鈣錳氧單晶之磁性與介電行為研究

Abstract

CaMn2O4是自然界中天然的礦物，俗稱黑鈣錳礦(Marokite)，因殘留在固態合成法所製成的龐磁阻錳氧化物中，而被發現進而研究其磁特性，文獻指出CaMn2O4的尼爾溫度在220 K，會由順磁態轉變成反鐵磁態，且在低於75 K以下有一個未知的順磁訊號造成磁化率增加，這個現象十分有趣，且在CaMn2O4的電性上也很少有人討論。 本論文主要研究CaMn2O4單晶的電傳輸與介電性質對溫度與頻率的關係，本論文研究之單晶是利用高溫溶液生長法製成，大小約為1x2x0.2 mm3。首先我們利用X光繞射與磁化率量測的結果證實樣品為高純相且具正交結構的CaMn2O4樣品，也確認出單晶的軸向。接著，本論文將討論CaMn2O4單晶沿著c軸的方向，其介電常數、介電損耗及電導率與溫度和頻率的關係，進而與磁特性做比較。上述這些實驗顯示反鐵磁性相變會引發磁介電效應，本論文也會對誘發此效應的機制做討論。

Marokite, CaMn2O4 a natural mineral, was identified as an impurity phase ubiquitously existing in colossal magnetoresistive doped rare-earth manganites prepared by solid-state reaction synthesis method. Nevertheless, owing to the interesting magnetic properties unveiled subsequently, it has since received some renewed interest. In addition to several novel antiferromagnetic structures being identified, the unknown paramagnetic signal below 75 K has been observed. In this study, high-quality single crystals with dimension of 1 mm x 2 mm x 0.2 mm of CaMn2O4 grown by flux melt growth method were used to further delineate the electric transport and dielectric properties as functions of temperature and frequency. The preliminary X-ray diffraction and magnetic susceptibility measurements evidently confirmed that the obtained material is indeed the stoichiometric orthorhombic structured (space group Pbcm) marokite crystal, exhibiting a clear antiferromagnetic ordering below the Néel temperature of 220 K. Further characterization on the temperature dependence of the permittivity, dielectric loss and conductivity will be carried out and the correlations with the magnetic phase transition will be presented and discussed. These experimental results suggest that the magnetodielectric effect response is very strong in CaMn2O4.