Taro Nagahama

High Tunnel Magnetoresistance at Room Temperature in Fully Epitaxial Fe/MgO/Fe Tunnel Junctions due to Coherent Spin-Polarized Tunneling

We fabricated fully epitaxial Fe(001)/MgO(001)/Fe(001) magnetic tunnel junctions (MTJs) and observed a magneto-resistance (MR) ratio of 88% at T = 293 K (146% at T = 20 K), the highest value yet reported. The origin of the high MR ratio is not the diffusive tunneling of Jullieres model but the coherent spin-polarized tunneling in epitaxial MTJs, in which only the electrons with totally symmetric wave functions with respect to the barrier-normal axis can tunnel. The bias-voltage dependence of the MR was very small, resulting in a high output voltage of 380 mV. This high voltage will help overcome problems in the development of high-density magnetoresistive random-access-memory (MRAM).

Ultrathin Co/Pt and Co/Pd superlattice films for MgO-based perpendicular magnetic tunnel junctions

Ultrathin [Co/Pt]n and [Co/Pd]n superlattice films consisting of 0.14–0.20-nm-thick Co and Pt(Pd) layers were deposited by sputtering. A large perpendicular magnetic anisotropy [(3–9)×106 ergs/cm3] and an ideal square out-of-plane hysteresis loop were attained even for ultrathin superlattice films with a total thickness of 1.2–2.4 nm. The films were stable against annealing up to 370 °C. MgO-based perpendicular magnetic tunnel junctions with this superlattice layer as the free layer showed a relatively high magnetoresistance ratio (62%) and an ultralow resistance-area product (3.9 Ω μm2) at room temperature. The use of these films will enable the development of gigabit-scale nonvolatile memory.

High Magnetoresistance Ratio and Low Resistance--Area Product in Magnetic Tunnel Junctions with Perpendicularly Magnetized Electrodes

We fabricated perpendicularly magnetized MgO-based magnetic tunnel junctions (p-MgO-MTJs) with a [Co/Pt]n/CoFeB/CoFe bottom electrode layer (free layer) and a CoFe/CoFeB/TbFeCo top electrode layer (reference layer). The insertion of thin CoFeB/CoFe layers at the barrier/electrode interfaces and post-annealing at a relatively low temperature of 225 °C simultaneously yielded high magnetoresistance (MR) ratios of up to 85% at room temperature and a low resistance–area (RA) product of 4.4 Ω µm2. Such a high MR ratio in low-RA p-MgO-MTJs is the key to developing ultrahigh-density spin-transfer-torque magnetoresistive random access memories (MRAMs).

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.

Oscillation of giant tunneling magnetoresistance with respect to tunneling barrier thickness in fully epitaxial Fe∕MgO∕Fe magnetic tunnel junctions

The authors fabricated fully epitaxial Fe(001)∕MgO(001)∕Fe(001) magnetic tunnel junctions (MTJs) with various MgO thicknesses (tMgO) and investigated spin-dependent transport properties. Both the tunneling resistance in the parallel magnetic state (RP) and that in the antiparallel magnetic state (RAP) exhibited short-period oscillations as functions of tMgO with the same period of 3.2A and different phases. RAP also showed a long-period oscillation with a period of 9.9A. As a result, tMgO dependence of magnetoresistance is expressed as a superposition of the short- and long-period oscillations. These results provide important clues for understanding the oscillatory tMgO dependence of the tunneling magnetoresistance effect.

Quantum-well effect in magnetic tunnel junctions with ultrathin single-crystal Fe(100) electrodes

We studied the tunnel spectra of magnetic tunnel junctions with a single-crystal ultrathin Fe(100) electrode. The tunnel spectra show oscillations of the differential conductivity and the differential tunnel magnetoresistance. The positions of the maxima of the oscillations move systematically with the change in the Fe(100) electrode’s thickness, indicating that the oscillations originate from the quantum-well states in the ultrathin Fe(100) electrode. This effect provides us with an opportunity to create voltage-controlled spin functional devices.

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.

Quantum size effect in magnetic tunnel junctions with ultrathin Fe(001) electrodes

The transport properties of magnetic tunnel junctions with single-crystalline ultrathin Fe(001) electrodes are studied. The tunnel spectra and the bias dependence of the differential magnetoresistance show quantum-well oscillations. This effect provides evidence of ballistic transport through the Al–O barrier and shows that magnetic tunnel junctions with quantum-well states can be a tool for studying spin-dependent transport mechanisms in magnetic tunnel junctions.

Inelastic tunneling spectra of MgO barrier magnetic tunneling junctions showing large magnon contribution

We investigated bias-voltage and temperature dependence of conductivity arising from the magnon contribution in MgO-based magnetic tunneling junctions (MTJs) with different ferromagnetic electrodes. Second derivative conductance curves showed broad peak structure, which extends from 5 to 200 mV, accompanied by additional peaks at around 23, 54, and 85 mV. The peak intensities were larger for antiparallel configuration than for parallel configuration except that at 85 mV. This difference in the peak intensity was observed to be larger for the MTJs having higher tunneling magnetoresistance ratio, indicating a magnetic origin of these peaks. Abrupt increase in the second derivative conductance at very low biasing voltage in the antiparallel configuration suggests the important role of the surface magnon excitation.

Atomically flat aluminum-oxide barrier layers constituting magnetic tunnel junctions observed by in situ scanning tunneling microscopy

Observation using in situ scanning tunneling microscopy of the layers constituting a magnetic tunnel junction with a naturally oxidized aluminum barrier layer revealed an extremely flat aluminum-oxide surface. It was clarified from line-scan images that the aluminum-oxide barrier layer has atomic steps. This flatness, which is surprising given that the aluminum-oxide film is amorphous, reduced electron scattering within the barrier, leading to momentum-dependent tunneling, which should enable the fabrication of advanced devices, such as spin-polarized resonant tunneling transistors.