Thesis (Ph.D.)--University of Western Australia, 2004.

by Ku, Nim Chung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1997. / Includes bibliographical references. / by Ku, Nim Chung. / Acknowledgment --- p.i / Abstract --- p.ii / Table of contents --- p.iv / List of figures --- p.viii / List of tables --- p.xiv / Chapter Chapter One --- Introduction / Chapter 1.1 --- History of permanent magnet materials --- p.1 / Chapter 1.2 --- NdFeB intermetallic compound --- p.4 / References --- p.8 / Chapter Chapter Two --- Background / Chapter 2.1 --- Corrosion and corrosion control --- p.9 / Chapter 2.1.1 --- Electrochemical mechanisms --- p.10 / Chapter 2.1.2 --- Types of corrosion cell --- p.12 / Chapter 2.1.3 --- Types of corrosion --- p.14 / Chapter 2.1.4 --- Corrosion control --- p.15 / Chapter 2.2 --- Intermetallic compounds and phases --- p.19 / References --- p.26 / Chapter Chapter Three --- Instrumentation / Chapter 3.1 --- Introduction --- p.27 / Chapter 3.2 --- Vacuum evaporator --- p.28 / Chapter 3.3 --- Vacuum furnace --- p.29 / Chapter 3.4 --- Scanning electron microscopy (SEM) & Energy dispersive analysis (EDX) --- p.29 / Chapter 3.5 --- Corrosion chamber --- p.31 / Chapter 3.6 --- Vibrating sample magnetometer (VSM) --- p.31 / References --- p.43 / Chapter Chapter Four --- Moisture corrosion test on a bare NdFeB permanent magnet (without any coating) / Chapter 4.1 --- Introduction --- p.44 / Chapter 4.2 --- Experimental procedures --- p.45 / Chapter 4.3 --- Results from visual inspection --- p.45 / Chapter 4.4 --- Corrosion behavior --- p.46 / Chapter 4.5 --- Measurement of the saturated magnetization --- p.47 / Chapter 4.6 --- The effect of corrosion on the magnetic property of the NdFeB magnet --- p.48 / Chapter 4.7 --- Conclusions --- p.49 / References --- p.57 / Chapter Chapter Five --- Interfacial reaction between the A1 coating and the NdFeB magnet / Chapter 5.1 --- Introduction --- p.58 / Chapter 5.2 --- Experiments --- p.59 / Chapter 5.3 --- The Interaction between the A1 coating and Magnet --- p.60 / Chapter 5.4 --- Conclusions --- p.63 / References --- p.67 / Chapter Chapter Six --- Corrosion behavior of A1 coated NdFeB magnet in moist environment / Chapter 6.1 --- Introduction --- p.68 / Chapter 6.2 --- Experiments --- p.69 / Chapter 6.3 --- Results --- p.69 / Chapter 6.3.1 --- The annealed samples before corrosion test --- p.70 / Chapter 6.3.2 --- Microstructure changes after 3 days of corrosion --- p.70 / Chapter 6.3.3 --- Corrosion process in the annealed samples --- p.71 / Chapter 6.4 --- Corrosion behavior --- p.73 / Chapter 6.4.1 --- Effects of NaCl on the samples --- p.73 / Chapter 6.4.2 --- Effects of the inter-diffusion --- p.74 / Chapter 6.5 --- Conclusions --- p.75 / References --- p.79 / Chapter Chapter Seven --- Dual layer coating of either Ti/Al or Ni/Al on the NdFeB permanent magnet / Chapter 7.1 --- Introduction --- p.80 / Chapter 7.2 --- Experiments --- p.81 / Chapter 7.3 --- The Interaction between the A1 and Magnet --- p.81 / Chapter 7.3.1. --- The Ti/Al coated magnets --- p.82 / Chapter 7.3.2. --- The Ni/Al coated magnets --- p.83 / Chapter 7.4 --- Volume contraction of the intermetallic coatings --- p.84 / Chapter 7.5 --- Conclusions --- p.85 / References --- p.90 / Chapter Chapter Eight --- Conclusions and suggestions for further studies --- p.91

The field of molecular magnetism has expanded rapidly since the discovery of single molecule magnets (SMMs) in the 1990’s and has witnessed extraordinary advances in the last several decades. One of the current trends in molecular magnetic research is to incorporate metal ions that have pronounced single-ion anisotropy in an effort to improve magnetic exchange interactions. The 4d and 5d transition metal ions have large spin-orbit coupling parameters which contribute to the orbital angular momentum effects that lead to anisotropic behavior. The work herein describes efforts to synthesize and characterize new cyanide-bridged molecular materials incorporating 4d and 5d transition metal ions, specifically the [Os(CN)_(6)]^(3-), [Mo(CN)_(6)]^(3-) and [W(CN)_(8)]^(3-) ions. The 5d hexacyanometallate [Os(CN)_(6)]^(3-) was incorporated into a trinuclear cyanide bridged molecule and the [Fe(CN)_(6)]^(3-) analog was prepared as a reference compound for assessing the effect of the 5d versus 3d metal ion on the magnetic properties. Both molecules exhibit SMM bistability with a pronounced increase (~90 %) in the blocking temperature (TB) of the OsIII analogue. In addition to typical SMM behavior, both compounds exhibit exchange-biased SMM behavior, a shift in the quantum tunneling of the magnetization (QTM) from zero field. This exchange-bias can be turned “on” or “off” depending on the presence of interstitial methanol molecules. New trigonal bipyramidal (TBP) molecules incorporating the rarely studies hexacyanomolybdate(III) ion are presented in chapter III of this dissertation. The molecules of general formula [M(tmphen)_(2)]_(3)[Mo(CN)_(6)]_(2) (M = V^(II), Mn^(II) and Fe^(II); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline), represent additions to a large homologous family of TBP molecules reported by the Dunbar group over the years. The [Mo(CN)_(6)]^(3-) ion was prepared in situ by loss of one cyanide ligand from [Mo(CN)_(7)]^(4-). Of particular interest among the compounds reported is the V_(3)Mo_(2) analog which exhibits extraordinarily strong antiferromagnetic coupling (estimated J = -134 cm^(-1). The observed exchange coupling parameter is more than twice the current record for the antiferromagnetic coupling parameter for a cyanide-bridged magnetic molecule. Another set of results were obtained using the octacyanometallate anion [WV(CN)_(8)]^(3-) as a building block for the synthesis and magnetic studies of a family of new cyanide-bridged magnetic materials. The compounds exhibit several different structural motifs including three 0-D molecular compounds (two pentanuclear molecules and a linear trinuclear molecule) and a 1-D chain, findings that illustrate the structural versatility of the octacyanotungstate(V) ions. The TBP molecule, [Mn(tmphen)_(2)]_(3)[W(CN)_(8)]_(2), exhibits evidence for an out-of-phase signal when subjected to ac measurements in zero applied field. The 1-D chain also reveals evidence for the beginning of an out-of-phase signal under zero applied field which hints at single chain magnet behavior.

Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1992. / Vita. Abstract. Includes bibliographical references (leaves 291-297). Also available via the Internet.

Magnetization processes in a sintered Nd-Fe-B permanent magnet (NEOMAX-35) were examined on a small scale using a Hall-effect microprobe with an active area 75 μm on a side. Probes were made by evaporating bismuth through a stencil mask onto glass slides. Experiments were performed by placing a probe onto the polished pole face of a Nd-Fe-B magnet and inserting the probe-magnet assembly into an electromagnet. Barkhausen steps, indicating rapid domain wall motion, were observed (superimposed upon the blank probe signal) in the demagnetization of a fully magnetized magnet. Magnetization traces for a thermally demagnetized Nd-Fe-B magnet did not exhibit measurable Barkhausen steps until a field of approximately 1.2 T was applied. The following observations were made for two thermally demagnetized samples which were cycled through minor hysteresis loops (maximum applied field of approximately 2 T): (1) virgin magnetization traces did not contain measurable Barkhausen steps, however all other forward and reverse magnetization traces did; (2) the initial reverse magnetization trace exhibited more and larger Barkhausen steps than subsequent traces; and (3) some Barkhausen steps were repeatable, that is, occurring at approximately the same field on each subsequent forward or reverse trace. Hall voltage signals were on the order of millivolts for probe currents of 10 mA.

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