The physical properties of metallic nanostructures with layer thicknesses at the nanometer scale are different from the bulk materials. Their behaviour is determined by the fact that characteristic composition modulation length is smaller than the mean free path of the electrons in the bulk form of any of the constituents. Particularly interesting are the multilayers composed of alternating (ferro)magnetic and non-magnetic layers with near-nanometer thicknesses. In such multilayers, electrons crossing the boundary of the layers may undergo spin-dependent scattering events. Due to this spin-dependent scattering, the resistivity of these structures strongly depend on the externally applied magnetic field. This phenomenon is called giant magnetoresistance (GMR).
In our research group, multilayers with GMR are produced by electrodeposition. We have already dealt with a great variety of electrodeposited multilayers, such as Ni-Cu/Cu, Co-Cu/Cu, Co-Ni-Cu/Cu, Co-Fe-Cu/Cu, Co-Ru/Ru and Co-Ag/Ag. The most important achievements of the group canbe summarized as follows:
- Results concerning the electrodeposition itself:
♦ Besides the current-controlled and potential-controlled methods used earlier, we introduced the deposition mode with mixed control. Hence, the deposition mode of each layer can be chosen the optimal way and the entire deposition procedure can be better optimized.
V. Weihnacht, L. Péter, J. Tóth, J. Pádár, Zs. Kerner, C. M. Schneider, I. Bakonyi; Giant magnetoresistance in Co-Cu/Cu multilayers prepared by various electrodeposition control modes; J. Electrochem. Soc. 150 (2003) C507-C515.
♦ We have elaborated the optimization method for the deposition of the non-magnetic layer.
L. Péter, Q. X. Liu, Zs. Kerner, I. Bakonyi, Relevance of the potentiodynamic method in parameter selection for pulse-plating of Co-Cu/Cu multilayers, Electrochim. Acta 49 (2004) 1513-1526.
- Physical properties of the electrodeposited multilayers:
♦ We have shown that the magnetoresistance of the multilayers can be divided into a ferromagnetic (FM) and a superparamagnetic (SPM)contribution.
I. Bakonyi, L. Péter, Z. Rolik, K. Kis-Szabó, Z. Kupay, J. Tóth, L. F. Kiss, J. Pádár; Decomposition of the magnetoresistance of multilayers into ferromagnetic and superparamagnetic contributions, Phys. Rev. B 70 (2004) 054427(1-10).
♦ We have determined the FM and SPM contribution to the GMR in a large number of multilayers prepared with different electrolytes and exhibiting various layer compositions and layer thicknesses.
Qun-Xian Liu, László Péter, József Pádár and Imre Bakonyi; Ferromagnetic and Superparamagnetic Contributions in the Magnetoresistance of Electrodeposited Co-Cu/Cu Multilayers; J. Electrochem. Soc. 152 (2005) C316-C323.
L. Péter, V. Weihnacht, J. Tóth, J. Pádár, L. Pogány, C.M.Schneider, I. Bakonyi; Influence of superparamagnetic regions on the giant magnetoresistance of electrodeposited Co-Cu/Cu multilayers; J. Magn. Magn. Mater. 312 (2007) 258-265.
♦ We have measured the temperature dependence of the FM and SPM contributions to the GMR for several samples. For the elucidation of the measurements, we adapted the model of interacting superparamagnetic particles to the electrodeposited multilayers.
L. Péter, Z. Rolik, L.F. Kiss, J. Tóth, V. Weihnacht, C.M.Schneider and I. Bakonyi, Temperature dependence of the giant magnetoresistance and magnetic properties in electrodeposited Co-Cu/Cu multilayers: the role of superparamagnetic regions; Phys. Rev. B 73 (2006) 174410/1-10.
♦ We have achieved new results concerning the interrelation of the structure and GMR of electrodeposited multilayers.
Á. Cziráki, L. Péter, V. Weihnacht, J. Tóth, E. Simon, J. Pádár, L. Pogány, C.M.Schneider, T. Gemming, K. Wetzig, G. Tichy, I. Bakonyi, Structure and giant magnetoresistance behaviour of Co-Cu/Cu multilayers electrodeposited under various deposition conditions; J. Nanosci. Nanotechnol. 6 (2006) 2000-2012.
D. Rafaja, C. Schimpf, V. Klemm, G. Schreiber, I. Bakonyi and L. Péter; Formation of microstructure defects in electrodeposited Co/Cu multilayers; Acta Materialia, accepted for publication.
Depth profile composition analysis of electrodeposited "bulk" metals and multilayers (László Péter; from 2007)
We have recognized that a significant composition gradient can form in electrodeposited alloys along the growth direction. This composition gradient is indicated indirectely by the change of the overall composition with the layer thickness. This composition gradient has also been shown by direct depth profile analysis. This was performed by using the secondary neutral mass spectrometry (SNMS) facility of the Nuclear Research Institute of HAS (in Debrecen, contact person: Kálmán Vad).
The most important results in the field of depth profile analysis of electrodeposited materials are as follows:
♦ We introduced a new procedure for the depth profile analysis of electrodeposited materials. The most important feature of this method is that after removing the deposit from the substrate, the analysis is started from the substrate side of the deposit. Hence, the layers deposited first can be analysed with the highest accuracy.
A. Csik, K. Vad, G.A. Langer, G.L. Katona, E. Tóth-Kádár, L. Péter; Analysis of Co/Cu multilayers by SNMS reverse depth profilig; Vacuum, Accepted for publication.
♦ We have shown that the composition gradient in electrodeposited alloys of the Fe-group elements can be explained by the symultanous effect of the deposition preference and the depletion of the electrolyte.
László Péter, Gábor L. Katona, Zoltán Berényi, Kálmán Vad, Gábor A. Langer, Enikő Tóth-Kádár, József Pádár, Lajos Pogány and Imre Bakonyi; Electrodeposition of Ni-Co-Cu/Cu multilayers. 2. Calculations of the element distribution and experimental depth profile analysis; Electrochimica Acta, 53 (2007) 837-845.
♦ We have demonstrated that during the d.c. current electrodeposition of Fe-Co-Ni alloys an approximately 400-nm thick transition zone occurs, in which the composition changes significantly. Occasionally, especially at high current densities, oscillation of the composition was also observed.
Attila Csik, Kálmán Vad, Enikő Tóth-Kádár, László Péter;Spontaneous near-substrate composition modulation in electrodeposited Fe-Co-Ni alloys; Electrochemistry Communications , közlésre elfogadva.
♦ We have shown that the result of the depth profile analysis of electrodeposited multilayers can be fully explained by the increase in surface roughness of the deposit with the deposit thickness. A calculation method was also suggested to take into account the impact of the change of surface roughness.
András Bartók, Attila Csik, Kálmán Vad, György Molnár, Enikő Tóth-Kádár, László Péter; Application of Surface Roughness Data for the Evaluation of Depth Profile Measurements of Nanoscale Multilayers; J. Electrochem. Soc., accepted for publication.
Earlier studies: Electrodeposition of nanocrystalline metals (Imre Bakonyi; 1990-2000.)
Elektrokémiai úton leválasztott nanokristályos nikkel és kobalt rétegeken, valamint gyorshűtött ötvözeteken vizsgáltuk a mikroszerkezet változásait. A szerkezeti változások követése az elektromos transzport tulajdonságok, illetve az elekrolitikusan bevitt hidrogén okozta hatások mérésével történt.
1 és 10 μm közötti vastagságú porozitás-mentes nikkel réteget állítottunk elő egyenáramú és impulzus leválasztással. A hordozóréteg eltávolítása után a leválasztási körülmények, a fürdő összetétele, hordozó anyaga (Cu vagy Ti) és az alkalmazott áramsürűség függvényében tanulmányoztuk nanokristályos nikkel réteg felületének morfológiáját (SEM), mikroszerkezetét (elekron és röntgen-diffrakcióval), transzport tulajdonságait, Curie-hőmérsékletét és termikus stabilitását. Az egyenáramú leválasztás során a szemcseméretet 30 nm-re sikerült leszorítani. Impulzusos leválasztások során a pulzus időtartamát 1 msec és 10 sec között változtatva a nikkel réteg maradékellenállása és az ellenállás hőmérsékletfüggése széles határok között változtaható. Arra következtettünk, hogy a nagy maradékellenállás nem csak a szemcsehatárok, hanem az előállítás során keletkező más rácshibák következménye is.
Other electrochemical studies: Electrochemical hydrogen permeation (László Péter, 2000-2004.)
We studied the hydrogen absorbance and hydrogen permation of enamel-grade steels. This work was sponsored by an industrial partner. A theoretical model was elaborated to formulate the rate of hydrogen absorbance. A complete experimental workstation was designed, including the electrochemical cell, the solution compositions with in situ exit side activation, the entire measurement protocol for multistep permeation measurements as well as the specification of the instrument needed. For the evaluation of the model parameters, a computer program was developed. The procedure implemented was awarded with the Applied Research Award of our institute in 2003.
Miscellaneous works other than electrochemistry: Micro- and nanostructures in rocks (Júlia Dégi, from 2008)
Remnants of numerous processes which have not reached thermodynamic equilibrium are preserved in rocks. They provide useful information on the evolution of the lithosphere and interactions between different geoshperes. In addition, they play an important role in the understanding of slow chemical reactions in solid phase material, since the duration of laboratory experiments are limited. Our research interest is focused on the mechanisms and kinetics of microstructure evolution and the development of submicron-sized chemical heterogeneities during chemical reactions between solid phases.
Rock fragments (xenoliths) of lower crustal origin brought up to the surface by alkali basalts in the Balaton Highlands contain excellent examples of incomplete mineral reactions. In a previous project we have shown that mineral reactions and fluid-rock interactions observed in these xenoliths are closely related to the tectonic processes acting during the formation of the Carpatho-Pannonian Basin (See: Török, K., Dégi, J., Marosi, Gy. Szép, A. (2005): Reduced carbonic fluids in mafic granulite xenoliths from the Bakony-Balaton Highland Volcanic Field, W-Hungary. Chemical Geology, 223, 93-108., & Dégi, J., Török, K. (2003): Petrographic evidence of crustal thinning in Bakony-Balaton Highland Volcanic Field. (in Hungarian with English abstract), Magyar Geofizika, 44/4. 125-133.)
♦ Submicron-sized chemical zoning patterns have been found in Fe-Ti-oxides in these xenoliths by means of high resolution electron beam microanalytical techniques. The diffusion-controlled exchange of Fe and Ti within these oxides has been modelled. We have shown that the interaction of the xenoliths and the hosting alkali basalt has lasted 9-20 hours at 1200°C. Using these data the Fe-Ti interdiffusion coefficient of ilmenite has been estimated.
Dégi, J., Abart, R., Török, K., Rhede, D., Petrishcheva, E. (2009): Evidence for xenolith - host basalt interaction from chemical patterns in Fe-Ti-oxides from mafic granulite xenoliths of the Bakony-Balaton Volcanic field (W-Hungary). Mineralogy and Petrology, 95(3-4), 219-234.
♦ We have studied in details the breakdown of garnet in these rocks. This process includes the replacement of a homogeneous precursor phase along a moving interphase boundary by three nanometer-sized phases forming vermicular intergrowths (symplectitic microstructure). Oriented TEM foils were prepared from the reaction products parallel with and perpendicular to the reaction interface by focused ion beam (FIB) technique. Transmission electron microscopy of these samples provided new information on the mechanism of phase separation along a moving reaction front in solid phase.
Dégi, J., Abart, R., Török, K., Bali, E., Wirth R., Rhede, D. (2009) Symplectite formation during decompression induced garnet breakdown in lower crustal mafic granulite xenoliths: mechanisms and rates. Submitted to Contributions to Mineralogy and Petrology.