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研究生:溫增明
研究生(外文):Wen, Zeng-Ming
論文名稱:鎳鋅鐵淦氧磁物系統磁性的梅思堡效應研究
論文名稱(外文):MOSSBAUER EFFECT STUDIES ON THE MAGNETIC PROPERTIES OF NI-ZN FERRITE SYSTEM
指導教授:鄭伯昆鄭伯昆引用關係
指導教授(外文):Zheng, Bo-Kun
學位類別:博士
校院名稱:國立清華大學
系所名稱:物理研究所
學門:自然科學學門
學類:物理學類
論文種類:學術論文
畢業學年度:68
語文別:中文
論文頁數:181
中文關鍵詞:梅思堡效應倪耳溫度平均自旋鬆弛率臨界指數磁偶交作用物理
外文關鍵詞:MOSSBAUER-EFFECTNEEL-TEMPERATUREAUERAGE-SPIUSRELAXATION-RATEORITICAL-EXPONENITDIPOLE-DIPOLE-INTERACTIONPHYSICS
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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-zinc
ferrite 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 magnetic
meighgors) 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-dipole
interaction) 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(anisotropic
part)。
///////
ABSTRACT
The nickel-zinc ferrite system (Zn)★(Ni)★Fe★O★, 0.20≦X≦0.75, has
been studied by means of the mossbauer effect. The spectra taken at
temperatures near and below the neel ten-perature T★ were explained
quantitatively by using the pertur-bation theory of relaxation lineshape
for magnetically orderd system. As the results of the successful fitting
of the experimental spectrum, the average spins and the longitudinal
relaxation rates of both A- and B-site ferric ions were obtained.
The average spins as functions of both tempertature and zinc-content have
been 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 greater
than that of the B-site ferric ion for a given zinc - content and a given
temperature.
(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 that
of the B-site ferric ion decreases faster.
(c) The temperature dependences of the average spins follow the simple
power relation
<S★>α (1-T/T★)★ β > 0
in the temperature ranges under consideration.
(d) The exponent β for the A-site ferric ion does not de-pend on the
zinc-content. The weighted average is 0.33 which is very close to the
critical exponent of the zero-field magne-tization for many magnetic
systems.
(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 were
interpreted qualitatively based on the consideration of nearest
magnetic-neighbors of the ferric ions.
The longitudinal relaxation rates of the A- and B-site ferric ions has
been investigated as functions of both tempera-ture and zinc-content. It
was observed that:
(a) The longitudinal relaxation rates are considerably greater than that
induced by the dipole-dipole interaction.
(b) For a given zinc-content and a given temperature, the longitudinal
relaxation rate of the A-site ferric ion is considerably greater than that
of the B-site ferric ion.
(c) The longitudinal relaxation rates of both A- and B-sit ferric ions
decrease when the zinc-content increases, but that of the B-site ferric
ion decreases faster.
(d) The longitudinal relaxation rates depend weakly on the temperature
when the temperature is far below the Neel tempera-ture. However, they
diverge when T★ is approached. The tem-perature dependences of the
longitudinal relaxation rates can be described by the relation
<R★>(T) α (1-T/T★)★ γ > 0
in the temperature ranges under consideration for most cases.
(e) The relaxation rate of A-site ferric ion diverges more sharply than
that of B-site ferric ion.
(f) The sharpness of the divergences of the A- and B-site relaxation rates
decrease when the zinc-content increases, but that of the B-site
relaxation rate decreases faster. For the A-site ferric ion, γ increases
from 0.1±0.1 for X=0.30 to 0.4±0.3 for X=0.07. For the B-site ferric
ion, 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 dominant
relaxation mechanism involves the modulation of the anisotropic part of
the A-O-B superexchange interaction.
COVER,ABSTRACT (in Chinese),ABSTRACT,謝詞,table of contents
Ⅰ. Introduction
Ⅱ. Physical Properties of Ni-Zn ferrites
1. Crystal Structure
2. Magnetic Properties
3. Magnetocrystalline Anisortropy
Ⅲ. Ferromagnetic Relaxation in Magnetic Dielectrics
1. Longitudinal and Transverse Relaxation Rates of the Spin
2. Time Correlation Functions of the Spin
3. Relaxation Processes in Magnetic Dielectrics
4. The Effect of Polycrystallinity on the Transverse Relaxation
Ⅳ. Mossbauer Spectra in the Presence of Electron Spin Relaxation
1. Hyperfine Interactions
2. General Expression for the Mossbauer Line-shape
3. Stochastic Theory of Mossbauer Relaxation Line-shape
4. Perturbation Theories of Mossbauer Relaxation Line-shape
Ⅴ. Experimental Apparatuses and Procedures
1. Mossbauer Spectrometer and Velocity Calibration
2. Temperature Control System and Temperature Measurement
3. Sample Preparation and Examination
Ⅵ. Data Analysis and Discussion
1. The Mossbauer Spectra of Nickel-Zinc ferrites
2. The Fitting of Mossbauer Relaxation Spectra
3. Average Spins of Ferric Ions
4. Longitudianl Relaxation Rates of Ferric Ions
Ⅶ. Conclusions
REFERENCES
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