Increasing Gd Concentration Research Articles

Magnetic properties of (La, Gd)Al2 alloys

Theac anddc susceptibilities of (La1−x Gd x )Al2 alloys were measured for moderate Gd concentrations (0.01≦x≦0.08) between 0.05 and 4.2 K. Sharp spin glass like maxima inχ AC are observed at temperaturesT f which scale asT f /x ≃ 26 K for all samples. However, scaling ofχ AC is not observed indicating that the RKKY interaction at short distances which becomes more important with increasing Gd concentration, is ferromagnetic in this system. The amplitudeV 0 of the RKKY interaction is obtained from both the approach to the saturation magnetization andχ AC forT→0, yielding a mean value $$\bar V$$ 0=(1.5±0.3) ⋅ 10−26 eV cm3 which is in agreement with that observed in other systems with Gd impurities. Furthermore, the inferred value of the exchange integral is in accord with other measurements (T c depression, reverse resistance anomaly) on (La, Gd)Al2. A superposeddc magnetic field rounds the maximum ofχ AC and shifts it to lower temperatures.χ AC is completely flat below 4.2 K in a field which saturates the isothermal remanent magnetization, showing once again the correlation between the peak inχ AC and the onset of irreversible properties.

Microwave magnetic resonance in amorphous GdAl films

The microwave absorption spectrum of amorphous sputtered ${\mathrm{Gd}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}$ films at 9.13 GHz exhibits a single broad line which is studied as a function of temperature and composition $x$. In the paramagnetic regime, the $g$ factor has a composition-independent positive shift of 0.006\ifmmode\pm\else\textpm\fi{}0.004 above 1.992 of the free ${\mathrm{Gd}}^{3+}$ ion, in contrast to negative shifts previously reported for crystalline GdAl. The shift is interpreted as arising from an exchange interaction ${J}_{\mathrm{df}}$ of the Gd $f$ shell with electrons of $d$ character. The $g$-shift contribution of the exchange interaction ${J}_{\mathrm{sf}}$ with $s$ electrons is presumed to be hidden because of a thermal bottleneck. This latter exchange interaction has a minimum value of 0.1 eV, as deduced from paramagnetic Curie temperatures. Using the observed temperature dependence of the resonance linewidth plus approximate band parameters from analogous dilute crystalline systems, the spin-lattice relaxation rate of amorphous ${\mathrm{Gd}}_{x}{\mathrm{Al}}_{1\ensuremath{-}x}$ is estimated to be ${10}^{13}x$ Hz, in order-of-magnitude agreement with dilute crystalline systems. At lower temperature the field for resonance shifts down, strongly for $x=0.37$ but less strongly for higher Gd concentration. This shift is interpreted as an effective anisotropy field arising from local demagnetizing fields of an inhomogeneous (spin-glass) system. The composition dependence of the shift indicates that with increasing Gd concentration, the alloy becomes more magnetically homogeneous, gradually approaching simple ferromagnetism.

Exchange optics in Gd-doped EuO

This paper is concerned with magneto-optical properties of EuO single crystals, doped with up to 5-at.% Gd. The optical absorption and the Faraday rotation have been measured above and below the magnetic-ordering temperatures and in various magnetic fields. In addition the reflectivity between 13 eV and 250 \ensuremath{\mu}m has been observed and the Curie temperature has been determined. The measurements permit the evaluation of the magnetic short-and long-range order. The exchange interaction between free electrons and $4f$ spins can be determined and is found to decrease with increasing doping. The analysis of the optical data permits the derivation of the carrier concentration, their mobility, and temperature dependence. The carrier concentration shows an exchange-induced ionization near ${T}_{C}$ for nondegenerate samples and the mobility has a sharp minimum at ${T}_{C}$. The transport parameters also reflect the fact that the electron-$4f$-spin interaction decreases with increasing Gd concentration, probably owing to electrostatic-shielding effects.

Giant Negative Magnetoresistance in Ferromagnetic Eu1−xGdxSe

The NaCl-type compounds Eu1−xGdxSe form a continuous series of solid solutions between the ferromagnetic insulator EuSe and the antiferromagnetic metal GdSe. We have studied the temperature dependence of electrical resistivity ρ, in Eu1−xGdxSe single crystals and polycrystals between 2.5° and 300°K; we find, in qualitative agreement with earlier observations, for small x, an exponential increase in ρ similar to normal semiconductors but a very sharp drop in ρ below the magnetic transition temperature. The resulting peak in ρ decreases sharply with applied magnetic field strength and increasing Gd concentration. The temperature dependence of the Hall effect in single-crystal samples of Eu0.95Gd0.05Se indicates as origin of the resistivity anomaly an unusually large effect of the magnetic order on the carrier mobility. Preliminary thermoelectric power measurements support this conclusion. The possibilities of describing strong interactions between conduction electrons and localized spins by scattering theory and by magnetic self-trapping of electrons in the spin lattice are discussed.