Charles R. Burr

Magnetic susceptibility of CoCl2−cBrc

Abstract A bond-random spin system CoCl2−cBrc(0 ≤ c ≤2) undergoes an antiferromagnetic phase transition at a Neel temperature TN, below which the 2D ferromagnetic Co2+ layers are antiferromagnetically stacked along the c-axis. The d.c. magnetic susceptibility shows a sharp peak just above TN. The value of TN linearly decreases with increasing Br concentration c from 24.5 K (c = 0) to 18.9 K (c = 2). The susceptibility obeys the Curie-Weiss law in the temperature range between 150 and 300 K. The Curie-Weiss temperature θ linearly decreases as c increases from 40.63 K (c = 0) to 8.37 K (c = 2). The ratio T N θ monotonically increases with increasing c, and becomes larger than 1 for c > 1, contrary to the prediction from the molecular field theory. The cause for T N θ > 1 for c > 1 is partly due to antiferromagnetic interplanar exchange interaction whose magnitude increases with increasing c as a result of a possible change in structure from CdCl2 type to CdI2 type.

Magnetic properties of stage 2 CocNi1-cCl2 graphite intercalation compounds

Magnetic properties of stage 2 CocNi1-cCl2 graphite intercalation compounds (GICs) for 0<or=c<or=1 have been studied by DC magnetic susceptibility. These compounds can be considered to approximate two-dimensional randomly mixed ferromagnets with XY spin symmetry. As a function of Co concentration c the average effective magnetic moment Peff, and the Curie-Weiss temperature Theta have been determined. The measurements indicate that Co2+ and Ni2+ spins are distributed randomly on triangular lattice sites of each intercalate layer. It is found that the intraplanar exchange interaction J(Co-Ni) between the different spins is larger than that between like spins, J(Co-Co) or J(Ni-Ni):J(Co-Ni):=1.2(J(Co-Co)J(Ni-Ni))12/. As a function of concentration, the susceptibility shows a rounding effect at the magnetic phase transition for a series of compounds with c=0.1, 0.19, 0.35, 0.40, 0.52, 0.63, 0.80 and 1.0. The authors have assumed a Gaussian distribution of transition temperatures to determine the average value of transition temperature (Tc), the distribution of transition temperature sigma and the critical exponent beta . It appears that the critical exponent beta is unchanged for 0.35<or=c<or=1.0: beta =0.09+or-0.01. They obtain for the initial slope of (Tc) versus c at c=0 the value -1.21+or-0.02. The ratio sigma /(Tc) shows a broad peak centred at c=0.5 which is explained here qualitatively by both finite size effect and macroscopic Co concentration gradient within the intercalate layer.

Magnetic Properties of Random-Mixture Graphite Intercalation Compounds

Abstract Random-mixture GICs such as stage-2 NicMn1-cCl2 GICs and stage-2 CocMn1-cCl2 GICs provide model systems for studying 2D random spin systems with spin frustration effects. The magnetic properties of these compounds have been studied by dc and ac magnetic susceptibility, and low field SQUID magnetization measurements. An irreversible effect of magnetization for stage-2 NicMni1-cCl2 GICs (0.8 ≤ c ≤ 1) and stage-2 CocNi1-cCl2 GICs (0 ≤ c ≤ 1) indicates an occurrence of cluster glass phase below a critical temperature where the spin directions of ferromagnetic clusters are frozen because of frustrated interisland interactions.

Structural and magnetic properties of stage 2 CocMn1−cCl2‐graphite intercalation compounds

Stage 2 CocMn1−cCl2‐graphite intercalation compounds (0≤c≤1) approximate two‐dimensional site‐random XY spin systems with a competition between ferromagnetic [J(Co–Co) and J(Co–Mn)], and antiferromagnetic exchange interactions [J(Mn–Mn)]. The magnetic phase diagram and critical behavior of these compounds are studied by means of ac and dc magnetic susceptibility. For c≥ (R18)0.45 the phase transition occurs between the paramagnetic phase and the ferromagnetic phase. The critical exponents of γ and β do not change with dilution of Mn atoms: γ=2.31 and β=0.085 for c=0.90.

Magnetic properties of random-mixture graphite intercalation compounds

The magnetic properties of a quasi‐two‐dimensional mixed ferromagnetic and antiferromagnetic system, stage‐2 CocMn1−cCl2 ‐graphite intercalation compounds (GICs), have been studied by dc magnetic susceptibility measurements. The Curie–Weiss temperature determined as a function of Co concentration indicates that the intraplanar exchange interaction between Co2+ and Mn2+ is ferromagnetic and that J(Co‐Mn)=1.3 [J(Co‐Co)‖J(Mn‐Mn)‖]1/2. With decreasing Co concentration c, the transition temperature tends to decrease to zero as c approaches 0.5 (the percolation concentration for the triangular lattice sites), but is not reduced to zero even below c=0.5. There may be a possibility of a spin‐glass phase for 0≤c<0.4 arising from the competition between J(Mn‐Mn), J(Co‐Mn), and J(Mn‐Mn).

Hysteresis and time dependence of the magnetic properties of a dilute MgYb alloy

Abstract Measurements of the magnetic susceptibility of a dilute MgYb alloy show that the Yb ions are diamagnetic at room temperature but become paramagnetic at low temperature. In addition to this configuration crossover of the Yb ions, hysteresis and time dependence of the magnetic susceptibility were observed.

Magnetic Properties of Graphite Biintercalation Compounds

Abstract The magnetic properties of CocMn1-cCl2-FeCl3 GBICs have been studied by using dc magnetic susceptibility and low and high field SQUID magnetization measurements. The effect of biintercalation on the magnetic properties is discussed in comparing the experimental results of CocMn1-cCl2-FeCl3 GBICs with those of stage-2 CocMn1-cCl2 GICs.

Percolation Behavior in CocMg1-cCl2 and Stage-2 CocMg1-cCl2 Graphite Intercalation Compounds

Abstract The magnetic phase transition of bulk CocMg1-cCl2 and stage-2 CocMg1-cCl2 GIC has been studied by dc and ac magnetic susceptibility, and low field SQUID magnetization measurements. The critical temperature decreases with the dilution of nonmagnetic Mg2+ ions and reduces to zero at the percolation threshold cp: cp ≈ 0.5 for stage-2 CocMg1-cCl2 GIC and Cp ≈ 0.36 for CocMg1-CC12. A possible one-dimensional character of stage-2 CocMg1-cCl2 GIC near c = cp and the irreversible effect of magnetization below the critical temperature for stage-2 CocMg1-cCl2 GIC (0.74 ≤ c ≤ 1) are discussed.