arXiv subjects

Anna Delin

Publications and source records attributed to Anna Delin.

At least 73 records · Page 4Linked to original sources

arXiv2018

Carbon Vacancy Formation in Binary Transition Metal Carbides from Density Functional Theory

We have investigated the trends in the formation of carbon vacancies in binary transition metal (TM) carbides using density functional calculations for two common TM carbide crystal structures, namely the B1 (rock-salt) and WC structure types. The TM are taken from group IV (Ti, Zr, Hf), V (V, Nb, Ta) and VI (Cr, Mo, W) of the periodic table, as well as Sc from group III. For B1-structured TM carbides, the general trend is that it is easier for C vacancies to be formed as the number of valence electrons in the system increases. The exception is ScC where C vacancies are rather easy to form. It is also clear that the formation of C vacancies depends on the growth conditions: For TM-rich conditions, B1-structured carbides will always favour C vacancy formation. For C-rich conditions it is energetically favourable to form C vacancies in ScC, VC, NbC, CrC, MoC and WC. Experimentally large C vacancy concentrations are found in B1-structured TM carbides and our calculations are in line with these observations. In fact, TiC, ZrC, HfC and TaC are the only B1-structured TM carbides that do not spontaneously favour C vacancies and then only for C-rich conditions. C vacancies may still be present in these systems, however, due to the high temperatures used when growing TM carbides. In the case of WC-structured TM carbides, C vacancy formation is not energetically favourable irrespective of the growth conditions, which is the reason why this structure is only found close to a one-to-one metal-to-carbon ratio. ...

cond-mat.mtrl-sci↗

arXiv2018

General method for atomistic spin-lattice dynamics with first principles accuracy

We present a computationally efficient general first-principles based method for spin-lattice simulations for solids. Our method is based on a combination of atomistic spin dynamics and molecular dynamics, expressed through a spin-lattice Hamiltonian where the bilinear magnetic term is expanded to second order in displacement, and all parameters are computed using density functional theory. The effect of first-order spin-lattice coupling on the magnon and phonon dispersion in bcc Fe is reported as an example, and is seen to be in good agreement with previous simulations performed with an empirical potential approach. In addition, we also illustrate the abilities of our method on a more conceptual level, by exploring dissipation-free spin and lattice motion in small magnetic clusters (a dimer, trimer and quadmer). Our method opens the door for quantitative description and understanding of the microscopic origin of many fundamental phenomena of contemporary interest, such as ultrafast demagnetization, magnetocalorics, and spincaloritronics.

cond-mat.mtrl-sci↗

arXiv2017

Micromagnetic simulations of spin-torque driven magnetisation dynamics with spatially resolved spin transport and magnetisation texture

We present a simple and fast method to simulate spin-torque driven magnetisation dynamics in nano-pillar spin-valve structures. The approach is based on the coupling between a spin transport code based on random matrix theory and a micromagnetics finite-elements software. In this way the spatial dependence of both spin transport and magnetisation dynamics is properly taken into account. Our results are compared with experiments. The excitation of the spin-wave modes, in- cluding the threshold current for steady state magnetisation precession and the nonlinear frequency shift of the modes are reproduced correctly. The giant magneto resistance effect and the magnetisa- tion switching also agree with experiment. The similarities with recently described spin-caloritronics devices are also discussed.

cond-mat.mes-hall↗

arXiv2017

Breakdown of Polarons in Conducting Polymers at Device Field Strengths

Conducting polymers have become standard engineering materials, used in manyelectronic devices. Despite this, there is a lack of understanding of the microscopicorigin of the conducting properties, especially at realistic device field strengths. Wepresent simulations of doped poly(p-phenylene) (PPP) using a Su-Schrieffer-Heeger(SSH) tight-binding model, with the electric field included in the Hamiltonian througha time-dependent vector potential via Peierls substitution of the phase factor. We findthat polarons typically break down within less than a picosecond after the field hasbeen switched on, already for electric fields as low as around 1.6 mV/Å. This is a fieldstrength common in many flexible organic electronic devices. Our results challenge therelevance of the polaron as charge carrier in conducting polymers for a wide range ofapplications.

cond-mat.mtrl-sci↗

arXiv2017

Extended spin model in atomistic simulations of alloys

An extended atomistic spin model allowing for studies of the finite temperature magnetic properties of alloys is proposed. The model is obtained by extending the Heisenberg Hamiltonian via a parameterization from a first principles basis, interpolating from both the low temperature ferromagnetic and the high temperature paramagnetic reference states. This allows us to treat magnetic systems with varying degree of itinerant character within the model. Satisfactory agreement with both previous theoretical studies and experiments are obtained in terms of Curie temperatures and paramagnetic properties. The proposed model is not restricted to elements but is also applied to binary alloys, such as the technologically important material Permalloy, where significant differences in the finite magnetic properties of Fe and Ni magnetic moments are found. The proposed model strives to find the right compromise between accuracy and computational feasibility for accurate modeling, even for complex magnetic alloys and compounds.

physics.comp-ph↗

arXiv2016

Nanoscale control of heat and spin conductance in artificial spin chains

We describe a mechanism to control energy and magnetisation currents in an artificial spin-chain, consisting of an array of Permalloy nano-disks coupled through the magneto-dipolar interaction. The chain is kept out of equilibrium by two thermal baths with different temperatures connected to its ends, which control the current propagation. Transport is enhanced by applying a uniform radio frequency pump field resonating with some of the spin-wave modes of the chain. Moreover, the two currents can be controlled independently by tuning the static field applied on the chain. Thus we describe two effective means for the independent control of coupled currents and the enhancement of thermal and spin-wave conductivity in a realistic magnonics device, suggesting that similar effects could be observed in a large class of nonlinear oscillating systems.

cond-mat.stat-mech↗

arXiv2016

A systematic study of magnetodynamic properties at finite temperatures in doped permalloy from first principles calculations

By means of first principles calculations, we have systematically investigated how the magnetodynamic properties Gilbert damping, magnetization and exchange stiffness are affected when permalloy (Py) (Fe$_{0.19}$Ni$_{0.81}$) is doped with 4d or 5d transition metal impurities. We find that the trends in the Gilbert damping can be understood from relatively few basic parameters such as the density of states at the Fermi level, the spin-orbit coupling and the impurity concentration. % The temperature dependence of the Gilbert damping is found to be very weak which we relate to the lack of intraband transitions in alloys. % Doping with $4d$ elements has no major impact on the studied Gilbert damping, apart from diluting the host. However, the $5d$ elements have a profound effect on the damping and allows it to be tuned over a large interval while maintaining the magnetization and exchange stiffness. % As regards spin stiffness, doping with early transition metals results in considerable softening, whereas late transition metals have a minor impact. % Our result agree well with earlier calculations where available. In comparison to experiments, the computed Gilbert damping appears slightly underestimated while the spin stiffness show good general agreement.

cond-mat.mtrl-sci↗

arXiv2016

Magnetism and exchange interaction of small rare-earth clusters; Tb as a representative

Here we follow, both experimentally and theoretically, the development of magnetism in Tb clusters from the atomic limit, adding one atom at a time. The exchange interaction is, surprisingly, observed to drastically increase compared to that of bulk, and to exhibit irregular oscillations as a function of the interatomic distance. From electronic structure theory we find that the theoretical magnetic moments oscillate with cluster size in exact agreement with experimental data. Unlike the bulk, the oscillation is not caused by the RKKY mechanism. Instead, the inter-atomic exchange is shown to be driven by a competition between wave-function overlap of the 5d shell and the on-site exchange interaction, which leads to a competition between ferromagnetic double-exchange and antiferromagnetic super-exchange. This understanding opens up new ways to tune the magnetic properties of rare-earth based magnets with nano-sized building blocks.

physics.atm-clus↗

arXiv2016

Spin relaxation signature of colossal magnetic anisotropy in platinum atomic chains

Recent experimental data demonstrate emerging magnetic order in platinum atomically thin nanowires. Furthermore, an unusual form of magnetic anisotropy -- colossal magnetic anisotropy (CMA) -- was earlier predicted to exist in atomically thin platinum nanowires. Using spin dynamics simulations based on first-principles calculations, we here explore the spin dynamics of atomically thin platinum wires to reveal the spin relaxation signature of colossal magnetic anisotropy, comparing it with other types of anisotropy such as uniaxial magnetic anisotropy (UMA). We find that the CMA alters the spin relaxation process distinctly and, most importantly, causes a large speed-up of the magnetic relaxation compared to uniaxial magnetic anisotropy. The magnetic behavior of the nanowire exhibiting CMA should be possible to identify experimentally at the nanosecond time scale for temperatures below 5 K. This time-scale is accessible in e.g., soft x-ray free electron laser experiments.

cond-mat.mtrl-sci↗

arXiv2015

Resistive Graphene Humidity Sensors with Rapid and Direct Electrical Readout

We demonstrate humidity sensing using a change of electrical resistance of a single- layer chemical vapor deposited (CVD) graphene that is placed on top of a SiO2 layer on a Si wafer. To investigate the selectivity of the sensor towards the most common constituents in air, its signal response was characterized individually for water vapor (H2O), nitrogen (N2), oxygen (O2), and argon (Ar). In order to assess the humidity sensing effect for a range from 1% relative humidity (RH) to 96% RH, devices were characterized both in a vacuum chamber and in a humidity chamber at atmospheric pressure. The measured response and recovery times of the graphene humidity sensors are on the order of several hundred milliseconds. Density functional theory simulations are employed to further investigate the sensitivity of the graphene devices towards water vapor. Results from the interaction between the electrostatic dipole moment of the water and the impurity bands in the SiO2 substrate, which in turn leads to electrostatic doping of the graphene layer. The proposed graphene sensor provides rapid response direct electrical read out and is compatible with back end of the line (BEOL) integration on top of CMOS-based integrated circuits.

cond-mat.mes-hall↗

arXiv2015

Tunable damping, saturation magnetization, and exchange stiffness of half-Heusler NiMnSb thin films

The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization $M_S$, Gilbert damping $α$, and exchange stiffness $A$. $M_S$ and $A$ are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agreement between theory and experiment for $M_S$ and $α$. The calculated $A$ shows the same trend as the experimental data, but has a larger magnitude. Additionally to the unique in-plane anisotropy of the material, these tunabilities of the magnetodynamic properties can be taken advantage of when employing NiMnSb films in magnonic devices.

cond-mat.mtrl-sci↗

arXiv2015

Coherent energy transport in classical nonlinear oscillators: an analogy with the Josephson effect

The standard approach to non-equilibrium thermodynamics describes transport in terms of generalised forces and coupled currents, a typical example being the Fourier law that relates temperature gradient to the heat flux. Here we demonstrate the presence of persistent energy currents in a lattice of classical nonlinear oscillators with uniform temperature and chemical potential. In strong analogy with the well known Josephson effect, the currents are generated only by the phase differences between the oscillators. The phases play the role of additional thermodynamical forces, that drive the system out of equilibrium. Our results apply to a large class of oscillators and indicate novel ways to practically control the propagation of coupled currents and rectification effects in many different devices. They also suggest a simple, macroscopic setup for studying a phenomenon which hitherto have only been observed in microscopic, quantum-mechanical systems.

cond-mat.stat-mech↗

arXiv2015

Experimental and ab initio studies of the novel piperidine-containing acetylene glycols

Synthesis routes of novel piperidine-containing diacetylene are presented. The new molecules are expected to exhibit plant growth stimulation properties. In particular, the yield in a situation of drought is expected to increase. The synthesis makes use of the Favorskii reaction between cycloketones/piperidone and triple-bond containing glycols. The geometries of the obtained molecules were determined using nuclear magnetic resonance (NMR). The electronic structure and geometries of the molecules were studied theoretically using first-principles calculations based on density functional theory. The calculated geometries agree very well with the experimentally measured ones, and also allow us to determine bond lengths, angles and charge distributions inside the molecules. The stability of the OH-radicals located close to the triple bond and the piperidine/cyclohexane rings was proven by both experimental and theoretical analyses. The HOMO/LUMO analysis was done in order to characterize the electron density of the molecule. The calculations show that triple bond does not participate in intermolecular reactions which excludes the instability of novel materials as a reason for low production rate.

physics.chem-ph↗

arXiv2015

Energy and magnetisation transport in non-equilibrium macrospin systems

We investigate numerically the magnetisation dynamics of an array of nano-disks interacting through the magneto-dipolar coupling. In the presence of a temperature gradient, the chain reaches a non-equilibrium steady state where energy and magnetisation currents propagate. This effect can be described as the flow of energy and particle currents in an off-equilibrium discrete nonlinear Schrödinger (DNLS) equation. This model makes transparent the transport properties of the system and allows for a precise definition of temperature and chemical potential for a precessing spin. The present study proposes a novel setup for the spin-Seebeck effect, and shows that its qualitative features can be captured by a general oscillator-chain model

cond-mat.stat-mech↗

arXiv2014

Electronic Structure and Lattice Dynamics in the FeSb$_{3}$ Skutterudite from Density Functional Theory

We have performed density functional calculations of the electronic structure and lattice dynamics of the binary skutterudite FeSb$_{3}$. We find that the ground state of FeSb$_{3}$ is a near semi-metallic ferromagnet with $T_{c}=175$~K. Furthermore, we find that FeSb$_{3}$ is softer than CoSb$_{3}$ based on an analysis of the relation of the elastic constants and the shape of the phonon density of states in the two systems, which is in agreement with experimental observation. Based on these observations we find it plausible that FeSb$_{3}$ will have a lower thermal conductivity than CoSb$_{3}$. Additionally, our calculations indicate that FeSb$_{3}$ may be stable towards decomposition into FeSb$_{2}$ and Sb. Furthermore, for ferromagnetic FeSb$_{3}$ we obtain real-valued phonon frequencies and also a $c_{44}$ greater than zero, indicating that the system is mechanically as well as dynamically stable.

cond-mat.mtrl-sci↗

arXiv2014

Thermo-magnonic diode: rectification of energy and magnetization currents

We investigate the dynamics of two coupled macrospins connected to thermal baths at different temperatures. The system behaves like a diode which allows the propagation of energy and mag- netization currents in one direction only. This effect is described by a simple model of two coupled nonlinear oscillators interacting with two independent reservoirs. It is shown that the rectification phenomenon can be interpreted as a a stochastic phase synchronization of the two spin-oscillators. A brief comparison with realistic micromagnetic simulations is presented. This new effect yields promising opportunities in spin-caloritronics devices.

cond-mat.mes-hall↗

arXiv2014

Designing a Spin-Seebeck Diode

Using micromagnetic simulations, we have investigated spin dynamics in a spin-valve bi-layer in the presence of a thermal gradient. The direction and the intensity of the gradient allow to excite the spin wave modes of each layer selectively. This permits to synchronize the magnetization precession of the two layers and to rectify the flows of energy and magnetization through the system. Our study yields promising opportunities for applications in spin caloritronics and nanophononics devices.

cond-mat.mes-hall↗

arXiv2013

Thermally driven domain wall motion in Fe on W(1 1 0)

It has recently been shown that domain walls in ferromagnets can be moved in the presence of thermal gradients. In this work we study the motion of narrow domain walls in low-dimensional systems when subjected to thermal gradients. The system chosen is a monolayer of Fe on W(1 1 0) which is known to exhibit a large anisotropy while having a soft exchange, resulting in a very narrow domain wall. The study is performed by means of atomistic spin dynamics simulations coupled to first-principles calculations. By subjecting the systems to a thermal gradient we observe a temperature dependent movement of the domain wall as well as changes of the spatial magnetization profile of the system. The thermal gradient always makes the domain wall move towards the hotter region of the sample with a velocity proportional to the gradient. The material specific study is complemented by model simulations to discern the interplay between the thermal gradient, magnetic anisotropy and the exchange interaction, and shows that the larger DW velocities are found for materials with low magnetic anisotropy. The relatively slow DW motion of the Fe/W(1 1 0) system is hence primarily caused by its large magnetic anisotropy.

cond-mat.mtrl-sci↗