V. Meenakshi

Enhanced magnetic anisotropy of Mn12-acetate

Abstract Thin films of the single molecule magnet [Mn 12 O 12 (CH 3 COO) 16 (H 2 O) 4 ]·2CH 3 COOH·4H 2 O (Mn 12 -acetate) have been fabricated on a Si-substrate by the dip-and-dry method, a simple and robust technique. Atomic force microscopy and X-ray photoelectron spectroscopy characterizations reveal that homogeneous, thin films of a few molecular layers with smoothness at the molecular level are deposited. Significant changes in magnetic properties of Mn 12 -acetate exposed to the same solvent were observed in zero field-cooled and field-cooled magnetization, as well as AC-susceptibility measurements. The blocking temperature was found to increase to T B > 10 K at low magnetic fields, indicating an enhanced magnetic anisotropy.

Some unusual features in the Raman spectrum of amorphous carbon films obtained by pyrolysis of maleic anhydride

Laser micro-Raman spectroscopic measurements were done on the amorphous conducting carbon films obtained from maleic anhydride by pyrolysis process. We have found a predominant broad peak around 1140 cm−1, in addition to the normally observed peaks in amorphous carbons around 1350 and 1600 cm−1, and peak of medium intensity around 800 cm−1. Here we discuss the possibility of conjugated polymer like bond alternating structure which can give rise to these unusual Raman features.

Nanopatterning of Mn12-acetate single-molecule magnet films

We report the fabrication of the artificial nanopatterns of Mn12-acetate films using e-beam lithography. Scanning electron micrographs and atomic force micrographs of the patterns reveal the minimum lateral size (∼50nm), height, and surface morphology of the patterns. X-ray photoelectron spectroscopy data indicate the presence of Mn12-acetate in the patterned structure. The thin film material indicates magnetic properties consistent with Mn12-acetate, supporting the conclusion that the lithographic chemicals used in this study do not interfere with the core properties of Mn12-acetate. The successful fabrication of Mn12-acetate nanopatterns enables a range of possibilities for designed hybrid systems with three-dimensional positional control on the nanometer scale.

Electronic properties of ion irradiated amorphous carbon films prepared by plasma assisted CVD method

Abstract Amorphous hydrogenated carbon films were prepared by plasma assisted CVD method. Their dc conductivity was studied as a function of temperature in the range of 300 K to 10 K. Films were then subjected to high energy (170 MeV) ion irradiation. After irradiation a marked change was observed in the conductivity and its temperature dependence. The conductivity decreased by 2–3 orders of magnitude and a gap appeared in the electronic structure. UPS studies of the material show a decrease in the π states of the electronic density of states spectrum. A change in the C1s peak shape was observed in XPS study of the irradiated carbon film.

Low temperature conductivity of irradiated amorphous conducting carbon

Abstract Conducting carbon films prepared by pyrolysis of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) were subjected to high energy ion irradiation with I 13+ ions (170 MeV). Different doses in the range 10 11 –10 13 were used. XRD observation (at room temperature) and electrical conductivity measurements down to 6.3 K were carried out on these films before and after irradiation. XRD shows that the structure remained amorphous as before. The temperature dependence of conductivity showed an interesting change below about 20 K, suggesting an emergence of a mobility gap in the electronic structure.

Challenges in Patterning Mn12‐acetate Thin Films by Electron‐Beam Lithography

We investigate how electron‐beam lithography can be applied to pattern Mn12‐acetate thin films produced by a solution evaporation technique. The low temperature magnetic ac‐susceptibility data of the powder sample extracted from a film dipped into acetone indicate that some of the molecules in the film are deformed by acetone treatment concluding that acetone is not a proper chemical for the lift‐off process, a crucial step in the patterning procedure.

Magnetic Relaxation and Magnetic Moment of Mn12‐acetate Film Material

Mn12‐acetate film material was made by evaporating a diluted solution of Mn12‐acetate powder in acetonitrile. Relaxation of the magnetization was measured at low temperature on the Mn12‐acetate film material, and slow relaxation was observed up to 5.5 K. A reduced magnetic moment of the film material was observed by applying a high magnetic field. These facts indicate an increased blocking temperature (TB) of the film material with reduced magnetic moments compared with the as‐produced Mn12‐acetate.

Structure of highly conducting amorphous carbon

We report on neutron diffraction study of a new form of conducting amorphous carbon up to Q similar to 14.5 Angstrom(-1). The bond distances from first two peaks in g(r) are 1.45 and 2.49 Angstrom, very similar to those in sputtered truly amorphous carbon films (Li and Lannin, Phys. Rev. Lett. 65 (1990) 1905). The first coordination number is 3.1 (+/- 0.1) indicating predominantly sp(2) hybridisation (ideal no. = 3). However, S(Q) itself shows vestiges of (0 0 2), (1 0) and (1 1) peaks, typical of glassy carbon (Mildner, J. Non-Cryst. Solids 47 (1982) 391)