Microstructure of a rapidly quenched nanocrystalline Hf11Ni89 alloy from X-ray diffraction

A rapidly quenched nanocrystalline Hf(11)Ni(89) alloy was produced by melt-spinning. The X-ray phase analysis shows that the as-quenched ribbon consists of mainly nanocrystalline fcc HfNi(5) although a small amount of Ni is also detected. The crystallite size distribution and the dislocation structure of the dominant HfNi(5) phase were determined by a recently developed method of diffraction profile analysis. In this procedure, by assuming spherical shape and log-normal size distribution of crystallites, the measured physical intensity profiles are fitted by the well established ab initio functions of size and strain peak profiles. The anisotropic broadening of peak profiles is accounted for by the dislocation model of the mean square strain in terms of average dislocation contrast factors. It was found that the median and the variance of the crystallite size distribution are 3.3 nm and 0.70, respectively. The dislocation density is 5.7x10(16) m(-2) and the character of dislocations is nearly pure screw

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

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

Magnetic and magnetoresistance studies of nanometric electrodeposited Co films and Co/Cu layered structures: influence of magnetic layer thickness

The magnetic properties and the magnetoresistance behavior were investigated for electrodeposited nanoscale Co films, Co/Cu/Co sandwiches and Co/Cu multilayers with individual Co layer thicknesses ranging from 1 nm to 20 nm. The measured saturation magnetization values supported reasonably the validity of the nominal layer thicknesses. All three types of layered structure exhibited anisotropic magnetoresistance for thick magnetic layers whereas the Co/Cu/Co sandwiches and Co/Cu multilayers with thinner magnetic layers exhibited giant magnetoresistance (GMR), the GMR magnitude being the largest for the thinnest Co layers. The decreasing values of the relative remanence and the coercive field when reducing the Co layer thickness down to below about 3 nm indicated the presence of superparamagnetic (SPM) regions in the magnetic layers which could be more firmly evidenced for these samples by a decomposition of the magnetoresistance vs. field curves into a ferromagnetic and an SPM contribution. For thicker magnetic layers, the dependence of the coercivity (Hc) on magnetic layer thickness (d) could be described for each of the layered structure types by the usual equation Hc = Hco + a/d n with an exponent around n = 1. The common value of n suggests a similar mechanism for the magnetization reversal by domain wall motion in all three structure types and hints, at the same time, for the absence of coupling between magnetic layers in the Co/Cu/Co sandwiches and Co/Cu multilayers

Preparation, structure and giant magnetoresistance of electrodeposited Fe Co/Cu multilayers

No systematic studies have been carried out on the giant magnetoresistance (GMR) of electrodeposited (ED) Fe-Co/Cu multilayers since the elaboration of a method for the optimization of the Cu layer deposition potential. In this paper, we present results on the electrochemical optimization of the Cu layer deposition potential which was found to depend on the relative iron concentration in the bath. An X-ray diffraction study of ED Fe5Co95(1.5 nm)/Cu(dCu) multilayers with dCu ranging from 0.8 nm to 10 nm revealed an fcc structure. For most of the multilayers, weak superlattice satellite reflections could be identified. The room-temperature magnetoresistance was studied in detail as a function of the individual layer thicknesses. Multilayers with Cu layer thicknesses above about 1.5 nm were found to exhibit a GMR behavior with a maximum GMR of about 5 % and a typical saturation field of 1 kOe. The GMR magnitude decreased with increasing Fe-content in the magnetic layer. The spacer layer thickness evolution of the MR data was established in detail after separating the ferromagnetic and superparamagnetic GMR contributions and no oscillatory GMR was found. A comparison with literature data on both physically deposited and ED Fe-Co/Cu multilayers is also made

A gyermek 14 (1920) 06-10

A gyermek A Magyar Gyermektanulmányi Társaság közlönye 14. évfolyam , 6-10. szám Budapest, 1920. A folyóirat 1908-ig a Gyermekvédelmi lap mellékleteként, 1909-től mint önálló lap jelent meg

Composition, morphology and electrical transport properties of Co-Pb electrodeposits

Bath compositions were elaborated for the codeposition of Co and Pb by taking into account the chemical compatibility of Co2+ and Pb2+ with the appropriate anions. Electrolytes containing either acetate, chloride or nitrate anions were tested, but only the acetate bath proved to be appropriate for the preparation of compact Co-Pb films. Deposits were obtained with constant current, with constant potential or by using various current and potential pulses in order to investigate the possibility of multilayer formation. The variation in deposit composition as a function of the deposition parameters was elucidated by using cyclic voltammetry, current transient measurements and gravimetry. X-ray diffraction (XRD) patterns recorded for two-pulse plated deposits revealed a nanocrystalline structure with grain sizes in the range 5 to 20 nm. The XRD peaks could be well indexed to pure face-centered cubic Co and Pb, indicating that the Pb codeposited with Co is not dissolved in Co but is segregated. Both the d.c.-plated and the two-pulse plated deposits exhibited anisotropic magnetoresistance without an indication for a noticeable giant magnetoresistance contribution. This means that the observed magnetoresistance arises from spin-dependent electron scattering events dominantly within the sufficiently large Co regions and not along electron paths between two Co regions via the Pb regions. Low-temperature resistivity measurements revealed a superconducting transition slightly below that of pure Pb. This may be ascribed to a proximity effect: the ferromagnetic Co grains suppress somewhat the superconductivity of the Pb phase due to the nanoscale phase mixture of the two constituents