effect of ferromagnetic layer thickness on the giant magneto resistance

8/8/2019 Effect of Ferromagnetic Layer Thickness on the Giant Magneto Resistance

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Proceedings of The 3rd Asian Physics Symposium (APS 2009)July 22 – 23, Bandung, Indonesia

65

Effect of Ferromagnetic Layer Thickness on the Giant Magnetoresistance

Properties of NiCoFe/Cu/NiCoFe Sandwich

Ramli1,3 , Mitra Djamal 1 , and Khairurrijal 2 1Theoretical High Energy Physics and Instrumentation Research Group, Faculty of Mathematics and Natural

Sciences, Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132, Indonesia2Physics of Electronic Material Research Group, Faculty of Mathematics and Natural Sciences, Institut Teknologi

Bandung, Jl. Ganesa 10, Bandung 40132, Indonesia3 Department of Physics, Faculty of Mathematics and Natural Sciences, Universitas Negeri Padang,

Jl. Prof. Dr. Hamka Airtawar Padang 25131, Indonesia

Email:[email protected]

AbstractWe have grown NiCoFe/Cu/NiCoFe sandwich onto Si (111) substrates by opposed target magnetron

sputtering (OTMS) technique. The growth paramaters are: temperature of 100 oC, applied voltage of 600 Volt, flow

rate of the Argon of 100 sccm, and growth pressure of 5.2 x 10 -1 Torr. The effects of ferromagnetic layer thickness

on giant magnetoresistance (GMR) properties of NiCoFe/Cu/NiCoFe sandwich were studied. At room temperature,

we have found the changes in the magnitudo of giant magnetoresistance (GMR) ratio of the samples as the thickness

ferromagnetic was varied. We observed that the GMR ratio of NiCoFe/Cu/NiCoFe sandwich was change with

different ferromagnetic layer thicknes (t NiCoFe). The magnitudo of GMR ratio with constant Cu layer thikness (t Cu =

13.2 nm) are; 10.5%, 24.4%, 70.0% and 30.1% for 40.0 nm, 47.5 nm, 62.5 nm, and 100.0 nm of NiCoFe layer

thickness ,respectively. In this paper, the influence of spin-dependent scattering to GMR value were studied. The

decrease in GMR at low thickness of NiCoFe is due to the scattering at the outer boundaries (substrate, buffer layer

or capping layer). This scattering significantly affects GMR when the thickness of the ferromagnetic layer becomes

smaller than the longer of the two mean-free paths associated with the spin up and spin down of electrons.

Keywords : Giant magnetoresistance, Opposed Target Magnetron Sputtering, NiCoFe, Spin-dependent scattering

1. Introduction

Giant magnetoresistance (GMR) effect

observed in magnetic multilayer films consisting of alternating magnetic and nonmagnetic layers

represents a relatively large change in the electrical

resistance when an external magnetic field is applied

to the films. The effects of GMR has been a focus of

intensive study for two decades, both for interesting

of fundamental physics [1,2] and important industrial

applications as sensors, memory, and read head

applications [3,4].

GMR can be qualitatively understood using

the series-resistor model. The series-resistor model

was initially developed using the two-current model

proposed by Mott [5]. The Mott model assumes that

no spin-flip scattering occurs during the transport of

electrons and that the currents through the twodifferent spin channels (spin up and spin down) are

described independently. Under these assumptions,

the conduction through these spin channels occur in

parallel.

Valet and Fert [6] proposed a different

model that modified the series-resistor model and

took into account the spin relaxation due to spin-flip

scattering in addition to the normal relaxation of

electrons associated with momentum. For the steady-

state spin transport in a magnetic multilayer, the spin

relaxation is balanced with the spin accumulation at

the layer interfaces when electrons move in the

direction perpendicular to the interfaces. The spin-

flip scattering is quantitatively described by the spin-

diffusion length. When the layer thicknesses are

much shorter than the spin diffusion length, the

Valet-Fert model simply represents the same as the

series-resistor model.

In this study, we are interested to sandwich

of NiCoFe/Cu/NiCoFe, which the NiCoFe alloy have

a soft magnetic properties. We investigate the GMR

effect with different thickness of NiCoFe layers at

room temperature.

2. Experiment

NiCoFe/Cu/NiCoFe sandwich were grownonto Si (111) substrate by dc-opposed targetmagnetron sputtering (dc-OTMS) technique. Thesputtering targets were NiCoFe and Cu. The NiCoFetarget was prepared by solid reaction method withmolar ratio Ni:Co:Fe = 60:30:10. Raw material forproducing NiCoFe target consists of 99.90% nickelpowder, 99.99% cobalt powder and 99.99% ironpowder. The Cu target also had been made by solidreaction from 99.50% copper powder.

Samples of the NiCoFe/Cu/NiCoFe

sandwich were deposited in several different time of



66 Ramli, Mitra Djamal, and Khairurrijal

growth, so that they had different thickness of

sandwich layers. Other deposition parameters are

fixed. This parameters are: flow rate of Ar gas is

about 100 sccm, the growth pressure is 5.2 x10-1

Torr, dc Voltage is 600 volt, and the temperature is1000C. The samples were characterized using SEM

(Scanning Electron Microscope) type JEOL JSM-

6360 LA, XRD (X-Ray Diffraction), and

magnetoresistance measurements were made using a

linear four-point probe method with current-

perpendicular to-plane.

3. Result and Discussion

From previous work [7], we found thatgrowth time influence the thin film thickness. Theincrease of growth time increases the atoms whichpatched on the substrate. The selected sample of NiCoFe/Cu/NiCoFe sandwich had characteristic of surface structure as shown in SEM image in Fig. 1.

Fig. 1. SEM image of NiCoFe/Cu/NiCoFe sandwich

with t NiCoFe = 62.5 nm.

Fig. 2 shows X-ray diffraction spectra forNiCoFe/Cu/NiCoFe sandwich with t NiCoFe = 62.5 nm.

Fig.2. XRD spectra for NiCoFe/Cu/NiCoFe sandwich

with t NiCoFe = 62.5 nm.

Fig. 3. GMR ratio curves for NiCoFe/Cu/NiCoFe

sandwich with various NiCoFe layer thickness

(t NiCoFe) and fixed Cu layer thickness (t Cu = 14,4 nm)

at room temperature.

The GMR ratio is calculated by usingdefinition, GMR ratio (%) = {( R H – R0)/ R0}x100%,where R H is the resistance in presence of magnetic

field and R0 is the resistance in absence of magneticfield. Fig. 3 shows the GMR ratio curves forNiCoFe/Cu/NiCoFe sandwich with various NiCoFelayer thickness (t NiCoFe) and fixed Cu layer thickness(t Cu = 14,4 nm) at room temperature. Results showthat magnitudo of both GMR ratio and saturation field( H s) is vary.

Fig. 4 describes the magnitudo of GMR ratioversus the magnetic layer thickness forNiCoFe/Cu/NiCoFe sandwich with a fixed nonmagnetic spacer layer thickness. The magnitudo GMR



Effect of Ferromagnetic Layer Thickness on the Giant Magnetoresistance ……………… 67

ratio possess a maksimum of 70.0 % at t NiCoFe = 62.5nm.

In sandwich structure, the deccrease in

magnitudo of GMR ratio at low thickness of NiCoFe

is due to the scattering on the outer surface likesubstrate or buffer layer [8]. This scattering

significantly affects GMR when the thickness of the

ferromagnetic layer becomes smaller than the longer

of the two mean-free paths associated with the spin

up and spin down of electrons.

At thickness of NiCoFe over 62.5 nm the

magnitudo GMR ratio decrease what could be

explained by the appearance of the inactive region

which shunts the current. For large NiCoFe layer

thickness, the inactive part appears in NiCoFe layer

and shunts the current. On the other hand, H s decrease

with increasing NiCoFe layer thickness, as observed

in Fig. 3. This result as same as phenomenon was

observed in NiFe/Cu multilayers [9] andelectrodeposited Co-Cu/Cu multilayers [10].

Fig. 4. GMR ratio as function of t NiCoFe for

NiCoFe/Cu/NiCoFe sandwich.

4. Conclusion

The NiCoFe/Cu/NiCoFe sandwich wassuccessful grown onto Si (111) substrate by opposedtarget magnetron sputtering. With this material, wehave successful to find material composition i.e.Ni60Co30Fe10, that has maximum GMR ratio at room

temperature. The magnitudo of GMR ratio changewith increase of ferromagnetic (NiCoFe) layerthickness and presents a maksimum of 70.0 % att NiCoFe = 62.5 nm.

AcknowledgmentThis work was supported by Asahi Glass Foundation(AGF) Grant, 2008.

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effect of ferromagnetic layer thickness on the giant magneto resistance

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