The studies of microstructures and magnetic properties in ferromagnetic/antiferromagnetic bilayers and [Cu/Co] thin films

Abstract

The exchange bias of CoFe/(Co,Fe)O and Mn-oxide/FePt bilayers, and the magnetic properties of Cu/Co thin films are studied in this thesis. In Part I, we investigate the effects of ion-beam bombardment on the exchange bias in CoFe/(Co,Fe)O bilayers using single crystalline MgO substrates. Results show that the ion-beam bombardment altered the magnitude of the effective anisotropy field for bilayers grown on an MgO(110) substrate and shifted the easy-plane direction for bilayers grown on an MgO(100) substrate. Our results reflect that the choice of substrate and bombardment conditions provide an efficient route for tuning both the effective unidirectional and uniaxial magnetic anisotropies in magnetic thin films. In Part II, we investigate the exchange coupling between the bottom FePt layer and the different structures of capped Mn-oxide layers in Mn-oxide/FePt bilayers. Results show that the magnetization reversal of the soft FePt layer is strongly influenced by the capped Mn-oxide layer (Mn, MnO, and Mn3O4), as revealed by the enhanced coercivities in the Mn-oxide/FePt bilayers. The typical temperature dependence of magnetization between the zero-field-cooling (ZFC) and field-cooling (FC) curves is observed in an Mn-oxide (8% O2/Ar)/FePt bilayer exhibiting a blocking temperature (TB ≈ 120 K) close to the TN of MnO. However, the Mn/FePt bilayer exhibited unusual the temperature dependence of M on T, implying an intermixing between the Mn and FePt interfaces. This in turn resulted in the formation of interfacial antiferromagnetic FeMn and may give rise to a high irreversibility temperature (Tirr ≈ 400 K), compared to the almost identical ZFC and FC curves in Mn-oxide (21% and 30% O2/Ar)/FePt bilayers that are attributed to the weaker exchange coupling with FePt. In Part III, we examine Cu/Co thin films are grown on SiO2 substrates using a reactive ion-beam deposition technique. The polycrystalline Cu/Co thin films formed as a result of intermixing. The Cu/Co thin films consisted of metallic Cu, Co, CuCo, and the oxides Cu2O and CoO. Magnetometry results show that the ferromagnetic and superparamagnetic behaviors strongly depend on the measuring temperature and the amount of cobalt. However, a much reduced magnetization onset temperature, Ton., which depended on the amount of cobalt in the film, is observed.本論文利用雙離子束濺鍍系統製備(1)CoFe/(Co,Fe)-oxide長在MgO(100)，(110)單晶基板上經由離子束轟擊(VEH= 130V)；(2)藉由不同氧含量改變Mn-oxide/FePt與 (3)[Cu/Co]薄膜之微結構與磁性質的研究。研究結果顯示(1) CoFe/(Co,Fe)-oxide底層經由21% O2/Ar氧含量轟擊下(Co,Fe)O為岩鹽結構，磁性質方面，藉由離子束轟擊過後，室溫下明顯發現矯頑磁力(Hc)的下降與磁滯曲線形狀的改變。經由場冷卻降至低溫(180K和50K)，低溫下有明顯交換偏壓(Hex)產生，並且交換偏壓大小與轟擊跟單晶基板相關。MvsT曲線結果顯示，不同的單晶基板導致MvsT曲線的改變，並且離子束轟擊過後造成不可逆溫度(Tirr)的下降，可以得知單晶基板與適當的轟擊能量可以改變CoFe/(Co,Fe)O磁晶異相性和上層鈷鐵的磁矩排列並導致交換偏壓的變化。(2)Mn-oxide/FePt雙層薄膜結果顯示，軟磁FePt的磁化翻轉強烈受到上層Mn-oxide(Mn,MnO,Mn3O4)的影響，典型的MvsT曲線可以從Mn-oxide(8%)發現，並且阻隔溫度(T¬B)~120K接近MnO的尼爾溫度(TN)。然而Mn(0%)/FePt雙層薄膜異常的MvsT曲線意味著Mn與FePt之間的界面相互混合(intermixing)產生部分的FeMn結構。(3) [Cu/Co]多層薄膜研究結果顯示，從TEM橫結面結果判斷Cu/Co薄膜為相互混合的情況，並且與XRD，TEM繞射環與XPS可以發現有Co，Cu，CuCo，Cu2O與少量的CoO存在，這意味著輔助離子槍在轟擊薄膜的過程中有少量的氧含量導致薄膜有部分的氧化物存在。磁性質結果顯示，隨著不同的Co含量的改變，會有超順磁與鐵磁的現象轉換。低溫180K下，部分超順磁受到熱震動的影響降低轉變成鐵磁性，MvsT行為可以發現隨著Co含量的提高而導致上升了磁化起始溫度(Ton.)，這可能是隨著不同的Co含量導致CuCo量的改變進而影響到Ton.，然而TB~ 50K並無隨任何Co含量有任何變化。此外XacvsT曲線更可以印證出多種不同的相存在導致的變化。Contents Chapter 1 Introduction 1 1.1 Background 1 1.2 Possible applications based on the exchange bias phenomenon 2 1.2.1 GMR and TMR 2 1.2.2 Magnetic Random Access Memory (MRAM) 4 1.3 Motivation 5 1.4 Review of earlier work on the exchange bias phenomenon 6 1.4.1 M vs T behavior 6 1.4.2 Ion-beam bombardment effect 8 1.4.3 Interfacial uncompensated antiferromagnetic spins 9 1.4.4 Diluted antiferromagnets in exchange bias 10 1.5 Related work by our research group 11 1.6 Material selection 12 1.7 References for the first chapter 16 Chapter 2 Basic Concepts 19 2.1 Magnetic anisotropy 19 2.2 Theoretical models of the exchange bias phenomenon 21 2.2.1 Ideal interface 24 2.2.2 Random interface 26 2.2.3 Planar AF domain wall 27 2.2.4 Perpendicular interfacial coupling 28 2.2.5 Frozen interface 29 2.2.6 Domain state 30 2.2.7 Thermally induced spontaneous magnetization reversal 32 2.3 References for the second chapter 33 Chapter 3 Experimental equipment 35 3.1 Experimental flowchart 35 3.2 Dual ion-beam deposition (DIBD) technique 35 3.3 Characterizations 39 3.3.1 X-Ray Diffractometer (XRD) 39 3.3.2 Transmission Electron Microscopy (TEM) 41 3.3.3 X-ray Photoelectron Spectroscopy (XPS) 43 3.3.4 Vibrating Sample Magnetometer (VSM) 45 3.4 References for the third chapter 46 Chapter 4 Results and discussion 47 4-1 The exchange bias field in CoFe/(Co,Fe)O bilayers using ion-beam bombardment and single crystalline substrates 47 4.1.1 XRD spectra 47 4.1.2 TEM 49 4.1.3 XPS 52 4.1.4 Magnetic hysteresis loop results at room temperature and low temperature 56 4.1.5 Magnetothermal behavior (Magnetization vs Temperature) 64 4-2 Exchange coupling effects in FePt/Mn-oxide bilayers with different O2/Ar ratios 70 4.2.1 XRD spectra 70 4.1.2 TEM 73 4.2.3 Magnetic hysteresis loops at room temperature and low temperature 75 4.2.4 Magnetothermal behavior (Magnetization vs Temperature) 83 4-3 The microstructure and magnetism of Cu-Co thin films modified by deposition processes 85 4.3.1 XRD spectra 85 4.3.2 TEM 87 4.3.3 XPS 94 4.3.4 Magnetic hysteresis loop results at 298 K and 180 K and magnetothermal behavior. 96 4.4 References for the fourth chapter 105 Chapter 5 Conclusions 106 Curriculum Vitae 10