Long-living terahertz magnons in ultrathin metallic ferromagnets.

The main idea behind magnonics is to use the elementary magnetic excitations (magnons) for information transfer and processing. One of the main challenges, hindering the application of ultrafast terahertz magnons in magnonics, has been the short lifetime of these excitations in metallic ferromagnets. Here, we demonstrate that the engineering of the electronic structure of a ferromagnetic metal, by reducing its dimensionality and changing its chemical composition, opens a possibility to strongly suppress the relaxation channels of terahertz magnons and thereby enhance the magnons' lifetime. For the first time, we report on the long-lived terahertz magnons excited in ultrathin metallic alloy films. On the basis of the first-principles calculations, we explain the microscopic nature of the long lifetime being a consequence of the peculiar electronic hybridizations of the species. We further demonstrate a way of tailoring magnon energies (frequencies) by varying the chemical composition of the film.

Relation between spin and orbital magnetism in excited states of ferromagnetic materials.

We report the first-principles study of the orbital magnetism, the magnetic anisotropy energy, the ratio of the spin, and the orbital moments in nano-sized systems perturbed from their magnetic ground state. We investigate one monolayer thick films of Co, Fe, and FePt. Two types of the perturbation are studied. First, the collinear spin structure is rotated continuously between the easy and hard axes. Second, the non-collinear spin structures are considered varying in both the angles between spin moments and the direction of the net magnetization. In agreement with the experiment we obtain a variety of behaviours. We show that the magnetic anisotropy energy can both increase and decrease with increasing magnetic disorder. The type of behaviour depends on the variation of the electronic structure with increasing angles between atomic moments. We obtain the effect of band narrowing accompanying the spin disorder that correlates with the band narrowing obtained experimentally in a laser irradiated system. In agreement with this experiment we show that the ratio of the spin and orbital moments can both remain unchanged and vary strongly. We analyse the applicability of Bruno's picture, which suggests proportionality between magnetic-anisotropy energy and orbital moment anisotropy for non-collinear spin configurations. We study the non-collinearity of the atomic spin and orbital moments and demonstrate that the response of the orbital moments to the variation of the spin structure can be unexpected and spectacular.

Direct probing of the exchange interaction at buried interfaces.

The fundamental interactions between magnetic moments at interfaces have an important impact on the properties of layered magnetic structures. Hence, a direct probing of these interactions is highly desirable for understanding a wide range of phenomena in low-dimensional solids. Here we propose a method for probing the magnetic exchange interaction at buried interfaces using spin-polarized electrons and taking advantage of the collective nature of elementary magnetic excitations (magnons). We demonstrate that, for the case of weak coupling at the interface, the low-energy magnon mode is mainly localized at the interface. Because this mode has the longest lifetime of the modes and has a finite spectral weight across the layers on top, it can be probed by electrons. A comparison of experimental data and first-principles calculations leads to the determination of the interface exchange parameters. This method may help the development of spectroscopy of buried magnetic interfaces.

Impact of atomic structure on the magnon dispersion relation: a comparison between Fe(111)/Au/W(110) and Fe(110)/W(110).

We present a combined experimental and theoretical study of the interplay between the atomic structure and the magnon excitations in low dimensional ferromagnets. Two monolayer thick Fe films on W(110) with and without a Au buffer layer are investigated. Our experiments show that adding the Au layer leads to a significant softening of the magnons. First-principles calculations confirm the experimental results revealing a strong dependency of exchange interactions on the atomic structure. It is observed that the intralayer exchange interactions increase with increasing distance between Fe layers. This unusual relationship is attributed to the complexity of the electronic structure and the contribution of different orbitals to the hybridization and exchange interaction. Our results suggest a way of tailoring magnetic excitations in low-dimensional magnetic structures.

Ultrafast magnon generation in an Fe film on Cu(100).

We report on a combined experimental and theoretical study of the spin-dependent relaxation processes in the electron system of an iron film on Cu(100). Spin-, time-, energy- and angle-resolved two-photon photoemission shows a strong characteristic dependence of the lifetime of photoexcited electrons on their spin and energy. Ab initio calculations as well as a many-body treatment corroborate that the observed properties are determined by relaxation processes involving magnon emission. Thereby we demonstrate that magnon emission by hot electrons occurs on the femtosecond time scale and thus provides a significant source of ultrafast spin-flip processes. Furthermore, engineering of the magnon spectrum paves the way for tuning the dynamic properties of magnetic materials.

Mn-stabilized zirconia: from imitation diamonds to a new potential high-Tc ferromagnetic spintronics material.

From the basis of ab initio electronic structure calculations which include the effects of thermally excited magnetic fluctuations, we predict Mn-stabilized cubic zirconia to be ferromagnetic above 500 K. We find this material, which is well known both as an imitation diamond and as a catalyst, to be half-metallic with the majority and minority spin Mn impurity states lying in zirconia's wide gap. The Mn concentration can exceed 40%. The high-Tc ferromagnetism is robust to oxygen vacancy defects and to how the Mn impurities are distributed on the Zr fcc sublattice. We propose this ceramic as a promising future spintronics material.

First-principles investigation of magnetism of U films and U(001)(1)/Fe(110)(3) multilayers.

The magnetism of surfaces and interfaces can differ strongly from the magnetism of corresponding bulk materials. In the case of actinide systems the study of the surfaces and interfaces is still at the very beginning. In this work, we investigated the electronic and magnetic properties of U films and U(001)(1)/Fe(110)(3) multilayers within the framework of the density functional theory. We report both scalar-relativistic and fully relativistic calculations. The exchange correlation potential was treated in the generalized gradient approximation (GGA). In agreement with previous calculation by Stojic et al (2003 Phys. Rev. B 68 094407) we obtained the surface layer of the U films to be magnetic for the bulk lattice parameter. The dependence of the magnetic properties of the U films on the lattice parameter was studied. It is shown that decreasing distances between U atoms lead to decreasing magnetic moment and finally to the nonmagnetic ground state. The variation of the magnetic moment as a function of the lattice parameter is discontinuous. Using the frozen-magnon approach we evaluated the parameters of the inter-atomic exchange interactions and estimated the Curie temperature. The calculation for U(001)(1)/Fe(110)(3) multilayers showed that the U layer is magnetic with the direction of the U moments opposite to the Fe moments. The importance of the U-Fe hybridization is revealed. Both the intra-layer (U-U, Fe-Fe) and inter-layer (U-Fe) exchange interactions were evaluated. The temperature dependence of the layer magnetizations was studied within the random-phase approximation for the Heisenberg Hamiltonian for classical spins.

An x-ray emission and density functional theory study of the electronic structure of Zn(1-x)Mn(x)S.

Mn 3d electronic states in the dilute magnetic semiconductor Zn(1-x)Mn(x)S (x = 0.1-0.3) are studied using soft x-ray emission (XES) measurements and density functional theory (DFT). Mn L(2,3) emission spectra of Zn(1-x)Mn(x)S (x = 0.1-0.3) suggest that the Mn impurities do not form clusters in the host ZnS lattice, in agreement with previous models. A shift in the position of a Mn L(3) XES feature suggests a change in the nature of the hybridization between the Mn 3d(3/2) and S 3p states as a function of x. Our DFT calculations reproduce the weak interatomic exchange interaction, as well as the strong intra-atomic exchange splitting that is expected from observations of Zeeman splitting in such materials.

Weakly dispersive band near the fermi level of GaMnAs due to Mn interstitials.

The nature of the weakly dispersive electronic band near the Fermi level observed in photoemission experiments on the diluted magnetic semiconductor GaMnAs is investigated theoretically. The combination of experimental features appears puzzling. We show that the formation of the band is closely related to the presence of the Mn interstitial impurities. The states forming the band have predominantly minority-spin Mn-3d character. The low experimental Mn-3d intensity is explained by the low content of the interstitial Mn impurities. The features of the band are robust with respect to the calculational technique [local density approximation (LDA), LDA + U].

Stability of ferromagnetism in the half-metallic pnictides and similar compounds: a first-principles study.

Based on first-principles electron structure calculations and employing the frozen-magnon approximation, we study the exchange interactions in a series of transition-metal binary alloys crystallizing in the zinc-blende structure and calculate the Curie temperature within both the mean-field approximation and random-phase approximation. We study two Cr compounds, CrAs and CrSe, and four Mn compounds, MnSi, MnGe, MnAs and MnC. MnC, MnSi and MnGe are isovalent to CrAs and MnAs is isoelectronic to CrSe. Ferromagnetism is particular stable for CrAs, MnSi and MnGe: all three compounds show Curie temperatures around 1000 K. On the other hand, CrSe and MnAs show a tendency to antiferromagnetism when compressing the lattice. In MnC the half-metallic gap is located in the majority-spin channel, in contrast to the other five compounds. The large half-metallic gaps, very high Curie temperatures, the stability of the ferromagnetism with respect to the variation of the lattice parameter and a coherent growth on semiconductors make MnSi and CrAs the most promising candidates for use in spintronics devices.