Giant interfacial perpendicular magnetic anisotropy in MgO/CoFe/capping layer structures

Tuning the crystalline field leads to giant regulated perpendicular magnetic anisotropy by depositing Bi atom onto the MgO surface

Two-dimensional ferromagnetic semiconductors of rare-earth monolayer GdX2 (X = Cl, Br, I) with large perpendicular magnetic anisotropy and high Curie temperature

Tetragonally distorted Heusler alloys are the promising class of materials for developing efficient spintronic devices such as magnetic tunnel junctions used for spin-transfer torque magnetic random-access memory due to their potentially large perpendicular magnetic anisotropy (PMA), high spin polarization, high Curie temperature, and small lattice mismatch with MgO tunnel barriers. Motivated by the previous theoretical prediction [ J. Phys. D: Appl. Phys. 50 , 095002 (2017)], we aim to experimentally demonstrate large PMA in tetragonally distorted Fe 2 PtGe Heusler alloy thin films on MgO(001) substrates. Although a relatively large PMA is obtained, microstructure analysis reveals the rejection of Ge atoms from the parent matrix of Fe 2 PtGe and the separation into Ge-poor and Ge-rich L 1 0 -ordered FePt regions by heating in the film deposition process. Our negative but preliminary results would provide experimental scientists useful knowledge for realizing theoretically-predicted tetragonally distorted Heusler alloys in the future.

Large tunnel magnetoresistance in magnetic tunnel junctions with magnetic electrodes of metastable body-centered cubic CoMnFe alloys

Magnetic tunnel junctions with perpendicular anisotropy form the basis of the spin-transfer torque magnetic random-access memory (STT-MRAM), which is non-volatile, fast, dense, and has quasi-infinite write endurance and low power consumption. Based on density functional theory (DFT) calculations, we propose an alternative design of magnetic tunnel junctions comprising Fe(n)Co(m)Fe(n)/MgO storage layers with greatly enhanced perpendicular magnetic anisotropy (PMA) up to several mJ/m2, leveraging the interfacial perpendicular anisotropy of Fe/MgO along with a stress-induced bulk PMA discovered within bcc Co. This giant enhancement dominates the demagnetizing energy when increasing the film thickness. The tunneling magnetoresistance (TMR) estimated from the Julliere model is comparable with that of the pure Fe/MgO case. We discuss the advantages and pitfalls of a real-life fabrication of the structure and propose the Fe(3ML)Co(4ML)Fe(3ML) as a storage layer for MgO-based STT-MRAM cells. The large PMA in strained bcc Co is explained in the framework of Bruno's model by the MgO-imposed strain and consequent changes in the energies of dyz and dz2 minority-spin bands.

Two-dimensional (2D) ferromagnets with high Curie temperatures (TC) and large perpendicular magnetic anisotropy (PMA) are rare, but have great potential in the field of spintronics. The present work investigates the electronic and magnetic properties of GdX (X = Cl and Br) monolayers and the two layers of GdCl/GdBr heterojunctions by conducting in-depth theoretical calculations. As expected, both GdCl and GdBr monolayers are ferromagnetic and have large magnetic moments of ∼8 μB per Gd atom. Besides, they both show large PMA with magnetic anisotropy energies of 0.88 and 0.85 meV/unit cell and ultrahigh TC of 826 and 704 K, respectively. Significantly, they both have topological properties and spin-orbit-induced energy band reversion. In addition, we found two stable stacking configurations for the GdCl/GdBr bilayer heterojunction. The ultrahigh TC and topological properties are maintained in the two stacking configurations. Their easy magnetization axes shift from out-of-plane to in-plane directions as a result of the competition mainly between the Gd p and Gd d orbitals. These properties endow GdCl and GdBr monolayers with great potential for practical applications in the field of spintronic devices at the nanoscale.

Materials with large perpendicular magnetic anisotropy (PMA) are candidates for spintronic devices, such as magnetic random-access memory, etc., due to their stable magnetic reference states. Because of shape anisotropy, the magnetic easy axis of oxide thin films favors in-plane orientation. In this Paper, we demonstrate a convenient means to control the magnetic anisotropy of ${\mathrm{Sr}\mathrm{Ru}\mathrm{O}}_{3}$ (SRO) ultrathin layers from in-plane to out-of-plane by capping with nonmagnetic materials. Tuning the anisotropy is achieved by imposition of symmetry mismatch at the interface-induced structural transition of SRO with suppressed octahedral tilt. These results suggest a potential direction for engineering magnetic oxide thin films with flexible tunable PMA using capping-layer-induced dissimilar symmetry.

Spin transfer torque magnetic random access memory (STT-MRAM) is a promising candidate for next generation memory as it is non-volatile, fast, and has unlimited endurance. Another important aspect of STT-MRAM is that its core component, the nanoscale magnetic tunneling junction (MTJ), is thought to be radiation hard, making it attractive for space and nuclear technology applications. However, studies on the effects of ionizing radiation on the STT-MRAM writing process are lacking for MTJs with perpendicular magnetic anisotropy (pMTJs) required for scalable applications. Particularly, the question of the impact of extreme total ionizing dose on perpendicular magnetic anisotropy, which plays a crucial role on thermal stability and critical writing current, remains open. Here we report measurements of the impact of high doses of gamma and neutron radiation on nanoscale pMTJs used in STT-MRAM. We characterize the tunneling magnetoresistance, the magnetic field switching, and the current-induced switching before and after irradiation. Our results demonstrate that all these key properties of nanoscale MTJs relevant to STT-MRAM applications are robust against ionizing radiation. Additionally, we perform experiments on thermally driven stochastic switching in the gamma ray environment. These results indicate that nanoscale MTJs are promising building blocks for radiation-hard non-von Neumann computing.

THz spintronics is an emergent area of research aimed at bridging the gap between fifth- and sixth-generation wireless telecommunications by utilizing spintronic devices such as magnetic spin torque oscillators as a source of low powered THz emission. The realization of such devices using ferromagnetic metal thin films however requires magnetic materials with both large perpendicular magnetic anisotropy (PMA) and low Gilbert damping constants. In this Letter, we report on the development of L10-ordered FePt with an effective Gilbert damping constant as low as 0.033. Using time-resolved magneto-optical Kerr effect, we characterized the magnetization dynamics of continuous L10-ordered FePt grown on MgO and SrTiO3 substrates. By changing the substrate on which FePt is grown, the lattice mismatch and subsequent number of misfit dislocations at the interface and L10-ordering can be controlled. We found that fewer misfits and improved ordering in FePt lead to a reduced Gilbert damping constant due to reduced electron scattering but that FePt grown on SrTiO3 also shows robust perpendicular magnetic anisotropy. Importantly, these results demonstrate the ability to control the damping in FePt and similar materials by changing the number of misfit dislocations at the interface and the smaller damping in FePt opens up the possibility of using this material in spintronic materials in the THz wave range.

A perpendicularly magnetized spin injector with a high Curie temperature is a prerequisite for developing spin optoelectronic devices on two-dimensional (2D) materials working at room temperature (RT) with zero applied magnetic field. Here, we report the growth of Ta/CoFeB/MgO structures with large perpendicular magnetic anisotropy (PMA) on full-coverage monolayer (ML) molybdenum disulfide (MoS2). A large perpendicular interface anisotropy energy of 0.975 mJ/m2 has been obtained at the CoFeB/MgO interface, comparable to that observed in magnetic tunnel junction systems. It is found that the insertion of MgO between the ferromagnetic (FM) metal and the 2D material can effectively prevent the diffusion of the FM atoms into the 2D material. Moreover, the MoS2 ML favors a MgO(001) texture and plays a critical role in establishing the large PMA. First-principles calculations on a similar Fe/MgO/MoS2 structure reveal that the MgO thickness can modify the MoS2 band structure, from a direct band gap with 3ML-MgO to an indirect band gap with 7 ML-MgO. The proximity effect induced by Fe results in splitting of 10 meV in the valence band at the Γ point for the 3ML-MgO structure, while it is negligible for the 7 ML-MgO structure. These results pave the way to develop RT spin optoelectronic devices based on 2D transition-metal dichalcogenide materials.

We have used conventional and anomalous dispersion x-ray scattering to study the near-neighbor atomic environments in sputter-deposited amorphous Fe-Tb thin films with a large perpendicular magnetic anisotropy. The as-deposited films show a clear structural anisotropy, with more Fe-Tb near neighbor pairs in the out-of-plane direction. Upon annealing, the magnetic anisotropy drops significantly, and we see a corresponding reduction in the structural anisotropy. The number of Fe-Tb near-neighbors increases in the in-plane direction, but does not change in the out-of-plane direction. Therefore, the distribution of Fe-Tb near neighbors becomes more uniform upon annealing. We conclude that the observed reduction in perpendicular magnetic anisotropy energy is a result of this change in structure. \textcopyright{} 1996 The American Physical Society.

Thin films with perpendicular magnetic anisotropy (PMA) have been widely studied for perpendicular recording media since the middle of the 1970s when Prof. Iwasaki suggested this approach [1]. Since them, a great effort has been done by the research community for the use of this kind of nanostructures with PMA in other technological applications such as in high-density spin-transfer torque magnetic random access memories (STT-MRAM) [2], skyrmion-based devices [3] or even in synthetic antiferromagnets for biomedical applications [4].PMA in thin films can be usually achieved through different strategies such as by using materials with large magnetocrystalline anisotropy energy [5] as well as multilayer thin films with high surface anisotropy energy contributions [6]. Regarding the last alternative, Ikeda et al. [2] demonstrated in 2010 that amorphous CoFeB ultrathin films can show PMA and it is supported by the CoFeB/MgO interfacial anisotropy contribution. Since then, the development of non-magnetic/ferromagnetic superlattices with PMA, using CoFeB alloys as the ferromagnetic element, has received great attention. In particular, PMA in CoFeB/Pd was first demonstrated in bilayers by Fowley et al. [7] as well as in multilayers by Jung et al. [8]. Literature has reported that the effective anisotropy energy depends on the thickness of magnetic (tCoFeB) and non-magnetic (tPd) layers as well as on the number of bilayers (N).In this work, the dynamical response of [CoFeB (tCoFeB) / Pd (10 Å)]5 multilayered ultrathin films (1 Å ≤ tCoFeB ≤ 5 Å) were studied by using two complementary methods: broadband ferromagnetic resonance (Figure 1 a) and time-resolved magneto-optical Kerr effect (Figure 1 b). The perpendicular magnetization was confirmed for multilayers with tCoFeB ≤ 4 Å, while magnetization became in-plane oriented for tCoFeB ≥ 5 Å. This behaviour was explained by considering competing contributions from surface and magnetoelastic anisotropies. In addition, we have observed that the effective damping parameter is reduced with increasing tCoFeB (Figure 1 c), and the lower parameter, αeff ≈ (0.019 ± 0.001), was determined for the multilayer with 4 Å CoFeB thickness. We have suggested that for tCoFeB ≥ 4 Å the layer is continuous and the main contribution to effective damping is coming from spin-pumping when for lower thicknesses αeff is dominated by two-magnon scattering.Afterwards, we have studied the magnetic behaviour of ultrathin [CoFeB(tCoFeB)/Pd(10 Å)]N films. Intending to ensure that the system shows PMA, tCoFeB (3 or 4 Å) was fixed, while the number of bilayers (N) was ranged from 3 to 15. We observed that our samples show PMA and it is generally improved with N. In addition, we studied the dependence of αeff with the number of CoFeB/Pd repeats (N). Up today, literature in superlattices with PMA reported that αeff is proportional, inversely proportional or even, independent of N [9]. Therefore, the reported relationship between the damping parameter and N in superlattices with PMA is still unclear. We show that the evolution of αeff with N shows a complex behaviour that we have correlated with the magnetization process: larger αeff for samples with square hysteresis loops while αeff is independent of N for samples with bow-tie-shaped hysteresis loops. **

Perpendicular magnetic anisotropy (PMA) is becoming increasingly important in spintronics research, especially for high-density magnetoresistive random access memories (MRAMs). The PMA induced at an Fe/MgO interface is widely used in magnetic tunnel junctions. Here, we propose inserting an ultrathin LiF layer at the interface in an epitaxial Fe/MgO junction. With a 0.3 nm-thick LiF layer, a large intrinsic interface PMA energy, Ki,0, of 2.8 mJ/m2 was achieved. We also found that the LiF/MgO bilayer tunneling barrier exhibited a large tunnel magnetoresistance (TMR) effect, suggesting that a coherent spin-dependent tunneling process was maintained in the ultrathin LiF layer. Atomic-scale interface engineering using fluoride can further improve the PMA and TMR properties of spintronic devices.

<sec><p indent="0mm">Magnetic random-access memory (MRAM) has emerged as a leading candidate for next-generation storage technology, offering high speed, high density, non-volatility, low power consumption, and radiation resistance. MRAM has evolved through three generations: field-driven toggle MRAM, spin-transfer torque (STT)-MRAM, and the experimental spin-orbit torque (SOT)-MRAM. The current commercial MRAM products primarily rely on STT-MRAM, which is based on Ta/CoFeB/MgO multilayers with the interfacial perpendicular magnetic anisotropy (PMA). However, STT-MRAM performance deteriorates at sub<sc>-10 nm</sc> size due to thermal instability. In contrast, the tetragonal Mn-based binary alloys exhibit stable bulk PMA and enhanced properties for sub<sc>-10 nm</sc> tunnel junction devices. Previous research has demonstrated the potential of these alloys for high-density MRAM, with magnetization switching achieved via SOT without the need for external magnetic fields. Among Mn-based binary alloys, <italic>L</italic>1₀-MnAl stands out due to its strong PMA, high spin polarization, and low magnetic damping, making it a promising material for next-generation MRAM. On the other side, the <italic>B</italic>2-CoGa alloy serves as both an ideal buffer layer and a spin current source for the epitaxial growth of PMA <italic>L</italic>1₀-MnAl thin films on GaAs substrates. Our study demonstrates that doping <italic>B</italic>2-CoGa with Pt significantly enhances the SOT efficiency in (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic>/<italic>L</italic>1₀-MnAl bilayers while preserving the excellent PMA properties of <italic>L</italic>1₀-MnAl. </sec><sec> Experimental results show that Pt doping up to <italic>x</italic> = 0.1 keeps the PMA characteristics while simultaneously reducing the critical current density required for magnetization switching from <sc>4.63×10⁷ A/cm²</sc> to <sc>2.59×10⁷ A/cm².</sc> SOT efficiency increases from 0.042 to 0.080. This enhancement is attributed to the increased resistivity of the (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> layer, which does not compromise its effective spin Hall conductivity. The optimized bilayer, with <italic>x</italic> = 0.075, supports magnetic tunnel junctions with lateral size as small as <sc>8.09 nm,</sc> reducing power consumption by 37%. The SOT effect utilizes spin currents generated by phenomena such as the spin Hall effect to switch local magnetization in adjacent ferromagnetic layers. The damping-like SOT efficiency (<italic>ξ</italic>_DL), a crucial parameter for reducing device power consumption, is influenced by both spin Hall conductivity and resistivity of the material. The introduction of Pt impurities into (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> bilayers optimizes this balance, significantly enhancing <italic>ξ</italic>_DL. Compared to traditional heavy metals used as buffer layers, the (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> layer provides better lattice matching with Mn-based binary alloys, preserving their crystalline quality and PMA. This work systematically investigates the structural, magnetic, and SOT-driven magnetization switching properties of epitaxially grown (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic>/<italic>L</italic>1₀-MnAl bilayers, focusing on the dependence of magnetization switching on Pt concentration. Deterministic SOT-induced switching is achieved for <italic>x</italic> ≤ 0.1, with optimal PMA and SOT performance observed at <italic>x</italic> = 0.075. These results demonstrate the feasibility of incorporating Pt-doped CoGa layers into MRAM designs to enhance performance and scalability. This study provides valuable insights into improving SOT efficiency in Mn-based MRAM structures by incorporating Pt-doped CoGa buffer layers. These findings contribute to the development of high-density, high-performance MRAM devices with reduced power consumption. </sec>

Spin-transfer torque magnetic random access memory (STT-MRAM) requires large interfacial perpendicular magnetic anisotropy (iPMA) to be maintained even after many microfabrication steps. The data retention of STT-MRAM depends on the quality of the interface between the ferromagnetic layer and the insulating layer in magnetic tunnel junction (MTJ). We have investigated the effects of the grain boundary (GB) in the MgO layer on the iPMA using first-principles calculations. The results show that the iPMA is reduced by the presence of GBs, even though some iPMA remains. This is explained by a decrease in the peak of density of states (DOS) just above the Fermi energy, and that the DOS at high energy is increased in the down spin channel for <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${d}_{xz}$ </tex-math></inline-formula> and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$d_{yz}$ </tex-math></inline-formula> orbitals. Moreover, we found that the bond structure of Fe-d orbitals around the GB is a key factor controlling the iPMA, in addition to the well-known role of interfacial Fe–O bonds.