[Paper Review] Vital role of anisotropy in cubic chiral skyrmion hosts

M. Preißinger, Kosuke Karube+11 more|arXiv (Cornell University)|Nov 11, 2020

Magnetic properties of thin films45 references4 citations

TL;DR

This study experimentally quantifies the role of cubic magnetic anisotropy in CoZnMn-based cubic chiral magnets, revealing that anisotropy strength and character (from ⟨100⟩ to ⟨111⟩ easy axes) dramatically change with temperature and Co/Mn ratio. The enhanced anisotropy at low temperatures governs the reorientation of helical q-vectors, drives lattice distortions in metastable skyrmion lattices (from hexagonal to square or rhombohedral), and correlates with increased spin relaxation rates and anharmonicity.

ABSTRACT

The impact of magnetic anisotropy on the skyrmion lattice (SkL) state in cubic chiral magnets has been overlooked for long, partly because a semi-quantitative description of the thermodynamically stable SkL phase pocket forming near the Curie temperature could be achieved without invoking anisotropy effects. However, there has been a range of phenomena reported recently in these materials, such as the formation of low-temperature tilted conical and SkL states as well as temperature-induced transformations of lattice geometry in metastable SkL states, where anisotropy was suspected to play a key role. To settle this issue on experimental basis, we quantified the cubic anisotropy in a series of CoZnMn-type cubic chiral magnets. We found that the strength of anisotropy is highly enhanced towards low temperatures in all the compounds, moreover, not only the magnitude but also the character of cubic anisotropy drastically varies upon changing the Co/Mn ratio. We correlate these changes with temperature- and composition-induced variations of the helical modulation vectors, the anharmonicity and structural rearrangements of the metastable SkLs and the spin relaxation rates. Similar systematic studies on magnetic anisotropy may not only pave the way for a quantitative and unified description of the stable and metastable modulated spin textures in cubic chiral magnets but would also help exploring further topological spin textures in this large class of skyrmion hosts.

Motivation & Objective

• To resolve the long-ignored role of magnetic anisotropy in stabilizing and shaping skyrmion lattices (SkLs) in cubic chiral magnets.
• To investigate how anisotropy influences the orientation of helical q-vectors and the symmetry of metastable SkL states.
• To correlate changes in anisotropy with structural and dynamic properties such as spin relaxation rates and lattice anisotropy.
• To establish a unified framework linking anisotropy, SkL geometry, and spin texture evolution in metastable states.

Proposed method

• Measured magnetic anisotropy using field-dependent neutron diffraction and ferromagnetic resonance (FMR) on (Co0.5Zn0.5)20−xMnx single crystals.
• Tracked temperature and composition dependence of cubic anisotropy constants (K1, K2, K4, K6) via analysis of q-vector orientation and lattice symmetry.
• Used FMR to extract Gilbert damping (α) as a probe of spin dynamics and anharmonicity in SkL states.
• Correlated anisotropy evolution with changes in helical modulation vectors, structural distortions, and spin relaxation rates.
• Applied group theory and symmetry analysis to link anisotropy character (⟨100⟩ vs ⟨111⟩) to SkL lattice geometry (hexagonal, square, rhombohedral).
• Performed Laue diffraction and magnetization measurements to confirm sample quality and magnetic transitions.

Experimental results

Research questions

• RQ1 How does magnetic anisotropy influence the orientation of helical q-vectors in cubic chiral magnets?
• RQ2 What is the temperature and composition dependence of cubic anisotropy in (Co0.5Zn0.5)20−xMnx systems?
• RQ3 How does enhanced anisotropy at low temperatures drive the transformation of metastable skyrmion lattices from hexagonal to square or rhombohedral symmetry?
• RQ4 What is the role of anisotropy in inducing anharmonicity and spin relaxation in metastable SkL states?
• RQ5 Can Gilbert damping serve as a reliable indicator of topological spin texture distortions in skyrmion lattices?

Key findings

• Cubic anisotropy strength increases significantly toward low temperatures, with the most pronounced enhancement in Co9Zn9Mn2 and Co10Zn10.
• The character of anisotropy evolves from ⟨100⟩-type to ⟨111⟩-type easy axes with increasing Co content, particularly in Co10Zn10.
• In Co10Zn10, the metastable skyrmion lattice transforms from hexagonal to rhombohedral due to q-vectors aligning along ⟨111⟩ directions, driven by high-order anisotropy terms.
• In Co7Zn7Mn6, Co8Zn8Mn4, and Co9Zn9Mn2, the metastable SkL evolves into a square lattice, with skyrmions elongated along ⟨100⟩ axes due to enhanced anisotropy.
• The Gilbert damping (α) increases at low temperatures in compounds with distorted metastable SkLs, correlating strongly with q-vector anharmonicity and lattice distortion.
• The temperature dependence of α and |q| vectors shows similar trends, suggesting a common microscopic origin linked to enhanced anisotropy and Mn spin fluctuations.