Ultrafast resonant magnetic scattering at the soft X-ray free-electron laser FLASH

Abstract

The quest for smaller and faster magnetic storage devices is a formidable challenge in modern magnetism. Ideally, one would like to probe elementary magnetization dynamics such as spin-flip processes and their coupling to the electronic system on their intrinsic time scales in the femtosecond (fs) regime. At the same time nanometer spatial resolution and element-specific information is required in order to account for the complex composition of technologically relevant magnetic media and devices. Simultaneous fulfilment of these requirements mandates ultrafast magnetic scattering experiments using flashes of resonantly tuned soft X-rays, in particular for the technologically relevant transition metals Cr, Mn, Fe, Co, and Ni with 2p electron binding energies between approximately 550 and 900 eV. Such experiments can be anticipated in the near future given the current construction of X-ray free-electron lasers (FEL) in the USA, Japan, and Germany. At present the world's most powerful FEL -FLASH in Hamburg, Germany -provides uniquely intense coherent short pulses in the extreme ultraviolet (EUV) energy range with the shortest fundamental wavelength of 6.5 nm. Here we report on the first ultrafast resonant magnetic scattering experiments using the free electron laser facility FLASH at DESY in Hamburg. The experiments have been performed at the Co L3 edge with FLASH lasing at the fifth harmonic and at the Co M edge using the fundamental wavelength at 20.9 nm. We show that a magnetic small angle scattering signal from a Co/Pt multilayer can be recorded with a single FEL pulse of 30 femtosecond duration. Damage thresholds and FEL induced changes in the magnetic properties of the samples as evident in the SAXS signal will be discussed. First experimental results on pump-probe experiment using 120 fs optical pump pulses and 30 fs long FEL probe pulses will be reported [1], [2].[1] Gutt, C. ; et al. Phys Rev B 79, 212406 (2009 The majority of rare earth metal -coinage metal -dipnictides LnMX 2 (Ln = La, Ce -Lu; M = Cu, Ag, Au; X = P, As, Sb) shows antiferromagnetic order at temperatures belowSimultaneously, characteristic upturns in the resistivity curves were observed for some of these compounds below T N which are discussed as possible Kondo systems [1]. For none of the LnMX 2 compounds the magnetic structure has been reported yet. Starting with CeAgAs 2 neutron diffraction experiments on powder samples were performed to elucidate its magnetic structure. CeAgAs 2 adopts a distorted HfCuSi 2 -type crystal structure (space group Pmca, [3]) and orders antiferromagnetically at T N = 6 K. In susceptibility measurements an additional metamagnetic transition at T m = 4.8 K was observed as well as a field dependence of the susceptibility [2]. In our diffraction experiments in the temperature interval 3.5 ≤ T ≤ 295 K no peak splitting or broadening indicating a nuclear phase transition were observed. In the neutron scattering studies the magnetic ordering can be monitored by superstructure reflections which appear at T N and inc...