SPEEM: The photoemission microscope at the dedicatedmicrofocus PGMbeamlineUE49-PGMa at BESSY II

The UE49-PGMa beamline hosts a photoemission electron microscope (PEEM) dedicated to spectromicroscopy and element-selective magnetic imaging on the nanometer scale. The instrument is an Elmitec PEEM III equipped with energy lter and Helium cooled manipulator. Laser driven excitations can be studied using an attached Ti:Sa laser. A variety of customized sample holders is available for imaging in moderate magnetic / electric eld, temperature control, or local laser excitations. With x-rays the instrument is capable of 30 nm spatial resolution.


Introduction
Magnetic nanostructures are at the heart of modern data storage technology.Typical dimensions of magnetic bits are in the sub-100 nm region.In addition novel magnetoelectronics devices such as magnetic random access memory junctions are operated on the sub-100 nm m scale.An understanding magnetic properties of such low-dimensional structures is only accessible to spectro-microscopy tools capable of appropriate lateral resolution.This goal is achieved by combining a photoemission microscope (SPEEM) with a dedicated microfocus PGM beamline (UE49 PGM).High photon ux in combination with full polarization control makes this setup the ideal tool for space resolved and element selective investigation of nanostructures by means of chemical maps (X-ray absorption spectroscopy (XAS)) and magnetic imaging (X-ray magnetic circular dichroism (XMCO) and X-ray magnetic linear dichroism (XMLD)).

Instrument application
The particular strength of this instrument is the element speci city and quantitative magnetic contrast at high spatial resolution in combination with a variable sample environment.The instrument has been equipped with a LHe cryostat for sample temperatures down to 45 K. Special sample holders have been developed.Some of them combine temperature control in a range from 45 K to 600 K with the application of magnetic elds of up to 75 mT and a voltage applied to the sample during imaging (Sandig et al., 2012).Customized power supplies and a dedicated software control allows for special features, such as lens tracking during application of magnetic or electric eld, on-the-y data recording, sub Kelvin temperature control and stabilization, or macro based data acquisition.A Ti:Sa laser system attached to the microscope can be used to study laser driven e ects such as phase changes or magnetic switching.Di raction limited laser spot sizes can be reached with a dedicated sample holder (Gierster, Pape, et al., 2015).
Di erent modes of operation are possible.Imaging of secondary electrons allows for XAS spectroscopy or magnetic imaging with XMCD or XMLD contrast.Due to an energy lter also spatially resolved photo electron spectroscopy (XPS) and even angle resolved photo emission spectroscopy (micro ARPES) at kinetic energies of up to 1000 eV is possible.Even depth resolved XPS using the standing wave technique can be done (Gray et al., 2010;Kronast et al., 2008).At typical working conditions of the microscope the eld of view is about 3 -10 µm and matches ideally with the x-ray spot size of 10x20 µm.The photon ux provided by the 1200 l/mm grating allows electron count rates close to the space charging limit and is su cient to optimize spatial resolution and collection e ciency.Frame rates of 1-3 s at 5 µm eld of view are possible.Some examples of typical applications are listed below: • Chemical maps by XAS and XPS (Fang et al., 2014;Moreno et al., 2010) • Magnetic domain imaging by XMCD and XMLD (Boeglin et al., 2009) • Phasetransitions / temperature dependent measurements (Ewerlin et al., 2013) • Field dependent measurements (Kronast et al., 2011) • Micro-spectroscopy on nanostructures, magnetic responses and interactions, probing of core shell structures (Kimling et al., 2011) • Magnetic transport and spin torque (Heyne et al., 2010) • Magnetic/magnetoelectric coupling in thin lms and multifoerroics (Cheri et al., 2010) • Laser induced magnetic switching or phase changes (Gierster, Ünal, et al., 2015) • Time-resolved magnetization dynamics (fs-laser pump, X-ray probe) (Miguel et al., 2009) 3 Source The insertion device is the elliptical undulator UE49 with the following parameters:

Optical design
The UE49-PGMa beamline is one of three branches at the UE49 insertion device, an Apple II-type undulator with full polarization control.For highest brilliance the UE49 is located in one of the low-beta sections of the BESSY II storage ring.A schematic layout of the UE49-PGMa beamline is shown in Figure 2. The beamline comprises ve optical elements, four mirrors and one grating.The cylindrical mirror M1 and the toriodal mirror M3 serve as switching mirror units and distribute the beam to neighboring branches.M4 is an ellipsoidal refocussing mirror, optimized for high transmittance and small spot size.The x-ray spot on the sample is a de-magni ed image of the exit slit.At an incidence angle of 74°and a slit opening of 200 µm the spot measures 20 µm in horizontal and 10 µm in vertical direction (FWHM).The beamline is equipped with a plane grating monochromator that covers an energy range from 80 to 1800 eV.With the nest grating (1200 l/mm) a spectral resolution (E/∆E) of 10000 at 700 eV can be achieved.The photon ux ranges from 10 11 to 10 13 ph/s/100 mA.A detailed ux table for this grating is shown in Figure 3. Using di erent gratings with a lower line density (600 l/mm and 300 l/mm) the photon ux can be increased at the expense of spectral resolution.Main parameters of the UE49-PGMa beamline are summarized in Table 1 and Table 2.

Table 2 :
Technical parameters for the SPEEM station and the UE49-PGMa beamline