Abstract
Remarkable magnetic and spin-dependent transport properties arise from well-designed spintronic materials and heterostructures. Half-metallic Heusler alloys with high spin polarization exhibit properties that are particularly advantageous for the development of high-performance spintronic devices. Here, we report fully (001)-epitaxial growth of a high-quality half-metallic NiMnSb half-Heusler alloy films, and their application to current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with Ag spacer layers. Fully (001)-oriented NiMnSb epitaxial films with very flat surface and high magnetization were prepared on Cr/Ag-buffered MgO(001) single crystalline substrates by changing the substrate temperature. Epitaxial CPP-GMR devices using the NiMnSb films and a Ag spacer were fabricated, and room-temperature (RT) CPP-GMR ratios for the C1b-type half-Heusler alloy were determined for the first time. A CPP-GMR ratio of 8% (21%) at RT (4.2 K) was achieved in the fully epitaxial NiMnSb/Ag/NiMnSb structures. Furthermore, negative anisotropic magnetoresistance (AMR) ratio and small discrepancy of the AMR amplitudes between RT and 10 K were observed in a single epitaxial NiMnSb film, indicating robust bulk half metallicity against thermal fluctuation in the half-Heusler compound. The modest CPP-GMR ratios could be attributed to interface effects between NiMnSb and Ag. This work provides a pathway for engineering a new class of ordered alloy materials with particular emphasis on spintronics.
Highlights
We open a new field in Heusler spintronics, i.e. half-Heusler compounds for current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices, and room-temperature CPP-GMR ratios are achieved in fully epitaxial NiMnSb/Ag/NiMnSb structures for the first time
When the Tsubstrate increases to 473− 673 K, the single phase and buffered MgO(001) single crystalline substrates at Tsubstrate varying from 300 to 773 K. (f) Extracted out-ofplane X-ray diffraction (XRD) data in a zero-dimensional region for the samples. (g,h) In-plane (φ-scan) XRD patterns of the NiMnSb(202) plane and NiMnSb(111) plane for the sample with the structure of MgO(001)-substrate//Cr(20)/ Ag(40)/NiMnSb(50) at Tsubstrate = 573 K, which were obtained by tilting the sample plane to χ = 45° and 54.7°, respectively
Well-grown NiMnSb films are of particular significance for maintaining the half metallic property as well as reducing the spin-flip scattering at the NiMnSb/Ag interface for successful demonstration of CPP-GMR devices
Summary
For half-Heusler alloys, the origin of the bandgap is the hybridized states of elements X and Y, which can form a very large bandgap between the bonding (t2g) and antibonding (eg) degenerates. Previous studies have demonstrated that the half-metallic properties of half-Heusler alloys can be very degraded by disordering and the presence of defects and/or surface segregation and termination atoms, and a few percent CPP-GMR ratio was only observed at low temperature (4.2 K) in a polycrystalline NiMnSb/Cu/NiFe system. Previous studies have demonstrated that the half-metallic properties of half-Heusler alloys can be very degraded by disordering and the presence of defects and/or surface segregation and termination atoms, and a few percent CPP-GMR ratio was only observed at low temperature (4.2 K) in a polycrystalline NiMnSb/Cu/NiFe system25 This indicates that a precise control over the ordering and structure of half-Heusler alloys is highly desired. Anisotropic magnetoresistance (AMR) measurement in a single epitaxial NiMnSb film indicates robust half metallicity against thermal fluctuation in the half-Heusler compound
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