Characteristic temperature in magnetically doped amorphous semiconductors

Abstract

The introduction of magnetic moments such as Gd into amorphous Si produces dramatic effects in electrical transport below a characteristic temperature T * . Below T * , the conductivity of the magnetically doped systems is strongly suppressed compared to equivalent nonmagnetic Y doped samples, and displays enormous negative magnetoresistance. T * occurs at relatively high temperatures ͑ϳ10-100 K͒ and decreases sharply with increasing Gd concentration, passing smoothly through the metal-insulator transition. In ternary samples with both Gd and nonmagnetic Y, T * decreases strongly with increasing metallization, whether due to the addition of Gd alone or a mixture of Gd and Y. These results cannot be explained by simple magnetic interaction models, suggest the crucial role of electron screening and are reminiscent of mass enhancement behavior.Amorphous metal-semiconductor alloys ͑a-M-Si͒ offer unique insight into the metal insulator transition ͑MIT͒ as a comparison to doped crystalline materials. It has been widely documented that amorphous alloys undergo a MIT with identical low temperature behavior but at much greater dopant concentration compared to their crystalline counterparts due to significant additional disorder. The doping of local magnetic moments into semiconductors near the MIT causes dramatic effects in the magnetic and transport properties, including enormous negative magnetoresistance, fielddependent anomalous ͑nonspectral weight conserving͒ optical conductivity, and a magnetic susceptibility with a near-Curie law temperature dependence but a nonmonotonic dependence on composition, including a large peak at the MIT. 1-3 The enormous magnetic field dependence has allowed measurements of scaling behavior continuously through the 3D MIT on a single sample, including tunneling determination of the electron density of states. 4,5 Strong similarities exist between the a-M-Si systems studied here and both dilute magnetic semiconductor systems ͑DMS͒, such as ͑Ga,Mn͒As and the perovskite manganites. In all these systems, there are indications of strong coupling of electrical conductivity, magnetic properties, and even the structural or lattice system, suggesting the possibility of an all-encompassing theoretical description. Distinct differences in our system, e.g., the strong disorder, and magnetic moments from f rather than d-shell electrons, offer unique insights into the underlying physics. In these systems the separate control of electron and moment concentrations is critical to understanding the underlying physics.While there has been some success in describing the low temperature properties of amorphous doped semiconductors on both the metallic and insulating sides of the MIT, including the magnetically doped semiconductors, 6-8 a strikingly unresolved question is the nature of the higher temperature behavior where the effects upon the charge carriers due to the magnetic dopants "turns on." This temperature, which we call T * , is clearly seen in a sharp decrease of dc conductivity dc ͑T͒ for the magn...

Magnetoelectronic properties of Gd-implanted tetrahedral amorphous carbon

“…This increase is in stark contrast to the decreasing T* (and T') with increasing x seen in a-Gd x Si 1-x and a-Gd x Ge 1-x , as well as ternary a-Gd x Y y Si 1-x alloys for constant x, and increasing y, which was interpreted as a consequence of increased screening of electron-moment interactions with the increasing metallicity associated with increasing x. 21 The dependence on x seen in Fig. 11 suggests that screening plays little or no role in the T-dependence of MR for ta-C 1-x :Gd x and sputtered a-Gd x C 1-x , unlike in a-Gd x Si 1-x and a-Gd x Ge 1-x .…”

“…10 -3 , 1% or 10%) OR by comparing zero-field σ(T) to σ(T) for non-magnetically doped analogues a-Y x Si 1-x or a-Y x Ge 1-x . 6,21 The latter method is very effective for metallic samples (x>0.14 in a-Gd x Si 1-x and a-Gd x Ge 1-x ) because it is a zero field measurement and provides an unambiguous characteristic temperature which we defined as T*, but is ineffective for insulating samples, briefly because the temperature dependence is not a simple power law and the dependence on x is far too strong to allow simple comparison to nonmagnetic analogs. The former method, based on magnetoresistance, gives a temperature which we will here call T'; T' depends on the magnitude chosen for the cutoff MR, but since MR vanishes exponentially with increasing temperature in both a-Gd x Ge 1-x and a-Gd x Si 1-x , the choice is not significantly important.…”

Tracking spin-glass barriers versus field and temperature ina-Gd0.19Si0.81

“…13,14 Previ-ous studies [13][14][15] showed that for 0.04Ͻ x Ͻ 0.19 such films are classical spin glasses with sharp cusps in susceptibility ͓at about T = 6.5 K for x = 0.19 ͑Ref. 13͔͒ driven by mixed ferromagnetic and antiferromagnetic Gd-Gd interactions mediated by conduction electrons.…”

Effects of annealing on amorphous GdxSi1−x near the metal-insulator transition

“…The introduction of transition metal elements such as Nb, Mo, or Fe into amorphous semiconductors such as Si leads to increased conductivity and a metal-insulator transition ͑MIT͒ for metal concentrations around 10-15 at. 6 Amorphous Si alloys have been studied after thermal treatment and show the formation of nanometer-sized grains. 1 Introduction of the magnetic element Gd has similarly been shown to lead to a MIT for amorphous Gd-Si alloys ͑a-Gd x Si 1−x ͒ with x = 14.0 at.…”