Magnetic Layer Composition Effect
on Giant Magnetoresistance in (NiFeCo/Cu) Multilayers
D. Meziane Mtalsi1) (a),M. El Harfaoui(a),A. Qachaou(a),M. Faris(a), J. Ben Youssef(b), andH. Le Gall(b)
(a) Laboratoire de Physique de la Matie`re Condense´e (LPMC), B.P. 133-14000, Ke´nitra, Maroc
(b) CNRS, LPS, Groupe de Magne´tisme, F-92 195 Meudon-Bellevue, France
(Received February 5, 2001; in revised form June 27, 2001; accepted July 30, 2001) Subject classification: 68.65.Ac; 75.70.Pa; S1.1; S1.2
In this work, we present a study of the structural, magnetic and magnetotransport properties in magnetic multilayered structures (NiFeCo/Cu) at room temperature. The magnetic (m) and non- magnetic (nm) layer thickness (tm, tnm) and the composition of the magnetic layer effect on the magnetoresistance (MR) have been investigated experimentally and discussed theoretically in the framework of the Johnson-Camley semiclassical approach, based on the Boltzmann transport equa- tion. The observed MR ratio oscillates for the Cu layer thickness with an average period of 12A and decreases from one system to another less rich in Co. The MR peak position depends on the magnetic layer composition. The materials become harder with increasing Co concentration.
1. Introduction
Among the very interesting phenomena that have been discovered in magnetic sand- wich and multilayers, giant magnetoresistance (GMR) is one of the most attractive phe- nomena for fundamental physics as well as for applications. This effect, first discovered in ferromagnetic multilayers (Fe/Cr) [1] with antiferromagnetic coupling, has been ob- served later in other layered structures [2, 3].
Various theoretical interpretations have been proposed for the observed magnetoresis- tive phenomena [4–6]. Most of the theories are based on the spin dependent scattering of electrons. Two main sources of scattering are considered; namely the scattering at inter- faces between magnetic and non-magnetic layers and the scattering in the bulk of mag- netic layers. Most systems studied have shown MR effects, the origin of which is attributed to exchange coupling oscillations. In alternating ferromagnetic (F) and antiferromagnetic (AF) layers, the exchange coupling gives rise to oscillations of MR with the same period.
A maximum of MR corresponds to AF coupling, and a minimum to F coupling.
In this paper, we investigate the structural and magnetic behaviour of (NiFeCo/Cu) multilayers prepared by the rf diode sputtering method. Our study will focus on the dependence of the magnetoresistance on the Cu layer thickness, the magnetic layer thickness and the composition of the magnetic layer.
2. Experimental
(NiFeCo/Cu) multilayers with 12 periods were prepared at room temperature using rf diode sputtering at a base pressure of 410––7mbar. The substrates consist of Corning
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7059 glass behind which a permanent magnet field (300 Oe) was held to induce uniax- ial anisotropy. The NiFeCo layers were obtained from a Co disk target covered by NiFe chips. Both low and high angle X-ray diffraction spectra indicate a good multilayer structure. Sputtering rates were estimated from thickness measurements made with a Tencor-1 profilometre of reference thick films and the chemical composition of the NiFeCo layer was obtained from an Electron Probe Microanalyser. A linear four-point probe method with the current flowing in the film plane and along the induced easy- axis direction enabled MR measurements.
3. Results and Discussion
The composition of the different magnetic layers of multilayers referred to by A, B, C and D is given in Table 1. These multilayers are made of an alloy of NiFeCo layered with Cu spacers.
3.1 Structural properties
Figure 1 shows the high angle X-ray diffraction spectra for the four samples belonging to the A, B, C and D series with thicknesses corresponding to the first magneto-resis- tance oscillation. The spectra show that all these multilayers are well crystallized with a (111) preferential orientation. The intensity of the line (111) decreases gradually with the Co concentration (XCo) except for a concentration of 41%.
3.2 Magnetic properties
MR properties were first investigated as a function of the Cu spacer thickness in the four series of (NiFeCo(tm)/Cu(tnm))12 multilayers. The tnm for each of the series was varied from 7 to 31A, while the magnetic layer thickness was held fixed attmfor each series.
The plots of MR versustnmare de- picted in Fig. 2. The curves show Ta b l e 1
Composition of magnetic layers
Ni (%) Fe (%) Co (%)
A 44 10 45
B 48 10.9 41
C 57.5 13 29
D 69 15.5 23
Fig. 1. High angle X-ray spectra of (NiFeCo/Cu) multilayers for various mag- netic layer compositions
MR oscillations with an average period of 12A. This would imply a MR origi- nating in the oscillating interlayer ex- change coupling between the magnetic layers across the Cu layer. The MR curves show a first maximum for the non magnetic spacer layer of low thick- ness (tnm1max¼9:7; 10.3; 10.5 and 11.3A for A, B, C, and D multilayers, respectively).
When the Co content of the magnetic layer is important, the first peak position is shifted approaching t1maxnm ¼9A observed for (Co/Cu) multilayers [7, 8]. The exchange coupling between adjacent magnetic layers is indeed AF. For all multilayers, the MR ratio presents a second maximum at about the same thickness. Such oscillations were also observed in other systems, such as: (Fe/Cr) [1] and (Co/Cu) [9].
The various hysteresis loops corresponding to the four studied sample series are pre- sented in Fig. 3. The film saturating field, which is proportional to the AF exchange coupling on the origin of the MR, increases with the Co content of the magnetic layer Fig. 2. Measured magnetoresistance versus thickness of the non-magnetic spacer layer of (NiFeCo/Cu) multilayers for different mag- netic layer compositions
Fig. 3. Hysteresis loops for various magnetic layer compositions in (NiFeCo/Cu) multilayers
(Fig. 4a). It passes from 1200 Oe for the concentration XCo ¼ 45% (sample D) to 3000 Oe forXCo¼23% (sample A). The saturation is reached more easily in the sam- ples with weak Co concentration.
The values of the coercive fieldHcdeduced from the hysteresis loops vary according to the composition of the magnetic layer between 40 Oe and 75 Oe (Fig. 4b). This re- veals that the coercive fieldHc increases with the Co concentration in the NiFeCo alloy and tends towards a value of 80 Oe measured on Co thin films where the coupling is ferromagnetic [8]. This suggests that the materials become increasingly hard with in- creasing Co concentration. This result is predictable knowing that the fraction of the soft layer of NiFe decreases.
The Mr/Ms ratio, where Mr and Ms are remanent and saturation magnetization re- spectively, is defined as the proportion of the zones coupled ferromagnetically. Fig- ure 4c shows aMr/Msdecrease tendency whenXCois growing. The relatively low values of this ratio, ranging between 30% and 50% for the studied systems, seem coherent with the existence of an antiferromagnetic coupling because the thicknesses are fixed at those corresponding to a maximum of magnetoresistance.
Figure 5 presents the evolution of the MR versus the proportion of the antiferromag- netic zones (1Mr/Ms). It confirms this tendency to privilege antiferromagnetic inter- actions with the introduction of Co into the studied systems.
The consequence of the decrease of the saturating field with the decrease of Co con- centration is highly evidenced by the evolution of the magnetoresistance sensitivity S¼DM R/DH. Indeed, Fig. 6 shows that S increases when the Co concentration de- Fig. 4. Co concentration dependence of a) saturation field, b) coercive field Hc and c) Mr/Msin (NiFeCo/Cu) multilayers
creases. For the first MR oscillation obtained at low Cu layer thicknesses, the sensitivity Spasses from 12%/kOe for sample A to 37%/kOe for sample D. For the samples belonging to the B and C series, one finds almost the same sensitivities approaching 20%/
kOe. For the second MR oscillation, corresponding to weaker saturating fields, the MR sensitivity is better. It increases clearly when the Co concentration decreases passing from 51.4%/kOe for the system with greater Co concentration to 135.7%/kOe for the system with weaker Co concen- tration.
The greatest value of the sensitivity observed at the second peak may result from a double effect acting onMRmax and Hs(Smax ¼ MRmax/HS). Indeed, when tCuincreases, the maximum of the MR ratio decreases, on the other hand, the AF coupling becomes weaker, thus inducing a parallel alignment of neighbour magnetic layer magnetizations which in turn decreases the saturation field Hs. This suggests that for the second peak, Hsdecreases swiftly and thereby (Smax)first peak<(Smax)second peak.
As shown in Fig. 7, MR appears clearly to depend on the Co content of the magnetic layer. The value of the MR ratio becomes large with increasing Co content. In addition, the increase of MR could be also, in coherence with the evolution of antiferromagnetic zones (Fig. 5), connected to the fact that Co contributes more to the spin dependent scattering (SDS) at the interfaces contrary to NiFe. Co is more miscible with Cu at the Co/Cu interface than NiFe at the NiFe/Cu interface [10]. The SDS in Co/Cu multilayers occurs in the bulk and within the interfaces, whereas in NiFe/Cu multilayers, it only takes place in the bulk.
In order to study more specifically the effect of magnetic (m) and non magnetic (nm) layer thickness and the concentration of Fe, Co and Ni on the MR, we have used the Johnson-Camley semiclassical model [6]. It is assumed in this model that the electron transport through the multilayer is gov- erned by the Boltzmann equation for the difference between the equilibrium state population f0"ð#ÞðvÞ and the per- turbed state population f"ð#Þðz; vÞ, ow- Fig. 5. The MR variation versus the AF fraction (1Mr/Ms)
Fig. 6. The MR sensitivity variation, at the first and second peak, as a function of the Co content
ing to the interfaces and the electric field:
g"ð#Þðz; vÞ ¼f"ð#Þðz; vÞ f0"ð#ÞðvÞ: ð1Þ
The arrows refer to the distributions for spin-up and spin- down electrons.
The general solution of the Boltzmann equation can be written as:
g"ð#Þ ¼eEt"ð#Þ m
@f0
@vx
1þF"ð#Þexp z
t"ð#Þvz
; ð2Þ
where Fis an arbitrary function of the velocity v, determined by the boundary condi- tions,eandmdenote the electron charge and electron effective mass, respectively,E is the applied electric field, andtis the spin- dependent relaxation time. Once theF’s are known, and thus the g’s, the current density in each region is obtained by integrating g"ð#Þðz; vÞ.
The current in the whole structure may be easily calculated by integrating the current density over z, and therefore the effective resistivity for the entire structure may be found.
Finally, the magnetoresistance is given by:
MR¼rAPrP
rP ¼IPIAP
IAP : ð3Þ
We have supposed that interfaces constitute mixed layers containing an “alloying phase” formed through an interdiffusion of magnetic and non-magnetic layers, as sug- gested in our previous paper for (CoZr/Cu) multilayers [11]. Furthermore, to obtain the mean free path (m.f.p.) for spin " ð#Þ electrons in the m and nm layers and at the interfaces ðl"ð#Þm ;lnm;l"ð#Þi Þ required by the Johnson-Camley model, we have used expressions of spin dependent in- terface and bulk resistivities ðri;rbÞ gi- Fig. 7. The MR ratio variation, at the first peak, as a function of Co content
Fig. 8. Calculated MR versus the thickness of the non-magnetic spacer layer of A, B, C and D magnetic multilayers
ven by Miyazaki et al. [12]:
rsi¼cðDsiðFeÞxFeþDsiðCoÞxCo
þDsiðNiÞxNiÞ;
r#b¼ ðxFexCoþxCoxNiþ4xNixFeÞR#; ð4Þ where c and R# are constants, x is the fraction of Ni, Fe and Co in the NiFeCo alloy. Dsi are the densities of states at the Fermi level for Ni, Fe and Co im- purities in Cu respectively, whose val- ues are obtained from the previous cal- culation [13]:
D"iðFeÞ ¼4; D"iðCoÞ ¼4;
D"iðNiÞ ¼6; D#iðFeÞ ¼59;
D#iðCoÞ ¼67; D#iðNiÞ ¼21: The values of constants c and R# are evaluated from the best fitting of the MR experimental results in our multi- layers. The correspondence between the resistivity r and the m.f.p. l has been established using bulk Fe as a re- ference and assuming the product
rðl"þl#Þ constant for elements close
to Fe [5].
The calculated dependence of MR on Cu layer thickness for each ferro- magnetic composition, given by Eq. (3), is shown in Fig. 8. It is only meaningful to compare the calculated results with the envelope of the observed MR maxima, seen in Fig. 2. Then the calculated MR relates only the data for antiferromagnetic coupled multilayers.
For all the studied systems, these calculated MR decrease as tnm increases. In fact, when the separating layer is thick, the conductivity through it becomes predominant and the spin dependent scattering effect becomes less effective. Consequently the MR decreases. Similarly, the MR decreases from one system to another less rich in Co. This could be explained by values of the density of states at Fermi level of electrons with spin down which are different for Co, Fe and Ni. Indeed, since the density of Co is the Fig. 9. The magnetoresistance ratio variation with the magnetic layer thickness at the first peak (tCu¼tnm1max) for systems with Co con- centrations of a) 45%, b) 41% and c) 23%.
Solid lines are our calculations, and symbols are experimental results
greatest, the substitution of Co by Fe or Ni atoms leads to a diminution of the number of spin down carriers, and then, the passage from a Co rich system to another with weak Co concentration provokes a decrease of MR.
The effect of the magnetic layer thickness on the MR of (NiFeCo/Cu) multilayers with A, B and D ferromagnetic composition is also investigated. The Cu layer thickness is fixed for each sample att1maxnm corresponding to the first MR oscillation. The calculated changes in MR ratio with the magnetic layer thickness are shown in Fig. 9 together with the experimental results. For each of the three systems, the MR increases with the magnetic layer thickness tm to reach a readily observable maximum at tmaxm : ðtmaxm;A¼19A,tmaxm;B¼18Aandtm;Dmax ¼10AÞ. These latter values are comparable to mea- sured values:ðtm;Amax ¼21:2A,tmaxm;B¼22:3Aandtm;Dmax ¼10AÞ.
The MR ratio decreases with increasing magnetic layer thickness tm over tmaxm . In- deed, for a large thickness tm, the magnetic layer can be divided into an active part, contributing to the MR, and an inactive part distant from interfaces. This inactive part shunts the current and then decreases the magnetoresistance.
The MR maximum and its corresponding magnetic layer thickness are greater in Co rich multilayers. This is in agreement with the MR evolution versus Co concentration previously shown in Fig. 8 and attributed to the difference between densities of states for Ni, Fe and Co.
4. Conclusion
The samples studied in this work are well textured preferentially in (111) direction. The investigated magnetic properties show that Co introduction into (NiFe/Cu) multilayers makes them magnetically hard.
The Johnson-Camley semiclassical approach was called upon to reproduce the behav- iour of the giant magnetoresistance in the (NiFeCo/Cu) magnetic multilayers. The find- ing allowed us to confirm the great sensitivity of the MR to magnetic and non-magnetic layer thickness, as well as to the composition of the magnetic layer.
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