## 1 Introduction  
Recently, extensive research has been conducted on CoFeB thin films, which are characterized by low Gilbert damping constant (\(\alpha\)) and high spin polarization [1; 2; 3; 4; 5; 6]. Their exceptional magnetic properties, which include high saturation magnetization and spin polarization, has resulted in the emergence of spin-transfer torque magnetic random-access memory (STT-MRAM) and spin-orbit torque magnetic random-access memory (SOT-MRAM) devices [7; 8; 9; 10; 11]. These devices feature a magnetic tunnel junction (MTJ) structure that is composed of a thin oxide tunnel layer positioned between two ferromagnetic metal layers [12; 13; 14]. Owing to their non-volatility and high-performance information storage characteristics, STT-MRAM and SOT-MRAM have garnered attention as promising candidates for next-generation memory technologies [14; 15]. In a MTJ, tunneling electrons are spin-polarized, resulting in a higher tunneling probability when the two ferromagnetic metal layers exhibit an alignment of magnetization directions in a parallel configuration, and a lower probability when they are aligned in an antiparallel configuration. CoFeB thin films, known for their magnetic effects, including giant magnetoresistance (GMR) [16; 17; 18], tunneling magnetoresistance (TMR) [19; 20], the Hall effect [21; 22], and anisotropic magnetoresistance (AMR) [23; 24], play a pivotal role in these structures owing to their high saturation magnetization, Curie temperature, and low coercive field, rendering them suitable for use in various spintronic devices [25]. A spintronic device utilizes the spin degree of freedom of an electron to store and transmit information [15], offering advantages such as reduced energy consumption, rapid operation, and high density [26]. MTJs, comprising multiple magnetic and non-magnetic layers, are a prime example of spintronic devices. Although the TMR effect was initially discovered in Fe/Ge/Co-based MTJ structures in 1975, the performance of the MTJ was limited [27]. When (100)-oriented crystalline MgO was used instead of amorphous Al\({}_{2}\)O\({}_{3}\) as a tunnel barrier in MTJ, superior interfacial properties and TMR ratios were observed [1]. Subsequently, the introduction of soft CoFeB thin films has considerably broadened the scope of their enhanced performance. Notably, research has revealed that MTJs constructed with a Ta/CoFeB/MgO/CoFeB/Ta structure exhibit a remarkable TMR ratio, of 604%, at room temperature [1; 28; 29; 30; 31]. Furthermore, recent studies have contributed to optimizing MTJ structures by incorporating specific heavy metals, including Ta, Pt, Hf, Mo, Ru, and W, as buffer and capping layers [32; 33; 34; 35; 36; 37]. In particular, a CoFeB/MgO frame with W buffer and capping layers demonstrated higher annealing stability compared to a CoFeB/MgO frame with Ta buffer and capping layers [36]. However, only a few reports have examined the effect of high temperature on the magnetic properties of CoFeB thin films with various buffer and capping layers. A few studies have investigated the temperature dependence of magnetic properties in a variety of MTJ and magnetic multi-layer structures, revealing a decrease in thermal stability with increasing temperatures [38; 39; 40; 41; 42; 43; 44; 45]. Therefore, analysis of the high-temperature characteristics of magnetic thin films from various perspectives such as materials, interfaces, structures, and annealing conditions is essential. For widespread application of spintronic devices, such as STT-MRAM, across various settings, it is necessary for their characteristics to remain stable across a broad range of operating temperatures. Therefore, it is essential to minimize any fluctuation in the magnetic properties of thin films [43]. Additionally, thermal stability is crucial for increasing the storage capacity of spintronic devices, including MTJs [2; 46; 47; 48; 49; 50; 51].