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

Liu, M.; Hellman, F.

doi:10.1103/physrevb.67.054401

Cited by 10 publications

(5 citation statements)

References 17 publications

(24 reference statements)

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“…5) as is found in other doped disordered semiconductors (Ref. 17,18,19). At low T , all samples show insulating behavior, with vanishing σ DC as T →0 K. The transport mechanism is that of the variable-range-hopping (VRH) type:…”

Section: Physical Properties a DC Transportsupporting

confidence: 56%

Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

et al. 2007

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The magnetic rare earth element gadolinium (Gd) was doped into thin films of amorphous carbon (hydrogenated a-C:H, or hydrogen-free a-C) using magnetron co-sputtering. The Gd acted as a magnetic as well as an electrical dopant, resulting in an enormous negative magnetoresistance below a temperature (T ′ ). Hydrogen was introduced to control the amorphous carbon bonding structure.High-resolution electron microscopy, ion-beam analysis and Raman spectroscopy were used to characterize the influence of Gd doping on the a-Gd x C 1−x (:H y ) film morphology, composition, density and bonding. The films were largely amorphous and homogeneous up to x=22.0 at.%. As the Gd doping increased, the sp 2 -bonded carbon atoms evolved from carbon chains to 6-member graphitic rings. Incorporation of H opened up the graphitic rings and stabilized a sp 2 -rich carbonchain random network. The transport properties not only depended on Gd doping, but were also very sensitive to the sp 2 ordering. Magnetic properties, such as the spin-glass freezing temperature and susceptibility, scaled with the Gd concentration.Amorphous carbon (a-C) thin films are amorphous semiconductors with a tunable band gap. Unlike other group IV amorphous semiconductors, such as a-Si or a-Ge, which have a tetrahedral sp 3 -bonded random-network matrix, a typical a-C film has a mixture of sp 2 and sp 3 bonding. Changing the sp 2 /sp 3 ratio tunes the band gap of a-C between that of diamond (100% sp 3 , band gap E g = 5.4eV ) and graphite (100% sp 2 , semimetal with E g = 0). Studies of a-C, diamond-like materials and carbon nanotubes have long focused on the superior mechanical properties. However, recent developments in novel electronic and spintronic materials have drawn attention to the electrical, optical and magneto-electronic properties [1]. Room-temperature positive magnetoresistance (MR∼60%) was found in some magnetically-doped carbon thin films such as Co x C 1−x [2] and Ni x C 1−x [3], with granular magnetic particles in an amorphous carbon matrix. Room-temperature positive MR was also reported for non-magnetically-doped (B-doped) polycrystalline diamond thin films (MR∼100%) [4, 5] and undoped a-C/n-Si (MR∼12%) two-layer structures [6]. These results suggest potential technological applications of carbon-based thin films for electronics and spintronics applications.In this work, we have co-sputtered a-C and the magnetic rare earth element Gd (a-Gd x C 1−x (:H y )) with a wide range of x, and studied the magnetic and magneto-transport properties. Gd has a half-filled f electron shell and is trivalent in solids, which gives rise to a large local moment with J =S =7/2 and three valence electrons. Previous studies on Gd-doped e-beam co-evaporated amorphous Si have shown strong interactions between the magnetic moments and carriers leading to very large negative MR (e.g. 10 5 at 1 K), anomalous magneto-optical properties and spin-glass freezing [Refs 7,8,9,10]. The magnitude of this temperature-dependent negative MR below a characteristic temperature (T * ) w...

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Section: Physical Properties a DC Transportsupporting

confidence: 56%

Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

et al. 2007

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“…7 Previous work has investigated the effect of substituting a different rare earth ion ͑Tb͒ for Gd in a-Si. 11 This system has a large negative MR, similar to but slightly smaller than a-Gd-Si with more complex magnetic behavior due to the nonzero orbital moment of Tb, which results in strong randomly oriented local magnetic anisotropy.…”

Section: Introductionmentioning

confidence: 90%

Magnetic and transport properties of amorphousGdxGe1−xalloys near the metal-insulator transition

et al. 2007

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The temperature and field dependence of magnetization and conductivity of amorphous Ge doped with Gd ͑a-Gd x Ge 1−x ͒ has been measured for a wide range of x ͑0.08Ͻ x Ͻ 0.25͒ near the metal-insulator transition. Magnetization and magnetic susceptibility measurements show strong magnetic interactions and a low temperature spin-glass freezing. High field magnetization and susceptibility per Gd atom in the paramagnetic state are significantly suppressed below that of noninteracting Gd, as observed previously for a-Gd-Si alloys. However, unlike a-Gd-Si, the low field susceptibility does not fit a Curie-Weiss law and shows no significant dependence on composition. Conductivity measurements show that Gd causes localization of charge carriers below a characteristic temperature T * , which also marks the onset of significant negative magnetoresistance. Both T * and the magnitude of the MR are significantly lower in a-Gd-Ge than in comparable a-Gd-Si alloys. It is proposed that the large effects of the host matrix ͑Ge vs Si͒ are due to differences in both the band gap and dielectric constant, which cause changes in screening, thereby altering the effect of Gd magnetic moments on the localization of carriers and on the indirect mediated Gd-Gd exchange interactions.

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“…Further details on sample preparation and characterization can be found in the literature. 9 DC conductivity data from room temperature to as low as 300 mK for some measurements were taken using a routine four probe technique. Figure 1͑a͒ shows dc ͑T͒ for several metallic a-Gd-Si samples and data for the critical concentration, x c Х 14 at.…”

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Characteristic temperature in magnetically doped amorphous semiconductors

et al. 2005

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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...

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

Cited by 10 publications

References 17 publications

Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

Microstructure, magnetotransport, and magnetic properties of Gd-doped amorphous carbon

Magnetic and transport properties of amorphousGdxGe1−xalloys near the metal-insulator transition

Characteristic temperature in magnetically doped amorphous semiconductors

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