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Papers - Nii, Yoichi

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16aCD-8 Large elastic response in a chiral magnet MnGe

Nii Y., Kanazawa N., Zhang X. -X., Nagaosa N., Tokura Y., Iwasa Y.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 70 1661 - 1661 2015

ISSN 2189-079X

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19pCD-2 Temperature dependence of magnetic excitation in spinel oxide MnV_2O_4

Matsuura K., Sagayama H., Nii Y., Arima T., Kamazawa K., Kajimoto R.

Meeting Abstracts of the Physical Society of Japan (The Physical Society of Japan (JPS)) 70 2198 - 2198 2015

ISSN 2189-079X

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Elastic Stiffness of a Skyrmion Crystal

Y. Nii, A. Kikkawa, Y. Taguchi, Y. Tokura, Y. Iwasa

PHYSICAL REVIEW LETTERS (AMER PHYSICAL SOC) 113 ( 26 ) 267203 2014.12

Lead author, Corresponding author, Accepted, ISSN 0031-9007

We observe the elastic stiffness and ultrasonic absorption of a Skyrmion crystal in the chiral-lattice magnet MnSi. The Skyrmion crystal lattice exhibits a stiffness 3 orders of magnitude smaller than that of the atomic lattice of MnSi, being as soft as the flux line lattice in type-II superconductors. The observed anisotropic elastic responses are consistent with the cylindrical shape of the Skyrmion spin texture. Phenomenological analysis reveals that the spin-orbit coupling is responsible for the emergence of anisotropic elasticity in the Skyrmion lattice.

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TEM studies of domain formation mechanisms in MnV2O4

Y. Murakami, Y. Nii, T. Arima, D. Shindo, K. Yanagisawa, A. Tonomura

JOURNAL OF ALLOYS AND COMPOUNDS (ELSEVIER SCIENCE SA) 577 S731 - S735 2013.11

Accepted, ISSN 0925-8388

Crystallographic and magnetic domains produced in a spinel-type compound MnV2O4, which exhibits a type of giant magnetostriction attributed to twin boundary motion, have been studied using transmission electron microscopy techniques. Although MnV2O4 undergoes a displacive cubic-to-tetragonal transformation upon cooling, it does not show a well-defined habit plane (i.e. the plane with a specific index that is favored for minimizing the transformation) due to the small elongation/contraction in the lattice. Electron holography demonstrates a considerable reduction in the magnetic signal by cooling the tetragonal phase to 40 K. Despite the elimination of micrometer-scale ferrimagnetic domains, weak magnetic contrast still remained, indicating small residual magnetic domains in particular portions, such as in the crosshatch of twinning pairs. (C) 2012 Elsevier B.V. All rights reserved.

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Interplay among spin, orbital, and lattice degrees of freedom in a frustrated spinel Mn3O4

Y. Nii, H. Sagayama, H. Umetsu, N. Abe, K. Taniguchi, T. Arima

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 87 ( 19 ) 2013.05

Accepted, ISSN 1098-0121

We have investigated magnetic-field (H parallel to[100]) effect on a spin-frustrated tetragonally distorted spinelMn3O4 by measurements of magnetization, dielectric constant, and strain as well as synchrotron x-ray and neutron diffraction. In two low-temperature phases below 40 K, long-wavelength lattice modulations coupled with magnetic order have been confirmed, revealing spin-lattice coupling caused by the exchange striction in the geometrically frustrated Mn3+ spin moments on the distorted pyrochlore lattice. A magnetic-field-induced phase transition has been found at low temperatures, which involves an abrupt change in both structural and magnetic configurations. The magnetostructural phase transition by the application of a magnetic field H parallel to[100] is discussed in terms of interplay among spin, orbital, and lattice degrees of freedom.

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Orbital-spin-coupled fluctuations in spinel vanadate MnV2O 4

Yoichi Nii, Nobuyuki Abe, Taka-Hisa Arima

Physical Review B - Condensed Matter and Materials Physics 87 ( 8 ) 2013.02

Accepted, ISSN 1098-0121

The elastic properties of a spinel vanadate MnV2O4 that has an orbital degree of freedom in the triply degenerate t2g orbital at V3+(d2) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C 11-C12)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between dyz and dzx. Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV2O4. © 2013 American Physical Society.

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Orbital-spin-coupled fluctuations in spinel vanadate MnV2O4

Yoichi Nii, Nobuyuki Abe, Taka-hisa Arima

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 87 ( 8 ) 2013.02

Accepted, ISSN 2469-9950

The elastic properties of a spinel vanadate MnV2O4 that has an orbital degree of freedom in the triply degenerate t(2g) orbital at V3+(d(2)) sites are investigated by ultrasonic pulse-echo measurement. Considerable softening of the shear elastic constant (C-11 - C-12)/2 is observed as the temperature approaches the first-order transition from the high-temperature orbital-disordered cubic phase to the low-temperature orbital-ordered tetragonal phase. The softening is attributed to fluctuations between d(yz) and d(zx). Moreover, the elastic anomaly is found to be sensitive to the external magnetic field, revealing a coupling between spin and orbital fluctuations. The elastic anomaly is well reproduced theoretically based on Landau theory and elucidates a characteristic precursor phenomenon in MnV2O4. DOI: 10.1103/PhysRevB.87.085111

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Orbital structures in spinel vanadates AV(2)O(4) (A = Fe, Mn)

Y. Nii, H. Sagayama, T. Arima, S. Aoyagi, R. Sakai, S. Maki, E. Nishibori, H. Sawa, K. Sugimoto, H. Ohsumi, M. Takata

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 86 ( 12 ) 2012.09

Accepted, ISSN 1098-0121

Spinel FeV2O4 exhibits successive structural phase transitions, reflecting the interplay between the Fe2+ (3d(6)) and V3+ (3d(2)) ions, both of which have orbital and spin degrees of freedom. The temperature-dependent orbital shapes of Fe2+ and V3+ were investigated by means of single-crystal structure analysis, and were compared with those in MnV2O4, where only the V3+ ions are Jahn-Teller active. The highest-temperature transition from the cubic to the high-temperature tetragonal phase was driven by a ferroic Fe2+ 3z(2)-r(2) orbital order (OO). At 110 K, where the ferrimagnetic transition takes place, the magnetic order modified the orbital shape through intratomic spin-orbit coupling, causing an orthorhombic distortion. The V3+ orbital order (V-OO) contributed to the lowest temperature transition from the orthorhombic to the low-temperature tetragonal phase. The V-OO in FeV2O4 was qualitatively different from that in MnV2O4. We propose that ferro-OO contains a complex orbital in FeV2O4 in contrast to the V-OO of real orbitals observed in MnV2O4.

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Magnetic shape memory effect in orbital-spin-coupled system MnV2O4

Y. Nii, N. Abe, K. Taniguchi, T. Arima

APPLIED PHYSICS LETTERS (AMER INST PHYSICS) 100 ( 5 ) 2012.01

Accepted, ISSN 0003-6951

Magnetic-field(H)-induced shape memory effect without temperature variation is demonstrated by a combination of H-sweep and H-rotation in an orbital-spin-coupled spinel-type insulating system MnV2O4. A rotation of the direction of a magnetic field of 5 T in the low-temperature tetragonal phase gives rise to a large macroscopic strain up to 1%, which stems from a 90 degrees rotation of tetragonal domains. The results show several possible ways of macroscopic shape control in this class of matter, and may open a possibility of high-speed actuators free from the eddy current. (C) 2012 American Institute of Physics. [doi: 10.1063/1.3681581]

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Magnetic domain structure in the orbital-spin-coupled system MnV2O4

Y. Murakami, Y. Nii, T. Arima, D. Shindo, K. Yanagisawa, A. Tonomura

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 84 ( 5 ) 2011.08

Accepted, ISSN 1098-0121

TEM studies have revealed the temperature dependence of the magnetic domain structure in the crystal MnV2O4, which has attracted considerable attention because of the intriguing coupling of structural orders (magnetism, orbital, and lattice) and potential applications of these phenomena. Although this crystal forms well-defined magnetic domains because of long-range ferrimagnetic order, in both the cubic and the tetragonal phases, the magnetic information is dramatically reduced at temperatures below similar to 42 K. The results provide useful information for understanding magnetic anomalies, such as glassy behavior in the thermomagnetization curves, which have been reported in this temperature range.

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Effect of randomness on ferroelastic transitions: Disorder-induced hysteresis loop rounding in Ti-Nb-O martensitic alloy (vol 82, 214104, 2010)

Yoichi Nii, Taka-hisa Arima, Hee Young Kim, Shuichi Miyazaki

PHYSICAL REVIEW B (AMER PHYSICAL SOC) 82 ( 21 ) 2010.12

Accepted, ISSN 1098-0121

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