Abstract
ABSTRACT 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.
Highlights
Spintronics has been initiated by the discovery of giant magnetoresistance (GMR) by Fert et al [1] and Grünberg et al [2] independently
Ni2MnGe(001)/GaAs(001) [163,165] and Ni2 MnIn(001)/InAs(001) [166] hybrid structures are fabricated for evaluation. Their interfaces are reported to be very sensitive to the growth temperature: Interfacial mixture occurs at the growth temperature of 373 K, while a large number of planer defects are formed at 433 K for Ni2MnGe/GaAs [163]
Mössbauer spectroscopy can be applied for studies of interfaces in layered film samples, which is useful for optimisation of spintronic devices, because magnetic properties at the interface sensitively affect the spin-dependent transport in magnetic tunnel junction (MTJ) or CPP-GMR junctions
Summary
Spintronics has been initiated by the discovery of giant magnetoresistance (GMR) by Fert et al [1] and Grünberg et al [2] independently. Further improvement has been made to satisfy 10 Gbit MRAM target [22], with satisfying a TMR ratio >100% and RA ~ 2 Ω·μm2 These MTJs under development are expected to replace the current-generation 256 Mbit MRAM with perpendicular magnetic anisotropy produced by Everspin [23]. For further improvement in the MR junctions to meet the requirements beyond 10 Gbit MRAM and 2 Tbit/in HDD, a half-metallic ferromagnet (HMF) needs to be developed to achieve 100% spin polarisation at the Fermi energy level (EF) at RT [27], leading to an infinite MR ratio using Eq (1). The Heusler alloy films can be the most promising candidate for the RT halfmetallicity due to their lattice constant matching with major substrates, high TC and large δ at EF in general as detailed
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