Magnetization Processes and Magnetic Domain Structures in Ta/CoFeB/MgO Stacks

Abstract

In this work, we present the results of polar magneto-optical Kerr effect (PMOKE) magnetometry and microscopy study of Ta/CoFeB/MgO stacks with a prototypical perpendicular easy axis system in spintronics applications [1]. Multilayer films with a structure of Ta (5 nm)/(Co0.25Fe0.75)75B25 (d=1.24÷1.60 nm)/MgO (2 nm)/Ta (2 nm) were deposited by dc/rf magnetron sputtering on thermally-oxidized Si substrates and after were annealed at 300 oC for 1 h under a perpendicular magnetic field 0.4 T in vacuum [2,3]. The magnetization processes statics, magnetization reversal dynamics and domain structures evolution driven by magnetic field were studied as a function of CoFeB thickness d.Magnetization curves are measured for samples with various d applying PMOKE magnetometry. For d=1.24 to 1.36 nm square-shaped hysteresis loops with full remanence, indicating an out-of-plane magnetization easy axis, are observed. For d=1.40 nm the shape of magnetization curve corresponds to the transitional state (at the edge of out-of-plane and in-plane magnetization states). For d =1.44 nm the magnetization curves demonstrate no hysteresis, which indicates on the reversible magnetization rotation and in-plane magnetization. The values of magnetic anisotropy parameters were deduced from the fitting of hard axis hysteresis curves using macrospin approximation model. In the case of the samples without in-plane magnetization additional 0.05 T magnetic field was applied to prevent domain structure formation.In the case of square hysteresis loop (d=1.24÷1.36 nm) magnetization reversal dynamics was studied exploiting PMOKE microscopy imaging technique driven by sequential magnetic field pulses (duration 200 ms) with a constant amplitude. Magneto-optical images were acquired in zero field after each pulse. The derived normalized magnetization m can be obtained as a function of number of field pulses expressed as time t using image processing techniques [4]. The dependence of magnetization m for direct process (starting from full saturated state) on time t for the sample with d= 1.24 nm is shown in Fig. 1. The logarithm of time t1/2, time when the sample has magnetization m=0, depends linearly on amplitude of pulses, which indicates on thermal activation of jump-like domain wall motion [5] and allows to calculate Barkhausen length lB » 120 nm [6].Magnetization reversal in the stacks with out-of-plane anisotropy (d = 1.24÷1.36 nm) goes through domain nucleation and domain wall propagation. In the case of the samples with d = 1.24, 1.28 nm, while increasing H few nucleation point appears initially in the field of view and later quickly expands covering all the area excluding narrow domain between neighboring expanding domains named as narrow stripe domains (NSDs). NSDs are stabilized due to magnetostatic repulsion between there opposite domain walls and finite value of coercivity [7]. The increase of thickness d lead to significant reduction of straights cuts of NSDs, and for d = 1.32, 1.36 nm (close to dSRT) NSDs network is converted to the more complex dendrite like structures.The evolution of NSDs network in external out-of-plane magnetic field was studied exploiting PMOKE microscopy imaging technique driven by sequential magnetic field pulses (duration 500 ms) with a growing amplitude. The digital image processing of acquired images yielded normalized magnetization and NSDs length in the field of view. The obtained dependences of normalized magnetization and NSDs length for the stack with d=1.24 nm are presented in Fig. 2 as the functions of the amplitude of applied field pulses. NSDs appear after beginning of nucleation process at 1.25 mT, quickly reach maximum at 1.45 mT (when normalized magnetization mL » 0.92), then gradually annihilate and disappear after 5.5 mT. So saturation field is few times bigger than coercivity due to the NSDs.In conclusion, we have preformed an experimental study of the static and dynamic magnetization processes in ultrathin FeCoB films depending on their thickness. We find that magnetization reversal is occurred via thermal activated domain wall motion and is influenced by NSDs. The dependence of NSDs network length on applied out-of-plane magnetic field was studied. **