spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets

2
Physica B 280 (2000) 331}332 Spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets A. Gerber* School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978, Tel Aviv, Israel Abstract Can spin-dependent scattering mimic spin-dependent tunneling in heterogeneous ferromagnets? The discussion is triggered by the surprising similarity in magnetoresistance properties of granular ferromagnets with metallic, insulating or point-contact spacers. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 75.70.Pa; 75.50.Tt; 75.70.-i Keywords: Giant magnetoresistance; Heterogeneous ferromagnets; Spin-dependent tunneling Two types of heterogeneous ferromagnetic systems are known today to demonstrate the giant magnetoresis- tance e!ect. The "rst is metallic, where ferromagnetic regions, layers or grains, are separated by normal metal spacers. The second is semiconducting or insulating, where the spacer is insulating and electronic transport is provided by tunneling or temperature activated hopping. Sensitivity of electronic transport to applied magnetic "eld in both systems is associated to the relative orienta- tion of magnetic moments: high resistance for antiparal- lel orientation and low resistance for the parallel one. The mechanisms of transport are di!erent and there is no reason a priori to expect any quantitative correlation between metallic and insulating systems. We are there- fore surprised to "nd such correlations, as presented in this paper. For the following discussion, we choose granular fer- romagnets where the details of granular geometry and intergranular coupling are statistically averaged and samples can be considered superparamagnetic. Two pairs of materials were studied in a wide range of con- centrations: Ni}Ag versus Ni}SiO 2 , and Co}Ag versus Co}SiO 2 . Structural and magnetic properties of each * Corresponding author. Tel.: #972-3-6408193; fax: #972- 3-6422979. E-mail address: gerber@ccsg.tau.ac.il (A. Gerber) pair are very close for any given concentration, which makes us assume that we study the same structures of ferromagnetic grains immersed in two matrices: metallic and insulating. Fig. 1 presents the normalized isotropic negative magnetoresistance, of the GMR type [1] as a function of the ferromagnet component concentration x for the two pairs: Ni}Ag versus Ni}SiO 2 , and Co}Ag versus Co}SiO 2 . Two major points should be mentioned: (i) the largest e!ect is found in the vicinity of the same ferromagnetic percolation threshold (which is also a conductive thre- shold in insulating systems (Ni/Co}SiO 2 )); (ii) for a given transition metal (Ni or Co) the e!ect is of almost the same magnitude for both metallic (Ag) and insulating (SiO 2 ) matrices, despite a huge di!erence in resistivities of ferromagnet-metal and ferromagnet-insulator systems below the threshold (1 : 107). On the other hand, changing Ni by Co increases the e!ect by an order of magnitude in both metallic and insulating matrices. Resistance of het- erogeneous, in particular granular, systems is dominated by intergranular barrier for the case of insulating matrix and by boundary scattering in the case of metallic matrix (resistivity of granular metals can be by orders of magni- tude higher than crystals). It seems, therefore, that for a given structure of the magnetic component the e!ect is only marginally dependent on resistivity of the inter- granular matrix and on the mechanism of electronic transport. In an attempt to get more information on this puzzling behavior we have developed [2] a new GMR system 0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 7 2 3 - 8

Upload: a-gerber

Post on 04-Jul-2016

214 views

Category:

Documents


0 download

TRANSCRIPT

Page 1: Spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets

Physica B 280 (2000) 331}332

Spin-dependent scattering versus spin-dependent tunnelingin heterogeneous ferromagnets

A. Gerber*

School of Physics and Astronomy, Tel Aviv University, Ramat Aviv 69978, Tel Aviv, Israel

Abstract

Can spin-dependent scattering mimic spin-dependent tunneling in heterogeneous ferromagnets? The discussion istriggered by the surprising similarity in magnetoresistance properties of granular ferromagnets with metallic, insulatingor point-contact spacers. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 75.70.Pa; 75.50.Tt; 75.70.-i

Keywords: Giant magnetoresistance; Heterogeneous ferromagnets; Spin-dependent tunneling

Two types of heterogeneous ferromagnetic systems areknown today to demonstrate the giant magnetoresis-tance e!ect. The "rst is metallic, where ferromagneticregions, layers or grains, are separated by normal metalspacers. The second is semiconducting or insulating,where the spacer is insulating and electronic transport isprovided by tunneling or temperature activated hopping.Sensitivity of electronic transport to applied magnetic"eld in both systems is associated to the relative orienta-tion of magnetic moments: high resistance for antiparal-lel orientation and low resistance for the parallel one. Themechanisms of transport are di!erent and there is noreason a priori to expect any quantitative correlationbetween metallic and insulating systems. We are there-fore surprised to "nd such correlations, as presented inthis paper.

For the following discussion, we choose granular fer-romagnets where the details of granular geometry andintergranular coupling are statistically averaged andsamples can be considered superparamagnetic. Twopairs of materials were studied in a wide range of con-centrations: Ni}Ag versus Ni}SiO

2, and Co}Ag versus

Co}SiO2. Structural and magnetic properties of each

*Corresponding author. Tel.: #972-3-6408193; fax: #972-3-6422979.

E-mail address: [email protected] (A. Gerber)

pair are very close for any given concentration, whichmakes us assume that we study the same structures offerromagnetic grains immersed in two matrices: metallicand insulating. Fig. 1 presents the normalized isotropicnegative magnetoresistance, of the GMR type [1] asa function of the ferromagnet component concentrationx for the two pairs: Ni}Ag versus Ni}SiO

2, and Co}Ag

versus Co}SiO2.

Two major points should be mentioned: (i) the largeste!ect is found in the vicinity of the same ferromagneticpercolation threshold (which is also a conductive thre-shold in insulating systems (Ni/Co}SiO

2)); (ii) for a given

transition metal (Ni or Co) the e!ect is of almost the samemagnitude for both metallic (Ag) and insulating (SiO

2)

matrices, despite a huge di!erence in resistivities offerromagnet-metal and ferromagnet-insulator systemsbelow the threshold (1 : 107). On the other hand, changingNi by Co increases the e!ect by an order of magnitude inboth metallic and insulating matrices. Resistance of het-erogeneous, in particular granular, systems is dominatedby intergranular barrier for the case of insulating matrixand by boundary scattering in the case of metallic matrix(resistivity of granular metals can be by orders of magni-tude higher than crystals). It seems, therefore, that fora given structure of the magnetic component the e!ect isonly marginally dependent on resistivity of the inter-granular matrix and on the mechanism of electronictransport.

In an attempt to get more information on this puzzlingbehavior we have developed [2] a new GMR system

0921-4526/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 1 7 2 3 - 8

Page 2: Spin-dependent scattering versus spin-dependent tunneling in heterogeneous ferromagnets

Fig. 1. Normalized GMR magnetoresistance as a function ofthe ferromagnet component concentration x. The results forNi-based series are multiplied by 10.

Fig. 2. Spin-dependent magnetoresistivity *o"o0!o(10 kOe)

of metallic Ni "lms (solid symbols) and insulating Ni}SiO2

mixtures (open symbols) as a function of their resistivity.

free of a non-magnetic spacer where a sharp reversalof local magnetic moments is achieved by dimensionalcon"nement. We have found that weakly coupled thinnanogranular Ni "lms above the conductance percola-tion threshold but below the macroscopic ferromagneticthreshold demonstrate isotropic negative magnetoresis-tance identi"ed as the GMR-like spin-dependent scatter-ing. The e!ect is observed in metallic samples and is notdue to spin-dependent tunneling across some shuntedinsulating barriers. We show in Fig. 2 the magnetoresis-tivity of both metallic Ni "lms demonstrating the e!ectand Ni}SiO

2samples as a function of their resistivity.

A shortcoming of this presentation is that all sampleshave di!erent topology which is particularly importantin the vicinity of the conductance percolation threshold.Nevertheless, magnetoresistivity appears to be roughlyproportional to resistivity over more than six decades ofresistivity change in a rather smooth transition betweenweakly coupled but continuous Ni "lms to discontinuousNi}SiO

2below the percolation threshold.

The apparent correlation between metallic and insu-lating GMR systems is surprising since two totally di!er-ent conductance mechanisms are involved: spin-depen-dent scattering and spin-dependent tunneling. Assumingthat the shown data correlation is not accidental, we staywith a challenge to "nd an underlying link for a generaldescription of the GMR phenomena.

Acknowledgements

This research was supported in part by the IsraelScience Foundation, under Grant No. 238/96.

References

[1] A. Milner et al., Phys. Rev. Lett. 76 (1996) 475.[2] A. Gerber et al., Europhys. Lett. 49 (2000) 383.

332 A. Gerber / Physica B 280 (2000) 331}332