Spin Chain Ferrimagnetism with Competing Exchange Interactions

A quantum spin model of a delta chain with exchange interactions of ferro- (F) and antiferromagnetic (AF) types is studied. The model properties significantly depend on the frustration parameter α (the ratio of the constants of AF and F interactions). When α < 1/2, the ground state is ferromagnetic, and at α = 1/2, a quantum phase transition occurs to another phase. Based on the numerical calculations and analysis of the model at large values of α it is concluded that the ground state at α > 1/2 is magnetically ordered and the total spin of the ground state is half the maximum possible spin. Comparison of the properties of the ground state of the classical and quantum models shows that the considered model is an example of a system in which quantum effects are responsible for the appearance of order from disorder.