GIANT MAGENTORESISCANCE AND MAGNETIC PROPERTIES OF ELECTRODEPOSITED Ni-Co-Cu/Cu MULTILAYERS

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GIANT MAGENTORESISCANCEAND MAGNETIC
PROPERTIESOF ELECTRODEPOSITEDNi-Co-Cu/Cu
MULTILAYERS
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G. NabiyouniDepartment of Physics, University of
Arak, Arak 38156, IranI.BakonyiResearch
Institute for Solid State Physics and Optics,
Hungarian Academy of Sciences. H-1525 Budapest,
P.O.B. 49, HungaryW. Schwarzacher,H. H. Wills
Physics Laboratory, Bristol BS8 1TL, UK,
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Multilayer structures are most commonly
prepare by vacuum based techniques such as
evaporation, sputtering and molecular beam
epitaxy (MBE). However, it has been well
established that nanoscale metallic multilayers
can also be successfully produced by
electrodeposition and at appropriate element and
layer thickness combinations, they can exhibit a
significant giant magnetoresistance (GMR) effect
as demonstrated for the systems Ni-Cu/Cu,
Co-Cu/Cu and Ni-Co-Cu/Cu.
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Ni-Co-Cu/Cu multilayers were electrodeposited
from a single bath under potentiostatic control
on (100)-textured polycrystalline Cu substrates
(this texture proved to be superior to the (110)
and (111) textures for the GMR effect). The
films consist of 100 repeats of
Ni-Co-Cu(3nm)/Cu(1nm) and Ni-Co-Cu(3nm)/Cu(2nm)
respectively.
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The starting electrolyte composition (in units of
g/litre H20) was as follows 742 g/l
Ni-sulphamate (135 g/l Ni as metal),13.5 g/l
CuSO4 (3.5 g/l Cu as metal)and 30 g/l H3BO3.
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Co metal concentration in solution was as
followX 0, 1, 2, 3, 5, 7, 10, 14, 18, 21 and
24 g/l The Co-content of the solution is
denoted by X referring to the amount of Co metal
(in g/l units) in the solution To introduce Co,
CoSO4 was added to the solution. (1 g/l Co metal
corresponds to 4.46 g/l CoSO4).
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The magnetoresistance (MR) measurements were
performed at room temperature for magnetic fields
up to H 8 kOe in the current-in-plane/field-in
plane (CIP/FIP) configuration. The MR ratio was
defined as ?R/R0 (RH-R0)/R0 where R0 R(H0)
is the resistance without a magnetic field and RH
R(H) that in a magnetic field H.
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The magnetoresistance was measured with four
point-probes arranged in a square making pressure
contacts with the sample. The MR measurements
were performed with the current either
predominantly parallel (longitudinal
magnetoresistance, LMR) or perpendicular
(transverse magnetoresistance, TMR) to the
applied magnetic field
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In the multilayers prepared by sputtering the
room-temperature GMR increases continuously from
about 7 to 55 when varying the Co-content of
the magnetic layers in Ni-Co/Cu multilayers from
0 to 100 . For sputtered Ni-Co/Cu multilayers
an optimum GMR behaviour was found when the
Co-content of the magnetic layer was around 20 to
40 at. Co.In the GMR data for electrodeposited
multilayers, also a significant increase has been
found when going fromNi-Cu/Cu multilayers to
the Co-Cu/Cu system.
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Due to the differences in the electrochemical
behaviour of Ni and Co atoms, the
electrodeposition of Co-Cu/Cu multilayers is less
favourable than that of Ni-Cu/Cu multilayers.
This is because the exchange reaction between Co
and Cu is stronger than between Ni and Cu.
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All these considerations led us to perform the
present work aimed at investigating the influence
of Co-content on the GMR and magnetic properties
of Ni-Co-Cu/Cu multilayers by systematically
increasing the Co ion concentration in the
electrolyte.
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It was also of interest to correlate the magnetic
properties and GMR parameters for a series of
samples with widely differing magnetoresistance
behaviour.
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The overall chemical composition (CNi, CCo and
CCu) of the electrodeposited multilayers was
established for both A and B type samples by
electron probe microanalysis (EPMA) in a scanning
electron microscope (SEM) after removing the
substrates (table 12).
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The overall Cu-content didnt show a systematic
dependence on electrolyte Co-content (X) and CCu
was 40 3 at. and 51 4 at. for series A and
B, respectively
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The higher Cu-content in series B reflects the
larger Cu-layer thickness (dCu 2 nm) in
comparison with series A (dCu 1 nm).
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AcknowledgementG.Nabiyouni (corresponding
author) acknowledges the University of Arak for
financial supports.