Ferromagnetic Gadolinium Research Articles

Ultrafast spin transfer and its impact on the electronic structure.

Optically induced intersite spin transfer (OISTR) promises manipulation of spin systems within the ultimate time limit of laser excitation. Following its prediction, signatures of ultrafast spin transfer between oppositely aligned spin sublattices have been observed in magnetic alloys and multilayers. However, it is known neither from theory nor from experiment whether the band structure immediately follows the ultrafast change in spin polarization or whether the exchange split bands remain rigid. We show that ultrafast spin transfer occurs even in ferromagnetic gadolinium metal. Charge transfer between localized surface and extended valence-band states leads to a decrease of the surface spin polarization. This synchronously alters the exchange splitting of the bulk valence bands during laser excitation. Moreover, the onset of demagnetization can be tuned by over 200 fs by changing the temperature-dependent spin mixing. Our results show a promising route to ultrafast control of the magnetization, widening the impact and applicability of OISTR.

Antiperovskite Gd3 SnC: Unusual Coexistence of Ferromagnetism and Heavy Fermions in Gd Lattice.

Inverted structures of common crystal lattices, referred to as antistructures, are rare in nature due to their thermodynamic constraints imposed by the switched cation and anion positions in reference to the original structure. However, a stable antistructure formed with mixed bonding characters of constituent elements in unusual valence states can provide unexpected material properties. Here, a heavy-fermion behavior of ferromagnetic gadolinium lattice in Gd3 SnC antiperovskite is reported, contradicting the common belief that ferromagnetic gadolinium cannot be a heavy-fermion system due to the deep energy level of localized 4f-electrons. The specific heat shows an unusually large Sommerfeld coefficient of ≈1114 mJ mol-1 K-2 with a logarithmic behavior of non-Fermi-liquid state. It is demonstrated that the heavy-fermion behavior in the non-Fermi-liquid state appears to arise from the hybridized electronic states of gadolinium 5d-electrons participating in metallic GdGd and covalent GdC bonds. These results accentuate the unusual chemical bonds in CGd6 octahedra with the dual characters of gadolinium 5d-electrons for the emergence of heavy fermions.

The role of ultrafast magnon generation in the magnetization dynamics of rare-earth metals.

Ultrafast demagnetization of rare-earth metals is distinct from that of 3d ferromagnets, as rare-earth magnetism is dominated by localized 4f electrons that cannot be directly excited by an optical laser pulse. Their demagnetization must involve excitation of magnons, driven either through exchange coupling between the 5d6s-itinerant and 4f-localized electrons or by coupling of 4f spins to lattice excitations. Here, we disentangle the ultrafast dynamics of 5d6s and 4f magnetic moments in terbium metal by time-resolved photoemission spectroscopy. We show that the demagnetization time of the Tb 4f magnetic moments of 400 fs is set by 4f spin-lattice coupling. This is experimentally evidenced by a comparison to ferromagnetic gadolinium and supported by orbital-resolved spin dynamics simulations. Our findings establish coupling of the 4f spins to the lattice via the orbital momentum as an essential mechanism driving magnetization dynamics via ultrafast magnon generation in technically relevant materials with strong magnetic anisotropy.

Enhancement of microwave absorption bandwidth of polymer blend using ferromagnetic gadolinium silicide nanoparticles

Ferromagnetic gadolinium silicide (Gd5Si4) nanoparticles significantly enhance the microwave absorption bandwidth of a polymer blend (PVB-PEDOT:PSS). These materials are critically needed for various military and civilian applications such as X-band (8.2–12.4 GHz) and Ku-band (12.4–18 GHz) absorption. A single 1.2 mm thick layer of PVB-PEDOT:PSS-Gd5Si4 (PPGS) nanocomposite film shows the most promising bandwidth (8.2–18 GHz) with a minimum reflection loss of −14 dB. Mechanistically, dielectric loss (tanδe≈2.4) and magnetic loss (tanδm≈1.1) contributes more efficiently, and standard microwave simulation confirms the stored energy is predominant in PPGS nanocomposite which enhances the bandwidth.

Spin polarization of conduction electrons and electronic structure of gadolinium

The structure of magnetic moment in metallic gadolinium is studied with special attention paid to the conduction electron spin-polarization. The theoretical electronic structure, Fermi surface topology, neutron magnetic form-factor and magnetic Compton profiles of ferromagnetic gadolinium are analysed. The calculated band structure is in good agreement with the results of a photoemission experiment, especially in the position of the 4f band. Some tendency to parallelization behaviour in the Fermi surface was observed. The maximum entropy reconstruction of the magnetization density from neutron diffraction data is also presented and compared with the theoretically predicted distribution. It is shown that in contrast to the old interpretation of the magnetic form factor data, the negative spin polarization in the interstitial regions may contain contribution coming from d-type electrons. The negative magnetization within the muffin-tin should characterize semi-core electrons 5s and 5p. Presence of this magnetization can hardly be found in neutron measurements, yet it is possible to observe it in accurate magnetic Compton profiles. The latter are calculated and discussed.

Nonlinear susceptibilities of gadolinium near curie temperature

The ac linear (X0) and nonlinear (X2) susceptibilities of an elemental ferromagnetic gadolinium have been studied at temperatures near the Curie temperature TC to clarify the feature of the magnetic phase transition, with special emphasis on the behavior at temperatures below TC. The temperature dependence of X0 is in good agreement with an experimental result reported in several references. The first observation for the critical behavior of X2 in gadolinium has been made at various field amplitudes. The behavior of nonlinear susceptibility is very sensitive to the field’s amplitude. The temperature dependence of X2 at temperatures below TC is not in agreement with some theoretical expectations and several experimental results for other ferromagnets.

MAGNONS HEAT TRANSPORT AT AN INTEGRATED NANOSTRUCTURE IN ULTRATHIN HEISENBERG FERROMAGNETIC FILMS

A theoretical model is presented for the study of magnons heat transfer across an integrated nanostructure acting as interface material between two ultrathin Heisenberg ferromagnetic films. This is done by calculating the transmission rates of the spin wave modes through the consideration of the magnon group velocity modification in the system. The group velocities are calculated explicitly for all propagating frequencies and spin wave incidence angles. The matching method is used with nearest neighbor ferromagnetic interactions to calculate the coherent transmission coefficients for magnons incident on the nanostructure boundaries. They are investigated for different nanostructure thickness over the entire propagating frequency range of the system, in order to calculate the heat transport across the nanostructure. The model is applied to a system of three Iron ferromagnetic atomic layers and Gadolinium integrated nanostructure. The thermal conductivities are numerically calculated for individual magnon modes of the system. The results yield an understanding of the relationship between the magnons thermal conductivity of the system and the structural configuration of the integrated nanostructure.

Spin-wave-mediated quantum corrections to the conductivity of thin ferromagnetic films of gadolinium

We present a study of quantum corrections to the conductivity of thin ferromagnetic gadolinium films. In situ magneto-transport measurements were performed on a series of thin films with thickness d < 135A. For sheet resistances R0 < 4011 Ohm and temperatures T < 30K, we observe a linear temperature dependence of the conductivity in addition to the logarithmic temperature dependence expected from well known quantum corrections in two dimensions. We show that such a linear T-dependence can arise from a spin-wave mediated Altshuler-Aronov type correction.

Thermodynamic simulation of the magnetocaloric effect in simple ferromagnets

The traditional Landau theory of second-order phase transitions is used to analyze the behavior of the magnetothermal properties of simple ferromagnets as functions of the temperature, the magnetic field, and the thermodynamic theory parameters that have not been thermodynamically interpreted earlier. A number of generally accepted concepts of the magnetocaloric effect (MCE) are shown to require refinement. The thermodynamic consideration is comprehensively illustrated by model calculations performed on real temperature and magnetic-field scales, and the calculation results are compared with the experimental data obtained for ferromagnetic gadolinium. Thermodynamic conditions required for reaching the maximum MCE in simple ferromagnets have been formulated.

Perturbed angular correlations for Gd in gadolinium: In-beam comparisons of relative magnetizations

Perturbed angular correlations were measured for Gd ions implanted into gadolinium foils following Coulomb excitation with 40MeV 16O beams. A technique for measuring the relative magnetizations of ferromagnetic gadolinium hosts under in-beam conditions is described and discussed. The combined electric-quadrupole and magnetic-dipole interaction is evaluated. The effect of nuclei implanted onto damaged or non-substitutional sites is assessed, as is the effect of misalignment between the internal hyperfine field and the external polarizing field. Thermal effects due to beam heating are discussed.

Low-level structure ofGe70from lifetime andg-factor measurements following α transfer to aZn66ion beam

The $g$ factor of the ${2}_{1}^{+}$ state in $^{70}\mathrm{Ge}$ was remeasured using a different experimental approach. Furthermore, for the first time an experimental value (although with a large uncertainty) was obtained for the $g$ factor of the ${2}_{2}^{+}$ state in $^{70}\mathrm{Ge}$. All this was accomplished by employing the technique of \ensuremath{\alpha} transfer to an energetic $^{66}\mathrm{Zn}$ ion beam in inverse kinematics combined with transient magnetic fields in ferromagnetic gadolinium. The value of the $g({2}_{1}^{+})$ factor obtained ranges from +0.32(11) to +0.43(12), subject to certain assumptions. This range of values is in general agreement with the range of values in the literature, where Coulomb excitation and different IMPAC techniques were used. Lifetimes of several low-lying states were redetermined using the Doppler-Shift-Attenuation-Method. The deduced $B(E2)$ values and the $g({2}_{1}^{+})$ factor are discussed within the framework of large-scale full $\mathit{fp}$ shell model calculations with a closed $^{40}\mathrm{Ca}$ core and including excitations from the ${f}_{7/2}$ orbital. The results are compared with recent data for $^{68}\mathrm{Ge}$ and $^{68}\mathrm{Zn}$.

Isolation and characterization of a ferromagnetic gadolinium (III) citrate chain

Isolation and characterization of a ferromagnetic gadolinium (III) citrate chain

Dynamic critical behavior in models of ferromagnetic gadolinium

A numerical technique combining Monte Carlo and molecular dynamics simulations is used for the first time to examine the complex critical dynamics of models of ferromagnetic gadolinium in which both strong exchange interactions and relativistic effects of several different types are taken into account. A finitesize scaling technique is used to calculate the corresponding dynamic critical exponents. The role played by isotropic dipole-dipole interaction in the critical behavior of gadolinium is evaluated. The results obtained provide an explanation for the anomalous dynamic critical behavior of gadolinium.

Evidence for the ion-induced electronic spike on fs and nm scales from transient field measurements

Measurements of nuclear spin precessions in the transient field (TF) of ferromagnetic gadolinium have provided for the first time evidence for demagnetization effects induced by the energy loss of traversing Mg ions. The primary electronic spike associated with the host electrons is probed in space and time on a nanometer and femtosecond scale by Coulomb-excited 76,80Se ions recoiling in and near the tracks of preceding 26Mg ions. The demagnetization that reduces the TF strength is observed via the spin precession of the Se probe ions. The present measurements are sensitive solely to the electronic spike and not to a succeeding lattice spike, as the lattice temperature is substantially lower than the Curie temperature and its build-up occurs significantly later in time than the interaction of the probe ions in the ferromagnet.

Adherence problem for carbon films of up to 0.5 mg/cm 2 on vacuum-deposited thick ferromagnetic Gd targets

For high precision measurements of magnetic moments and reduced transition probabilities by the combined technique of projectile Coulomb excitation in inverse kinematics and transient magnetic fields sandwich targets of carbon and gadolinium were required. First preparations revealed a lack of adhesion between the Gd film and the vacuum-deposited C layer. Either the adhesion was generally poor or good results could not be reproduced. Now on a 4 mg/cm 2 Gd target 0.5 mg/cm 2 of natC should be deposited. The Gd was deposited on 1–1.6 mg/cm 2 Ta backings and Cu films of 3.5–7 mg/cm 2 were deposited on the reverse side of the Ta backings. The adhesive properties of evaporated carbon on ferromagnetic gadolinium were investigated. For this either substrate cooling or the deposition of a 5 μg/cm 2 Ti film as adhesion agent were applied.

Spin-resolved photoemission study of photohole lifetimes in ferromagnetic gadolinium

High resolution spin-resolved photoemission is used to probe the properties of a Gd(0001) surface state. The state shows both a spin-mixing behavior reflecting the exchange of magnons with the local moments and a reduction of the exchange splitting with increasing temperature. The surface state polarization at low T suggests that the surface layer has an enhanced Tc of 365K or greater. Measurements of the photoemission linewidths show that at low temperatures, the lifetime of a majority spin photohole is predominantly limited by electron-phonon scattering and that of a minority spin photohole by electron-magnon scattering. Since similar behavior may be expected for bulk states close to the Fermi level, the transport properties of this material will also be determined by different decay mechanisms in the two channels.

Monte Carlo investigation of critical phenomena in models of real magnetics with crossovers

Microscopic models of real ferromagnetic gadolinium are proposed, and their critical properties are studied by the single-cluster Monte Carlo algorithm. The critical exponents of these models with crossovers are determined by the finite-size scaling theory. The crossover from the Heisenberg critical behavior to the dipole one are observed.

Cluster algorithms of the Monte Carlo method, finite-size scaling, and critical exponents of complex lattice models

For the first time, cluster algorithms of the Monte Carlo method are used to investigate critical properties of microscopic models of real ferromagnetic gadolinium. On the basis of the finite-size scaling theory, the critical exponents of the heat capacity α, magnetization β, susceptibility γ, and Fisher index η are calculated. Specific features, character, and the degree of influence of two types of weak relativistic interactions on the critical properties of gadolinium models are determined when both these interactions are taken into account. It is shown that cluster algorithms of the Monte Carlo method provide an effective tool for studying critical properties of complex models involving crossovers.

Sandwich targets of 0.5 mg/cm 2 C with 4 mg/cm 2 ferromagnetic Gd on 3 mg/cm 2 Ta support with 3.5 mg/cm 2 Cu backing

For high precision measurements of magnetic moments and reduced transition probabilities by the combined technique of projectile Coulomb excitation in inverse kinematics and transient magnetic fields sandwich targets of carbon and gadolinium had to be developed. A Gd layer of 4 mg/cm 2 was evaporated onto 1–3 mg/cm 2 Ta backings heated to 800 K for optimum ferromagnetic properties of the Gd. A Cu film of 3.5 mg/cm 2 was then deposited on the reverse side of the Ta backing. The Cu was required for stopping the Coulomb excited beam ions in a hyperfine interaction-free medium and for cooling the Gd film below its Curie temperature. The sandwich was then completed with 0.5 mg/cm 2 natC deposited onto the Gd. The adherence of evaporated carbon on the ferromagnetic gadolinium was found to be poor, and good results were not easily reproduced. Therefore, deposition of carbon by laser plasma ablation was also investigated.

Electron spin-flip relaxation by one magnon processes: Application to the gadolinium surface band

The "s-f model", also known as the ferromagnetic Kondo lattice, contains a description of band electrons coupled to localized spins which is an appropriate description of the magnetic part of the low-energy physics of Gd metal. Here the model is used to estimate the lifetime broadening of the minority spin component of the surface electron band in ferromagnetic gadolinium metal at temperatures below the Curie temperature. The low temperature result 0.10 eV agrees nicely with a measurement by Fedorov et al.