Electronic materials Ultrahigh Spatial Resolution Magnetic Imaging of Oxide Surfaces and Interfaces by the Development of Laser-PEEM

An entirely new PEEM having the world’s highest resolution

In order to clarify fine structures and properties such as magnetism on the nanoscale, a photoemission electron microscope (PEEM) using synchrotron radiation has commonly been employed to achieve spatial resolutions on the order of 20 nm. As an upgrade to the PEEM, we developed an entirely new photoemission electron microscope incorporating both PEEM and an ultraviolet laser (laser PEEM). Since use of a continuous wave laser minimizes the effect that repulsion among emitted electrons has on spatial resolution (the space charge effect), we succeeded in achieving a spatial resolution of 2.6 nm.

Confirming ferromagnetism in oxides with the laser PEEM

The two oxides lanthanum aluminate (LaAlO₃) and strontium titanate (SrTiO₃) are both insulators that do not conduct electricity. However, we know that when the two oxides are laminated together to form a LaAlO₃/SrTiO₃ thin film, a two-dimensional layer with high electron mobility (two-dimensional electron gas) emerges at the interface of the thin film as the thickness of LaAlO₃ increases and exhibits superconductivity and ferromagnetism simultaneously (Figure 1). Observing this structure with the laser PEEM revealed that the LaAlO₃/SrTiO₃ thin film interface is densely packed with ferromagnetic domains between 30 and 40 nm in size, indicating the presence of ferromagnetism at the interface.

On the other hand, it is known that two-dimensional electron gas is produced on the surface of SrTiO3 that has been flash-annealed (a rapid heat treatment) in vacuum. Our research showed that flash-annealed SrTiO₃ possesses ferromagnetic domains that are 30–40 nm in size, as illustrated in Figure 2, with a ferromagnetic transition temperature of approximately 900K. These results provide evidence that the interface of LaAlO₃/SrTiO₃ and the surface of SrTiO₃ exhibit ferromagnetism.

Utilizing the ferromagnetism of these oxides offers the potential to develop low-cost, high-capacity magnetic storage devices free of rare-earth metals.

Figure 1　Band structures at the interface of LaAlO₃/SrTiO₃ (left) and the annealed SrTiO₃ surface (right)

Figure 2　a: PEEM image of the annealed SrTiO₃ surface (dark lines correspond to a surface step structure), b: magnetic circular dichroism (MCD) PEEM image of the flash-annealed SrTiO₃ surface in vacuum, c: images in a and b superimposed (dark lines correspond to the step structure), d: PEEM image of the SrTiO₃ surface at 1,000K (higher than the Tc) in which the colors indicating MCD signals have disappeared