Simultaneous tuning of magnetocrystalline anisotropy and spin reorientation transition via Cu substitution in Mn-Ni-Ga magnets for nanoscale biskyrmion formation

Abstract

Skyrmions with multiple helicity or topology in centrosymmetric crystals are intriguing magnetic-domain objects due to their diverse dynamics under external stimuli. Here we illustrate how the two key gradients of magnetocrystalline anisotropy (MCA) and spin reorientation transition (SRT) affect the skyrmion formation and topology by Cu substitution in the biskyrmion-host MnNiGa alloy. The MCA and SRT are simultaneously tuned in a large scope, while the original high Curie temperature (TC) is retained. Detailed neutron-scattering studies revealed the construction of a noncollinear canted magnetic structure below the SRT temperature (TSR), which effectively correlates the SRT with the evolution of the MCA, as well as the exchange interaction. The Cu substitution raises the TSR to merge with the TC, and meanwhile, reduces the c-axis anisotropy. Lorentz transmission electron microscopy revealed the formation of stacked biskyrmions from above room temperature to lower temperatures in MnNi1-xCuxGa(x=0-0.3) in the presence of proper MCA. Micromagnetic simulations further confirmed the great effect of uniaxial anisotropy on the stabilization of biskyrmions. Our work has helped clarify the evolution of magnetic structures and their correlation to the SRT, providing an account of the effect of MCA and exchange interaction on the biskyrmion formation.

Uncovering the skyrmion bubble magnetic states in centrosymmetric kagome magnet Mn4Ga2Sn

“…It looks like a circular pattern consisting of two incomplete circles of dark and bright contrast. A similar kind of LTEM intensity variation has also been found in other uniaxial anisotropy based centrosymmetric materials [14,15,17,19,22,32,33]. In some of the earlier reports, this magnetic texture is explained as a type-II bubble with topological number zero, as schematically shown in Figure 1(c) and (g) [13,17,21,22].…”

“…In some of the earlier reports, this magnetic texture is explained as a type-II bubble with topological number zero, as schematically shown in Figure 1(c) and (g) [13,17,21,22]. Few other reports also claim that the given magnetic texture is composed of two magnetic skyrmions with opposite helicity, named as biskyrmion, resulting in a skyrmion number of two [schematically shown in the Figure 1(d) and (h)] [14,15,19,32]. The zoomed view of the additional magnetic texture found at 200 K (enclosed in solid boxes) can be interpreted as a type-I magnetic bubble with spin rotation of a Blochskyrmion [Figure 1(m)].…”

Spin dynamics in bulk MnNiGa and Mn1.4Pt0.9Pd0.1Sn investigated by muon spin relaxation

“…At lower temperature in the µSR data, we see a further drop in the relaxing asymmetry at 200 K, which coincides with a minimum in dM /dT . This is consistent with the temperature of the spin-reorientation transition to a non-colinear canted ferromagnetic state that has been suggested from neutron scattering measurements [18]. The additional drop in the relaxing asymmetry would, for example, occur if a larger fraction of the static magnetic moments in the material are canted away from the applied field direction below 200 K, which would increase the fast-relaxing component of the µSR signal, leading to the additional missing asymmetry we observe.…”

“…In Mn 1.4 Pt 0.9 Pd 0.1 Sn, the transition is suggested to be a spin reorientation by analogy with similar materials such as Mn 1.4 PtSn [4,15] and Mn 2 RhSn [16]. Neutron diffraction measurements on MnNiGa show that the low-temperature transition in this material also involves spin reorientation, in this case introducing an antiferromagnetic component to create a canted non-collinear state [17,18].…”

Spin dynamics in bulk MnNiGa and Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn investigated by muon spin relaxation

“…In Mn 1.4 Pt 0.9 Pd 0.1 Sn, this transition is suggested to be a spin reorientation from analogy with similar materials such as Mn 1.4 PtSn 3,15 and Mn 2 RhSn 16 . Direct neutron diffraction measurements on MnNiGa have shown that the low-temperature transition in this material is also a spin-reorientation transition, but one that introduces an antiferromagnetic component to create a canted non-collinear state 17,18 .…”