of a series of nearly uncorrelated, linear, antiferromagnetic chains along the monolayer b axis. ... antiferromagnetic order.2,3,7-9 For T> 11.9 K, a new incom-.
Thermodynamic behavior of the structures and magnetic order of O2 monolayers on graphite O. M. B. Duparc and R. D. Etters Citation: The Journal of Chemical Physics 86, 1020 (1987); doi: 10.1063/1.452336 View online: http://dx.doi.org/10.1063/1.452336 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/86/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Electronic structure and magnetic ordering of MgV 2 O 4 AIP Conf. Proc. 1512, 832 (2013); 10.1063/1.4791296 Thermodynamic properties of fluorine monolayers deposited on graphite J. Chem. Phys. 94, 5190 (1991); 10.1063/1.460702 Summary Abstract: Structure determination for the S2 monolayer phase of ethane physisorbed on graphite J. Vac. Sci. Technol. A 6, 768 (1988); 10.1116/1.575105 Epitaxial orientations of O2 monolayers on graphite by superlattice formation J. Chem. Phys. 84, 4657 (1986); 10.1063/1.449992 Predictions for partial and monolayer coverages of O2 on graphite J. Chem. Phys. 77, 1035 (1982); 10.1063/1.443915
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.104.29.2 On: Sat, 21 Nov 2015 14:19:09
Thermodynamic behavior of the structures and magnetic order of O2 monolayers on graphite8 )
o. M. B. Duparcb ) and R. D. Etters Physics Department, Colorado State University, Fort Collins, Colorado 80523 (Received 15 September 1986; accepted 3 October 1986) A Monte Carlo procedun, with continuously deformable boundary conditions is used to calculate the thermodynamic behavior of O2 monolayers on graphite. At temperatures 0 To the spins order into a quasihelical, antiferromagnetic arrangement identical to that observed 26 for the basal plane of bulk /3-02 , On this triangular lattice the six nearest neighbors to each molecule i have spins oriented at A an angle 21T/3 with respect to S;. This spin system can be described in terms of three interpenetrating rectangular sub• A A lattIces. Because of thermal fluctuations, however, (S; . ~) for nearest neighbor pairs is considerably reduced from its value of - 0.5 for a perfect quasihelical structure, and the spins become increasingly disordered with increasing temperature. The range of the spin-spin correlations also decrease with increasing temperature. These features are evident in Table I. The expectation value of the distance between the molecular centers and the substrate is (Z) = 3.73 A, and is virtually independent oftemperature.
TABLE I. The thermodynamic expectations value of the spin orientations (S, 'S,) in E and ;-02 vs temperature. The index n identifies molecules (i,j) as the first through sixth nearest neighbors. The first row represents (S, ..Sj ) for the static spin orientations of the ideal E-phase magnetic order and row eight gives the same quantities for the ;-phase quasihelical spin structure.
T(K) n= Ideal 0.1 4.0 6.0 11.0 11.5 11.8 Ideal 12.0 14.0 18.0
2
-1.00 -1.00 -0.96 -0.94 -0.81 -0.81 -0.76 -0.50 -0.32 -0.29 -0.27
1.00 1.00 0.94 0.90 0.73 0.72 0.57 1.00 0.43 0.38 0.19
3
4
1.00 -1.00 1.00 -1.00 0.96 -0.92 0.93 -0.88 0.79 -0.63 0.79 -0.64 0.69 -0.47 -0.50 -0.50 -0.22 -0.16 -0.18 -0.12 0.06 -0.08
5
6
1.00 1.00 0.94 0.89 0.71 0.71 0.56
1.00 1.00 0.92 0.87 0.52 0.58 0.44
J. Chem. Phys., Vol. 86, No.2, 15 January 1987 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.104.29.2 On: Sat, 21 Nov 2015 14:19:09
O. M. B. Duparc and R. D. Etters: O. monolayers on graphite
At temperature T> 18 K the high density structure becomes unstable and the system relaxes into the low density phase in which all the molecular axes lie parallel to the substrate plane. This instability occurs because, in the absence of an external spreading pressure, the high density phase is metastable only at temperatures below which thermal excitations breech the potential barrier between the low and high density energy minima. This barrier height is about 45 K. It is not possible to calculate properties under thermodynamic conditions identical to experiment because measurements are not constrained to constant volume or constant spreading pressure, and are often not known. Nevertheless, to examine the behavior of the high density monolayer at 18
