Structures And Magnetic Ordering Of Bimetallic Transition Metal Clusters

Magnetic transition metal (Mn, Co, Fe, Ni) clusters are very attractive due to their unique electronic structures and magnetic properties. Bimetallic transition metal clusters offer the opportunity to tailor the magnetic properties through the choice of atom types and compositions, potentially leading to new behavior that is not possible for single-component clusters. Thus, increasingly sophisticated studies have been focused on the magnetic of binary clusters systems.In this paper, we have performed a global minimization of the total energy for MnnX (X =Fe, Co, Ni, La) and XnMn (X =Fe, Co, Ni) (n=1-12) clusters using the all-electron density functional theory (DFT) implemented in the DMol package and employing a double-numerical basis with polarized functions (DNP) and the BLYP exchange-correlation functional. The geometric optimization is performed for the abundant low-lying isomers of the clusters. The electronic and magnetic properties of the binary clusters have been investigated. The results show:(1) The MnnFe (n=1-12) clusters undergo a change in magnetic behavior from ferromagnetic ordering for the smallest size to ferrimagnetic ordering for intermediate sizes and beyond. Ferromagnetic ordering is clearly favored for n=1, but ferromagnetic and ferrimagnetic states are nearly degenerate for n=2, 3, and 4. A radical change occurs at n=5 where the ferrimagnetic states completely prevail.(2) For MnnCo (n=1-12) clusters, the lowest-energy structures of the MnCo,Mn2Co,Mn3Co,Mn4Co clusters all have FM ordering. There exist their low-lying isomers with AFM Mn-Mn couplings. For n≥5, the Mn-Mn couplings of the binary Co-Mn clusters are all with AFM ordering.(3) For MnnNi (n=1-12) clusters, the lowest-energy structure of MnNi possesses FM ordering. The Mn-Mn couplings of the lowest-energy structure for Mn2Ni and Mn3Ni clusters are with AFM ordering. However, they have low-lying isomers with FM ordering. For n≥4, the Mn-Mn couplings of the binary Ni-Mn clusters are all with AFM ordering.(4) The transition range of magnetic ordering from ferromagnetic to ferrimagnetic for MnnFe, MnnCo and MnnNi (n=1-12) clusters not only exists, but also occurs early compared to the pure Mnn+1(n=1-12) clusters. It is worth pointing out that there does not exist the magnetic transition from FM to AFM ordering in the MnnLa (n=1-12) clusters because of their AFM Mn-La couplings. The doped Fe, Co and Ni atoms prefer to accept electrons, while the La atom loses electrons.(5) The Fe-Mn couplings in the lowest-energy structures of FenMn (n=1-12) clusters undergo a change from ferromagnetic ordering for the smallest (n=1, 2) clusters to ferrimagnetic ordering for the intermediate (n=3-6) clusters. Starting from n=7, however, ferromagnetic Fe-Mn couplings completely prevail. The low coordination number of the doped Mn atom results in its having a high spin moment in the Fe-Mn binary clusters, which exhibits a marked magnetic moment surface enhancement effect.(6) The total spin moments of ConMn (n=1-9) clusters, in which the doped Mn atoms occupy the surface site, are found to be enhanced compared to that of pure Co clusters of same size. Moreover, the enhancement (2μB) is independence of cluster size. However, the enhancement of the total spin moment with Mn substitution is no longer observed for the Co₁₁, Co₁₂ and Co₁₃ clusters. The Mn-centered lowest-energy structures for the ConMn (n=10-12) clusters are obtained here.(7) An enhancement in the magnetic moment is obtained for small binary Ni-Mn clusters compared to that of pure Ni clusters of same size. The Ni10Mn cluster is an exception. In this case, the magnetic properties appear to depend on the placement of the doped Mn atom.