X. M. Zhang

Effect of Mg interlayer on perpendicular magnetic anisotropy of CoFeB films in MgO/Mg/CoFeB/Ta structure

The effects of Mg metallic interlayer on the magnetic properties of thin CoFeB films in MgO/Mg (tMg)/CoFeB (1.2 nm)/Ta structures were studied in this letter. Our experimental result shows that the CoFeB film exhibits perpendicular magnetic anisotropy (PMA) when the CoFeB and MgO layers are separated by a metallic Mg layer with a maximum thickness of 0.8 nm. The origin of PMA was discussed by considering the preferential transmission of the Δ1 symmetry preserved by the Mg interlayer in crystallized MgO/Mg/CoFeB/Ta. In addition, the thin Mg interlayer also contributes to enhancing the thermal stability and reducing the effective damping constant and coercivity of the CoFeB film.

Influence of microstructure on soft magnetic properties of low coreloss and high Bs Fe85Si2B8P4Cu1 nanocrystalline alloy

Microstructure and magnetic properties of high Bs Fe85Si2B8P4Cu1 nano-crystalline alloy were tailored. The Bs ∼ 1.85 T and low coercivity Hc ∼ 6 A/m were obtained by conventional annealing (i.e., one step) at a heating rate of ∼400 °C/min. While one step annealing process produces a fairly stable nanostructure, for short times at temperatures <425 °C, the nanostructure and concomitant magnetic properties of this alloy are shown to be tunable by two step annealing process. Nanocrystalline ribbons with grain size (D) ranging from ∼15 to 32 nm, Hc ∼ 6 to 140 A/m and core-loss at 1.5 T, 50 Hz ∼ 0.39 ∼ 5.0 W/kg were obtained. The Hc of Fe85Si2B8P4Cu1 alloy is shown to vary as D6, but a small deviation was noticed, which may be due to weak anisotropy.

Artificially engineered Heusler ferrimagnetic superlattice exhibiting perpendicular magnetic anisotropy

To extend density limits in magnetic recording industry, two separate strategies were developed to build the storage bit in last decade, introduction of perpendicular magnetic anisotropy (PMA) and adoption of ferrimagnetism/antiferromagnetism. Meanwhile, these properties significantly improve device performance, such as reducing spin-transfer torque energy consumption and decreasing signal-amplitude-loss. However, materials combining PMA and antiferromagnetism rather than transition-metal/rare-earth system were rarely developed. Here, we develop a new type of ferrimagnetic superlattice exhibiting PMA based on abundant Heusler alloy families. The superlattice is formed by [MnGa/Co2FeAl] unit with their magnetizations antiparallel aligned. The effective anisotropy (Kueff) over 6 Merg/cm3 is obtained, and the SL can be easily built on various substrates with flexible lattice constants. The coercive force, saturation magnetization and Kueff of SLs are highly controllable by varying the thickness of MnGa and Co2FeAl layers. The SLs will supply a new choice for magnetic recording and spintronics memory application such as magnetic random access memory.

Interfacial exchange coupling in cubic Heusler Co2FeZ (Z = Al and Si)/tetragonal Mn3Ga bilayers

We have fabricated bilayer films of tetragonal Heusler-like D022 Mn3Ga and cubic Heusler Co2FeZ (Z = Si and Al) on (100) single-crystalline MgO substrates and investigated their structural and interfacial exchange coupling. The coupling in the Mn3Ga/Co2FeAl bilayer was either ferromagnetic or antiferromagnetic, depending on annealing temperature, whereas only antiferromagnetic exchange coupling was observed in the Mn3Ga/Co2FeSi bilayers. The effects of annealing on the structure and coupling strength in the bilayers are discussed.

Magnetization dynamics for L10 MnGa/Fe exchange coupled bilayers

The precessional magnetization dynamics of L10 MnGa/Fe bilayers with a large perpendicular magnetic anisotropy are investigated using vector network analyzer ferromagnetic resonance (VNA-FMR) and time-resolved magneto-optical Kerr effect (TRMOKE). The MnGa/Fe(1 nm) bilayer exhibited perpendicular magnetization and MnGa/Fe(2 and 5 nm) bilayers showed in-plane magnetizations. The VNA-FMR and TRMOKE data for these bilayers are well explained from calculations, based on the coupled Landau-Lifshitz equations, taking into account the interfacial exchange coupling of 2.4 erg/cm2.

Tunable ferromagnetic and antiferromagnetic interfacial exchange coupling in perpendicularly magnetized L10-MnGa/Co2FeAl Heusler bilayers

In this work, we report a tailorable exchange coupling (Jex) at the Mn62Ga38/Co2FeAl interface, where Mn62Ga38 and Co2FeAl alloys are tetragonal Heusler alloy with high perpendicular magnetic anisotropy and typical cubic Heusler alloy with soft magnetism, respectively. As the post annealing temperature (Ta) is lower than 375 °C, the Jex is ferromagnetic with strength controllable from 7.5 to 0.5 erg/cm2. Interestingly, as Ta increases higher than 400 °C, an antiferromagnetic Jex of −5.5 erg/cm2 is observed. The ferromagnetic/antiferromagnetic transition is further evidenced by the spin dependent transport property of the magnetic tunnel junctions with Mn62Ga38/Co2FeAl as electrode. Based on structure characterization, the variation of Jex during annealing is discussed.

Spin dynamics induced by ultrafast heating with ferromagnetic/antiferromagnetic interfacial exchange in perpendicularly magnetized hard/soft bilayers

The laser-induced spin dynamics of FeCo in perpendicularly magnetized L10-MnGa/FeCo bilayers with ferromagnetic and antiferromagnetic interfacial exchange coupling (IEC) are examined using the time-resolved magneto-optical Kerr effect. We found a precessional phase reversal of the FeCo layer as the IEC changes from ferromagnetic to antiferromagnetic. Moreover, a precession-suspension window was observed when the magnetic field was applied in a certain direction for the bilayer with ferromagnetic IEC. Our observations reveal that the spin dynamics modulation is strongly dependent on the IEC type within the Landau-Lifshitz-Gilbert depiction. The IEC dependence of the precessional phase and amplitude suggests the interesting method for magnetization dynamics modulation.

Magnetoresistance Enhancement in Mn

The magnetoresistance effects of the perpendicular magnetic tunnel junctions (p-MTJs) based on Mn-Ga ordered alloys are reported. By tuning the Mn-Ga composition, the MTJs based on and structured Mn-Ga alloys were achieved in the MTJ stack structure of Cr(40)/MnxGa100-x(30)/Mg(0.4)/MgO(2.2)/CoFeB(1.2)/Ta(5)/Ru(7) (nm). The values of magnetoresistance (MR) ratio at room temperature for different Mn-Ga composition are around 5%. In order to enhance the MR ratio, a thin CoFeB layer was introduced between the Mn-Ga and the MgO barrier. The MR effect shows a strong Mn-Ga composition dependent as CoFeB interlayer thickness increases. An MR ratio of 50% was obtained at room temperature when the CoFeB thickness is 1.5 nm for Mn0.2Ga38 based MTJs.

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The post annealing temperature and Co thickness dependence of tunnel magnetoresistance (TMR) ratio was investigated for <i>L</i>1₀-MnGa/Co(<i>t</i>_Co)/MgO/CoFeB perpendicular magnetic tunnel junctions (MTJs). The MnGa/MgO interface optimization by Co insertion was shown to be an effective way for improving the TMR ratio. The TMR value increases with the annealing temperature (<i>T</i>_a), and exhibits maxima at 325 ^°C for <i>t</i>_Co = 1.0 - 5.0 nm. Thermal annealing process improves the structure of MTJs, but also causes elements diffusion. In this work, the annealing effect on MTJs with different Co thicknesses was discussed in detail along with a fast annealing method.