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What We Do

Why ultrathin films?

The magnetic properties of ultrathin film structures (typically of a few atomic layers in thickness) depend on their atomic scale structure: the presence of atomic steps, islands, etc. can have a surprisingly large effect on the magnetic properties. For example, the preferred orientation of atomic spins in the vicinity of atomic steps can be changed by 90 degrees due to the surprisingly large magnetic field associated with the steps. Increasingly, it is recognised that precise control of the structure and composition of interfaces at the atomic level is required in order to both investigate the fundamental magnetic properties of ultrathin films and to control magnetic properties for applications. Examples of key applications include: giant magnetoresistive sensors based on ultrathin magnetic multilayer structures (spin valve structures) for miniature hard disc read heads, spin valve structures for magnetic random access memories, perpendicular anisotropy films for magneto-optic media.

Our approach is to control the structure of the evolving magnetic film using advances growth and structural characterisation techniques together with state of the art, highly sensitive magnetic measurements during the growth. The main issue addressed by these in-situ studies of MBE-grown epitaxial ultrathin films is how do magnetic (and magneto-transport) properties evolve at the interface? By correlating the evolution of magnetic properties with structural parameters (film thickness, strain, crystallographic structure) it is possible to elucidate the main mechanisms which give rise to novel magnetic properties.

For an introduction to this field see "Ultrathin Magnetic Structures," 2 Volumes, J.A.C. Bland and B. Heinrich, Springer Verlag, Berlin (1994); ISBN 3-540-57407-7, 3-387-57407-7.

Recent research highlights of the TFM group (see recent publications):

  • Spin engineering in ultrathin Co/Cu(110) epitaxial thin films
  • Strain induced anisotropies in Co/Ni structures
  • Evidence for spin transport at the ferromagnet/semiconductor interface
  • Epitaxial growth of Fe/InAs ohmic contacts with in-plane magnetic anisotropy

Why nanoscale structures?

Much current technological and scientific interest now focuses on the properties of nanostructures (artificial structures with dimensions on the nm scale) created by lithographic techniques originally developed for semiconductor technology, or by advanced growth techniques such as self organisation or manipulation using the tip of STM or AFM. For example, as data densities increase, the characteristic dimensions for magnetic bits written in computer hard discs are already measured in nm. Interest in developing magnetic sensors for future ultra high density magnetic storage media, non-volatile random access memory, etc. now requires that the magnetic properties of sub-micron structures needs to be both controlled and understood. Additionally, the dynamic magnetic reversal process is dramatically changed as the characteristic dimensions approach te superparamagnetic stability limit (corresponding to mean bit dimensions in magnetic media of around 50-100 nm). Interest therefore shifts to the possibility of developing nanostructured magnetic materials with dimension in the nm range, whereby the shape, thickness and magnetic properties are controlled in order to create ultrasmall magnetic structures which remain stable. Such artificial structures can be envisioned as 'giant spins' in which the atomic spins look together to create one single giant spin which is stable due to the shape and surface magnetic anisotropy barriers, in analogy with the single domain particle in 3D. Each such nanomagnet could encode one 'bit' of information. Such structures could also be used in magnetoelectronic devices and magnetic random memory.

The main issue in our magnetic mesostructure studies is: how does the lateral size of high quality magnetic thin film structures in sub-micron to nm range (lithographically defined elements) control magnetic and transport properties? By using magnetic microscopies (MFM, Kerr microscopy) in conjunction with magnetotransport measurements which are highly sensitive to the local spin configuration, the spin configurations of artificial magnetic mesostructures are being studied. A key goal is to create spin coherent structures in which conventional domains are removed and a few or single domain walls act as quasiparticles which can be trapped or manipulated using external magnetic fields. An important part of our effort is to carry out dynamical studies up to the GHz range. In parallel, computational micromagnetic simulations are being carried out in order to interpret the experimental results.

Recent research highlights of the TFM group (see recent publications):

  • Domain wall trapping in a NiFe cross structure
  • Dynamic scaling of the magnetic hysteresis in permalloy dot structures
  • Geometric scaling of the transition from single domain to multidomain states in Fe(001) planar squares
Bio Project

 
Co/Cu(001) and Ni/Cu(001) epitaxial films

 
Commercial exploitation

 
Crystallization of Au thin films on Si-based substrates by annealing for self assembly monolayer

 
Ex situ and in situ Brillouin light scattering

 
Fe-semiconductor interface magnetic moments

Polarized neutron reflection studies in continuous films: Fe magnetic moment at a GaAs and InAs semiconductor interface

Ferromagnetic rings

Investigation of the magnetic states and switching processes in ferromagnetic rings.

Functionalization of high critical temperature cuprate superconductors for nanoelectronic devices

 
Global Encoding and Magnetic Characterisation

 
In situ magnetoresistance of nanostructures

Measurement of the tunnelling magnetoresistance in Fe/GaAs nanoclusters for various temperatures (100 - 300 K).

Magnetic Mesostructures

 
Magnetic phase plate for transmission electron microscopy

 
Magnetic Tag Designs: Planar and Pillar Structures

 
Measuring spin-injection and detection

 
Mesoscopic structures

 
Micro and nano magnetic particles in liquid suspension

 
Microfluidic integration - reading in flow

 
Nanogaps and nanocontacts for nanoelectronics

 
Perpendicular anisotropy nanodots

Theoretical/computational and experimental study of high-symmetry stable states in disc-shaped FePt particles.

Previous Projects

 

Also in this section

An introduction to nanomagnets

 
Data storage in computer technology

 
What is giant magnetoresistance?