Solid State Communications, Vol. 37, pp. 133H37.
Pergamon Press Ltd. 1981. Printed in Great Britain.
MAGNETIC STRUCTURES DETERMINED BY NEUTRON DIFFRACTION IN THE EuB
C
SYSTEM
6
-x x
+
J.M. Tarascon, J.L. Soubeyroux
Laboratoire de Chimie du Solide du CNRS, Universit~de Bordeaux I,
51 Cours de la Libe’ration, 33405 Talence Cedex, France.
,
J. Etourneau and R. Georgos
3
+
Institut Max Von Laue
-
Paul Langevin l5ôX
-
38042 Grenoble Cedex, France.
J.M.D. Coey
Department of Pure and Applied Physics, University of Dublin,
Trinity College, Dublin 2, Ireland.
0. Massenet
Groupe des Transitions de Phases du CNRS, 166X, 38042 Grenoble Cedex, France
Received 8 October 1980 by E. F. Bertaut)
(
Magnetic structures of pure and carbon-doped europium hexaboride
EuB6_xCx, were determined by neutron diffraction on powders prepared from
1
1B and 153Eu. EuB is a simple ferromagnet, whereas the x
=
0.20 compound
(0.28,
, 0). Data for a magnetically inhonogenecus intermediate composition, x
.05, indicate a mixture of ferromagnetic and helimagnetic domains with T
has an incommensurate spiral structure with propagation vector
T
=
0
=
=
0
(
~,0, 0.104). Helimagnetism in EuB6_xC~arises from a competition between
ferromagnetic near-neighbour exchange and antiferromagnetic interactions
due to ccjnduction electrons.
The relation between the magnetic
properties and the electronic structure
of europium hexaboride EuB6 has been the
mum around x
=
0.06. The ordering tempe—
rature was defined by appearance either
of magnetic hyperfine splitting in the
subject of many experimental and theore—
151Eu Mössbauer spectrum4 or of a peak
in the magnetization measured in a cons—
tant small field3. This cusp disappears in
tical studies1
1 0 ~ It was difficult to
decide whether the compound is intrinsi—
cally a semiconductor or a semimetal,
because of the small gap (~0.1 eV).
Furthermore the role of the impurities
fields of 1.2
—
3 kOe when x
0.10, but
persists in fields greater than 10 kOe
for x
=
0.21.
and possible cation vacancies could not
be neglected1. Nevertheless a recent
work suggests that EuB6 is a narrow—gap
intrinsic semiconductor in the parama—
gnetic state, and becomes a semimetal or
a metal when it orders magnetically7’ 9 ~
Our Mössbauer study revealed a tein—
perature—dependent broadening of magnetic
hyperfine lines for x
=
0.05 (absent for
x
=
0 and x 0.21), which implies that
=
the exchange interactions a r ~ inhomoge—
neous on a microscopic scale
.
This result
Semiconducting behaviour arises
for M2~B6hexaborides because the bon-
ding bands of the boron network are
filled up by transfer of two electrons
from the cation11’ 12~.
together with magnetization curves at
4.2 K, which show that 90 % of the satu—
ration is achieved in fields of less than
5 kOe for EuB
,
but that 100 kOe is r e -
quired to pro~uce the same degree of satu—
EuB can be doped either by subs-
titution o ~ a tn— or tetravalent cation
or by replacing some boron atoms by
carbon2, 4, 6, 13, 14, In both cases,
substitution yields n—type metallic
conductors6. The conduction electrons
modify the magnetic interactions and
ration in the x 0.21 sample3 suggest
,
strongly that the magnetic order evolved
from ferromagnetism (x 0) to antiferro—
magnetism (x 0.21), via an intermedia-
te mictomagnetic phase.
To test these hypotheses, a series
of neutron diffraction measurements has
=
=
induce a sign change of the paramagnetic
been undertaken on samples with x
=
0,
Curie temperature e~.
0.05 and 0.20. Powder samples of EuB6_xCx
Data shown in Fig. 1 concern the
ternary compound EuB~_~Cx.The lattice
parameter decreases trom 4.1855 A for
were prepared by reduction of Eu2O3
(99.999 %) with boron or boron-carbon
mixtures2. Since natural boron and euro—
pium are both intense absorbers of ther-
mal neutrons, all three samples were made
from boron enriched to 99.3 % in ~1B.
Furthermore we also used a single euro—
x
=
0 to 4.1685 A for x
=
0.21, the limit
of the single phase range. O~,changes
sign at x 0.13, but the magnetic orde—
ring temperature passes through a mini—
133
Tarascon, J. M.
