Electromagnetic Radiation Effects on MgO-Based Magnetic Tunnel Junctions: A Review

Abstract

Magnetic tunnel junctions (MTJs) have been widely utilized in sensitive sensors, magnetic memory, and logic gates due to their tunneling magnetoresistance. Moreover, these MTJ devices have promising potential for renewable energy generation and storage. Compared with Si-based devices, MTJs are more tolerant to electromagnetic radiation. In this review, we summarize the functionalities of MgO-based MTJ devices under different electromagnetic irradiation environments, with a focus on gamma-ray radiation. We explore the effects of these radiation exposures on the MgO tunnel barriers, magnetic layers, and interfaces to understand the origin of their tolerance. This review enhances our knowledge of the radiation tolerance of MgO-based MTJs, improves the design of these MgO-based MTJ devices with better tolerances, and provides information to minimize the risks of irradiation under various irradiation environments. This review starts with an introduction to MTJs and irradiation backgrounds, followed by the fundamental properties of MTJ materials, such as the MgO barrier and magnetic layers. Then, we review and discuss the MTJ materials and devices' radiation tolerances under different irradiation environments, including high-energy cosmic radiation, gamma-ray radiation, and lower-energy electromagnetic radiation (X-ray, UV-vis, infrared, microwave, and radiofrequency electromagnetic radiation). In conclusion, we summarize the radiation effects based on the published literature, which might benefit material design and protection.

Keywords: irradiation; magnetic tunnel junction; review.

Figure 1

Historical development of MR ratio of MgO-based MTJs at room temperature. The data of AlO-based MTJs are also plotted for comparison. Reproduced with permission [15]. Copyright 2008, the Physical Society of Japan.

Figure 2

(a) TEM and (b) HRTEM images of an Fe(001)/MgO(001)/Fe(001) MTJ. Reproduced [2]. Copyright 2004, Springer Nature.

Figure 3

Structure of an MRAM cell (courtesy of Freescale).

Figure 4

Radiation (a) in outer space and (b) on Earth. Satellites are orbiting in the radiation zone of the Van Allen belts whose cross-sectional shape and intensity are shown in (a). From nasa.gov (accessed on 7 August 2017).

Figure 5

Crystallographic structure of MgO.

Figure 6

(a) Top view (along <001> direction) and (b) side view (along <100> direction) of a MgO (001) monolayer.

Figure 7

Coupling of wave functions between the Bloch states in ferromagnetic Fe(001) layers and the evanescent states in the MgO(001) barrier for $k_{\Vert }=0$ direction. ${\Delta }_1:s-p-d$; ${\Delta }_2:d$; ${\Delta }_5:p-d$. Reproduced with permission [15]. Copyright 2008, the Physical Society of Japan.

Figure 8

Dose dependence of TL intensity of MgO nanomaterials irradiated by a pulsed electron beam. Reproduced with permission [157]. Copyright 2015, Elsevier Ltd.

Figure 9

(a) TL intensity of MgO powder irradiated by gamma rays. Replotted from [165]. (b) TL response of MgO powder with gamma-ray dose. Reproduced with permission [165]. Copyright 2009, Taylor & Francis Group.

Figure 10

Real-time capacitance versus $\gamma$-ray radiation dose for Ag/MgO/Ag capacitors. Replotted from Ref. [121]. Copyright 2005, Springer.

Figure 11

(a) Thermal conductivity and (b) spectra of optical absorption of MgO crystals before and after $\gamma$-ray irradiation. Reproduced with permission [163]. Copyright 1981, John Wiley and Sons.

Figure 12

(a) TSL curves of MgO single crystals with OH${}^{-}$ impurity of $4.9\times {10}^{17}/{\mathrm{cm}}^3$ under $\gamma$-ray irradiation under different temperatures. (b) TSL intensity dependence of $\gamma$-ray irradiation dose at $450\mathrm{K}$ [160]. Copyright 2011, David Publishing Company.

Figure 13

Temperature-dependent photoconductance per unit length of MgO polycrystals before, during, and after $\gamma$-ray irradiation [170]. Copyright 1975, Canadian Science Publishing.

Figure 14

M–H hysteresis loops of MgO-based MTJs measured in an in-plane magnetic field before and after irradiation with a TID of 20 Mrad (SI) [120]. Copyright 2019, IEEE.

Figure 15

Optical surface images of MgO-based MTJs (a) before irradiation, (b) after 20 Mrad (Si) irradiation, and (c) after 247 Mrad (Si) irradiation [120]. Copyright 2019, IEEE.

Figure 16

TMR of a single MgO-based MTJ before and after irradiation. Inset: Cross-sectional TEM image [166]. Copyright 2010, International Training Institute for Materials Science. Reproduced with permission [158]. Copyright 2011, IOP Publishing Ltd.

Figure 17

(a) Hysteresis loop of a single MgO-based MTJ and (b) H${}_c$ and TMR of a series of MgO-based MTJs before and after exposure to $\gamma$-ray radiation with a dose rate of ∼$10\mathrm{rad}/\mathrm{s}$ and energy of $1.25\mathrm{MeV}$. Inset: Illustration of the MTJ stack [152]. Copyright 2012, IEEE.

Figure 18

Transmission of electromagnetic radiation through an MTJ device. (a) Structure [180] and (b) HRTEM cross-sectional image [171] of an MTJ device used for penetration calculations of various types of radiation. Calculated radiation intensity through electrodes (c) and sublayers (d), including MgO barriers, under various radiation energies. The linear attenuation coefficients of the materials were obtained from https://www.physics.nist.gov (accessed on 17 September 2009). Reproduced with [180] with copyright 2010, Springer Nature. Reproduced with permission [171] with copyright 2016, American Chemical Society.

Figure 19

Transmission of $\gamma$-radiation through iron. ▲: ${}^{137}$Cs radiation of $0.66\mathrm{MeV}$; •: ${}^{60}$Co radiation of $1.17\mathrm{MeV}$ and $1.33\mathrm{MeV}$; ■: ${}^{24}$Na radiation of $1.38\mathrm{MeV}$ and $2.76\mathrm{MeV}$. Reproduced with permission [117]. Copyright 1953, American Physical Society.

Figure 20

Schematic illustrations of electron tunneling through (a) a crystalline barrier and (b) an irradiated barrier. ${\Delta }_1:s-p-d$; ${\Delta }_2:d$; ${\Delta }_5:p-d$. Replotted from Ref. [31]. Copyright 2007, IOP Publishing Ltd.

Figure 21

(a) Effect of room temperature restoration of irradiated MgO powder measured with a delay period of 75 days ($t=75\mathrm{d}$) and without a delay ($t=0\mathrm{d}$). (b) Relative thermoluminescence as a function of restoration time for irradiated MgO. Reproduced with permission [165] with copyright 2009, Taylor & Francis Group.

Figure 22

TL response of four MgO crystals as a function of UV exposure at 295 nm. Impurity of PA sample: <0.026; impurity of NA sample: 0.068; impurity of NB sample: 0.082; impurity of NC sample: <0.047. Reproduced with permission [186] with copyright 1976, Am. Assoic. Phys. Med.

Figure 23

Cross-sectional HRTEM images (a–d) and ADF-STEM images and corresponding elemental EELS mappings (e–h) using O-K, Fe-L${}_{3,2}$, Co-L${}_{3,2}$ and B-K ionization edges taken from the Ta/CoFeB/MgO/CoFeB/Ta MTJ (a,b,e,f) and W/ CoFeB/MgO/CoFeB MTJ (c,d,g,h) at $300{}^{\circ }\mathrm{C}$ (a,c,e,g) and $400{}^{\circ }\mathrm{C}$ (b,d,f,h). Reproduced with permission [193] with copyright 2018, Elsevier.