Quasispins of vacancy defects in Ising chains with nearest- and next-to-nearest-neighbour interactions

TL;DR

This paper investigates how vacancy defects influence the magnetic susceptibility of 1D Ising chains with competing interactions, revealing the conditions under which vacancies act as free quasispins or mainly reduce chain length.

Contribution

It introduces the concept of quasispins in Ising chains with NN and NNN interactions, analyzing their behavior and impact on magnetic properties in the presence of vacancies.

Findings

Quasispins act as free spins with Curie-like susceptibility in certain regimes.

Vacancy effects depend on the nature of short-range order and interaction signs.

Quasispin contributions dominate in antiferromagnetic short-range order regimes.

Abstract

Motivated by frustrated magnets and quasi-one-dimensional magnetic materials, we study the magnetic properties of 1D Ising chains with nearest-neighbour (NN) and weaker next-to-nearest neighbour (NNN) interactions in the presence of vacancy defects. The effect of a vacancy on the magnetic susceptibility of a spin chain is two-fold: it reduces the length of the chain by an effective ``vacancy size'' and may also act as a free spin, a ``quasispin'', with a Curie-type $\chi_\text{quasi}=\langle S^2\rangle/T$ contribution to the susceptibility. In chains with antiferromagnetic short-range order, the susceptibility of vacancy-free chains is exponentially suppressed at low temperatures, and quasispins dominate the effect of impurities on the chains' magnetic properties. For chains with antiferromagnetic NN interactions, the quasispin matches the value $\langle S^2\rangle=1$ of the Ising spins in the chain for ferromagnetic NNN interactions and vanishes for antiferromagnetic NNN interactions. For chains with ferromagnetic short-range order, quasispin effects are insignificant due to exponentially large low-temperature susceptibilities, and the dominant effect of a vacancy is effectively changing the length of the chain.