Doping Control of Magnetic Anisotropy for Stable Antiskyrmion Formation in Schreibersite (Fe,Ni)₃P with S₄ symmetry

Abstract

have been extensively studied in the last decade, both in the fields of fundamental science and applications to spintronics devices. [1,2] One of the well-known mechanisms of skyrmion formation is the competition between ferromagnetic exchange interaction and the Dzyaloshinskii-Moriya interaction (DMI) arising from the lack of inversion symmetry. Skyrmions with helical (Bloch type) and cycloidal (Néel type) spin configurations have been observed in non-centrosymmetric magnets with chiral (T or O class) and polar (C nv class) structures, respectively. [3][4][5][6][7][8][9] Recently, a new topological spin texture, the antiskyrmion, with opposite sign of N sk for the same polarity has attracted much attention. The antiskyrmion consists of both Bloch and Néel walls with the opposite helicities along two orthogonal axes, and its formation is attributed to the anisotropic DMI present in non-centrosymmetric tetragonal crystals belonging to the D 2d or S 4 symmetry group both containing fourfold rotoinversion (4). [10,11] In real materials, antiskyrmions were first found in Heusler compounds with D 2d symmetry, Mn 1.4 PtSn and Mn 1.4 Pt 0.9 Pd 0.1 Sn. [12,13] More recently, the formation of antiskyrmions was also found in Fe 1.9 Ni 0.9 Pd 0.2 P [Pd-doped (Fe,Ni) 3 P] with S 4 symmetry [14] and Fe/Gd-based multilayers. [15] Lorentz transmission electron microscopy (LTEM) observation for thin plates of these Magnetic skyrmions, vortex-like topological spin textures, have attracted much interest in a wide range of research fields from fundamental physics to spintronics applications. Recently, growing attention is also paid to antiskyrmions emerging with opposite topological charge in non-centrosymmetric magnets with D 2d or S 4 symmetry. In these magnets, complex interplay among anisotropic Dzyaloshinskii-Moriya interaction, uniaxial magnetic anisotropy, and magnetic dipolar interactions generates various magnetic textures. However, the precise role of these magnetic interactions in stabilizing antiskyrmions remains to be elucidated. In this work, the uniaxial magnetic anisotropy of schreibersite (Fe,Ni) 3 P with S 4 symmetry is controlled by doping and its impact on the stability of antiskyrmions is investigated. The authors' magnetometry study, supported by ferromagnetic resonance spectroscopy, shows that the variation of the Ni content and slight doping with 4d transition metals considerably change the magnetic anisotropy. In particular, doping with Pd induces easy-axis anisotropy, giving rise to formation of antiskyrmions, while a temperature-induced spin reorientation is observed in an Rh-doped compound. In combination with Lorentz transmission electron microscopy and micromagnetic simulations, the stability of antiskyrmion as functions of uniaxial anisotropy and demagnetization energy is quantitatively analyzed, and demonstrated that subtle balance between them is necessary to stabilize the antiskyrmions.

Formation and Control of Zero‐Field Antiskyrmions in Confining Geometries

“…We then apply a 52-mT field cooling (FC) procedure (the right panel of Figure 1c) with the magnetic field and temperature passing through the thermodynamic equilibrium phase (pink ellipse), and carry out L-TEM observation at RT. This process can largely extend the temperature-magnetic field stable region of antiskyrmions, which can persist at zero field, in contrast to the previous studies [21,22,29] where the antiskyrmions were formed at a high field. Metastable antiskyrmions are generated at RT after FC at 52 mT (Figure 1e).…”

“…[31][32][33][34][35][36][37][38] ) that are shared by Fe, Ni, and Pd, whereas the gray spheres correspond to the P element. [21,22] b) A scanning electron microscopy image of a Fe 1.9 Ni 0.9 Pd 0.2 P thin plate, in which the border with tungsten W defines a square geometry. The sample edges are aligned to the [110] and [ 110] crystal axes.…”

“…Antiskyrmions containing alternating Bloch‐ and Néel‐type spin twists have been observed in noncentrosymmetric magnets with D 2d [ 4 , 5 , 19 , 20 ] and S 4 [ 21 , 22 ] symmetries. The anisotropic DMI is essential for the formation of the opposite helicity of the helix and hence antiskyrmions, while the magnetic dipole interaction is important for the square shape of antiskyrmions as it favors Bloch‐type twists and suppresses the Néel‐type parts (Bloch lines).…”

“…[ 5 , 19 , 20 , 21 , 23 ] Tuning the external magnetic field, temperature, and sample geometry induces transformations among the antiskyrmions, elliptical skyrmions, and non‐topological bubbles via the creation, propagation, and annihilation of Bloch line pairs. [ 5 , 21 ] To create antiskyrmion lattice at room temperature (RT), previous studies [ 5 , 21 , 22 , 24 ] has to vary the temperature as well as both the magnitude and direction of the applied magnetic field. Only sparse antiskyrmions can be created by applying the magnetic field normally to the thin plate because the ferromagnetic state with a stripe domain structure is thermodynamically stable at RT.…”

“…The a-b plane projection is shown at the right. Blue, green, and orange spheres represent three crystallographically inequivalent metal sites (M1, M2, M3) that are shared by Fe, Ni, and Pd, whereas the gray spheres correspond to the P element [21,22]. b) A scanning electron microscopy image of a Fe 1.9 Ni 0.9 Pd 0.2 P thin plate, in which the border with tungsten W defines a square geometry.…”