Invited Paper SP+AS+MI+NS+SS-TuA9
Spin-polarized Scanning Tunneling Microscopy on Surfaces Prepared by Molecular Beam Epitaxy

Tuesday, November 8, 2016, 5:00 pm, Room 104A

Spin-polarized scanning tunneling microscopy (SP-STM) has proven to be a powerful in-situ technique for obtaining detailed information about spin structures at surfaces down to atomic scale.¹ It has been applied extensively to investigate pristine ferromagnetic and antiferromagnetic (aFM) transition metal surfaces, with many great results in the case of model systems such as nano-sized magnetic islands and single magnetic monolayers.² This has led to fascinating discoveries of nanoscale magnetic domains, domain walls, spin spirals, spin skyrmions, and much more.^3,4 Although not simple in practice, SP-STM can in principle also yield unprecedented spin characterization on a broad spectrum of material surfaces, including practical, real world systems. For example, it could be applied to investigate surfaces of intermetallic compounds, superconductors, complex magnetic oxides, and magnetic semiconductors.

We are applying SP-STM to study various magnetic systems grown in-situ by molecular beam epitaxy, including transition metal nitrides,⁵ magnetic-doped nitride semiconductors, and several bi-metallic magnetic systems. I will present our recent work using STM and SP-STM, beginning with a discussion of manganese nitrides, including our work on aFM θ-phase MnN and ferrimagnetic ε-phase Mn₄N. The θ-phase films are very complex due to the expectation of canted spins within each atomic layer with four unique canting angles, while the ε-phase films contain two types of spins (Mn^I and Mn^II) with equally complex spin arrangements.

A second material we are working on is the chromium nitride system in which we investigate its electronic and spin properties in a low-temperature SP-STM system. Spectroscopy results to date suggest a d-wave resonance on the surface and a Kondo signature for nanoscale iron islands grown on atomically-smooth CrN surfaces.

I will also present results for Mn δ-doped semiconducting gallium nitride surfaces in which we find atomic layer ferromagnetism within a unique and stable √3 x √3 - R30° MnGaN surface reconstruction. Spectroscopy clearly reveals spin-polarized and spin-split Mn states, as predicted by first principles theory calculations. SP-STM measurements map out ferromagnetic domains at room temperature, and the additional presence of magnetic rim states seen at the edges of ferromagnetic islands, as well as magnetic hysteresis, give further interest to this intriguing system.

¹ R. Wiesendanger, Rev. Mod. Phys. 81, 1495 (2009).

² M. Bode et al., Phys. Rev. Lett. 92, 67201 (2004).

³ P. Ferriani et al., Phys. Rev. Lett. 101, 027201 (2008).

⁴ S. Loth et al. Science 335, 196 (2012).

⁵ K.K. Wang and A.R. Smith, Nano Lett. 12, 5443 (2012).