Photon Factory Activity Report 2005 #23 Part B (2006)
Materials Science

3A/2005G140

Crystal distortion accompanied by ferrimagnetic transition in Laves phase ErCo2
Hiroki ISHIBASHI1*, Takao NAKAMA2, Shiko NOTSU2, Yoshinao TAKAESU2,Masataka TAKEDA2, Kei HASEGAWA2, Katsuma YAGASAKI2
1Osaka Prefecture Univ., Sakai, Osaka 599-8531, Japan
2Univ. Ryukyus, Nishihara, Okinawa 903-0213, Japan
Introduction
Laves compound ErCo2 is known to show ferrimagnetic
ordering below TC ~ 32 K. At room temperature ErCo2 hasa cubic Laves structure with a space group Fd3m. BelowTC, rhombohedrally distorted structure was reported [1],however, the precise crystal structure of the ferrimagnetic
phase has not been determined because of its small lattice
distortion. Recently, it was found that the magnetic field
induces the structural transition, i.e., the crystal symmetry
increases to cubic in external magnetic field at 5 T [2]. In
the present study, we investigate the detailed crystal
structure under zero field at low temperatures by highresolution x-ray powder diffraction measurements tounderstand the mechanism of the magnetic field induced
structural transition in ErCo2.
Experimental
ErCo2 compound was prepared by tetra-arc melting of
stoichiometric amount of components followed by singlecrystallization by Czochoralski method. The sample wascarefully ground in acetone and fine powder was used for
diffraction experiments. The powder diffraction
experiments were performed using high-resolution
diffractometer installed at a beam line of BL-3A station. A
wavelength of incident beam used was 0.90 Å. A flat
Si(111) crystal analyzer was used in order to obtain thedata with high angular resolution. The temperaturedependence of the diffraction profiles of specificreflections and whole powder patterns at 20 K and 40 K
were measured.
Results and Discussion
Figure 1 shows the temperature dependence of powder
diffraction pattern in the 2θ range around 222 and 440
reflections, where the indices of Bragg reflections arebased on the cubic structure. The structural transition isclearly observed from cubic phase to distorted one at~32.5 K, which is the same temperature as theferrimagnetic ordering one. From Fig. 1, the Bragg peaks
for both 222 and 440 reflections split into two peaks
below the transition temperature, therefore the crystal
symmetry is considered to be rhombohedral. It is notedthat there exist two phases, the cubic phase and the
distorted one, between 32.0 and 32.5 K, therefore thephase transition is of first order.
In order to determine the crystal system and the lattice
constant below the transition temperature, we carefully
checked the peak splitting using the whole powder patternFig. 1. Temperature dependence of powder x-ray
diffraction patterns of ErCo2 on heating in the2θ range around (a) 222 and (b) 440
reflections.

Fig. 2. X-ray diffraction pattern at 20 K. The indices
are based on the rhombohedral symmetry withthe lattice constant of a = 5.0605 Å and c =12.351 Å.
at 20 K. As the result, all Bragg peaks can be indexed bya rhombohedral symmetry with the lattice constant of a =5.0605 Å and c = 12.351 Å. The result of the indices isshown in Fig. 2. The rhombohedral symmetry implies thatthe lattice distorts a [111] direction, which is consistent
with the easy direction of magnetization. The detailed
structural analysis is now in progress.
References[1] A. S. Markosyan, Sov. Solid State 23, 965 (1981).
[2] K. Yagasaki et al., Physica B 378-380, 1089 (2006)*hiroki@p.s.osakafu-u.ac.jp

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