Giant magnetoresistance in amorphous magnetic rare-earth silicon alloys (abstract)

Abstract

We explore the influence of local magnetic moments on transport and magnetic properties in a new class of diluted magnetic semiconductors, amorphous RExSi1−x (RE=Gd, or Tb, 0.1<x<0.15). As a comparison, nonmagnetic a-YxSi1−x was also studied. The Gd, Tb, and Y ions have similar size and electronic structure, except for the 4f electrons in Gd and Tb. For 50 K<T<300 K, the conductivity σ rises linearly with increasing T with nearly identical slopes for these three alloys. Below 50 K, they have dramatically different conductivities. a-YxSi1−x shows the usual metal-to-insulator transition observed in doped semiconductors as the Y content is lowered. For a-GdxSi1−x and a-TbxSi1−x, however, σ rapidly decreases such that compositions that are metallic in a-YxSi1−x are insulating for a-GdxSi1−x and a-TbxSi1−x. Applying a magnetic field H has the usual relatively small effects on σ of a-YxSi1−x but produces a large increase in conductivity for the a-GdxSi1−x alloys. For example, in a-Gd0.12Si0.88, σ increases by 60 times at T=2.4 K and H=6 T. This giant negative magnetoresistance drops as T rises, disappearing above 50 K. Magnetization M as a function of H shows no hysteresis. M(H/T) for different fields and temperatures even for a single sample do not fit a Brillouin function, showing no paramagnetism or superparamagnetism. The magnetic susceptibility is much smaller than expected for isolated moments, indicating antiferromagnetic interactions between local moments. However, no Curie, Neel, or spin-glass freezing temperatures are observed. The magnetization and transport data are presumably the result of an interaction between nearly localized conduction electrons and the local moments of the Gd(J=7/2) and Tb (J=6) ions, i.e., a magnetic polaron forms that increases the conduction electron effective mass, causing it to localize. The amorphous nature of these alloys makes conduction significantly different than in the usual crystalline diluted magnetic semiconductors.

Magnetic and transport properties of amorphous Tb-Si alloys near the metal-insulator transition

“…Using this definition of T*, which depends on concentration x, T* does not vary significantly between Tb-Si and Gd-Si, contrary to our own suggestion in the earliest work on this subject. 1 By contrast, preliminary measurements on amorphous Gd-Ge alloys suggest a strong dependence of T* on the semiconductor. 17 To summarize, we have shown that ͑1͒ the characteristic temperature below which the conductivity is strongly affected by the local moments does not appear to depend strongly on magnetic ion ͑Tb vs Gd͒, ͑2͒ the effective moment p e f f in the amorphous Tb-Si system shows the same dependence on composition as it did in the amorphous Gd-Si system, with a strong peak at the M-I transition and significant suppression below the expected Tb 3ϩ value for both metallic and insulating samples, ͑3͒ the strengths of exchange interactions are comparable for amorphous Tb-Si and amorphous Gd-Si, in contrast to the different de Gennes factor, but are still nearly perfectly balanced ferromagnetic and antiferromagnetic, and ͑4͒ the random anisotropy of the L ϭ0 Tb ion has a strong effect on the magnetic freezing, causing a poorly defined and strongly frequency-dependent transition more reminiscent of a blocking temperature than the spin-glass freezing seen in amorphous Gd-Si.…”

“…Transport measurements on a-Gd-Si show strong localization of extended-state carriers caused by the presence of the Gd local moments and many orders of magnitude negative magnetoresistance. 1,2 This extremely large magnetoresistance ͑MR͒ has allowed tunneling spectroscopy studies of the electron density of states through the three-dimensional M-I transition on a single sample for the first time. 3 The localizing effects of the Gd local moments and the negative magnetoresistance occur below a characteristic temperature T* which is of order 50-100 K. Magnetic studies show strong balanced ferromagnetic and antiferromagnetic exchange interactions which suppress the magnetization well below the noninteracting Brillouin function and lead to a spin-glass freezing at temperatures below 10 K ͑dependent on composition͒.…”

XAFS study of local disorder in thea−GdxSi1−xamorphous magnetic semiconductor

“…1,3 Attempts to include Gd atoms near the first coordination shell distance of Ϸ2.4 Å resulted in significantly poorer fits and a fitted Gd coordination of 0Ϯ0.5. The Si-Gd correlations occur at the much longer distance of Ϸ3.0 Å ͑beyond the first shell͒ as determined for Gd-Si in the Gd XAFS analysis.…”

“…2,3 Strong frustrated magnetic interactions between Gd moments lead to a spin glass freezing at low temperatures (Շ10 K) and suppression of the magnetization below the noninteracting Brillouin function. 1 The magnetic susceptibility in the paramagnetic state shows a strong peak at the composition of the zero-field insulator-metal transition, indicative of polarization of carriers in a nontrivial way.…”