ELSEVIER Thin Solid Films 304 (i997) 319-322

Magnetoresistance of granular ferromagnet-insulator films

A. Gerber a,*, A. Milner a, B. Groisman a, M. Karpovsky a, A. Sulpice u School of Physics and Astronomy, Raymond and Beuerly SackIer Fac~dty of Exact Sciences, Tel AL, i~, UniuersiO; Ramat AL, iu, 69978 Tel Arty, Israel

b CRTBT, CNRS, BP 166, 25 av. des Martyrs, F-38042 Grenoble Cedex 09, France

Received I4 August 1996; accepted 7 January 1997

Abstract

We report the GMR-like magnetoresistive behavior of granular ferromagnet-insulator mixtures beIow the percolation threshold. In contrast with heterogeneous ferromagnet-normal metal systems, the absolute magnitude of the magnetoresistance increases at lower concentrations of the ferromagnetic component, and reaches a field sensitivity A p / A H of the order of 10 t2 cm T-~. Characteristic features of the GMR in highly resistive granular ferromagnets are discussed. © 1997 Elsevier Science S.A.

Ko~,.ords: Conductivity; Electrical properties and measurements; Ferromagnet-insnlator mixture films; Giant magnetoresistance; Magnetic properties and measurements; Sensors

1. Introduct ion

This paper describes the magnetotransport properties of granular ferromagnet-insulator mixtures, where nanome- ter-sized polycrystalline grains of the ferromagnetic com- ponent are embedded in either a crystalline or an amor- phous insulating matrix. Both the electrical and magnetic properties of these materials depend on the ferromagnetic metal volume fraction x. When x is large, the metallic grains coalesce and form a continuous network, so that electrons can percolate along the metallic channels. The magnetic phase also percolates throughout the material and the sample appears to be a normal ferromagnet [1]. When x is small, the metallic grains form isolated dispersions in an insulating matrix. Electrical conduction in this dielectric regime is by a hopping mechanism in which the charge carriers are transported from one grain to another via thermally activated tunneling. The transition from the metallic to the dielectric regime is characterized by a percolation threshold at which the metallic network first becomes disconnected. The temperature coefficient of the resistivity changes its sign to negative at the composition and temperature at which the contribution to the electrical conductivity due to thermally activated tunneling becomes

* Corresponding author. Fax: + 972 3 6422979; email gerber@ccsg.tau.ac.il

0040-6090/97/$I7.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0040-6090(97)0001 i-4

comparable to the contribution along the percolating metal- lic channels.

If the sizes of individual grains are sufficiently large for the existence of intragranular magnetic order, the isolated nanometer-sized grains are single-domain ferromagnets. For a random spatial orientation of intragranular easy magnetic axes, the system below the percolation threshold behaves as a superparamagnet [2]. When the layer of insulator between neighboring ferromagnetic grains is thin enough, the spin-dependent tunneling or temperature- activated hopping can take place. As predicted by Helman and Abeles [3], the spin-conserved tunneling probability depends on the difference between the intragranular elec- tronic magnetic exchange energies. The external magnetic field aligns the granular magnetic moments and thus af- fects the intergranular conductance. Since it was first observed more than two decades ago [4], the effect has recently been rediscovered [5,6] in a number of ferromag- net-insulator mixtures. Our preliminary comparison [6] of the magnetotransport properties of ferromagnet-insulator mixtures with the GMR effect, as known in heterogeneous ferromagnet-normal metal alloys [7], revealed surprisingly similar field-dependent effects in both types of systems, an intriguing fact that calls for a common description of magnetoresistive phenomena in any granular ferromagnet. Here, we wish to draw attention to the specific features of the GMR-like effect in ferromagnet-insulator mixtures

320 A. Gerber et al. / Thin Solid Films 304 (]997) 319-322

which make these materials promising candidates for a range of practical applications.

2. Procedure and results

Thin granular films of Ni-SiO 2 and Co-SiO z were prepared by co-evaporation of the starting materials using two independent electron beam guns. The results presented in this paper were obtained on samples deposited on a room-temperature glass substrate without any post-deposi- tion thermal processing. Usually, a set of up to 24 samples was deposited simultaneously. The relative concentration of the components varied smoothly due to a shift in the geometrical location relative to the material evaporation sources. Samples with Ni(Co) volume concentrations from 30% up to 100% were prepared. The absolute concentra- tion accuracy defined by the deposition rate control was about + 5%. The relative concentration accuracy of sam- ples of the same series, as defined by their geometrical location, was better than __.0.3%. Most of the results reported here were obtained on 100 nm thick films. Trans- mission electron microscopy was performed on 15-30 nm thick films deposited on graphite substrates. Ni and Co were found to be crystalline with typical grain sizes of the order of 30-40 A. The SiO 2 matrix was found to be amorphous.

The room-temperature magnetization of four Co-SiO 2 samples with different Co concentrations are plotted in Fig. 1 as a function of the applied magnetic field. Samples with high Co concentrations (above 50%) are ferromag- netic, whereas the two samples with lower Co concentra- tions (42.3% and 39.4%) are superparamagnetic.

Co-SiO z and Ni-SiO 2 films with transition metal con- centrations below the percolation threshold demonstrate negative magnetoresistance, as shown in Fig. 2. The be- havior is essentially isotropic, i.e. it does not depend on the relative orientation of the magnetic field with respect to

400!

200

,V - - D - - 39,4%

,7 .~- --~-- 52% T / --v-- 5s.5%

/ = /

[d 0

0 5 10 15 20 [4 (kOe)

Fig. t. Room-temperature magnetization of Co-SiO 2 samples as a func- tion of magnetic field for four different Co volume concentrations.

1.00 - ~ V ~ V

0 97 \ ~ " " ~ %

0.96 ~ x ~ %

i r I t 1 I ~ I r r

2 4 6 8 t0 H {kOe)

Fig. 2. Normalized room-temperature magnetoresistance of Co-SiO 2 granular ferromagnets for various Co volume concentrations x. The magnetic field is applied perpendicular to the film plane. P0 is the zero-field resistivity.

the current flow direction. The percolation threshold is def'med at a N i /Co concentration x = 49%. The resistivity of the samples at this concentration is about 10-1 fZ cm. As shown in Fig. 2, magnetoresistance is found below the percolation threshold only, i.e. in superparamagnetic sam- ples with high zero-field resistivity. The isotropic negative magnetoresistance is the trademark of the phenomenon of the giant magnetoresistance (GMR) as known in metallic heterogeneous magnetic structures. We have previously demonstrated [6] that the mechanism of spin-dependent magnetotransport in granular ferromagnets is to a large extent independent of the nature of the non-magnetic intergranular matrix. Nevertheless, a high background re- sistivity and the restrictions imposed on the current flow profiles in the bulk of ferromagnet-insulator mixtures are responsible for a number of important characteristic prop- erties.

2.1. Absolute magnetoresistance values

In all-metallic heterogeneous systems, the GMR is usu- ally characterized by its normalized value (Po- P)/Po, where P0 and p are the resistivities measured at zero and high fields, respectively. In any granular ferromagnet, including ferromagnet-insulator mixtures, the effect devel- ops at the percolation threshold, its normalized value A p/p reaches a maximum at a certain concentration below the threshold, and then continuously decreases at lower transi- tion metal concentrations [6]. For Ni-based mixtures the

A. Gerber et aL /Th in Solid Films .304 (1997) 319-322 3 2 I

largest normalized magnetoresistance of about 0.6% is found in the vicinity of 4% below the percolation thresh- 0 old, whereas in Co-based mixtures A p / p reaches 4.5% at a concentration about 7% below its threshold. Although -io the relative GMR-like magnetoresistance in ferromagnet- normal metal and ferromagnet-insulator mixtures appears to be identical, the absolute values of the magnetoresis- ~ ~-20 tance demonstrate different tendencies at low transition metal concentrations. In ferromagnet-nonnal metal mix- -30 nares such as Ni -Ag and Co-Ag, the zero-field resistivity remains roughly constant over the entire range of compos- -40 ire concentrations. Therefore, both the absolute and the normalized magnetoresistance show the same dependence on the ferromagnet concentration. The behavior is different in ferromagnet-insulator mixtures. Fig. 3 shows the abso- lute values of the magnetoresistance A p as a function of the room-temperature resistivity p for Ni-SiO 2 and C o - SiO 2 samples. The absolute magnetoresistance values of the Ni(Co)-SiO 2 mixtures show no tendency to decrease at low metal concentrations. In contrast, A p continues to increase with decreasing metal concentrations, at least in the range we were able to measure.

The difference can be understood by analyzing the current profiles in two types of systems at low concentra- tions of the ferromagnetic grains. In heterogeneous metal- lic systems the current density remains roughly uniform at any relative concentration of the two components. There- fore, at low ferromagnet concentrations the current passes mainly along the metallic non-magnetic matrix and avoids ferromagnetic grains. In metal-insulator mixtures the situ- ation is different, since the current is always confined to flow through the metallic ferromagnetic grains and across the shortest intergranular gaps, and not along the insulating matrix. Here, field-dependent tunneling remains the only mechanism responsible for the electric transport. Accord- ingly, we found no evidence for the disappearance of

I0 = -- / f f l

zo: --O- Nisio2 ~,.D 36% --I'I-- CoSiO 2 [3[~ []

10 0 /

I0 "1 4 6 % ~ . O O / 4 0 0 %

~o. a~. to ~ ~~45%/o--///..o- 10 ..4 48 °/"~7'' 104

i 0 -~ . . . . ~ d 51 °,/° , . . . . . . , . . . . . . . . , . . . . . . . . , . . . . . . . . , . . . . . . . . ,

10 .3 10-2 10-1 100 101 I02 103

Po (acre)

Fig. 3. Absolute values of the GMR-like magnetoresistance (Ap = P0 - Plo kOe) as a function of the zero-field resistivity P0 for Ni--SiO2 (circles) and Co-SiO2 (squares) samples.

- Q

Q

r 0 2 4 6 8 10

H (kOe)

Fig. 4. Typical magnetic field sensitivity Ap/AH for a Co-SiO 2 sampIe with Co volume fraction 37% and zero-field resistivity i kK2cm.

GMR at low ferromagnet component concentrations in our experiments.

2.2. Magnetic field sensitivity

The sensitivity of the magnetoresistance, i.e. the change in resistivity compared with the field required to produce this change, is an important parameter for any potential application of GMR materials. While the magnetoresis- tance found in antiferromagnetically coupled F e / C r super- lattices is large (about 21 txD cm at 4.2 K) [8], the fields needed to achieve this are of the order of 1-2 T, and the ratio Al/Hsa t is of the order of 10 -3 b ~ cm G - I . For uncoupled structures one is able to produce A p = 0.88 tx12cm with a field of only 2 G, i.e. A p / A H = 0.44 ix[2 cm G -I [9]. Our data can be compared favorably with those obtained in the best ferromagnet-normal metal mul- tilayers quoted above. Fig. 4 demonstrates the field sensi- tivity A p / A H as a function of the magnetic field for a Co-SiO 2 sample with a Co volume concentration of 36%. The zero-field resistivity of the sample is about 1 k12 cm, and A o /A H at room temperature reaches 40 [2 cm T - (4 m12 cm G -1) at H = 0.4 T, and is about 25 12 cm T - I at H = 1 T. This sensitivity is four orders of magnitude higher than the value quoted above for the best magneti- cally uncoupled multilayers. Moreover, the magnetoresis- tance of the Co-SiO 2 sample does not saturate up to 1 T, and the mentioned sensitivity is typical for a wide field range. The high-field limits are currently under study. Evidently, in order to benefit from this high-field resolu- tion, the measurements should be performed in a bridge geometry with balanced high zero-field resistance.

2.3. Diagnostics

Microscopic geometric deviations and imperfections of multilayered and heterogeneous ferromagnet-norrnal metal systems are known to affect the GMR effect strongly.

322 A. Gerber et at. / Thin Solid Films 304 (1997) 319-322

3

o o o o

o

o

0 • o ~ • I ~ T I I P I 1 , H , I t n t ~ ' , ' * ] ~ ' b r r l l t l ' I ' l l l r l l I ' ~ l q r ; n l I ' I I r H I

i0 -S 10 -2 10 -I 10 o I0 t I0 2 10 a p (tuna)

Fig. 5. Normalized magnetoresistance of Co-SiO 2 samples as a function of their zero-field resistivity. Different symbols indicate different deposi- tion series.

However, prior to the actual measurement of the sample magnetoresistance, no simple diagnostic procedure has been found to indicate the quality of the sensor and the magnitude of the effect to be expected. Here, the ferro- magnet-insulator compounds have a strong advantage compared with all-metal systems. The dependence of the normalized GMR effect on the sample resistivity is plotted in Fig. 5 for a number of different deposition series. As can be seen, magnetoresistance is highly reproducible and, to a large extent, is a well defined function of the zero-field resistivity. The measurement of the latter can serve as a diagnostic tool for the expected GMR effect.

3. Summary

The granular ferromagnet-insulator mixtures Ni -S iO 2 and Co-S iO z below the ferromagnet component percola-

tion threshold demonstrate isotropic negative magnetore- sistance similar to the GMR effect. The values of the magnetoresistance normalized to the zero-field resistivity are in the range of a few percent. Nevertheless, due their high background resistivity, the field sensitivity A p / A H

can be as high as few m[2 cm G - t in a wide field range, which is orders of magnitude higher than that of any other magnetic field sensor. In practical sensors, a balancing of the high background resistance wilt be required in order to benefit from this extraordinary sensitivity.

Acknowledgements

This research was supported by the Israel Science Foun- dation founded by The Israel Academy of Sciences and Humanities. B.G. acknowledges the support granted by Levi Eshkol Foundation, the Israel Ministry of Science and the Arts.

References

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219. [6] A. Milner, A. Gerber, B. Groisman, M. Karpovsky, A. Gladkikh,

Phys. Rev. Lett. 76 (1996) 475. [7] See, for example, C.L. Chien, Ann. Rev. Mater. Sci. 25 (1995) 129,

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Appl. Phys. Lett. 63 (1993) 1699. [9] B. Dieny, V.S. Speriosu, S. Metin, S.S.P. Parkin, B.A. Gurney, P.

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