Enhancement Of Perpendicular Magnetic Anisotropy Research Articles

The modulation of perpendicular magnetic anisotropy and spin–orbit toque in Pt/Co/Pt multilayers with interfacial decoration by insertion of a Bi layer

Bismuth is a well-known semimetal material with strong spin–orbit coupling and has been confirmed to exhibit a larger spin Hall angle in CuBi and PtBi alloys with low Bi-doping concentration. In this study, we investigated perpendicular magnetic anisotropy (PMA) and spin–orbit torques (SOTs) in Pt/Co/Pt multilayers by inserting a Bi layer with a thickness of 2 nm at the Co/Pt interface. Two types of PMA multilayers, Pt (3 nm)/Co (1 nm)/Pt (5 nm) and Pt (3 nm)/Co (1 nm)/Pt (1 nm), with different spin accumulations, were designed and prepared by varying the top Pt thickness. A significant enhancement of PMA and SOT efficiency is observed in the Pt (3 nm)/Co (1 nm)/Bi (2 nm)/Pt (5 nm) multilayer via a Bi-layer interfacial decoration. However, for the Pt (3 nm)/Co (1 nm)/Bi (2 nm)/Pt (1 nm) multilayers, both PMA and SOT efficiency decrease with decoration of the Bi layer at the Co/Pt interface. Meanwhile, the sign of SOTs changes in the Pt (3 nm)/Co (1 nm)/Pt (5 nm) and Pt (3 nm)/Co (1 nm)/Pt (1 nm) multilayers when introducing a Bi layer. This completely opposite behavior illustrates that the Bi/Pt interface plays an important role in modulating the PMA and SOT efficiency of Pt/Co/Bi/Pt multilayers. Optimizing the alloying of Bi/Pt could be an effective approach to increase the PMA and SOT efficiency of Pt/Co/Bi/Pt multilayers.

Influence of Pt Ultrathin Interlayers on Magnetic Anisotropy in Ni/NiO Multilayers

Perpendicular magnetic anisotropy at transition metal/oxide interfaces plays a significant role in technological applications such as magnetic storage and spintronics. In this study, we investigate the effects of thermal annealing and Pt ultrathin interlayers on the magnetic anisotropy in Ni/NiO multilayers. Ni/NiO/Pt multilayers were fabricated via radiofrequency magnetron sputtering and natural oxidation. The static magnetic properties of the samples were studied using temperature-dependent SQUID magnetometry. We focus on a sample with a Nickel thickness of 6.7 nm in each multilayer period. This multilayer in Ni/NiO form showed the maximum enhancement of perpendicular magnetic anisotropy after mild thermal annealing in past work. In this work, we study the effects of ultrathin Pt interlayers on the magnetic properties of such a Ni/NiO multilayer before and after annealing. We have observed a further increase in perpendicular magnetic anisotropy, and we study the temperature-dependent magnetic properties of this system, which combines the favorable magnetic properties of Ni/Pt and Ni/NiO multilayers.

A Strategy for Enhancing Perpendicular Magnetic Anisotropy in Yttrium Iron Garnet Films.

In future information storage and processing, magnonics is one of the most promising candidates to replace traditional microelectronics. Yttrium iron garnet (YIG) films with perpendicular magnetic anisotropy (PMA) have aroused widespread interest in magnonics. Obtaining strong PMA in a thick YIG film with a small lattice mismatch (η) has been fascinating but challenging. Here, a novel strategy is proposed to reduce the required minimum strain value for producing PMA and increase the maximum thickness for maintaining PMA in YIG films by slight oxygen deficiency. Strong PMA is achieved in the YIG film with an η of only 0.4% and a film thickness up to 60nm, representing the strongest PMA for such a small η reported so far. Combining transmission electron microscopy analyses, magnetic measurements, and a theoretical model, it is demonstrated that the enhancement of PMA physically originates from the reduction of saturation magnetization and the increase of magnetostriction coefficient induced by oxygen deficiency. The Gilbert damping values of the 60-nm-thick YIG films with PMA are on the order of 10-4 . This strategy improves the flexibility for the practical applications of YIG-based magnonic devices and provides promising insights for the theoretical understanding and the experimental enhancement of PMA in garnet films.

Interfacial Fe segregation and its influence on magnetic properties of CoFeB/MgFeO multilayers

Abstract We investigated effects of Fe segregation from partially Fe-substituted MgO (MgFeO) on the magnetic properties of CoFeB/MgFeO multilayers. X-ray photoelectron spectroscopy and magnetic measurements revealed that segregated Fe reduced to metallic Fe and ferromagnetism was exhibited at the CoFeB/MgFeO interface. The CoFeB/MgFeO multilayer showed a more than 2-fold enhancement in perpendicular magnetic anisotropy (PMA) energy density compared with that of a standard CoFeB/MgO multilayer. The PMA energy density was further enhanced by insertion of an ultrathin MgO layer between the CoFeB and MgFeO layers. Ferromagnetic resonance measurements also revealed a remarkable reduction of magnetic damping in the CoFeB/MgFeO multilayers.

Thermal Annealing Driven Enhancement of Perpendicular Magnetic Anisotropy and the Interfacial Dzyaloshinskii–Moriya Interaction in Ultrathin Ru/Co/W/Ru Films

Spin-related effects discovered in ultrathin magnetic films generally have an interfacial nature. The quality of interfaces between a heavy metal and a ferromagnet is the main driver of surface anisotropy, antisymmetric exchange, chiral damping, spin Hall effect, and spin–orbit torques. To understand the physics and usage of these phenomena for future atomic-scale devices, one has to study systems with disordered interfaces and search there for novel practically desirable effects, which lie beyond the atomic order and surface flatness. Here, we report the interface-related effects in Ru/Co/W/Ru ultrathin films treated by thermal annealing in vacuum. The surface roughness and degree of atomic intermixing at Ru/Co and Co/W interfaces change with increasing annealing temperature up to 240 °C, promoting an enhancement of perpendicular magnetic anisotropy and the interfacial Dzyaloshinskii–Moriya interaction (i-DMI). Further annealing at higher temperatures brings the interface deterioration and, consequently, a drastic degradation of the magnetic properties. Our first-principles calculations qualitatively support the experimental findings, providing an understanding of the i-DMI enhancement nature.

Heteroepitaxy of 2D CuCr2 Te4 with Robust Room-temperature Ferromagnetism.

Magnetic materials in 2D have attracted widespread attention for their intriguing magnetic properties. 2D magnetic heterostructures can provide unprecedented opportunities for exploring fundamental physics and novel spintronic devices. Here, the heteroepitaxial growth of ferromagnetic CuCr2 Te4 nanosheets is reported on Cr2 Te3 and mica by chemical vapor deposition. Magneto-optical Kerr effect measurements reveal the thickness-dependent ferromagnetism of CuCr2 Te4 nanosheets on mica, where a decrease of Curie temperature (TC ) from 320 to 260K and an enhancement of perpendicular magnetic anisotropy with reducing thickness are observed. Moreover, lattice-matched heteroepitaxial ultrathin CuCr2 Te4 on Cr2 Te3 exhibits an enhanced robust ferromagnetism with TC up to 340K due to the interfacial charge transfer. Stripe-type magnetic domains and single magnetic domain are discovered in this heterostructure with different thicknesses. The work provides a way to construct robust room-temperature 2D magnetic heterostructures for functional spintronic devices.

Modulation of Spin-Orbit Torque and Large Enhancement of Perpendicular Magnetic Anisotropy in W/Co20Fe60B20/MgO by Ir Insertion Layer

Iridium ($\mathrm{Ir}$) is a 5d heavy metal with strong spin-orbit coupling (SOC), which introduces various intriguing phenomena when interacting with a ferromagnetic layer. In this study, we utilize $\mathrm{Ir}$ as an insertion layer in $\mathrm{W}/\mathrm{Ir}({t}_{\mathrm{Ir}})/{\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}/\mathrm{Mg}\mathrm{O}$ heterostructures to tune SOC and study its influence on the spin-orbit torque (SOT), perpendicular magnetic anisotropy (PMA), and Dzyaloshinskii-Moriya interaction (DMI). The SOT efficiency decreases first and then increases with an increase of $\mathrm{Ir}$ insertion layer thickness because of the opposite spin Hall angle between $\mathrm{W}$ and $\mathrm{Ir}$, and the sign changes when the thickness of $\mathrm{Ir}$ is 1.5 nm, which is close to the spin diffusion length of $\mathrm{Ir}$ of 1.35 nm. Importantly, a 0.25-nm-thick $\mathrm{Ir}$ layer enhances largely the effective PMA by 140%--350% under different annealing temperatures compared with that of a sample without insertion. Meanwhile, the DMI constant is also enhanced. The enhancement of PMA and DMI is likely due to the enhancement of interfacial SOC when $\mathrm{Ir}$ diffuses into ${\mathrm{Co}}_{20}{\mathrm{Fe}}_{60}{\mathrm{B}}_{20}$ and leads to the formation of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}$. These results provide an effective method to tune SOC by inducing a dusting layer with strong SOC to meet different requirements in spintronic devices like small magnetic tunnel junctions.

First-principles study of enhancement of perpendicular magnetic anisotropy obtained by inserting an ultrathin LiF layer at an Fe/MgO interface

Perpendicular magnetic anisotropy (PMA) is a key property of magnetoresistive random access memory (MRAM). To increase areal density of MRAM it is important to find a way to enhance the PMA. Recently a strong enhancement of the PMA by inserting an ultrathin LiF layer at an Fe/MgO interface was reported [T. Nozaki et al., NPG Asia Materials (2022) 14: 5]. To understand the origin of the observed enhancement of the PMA we perform first-principles calculations of magnetocrystalline anisotropy energy (MAE) of the following four kind of multilayer structures: Fe/MgO, Fe/LiF/MgO, Fe/FeO/MgO, and Fe/FeF/LiF/MgO. We find that the MAEs of the Fe/LiF/MgO and the Fe/FeF/LiF/MgO structures are almost the same as that of the Fe/MgO structure, while the MAE of the Fe/FeO/MgO structure is less than a half of that of the Fe/MgO structure. The results show that the major origin of the enhancement of the PMA obtained by inserting an ultrathin LiF layer at an Fe/MgO interface is the suppression of the mixing of Fe and O atoms at the interface. We also find that the in-plane Fe–F coupling gives a positive contribution to the MAE while the in-plane Fe–O coupling gives a negative contribution. The results are useful for designing of high-PMA materials.

Enhancement of perpendicular magnetic anisotropy and formation of interface in Nd-Dy-Fe-Co-B thick film with Ta spacer layer

The perpendicular magnetic anisotropy of Nd-Fe-B films decreases when the film thickness increases above a critical value, owing to the formation of the in-plane texture. To avoid this dilemma, the influences of Ta spacer layers on the microstructure and magnetic properties of Nd-Dy-Fe-Co-B thick films are systematically investigated by varying the number and thickness of Ta spacer layers. With increasing the number and thickness of Ta spacer layers, a great enhancement of the perpendicular magnetic anisotropy is found, while the coercivity maintains around 2 T. According to the analysis of microstructural results, some interfaces are formed, with the Nd-Fe-B columnar crystal and the Nd element aggregation area being the two sides of the interface. It is found that the Ta spacer layer facilitates the formation of the out-of-plane texture. In addition, the Ta spacer layer has the effect of enhancing the pinning effect leading to a high coercivity. Based on first-order reversal curve analysis, the Ta spacer layer can enhance the dipole interaction and reduce the exchange interaction, which reveals that Nd-Fe-B grains are isolated by the Nd-rich phase. This work can help us to prepare thick Nd-Fe-B-based films with high perpendicular magnetic anisotropy and high coercivity for applications in micromechanical systems.

Growth, Magnetic Anisotropies and Exchange Bias of Thin Ni0.95Fe0.05/NiFeO Multilayers

Ni0.95Fe0.05/NiFeO multilayers were fabricated by radio frequency magnetron sputtering and natural oxidation. Doping of Ni by only 5 at. % Fe results in enhanced layering quality as X-ray reflectivity reveals. Due to magnetostatic anisotropy, the multilayers were found to be in-plane magnetized. The influence of mild thermal annealing (T = 525 K) on the magnetic properties of NiFe/NiFeO multilayers is also investigated. Annealing results in the enhancement of perpendicular magnetic anisotropy, mainly due to an increase in the uniaxial volume anisotropy term. Temperature-dependent hysteresis measurements between 4–400 K revealed considerable enhancement of coercivity and appearance of exchange bias effect.

Plasma Etching‐Assisted Perpendicular Magnetic Anisotropy

Ferromagnetism with perpendicular magnetic anisotropy (PMA) receives continuous attention. But, the PMA is always limited by materials system, the assistance from suitable buffer layers such as Pt and Ta is essential. A general method for the generation of PMA on various systems is pursued. Herein, it is shown that preparatory plasma etching of oxide substrate produces robust PMA in subsequent grown ferromagnetic multilayers, out‐of‐plane hysteresis loop is rectangular with remanence close to 100%. In contrast, without the preparatory plasma etching, out‐of‐plane hysteresis loop of subsequent grown ferromagnetic multilayers is very oblique, the calculated uniaxial anisotropic constant is one order of magnitude smaller than the former one. The enhancements of PMA are observed in distinct oxide substrates including Si/SiO2, Al2O3, LaAlO3, and MgO, indicating the interfacial plasma etching technique can be used for various oxide substrates to obtain PMA. The promotion of Co–O hybridization is proposed to be the dominant reason for the enhancement of PMA, which is supported by the control experiment. This finding provides opportunity for the formation of interfacial PMA on crystalline oxide substrates, and expands the material selection for achieving PMA.

Oxygen Vacancy Engineering for Highly Tunable Ferromagnetic Properties: A Case of SrRuO3 Ultrathin Film with a SrTiO3 Capping Layer

AbstractOxide heterostructures have great potential for spintronics applications due to their well‐defined heterointerfaces and vast functionalities. To integrate such compelling features into practical spintronics devices, effective control of the magnetic switching behavior is key. Here, continuous control of the magnetic coercive field in SrTiO3/SrRuO3 ultrathin heterostructures is achieved by oxygen vacancy (VO) engineering. Pulsed laser deposition of an oxygen‐deficient SrTiO3 capping layer can trigger VO migration into the SrRuO3 layer while avoiding the formation of Ru vacancies. Moreover, by varying the thickness and growth conditions of the SrTiO3 capping layer, the value of the coercive field (HC) in the ferromagnetic SrRuO3 layer can be continuously tuned. The maximum enhancement of HC at 5 K is 3.2 T. Such a wide‐range tunability of HC may originate from a VO‐induced enhancement of perpendicular magnetic anisotropy and domain wall pinning. This study offers effective approaches for controlling physical properties of oxide heterostructures via VO engineering, which may facilitate the development of oxide‐based functional devices.

Enhancement of perpendicular magnetic anisotropy and Dzyaloshinskii\u2013Moriya interaction in thin ferromagnetic films by atomic-scale modulation of interfaces

To stabilize nontrivial spin textures, e.g., skyrmions or chiral domain walls in ultrathin magnetic films, an additional degree of freedom, such as the interfacial Dzyaloshinskii–Moriya interaction (IDMI), must be induced by the strong spin-orbit coupling (SOC) of a stacked heavy metal layer. However, advanced approaches to simultaneously control the IDMI and perpendicular magnetic anisotropy (PMA) are needed for future spin-orbitronic device implementations. Here, we show the effect of atomic-scale surface modulation on the magnetic properties and IDMI in ultrathin films composed of 5d heavy metal/ferromagnet/4d(5d) heavy metal or oxide interfaces, such as Pt/CoFeSiB/Ru, Pt/CoFeSiB/Ta, and Pt/CoFeSiB/MgO. The maximum IDMI value corresponds to the correlated roughness of the bottom and top interfaces of the ferromagnetic layer. The proposed approach for significant enhancement of PMA and the IDMI through interface roughness engineering at the atomic scale offers a powerful tool for the development of spin-orbitronic devices with precise and reliable controllability of their functionality.

Enhancing perpendicular magnetic anisotropy through dead layer reduction utilizing precise control of Mg insertions

Using an ultrathin metallic layer of Mg at the CoFeB|MgO interface has been emerged as an appealing solution to efficiently enhance the tunneling magnetoresistance and thermal stability while reducing the resistance-area product and effective damping of CoFeB films for efficient and low-power dissipated MTJ. However, it is still a great challenge to significantly enhance the perpendicular magnetic anisotropy (PMA) using Mg interlayer in CoFeB|MgO structures and reveal the behind mechanism, for continuously scaling down the MTJ structure. Here, we investigate the effect of ultrathin Mg insertions of 0.2 nm, 0.4 nm and 0.6 nm on PMA at CoFeB|MgO interface for the CoFeB thickness range of 1–1.5 nm and compare the results with an identical structure without insertion. We observed that upon inserting ultrathin Mg layer, the effective anisotropy turned to out-of-plane even for thicker 1.5 nm CoFeB layers which favors in-plane anisotropy without Mg insertion. We also observe a reasonable increase in the interfacial anisotropy energy along with the effective CoFeB thickness for the structures with Mg insertion. The effect that Mg insertion layer helps reducing the dead layer was revealed accounting for the PMA enhancement mechanism. Our results provide a more detailed explanation for the enhancement of PMA through Mg insertion, which may offer a guidance for the development of CoFeB|MgO based MTJ structure with higher thermal stability and lower switching current.

Large spin Hall effect and increase in perpendicular magnetic anisotropy in artificially synthesized amorphous W/Hf multilayer/CoFeB system

We studied the spin-Hall effect and perpendicular magnetic anisotropy in W/Hf multilayer/CoFeB/MgO and W80Ta20/Hf multilayer/CoFeB/MgO systems and compared them with those in the β-W/CoFeB/MgO system. From the cross-sectional high-resolution transmission electron microscopy images, (i) the amorphous structure of W/Hf and W80Ta20/Hf multilayers, (ii) the flat interface between heavy metals and CoFeB, and (iii) highly (100) texture of MgO (1.0 nm) on CoFeB were observed in those multilayer systems. A higher spin-Hall effect and enhancement of perpendicular magnetic anisotropy in the W/Hf multilayer/CoFeB/MgO system can be achieved compared to the β-W/CoFeB/MgO system. In addition, we found that the resistivity in amorphous W/Hf multilayers is low compared to that in β-W. These results suggest that the artificially synthesized multilayer system is one of the avenues for realizing the heavy metal with a large spin-Hall effect and low resistivity.

Enhancement of perpendicular magnetic anisotropy and spin-orbit torque in Ta/Pt/Co/Ta multi-layered heterostructures through interfacial diffusion

Heavy metal/ferromagnetic metal bi-layered structures that exhibit both strong perpendicular magnetic anisotropy (PMA) and large spin-orbit torque (SOT) efficiency have high potential in high-density, low-power memory, and logic device applications. Here, we report the enhancement of PMA and SOT in Ta/Pt/Co/Ta multi-layered heterostructures through interfacial diffusion. The structures can exhibit PMA fields of 9100 Oe at 300 K and 14100 Oe at 10 K and an effective spin Hall angle (SHA) of 0.61 ± 0.03 at 300 K. These values are larger than the corresponding values reported previously for similar heterostructures. The current-induced magnetization switching was demonstrated. The critical switching current density is on the order of 106 A/cm2, and the corresponding switching efficiency is higher than that reported for similar structures. X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy analyses suggest a strong correlation between the observed PMA and SOT enhancement and the interfacial diffusion during the sputtering growth of the samples. It is very likely that the interfacial diffusion gives rise to enhanced spin-orbit coupling at the interface, while the latter results in enhancement in the PMA, SHA, and switching efficiency in the structure.

Role of the Interfacial Rh-layer on Robust Ferromagnetism and Large Perpendicular Magnetic Anisotropy of FeRh Films on MgO(0 0 1)

Abstract Employing first-principles calculations we reveal the strong interface termination dependences of the magnetic order and perpendicular magnetic anisotropy (PMA) in FeRh films on MgO(0 0 1). While the magnetic ground state of the Fe-terminated FeRh(0 0 1) is an antiferromagnetic, the Rh-termination favors ferromagnetism within the underlying FeRh layers. This ferromagnetism can persist even at the interface with MgO, which in turn leads to a few times enhancement of PMA compared with the Fe-interface in FeRh/MgO(0 0 1). The underlying mechanism for this large PMA is an interplay between the spin-polarized Rh d xy and d x 2 - y 2 orbital states with large spin-orbit coupling at the interface. These results unveil a crucial role of the interface termination on magnetism of FeRh in antiferromagnet spintronics.

Unexpected Development of Perpendicular Magnetic Anisotropy in Ni/NiO Multilayers After Mild Thermal Annealing

We report on the significant enhancement of perpendicular magnetic anisotropy of Ni/NiO multilayers after mild annealing up to 90 min at 250 °C. Transmission electron microscopy shows that after annealing, a partial crystallization of the initially amorphous NiO layers occurs. This turns out to be the source of the anisotropy enhancement. Magnetic measurements reveal that even multilayers with Ni layers as thick as 7 nm, which in the as-deposited state showed in-plane anisotropy with square hysteresis loops, show reduced in-plane remanence after thermal treatment. Hysteresis loops recorded with the field in the normal-to-film-plane direction provide evidence for perpendicular magnetic anisotropy with up and down magnetic domains at remanence. A plot of effective uniaxial magnetic anisotropy constant times individual Ni layer thickness as a function of individual Ni layer thickness shows a large change in the slope of the data attributed to a drastic change of volume anisotropy. Surface anisotropy showed a small decrease because of some layer roughening introduced by annealing.

Strain-tunable electric structure and magnetic anisotropy in monolayer CrSI.

Two-dimensional (2D) ferromagnetic semiconductors provide platforms for studying novel physical phenomena in low dimensional materials. Using first-principles calculations, we systematically investigate the effect of strain on the electronic structure and magnetic anisotropy energy (MAE) of monolayer CrSI. The results demonstrate that the easy axis of unstrained monolayer CrSI is parallel to the in-plane [100] axis and the MAE of monolayer CrSI is mainly contributed by the spin-polarized p-orbitals of nonmetallic I atoms. Remarkably, the strain transforms the ground state of monolayer CrSI from a ferromagnetic semiconductor to ferromagnetic metal. More importantly, the external strain can switch the direction of the easy axis of monolayer CrSI and compressive strain significantly enhances the MAE of the I atom to reach 0.52 meV per atom, which is comparable to that of metallic Fe atoms at Fe/MgO interfaces. Furthermore, we elucidate that the increase of the positive contributions of matrix element differences between the spin-up px and py orbitals as well as spin-up py and pz orbitals of the I atom to MAE with respect to compressive strain is the main cause of the significant enhancement in perpendicular magnetic anisotropy of monolayer CrSI under -10% compressive strain. Our research proves that monolayer CrSI has a good application prospect in magnetic storage devices.

Enhancement of Perpendicular Magnetic Anisotropy Through Fe Insertion at the CoFe/W Interface

Magnetic tunnel junctions (MTJs) based on MgO/CoFeB/W structures are of particular interest for magnetic random access memories due to their high tunnel magnetoresistance, low damping, and stable properties at high annealing temperature. One of the major challenges is to obtain a strong interfacial perpendicular magnetic anisotropy (PMA) for sufficient thermal stability during data storage. Here we investigate magnetic anisotropy energies (MAEs) in the ferromagnet (FM)/W structure with different FM materials and interface insertions. We find that the Fe/W interface exhibits a remarkable PMA, whereas the Co/W interface displays a large in-plane anisotropy, which reduces the PMA in the MgO/CoFeB/W structure. By inserting a thin Fe layer at the CoFe/W interface, the interfacial PMA can be enhanced to 3.34 mJ/m2. Microscopic mechanism behind these phenomena is explained with the changes of orbital-resolved MAEs due to the charge redistribution at the interface. These findings pave a new way for the enhancement of PMA and point toward the possibility of achieving high thermal stability in small-node MTJs.