[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Ever since its discovery, the Berry phase has permeated through all branches of physics. Over the last three decades, it was gradually realized that the Berry phase of the electronic wave function can have a profound effect on material properties and is responsible for a spectrum of phenomena, such as ferroelectricity, orbital magnetism, various (quantum/anomalous/spin) Hall effects, and quantum charge pumping. This progress is summarized in a pedagogical manner in this review. We start with a brief summary of necessary background, followed by a detailed discussion of the Berry phase effect in a variety of solid state applications. A common thread of the review is the semiclassical formulation of electron dynamics, which is a versatile tool in the study of electron dynamics in the presence of electromagnetic fields and more general perturbations. Finally, we demonstrate a re-quantization method that converts a semiclassical theory to an effective quantum theory. It is clear that the Berry phase should be added as a basic ingredient to our understanding of basic material properties.

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Berry phase effects on electronic properties

Spin currents can apply useful torques in spintronic devices. The spin Hall effect has been proposed as a source of spin current, but its modest strength has limited its usefulness. We report a giant spin Hall effect (SHE) in β-tantalum that generates spin currents intense enough to induce efficient spin-torque switching of ferromagnets at room temperature. We quantify this SHE by three independent methods and demonstrate spin-torque switching of both out-of-plane and in-plane magnetized layers. We furthermore implement a three-terminal device that uses current passing through a tantalum-ferromagnet bilayer to switch a nanomagnet, with a magnetic tunnel junction for read-out. This simple, reliable, and efficient design may eliminate the main obstacles to the development of magnetic memory and nonvolatile spin logic technologies.

Spin-torque switching with the giant spin hall effect of tantalum

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Electrons have a charge and a spin, but until recently these were considered separately. In classical electronics, charges are moved by electric fields to transmit information and are stored in a capacitor to save it. In magnetic recording, magnetic fields have been used to read or write the information stored on the magnetization, which 'measures' the local orientation of spins in ferromagnets. The picture started to change in 1988, when the discovery of giant magnetoresistance opened the way to efficient control of charge transport through magnetization. The recent expansion of hard-disk recording owes much to this development. We are starting to see a new paradigm where magnetization dynamics and charge currents act on each other in nanostructured artificial materials. Ultimately, 'spin currents' could even replace charge currents for the transfer and treatment of information, allowing faster, low-energy operations: spin electronics is on its way.

/pdf/the-emergence-of-spin-electronics-in-data-storage-1vrnm5549k.pdf

The emergence of spin electronics in data storage

Reversible spin Hall effect comprising the direct and inverse spin Hall effects was electrically detected at room temperature. A platinum wire with a strong spin-orbit interaction is used not only as a spin current absorber but also as a spin-current source in the specially designed lateral structure. The obtained spin Hall conductivities are $2.4\ifmmode\times\else\texttimes\fi{}{10}^{4}\text{ }\text{ }(\ensuremath{\Omega}\mathrm{m}{)}^{\ensuremath{-}1}$ at room temperature, ${10}^{4}$ times larger than the previously reported values of semiconductor systems. Spin Hall conductivities obtained from both the direct and inverse spin Hall effects are experimentally confirmed to be the same, demonstrating the Onsager reciprocal relations between spin and charge currents.

https://arxiv.org/pdf/cond-mat/0609304.pdf

Room-temperature reversible spin Hall effect.

We have theoretically and numerically studied the dynamic properties of the vortex magnetic state in soft submicron ferromagnetic dots with variable thickness and diameter. To describe the vortex translation mode eigenfrequencies, we applied the equation of motion for the vortex collective coordinates. We calculated the vortex restoring force with an explicit account of the magnetostatic interaction on the bases of the “rigid” vortex and two-vortices “side charges free” models. The latter model well explains the results of our micromagnetic numerical calculations. The translation mode eigenfrequency is inversely proportional to the vortex static initial susceptibility and lies in GHz range for submicron in-plane dot sizes.

/pdf/eigenfrequencies-of-vortex-state-excitations-in-magnetic-3u0b3tlq06.pdf

Eigenfrequencies of vortex state excitations in magnetic submicron-size disks

The anomalous Nernst effect is usually associated with ferromagnets — enabling a temperature gradient to generate a transverse electric field — but the Berry curvature associated with Weyl points can drive this phenomenon in chiral antiferromagnets. A temperature gradient in a ferromagnetic conductor can generate a transverse voltage drop perpendicular to both the magnetization and heat current. This anomalous Nernst effect has been considered to be proportional to the magnetization1,2,3,4,5,6,7, and thus observed only in ferromagnets. Theoretically, however, the anomalous Nernst effect provides a measure of the Berry curvature at the Fermi energy8,9, and so may be seen in magnets with no net magnetization. Here, we report the observation of a large anomalous Nernst effect in the chiral antiferromagnet Mn3Sn (ref. 10). Despite a very small magnetization ∼0.002?μB per Mn, the transverse Seebeck coefficient at zero magnetic field is ∼0.35?μV?K−1 at room temperature and reaches ∼0.6?μV?K−1 at 200?K, which is comparable to the maximum value known for a ferromagnetic metal. Our first-principles calculations reveal that this arises from a significantly enhanced Berry curvature associated with Weyl points near the Fermi energy11. As this effect is geometrically convenient for thermoelectric power generation—it enables a lateral configuration of modules to cover a heat source6—these observations suggest that a new class of thermoelectric materials could be developed that exploit topological magnets to fabricate efficient, densely integrated thermopiles.

/pdf/large-anomalous-nernst-effect-at-room-temperature-in-a-by0bsn46dh.pdf

Large anomalous Nernst effect at room temperature in a chiral antiferromagnet

We have investigated spin Hall effects in 4$d$ and 5$d$ transition metals, Nb, Ta, Mo, Pd, and Pt, by incorporating the spin absorption method in the lateral spin valve structure, where large spin current preferably relaxes into the transition metals, exhibiting strong spin-orbit interactions. Thereby nonlocal spin valve measurements enable us to evaluate their spin Hall conductivities. The sign of the spin Hall conductivity changes systematically depending on the number of $d$ electrons. This tendency is in good agreement with the recent theoretical calculation based on the intrinsic spin Hall effect.

Indication of intrinsic spin Hall effect in 4d and 5d transition metals

A series of interstitial ternary nitrides R2Fe17Ny with 23<y<29 has been prepared for R=Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu and Y The nitrides have structures related to those of Th2Zn17 or Th2Ni17, but the unit cell volumes are 6-7% greater, and the magnetic ordering temperatures some 400 K higher than for the 2:17 parent compounds The Fe-Fe exchange interactions are more than doubled by nitrogenation, whereas the R-Fe exchange interactions are little changed All compounds exhibit easy-plane anisotropy at room temperature, except for Sm2Fe17Ny, which is easy-axis

https://iopscience.iop.org/article/10.1088/0953-8984/2/30/013/pdf

Yoshichika Otani

Papers

Room-temperature reversible spin Hall effect.

Eigenfrequencies of vortex state excitations in magnetic submicron-size disks

Large anomalous Nernst effect at room temperature in a chiral antiferromagnet

Indication of intrinsic spin Hall effect in 4d and 5d transition metals

Magnetic properties of a new series of rare-earth iron nitrides: R2Fe17Ny(y approximately 2.6)