Adiabatic demagnetization is currently gaining strong interest in searching for alternatives to ³He-based refrigeration techniques for achieving temperatures below 2 K. The main reasons for that are the recent shortage and high price of the rare helium isotope ³He. Here we report the discovery of a large magnetocaloric effect in the intermetallic compound YbPt₂Sn, which allows adiabatic demagnetization cooling from 2 K down to 0.2 K. We demonstrate this with a home-made refrigerator. Other materials, for example, paramagnetic salts, are commonly used for the same purpose but none of them is metallic, a severe limitation for low-temperature applications. YbPt₂Sn is a good metal with an extremely rare weak magnetic coupling between the Yb atoms, which prevents them from ordering above 0.25 K, leaving enough entropy free for use in adiabatic demagnetization cooling. The large volumetric entropy capacity of YbPt₂Sn guarantees also a good cooling power.

The $f$-electron compound CeAuSb${}_2$, which crystallizes in the ZrCuSi${}_2$-type tetragonal structure, orders antiferromagnetically between 5 and 6.8 K, where the antiferromagnetic transition temperature $T_N$ depends on the occupancy of the Au site. Here we report the electrical resistivity and heat capacity of a high-quality crystal CeAuSb${}_2$ with $T_N$ of 6.8 K, the highest for this compound. The magnetic transition temperature is initially suppressed with pressure, but is intercepted by a new magnetic state above 2.1 GPa. The new phase shows a dome shape with pressure and coexists with another phase at pressures higher than 4.7 GPa. The electrical resistivity shows a $T^2$ Fermi-liquid behavior in the complex magnetic state, and the residual resistivity and the $T^2$ resistivity coefficient increases with pressure, suggesting the possibility of a magnetic quantum critical point at a higher pressure.

We report on thermodynamic, magnetization, and muon spin relaxation measurements of the strong spin-orbit coupled iridate ${\mathrm{Ba}}_3{\mathrm{IrTi}}_2{\mathrm{O}}_9$, which constitutes a distinct frustration motif made up of a mixture of edge- and corner-sharing triangles. In spite of a strong antiferromagnetic exchange interaction of the order of 100 K, we find no hint for long-range magnetic order down to 23 mK. The magnetic specific heat data unveil $T$-linear and $T$-squared dependences at low temperatures below 1 K. At the respective temperatures, the zero-field muon spin relaxation features a persistent spin dynamics, indicative of unconventional low-energy excitations. A comparison to the $4d$ isostructural compound ${\mathrm{Ba}}_3{\mathrm{RuTi}}_2{\mathrm{O}}_9$ suggests that a concerted interplay of compasslike magnetic interactions and frustrated geometry promotes a dynamically fluctuating state in a triangle-based iridate.

We present the structural, magnetic, thermodynamic and transport properties of the two new compounds YbPt(2)Sn and YbPt(2)In. X-ray powder diffraction shows that they crystallize in different structure types, the hexagonal ZrPt(2)Al and the cubic Heusler type, respectively. Despite quite different lattice types, both compounds present very similar magnetic properties: a stable trivalent Yb(3+), no evidence for a sizeable Kondo interaction and very weak exchange interactions with a strength below 1 K as deduced from specific heat C(T). Broad anomalies in C(T) suggest short range magnetic ordering at about 250 mK and 180 mK for YbPt(2)Sn and YbPt(2)In, respectively. The weak exchange and the low ordering temperature result in a large magnetocaloric effect as deduced from the magnetic field dependence of C(T), making these compounds interesting candidates for magnetic cooling. In addition we found in YbPt(2)In evidences for a charge density wave transition at about 290 K. The occurrence of such transitions within several RET2X compound series (RE = rare earth, T = noble metal, X = In, Sn) is analyzed.

We present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnet α − RuCl₃ using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic ( C 2 / m ). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral ( R 3 ˉ ) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops below T N = 6 ∼ 6.6 K. The critical exponent of the magnetic order parameter was determined to be β = 0.11 ( 1 ) , which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within the a b − plane. These findingsreflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.