• PDF / 3,979,948 Bytes
• 6 Pages / 612 x 792 pts (letter) Page_size
• 49 Downloads / 249 Views

DOWNLOAD

REPORT

---

0998-J03-05

Positive Magnetisation in Carbon Nanoclusters Produced by High-Repetition-Rate Laser Ablation Andrei V. Rode1, Denis Arcon2, Andrej Zorko2, Zvonko Jaglicic3, Andrew G. Christy1, Nathan R. Madsen1, Barry Luther-Davies1, Desmond W. M. Lau4, and Dougal G. McCulloch4 1 The Australian National University, Canberra, 0200, Australia 2 Institute Jozef Stefan, Ljubljana, 1000, Slovenia 3 University of Ljubljana, Ljubljana, 1000, Slovenia 4 RMIT University, Melbourne, 3000, Australia

ABSTRACT Carbon nanoclusters produced by high-repetition-rate laser ablation of graphite and glassy carbon in Ar exhibits para- and ferromagnetic behaviour at low temperature. The results show that the degree of remanent order is strongly dependent on the magnetic history, i.e. whether the samples were cooled under zero-field or field conditions. Such behaviour is typical for a spin glass structure where the system can exist in many different roughly equivalent spin configurations. The spin-freezing temperature is unusually high (50 - 300 K) compared with ≤ 15 K for typical spin glasses. The maximum in the zero-field magnetic susceptibility experiments and their field dependence indicate that there is competition between ferromagnetic and antiferromagnetic exchange pathways, accounting for the spin glass behavior and/or a low-dimensionality of the system.

INTRODUCTION Recent observations of macroscopic magnetic ordering in proton irradiated graphite [1-4] and fullerenes [5,6], the discovery of ferromagnetic response from cluster-assembled carbon nanofoam [7-9] and controversial reports on ferromagnetism in polymerized C60 forms [10-12,6] throws new light on carbon-based ferromagnetism. Carbon nanofoam was synthesized by high-repetition-rate laser-ablation of a graphite or glassy carbon target in an argon atmosphere [13,14]. Electron diffraction of the nanofoams suggests the presence of hyperbolic “schwarzite” layers [79,13,14]. Schwarzite layers are saddle-shaped graphitic layers in which the negative Gaussian curvature arises from the presence of carbon rings larger than six members. The carbon nanofoam shows strong magnetization immediately after production but most of this decays over several minutes and a fraction of the magnetization is stable even on a very long time scale. A saturation magnetization of Ms= 0.42 emu/g at 1.8 K can be measured even 12 months after synthesis [7,8]. Here we present the results of a systematic study of the magnetic susceptibility and electron paramagnetic resonance (EPR) of carbon nanofoam samples produced at different Ar pressures, conducted with the aim of elucidating the nature and ordering behavior of the spin centers in the carbon nanofoam.

EXPERIMENT Sample preparation Carbon nanofoam samples were prepared using high power frequency doubled Nd:YVO4 laser operating at 532 nm, with repetition rate 1.5 MHz, pulse duration 12 ps, and focused down to a spot size ~15 µm [15]. This gave a maximum incident intensity of 7×1011 W/cm2 with corresponding fluence 8.6 J/cm2. The ablated mass per pulse