Y. Tokunaga

A new class of chiral materials hosting magnetic skyrmions beyond room temperature

Skyrmions, topologically protected vortex-like nanometric spin textures in magnets, have been attracting increasing attention for emergent electromagnetic responses and possible technological applications for spintronics. In particular, metallic magnets with chiral and cubic/tetragonal crystal structure may have high potential to host skyrmions that can be driven by low electrical current excitation. However, experimental observations of skyrmions have been limited to below room temperature for the metallic chiral magnets, specifically for the MnSi-type B20 compounds. Towards technological applications, transcending this limitation is crucial. Here we demonstrate the formation of skyrmions with unique spin helicity both at and above room temperature in a family of cubic chiral magnets: β-Mn-type Co-Zn-Mn alloys with a different chiral space group from that of B20 compounds. Lorentz transmission electron microscopy, magnetization and small-angle neutron scattering measurements unambiguously reveal formation of a skyrmion crystal under application of a magnetic field in both thin-plate and bulk forms.

Versatile helimagnetic phases under magnetic fields in cubic perovskite SrFeO 3

A helical spin texture is of great current interest for a host of novel spin-dependent transport phenomena. We report a rich variety of nontrivial helimagnetic phases in the simple cubic perovskite

Colossal electroresistance effect at metal electrode/La1−xSr1+xMnO4 interfaces

{\mathrm{SrFeO}}_{3}

Imaging oxygen defects and their motion at a manganite surface

under magnetic fields up to 42 T. Magnetic and resistivity measurements revealed that the proper-screw spin phase proposed for

Current oscillation and low-field colossal magnetoresistance effect in phase-separated manganites.

{\mathrm{SrFeO}}_{3}

Robust metastable skyrmions and their triangular–square lattice structural transition in a high-temperature chiral magnet

can be subdivided into at least five kinds of ordered phases. Near the multicritical point, an unconventional anomalous Hall effect was found to show up and was interpreted as due to a possible long-period noncoplanar spin texture with scalar spin chirality.

One-way transparency of four-coloured spin-wave excitations in multiferroic materials

We have studied the current-voltage (I-V) characteristics and resistance switching at the interface between metal electrodes M (=Pt, Au, Ag, Al, Ti, and Mg) and atomically flat cleaved (001) surfaces of La1−xSr1+xMnO4 (x=0–1.0) single crystals by using a three-probe method. Hysteretic I-V characteristics, indicating the appearance of the resistance switching, were observed in the junctions for M=Mg, Al, and Ti, which have relatively shallow work functions. The resistance switching ratio depends on the hole doping x and the optimal doping level is around x=0.5, verifying that the resistance switching property can be controlled by the doping level.

Complex Magnetism and Magnetic-Field-Driven Electrical Polarization in Co3TeO6

Manganites are technologically important materials, used widely as solid oxide fuel cell cathodes; they have also been shown to exhibit electroresistance. Oxygen bulk diffusion and surface exchange processes are critical for catalytic action, and numerous studies of manganites have linked electroresistance to electrochemical oxygen migration. Direct imaging of individual oxygen defects is needed to underpin understanding of these important processes. Currently, it is not possible to collect the required images in bulk, but scanning tunnelling microscopy (STM) could provide such data for surfaces. Here, we report the first atomic resolution images of oxygen defects at a manganite surface. Our experiments also reveal defect dynamics, including oxygen adatom migration, vacancy-adatom recombination and adatom bistability. Beyond providing an experimental basis for testing models describing the microscopics of oxygen migration at transition-metal oxide interfaces, our work resolves the long-standing puzzle of why STM is more challenging for layered manganites than for cuprates.

Cupric chloride CuCl2 as an S=1/2 chain multiferroic

Current-induced switching from a metallic to an insulating state is observed in phase-separated states of (La(1-y)Pr(y))0.7Ca0.3MnO3 (y=0.7) and Nd(0.5)Ca(0.5)Mn(1-z)Cr(z)O3 (z=0.03) crystals. The application of magnetic fields to this current-induced insulating state causes a pronounced low-field negative magnetoresistance effect [rho(H)/rho(0)=10(-3) at H=1 kOe]. The application of a constant voltage also causes the breakdown of the Ohmic relation above a threshold voltage. At voltages higher than this threshold value, oscillations in currents are observed. This oscillation is well reproduced by a simple model of local switching of a percolative conduction path.

Nonvolatile multivalued memory effects in electronic phase-change manganites controlled by Joule heating

Skyrmions, topologically protected nanometric spin vortices, are being investigated extensively in various magnets. Among them, many structurally chiral cubic magnets host the triangular-lattice skyrmion crystal (SkX) as the thermodynamic equilibrium state. However, this state exists only in a narrow temperature and magnetic-field region just below the magnetic transition temperature Tc, while a helical or conical magnetic state prevails at lower temperatures. Here we describe that for a room-temperature skyrmion material, β-Mn-type Co 8Zn 8Mn 4, a field-cooling via the equilibrium SkX state can suppress the transition to the helical or conical state, instead realizing robust metastable SkX states that survive over a very wide temperature and magnetic-field region. Furthermore, the lattice form of the metastable SkX is found to undergo reversible transitions between a conventional triangular lattice and a novel square lattice upon varying the temperature and magnetic field. These findings exemplify the topological robustness of the once-created skyrmions, and establish metastable skyrmion phases as a fertile ground for technological applications.