Evidence for magnon excitation contribution to the magnetoresistance behavior during thermal annealing in CoFeB/MgO/CoFeB magnetic tunnel junctions

Abstract

For sputtered CoFeB/MgO/CoFeB magnetic tunnel junctions, it is well known that the tunnel magnetoresistance (TMR) ratio increases with increasing annealing temperature (T a ) up to a critical value (T p ), and then decreases with further increasing T a , resulting in a peak around T p . The improved crystallinity of the MgO barrier and CoFeB electrodes due to annealing has been considered as the main reason for the enhancement of the TMR ratio, especially for T a < T p . In this work, the evidence is provided that the magnon excitation plays a great contribution to the magnetoresistance (MR) behavior in annealed samples based on the measurement of dynamic conductance and inelastic electron tunneling (IET) spectra. The magnon activation energy (E c ) obtained from the fits for IET spectra exhibits a similar temperature dependence with that of the TMR ratio. A detailed analysis shows that the magnon excitation, together with improved crystallinity of the MgO barrier and CoFeB layers, is the main contribution to the annealing-temperature-dependent MR behavior. PACS number(s): 75.70.Cn, 75.30.Hx MgO-based magnetic tunnel junctions (MTJs) have been investigated widely due to their interesting fundamental physics and potential applications. 1-8 Thermal annealing is a critical process for sputtered CoFeB/MgO/CoFeB junctions because the tunnel magnetoresistance (TMR) ratio of junctions annealed at an appropriate temperature increases dramatically. 9-13 Annealing greatly improves the crystallinity of the MgO barrier and ferromagnetic (FM) CoFeB electrodes as well as the interfaces between the MgO and CoFeB layers, 10,12 which enhances spin-dependent tunneling across the MgO barrier. The effects of thermal annealing were studied using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS), and x-ray diffraction (XRD), 10,12,14-16 with a focus on the structural characterization.For MTJs, the measurement of dynamic conductance (dI/dV ) and the inelastic electron tunnel (IET) spectrum (d 2 I/dV 2 ) has been proven to be a powerful tool to study spindependent tunneling. [16][17][18][19][20][21][22][23][24] Using this method, the influence of the density of states (DOS) and the inelastic scattering process on conductance can be clarified by measuring the first and second derivative of conductance. 24,25 Here, we report a detailed investigation of the dynamic conductance and IET spectra in CoFeB/MgO/CoFeB junctions as a function of annealing temperature T a , with a focus on the magnetoresistance (MR) behavior. Our results show that magnon excitation contributes greatly to the annealing-temperature-dependent MR behavior.Multilayers with a core structure of NiFe(5)/IrMn(10)/ CoFe(2)/Ru(0.8)/CoFeB(3)/MgO(2.5)/CoFeB(3) (in nm) were grown in a Shamrock sputtering system, where NiFe, IrMn, CoFe, and CoFeB stand for Ni 81 Fe 19 , Ir 22 Mn 78 , Co 90 Fe 10 , and Co 40 Fe 40 B 20 , respectively. The bottom synthetic NiFe/IrMn/CoFe/Ru structure is used to pin the CoFeB layer via exchange bias. Thermal annealin...

Spin-dependent tunneling spectroscopy in MgO-based double-barrier magnetic tunnel junctions

“…The details of IET spectra measurement can be found in our previous papers. [9][10][11] III. RESULTS AND DISCUSSIONS Figure 1 shows the T a dependences of the TMR ratios at RT for the DMTJs and SMTJ.…”

“…For MTJs, the measurement of dynamic conductance (dI/dV) and the inelastic electron tunnel spectroscopy (IETSd 2 I/dV 2 ) has been proven to be a very powerful tool to investigate spin-dependent tunneling. 9,10 There has been much work devoted to the dI/dV and IET spectra in MgO SMTJs. [9][10][11][12][13][14][15][16] However, much less attention has been paid to MgO DMTJs.…”

“…9,10 There has been much work devoted to the dI/dV and IET spectra in MgO SMTJs. [9][10][11][12][13][14][15][16] However, much less attention has been paid to MgO DMTJs. In this study, the measurement of dynamic conductance and IET spectra in MgO-based DMTJs with a Co 50 Fe 50 /Co 40 Fe 40 B 20 free layer was carried out using a SMTJ as a reference.…”

Symmetry-dependent electron-electron interaction in coherent tunnel junctions resolved by measurements of zero-bias anomaly

“…Related to ZBA is the ZB peak in the second derivative of the current-voltage curves, which is also called inelastic electron tunneling spectroscopy (IETS) and is frequently used to investigate inelastic tunneling processes. In IETS of MTJs [29][30][31][32][33], the ZB peak has been ascribed to various mechanisms, including magnon excitation [34], magnetic impurity scattering [35], and a combination of magnon scattering and EEI [27]. Although some earlier works [25,36] did not distinguish between ZBA and magnon-induced reduction of TMR over a much wider bias range, it was later shown theoretically that interface magnon scattering does not yield sharp peaks at all [37], contradicting the interpretation that the ZBA and the sharp peak in the IETS near zero bias are due to magnon excitation.…”

Manipulation of magnetization switching and tunnel magnetoresistance via temperature and voltage control

“…From the hard axis hysteresis measurement, we determined the effective perpendicular anisotropy is 3.1 × 10 6 erg/cm 3 for Ta/CoFeB (1.2)/MgO and 5.4 × 10 6 erg/cm 3 for Ta/MgO/CoFeB (1.4) at RT. Figure 1b shows differential tunnel conductance dI/dV in P configuration at 2 K. The zero field anomaly as well as the two shoulders are typical of spin dependent tunneling through MgO barrier 21 22 23 . Comparing the above transport and magnetic properties with previous measurement by other groups confirms that our samples are among the highest quality 11 18 24 25 26 .…”