低維度磁性材料SrCo2V2O8的核磁共振研究

Abstract

本論文主要是利用核磁共振(NMR)的技術來探討準一維自旋鏈(spin-chain)材料SrCo2V2O8 中所出現的反鐵磁相變。我們量測樣品中V51的NMR訊號，觀測其NMR頻譜於不同溫度及磁場下的變化。在固定磁場(B~26.8KG)下，磁場平行於晶格的c軸時，我們發現此樣品在T=4K，開始從高溫的順磁態進入到低溫的反鐵磁態，但在T=3.85-3.9K，出現順磁-反鐵磁共存的態，然後又在T=3.3K~3.7K，出現spin-flop的相轉變，最後才達到穩定的反鐵磁態。從頻譜的結構分析，我們認為此反鐵磁態中，Co+2的磁矩排列為commensurate。另外，為了探討樣品中在磁場下所出現的metamagnetic相變，我們固定溫度(T=2.5K)，量測樣品在不同磁場下的頻譜，發現此metamagnetic相變主要是由於spin-flop transition所引起，而非單純磁場所引發的順磁-反鐵磁相變。藉由本實驗的結果並與其他類似的準一維自旋鏈材料做比較，將有助於更深入的瞭解自旋鏈材料的磁性。

The field of low-dimensional magnetism has received considerable attention in recent decades, as the theoretical modeling of interactions at low dimensions can be done easily, and novel phenomena often occur experimentally. This thesis reports a nuclear magnetic resonance (NMR) study of the quasi-one-dimensional antiferromagnetic (AFM) spin-chain system, SrCo2V2O8, including a detailed study of its magnetic phase diagram. We measured the V51 NMR spectra at different temperatures and magnetic fields. For a fixed NMR field (~26.8kG) applied parallel to the c axis, the temperature-swept spectra show an onset of AFM transition at 4K, but there is a coexistence of paramagnetic and antiferromagnetic phases down to 3.9K. Following that is a spin-flop transition in the range of T=3.3-3.9K. The stable AFM state finally occurs below 3.3K. By analyzing the spectra, we found that the antiferromagnetic ordering of Co+2 moments is commensurate with the lattice. We also investigated the field-induced metamagnetic transition by taking the V51 spectrum at several different fields across the phase boundary at 2.5K. We found that the metamagnetism is caused by the spin-flop transition, and not the paramagnetic-antiferromagnetic transition. Our findings will also facilitate a better understanding of similar quasi-one-dimensional spin-chain systems such as PbCo2V2O8, as well as related theoretical studies.