NMR Study of Magnetism and Superparamagnetism

The research described in this dissertation is concerned with two different types of magnetic materials. Both types of systems involve competing interactions between transition metal ions. New approaches involving magnetic resonance in the large hyperfine fields at nuclear sites have been developed. The interactions responsible for the properties that have been investigated in the materials studied are geometric frustration in an insulator and ferromagnetic and antiferromagnetic interactions in a metal alloy. Further details are given below. The extended kagome frustrated system YBaCo4O7 has 2D kagome and triangular lattices of Co ions stacked along the c-axis. Antiferromagnetic (AF) ordering accompanied by a structural transition has been reported in the literature. From a zero field (ZF) NMR single crystal rotation experiment, we have obtained the Co spin configurations for both the kagome and triangular layers. A 'spin-flop' configuration between the spins on the kagome layer and the spins on the triangular layer is indicated by our results. Our NMR findings are compared with neutron scattering results for this intriguing frustrated AF spin system. The non-stoichiometric oxygenated sister compound YBaCo4O7.1 has application potential for oxygen storage. While, its' magnetic properties are quite different from those of the stoichiometric compound, in spite of their similar structures of alternating kagome and triangular Co layers. Various techniques, including ZF NMR have been used to investigate the spin dynamics and spin configuration in a single crystal of YBaCo4O7.1. A magnetic transition at 80 K is observed, which is interpreted as the freezing out of spins in the triangular layers. At low temperatures (below 50 K), the spin dynamics persists and a fraction of spins in the kagome layers form a viscous spin liquid. Below 10 K, a glass-like spin structure forms and a large distribution of spin correlation times are suggested by nuclear spin lattice relaxation behavior. The magnetic shape memory alloys Ni-Mn-Sn exhibit interesting properties including, field induced transformations, conventional and inverse magnetocaloric effects. They have potential for use as sensors, actuators and energy conversion devices. The Heusler alloy, Ni50Mn50 xSnx with x=10 is one of these materials. It undergoes a transition from an austenite phase to a martensitic phase at 400 K, with the emergence of rich interesting magnetic properties below the transition. Coexistence of ferromagnetic (F) and AF spin configurations is reported in these compounds. 55Mn NMR has been used as a local probe to study the magnetic properties of this alloy. Rich peak features are observed with the various components assigned to nanoscale F or AF regions. Our results have provided detailed information on the AF regions, which has not been provided by other techniques. Measurements of the temperature dependence of the NMR spectra, in ZF and in a perturbing field were made. The spin-lattice relaxation dependence on T provides detailed information on the nanocluster size distribution and relative concentrations of the F and AF regions. Recently, the Heusler alloy Ni50-xCoxMn40Sn10, with 5 ≤ x ≤ 8, have attracted interest because the low thermal hysteresis and the large change in magnetization which they exhibit at the martensitic transition. Evidence for phase separation of ferromagnetic and antiferromagnetic regions at low temperatures is provided by magnetization and small angle neutron scattering measurements. Superparamagnetism and intrinsic exchange bias effects have been detected below 50 K. Zero field 55Mn NMR has provided detailed information on the nanoscale magnetic properties of samples with x = 7 and, for comparison, x = 14. For x = 7 F and AF regions, with a broad size distribution are identified and our results show that F clusters with the highest blocking temperatures are associated with regions rich in Co ions.