In a recent letter [Phys. Rev. Lett. 82, 1012(1999] it was found that the Tb 3+ magnetic moments in the Tb2Ti2O7 pyrochlore lattice of corner-sharing tetrahedra remain in a collective paramagnetic state down to 70mK. In this paper we present results from d.c. magnetic susceptibility, specific heat data, inelastic neutron scattering measurements, and crystal field calculations that strongly suggest that (1) the Tb 3+ ions in Tb2Ti2O7 possess a moment of approximatively 5µB, and (2) the ground state g−tensor is extremely anisotropic below a temperature of O(10 0 )K, with Ising-like Tb 3+ magnetic moments confined to point along a local cubic 111 diagonal (e.g. towards the middle of the tetrahedron). Such a very large easy-axis Ising like anisotropy along a 111 direction dramatically reduces the frustration otherwise present in a Heisenberg pyrochlore antiferromagnet. The results presented herein underpin the conceptual difficulty in understanding the microscopic mechanism(s) responsible for Tb2Ti2O7 failing to develop long-range order at a temperature of the order of the paramagnetic Curie-Weiss temperature θCW ≈ −10 1 K. We suggest that dipolar interactions and extra perturbative exchange coupling(s) beyond nearest-neighbors may be responsible for the lack of ordering of Tb2Ti2O7.

Diluted magnetic semiconductors have received much attention due to their potential applications for spintronics devices. A prototypical system (Ga,Mn)As has been widely studied since the 1990s. The simultaneous spin and charge doping via hetero-valent (Ga 3 þ ,Mn 2 þ ) substitution, however, resulted in severely limited solubility without availability of bulk specimens. Here we report the synthesis of a new diluted magnetic semiconductor (Ba 1 À x K x )(Zn 1 À y Mn y ) 2 As 2 , which is isostructural to the 122 iron-based superconductors with the tetragonal ThCr 2 Si 2 (122) structure. Holes are doped via (Ba 2 þ , K 1 þ ) replacements, while spins via isovalent (Zn 2 þ ,Mn 2 þ ) substitutions. Bulk samples with x ¼ 0.1 À 0.3 and y ¼ 0.05 À 0.15 exhibit ferromagnetic order with T C up to 180 K, which is comparable to the highest T C for (Ga,Mn)As and significantly enhanced from T C up to 50 K of the '111'-based Li(Zn,Mn)As. Moreover, ferromagnetic (Ba,K)(Zn,Mn) 2 As 2 shares the same 122 crystal structure with semiconducting BaZn 2 As 2 , antiferromagnetic BaMn 2 As 2 and superconducting (Ba,K)Fe 2 As 2 , which makes them promising for the development of multilayer functional devices.

No abstract

We report low-temperature muon spin relaxation/rotation (µSR) measurements on single crystals of the actinide superconductor UTe2. Below 5 K we observe a continuous slowing down of magnetic fluctuations, which persists through the superconducting transition temperature (Tc = 1.6 K). The temperature dependence of the dynamic relaxation rate down to 0.4 K agrees with the self-consistent renormalization theory of spin fluctuations for a three-dimensional weak itinerant ferromagnetic metal. However, we find no evidence of long-range or local magnetic order down to 0.025 K. Weak transverse-field µSR measurements indicate that the superconductivity coexists with the magnetic fluctuations.