In this paper we have considered spectral and trans- port properties of a single-level quantum dot connected to external ferromagnetic leads and exchange-coupled to a nonmagnetic reservoir. Using the numerical renor- malization group method we have calculated the zero- temperature d -level spectral function and the conduc- tance through the dot. We have shown that in the an- tiparallel configuration, depending on the strength of the exchange interaction J , the Kondo singlet ground state can form, in which the conduction electrons either in the ferromagnetic leads or in the nonmagnetic reservoir are involved. In the former case, the conductance is maxi- mum, whereas in the latter case the conductance becomes fully suppressed. For a certain critical value of J , J AP c , both electron channels try to screen the dot’s spin and the conductance is equal to a half of its maximum value. The boundary between the two ground states is a quan- tum phase transition. 

In the parallel magnetic configuration, on the other hand, the Kondo effect is generally destroyed due to an effective spin splitting of the dot level caused by the pres- ence of ferromagnetic leads. However, there are still small side peaks – reminiscent of the Kondo effect – which oc- cur on both sides of the Fermi level for energies of the order of effective exchange field. Nevertheless, with in- 

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## IV. CONCLUSIONS 

creasing the exchange constant J , these peaks become suppressed. 

We have also considered the influence of an external magnetic field on the d -level spectral function and shown that in the parallel configuration the Kondo effect can be restored by applying appropriately tuned compensating magnetic field for J < ˜ J P c , where ˜ J P c is the critical value of J in the compensating magnetic field. If, however, J > ˜ J P c , the full Kondo effect cannot be restored by a magnetic field. In addition, we have found that the quan- tum critical behavior, which is suppressed in the parallel configuration, can also be recovered by tuning the exter- nal magnetic field. 

This work was supported by funds of the Polish Min- istry of Science and Higher Education as a research project for years 2006-2009. I.W. also acknowledges sup- port from the Alexander von Humboldt Foundation, the Foundation for Polish Science and funds of the Polish Ministry of Science and Higher Education as a research project for years 2008-2010. Financial support by the Ex- cellence Cluster ”Nanosystems Initiative Munich (NIM)” is gratefully acknowledged. 

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## Acknowledgments 

