Zhenchao Wen

Perpendicular magnetization of Co2FeAl full-Heusler alloy films induced by MgO interface

The perpendicular magnetization of Co2FeAl (CFA) full-Heusler alloy films was achieved in the structures of CFA/MgO and MgO/CFA with the perpendicular magnetic anisotropy energy density (KU) of 2–3×106 erg/cm3, which can be used as the perpendicular ferromagnetic electrodes of MgO-based magnetic tunnel junctions (MTJs) with high thermal stability at sub-50-nm dimension. The CFA thickness dependence of KU was investigated at different annealing temperatures, indicating that the perpendicular anisotropy of CFA is contributed by the interfacial anisotropy between CFA and MgO. This letter will open up a way for obtaining perpendicular magnetization of Co-based full-Heusler alloys, which is promising for further reduction in the critical current of current induced magnetization switching in MgO-based MTJ nanopillars with perpendicular full-Heusler alloy electrodes.

A 4‐Fold‐Symmetry Hexagonal Ruthenium for Magnetic Heterostructures Exhibiting Enhanced Perpendicular Magnetic Anisotropy and Tunnel Magnetoresistance

A 4-fold-symmetry hexagonal Ru emerging in epitaxial MgO/Ru/Co2 FeAl/MgO heterostructures is reported, in which an approximately Ru(022¯3) growth attributes to the lattice matching between MgO, Ru, and Co2 FeAl. Perpendicular magnetic anisotropy of the Co2 FeAl/MgO interface is substantially enhanced. The magnetic tunnel junctions (MTJs) incorporating this structure give rise to the largest tunnel magnetoresistance for perpendicular MTJs using low damping Heusler alloys.

Large perpendicular magnetic anisotropy at Fe/MgO interface

A large perpendicular magnetic anisotropy (PMA) of 1.4 MJ/m3 was observed from ultrathin Fe/MgO(001) bilayers grown on Cr-buffered MgO(001). The PMA strongly depends on the surface state of Fe prior to the MgO deposition. A large PMA energy density of 1.4 MJ/m3 was achieved for a 0.7 nm thick Fe layer having adsorbate-induced surface reconstruction, which is likely to originate from oxygen atoms floating up from the Cr buffer layer. This large magnitude of PMA satisfies the criterion that is required for thermal stability of magnetization in a few tens nanometer-sized magnetic memory elements.

Magnetic Tunnel Junctions with Perpendicular Anisotropy Using a Co2FeAl Full-Heusler Alloy

We fabricated perpendicularly magnetized magnetic tunnel junctions (p-MTJs) with an ultrathin Co2FeAl (CFA) full-Heusler alloy electrode having large interface magnetic anisotropy of CFA/MgO. An out-of-plane tunnel magnetoresistance (TMR) ratio of 53% at room temperature was observed in CFA/MgO/Co20Fe60B20 p-MTJs. By inserting a 0.1-nm-thick Fe (Co50Fe50) layer between the MgO and Co20Fe60B20 layers, The TMR ratio was significantly enhanced to 91% (82%) due to the improved interface. The bias voltage dependence of differential conductance did not clearly show coherent tunneling characteristics for ultrathin CFA-MTJs, suggesting that a higher TMR ratio may be achieved by improving the B2 ordering of CFA and/or interface structure.

Spin-transfer switching in full-Heusler Co2FeAl-based magnetic tunnel junctions

We demonstrated spin-transfer magnetization switching using magnetic tunnel junctions (MTJs) with a full-Heusler alloy Co2FeAl (CFA). We prepared CFA (1.5 nm)/MgO/CoFe (4 nm) (“CFA-free”) and CFA (30 nm)/MgO/CoFeB (2 nm) (“CFA-reference”) MTJs on a Cr(001) layer. The intrinsic critical current density (Jc0) of the CFA-free (CFA-reference) MTJ was 29 MA/cm2 (7.1 MA/cm2). The larger Jc0 of the CFA-free MTJ is attributed to the significant enhancement of the Gilbert damping factor (∼0.04) of the CFA due to the Cr layer. The Jc0 of the CFA-reference is as small as that reported for typical CoFeB/MgO/CoFeB MTJs.

Tunnel magnetoresistance in textured Co2FeAl/MgO/CoFe magnetic tunnel junctions on a Si/SiO2 amorphous substrate

Magnetic tunnel junctions with B2-ordered Co2FeAl full Heusler alloy as a ferromagnetic electrode were fabricated by sputtering on thermally oxidized Si/SiO2 amorphous substrates. A Co2FeAl/MgO/Co50Fe50 structure showed a highly (001)-textured structure and the tunneling magnetoresistance (TMR) ratio of 166% at room temperature and 252% at 48 K were achieved. The temperature dependence of TMR can be fitted with spin wave excitation model, and the bias voltage dependence of differential conductance demonstrated that the high TMR was mainly contributed by coherent tunneling. This work suggests the B2-Co2FeAl is one of the promising candidates for practical spintronic applications.

Large anisotropic Fe orbital moments in perpendicularly magnetized Co2FeAl Heusler alloy thin films revealed by angular-dependent x-ray magnetic circular dichroism

Perpendicular magnetic anisotropy (PMA) in Heusler alloy Co2FeAl thin films sharing an interface with a MgO layer is investigated by angular-dependent x-ray magnetic circular dichroism. Orbital and spin magnetic moments are deduced separately for Fe and Co 3d electrons. In addition, the PMA energies are estimated using the orbital magnetic moments parallel and perpendicular to the film surfaces. We found that PMA in Co2FeAl is determined mainly by the contribution of Fe atoms with large orbital magnetic moments, which are enhanced at the interface between Co2FeAl and MgO. Furthermore, element specific magnetization curves of Fe and Co are found to be similar, suggesting the existence of ferromagnetic coupling between Fe and Co PMA directions.

Tunnel Magnetoresistance and Spin-Transfer-Torque Switching in PolycrystallineCo2FeAlFull-Heusler-Alloy Magnetic Tunnel Junctions on AmorphousSi/SiO2Substrates

We studied polycrystalline B2-type Co2FeAl (CFA) full-Heusler alloy based magnetic tunnel junctions (MTJs) fabricated on a Si/SiO2 amorphous substrate. Polycrystalline CFA films with a (001) orientation, a high B2 ordering, and a flat surface were achieved using a MgO buffer layer. A tunnel magnetoresistance (TMR) ratio up to 175% was obtained for an MTJ with a CFA/MgO/CoFe structure on a 7.5-nm-thick MgO buffer. Spin-transfer torque induced magnetization switching was achieved in the MTJs with a 2-nm-thick polycrystalline CFA film as a switching layer. Using a thermal activation model, the intrinsic critical current density (Jc0) was determined to be 8.2 x 10^6 A/cm^2, which is lower than 2.9 x 10^7 A/cm^2, the value for epitaxial CFA-MTJs [Appl. Phys. Lett. 100, 182403 (2012)]. We found that the Gilbert damping constant evaluated using ferromagnetic resonance measurements for the polycrystalline CFA film was ~0.015 and was almost independent of the CFA thickness (2~18 nm). The low Jc0 for the polycrystalline MTJ was mainly attributed to the low damping of the CFA layer compared with the value in the epitaxial one (~0.04).

Lattice-matched magnetic tunnel junctions using a Heusler alloy Co2FeAl and a cation-disorder spinel Mg-Al-O barrier

Perfectly lattice-matched magnetic tunnel junctions (MTJs) consisting of a Heusler alloy B2-Co2FeAl (CFA) electrode and a cation-disorder spinel (Mg-Al-O) barrier were fabricated by sputtering and plasma oxidation. We achieved a large tunnel magnetoresistance (TMR) ratio of 228% at room temperature (RT) (398% at 5 K) for the epitaxial CFA/MgAl-O/CoFe(001) MTJ, in which the effect of lattice defects on TMR ratios is excluded. With inserting a ultrathin (≤1.5 nm) CoFe layer between the CFA and Mg-Al-O, the TMR ratio further increased up to 280% at RT (453% at 5 K), which reflected the importance of controlling barrier-electrode interface states other than the lattice matching.

Fully epitaxial C1b-type NiMnSb half-Heusler alloy films for current-perpendicular-to-plane giant magnetoresistance devices with a Ag spacer.

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.