The role of nucleation in the evolution of giant magnetoresistance with layer thicknesses in electrodeposited Co–Cu/Cu multilayers

The giant magnetoresistance (GMR) and structure was investigated for electrodeposited Co/Cu multilayers prepared by a conventional galvanostatic/potentiostatic pulse combination from a pure sulfate electrolyte with various layer thicknesses, total multilayer thickness and Cu deposition potential. X-ray diffraction (XRD) measurements revealed superlattice satellite reflections for many of the multilayers having sufficiently large thickness (at least 2 nm) of both constituent layers. The bilayer repeats derived from the positions of the visible superlattice reflections were typically 10 -20% higher than the nominal values. The observed GMR was found to be dominated by the multilayer-like ferromagnetic (FM) contribution even for multilayers without visible superlattice satellites. There was always also a modest superparamagnetic (SPM) contribution to the GMR and this term was the largest for multilayers with very thin (0.5 nm) magnetic layers containing apparently a small amount of magnetically decoupled SPM regions. No oscillatory GMR behavior with spacer thickness was observed at any magnetic layer thickness. The saturation of the coercivity as measured by the peak position of the MR(H) curves indicated a complete decoupling of magnetic layers for large spacer thicknesses. The GMR increased with total multilayer thickness which could be ascribed to an increasing SPM contribution to the GMR due to an increasing surface roughness, also indicated by the increasing coercivity. For multilayers with Cu layers deposited at more and more positive potentials, the GMR FM term increased and the GMR SPM term decreased. At the same time, a corresponding reduction of surface roughness measured with atomic force microscopy indicated an improvement of the multilayer structural quality which was, however, not accompanied by an increase of the superlattice reflection intensities. The present results underline that whereas the structural quality as characterized by the surface roughness generally correlates fairly well with the magnitude of the GMR, the microstructural features determining the amplitude of superlattice reflections apparently do not have a direct influence on the GMR. Due to the large giant magnetoresistance (GMR) effect observed in physically deposited Co/Cu multilayers, 1-3 a lot of efforts have been devoted to the study of GMR also on electrodeposited (ED) Co/Cu multilayers (for detailed references, see a recent review).4 A variety of baths have been used for the preparation of ED Co/Cu multilayers, 4 the simplest one containing merely CoSO 4 and CuSO 4 . Over the last two decades, numerous reports have been published on studying the GMR characteristics of ED Co/Cu multilayers from the pure sulfate bath (containing at most some buffering agents). [5][6][7][8][9][10][11][12][13][14][15] In this list of references, we have included only those works from the much larger number of reports 4 in which the ED Co/Cu multilayers were prepared from the sulfate bath at or close to the electrochemically optimized Cu depositi...

mentioning

confidence: 81%

Giant Magnetoresistance and Structure of Electrodeposited Co/Cu Multilayers: The Influence of Layer Thicknesses and Cu Deposition Potential

Rajasekaran

Mani

Tóth

et al. 2015

“…The formation of a significant amount of SPM regions in ED multilayers often occurs if both kinds of constituent layers are fairly thin. 27,28 According to Figs. 7c and 7d, for a sufficiently thick (here: 2.4 nm) spacer layer, the observed magnetoresistance indeed mainly arises due to spin-dependent scattering events between well separated FM layers although a small SPM contribution still persists.…”

Section: Gmr In Ed Fe-co/cu Multilayersmentioning

confidence: 99%

Preparation, Structure and Giant Magnetoresistance of Electrodeposited Fe-Co/Cu Multilayers

Tóth

Péter

Pogány

et al. 2014

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 Fe 5 Co 95 (1.5 nm)/Cu(d Cu ) multilayers with d Cu 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. Since bulk Fe-Co alloys have good soft magnetic properties, 1 multilayers with this kind of alloy as the magnetic layer can have favorably low coercivity. A detailed giant magnetoresistance study (GMR) study of sputtered TM/Cu multilayers by using binary and ternary alloys of Fe, Co and Ni as magnetic layers has been carried out by Miyazaki et al.2 who also mapped out the anisotropic magnetoresistance (AMR) of bulk alloys of the Fe-Co-Ni system. As to the GMR of physically deposited Fe-Co/Cu multilayers, it was shown that they can, indeed, have a significant room-temperature GMR (Refs. 2 and 3) with fairly low saturation fields. 3Inomata and Saito 3 investigated sputtered Fe-Co/Cu multilayers with Fe-concentrations of 10, 20 and 50%. In their study, the Cu layer thickness was varied between 0.9 and 3.8 nm and three oscillatory GMR peaks were found as a function of the spacer layer thickness. Rafaja et al. 4 found about 25% GMR on sputtered [Fe 10 Co 90 (1.1 nm)/Cu(2.2 nm)]x20 multilayers but a detailed study on the influence of the individual layer thicknesses was not carried out. Gangopadhyay et al.5 also studied sputtered Fe 10 Co 90 /Cu multilayers. The Cu layer thickness was varied between 0.5 and 3.1 nm and an oscillatory GMR was found, but contrary to the typical result on sputtered multilayers, the second peak was found to be the larger.There are contradictory experimental data in the literature on physically deposited FM/Cu multilayers, whether a pure Co magnetic layer or a Fe-Co alloy shows larger GMR value. Kataoka et al. 6 and Inomata and Saito 3 present data according to which for [Fe 20 Co 80 (1.0 nm)/Cu(1.0 nm)]x15 multilayers the GMR is higher than for multilayers with pure Co as magnetic layer, while the work of Miyazaki et al. 7 shows that...

“…The higher roughness values for small Cu layer thicknesses may arise from the fact that nucleation of Cu on Co proceeds in an island-like manner 20 and a sufficiently high effective Cu layer thickness should be achieved to ensure a uniform coverage. This mechanism explains clearly the evolution of roughness with t Cu for the G/P series multilayers.…”

Section: D342mentioning

confidence: 99%

Structure and Giant Magnetoresistance of Electrodeposited Co/Cu Multilayers Prepared by Two-Pulse (G/P) and Three-Pulse (G/P/G) Plating

Rajasekaran

Pogány

Révész

et al. 2014

The giant magnetoresistance (GMR) was investigated for electrodeposited Co/Cu multilayers. In order to better understand the formation of individual layers and their influence on GMR, multilayers produced by two different deposition strategies were compared. One series of Co(2 nm)/Cu(t Cu ) multilayers with t Cu ranging from 0.5 nm to 6 nm was produced with the conventional two-pulse plating by using a galvanostatic/potentiostatic (G/P) pulse combination for the magnetic/non-magnetic layer deposition, respectively, whereby the Cu layer deposition was carried out at the electrochemically optimized potential. Another Co(2 nm)/Cu(t Cu ) multilayer series with the same t Cu range was prepared with the help of a G/P/G pulse combination. In this latter case, first a bilayer of Co(2 nm)/Cu(6 nm) was deposited in each cycle as in the G/P mode after which a third G pulse was applied with a small anodic current to dissolve part of the 6 nm thick Cu layer in order to ensure the targeted t Cu value. The comparison of the two series revealed that the G/P/G pulse combination yields multilayers for which GMR can be obtained even at such low nominal Cu layer thicknesses where G/P multilayers already exhibit bulk-like anisotropic magnetoresistance only. Surface roughness measurements by atomic force microscopy revealed that the two kinds of pulse combination yield different surface roughness values which correlate with the structural quality of the multilayers as indicated by the absence or presence of multilayer satellite reflections in the X-ray diffraction patterns. A separation of the superparamagnetic (SPM) contribution from the total observed GMR provided useful hints at the understanding of differences in layer formation between samples prepared with the two kinds of pulse combination. The results of multilayer chemical analysis revealed that mainly an increased Cu content of the magnetic layer is responsible for the onset of SPM regions in the form of Co segregations in the G/P/G multilayers with small Cu layer thicknesses. Magnetization measurements provided coercive force and remanence data which gave further support for the above interpretation of the GMR data. The giant magnetoresistance (GMR) effect in electrodeposited (ED) multilayer films was extensively studied in the last two decades.