Analysis of the microstructure of bulk MgB2 using TEM, EBSD and t-EBSD

EBSD analysis can provide information about grain orientation, texture and grain boundary misorientation of bulk superconducting MgB₂ samples intended for supermagnet applications. However, as the grain size of the MgB₂ bulks is preferably in the 100-200 nm range, the common EBSD technique operating in reflection mode works only properly on highly dense samples. In order to achieve reasonably good Kikuchi pattern quality on all types of MgB₂ samples, we apply here the newly developed transmission EBSD (t-EBSD) technique to spark-plasma sintered MgB₂ samples. This method requires the preparation of TEM slices by means of focused ion-beam milling, which are then analysed within the SEM, operating with a custom-built sample holder. To obtain multiphase scans, we identified the Kikuchi pattern of the MgB₄ phase which appears at higher reaction temperatures and may act as additional flux pinning sites. We present here for the first time EBSD mappings of multiple phases, which include MgB₂ , MgB₄ and MgO. LAY DESCRIPTION: The electron backscatter diffraction (EBSD) technique operating in the scanning electron microscope provides information on the crystallographic orientation the material by recording Kikuchi patterns. In polycrystalline samples, it becomes possible to analyse the orientations of the grains to each other. The metallic superconductor with the currently highest superconducting transition temperature, MgB₂ with a T_c of 38.5 K, can be used in applications in polycrystalline form. One such application of interest are trapped field magnets or supermagnets, where the superconductor cooled in an applied magnetic field can trap the magnetic field as vortices at numerous flux pinning sites in the sample. When the external magnetic field is removed, the sample will stay magnetised as long as it is kept cool, and importantly, the trapped magnetic fields can be much higher as for any permanent magnet. However, the small size of the MgB₂ grains in the 100-200 nanometre range requires a different approach when using the EBSD technique on such samples. The recently developed EBSD technique working in transmission mode (t-EBSD) helps considerably to image such materials. In this approach, a tiny TEM slice has to be milled out from the original sample by using focused ion beam milling. To understand the properties of the flux pinning in the spark-plasma sintered MgB2 sample, we had to identify the Kikuchi pattern of MgB₄ , which is another, non-superconducting phase appearing at higher reaction temperatures required to compact the material. Using this information, we could perform EBSD scans using three different phases, MgB₂ , MgB₄ and MgO. The EBSD mappings enable to see where the secondary phase particles are located in the sample, and to judge if the particles could work as flux pinning sites.

Topochemical growth of textured polycrystalline barium hexaferrite from oriented antiferromagnetic alpha-FeOOH nanorods

Nanorods of goethite, i.e. alpha-FeOOH, were mixed with BaCO3, dispersed in a polymer solution, and oriented under a 90 kOe magnetic field during polymerization. The orientation arose principally from the interaction of the magnetic field with the anisotropic antiferromagnetism of the goethite particles. The oriented antiferromagnetic particles act as seeds for the topochemical growth of BaFe12O19 ferrite grains along the [0001] direction. The degree of grain orientation was determined using magnetic measurements and orientation distribution functions and pole figures determined by electron backscatter diffraction analysis.

Advanced cantilevers for magnetic force microscopy and high frequency magnetic force microscopy

In order to improve the spatial resolution achieved by magnetic force microscopy (MFM) technique and its derivatives, we employ here advanced MFM tips fabricated by means of focused ion beam (FIB) milling. The magnetic coating applied on these tips is a CoCr film of 10 nm thickness. The MFM measurements on hard disk test samples reveal the achieved high resolution, and the measurement on a garnet film demonstrates the low invasiveness. High-frequency MFM (HF-MFM) is a development of the MFM technique to observe the HF stray fields emerging from magnetic recording writer poles at their operating conditions. By means of HF-MFM, magnetic recording writer poles are characterized in the frequency range 100-1,000 MHz. Up to now, all HF-MFM experiments conducted were using standard MFM cantilevers. From the HF-MFM images obtained using the advanced MFM cantilevers, it is clearly seen that the spatial resolution is considerably improved over the images obtained using standard MFM tips. However, the 10 nm thick magnetic coating of the cantilevers is found to work properly only at frequencies of up to about 500 MHz.

Recent advances in magnetic force microscopy

During the past ten years magnetic force microscopy (MFM) has become probably the most powerful general-purpose method for magnetic imaging. MFM can be applied under various environmental conditions and requires only little sample preparation. Basic research on magnetic materials as well as the mentioned industrial applications create an increasing demand for high-resolution magnetic imaging methods. This contribution will review some new concepts which have been realized in the field of advanced probe preparation, based on electron beam methods in order to improve the spatial resolution beyond 100nm. It is shown that the advanced probes allow high-resolution imaging of magnetic fine structures within thin film permalloy elements exhibiting a complicated cooperative magnetization reversal process. These investigations are of importance for various concepts underlying modern magnetic data storage developments. Furthermore, we present some developments of MFM to suit the needs of the magnetic recording industry.