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

Abstract

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...

Superconductivity and unusual magnetic behavior in amorphous carbon

“…A growing number of carbon-based materials (CBM) have been found to exhibit ferromagnetic (FM) or a FM-like behavior, which may be attributed to the mixture of carbon atoms with sp 2 and sp 3 bonds as discussed above. [7][8][9] It is still unclear, which are the most crucial factors that stabilize the FM-like state. The experimental evidences accumulated so far indicate that structural disorder, topological defects produced by proton irradiation, [9] or alternatively graphitic fragments with positive or negative curvatures, [10] as well as adsorbed foreign atoms such as hydrogen [11] and/or oxygen [12] may be the FM origin for FM in CBM.…”

Raman microspectroscopy of melanosomes: the effect of long term light irradiation

“…Also a contribution due to stretching C¼O vibration is probable at this wavelength [26]. The main contribution to the peak at 1390 cm À1 comes from the disorder activated Raman mode similar to the A 1g mode in graphitic domains ( Figure 3) [27]. The other peak at 1450 cm…”

Magnetoelectronic properties of Gd-implanted tetrahedral amorphous carbon

“…and a-Gd x Ge 1-x , where a monotonic increase in σ RT was found, passing smoothly through the insulator to metal transition, for x up to 0.25 (above which TEM showed signs of Gd clustering), but similar to what was observed in sputtered a-Gd x C 1-x films where the non-monotonic dependence was attributed to a competition between increasing dopant concentration and increasing modifications to the C-matrix. 7 However, unlike sputtered a-Gd x C 1-x films, no correlation between the I D /I G ratio and σ RT as a function of x is found. This non-monotonic conductivity dependence on x will be further discussed below.…”

“…More sample information is displayed in Table I, and more experimental details are available in Ref. 7 .…”

“…Instead, by analogy to T' above, for all the ta-C 1-x :Gd x films in this study, as well as in the previous work on sputtered a-GdC 1-x , 7 we define T' as the temperature at which MR switches from positive to negative in a 70 kOe field; Fig. 9 inset showed an example.…”