Investigating magnetisation dynamics in magnetic thin films and micro-structures

Data storage is the fulcrum on which technology advances. As the rate of consumption of data increases there is growing demand from the storage industry for faster, more efficient storage solutions which is capable of storing more information per unit area. Non-volatile magnetic memory based on spin-electronics is one of the key proponents for addressing this demand. In order to develop such a memory it is necessary to attain a better understanding of the magnetic properties on which this is based. This thesis aims to develop a tool to study the magnetisation dynamics of the free layer in a Pseudo Spin-Valve (PSV) of the form Co/Cu/NiFe utilising both the Giant Magneto-Resistance (GMR) and the Anisotropic Magneto-Resistance (AMR) effects. The key difference in the resistance responses of each of these effects is exploited here to showcase a proof of concept device which can be used as another method to aid in the study of magnetisation dynamics. GMR’s resistance response is dependent on the relative orientations of magnetisation between the free layer and the pinned layer. The AMR’s resistance response however, depends on the relative orientations of the magnetisation and the current direction. However, the resistance response of AMR is independent of magnetisation parallel or anti-parallel to current. This independence and the dependence of GMR on the relative magnetisations in each layer are used in combination to study the evolution of the magnetisation in a free layer PSV. The magneto-resistance of the Co/Cu/NiFe spin-valve was measured in as deposited samples and samples deposited in the presence of an in-plane field. The magneto-resistance was compared to a PSV with an insertion of a Co layer at the Cu/NiFe interface. An L-shaped device was designed and patterned which could be used to study the magnetisation reversal in the NiFe free layer. This was done by tuning the RKKY coupling between the Co and NiFe, shape anisotropy of the patterned micro-wire and the Anisotropic Magneto-Resistance ratio of the NiFe in the pseudo-spin-valve

Spin transport across antiferromagnets induced by the spin Seebeck effect

For prospective spintronics devices based on the propagation of pure spin currents, antiferromagnets are an interesting class of materials that potentially entail a number of advantages as compared to ferromagnets. Here, we present a detailed theoretical study of magnonic spin current transport in ferromagnetic-antiferromagnetic multilayers by using atomistic spin dynamics simulations. The relevant length scales of magnonic spin transport in antiferromagnets are determined. We demonstrate the transfer of angular momentum from a ferromagnet into an antiferromagnet due to the excitation of only one magnon branch in the antiferromagnet. As an experimental system, we ascertain the transport across an antiferromagnet in YIG$|$Ir$_{20}$Mn$_{80}|$Pt heterostructures. We determine the spin transport signals for spin currents generated in the YIG by the spin Seebeck effect and compare to measurements of the spin Hall magnetoresistance in the heterostructure stack. By means of temperature-dependent and thickness-dependent measurements, we deduce conclusions on the spin transport mechanism across IrMn and furthermore correlate it to its paramagnetic-antiferromagnetic phase transition.Comment: 10 pages, 6 figure

Gilbert damping of CoFe-alloys

We report structural, magnetic and dynamic properties of polycrystalline coxfe1-x-alloy films on sapphire, silicon, and mgo substrates across the full composition range, by using a vector network analyser ferromagnetic resonance measurement technique (vna-fmr), superconducting quantum interference device magnetometry (squid) and x-ray diffraction (xrd). in the approximate vicinity of 28% co, we observe a minimum of the damping parameter, associated with a reduction in the density of states to a minimum value at the fermi energy level. for films on all substrates, we find magnetic damping of the order of 4-5 . 10(-3), showing that the substrate does not play a major role. using a post-annealing process, we report a decrease of the magnetic damping down to 3-4 . 10(-3). we find that the saturation magnetization depends approximately reciprocally on the damping parameter with varying alloy composition

Current induced chiral domain wall motion in CuIr/CoFeB/MgO thin films with strong higher order spin-orbit torques

We investigate the Dzyaloshinskii-Moriya interaction (DMI) and spin-orbit torque effects in cuir/cofeb/mgo heterostructures. to this end, harmonic hall measurements and current induced domain wall motion experiments are performed. the motion of domain walls at zero applied field due to current demonstrates the presence of dmi in this system. we determine the strength of the dmi to be d= 5 /- 3 mu j/m2 and deduce right-handed chirality in domain walls showing a partial neel type spin structure. to ascertain the torques, we perform a second harmonic measurement to quantify the damping- and field-like current induced effective fields as a function of the magnetization direction. from the angular dependent analysis, we identify non-negligible higher order terms for polar magnetization angles theta>0, which need to be included when considering the effective manipulation of spins by current

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy.

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current js arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto js because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge