The physical properties of metamagnetic Fe$_3$Ga$_4$ single crystals are investigated to explore the sensitivity of the magnetic states to temperature, magnetic field, and sample history. The data reveal a moderate anisotropy in the magnetization and
the metamagnetic critical field along with features in the specific heat at the magnetic transitions $T_1=68$ K and $T_2=360$ K. Both $T_1$ and $T_2$ are found to be sensitive to the annealing conditions of the crystals suggesting that disorder affects the competition between the ferromagnetic (FM) and antiferromagnetic (AFM) states. Resistivity measurements reveal metallic transport with a sharp anomaly associated with the transition at $T_2$. The Hall effect is dominated by the anomalous contribution which rivals that of magnetic semiconductors in magnitude ($-5 mu Omega$ cm at 2 T and 350 K) and undergoes a change of sign upon cooling into the low temperature FM state. The temperature and field dependence of the Hall effect indicate that the magnetism is likely to be highly itinerant in character and that a significant change in the electronic structure accompanies the magnetic transitions. We observe a contribution from the topological Hall effect in the AFM phase suggesting a non-coplanar contribution to the magnetism. Electronic structure calculations predict an AFM ground state with a wavevector parallel to the crystallographic $c$-axis preferred over the experimentally measured FM state by $approx$ 50 meV per unit cell. However, supercell calculations with a small density of Fe-antisite defects introduced tend to stabilize the FM over the AFM state indicating that antisite defects may be the cause of the sensitivity to sample synthesis conditions.
We generalize the typical medium dynamical cluster approximation (TMDCA) and the local Blackman, Esterling, and Berk (BEB) method for systems with off-diagonal disorder. Using our extended formalism we perform a systematic study of the effects of non
-local disorder-induced correlations and of off-diagonal disorder on the density of states and the mobility edge of the Anderson localized states. We apply our method to the three-dimensional Anderson model with configuration dependent hopping and find fast convergence with modest cluster sizes. Our results are in good agreement with the data obtained using exact diagonalization, and the transfer matrix and kernel polynomial methods.
We develop a systematic typical medium dynamical cluster approximation that provides a proper description of the Anderson localization transition in three dimensions (3D). Our method successfully captures the localization phenomenon both in the low a
nd large disorder regimes, and allows us to study the localization in different momenta cells, which renders the discovery that the Anderson localization transition occurs in a cell-selective fashion. As a function of cluster size, our method systematically recovers the re-entrance behavior of the mobility edge and obtains the correct critical disorder strength for Anderson localization in 3D.
We report both experimental and theoretical investigations of the physical properties of Ba$_mathrm{2}$Mn$_mathrm{2}$Sb$_mathrm{2}$O single crystals. This material exhibits a hexagonal structure with lattice constants: a = 4.7029(15) AA{} and c = 19.
9401(27) AA{}, as obtained from powder X-ray diffraction measurements, and in agreement with structural optimization through density functional theory (DFT) calculations. The magnetic susceptibility and specific heat show anomalies at T$_mathrm{N}$ = 60 K, consistent with antiferromagnetic ordering. However, the magnitude of T$_mathrm{N}$ is significantly smaller than the Curie-Weiss temperature ($mid$$mathrm{Theta_{CW}}$$mid$ $approx$ 560 K), suggesting a magnetic system of reduced dimensionality. The temperature dependence of both the in-plane and out-of-plane resistivity changes from an activated at $T$ $>$ T$_mathrm{x}$ $sim$ 200 K to a logarithmic at $T$ $<$ T$_mathrm{x}$. Correspondingly, the magnetic susceptibility displays a bump at T$_mathrm{x}$. DFT calculations at the DFT + U level support the experimental observation of an antiferromagnetic ground state.
