Spintronics, Interface

Fujimori groupResearchResearch subjectsHigh-temperature superconductivity, Spintronics, Interface

What is the spintronics?

Spintronics, the combined technology of both electronics (charge control) and magnetics (spin control), is receiving a lot of attention. We are studying half-metals and diluted magnetic semiconductors as materials contributing to spintronics devices.

Tunnel magnetoresistance device

Tunnel magnetoresistance device

Electronic structure of a half-metal

Electronic structure of a half-metal

Half-metal

The dependence of XMCD spectra upon Mn content of the Heusler alloy Co2MnβGe0.38

The dependence of XMCD spectra upon Mn content of the Heusler alloy Co2MnβGe0.38.

Crystal structure of the Heusler alloy X2YZ

Crystal structure of the Heusler alloy X2YZ


Half-metallicity was increased with decreasing Mn content.

A half-metal has a specific electronic structure. The valence band of electrons with one spin orientation crosses the Fermi level (metallic), while another band for electrons with opposite-oriented spin has a band gap at the Fermi level (semiconductive). Therefore, a half-metal is a promising material as the source of perfectly spin-polarized electrons. Heusler alloys and La1-xSrxMnO3 are examples of half-metal.

Angle dependent XMCD of the half-metallic oxide thin film

Angle-dependent XMCD spectra of the half-metallic oxide La0.6Sr0.4MnO3 thin film.

Angle-dependent XMCD spectra of the half-metallic oxide La0.6Sr0.4MnO3 thin film.

This figure shows angle-dependent XMCD spectra of a La0.6Sr0.4MnO3 thin film, a half-metallic oxide. The XMCD spectra change the sign at the magnetic-field angle θH = 60º, which indicates that the electron spins are aligned perpendicular to the incident x-ray. This result show the tendency that the electron spins are more preferentially oriented along the film in-plane direction than the out-of-plane direction. Also, the observed angular dependence of the XMCD intensity is quantitatively reproduced by a simulation incorporating two types of magnetic anisotropy: a macroscopic electromagnetic effect [shape anisotropy (SA)] and an effect originating from microscopic electronic structure [magnetocrystalline anisotropy (MCA)].

Diluted magnetic semiconductor

XAS, XMCD spectra of the diluted magnetic semiconductor Zn1-xCoxO.

XAS, XMCD spectra of the diluted magnetic semiconductors
Zn1-xCoxO.

Diluted magnetic semiconductors, in which a portion of atoms of the nonmagnetic semiconductor hosts are replaced by magnetic ions, are key materials for spintronics, which is intended to manipulate both the spin and charge degrees of freedom by use of coupling between the spins of the magnetic ions and the charge carriers of the host semiconductors.This figure shows comparison between the experimental XMCD spectra and theoretical XMCD spectra calculated using atomic multiplet theory. The valence of metal ion, crystal structure and interaction between ligands are determined to reproduce the spectra.

n-type diluted magnetic semiconductor, (In,Fe)As:Be

XAS, XMCD spectra of the n-type diluted magnetic semiconductor(In,Fe)As:Be.

XAS, XMCD spectra of the n-type diluted magnetic semiconductor(In,Fe)As:Be.

(In,Fe)As:Be is a recently discovered rarely reported n-type diluted magnetic semiconductor. Be atoms are doped as double donor, and ferromagnetism is introduced due to the presence of doped electrons which mediate magnetic coupling between the Fe ions. Our XMCD study revealed the existence of nano-scale ferromagnetic domains originating in a spatial fluctuations of Fe concentration on the nano-scale even above the Curie temperature.

High MCA originating in L10 order

XMCD study of the SiO2-coated FePt nanoparticles.

XMCD study of the SiO2-coated FePt nanoparticles.

In order to increase the storage density of hard disk drive (HDD), materials with high magnetocrystalline anisotropy (MCA) have been intensively studied. Especially, FePt in the L10-ordered phase is receiving high attention as a promising material for next generation HDD because of the high MCA originating from the crystal structure.We have investigated Fe in SiO2-coated FePt nanoparticles by using high magnetic field XMCD spectroscopy and revealed an element specific field dependence of magnetization up to saturation region. The shape of the M-H curve is well explained by the Stoner-Wohlfarth model for non-interacting single-domain nanoparticles. We conclude that the origins of the high coercivity (Hc) are the formation of single domains in nanoparticles and the weakness of interactions between the nanoparticles.


What is the interface between correlated materials ?

The material containing electrons with strong electron-electron interaction is called strongly correlated material. The high temperature superconductivity in cuprates or iron arsenides and the colossal magnetoresistance in manganese oxides are well-known phenomena found in strongly correlated materials. Nowadays, development of a technology of the film growth makes it possible to study the thin films of strongly correlated materials, therefore investigations of completely new physical properties emerged at interfaces between different strongly correlated materials are increasing.

Interplay between spin, charge and orbital degrees of freedom.

Interplay between spin, charge, and orbital degrees of freedom.

Typical phase diagram of strongly correlated materials.

Typical phase diagram of strongly correlated materials.

Interface between strongly correlated materials.

Interface between strongly correlated materials.

Pulsed laser deposition technique.

Pulsed laser deposition technique.

A target is evaporated by laser pulses and vapor of the target is to be deposited and forms a thin film. This technique can control the layer-by-layer growth and, therefore, high-quality single crystalline films and superlattice films can be obtained.

Studies on thin films and interfaces grown by the pulsed laser deposition (PLD) technique

Angle resolved photoemission spectroscopy of the SrVO<sub>3</sub> thin film

Angle-resolved photoemission spectroscopy of SrVO3 thin films.

Precise observations of the Fermi surfaces and the quasi-particle band dispersions in SrVO3 are enabled by thin film growth of this material. A kink at the phonon energy was observed in quasi-particle band dispersions, therefore, strong electron-phonon interaction was confirmed.

XMCD spectra of the La<sub>0.6</sub>Sr<sub>0.4</sub>MnO<sub>3</sub> thin films

XMCD spectra of La0.6Sr0.4MnO3 thin films.

A decrease of magnetization of the ferromagnetic metal La0.6Sr0.4MnO3 corresponding to the decrease of the film thickness was observed.