Magnetic phase transitions in Ta/CoFeB/MgO multilayers

Abstract

We study thin films and magnetic tunnel junction nanopillars based on Ta/Co20Fe60B20/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co20Fe60B20/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories.The ferromagnet/oxide (FM/Ox) interfaces play an important role in modern spintronics. Insulating oxide layers sandwiched between two metallic ferromagnets form magnetic tunnel junctions (MTJs) [1][2][3] which find applications in magnetic sensors [4][5][6][7], spin torque oscillators [8][9][10][11], and non-volatile spin torque memory (STT-RAM) [12]. A number of FM/Ox interfaces exhibit large perpendicular magnetic anisotropy (PMA) [13][14][15][16] needed for enhancing thermal stability of . Furthermore, some FM/Ox interfaces exhibit magneto-electric coupling that allows control of the interfacial PMA with an electric field applied perpendicular to the interface. This effect, known as voltagecontrolled magnetic anisotropy (VCMA) [20][21][22][23], can be used for energy-efficient voltage-driven switching of magnetization at low current densities [15,24,25].The interface between Co x Fe y B z (CoFeB) ferromagnet and MgO insulator is one of the most important interfaces in spintronics because the STT-RAM technology is based on CoFeB/MgO/CoFeB MTJs [26]. Comprehensive understanding of structural, magnetic and electronic properties of CoFeB/MgO-based multilayers is at the forefront of applied spintronics research. In this Letter, we report magnetometry and electrical transport studies of (i) CoFeB/MgO/CoFeB nanoscale magnetic tunnel junctions and (ii) CoFeB/MgO based multilayer films. These studies reveal surprising anomalies in the temperature dependence of resistance and magnetization of the CoFeB films interfaced with MgO. Our data suggest that ultrathin magnetic oxide layers are formed at the CoFeB/MgO interfaces prepared under typical deposition and annealing conditions employed in fabrication of CoFeB/MgO/CoFeB MTJs with high tunneling magnetoresistance (TMR) [1,2]. Interestingly, the high TMR observed in these MTJs seems to be largely unaffected by the interfacial oxide formation.We Prior to patterning, the multilayers are annealed for 2 hours at 300• C in a 1 Tesla in-plane magnetic field that sets the exchange bias direction for the SAF bottom layer parallel to the long (easy) axis of the nanopillar.The CoFeB/MgO based multilayer film composition is Si/ SiOx/ Ta(5 nm)/ Co 20 Fe 60 B 20 (d)/ MgO(1.1 nm) /Ta(1 nm)/ Ru(2 nm), where the CoFeB layer thickness d ranges from 0.9 nm to 2.5 nm. In order to minimize the sample-to-sample variations, the films were grown in a single run without changing the deposition parameters. The films w...

Second order anisotropy contribution in perpendicular magnetic tunnel junctions

“…Besides, several studies have pointed out the possible influence of strain on second order anisotropy in thin magnetic films 27 28 . For instance in ref.…”

“…Besides, several studies have pointed out the possible influence of strain on second order anisotropy in thin magnetic films [26,27]. For instance in Ref [27], the authors observed an anomaly in the tunneling conductance in Ta/CoFeB/MgO based MTJ at low temperatures (T=160K), that they interpreted as a structural-magnetic phase transition of a magnetic oxide formed at the interface between MgO and CoFeB. Magnetic phase transition alters both spintronic and magnetic properties of the MTJ stack.…”

Magnetic, thermal, electrical properties and crystallization kinetics of Co60Fe20B20 alloy films

“…The alloy can be used as ferromagnetic electrodes in combination with MgO or AlOx barriers to fabricate magnetic tunnel junctions (MTJs) with a very high tunnel magnetoresistive (TMR) ratio [7][8][9][10][11]. It has been known that the crystalline CoFeB film in MTJs has enhanced spin tunneling efficiency and hence can exhibit a higher TMR ratio than the corresponding amorphous one [12].…”

High temperature ferromagnetic resonance study on pMTJ stacks with diffusion barrier layers

“…Noting that equation (6) is based on the assumption that ∂M ∂T is constant in order for Γ to be 2, we should expect the fit of ∂Heff ∂T against H eff (T = 0 K) to pass through the origin for samples with the same free layer composition (and therefore the same M s at T = 0 K). Indeed, by using the gradients and y-intercepts of the H eff plot from figure 4(a) to compare ∂Heff ∂T as a function of H eff (T = 0 K), the experimental results in figure 4(b) are in excellent agreement with equation (6) with the case of proportionality constant, Γ = 2 for CoFeB.…”