Peter Mohn

Electronic and magnetic structure of BCC Fe-Co alloys from band theory

The connection between electronic structure and ferromagnetism is studied for the Fe-Co alloy system by means of self-consistent local spin-density functional calculations. The local environment of these BCC-substitutional alloys is modelled by different ordered compounds: besides BCC Co and Fe these are Co3Fe and Fe3Co in the Fe3Al structure and FeCo in the CsCl and NaTl structures. FeCo in the CsCl structure is studied in great detail giving the ferromagnetic band structure, decomposed state densities, Wigner delay times and the spatial form of the spin density. The role of the nearest-neighbour coordination in determining the value of the magnetic moments is discussed for all the compounds studied. The calculated and measured magnetic moments are compared and trends are analysed and explained.

The order of the magnetic phase transitions in RCo2 (R = rare earth) intermetallic compounds

It has been found experimentally that the order of the magnetic phase transitions in RCo2 compounds (R standing for rare-earth metals) at Tc changes from second order for the light-rare-earth series up to TbCo2 to first order for the heavier-rare-earth compounds DyCo2, HoCo2 and ErCo2. On the basis of results of fixed-spin-moment band-structure calculations for the isostructural compound YCo2 at different lattice constants, we propose an explanation for this behaviour. In contrast to the widely accepted Inoue-Shimizu theory for this class of compounds, our explanation also includes Pr, Nd which were thought to behave differently due to the influence of crystal-field effects. We show that an itinerant-electron metamagnetic transition in these compounds can occur only over a certain range of lattice constants and that the possibility of a first-order phase transition is connected to features of the electronic structure rather than to the magnitude of the transition temperature as conjectured earlier. The influence of the latter is only important if the transition takes place at elevated temperatures, where effects of spin fluctuations can suppress a first-order transition.

A CENTURY OF ZERO EXPANSION

Metallic mixtures of iron and nickel that do not change volume when heated were discovered more than a century ago. These ‘invar’ alloys have found many uses in heat-sensitive devices. A new quantum mechanical explanation of their behaviour describes the changing nature of magnetically ordered states in the alloy.

Binding mechanism and itinerant magnetism of ZrFe2 and YFe2

Abstract Self-consistent spin-polarized energy band calculations for the two Laves phases show that a ferrimagnetic state is more stable than the paramagnetic one, but at a slightly larger volume. For ZrFe2 the interaction between the low lying Zr and the exchange-split Fe states leads to covalent magnetism which is discussed using site- and spin-projected densities of states. The different interaction between Fe and Zr for majority and minority spin is responsible for the ferrimagnetic ordering which is analyzed in terms of spin densities. The resulting magnetic moment of about 1.9 μB for Fe is spatially localized near the Fe site, while around Zr a small but extended negative spin density causes (through the large volume) a moment of about -0.56 μB a prediction which should be verified experimentally. YFe2 is qualitatively similar, but contrasts YCo2 which is near a possible metamagnetic transition.

Total energy surfaces in the M-V plane for bcc and fcc cobalt

Abstract We use band calculations to study the different structural and magnetic phases of cobalt over a 30% range in volume. In this range, we find total energy surfaces and relative stabilities of different phases and show that bcc cobalt only exists as a ferromagnet while fcc cobalt shows the coexistence of a ferromagnetic and a nonmagnetic phase.

Magnetism in the Huesler alloys: Co2TiSn and Co2TiAl

In the weak itinerant ferromagnets Co 2 TiSn and Co 2 TiAl the Co atom carries a small magnetic moment and the spin density is strongly aspherical. This causes the ASW method to yield a non-magnetic ground state, while the FPLAPW method finds the magnetic ground state in agreement with experiment

Layered antiferromagnetism with high Neel temperature in the intermetallic compound Mn2Au

On the basis of earlier experimental studies the intermetallic compound Mn2Au has been characterized as a nonmagnetically ordered material. Here we report the results of first-principles calculations based on local spin-density approximation that describes Mn2Au to have a narrow band ground state with rigid local moments on the Mn sites. Calculations of the interatomic exchange constants based on the magnetic force theorem and a Monte Carlo modeling of the resulting Heisenberg-like Hamiltonian predict a high Neel temperature of ∼1600 K. This temperature is considerably higher than for the other known high-temperature antiferromagnetic L10-type Mn based binary alloys used in magnetic storage applications.

Adhesion and material transfer between contacting Al and TiN surfaces from first principles

A series of density functional theory (DFT) simulations was performed to investigate the approach, contact, and subsequent separation of two atomically flat surfaces consisting of different materials. Aluminum (Al) and titanium nitride (TiN) slabs were chosen as a model system representing a metal-ceramic interface and the interaction between soft and hard materials. The approach and separation were simulated by moving one slab in discrete steps normal to the surfaces allowing for electronic and atomic relaxations after each step. Various configurations were analyzed by considering (001), (011), and (111) surfaces as well as several lateral arrangements of these surfaces at the interface. Several tests were conducted on the computational setup, for example, by changing the system size or using different approximations for the exchange correlation functional. The performed simulations revealed the influences of these aspects on adhesion, equilibrium distance, and material transfer. These interfacial properties depend sensitively on the chosen configuration due to distinct bond situations. Material transfer, in particular, was observed if the absolute value of the adhesion energy for a given configuration is larger than the energy cost to remove surface layers. This result was found to be independent of the employed exchange correlation functional. Furthermore, it was shown that a simple comparison of the surface energies of the slabs is not sufficient to predict the occurrence of material transfer.

Magnetism and structural ordering on a bcc lattice for highly magnetostrictive Fe–Ga alloys: A coherent potential approximation study

We calculate the electronic structure, magnetic moments, and ordering energies of highly magnetostrictive Fe1−xGax alloys from first-principles in the composition range up to x=0.25. The coherent potential approximation was used to treat effects of chemical disorder. Given an underlying bcc lattice in whole range compositions investigated, the DO3 type of ordering is found to have a lower energy than A2- and B2-type structures. We find that ordering energies strongly depend on the state of magnetic order such that thermal magnetic disorder strongly favors chemical ordering (DO3 and B2). The values of the magnetic moments of Fe on different sublattices of ordered structures are found to have a distinctive dependency on the Ga concentration. By taking into account the results of earlier fully relativistic and full potential calculations of magnetostriction for ordered stoichiometric Fe3Ga compounds and available experimental phenomenology, our results for disordered alloys suggest an eventually more complex...

Formation of a weak ferromagnetic state in Y(Co1-xAlx)2 compounds: a coherent potential approximation study

The band structure of substitutionally disordered Y(Co1-xAlx)2 has been calculated for various concentrations in the range 0≤x≤0.25 employing the coherent potential approximation embodied in an all-electron tight-binding linear muffin-tin orbital method. On the basis of the results, we provide a new explanation for the formation of weak ferromagnetic moments in these compounds. The discussion of the non-spin-polarized calculated densities of states is supported by direct evidence of the weak ferromagnetic states for certain lattice constants and Al concentrations. The roles of the disorder and the volume effects are discussed.