Magnetic structure of cobalt formate dihydrate : a single crystal determination

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Magnetic structure of cobalt formate dihydrate : a single

crystal determination

P. Burlet, J. Rossat-Mignod, M. Matsuura

To cite this version:

P. Burlet, J. Rossat-Mignod, M. Matsuura. Magnetic structure of cobalt formate dihydrate : a

sin-gle crystal determination. Journal de Physique Lettres, Edp sciences, 1979, 40 (17), pp.455-457.

�10.1051/jphyslet:019790040017045500�. �jpa-00231664�

L-455

Magnetic

structure

of cobalt formate

_{dihydrate :}

a

single

crystal

determination

P.

_Burlet,

J.

_{Rossat-Mignod}

and M. Matsuura

_(*)

DRF/DN, Centre d’Etudes Nucléaires, 85 X, 38041 Grenoble _Cedex, France

(*) Osaka _{University, Faculty} of Engineering Sciences, Toyonaka, Osaka, Japon (Re~u le 19 avril 1979, revise le 2 juillet 1979, accepte le _{6 juillet 1979)}

Résumé. 2014

L’étude par diffraction de neutrons non

_polarisés

d’un monocristal de formiate de cobalt montre _que sa structure

_magnétique

est _{décrite par} un mode

_{antiferromagnétique}

_{Axz pour les ions A} et B. Le _couplage entre

ces deux _types d’ions est

_{ferromagnétique.}

Ces résultats, en accord avec ceux _{d’une étude par} RMN, conduisent

à une structure

_magnétique

différente de celle

_{précédemment}

_{déterminée pour} un échantillon

_{polycristallin}

deutéré à une _température très inférieure à _TN.

Abstract. 2014 An

unpolarized

neutron diffraction _study of a

_single

_crystal of cobalt formate

_Co(COOH)2

2 _H2O

indicates that the

_magnetic

structure is described _by an

_{antiferromagnetic}

mode Axz for both the A and B ions.

The

_coupling

between these two _types of ions is

_{ferromagnetic.}

These results are consistent with the NMR study

but not with the _previous determination of the _magnetic structure on a _powdered deuterated _sample at a very low

temperature far below _TN. LE JOURNAL DE PHYSIQUE - LETTRES

TOME _40, ler SEPTEMBRE _1979, P

Classification

Physics Abstracts 75.25

1. Introduction. - The

series of

_isomorphous

formates of transition metal

_M(HCOO)2

_{2 H20}

(M

=

Mn, Fe, Co,

Ni)

has been studied

by

_neutron

diffraction

[1],

[2].

The

_experiments

were

_performed

on

_{polycrystalline}

deuterated

_samples

from which the

_magnetic

structures were derived.

Recently

[3],

an NMR

_study

on

single crystals

of the cobalt

for-mate

_dihydrate

concluded a different

_magnetic

struc-ture to that

_{previously reported.}

Therefore a

redeter-mination of the

_magnetic

structure of this salt has been undertaken

_by

neutron diffraction

_experiments

on a

_hydrated

single crystal.

The

_dihydrated

formates of the transition group metals

_belong

to the monoclinic space group

_?2i/c

with four formula units _per unit cell. The

crystallo-graphic

structure of the cobaltous salt has not been determined but it seems a

good approximation

to

assume that the atomic

_parameters

of cobaltous formate are almost the same as for _manganous

formate

_{[4, 5].}

_Concerning

the

_magnetic

ions,

the

structure can be schematized as in

_figure

1. In the

monoclinic unit cell the four cobalt atoms are on

two sites : the A ions at

_(0,

_0,

₀₎

_(AI)

and at

_{(0, 1/2,}

1 /2)

(A2)

and the B ions at

_(1/2,

_0,

_1/2)

_{(B 1 )}

and at

(1/2, 1/2, 0)

(B2).

The A ions are

_magnetically

linked

only

in the

₍₁₀₀₎

_{plane by}

_exchange

interaction via C-0-0 bonds and no direct

coupling

exists

between the

_planes,

since the A ions are

_{only coupled}

to B ions which are not

_coupled

_together.

The

_system

Fig. 1. - Schematic

representation of the structure of Co formate.

then can be described

_by

a

_stacking

of

plane

of A

ions

_separated

_by

_paramagnetic

_planes

of B ions. In the two

_proposed

models the

_magnetic

cell is identical to the chemical cell and the

_ordering

consists of an

_{antiferromagnetic}

mode noted A and a

ferromagnetic

mode F for both A and B ions.

L-456 JOURNAL DE _{PHYSIQUE -} LETTRES

From the

_previous

_powder

diffraction

_experiments,

the

antiferromagnetic

component has been found to

be

_parallel

to the monoclinic b axis

_(Ay)

and the

ferro-magnetic

component to be in the x-z

plane (Fx.,)

[1, 2],

while,

from NMR the model consists of an

anti-ferromagnetic

component

~4~

and a

ferromagnetic

component

Fy

[3].

Besides,

powder

neutron dif-fraction

_experiments

lead to an

_{antiferromagnetic}

coupling

between A and B ions whereas NMR results

are consistent with a

ferromagnetic coupling.

2.

_Experiments

and results. -

Single crystals

of cobalt formate

_dihydrate

have been _grown

_by

a

slow

_evaporation

method as described in

[3].

The

crystal

used has the form and size indicated in

_figure

2. Neutron diffraction

_experiments

were

performed

at

the reactor Siloe of the CEN-Grenoble

using

a double

axis

_{spectrometer.}

The

_crystal

was

_placed

inside a

cryostat which enables

_temperatures

to be controlled in the _{range 300} to 1.5 K.

Fig. 2. - Size and

morphology of the simple crystal used in neutron

diffraction _experiments.

Nuclear

_{Bragg peak}

intensities have been

mea-sured at T = 16 K i.e. above the Neel

_temperature

TN

= 5.1 K. The

comparison

of these intensities with the _square of the structure factors calculated

with the atomic

_positions

of _manganese formate leads to a

scaling

factor K = 1.83 x

10-3

and a

reliability

factor R = 0.13. The calculated and observed intensities are listed in table I. The _agreement

is

_{satisfactorily}

_good

_indicating

that too severe

extinction effects are not

_present

in

_spite

of the

_large

size of the

_crystal.

If the

_magnetic

structure is described

_by

an

anti-Table I. - Normalized nuclear intensities

(in barn/unit

cell)

measured at T = 16 K

compared

with the cal-culated values

_using

the atomic

_positions

_of

the

man-ganese

formate.

hkl Observed F2 Calculated F2 002 12.4 11.6 100 4.0 4.3 200 18.3 16.2 300 5.5 5.0 102 6.9 6.0 202 11.8 14.2 020 17.9 14.8 210 45.8 51.0 110 1.9 1.7

202

1.2 1.16

ferromagnetic

mode A and a

ferromagnetic

mode F,

the

_magnetic

structure factor can be

_expressed

_by

Under the

_hypothesis

that the dominant term of the Hamiltonian is a bilinear term, the

_components

A

and F must be

_{mutually perpendicular.}

Therefore the

_magnetic

intensities which _appear on reflections

characterized

_{byA;+/=2~+l depend only}

on

the

_{antiferromagnetic}

_{component A,}

whereas reflec-tions with k + I = 2 n

depend only

_on the

ferro-magnetic

component.

As in the ordered state there is no increase of the

intensity

of the reflections with k + I = 2 n _{we can} conclude that the

_{ferromagnetic component F}

is

too weak to be detected in an

_unpolarized

neutron

experiment (,uF

~0.7

_~p)’

Some reflections

_{corresponding}

to k + I = 2 n + 1 were measured at T = 1.5 K.

They

indicate that the main

_part

of the moment is the

_{antiferromagnetic}

mode A.

The observation of the

₍₀₁₀₎

reflection indicates

unambiguously

that the AF

_component

lies in the

(a, c)

plane

(~z)’

The

_intensity

of an

antiferromagnetic

reflection

(hkl )

is

_{given by}

where

_{f (h,}

k,

1)

is the form factor of the

C02+

ions

_[6],

a is the

angle

between

_AA

or

_AB

and the

scattering

vector, the

positive sign

in

_{(2) corresponds}

to a

ferro-magnetic coupling

of A and B ions and the

_negative

sign

to an

_{antiferromagnetic coupling.}

The best _agreement between observed and cal-culated intensities is obtained for

a

_{ferromagnetic}

coupling

of A and B ions and a

moment

direction

making

an

_angle

of 35 ± 50 with the c axis. The

L-457 MAGNETIC STRUCTURE OF COBALT FORMATE DIHYDRATE

AA

= 2.19 _±

0.5 ~

and

_{A B}

= 0.71 _:t 0.5 PB. The observed and calculated intensities are

_compared

in table II. Table II. -

Magnetic

intensities

_{(in barn/unit cell)}

compared

with the calculated values

hkl I Obs.

_(barn)

I Cal.

_(barn)

001 0.44 + 012 0.47 003 0.47 _{+ 013} 0.41 101 0.13

_{+ Ol l}

0.14 201 0.55 _{+ 013} 0.47

101

0.47 + 013 0.56 201 1.90 + 020 1.90 010 0.67 + 015 0.60 110

_{1.80 + 0.19}

1.73

AA

= 2.19 _:t

0.50 ~

AB

= 0.71 ±

0.50 ~B.

3. Discussion. - The results of this neutron

diffrac-tion

_experiment

on

single crystal

cobalt formate

dihydrate

are consistent with those of a NMR

_study

but not with those obtained from neutron diffraction

on deuterated

_{powder samples}

at 0.4 K. NMR and

neutron diffraction on a

_hydrated

_{single crystal}

both

lead to an

antiferromagnetic

mode

_~4~

in which the

moments make an

_angle

of 350 with the c axis. A

ferromagnetic

component

is deduced from NMR but its value is too small to be measured in an

unpola-rized neutron

_experiment.

The moment value and the ratio of

_{magnetization}

of A and B ions derived from the two

_experiments

are consistent within

the limits of

_experimental

accuracy. The

coupling

between the A and B ions is

_{ferromagnetic}

in the two

determinations. The

_magnetic

structure of cobalt formate

_dihydrate

at

_temperature

_higher

than 1.3 K is described

_by

the mode

_A.,,

_Fy

magnetic

group

_?2i/c

and a

ferromagnetic coupling

between A and B ions.

The observed value of the

magnetic

moment of

Co 2 +

ions

_{(2.2 ,uB)}

is rather small and leads for an

effective

_spin

value S =

1/2

_{to a}

g-factor (g

=

4.4)

comparable

to that

_(g

= 4.7

,uB)

derived from

sus-ceptibility experiment

17]

performed

in the

tempe-rature _{range 10} to 20 K. The reduction of the moment

value _may be

_explained

in cobalt formate

_by

the, effect of the

_{spin-orbit coupling}

and the

_crystal

field energy which

give

rise to _very

_{anisotropic g}

tensor

and

_{small g}

value

_{[8, 9].}

Acknowledgments.

- The

authors express their thanks to Dr. Yamakawa for the

_preparation

of the

sample

and for

_helpful

discussions.

References

[1] BURLET, P., Thèse Grenoble _(1975).

[2] BURLET, P., BURLET, P., ROSSAT-MIGNOD, J., DE _{COMBARIEU, A.,}

BEDIN, E., Phys. Status Solidi. b71 ₍₁₉₇₅₎ 675.

[3] YAMAKAWA, H., MATSUURA, M., J. Phys. Soc. Japan 41 (1976)

798.

[4] OKASAKI, K., NAKAI, Y. and _{WATANABE, T.,} J. Phys. Soc.

Japan 19 (1964) 717.

[5] IKAY, M., ALMODOVAR, I., KAPLAN, S. F., Acta Crystallogr. B 24 (1968) 1312.

[6] WATSON, R. E., FREEMAN, A. J., Acta _{Crystallogr. 14 (1961)} 27.

[7] TAKEDA, K., MATSUKAWA, S., J. _Phys. Soc. Japan 30 (1971)

887.

[8] URYU, N., SKALYO, J., FRIEDBERG, S. _{A., Phys.} Rev. 150 ₍₁₉₆₆₎

226.