First-principles study of the exchange interactions and Curie temperature in Heusler alloys ( Dissertation )

  Abstract

This work contains the theoretical investigations of the exchange interactions and Curie temperature in various Heusler alloys. The calculations are performed within the parameter-free density-functional theory with the state-of-the-art ASW-method. The calculation of the exchange parameters is based on the frozen-magnon approach. The Curie temperature is estimated within mean-field (MF) and random-phase-approximation (RPA) approaches. The obtained results are divided in three parts. In the first part, a systematic study of the intra-sublattice and inter-sublattice exchange interactions as well as Curie temperatures for experimentally well-established Heusler alloys is presented. We focused on the microscopical mechanisms of the formation of the long range magnetic order. For Ni-based compounds Ni₂MnZ (Z = Ga, In, Sn, Sb) we obtain a strong dependence of the exchange interactions on the Z constituent (sp-atom) despite the closeness of calculated and experimental Curie temperatures of all four compounds. The role of the sp-electrons in mediating exchange interactions between Mn atoms is further revealed by studying non-stoichiometric compositions of Pd- and Cu-based semi and full Heusler alloys. We found that an important factor strongly influencing the electronic properties of the Heusler alloys is the spin polarization of the sp-electrons. We obtained a clear relationship between the strength of the exchange interaction and the sp-electron spin polarization. In all considered systems the Mn-Mn exchange interaction is long-ranged and shows RKKY-type oscillations.
In the second part, a detailed study of magnetism of half-metallic Heusler compounds promising for magneto-electronics applications is presented. We study both ferromagnetic and ferrimagnetic compounds. The role of the inter-sublattice exchange interactions in formation of the very high Curie temperature is revealed. We investigate the effect of the half-metallic gap on the stability of exchange interactions and Curie temperature. We study spin wave spectra and temperature dependence of the magnetization employing multi-sublattice Green function technique within Tyablikov decoupling scheme. We predicted new semi Heusler compounds with very high T_C values.
In the last part, we study the pressure dependence of electronic structure, exchange interactions, and Curie temperature in the ferromagnetic Heusler alloy Ni₂MnSn. In agreement with the experiment we obtain an increase of Curie temperature from 362 K at ambient pressure to 396 K at 12 GPa. Extending the variation of the lattice parameter beyond the range studied experimentally, we obtained non-monotonic pressure dependence of the Curie temperature and metamagnetic transition. We relate the theoretical dependence of T_C on the lattice constant to the corresponding dependence predicted by the empirical interaction curve. The Mn-Ni atomic interchange observed experimentally is simulated to study its influence on the Curie temperature.

Online-Dokument im PDF-Format 2.000 KB) mit integrierter Gliederung. Inhaltsverzeichnis

 
Front page, List of publications, Contents, Abstract (3, i-vi)
Introduction (1-3)
Part I Methods and Materials
1 Density Functional Theory of Magnetic Systems (4-15)
2 First-Principles Calculation of Exchange Interactions (16-24)
3 Magnetism at Finite Temperatures (25-32)
4 Heusler Alloys: Experimental and Theoretical Background (33-45)
Part II Results of Calculations
5 Exchange Mechanisms in Heusler Alloys (46-63)
6 Magnetism of Half-Metallic Heusler Compounds (64-84)
7 Effect of Pressure on Magnetic Properties of Heusler Alloys (85-94)
8 Summary and Conclusions (95-96)
Zusammenfassung (97-98)
Bibliography (99-105)