Date of Award
Doctor of Philosophy
Physics and Astronomy
This thesis emphasizes two frequency-domain techniques which uniquely employ radio frequency (RF) excitations to investigate the static and dynamic properties of novel magnetic and superconducting materials. The first technique is a tunnel-diode resonator (TDR) which detects bulk changes in the dynamic susceptibility, &chi = dM/dH. The capability of TDR to operate at low temperatures (less than 100 mK) and high fields (up to 65 T in pulsed fields) was critical for investigations of the antiferromagnetically correlated magnetic molecules Cr12Cu2 and Cr12Ln4 (Ln = Y, Eu, Gd, Tb, Dy, Ho, Er, Yb), and the superconductor SrFe2(As1-xPx)2 (x = 0.35). Investigations of Cr12Cu2 and Cr12Ln4 demonstrates the first implementation of TDR to experimentally investigate the lowlying energy spectra of magnetic molecules in pulsed magnetic elds. Zeeman splitting of the quantum spin states results in transitions between field-dependent ground state energy levels observed as peaks in dM/dH at 600 mK, and demonstrate good agreement with theoretical calculations using a isotropic Heisenberg spin Hamiltonian. Increasing temperature to 2.5 K, TDR reveals a rich spectrum of frequency-dependent level crossings from thermally populated excited states which cannot be observed by conventional static magnetometry techniques. The last study presented uses TDR in pulsed fields to determine the temperature-dependent upper-critical field Hc2 to investigate the effects of columnar defects arising from heavy ion irradiation of SrFe2(As1-xPx)2. Results suggest irradiation uniformly suppresses Tc and Hc2, and does not introduce additional features on Hc2(T) and the shapes of the anisotropic Hc2 curves indicates a nodal superconducting gap. The second technique is nuclear magnetic resonance (NMR) which yields site specic magnetic and electronic information arising from hyperfine interactions for select magnetic nuclei. NMR spectra and nuclear spin-lattice relaxation measurements are reported for the geometrically frustrated magnetic molecule W72V30, and for BaMn2As2 and Ba1-xKxMn2As2 (with K-concentration x = 0.04 - 0.40) which are analogs of the high Tc iron arsenides. For the magnetic molecule W72V30, 1H and 51V NMR and DC magnetization were used to investigate geometric frustration arising from antiferromagnetic interactions between 30 V4+ ions occupying the edge sites of an icosidodecahedron. This system serves as a molecular representation of the 2-dimensional kagome lattice whose finite-size allows precise quantum calculations. Analysis of W72V30 data suggests a large distribution of exchange values are necessary to characterize the field and temperature-dependent magnetic properties. For the insulating BaMn2As2 and hole-doped metallic Ba1-xKxMn2As2, both local moment antiferromagnets, 55Mn and 75As NMR spectra and spin-lattice relaxation rates 1/T1 were conducted to investigate the local magnetic and electronic properties as a function of K-concentration x. NMR independently confirms G-type antiferromagnetism from spectra measurements, while a Korringa relation in 1/T1 indicates conduction electrons in both the Mn-3d and As-4d orbitals. The observation of ferromagnetic enhancement of the 55Mn NMR signal and no appreciable shift observed in the 75As spectra, combined with the absence of a structural phase transition in neutron diraction measurements suggests, the K-doped system may exhibit a previously unseen coexistence of local-moment antiferromagnetism from the Mn2+ moments and weak ferromagnetism, possibly arising from the Mn-3d orbitals. In summary, the data presented in this work demonstrates the diversity of novel materials and physical properties which can be investigated by the RF techniques TDR and NMR.
Steven Lee Yeninas
Yeninas, Steven Lee, "Tunnel-diode resonator and nuclear magnetic resonance studies of low-dimensional magnetic and superconducting systems" (2013). Graduate Theses and Dissertations. 13374.