Abstract
本文是作者用梅思堡效應(Mossbauer effect)研究鎳鋅鐵淦氧磁物系統(nickel-zincferrite system)磁學性質的詳細報告。作者攝取了鎳鋅鐵淦氧磁物系統─(Zn)★(Ni)★Fe★O★0.20≦x≦0.75─在倪耳溫度(Neel temperature) T★附近以及以下各溫度的梅思堡能譜, 並用磁性有序系統鬆弛線形的微擾理論來定量解釋這些所得的能譜。應用這個理論所計算出來的能譜能夠成功地吻合實驗所得的數據。由於此項成功的吻合, 作者粹取了AB 兩晶鐵離子的平均自旋(average spins)以及縱向鬆弛率(longitudinal relaxation rates)。對AB兩晶座鐵離子平均自旋的溫度相依性和鋅含量相依性所作的研究發現:(a) 同一鋅含量, 同一溫度下, A 晶座鐵離子的平均自旋較B 晶座鐵離子的為大。(b) 在同一相對溫度T/T★下, AB 兩晶座鐵離子的平均自旋都隨鋅含量的加而減少,但是 B 晶座鐵離子的平均自旋減少較快。(c) 在接近倪耳溫度時, 平均自旋的溫度相依性可用下式來描述:<S★>(T)α(1-T/T★)★ B>0 。(d) A 晶座鐵離子的β指數和鋅含量無關, 其計權平均(weighted average)為0.33°此數值與許多磁性系統的零場磁化強度(zero-field magnetization) 的臨界指數(critical exponent)極為接近。(e) B 晶座鐵離子的β指數首先從 x=0.20的0.48±0.01 增至 x=0.60的0.97±0.07,然後又降到 x=0.75 的 0.52±0.10 。對平均自旋的這些行為, 作者經由考慮鐵離子的最近磁性鄰居(nearest magneticmeighgors) 而加以定性的解釋。對 AB 兩晶座鐵離子縱離子縱向鬆弛率的溫度相依性以及鋅含量相依性所作的研究顯示:(a) AB 兩晶座鐵離子的縱向鬆弛率都高於磁偶 ─ 磁偶交互作用(dipole-dipoleinteraction) 所引發者。(b) 同一鋅含量, 同一溫度下, A 晶座鐵離子的縱向鬆弛率比 B 晶座鐵離子的高。(c) 鋅含量增加時, AB 兩晶座鐵離子的縱向鬆弛率都告降低, 但是 B 晶座鐵離子的縱向鬆弛率降低比較快。(d) 在溫度遠低於倪耳溫度時, 縱向鬆弛率對溫度的相依性較弱。然而在接近倪耳溫度時, 縱向鬆弛率劇烈增加, 趨近無窮大。換言之, 在接近倪耳溫度時, 縱向鬆弛率就發散(diverge) 了。在大多數情況下, 縱向鬆弛率可用下式描述:R★(T)α (1-T/T★)★ γ>0(e) A 晶座鐵離子縱向鬆弛率的發散比 B 晶座鐵離子縱向鬆弛率的發散尖銳。也就是 A 晶座鐵離子的γ指數比 B 晶座鐵離子的小。(f) AB 兩晶座鐵離子縱向鬆弛率的尖銳度隨鋅含量的增加而降低較快。對 A 晶座鐵離子來說, γ指數從 x=0.30 的 0.1±0.1 緩增到 x=0.70 的 0.4±0.3 。對 B 晶座鐵離子而言, γ指數從 x=0.30 的 0.21±0.06 急增到 x=0.75 的 0.90±0.17。根據以上的結果, 作者相信在鎳鋅鐵淦氧磁物系統中, 最重要的縱向鬆弛機制牽涉到A-O-B 超交換交互作用(superexchange interaction)的非均向性部分(anisotropicpart)。///////ABSTRACTThe nickel-zinc ferrite system (Zn)★(Ni)★Fe★O★, 0.20≦X≦0.75, hasbeen studied by means of the mossbauer effect. The spectra taken attemperatures near and below the neel ten-perature T★ were explainedquantitatively by using the pertur-bation theory of relaxation lineshapefor magnetically orderd system. As the results of the successful fittingof the experimental spectrum, the average spins and the longitudinalrelaxation rates of both A- and B-site ferric ions were obtained.The average spins as functions of both tempertature and zinc-content havebeen studied for both A- and B-site ferric ions. It was found that:(a) The average spin of the A-site ferric ion is consider-ably greaterthan that of the B-site ferric ion for a given zinc - content and a giventemperature.(b) For a given relative temperature T/ T★, the average spins of both A -and B-site ferric ions decrease when the zinc-content increases, but thatof the B-site ferric ion decreases faster.(c) The temperature dependences of the average spins follow the simplepower relation<S★>α (1-T/T★)★ β > 0in the temperature ranges under consideration.(d) The exponent β for the A-site ferric ion does not de-pend on thezinc-content. The weighted average is 0.33 which is very close to thecritical exponent of the zero-field magne-tization for many magneticsystems.(e) The exponent β for the B-site ferric ion first increase from 0.48±0.01 for X=0.20 to 0.97±0.07 for X=0.06 and then de-creases to 0.552±0.10 for X=0.75.The behaviors of the average spins versus the zinc-content wereinterpreted qualitatively based on the consideration of nearestmagnetic-neighbors of the ferric ions.The longitudinal relaxation rates of the A- and B-site ferric ions hasbeen investigated as functions of both tempera-ture and zinc-content. Itwas observed that:(a) The longitudinal relaxation rates are considerably greater than thatinduced by the dipole-dipole interaction.(b) For a given zinc-content and a given temperature, the longitudinalrelaxation rate of the A-site ferric ion is considerably greater than thatof the B-site ferric ion.(c) The longitudinal relaxation rates of both A- and B-sit ferric ionsdecrease when the zinc-content increases, but that of the B-site ferricion decreases faster.(d) The longitudinal relaxation rates depend weakly on the temperaturewhen the temperature is far below the Neel tempera-ture. However, theydiverge when T★ is approached. The tem-perature dependences of thelongitudinal relaxation rates can be described by the relation<R★>(T) α (1-T/T★)★ γ > 0in the temperature ranges under consideration for most cases.(e) The relaxation rate of A-site ferric ion diverges more sharply thanthat of B-site ferric ion.(f) The sharpness of the divergences of the A- and B-site relaxation ratesdecrease when the zinc-content increases, but that of the B-siterelaxation rate decreases faster. For the A-site ferric ion, γ increasesfrom 0.1±0.1 for X=0.30 to 0.4±0.3 for X=0.07. For the B-site ferricion, γincrease from 0.21±0.06 for X=0.30 to 0.90±0.17 for X=0.75.Basing on the above observations, the author believes that the dominantrelaxation mechanism involves the modulation of the anisotropic part ofthe A-O-B superexchange interaction.