Journal of Magnetism and Magnetic Materials 140-144 (1995) 1381-1382
Muon spin rotation spectroscopy on a UNiGa single crystal
K. Proke~ a,* p. Svoboda b V. Sechovsky b E. Briick a, A. Amato c
R. Feyerherm c, F.N. Gygax c, A. Schenck c, H. Maletta d, F.R. de Boer a
a Van der Waals-Zeeman Laboratory, UniL'ersity of Amsterdam, 1018 XE Amsterdam, The Netherlands
b Department of Metal Physics, Charles Unit~ersity, 121 16 Prague 2, The Czech Republic
c Institute of Particle Physics, ETH Ziirich, CH-5232 Villigen PSI, Switzerland
d Hahn-Meitner Institute, BENSC, D-14109 Berlin 39, Germany
UNiGa orders antiferromagnetically (AF) below 39 K with four different AF phases appearing with decreasing
temperature. These phases are reflected in taSR spectra by different sets of frequencies. In paramagnetic range three different
Knight shift signals were detected. The results can be explained assuming three different stopping sites.
UNiGa is one of the UTX compounds crystallizing in
the hexagonal ZrNiAI structure and consists of U-Ni and
Ni-Ga layers alternating along the c-axis. The huge uniax-
ial magnetic anisotropy in UNiGa is caused by strong
5f-electron bonding within the U-Ni planes and the con-
siderable U 5f-orbital moments . This mechanism locks
the U magnetic moments along the c-axis (a negligible
induced magnetization on the Ni sites is expected). The
bonding anisotropy results in a strong ferromagnetic cou-
pling of U magnetic moments within U-Ni planes. The
interlayer coupling is weaker and yields the stacking se-
quence of + or - oriented moments along the c-axis
which can be changed easily.
A single crystal of UNiGa was grown in FOM-ALMOS
at the University of Amsterdam by a modified Czrochalski
technique from a stoichiometric melt. A small portion of
resulting single-crystalline sample was checked by means
of X-ray Laue diffraction and by micro-probe analysis and
was found to be stoichiometric and single crystalline within
the experimental error. For IxSR spectroscopy studies,
plates oriented perpendicular to the c-axis with thickness
of 0.4 mm were spark eroded and mounted on a silver
sample holder covering about 300 mm 2 in total.
The txSR studies were performed on the General Pur-
pose Surface-Muon Spectrometer arM3 at the Paul Scher-
rer Institute. The recorded depolarization functions could
be described by combining several exponential functions
and were thus characterized by initial asymmetries, damp-
ing rates and characteristic frequencies.
author. Fax: +31-20-525-5788; email:
The magnetic phase diagram of UNiGa determined by
neutron diffraction experiments on a single crystal  is
rather complex and we have to note that some of the
magnetic phases are stable only in very small temperature
and magnetic field regions. As all the AF structures consist
of ferromagnetic basal-plane layers of U magnetic mo-
ments oriented along the c-axis, no oscillatory signal was
detected in the longitudinal geometry. In transversal geom-
etry in zero field a different number of signals with distinct
values of characteristic frequencies were detected for each
antiferromagnetic structure. The AF structure below 34.8
K is characterized by a magnetic unit cell with
+ +- +-- stacking of ferromagnetic U-Ni planes
along the c-axis. The recorded spectra can be decomposed
into three signals with characteristic frequencies of 0, 47
and 56 MHz. From the intensity of the signal we can
deduce the weight of the signals to be approximately
3.5 : 6 : 1.5. The signal without oscillation, which is associ-
ated with a more than doubled relaxation rate, arises from
symmetrical muon stopping sites with respect to the actual
AF structure, where the local magnetic field is zero. These
sites are located in Ni-Ga planes between U-Ni planes
with + and - orientation of magnetic moments.
The non-zero frequency signals are arising from sites
which are located between the U-Ni and the Ni-Ga
planes, within U-Ni planes and within Ni-Ga planes
situated between two U-Ni planes with parallel orientation
of magnetic moments. The number of interstitial sites
between U-Ni and Ni-Ga planes is larger and thus the
frequency of 47 MHz can be assigned to them.
As one approaches the magnetic phase transitions new
frequencies appear in the recorded depolarization function.
The abrupt changes of the txSR frequencies without con-
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K. Proke~ et al. /Journal of Magnetism and Magnetic Materials 140-144 (1995) 1381-1382
siderable variation of associated relaxation rates at 34.5
(into the AF phase characterized by the + + + - - + -
sequence) and at 36.1 K show that the magnetic phase
transitions in UNiGa between different AF phases are of
first-order type without additional fluctuation effects (Fig.
1). Puzzling is the fact that in the AF phase between 36.1
and 37 K, which can be characterized by the sequence
+ 0 - (0 - zero moment plane), a frequency of 66 MHz
appears. A possible explanation is that the competing
nearest-neighbour and next-nearest-neighbour AF interac-
tions which frustrate each third U moment along the c-axis
change drastically the distribution of effective fields and
thus the local magnetic field at a particular muon stopping
site can now be larger than at low temperatures. In this
temperature region, the depolarization function can be
decomposed into six signals, including the zero frequency.
Apparently, this is caused by the magnetic inequivalency
of crystallographically equivalent stopping sites. Above 37
K, where an incommensurate AF structure with k =
(0, 0, 0.36) exists, the relaxation rates start to increase and
diverge at T N. Consistent with the expectation, no zero
frequency is observed in this temperature region. Surpris-
ingly, we have found oscillatory signals above TN, which
can be attached to the presence of significant slow AF
fluctuations in the vicinity of T N.
We have also studied p~SR signals in fields up to 0.6 T
with the initial muon spin polarization perpendicular to the
applied external field and to the c-axis. Three different
,: , ,
50 = : : "
~ 40 [ : ......
i0 20 38 40 42 44
Fig. 1. Temperature dependencies of characteristic zero-field muon
rotation frequencies in a UNiGa single crystal in transversal
geometry. Open symbols denote strongly dumped signals reflect-
ing fluctuations of magnetic moments.
" UNiGa ]
t • I
B = O . 6 T
B Ii c lp~
-I0 ! i
Fig. 2. Temperature dependencies of the Knight shifts in UNiGa
in transversal geometry in a field of 0.6 T parallel to c-axis.
Knight shift signals (KS) with two distinct types of tem-
perature dependencies were observed in the temperature
range 40-250 K (Fig. 2). One KS scales with the bulk
magnetic susceptibility along the c-axis, the other two
(above 50 K) rather scale with the susceptibility in the
Combining magnetic susceptibility measurements, re-
suits from p~SR studies and crystallographic considera-
tions, we conclude, that muons are stopped at three differ-
ent sites. One of these sites is located in the U-Ni plane,
the second in the Ni-Ga plane and the third is situated
between the U-Ni and Ni-Ga planes. Supposing that these
stopping sites are the same at different temperatures, we
conclude that the local magnetic fields at stopping sites
undergoes sudden changes without additional fluctuations
in the vicinity of magnetic phase transitions.
Acknowledgements: This work has been supported by
the 'Stichting voor Fundamenteel Onderzoek der Materie
(FOM)'. The work of V.S. and P.S. was supported by the
Grant Agency of Czech Republic (grant no. 202/93/
 V. Sechovsky, L. Havela, H. Nakone, F.R. de Boer and E.
Briick, J. Alloys Comp. 207/208 (1994) 221.
 V. Sechovsky, L. Havela, P. Svoboda, A.V. Andreev, P.
Burlet, K. Proke~, H. Nakotte, F.R. de Boer, R.A. Robinson
and H. Maletta, J. Magn. Magn. Mater. 140-144 (1995) 1379