Magnetic force microscopy was used to observe the magnetic microstructure of Fe3GeTe2 at 4 K on the (001) surface. The surface magnetic structure consists of a two-phase domain branching pattern that is characteristic for highly uniaxial magnets in the plane perpendicular to the magnetic easy axis. The average surface magnetic domain width Ds = 1.3 μm determined from this pattern, in combination with intrinsic properties calculated from bulk magnetization data (the saturation magnetization Ms = 376 emu/cm3 and the uniaxial magnetocrystalline anisotropy constant Ku = 1.46 × 107 erg/cm3), was used to determine the following micromagnetic parameters for Fe3GeTe2 from phenomenological models: the domain wall energy γw = 4.7 erg/cm2, the domain wall thickness δw = 2.5 nm, the exchange stiffness constant Aex = 0.95 × 10−7 erg/cm, the exchange length lex = 2.3 nm, and the critical single domain particle diameter dc = 470 nm.

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...

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