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A rare ferromagnet-antiferromagnet transition is known to exist in the solid-solution Y${}_{1\ensuremath{-}x}$La${}_{x}$TiO${}_{3}$ at $x$ \ensuremath{\approx} 0.3. Here, the authors study this transition using a unique battery of techniques, including magnetometry, x-ray absorption spectroscopy, x-ray magnetic circular dichroism, muon spin rotation, neutron diffraction, and small-angle neutron scattering. The thermal magnetic phase transitions are observed not to be of conventional second-order type, but rather to be first order in nature. Furthermore, complex changes in the magnetic ground state are revealed.

We report the successful synthesis of a 122 diluted ferromagnetic semiconductor with $n$-type carriers, $\mathrm{Ba}{(\mathrm{Zn},\mathrm{Co})}_{2}{\mathrm{As}}_{2}$. Magnetization measurements show that the ferromagnetic transition occurs up to ${T}_{C}\ensuremath{\sim}45\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Hall effect and Seebeck effect measurements jointly confirm that the dominant carriers are electrons. Through muon spin relaxation, a volume-sensitive magnetic probe, we have also confirmed that the ferromagnetism in $\mathrm{Ba}{(\mathrm{Zn},\mathrm{Co})}_{2}{\mathrm{As}}_{2}$ is intrinsic and the internal field is static.

We study spinful non-interacting electrons moving in two-dimensional materials which exhibit a spectral gap about the Fermi energy as well as time-reversal invariance. Using Fredholm theory we revisit the (known) bulk topological invariant, define a new one for the edge, and show their equivalence (the bulk-edge correspondence) via homotopy., Comment: 18 pages

Muon spin rotation and relaxation studies have been performed on a "111" family of iron-based superconductors NaFe_1-xNi_xAs. Static magnetic order was characterized by obtaining the temperature and doping dependences of the local ordered magnetic moment size and the volume fraction of the magnetically ordered regions. For x = 0 and 0.4 %, a transition to a nearly-homogeneous long range magnetically ordered state is observed, while for higher x than 0.4 % magnetic order becomes more disordered and is completely suppressed for x = 1.5 %. The magnetic volume fraction continuously decreases with increasing x. The combination of magnetic and superconducting volumes implies that a spatially-overlapping coexistence of magnetism and superconductivity spans a large region of the T-x phase diagram for NaFe_1-xNi_xAs . A strong reduction of both the ordered moment size and the volume fraction is observed below the superconducting T_C for x = 0.6, 1.0, and 1.3 %, in contrast to other iron pnictides in which one of these two parameters exhibits a reduction below TC, but not both. The suppression of magnetic order is further enhanced with increased Ni doping, leading to a reentrant non-magnetic state below T_C for x = 1.3 %. The reentrant behavior indicates an interplay between antiferromagnetism and superconductivity involving competition for the same electrons. These observations are consistent with the sign-changing s-wave superconducting state, which is expected to appear on the verge of microscopic coexistence and phase separation with magnetism. We also present a universal linear relationship between the local ordered moment size and the antiferromagnetic ordering temperature TN across a variety of iron-based superconductors. We argue that this linear relationship is consistent with an itinerant-electron approach, in which Fermi surface nesting drives antiferromagnetic ordering.