Observations of ferromagnetic correlations at high temperatures in paramagnetic iron

Polarised neutron scattering with polarisation analysis has been used to obtain a unique measurement of the paramagnetic fluctuations in iron at temperatures between 1273 and 1573 K. The results clearly demonstrate almost complete ferromagnetic correlation over distances up to 15 Å. The average moment per atom taking part in the correlation and giving rise to paramagnetic scattering is about 1.3μ_B. These findings should lead to a better understanding of paramagnetism in metals.     

Hyperfine fields at nonmagnetic impurities in ferromagnetic GdZn and GdCd

Hyperfine fields at sp impurities in ferromagnetic CsCl type compounds GdZn and GdCd have been measured by NMR method. The impurity hyperfine fields are always negative, indicating the negative polarization of s-like conduction electrons at impurities on the Zn(Cd) sites, and are smaller at the beginning and end of each sp series. This trend of the hyperfine fields is similar to that in Gd metal and is qualitatively understood by the Daniel-Friedel mechanism.     

The magnetic hyperfine field at Hg impurities in ferromagnetic Gd

The magnetic hyperfine field at dilute Hg impurities in Gd has been investigated by the conversion electron (e ^–)--time differential perturbed angular correlation (TDPAC) technique. The radioactivities^197m Hg and¹⁹⁹Tl were implanted into Gd foils by means of an isotope separator. TDPAC measurements were performed with the 165 keV-L-conversion electron—134 keV--cascade of¹⁹⁷Hg at different temperatures and with the 334 keV--158 keV-K-conversion electron cascade of¹⁹⁹Hg at 200 K.The regular site occupation probabilities were found to be 15(3)% for an annealed^197m HgGd sample and 29(5)% in unannealed¹⁹⁹TlGd samples.From the magnetic hyperfine interaction frequencies measured for the regular sites at 200 K the magnetic hyperfine fields |H _hf(¹⁹⁷HgGd; 200 K)|=256(13) kG and |H _hf(¹⁹⁹HgGd; 200 K)|=267(7) kG were deduced.On leave from the University of Lisboa, Portugal     

Theory of zero bias anomalies due to paramagnetic impurities

The method developed by one of the authors is generalized to include paramagnetic impurities in order to explain zero bias tunneling anomalies. The self-energy of the electrons is supposed to be a local function in space; nonlocal or assisted tunneling effects are beyond the scope of the present theory. The tunneling current is expressed in terms of the local density of states at the barrier which in turn is given in terms of the life-time of the conduction electrons. The change of the local density of states depends on the positions of the impurities relative to the metal-oxide interface. This dependence is thoroughly investigated.     

Aggregation of ferromagnetic and paramagnetic atoms at edges of graphenes and graphite

In this work we report that when ferromagnetic metals (Fe,Co and Ni) are thermally evaporated onto n-layer graphenes and graphite,a metal nanowire and adjacent nanogaps can be found along the edges regardless of its zigzag or armchair structure.Similar features can also be observed for paramagnetic metals,such as Mn,Al and Pd.Meanwhile,metal nanowires and adjacent nanogaps cannot be found for diamagnetic metals (Au and Ag).An external magnetic field during the evaporation of metals can make these unique features disappear for ferromagnetic and paramagnetic metal;and the morphologies of diamagnetic metal do not change after the application of an external magnetic field.We discuss the possible reasons for these novel and interesting results,which include possible one-dimensional ferromagnets along the edge and edge-related binding energy.     

Static and time dependent fields at nonmagnetic impurities in ferromagnetic samples

Several techniques are used for the measurement of hf-interactions. One of them, the PAC-method, gives the value of the effective field acting on a decaying nucleus, assuming a static force. Using the Master Equation, this paper shows if that there exists in addition a strong fluctuating field, the angular correlation is wiped out.     

Nuclear relaxations due to paramagnetic impurities in two-level systems

The rates of two types of nuclear spin-lattice relaxation are compared. Transverse relaxation of nuclear spins interacting with paramagnetic centers is also examined under the assumption that the paramagnetic centers form two-level tunneling systems. The transverse relaxation rate is calculated and it is shown that at certain temperatures the transverse relaxation rate is governed by the two-level systems. Fiz. Tverd. Tela (St. Petersburg) 39, 1210–1212 (July 1997) Deceased     

Effect of substrate impurities on layering transitions

We investigate the influence on thin film growth of substrate impurities distributed throughout the substrate bulk. In particular, using a method discussed in a previous paper, we show that the case of correlated disorder will enhance film growth via two mechanisms. Both the chemical potential, and the adatom-adatom interactions are effectively shifted in the intermediate temperature range, thus having the effect of encouraging film growth.     

Effect of paramagnetic fluctuations on superconductivity in ferromagnetic superconductors

The superconducting transition temperature T_c is calculated for ferromagnetic superconductors taking into account explicitly elastic scattering between electrons and paramagnetic fluctuations. Depending on the strength and range of the Heisenberg interaction between localized magnetic moments, the superconducting gap may or may not remain finite at the onset of long range ferromagnetic ordering. In a dirty system, the mean free path reduces the depairing effect of fluctuations. The Heisenberg interaction is assumed to be of other than RKKY origin.     

Spin-wave excitations of paramagnetic impurities in superconductors

The existence of spin-wave excitations of paramagnetic impurities in a superconductor in an external magnetic field due to the long-range exchange interactions is predicted. The damping has been calculated and possible manifestations of this collective excitations in ESR experiments are analysed.     

Electrically driven spin dynamics of paramagnetic impurities

The spin dynamics of dilute paramagnetic impurities embedded in a semiconductor GaAs channel of a conventional lateral spin valve has been investigated. It is observed that the electron spin of paramagnetic Mn atoms can be polarized electrically when driven by a spin valve in the antiparallel configuration. The transient current through the MnAs/GaAs/MnAs spin valve bears the signature of the underlying spin dynamics driven by the exchange interaction between the conduction band electrons in GaAs and the localized Mn electron spins. The time constant for this interaction is observed to be dependent on temperature and is estimated to be 80 ns at 15 K.     

The influence of paramagnetic molecules on singlet-triplet transitions

It is shown that singlet-triplet transitions may obtain a finite probability when exchange interaction occurs between the absorbing molecule and a paramagnetic molecule.

According to this mechanism no quantitative relation exists between the transition dipole strength and the paramagnetic susceptibility of the perturbing molecule.     

Influence of pressure and paramagnetic impurities on properties of superconductors

An investigation is made of the influence of a Fermi-surface topology change under pressure and paramagnetic impurities on the superconducting transition temperature, on the order parameter and on the energy gap at zero temperature. The critical impurity concentrations are not the universal values and depend on the valence difference between the impurity atoms and the metal atoms.     

Nuclear spin-lattice relaxation of impurities in ferromagnetic iron

Due to the development of Green's function method the calculation of the nuclear spin-lattice relaxation time of impurities in ferromagnets has become feasible in the last years. We present the result of calculations for allsp andd impurities in ferromagnetic iron. The calculations are based on the density functional formalism. They well, reproduce the experimental trend of the relaxation timeT ₁ for bothsp andd impurities. By decomposing the relaxation rate into various contributions, we explain the observed systematic behavior ofT ₁ T in terms of the local electronic structure.     

Low-temperature ferromagnetic resonance in epitaxial garnet films on paramagnetic substrates

The temperature dependence (4.2–300 K) of ferromagnetic resonance (FMR) in the 3-cm range in films of yttrium-iron garnet (YIG)1.7–3 m thick, epitaxially grown on a paramagnetic gadolinium-gallium garnet (GGG) paramagnetic substrate is studied. It is found that with decrease in temperature, beginning at T 70 K, the temperature dependences of the resonant FMR fields _res and _res for magnetic fields applied parallel and perpendicular to the normal to the film deviate from similar dependences for a free thin YIG plate, which approach the value of / for YIG. This deviation increases approximately in proportion to T^–1, reaching hundreds of Oe at T=4.2 K. It is established that the given effect is related to the additional static magnetic field H_p created in the YIG film by the paramagnetic substrate, and is dependent on the form of the latter.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 48–53, April, 1989.     

Electron paramagnetic resonance of impurities in uranium trichalcogenides

Uranium trichalcogenides US₃ and UTe₃ were examined by EPR. The measurements were performed at the X-band over temperature range 4.2–300K and with the magnetic field applied perpendicular and parallel to the (001) plane. The EPR spectra consist of two lines with g-factors about 2 and 4 respectively. Results are discussed in terms of interactions of the U⁴⁺ ion with impurities. A magnetic ordering in US₃ below 50 K is suggested.     

How to deal with negative surface heat capacities

The European Physical Journal Special Topics - Negative surface heat capacities are observed for many liquids, at least in certain temperature regimes. Since thermodynamic stability of a system...