PHYSICAL REVIEW B 99, 144427 (2019) Spin fluctuations, geometrical size effects, and zero-field topological order in textured MnSi thin films J. López-López, 1 Juan M. Gomez-Perez, 1 A. Álvarez, 1 Hari Babu Vasili, 2 A. C. Komarek, 3 L. E. Hueso, 1, 4 Fèlix Casanova, 1, 4 and S. Blanco-Canosa 1, 4, 5 , * 1 CIC nanoGUNE, 20018 Donostia-San Sebastian, Basque Country, Spain 2 ALBA Synchrotron Light Source, Cerdanyola del Vallès, 08290 Barcelona, Catalonia, Spain 3 Max Planck Institute for Chemical Physics of Solids, Nöthnitzerstrasse 40, 01187 Dresden, Germany 4 IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Basque Country, Spain 5 Donostia International Physics Center, DIPC, 20018 Donostia-San Sebastian, Basque Country, Spain (Received 18 August 2018; revised manuscript received 22 March 2019; published 29 April 2019) Noncollinear magnetic structures with broken inversion symmetry have recently revolutionized the field of spintronics. Among them, transition metal monosilices and germanides are potential candidates due to their nontrivial spin textures. Here we report the growth and magnetic and electronic characterization of high-quality (111)-oriented thin films of MnSi by magnetron sputtering. While thicker films order magnetically similar to their bulk counterpart and according to previous reports in literature, x-ray magnetic circular dichroism measurements indicate that 30-nm-thick films do not develop long-range magnetic order, presumably due to magnetic frustration introduced by domains of different chiralities and spin disorder. X-ray absorption spectroscopy shows a stabilization of Mn + oxidation state in epitaxial thin films evidencing different electronic structure as compared with bulk MnSi. Field- and angular-dependent magnetoresistance support a magnetically disordered scenario, while Hall resistivity develops features of zero-field topological order formerly ascribed to skyrmions. Our results highlight the importance of geometrical size effects and spin disorder in noncentrosymmetric magnets and suggest that the topological Hall effect in thin films cannot be taken as the sole criterium for the assignment of nontrivial magnetic structures. DOI: 10.1103/PhysRevB.99.144427 I. INTRODUCTION The discovery of a skyrmion lattice [1] and its manipu- lation with low current densities [2] promoted noncollinear magnetic structures as promising candidates for information processing and data storage [3]. Special attention is devoted to the B20 crystals [4,5], multiferroics [6], alloys [7,8], and arti- ficial structures lacking inversion symmetry. From the micro- scopic point of view, broken inversion symmetry introduces a Dzyaloshinkii-Moriya interaction (DMI) D S M · (∇× M), where D is the Dzyaloshinkii constant and M the magnetiza- tion, which allows for orthogonal spin interactions. Besides, the isotropic Heisenberg exchange interaction favors parallel spin alignments, thus, the competition among these quantum mechanical interactions, thermal and magnetic energies leads to complex magnetic structures with nontrivial topological magnetotransport properties [9]. In noncentrosymetric cubic monosilices and monoger- manides [A(Si,Ge), A= Mn, Fe, Co], the magnetic phase dia- gram encompasses magnetic phases ranging from helicoidal, conical to fully polarized states depending on the strength of the magnetic field [4,5,10]. For the particular case of MnSi at zero magnetic field, the competition between the DMI and the Heisenberg exchange interaction produces a helical magnetic order below the Curie temperature, T C = 30 K, * sblanco@dipc.org with a wavelength of the helix λ D ∼ 18 nm [11] determined by the ratio of the energy terms 2π/Q ∝ A/D, where A is the exchange stiffness and Q is the propagation vector. Q points along [111] direction with the magnetic moments lying perpendicular to it. If an external magnetic field is applied perpendicular to [111], Q rotates in the direction of the magnetic field and becomes parallel to field at H ‖ s = 0.1 T. Above the critical field applied along the [111] direction, H ⊥ c , the magnetic moments form a conical phase, which collapses into a ferromagnetic state at H ⊥ s = 0.6 T. In a small region of the H -T phase diagram, strong evidences for the presence of solitonic states, termed skyrmions, have been revealed by means of neutron scattering [1], ac susceptibility [12], and Hall effect [13]. Lorentz microscopy imaged the helimagnetic and skyrmion phase under a moderate normal magnetic field in thin films [14] and nanometer-polished MnSi [15]. This suggests that size effects, strain and magnetic anisotropies play a crucial role in stabilizing these spin textures. In order to increase the skyrmion region, theory predicts that uniaxial strain introduced by the epitaxial growth sup- presses the helical order and thermodynamically stabilizes skyrmion lattices. Micromagnetic simulations show that he- licoids and skyrmion states are the most stable solutions for large values of the anisotropy ratio (K/K 0 )[16], K stands for the uniaxial anisotropy and K 0 is the anisotropy value defined as K 0 = H d /2M, where H d is a characteristic field depending on the D and A ratio and M is the magnetization. There- fore, tuning the magnetic anisotropy of noncentrosymmetric 2469-9950/2019/99(14)/144427(12) 144427-1 ©2019 American Physical Society