ~ O are in pro- gress in order to verify the anomalous magnitude r f

JIH as well as the absence of clustering phenomena. To this aim also susceptibility measurements will be undertaken. If the value of JIH proves to have the same order of magnitude as JHH and no cluste- ring occurs, (CH3)3NHCol-xC~xC13.2H20 seems the first realisation of a bond impurity system.

References

/I/ Hone,D., Montano,P.A., T0negawa.T. and Imry,Y.. Phys. Rev.

B12

/ 2 / See for instance

Dupas,C. and Renard,J.P., Phys. Lett. e(1975) 181

Steiner.M. and Axmann.A., Solid State Coormun. 19 (1976) 115

Takeda,K., J. Phys. Soc. Japan

40

131 Losee,D.B., McElearny,J.N., Shankle,G.E., Carlin, R.L., Cresswel1,P.J. and Robinson,W.T., Phys.

Rev. (1973) 2185