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We report the observation of an extreme magnetoresistance (XMR) in HoBi with a large magnetic moment from Ho f-electrons. Neutron scattering is used to determine the magnetic wave vectors across several metamagnetic (MM) transitions on the phase diagram of HoBi. Unlike other magnetic rare-earth monopnictides, the field dependence of resistivity in HoBi is non-monotonic and reveals clear signatures of every metamagnetic transition in the low-temperature and low-field regime, at T < 2 K and H < 2.3 T. The XMR appears at H > 2.3 T after all the metamagnetic transitions are complete and the system is spin-polarized by the external magnetic field. The existence of an onset field for XMR and the intimate connection between magnetism and transport in HoBi are unprecedented among the magnetic rare-earth monopnictides. Therefore, HoBi provides a unique opportunity to understand the electrical transport in magnetic XMR semimetals.
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The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe$_{1-x}$Co$_x$Ga$_3$ was studied by means of electrical resistivity, magnetization, ab-initio band structure calculations, and nuclear spin-lattice relaxation $1/T_1$ of the $^{69,71}$Ga nuclei. Temperature dependencies of the electrical resistivity reveal that the evolution from the semiconducting to the metallic state in the Fe$_{1-x}$Co$_x$Ga$_3$ system occurs at $0.025<x<0.075$. The $^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of $2!-!300$ K for the concentrations $x = 0.0,$ $0.5,$ and $1.0$. In the parent semiconducting compound FeGa$_3$, the temperature dependence of the $^{69}(1/T_1)$ exhibits a huge maximum at about $T!sim!6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa$_3$, demonstrates a metallic Korringa behavior with $1/T_1$ $propto T$. In Fe$_{0.5}$Co$_{0.5}$Ga$_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with both magnetization measurements, showing localized magnetism with a relatively low effective moment of about 0.7 $mu_B$/f.u., and ab initio band structure calculations, where a ferromagnetic state with an ordered moment of 0.5 $mu_B$/f.u. is predicted. The results are discussed in terms of the interplay betwen the localized and itinerant magnetizm including in-gap states and spin fluctuations.
The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.
Through comprehensive density functional calculations, the crystallographic, magnetic and electronic properties of $Na_xCoO_2$ ($x$ = 1, 0.875, 0.75, 0.625 and 0.50) were investigated. We found that all Na ions in $NaCoO_2$ and $Na_{0.875}CoO_2$ share the basal coordinates with O ions. However, as $x$ decreases, some of Na ions move within the basal plane in order to reduce the in-plane Na$-$Na electrostatic repulsion. Magnetically, there was strong tendency for type A antiferromagnetism in the $Na_{0.75}CoO_2$ system, while all other Na deficient systems had a weaker ferromagnetic tendency. The results on magnetism were in excellent agreement with the experiments.
Using ab initio methods based on density functional theory, the electronic and magnetic structure of layered hexagonal NbSe$_{2}$ is studied. In the case of single-layer NbSe$_{2}$ it is found that, for all the functionals considered, the magnetic solution is lower in energy than the non-magnetic solution. The magnetic ground-state is ferrimagnetic with a magnetic moment of 1.09 $mu_{B}$ at the Nb atoms and a magnetic moment of 0.05 $mu_{B}$, in the opposite direction, at the Se atoms. Our calculations show that single-layer NbSe$_{2}$ does not display a charge density wave instability unless a graphene layer is considered as a substrate. Then, two kinds of 3$times$3 charge density waves are found, which are observed in our STM experiments. This suggest that the driving force of charge instabilities in NbSe$_{2}$ differ in bulk and in the single-layer limit. Our work sets magnetism into play in this highly-correlated 2D material, which is crucial to understand the formation mechanisms of 2D superconductivity and charge density wave order.
A new ternary intermetallic compound, namely, YbMn2Sb2, has been synthesized and its magnetic and electrical transport properties have been studied in the temperature range of 2 to 300 K. This compound crystallizes in a trigonal, La2O2S type structure (space group P3bm1, No. 164) and is found to be ferromagnetically ordered at room temperature. The magnetism is attributed to the ordering of Mn sublattice. M5 xray absorption spectrum of YbMn2Sb2 obtained at room temperature suggests that the valency of Yb in this compound is close to 2. Electrical resistivity of this compound is metal like and a positive magnetoresistance of 13 percent is observed at 5 K in an applied field of 9T. Key words Rare earth intermetallics and alloys, Magnetic properties, Xray absorption spectroscopy, Electrical transport.
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