Effects of Lower-Energy Irradiation on MgO-based Magnetic Tunnel Junctions: Comparison
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Please note this is a comparison between Version 2 by Wendy Huang and Version 1 by Yucheng Lan.

Electromagnetic waves with wavelengths longer than gamma rays are commonly known as lower-energy waves, such as X-rays, ultraviolet radiation (UV), visible light, infrared radiation, microwaves, and radio waves. These electromagnetic waves have less energy compared to gamma rays, and are generally classified as non-ionizing radiation, with the exception of X-rays.  This discussion revolves the impacts of lower-energy electromagnetic waves on magnetic tunnel junctions (MTJs) that comprise two ferromagnetic layers separated by a thin insulating barrier.

  • X-rays
  • ultraviolet radiation
  • infrared radiation
  • microwaves
  • Magnetic Tunnel Junctions
  • Radiofrequency

1. X-ray Irradiation

The energy of X-rays ranges from several tens of electron volts to hundreds of kiloelectron volts. The intensity of X-rays decreases exponentially from the surfaces of MTJs, as described by the Beer–Lambert law in Equation (1).
I = I 0 e − μ z where I is the intensity of electromagnetic radiation transmitted over a distance z, I 0 is the incident electromagnetic wave intensity, μ is the linear attenuation coefficient in cm−1, μ = n σ = n ( σ p h o t o e l e c t r i c + σ C o m p t o n + σ P a i r ) (n: the number of atoms/cm3; σ : proportionality constant that reflects the probability of an electromagnetic wave photon being scattered or absorbed), and z is the distance traveled by the radiation in cm. For multilayered films, the electromagnetic intensity is proportional to both the attenuation coefficient and the thickness of each layer through which it passes [179][1].  X-ray radiation typically only penetrates a few microns into materials, depending on its energy and the material’s composition. MgO-based MTJs are typically sandwiched by electric electrodes made of materials such as gold or tantalum. These metal electrodes are usually thick enough to prevent X-rays from penetrating through to the MgO barriers and ferromagnetic layers of the MTJs. The detailed screening effect can be calculated. X-ray radiation typically only penetrates a few microns into materials, depending on its energy and the material’s composition. MgO-based MTJs are typically sandwiched by electric electrodes made of materials such as gold or tantalum. These metal electrodes are usually thick enough to prevent X-rays from penetrating through to the MgO barriers and ferromagnetic layers of the MTJs. The detailed screening effect can be calculated. Hard X-rays can fully penetrate MgO-based MTJs with weak absorption, therefore affecting the physical and chemical properties of both the MgO barriers as well as the ferromagnetic layers. The MgO barrier layers should be affected by X-ray radiation in a similar way to two-dimensional materials, such as MoS2 monolayers [112,140]. In this case, the effects of X-ray radiation on MgO-based MTJs are very similar to those of monolayers [2][3]. In this case, the effects of X-ray radiation on MgO-based MTJs are very similar to those of γ -ray radiation. These X-ray effects may be temporary and only detectable through real-time measurements. Soft X-rays with energies of ten kiloelectron volts or less would be strongly screened by metal electrodes, with penetration through to the MgO barriers and ferromagnetic layers of MTJs being prevented. Consequently, the effects of soft X-ray irradiation can be disregarded. Up to now, there have been few studies of X-ray radiation on MgO-based MTJs.
 

2. UV–Vis Irradiation

The energy of ultraviolet–visible (UV–vis) electromagnetic waves ranges from 1 eV to several tens of electron volts, with wavelengths of 10–400 nm. UV and visible electromagnetic waves cannot penetrate through metal layers to reach ferromagnetic and MgO layers. Additionally, metallic electrodes reflect UV–vis radiation, making MgO-based MTJs highly resistant to such radiation.  However, heat produced by UV–vis radiation may degrade MgO-based MTJs. If heating effects are avoided, MgO-based MTJs should be highly tolerant to UV–vis radiation. To date, there is no literature available on the subject of the effects of UV–vis radiation on MgO-based MTJs.

3. Infrared Radiation and Thermal Annealing

Heat radiation or thermal radiation is a well-known term for infrared radiation. Pulsed thermal radiation, with a long wavelength of 1–20 microns and energy of 1–24 eV, can be efficiently screened by metallic electrodes. However, continuous thermal radiation, also known as heat, can penetrate MTJ devices during prolonged exposure to high temperatures, resulting in thermal annealing and thermal equilibrium. Thus, infrared radiation is somewhat different to other types of radiation.
Typically, thermal annealing (using infrared radiation) has a positive effect on the crystallization of MgO barriers, which enhances the performance of MTJs. Shen et al. investigated MgO-based MTJs [189] and their investigation indicated that thermal annealing at 425 Shen et al. investigated MgO-based MTJs [4] and their investigation indicated that thermal annealing at 425 ∘C enhanced the crystallization of CoFeB layers at the interfaces with MgO, affecting the magnetoresistance of MgO-based MTJs.  Ikeda et al. investigated the effect of thermal annealing on MTJs at temperatures higher than 500 ∘C [30]. The MTJs have a structure of Ta(5)/Ru(10)/Ta(5)/Co[5]. The MTJs have a structure of Ta(5)/Ru(10)/Ta(5)/Co20Fe60B20 (5)/MgO(2.1)/Co20Fe60B20(4)/Ta(5)/Ru(5) (in nm). It was reported that the annealing process led to the relaxation of residual stress and an improvement in the (001) orientation of the MgO barriers, resulting in an enhanced TMR ratio. Wang et al. studied both in-situ and ex-situ measured TMR values at 380 ∘C [190]. The TMR structure consisted of Si/SiO[6]. The TMR structure consisted of Si/SiO2/Ta(7)/Ru(20)/Ta(7)/CoFe(2)/IrMn(15)/CoFe(2) /Ru(1.7)/CoFeB(3)/MgO(1.5–3)/CoFeB(3)/Ta(8)/Ru(10), with the numbers indicating the layer thicknesses in nanometers. It was found that the amorphous CoFeB layers underwent crystallization, and the quality of the MgO barriers’ crystallinity improved in less than 10 minutes of annealing, resulting in a TMR value larger than 200%. The crystallization was further experimentally confirmed through their HRTEM work [171]. of annealing, resulting in a TMR value larger than 200%. The crystallization was further experimentally confirmed through their HRTEM work [7].
However, thermal annealing also accelerates interface diffusion between MgO barriers and ferromagnetic layers, leading to degradation of MTJ performance [192][8].  Liu et al. investigated the thermal stability of MTJs with MgO barriers at temperatures up to 500 ∘C [191][9]. The MTJs consisted of Ta(30)/[Co50Fe50]×3/IrMn(15)/[Co50Fe50]×2/ Ru(0.8)/[Co40Fe40B20]×3/MgO(1.2)/[Co40Fe40B20]×3/Ta(10)/Ru(5). The study observed the irreversible loss of magnetoresistance at high temperatures.
In order to explain the effects of thermal annealing, Xu et al. employed transmission electron microscopy and electron energy loss spectroscopy to investigate the microstructures of the MgO-CoFeB interfaces of MTJs [193][10]. Thermal annealing indeed crystallized MTJ layers, and caused boron diffusion. Boron diffusion led to the growth of CoFe nanocrystals from CoFeB layers under annealing, while the crystallization did not significantly affect the MR properties. Instead, the MR ratio was predominantly determined by grain boundary transport caused by boron distribution. If boron diffused to metallic underlayers from the inside to the outside, the MR ratio would be improved. Conversely, annealing may result in boron diffusing into grain boundaries of the MgO barriers from the outside to the inside, leading to a decrease in the MR ratio. The interfacial properties of MTJs regulated the diffusion of boron and affected the effect of thermal annealing. Thus, the effect of thermal radiation on MTJ devices depends on the annealing temperature, the duration, and the structure of the MTJs. Thermal irradiation can either benefit or degrade MTJs’ performance.
It is important to note that radiation other than infrared radiation can also produce heat, particularly at high-dose rates, which can lead to an increase in the temperatures of MTJs and produce similar annealing effects. Under such circumstances, high-energy radiation, such as γ-ray and hard X-ray radiation, may cause additional annealing effects. To study the effects of irradiation, it is crucial to investigate MTJs at constant temperatures or monitor the internal temperatures of MTJs, particularly the temperatures of the MgO and ferromagnetic layers.

4. Microwave Irradiation

The penetration depth of microwaves into conductive metal surfaces is typically less than one micron [194][11]. Therefore, the metallic electrodes of MTJs can efficiently reflect microwaves. In other words, microwaves should not penetrate through the electrodes to irradiate the MgO barriers and ferromagnetic layers. Therefore, the microwave irradiation effect can be ignored, and microwave radiation should not have any significant impact on the performance of MTJs. Although microwave radiation is not expected to penetrate through the electrodes of MTJs to affect the MgO barriers and ferromagnetic layers, microwave radiation can cause a significant increase in the temperature of metal layers. Research has shown that microwave irradiation can produce a high temperature, of up to 500 ∘C, in Au films in less than 10 s [195][12]. Therefore, microwave irradiation could generate a high temperature locally in ferromagnetic fixed-/free-layes of MTJs, which could have a significant impact on the performance of MTJs. Up to now, there have been limited reports on the impact of microwave radiation on MgO-based MTJs. Some groups have employed MgO-based MTJs to detect microwave irradiation [94][13]. Unfortunately, it was not stated whether the MgO-based MTJs were damaged under microwave irradiation.

5. Radiofrequency Electromagnetic Irradiation

Radiofrequency (RF) electromagnetic radiation, with energy of less than a few milli-electronvolts,  can be shielded by conductive or magnetic materials, which is known as RF shielding.  MTJ metal electrodes should block / shield total RF radiation and therefore MTJ performance should not be affected as RF radiation. Similar to microwaves, RF irradiation can also induce heating in metals, leading to high temperatures locally in MTJ electrodes. However, the induced temperature is expected to be low due to the extremely low energy of RF radiation. Therefore, the effects of radiofrequency can be ignored. MgO-based MTJs should be highly tolerant to this radiation.

References

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