The aim of the study, which was carried out in two main parts, was to investigate the atomic and magnetic ordering in various full Heusler alloy systems.In the theoretical part of the thesis, investigations based on the statiscothermodynamical theory of ordering by means of Bragg-Williams-Gorsky (BWG) method combined with electronic theory in the pseudopotential approximation have been conducted to model order-order (L21&amp / #8596 / B2) and order-disorder (B2&amp / #8596 / A2) phase transformations in Ni-Mn-C (C=Ga, In, Sb) and A2BGa (A=Fe, Ni, Co / B=Ni, Mn, Co, Fe) Heusler alloys. The partial ordering energies, calculated according to the electronic theory of alloys in pseudopotential approximation for the first two coordination spheres were utilized as input data for the theoretical superlattice formation models based on BWG approximation. Furthermore, the trends of L21&amp / #8596 / B2 transition temperatures with electron concentration at A, B and C atomic sites were determined. In the experimental part of the thesis, structural and magnetic properties of Ni-Mn-Ga and Ni-Mn-Al Heusler alloys were investigated. For the Ni-Mn-Ga Heusler alloy system, it was found that the martensitic (TM) and Curie temperatures (TC) merge for Ni54Mn20Ga26 and Ni56Mn18Ga26 alloys by compositional tuning. For the Ni-Mn-Al Heusler alloy system, it was found that ferromagnetism was introduced into these alloys by low temperature aging, as a result of the stabilization of the L21 phase. In addition to that, magnetocaloric effect (MCE) was determined in the vicinity of the first order magneto-structural transition for Ni-rich Ni-Mn-Ga alloys and near the second order magnetic transition for the Ni2MnAl alloy.

No description available.

Abstract Methyl Viologen is a kind of organic molecular magnet of current interest which comprises with organic radicals (unpaired electrons). The molecular formula of the sample is (C12H14N2)2+X2-, where ¡§X2-¡¨ denotes a non-magnetic anion (CdI42-, I3-I- etc.). We perform the magnetization (using SQUID magnetometer) and electron paramagnetic resonance (EPR) measurements. Two absorption peaks are observed in EPR spectrum indicating that there are two radical forms in our samples. One of the radicals exists in every sample having the g1-factor value 2.004. The other radical which has the g2-factor 2.001 exists only in certain samples. Interestingly, we find that these samples exhibit ferromagnetism at room temperature. We use Lorentzian profile to quantitatively analyze the EPR spectrum of the samples and derive the ratio (A2/A1) of two different absorption peaks, which represents the amount of g2 radical. The remanent magnetization and saturation magnetization are found to increase as the ratio value increases, indicating that the strength of ferromagnetism is strongly correlated with g2 radical.

Magnetic force microscopy studies in varying temperature and applied external magnetic field of magnetic thin films of LaCoO₃ under strain and single crystal UMn₂Ge₂ have been performed. In the case of LaCoO₃ thin films the aim is an understanding of the response of the magnetic microstructure to different signs and degrees of strain and a further attempt to distinguish the effect of defects from strain-induced effects. In UMn₂Ge₂ the magnetic microstructure is imaged for the first time and signatures of a possible phase transition at 150 K and crystalline anisotropy are explored. The first portion of this dissertation focuses on the synthesis methods used to produce the samples investigated and the critical role of synthesis in producing high-quality samples. This is followed by a discussion of characterization techniques used to obtain local and global magnetic and structural characteristics, with particular emphasis on magnetic force microscopy including noise characteristics and a discussion of achieving a high force gradient sensitivity by optimizing the fiber-optic interferometer used for cantilever deflection detection. Design elements and features of the multi-mode variable-temperature atomic force microscope used to obtain magnetic force microscopy images are presented and results for LaCoO₃ and UMn₂Ge₂ are discussed. / text

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A Barkhausen noise (BN) testing system was developed for the non-destructive evaluation (NDE) of residual stresses in CANDU reactor feeder pipes. The system consists of a four-channel arbitrary waveform flux control system (FCS), and the spring-loaded tetrapole prototype (SL4P) BN probe. The combination of the FCS and SL4P was shown to provide repeatable BN measurements on feeder pipe samples, with variations in air gaps between the SL4P poles and the sample from 0.43 mm to 1.29 mm, and typical pickup coil coupling uncertainties for the total BN energy from ±2% to ±7%. Precision for elastic strain estimation in feeder pipes was found to be between ±7 MPa and ±9 MPa in tension, depending on the excitation field configuration, and negligible in compression. Modelling of the BN penetration depth as a function of the excitation field was used to estimate the BN penetration depth between 5 μm at 300 kHz to a maximum of 500 μm at 3 kHz. The modelling, engineering, and procedures developed for this BN testing system provide an improved basis for the future advancement of BN testing, and ferromagnetic NDE in general. / Thesis (Ph.D, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-07-22 15:34:28.967

I have measured the magnetic and transport properties of Fe and Co in the variable SDW host, CrMo. Analyzing the behaviour of the Neel temperature shows that Co interacts with the SDW, almost an order of magnitude more strongly than does Fe. Also that it magnetically polarizes the SDW more strongly than Fe. Resistivity measurements show that Co reduces the nesting area of the Fermi surface while Fe increases it. Overall, the Neel temperature is strongly reduced by Fe and much less reduced by Co. / Magnetic susceptibility measurements show that Fe supports an effective magnetic moment below T(,N) while Co does not. However, as suggested by Friedel, only nearest neighbour Fe pairs contribute to the effective Fe moment, while isolated Fe moments, like Co, are locked into the SDW antiferromagnetism. / In the absence of SDW formation, the strong interaction between Co and the CrMo host leads to a Kondo state, while the smaller interaction with Fe allows the onset of spin glass ordering.

No description available.

This thesis presents the results from Monte Carlo calculations on classical vector spins in face-centered-cubic (FCC) lattices. The goal of the study was to understand the effect of interlayer coupling, dilution of magnetic atoms in the lattice, and symmetry-changing strain. Experimental work by T. M. Giebultowicz et al. and J. A. Borchers et al. greatly inspired my work [1, 2]. J. A. Borchers's group studied NiO/CoO superlattices and observed that the magnetic order of CoO persisted above its Neel temperature due to the effect of interlayer coupling with NiO, which has a higher Neel temperature than CoO [1]. Simulating on a model of NiO/CoO bilayer reproduced the experimental results from Borchers et al. [1]. I concluded that exchange pinning on the NiO/CoO interface preserves the magnetic order of CoO above its Neel temperature significantly. Building on this initial result, a ferromagnet/antiferromagnet/ferromagnet (FM/AFM/FM) trilayer model was studied, where the ferromagnetic (FM) layers were antiferromagnetically coupled. First, I calculated the strength of the AF coupling as a function of the number of antiferromagnetic (AFM) spacer monolayers and concluded that the strength of AFM coupling decreases as the number of AFM spacer monolayers increases. Secondly, I added a uniaxial anisotropy to the model and obtained magnetization curves which exhibited hysteresis-like features with an external field and a first order magnetic transition. Lastly, I diluted the AFM spacer layer in the FM/AFM/FM trilayer by replacing magnetic spins with zero spins in the model. The dilution of AFM spacer layer caused fluctuations in the magnetization curves with external field but the strength of AFM coupling decreases as the number of AFM monolayers increases as in the nondiluted cases. The experimental results from T.M. Giebultowicz's group on MnSe/ZnTe superlattices by neutron scattering showed incommensurate helical spin order in MnSe, where MnSe layers were under tensile strain due to a small mismatching in the lattice parameter [2]. In addition, they observed that the pitch of the spin helix increased as the temperature increased [2]. I modeled the MnSe/ ZnTe system with Monte Carlo method and found that the pitch of the spin helix increased with temperature. In fact, the dependence of helix pitch on temperature was present regardless of the thickness of the sample, so I concluded that this pitch increase is not from the weakening of coupling of surface spins / Graduation date: 2013 / Access restricted to the OSU Community at author's request from Jan. 18, 2013 - July 18, 2013

[Truncated abstract] In this dissertation we present a theoretical study of electron transport through domain walls, with a particular focus on conductance properties, in order to understand various transport measurements that have been carried out recently on ferromagnetic nanowires. The starting point for our work is a ballistic treatment of transport through the domain wall. In this case conduction electrons are generally only weakly reflected by the domain wall, and the principal effect is a mixing of transmitted electron spins between up and down states. For small spin-splitting of conductance electrons the latter can be characterized by an appropriate adiabaticity parameter. We then incorporate the effect of spin-dependent scattering in the regions adjacent to the domain wall through a circuit model based on a generalization of the two-resistor theory of Valet and Fert. Within this model we find that the domain wall gives rise to an enhancement of resistance similar to the giant magnetoresistance effect found in ferromagnetic multilayer systems. The effect is largest in the limit of an abrupt wall, for which there is complete mistracking of spin, and decreases with increasing wall width due to the reduction of spin mistracking. For reasonable physical parameter values we find order-of-magnitude agreement with recent experiments. Going beyond the assumption of ballistic transport, we then consider the more realistic case of a domain wall subject to impurity scattering. A scattering matrix formalism is used to calculate conductance through a disordered region with either uniform magnetization or a domain wall. By combining either amplitudes or probabilities we are able to study both coherent and incoherent transport properties. The coherent case corresponds to elastic scattering by static defects, which is dominant at low temperatures, while the incoherent case provides a phenomenological description of the inelastic scattering present in real physical systems at room temperature. It is found that scattering from impurities increases the amount of spin mistracking of electrons travelling through a domain wall. This leads, in the incoherent case, to a reduction of conductance through the domain wall as compared to a uniformly magnetized region. In the coherent case, on the other hand, a reduction of weak localization and spin-reversing reflection amplitudes combine to give a positive contribution to domain wall conductance, which can lead to an overall enhancement of conductance due to the domain wall in the diffusive regime. A reduction of universal conductance fluctuations is found in a coherent disordered domain wall, which can be attributed to a decorrelation between spin-mixing and spin-conserving scattering amplitudes.