Mn-Ga binary alloys show strong magnetism and large uniaxial magnetic anisotropy even though these alloys do not contain any noble, rare-earth metals or magnetic elements. We investigate the composition dependence of saturation magnetization ${M}_{\mathrm{S}}$ and uniaxial magnetic anisotropy ${K}_{\mathrm{u}}$ in epitaxial films of Mn${}_{x}$Ga${}_{1\ensuremath{-}x}$ alloys ($x\ensuremath{\sim}0.5$--0.75) grown by magnetron sputtering. The ${M}_{\mathrm{S}}$ values decrease linearly from approximately 600 to 200 emu/cm${}^{3}$ with increasing $x$, whereas the ${K}_{\mathrm{u}}$ values decrease slightly from approximately 15 to 10 Merg/cm${}^{3}$ with increasing $x$. These trends are distinct from those for known tetragonal hard magnets obtained in a limited composition range in Mn-Al and Fe-Pt binary alloys. These data are analyzed using a localized magnetic moment model.

The tunnel magnetoresistance (TMR) effect with a perpendicularly magnetized D0(22)-Mn3-delta Ga (delta = 0.6) electrode was investigated in epitaxially grown D0(22)-Mn3-delta Ga (30)/Mg (d(Mg))/MgO (2)/CoFe (2.5) (nm) magnetic tunnel junctions (MTJs). The maximum TMR ratio of 9.8% (22.1%) was achieved at 300 K (10 K) with d(Mg) = 0: 4 nm. The bias voltage dependence of differential conductance spectra suggests the existence of a coherent tunneling process in the MTJs. First principles calculations of band dispersion relations and tunneling transmittance in a Mn3Ga/MgO/Mn3Ga structure were also performed. The results revealed the existence of Delta(1)-bands in Mn3Ga and demonstrated the possibility of a coherent tunneling process existing in the MTJ. (C) 2011 The Japan Society of Applied Physics

In this study, influence of composition on structure and magnetic properties of epitaxial Mn-Ga films were investigated. The epitaxial Mn-Ga films with different composition were grown on metal (Pt and Cr) buffered MgO substrates by co-sputtering technique. By use of the Pt buffered MgO substrates, the D0(22) phases can be obtained in the Mn-Ga films with the composition ranging from Mn65Ga35 to Mn75Ga25. The crystalline quality and perpendicular magnetic anisotropic properties of the Mn-Ga films were deteriorated with increase of Mn content. The saturation magnetization decreased with the increase of Mn content, which is consistent with the reported results of bulk alloys.

Spin precession with frequencies up to 280 GHz is observed in Mn(3-delta)Ga alloy films with a perpendicular magnetic anisotropy constant K(u) similar to 15 Merg/cm(3). The damping constant alpha, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the delta = 1.46 (0.88) film. Those are about one-tenth of alpha values for known materials with large K(u). First-principles calculations well describe both low alpha and large K(u) for these alloys.

Effects of metallic Mg insertion on tunnel magnetoresistance (TMR) effect were investigated in D0(22)-Mn2.4Ga/Mg/MgO/CoFe magnetic tunnel junctions (MTJs). The thickness of Mg (d(Mg)) was varied from 0 to 1.4 nm. TMR ratio exhibited maximum value of 22% with the d(Mg) = 0.4 nm and a negative value of 14% with the d(Mg) = 1.4 nm at 10 K. The dependence of resistance area products (R x A) on the d(Mg) showed similar trend compared with those of what reported in conventional CoFeB-MgO based MTJs. Bias voltage dependences of differential conductance (dI/dV) and TMR ratio exhibited asymmetry with respect to the zero-bias. The difference of the zero-bias anomaly in the dI/dV spectra was also discussed, and an indication of reducing the inelastic tunneling process was found, implying the improvement of barrier/magnetic-layer interfaces by the Mg insertion. Considering a theoretical work done by Wang et al. [Phys. Rev. B 82, 054405 (2010)], the inversion of the sign and the asymmetric bias voltage dependence of TMR ratio were inferred to be attributed to the minority spin tunneling via a quantum well state in the thin metallic Mg layer between Mn2.4Ga and MgO. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3603034]

Effects of metallic Mg insertion on tunnel magnetoresistance (TMR) effect were investigated in D022-Mn2.4Ga/Mg/MgO/CoFe magnetic tunnel junctions (MTJs). The thickness of Mg (dMg) was varied from 0 to 1.4 nm. TMR ratio exhibited maximum value of 22% with the dMg = 0.4 nm and a negative value of 14% with the dMg = 1.4 nm at 10 K. The dependence of resistance area products (R × A) on the dMg showed similar trend compared with those of what reported in conventional CoFeB-MgO based MTJs. Bias voltage dependences of differential conductance (dI/dV) and TMR ratio exhibited asymmetry with respect to the zero-bias. The difference of the zero-bias anomaly in the dI/dV spectra was also discussed, and an indication of reducing the inelastic tunneling process was found, implying the improvement of barrier/magnetic-layer interfaces by the Mg insertion. Considering a theoretical work done by Wang et al. [Phys. Rev. B 82, 054405 (2010)], the inversion of the sign and the asymmetric bias voltage dependence of TMR ratio were inferred to be attributed to the minority spin tunneling via a quantum well state in the thin metallic Mg layer between Mn2.4Ga and MgO.

The magnetic properties of CoFeB buffered [Co(0.3 nm)Pd(x nm)]6 multilayer films have been investigated. It is found that the magnetic properties of the multilayer depended on the Pd thickness and for thickness below 0.5 nm no perperdicular anisotropy is observed. Magnetization dynamics in perpendicularly magnetized CoFeB-[Co/Pd] multilayer films are investigated using time-resolved magneto-optical Kerr effect (TRMOKE). The variation of precession frequency with external magnetic field for different palladium thickness is quantitatively understood using the macrospin approximation of the Landau-Lifshitz-Gilbert equation of motion. The Gilbert damping constant ¿, in the range 0.04-0.1, varying with the palladium thickness is reported. The observed ¿ value is comparable to the damping coefficient of bulk Ni, and much lower than the reported values for perpendicularly magnetized films. The CoFeB buffer layer with in-plane anisotropy appears to significantly affect the precession frequency and thus the damping constant of the films.

We report electrical transport properties of perpendicular magnetized Mn-Ga epitaxial films with various thicknesses. The maximum extraordinary Hall resistivity and Hall angle is 11.5 μΩ cm and 5.7%, respectively, which is comparable to the highest value reported in amorphous Fe0.79Gd0.21 alloy. In the low temperature region, resistivity was proportional to T2.9 owing to the unconventional one-magnon scattering processes, indicating high spin polarization of this material.

The influence of annealing temperature and film thickness on structural and magnetic properties of Mn2.5Ga films were investigated in this work. The annealing temperature of 400 degrees C was found to be the optimum condition to obtain the films with high perpendicular magnetic anisotropy (PMA) (K-u(eff) = 7.8 x 10(6) erg/cm(3)) and smooth surface (R-a approximate to 0.15 nm). The PMA property was maintained in the 5 nm thick film, and deterioration of the PMA properties with decreasing film thickness can be ascribed to the tensile strain existed in the thin Mn2.5Ga films.

In this work polycrystalline and epitaxial Mn2.5Ga films were prepared by dc magnetron sputtering technique. The lower sputtering power and higher annealing temperature are found to be beneficial for increasing ratio of DO22 phase in the polycrystalline Mn2.5Ga films, leading to the improved magnetic properties although perpendicular magnetic anisotropy (PMA) properties cannot be observed. On the contrary, (001)-oriented epitaxial Mn2.5Ga films grown on Cr buffered MgO substrates possess low saturation magnetization and giant PMA properties simultaneously.

Interlayer exchange coupling in synthetic ferrimagnet structures consisting of perpendicularly magnetized CoCrPt and in-plane magnetized CoFeB layers, which are coupled by a Ru thin spacer, were investigated. The magnetization of the CoFeB layer turned perpendicular to the film plane after annealing at 300°C because of the appearance of interlayer coupling from the CoCrPt layer. The coupling varied between antiferromagnetic and ferromagnetic depending on the Ru spacer thickness. The sign and strength of the coupling were also observed through analyses of magnetization curves and ferromagnetic resonance spectra.

To investigate the correlation between perpendicular magnetic anisotropy and intrinsic Gilbert damping, time-resolved magneto-optical Kerr effect was measured in Pt/Co(dCo)/Pt films. These films showed perpendicular magnetization at dCo=1.0 nm and a perpendicular magnetic anisotropy energy Kueff that was inversely proportional to dCo. With an analysis based on the Landau–Lifshitz–Gilbert equation, the intrinsic Gilbert damping constant α was evaluated by parameter-fitting of frequency and lifetime expressions to experimental data of angular variations in spin precession frequency and life-times. The α values increased significantly with decreasing dCo but not inversely proportional to dCo.

We report on epitaxial growth and magnetic properties of Mn2.5Ga thin films, which were deposited on Cr/MgO single crystal substrates by magnetron sputtering. X-ray diffraction results revealed the epitaxial relationships as Mn2.5Ga(001)[100]∥Cr(001)[110]∥MgO(001)[100]. The presence of (002) and (011) superlattice peaks indicates that the films were crystallized into DO22 ordered structures. The perpendicular magnetic anisotropy (PMA) properties were found to be related to the extent of DO22 chemical ordering. A giant PMA (Kueff=1.2×107 erg/cm3) and low saturation magnetization (Ms=250 emu/cm3) can be obtained for the film with highest chemical ordering parameter (S=0.8).