On the basis of magnetic, transport, and optical measurements performed on single crystals, we report CaMn2Sb2 to be an antiferromagnetic insulator that exhibits weak ferromagnetic order above the Néel temperature. Magnetic susceptibility measurements reveal the magnitude of the high temperature Curie-Weiss moment to be only half as large as the ground state ordered moment, while electronic structure calculations based on crystallographic measurements suggest a crystal-field induced spin state transition does not occur. The antiferromagnetic state is relatively insensitive to both doping and modest pressures, while the ferromagnetism can be readily tuned by either. Infrared transmission and pressure dependent resistivity measurements suggest proximity to an electronic delocalization transition. We suggest the ferromagnetic state may be the signature of magnetic polarons.

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We present inelastic neutron scattering measurements of the antiferromagnetic insulator CaMn2Sb2, which consists of corrugated honeycomb layers of Mn. The dispersion of magnetic excitations has been measured along the H and L directions in reciprocal space, with a maximum excitation energy of ≈ 24 meV. These excitations are well described by spin waves in a Heisenberg model, including first and second neighbor exchange interactions, J1 and J2, in the Mn plane and also an exchange interaction between planes. The determined ratio J2/J1 ≈ 1/6 suggests that CaMn2Sb2 is the first example of a compound that lies very close to the mean field tricritical point, known for the classical Heisenberg model on the honeycomb lattice, where the Néel phase and two different spiral phases coexist. The magnitude of the determined exchange interactions reveal a mean field ordering temperature ≈ 4 times larger than the reported Néel temperature TN = 85 K, suggesting significant frustration arising from proximity to the tricritical point.

We report the discovery of CaMn2Al10, a metal with strong magnetic anisotropy and moderate electronic correlations. Magnetization measurements find a Curie-Weiss moment of 0.83 µB/Mn, significantly reduced from the Hund's rule value, and the magnetic entropy obtained from specific heat measurements is correspondingly small, only ≈ 9 % of Rln 2. These results imply that the Mn magnetism is highly itinerant, a conclusion supported by density functional theory calculations that find strong Mn-Al hybridization. Consistent with the layered nature of the crystal structure, the magnetic susceptibility χ is anisotropic below 20 K, with a maximum ratio of χ [010] /χ [001] ≈ 3.5. A strong power-law divergence χ(T ) ∼ T −1.2 below 20 K implies incipient ferromagnetic order, and an Arrott plot analysis of the magnetization suggests a vanishingly low Curie temperature TC ∼ 0. Our experiments indicate that CaMn2Al10 is a rare example of a Mn-based weak itinerant magnet that is poised on the verge of ferromagnetic order.

We present a study of the crystal structure and physical properties of single crystals of a new Fe-based ternary compound, Zr2-xFe4Si16-y(x = 0.81, y = 6.06). Zr1.19Fe4Si9.94 is a layered compound, where stoichiometric β-FeSi2-derived slabs are separated by Zr-Si planes with substantial numbers of vacancies. High resolution transmission electron microscopy (HRTEM) experiments show that these Zr-Si layers consist of 3.5 nm domains where the Zr and Si vacancies are ordered within a supercell sixteen times the volume of the stoichiometric cell. Within these domains, the occupancies of the Zr and Si sites obey symmetry rules that permit only certain compositions, none of which by themselves reproduce the average composition found in x-ray diffraction experiments. Magnetic susceptibility and magnetization measurements reveal a small but appreciable number of magnetic moments that remain freely fluctuating to 1.8 K, while neutron diffraction confirms the absence of bulk magnetic order with a moment of 0.2μB or larger down to 1.5 K. Electrical resistivity measurements find that Zr1.19Fe4Si9.94 is metallic, and the modest value of the Sommerfeld coefficient of the specific heat γ = C/T suggests that quasi-particle masses are not particularly strongly enhanced. The onset of superconductivity at Tc ≃ 6 K results in a partial resistive transition and a small Meissner signal, although a bulk-like transition is found in the specific heat. Sharp peaks in the ac susceptibility signal the interplay of the normal skin depth and the London penetration depth, typical of a system in which nano-sized superconducting grains are separated by a non-superconducting host. Ultra low field differential magnetic susceptibility measurements reveal the presence of a surprisingly large number of trace magnetic and superconducting phases, suggesting that the Zr-Fe-Si ternary system could be a potentially rich source of new bulk superconductors.