M.Sc. (Experimental Physics) / Please refer to full text to view abstract

M.Sc. / An experimental study on the magnetic properties of the (Cr86Ru14)1–x(V,Mn)x alloy system is reported in this thesis. The work focusses mainly on two aspects of the magnetic behaviour of this alloy system; firstly that of the base alloy Cr86Ru14 having a Ru concentration on the magnetic phase diagram of the Cr-Ru system that is close to a putative quantum critical point (QCP) (xc ≈ 17 at.% Ru), and secondly, of the effects of tuning this base alloy towards (V-addition) or away (Mn-addition) from the QCP. For the Cr86Ru14 base alloy, measurements of the electrical resistivity and elastic constants as functions of temperature and pressure are reported. These are supplemented by thermal expansion and neutron diffraction measurements as a function of temperature through the Néel transition temperature, TN.

Ph.D. (Physics) / Please refer to full text to view abstract.

M.Sc. (Physics) / Please refer to full text to view abstract

Several workers have previously investigated the magnetic properties of copper and aluminium, but in most cases too small regard has been paid to the changes in magnetic susceptibility brought about by mechanical and thermal treatments previous to measurements. In the present work I have attempted to correlate changes in the magnetic susceptibilities of copper and aluminium with metallurgical changes in the metal lattices. A somewhat tentative theory is outlined covering this correlation. Previous investigations by Kussmann and Seeman led to the conclusion that iron impurity in the pure copper was the sole cause of the various magnetic changes recorded. This it is hoped to disapprove. To get some insight into the changes caused by such ferromagnetic impurity a series of magnetic measurements were carried out on low iron content iron-copper alloys, very kindly supplied by Imperial Chemical Industries Ltd., “Metals Division”. The thesis falls naturally into four main parts, and appendices are added at the end to avoid interpolation of the work with definitions etc. Throughout the thesis (Parts III and IV) only a few experimental values, typical of the many results obtained, are quoted.

The magnetocrystalline anisotropy energy (MAE) for fcc Ni and bcc Fe is calculated as the difference of single particle energy eigenvalue sums using a tight-binding model. For nickel we predict a MAE of -0.15 eV and the wrong easy axis, for iron we find a MAE of -0.7 eV with the easy axis in agreement with experiment. Our results compare favorably with previously reported first-principles calculations based on density functional theory and the local spin density approximation. The inclusion of an orbital polarization correction improves the magnitude of the MAE for iron, but fails to bring the result for nickel closer to the experimental value. The outstanding feature of our calculations is the careful handling of the necessary Brillouin zone integrals. Linear interpolation schemes and methods based on Fermi surface smearing were used and analyzed. An alternative method of calculating the MAE based on the torque on a magnetic moment centered on an atom is found to be equivalent to the calculation of the MAE in terms of energy differences. / Graduation date: 1999

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This thesis contains three major areas of work. Chapter 1 describes the various film deposition techniques available for the growth of thin films. These different growth methods, substrate preparation and vacuum techniques, are discussed in the context of the effect of the growth process and deposition environment on film structure and quality. Chapter 2 describes the two film growth chambers. Chapter 3 describes the origin of giant magnetoresistance (GMR) in magnetic multilayered structures, and investigates the magnetoresistive properties of ion-beam sputtered CoCu multilayered thin films. Different film structures and deposition conditions have been investigated in order to maximise the size of negative magnetoresistance seen. Chapter 4 investigates the magnetic and giant magnetoresistive properties of sputtered CoCu and CoAg heterogenous alloys. The magnitude of the GMR effect is found to depend on the Co concentration and Co particle size. Chapter 5 describes an investigation of molecular beam epitaxy (m.b.e.) grown Dy/Y multilayer structures. The temperature dependences of helical magnetic structure in the Dy layers is investigated in two samples of different multilayer structures. Chapter 6 describes an acoustic interferometer which has been used to measure velocity and attenuation changes in an ultrasonic signal at frequencies of order IGHz. Also described is the production of the thin film piezoelectric transducers used in this technique, which are grown directly onto a single crystal of the material under investigation. The acoustic interferometer is used to investigate the dynamics of of a relaxation process between two time-reversed antiferromagnetic states in DyAlG, and a model for domain growth in this system is postulated. Chapter 7 investigates the phase diagram of DyPO₄ using the acoustic interferometry technique. The observations of hysteresis effects are used to define a mixed phase region below the tricritical point.

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M.Sc. / Please refer to full text to view abstract.