A. K. Petford-Long

Atomic scale structure of sputtered metal multilayers

A combined theoretical and experimental approach has been used to study nanoscale CoFe/Cu/CoFe multilayer films grown by sputter deposition. Such films have applications in sensors that utilize the giant magnetoresistance effect, for example, read heads in high-density information storage devices. Atomistic simulations based on a molecular dynamics approach and an alloy form of the embedded atom method have been developed to accurately model the sputter deposition of the CoFe/Cu/CoFe multilayers. The simulations show that relatively flat interfaces are formed because of the energetic deposition conditions. However, significant intermixing at the CoFe-on-Cu interface, but not at the Cu-on-CoFe interface, was observed. An abnormal Fe depletion zone is also revealed at the CoFe-on-Cu interface. A three-dimensional atom probe method has been used for a nanoscale chemical analysis of the films. It provided direct verification of the simulations. The simulations have then been used to understand the mechanism responsible for the formation of the intermixing defects observed in the multilayers. A novel deposition technique is proposed which reduces both interfacial mixing and Fe depletion by controlling the incident adatom energies.

Field-ion specimen preparation using focused ion-beam milling

Preparation of field-ion specimens from various materials has been accomplished using focused ion-beam milling in either a simple cutting mode or by application of an annular-shaped ion-milling pattern. These specimens have been investigated using field-ion microscopy and three-dimensional atom probe analysis. In the cutting mode, gallium implantation is minimised when using a lower beam energy. However, with annular milling, using 30 keV ions opposed to 10 keV ions results in less gallium implantation and produces a smaller shank angle and a sharper apex radius. High-dose ion imaging at 30 keV ion energy, even with relatively low beam currents, results in excessive implantation during field-ion specimen fabrication. Focused ion-beam milling provides not only an alternative method of field-ion sample preparation, but also, in conjunction with atom probe analysis, allows the quantitative investigation of the gallium implantation and damage which occurs during the milling.

Determination of the isothermal nucleation and growth parameters for the crystallization of thin Ge2Sb2Te5 films

The isothermal crystallization of thin amorphous Ge2Sb2Te5 films, sandwiched between Si3N4 dielectric layers, was followed in real time using in situ transmission electron microscopy. A temperature-dependent incubation time is observed. After this incubation time, the crystallization is found to follow Johnson–Mehl–Avrami–Kolmogorov (JMAK) transformation kinetics. The JMAK parameters were determined, as well as the individual nucleation and growth parameters. The relationships between the JMAK parameters and the nucleation and growth parameters were tested and found to be valid. Nucleation was found to occur at the interfaces with the dielectric layers. The average grain size after crystallization did not show appreciable temperature dependence in the temperature range investigated.

Spin injection from the Heusler alloy Co2MnGe into Al0.1Ga0.9As∕GaAs heterostructures

Electrical spin injection from the Heusler alloy Co2MnGe into a p-i-nAl0.1Ga0.9As∕GaAs light emitting diode is demonstrated. A maximum steady-state spin polarization of approximately 13% at 2 K is measured in two types of heterostructures. The injected spin polarization at 2 K is calculated to be 27% based on a calibration of the spin detector using Hanle effect measurements. Although the dependence on electrical bias conditions is qualitatively similar to Fe-based spin injection devices of the same design, the spin polarization injected from Co2MnGe decays more rapidly with increasing temperature.

Lorentz transmission electron microscopy of focused ion beam patterned magnetic antidot arrays

A focused-ion beam has been used to pattern magnetic antidot arrays containing different sized antidots and spacings in Ni80Fe20 films. Their magnetic domain structure has been studied using Lorentz transmission electron microscopy, which has shown that the edges of the antidots pin the domain walls during magnetization reversal. The remanent domain structure was found to depend strongly on the antidot size and spacing, from domains pinned between the corners of adjacent rows of antidots for large sizes to domains pinned between the edges of adjacent antidots for small sizes. The relevance of such structures for high density recording is discussed.

Structural studies of Ag nanocrystals embedded in amorphous Al2O3 grown by pulsed laser deposition

Thin films consisting of layers of Ag nanocrystals (NCs) embedded in amorphous Al2O3 were grown by pulsed laser deposition. High-resolution electron microscopy was used to characterize the structure of the films. The growth kinetics of the NCs were studied by varying the Ag content of the films between 0.8 and 12.4×1015 atoms cm-2 which produced NCs with average diameters of between 1.1 and 9.6 nm. At low Ag content the NCs have a spherical in-plane shape with a narrow size distribution but they become more elongated with a broader size distribution as the Ag content increases due to coarsening and coalescence of the NCs. Underneath each layer of NCs there is a continuous layer of what is believed to be Ag implanted into the amorphous Al2O3 due to the high kinetic energy, of the order of 100 eV, of the Ag species produced during laser ablation.

Atomic-scale analysis of CoFe/Cu and CoFe/NiFe interfaces

Internal interfaces in metallic multilayers grown on planar silicon substrates have been chemically analyzed with atomic resolution using three-dimensional atom probe microscopy. The structure studied was a NiFe/CoFe/Cu/CoFe multilayer grown with (111) texture. Atom probe measurements across the NiFe/CoFe interfaces yield widths of 1.1±0.2 nm for NiFe grown on CoFe and 1.7±0.2 nm for CoFe grown on NiFe. The widths of interfaces between CoFe and Cu layers vary as well, with values of 0.82±0.10 nm for CoFe grown on Cu, but only 0.47±0.15 nm for Cu grown on CoFe. In addition, the Fe concentration is enriched at the interface where Cu is grown on CoFe, and depleted where CoFe is grown on Cu. These results indicate that the Fe segregates to the surface during the deposition of CoFe so that the composition at the top of this layer is Fe rich.

Effect of Ga implantation on the magnetic properties of permalloy thin films

Several regions of a 30 nm thick Ni80Fe20 (permalloy) thin film have been implanted using a focused ion-beam system with varying doses of Ga ions. Changes to their magnetization reversal have been investigated by Lorentz microscopy. Implantation with a dose of 1015 Ga+ cm−2 and 1016 Ga+ cm−2 has increased coercivity of the implanted region while no difference has been observed with a dose of 1014 Ga+ cm−2. Domain wall pinning was shown to be a major mechanism for the increase in coercivity. Also changes to the lattice parameter, composition, thickness and grain size in relation to magnetic properties were investigated using various transmission electron microscopy techniques. The lattice parameter change within the implanted region indicates that the stress can be a major contributor to the increase in coercivity. Relative contributions of changes in other factors such as Ni/Fe ratio, thickness, and grain size within the implanted area have also been discussed.

Quantitative mapping of the magnetic induction distribution using Foucault images formed in a transmission electron microscope

A method of producing quantitative maps of the magnetic induction distribution using a conventional transmission electron microscope (TEM) is presented. The magnetic induction maps are constructed from four series of Foucault images taken with small incremental tilts of the incident illumination. By reciprocity, this is equivalent to differential phase contrast (DPC) which is usually performed in a modified scanning transmission electron microscope (STEM), and whilst maintaining all the benefits of its STEM counterpart, the DPC images formed by the method presented in this paper are particularly suited to studying thick magnetic films. Unlike the other quantitative Lorentz imaging techniques, the magnetic induction maps constructed from Foucault images can be produced on a standard TEM modified with a low-field pole-piece. Images from two types of cross-tie domain wall are shown as examples which have a spatial resolution of approximately 8 nm.

Focused ion-beam specimen preparation for atom probe field-ion microscopy characterization of multilayer film structures

Focused ion-beam milling with a sub-10 nm diameter beam of gallium ions has been used to fabricate field-ion specimens from a multilayer film nanostructure containing 100 repetitions of a bilayer deposited directly onto a planar substrate. Successful field-ion specimen preparation has allowed the observation of these layers on the atomic scale by both field-ion imaging and atom probe compositional analysis.