Short-Range Correlations and Frustration in the Stuffed-Pyrochlore Tb2+xTi2-2xNbxO7 and Heavy-Fermion CeCu4Ga

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Date/Time:Thursday, 13 Sep 2012 - Thursday, 13 Sep 2012
Location:PHYSICS Hall Room 5
Phone:515-294-5630
Channel:Condensed Matter Physics
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Benjamin G. Ueland
MPA-CMMS Los Alamos National Laboratory

Cooperative-paramagnets possess strongly-correlated magnetic moments (spins) that can continue fluctuating as T~0 K. In this talk, I will present results from neutron scattering and magnetization measurements on the geometrically frustrated magnet Tb2+xTi2-2xNbxO7, x=0-1, which for x=0 is a pyrochlore possessing a magnetic lattice of corner-sharing tetrahedra that exhibits cooperative-paramagnetic behavior. As magnetic Tb and non-magnetic Nb are substituted (“stuffed”) into the lattice, the magnetic lattice changes to that of side-sharing tetrahedra--another frustrated geometry. Surprisingly, we find that cooperative-paramagnetism exists for all values of x. For x=1, cooperative-paramagnetism coexists with antiferromagnetic order below TN=2.2 K producing a unique example of concomitant liquid-like fluctuations and long-range order without applying any external fields or pressure. I will also present more recent results from neutron scattering, resistivity, heat capacity, ac susceptibility, and magnetization measurements on the heavy-fermion metal CeCu4Ga which suggest that its hexagonal lattice and disorder due to Ga substitution may frustrate formation of the long-range magnetic order found in its parent CeCu5. The absence of magnetic Bragg peaks, field-dependent specific heat data, a Weiss temperature of ~10 K, and diffuse scattering below ~1 K which can be fit to an isotropic model describing spin-spin correlations between second through fifth nearest-neighbors support this suggestion. Kondo behavior also plays a role in determining the physical properties of CeCu4Ga.
B.G.Ueland et al., Phys. Rev. B 81, 060408(R) (2010).