Fundamentals of half-metallic Full-Heusler alloys

Intermetallic Heusler alloys are amongst the most attractive half-metallic systems due to the high Curie temperatures and the structural similarity to the binary semiconductors. In this review we present an overview of the basic electronic and magnetic properties of the half-metallic full-Heusler alloys like Co$_2$MnGe. Ab-initio results suggest that the electronic and magnetic properties in these compounds are intrinsically related to the appearance of the minority-spin gap. The total spin magnetic moment in the unit cell, $M_t$, scales linearly with the number of the valence electrons, $Z_t$, such that $M_t=Z_t-24$ for the full-Heusler alloys opening the way to engineer new half-metallic alloys with the desired magnetic properties. Moreover we present analytical results on the disorder in Co$_2$Cr(Mn)Al(Si) alloys, which is susceptible to destroy the perfect half-metallicity of the bulk compounds and thus degrade the performance of devices. Finally we discuss the appearance of the half-metallic ferrimagnetism due to the creation of Cr(Mn) antisites in these compounds and the Co-doping in Mn$_2$VAl(Si) alloys which leads to the fully-compensated half-metallic ferrimagnetism.Comment: Submitted for a book entitled "Spintronics: Materials, Applications and Devices" to be published by Nova Publisher

Engineering the electronic, magnetic and gap-related properties of the quinternary half-metallic Heusler alloys

We review the electronic and magnetic properties of the quinternary full Heusler alloys of the type Co$_2$[Cr$_{1-x}$Mn$_x$][Al$_{1-y}$Si$_y$] employing three different approaches : (i) the coherent potential approximation (CPA), (ii) the virtual crystal approximation (VCA), and (iii) supercell calculations (SC). All three methods give similar results and the local environment manifested itself only for small details of the density of states. All alloys under study are shown to be half-metals and their total spin moments follow the so-called Slater-Pauling behavior of the ideal half-metallic systems. We especially concentrate on the properties related to the minority-spin band-gap. We present the possibility to engineer the properties of these alloys by changing the relative concentrations of the low-valent transition metal and $sp$ atoms in a continuous way. Our results show that for realistic applications, ideal are the compounds rich in Si and Cr since they combine large energy gaps (around 0.6 eV), robust half-metallicity with respect to defects (the Fermi level is located near the middle of the gap) and high values of the majority-spin density of states around the Fermi level which are needed for large values of the perfectly spin-polarized current in spintronic devices like spin-valves or magnetic tunnel junctions.Comment: 17 pages, 10 figure

Defects in CrAs and related compounds: a route to half-metallic ferrimagnetism

Half-metallic ferrimagnetism is crucial for spintronic applications with respect to ferromagnets due to the lower stray fields created by these materials. Studying the effect of defects in CrAs and related transition-metal chalcogenides and pnictides crystallizing in the zinc-blende structure, we reveal that the excess of the transition-metal atoms leads to half-metallic ferrimagnetism. The surplus of these atoms are antiferromagnetically coupled to the transition-metal atoms sitting at the perfect lattice sites. The needed condition to achieve half-metallic ferrimagnetism is to prevent the migration of the $sp$ atoms to other sites and the atomic swaps

Above-room-temperature ferromagnetism in half-metallic Heusler compounds NiCrP, NiCrSe, NiCrTe and NiVAs: A first-principles study

We study the interatomic exchange interactions and Curie temperatures in half-metallic semi Heusler compounds NiCrZ (Z=P, Se, Te) and NiVAs. The study is performed within the framework of density functional theory. The calculation of exchange parameters is based on the frozen-magnon approach. It is shown that the exchange interactions in NiCrZ vary strongly depending on the Z constituent. The Curie temperature, Tc, is calculated within the mean field and random phase approximations. The difference between two estimations is related to the properties of the exchange interactions. The predicted Curie temperatures of all four systems are considerably higher than room temperature. The relation between the half-metallicity and the value of the Curie temperature is discussed. The combination of a high spin-polarization of charge carriers and a high Curie temperature makes these Heusler alloys interesting candidates for spintronics applications.Comment: 6 pages, 3 figure

Ferrimagnetism and antiferromagnetism in half-metallic Heusler alloys

Half-metallic Heusler alloys are among the most promising materials for future applications in spintronic devices. Although most Heusler alloys are ferromagnets, ferrimagnetic or antiferromagnetic (also called fully-compensated ferrimagnetic) alloys would be more desirable for applications due to the lower external fields. Ferrimagnetism can be either found in perfect Heusler compounds or achieved through the creation of defects in ferromagnetic Heusler alloys.Comment: To be considered for the proceedings of the International Conference on Nanoscale Magnetism (ICNM 07

Generalized Slater-Pauling rule for the inverse Heusler compounds

We present extensive first-principles calculations on the inverse full-Heusler compounds having the chemical formula X$_2$YZ where (X = Sc, Ti, V, Cr or Mn), (Z = Al, Si or As) and the Y ranges from Ti to Zn. Several of these alloys are identified to be half-metallic magnets. We show that the appearance of half-metallicity is associated in all cases to a Slater-Pauling behavior of the total spin-magnetic moment. There are three different variants of this rule for the inverse Heusler alloys depending on the chemical type of the constituent transition-metal atoms. Simple arguments regarding the hybridization of the d-orbitals of neighboring atoms can explain these rules. We expect our results to trigger further experimental interest on this type of half-metallic Heusler compounds.Comment: 5 pages, 3 figures, 1 tabl

Doping and disorder in the Co2_2MnAl and Co2_2MnGa half-metallic Heusler alloys

We expand our study on the full-Heusler compounds [I. Galanakis \textit{et al.}, Appl. Phys. Lett. \textbf{89}, 042502 (2006)] to cover also the case of doping and disorder in the case of Co$_2$MnAl and Co$_2$MnGa half-metallic Heusler alloys. These alloys present a region of very small minority density of states instead of a real gap. Electronic structure calculations reveal that doping with Fe and Cr in the case of Co$_2$MnAl retains the half-metallicity contrary to the Co$_2$MnGa compound. Cr impurities present an unusual behavior and the spin moment of the Cr impurity scales almost linearly with the concentration of Cr atoms contrary to the Co$_2$MnZ (Z= Si, Ge, Sn) where it was almost constant. Half-metallicity is no more preserved for both Co$_2$MnAl and Co$_2$MnGa alloys when disorder occurs and there is either excess of Mn or $sp$ atoms

Defects-driven appearance of half-metallic ferrimagnetism in Co-Mn--based Heusler alloys

Half-metallic ferromagnetic full-Heusler alloys containing Co and Mn, having the formula Co$_2$MnZ where Z a sp element, are among the most studied Heusler alloys due to their stable ferromagnetism and the high Curie temperatures which they present. Using state-of-the-art electronic structure calculations we show that when Mn atoms migrate to sites occupied in the perfect alloys by Co, these Mn atoms have spin moments antiparallel to the other transition metal atoms. The ferrimagnetic compounds, which result from this procedure, keep the half-metallic character of the parent compounds and the large exchange-splitting of the Mn impurities atoms only marginally affects the width of the gap in the minority-spin band. The case of [Co$_{1-x}$Mn$_x$]$_2$MnSi is of particular interest since Mn$_3$Si is known to crystallize in the Heusler $L2_1$ lattice structure of Co$_2$MnZ compounds. Robust half-metallic ferrimagnets are highly desirable for realistic applications since they lead to smaller energy losses due to the lower external magnetic fields created with respect to their ferromagnetic counterparts