12 research outputs found
Time resolved and time average imaging of magnetic nano-structures
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
Time resolved scanning Kerr microscopy of flux beam formation in hard disk write heads
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.The underlying research dataset supporting this publication is available under a Creative
Commons Attribution-ShareAlike 4.0 International License (https://creativecommons.
org/licenses/by-sa/4.0/) and can be publicly accessed in Open Research Exeter via the
following persistent identifier: http://hdl.handle.net/10871/21108.To meet growing data storage needs, the density of data stored on hard disk drives
must increase. In pursuit of this aim the magnetodynamics of the hard disk write head
must be characterized and understood, particularly the process of “flux beaming”. In
this study, seven different configurations of perpendicular magnetic recording (PMR)
write heads were imaged using time-resolved scanning Kerr microscopy, revealing
their detailed dynamic magnetic state during the write process. It was found that
the precise position and number of driving coils can significantly alter the formation
of flux beams during the write process. These results are applicable to the design and
understanding of current PMR and next-generation heat-assisted magnetic recording
(HAMR) devices, as well as being relevant to other magnetic devices.The authors gratefully acknowledge financial support from the Seagate Plan
Ultrafast time-evolution of chiral N\'eel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering
Non-collinear spin textures in ferromagnetic ultrathin films are attracting a
renewed interest fueled by possible fine engineering of several magnetic
interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This
allows the stabilization of complex chiral spin textures such as chiral
magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report
here on the ultrafast behavior of chiral DWs after optical pumping in
perpendicularly magnetized asymmetric multilayers, probed using time-resolved
circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe
a picosecond transient reduction of the CD-XRMS, which is attributed to the
spin current-induced coherent and incoherent torques within the continuously
dependent spin texture of the DWs. We argue that a specific demagnetization of
the inner structure of the DW induces a flow of hot spins from the interior of
the neighboring magnetic domains. We identify this time-varying change of the
DW textures shortly after the laser pulse as a distortion of the homochiral
N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a
direction transverse to the DW. Our study highlights how time-resolved CD-XRMS
can be a unique tool for studying the time evolution in other systems showing a
non-collinear electric/magnetic ordering such as skyrmion lattices,
conical/helical phases, as well as the recently observed antiskyrmion lattices,
in metallic or insulating materials
Chiral spin spiral in synthetic antiferromagnets probed by circular dichroism in x-ray resonant magnetic scattering
International audienc
Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads (dataset)
Data behind the published paper "Time-resolved scanning Kerr microscopy of flux beam formation in hard disk write heads"Seagate Technology PL
Tilted X-Ray Holography of Magnetic Bubbles in MnNiGa Lamellae
Nanoscopic lamellae of centrosymmetric ferromagnetic alloys have recently been reported to host the biskyrmion spin texture; however, this has been disputed as the misidentication of topologically trivial type-II magnetic bubbles. Here we demonstrate resonant soft X-ray holographic imaging of topological magnetic states in lamellae of the centrosymmetric alloy (Mn1–xNix)0.65Ga0.35 (x = 0.5), showing the presence of magnetic stripes evolving into single core magnetic bubbles. We observe rotation of the stripe phase via the nucleation and destruction of disclination defects. This indicates the system behaves as a conventional uniaxial ferromagnet. By utilizing the holography with extended reference by autocorrelation linear dierential operator (HERALDO) method, we show tilted holographic images at 30° incidence confirming the presence of type-II magnetic bubbles in this system. This study demonstrates the utility of X-ray imaging techniques in identifying the topology of localized structures in nanoscale magnetism
Zadrževanje ključnih kadrov v podjetju X
International audienceTime-averaged images of the magnetization within single-layer spin-transfer oscillators have been obtained using the holography with extended reference by autocorrelation linear differential operator technique. Transport measurements on a Pd(5)-Cu(20)-Ni81Fe19(7)-Cu(2)-Pd(2) (in nanometers) stack with a 100 nm diameter nanocontact reveal the presence of vortex dynamics. Magnetic images of the device for injected current values of 24 and -24 mA suggest that a vortex has been ejected from the nanocontact and become pinned at the edge of the region that is visible through the Au mask
Do Images of Biskyrmions Show Type-II Bubbles?
The intense research effort investigating magnetic skyrmions and their applications for spintronics has yielded reports of more exotic objects including the biskyrmion, which consists of a bound pair of counter‐rotating vortices of magnetization. Biskyrmions have been identified only from transmission electron microscopy images and have not been observed by other techniques, nor seen in simulations carried out under realistic conditions. Here, quantitative Lorentz transmission electron microscopy, X‐ray holography, and micromagnetic simulations are combined to search for biskyrmions in MnNiGa, a material in which they have been reported. Only type‐I and type‐II magnetic bubbles are found and images purported to show biskyrmions can be explained as type‐II bubbles viewed at an angle to their axes. It is not the magnetization but the magnetic flux density resulting from this object that forms the counter‐rotating vortices