28 Search Results

Here, studies on the phenomenon of magnetoresistance (MR) have produced intriguing and application-oriented outcomes for decades–colossal MR, giant MR and recently discovered extremely large MR of millions of percents in semimetals can be taken as examples. We report here the discovery of novel multiple sign changes versus applied magnetic field of the MR in the cubic intermetallic compound GdPd₃. Our study shows that a very strong correlation between magnetic, electrical and magnetotransport properties is present in this compound. The magnetic structure in GdPd₃ is highly fragile since applied magnetic fields of moderate strength significantly alter the spin arrangement within themore » system–a behavior that manifests itself in the oscillating MR. Intriguing magnetotransport characteristics of GdPd₃ are appealing for field-sensitive device applications, especially if the MR oscillation could materialize at higher temperature by manipulating the magnetic interaction through perturbations caused by chemical substitutions.« less

Crystallographic, electronic transport, thermal, and magnetic properties are reported for SrMn₂As₂ and CaMn₂As₂ single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal CaAl₂Si₂-type structure (space group $$P\bar{3}$$ m1), in agreement with the literature. Electrical resistivity ρ versus temperature T measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for SrMn₂As₂ and 61 meV for CaMn₂As₂. In a local-moment picture, the Mn⁺² 3d⁵ ions are expected to have high-spin S=5/2 with spectroscopic splitting factor g≈2. Magnetic susceptibility χ and heatmore » capacity Cp measurements versus T reveal antiferromagnetic (AFM) transitions at T_N=120(2) K and 62(3) K for SrMn₂As₂ and CaMn₂As₂, respectively. The anisotropic χ(T≤T_N) data indicate that the hexagonal c axis is the hard axis and hence that the ordered Mn moments are aligned in the ab plane. Finally, the χ(T) data for both compounds and the Cp(T) for SrMn₂As₂ show strong dynamic short-range AFM correlations from T_N up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the ab plane.« less

The metallic compound EuCo₂P₂ with the body-centered tetragonal ThCr₂Si₂ structure containing Eu spins-7/2 was previously shown from single-crystal neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM) structure below T_N=66.5 K with the helix axis along the c axis and with the ordered moments aligned within the ab plane. Here we report crystallography, electrical resistivity, heat capacity, magnetization, and magnetic susceptibility measurements on single crystals of this compound. We demonstrate that EuCo₂P₂ is a model molecular-field helical Heisenberg antiferromagnet from comparisons of the anisotropic magnetic susceptibility χ, high-field magnetization, and magnetic heat capacity of EuCo₂P₂ single crystals at temperature T≤TNmore » with the predictions of our recent formulation of molecular-field theory. Values of the Heisenberg exchange interactions between the Eu spins are derived from the data. The low-T magnetic heat capacity ~T³ arising from spin-wave excitations with no anisotropy gap is calculated and found to be comparable to the lattice heat capacity. The density of states at the Fermi energy of EuCo₂P₂ and the related compound BaCo₂P₂ are found from the heat capacity data to be large, 10 and 16 states/eV per formula unit for EuCo₂P₂ and BaCo₂P₂, respectively. These values are enhanced by a factor of ~2.5 above those found from DFT electronic structure calculations for the two compounds. Additionally, the calculations also find ferromagnetic Eu–Eu exchange interactions within the ab plane and AFM interactions between Eu spins in nearest- and next-nearest planes, in agreement with the MFT analysis of χ_ab(T≤TN).« less

BaMn₂ As₂ is an antiferromagnetic insulator where a metal-insulator transition occurs with hole doping via the substitution of Ba with K. The metal-insulator transition causes only a small suppression of the Néel temperature (T_N) and the ordered moment, suggesting that doped holes interact weakly with the Mn spin system. Powder inelastic neutron scattering measurements were performed on three different samples of Ba_{1 - x}K_xMn₂ As₂ with x = 0 , 0.125, and 0.25 to study the effect of hole doping and metallization on the spin dynamics. We compare the neutron intensities to a linear spin-wave theory approximation to the J₁more » $$-$$ J₂ $$-$$ J_c Heisenberg model. Hole doping is found to introduce only minor modifications to the exchange energies and spin gap. Lastly, the changes observed in the exchange constants are consistent with the small drop of T_N with doping.« less

Half-metallic ferromagnetism (FM) in single-crystal Ba_0.39(1)Rb_0.61(1)Mn₂As₂ below its Curie temperature T_C = 103(2) K is reported. The magnetization M versus applied magnetic field H isotherm data at 1.8 K show complete polarization of the itinerant doped-hole magnetic moments that are introduced by substituting Rb for Ba. Here, the material exhibits extremely soft FM, with unobservably small remanent magnetization and coercive field. Surprisingly, and contrary to typical itinerant FMs, the M(H) data follow the Arrott-plot paradigm that is based on a mean-field theory of local-moment FMs. The in-plane electrical resistivity data are fitted well by an activated-T² expression for T ≤more » T_C, whereas the data sharply deviate from this model for T > T_C. Hence the activated-T² resistivity model is an excellent diagnostic for determining the onset of half-metallic FM in this compound, which in turn demonstrates the presence of a strong correlation between the electronic transport and magnetic properties of the material. Together with previous data on 40% hole-doped Ba_0.6K_0.4Mn₂As₂, these measurements establish 61%-doped Ba_0.39Rb_0.61Mn₂As₂ as a prototype for a class of half-metallic ferromagnets in which all the itinerant carriers in the material are ferromagnetic.« less

Magnetization, neutron diffraction, and high-energy x-ray diffraction results for Sn-flux grown single-crystal samples of Ca(Co_1–xFe_x)_yAs₂, 0 ≤ x ≤ 1, 1.86 ≤ y ≤ 2, are presented and reveal that A-type antiferromagnetic order, with ordered moments lying along the c axis, persists for x ≲ 0.12(1). The antiferromagnetic order is smoothly suppressed with increasing x, with both the ordered moment and Néel temperature linearly decreasing. Stripe-type antiferromagnetic order does not occur for x ≤ 0.25, nor does ferromagnetic order for x up to at least x = 0.104, and a smooth crossover from the collapsed-tetragonal (cT) phase of CaCo_1.86As₂ tomore » the tetragonal (T) phase of CaFe₂As₂ occurs. Furthermore, these results suggest that hole doping CaCo_1.86As₂ has a less dramatic effect on the magnetism and structure than steric effects due to substituting Sr for Ca.« less

Here, we report the electronic structure, synthesis, and measurements of the magnetic, transport, and thermal properties of the polycrystalline noncentrosymmetric compound Re₆Zr . We observed a bulk superconducting transition at temperature T_c ~ 6.7 K, and measured the resistivity, heat capacity, thermal conductivity, and the London penetration depth below the transition, as well as performed doping and pressure studies. From these measurements we extracted the critical field and the superconducting parameters of Re₆Zr. Lastly, our measurements indicate a relatively weak to moderate contribution from a triplet component to the order parameter, and favor a full superconducting gap, although we cannotmore » exclude the existence of point nodes based on our data.« less

In nonsuperconducting, metallic paramagnetic SrCo₂As₂, inelastic neutron scattering measurements have revealed strong stripe-type q=(π,0) antiferromagnetic (AFM) spin correlations. Using nuclear magnetic resonance (NMR) measurements on ⁵⁹Co and ⁷⁵As nuclei, we demonstrate that stronger ferromagnetic (FM) spin correlations coexist in SrCo₂As₂. Our NMR data are consistent with density functional theory (DFT) calculations which show enhancements at both q=(π,0) and the in-plane FM q=0 wave vectors in static magnetic susceptibility χ(q). We suggest that the strong FM fluctuations prevent superconductivity in SrCo₂As₂, despite the presence of stripe-type AFM fluctuations. Since DFT calculations have consistently revealed similar enhancements of the χ(q) at bothmore » q=(π,0) and q=0 in the iron-based superconductors and parent compounds, our observation of FM correlations in SrCo₂As₂ calls for detailed studies of FM correlations in the iron-based superconductors.« less

We present high-energy x-ray diffraction data under applied pressures up to p = 29GPa, neutron diffraction measurements up to p = 1.1GPa, and electrical resistance measurements up to p = 5.9GPa, on SrCo₂As₂. Our x-ray diffraction data demonstrate that there is a first-order transition between the tetragonal (T) and collapsed-tetragonal (cT) phases, with an onset above approximately 6 GPa at T = 7K. The pressure for the onset of the cT phase and the range of coexistence between the T and cT phases appears to be nearly temperature independent. The compressibility along the a axis is the same for themore » T and cT phases, whereas, along the c axis, the cT phase is significantly stiffer, which may be due to the formation of an As-As bond in the cT phase. Our resistivity measurements found no evidence of superconductivity in SrCo₂As₂ for p ≤ 5.9 GPa and T ≥ 1.8 K. The resistivity data also show signatures consistent with a pressure-induced phase transition for p ≳ 5.5 GPa. Single-crystal neutron diffraction measurements performed up to 1.1 GPa in the T phase found no evidence of stripe-type or A-type antiferromagnetic ordering down to 10 K. Spin-polarized total-energy calculations demonstrate that the cT phase is the stable phase at high pressure with a ca ratio of 2.54. As a result, these calculations indicate that the cT phase of SrCo₂As₂ should manifest either A-type antiferromagnetic or ferromagnetic order.« less