Depth Profiling Photoelectron-Spectroscopic Study of an Organic Spin Valve with a Plasma-Modified Pentacene Spacer

[[abstract]]We report an enhanced magnetoresistance (MR) in an organic spin valve with an oxygen plasma-treated pentacene (PC) spacer. The spin valve containing PC without the treatment shows no MR effect, whereas those with moderately plasma-treated PC exhibit MR ratios up to 1.64% at room temperature. X-ray photoelectron spectroscopy with depth profiling is utilized to characterize the interfacial electronic properties of the plasma-treated PC spacer which shows the formation of a derivative oxide layer. The results suggest an alternative approach to improve the interface quality and in turn to enhance the MR performance in organic spin valves.[[incitationindex]]SCI[[booktype]]電子

Fabrication and characterization of pseudo-spin valve with organic spacer

我們試著去製作以有機分子做為位障的準自旋閥，並使用兩種方法形成有機薄膜，分別是形成兩性分子的 Langmuir-Blodgett (LB) 膜以及利用蒸鍍方法製作五苯。本實驗的目的是在有機接面觀測磁阻效應，並量測其電性、磁性和表面特性。本文藉由改變其位障的結構以量測磁電阻的改變。首先在 LB 膜上覆蓋一層氧化鋁(~1 nm)，觀察其電阻的變化，可發現其接面的電阻能有效地提高。再而改變有機分子位障，由 LB 膜改變成五苯，並利用氧化以改變其性質，比較氧化前後接面的電阻改變，進而改變氧化時間、厚度，加熱溫度，找出和磁性組態一致的磁阻效應。In this study, we try to fabricate magnetic tunnel junctions with organic spacer. Two processing techniques, Langmuir-Blodgett (LB) method and thermal evaporation, are applied for the preparation of the organic layer. While the stearic acid (SA) and BEDT-TTF (ET) are deposited by LB method, pentacene (Pn) film is prepared by thermal evaporation system. In the first part of this thesis, we grow ET and SA by LB film method. With the help of four-terminal measurement, the measured junction resistance (Rj) is positive as the spacer is four mono-layer (ML) and negative if the spacer is less than three ML. The negative measured resistance is a result of the smaller junctions’ resistance as compared to the resistance of electrodes. In addition, magnetoresistance (MR) effect can not be observed in these junctions. In order to increase Rj, Al2O3 of 1 nm thickness is deposited prior to the subsequent growth of LB films. By introducing the Al2O3 to the junctions, Rj may increase to the range of 102 ohm and 106 ohm, while the LB-prepared spacer are one and two ML, respectively. In the second part, the Pn spacer layer is deposited by thermal evaporation. However, the Rj is negative and there is no MR effect. Therefore, we intentionally exposed Pn to oxygen plasma in order to remodel the Pn films. As a result, the Rj increases and the two-step R-H loop is found to consist with M-H loop. Hence, we tried the fine tunings of preparation conditions, such as the exposure time in oxygen plasma, thickness of Pn and annealing temperature. MR effect can then be observed. Moreover, the MR ratio may be up to 0.5 %, if Pn films of thickness 14 nm are annealed together with CoFe layer at 130 ℃ and 157 ℃ for 1 hour and exposed to oxygen plasma for 4 minutes at room temperature. The change of Pn to “6, 13-pentacenequinone”, an endoperoxide or the formation of peroxy bridge between Pn molecules are thought responsible for the results.Contentsbstract …………………..……………………………..………….…………… ivist of Figures ……………………………………………………………… viiiist of Tables ………………………………………………………………..... xii Introduction ……………………...…………………………………………. 1 Basic concepts …………………………………………………….………… 5.1 Introduction to Magnetoresistance ……………………………………………. 5.1.1 Giant magnetoresistance (GMR) ………………………………………... 7.1.2 Tunneling magnetoresistance (TMR) ………………………………….. 10.2 Organic Material for Tunneling Junctions …………………………………….. 13.2.1 Electric transport in organic semiconductors ...…………………………… 13.2.2 Organic materials ………………………………………………………...15 Experimental instruments …………………………………………… 19.1 Fabrication Apparatus ………………………………………………………... 19.1.1 Ultra high vacuum system ……………………………………………... 19.1.2 Pumping system ……………………………………………………….. 22.1.3 Magnetron sputtering system …………………………………………... 22i.1.4 Thermal evaporation …………………………………………………… 24.1.5 Langmuir-Blodgett (LB) method ……….……………………………… 25.2 Specimen Characterization ……………………………………………………. 27.2.1 Atomic force microscopy (AFM) ……………………………………… 27.2.2 X-ray diffraction (XRD) …………………………..…….……………... 29.2.3 Four-probe resistance measurement ……………..…………………… 31.2.4 Magneto-optical Kerr effect (MOKE) …………………………………. 33. Tunnel barrier prepared by LB method ………………………..35.1 Sample Preparation …………………………………………………………... 35.2 NiFe/CoFe/ET+SA/CoFe …..………………………………………………... 37.3 NiFe/CoFe/Al2O3/ET+SA/CoFe ……………………………………………... 39 Pseudo-spin valve with pentacene (Pn) spacer ………………. 43.1 Sample Preparation …………………………………………………………... 43.2 Substrate Effects …………………………………………………………… .. 46.3 NiFe/CoFe/Pn (d nm)/Al2O3 (n nm)/CoFe …………………………...……….47.4 Exposure of Pentacene to Radical Oxygen .…………………………………. 49.4.1 Exposure time ………………………………………………………….. 50iFe/CoFe/Pn (20 nm) + oxygen gas (6 mins)/ CoFeiFe/CoFe/Pn (20 nm) + oxygen plasma (toxi mins)/ CoFe.4.2 Dependence on pentacene thickness ………………………………… .. 53iFe/CoFe/Pn (d nm) + oxygen plasma 2 mins/ CoFeiiiFe/CoFe/Pn (d nm) + oxygen plasma 4 mins/ CoFe.5 Annealing Effect …………………………………………………………… .. 57.6.1 Annealing temperature …………………………………………………. 57iFe/CoFe/Pn (20 nm) annealed 1 hr (TA ℃)+ oxygen plasma 4 mins/CoFe.6.2 Thickness of pentacene ……………………………………………… .. 62iFe/CoFe/Pn (d nm) annealed 1 hr (157 ℃)+ oxygen plasma 4 mins/CoFe Conclusion …………………………………………………………………. 64ibliography ………………………………………………………………… 6