在自旋不穩態系統中，晶體結構和磁矩排列扮演著十分重要的角色，而磁場和壓力則是能讓磁矩有序和晶體型變最關鍵的誘因。近年來透過勞倫茲電子顯微鏡和中子散射在矽化錳(MnSi)發現了新物理現象skyrmion，skyrmion是一種在特定磁場、溫度下磁矩會呈現漩渦狀的現象，令我們產生研究的興趣。
本論文中，我們探討兩種自旋不穩態系統，分別為立方亞鐵磁材料硒銅氧(Cu2OSeO3) 和準一維雙鏈反鐵磁材料鈉鉻氧(NaCr2O4)。硒銅氧的亞鐵磁相變發生在60 K附近，在壓力下的交流磁化率呈(dT_C)⁄(dP=0.3 K⁄kbar) 上升。另一方面，鈉鉻氧為巨磁阻半導體，伴隨著鉻離子混價Cr3+、Cr4+的狀態，反鐵磁相變溫度Tpeak為123 K，在壓力下量交流磁化率可觀察到 (dT_peak)⁄dP=-0.17K⁄kbar的速率往低溫區移動的趨勢，並且磁滯曲線磁場接近35 kOe有自旋反轉的現象。本論文所探討的兩個樣品的磁化強度在高磁場下都被強化同時都有自旋反轉的現象發生，但交流磁化率在高壓的環境下則具有相反的趨勢。最後我們量測交流磁化率檢驗此兩種樣品是否為Skyrmion樣品。

Both the spin configuration and lattice geometry play a important role in geometrically frustrated spin systems. Magnetic field and hydrostatic pressure on the two frustrated systems were performed, resulting in enhanced spin alignment and lattice deformation. There is a considerable research interest in skyrmion whose magnetic properties have a remarkable characteristic as a vortex-like spin orientation. Recently, neutron scattering and Lorentz TEM measurements showed that cubic MnSi exists a skyrmion state.
In this thesis, two kinds of interesting materials are investigated. One is quasi-1D double chain antiferromagnet NaCr2O4, the other is cubic-like ferrimagnet Cu2OSeO3. For Cu2OSeO3, the peak temperature increases with a rate of (dT_C)⁄(dP=0.3 K⁄kbar). Meanwhile, we observe a second metastable spin-ordered state from dM⁄(dH vs H). In the polycrystalline compound NaCr2O4, it shows similar transition to that of ferromagnet at 123 K, but it is antiferromagnetic duo to its interaction. The peak temperature decreases linearly with a rate of (dT_N)⁄dP=-0.17K⁄kbar under pressure.
From magnetic properties, their common similarities including TC is enhanced by applied magnetic fields and spin-flop exists in hysteresis loop. However, their TC under pressure is the opposite. Finally, we have measured ac susceptibility for skyrmion behavior by scaning magnetic field. The physical significance for the interaction of skyrmion is discussed.

致謝 i
論文摘要 ii
Abstract iii
目錄 v
圖目錄 vii
表目錄 viii
第一章 簡介 1
1-1 多鐵性介紹： 1
1-2 自旋不穩態 7
1-3 樣品介紹： 9
Cu2OSeO3系統： 9
NaCr2O4系統： 12
1-4研究動機： 21
第二章 實驗儀器介紹 23
2-1樣品製備： 23
Cu2OSeO3系統： 23
NaCr2O4系統： 23
2-2 磁性量測儀器： 25
2-3 壓力效應： 28
第三章 實驗與結果 33
3-1 Cu2OSeO3系統： 33
3-2 NaCr2O4系統： 41
第四章 結論 47
參考文獻 49
附錄一：Cu2OSeO3多晶樣品製備 56

J. P. Velev, S. S. Jaswal, and E. Y. Tsymbal, “Multi-ferroic and magnetoelectric materials and interfaces”, Phil. Trans. R. Soc. A 369, 3069 (2010)
N. A. Spalddin and M. Fiebig, “The renaissance of magnetoelectric multiferroic”, Science 309, 391 (2005)
K. Yamauchi and S. Picozzi, “Interplay between charge order, ferroelectricity, and ferroelasticity：tungsten bronze structures as a playground for multiferroicity”, Phys. Rev. Lett. 105, 107202 (2010)
D. Khomskii, “Classifying multiferroics: mechanisms and effects”, Physics 2, 20 (2009)
C. C. Chou, C. L. Huang, S. Mukherjee, Q. Y. Chen, H. Sakurai, A. A. Belik, E. Takayama-Muromachi, and H. D. Yang, “Multiple magnetic transitions in multiferroic BiMnO3”, Phys. Rev. B 80, 184426 (2009)
A. A. Belik and E. Takayama-Muromachi, “Ac susceptibility studies of multiferroic BiMnO3 and solid solutions between BiMnO3 and BiScO3”, J. Phys.: Condens. Matter 20, 025211 (2008)
V. Goian, S. Kamba, M. Savinov, D. Nuzhnyy, F. Borodavka, P. Vane ̌k, and A. A. Belik, “Absence of ferroelectricity in BiMnO3 ceramics”, J. Appl. Phys. 112, 074112 (2012)
T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, “Magnetic control of ferroelectric polarization”, Nature 426, 55 (2003)
N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, and S.-W. Cheong, “Electric polarization reversal and memory in a multiferroic material induced by magnetic fields”, Nature 429, 392 (2004)
S.-W. Cheong and M. Mostovoy, “Multiferroics：a magnetic twist for ferroelectricity”, Nature materials 6, 13 (2007)
S. Nakatsuji, Y. Nambu, H. Tonomura, O. Sakai, S. Jonas, C. Broholm, H. Tsunetsugu, Y. Qiu, Y. Maeno, “Spin disorder on a triangular lattice”, Science 309, 1697 (2005)
I. Terasaki, Y. Sasago, and K. Uchinokura, “Large thermoelectric power in NaCo2O4 single crystals“, Phys. Rev. B 56, R12685 (1997)
D. Grohol, K. Matan, J.-H. Cho, S.-H. Lee, J. W. Lynn, D. G. Nocera, and Y. S. Lee, “Spin chirality on a two dimensional frustrated lattice”, Nature materials 4, 323 (2005)
T. M. McQueen, D. V. West, B. Muegge, Q. Huang, K. Noble, H. W. Zandbergen, and R. J. Cava, “Frustrated ferroelectricity in niobate pyrochlores”, J. Phys.: Condens. Matter 20, 235210 (2008)
M. Pregelj, O. Zaharko, A. Zorko, Z. Kutnjak, P. Jeglic ̌, P. J. Brown, M. Jagodic ̌, Z. Jaglic ̌ic ́, H. Berger, and D. Arc ̌on, “Spin amplitude modulation driven magnetoelectric coupling in the new multiferroic FeTe2O5Br”, Phys. Rev. Lett. 103, 147202 (2009)
Jan-Willem G. Bos, C. V. Colin, and T. T. M. Palstra, “Magnetoelectric coupling in the cubic ferrimagnet Cu2OSeO3”, Phys. Rev. B 78, 094416 (2008)
M. Belesi, I. Rousochatzakis, M. Abid, U. K. Ro ̈βler, H. Berger, and J.-Ph. Ansermet, “Magnetoelectric effects in single crystals of the cubic ferrimagnetic helimagnet Cu2OSeO3”, Phys. Rev. B 85, 224413 (2012)
I. Z ̌ivkovic ́, D. Pajic ́, T. Ivek, and H. Berger, “Two-step transition in a magnetoelectric ferrimagnet Cu2OSeO3”, Phys. Rev. B 85, 224402 (2012)
S. Seki, X. Z. Yu, S. Ishiwata, and Y. Tokura, “Observation of skyrmions in a multiferroic material”, Science 336, 198 (2012)
V. P. Gnezdilov, K.V. Lamonova, Y. G. Pashkevich, P. Lemmens, H. Berger, F. Bussy, and S. L. Gnatchenko, “Magnetoelectricity in the ferrimagnetic Cu2OSeO3：symmetry analysis and Raman scattering study”, Low Temp. Phys. 36, 550 (2010)
M. I. Kobets, K. G. Dergachev, E. N. Khatsko, A. I. Rykova, P. Lemmens, H. Berger, and D. Wulferding, “Microwave absorption in the frustrated ferrimagnet Cu2OSeO3”, Low Temp. Phys. 36, 176 (2010)
K. Kohn, “A new ferrimagnet Cu2SeO4”, J. Phys. Soc. Jpn. 42, 2065 (1977)
H. Sakurai, T. Kolodiazhnyi, Y. Michiue, E. Takayama-Muromachi, Y. Tanabe, and H. Kikuchi, “Unconventional colossal magnetoresistance in sodium chromium oxide with a mixed-valence state”, Angew. Chem. Int. Ed. 51, 6653 (2012)
A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, and Y. Tokura, “Insulator-metal transition and giant magnetoresistance in La1-xSrxMnO3”, Phys. Rev. B 51, 14103 (1995)
S. Nakamura, Y. Tadokoro, Y-J Shan, and T. Nakamura, “Magnetic field-induced insulator-to-metal transition in perovskite manganites Eu1-xSrxMnO3”, J. Phys. Soc. Jpn. 68, 1485 (1999)
Y. Tomioka, A. Asamitsu, H. Kuwahara, and Y. Moritomo, “Magnetic-field-induced metal-insulator phenomena in Pr1-xCaxMnO3 with controlled charge-ordering instability”, Phys. Rev. B 53, R1689 (1996)
Y. Tomioka, A. Asamitsu, Y. Moritomo, and Y. Tokura, “Anomalous magnetotransport properties of Pr1-xCaxMnO3”, J. Phys. Soc. Jpn. 64, 3626 (1995)
Y. Tokura, H. Kuwahara, Y. Moritomo, Y. Tomioka, and A. Asamitsu, “Competing instabilities and metastable states in (Nd,Sm)1/2Sr1/2MnO3”, Phys. Rev. Lett. 76, 3184 (1996)
K. Yamaura, M. Arai, A. Sato, A. B. Karki, D. P. Young, R. Movshovich, S. Okamoto, D. Mandrus, and E. Takayama-Muromachi, “NaV2O4: a quasi-1D metallic antiferromagnet with half-metallic chains”, Phys. Rev. Lett. 99, 196601 (2007)
M. Itoh, H. Takeda, Y. Shimizu, H. Sakurai, M. Isobe, and Y. Ueda, “Magnetic interactions and orbital state in double chain systems investigated by NMR measurements”, J. Phys.: Conf. Series 320, 012070 (2011)
S. Mu ̈hlbauer, B. Binz, F. Jonietz, C. Pfleiderer, A. Rosch, A. Neubauer, R. Georgii, and P. Bo ̈ni, “Skyrmion lattice in a chiral magnet”, Science 323, 915 (2009)
N. Potapova, V. Dyadkin, E. Moskvin, H. Eckerlebe, D. Menzel, and S. Grigoriev, “Magnetic ordering in bulk MnSi crystals with chemically induced negative pressure”, Phys. Rev. B 86, 060406® (2012)
W. Mu ̈nzer, A. Neubauer, T. Adams, S. Mu ̈hlbauer, C. Franz, F. Jonietz, R. Georgii, P. Bo ̈ni, B. Pedersen, M. Schmidt, A. Rosch, and C. Pfleiderer, “Skyrmion lattice in the doped semiconductor Fe1-xCoxSi”, Phys. Rev. B 81, 041203(R) (2010)
S. X. Huang and C. L. Chien, “Extended skyrmion phase in epitaxial FeGe(111) thin films”, Phys. Rev. Lett. 108, 267201 (2012)
I. A. Sergienko and E. Dagotto, “Role of the Dzyaloshinskii-Moriya interaction in multiferroic perovskites”, Phys. Rev. B 73, 094434 (2006)
J. H. Yang, Z. L. Li, X. Z. Lu, M.-H. Whangbo, S.-H. Wei, X. G. Gong, and H. J. Xiang, “Strong Dzyaloshinskii-Moriya interaction and origin of ferroelectricity in Cu2OSeO3”, Phys. Rev. Lett. 109, 107203 (2012)
C. D. Hu, “The Dzyaloshinskii-Moriya interaction in metals”, J. Phys.: Condens. Matter 24, 086001 (2012)
S. Mukherjee, A. K. Pal, S. Bhattacharya, and S. Chattopadhyay, “Field-induced spin-flop transitions of interacting nanosized α-Fe2O3 particles dispersed in silica glass matrix”, J. Phys.: Condens. Matter 20, 055204 (2008)
Y. Tokura and S. Seki, “Multiferroics with Spiral Spin Orders”, Adv. Mater. 22, 1554 (2010)
K. H. Miller, X. S. Xu., H. Berger, E. S. Knowles, D. J. Arenas, M. W. Meisel, and D. B. Tanner, “Magnetodielectric coupling of infrared phonons in single-crystal Cu2OSeO3”, Phys. Rev. B 82, 144107 (2010)
M. Belesi, T. Philippe, I. Rousochatzakis, H. C. Wu, H. Berger, S. Granville, I. V. Shvets, and J.-Ph. Ansermet, “Magnetic properties of the magnetoelectric compound Cu2OSeO3：magnetization and 77Se NMR study”, J. Phys.: Conf. Series 303, 012069 (2011)
M. Belesi, I. Rousochatzakis, H. C. Wu, H. Berger, I. V. Shvets, F. Mila, and J. P Ansermet, “Ferrimagnetism of the magnetoelectric compound Cu2OSeO3 probed by 77Se NMR”, Phys. Rev. B 82, 094422 (2010)
B. Mart1 ́nez, V. Laukhin, M. Hernando, and J. Fontcuberta, “Enhancement of antiferromagnetic coupling in the quasi-one-dimensional Ca3Co2O6 ferrimagnet”, Phys. Rev. B 64, 012417 (2001)
C. Thessieu, C. Pfleiderer, A. N. Stepanov and J. Flouquet, “Field dependence of the magnetic quantum phase transition in MnSi”, J. Phys.: Condens. Matter 9, 6677 (1997)
K. Koyama, T. Goto, T. Kanomata, and R. Note, “Observation of an itinerant metamagnetic transition in MnSi under high pressure”, Phys. Rev. B 62, 986 (2000)
C. L. Huang, K. F. Tseng, C. C. Chou, S. Mukherjee, J. L. Her, Y. H. Matsuda, K. Kindo, H. Berger, and H. D. Yang, “Observation of a second metastable spin-ordered state in ferrimagnet Cu2OSeO3”, Phys. Rev. B 83, 052402 (2011)
T. Adams, A. Chacon, M. Wagner, A. Bauer, G. Brandl, B. Pedersen, H. Berger, P. Lemmens, and C. Pfleiderer, “Long-wavelength helimagnetic order and skyrmion lattice phase in Cu2OSeO3”, Phys. Rev. Lett. 108, 237204 (2012)
A. Bauer, A. Neubauer, C. Franz, W. Mu ̈nzer, M. Garst, and C. Pfleiderer, “Quantum phase transitions in single-crystal Mn1-xFexSi and Mn1-xCoxSi: crystal growth, magnetization, ac susceptibility, and specific heat”, Phys. Rev. B 82, 064404 (2010)
F. Damay, C. Martin, V. Hardy, A. Maignan, G. Andre ́, K. Knight, S. R. Giblin, and L. C. Chapon, “Zigzag ladders with staggered magnetic chirality in the S=3/2 compound β-CaCr2O4”, Phys. Rev. B 81, 214405 (2010)
A. V. Andreev, M. D. Kuz^，min, Y. Skourski, N. V. Kudrevatykh, K. Kindo, F. R. de Boer, and J. Wosnitza, “High-field magnetization study of a Tm2O17 single crystal”, Phys. Rev. B 81, 134429 (2010)
C. C. Lin, “Electric and magnetic properties of Spinel CdCr2S4”, Department of Physics National Sun Yat –Sen University Master thesis, (2008)