近年來多鐵電(multiferroic)材料的題材受到熱烈地討論，因為其同時具有鐵電與鐵磁等行為，可以利用作為新一代的微電子元件。CdCr2S4是目前被宣稱少數具有多鐵電行為的尖晶石結構(spinel structure)化合物，其展現出複雜的晶格、自旋、電偶極交互作用極為有趣。為了更深入研究，我們對單晶CdCr2S4樣品進行磁性與電性的量測。在磁性量測上，樣品之鐵磁相變溫度TC=84 K，且隨磁場增加而升高，由磁滯曲線可以看出軟鐵磁行為。在電性量測上，電阻率從剛開始的半導體行為，經由施加高電場而引發金屬-絕緣相變，且觀察到巨磁阻的特性；觀察介電常數量測，發現其具有兩種有趣的電偶極有序行為，在84 K有鐵磁有序所產生的玻璃態電偶級矩，54 K有一電場誘導的鐵電行為。此外，在外加電、磁場下測量磁化率與介電常數，有助於進一步瞭解自旋-電偶極(spin-dipole)之交互作用。

Multiferroic materials, showing strong coupling between ferroelectric and ferromagnetic interaction, have been the subject of intense research recently. These materials are potential candidates for the next generation in microelectronic device. CdCr2S4, one of the few multiferroics with spinel structure proposed in 2005, is in the limelight of scientific research, because of the complex interaction among lattice, spin and dipole. To investigate further, we have measured the magnetic and electric properties of single crystal CdCr2S4. The spins are coupled ferromagnetically below Curie temperature TC = 84 K. With increasing magnetic field, ferromagnetic transition is found to be shifted towards higher temperature. The magnetic hysteresis is satisfied with the characteristic of soft ferromagnet. Electric field induced permanent metal-insulator transition (MIT) and colossal magnetoresistance (CMR) are observed in the resistivity measurement. From dielectric constant measurement, two interesting dipolar ordering states have been observed. The peak near TC ~ 84 K is due to the glassy type dipolar state stimulated by the onset of ferromagnetic ordering and the other near 54 K, might be the ferroelectric ordered state induced by externally applied electric field. Both electric and magnetic field dependent physical properties have been studied and reveal significant evidences of strong spin-dipole coupling in the present system.

圖目錄…4
表目錄…7
第一章 簡介
第一節 尖晶石結構的特性介紹…8
第二節 CdCr2S4樣品介紹 …10
第二章 CdCr2S4之磁性研究
第一節 磁性介紹…15
第二節 儀器介紹…21
第三節 實驗結果…26
第三章 CdCr2S4之電性研究
第一節 電性介紹…37
第二節 測量方法…40
第三節 實驗結果…43
第四章 討論與結論…55
參考文獻…59
附錄 …60

[1]S. Satpathy and R.M. Martin, Phys. Rev. B 36, 7269 (1987).
[2]C. P. Sun, J. –Y. Lin, Y. C. Kang, S. Mollah, H. D. Yang, F. C. Xu, Y. C. Liao, and, M. K. Wu, Phys. Rev. B 70, 54519 (2004).
[3]Heinrich Hohl, Christian Kloc, and Ernst Bucher, J. Solid State Chem. 125, 216 (1996).
[4]S. Kondo, D. C. Johnston, C. A. Swenson, F. Borsa, A. V. Mahajan, L. L. Miller, T. Gu, A. I. Goldman, M. B. Maple, D. A. Gajewski, E. J. Freeman, N. R. Dilley, R. P. Dickey, J. Merrin, K. Kojima, G. M. Luke, Y. J. Uemura, O. Chmaissem, and J. D. Jorgensen, Phys. Rev. Lett. 78, 3729 (1997).
[5]P. K. Baltzer, P. J. Wojtowicz, M. Robbins, and E. Lopatin, Phys. Rev. 151, 367 (1966).
[6]H. W. Lehmann and M. Robbins, J. Appl. Phys. 37, 1389 (1966).
[7]W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature 442, 759 (2006).
[8]J. Hemberger, P. Lunkenheimer, R. Fichtl, H.-A. Krug von Nidda, V. Tsurkan, amd A. Loidl, Nature 434, 364 (2005).
[9]P. Lunkenheimer, R. Fichtl, J. Hemberger, V. Tsurkan, and A. Loidl, Phys. Rev. B 72, 060103 (2005).
[10]J. Hemberger, P. Lunkenheimer, R. Fichtl, S. Weber, V. Tsurkan, and A. Loidl, Physica B 378-380, 363 (2006).
[11]Gustau Catalan and James F. Scott, Nature 448, E4 (2007).
[12]L. Q. Yan, J. Shen, Y. X. Li, F. W. Wang, Z. W. Jiang, F. X. Hu, J. R. Sun, and B. G. Shen, Appl. Phys. Lett. 90, 262502 (2007).
[13]Vishnu C. Srivastava, J. Appl. Phys. 40, 1017 (1969).