Ferromagnetism in CeCo3B2

Communications in Physics, Vol. 17, No. 4 (2007), pp. 241-245
FERROMAGNETISM IN CeCo3B2
NGUYEN MINH HONG
Faculty of Electrical and Electronic Engineering,
Hung Yen University of Technical Teacher Education,
Khoai Chau district, Hung Yen Province, Viet Nam
Abstract. The observation of ferromagnetism in CeCo3B2 with a high Curie temperature TC
= 140 K and a magnetic moment 8 × 10−3 µB /F.U has been presented and explained by 4f
itinerant magnetism with strong 4f-4f direct exchange interaction. The observed small magnetic
moment can be understood by a cancellation of 4f orbital and spin contributions and of 5d polarized
contribution to magnetic moment.
I. INTRODUCTION
Intermetallic compounds under a common formular expression Cen+1Co3n+5B2n
with n = 1, 2, 3 and ∞ have attracted attention during the last decade due largely to
their rich variety of magnetic phenomena [1-4]. The crystallographic structures of these
compounds are hexagonal and closely related to each other and to CeCo5 (CaCu5 type of
structure) by ordered substitution of Co at the 2c sites by B atoms [5]. Upon increasing
n, the lattice parameters a are found almost constant, while the normalized (per formular
unit) c parameters decrease, resulting in a systematic decrease of Ce-Ce interatomic dis-
tance along the c axis from ∼ 4 A˚ in CeCo5 to ∼ 3 A˚ in CeCo3B2(n = ∞), while in the
basal plane Ce-Ce distance is of little change, of about 5 A˚. For CeCo3B2, all 2c sites are
occupied by B atoms, and Co atoms are found only at 3g sites. Thus the structure of this
compound is built by alternatively stacking of the two layers, a closely packed Co atom
layer and the one containing Ce and B atoms and the difference in Ce-Ce distances along
the c-axis and in the basal plane makes Ce atoms to form a pseudo-1D chain along the
c-axis.
Magnetic measurements pointed out that all compounds are ferromagnetic. CeCo4B
(n = 1) and Ce3Co11B4 (n = 2) ordered ferromagnetically below 280 K and 270 K,
respectively. The later compound behaves as a simple ferromagnet, while in the former
there appears an isostructural phase transition at about 120 K [6]. CeCo7B3 (n = 3)
exhibits metamagnetic-like behaviours in the whole range of ordering temperature below
250K. Its is likely that upon increasing temperature, the compound endures a transition
from ferrimagnetic to antiferromagnetic states at around 150K.
For a long time, CeCo3B2 (n = 4) had been believed to be paramagnetic, in consis-
tent with paramagnetic behaviours of YCo3B2 and LuCo3B2 [7,8]. In a previous publica-
tion, we have briefly reported on the observation of ferromagnetism in CeCo3B2 below 140
K with an easy axis anisotropy [1], however, no experimental details had been presented.
In a recent announcement, Long Pham et al. [4] described CeCo3B2 as a weak ferromagnet
242 NGUYEN MINH HONG
below 210 K with a magnetic moment of 0.01 µB at low temperature. However, over a year
has past since their announcement, details of their experiments have not been published
and available.
In this communication, we report on the first magnetic measurements on a CeCo3B2
single crystal and discuss the obtained results in term of Ce 4f delocalized electron contri-
butions to magnetism.
II. EXPERIMENTAL RESULTS
Experiments were carried out on facilities of Laboratoire du Magnetisme Louis Ne´el
(CNRS, Grenoble, France). CeCo3B2 single crystal had been grown from ingots with
norminal composition Ce1+δCo3B2 by Czochralski method in an induction furnace with
cold crucible. An access in Ce, denoted by atomic percent δ, had been appropriately chosen
to compensate the Ce lost during the melting and crystal growing process. Tiny good
crystals with weights of a few milligrams were obtained and the quality and orientation
of crystal were checked by Laue method. Lattice parameters obtained by Debye- Scherrer
method at room temperature are a = 5.056 ± 0.003 A˚ and c = 3.038 ± 0.002 A˚, in good
agreement with literature [5].
AC susceptibility (in zero DC field) and magnetization measurements (with mag-
netic fields up to 50 kOe) along the c-axis and in the basal plane of a crystal of 1 mg in
mass have been performed on a Quantum Design Magnetometer. The AC susceptibility
results are shown in Fig. 1. A ferromagnetic transition is clear seen at TC = 140±1 K. The
measured susceptibility is larger along the c-axis than in the (a,b)-plane indicating an easy
axis anisotropy of CeCo3B2, in consistent with the magnetization measurement presented
below. We note that below 30 K, there is a pronounced decrease in susceptibility along
both two directions. If this is a spin reorientation, a decrease in susceptibility along the
c-axis would imply an increase in the susceptibility in the basal plane, which is clearly not
the case. It is likely, thus, this is a transition associated not with the magnetic moment
orientation, but with its amplitude or with the magnetic hardness of the compound in
both directions. Further experiments are needed to clarify the nature of this phenomenon.
Magnetization curves at 4.2 K are presented in Fig. 2. The easy direction is indeed
along the c-axis from which a magnetic moment of 8 × 10−3 µB/F.U. is deduced. The
curve measured in the basal plane does not reach saturation up to 50 kOe, however the
anisotropy is relatively low, because of small value of magnetic moment.
III. DISCUSSIONS
Structural and magnetic property investigations of Yn+1Co3n+5B2n by neutron
diffraction had revealed a close relationship between amplitude of Co magnetic moments
and the presence of boron in the neighborhood of the cobalt atoms [8]. It is stipulated that
the hybridization of the cobalt 3d electronic state with the boron 2p state plays a major
role in the determination of the magnitude of Co magnetic moments and this accounts
for the paramagnetic state in YCo3B2 where each Co atom is surrounded by up to four B
atoms.
FERROMAGNETISM IN CeCo3B2 243
Fig. 1. AC susceptibility measurements on CeCo3B2 crystal with AC magnetic
field applied along the c axis and in the basal plane: real part χ’, imaginary part
χ”.
Fig. 2. Magnetization measurements on CeCo3B2 crystal with applied magnetic
fields along c axis (filled circles: 4.2 K, open circles: 15 K) and in the basal plane
(crosses: at 4.2 K and 15 K, the two curves are almost indentical).
Ferromagnetism discovered in CeCo3B2 single crystal is, therefore, not expected
because on one hand Co electrons in this kind of compound is naturally expected to be in
a paramagnetic state like in YCo3B2and Ce is believed to be in a mixed valence state (due
to CeCo3B2 unit cell volume is much smaller than expected from lantanide constraction)
244 NGUYEN MINH HONG
and thus possesses no localized magnetic moment; on the other hand, measurement on
CeCo3B2 polycrystalline samples revealed almost no spontaneous magnetization at low
temperature [7]. With our discovery of a very low magnetic moment and considerable
anisotropy in single crystals, the later fact can be understood by the cautious suspicion of
most researchers dealing with the observation of tiny magnetic moment in this compound.
It is interesting to point out that among Ce boride compound with similar crystallo-
graphic structure, CeCo3B2 is not uniquely ferromagnetically ordered. CeRh3B2 is found
to be ferromagnetic below 113- 115 K with a magnetic moment of 0.37 – 0.4 µB /F.U.,
depending on authors, and an easy plane anisotropy [7, 9].
Ferromagnetism in CeCo3B2 can be understood if we acknowledge the existence
of itinerant states of Ce 4f electrons and their contribution to magnetism. In CeCo3B2,
Ce-Ce interatomic distance along the c-axis is so short (∼ 3 A˚) that 4f electronic states
form a narrow 4f band with large density of states at Fermi level and 4f-4f direct exchange
interactions become strong enough for a band splitting to take place. And there is a strong
4f-5d intratomic hybridization which results in a negative polarized 5d moments [10]. Due
to its shorter Ce-Ce distance, CeCo3B2 is expected to have a broader 4f band, stronger
crystal electric field and stronger 4f-5d hybridization than that of CeRh3B2. The differ-
ences in properties of CeCo3B2 and CeRh3B2,thus can be understood as a consequence
of the particular 4f states at the Fermi level of these two compounds which results in
a different magnetic anisotropy and a low appearance value of Ce magnetic moment in
CeCo3B2. In another word, we observed in CeCo3B2 an almost cancellation of magnetic
moment by opposite 4f orbital and spin contributions and 5d spin polarized contribution.
Ce orbital magnetism has been proved by ab-initio band structure calculations [11].
Nordstro¨m et al. performed calculations for CeCo5 in the hexagonal CaCu5 structure
and, for comparison, for the isostructural compounds YCo5 and LaCo5. The authors have
shown that the properties of CeCo5, in particular its reduced magnetic moment relative to
YCo5 and LaCo5, can be understood as due to itinerant 4f states. An important concept
in the electronic structure of CeCo5 is the hybridization between the 3d states of cobalt
and the 4f states of cerium in the Ce containing plane. With spin-orbit coupling included
in the calculation, good agreement with experiment for the total magnetic moment is
achieved. A non-negligible 4f magnetic moment composed of both spin and orbital oppo-
site contributions is found on the cerium site. This manifests the existence of itinerant 4f
magnetism in this compound. We note, however, that unlike Ce magnetism in CeCo3B2or
CeRh3B2 where ferromagnetism discovered is solely due to Ce electronic states and spon-
taneous Ce magnetic moment occurs, the calculated Ce magnetism in CeCo5 is induced
by the presence of Co sublattice strong ferromagnetism.
There are experiments that probe local (atomic level) magnetic properties. Mag-
netic circular X-ray dichroism experiments at the M4,5 absorption edges of cerium in the
intermetallic compounds CeCuSi, CeRh3B2, and CeFe2 had been reported by Schille´ et
al. [12]. The authors showed that these experiments were able to yield both the magni-
tude and the direction of the 4f magnetic moment on Ce and demonstrated the existence
of a 4f magnetic moment on Ce in CeFe2 and confirmed the extreme sensitivity of the
4f orbital contribution to the degree of localization of the 4f electrons. This 4f orbital
FERROMAGNETISM IN CeCo3B2 245
contribution has been found significantly higher than the one predicted from spin-resolved
band-structure calculations.
To conclude, we have presented the observation of ferromagnetism in CeCo3B2 with
a high Curie temperature TC = 140 K and magnetic moment 8 × 10−3 µB/F.U. The
observed small magnetic moment can be explained by a cancellation of various 4f and 5d
electron contributions to magnetic moment. Further experiments to explore microscopic
magnetic properties, such as magnetic circular x-ray dichroism or muon spin relaxation,
are needed to verify the nature of ferromagnetism in this compound. Finally, we note that
although our measured magnetic moment is of equivalent to that done by Long Pham
et al. [4], the Curie temperature from the two works differs substantially. This sample
dependence could indicate the sensitivity of Ce magnetism on the sample conditions, i.e.
the crystal imperfections or due to the stoichiometric composition which again requires
further single crystal experiments to resolve.
ACKNOWLEDGEMENT
The author expresses his sincere thanks to Dr. D. Givord for providing the author
the opportunity to work in the Laboratoire Louis Neel to accomplish the single crystal
grow and magnetic measurement and to Dr. R. Ballou for his experimental assistance and
discussions.
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Received 07 September 2007.