S.A.M. Mentink

Antiferromagnetism and crystal-field effects in polycrystal CePd2Al3

Abstract We present an experimental study of the structural and magnetic properties of various polycrystalline samples of the new heavy-fermion antiferromagnet CePd2Al3, TN = 2.7 K. The relevant energy scales of the Kondo effect and crystal-field splitting are determined by comparing them with nonmagnetic LaPd2Al3. The critical dependence of the magnetic properties of rare-earth heavy-fermions on sample preparation is discussed using Doniachs Kondo lattice model.

Kondo effect and quadrupolar interactions in single-crystal CePd2Al3

Abstract We have studied the magnetic anisotropy on a single crystal of the new heavy-fermion compound CePd 2 Al 3 . The magnetic moments lie in the hexagonal ( a , b )-plane, but do not order antiferromagnetically, in contrast to annealed polycrystalline samples. The Kondo effect can account for the large specific heat at low temperatures. The small crystal-field splitting causes a field-induced electric-quadrupole moment, which orders in fields larger than 4.3 T.

Crystal symmetry and electronic properties of heavy-fermion MPd2Al3 (M = Ce, U)☆

Abstract We have determined the Kondo temperature and crystal-field splitting of the new heavy-fermion compound CePd2Al3 by thermopower and inelastic neutron scattering. The absence of static, magnetically ordering cerium moments in single crystals is demonstrated by zero-field μ+SR. Small deviations of aluminum stoichiometry yield significant variations of TN of CePd2Al3 and Tc of UPd2Al3 in different samples.

Reduced-moment antiferromagnetism in single-crystal UNi4B

Abstract We present an extensive experimental study on a single-crystalline sample of the new hexagonal compound UNi4B. The magnetic anisotropy as observed in susceptibility and resistivity is huge, with small antiferromagnetically ordered U-moments lying in the (a,b)-plane. Below TN = 20 K, a large, nearly field-independent increase of the linear specific-heat coefficient up to 250 mJ/mol K2 is observed, comparable to that of heavy-fermion UCu5.

Magnetic and thermal properties of CeT2X2 compounds (T=Ni, Cu; X=Sn, Sb)

Abstract We present an experimental study of the tetragonal heavy-fermion materials CeT 2 Sn 2 (T ue5fb Ni, Cu), CeNi 2 Sb 2 and CeCu 1.3 Sb 2 . Their large low-temperature specific heat partly results from a small crystalline electric field splitting, which decreases in the CeT 2 X 2 (X ue5fb Si, Ge, Sn, Sb) series with increasing size of the X-atom.

Anisotropic f-electron magnetism in Uni4B

Abstract The hexagonal uranium compound UNi4B has been shown to exhibit strongly anisotropic hybridization of f- and d-electron states. We performed low-field (3 mT) and high-field (40 T) magnetization measurements from 1.4 K to 30 K on a single crystal of this antiferrromagnetically ordering compound, TN=20 K, to further investigate the magnetic anisotropy. A phase diagram, including a spin-flop transition around 9 T, is presented. The observed large values of the electronic specific heat at low temperatures are attributed to in-plane magnetic fluctuations, which persist far below TN. No superconductivity was found down to 40 mK. The relevance of strong 5f−3d hybridization is confirmed by extensive experiments on a series of diluted compounds UCoxNi4−xB(0≤×≤4).

Antiferromagnetic order and spin frustration in UNi4B

Abstract We present the novel magnetic ordering structure of the hexagonal antiferromagnet UNi4B (TN = 20 K) as determined by single-crystal neutron diffraction. Uranium moments of 1.2(2)μB are oriented perpendicular to c, forming ferromagnetic chains with antifferomagnetic in-plane coupling. Surprisingly, 1 2 of the U-spins remain paramagnetic. Large amplitude, one-dimensional ferromagnetic spin-wave excitations in these free chains are proposed to cause the highly anomalous low-temperature behavior of the ordered state.

μ+SR in antiferromagnetic UNi4B

Abstract Positive-muon spin rotation (μ + SR) has been carried out in a single crystal of the hexagonal antiferromagnet UNi 4 B, for which the magnetic structure has recently been resolved by Mentink et al. No spontaneous precession was observed in zero field with the μ + spin directed along the rhombohedral b -axis; instead, rapidly and slowly relaxing components (comprising 70% and 30% volume fraction, respectively) were found. On the other hand, a frequency of 5 MHz was detected when the μ + spin was directed along the c -axis. Transverse-field measurements indicate that the μ + is not stopped at a highly symmetric site. The experimental results are compared with calculations based on the magnetic structure.

Specific heat of CePd2Al3 under high pressure

Abstract The Kondo lattice compound CePd 2 Al 3 appears to lie on the borderline between magnetic and nonmagnetic behavior. This has recently been concluded from the fact that the long range magnetic ordering is very sensitive to the metallurgical state of the sample. Single crystals of CePd 2 Al 3 exhibit no magnetic order down to 0.3 K, while various polycrystalline samples are antiferromagnetic at T N = 2.7 K. Here, we present results of specific heat measurements on polycrystalline material under high pressure. Surprisingly, a weak initial increase of T N ∼20 mK/kbar with pressure is observed, which is rather exceptional in Kondo lattice systems.

NMR studies of frustrated spin structure in UNi4B

We have measured 11B NMR spectra in the hexagonal antiferromagnetic compound UNi4B (Neel temperature TN = 20 K) over the temperature range 2–260 K. Spectra in the paramagnetic state yield a quadrupole splitting frequency of 0.215 ± 0.016 MHz and temperature-dependent Knight shifts. For applied field in the basal plane a Knight shift versus susceptibility plot yields a total hyperfine field of 1.14 ± 0.15 kOe/μB. The observed 11B line below TN in a c-axis field-oriented powder sample exhibits a partial powder pattern due to three magnetically inequivalent boron sites. The width of this line is consistent with the magnetic structure determined by neutron diffraction, using the transferred hyperfine field derived from paramagnetic-state 11B spectra, and therefore confirms the presence of the frustrated U spins.