Low-temperature crystallisation of Heusler alloy films with perpendicular magnetic anisotropy

Science and Technology of Advanced Materials

et al. 2021

Self Cite

229

Heusler alloys are theoretically predicted to become half-metals at room temperature (RT). The advantages of using these alloys are good lattice matching with major substrates, high Curie temperature above RT and intermetallic controllability for spin density of states at the Fermi energy level. The alloys are categorised into half- and full-Heusler alloys depending upon the crystalline structures, each being discussed both experimentally and theoretically. Fundamental properties of ferromagnetic Heusler alloys are described. Both structural and magnetic characterisations on an atomic scale are typically carried out in order to prove the half-metallicity at RT. Atomic ordering in the films is directly observed by X-ray diffraction and is also indirectly probed via the temperature dependence of electrical resistivity. Element specific magnetic moments and spin polarisation of the Heusler alloy films are directly measured using X-ray magnetic circular dichroism and Andreev reflection, respectively. By employing these ferromagnetic alloy films in a spintronic device, efficient spin injection into a non-magnetic material and large magnetoresistance are also discussed. Fundamental properties of antiferromagnetic Heusler alloys are then described. Both structural and magnetic characterisations on an atomic scale are shown. Atomic ordering in the Heusler alloy films is indirectly measured by the temperature dependence of electrical resistivity. Antiferromagnetic configurations are directly imaged by X-ray magnetic linear dichroism and polarised neutron reflection. The applications of the antiferromagnetic Heusler alloy films are also explained. The other non-magnetic Heusler alloys are listed. A brief summary is provided at the end of this review.

Section: Introductionmentioning

confidence: 99%

Heusler alloys for spintronic devices: review on recent development and future perspectives

Elphick

Science and Technology of Advanced Materials

et al. 2021

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229

“…The CFAS(220) diffraction ring was observed at 5.1 nm −1 from the centre spot. Therefore, the lattice constant of CFAS is estimated to be 0.57 nm, which is 101.3% of the CFAS film grown at 673 K estimated by the corresponding XRD result previously 5 . The structural analysis via TEM imaging confirmed CFAS pillars after the current introduction was partially crystallised and maintained a smooth interface of < 1 nm roughness in the GMR junction.…”

Section: Resultsmentioning

confidence: 76%

“…A multilayer of W (20)/Co 2 FeAl 0.5 Si 0.5 (CFAS) Heusler-alloy (10)/W (3)/CFAS (5)/Ru (3) (thickness in nm) as optimised previously 5 was grown on a thermally oxidised Si substrate using a high target utilisation sputtering system (PlasmaQuest, HiTUS) at room temperature. The multilayer was patterned into a nanopillar junction using a combination of electron-beam lithography and Ar-ion milling using the same process as previously reported 4 , 5 . Here, a seed layer, Cr/W, was patterned into a bottom electrode with a width of 100–200 nm.…”

Section: Methodsmentioning

confidence: 99%

“…We have recently found a process to lower the crystallisation temperature of a ternary/quaternary Heusler alloy film by growing on the (110) plane 3 . Our latest study demonstrates that almost 85% of a Co 2 FeAl 0.5 Si 0.5 (CFAS) Heusler-alloy film can be crystallised at 353 K for 2 min 4 , 5 . In this study, we have demonstrated the crystallisation of a Heusler alloy film with the (110) surface in a giant magnetoresistive (GMR) junction by introducing a current pulse, of which amplitude is greater than the typical sensing current.…”

Section: Introductionmentioning

confidence: 95%

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Current-induced crystallisation in Heusler alloy films for memory potentiation in neuromorphic computation

Elphick

et al. 2021

Sci Rep

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The current information technology has been developed based on von Neumann type computation. In order to sustain the rate of development, it is essential to investigate alternative technologies. In a next-generation computation, an important feature is memory potentiation, which has been overlooked to date. In this study, potentiation functionality is demonstrated in a giant magnetoresistive (GMR) junction consisting of a half-metallic Heusler alloy which can be a candidate of an artificial synapse while still achieving a low resistance-area product for low power consumption. Here the Heusler alloy films are grown on a (110) surface to promote layer-by-layer growth to reduce their crystallisation energy, which is comparable with Joule heating induced by a controlled current introduction. The current-induced crystallisation leads to the reduction in the corresponding resistivity, which acts as memory potentiation for an artificial GMR synaptic junction.

“…A multilayer of W (10)-CFAS (12. using a combination of electron-beam lithography and Ar-ion milling using the same process as previously reported [19]. Here, a seed layer, Cr-W, was patterned into a bottom electrode with a width of 100-200 nm.…”

Section: Methodsmentioning

confidence: 99%

Perpendicular Anisotropy Controlled by Seed and Capping Layers of Heusler-Alloy Films

IEEE Trans. Electron Devices

Hirohata

2022

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Half-metallic Heusler alloys typically have in-plane magnetic anisotropy, which can be converted to perpendicular by attaching MgO or heavy metal, e.g., Pt, layers as similarly applied for conventional ferromagnets. Recently we have found body-centered cubic (bcc) seed layers, e.g., V and W, to induce perpendicular anisotropy in Heusler-alloy films above, however, they show small giant magnetoresistive (GMR) ratios in spin-valve structures to date. This is partially because of the large resistivity of the seed layer and the nonmagnetic layer in the spinvalve. In this study, we have systematically investigated nonmagnetic overlayers and have found that a Ag layer best maintains the perpendicular anisotropy. The corresponding GMR devices have then been fabricated and characterized, achieving the GMR ratio of ∼0.03% at room temperature. Such bcc seed layers can offer an alternative method for perpendicularly magnetized GMR junctions for applications.