Theory of the helical spin crystal: a candidate for the partially ordered state of MnSi

TL;DR

This paper proposes a helical spin crystal model, especially a bcc structure, to explain the partially ordered magnetic state of MnSi under pressure, supported by theoretical analysis and experimental data comparison.

Contribution

It introduces a novel helical spin crystal framework, particularly a bcc structure, as a candidate for MnSi's partially ordered state, with a Landau theory analysis.

Findings

The bcc helical spin crystal matches neutron scattering data.

Symmetry-protected line nodes are identified in the proposed structure.

The model's predictions align with magnetic field and disorder effects observed in MnSi.

Abstract

MnSi is an itinerant magnet which at low temperatures develops a helical spin density wave. Under pressure it undergoes a transition into an unusual partially ordered state whose nature is debated. Here we propose that the helical spin crystal (the magnetic analog of a solid) is a useful starting point to understand partial order in MnSi. We consider different helical spin crystals and determine conditions under which they may be energetically favored. The most promising candidate has bcc structure and is reminiscent of the blue phase of liquid-crystals in that it has line-nodes of magnetization protected by symmetry. We introduce a Landau theory to study the properties of these states, in particular the effect of crystal anisotropy, magnetic field and disorder. These results compare favorably with existing data on MnSi from neutron scattering and magnetic field studies. Future experiments to test this scenario are also proposed.