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Magnetismus Poster: Do., 13:00–15:30 D-P249
Forbidden magnetic reflections in UNiGa
Karel Prokes 1 , Ralf Feyerherm 1 , Vladimir Sechovsky 2 , Marian Mihalik 3
1 Hahn-Meitner-Institute, SF-2, Glienickerstr. 100, 141 09 Berlin, Germany –
2 Department of Electronic Structures, Charles University, 121 16 Prague 2, The Czech
Republic – 3 Institute of Experimental Physics, SAS , Watsonova 47, Kosice, Slovakia
UNiGa crystallizes in the hexagonal ZrNiAl structure and undergoes a sequence of
magnetic phase transitions in the 35 - 39.5 K temperature range. The ground-state
antiferromagnetic (AF) structure is characterized by three propagation vectors q1=(0,
0, 1/2), q2=(0, 0, 1/3) and q3=(0, 0, 1/6) corresponding to the c-axis (up-up-downup-down-down)
stacking of U-moments that are ordered ferromagnetically within the
hexagonal basal planes [1]. A strong uniaxial anisotropy keeps the moments aligned
along c in all the magnetic structures. A magnetic field applied along the c axis induces
a metamagnetic-like transition leading to a field-forced ferromagnetic stacking.
This transition, which is of the first-order type with considerable hysteresis at T = 4.2
K and appears at Bc≈ 0.8 T, splits into two steps for the temperature above 15 K.
In the intermediate fields the structure of the (up-up-down) type was detected. The
size of U-moments (1.35 µB at T = 4.2 K) is practically independent of the type of
structure. A strong uniaxial anisotropy in this system locks the U moments along the
c-axis (which is also the easy-magnetization axis). While low field applied at low temperatures
along the hexagonal axis breaks the AF coupling and forces the U magnetic
moments to align parallel, magnetic field applied perpendicular to the c axis has no
significant influence on the low-field magnetic structure of UNiGa. Previous high-field
magnetization studies at 4.2 K revealed that even magnetic fields as high as 35 T are
not sufficient to tilt the moments significantly away from the hexagonal c axis. From
these data one can roughly estimate the magnetic anisotropy field to be at least 300
T. It is therefore surprising that among the observed ground-state AF reflections are
also reflections of the (0 0 +/- qn) type, i.e. reflections for which is the projection
of magnetic moment on the plane perpendicular to the scattering vector zero. There
is no doubt that the ground state AF structure of UNiGa published in the literature
on the basis of previous unpolarised and polarised neutron diffraction experiments is
correct. The apparent disagreement between the experiment and theory calls for an
explanation. In this contribution we describe combined results of neutron and X-ray
diffraction performed on a good-quality single crystal to explain the possible reason for
such an observation.
[1] K. Prokes, E. Bruck, F. R. de Boer, M. Mihalik, A.A. Menovsky, P. Burlet, J.M.
Mignot, L. Havela, V. Sechovsky, J. Appl. Phys. 79 (1996) 6396.