Akimasa Sakuma

Giant tunneling magnetoresistance in Co2MnSi∕Al–O∕Co2MnSi magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) with a stacking structure of Co2MnSi∕Al–O∕Co2MnSi were fabricated using magnetron sputtering system. Fabricated MTJ exhibited an extremely large tunneling magnetoresistance (TMR) ratio of 570% at low temperature, which is the highest TMR ratio reported to date for an amorphous Al–O tunneling barrier. The observed dependence of tunneling conductance on bias voltage clearly reveals the half-metallic energy gap of Co2MnSi. The origins of large temperature dependence of TMR ratio were discussed on the basis of the present results.

Magnetic and Mössbauer studies of single‐crystal Fe16N2 and Fe‐N martensite films epitaxially grown by molecular beam epitaxy (invited)

Single‐phase, single‐crystal Fe16N2(001) films and Fe‐11 at. %N martensite films of 200–900 A thickness have been epitaxially grown on In0.2Ga0.8As(001) substrates by evaporating Fe in an atmosphere of mixed gas of N2 and NH3, followed by annealing. The saturation magnetizations 4πMs’s for Fe16N2 and Fe‐N martensite films have been measured to be around 29 and 24 kG at room temperature, respectively, and almost constant in the above thickness range by using a vibrating sample magnetometer. 4πMs for Fe‐N martensite films has been increased with ordering of N atoms caused by annealing and finally reached around 29 kG for Fe16N2. Mossbauer spectra have been measured for those films. The spectrum for Fe‐N martensite films was a superposed one with hyperfine fields of 360, 310, and 250 kOe, similar to those previously reported for martensite. While the spectrum became simpler with ordering, finally reaching a single hyperfine field of 330 kOe for Fe16N2. 4πMs of 29 kG for Fe16N2 (3.2 μB/Fe atom) and 4πMs of 24...

Self-consistent calculations for the electronic structures of iron nitrides, Fe3N, Fe4N and Fe16N2

Abstract The spin-polarized band calculations for the iron nitrides, Fe 3 N, Fe 4 N and Fe 16 N 2 , have been performed with use of the LMTO-ASA method in the frame of local spin density functional formalism. The results show that the most distant Fe atoms from N have the largest magnetic moment. This can be ascribed mainly to the transfer of the down spin electrons, which is caused by the electronic interference due to the interstitial N atoms. The central role of the N atom is to bring about the large magnetic moments through the lattice expansion. Concurrently, the N atoms promote an itineracy of electrons and then in turn prevent the exchange-splitting. This leads to the behavior that the lower N concentration gives the larger magnetic moments. Quantitatively, the results are in fair agreement with the experimental results except for Fe 16 N 2 whose measured value is considerably larger than the calculated result.

Huge Spin-Polarization of L21-Ordered Co2MnSi Epitaxial Heusler Alloy Film

Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial Co2MnSi/Al–O barrier/poly-crystalline Co75Fe25 were fabricated using an ultrahigh vacuum sputtering system. The epitaxial Co2MnSi bottom electrode exhibited highly ordered L21 structure and very smooth surface morphology. Observed magnetoresistance (MR) ratios of 70% at room temperature (RT) and 159% at 2 K are the highest values to date for MTJs using a Heusler alloy electrode. A high spin-polarization of 0.89 at 2 K for Co2MnSi obtained from Jullieres model coincided with the half-metallic band structure that was predicted by theoretical calculations.

First Principle Calculation of the Magnetocrystalline Anisotropy Energy of FePt and CoPt Ordered Alloys

The spin-polarized band calculations including spin-orbit interaction for L1 0 -FePt and CoPt ordered alloys have been performed with LMTO-ASA method in the frame of local spin density functional approximation. It has been shown that strong uniaxial magnetic anisotropy of both alloys is brought about by a large spin-orbit coupling of Pt atom and a strong hybridization of Pt d bands with highly polarized Fe (Co) d bands. The obtained magnetocrystalline anisotropy energy (MAE) is about 16×10 6 J/m 3 for FePt and 9×10 6 J/m 3 for CoPt. It is also found that both MAEs have a trend of increase with increasing axial ratio c / a in the vicinity of measured c / a . This can be regarded as being associated with the behavior that the MAEs decrease with increasing band filling.

Magnetic Damping in Ferromagnetic Thin Films

We determined the Gilbert damping constants of Fe–Co–Ni and Co–Fe–B alloys with various compositions and half-metallic Co2MnAl Heusler alloy films prepared by magnetron sputtering. The ferromagnetic resonance (FMR) technique was used to determine the damping constants of the prepared films. The out-of-plane angular dependences of the resonance field (HR) and line width (ΔHpp) of FMR spectra were measured and fitted using the Landau–Lifshitz–Gilbert (LLG) equation. The experimental results fitted well, considering the inhomogeneities of the films in the fitting. The damping constants of the metallic films were much larger than those of bulk ferrimagnetic insulators and were roughly proportional to (g-2)2, where g is the Lande g factor. We discuss the origin of magnetic damping, considering spin–orbit and s–d interactions.

Magnetic tunnel junctions using B2-ordered Co2MnAl Heusler alloy epitaxial electrode

Magnetic tunnel junctions were fabricated with epitaxially grown Co2MnAl bottom electrodes combined with an Al–O tunnel barrier using a magnetron sputtering system. The epitaxial Co2MnAl electrode had very low surface roughness of 0.2nm and a highly ordered B2 structure. Magnetic tunnel junctions (MTJs) with a stacking structure of epitaxial-Co2MnAl∕Al–O∕CoFe∕IrMn exhibited large tunnel magnetoresistance (TMR) ratios of 65% at room temperature and 83% at 10K. The TMR ratios were larger than those of a MTJ with a Co2MnAl polycrystalline electrode.

Magnetic properties and band structures of half-metal-type Co2CrGa Heusler alloy

The saturation magnetic moment Ms and the Curie temperature TC of the half-metal-type Co2CrGa Heusler alloy have been investigated. The value of Ms at 4.2K for the L21-type Co2CrGa alloy is 3.01μB∕f.u., in agreement with the generalized Slater-Pauling line and the theoretical calculation. The Curie temperature TC and the phase-transformation temperature Tt from the L21 to B2-type structure are 495 and 1050K, respectively. The band calculations disclose that the spin polarization is also relatively high in the B2-type structure, although its estimated TC is about 100K lower than that of the L21-type structure.

Direct observation of half-metallic energy gap in Co2MnSi by tunneling conductance spectroscopy

Magnetic tunnel junctions with a Co2MnSi∕Al–O∕CoFe structure are prepared by magnetron sputtering and investigated with respect to the energy gap near the Fermi energy level. The plasma oxidation time for the Al–O barrier is found to affect the condition of the Co2MnSi∕Al–O interface. The optimized sample (50s oxidation time) exhibits a magnetoresistance ratio of 159% and tunneling spin polarization of 0.89 at 2K. The bias voltage dependence of tunneling conductance (dI∕dV−V) reveals a clear half-metallic energy gap at 350–400meV for Co2MnSi, with an energy separation of just 10meV between the Fermi energy and the bottom edge of conduction band.

Magnetic properties of exchange-coupled α-Fe/Nd–Fe–B multilayer thin-film magnets

The multilayer thin films having the form of Ti(30 nm)/Fe(dFe)/[Nd–Fe–B(dNd–Fe–B)/Fe(dFe)] ×5/Ti(30 nm)/glass were fabricated on glass substrates by means of radio frequency (rf) sputtering, with dFe varied from 0 to 50 nm and dNd–Fe–B from 0 to 100 nm. Magnetization measurements have revealed that the minor loops of the films are reversible in a certain range of demagnetization fields except for the films with dFe∼0 nm or dNd–Fe–B<10 nm. Micromagnetic calculation of magnetization curves has been performed, which has well reproduced the observed hysteresis loops including spring-back behavior and the dependence of the coercive field HcJ on dFe and dNd–Fe–B. From the comparison between experiment and calculation, we can infer that the interlayer exchange-coupling strength is about 10% of the intralayer couplings, which are found to be almost independent of dFe and dNd–Fe–B.