AVS 67 Session LS-Invited On Demand: New Trends in Structural Electronic Characterization of Materials, Interfaces, and Surfaces Using Synchrotron and FEL Based Radiation Sources Invited On Demand Session

Soft x-ray resonant inelastic x-ray scattering (RIXS) spectroscopy has made many advances in the past few years mainly due to improved experimental facilities at third generation synchrotron sources [1].

In this talk the technique will be introduced and the current possibilities offered by soft x-ray RIXS will be demonstrated using results from the ESRF. In particular, some of the work on transition metal oxides like the nickelates [2,3], the cuprates [4] and iron [5] will be highlighted.

[1] Brookes et al. NIMA 903, 175 (2018).

[2] Betto et al. PRB 96, 020409(R) (2017).

[3] Lu et al. PRX 8, 031014 (2018).

[4] Peng et al. Nat. Phys. 13, 1201 (2017)

Photoelectron momentum microscopy (MM) is an alternative approach for ARPES, combining full-field k-space imaging with hemispherical or time-of-flight (ToF) energy analyzers. The heart of ToF-MM is a fast delay-line detector (DLD), recording position and time t of each counting event (at rates up to ~5x10⁶ cps). 3D (k_x,k_y,t)-recording is advantageous in low-intensity experiments with soft or hard X-rays, spin mapping, or time-resolved ARPES at FEL sources. This contribution gives an overview with examples and an outlook on ongoing developments.

MM with X-ray excitation gives access to 4D bulk spectral functions ρ(E_B,k), yielding bands, Fermi surfaces and -velocity distributions [1]. The band structure is captured in a tomographic-like mode via direct transitions to a final-state sphere, whose k-radius is tuned via the photon energy. PETRA III (Hamburg) provides 50ps photon pulses with 192ns period (40-bunch mode) at beamlines P04 (hν=300-1700eV; resolving power >10⁴) and P22 (2300-7500eV; r. p. up to 10⁵ with Si(333) monochromator). Information depths of 10-20nm allow studying buried structures like a 2D e-gas at an inner interface or bulk bands of a Heusler compound capped with 2nm MgO or 1nm Au.

Imaging spin filters are powerful tools complementing MM [2]; the effective figure-of-merit increases when exploiting ToF as third “coordinate” [3]. Using circ.-pol. soft X-rays, we uncovered a relation between the Fano spin component (along the photon helicity), the perpendicular spin component (oriented like spin in Mott scattering) and the circular dichroism [4]. Hard X-ray spin-MM allowed quantifying the spin gap in the half-metallic Heusler ferromagnet Co₂MnSi and the degree of (bulk) spin polarization close to E_F in magnetite, being controversially discussed for decades.

First experiments at FLASH (DESY, Hamburg) suggest a large potential of ToF-MM for fs pump-probe photoemission [5]. Event coordinates (E_B,k), arrival time and intensity of the corresponding FEL pulse, delay and parameters of the pump pulse are streamed in real-time and sorted into a multidimensional histogram memory. Current technical improvements concern a new objective lens without extractor field, correction and suppression of space-charge shifts, and a novel dispersive-plus-ToF hybrid MM. A drift section and fast DLD (<80ps time res.) behind a large single hemisphere will facilitate ToF at synchrotrons with 500MHz multibunch filling.

[1] K. Medjanik et al., Nat. Mat.16, 615 (2017) and J. Synchr. Rad.26, 1996 (2019); [2] C. Tusche et al., Ultramicrosc. 159, 520 (2015); [3] G. Schönhense et al., Ultramicrosc. 183, 19 (2017); [4] D. Vasilyev et al., J. Phys. C 32,135501 (2020); [5] D. Kutnyakhov et al., Rev. Sci. Instrum. 91, 0130109 (2020)

In many areas of science and the world, competition is seen as an opportunity to obtain improved performance or results. Utilizing many techniques (bulk magnetometry, neutron reflectometry and resonant x-ray magnetic scattering), we have discovered and explored the existence of competing magnetic phases in many single layer thin films that results in giant negative magnetization. We have focused on the system of complex oxide La_0.7Sr_0.3MnO_3. While transmission electron microscopy images show pristine epitaxial growth, the data supports that there are regions of different magnetic order. This results in interesting magnetic measurements, that share similarities with ferrimagnets with competing magnetic lattices. This competition results in spontaneous negative magnetization that aligns counter to a small applied magnetic field and inverted hysteresis loops near room temperature. This behavior has much in common with superparamagnetic nanoparticles. In this talk, the time, field and temperature dependence of these samples will be discussed to help understand this phenomenon. The switch from negative to positive magnetization effectively doubles the change in magnetization, important for some types of devices. We acknowledge funding support from NSF (DMR-1608656) for growth and optimization and DOE (DE-SC0016176) for the magnetic characterization of our films.