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In films of a-Fel "Zr with x =0.08 two distinct transitions were found in the electrical noise. A high-temperature transition produced a regime of strongly magnetic-field-dependent noise showing non-Gaussian effects, as would be expected at the ferromagnetic transition. Observations of single-domain switching events give a direct estimate of a dynamical coherence length, large enough to show true ferromagnetism in the films. At lower temperature, a sharp, field-dependent rise in noise appeared near the suspected spin-glass transition, as in other conducting spin glasses. The temperature and frequency dependences of the noise at the lower transition imply cooperativity.

In mesoscopic samples of disordered Fe&oo "Zr"evaporated films with x =8, complex discrete resistance switching due to collective rearrangements of dissolved hydrogen appeared at temperatures from 4 to 300 K. In larger samples, 1% irreproducible hysteretic changes in the resistance as a function of temperature were observed near the nominal ferromagnetic transition, T& =250 K, also apparently due to clustering of dissolved H, coupled to the ferromagnetism. The unidirectional anisotropy of the lowtemperature reentrant spin-glass magnetic regime was detected via magnetoresistance in mesoscopic samples with hydrogen removed. However, mesoscopic samples exhibiting such anisotropy still showed no detectable spin-glass resistance noise, unlike larger samples. INTRODUCTIONThe phenomenological magnetic phase diagram of amorphous Fe&00 "Zr" for x in the range of 6 -15, resernbles that predicted for the mean-field solution of a model of an infinite-range Heisenberg spin glass, including reentrance from a (more-or-less) ferromagnetic phase to a lower-temperature phase which preserves ferromagnetic order while acquiring what appears to be some frozen transverse random structure. ' Despite the similarity between the model and the observed magnetic phase diagram, it has proved to be difficult experimentally to reach consensus regarding the most appropriate description(s) of the magnetic phase(s) of a-Fezr. In this paper we explore the magnetic structure of aFeZr films by mesoscopic resistance fluctuations, a technique that has proved effective on other disordered magnetic materials. ' We shall describe two main results.The first is an unexpected presence of large collective fluctuations in the arrangements of dissolved H. The second is an unexpected absence of noise from the reentrant spin-glass effects, in contrast to the behavior of large samples of similar films, despite clear evidence in magnetoresistivity effects of unidirectional anisotropy characteristic of spin-glasses. A more complete account of experimental details, etc. , may be found in Ref. 7. BACKGROUND

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