T. Katayama

Characterization of growth and crystallization processes in CoFeB∕MgO∕CoFeB magnetic tunnel junction structure by reflective high-energy electron diffraction

We performed reflective high-energy electron diffraction observations to investigate the growth and crystallization processes of Co60Fe20B20∕MgO∕Co60Fe20B20 magnetic tunnel junction structures. A MgO layer grown on an amorphous CoFeB layer has an amorphous structure up to the MgO thickness (tMgO) of 4 monoatomic layers (ML) and begins to crystallize with (001) preferred orientation when tMgO⩾5ML. By annealing, an amorphous CoFeB layer grown on MgO(001) crystallizes in a body-centered-cubic structure with (001) orientation because MgO(001) acts as a template to crystallize CoFeB. The results give important information for understanding the mechanism of giant tunneling magnetoresistance effect in CoFeB∕MgO∕CoFeB MTJs.

Magnetic properties of Bi3Fe5O12 garnet

Nonthermodynamical garnet Bi3Fe5O12 was synthesized by direct epitaxial growth via vapor phase using a reactive ion beam sputtering technique. The lattice constant was 12.631 A. The saturation magnetization of the film grown onto a substrate (a=12.495 A) was 1500 G at room temperature. The uniaxial anisotropy energy was +7.2×104 erg/cm3. The conversion electron Mossbauer spectroscopy measurement indicates that the internal fields of 24d and 16a sites in the nearly lattice‐matched film were 420 and 490 kOe, respectively. The internal field of the 24d site was larger by 30 kOe than that in YIG. The angle between the direction of the internal field and the film normal was 34.8°.Nonthermodynamical garnet Bi3Fe5O12 was synthesized by direct epitaxial growth via vapor phase using a reactive ion beam sputtering technique. The lattice constant was 12.631 A. The saturation magnetization of the film grown onto a substrate (a=12.495 A) was 1500 G at room temperature. The uniaxial anisotropy energy was +7.2×104 erg/cm3. The conversion electron Mossbauer spectroscopy measurement indicates that the internal fields of 24d and 16a sites in the nearly lattice‐matched film were 420 and 490 kOe, respectively. The internal field of the 24d site was larger by 30 kOe than that in YIG. The angle between the direction of the internal field and the film normal was 34.8°.

Interlayer exchange coupling in Fe∕MgO∕Fe magnetic tunnel junctions

Interlayer exchange coupling (IEC) in fully epitaxial Fe∕MgO∕Fe(001) tunnel junctions with wedge-shaped MgO layers is measured at room temperature from the unidirectional shift of the Kerr hysteresis loop. It is found that the IEC is antiferromagnetic for small MgO thickness but changes sign at 0.8nm. Ab initio calculations of IEC show that this behavior can be explained by the presence of O vacancies in the MgO barrier which makes IEC antiferromagnetic for thin barriers. With increasing MgO thickness the resonance contribution to IEC from localized defect states is reduced resulting in the ferromagnetic coupling typical for perfect MgO barriers.

Giant tunneling magnetoresistance in fully epitaxial body-centered-cubic Co∕MgO∕Fe magnetic tunnel junctions

Fully epitaxial bcc Fe1−xCox(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (x=0, 0.5, 1) were fabricated with molecular-beam epitaxy and microfabrication techniques. While the bcc Fe(001) and Fe0.5Co0.5(001) electrodes had similar magnetoresistance (MR) ratios of about 180% at room temperature, the bcc Co(001) electrode exhibited a higher MR ratio up to 271% at room temperature (353% at 20 K). The fact that the MR ratio for a bcc Co electrode is much higher than that for a bcc Fe electrode is consistent with first-principle calculations, indicating the importance of electrode band structure in the k‖=0 direction.

Preparation of polycrystalline Bi3Fe5O12 garnet films

Polycrystalline films of nonthermodynamical garnet Bi3Fe5O12 were synthesized by direct epitaxial growth from the vapor phase. Polycrystalline thin layers of various kinds of thermodynamical garnets prepared on fused quartz substrates were employed as the substrate materials. The saturation magnetization of the film grown onto the polycrystalline GGG layer was 140 mT at 5 K. The films showed quite large magneto‐optical effects. In the film on the GGG layer, the Faraday rotation angle at a wavelength of 633 nm was −5.5 deg/μm and the magneto‐optical Kerr rotation angle at 455 nm was −0.9°.

X-ray Absorption and X-ray Magnetic Circular Dichroism Studies of a Monatomic Fe(001) Layer Facing a Single-Crystalline MgO(001) Tunnel Barrier

We studied the electronic and magnetic states of a monatomic bcc Fe(001) layer facing a single crystalline MgO(001) tunnel barrier by using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) at the Fe L2,3 core edges in order to clarify the origin of the huge tunnel magnetoresistance (TMR) effect observed in Fe/MgO/Fe magnetic tunnel junctions. Both the XAS and XMCD revealed that the 1 ML-Fe is not oxidized, which is crucial to the huge TMR effect. A sum-rule analysis of the XMCD shows that the 1 ML-Fe(001) has an enhanced total magnetic moment of about 2.6 µB per Fe atom compared with that of bulk Fe.

CRYSTAL-ORIENTATION DEPENDENCE ON MAGNETIC CIRCULAR DICHROISM SPECTRA OF MNSB EPITAXIAL FILM

We present the crystal‐orientation dependence of the magnetic circular dichroism (MCD) spectra of ferromagnetic (1101) and (0001) MnSb epitaxial films grown by molecular beam epitaxy on (001) GaAs and (0001) sapphire substrates, respectively, where very large anisotropy is clearly observed at around 3.0 and 4.7 eV. The anisotropy of the MCD spectra is discussed in connection with the magnetocrystalline anisotropy.

Exchange coupling of NiFe/FeRh–Ir thin films

We report unusually large room temperature coercive field Hc induced in Ni0.81Fe0.19 films grown on the (001) magnetically compensated surface of antiferromagnetic single crystal CsCl-type FeRh0.95Ir0.05 alloy thin layers on MgO(001) substrates. The specimens were prepared by conventional rf sputtering in Kr atmosphere, at 450–500u2009°C for FeRh–Ir and 20–200u2009°C for NiFe. The epitaxial growth of NiFe/FeRh–Ir and atomically flat terraces of FeRh–Ir were confirmed by x-ray reflectometry and scanning tunneling microscopy. For a NiFe(2.7 nm)/FeRh–Ir(22 nm) film, a maximum Hc of 810 Oe is observed. The 1/tNiFe dependence of Hc for larger NiFe thicknesses indicates that the large coercive field originates from the magnetic interaction at NiFe/FeRh–Ir interface. The large Hc combined with high corrosion resistance make FeRh based alloys a promising candidate for application as pinning layers in spin-valve devices.

Magneto‐optical response of nanoscaled cobalt dots array

Magneto‐optical (MO) spectroscopic studies were carried out on submonatomic layer coverage of Co atoms deposited on the reconstructed Au(111) surface forming two‐dimensional lattice of nanometer scaled Co dots. The MO spectral features were considerably affected by the shape of the Co dots. Strong dependence of the MO peak intensity on the change of the Co dot shape was observed at around the plasma edge of Au.

Spin-Transfer Switching and Thermal Stability in an FePt/Au/FePt Nanopillar Prepared by Alternate Monatomic Layer Deposition

We fabricated a current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) nanopillar with a 1-nm-thick FePt free layer having perpendicular anisotropy using the alternate monatomic layer deposition method. Nanopillars consisting of [Fe (1 monolayer (ML))/Pt (1 ML)]n (n: the number of the alternation period) ferromagnetic layers and an Au spacer layer showed spin-transfer induced switching at room temperature. An average critical switching current density (Jc0) of 1.1×107 A/cm2 with a large thermal stability parameter (Δ) of 60 was obtained in a nanopillar with a free-layer thickness of 1.02 nm (n=3) and a pillar diameter of 110 nm. The ultrathin free-layer with high perpendicular anisotropy is effective to obtain both large Δ and small Jc.