Semester

Summer

Date of Graduation

2011

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Physics and Astronomy

Committee Chair

Mohindar S. Seehra.

Abstract

Investigations of properties of materials with lower lattice dimensionality (one and two dimensions) often provides good opportunity to advance physics since solutions to corresponding theoretical models are more easily obtained. In this work, results on magnetic properties of two quasi two-dimensional (2D) systems viz. the layered hydroxides of nickel, beta-Ni(OH)2 and alpha-Ni(OH)2, are reported. For beta-Ni(OH)2, there have been conflicting reports in the literature whether the magnetic ordering is ferromagnetic or antiferromagnetic. For the less stable alpha-Ni(OH) 2 with the larger spacing between the layers, the nature of magnetism is largely unexplained. Therefore, the results and their interpretation present here contribute significantly to the understanding of the magnetic properties of these quasi 2D systems.;Synthesis of the two hydroxides were done by the sol-gel and hydrothermal techniques followed by structural characterizations by x-ray diffraction (XRD), transmission electron microscopy (TEM)/scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and FTIR spectroscopy. Magnetic properties were investigated using an in-house SQUID magnetometer for the temperature range of 2 K to 350 K in magnetic fields up to +/- 65 kOe. Additional magnetic studies on beta-Ni(OH)2 for H up to 180 kOe were carried out at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Fl.;For the beta-phase with the formula Ni(OH)2 · 0.144 H2O, the lattice constants of the hexagonal lattice are a = 3.12 A and c = 4.67 A. Temperature dependence of the magnetization showed a two step transition: ferromagnetic (FM) ordering of the (001) layers with exchange coupling J1/kB = 3.25 K at 25 K followed by 3D antiferromagnetic (AFM) ordering at TN = 23 K triggered by the interplane AFM exchange constant J2/kB = -0.32 K. Both bulk-like and nanosize beta-Ni(OH)2 show a magnetic field induced metamagnetic transition to ferromagnetism at HC2 ≃ 55 kOe. In the nanosize beta-Ni(OH) 2 only, a second weaker transition at HC2/2 is observed which is shown to be due to magnetic field induced flipping of the surface Ni 2+ spins observable only on the nanosize particles. The associated magnetic moment per Ni2+ spin is determined to be 2.92mu B (3.33muB) for nanosize (bulk) beta-Ni(OH)2. The observed saturation magnetization MS = 118 emu/g above 150 kOe is shown to be consistent with the theoretical model of AFM order and a metamagnetic transition to FM order above HC2.;The two samples of alpha-Ni(OH)2 studied here give nearly identical results for characterization and magnetization. The sample with formula Ni(OH)1.23(CH3COO)0.77 · 0.141 H2O gives a hexagonal lattice with a = 3.01 A and c = 8.6 A. The morphology, based on the SEM images, shows a flower-like structure with petal thickness of ∼ 10 nm, while the particle is larger, around 200 nm.;The temperature variation of the magnetization in the alpha-phase shows a peak temperature at 16 K for H = 50 Oe corresponding to 2D ferromagnetic ordering. As such, the M vs. T ZFC data for T > 50 K was fit to the 2D triangular Ising lattice S = 1 high temperature series giving g = 2.29 and the in-plane ferromagnetic interaction of J1/kB ≃ 4.38 K. Based on the Heisenberg 2D to 3D transition of the ordering temperature, the interplanar exchange interaction was determined to be J2/kB ≃ 0.14 K. The ac susceptibility gives evidence for two magnetic regimes below the 2D FM ordering at Tc ≃ 16 K. For temperatures below T p ≃ 8 K particle size effects are seen with a blocking temperature highly dependent on the applied magnetic field and the measuring frequency. The system also exhibits magnetic annealing behavior due to surface spin interactions below Tm ≃ 3.5 K, giving rise to exchange bias and hysteresis loop characteristic of pinned spins.

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