Interatomic Exchange Coupling Research Articles

Laser induced ultrafast Gd 4f spin dynamics in Co100-xGdx alloys by means of time-resolved XMCD

We have studied the laser induced ultrafast quenching of Gd 4f magnetic order in ferrimagnetic Co100-xGdx alloys to highlight the role of the inter-atomic exchange coupling. We have taken advantage of the ultrashort soft X-ray pulses deliver by the femtoslicing beamline at the BESSY II synchrotron radiation source at the Helmholtz-Zentrum Berlin to perform element- and time-resolved X-ray Magnetic Circular Dichroism spectroscopy.Our results show that the laser induced quenching of Gd 4f magnetic order occurs on very different time-scales for the Co72Gd28, the Co77Gd23 and the Co79Gd21 alloys. Most of the magnetic moment losses occur within the first picosecond (ps) while the electron distribution is strongly out of equilibrium. After the equilibration of the electrons and lattice temperatures (t > 1 ps), the magnetic losses occur on slower rates that depend on the alloy composition: increasing the Co composition speeds up the demagnetization of Gd 4f sublattice. The strength of the inter-atomic exchange coupling which depends on composition, determines the efficiency of the angular momentum flow from the Gd 4f spin towards the lattice. Our results are in qualitative agreements with the predictions of the microscopic three temperatures model for ferrimagnetic alloys.

Magnetism between magnetic adatoms on monolayer NbSe2

In this work, we report on an ab-initio computational study of the electronic and magnetic properties of transition metal adatoms on a monolayer of NbSe2. We demonstrate that Cr, Mn, Fe and Co prefer all to sit above the Nb atom, where the d states experience a substantial hybridization. The inter-atomic exchange coupling is shown to have an oscillatory nature accompanied by an exponential decay, in accordance with what theory predicts for a damped Ruderman–Kittel–Kasuya–Yosida interaction. Our results indicate that the qualitative features of the magnetic coupling for the four investigated adatoms can be connected to the fine details of their Fermi surface. In particular, the oscillations of the exchange in Fe and Co are found to be related to a single nesting vector, connecting large electrons and hole pockets. Most interestingly, this behavior is found to be unaffected by changes induced on the height of the impurity, which makes the magnetism robust to external perturbations. Considering that NbSe2 is a superconductor down to a single layer, our research might open the path for further research into the interplay between magnetic and superconducting characteristics, which could lead to novel superconductivity engineering.

Magnetic regulation in the off-stoichiometric L10-MnGa thin film by Fe doping

L10-MnGa alloy film is a kind of promising material for application of spintronics device and permanent magnet. In this paper, we prepare the Fe-doped L10-MnGa films on thermal MgO (100) substrates with different annealing temperatures and report magnetic regulation of the off-stoichiometric L10-MnGa thin film by tuning Fe doping amount and annealing temperature. It is found that the 4.6% Fe-doped L10-MnGa film has (001) orientation, large coercivity of 6.14 kOe, high effective anisotropy constant of 9.9 Merg/cc and low saturation magnetization of 482 emu/cc. The film annealed at 500 °C has a maximum magnetic energy product of 9.21 MGOe. The production of tensile strain and the change of interatomic exchange coupling state by Fe doping and annealing temperature enable the L10-MnGa film to apply in many fields. It is noteworthy that there are the different doping results compared to the Pt-doped L10-MnGa film in the previous work. These research results are valuable for the design and application of L10-MnGa film.

Persistent half-metallic ferromagnetism in a (111)-oriented manganite superlattice

We employ electronic structure calculations to show that a (111)-oriented (LaMnO3)12∣(SrMnO3)6 superlattice retains a half-metallic ferromagnetic character despite its large thickness. We link this behaviour to the strain and the octahedral connectivity between the layers. This also gives rise to breathing modes, which are coupled to charge and spin oscillations, whose components have a pure eg character. Most interestingly, the magnetisation reaches its maximum value inside the LaMnO3 region and not at the interface, which is fundamentally different from what observed for the (001) orientation. The inter-atomic exchange coupling shows that the magnetic order arises from the double-exchange mechanism, despite competing interactions inside the SrMnO3 region. Finally, the van Vleck distortions and the spin oscillations are crucially affected by the variation of Hund’s exchange and charge doping, which allows us to speculate that our system behaves as a Hund’s metal, creating an interesting connection between manganites and nickelates.

Paramagnetic molecule induced strong antiferromagnetic exchange coupling on a magnetic tunnel junction based molecular spintronics device

This paper reports our Monte Carlo (MC) studies aiming to explain the experimentally observed paramagnetic molecule induced antiferromagnetic coupling between ferromagnetic (FM) electrodes. Recently developed magnetic tunnel junction based molecular spintronics devices (MTJMSDs) were prepared by chemically bonding the paramagnetic molecules between the FM electrodes along the tunnel junction’s perimeter. These MTJMSDs exhibited molecule-induced strong antiferromagnetic coupling. We simulated the 3D atomic model analogous to the MTJMSD and studied the effect of molecule’s magnetic couplings with the two FM electrodes. Simulations show that when a molecule established ferromagnetic coupling with one electrode and antiferromagnetic coupling with the other electrode, then theoretical results effectively explained the experimental findings. Our studies suggest that in order to align MTJMSDs’ electrodes antiparallel to each other, the exchange coupling strength between a molecule and FM electrodes should be ∼50% of the interatomic exchange coupling for the FM electrodes.

Role of spin-lattice coupling in the ultrafast demagnetization ofGd1−xTbxalloys

After excitation by femtosecond laser pulses, Gd and Tb exhibit ultrafast demagnetization in two steps, with the time constant of the second step linked to the coupling strength of the $4f$ magnetic moments to the lattice. In time-resolved magneto-optical Kerr effect measurements of ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ alloys, we observe a decrease in this time constant from 33 to 9 ps with Tb content $x$ increasing from 0 to 0.7. We explain this behavior by the stronger spin-lattice coupling of Tb compared to Gd, which increases the effective spin-lattice coupling in ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ with $x$. In contrast, the faster time constant of the first demagnetization step exhibits no dependence on $x$. Additional time- and element-resolved x-ray magnetic circular dichroism measurements show a two-step demagnetization of Gd and Tb in ${\mathrm{Gd}}_{0.6}{\mathrm{Tb}}_{0.4}$ with an equivalent time scale of the second step but a different magnitude of demagnetization which persists for 15 ps. We explain this by an increased coupling of the Gd $4f$ magnetic moments to the lattice compared to pure Gd, via interatomic exchange coupling to the neighboring Tb $4f$ moments mediated by $5d$ electrons, which has limited efficiency and allows an estimation of a characteristic time scale of the interatomic exchange coupling. We assign the first demagnetization step to the dynamics of the laser-excited $5d$ electrons, while the second demagnetization step is dominated by the strength of spin-lattice coupling of the $4f$ electrons.

Tailored Exchange in Binary Manganese—Noble Metal Alloys

A general phenomenological model is presented that predicts the evolution of magnetic order and transition temperatures found in binary noble metal (NM) alloys with high Mn content. The model is derived from magnetic measurements and analysis of rapidly solidified Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> (0.25 ≤ x ≤ 0.4) alloys, also describing the magnetic attributes of CuMn and AuMn alloys, as reported in the literature. Mn atomic substitution in the face-centered-cubic noble metal lattice results in a tunable average Mn-Mn characteristic interatomic distance estimated from the average Mn concentration that governs the evolution of the magnetic ordering temperature. This evolution is found to be a consequence of the competition between positive ferromagnetic (FM) and negative antiferromagnetic (AF) interatomic exchange coupling revealing a Bethe-Slater-type behavior. Considering the composition range analyzed in this letter and the previous literature ( x = 0.02-0.40), the ordering temperature is maximized at x ≈ 0.17 for Ag-Mn and Cu-Mn alloys, with a progressive decrease at higher Mn concentration that is indicative of an increasing AF character. These predictive results enable the development of potential pathways to manipulate and tailor magnetic attributes in Mn-based alloys that are of current interest for tunable exchange-biased systems.

Magnetic properties of Co- and FePt-clusters

The electronic and magnetic properties of free FePt-clusters as well as of Co-clusters supported by a Pt(111)-substrate has been studied by means of the spin-polarised relativistic version of the Korringa–Kohn–Rostoker (KKR) multiple scattering formalism. This approach gives access to the individual spin as well as spin-orbit induced orbital magnetic moments, that were found to be in rather satisfying agreement with available experimental data. In addition, the interatomic exchange coupling constants Jij have been calculated, that indicate a rather high thermal stability of the total magnetic moments for the investigated clusters.

Pairs of 3d impurities in Au: local moments and exchange coupling from first-principles

We use a first-principles approach, implemented directly in real space, to obtain the electronic structure and magnetic behavior of a pair of adjacent 3d impurities, one of them Fe, in Au. We find that the inter-atomic exchange coupling J between the Fe and the 3d impurity in the pair is negative for Ti, V and Cr leading to an antiferromagnetic arrangement, while in all other cases J is positive and a ferromagnetic configuration is favored. The local moment of Fe is not significantly affected by the presence of the 3d neighbors. Therefore, the strongly modified values of hyperfine field observed in some of these systems cannot be explained by changes in the moment of the Fe probe.

A model that gives rise to effective dry friction in micromagnetics

Since dissipation requires motion, the velocity-independent friction of macroscopic systems must result from quick motions of some of its components. A model of coupled spins, some of which have irregular anisotropy directions, presents velocity independent loss. During a collective motion of the spin ensemble, an irregular spin can fall into its potential minimum, from which it is subsequently dragged out again. The effect is pronounced for a range of the ratio between the anisotropy and interatomic exchange coupling constants which is typical for rare-earth ions.

Cálculo da constante de dureza em sistemas ferromagnéticos

A two-band model for the magnetism is used to calculate the stiffness constant in ferromagnetic systems. One band is narrow and degenerated, representing the "quasi-localized" electrons. The second one is a wide band containing very few itinerant electrons. The transverse dynamical susceptibility is calculated within the random-phase approximation (RPA). The value of the stiffness constant of the acoustical spin wave is essential to satisfy the criterion of ferromagnetic stability at T = 0K. From the poles of the susceptibility two spin-wave branches are derived: one acoustic and one optic. It is shown that taking into account the interatomic exchange coupling between electrons of the narrow bands is necessary for the spin-wave stiffness constant to assume the experimental value.

Two-band model for the magnetism of iron.

We introduce a two-band model for the magnetism of Fe and report results in fairly good agreement with many experimental facts both at low and high temperatures. One band is narrow and degenerate, representing the ``quasilocalized'' electrons. The second one is a wide band containing very few itinerant electrons. The occupation of these bands is suggested by positron annihilation data and the observed saturation moment. The one-particle excitations and the transverse dynamical susceptibility are calculated within the Hartree-Fock and the random-phase approximations, respectively. An exchange coupling between electrons of both bands plays a crucial role in determining the magnetic state of the system, as evidenced in the derived modified Stoner criterion. On the other hand, an effective interatomic exchange coupling between electrons of the narrow band is necessary for the spin-wave stiffness constant to assume the extrapolated ``experimental value'' at zero temperature. Moreover, an excellent fitting of the temperature dependence of the static paramagnetic susceptibility is obtained. The results are analyzed in view of Stearns's ideas concerning the origin of the magnetism of Fe.

Theory of Invar and Heusler alloys

The physical picture of Invar and Heusler alloys provided by parameter-free, self-consistent, spin-polarized energy-band calculations is described. In our view, Invar differs from other itinerant magnets in possessing a nonmagnetic state that can be reached by thermal excitations. Calculations for ordered Fe-Ni compounds containing 0, 25, 50, 75 and 100% Ni agree with zero-temperature measurements, and show the total-energy separation of the magnetic and nonmagnetic states to vanish in the Invar composition range, where both states are stable, but at different volumes. Heusler alloys (X 2MnY) possess localized magnetic moments (on the Mn atoms) that are not in nearest-neighbor contact, and therefore provide a testing ground for theories of indirect magnetic coupling. Our calculations indicate that the majority-spin electrons are completely itinerant and that the localized moments result from the exclusion of the minority-spin electrons from the interior of the Mn atoms. Interatomic exchange coupling is studied by comparing calculations for ferromagnetic and antiferromagnetic moment alignments.

Magnetism of iron. II

A previously developed theory of the magnetism of iron, based upon the notion of a randomized exchange field, is modified and extended. It is shown that Coulomb fields arise in association with the randomized exchange field; the modification allows one to take into account these fields, which are found to change the details of the previous results without affecting the main conclusions. The theory has been extended to calculate the properties of the model at finite temperatures: the Curie temperature (1840 K), the magnetization curve, the paramagnetic susceptibility (a Curie-Weiss law), and the effective interatomic exchange coupling are calculated for iron. The magnitudes of the atomic spin moments were found to vary little up to 1.5 times the Curie temperature.