Role of conduction electrons in mediating exchange interactions in Mn-based Heusler alloys

Abstract

Because of the large spatial separation of the Mn atoms in Heusler alloys ͑d Mn-Mn Ͼ 4 Å͒, the Mn 3d states belonging to different atoms do not overlap considerably. Therefore, an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi-and full-Heusler alloys, we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The Curie temperature is estimated within the mean-field approximation. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction sp electrons, their spin polarization, and the position of the unoccupied Mn 3d states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The magnetic phase diagram is determined at zero temperature. The results of the calculations are in good agreement with available experimental data. Anderson's s-d model is used to perform a qualitative analysis of the obtained results. The conditions leading to a diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied Mn 3d states lie well above the Fermi level, a Ruderman-Kittel-Kasuya-Yoshida-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied Mn 3d states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculation results are in good correlation with the conclusions made on the basis of the Anderson s-d model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.

Ab initio study of effect of Co substitution on the magnetic properties of Ni and Pt-based Heusler alloys

“…34 Previously in literature 20 it was mentioned for A 2 MnC systems (A=Cu, Pd; C=Al, In, Sb), that the conduction electrons of the C atom take role in the coupling between Mn atoms. But in a recent study 22 the role of conduction electron of A atom has also been confirmed for a number of Mn-based Heusler alloys. In our studied systems here, for a given series of materials, C-atom is fixed which is Ga for Ni 2−x Co x MnGa, Ni 2−x Co x FeGa, Pt 2−x Co x MnGa and Sn for Pt 2−x Co x MnSn.…”

“…20 Additionally, there is also an indirect RKKYtype exchange mechanism 21 between the B atoms, primarily mediated by the electrons of the C atoms, which also plays an important role in defining the magnetic properties of these materials. 20,22 Staunton et al 23 reported the role of RKKY interaction behind the origin of magnetic anisotropy of a system. For the magnetic shape memory alloys, magnetic anisotropy energy plays an important role.…”

Spin-Resolved Fermi Surface of the Localized Ferromagnetic Heusler CompoundCu2MnAlMeasured with Spin-Polarized Positron Annihilation

“…In particular, the interplay between the localized d electrons and the delocalized electrons in Mn based Heusler systems is still under discussion, and in this context the shape of the Fermi surface is a key ingredient of the Ruderman-KittelKasuya-Yosida (RKKY) interaction [7,[11][12][13]. Besides these fundamental questions, Cu 2 MnAl has also a large relevance for applied physics as a neutron polarizer and monochromator material [14,15].…”

“…Being the prototype of all Heusler alloys [8], Cu 2 MnAl has also become a model system for understanding the electronic correlations in this class of materials [2,7,9,10]. In particular, the interplay between the localized d electrons and the delocalized electrons in Mn based Heusler systems is still under discussion, and in this context the shape of the Fermi surface is a key ingredient of the Ruderman-KittelKasuya-Yosida (RKKY) interaction [7,[11][12][13].…”

“…The nature of the electronic interactions in Heusler compounds is known to be rather delicate. Detailed knowledge of the electronic structure is vital for tailoring specific physical properties such as magnetism and electron spinpolarization, since features in the band structure depend very sensitively on the composition [2,7].…”