Markus Garst

Spin transfer torques in MnSi at ultralow current densities

Spin Control Controlling and manipulating the spin of an electron is a central requirement for applications in spintronics. Some of the challenges researchers are facing include efficient creation of spin currents, minimization of Joule heating, and extending the lifetime of electronic spins, which is especially important for quantum information applications. Costache and Valenzuela (p. 1645) address the first challenge by designing and fabricating an efficient and simple superconducting-based single-electron transistor that can produce spin current with controlled flow. Key to the design is asymmetric tunneling, which leads to a ratchet effect (or diode-like behavior), allowing the separation of up and down spins. Jonietz et al. (p. 1648) use electric currents five orders of magnitude smaller than those used previously in nanostructures to manipulate magnetization in a bulk material, MnSi, pointing the way toward decreased Joule heating in spintronic devices. This so-called spin-torque effect causes the rotation of the skyrmion lattice of spins, characteristic of MnSi, which is detected by neutron scattering. Finally, McCamey et al. (p. 1652) extend the short lifetime of an electronic spin of a phosphorous dopant by mapping it onto the much longer lived nuclear spin of the atom. Mapping the nuclear spin back onto the electronic spin allows production of a spin memory with a storage time exceeding 100s, which should prove useful for future practical applications. A complicated spin texture lattice in a bulk material rotates under the influence of a tiny electrical current. Spin manipulation using electric currents is one of the most promising directions in the field of spintronics. We used neutron scattering to observe the influence of an electric current on the magnetic structure in a bulk material. In the skyrmion lattice of manganese silicon, where the spins form a lattice of magnetic vortices similar to the vortex lattice in type II superconductors, we observe the rotation of the diffraction pattern in response to currents that are over five orders of magnitude smaller than those typically applied in experimental studies on current-driven magnetization dynamics in nanostructures. We attribute our observations to an extremely efficient coupling of inhomogeneous spin currents to topologically stable knots in spin structures.

Emergent electrodynamics of skyrmions in a chiral magnet

When an electron moves in a smoothly varying non-collinear magnetic structure, its spin orientation adapts constantly, thereby inducing forces that act both on the magnetic structure and on the electron. These forces may be described by electric and magnetic fields of an emergent electrodynamics1, 2, 3, 4. The topologically quantized winding number of so-called skyrmions—a type of magnetic whirl discovered recently in chiral magnets5, 6, 7—has been predicted to induce exactly one quantum of emergent magnetic flux per skyrmion. A moving skyrmion is therefore expected to induce an emergent electric field following Faraday’s law of induction, which inherits this topological quantization8. Here we report Hall-effect measurements that establish quantitatively the predicted emergent electrodynamics. We obtain quantitative evidence for the depinning of skyrmions from impurities (at current densities of only 106 A m−2) and their subsequent motion. The combination of exceptionally small current densities and simple transport measurements offers fundamental insights into the connection between the emergent and real electrodynamics of skyrmions in chiral magnets, and might, in the long term, be important for applications.

Partial order in the non-Fermi-liquid phase of MnSi

Only a few metallic phases have been identified in pure crystalline materials. These include normal, ferromagnetic and antiferromagnetic metals, systems with spin and charge density wave order, and superconductors. Fermi-liquid theory provides a basis for the description of all of these phases. It has been suggested that non-Fermi-liquid phases of metals may exist in some heavy-fermion compounds and oxide materials, but the discovery of a characteristic microscopic signature of such phases presents a major challenge. The transition-metal compound MnSi above a certain pressure (pc = 14.6 kbar) provides what may be the cleanest example of an extended non-Fermi-liquid phase in a three-dimensional metal. The bulk properties of MnSi suggest that long-range magnetic order is suppressed at pc (refs 7–12). Here we report neutron diffraction measurements of MnSi, revealing that sizeable quasi-static magnetic moments survive far into the non-Fermi-liquid phase. These moments are organized in an unusual pattern with partial long-range order. Our observation supports the existence of novel metallic phases with partial ordering of the conduction electrons (reminiscent of liquid crystals), as proposed for the high-temperature superconductors and heavy-fermion compounds.

Fluctuation-induced first-order phase transition in Dzyaloshinskii-Moriya helimagnets

Two centuries of research on phase transitions have repeatedly highlighted the importance of critical fluctuations that abound in the vicinity of a critical point. They are at the origin of scaling laws obeyed by thermodynamic observables close to second-order phase transitions resulting in the concept of universality classes, that is of paramount importance for the study of organizational principles of matter. Strikingly, in case such soft fluctuations are too abundant they may alter the nature of the phase transition profoundly; the system might evade the critical state altogether by undergoing a discontinuous first-order transition into the ordered phase. Fluctuation-induced first-order transitions have been discussed broadly and are germane for superconductors, liquid crystals, or phase transitions in the early universe, but clear experimental confirmations remain scarce. Our results from neutron scattering and thermodynamics on the model Dzyaloshinskii-Moriya (DM) helimagnet (HM) MnSi show that such a fluctuation-induced first-order transition is realized between its paramagnetic and HM state with remarkable agreement between experiment and a theory put forward by Brazovskii. While our study clarifies the nature of the HM phase transition in MnSi that has puzzled scientists for several decades, more importantly, our conclusions entirely based on symmetry arguments are also relevant for other DM-HMs with only weak cubic magnetic anisotropies. This is in particular noteworthy in light of a wide range of recent discoveries that show that DM helimagnetism is at the heart of problems such as topological magnetic order, multiferroics, and spintronics.

Universal helimagnon and skyrmion excitations in metallic, semiconducting and insulating chiral magnets

Nearly seven decades of research on microwave excitations of magnetic materials have led to a wide range of applications in electronics. The recent discovery of topological spin solitons in chiral magnets, so-called skyrmions, promises high-frequency devices that exploit the exceptional emergent electrodynamics of these compounds. Therefore, an accurate and unified quantitative account of their resonant response is key. Here, we report all-electrical spectroscopy of the collective spin excitations in the metallic, semiconducting and insulating chiral magnets MnSi, Fe1-xCoxSi and Cu2OSeO3, respectively, using broadband coplanar waveguides. By taking into account dipolar interactions, we achieve a precise quantitative modelling across the entire magnetic phase diagrams using two material-specific parameters that quantify the chiral and the critical field energy. The universal behaviour sets the stage for purpose-designed applications based on the resonant response of chiral magnets with tailored electric conductivity and an unprecedented freedom for an integration with electronics.

Rotating skyrmion lattices by spin torques and field or temperature gradients

Chiral magnets like MnSi form lattices of skyrmions, i.e., magnetic whirls, which react sensitively to small electric currents j above a critical current density jc. The interplay of these currents with tiny gradients of either the magnetic field or the temperature can induce a rotation of the magnetic pattern for j

Magnon-skyrmion scattering in chiral magnets

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Long-range crystalline nature of the skyrmion lattice in MnSi

jc. Either a rotation by a finite angle of up to 15

Specific heat of the Skyrmion lattice phase and field-induced tricritical point in MnSi.

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Current-induced rotational torques in the skyrmion lattice phase of chiral magnets

or\textemdash{}for larger gradients\textemdash{}a continuous rotation with a finite angular velocity is induced. We use Landau-Lifshitz-Gilbert equations extended by extra damping terms in combination with a phenomenological treatment of pinning forces to develop a theory of the relevant rotational torques. Experimental neutron scattering data on the angular distribution of skyrmion lattices suggest that continuously rotating domains are easy to obtain in the presence of remarkably small currents and temperature gradients.