ISOTOPE EFFECT ON THE MAGNETIC STRUCTURE OF CoCl2, 6 H2O

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ISOTOPE EFFECT ON THE MAGNETIC STRUCTURE OF CoCl2, 6 H2O

H. Benoit, W. Ghidalia, J.-P. Legrand, J.-P. Renard

To cite this version:

H. Benoit, W. Ghidalia, J.-P. Legrand, J.-P. Renard. ISOTOPE EFFECT ON THE MAGNETIC

STRUCTURE OF CoCl2, 6 H2O. Journal de Physique Colloques, 1971, 32 (C1), pp.C1-1151-C1-

1153. �10.1051/jphyscol:19711412�. �jpa-00214454�

JOURNAL DE

PHYSIQUE Colloque C 1, supplPment au no 2-3, Tome 32, Fkorier-Mars 1971, page C 1 - 1 1 5 1

ISOTOPE EFFECT ON THE MAGNETIC STRUCTURE OF COCI,, 6

H. BENOIT, W. GHIDALIA

Facult6 des Sciences, 9, quai Saint-Bernard, Paris 5 c , France J.-P. LEGRAND, J.-P. RENARD

Institut d'Electronique Fondamentale, Laboratoire associC a u C. N . R. S., BBt. 220 Faculte des Sciences, 91, Orsay, France

Rhume. - On a etudie la resonance magnetique nucleaire des protons et de 3sCI ainsi que 1aJrksonance antiferro- magnetique dans CoC12, 6 Hz0 antiferromagnetique deuterie dans la proportion

En dessous de 2,28 OK, CoC12,6 Hz0 a une structure antiferromagnbtique h deux sous-reseaux magnktiques avec une direction de facile aimantation voisine dell'axe c. Quand x est superieur a 3,5 %, une modification de la plage de resonance antiferromagdtique et un doublernent du nombre de champs magnetiques internes vus par les noyaux sont observes. Ces effets sont expliquh par un changement de structure magnetique dil a la deutkriation. La nouvelle structure est une structure croisie A quatre sous-reseaux, chacun d'entre eux faisant un angle

avec le plan ac. Deux arrangements possibles des spins de Co2+ sont envisagb.

Abstract. - The nuclear magnetic resonance (NMR) of H and Cl35 and the antiferromagnetic resonance (AFMR) have been studied in antiferromagnetic CoC12, 6 Hz0 when hydrogen is replaced by deuterium in a proportion x. Below T

2.28

CoC12, 6 Hz0 has an antiferrornagnetic structure with two sublattices, the spin easy direction being near the c crystal axis. When x exceeds ^3.5 %, a modification of the A F M R pattern and a doublingof thenumber of nuclear internal fields are observed. These effects are explained by a change of the magnetic structure due to deuteration, the new one being a

cross structure

with four sublattices, each one being tilted of an angle

from ac plane. Two possible Co2+ spin arrangements are discussed.

I. Introduction.

The effect of replacing the pro- tons by deuterons on the NCel temperature has already been studied in some antiferromagnetic salts such as CuCI,, 2 H 2 0 [I] and MnCI,, 4 H,O [2]. In these antiferromagnets, a decrease of TN of a few per cent was observed and was related to a diminution of the exchange interactions which are due to superexchange paths including a hydrogen bond. In CoCl,, 6 H 2 0 , Sahri and Bloom [3] observed an increase of TN on deuteration, using proton NMR in the paramagnetic phase. In this paper we report NMR and AFMR study of the isotope effect in the ordered phase of the well known salt [4] CoCl,, 6 H20. AFMR and NMR apparatus and methods are described in preceding papers [5, 61.

11. Antiferromagnetic resonance results. - A de- tailed AFMR study of CoCl,, 6 H,O was performed by Date [7]. Its experimental results indicate a spin easy axis along c and strongly anisotropic exchange interaction between the Co2+ spins. Our experimental results are in agreement with the Date's one for CoCI,, 6 H 2 0 ; two resonance lines are observed one below and the other above the spin-flop field.

The angular variation of the resonance fields when the magnetic field H is applied in the be plane is shown in figure 1. A drastic change in the angular diagrams occurs for x ² 0.035 and there appears new resonances with the H direction quite far from the c axis, which may be explained only by a change of the magnetic structure. When H is outside the bc plane, the resonances are associated' only with the projection of H on the bc plane. This points out that the Co2+ spin directions remain in the bc plane.

111. Nuclear magnetic resonance results. - 111. 1 CHLORINE NMR. - In antiferromagnetic CoCI,,

FIG. 1. -

Angular dependence of the antiferromagnetic rew- nance field in the bc plane at 10 GHz and

for single crystals of cobaltous chloride containing a deuterium propor- tion

curve

x

0 ; curve (2)

x

0.035 ; curve (3) :

x

0.20.

6 H,O, two internal fields of the same magnitude (12 676 Oe at 0 OK) and of opposite directions were observed [6]. Their direction 6 is approximately along c in the mirror ac plane. In CoCI,, 6 D 2 0 , there are four internal C1 fields of the same magnitude (13 180 Oe at 0 OK) which are related by inversion and symmetry with respect to ac. They are tilted of 320 from ac and their projections on ac coincide within lo with 6 .

111.2 PROTON NMR. - In zero field, the measured proton NMR frequencies a t low temperature in figure 2 show a striking change due to deuteration when x exceeds 0.035. Each of the two proton I lines observed for x c 0.035 gives rise to two lines for x > 0.035, the frequencies of which vary rapidly with x while the proton I1 lines are not doubled.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19711412

C 1 - 1152 H. BENOIT, W. GHIDALIA, J.-P. LEGRAND, J.-P. RENARD

FIG. 2. - Zero field proton NMR

chloride versus deuterium proportion

protons I

pro-

tons 11.

The proton I1 lie in the mirror ac plane, therefore quite similar effects of deuteration as for C1 were observed. The two non equivalent HI[ in the chemical cell give rise to two sets of internal fields. In CoCI,, 6 H 2 0 each set comprises two opposite internal fields located in ac. For x > 0.035 each set comprises four internal fields related by inversion and symmetry with respect to ac. The directions of the internal fields projections on ac are unchanged with deuteration.

For the protons I which are off the mirror plane, two sets in CoCl,, 6 H 2 0 and four sets in deuterated samples, each one comprising four fields related by inversion and symmetry with respect to ac were observed. Finally the total number of internal proton fields is doubled with deuteration for x > 0.035.

Our results are summarized in Tables I and 11.

In Table I, the experimental proton fields in CoCI,, 6 H 2 0 are compared with calculated values obtained from the simple point dipole model. Using the proton atomic parameters deduced from a deuteron NMR

Measured and calculated sets of proton internal magneticjields

in CoCl,, 6 H 2 0

Pro- Hm

HC

ton (Oe) ( 0 e )

-

-

-

H: 1 987 106O 520 2 155.5 103O 30' 500 45' H: ¹ 582 960 830 1743.9 101018' 82045' H~ ¹ 630 79O

00 I 780.7 800 30' 0°

HII 1 202 119" 30' 0" 1286.5 121° 36' O0

study of CoCI,, 6 D 2 0 at 77 OK [8] and a spin easy axis 60 from the c axis toward a, a better agreement with experience than in a previous computation [9]

is obtained.

IV. Discussion.

Our N M R and AFMR results, as previous susceptibility measurements [lo], indicate a change of the magnetic structure of CoCI,, 6 H,O with deuteration when D concentration exceeds 0.035.

The doubling of the number on nuclear internal fields suggests an antiferromagnetic

cross structure

with four magnetic sublattices.

The projections of the Co2+ spins on the mirror ac plane remain unchanged by deuteration and, due to crystal symmetry, each sublattice magnetization must be at the same angle ^cp from ac.

TABLEAU III

Experimental q~ values versus deuterium proportion x

x 0 0.04 0.06 0.1 0.2 1

cp 00 60(1) 120(1) 180(1) 250(1) 320(2) 33.50 (3) Type of measurement :

(1) AFMR.

(2) ClNMR.

(3) Susceptibility.

R. Kleinberg kindly performed a preliminary neu- tron diffraction experiment on our CoCI,, 6 D 2 0 single crystals. Its measurements point out that there is no additional doubling of the magnetic cell with deutera-

Measured and calculated sets of proton internal magneticfiel& in CoCI,, 6 D,O

Measured fields proton _- &) ^{om q m}

-

- -

H I 2 784.5 790 40' 500 50' 1 393 1550 55' 360 55'

H: ^{2 213} ₁₁₁₂ ^116020' _{500 55'} ^76040' _{890 20'}

H;I

1316 8

40' 50 5'

H; 1 218 114045' 350 5'

Structure A

Calculated fields

Structure B

The magnitudes of internal fields are given at 0 OK, H, 0 and cp are the ordinary spheric coordinates in

the crystal axis system a' b c (a' I b, c).

ISOTOPE EFFECT ON THE MAGNETIC STRUCTURE OF CoC12, 6 Hz0 tion and that the magnetic structures of the deuterate

and the hydrate are not basically different.

Only two magnetic structures with four sublattices, which coincide with the CoCl,, 6 H 2 0 magnetic structure for p = 0, are consistent with the crystal symmetry (Fig. 3) : A with antiferromagnetic ab pla- nes, B with antiferromagnetic bc planes.

A computation of the proton internal fields for these two structures was performed, using a cp value equal to 330 which corresponds to CoCl,, 6 D,O (Table 111). For each one, six sets of proton internal fields were obtained, each set comprising four internal fields of the same magnitude related by inversion and symmetry with respect to ac, in agreement with expe- rience. Unfortunately the calculated fields do not differ greatly from one structure to the other. In fact, a slightly better agreement with experience is found with the structure A (Table 111).

This structure is probably the right one in agreement with Shinoda et al. [ll] which assume strong super- exchange interactions between the Co2' within the ab planes and rather weak couplings between neigh- bouring ab planes. Nevertheless, the intricate form of the AFMR patterns obtained in the deuterated samples points out that these last couplings are not completely negligible.

By no means, the strong effect of deuteration on the magnetic structure of cobaltous chloride cannot be explained by the simple assumptions of Sahri and Bloom [3] even with anisotropic exchange interactions.

FIG. 3. - The two possible arrangements of the Coz+ spins in CoCI2, 6 DzO. The spins are approximately located in bc planes

at

from ac.

Acknowledgments.

We wish to thank Dr. R. Klein- berg for performing neutron diffraction on CoCl,, 6 D,O and for helpful suggestions on the magnetic structure of this crystal.

References BENOIT (H.), DROCOURT (J. M.), LEGRAND (J.-P.),

RENARD (J.-P.), J. Appl. Phys., 1968, 39, 1015.

TURRELL (B. G.), YUE (C. L.), Can. J. Phys., 1969, 47. 2575.

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[S] BENOIT (H.) et al., Colloque Ampkre, Bucarest, 1970.

[9] LEGRAND (J.-P.), RENARD (J.-P.), C. R. Acad. Sci.

Paris, 1969, 268, 1197.

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