| 1. |
Kei Soejima, Kensuke Otsubo, Takayuki Ohnishi, Hirofumi Wada, Large magnetocaloric effect of Ge-doped (MnFeRu)2(PSi) above room temperature, J. Supercond. Nov. Magn., https://doi.org/10.1007/s10948-021-05995-8, 34, 4, 2879-2884, 2021.08, The magnetocaloric properties of (MnFeRu)2(PSiGe) have been studied by monitoring the differential scanning calorimetry (DSC) and the adiabatic temperature change. The compounds have the Curie temperature of 340 – 360 K. The DSC curves show sharp peaks, indicating a first-order magnetic transition. The thermal hysteresis is less than 2.3 K. Using our previous data, we examined the correlation between the DSC peak and the magnetic entropy change Delta_SM. A strong positive correlation between these two quantities was is observed. The analyses also suggest that Delta_SM of (MnFeRu)2(PSiGe) reaches 9~20 J/ K kg in a field change of 1.5 T. The direct measurements of the adiabatic temperature change Delta_Tad have revealed Delta_Tad of 1.7 – 1.8 K at 1 T for (MnFeRu)2(PSiGe). These results suggest that (MnFeRu)2(PSiGe) compounds are potential candidates for magnetic refrigerant materials above room temperature up to 90 C (~360 K).. |
| 2. |
Masanobu Shiga, Isao Maruyama, Kengo Okimura, Takurou Harada, Takuya Takahashi, Akihiro Mitsuda, Hirofumi Wada, Yuji Inagaki and Tatsuya Kawae, Evolution of lattice coherence in the intermediate-valence heavy-fermion compound EuNi2P2 studied by point contact spectroscopy, Phys. Rev. B, DOI: 10.1103/PhysRevB.103.L041113, 103, 4, L041113 (6 pages), 2021.01, We performed a point contact spectroscopy study in an intermediate-valence heavy-fermion (HF) compound EuNi2P2. Above the Kondo temperature TK, the differential conductance spectra show a broad peak due to the Kondo resonance at the Eu site. Below TK, the broad peak splits into two peaks that can be reproduced by the
summation of two Fano curves with the same parameters, indicating the emergence of the indirect hybridization gap in an Anderson lattice. With decreasing temperature, the separation between the two peaks increases and saturates at very low temperatures. The results reveal the evolution of the electronic structure in the HF state due to the development of the lattice coherence.. |
| 3. |
Satoshi Tsutsui, Takumi Hasegawa, Akihiro Mitsuda, Masaki Sugishima, Kohei Oyama, Masaichiro Mizumaki, Norio Ogita, Hirofumi Wada, and Masayuki Udagawa, Lattice instability coupled with valence degrees of freedom in valence fluctuation compound YbPd, Phys. Rev. B, DOI: 10.1103/PhysRevB.102.245150, 102, 24, 245150 (9 pages), 2020.12. |
| 4. |
Kohei Oyama, Akihiro Mitsuda, Hirofumi Wada, Yasuo Narumi, Masayuki Hagiwara, Ryunosuke Takahashi, Hiroki Wadati, Hiroyuki Setoyama, and Koichi Kindo, Ga Substitution Effect on the Valence Transition of Eu2Pt6Al15, J. Phys. Soc. Jpn., https://doi.org/10.7566/JPSJ.89.114713, 89, 11, 114713 (7 pages), 2020.10. |
| 5. |
Akihiro Mitsuda, Hirofumi Wada, Ryo Masuda, Shinji Kitao, Makoto Seto, Yoshitaka Yoda, and Hisao Kobayashi, Valence Transition of EuRh2Si2 Studied by Synchrotron Mössbauer Spectroscopy, J. Phys. Soc. Jpn., https://doi.org/10.7566/JPSJ.89.104703, 89, 10, 104703 (5 pages), 2020.09. |
| 6. |
Kohei Oyama, Kousuke Tanabe, Akihiro Mitsuda, Hirofumi Wada, Naohisa Hirao, Saori Imada Kawaguchi, Yasuo Ohishi, Jun Gouchi, and Yoshiya Uwatoko, Pressure-induced Cubic Valence Fluctuating Ground State in YbPd, JPS Conf. Proc., https://doi.org/10.7566/JPSCP.30.011141, 30, 011141 (6 pages), 2020.03. |
| 7. |
Ryohei Shimokasa, Naomi Kawamura, Taku Kawabata, Gen Isumi,
Takayuki Uozumi, Akihiro Mitsuda, Hirofumi Wada, Fuminori Honda,
Masato Hedo, Takao Nakama, Yoshichika Onuki, Masaichiro MIzumaki,
and Kojiro Mimura, Electronic Structure of the Valence Transition System Eu(Rh1–xTx)2Si2 (T = Co, Ir) Studied by High-Energy Resolution Fluorescence Detection X-Ray Absorption Spectroscopy, JPS Conf. Proc. , https://doi.org/10.7566/JPSCP.30.011134, 30, 011134 (6 pages), 2020.03. |
| 8. |
Masanobu Shiga, Kengo Okimura, Hiroki Takata, Akihiro Mitsuda, Isao Maruyama, Hirofumi Wada, Yuji Inagaki, and Tatsuya Kawae, Observation of Kondo resonance in valence-ordered YbPd, Physical Review B, https://doi.org/10.1103/PhysRevB.100.245117, 100, 245117 (7 pages), 2019.12. |
| 9. |
Y. Yokoyama, K. Kawakami, Y. Hirata, K. Takubo, K. Yamamoto, K. Abe, A. Mitsuda, H. Wada, T. Uozumi, S. Yamamoto, I. Matsuda, S. Kimura, K. Mimura, H. Wadati, Photoinduced valence dynamics in EuNi2(Si0.21Ge0.79)2 studied via time-resolved x-ray absorption spectroscopy, Physical Review B, 10.1103/PhysRevB.100.115123, 100, 11, 2019.09, The photoinduced valence dynamics of EuNi2(Si0.21Ge0.79)2 were investigated using time-resolved x-ray absorption spectroscopy for Eu M5 edge. Through the pump-probe technique with synchrotron x-ray and Ti:sapphire laser pulses, a photoinduced valence transition was observed from Eu3+ to Eu2+. Because the lifetime of a photoinduced state can be up to 3 ns, a metastable state is considered to be realized. By comparing the experimental results with the theoretical calculations, the photoinduced valence transition between Eu 4f and conduction electrons was quantitatively evaluated.. |
| 10. |
Hirofumi Wada, Kosuke Tanabe, Ibuki Yamamoto, Akihiro Mitsuda, Magnetic transport properties of heavy fermionic EuNi2P2, Solid State Communications, 10.1016/j.ssc.2019.113665, 300, 2019.10, Electrical resistivity, magnetoresistance (MR) and Hall resistivity have been measured for Eu-based heavy fermion compound EuNi2P2 (exact composition is Eu1.2Ni2P2.1). It is found that the 4f electron contribution to the resistivity shows −log T above 200 K. The MR is positive at the lowest temperature and shows a negative minimum at around 60 K. The Hall resistivity shows a broad positive peak at around 120 K. These results are very similar to those of typical heavy fermions, CeCu6 and CeAl3. We discuss the results in terms of the Kondo model for heavy fermions. Measurements under high pressures suggest that the scaling of the Hall effect is valid below 1.5 GPa in EuNi2P2.. |
| 11. |
Hirofumi Wada, Kosuke Fukuda, Takayuki Ohnishi, Kei Soejima, Kensuke Otsubo, Keiichiro Yamashita, Thermal conductivity of giant magnetocaloric Mn compounds, Journal of Alloys and Compounds, 10.1016/j.jallcom.2019.01.188, 445-451, 2019.05. |
| 12. |
Akihiro Mitsuda, Eigo Kishaba, Takumi Fujimoto, Kohei Oyama, Hirofumi Wada, Masaichiro Mizumaki, Naomi Kawamura, Naoki Ishimatsu, Pressure and magnetic field effects on the valence transition of EuRh2Si2, Physica B: Condensed Matter, 10.1016/j.physb.2017.10.045, 536, 427-431, 2018.05. |
| 13. |
Kohei Oyama, Masaki Sugishima, Kousuke Tanabe, Akihiro Mitsuda, Hirofumi Wada, Kenji Ohoyama, Takeshi Matsukawa, Yukihiko Yoshida, Akinori Hoshikawa, Toru Ishigaki, Kazuaki Iwasa, Neutron diffraction studies on valence ordering compound YbPd, journal of the physical society of japan, 10.7566/JPSJ.87.114705, 87, 11, 2018.01. |
| 14. |
Hirofumi Wada, Yusuke Goki, Akihiro Mitsuda, Transport properties of EuNi2Ge2 under high pressure, Journal of the Physical Society of Japan, 10.7566/JPSJ.87.034707, 87, 3, 2018.01. |
| 15. |
Katsuya Ichiki, Kojiro Mimura, Hiroaki Anzai, Takayuki Uozumi, Hitoshi Sato, Yuki Utsumi, Shigenori Ueda, Akihiro Mitsuda, Hirofumi Wada, Yukihiro Taguchi, Kenya Shimada, Hirofumi Namatame, Masaki Taniguchi, Hard x-ray photoemission study of the temperature-induced valence transition system EuNi2(Si1-xGex)2, Physical Review B, 10.1103/PhysRevB.96.045106, 96, 4, 2017.07. |
| 16. |
Takayuki Ohnishi, Kei Soejima, Keiichiro Yamashita and Hirofumi Wada, Magnetocaloric properties of (MnFeRu)2(PSi) as magnetic refrigerants near room temperature, Magnetochemistry, 10.3390/magnetochemistry3010006, 3, 1, 6 (8 pages), 2017.02. |
| 17. |
Akihiro Mitsuda, Takumi Fujimoto, Eigo Kishaba, Suguru Hamano, Akihiro Kondo, Koichi Kindo, Hirofumi Wada, Large magnetoresistance and volume expansion associated with valence transition in Eu(Rh1-xIrx)2Si2, Journal of the Physical Society of Japan, 10.7566/JPSJ.85.124703, 85, 12, 2016.12. |
| 18. |
Hirofumi Wada, Yoshiro Maekawa, Daichi Kawasaki, High-field transport properties of itinerant electron metamagnetic Co(S1-xSex)2, Journal of Science: Advanced Materials and Devices, 10.1016/j.jsamd.2016.06.001, 1, 2, 179-184, 2016.06, The Co(S1-xSex)2 compounds are known to exhibit itinerant electron metamagnetism (IEM). We present field dependence of electrical resistivity and Hall effect of the compounds with 0 ≤ x < 0.15. It was found that the magnetoresistance shows a positive jump associated with the IEM. This jump is nearly independent of temperature. We also observed a jump in the field dependence of Hall resistivity, which is attributable to the anomalous Hall effect due to the onset of ferromagnetism. Our analyses revealed that the ordinary Hall coefficient decreases considerably by the IEM. These results are discussed in terms of the proposed electronic structure of CoS2, in which a highly spin polarized state is achieved.. |
| 19. |
Hirofumi Wada, T. Takahara, K. Katagiri, T. Ohnishi, K. Soejima, K. Yamashita, Recent progress of magnetocaloric effect and magnetic refrigerant materials of Mn compounds (invited), Journal of Applied Physics, 10.1063/1.4914120, 117, 17, 2015.05, Magnetocaloric and related properties of Ru and Ni substituted (MnFe)2(PSi) are presented. It is found that Ru and Ni are effective doping elements to reduce the thermal hysteresis of (MnFe)2(PSi). The origin of the thermal hysteresis is discussed on the basis of a thermodynamic model. It is shown that the elastic energy is responsible for the thermal hysteresis. We also show recent developments of the production process of Mn compounds in an industrial scale.. |
| 20. |
T. Koyama, T. Maruyama, K. Ueda, T. Mito, Akihiro Mitsuda, M. Umeda, M. Sugishima, Hirofumi Wada, Zero-field NMR and NQR studies of magnetically ordered state in charge-ordered EuPtP, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.91.094419, 91, 9, 2015.03, EuPtP undergoes two valence transitions and has two kinds of valence states of Eu ions at low temperatures. In the charge-ordered state, this compound shows an antiferromagnetic order ascribed to magnetic divalent Eu ions. We investigated the antiferromagnetically ordered state of EuPtP by nuclear magnetic resonance (NMR) measurement and nuclear quadrupole resonance (NQR) measurement in a zero external magnetic field. The observed Eu153 NMR signals of a magnetic divalent state and Eu151,153 NQR signals of a nonmagnetic trivalent state clearly demonstrate that the spins order in the hexagonal basal plane and the internal magnetic field is not canceled out, even at the Eu3+ layers which are in the middle of magnetic Eu2+ layers. In addition, the observation of 31P and Pt195 NMR spectra allowed us to discuss a possible magnetic structure. We also evaluated the nuclear quadrupole frequencies for both Eu2+ and Eu3+ ion states.. |
| 21. |
T. Mito, K. Nishitani, T. Koyama, H. Muta, T. Maruyama, G. Pristáš, K. Ueda, T. Kohara, Akihiro Mitsuda, M. Sugishima, Hirofumi Wada, NMR studies of ordered structures and valence states in the successive valence-transition system EuPtP, Physical Review B - Condensed Matter and Materials Physics, 10.1103/PhysRevB.90.195106, 90, 19, 2014.11, We have studied EuPtP, which undergoes two successive valence transitions at TA∼240 K and TB∼200 K by P31-nuclear magnetic resonance (NMR) measurements. From the analysis of NMR spectra, we obtained plausible ordered structures and Eu valence states in three phases divided by TA and TB. These ordered structures well explain observed inequivalent P sites and the intensity ratio of the NMR spectra arising from these P sites. The results are also in good accordance with mean Eu valence measured by the x-ray absorption spectroscopy. We also discuss Eu 4f states and the origin of the transitions from the measurements of nuclear spin lattice relaxation rate and hyperfine coupling constant.. |
| 22. |
K. Katagiri, K. Nakamura, Hirofumi Wada, Magnetocaloric properties and magnetic refrigerant capacity of MnFeP1-xSix, JOURNAL OF ALLOYS AND COMPOUNDS, 10.1016/j.jallcom.2012.11.127, 553, 286-290, 2013.03. |
| 23. |
M. Sugishima, K. Yamada, A. Mitsuda and H. Wada, Effects of magnetic field and pressure on the intermediate valence state of YbPd, J. Phys: Condens. Matter, 22, 37, 375601, 2011.08. |
| 24. |
A. Mitsuda, T. Okuma, K. Sato, K. Suga, Y. Narumi, K. Kindo and H. Wada, Pressure and magnetic field dependence of valence and magnetic transitions in EuPtP, J. Phys: Condens. Matter , 22 , 22, 226003, 2010.05. |
| 25. |
Y. H. Matsuda, Z. W. Ouyang, H. Nojiri, T. Inami, K. Ohwada, M. Suzuki, N. Kawamura,
A. Mitsuda and H. Wada
, X-ray Magnetic Circular Dichroism of a Valence Fluctuating State in Eu at High
Magnetic Fields
, Phys. Rev. Lett., vol. 103 (2009) 046402-1-4., 2009.07. |
| 26. |
A. Mitsuda, K. Yamada, M. Sugishima and H. Wada, Pressure effect on valence fluctuation and magnetic ordering in YbPd, Physica B, vol. 404 (2009) 3002-3004, 2009.07. |
| 27. |
H. Wada, S. Matsuo, and A. Mitsuda, Pressure dependence of magnetic entropy change and magnetic transition in MnAs1-xSbx, Phys. Rev. B, vol. 79 (2009) 092407-1-4, 2009.03. |
| 28. |
H. Wada, C. Funaba, T. Asano, M. Ilyn and A. M. Tishin, Recent progress of magnetocaloric effect of MnAs1-xSbx, Proc. Int. Conf. IIR on Magnetic Refrigeration at Room Temperature,
Montreux, Ed. P. Egolf, (IIR-IIF, Paris, 2005), 37-46, p37-p46., 2005.01. |
| 29. |
T. Morikawa and H. Wada, Effect of deviation from stoichiometry on magnetic and magnetocaloric properties in MnAs1-xSbx, J. Magn. Magn. Mater., vol. 272-276, Supplement 1, E583-584., 2004.08. |
| 30. |
T. Tohei, H. Wada and T. Kanomata, Negative magnetocaloric effect at the antiferromagnetic to ferromagnetic transition of Mn3GaC, J. Appl. Phys., vol. 94, 1800-1802., 2003.08. |
| 31. |
H. Wada, T. Morikawa, K. Taniguchi, T. Shibata, Y. Yamada and Y. Akishige, Giant magnetocaloric effect of MnAs1-xSbx in the vicinity of first-order
magnetic transition, Physica B, vol. 328, 114-116., 2003.04. |
| 32. |
H. Wada, H. Yoshioka, T. Goto, K. Koyama and K. Watanabe, Novel field-induced magnetic transitions of GdMn2Ge2, J. Phys. Soc. Japan, vol.72, No.12, 3197-3203., 2003.12. |
| 33. |
H. Wada, K. Taniguchi and Y. Tanabe, Extremely large magnetic entropy change of MnAs1-xSbx near room temperature, Mater. Trans., vol.43, No.1, 73-77., 2002.01. |
| 34. |
H. Wada and Y. Tanabe, Giant magnetocaloric effect of MnAs1-xSbx, Appl. Phys. Lett., vol. 79, No.20, 3302-3304., 2001.11. |
| 35. |
H. Wada, H. Gomi, A. Mitsuda and M. Shiga, Specific heat anomaly due to valence transition in Eu(Pd1-xPtx)2Si2, Solid State Commun., vol.117, 703-707., 2001.02. |
