Transverse and longitudinal magnetoresistance in graphite intercalated by Co

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Physica E 40 (2008) 2630–2634 www.elsevier.com/locate/physe

Transverse and longitudinal magnetoresistance in graphite intercalated by Co D.V. Matsuia, Yu.I. Prylutskyya,, L.Yu. Matzuya, F. Le Normandb, U. Ritterc, P. Scharffc a

Department of Physics, Kyiv National Shevchenko University, Volodymyrska Str., 64, Kyiv 01033, Ukraine Groupe Surfaces–Interfaces, Institut de Physique et Chimie des Mate´riaux, 23 rue du Loess BP 43, 67037 Strasbourg, France c Institute of Physics, Technische Universita¨t Ilmenau, Weimarer Str., 25, D-98684 Ilmenau, Germany

b

Available online 29 September 2007

Abstract Graphite intercalation compounds containing transition metal (Co) were successfully synthesized and their electrical properties investigated over a wide temperature range. These properties were found to be strongly dependent on the initial graphite material used. The results of the XRD investigations indicate that the intercalated compounds of graphite with transition metals could be obtained by the reduction of C8K-intercalated compounds by metal chlorides. The behavior of resistivity, Hall coefficient, transversal and longitudinal magnetoresistance for graphite–Co compounds based on different structure forms of pyrolitic graphites have been studied at 77–300 K temperature range in magnetic field of up to 2 T. And the anomalous Hall coefficient and onset of transversal magnetoresistance have been observed in Co–graphite intercalated compounds. This is caused by the formation of ferromagnetic Co layers in the studied compounds. The anisotropy of the transverse magnetoresistance at the inversion of magnetic field direction was observed. This is related with the onset of additional contribution linearly dependent on magnetic field. This contribution arises due to residual magnetization of ferromagnetic cobalt layers. r 2007 Elsevier B.V. All rights reserved. PACS: 61.10.i; 61.50.f; 75.50.y Keywords: Graphite intercalation compounds; X-ray diffractometry; Hall coefficient; Magnetoresistance

1. Introduction The possibility of controlling electron transport by means of the spin degree of freedom, in short spintronics, has drawn recent attention, as spintronics devices may have the potential for the applications in future commercial electronics and generate insight into fundamental properties of the electron spin physics in solids [1]. Spin-polarized transport in mesoscopic and low dimensional systems has gained particular interest. Giant and anisotropic magnetoresistance has been found in a variety of heterogeneous magnetic systems including heterostructures and multilayers [2,3], spin valves [4], and granular ferromagnets with non-magnetic or insulating matrices [5,6]. The common property for all these systems is that in their nonCorresponding author.

E-mail address: [email protected] (Y.I. Prylutskyy). 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.09.121

magnetized state, they consist of regions of magnetic material with different magnetization orientation separated by non-magnetic spacer. The magnetoresistance is associated with the alignment of magnetic moments under the applied magnetic field. Natural 2-dimensional (2D) multilayer system is a graphite intercalation compound (GIC). The possibility to produce by intercalating the multilayer structures with thin, monomolecular layers and the possibility to change the thickness, structure of layers using different types of graphite and intercalant open the wide perspective for such kind of materials. Using metals to intercalate graphite, especially using transition metals such as cobalt, iron, and nickel, is one of the synthesizing methods for nano-scaled ferromagnetic clusters. But now, the most of investigations are directed on study of carbon–transition metals systems based on carbon nanotubes. There are very restricted dates about the possibility of production and investigation of the

ARTICLE IN PRESS D.V. Matsui et al. / Physica E 40 (2008) 2630–2634

C8 K þ MeCl ! C2Me þ KCl

ðMe ¼ CoÞ:

(1)

As a result of the reaction between C8K and the transition metal chloride, K was replaced by the metal and the C8K–GIC was transformed into a transition metal GIC. Different types of graphite were used as supporter: highly ordered pyrolytic graphite (HOPG) and disordered pyrolytic graphite (DPG), which differ by the value of distance between the graphite layers character (d002), layerpacking parameter (g), the sizes of crystallizes (La, Lc) and, as consequence, have different transport properties (the structure parameters are presented in Table 1). The results of X-ray studies of the obtained compounds showed (Fig. 1) that a number of additional reflections were observed at the X-ray diffraction patterns to compare with pure graphite. The additional peaks, attributable to the intercalate layers with the identity spacing (Ic), were observed as HOPG–Co and DPG–Co (Fig. 1). These peaks indicated the formation of intercalated compounds. Meanwhile, the calculations performed for Ic and distance between the graphite layers containing Co layer (di)

10000 3(002)

8000 (002)C

6000 (004)C

600 500 400 300 200 100 0 -100 -200

2s(002) 1 3(008) 2s(002) 2 3(008)

3(007)

3

20

30

40

50 2θ, degrees.

60

70

Table 2 Structural parameters for the graphite–cobalt compounds Samples

Stage

di (nm)

Ic (nm)

r (  106 O m)

HOPG–Co DPG–Co

3 3

0.70 0.71

1.380 1.385

1.53 1.22

4.0

2.4 2.2 2.0

3 5

3.8

3.6

46 64 3.4

1.8

3.2

1

2

1.6

3 ln T

4

5

1

1.4 1.2

Table 1 Structural parameters for the pyrolytic graphite

80

Fig. 1. XRD patterns for pyrolytic graphite–Co: (1) HOPG, (2) HOPG–Co, (3) DPG–Co.

σ,104 Om-1 m-1

It is difficult to directly intercalate a transition metal into graphite. Thus, graphite–metal compounds were obtained by metal reduction of metal chlorides by C8K [8,9]. At the first stage, standard two-temperature method was used to produce C8K. Then the metal chloride reduction was performed in dry tetrahydrofurane (THF), which was thoroughly dried by boiling it over metallic sodium for several days. The metal chloride was first heated in a vacuum at 373 K and was then dissolved in 50 ml of THF before the C8K was added. The solution was mixed by a magnetic stirrer at room temperature for 3 days. The sediment was separated by filtration, washed several times with THF and then by a solution of water/alcohol (1:1 ratio). The obtained materials were dried in a vacuum. The reaction takes place according to the following scheme in THF:

I.a,u,

2. Results and discussion

resulted in the values 0.70 nm for HOPG–Co and 0.71 nm for DPG–Co (Table 2). The increase revealed for di was thought to be predominantly conditioned due to the structural parameters of the source graphite, namely due to the fact that d002 for these graphite is equal to 0.337 nm for HOPG and 0.342 nm for DPG. Studies of the resistivity temperature (r) dependencies were performed for pyrolytic graphite–cobalt compounds using the procedure described elsewhere [7]. The obtained data were presented in Fig. 2.

ρ,10-6 Om m

properties of ferromagnetic metal–graphite or metal– graphene structures. Our previous study has shown that the production of Co–GIC based on thermo-exfoliated and disperse forms of graphite [7] is principally possible. The aim of this work is to produce GIC with transition metal (Co) and to study the influence of the graphite supporter type on the resistivity and the magnetotransport characteristics of the metal GIC.

2631

2

1.0 6

Samples

d002 (nm)

La (nm)

Lc (nm)

g

r (  10

HOPG DPG

0.337 0.342

200 30

100 20

0.97 0.3

2.5 6.9

O m)

0

50

100

150 T, K

200

250

300

Fig. 2. r(T) dependence for HOPG–Co (1) and DPG–Co (2). Inset: the dependence s ¼ f(ln T) for HOPG–Co (3) and DPG–Co (4).

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Obviously, the resistivity of the obtained compounds was reduced, which is typical for an intercalation process. However, there are some peculiarities: in contrast with data for GIC with alkali metals and metal chlorides, the reduction of r was rather 2–3 times small only. Firstly, this could be connected with the accommodation index, in other words, it means that the portion of the additional carriers involved in the intercalate layer could be low. On the other hand, it could be results of increasing number of defects, which are formed in the process of intercalation. The results of investigation of r(T) dependencies under low temperature confirmed it. GIC, on the base of pyrolytic graphite, exhibited an abnormal character of the resistivity–temperature dependence in the low-temperature range: the resistivity increased as the temperature decreased. The analysis of r(T) dependencies for HOPG–Co and DPG–Co GIC showed the temperature variation of the resistivity in the low-temperature region and was well described by the logarithmic functions (Fig. 2, inset). This type of dependence was inherent to 2D disordered systems. If the results of investigations of resistivity demonstrated quite regular qualitative changes in behavior of resistivity of graphite, which took place under intercalation, the behavior of Hall coefficient, magnetoresistance for the studied systems, revealed a number of new conformities which substantially differs from that for the routine GIC. The studies of the Hall coefficient vs. temperature and magnetic field (within magnetic field ranged from 0 to 2 T) for HOPG and DPG specimens containing Co showed that graphite intercalated by Co results in the decreasing (about 5–8 times) of the Hall coefficient value. Fig. 3 shows the anomalous character of the dependency of the Hall coefficient vs. magnetic field Rx ¼ f(B). It is known that the Hall coefficient, which is conditioned by the action of the Lorentz force on the charge carriers in the magnetic field directed perpendicularly to their moving, does not depend on the magnetic field. The presence of the inner

RH10-7, m 3/coulomb

5.0 4.5

RH10-7, m3/coulomb

4.0 3.5

4

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

3 2 1

0.00

3.0

0.01

0.02 1/B, m-1

0.03

0.04

2.5 4

2.0 1.5

3

1.0

2

0.5 1

0.0 0

500

1000 B, mT

1500

2000

Fig. 3. RH(T) dependence for HOPG–Co (1, 3) and DPG–Co (2, 4): (1, 2) T ¼ 290 K; (3, 4) T ¼ 77 K.

magnetic field for the magnetic materials gives the additional term in the Hall coefficient expression [10]: Rx ¼ Rx0 þ

Ra m0 M s , B

(2)

where Rx0 is the normal Hall coefficient, Ra is the abnormal Hall coefficient, m0 is the magnetic permeability in vacuum and Ms is magnetization that depends on the magnetic field. The abnormal Hall coefficient arises due to spin– orbital interaction and scattering potential, which is conditioned by the impurities, crystal structure imperfections, phonons, magnons, etc. As it is seen from Fig. 3 (inset), which presents RH ¼ f(1/B) dependence, as temperature decreases the value Ram0Ms increases approximately 3 times for HOPG–Co (from 23.6  106 m3 mT/C at T ¼ 300 K to 77.8  106 m3 mT/C at T ¼ 77 K) and 1.5 times for DPG–Co (from 33.2  106 m3 mT/C at T ¼ 300 K to 59.9  106 m3 mT/C at T ¼ 77 K). The increasing of Ram0Ms value at temperature decrease could be conditioned both by increasing the Ms value and also by Ra increasing. However, the temperature dependence of Ra strongly correlates with the temperature dependence of the resistivity [11]. The skewness of the carriers’ scattering probability arises upon the scattering of electrons on the impurity centers due to the influence of spin–orbital interaction. This conditions the dependence of Raar+br2. As it is clearly seen from Fig. 2, the resistivity of Co–GIC is almost independent on the temperature within 77–300 K range. It is allowed to assume Ra being weakly dependent on the temperature for the GIC studied. The normal Rx0 is observed when the external magnetic field exceed the saturation field (Hsut) that is equal to 600 and 250 mT at T ¼ 77 and 293 K, respectively. Arising of the anomalous Hall effect, which is commonly observed for the ferromagnetic materials, indicates the layers of Co intercalant in Co–GIC are ferromagnetic. The behavior of magnetoresistance for the studied systems also substantially differs from that for the routine GIC. Thus, the studies of magnetoresistance for Cocontaining GIC at different directions of applied magnetic field with respect to current direction (H?I—transversal magnetoresistance and HJI—longitudinal magnetoresistance) have revealed the following regularities. Inserting Co monolayers between the graphite layers leaded to an essential change both in the value and the character of the Dr/r dependence on the temperature as well as the magnetic field. The influence of the magnetic field direction on the magnetoresistance was observed both for HOPG– Co and DPG–Co compounds. First of all, it exhibited longitudinal magnetoresistance (Dr/r)J studied for the referred compounds. (Dr/r)J was shown to be significant by magnitude and to exhibit complex dependence on the external magnetic field (Fig. 4). It is known that an anisotropy of magnetoresistance is observed in bulk ferromagnets with significant residual magnetization [12]. This is conditioned by strong spin–orbital scattering. The resistivity of such specimen in external magnetic field

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1.0 120 0.5

80 Δρ/ρ, % Δρ

0.0

δρ/ρ,%

1

100

-0.5

60 40

2

20

4 3

-1.0 0 -1500

-1.5

-1000

-500

0 B, mT

500

1000

1500

-2.0 -600

-400

-200

0 200 B, mT

400

600

1

4

800

3 2 4

Fig. 4. (Dr/r):(H) dependence for HOPG–Co at T ¼ 293 K.

      Dr Dr Dr ¼ þ , r ex r n r a

(3)

where (Dr/r)nH2 is the contribution, which is conditioned by the action of the Lorentz force, and (Dr/r)a is the addition component determining the behavior of magnetoresistance in GIC with Co. As it is seen from Fig. 6, the value of (Dr/r)a linearly depends on magnetic field. Namely, the presence of this component which increases under decreasing the temperature, and proportions between Lorentz and liner evidences determine the value and sing of magnetoresistance under changes of magnetic fields.

-2 -4 -6 2

-8 -1500

-1000

-500

0 B, mT

500

1000

1500

Fig. 5. (Dr/r)?(H) of pyrolytic graphite–Co under different temperatures—(a) HOPG: (1) T ¼ 293 K, (2) T ¼ 77 K; HOPG–Co: (3) T ¼ 293 K, (4) T ¼ 77 K; and (b) DPG: (1) T ¼ 293 K, (2) T ¼ 77 K; DPG–Co: (3) T ¼ 293 K, (4) T ¼ 77 K.

20 1 15 (Δρ Δρ/ρ )ex-(Δρ Δρ/ρ, )n, %

increases if H is parallel to electric current and this contribution to magnetoresistance trends to saturation if material is magnetically saturated. So, the observed longitudinal magnetoresistance (Dr/r)J as well as anomalous Hall effect could be considered as an evidence of formation of the layers of ferromagnetic phase in Co–GIC system. The analysis of transversal magnetoresistance (magnetic field is perpendicular to current) showed the presence of Co between graphite layers results not only in sharp reducing of magnetoresistance, but also in asymmetry of this dependence vs. the inversion of magnetic field that is in contrast to the source graphite. As it is seen from Fig. 5a, b, the change in Dr/r value is observed for HOPG–Co and DPG–Co at the inversion of magnetic field. The substantial difference in magnetoresistance was observed for specimens cooled in magnetic field 2 T and without field. Therewith, (Dr/r)?77 K value is almost the same as (Dr/r)?293 K value if the field does not exceed 600 mT. For GIC, based on disordered pyrolitic graphite, not only the value but the sign of (Dr/r)? are changing at the inversion of magnetic field for the third stage of intercalation. The analysis of Dr/r ¼ f(B) dependence showed that it could be described by equation of type

Δρ/ρ, % Δρ/ρ

0

10 2

5 3

0 -5 -10 -2000

-1000

0 B, mT

1000

2000

Fig. 6. (Dr/r)a(H) dependence for HOPG–Co: (1) T ¼ 77 K, (2) T ¼ 290 K; DPG–Co: (3) T ¼ 290 K.

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Table 3 Values of (Dr/r)n and (Dr/r)a for HOPG–Co and DPG–Co Samples

(Dr/r)n/H2 (%/mT2) T ¼ 290 K

HOPG–Co DPG–Co

3

2.6  10 1.8  103

(Dr/r)a/H (%/mT)

T ¼ 77 K 2

6.7  10

T ¼ 290 K 6

1.2  10 3.1  107

T ¼ 77 K 6.1  106

For an example, for HOPG–Co, both the value contributions do not differ much and only the difference in value of Dr/r is observed under changing the direction of magnetic field (Table 3). But for the DPG system, in which value of normal Dr/r is small, presence of (Dr/r)a leads to invention of sign of magnetoresistance under changes of magnetic fields. It is worst to note that value of (Dr/r)a does not clearly depends on the type of initial graphite but increase approximately 2 times under the decreasing temperature from 290 to 77 K. Such behavior of (Dr/r)a is in a good correlation with the behavior of anomalous contribution in Hall coefficient, where the value Ram0Ms also weakly depends on the type of initial graphite but increase under the decreasing temperature due to increasing magnetization. It is known that the residual magnetization promotes the anisotropic magnetoresistance effect, i.e. the differences in the magnetoresistance at different current/magnetic fields directions. The essential value of longitudinal magnetoresistance allowed us to assume the existence of non-zero magnetic moment (i.e., the existence of residual magnetization) due to the formation of 2D metallic clusters between graphite layers in these compounds. This is also thought to be the reason of an essential contribution to transverse magnetoresistance, which is linearly dependent on H. 3. Conclusions 1. The intercalated compounds of graphite with transition metal were shown to be obtained by the reduction of C8K-intercalated compounds by metal chlorides.

2. The behavior of resistivity, Hall coefficient, transversal and longitudinal magnetoresistance for graphite–Co compounds based on different structure forms of pyrolitic graphites have been studied at 77–300 K temperature range in magnetic field of up to 2 T. 3. For the first time, the anomalous Hall coefficient and onset of transversal magnetoresistance have been observed in Co–graphite intercalated compounds. This is caused by the formation of ferromagnetic Co layers in the studied compounds. 4. The anisotropy of the transverse magnetoresistance at the inversion of magnetic field direction was observed. This is related with the onset of additional contribution linearly dependent on magnetic field. This contribution arises due to residual magnetization of ferromagnetic cobalt layers.

Acknowledgments This work was supported partly by the Alsace/RussieUkraine Grant (#NN-4) and ‘‘Dnipro’’ Program. References [1] [2] [3] [4] [5]

[6] [7] [8] [9] [10] [11] [12]

Zˇ.J. Fabian, S.D. Sarma, Rev. Mod. Phys. 76 (2004) 323. C. Christides, J. Appl. Phys. 94 (2003) 2516. A. Jensen, J.R. Hauptmann, J. Nygard, P.E. Lindelof, Phys. Rev. B 72 (2005) 035419. B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. A.E. Berkowitz, J.R. Mitchell, M.J. Carey, A.P. Young, S. Zhang, F.E. Spada, F.T. Parker, A. Hutten, G. Thomas, Phys. Rev. Lett. 68 (1992) 3745. C. Grunberg, R. Coehoorn, H.A.M. van den Berg, in: U. Hartman, (Ed.), Multilayers and Giant Magnetoresistance, Springer, Berlin, 1999. L. Vovchenko, L. Matzui, O. Stelmakh, M. Zakharenko, Fullerenes, Nanotubes Carbon Nanostruct. 13 (2005) 491. A. Fu¨rstner, F. Hofer, H. Weidmann, Carbon 29 (1991) 915. A. Mastalir, F. Notheisz, M. Barto´k, J. Phys. Chem. Solids 57 (1996) 899. A. Gerber, A. Milner, I. Korenblit, M. Karpovsky, A. Gladkikh, A. Sulpice, Phys. Rev. B 57 (1998) 13667. A. Cr’epieux, P. Bruno, Phys. Rev. B 64 (2001) 01441. T.R. McGuire, R.I. Potter, IEEE Trans. Magn. 11 (1975) 1018.