Thermodynamics of a two-dimensional frustrated spin-1/2 Heisenberg ferromagnet

Using the spin-rotation-invariant Green's function method we calculate the thermodynamic quantities (correlation functions <S_0 S_R>, uniform static spin susceptibility \chi, correlation length \xi, and specific heat C_V) of the two-dimensional spin-1/2 J1-J2 Heisenberg ferromagnet for J2 < J2^c \approx 0.44|J1|, where J2^c is the critical frustrating antiferromagnetic next-nearest neighbor coupling at which the ferromagnetic ground state gives way for a ground-state phase with zero magnetization. Examining the low-temperature behavior of \chi and \xi, in the limit T \to 0 both quantities diverge exponentially, i.e., \chi \propto \exp(b/T) and \xi \propto\exp(b/2T), respectively. We find a linear decrease of the coefficient b with increasing frustration according to b=-(\pi/2)(J1+2J2), i.e., the exponential divergence of \chi and \xi is present up to J2^c. Furthermore, we find an additional low-temperature maximum in the specific heat when approaching the critical point, J2 \to J2^c.