Scaling Theory for Critical Phenomena in Coupled Magnetic Systems: Critical Exponents of πd Antiferromagnets

Shimahara Hiroshi

Journal of the Physical Society of Japan

The scaling theory for critical phenomena is extended to coupled magnetic systems that consist of two subsystems, and some extended relations of critical exponents are derived. It is shown that the extended theory practically reduces to the conventional scaling theory in ferromagnets and in non-ferromagnetic systems with β1 = β2; however, the extended form of the theory can be relevant otherwise, where β1 and β2 are exponents of the order parameters in subsystems 1 and 2, respectively. The theory is applied to a model of the organic πd antiferromagnet λ-(BETS)2FeCl4, which contains π- and d-spin subsystems, where BETS stands for bis(ethylenedithio)tetraselenafulvalene. It is shown that an effective Hamiltonian for the π spins is reduced to the two-dimensional Ising model in the vicinity of the critical temperature Tc. This supports a conjecture from a recent experimental observation. Consequently, β1 = 1/8 is obtained, where subsystems 1 and 2 correspond to the π- and d-spin systems, respectively. Additional relations α = 1 − β1 − β2 and β1 = β2 ≡ β are derived from specific features of the λ-(BETS)2FeCl4 system. These relations result in α = 1 − 2β, which was previously obtained in a free-energy functional model. Critical exponents below Tc are obtained as α = 3/4, β = 1/8, γ = 1, δ = 9, ψ = 1/5, and ν = 5/8. The value of α is close to a recent experimental result of α = 0.77 in λ-(BETS)2FeCl4.

eng

The Physical Society of Japan

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[ISSN] 0031-9015

[DOI] 10.7566/JPSJ.89.043001

[DOI] https://doi.org/10.7566/JPSJ.89.043001