Tunnelmagnetowiderstand zwischen Uebergangsmetallclustern getrennt durch isolierende Molekuele

In this thesis the influence of the cluster-surface/insulator interaction on the TMR in well-defined granular systems has been studied. Eight new granular systems have been prepared by the co-deposition of well-defined transition metal clusters Co and Ni (mean diameter 4.5 nm) and different insulating atoms or molecules onto a cold substrate. The prepared systems are: Co/CO, Co/C_2H_2, Co/C_2H_4, Co/C_6H_6, Co/C_6_0, Ni/CO and Ni/Kr(Xe). The resistivities of all samples obey the theoretical expected r #mu# exp(T_0/T)1/2-law. The magnetic-field dependence of the sample resistance was measured for different cluster volume fraction vCl. Expect for Co/C_2H_4 and Co/C_6_0, the TMR in all investigated systems has been found to be independent of #nu#_C_l, despite the fact that there are big differences in #nu#_Cl. In Co/C_2H_4 and Co/C_6_0 TMR decreases with increasing #nu#_C_l. The decrease of TMR with increasing #nu#_C_l in Co/C_2H_4 is due to change in the orientation of C_2H_4-molecules between the two clusters caused by changing in vCl. The reason for the observed strong decrease of the TMR with increasing #nu#_C_l in Co/C_6_0-system is the enhancement of the probability of spin-flip-process due to enhanced electron-doping of C_6_0 caused by a charge transfer process between C_6_0 and Co. In addition, the temperature dependence of the TMR has been investigated. We observed in Co/CO-system a TMR-value of about 50% at T=1.7 K, which to our knowledge is the highest TMR reported for a granular Co-system. The TMR(T)-data were fitted with a model, which assumes increasing spin disorder at the cluster surface with increasing temperature and provides TMR(0), the TMR at T=0. Using Julliere model we obtained the spin polarization P of the tunneling electrons. The obtained values for TMR(0) or P in systems composed of Co-clusters were strongly enhanced compared to the non-interacting Co/Kr(Xe)-systems. We explain this enhancement as caused by a direct contact between Co-clusters and C-atom, which leads to the formation of a p-bond and thereby to a charge transfer between the clusters an the insulating component. On the basis of TMR-investigation of Ni/Kr(Xe)-systems we have determined the spin polarization of tunneling electrons and compared it to bulk-Ni ones, which is in spite of numerous experimental and theoretical works not yet quite understood. For the spin polarization we obtained P=20%. The obtained P-value in Ni/CO-system (P=28%) showed an enhancement of about 40% compared to that of Ni/Kr(Xe). This enhancement can be assigned to the interaction between Ni-cluster surface and C-atoms.SIGLEAvailable from: http://deposit.ddb.de/cgi-bin/dokserv?idn=97355696 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekDEGerman

Spin-dependent transport in films composed of Co clusters and C

Granular films of Co-cluster/C60 mixtures have been prepared by the co-deposition of well defined Co clusters (mean diameter ≈4.5 nm) and C60 fullerenes onto a cold (≈35 K) substrate. Films having a Co cluster volume fraction $0.23 \leq v_{\rm Co} \leq 0.32$ show a resistivity $\rho \propto \exp(T_{0}/T)^{1/2}$, typical for tunneling with a Coulomb barrier. The tunneling magnetoresistance (TMR) has a value of TMR ($T \rightarrow 0) = 33$% for $v_{\rm Co} = 0.23$ and is decreasing by almost one order of magnitude going to $v_{\rm Co} = 0.32$. We explain this unusual decrease of the TMR with increasing vCo as caused by electron-doping of the C60 fullerenes due to the known charge transfer process occurring between transition metal surface and C60. Increasing electron doping may lead to an increasing probability for spin-flip processes within the tunneling barrier, resulting in a decrease of the TMR

Strongly enhanced tunneling magnetoresistance in granular films of

A new granular system, namely, \chem{Co} clusters embedded in solid \chem{CO_{2}}, has been prepared by the co-deposition of well-defined \chem{Co} clusters (mean diameter $L \simeq 4.2$ \un{nm}, and \chem{CO_{2}} molecules onto a cold substrate. This system shows a temperature-dependent tunneling magnetoresistance (TMR) which has a value of 31(2)% at 2.5\un{K}. Using Jullière's model this value corresponds to a spin-polarisation |P| = 0.68(2) of the tunneling electrons, compared to P = 0.42 for granular \chem{Co/Xe(Kr)} or planar \chem{Co/Al_{2}O_{3}/Co}. We interpret this enhancement of P as caused by a cluster-surface/matrix interaction