J. Sagar

Effect of grain size on exchange-biased Heusler alloys

We report on an investigation into the effects of grain size of both antiferromagnetic IrMn and ferromagnetic Heusler alloy layers on the magnetic properties of exchange-biased films. IrMn/Co2FeSi was grown by a HiTUS sputtering system which allows control of the grain size. We found that small IrMn grains (?7?nm) could not generate an exchange bias Hex, while those above 8?nm in size showed Hex between 100 and 200?Oe. Hex showed a minor decrease with increasing Co2FeSi grain sizes (up to 15?20?nm) but Ms gradually increased. Our results show that sharp interfacial matching is required between 8 and 10?nm IrMn grains and 15?nm Co2FeSi grains to exhibit both large Hex and Ms for device applications.

The Effect of Cobalt-Sublattice Disorder on Spin Polarisation in Co2FexMn1−xSi Heusler Alloys

In this work we present a theoretical study of the effect of disorder on spin polarisation at the Fermi level, and the disorder formation energies for Co2FexMn1−xSi (CFMS) alloys. The electronic calculations are based on density functional theory with a Hubbard U term. Chemical disorders studied consist of swapping Co with Fe/Mn and Co with Si; in all cases we found these are detrimental for spin polarisation, i.e., the spin polarisation not only decreases in magnitude, but also can change sign depending on the particular disorder. Formation energy calculation shows that Co–Si disorder has higher energies of formation in CFMS compared to Co2MnSi and Co2FeSi, with maximum values occurring for x in the range 0.5–0.75. Cross-sectional structural studies of reference Co2MnSi, Co2Fe0.5Mn0.5Si, and Co2FeSi by Z-contrast scanning transmission electron microscopy are in qualitative agreement with total energy calculations of the disordered structures.

Effect of Interface Structure on Exchange Biased Heusler Alloy Films

We report on the effects of grain size in antiferromagnetic IrMn layers exchange bias to Co2FeSi. We also report on an enhanced effect where Mn layers are inserted in the interface. The exchange-biased IrMn/Co2FeSi samples were grown by a HiTUS system, which allows us to control the grain size. The smaller IrMn grains were too small to give a large Hex while an Mn layer 0.5 nm thick dramatically increased Hex. This significant increase is attributed to optimization of the Mn concentration at the interface. This grain-size and interface tuning offers a way to control the exchange bias in such systems.

Over 50% reduction in the formation energy of Co-based Heusler alloy films by two-dimensional crystallisation

Crystalline formation of high magnetic-moment thin films through low-temperature annealing processes compatible with current semiconductor technologies is crucial for the development of next generation devices, which can utilise the spin degree of freedom. Utilising in-situ aberration corrected electron microscopy, we report a 235 °C crystallisation process for a Co-based ternary Heusler-alloy film whose initial nucleation is initiated by as few as 27 unit cells. The crystallisation occurs preferentially in the ⟨111⟩ crystalline directions via a two-dimensional (2D) layer-by-layer growth mode; resulting in grains with [110] surface normal and [111] plane facets. This growth process was found to reduce the crystallisation energy by more than 50% when compared to bulk samples whilst still leading to the growth of highly ordered grains expected to give a high degree of spin-polarisation. Our findings suggest that the 2D layer-by-layer growth minimises the crystallisation energy allowing for the possible implementation of highly spin-polarised alloy films into current chip and memory technologies.

The effect of interfaces on magnetic activation volumes in single crystal Co2FeSi Heusler alloy thin films

Structural and magnetization reversal studies have been carried out on single crystal Co2FeSi thin films grown on MgO (001) substrates. The films are highly L21 ordered after annealing above 500 °C. Magnetization reversal has been investigated by measurements of the activation volumes (Vact) within the films. This volume represents the unit of reversal in a magnetic material. Vact (∼4 × 103 nm3) has been found to be independent of the physical structure. Vact is found to correspond to an array of periodic misfit dislocations at the Co2FeSi/MgO interface. Such a small Vact potentially prevents coherent magnetization reversal as required for giant magnetoresistance or tunnel magnetoresistance devices.

Activation Volumes in Co

Magnetic measurements and TEM analysis have been carried out in order to investigate the activation volume and its correlation with physical grain size within plasma sputtered Co2FeSi thin films. This has led to a new technique for estimating the volumes of ordered and disordered interfacial regions within granular Heusler alloy films. It has been shown that the activation volume has very little grain-size dependence, while the physical grain volume is seen to increase with bias voltage. This suggests that reversal within the films is a domain wall process, and the multistage reversal seen in those films with larger grain sizes is due to pinning of domain walls within the grains.

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We have prepared polycrystalline Co2FeSi thin films on a number of seed layers to optimize their structural and magnetic properties. Using a Cr/Ag combined seed layer, films have been produced with extremely low interfacial roughness (<1 nm) and controllable coercivities in the range 12–27 Oe. Such a structure would be suitable for the free layer in a spintronic device. Using a NiCr seed layer and IrMn as an antiferromagnetic layer a small exchange bias of ~30 Oe has been achieved. However the use of a 0.5 nm Mn layer at the IrMn/Co2FeSi interface increases the exchange bias (Hex) to 375 Oe after annealing. This structure would be suitable for the pinned layer in a spintronic device.

FeSi Thin Films

We have fabricated and investigated IrMn3/Co2FeAl0.5Si0.5 stacks to meet the criteria for future spintronic device applications which requires low-temperature crystallisation ( 500 Oe). Such a system would form the pinned layer in spin-valve or tunnel junction applications. We have demonstrated that annealing at 300 °C which can achieve crystalline ordering in the Co2FeAl0.5Si0.5 layer giving ∼80% of the predicted saturation magnetisation. We have also induced an exchange bias of ∼240 Oe at the interface. These values are close to the above criteria and confirm the potential of using antiferromagnet/Heusler-alloy stacks in current Si-based processes.

Growth of polycrystalline Heusler alloys for spintronic devices

For the implementation of Heusler-alloy films into next-generation magnetic memories and storages, the optimization of their polycrystalline nature is critical. In this review, we identify two key parameters for the optimization; grain volume for interfacial magnetism and activation volume for magnetic dynamics. We establish correlations between these structural and magnetic volumes and magnetic behavior and then optimize these properties. The optimized polycrystalline films possess exchange bias of 250 Oe induced by low-temperature annealing (400°C for 30 min.), which is induced by the grain volume matching between the Heusler-alloy and antiferromagnetic layers. Despite the matching size measuring 7–8 nm in diameter, the magnetic activation volumes are estimated to be almost 80 nm which is one order of magnitude greater than that for epitaxial films. These values satisfy the requirements for next-generation spintronic device applications and unambiguously prove the great potential of the polycrystalline Heusler-alloy films for future spintronic applications.

Optimization of exchange bias in Co2FeAl0.5Si0.5 Heusler alloy layers

We report on the control of grain development and its effect on magnetic properties in polycrystalline Co2FeSi thin films through use of Ag layers. Thin film polycrystalline samples consisting of a 20 nm Co2FeSi layer deposited onto a 6 nm Ag seed layer have been fabricated using high target utilization sputtering. After thermal annealing at 300°C for 6 hours these films have median grain diameters of between 30 and 40 nm which are constrained by island like growth of the Ag Seed layer. This constraint of grain growth results in films with coercivities of no greater than 40 Oe and values of saturation magnetization exceeding 80% of the theoretical bulk value. The use of Ag seed layers is thus shown to reduce coercivities by almost 90% compared with previously reported similar materials making them much more suitable for device applications.