• PDF / 1,100,966 Bytes
• 13 Pages / 593.28 x 841.68 pts Page_size
• 2 Downloads / 262 Views

DOWNLOAD

REPORT

---

The structural and magnetic properties of the stage 2 CocNii_cCl2- and CocFei_cCl2-graphite intercalation compounds (GICs) for 0 ^ c «S 1 have been studied by x-ray scattering and dc magnetic susceptibility. The stage 2 CocNi!_cCl2-GICs approximate two-dimensional randomly-mixed ferromagnets with XY spin symmetry. The average effective magnetic moment Peii, the Curie-Weiss temperature 0, and the paramagnetic-to-ferromagnetic phase transition temperature Tc have been determined as continuously varying functions of Co concentration c. They indicate that the Co2+ and Niz+ spins are randomly distributed on the triangular lattice sites of each intercalate layer. They also show that the intraplanar exchange interaction /(Co-Ni) between the Co2+ and Ni2+ spins is enhanced and is larger than the interaction /(Co-Co) between two Co2+ spins and /(Ni-Ni) between two Ni2+ spins. This enhanced interaction, /(Co-Ni), can be expressed as /(Co-Ni) = 1.28 [/(Co-Co) • /(Ni-Ni)]1/2. The stage 2 CocFei-cCl2-GICs approximate two-dimensional randomly mixed ferromagnets with competing spin anisotropy. The dc magnetic susceptibility results suggest that Co2+, Fe3+ rather than Fe2+ are distributed in the intercalate layer. The repeat distance along the c-axis (^-spacing) versus Co concentration deviates from Vegard's law which states that therf-spacingis proportional to Co concentration. The broad peak ofrf-spacingobserved at c = 0.75 is discussed in terms of the double layer model developed by Jin and Mahanti.

I. INTRODUCTION

In recent years, the graphite intercalation compounds (GICs) have received considerable attention from both chemists and physicists. The GICs form a class of materials with a dimensionality that can be systematically controlled by the stage number N. These compounds provide ideal model systems for systematically studying the cross-over physics from three-dimensional (3D) to two-dimensional (2D). Recent studies of the 2D physics of GIC have been reviewed by Dresselhaus1"3 and Zabel and Chow.4 The GICs also offer an opportunity to synthesize exotic compounds and to explore their novel physical properties. Of particular interest among these compounds is the ternary GIC, where two distinct intercalant species form the intercalate layers. Recent studies of ternary GICs have been reviewed extensibly by Herold et al.5 and Solin and Zabel.6 Here, we will restrict ourselves to the magnetic ternary GICs such as the graphite bi-intercalation compound (GBIC) and the random mixture graphite intercalation compound (RMGIC). The c-axis stacking sequence of the magnetic GBIC is restricted to one species of intercalant per gallery, separated by one or more graphite layers. An example is the stacking sequence ...GIiGI2..., where two different mag422

http://journals.cambridge.org

J. Mater. Res., Vol. 5, No. 2, Feb 1990

Downloaded: 14 Mar 2015

netic intercalate layers (/i and I2) alternate with graphite layers (G) and both intercalants are of the acceptor type and may be compounds such as NiCl2, CoCl2, and FeCl3. These compounds fo