Kazuhisa Kakurai

The simultaneous application of high magnetic fields and high pressures for controlling magnetic ground states is important for testing our understanding of many-body quantum theory. However, the implementation for neutron scattering experiments presents a technical challenge. To overcome this challenge, we present an optimized pressure-cell design with a novel bullet shape, which is compatible with horizontal-field magnets, in particular the high-field magnet operating at the Helmholtz-Zentrum Berlin. The cell enabled neutron diffraction and spectroscopy measurements with the combination of three extreme conditions: high pressures, high magnetic fields, and dilution temperatures, simultaneously reaching 0.7 GPa, 25.9 T, and 200 mK, respectively. Our results demonstrate the utility of informed material choices and the efficiency of finite-element analysis for future pressure-cell designs to be used in combination with magnetic fields and dilution temperatures for neutron scattering purposes.

Magnon spin currents in the ferrimagnetic garnet Tb3Fe5O12 with 4f electrons were examined through the spin-Seebeck effect and neutron scattering measurements. The compound shows a magnetic compensation, where the spin-Seebeck signal reverses above and below Tcomp=249.5(4) K. Unpolarized neutron scattering unveils two major magnon branches with finite energy gaps, which are well explained in the framework of spin-wave theory. Their temperature dependencies and the direction of the precession motion of magnetic moments, i.e., magnon polarization, defined using polarized neutrons, explain the reversal at Tcomp and decay of the spin-Seebeck signals at low temperatures. We illustrate an example that momentum- and energy-resolved microscopic information is a prerequisite to understand the magnon spin current.

We investigate the spin rotational structure of magnetic skyrmions in a tetragonal polar magnet VOSe2O5 via polarized small-angle neutron scattering (SANS). Spin polarization analysis of the scatte...

We investigated the low-energy incommensurate (IC) magnetic fluctuations in Sr2RuO4 by the high-resolution inelastic neutron scattering measurements and random phase approximation (RPA) calculation...

Magnetic skyrmion textures are realized mainly in non-centrosymmetric, e.g. chiral or polar, magnets. Extending the field to centrosymmetric bulk materials is a rewarding challenge, where the released helicity/vorticity degree of freedom and higher skyrmion density result in intriguing new properties and enhanced functionality. We report here on the experimental observation of a skyrmion lattice (SkL) phase with large topological Hall effect and an incommensurate helical pitch as small as 2.8 nm in metallic Gd₃Ru₄Al₁₂, which materializes a breathing kagomé lattice of Gadolinium moments. The magnetic structure of several ordered phases, including the SkL, is determined by resonant x-ray diffraction as well as small angle neutron scattering. The SkL and helical phases are also observed directly using Lorentz-transmission electron microscopy. Among several competing phases, the SkL is promoted over a low-temperature transverse conical state by thermal fluctuations in an intermediate range of magnetic fields.

We report a wide-range reciprocal space observation with neutron diffraction in an iron vacancy controlled YbFe2O4 crystal, which is proposed as a prototype of electronic ferroelectrics arising fro...

Mutual control of the electricity and magnetism in terms of magnetic (H) and electric (E) fields, the magnetoelectric (ME) effect, offers versatile low power consumption alternatives to current data storage, logic gate, and spintronic devices. Despite its importance, E-field control over magnetization (M) with significant magnitude was observed only at low temperatures. Here we have successfully stabilized a simultaneously ferrimagnetic and ferroelectric phase in a Y-type hexaferrite single crystal up to 450 K, and demonstrated the reversal of large non-volatile M by E field close to room temperature. Manipulation of the magnetic domains by E field is directly visualized at room temperature by using magnetic force microscopy. The present achievement provides an important step towards the application of ME multiferroics.

Manipulating topological spin textures is a key for exploring unprecedented emergent electromagnetic phenomena. Whereas switching control of magnetic skyrmions, e.g., the transitions between a skyrmion-lattice phase and conventional magnetic orders, is intensively studied towards development of future memory device concepts, transitions among spin textures with different topological orders remain largely unexplored. Here we develop a series of chiral magnets MnSi_1−xGe_x, serving as a platform for transitions among skyrmion- and hedgehog-lattice states. By neutron scattering, Lorentz transmission electron microscopy and high-field transport measurements, we observe three different topological spin textures with variation of the lattice constant controlled by Si/Ge substitution: two-dimensional skyrmion lattice in x = 0–0.25 and two distinct three-dimensional hedgehog lattices in x = 0.3–0.6 and x = 0.7–1. The emergence of various topological spin states in the chemical-pressure-controlled materials suggests a new route for direct manipulation of the spin-texture topology by facile mechanical methods.

(2018). In memoriam: Nobuyoshi Wakabayashi (1938–2017) Neutron News: Vol. 29, No. 1, pp. 32-34.

(2017). The International Conference on Neutron Optics (NOP2017) in Nara. Neutron News: Vol. 28, No. 4, pp. 18-20.