Mikihiko Oogane

Large tunnel magnetoresistance in magnetic tunnel junctions using a Co2MnSi Heusler alloy electrode and a MgO barrier

A large tunnel magnetoresistance (TMR) ratio of 753% has been observed at 2 K in a magnetic tunnel junction (MTJ) using a Co2MnSi Heusler alloy electrode and a crystalline MgO tunnel barrier. This TMR ratio is the largest reported to date in MTJs using a Heusler alloy electrode. Moreover, we have observed a large TMR ratio of 217% at room temperature (RT). This TMR at RT is much larger than that of MTJs using an amorphous Al-oxide tunnel barrier. However, the temperature dependence of the TMR ratio is still large because of inelastic tunneling in the antiparallel magnetic configuration.

Epitaxial Mn2.5Ga thin films with giant perpendicular magnetic anisotropy for spintronic devices

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=1.2×107u2002erg/cm3) and low saturation magnetization (Ms=250u2002emu/cm3) can be obtained for the film with highest chemical ordering parameter (S=0.8).

Low damping constant for Co2FeAl Heusler alloy films and its correlation with density of states

Gilbert damping for the epitaxial Co2FeAl Heusler alloy films was investigated. Gilbert damping constant for the films was evaluated by analyzing the data of ferromagnetic resonance measured at the frequency of 2–20 GHz. Gilbert damping constant for the film without annealing was rather large, while it decreased remarkably with postannealing. Gilbert damping constant for the film annealed at 600u2009°C was ≃0.001. These behavior of Gilbert damping constant can be well explained by the fact that the density of states calculated from first principles decreases with increasing the degree of B2 order.

Large Tunnel Magnetoresistance of 1056% at Room Temperature in MgO Based Double Barrier Magnetic Tunnel Junction

CoFeB/MgO/CoFeB/MgO/CoFeB double barrier magnetic tunnel junctions (DBMTJs) with thin middle layers were fabricated on SiO2/Si(001) substrates by r.f. magnetron sputtering. We successfully obtained a large tunnel magnetoresistance of 1056% at room temperature in a DBMTJ with a middle CoFeB layer thickness of 1.2 nm that was fabricated at a relatively low post-deposition annealing temperature of 350 °C. This DBMTJ also realized sharp magnetization switching in the free middle CoFeB layer, which is attributed to strong antiferromagnetic coupling between the exterior CoFeB and PtMn layers. These favorable magnetoresistive properties offer interesting possibilities for developing practical spintronics applications and noble magnetotransport physics.

Influence of the L21 ordering degree on the magnetic properties of Co2MnSi Heusler films

We report on the influence of the improved L21 ordering degree on the magnetic properties of Co2MnSi Heusler films. Different fractions of the L21 phase are obtained by different postgrowth annealing temperatures ranging from 350u2009°C to 500u2009°C. Room temperature magneto-optical Kerr effect measurements reveal an increase of the coercivity at an intermediate annealing temperature of 425u2009°C. This is probably a result of an increasing number of pinning centers on the one hand and a drop of the cubic anisotropy constant K1 by a factor of 10 on the other for an increasing amount of the L21 phase. Furthermore, Brillouin light scattering studies show that the improvement of the L21 order in the Co2MnSi films is correlated with a decrease of the saturation magnetization by about 7%. The exchange stiffness constant of Co2MnSi, however, increases by about 8% when the L21 order is improved.

Enhancement in tunnel magnetoresistance effect by inserting CoFeB to the tunneling barrier interface in Co2MnSi/MgO/CoFe magnetic tunnel junctions

Tunnel magnetoresistance (TMR) effect was investigated in Co2MnSi/CoFeB(0–2u2002nm)/MgO/CoFe magnetic tunnel junctions (MTJs). TMR ratio was enhanced by inserting a thin CoFeB layer at the Co2MnSi/MgO interface. The MTJ with CoFeB thickness of 0.5 nm exhibited the highest TMR ratio. From the conductance-voltage measurements for the fabricated MTJs, we infer that the highly spin polarized electron created in Co2MnSi can conserve the polarization through the 0.5-nm-thick CoFeB layer. Furthermore, by insertion of the thin CoFeB layer, the temperature dependence of the TMR ratio was improved because of the suppression of the fluctuation of the magnetic moment at the Co2MnSi/MgO interface.

Structure, exchange stiffness, and magnetic anisotropy of Co2MnAlxSi1−x Heusler compounds

We have investigated the exchange constant A (exchange stiffness D) and the magnetic anisotropy of Co2MnAlxSi1−x (CMAS) Heusler compounds as a function of the film composition. In case of Co2MnSi, A was found to be 23.5 pJ/m (D=5.86u2002meVu2009nm2). Furthermore, A decreased with an increase in the Al content x. In the case of Co2MnAl, it was found to be 4.8 pJ/m (D=1.90u2002meVu2009nm2). Finally, the cubic anisotropy constant K1 of CMAS films was found to be small (below 10u2002kJ/m3) and did not exhibit simple dependence on x.

Direct Observation of Atomic Ordering and Interface Structure in Co2MnSi/MgO/Co2MnSi Magnetic Tunnel Junctions by High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy

The atomic ordering of Co2MnSi (CMS) full-Heusler film and the interface structure of CMS/MgO/CMS magnetic tunnel junctions (MTJs) were investigated by high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). We observed the atomic ordering of L21 and B2 structures of CMS from the atomic number (Z) contrast STEM images. We also confirmed that the interface structure consists of the layer next to the Co layer terminating in the CMS to MgO layer from the layer periodicity along the [001] direction, however, site-disorder exists between two atomic layers at the termination of CMS, including locally L21-ordered MnSi terminated structure.

Tunnel magnetoresistance effect in magnetic tunnel junctions using epitaxial Co2FeSi Heusler alloy electrode

Magnetic tunnel junctions (MTJs) using epitaxially grown (100)-oriented and (110)-oriented Co2FeSi Heusler alloy bottom electrodes and amorphous Al-oxide barriers were fabricated. The tunnel magnetoresistance (TMR) ratios were 80% at 2 K and 48% at room temperature for the MTJ with the (100)-Co2FeSi bottom electrode. The MTJ with the (100)-Co2FeSi bottom electrode had a smaller TMR ratio than the MTJ with (100)-Co2FeSi electrode. The TMR ratio in MTJs with Co2FeSi electrode is smaller than that of a MTJ with Co2MnSi electrode. Tunnel conductance characteristics were investigated, revealing no half-metallic character in MTJs with Co2FeSi electrode in the conductance-voltage curves.

Ferrimagnetism in epitaxially grown Mn2VAl Heusler alloy investigated by means of soft x-ray magnetic circular dichroism

Epitaxially grown Mn2VAl films were fabricated on MgO (001) single crystalline substrates by using a sputtering technique. The resulting Mn2VAl films with substrate temperatures of Ts=500 and 600u2009°C showed an L21-ordered structure. The saturation magnetization was 150u2002emu/cm3 at 300 K for a sample with Ts=600u2009°C. Ferrimagnetic coupling between the Mn and V magnetic moments in the L21-Mn2VAl film was clearly demonstrated by soft x-ray magnetic circular dichroism. In previous studies, this coupling was found only in polycrystalline bulk samples examined by nuclear magnetic resonance.