Contact person: Sára Judit Balogh,
Different devices are used to prepare non-equilibrium alloys from liquid, solid and vapour phase precursors. In our laboratory we use a special melt quenching unit which enables the handling of small mass (1-2 g) alloy pieces in vacuum or protective atmosphere, facilitating sample preparation even for expensive (e.g. isotopically enriched) materials. Melt quenching of larger quantities is possible in the Metallurgical Laboratory of our Institute.
Surface laser melting is a special case of non-equilibrium melt solidification. For this purpose we use a 0.5 mm diameter focused pulsed NdYAG laser beam (20 ns pulse length and 25 mJ energy) which is moved at the surface by a X-Y table, enabling the treatment of an approximately 10 x 10 mm area.
Non-equilibrium alloys can be prepared by evaporation in ultra vacuum (10-8 mbar) environment using an electron beam evaporation unit controlling two independent sources. The machine utilises a liquid nitrogen cooled substrate holder system and is also equipped with a computer controlled thickness monitor. The system is also capable to produce multilayers and the subsequent thermal treatment is one of the procedures used to produce non-equilibrium alloys in the solid state.
For this later purpose ball milling devices are also used: a Fritsch type planetary mill for the treatment in protective atmosphere and vibratory mills which are continuously pumped by a turbomolecular unit.
The non-equilibrium phases are studied by calorimetry, magnetic and Mössbauer spectroscopic measurements. The thermal behaviour is investigated in the 100-1000 K temperature range by a Perkin-Elmer type DSC-2 differential scanning calorimeter. A special home developed device enables full computer control of the measurement, including even temperature modulated measurements.
Besides the ac (0-100 kHz) and dc equipment for studying soft magnetic properties, a Foner-type vibrating sample magnetometer is also used to measure the magnetisation in the 15-800 K temperature and in the 0-1.8 T external magnetic field range. The 2*10-4 emu sensitivity of the magnetometer is appropriate for most of the studies except for thin film and multilayer investigations. For this purpose the infrastructural fund of the National Technical Development Board (OMFB) financed in May 1998 the installation of a Quantum Design type SQUID (Superconducting Quantum Interference Device) magnetometer attaining 10-8 to 10-7 emu sensitivity in the 1.8-400 K temperature and in the 0-5 T external magnetic field range. Besides the standards parts, the RSO measuring head unit (Reciprocating Sample Option) was also bought, enabling an order of magnitude higher sensitivity. Three years later (in March 2001) the fund made it possible to upgrade the SQUID magnetometer by purchasing two further options. The ac susceptometer option works in the 0.01-1000 Hz frequency and in the 0-2 Oe ac field amplitude range. The high temperature oven option operates in the 300-800 K temperature range. Financed by the investment fund of the institute a pressure cell was installed into the SQUID magnetometer in 2010 working below 350 K up to pressures of 12 kbar (1.2 GPa). The cell was fabricated by the High Pressure Laboratory of the Institute of Physics in Prague (see schematic view).
High pressure CuBe "SQUID' cell (High Pressure Laboratory of the Institute of Physics AS, Praha, Czech Republic)
1 - plug supporting bolt; 2 - plug with washer; 3 - sealing of the plug (Cu- and rubber-rings); 4 - sample on a holder; 5 - pressure cell; 6 - Pb pressure sensor;
7 - piston with the Bridgman mushroom; 8 - piston backup; 9 - piston supporting bolt
The device was tested by measuring the pressure dependence of the magnetization and Curie temperature for FeMnB amorphous alloys (see figure) and in 2011 we began to study the pressure dependence of the magnetic properties for sputtered Fe2Zr amorphous alloys.
Temperature dependence of magnetization for Fe60Mn20B20 measured in a magnetic field of 10 Oe
after zero-field cooling (ZFC, full symbols) and field cooling in 10 Oe (FC, open symbols) at different pressures indicated.
The Curie temperature decreases significantly on applying several kbar pressure to the sample.
Similar significant pressure effect can be observed on the saturation magnetization.
Mössbauer spectroscopy is used to study the local environment of Fe atoms in alloys and compounds. We have several transmission spectrometers in our laboratory which can investigate 1-50 micrometer thick samples in the 1.5-1000 K temperature range.
The APD closed cycle cryostat is especially suitable for long time measurements in the 15-300 K temperature range without the use of expensive cryogenic liquids. In the
1.5-300 K temperature range we can also perform measurements in external magnetic field up to 7T, using a Janis cryostat equipped with a superconducting split-coil magnet.
Near surface layers (100-300 nm thickness) can be studied by detecting the conversion electrons in a CEMS spectrometer. The cryosystem, which extends the temperature range of this measurement is under development.
The accumulated almost thirty years' experience in the field of Mössbauer spectroscopy and our self-developed computer programs for evaluating even the most complex spectra make possible to compare the local chemical, topological and magnetic structure of the non-equilibrium alloys with the respective environments well characterised in equilibrium systems by other (e.g. diffraction) methods. In the last years we were awarded some National Scientific Research Fund (OTKA), an Academic Research Fund (AKA) and a Copernicus project besides a number of Instrumental Development Grants.
During the past two decades several national and international scientific grants were awarded to our group. The yearly scientific funding is near to HUF 5 Million ( approximately 20 kEuro). The research personnel of the group is active in the solid state physics education at the Roland Eötvös and at the Technical University of Budapest. They are also involved in refereeing for the highest ranking journals of the field. They published several hundred papers which received above 3000 independent citations.
Magnetic properties of multilayer structures (2005 - 2008)
The connection between the nanoscale structure, the interface mixing and the magnetic properties were studied in Fe-Al and Fe-Ag multilayers and nanoscale composites. The unusually large magnetic anisotropy of ultrathin Fe-Al multilayers has been related to two monolayer thick Fe regions deduced from the low temperature Fe hyperfine fields. The size of Fe grains in Fe-Ag multilayers with discontinuous Fe layers could be determined from the superparamagnetic properties. Bulk magnetization and local Mössbauer spectroscopy measurements gave similar results and the decrease of the grain size with increasing Ag spacer thickness was deduced. Below certain grain-size a close to perpendicular magnetic anisotropy was observed, which was shown to be independent of mechanical stresses arising from heat dilatation or application of cover layer. Fe-Ag granular alloys prepared by co-evaporation of the components were found to contain Fe grains also in the nano-size range, but no perpendicular magnetic anisotropy could be observed. The notion of sequence permutated multilayers with more than two components was introduced and applied to investigate the top (Fe/B) and bottom (B/Fe) interfaces of Fe with B in Ag/Fe/B- and Ag/B/Fe sequence permutated multilayer pairs. The results show that both interfaces are amorphous, but B concentration of the top interface is higher.
(See publication list of Judit Balogh: No. 59., 60., 63., 64., 65., 66., 68., 69. and L.F. Kiss et al., J. Alloys and Compounds 509 (2011) S188.)
Magnetic anisotropy of structures with reduced dimension (2007 - 2010)
The size decrease of magnetic data-recording units is limited by the onset of superparamagnetism. The key factor that determines this phenomenon is the magnetic anisotropy, therefore it is of great importance to understand the underlying physics in objects with reduced dimensions. By means of low-temperature measurements performed by Mössbauer spectroscopy (occasionally in external magnetic field) and superconducting (SQUID) magnetometer, we studied ball-milled nanometer-size grains, nanocomposites produced by crystallization of amorphous alloys, multilayers and granular alloys fabricated by ultra-vacuum evaporation and sputtered thin layers. It was determined how the magnetic properties depend on the composition and structure of the constituting phases and the magnetic characteristics and interactions of the formed ? occasionally amorphous ? nanophases were explored. Our results make contribution to the goal of tailoring the magnetic characteristics of structures designed for special purposes in case they consist of interacting nanophases.
(See publication list of László Ferenc Kiss: No. 137., 138., 142., 144., 147., 148. and L.F. Kiss et al., J. Alloys and Compounds 509 (2011) S188.)
Non-equilibrium and Nanostructured Magnetic Materials Laboratory (NANOMAG)