13ª Semana Integrada do Instituto de Física de São Carlos

Magnetization processes in frustrated magnets

21 de ago. de 2023 09:00

1h 30m

Salão de Eventos USP

Prêmio YPM 9h00 - 10h30

Descrição

Frustrated magnets often display novel states, ranging from ordered states induced by fluctuation to disordered quantum spin liquids states. One remarkable effect of frustrated magnets in the presence of an external field is the stabilization of a magnetization plateau. This is a semiclassical effect, in contrast to the quantum hall effect, for example, and is observed experimentally in several triangular lattice compounds. Here we delve into the richness of this phenomenon, beginning with the study of the classical limit of the antiferromagnetic Heisenberg model in a triangular lattice. This model shows a massive accidental degeneracy in the ground state when there is an external magnetic field. We then introduce thermal fluctuations employing a real space perturbation theory, which lifts the degeneracy via an order-by-disorder mechanism. (1) In particular, this generates an effective biquadratic exchange term in the Hamiltonian. By minimizing the new Hamiltonian, we can determine the phases of the system in each interval of the magnetic field, and the phase transitions are studied in detail. Once the classical limit is understood, we use spin-wave theory (2) to move to the quantum case. The method consists of a $1/S$ series expansion $-$ where $S$ is the spin size $-$ with the starting point corresponding to the classical ground state. Because of the noncollinearity of the classical spin configuration, rotations to the local axis in each site are introduced. (3) Truncating the series in first order, we can qualitatively explain the appearance of the magnetization plateau. Magnetization curves are obtained for different values of $S$, demonstrating that the plateau width is a function of $S$. This approach illustrates how some insightful ideas can be developed in understanding the characteristic plateau in frustrated systems.

Referências

1 MARYASIN, V. Order by structural disorder and field effects in frutrated systems. 2015. 122 p. Thesis (PhD in Physique Theorique) - École doctorale de Physique, Universitè Grenoble Alpes, Alpes, 2015.

2 ZHITOMIRSKY, M. E.; NIKUNI, T. Magnetization curve of a square lattice Heisenberg antiferromagnet. Physical Review B, v. 57, n. 9, p. 5013-5016, 1998.

3 COLETTA, T. et al. Semiclassical theory of the magnetization process of the triangular lattice Heisenberg model. Physical Review B, v. 94, n. 7, p. 075136-1-075136-10, 2016.