Time Resolved and Time Average Imaging of Magnetic Nano-Structures

Abstract

The ability of a ferromagnet to maintain its magnetic state in the absence of an externalmagnetic field has made ferromagnetic materials an important subject of studyin physics since the end of the 19^th century. Moreover, ferromagnetic materials arethe cornerstone for data storage systems such as magnetic tapes, magnetic disk drivesand magnetic random access memory. The discovery of the Giant Magneto Resistance(GMR) in 1988 suggested that, since the magnetic state of the electrical conductor hasan important effect upon the current flow, there may also be an inverse influence ofthe current upon the magnetization. In this effect, predicted in 1989 [1] by Slonczewskiand called Spin Transfer Torque, angular momentum transferred by a spin polarizedcurrent can exert a torque on the magnetization of a ferromagnetic material, changingthe local magnetization and stimulating the precession of the magnetic moments,generating microwave signals. This provides a new method of manipulating magnetizationwithout applying an external field. Large polarized currents lead to spin transfereffects which are the driving force for the magnetic dynamics of devices knownas Spin Transfer Oscillators (STO). In this new kind of nano-device the emission ofmicrowaves is stimulated by a DC electrical current and measured as a change in theoutput voltage due the GMR effect. The specific characteristics of these devices such asworking frequency and DC current ranges, microwave emission linewidth, and maximumemission power among others, are given by the design and size of the device,and the nature of the magnetic oscillations producing the emission.

Among the multiple types of STO that now exist , I have focused my research uponthree of them: Spin Transfer Vortex Oscillators (STVO), Single Layer Spin Transfer Oscillators(SL-STO) and Orthogonal Pseudo Spin Valves. Within STVOs and SL-STOswe can nucleate what is called a magnetic vortex. A magnetic vortex is a curling of thein-plane of a magnetic layer with its centre pointing out of the magnetization plane.The gyration of this vortex due to STT produces a microwave emission < 1GHz witha greater emission power than that produced by the precession of magnetic momentsin STOs. The phase-locked synchronisation of multiple vortices is expected to exhibitenhanced microwaved power and phase stability compared to a single vortex device,providing a solution to the drawbacks of the STO in the low frequency regime. Onthe other hand, Orthogonal Pseudo Spin Valves promote the nucleation of magneticdissipative solitons, also called magnetic droplets. This type of magnetic structure hasan opposite out of plane magnetization to the layer that contains it. Compared to themicrowave emission of magnetic vortices , magnetic droplets have a higher frequencyrange and emission power. However, their nucleation is subject to large external fieldsbeing applied to the sample.

In this thesis, I electrically characterized these devices and applied magnetic imagingtechniques in order to go further in the understanding of the spatial features anddynamic behaviour of these magnetic structures. It is not possible to acquire thisknowledge by only using electrical characterization. Understanding the magnetizationdynamics in these devices is crucial for the design of STO based devices whileimaging studies are required to prove the existence of these magnetic structures, as incase of the magnetic droplet.