Microstructure formation in electrodeposited Co-Cu/Cu multilayers with GMR effect: influence of current density during the magnetic layer deposition

The influence of the current density applied during the deposition of the magnetic layers on the microstructure formation in electrodeposited Co-Cu/Cu multilayers and on their giant magnetoresistance (GMR) was investigated using a combination of magnetoresistance measurements, wide-angle and small-angle X-ray scattering, high-resolution transmission electron microscopy, atomic force microscopy and chemical analysis. The magnetoresistance measurements revealed that a reduction of the current density stimulates a transition from the formation of the magnetic layers with predominantly ferromagnetic character to the formation of superparamagnetic regions. As based on electrochemical considerations, it was supposed that such a change in the magnetic properties can be caused by an increased amount of Cu codeposited with Co at low current densities. It turned out from the structural studies that a pronounced segregation of Co and Cu occurs at low current densities. In accordance with their very low mutual solubility at room temperature, no atomic scale intermixing of Co and Cu could be detected. The segregation of Cu and Co was related to the fragmentation of the magnetic layers, to the enhancement of the local lattice strains, to the increase of the interface corrugations, to the partial loss of the multilayer periodicity and finally to the formation of Co precipitates in the Cu matrix

Giant magnetoresistance (GMR) in electrodeposited multilayer films: the influence of superparamagnetic regions

When preparing an alternating sequence of magnetic (Co or Ni) and non-magnetic (Cu) layers by electrodeposition using the two-pulse plating technique, a dissolution of the lessnoble magnetic Co and Ni atoms can take place during the deposition of the more noble and non-magnetic Cu atoms. This process results in changes of the actual sublayer thicknesses with respect to the nominal values and can also cause some chemical intermixing at the magnetic/non-magnetic interfaces. As a consequence, superparamagnetic (SPM) regions with “loose magnetic moments” can form as has been demonstrated for electrodeposited Ni-Cu/Cu multilayers. We have also shown recently for electrodeposited Co-Cu/Cu multilayers that if some fraction of the magnetic layers exhibits SPM behaviour then the observed giant magnetoresistance (GMR) can be quantitatively decomposed into a ferromagnetic (FM) and a SPM contribution. In this paper, the results of a similar GMR decomposition study are presented for two electrodeposited Co-Cu/Cu multilayers. In the multilayer with strongly non-saturated magnetoresistance curves, the dominant GMR term was due to SPM regions, whereas in the other multilayer for which the magnetoresistance is mostly saturated in magnetic fields around 1 to 2 kOe, the FM contribution to the GMR is much larger. At the same time, magnetic measurements on the first multilayer sample have also revealed the presence of a large SPM contribution to the magnetization

Temporary feeding shocks increase the productivity in a continuous biohydrogen-producing reactor

Continuous hydrogen production stability and robustness by dark fermentation were comprehensively studied at laboratory scale. Continuous bioreactors were operated at two different hydraulic retention times (HRT) of 6 and 10 hours. The reactors were subjected to feeding shocks given by decreases in the HRT, and therefore the organic loading increase, during 6 and 24 hours. Results indicated that the H2 productivity was significantly improved by the temporary organic shock loads, increasing the hydrogen production rate up to 40%, compared to the rate obtained at the steady-state condition. Besides, it was observed that after the shock load, the stability of the reactor (measured as the hydrogen production rate) was recovered attaining the values observed before the feeding shocks. The bioreactor operated at shorter HRT (6 h) showed better H2 productivity (17.3 ± 1.1 L H2/L-d) in comparison to the other one operated at 10 h HRT (12.4 ± 1.6 L H2/L-d)

Biohydrogen purification by membranes: An overview on the operational conditions affecting the performance of non-porous, polymeric and ionic liquid based gas separation membranes

Many types of membranes are available to enrich hydrogen. Nevertheless, there are some with special potential for biohydrogen purification such as the non-porous, polymeric and ionic liquid based membranes. The attractiveness of these membranes comes from the fact that they can be employed nearly under the conditions where biohydrogen formation taking place. Therefore, they appear as promising candidates to be coupled with hydrogen producing bioreactors and hence giving the chance for in situ biohydrogen concentration. It is known that the feasibility and efficiency of membrane technology - beside material selection and module design - significantly depend on the separation circumstances. Thus, the operation of membranes is a key issue and the most important factors to be considered for gas purification are the composition of gas to be separated, the pressure and temperature applied. The scope of this study is to give a comprehensive overview on the recent applications of non-porous, polymeric and ionic liquid supported membranes for biohydrogen recovery, placing emphasis on the operational conditions affecting membrane's behavior and performance. Furthermore, a novel concept for integrated biohydrogen production and purification using gas separation membranes is demonstrated and discussed. © 2013 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Mikroszerkezet és mágneses ellenállás kapcsolata nanométeres skálájú multirétegekben = Correlation of microstucture and magnetoresistance in nanoscale multilayers

Mindhárom kitűzött anyagcsaláddal kapcsolatban lefolytattuk a lehetséges vizsgálatokat: - A Co-Cu/Cu rendszerrel kapcsolatban feltártuk, hogy a mágneses rétegben tapasztalható szegregáció mértéke nő a Co réteg Cu koncentrációjának növekedésével. A Cu kiválás preferált helye a kristályhatárok mentén található. A mágneses ellenállás csökkenése csak jóval nagyobb Cu koncentrációknál észlelhető, mint ahol a kiválás szerkezetvizsgálati módszerekkel már ténylegesen kimutatható. - A Co-Ru rendszerrel kapcsolatban kimutattuk, hogy az elektrokémiai Co leválás a nemesebb Ru mellett anomális jellegű. A Co és Ru együtt leválására vonatkozóan számos oldatösszetételre meghatároztuk a leváló fém összetételét az áramsűrűség függvényében, és jellemeztük a képződő bevonat felületi morfológiáját a komponensek móltörtjének függvényében. Kis Ru tartalmú Co bevonat vizsgálatával igazoltuk, hogy a Ru szennyező nagyban csökkenti a Co mágneses ellenállását. - A Ni-Cu rendszerrel kapcsolatosan kimutattuk, hogy az alkalmazott vegyszer minősége meghatározó a képződő multirétegek sajátságaira vonatkozóan. A megvizsgált mintasorozatokban a szerkezetvizsgálati adatok egyértelműen igazolták a multiréteges szerkezet kialakulását. Az eredeti kutatási tervben kitűzött vizsgálatok mellett megvizsgáltuk számos elektrokémiai úton leválasztott ötvözet összetételét a leválás kezdeti szakaszában, továbbá magnetokalorikus hűtésre vonatkozó kísérletekhez mágneses multiréteg mintákat készítettünk. | All the three key element pairs have been studied: - For the Co-Cu/Cu system, it has been revealed that the segregation within the magnetic layer increases with the increase of Cu concentration in the magnetic layer. The preferred position for the Cu segregation is the crystal boundary. The decrease in magnetoresistance can be seen at a significantly higher Cu concentration only than for which the segregation can be detected with structural studies. - For the Co-Ru system, it has been observed that the electrodeposition of Co besides Ru is an anomalous process. The composition of the deposits has been determined for a number of solution compositions as a function of the current density, and the morphology of the deposits has been characterized as a function of the molar fraction of the components. It has been verified for alloys with low Ru content that the Ru impurity substantially decreases the magnetoresistance of Co. - It was observed for the Ni-Cu/Cu system that the quality of the chemicals used has a major impact on the features of the electrodeposited multilayers. For all sample series produced, the structural studies confirmed the formation of a multilayer structure. Beyond the goal of the original research plan, the composition of the near-substrate zone has been analyzed for several electrodeposited alloy systems. Magnetic mutilayer samples were also produced for experiments related to magnetocaloric refrigiration

Magnetoresistance and surface roughness study of electrodeposited Ni50Co50/Cu multilayers

Room-temperature transport properties (the zero-field resistivity, ρ0, and the GMR) were studied for ED Ni50Co50/Cu multilayers as a function of the individual layer thicknesses and total multilayer thickness. The Cu deposition potential was optimized in order to obtain the preset layer thicknesses. The surface roughness development was studied by AFM, which revealed an exponential roughening with total thickness. The Cu layer thickness strongly influenced the roughness evolution. As expected, ρ0 decreased with increasing Cu layer thickness whereas it increased strongly for large total multilayer thicknesses that could be ascribed to the observed deposit roughening. All multilayers with Cu layer thicknesses above about 1.5 nm exhibited a GMR behavior with a maximum GMR of about 5 %. The GMR decreased for total multilayer thicknesses above about 300 nm due to the strong increase of ρ0, the latter caused by the enhanced roughness. The GMR data indicated the appearance of a current at angle to plane type scattering due to the layer undulations. The thickness evolution of the MR data was analyzed in detail after separating the ferromagnetic and superparamagnetic GMR contributions. It could be established that ED Ni-Co/Cu multilayers do not exhibit an oscillatory GMR behavior with spacer thickness

Alagutazó mágneses ellenállás (TMR) ferromágneses/szigetelő nanoszerkezetekben = Tunnelling magnetoresistance (TMR) in ferromagnetic/insulator nanostructures

Az OTKA pályázat során folytatott kutatások közül az elektrolitikusan előálított mágneses/nem-mágneses (főleg Co-Cu/Cu) multirétegek óriás mágneses ellenállásának (GMR) megértésével kapcsolatos tevékenységünk igen eredményes volt (10 nemzetközi folyóiratpublikáció, 2 konferenciaközlemény, felkérés 1 könyvrészlet megírására és 13 meghívott előadás tartására). Kutatásaink során tisztáztuk a szakirodalomban sokat vizsgált, de korábban nem teljesen megértett elektrokémiai folyamatok (az ún. Co Cu cserereakció és a Co-visszaoldódás) és a leválasztási módok szerepét a multirétegek képződésében és hatásukat a GMR-ra. A kétimpulzusos multiréteg előállításnál eddig kizárólagosan használt áram/áram vagy potenciál/potenciál kombinációk mellett bevezettük az optimálisabb áram/potenciál impulzuskombinációt. Egy másik OTKA pályázatunk eredményeit felhasználva optimalizáltuk a kulcsfontosságú Cu-leválasztási potenciál értékét, így első ízben lehetett tanulmányozni a GMR függését a valódi rétegvastagságoktól elektrolitikus multirétegekben. Ezen eredmények alapján arra következtettünk, hogy a Cu nukleációja Co felületen más jellegű, mint a Co nukleációja Cu felületen. A mágneses ellenállás térfüggését analizálva megállapítottuk, hogy az elektrolitikus multirétegekben megfigyelt nagy telítési teret a mágneses rétegek egyes tartományainak szuperparamágneses viselkedése okozza, amelyek GMR járulékának kvantitatív meghatározására módszert dolgoztunk ki. | Our OTKA project activity was very successful in the field of electrodeposited (ED) magnetic/non-magnetic Co-Cu/Cu multilayers, especially concerning the study of their giant magnetoresistance (GMR) behaviour, as indicated by 10 papers in international journals and 2 in conference proceedings, and by the invitation to write a book chapter as well as to give 13 invited talks at international conferences. We have clarified the role of the previously intensively studied but not completely understood electrochemical processes (Co Cu exchange reaction and Co-dissolution) and of the deposition modes in multilayer formation and their influence on GMR. In addition to the current/current or potential/potential deposition modes formerly exclusively applied, the more advantageous current/potential pulse combination was introduced. Relying on our reuslts from another OTKA grant, we have optimized the Cu deposition potential, enabling for the first time a study of the dependence of GMR on true layer thicknesses in ED multilayers. It could be concluded from this study that there is an asymmetry in the nucleation behaviour of Co and Cu on top of each other. Analyzing the field dependence of the GMR, we have established that the large GMR saturation field usually observed in ED multilayers arises from some regions of the magnetic layers which exhibit superparamagnetic behaviour and we have elaborated a method to extract the latter contribution from the measured data