Effect of Annealing Temperature on Spectroscopic g Factor at CoFeB/MgO Interface

Abstract

Magnetic properties of an ultra-thin CoFeB film, particularly at the CoFeB/MgO interface, are of critical importance for the development of magnetic tunnel junction based magnetoresistive random access memory (MRAM) such as spin transfer torque MRAM (STT-MRAM) and voltage-controlled MRAM (VC-MRAM), because these MRAM cells have a CoFeB free layer with a thickness of around 1 nm in direct contact with a MgO tunnel barrier layer. In STT-MRAM cells, the CoFeB free layer is magnetized perpendicularly to the film plane by taking advantage of interfacial perpendicular magnetic anisotropy (PMA) induced at the interface with MgO [1,2]. Larger interfacial PMA is advantageous for shrinking the cell size while maintaining a sufficiently large thermal stability factor of the free layer magnetization. In VC-MRAM cells, the interfacial PMA is not only used to perpendicularly magnetize the free layer, but also actively modulated by the application of electric fields to trigger the so-called precessional toggle switching [3]. Therefore, thorough understanding of the physical mechanism of interfacial PMA is indispensable for material/process engineering of these MRAM technologies.The source of the interfacial PMA is considered to be preferential occupation of the dxy electron orbit that lies in-plane due to broken symmetry at the interface and spin-orbit interaction [4,5], which revives the orbital angular moment of the d-electrons that is almost completely quenched in the bulk. The magnitude of the orbital angular moment can be evaluated by the deviation of spectroscopic g factor from 2, i.e., g=2+L/S, where S and L are the spin and orbital angular moments, respectively. It has been observed that the magnitude of interfacial PMA is strongly dependent on the annealing temperature, suggesting that the crystalline state affects the electronic structure at the CoFeB/MgO interface, thus changing the interfacial PMA. According to these considerations, the spectroscopic g factor is expected to increase at the CoFeB/MgO interface with an annealing temperature dependence similar to that of the interfacial PMA.To examine this notion, we studied static and dynamic magnetic properties of CoFeB/MgO film stacks as a function of CoFeB thickness and annealing temperature. The samples are Ta(5) / CoFeB (t) / MgO(2) / Ta(5) / Ru(5) film stacks sputter deposited on a thermally oxidized Si substrate (from bottom to top, number in parentheses is the thickness of each layer in nm), where t is the nominal CoFeB thickness ranging from 1.1 nm up to either 1.5 or 5 nm for different annealing temperatures. These films were left as deposited or annealed between 150°C and 350°C for 1 hour in vacuum. We performed vibrating sample magnetometry (VSM) measurement for static characterization, and broadband ferromagnetic resonance (FMR) measurement for dynamic characterization, respectively.The CoFeB films have a magnetically dead layer at the Ta/CoFeB interface due to Ta diffusion with thicknesses depending on the annealing temperature, and the saturation magnetization Ms also varies on annealing temperature due to different degrees of crystallization. From the static M-H loop obtained by VSM, Ms as a function of t was first plotted, from which the effective thickness teff (t minus dead layer thickness) and effective Ms of the CoFeB layer were estimated for each annealing temperature. Next, the total PMA energy was calculated by integrating the static M-H loop. The interfacial PMA energy was estimated from the linear regression on the plot of total PMA energy as a function of teff for each annealing temperature. Figure 1 shows the interfacial PMA energy as a function of annealing temperature, which becomes maximum at 250°C.In the dynamic magnetic characterization, the broadband FMR measurement performed under out-of-plane bias magnetic field HB observed a Kittel mode FMR spectrum. The FMR frequency showed a liner relation on HB, from the slope of which the spectroscopic g factor was determined. The value of g factor showed a significant increase as teff decreased. Figure 2 shows the increase of g factor at the CoFeB/MgO interface from the bluk value as a function of annealing temperature, which becomes maximum at 200°C, and exhibits a very similar dependence on the annealing temperature to the interfacial PMA. These results support the notion that the increase of g factor and interfacial PMA have the same physical origin, i.e., revival of the orbital angular moment due to broken symmetry at the CoFeB/MgO interface.In the talk, we will present the detailed experimental and analytical procedures for studying the static and dynamic magnetic properties of the ultra-thin CoFeB film at MgO interface performed in this work, and possible physical origin to explain the observation results.This presentation was partly based on results obtained from a project, JPNP16007, commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan, and JSPS KAKENHI 18K14118. **