Magnetic Structure and Magnetization of Helical Antiferromagnets in High Magnetic Fields Perpendicular to the Helix Axis at Zero Temperature

TL;DR

This paper investigates the magnetic structures and transitions of helical antiferromagnets under high perpendicular magnetic fields at zero temperature, revealing complex behaviors including phase transitions and crossovers dependent on the helix turn angle.

Contribution

It provides a detailed theoretical analysis of magnetic moment angles, phase transitions, and magnetization behaviors in helical antiferromagnets using the classical J0-J1-J2 Heisenberg model, including comparison with experimental data.

Findings

First-order helix-to-fan transitions occur for 0 < kd < 4π/9.

Crossover behaviors with no phase transition are observed for 4π/9 < kd < π.

The model's predictions agree with experimental data on EuCo2P2.

Abstract

The zero-temperature angles of magnetic moments in a helix or sinusoidal fan confined to the xy plane, with respect to an in-plane magnetic field Hx applied perpendicular to the z axis of a helix or fan, are calculated for commensurate helices and fans with field-independent turn angles kd between moments in adjacent layers of the helix or fan using the classical J0-J1-J2 Heisenberg model. For 0 < kd < $4\pi/9$, first-order transitions from helix to a fan structure occur at fields Ht as previously inferred, where the fan is found to be approximately sinusoidal. However, for $4 \pi/9$ < kd < $\pi$, different behaviors are found depending on the value of kd and these properties vary nonmonotonically with kd. In this kd range, the change from helix to fanlike structure is usually a crossover with no phase transition between them, although first-order and second order transitions are found. We also calculated the average x-axis moment per spin $\mu_{x ave}$ versus Hx for helices and fans with crossovers and phase transitions between them. When smooth helix to fanlike crossovers occur in the range $4\pi/9$ < kd < $\pi$, $\mu_{x ave}$ exhibits an S-shape behavior with increasing Hx. This predicted behavior is consistent with $\mu_{x ave}$(Hx) data previously reported by Sangeetha, et al. [Phys. Rev. B 94, 014422 (2016)] for single-crystal EuCo2P2 possessing a helix ground state with kd = $0.85\pi$. The low-field magnetic susceptibility and the ratio Ht/Hc are calculated analytically or numerically versus kd for helices.