J. Filippi

Rotational and transverse dynamics of some random anisotropy magnets (invited)

After rapid survey of the behavior of random anisotropy magnets, the time and frequency evolution of the magnetization vector of a rotating disk of amorphous DyNi is focused on, as measured by a two‐SQUID detectors setup. The results show that the dynamics of this system is completely determined by two macroscopic variables: the magnetizations M1 and M2, respectively associated with short and long relaxation times. The first one is fixed with respect to the applied field and therefore responsible for dissipation (magnetic friction) whereas the second one is fixed with respect to the sample (rotating anisotropy). Furthermore, transverse ac susceptibility experiments are used to confirm some of the results obtained. In particular low‐temperature dynamical scaling (T→0) permits the existence of a new type of frequency crossover, when the volume samples by the ac field becomes of the order of Imry and Ma domains, to be shown.

Spin wave localization in random anisotropy systems : amorphous (DyxGd1-x)Ni

Magnetization measurements performed in the random anisotropy (amorphous) system (DyxGd1-x)Ni are analysed in terms of spin wave excitations in the temperature range 1.5 to 20 K and magnetic fields up to 60 kOe. Besides the Zeeman term, the field-dependent spin wave gap contains a contribution coming from the random magnetic anisotropy. This contribution decreases with H, from a zero-field value proportional to the random anisotropy field, D2/J. This observation, in quantitative agreement with the theory, constitutes the first evidence for the existence of a random anisotropy gap in amorphous alloys. The field decreasing dependence of this gap induces a progressive delocalization of spin waves with the application of a magnetic field, opposing to the effect of the disorder of local anisotropy directions. For zero field the excitations are localized in space, in regions of the order of the ferromagnetic correlation length (Imry and Ma domains). Field-induced reorientations of the magnetization of these domains favour the propagation of spin waves to much more extended regions (spin wave delocalization).

Modifications of weak localization contributions to magnetoresistance due to magnetic impurities in amorphous DyxY1−xNi alloys (invited)

High‐field magnetoresistance experiments performed on thick amorphous alloys offer a simple way to study three‐dimensional (3D) weak localization of conduction electrons. After the precursor work of Fert et al. on nonmagnetic amorphous alloys, we found it interesting to study how these effects would disappear under substitutions of magnetic impurities (1% to 10% of Dy) in a nonmagnetic amorphous alloy (YNi). The experiments, performed between 1.5 to 50 K and in magnetic fields up to 20 T, showed (i) in YNi, characteristic features of the magnetoresistance due to weak localization under strong spin‐orbit scattering and (ii) in DyxY1−xNi, a coexistence of weak localization effects with the classical contribution of spin alignment by the applied magnetic field, saturating at negative values. This last contribution dominates the behavior of Dy‐richer samples whereas weak localization is clearly observed for x≤3%. In all the samples a dramatic increase of the initial magnetoresistance slope Δρ/ρH2 (where ρ=res...

Weak Localization in Amorphous Alloys in the Presence of Magnetic Impurities: Enhancement of the Spin-Splitting Contribution

The effect of magnetic impurities on weak localization in amorphous metals is investigated with magnetoresistance in the series (DyxY1-x)Ni up to x = 0.12. In these samples we find that the weak-localization contribution to magnetoresistance is increased with respect to the nonmagnetic sample, YNi, due to an enhancement of the Zeeman spin splitting of the conduction bands by the exchange interaction with the magnetic ions. Over the whole series, the spin-splitting dependence on temperature and composition scales with the paramagnetic susceptibility of the samples. This shows its direct relation with the magnetic-exchange interaction of conduction electrons with Dy moments, which polarize the conduction band.

Transverse ac susceptibility of the random anisotropy system a-DyNi

Abstract The transverse ac susceptibility of amorphous DyNi has been measured around the freezing temperature. The data have been analysed using a characteristic temperature shift between the X ′ and X ″ curves. This leads to a frequency independent line for the onset of transverse correlations.

On the dynamical behaviour of the random anisotropy systems amorphous DyxGd1−xNi

Abstract Longitudinal ac susceptibility experiments have been performed on random anisotropy systems of the series Dy x Gd 1− x Ni. Usual dynamical scaling forms with a power-law size distribution failed to explain χ′(ω) and χ″(ω) variations. Furthermore the π/2 law is not obeyed in these systems because of non-monotonic distribution of relaxation times, and the divergence when ω→0 of the first and upper distribution moments shows that the distribution is slowly decaying. A Cole-Cole model fits fairly well our experimental results and we observe in these alloys a truncated divergence of the time constant τ c of the Cole-Cole susceptibility at a finite temperature T 0 .

Electron-electron interactions in the presence of magnetic impurities: Effect on the temperature dependence of resistivity

We investigate the effect of the addition of magnetic impurities on the temperature dependence of the electrical resistivity ϱ(T) of 3-dimensional amorphous alloys at low temperatures (T<20K). The non-magnetic sample YNi presents the √T dependence characteristic of electron-electron interactions effects. With the introduction of Dy impurities, the samples (DyxY1−x) Ni with x<0.12 present an additional contribution varying as 1/√T and proportional to the inelastic spin dephasing rate τs-1 (and x), determined independently through weak localization magnetoresistance. These results are analyzed in terms of the theories developed for the electron-electron interaction effects and localization corrections in the presence of magnetic impurities.

Quantum corrections in the magnetic susceptibility: Amorphous Y0.5Ni0.5

The magnetic susceptibility χ(H, T) of the paramagnetic Y0.5Ni0.5 amorphous alloy was measured with a SQUID at low fields (H<500Oe) and in the temperature range 1.3<T<25K. Besides a small field independent Curie-like contribution C/T due to about 100 ppm of magnetic rare-earth impurities in Y, we find that χ(H, T) presents a √T dependence: χ(H, T)=C/T+χo(H)−A(H)√T, A(H) being a linear function of χo(H). The results are analyzed in terms of the quantum corrections to the magnetic susceptibility in the weak localization regime in 3-dimensional systems.

Magnetoresistance and local magnetic order in random anisotropy ferromagnetic systems

Abstract In ferromagnetic systems with random anisotropy (RA), long range magnetic order is destroyed and the correlation function of the magnetization presents an exponential form e -r/R f . Also, the law of approach to saturation follows a 1/√H law (Ferromagnet with Wandering Axis regime). We calculated the effect of magnetic correlations on the resistivity of a RA ferromagnetic system at low temperatures. In the FWA regime the magnetoresistance has a logarithmic dependence on the magnetic field and the slope of that variation is inversely proportional to the correlation length, R f . This is verified by magnetoresistance measurements in amorphous alloys with RA (with Dy). The logarithmic dependence was observed in extended field ranges (1–50 kOe), and the values obtained for R f are in agreement with the ones determined from magnetic measurements.

Electrical resistivity and local magnetic order in random anisotropy amorphous ferromagnets

In ferromagnetic systems with random anisotropy (RA), long‐range magnetic order is destroyed and the correlation function of the magnetization exhibits an exponential form e−r/Rf, where the correlation length Rf corresponds to the size of Imry–Ma domains. Also, the law of approach to magnetic saturation follows a 1/√H law [ferromagnet with wandering axis (FWA) regime]. We have calculated the effect of magnetic correlations on the electrical resistivity and magnetoresistance of a RA ferromagnetic system at low temperatures. We find that in zero magnetic field RA introduces a positive contribution into the electrical resistivity, which increases with the anisotropy to exchange ratio, D/T.  In the FWA regime the magnetoresistance has a logarithmic dependence on the magnetic field with a slope Δρ/(ρ ln H) inversely proportional to Rf. These features have been observed in an experimental study of the magnetoresistance at low temperatures (T≊4 K) of the amorphous series (DyxGd1−x)Ni in which the D/T  ratio can ...