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The formation of magnetic order in solids is a complex and subtle issue. There are a wide range of different types of magnetisation, all of which may be favoured under different circumstances. In this thesis we consider a novel combination of ideas where the formation of spatially modulated magnetisation is linked to a metamagnetic transition. In this we are inspired by a general principle of modulated phases intervening as intermediate states in phase transitions. In particular we draw analogies with the Fulde-Ferrell-Larkin-Ovchinnikov state of spatially modulated superconductivity. We study a mean-field theory for itinerant magnetism where the crystal lattice drives the formation of a rich phase diagram. A peak in the electronic density of states due to a van Hove singularity creates ferromagnetism and a metamagnetic transition. Furthermore we find that a modulated magnetic phase - a spin-spiral, becomes favoured along the metamagnetic transition line. The appearance of this phase causes the metamagnetic transition to bifurcate to enclose the modulated region. The topology of this reconstructed phase diagram shows remarkable similarity to that observed in experiments on Sr₃Ru₂O₇. This material shows a metamagnetic transition which can be tuned by field angle towards zero temperature. Before this point is reached a new phase with high and anisotropic resistivity appears. We believe that this anomalous phase can be explained by the formation of a phase of modulated magnetisation caused by a peak in the electronic density of states. This mechanism may also apply in a range of other materials as it is driven by rather generic features of the bandstructure.

The recent discovery that a spin-polarized current can exert a large torque on a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of electrical current controlled manipulation of magnetic moment in nanoscale magnetic device structures. This so-called spin torque effect holds great promise for two applications, namely, spin torque oscillators (STOs) for wireless communication and radar communication, and spin transfer torque RAM (STT-RAM) for data/information storage.   The STO is a nanosized spintronic device capable of microwave generation at frequencies in the 1-65 GHz range with high quality factors. Although the STO is very promising for future telecommunication, two major shortcomings have to be addressed before it can truly find practical use as a radio-frequency device. Firstly, its very limited output power has to be significantly improved. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase the signal quality. Synchronization of serially connected STOs has been suggested in this thesis. In this configuration, synchronization relies on phase locking between the STOs and their self-generated alternating current. While this locking mechanism is intrinsically quite weak, we find that the locking range of two serially connected spin-valve STOs can be enhanced by over two orders of magnitude by adjusting the circuit I-V phase to that of an intrinsic preferred phase shift between the STO and an alternating current. More recently, we have also studied the phase-locking of STOs based on magnetic tunnel junctions (MTJ-STO) to meet the power specifications of actual application where the rf output levels should be above 0 dBm (1 mW). In addition to the spin torque terms present in GMR spin valves, MTJs also exhibit a significant perpendicular spin torque component with a quite complex dependence on both material choices and applied junction bias. We find that the perpendicular torque component modifies the intrinsic preferred I-V phase shift in single MTJ-STOs in such a way that serially connected STOs synchronize much more readily without the need for additional circuitry to change the I-V phase.   Secondly, equal attention has been focused on removing the applied magnetic field for STO operation, which requires bulky components and will limit the miniaturization of STO-based devices. Various attempts have been made to realize STOs operating in zero magnetic field. By using a tilted (oblique angle) polarizer (fixed layer) instead of an in-plane polarizer (standard STO), we show zero field operation over a very wide polarizer angle range without sacrificing output signal. In addition, the polarizer angle introduces an entirely new degree of freedom to any spin torque device and opens up for a wide range of additional phenomena.   The STT-RAM has advantages over other types of memories including conventional MRAM in terms of power consumption, speed, and scalability. We use a set of simulation tools to carry out a systematic study on the subject of micromagnetic switching processes of a device for STT-RAM application. We find that the non-zero k spin wave modes play an important role in the experimentally measured switching phase boundary. These may result in telegraph transitions among different spin-wave states, and be related to the back-hopping phenomena where the switching probability will decrease with increasing bias in tunnel junctions. / QC 20100819

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In this thesis the results of several different experimental techniques are presented. Time-resolved scanning Kerr microscopy and time-resolved optically pumped scanning optical microscopy measurements were made in Exeter on bi-component anti-dot lattices and permalloy films respectively. Magnetic transmission x-ray microscopy measurements were performed on cobalt nanostructures at the Advanced Light Source at the Lawrence Berkeley National Laboratory in Berkeley, California, USA. Time-resolved Kerr microscopy was used to study bi-component 400nm anti-dot lattices with a 1μm lattice constant. At 200 Oe the mode frequencies were obtained using time-resolved measurements. The mode frequencies of the cobalt filled anti-dots (binary sample) are 3 and 4.4 GHz. The 4.4 GHz mode has propagating character; the 3 GHz mode has non-propagating character. The mode frequencies of the air –filled anti-dot arrays (anti-dot reference sample) are 3.84 and 4.72 GHz. The 3.84 GHz mode has propagating character; the 4.72 GHz mode has non-propagating character. The alteration of the internal field by the air-filled anti-dots lowers the propagating mode frequency compared to the binary sample. Scanning Kerr microscopy was used to study the spatial character of the anti-dot modes. By fitting the spatial character the effective damping parameter(s) were determined. The effective damping parameter for the binary sample was 0.023 and 0.044 for the 4.4 and 3.04 GHz modes respectively. The 3.04GHz mode exists through the cobalt filled anti-dots leading to a high effective damping. The effective damping parameters for the anti-dot reference sample 0.026 and 0.028 for the 3.84 and 4.72 GHz modes respectively. Time-resolved optically pumped measurements have been performed on a continuous 20nm permalloy film. This is a new experimental technique developed during my PhD. Early data acquired on the optical microscope is compared with data measured later and attempts are made to explain the discrepancies. With a 500 Oe out-of-plane field initial time resolved signals show an oscillation at 17 GHz, the origin of the oscillation is unknown and is thought to originate from a magnon or phonon contribution. The experiment overlapping sub-micron pump and probe spots makes acquiring consistent time-resolved signals a difficult challenge. Images revealing circular lobe shapes are observed, the origin of these images is not fully understood. Later measurements are compared to the early measurements. In the recent measurements, images of spin waves with a wavelength and frequency of 2.5 μm and 4 GHz respectively are observed. These values are not reconcilable with the wavelengths and frequencies of 1μm and 17GHz seen in the images and time-resolved signals respectively. Recent measurements also revealed a strong dependence on the pump focus position on the measured images. Lastly, magnetic transmission x-ray microscopy measurements are presented on 700nm cobalt anti-dot structures overlaid on continuous permalloy films of thicknesses ranging from 20 to 60nm. The magnetic ground states of the nanostructures are investigated using L3 edge x-ray absorption and x-ray magnetic circular dichroism as a magnetic contrast mechanism. The reversal fields are determined and consistent with hysteresis loops measurements in Exeter. The dipolar fields from the complex shape of the cobalt anti-dots are expected to modulate the magnetic ground state of the permalloy. Reversal of the permalloy occurs suddenly over a consistent field window, starting and finishing between 13 ~ 17 Oe respectively. The reversal process in the cobalt occurs gradually and full saturation is not observed until fields of up to 350Oe.

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