Versatile strain-tuning of modulated long-period magnetic structures

Abstract

We report a detailed small-angle neutron scattering (SANS) study of the skyrmion lattice phase of MnSi under compressive and tensile strain. In particular, we demonstrate that tensile strain applied in the skyrmion lattice plane, perpendicular to the magnetic field, acts to destabilize the skyrmion lattice phase. This experiment was enabled by our development of a versatile strain cell, unique in its ability to select the application of either tensile or compressive strain in-situ by using two independent helium-actuated copper pressure transducers, whose design has been optimized for magnetic SANS on modulated long-period magnetic structures and vortex lattices, and is compact enough to fit in common sample environments, such as cryostats and superconducting magnets.Modulated long-period magnetic structures incommensurate to the underlying crystal structure have been demonstrated to be at the heart of a wide range of phenomena in solid-state physics such as the magnetoelectric coupling in multiferroic materials, 1 the magnetocaloric effect, 2 , the emergence of topologically-stabilized magnetic defects such as solitons 3 and skyrmions, 4 and unconventional superconductivity. In the latter case, long-period magnetic arrangements arise in two distinct forms, namely the underlying long-range ordered antiferromagnetic state whose critical fluctuations are believed to mediate unconventional superconductivity, 5 and the vortex lattice that forms in type-II superconductors, 6 which in some cases are intimately coupled to one another. 7 This makes long-period magnetic structures relevant for applications ranging from solid-state cooling to energy-applications to data storage and spintronics.Strain frequently represents an important tuning parameter in the optimization of the desired material response related to these phenomena. For example, in multiferroic materials, the ferroelectric polarization is strongly coupled to the underlying crystal structure, which can be tuned by strain, and will affect the magnetization via magnetostriction. 8 Likewise, uniaxial strain can be utilized to alter magnetic exchange integral overlap along a specific direction, and thus introduce or enhance magnetic anisotropy. It has also been demonstrated that magnetic anisotropy in magnetocaloric materials results in the rotating magnetocaloric effect, which is believed to facilitate the implementation of magnetic cooling. 9 Additionally, uniaxial magnetic anisotropy has been shown to increase the stability of skyrmion lattice phases. 10-14 Finally, in the iron pnictide family of materials the antiferromagnetic and unconventional superconducting states show extreme sensitivity to strain. 15 Small-angle neutron scattering (SANS) has proven itself to be one of the most powerful probes to study relevant microscopic parameters of modulated long-period magnetic structures, such as the order parameter and the period, and is a crucial tool for the study of superconducting vortex lattices. 6 However, most available insitu strain application options f...

Magnetic small-angle neutron scattering

“…β-Mn-type Co x Zn y Mn z (Karube et al, 2016) exhibits a transition from a conventuional triangular lattice to a metastable square SkL for low temperatures due to magneto-crystalline anisotropy. (iv) The destabilization of competing phases (Seki et al, 2012c;Yu et al, 2011Yu et al, , 2010, (v) strain (Fobes et al, 2017;Nii et al, 2015) and (vi) terms induced by free surfaces (Rybakov et al, 2015) and interface spin orbit effects (Heinze et al, 2011;Romming et al, 2013) play an important role, in particular with decreasing sample thickness. Here, SANS, GISANS and polarized neutron reflectometry (PNR) have been used to study the possible formation of skyrmionic structures in B20 thin films: While such textures have been claimed to exist in MnSi thin films (Karhu et al, 2012;Meynell et al, 2017;Wilson et al, 2013), based on magnetization and PNR and SANS measurements, recent GISANS studies (Wiedenmann et al, 2017) did not reveal any hints for SKL spin textures in thin epitaxial films of MnSi.…”

Tensile deformations of the magnetic chiral soliton lattice probed by Lorentz transmission electron microscopy

“…Lorentz transmission electron microscopy (TEM) studies of the magnetic configuration revealed that SkX distortions are amplified by two orders of magnitude in comparison with elastic strains in the crystal lattice [27]. This provides a new approach of skyrmion crystal manipulation by elastic lattice degrees of freedom [28][29][30][31][32][33].…”

Magnetostriction of helimagnets in the skyrmion crystal phase

“…As a magnetic texture, skyrmion is not only affected by application of electromagnetic fields such as bias magnetic field, electric current [15], and laser [16], but also sensitive to non-electromagnetic fields such as temperature gradient [17] and mechanical stresses [3,18]. While the interaction between skyrmions and the electromagnetic fields or the temperature gradient are well-understood both theoretically and experimentally, the effect of mechanical stress on the skyrmion is not well addressed, even though several previous experiments have proven that mechanical stress is an effective method for controlling the formation and stability of skyrmions [3,[18][19][20][21][22][23][24] and even their dynamic properties [25]. More specifically, the ultra-low emergent elastic stiffness [26] of the skyrmion crystal (SkX) observed in FeGe thin films [3] and in MnSi [18] makes it convenient to manipulate with mechanical stresses.…”