Towards the microscopic understanding of magnetic spin textures in chiral magnets: FeGe, MnSi and beyond B20
M. Baenitz, P. Khuntia, M. Majumder, H. Wilhelm1, U. Roessler2, H. Rosner, H. Yasuoka, M. Schmidt
1 Diamond Light Source Ltd., Chilton, Didcot, Oxfordshire, OX11 0DE,
2 IFW Dresden, Postfach 270116, 01171 Dresden, Germany
The B20 systems: FeSi, FeGe and MnSi
The electronic and magnetic properties of binary compounds crystallizing in the B20 structure, such as the monosilicides and monogermanides of Mn, Cr, Fe, and Co are an active topic of research in condensed matter physics. Among them, FeSi has attracted strong interest because it is a narrow gap semiconductor with Eg = 80 meV and a non-magnetic ground state that has led to its classification as a correlated semimetal, or 'Kondo insulator'. It shares these properties with FeSb2 and FeGa3 (see report on Fe-based systems with ferromagnetic quantum criticality) [1,2]. In contrast to FeSi, FeGe is a metallic helimagnet as a consequence of the broken inversion symmetry and the spin orbit coupling (Dzyaloshinskii - Moriya interaction) with an ordering temperature of 278.7 K .
In 2004 the idea to apply pressure on FeGe single crystals to study the crossover from the chiral ferromagnet to the non-magnetic correlated semi-metal FeSi was born and continuous efforts finally succeeded in 2006 in the growth of rather large FeGe single crystals of extremely high purity . So far these crystals made by M. Schmidt (from the Department of Inorganic Chemistry) are unique and this was the origin of a long standing in-depth research activity led from this institute on the various aspects on this fascinating room temperature chiral magnet. The surprising result of the pressure studies of H. Wilhelm and co-workers was that in contrast to the alloying study on FeSi1-xGex, the pressurized FeGe stays metallic up to the highest applied pressures. Furthermore a very pronounced non Fermi-liquid (NFL) behaviour in the temperature dependence of the resistivity (ρ proportional T 3/2) was found over a wide pressure range in the vicinity of the critical pressure pc . This behaviour was reminiscent to another relative in the B 20 class, MnSi, which has a much lower Tc of about 29 K. The latter attracted great attention in the context of ferromagnetic criticality  along with other weak ferromagnets like ZrZn2 , Ni3Al or CoS2. In early 2006 Kirkpatrik and co-workers already discussed the chiral aspect (chiral order parameter) to explain puzzling neutron scattering results near Tc and the extended NFL behaviour in the ρ(T)- data near pc (helimagnon scattering) [7,8].
Even more important was the publication by U. Roessler, A. Bogdanov and C. Pfleiderer about a “spontaneous skyrmion ground state in magnetic metals” which predicts a new long range ordered phase formed out of magnetic vortices (in analogy to the flux lattice in type II superconductors) in chiral magnets . Finally 2009 this new form of matter in the ordered phase was experimentally confirmed in MnSi single crystals. Small-angle neutron scattering (SANS) experiments at selected points in the H-T- phase diagram showed magnetic modulations which are transverse to the H direction and which produce a sixfold Bragg spot pattern . This was taken as an evidence for the hexagonal Skyrmion lattice (historically called “A phase”) in MnSi. The Skyrmion lattice is believed to be formed out of bundles of two dimensional spin textured objects called “Skyrmions” (SKY) forming magnetic vortices in analogy to type II superconductors. Such magnetic vortices were predicted as early as 1989 by Bogdanov and Yablonskii for helimagnets [11,12]. Based on Hall resistivity measurements, the complex spin texture in the ordered state and the impact of chiral excitations above Tc under pressure was discussed recently for MnSi .
In 2009 our research on FeGe was focused on A-phase signatures accompanied by the Skyrmion formation in this nearly room temperature chiral magnet (see schematic phase diagram Fig.1). In 2011 we discovered a “segmented A-Phase” with several pockets near Tc by a detailed ac magnetization study on oriented single crystals . This was the first hint towards the formation of a Skyrmion lattice in FeGe bulk material (Fig.2, left). One month later Y. Tokura and co-workers confirmed the Skyrmion lattice formation in FeGe thin films by Lorentz microscopy . In contrast to MnSi where the Skyrmions have a relatively small diameter (equals the helical pitch length) of about 180 Å for FeGe a diameter of about 700 Å was found which points towards large lateral regimes in the crystal with this unique chiral spin texture.
Figure 1: Schematic magnetic phase diagram of B20 helimagnets (left). Building blocks of the various phases (right): a) single Skyrmion. b) conical state c) helical state d) hexagonal Skyrmion lattice (“A phase”).[less]
Figure 1: Schematic magnetic phase diagram of B20 helimagnets (left). Building blocks of the various phases (right): a) single Skyrmion. b) conical state c) helical state d) hexagonal Skyrmion lattice (“A phase”).
Small angle neutron scattering measurements (SANS) were conducted in 2009 on our FeGe single crystals in collaboration with S. Grigoriev (Petersburg Nuclear Physics Institute,
A further goal of our efforts was to prepare 57Fe enriched FeGe crystals and to use the 57Fe resonance to probe the magnetic phase diagram and especially the Skyrmion lattice by zero field (or field modulated) NMR. Using the effect of the small internal field (created by the magnetic ion itself) in the ordered state on the 57Fe (I=1/2) or 55Mn(I=5/2) nuclear spin to probe chiral magnetic textures in a helimagnet is a unique approach. Over the past decade a unique set of solid state NMR techniques far beyond standard was developed at the CPFS. Especially powerful low field- (frequency-) techniques are improved strongly. In general our microscopic study is aimed to static and dynamic magnetism of the helimagnet and for Skyrmion matter in particular to probe the dipolar coupling between the electron spins and nuclear spins and the DM-interaction itself. Furthermore Skyrmion dynamics and helical excitations across the magnetic phase diagram are a matter of further studies.
In contrast to neutron studies or Fe-Moessbauer, “on site” 57Fe NMR has the capability for “line profiling” of the SKY lattice (due to the NMR line shape) and as a probe for spin dynamics of individual and collective Skyrmion excitations. The lateral dimension of about 700 Å of the FeGe-Skyrmion modulates the local field of about 150 Fe ions which should result in a significant NMR line- modulation or splitting. Our zero field NMR approach was very successful and 57Fe NMR probes the local magnetization (hyperfine field, see Fig.3), and the dynamic susceptibility (spin lattice relaxation rate) directly "on site" . It should be mentioned that so far there is no other helical SKY magnet where such studies could be performed. The reason for this is that the zero field NMR towards room temperature has to be performed in the internal fields far below 1 T (NMR frequency of about 1 MHz) which requires special NMR techniques. Here low field and/or zero field magnetic resonance is the method of choice.
55Mn on-site NMR studies on MnSi suffer from the broad NMR lines arising from the quadrupole interaction of 55Mn with its I = 5/2 spin . To overcome this problem we concentrated on 29Si zero field NMR on MnSi. 29Si has a I=1/2 nuclear spin but NMR studies were difficult because of the rather low natural abundance (NA=2.1 %) of the NMR active isotope. Therefore we started recently rather successful a 29Si zero field NMR study on 29Si enriched MnSi powder in close collaboration with the group of Professor C. Krellner from the Physics Institute at the Goethe University Frankfurt. Due to the spin of I= 1/2 the NMR lines of 29Si are rather narrow but on the other hand the hyperfine fields have a transferred nature which leads to rather small values and resonance frequencies on the Si nuclei. Nonetheless we were able to obtain rather sharp Si resonances in MnSi powder and in single crystals which is a promising start of this new project.
Beyond the B20 compounds
Apart from the B20 systems the cubic multiferroic Cu2OSeO3 is another interesting chiral magnet which hosts a Skyrmion lattice which shares the same space group P213 of the B20 family [20,21]. A case study on zero field NMR on the twin isotopes 63Cu (I=3/2, NA=69%) and 65Cu (I=3/2, NA=31%) on single crystals synthesized by M. Schmidt was very successful. In contrast to the B20 systems Cu2OSeO3 is a oxide which leads in general to a weaker hyperfine coupling between nucleus- and spin- moment. Finally it could be speculated that among the non centrosymmetric Mn-Heusler alloys there might be also candidates for non collinear chiral magnetic textures or Skyrmion lattices. We have shown for a Mn-Pt-Ga Heussler alloy that zero field NMR is a suitable tool to probe the local magnetism . Recently Y. Tokura and co-workers surprisingly reported on a β-Mn type alloy the presence of Skyrmions even above room temperature .
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