Magnetic structure and magnetization of helical antiferromagnets in high magnetic fields perpendicular to the helix axis at zero temperature

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 < 4π/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 4π/9 ≤ kd ≤ π, 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 transitions are found for kd = 3π/5 and 8π/11 and a second-order transition for kd = 3π/4. At a critical field Hc, the fan or fanlike structures exhibit a second-order transition to the paramagnetic state. The Hc for a helix undergoing a field-induced change to a fan or fanlike structure is found to be the same as for a sinusoidal fan with the same kd and interlayer interactions. Analytical expressions for Hc versus kd are presented. We also calculated the average x-axis moment per spin µxave versus Hx for helices and fans with crossovers and phase transitions between them. When smooth helix to fanlike crossovers occur in the range 4π/9 ≤ kd ≤ π, µxave exhibits an S-shape behavior with increasing Hx. This predicted behavior is consistent with µxave(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 ≈ 0.85π. The low-field magnetic susceptibility and the ratio Ht/Hc are calculated analytically or numerically versus kd for helices, and are shown to approach the respective known limits for kd → 0.

Helical antiferromagnetic ordering in EuNi1.95As2 single crystals

“…3(a) and 3(b), respectively. Metamagnetic transitions are seen to occur in χ ab (H, T ) for H ≥ 3 T, as might be expected from the helical magnetic structure [52,53]. One sees that χ c (T ) in Fig.…”

“…Theoretical studies of classical field-induced xy-plane (ab plane here) metamagnetic transitions in z-axis helices (c axis here) at T = 0 were presented by one of us for moments confined to the xy plane (infinite XY anisotropy) in Ref. [52] and more recently for finite XY anisotropy in Ref. [53] where the moments can flop from the xy plane into a three-dimensional spin-flop arrangement on one and/or two spherical ellipses.…”

Magnetic structure and magnetization ofz-axis helical Heisenberg antiferromagnets with XY anisotropy in high magnetic fields transverse to the helix axis at zero temperature

“…x → h * * − c , the stable phase for all values of kd is a single fan in the xy plane, which was studied in detail in Ref. [8]. The approximate values of h * * c versus kd listed in Figs.…”

“…The phase boundary between the xy helix phase and the xy fan phase of the helix when it occurs was determined previously in Ref. [8], where the energies of the xy helix and higher-field xy fan phases were determined versus h * * x in Ref. [8].…”

“…Previously, the magnetic structure and magnetization of a planar helical antiferromagnet in a high applied magnetic fields H perpendicular to the helix wave vector axis (z axis) at temperature T = 0 was calculated where the ordered magnetic moments were restricted to lie in the xy plane [7,8]. This is the plane in which the ordered moments reside in zero field as shown in Fig.…”