ELSEVIER Journal of Magnetism and Magnetic Materials 175 (1997) 33

~ H Journal of magnatlsm and magnetic materials

Spin-dependent tunneling in granular magnetic tunnel junctions

F. Pe t ro f f a'*, L.F. Schelp a, S.F. Lee a, F. F e t t a r a, P. H o l o d y a, A. J.L. M a u r i c e a, A. Fe r t a'b

a Unite Mixte de Physique CNRS/Thomson-LCR, 91404 Orsay, France b Universite Paris-Sud, 91405 Orsay, France

Vaures a,

We have studied the interplay between spin-dependent tunneling [J.S. Moodera, L.R. Kinder, T.M. Wong, R. Meservey, Phys. Rev. Lett. 74 (1995) 3273] and Coulomb blockade [J.B. Barner, S.T. Ruggiero, Phys. Rev. Lett. 59 (1987) 807] in Co/A1203/Co tunnel junctions in which the A1203 layer includes a unique layer of magnetic clusters. The tunneling structures are prepared by sputtering with typical cluster sizes of 3.5 nm but smaller or larger can also be prepared. We observe spin-dependent tunneling with, below about T = 50 K, typical Coulomb blockade effects induced by intermediate electron tunneling into clusters. The tunnel magnetoresistance ratio is approximately the same in the Coulomb blockade regime (low temperature range with very high tunnel resistance) and in the room temperature regime without Coulomb blockade. The magnetic field dependence of the tunnel resistance reflects the magnetization reversal of the electrodes and cobalt clusters. An interesting result is that the magnetoresistance ratio depends weakly on the applied voltage. We discuss the problems raised by the interplay between spin dependent tunneling and Coulomb blockade.

* Corresponding author.

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