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Lawrence Berkeley National Laboratory

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Rajesh Chopdekar

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last update: 03/30/2020
Tutorial - Cryo PEEM

Tutorial - Cryo PEEM

CoO

Domain image of antiferromagnetic CoO disks at low temperature.

Imaging of micron-scale phases undergoing a phase transition is one of the main applications of PEEM3. Materials often exist in different states or phases depending on an external parameter, for example pressure or temperature. Multiferroic materials exhibit several types of ordering that can coexist, compete, or couple. Magnetism is perhaps the most studied ordering phenomenon. The transition between a nonmagnetic to a magnetic state is a 2nd order phase transition which occurs at the Curie temperature (for a ferromagnet) or the Néel temperature (for an antiferromagnet). Other important ordering phenomena are superconductivity and ferroelectricity. Inhomogeneity or domain formation can result in a laterally different response of the material, which becomes visible using X-ray imaging. In coupled heterogeneous materials, for example multilayers or multiferroics, a phase transition of one component can drive a change of the other component. Utilizing element specific imaging X-PEEM isolates the response of one component to a change of the other. Temperature is often used as a driver for a phase transition and fine control of the temperature is essential. The temperature stage of the PEEM-manipulator has been designed with the goal of providing high temperature stability, fast transition speed, and small drift. In this section we will review two early applications of the low temperature imaging capability of the PEEM3 microscope.

The PEEM-3 sample manipulator, which aligns the region of interest of the sample under test in front of the objective lens of the imaging column, is a home built 5 axis device capable of ±5 mm of travel in three orthogonal linear directions and ±2 degrees of tilt about the plane of the sample to optimally align the surface normal to the optical axis of the microscope. The manipulator is capable of sub 100 nm steps for centering of the region of interest and vibrational stability better than the currently available resolution limit of the electron optics (~30 nm). The mechanism is motorized and encoded and is integrated into a fully automated transfer system that moves sample pucks between three parking docks and the microscope through a computer interface. One of the strengths of the instrument is the fairly large sample pucks that provide for almost a cubic inch of space above and beyond the baseplate needed for interfacing with the microscope manipulator. This space is used for a variety of exotic and yet to be defined custom sample holders as well as for standard heatable (20–400 °C) holders. Manipulator
Puck Cut away drawing showing a cryogenic sample holder and the interface to the sample manipulator and cooling. The sample under test is represented by the dark square in the central area of the drawing. This holder contains onboard diode and heater for temperature control, as well as an integrated electromagnet for applying magnetic field while cold. Visible in the bottom cut away is the clamping mechanism which grabs the silicon nose extending from the sample holder and connects it to the cryostat, first through a sapphire rod and then through a flexible copper braid (not shown).
Manipulator Peformance

Imaging performance of the PEEM3 under various room temperature and cold conditions. The PEEM image in the top of the figure is of a lithographically produced “star” pattern of Ni on Si imaged at the Ni L3 absorption resonance to maximize contrast. The circular arc visible near the right edge of the image is at a place where the radial features are 100 nm in size. The lithography starts to break down near the left edge of image near feature sizes of 20–30 nm. The lower left figure shows line profiles of the image intensity contrast across the radial lines at varying feature sizes as represented by the vertical boxes in the image. The lower right figure shows the reduction in imaging contrast as a function of spatial frequency for three conditions, a standard PEEM3 holder at room temperature, a PEEM3 cryogenic holder with coupling to the cryostat also at room temperature, and the same holder at liquid nitrogen temperatures with cryogen flowing. Contrast is defined as the difference over the sum of the peak to valley intensities plotted in the line profiles, with the contrast of the 150 nm features normalized to 1 for each of the three microscope conditions.

 

Exchange coupling

(a) Anti-ferromagnetic (with β = 90°) and (b) ferromagnetic domain images as a function of temperature for φ = 10°. The white lines outline the locations of a few anti-ferromagnetic domains and the edge of a white (black) stripe seen at temperature below 100 K.

The images above show the the ferromagnetic phase transition in thin ferromagnetic LSMO (La0.7Sr0.3MnO3) layers, exchange coupled to eptitaxial, antiferromagnetic LSFO (La0.7Sr0.3FeO3) layers. Ultrathin LSMO layers have a transition temperature of 150–200 K and are ferromagnets at low temperatures. LSFO films are antiferromagnets with a transition temperature above 400 K. Below the ferromagnetic transition temperature, interface exchange coupling results in a spin-flop or 90 degree arrangement of the magnetization of the LSMO layers and the magnetic axis of the LSFO layer. Above the transition temperature the LSMO becomes non-magnetic and the LSFO layers relax into their lowest energy state. X-ray Magnetic Linear Dichroism was used for imaging of the LSFO antiferromagnetic domain structure as the material is being heated through the Curie temperature of the LSMO layers.

Doran, A., M. Church, T. Miller, G. Morrison, A.T. Young, and A. Scholl, "Cryogenic PEEM at the Advanced Light Source," Journal of Electron Spectroscopy and Related Phenomena 185(10), 340-346 (2012).