Electronic structures of MgO/Fe interfaces with perpendicular magnetization revealed by hard X-ray photoemission with an applied magnetic field.

We have developed hard X-ray photoelectron spectroscopy (HAXPES) under an applied magnetic field of 1 kOe to study the electronic and magnetic states related to the MgO/Fe interface-induced perpendicular magnetic anisotropy (PMA). In this work, we used MgO (2 nm)/Fe (1.5 and 20 nm)/MgO(001) structures to reveal the interface-induced electronic states of the Fe film. Perpendicular magnetization of the 1.5-nm-thick Fe film without extrinsic oxidation of the Fe film was detected by the Fe 2p core-level magnetic circular dichroism (MCD) in HAXPES under a magnetic field, and easy magnetization axis perpendicular to the film plane was confirmed by ex situ magnetic hysteresis measurements. The valence-band HAXPES spectrum of the 1.5-nm-thick Fe film revealed that the Fe 3d electronic states were strongly modified from the thick Fe film and a reference bulk Fe sample due to the lifting of degeneracy in the Fe 3d states near the MgO/Fe interface. We found that the tetragonal distortion of the Fe film by the MgO substrate also contributes to the lifting of degeneracy in the Fe 3d states and PMA, as well as the Fe 3d-O 2p hybridization at the MgO/Fe interface, by comparing the valence-band spectrum with density functional theory calculations for MgO/Fe multilayer structures. Thus, we can conclude that the Fe 3d-O 2p hybridization and tetragonal distortion of the Fe film play important roles in PMA at the MgO/Fe interface. HAXPES with in situ magnetization thus represents a powerful new method for studying spintronic structures.

Ab initiostudy of electronic and magnetic properties of Mn2RuZ/MgO (001) heterojunctions (Z=Al, Ge).

We studied the applicability of Heusler alloys Mn2RuZ(Z= Al, Ga, Ge, Si) to the electrode materials of MgO-based magnetic tunnel junctions. All these alloys possess Hg2CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized Δ1band, which is robust against atomic disorder. Next we studied the electronic structure of Mn2RuAl/MgO and Mn2RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn2RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn2RuAl/MgO junction, high spin polarization of bulk Mn2RuAl is preserved independent of its termination.

The effect of the interface oxidation on tunneling conductance of Co(2)MnSi/MgO/Co(2)MnSi magnetic tunnel junction.

We investigate and discuss the effects of interfacial oxidation on electronic structures and tunnel conductance of the Co(2)MnSi/MgO/Co(2)MnSi(001) magnetic tunnel junction (MTJ) on the basis of first-principles calculations. It is found that the MnSi termination tends to be oxidized compared with the Co termination because of the relaxation of atomic positions in the MnSi-terminated interface. Furthermore, we found that the single oxide layer inserted on both sides of the junction greatly decreases the tunnel conductance of the MTJ in parallel magnetization. We concluded that the relaxation of the Mn atomic position in the oxidized junction reduces the coupling of the Δ(1) states between the Co(2)MnSi electrode and the MgO barrier and causes significant interfacial scattering of the majority-spin electrons with Δ(1) symmetry at the oxidized layer.

Half-metallic interface between a Heusler alloy and Si.

The interface between the half-Heusler alloy CoFeSi and Si is investigated by using first-principles density-functional calculations. Although CoFeSi has not been fabricated yet, its formation energy turns out to be negative. Within the generalized gradient approximation, CoFeSi shows nearly half-metallic properties, and its lattice constant is about 5.38 Å; this value is relatively close to the lattice constant of Si. We here chiefly investigate the CoFeSi/Si (110) interface, and find that the half-metallic properties are almost preserved at a specific (110) interface. Furthermore, the interfacial structure which leads to the high spin polarization has the lowest energy of the (110) interfacial patterns examined in this work. The half-metallicity at the interfaces is similarly observed in the densities of states projected onto delocalized sp states, and this suggests the relevance of the high spin polarization to transport properties.

Recent Progress in the Voltage-Controlled Magnetic Anisotropy Effect and the Challenges Faced in Developing Voltage-Torque MRAM.

The electron spin degree of freedom can provide the functionality of "nonvolatility" in electronic devices. For example, magnetoresistive random access memory (MRAM) is expected as an ideal nonvolatile working memory, with high speed response, high write endurance, and good compatibility with complementary metal-oxide-semiconductor (CMOS) technologies. However, a challenging technical issue is to reduce the operating power. With the present technology, an electrical current is required to control the direction and dynamics of the spin. This consumes high energy when compared with electric-field controlled devices, such as those that are used in the semiconductor industry. A novel approach to overcome this problem is to use the voltage-controlled magnetic anisotropy (VCMA) effect, which draws attention to the development of a new type of MRAM that is controlled by voltage (voltage-torque MRAM). This paper reviews recent progress in experimental demonstrations of the VCMA effect. First, we present an overview of the early experimental observations of the VCMA effect in all-solid state devices, and follow this with an introduction of the concept of the voltage-induced dynamic switching technique. Subsequently, we describe recent progress in understanding of physical origin of the VCMA effect. Finally, new materials research to realize a highly-efficient VCMA effect and the verification of reliable voltage-induced dynamic switching with a low write error rate are introduced, followed by a discussion of the technical challenges that will be encountered in the future development of voltage-torque MRAM.

Atomic ordering, magnetic properties, and electronic structure of Mn2CoGa Heusler alloy.

The magnetic properties and atomic arrangement of Mn2CoGa Heusler alloy were investigated experimentally and by theoretical calculations. The magnetic moment derived from spontaneous magnetization at 5 K was 2.06 µ B/f.u. and was close to the integer number of the expected value from theoretical calculation and the Slater-Pauling rule predicted by Galanakis et al. The Curie temperature and L21-B2 order-disorder phase transition temperature were 741 and 1047 K, respectively. Powder neutron diffraction experiment results suggested that the atomic arrangement prefers an L21b-type structure rather than that of Hg2CuTi, being consistent with our previous results of high-angle annular dark-field-scanning transmission electron microscopic observations. The magnetic moments obtained were in good agreement with the theoretical values in the model of the L21b-type structure. The density of states obtained by the first-principles calculation combined with the coherent potential approximation in Mn2CoGa with the L21b-type crystal structure maintained the half-metallic character, even though disordering by Mn and Co atoms was introduced.

The computational materials design of (Ga, Cr)N: effects of co-doping on exchange interactions.

We investigate the effect of O or Be co-doping on the exchange interaction between Cr spins in (Ga, Cr)N by means of first-principles calculations based on the density-functional theory. The ferromagnetic exchange interactions are reduced by doping Be around Cr. On the other hand, O doping reduces the ferromagnetic interaction remarkably only for the case of Cr-O-Cr complex formation. The enhancement of the ferromagnetic exchange interaction cannot be achieved by doping O or Be impurities. However, the O and Be impurities can help the clustering of Cr atoms due to the enhancement of the attractive interaction between Cr atoms.

Coherent tunnelling conductance in magnetic tunnel junctions of half-metallic full Heusler alloys with MgO barriers.

We have carried out electronic structure and transport calculations for magnetic tunnel junctions (MTJ) composed of MgO and a half-metallic full Heusler alloy Co(2)MnSi on the basis of the density functional theory and the Landauer formula. We find that the density of states of Co atoms at the Co(2)MnSi/MgO(001) interface shifts toward the higher energy side due to the reduced symmetry, leading to a reduction of the spin polarization at the interface. Furthermore, we show that the majority-spin transmittance as a function of the in-plane wavevector [Formula: see text] has a broad peak centred at [Formula: see text] due to the tunnelling from the Δ(1) channel of Co(2)MnSi, while the transmittance from the Δ(5) channel is three orders of magnitude smaller than that of the Δ(1) channel. These results indicate that coherent tunnelling through the Δ(1) band is dominant also in an MTJ with Co(2)MnSi and an MgO barrier, like in Fe/MgO/Fe(001) MTJ and related systems.

Induced perpendicular magnetization in a Cu layer inserted between Co and Pt layers revealed by x-ray magnetic circular dichroism.

We used x-ray absorption spectroscopy and x-ray magnetic circular dichroism to investigate the effects of inserting Cu into Co/Pt interfaces, and found that a 0.4-nm-thick inserted Cu layer showed perpendicularly magnetized properties induced by the proximity effect through the Co and Pt layers. The dependence of the magnetic properties on the thickness of the Cu layers showed that the proximity effects between Co and Pt with perpendicular magnetic anisotropy can be prevented by the insertion of a Cu layer with a nominal threshold thickness of 0.7 nm. Element-specific magnetization curves were also obtained, demonstrating that the out-of-plane magnetization is induced in the Cu layers of the Co/Cu/Pt structures.

Voltage controlled interfacial magnetism through platinum orbits.

Electric fields at interfaces exhibit useful phenomena, such as switching functions in transistors, through electron accumulations and/or electric dipole inductions. We find one potentially unique situation in a metal-dielectric interface in which the electric field is atomically inhomogeneous because of the strong electrostatic screening effect in metals. Such electric fields enable us to access electric quadrupoles of the electron shell. Here we show, by synchrotron X-ray absorption spectroscopy, electric field induction of magnetic dipole moments in a platinum monatomic layer placed on ferromagnetic iron. Our theoretical analysis indicates that electric quadrupole induction produces magnetic dipole moments and provides a large magnetic anisotropy change. In contrast with the inability of current designs to offer ultrahigh-density memory devices using electric-field-induced spin control, our findings enable a material design showing more than ten times larger anisotropy energy change for such a use and highlight a path in electric-field control of condensed matter.

Prevalence and risk factors for erectile dysfunction in Japanese diabetics.

We evaluated the prevalence and risk factors for erectile dysfunction (ED) and interest in ED treatment among Japanese men being treated for type 2 diabetes mellitus. Patients (40-79 years; n=1118) completed the 5-item version of the International Index of Erectile Function (IIEF-5), and questions related to interest in ED pharmacotherapy, subjective symptoms of diabetes, and general quality of life. A separate survey completed by physicians examined the relationships between age, diabetic treatments (insulin or oral), symptoms of diabetes (poor glycemic control, microangiopathy), complications of diabetes (hypertension, ischemic heart disease, cerebrovascular disease), and ED. The prevalence of ED in patients with diabetes was 90%, a rate double that of non-diabetic individuals. Multivariate analyses revealed that age, insulin therapy, microangiopathy, hypertension, history of cerebrovascular or cardiovascular disease, leg dysesthesia, dysuria, insomnia, and anorexia all represented significant risk factors for ED. Half of all respondents were interested (29%) or would consider pharmacotherapy for ED (21%). These findings suggest that ED is a significant problem in Japanese men with diabetes, and that specific risk factors increase the prevalence of ED. Furthermore, the survey results expose national attitudes toward treatment of ED.

Drastic change in density of states upon martensitic phase transition for metamagnetic shape memory alloy Ni2Mn(1+x)In(1-x).

We have unravelled the electronic structure of a class of metamagnetic shape memory alloy Ni2Mn1+x In1-x by combining bulk-sensitive hard x-ray photoelectron spectroscopy and first-principles density-functional calculations. A sharp drop in the Ni 3d e(g) density of states forming a pseudogap in the martensitic phase transition (MPT) for x = 0.36 has been observed near the Fermi level. As a feature of MPT, hysteretic behaviour of this drop has been confirmed in both cooling and warming. This pseudogap is responsible for the giant negative magnetoresistance. The experimental result is well reproduced by the first principle calculation. We have also clarified theoretically that the MPT is linked to a competition of ferromagnetic and anti-ferromagnetic coupling between ordinary and anti-site Mn atoms.

Measurement of intracavernosal catecholamine during a prostaglandin E1 test.

Background : Erectile dysfunction (ED) can be a cause of male infertility among young men. The prostaglandin E1 (PGE1) intracavernous injection test, an erectile function test, is known as an objective method of examining the penile vasculature system. However, some ED patients fail to sufficiently maintain an erection because of the stress load upon them during the test. Thus, we measured changes in catecholamine levels by stress loading when we performed dynamic infusion cavernosometry (DIC). Methods : Among the 221 ED patients undergoing a PGE1 test, 10 were selected as the subjects for the present study. These 10 patients were 25-56 years of age, with a mean of 32.8 years, whose maximal penile rigidity was normal, but in whom penile erection could not be maintained sufficiently in the PGE1 test. Stress loading via vibratory sensory measurement during the PGE1 test suppressed penile rigidity strongly. Catecholamine levels were measured by using blood samples obtained from the corpus cavernosum and the cubital vein when erection was suppressed by stress loading, pain caused by needle insertion, and also when erection recovered from the suppression. Dynamic infusion cavernosometry was performed after each blood sampling time, to check corporeal veno-occlusive function and cavernosal arterial flow. Results : Penile norepinephrine levels were 0.20 ± 0.06 ng/mL during a suppressed erection and 0.15 ± 0.03 ng/mL during an erection, showing a significantly higher level (Student's t-test, P = 0.0309) during suppressed erection. The results of the DIC measurement indicated a normal corporeal veno-occlusive function and cavernosal artery in all cases. Conclusion : The results of the present study revealed that corporeal veno-occlusive function and cavernosal arterial flow are normal in men who have normal maximum penile rigidity, but cannot sufficiently maintain the erection in the PGE1 test, suggesting the involvement of stress-induced penile norepinephrine in the suppression of erection maintenance. (Reprod Med Biol 2002; 1: 11-15).

The origin of perpendicular magneto-crystalline anisotropy in L1(0)-FeNi under tetragonal distortion.

We investigated the origin of perpendicular magneto-crystalline anisotropy (MCA) in L1(0)-ordered FeNi alloy using first-principles density-functional calculations. We found that the perpendicular MCA of L1(0)-FeNi arises predominantly from the constituent Fe atoms, which is consistent with recent measurements of the anisotropy of the Fe orbital magnetic moment of L1(0)-FeNi by means of x-ray magnetic circular dichroism. Analysis of the second-order perturbation of the spin-orbit interaction indicates that spin-flip excitations between the occupied majority-spin and unoccupied minority-spin bands make a considerable contribution to the perpendicular MCA, as does the spin-conservation term for the minority-spin bands. Furthermore, the MCA energy increases as the in-plane lattice parameter decreases (increasing the axial ratio c/a). The increase in the MCA energy can be attributed to further enhancement of the spin-flip term due to modulation of the Fe d(xy) and d(x(2) - y(2)) orbital components around the Fermi level under compressive in-plane distortion.