Time resolved and time average imaging of magnetic nano-structures
Burgos Parra, Erick Omar
Date: 3 August 2018
Publisher
University of Exeter
Degree Title
PhD in Physics
Abstract
The ability of a ferromagnet to maintain its magnetic state in the absence of an external
magnetic field has made ferromagnetic materials an important subject of study
in physics since the end of the 19th century. Moreover, ferromagnetic materials are
the cornerstone for data storage systems such as magnetic tapes, magnetic disk ...
The ability of a ferromagnet to maintain its magnetic state in the absence of an external
magnetic field has made ferromagnetic materials an important subject of study
in physics since the end of the 19th century. Moreover, ferromagnetic materials are
the cornerstone for data storage systems such as magnetic tapes, magnetic disk drives
and magnetic random access memory. The discovery of the Giant Magneto Resistance
(GMR) in 1988 suggested that, since the magnetic state of the electrical conductor has
an important effect upon the current flow, there may also be an inverse influence of
the current upon the magnetization. In this effect, predicted in 1989 [1] by Slonczewski
and called Spin Transfer Torque, angular momentum transferred by a spin polarized
current can exert a torque on the magnetization of a ferromagnetic material, changing
the local magnetization and stimulating the precession of the magnetic moments,
generating microwave signals. This provides a new method of manipulating magnetization
without applying an external field. Large polarized currents lead to spin transfer
effects which are the driving force for the magnetic dynamics of devices known
as Spin Transfer Oscillators (STO). In this new kind of nano-device the emission of
microwaves is stimulated by a DC electrical current and measured as a change in the
output voltage due the GMR effect. The specific characteristics of these devices such as
working frequency and DC current ranges, microwave emission linewidth, and maximum
emission 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 upon
three of them: Spin Transfer Vortex Oscillators (STVO), Single Layer Spin Transfer Oscillators
(SL-STO) and Orthogonal Pseudo Spin Valves. Within STVOs and SL-STOs
we can nucleate what is called a magnetic vortex. A magnetic vortex is a curling of the
in-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 with
a greater emission power than that produced by the precession of magnetic moments
in STOs. The phase-locked synchronisation of multiple vortices is expected to exhibit
enhanced 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. On
the other hand, Orthogonal Pseudo Spin Valves promote the nucleation of magnetic
dissipative solitons, also called magnetic droplets. This type of magnetic structure has
an opposite out of plane magnetization to the layer that contains it. Compared to the
microwave emission of magnetic vortices , magnetic droplets have a higher frequency
range and emission power. However, their nucleation is subject to large external fields
being applied to the sample.
In this thesis, I electrically characterized these devices and applied magnetic imaging
techniques in order to go further in the understanding of the spatial features and
dynamic behaviour of these magnetic structures. It is not possible to acquire this
knowledge by only using electrical characterization. Understanding the magnetization
dynamics in these devices is crucial for the design of STO based devices while
imaging studies are required to prove the existence of these magnetic structures, as in
case of the magnetic droplet.
In chapter 2 I will introduce the background concepts of magnetism that are relevant
to this thesis. I will go from the basics principles of ferromagnetism, its quantum
mechanical treatment, and the theory that explain the dynamics of the magnetisation. I
will also present the state of the art in experimental research in the field of spin transfer oscillators.
My aim is to give the basic background needed to understand the results presented in this thesis.
In chapter 3 I will introduce the two main experimental techniques used for imaging
the magnetisation of the devices presented: Holography with Extended Reference
by Autocorrelation Linear Differential Operator (HERALDO) and Time Resolved Scanning
Kerr Microscopy (TRSKM). I will revise the theoretical background concepts and
the development of the techniques in order to demostrate the uniqueness of each technique
and how they were used in this thesis. It is interesting to note that while MOKE
is a well-known and widely-used technique, far fewer laboratories in the world area
able to perform time resolved measurements using MOKE, with the University of Exeter
being one of them. Furthermore, HERALDO is a novel technique that is used for
the first time to image magnetic structures within multilayer systems in this thesis,
which is a milestone in the development of the techinque.
In chapter 4 I present an investigation of the magnetization dynamics of a SL-STO.
Electrical transport measurements provided an initial characterization of the device.
We then used HERALDO for the first time to investigate the magnetization dynamics
in an intermediate layer of a multilayer stack. We present time averaged measurements
of the magnetisation of a magnetic vortex formed underneath a nano contact (NC)
positioned on top of the multilayer, using a combination of x-ray holography and x-ray
magnetic circular dichroism.
In chapter 5 I present the first direct measurement at the time of a magnetic dissipative
droplet, using holography with extended reference autocorrelation by linear
differential operator (HERALDO). I studied the out of plane magnetisation of the free
layer under a NC within an orthogonal pseudo spin salve.
In chapter 6 I present and study STVO devices with pairs of NCs of 100 nm diameter
and centre-to-centre separation D = 200 to 1100 nm, by a combination of electrical
measurements and time-resolved scanning Kerr microscopy (TRSKM). It will be
shown that the dynamic behaviour of vortices and anti vortices changes when the distances
between the NCs within the devices is changed.
Doctoral Theses
Doctoral College
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