M. Ogura

First-principles calculation of the instability leading to giant inverse magnetocaloric effects

The structural and magnetic properties of functional Ni-Mn-Z (Z=Ga, In, Sn) Heusler alloys are studied by first-principles and Monte Carlo methods. The ab initio calculations give a basic understanding of the underlying physics which is associated with the strong competition of ferro- and antiferromagnetic interactions with increasing chemical disorder. The resulting d-electron orbital dependent magnetic ordering is the driving mechanism of magnetostructural instability which is accompanied by a drop of magnetization governing the size of the magnetocaloric effect. The thermodynamic properties are calculated by using the ab initio magnetic exchange coupling constants in finite-temperature Monte Carlo simulations, which are used to accurately reproduce the experimental entropy and adiabatic temperature changes across the magnetostructural transition.

First-principles calculation of the Curie temperature Slater–Pauling curve

It is well known that the magnetizations as a function of the valence electron number per atom of 3d transition metal substitutional alloys form the so-called Slater-Pauling curve. Similarly, the Curie temperatures of these alloys also show systematic behaviour against the valence electron number. Though this fact has long been known, no attempt has been made so far to explain this behaviour from first principles. In this paper we calculate T(C) of 3d transition metal alloys in the framework of first-principles electronic structure calculation based on the local density approximation.

Quaternary Ni?Mn?In?Y Heusler alloys: a way to achieve materials with better magnetocaloric properties?

The influence of nonmagnetic impurities (Y) on the magnetocaloric effect (MCE) in the Heusler alloy Ni50Mn34In16Y in which ferromagnetic and antiferromagnetic coupling coexists in the martensitic state is investigated by means of ab initio and Monte Carlo simulations. Different impurity configurations are considered. Either Mn on the Mn sublattice or Ni on the Ni sublattice is substituted randomly by Y or substitution may occur simultaneously on both sublattices. The magnetic and magnetocaloric properties are obtained as a function of temperature and magnetic field using a mixed type of Potts and Blume?Emery?Griffiths model where the model parameters are obtained from ab initio calculations. When varying the impurity concentration, the shape of the magnetization curves is drastically changed at the martensitic transformation temperature due to the complex competing magnetic interactions. We show that the inverse MCE effect increases if Mn is substituted by Y compared with Y substituting for Ni. For example, the alloy system Ni50Mn32.75Cu1.25In16 where 5?at% Mn is replaced by Cu exhibits an inverse MCE which is twice as large compared with Ni50Mn34In16. Such predictive simulations may be used to design Heusler composites which display larger MCEs.

Monte Carlo simulations of diluted magnetic semiconductors using ab initio exchange parameters.

Co doped ZnO (Zn(1-x)Co(x)O) is studied as a prototype material for transition metal doped II-VI diluted magnetic semiconductors (DMSs) from first-principles and Monte Carlo simulations. The exchange interactions are calculated using the Korringa-Kohn-Rostoker (KKR) Greens function method. The exchange coupling constants thus obtained are treated in the classical Heisenberg model and the magnetic phase transitions are studied by the Monte Carlo technique. Our results show that the defect free substitutional DMSs of Zn(1-x)Co(x)O do not sustain magnetization at low concentration. At high concentration, we find layered magnetic structures. Ferromagnetism, with Curie temperature below room temperature, is stable at intermediate Co concentrations. First-principles studies with the generalized gradient approximation (GGA) and the GGA together with the Hubbard U are discussed with respect to structural and electronic properties of ZnO.

New type of half-metallic antiferromagnet: transition metal pnictides.

The electronic structures of transition metal pnictides ABX(2), where A and B are the transition metal elements and X = N, P, As, Sb, and Bi, with the total valence d-electron number of the transition metal ions being ten, are investigated in the framework of the first-principles KKR Greens function method. Some possible crystal structures such as NiAs-type, NaCl-type, chalcopyrite, zinc-blende, wurtzite, and MnP-type structures are assumed. Similarly to chalcogenides, a new type of spin-compensated half-metallic ferrimagnet is found for the case of nitrides. The stability and magnetic transition temperature of these nitrides indicate that they are good candidates for spintronics materials. For other cases of pnictides such as P, As, Sb, and Bi, the half-metallicity seems not to be realized.

Calculated electronic structures and Néel temperatures of half-metallic diluted antiferromagnetic semiconductors

The possibility of half-metallic diluted antiferromagnetic semiconductors of II-VI compounds is investigated on the basis of first-principles electronic structure calculation. The electronic structures of ZnS, ZnSe, ZnO, CdS and CdSe doped with two kinds of 3d transition metal ions are calculated using the Korringa-Kohn-Rostoker (KKR) method and their magnetic transition temperatures are determined using a cluster-type approximation. It is predicted that II-VI compound semiconductors doped with two kinds of magnetic ions might be good candidates for half-metallic antiferromagnets.

Quadrupole moments of neutron-deficient 20,21Na

Abstract The electric-quadrupole coupling constant of the ground states of the proton drip line nucleus 20Na ( I π = 2 + , T 1 / 2 = 447.9 ms ) and the neutron-deficient nucleus 21Na ( I π = 3 / 2 + , T 1 / 2 = 22.49 s ) in a hexagonal ZnO single crystal were precisely measured to be | e q Q / h | = 690 ± 12 kHz and 939 ± 14 kHz , respectively, using the multi-frequency β-ray detecting nuclear magnetic resonance technique under presence of an electric-quadrupole interaction. An electric-quadrupole coupling constant of 27Na in the ZnO crystal was also measured to be | e q Q / h | = 48.4 ± 3.8 kHz . The electric-quadrupole moments were extracted as | Q ( Na 20 ) | = 10.3 ± 0.8 e fm 2 and | Q ( Na 21 ) | = 14.0 ± 1.1 e fm 2 , using the electric-coupling constant of 27Na and the known quadrupole moment of this nucleus as references. The present results are well explained by shell-model calculations in the full sd-shell model space.

A Possible Nuclear Spin Dewar. Hyperfine Interactions of Short-Lived β Emitter 8Li and 12B in TiO2

It was confirmed by detecting the β-NMR of 12B (Iπ=1+, T1/2=20.2 ms) in a TiO2 (rutile) crystal that the nuclear spin polarization of 12B was totally maintained in the crystal as produced through a nuclear reaction before implantation. Two locations, site 1 and site 2, were found with the relative populations 9 and 1, respectively, and the electric field gradients (EFGs) at those sites were obtained to be q(site 1)=+(37.1±0.5)1015 V/cm2, η(site 1)<0.03, q(site 2)=+(185±5)1015 V/cm2 and η(site 2)=0.62±0.02. We also found that about 30% of the initial polarization of 8Li (Iπ=2+, T1/2=838 ms) was maintained in the crystal. Since the polarizations of other β emitting nuclei, 12,16N, 13,19O, and 41Sc were also totally maintained in the crystal, the crystal can be a “Spin Dewar” in which many short-lived nuclides can be implanted with their polarizations totally maintained during their lifetimes for the studies not only on the electronic structure of impurities in it but also on the nuclear properties.

Electric-field-driven nuclear spin control using diluted magnetic semiconductors

We propose a technique for electric-field-controlled nuclear magnetic resonance (NMR) using a heterostructure constructed from diluted magnetic semiconductors. The effect on NMR of a bias voltage on an (In,Mn,As)As∕AlSb system is calculated using first-principles electronic structure methods. The calculations show that the hyperfine field at the antisite As is very sensitive to an external electric field. This technique can be used to control nuclear spins in quantum computer that uses nuclear spins as its quantum bits.

Test of the conserved vector current hypothesis by a β -ray angular distribution measurement in the mass-8 system

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