Tom Heitmann

Ba(1-x)K(x)Mn2As2: an antiferromagnetic local-moment metal.

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature T(N)=625 K and a large ordered moment μ=3.9μ(B)/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high T(N) and large μ. Ba(1-x)K(x)Mn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba(1-x)K(x)Mn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high T(c) superconductivity.

Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4spinel

Nuclear magnetic resonance (NMR), neutron diffaction (ND), x-ray diffraction, magnetic susceptibility and specific heat measurements on the frustrated A-site spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below Tn = 9.8(2) K. A high quality powder sample characterized by x-ray diffraction that indicates a relatively low Co-Al inversion parameter x = 0.057(20) in (Co1-xAlx)[Al2-xCox]O4, shows a broad maximum around 15 K in magnetic susceptibility and a sharp peak at Tn in heat capacity. The average ordered magnetic moment of Co^2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1) Bohr magneton from NMR and 1.9(5) Bohr magneton from ND which are smaller than the expected value of 3 Bohr magneton for S = 3/2 and g = 2. Antiferromagnetic spin uctuations and correlations in the paramagnetic state are revealed from the magnetic susceptibility, NMR and ND measurements, which are due to spin frustration and site inversion effects in the system. The ND data also show short-range dynamic magnetic ordering that persists to a temperature that is almost twice Tn.

Effects of Transition Metal Substitutions on the Incommensurability and Spin Fluctuations in BaFe2As2 by Elastic and Inelastic Neutron Scattering

The spin fluctuation spectra from nonsuperconducting Cu-substituted, and superconducting Co-substituted, BaFe(2)As(2) are compared quantitatively by inelastic neutron scattering measurements and are found to be indistinguishable. Whereas diffraction studies show the appearance of incommensurate spin-density wave order in Co and Ni substituted samples, the magnetic phase diagram for Cu substitution does not display incommensurate order, demonstrating that simple electron counting based on rigid-band concepts is invalid. These results, supported by theoretical calculations, suggest that substitutional impurity effects in the Fe plane play a significant role in controlling magnetism and the appearance of superconductivity, with Cu distinguished by enhanced impurity scattering and split-band behavior.

Crystal and Magnetic Structure of CaCo1.86As2 Studied by X-ray and Neutron Diffraction

Neutron and x-ray diffraction measurements are presented for powders and single crystals of CaCo2As2. The crystal structure is a collapsed-tetragonal ThCr2Si2-type structure as previously reported, but with 7(1)% vacancies on the Co sites corresponding to the composition CaCo1.86(2) As2. The thermal expansion coefficients for both the a and c axes are positive from 10 to 300 K.

Quasi-local critical nature of cooperative paramagnetic fluctuations in CaRuO

We report the observation of cooperative paramagnetic fluctuations of

_3

{\mathrm{Ru}}^{4+}

metal

spins that coexist with the non-Fermi-liquid state in

Magnetic fluctuations driven insulator-to-metal transition in Ca(Ir(1-x)Rux)O3.

{\mathrm{CaRuO}}_{3}

Cluster formation in quantum critical systems

perovskite below

Itinerant G-type antiferromagnetic order in SrCr2As2

T\ensuremath{\simeq}21