Interfacial Perpendicular Magnetic Anisotropy Research Articles

Engineering Mn3Ga/GaAs interfaces: a first-principles study on energetic stability and magnetic anisotropy

Mn3Ga-based ferrimagnets have emerged as a promising platform for energy-efficient spintronics. However, the challenge of identifying an appropriate substrate with minimal lattice misfit for the growth of high-quality Mn3Ga films while achieving the desired interfacial magnetic properties persists. Here we investigate the interfacial energetic stability and magnetic anisotropy at the Mn3Ga/GaAs interface through first-principles electronic structure calculations. Four types of interface models based on different configurations, including MnMn/As, MnMn/Ga, MnGa/As, and MnGa/Ga, are systematically examined. Our findings reveal that the MnGa/Ga interface exhibits the largest perpendicular magnetic anisotropy (PMA) but is energetically the least favorable. Conversely, the MnMn/As interface is energetically the most favorable but demonstrates a lower interfacial PMA. Our calculations align well with the experimentally observed formation of the secondary phase Mn2As. This study deepens our understanding of the interfacial properties in Mn3Ga/GaAs and offers some insights for their optimization through strategic interfacial engineering.

Intermixing of iron and cobalt with oxygen-rich magnesium oxide in CoFeB/MgO/CoFeB magnetic tunneling junctions

Atomic-scale spectroscopic imaging of sputtered magnetic tunnel junction structures with a thick oxygen-rich MgO barrier reveals the diffusion of iron and cobalt into the MgO barrier from CoFeB electrodes. First principles calculations are performed to (1) confirm that Fe diffusion through Mg vacancies is energetically favorable, (2) quantify the reduction of interfacial perpendicular magnetic anisotropy due to Fe diffusion into MgO, and (3) predict that the presence of Fe impurities in MgO causes an increased leakage and a tunneling magnetoresistance decrease. Through the chemical shift of the Fe L3 edge and the peak ratio Fe L3/Fe L2 measured by electron energy loss spectroscopy, we suggest that, within MgO, iron with mixed oxidation state Fe2+ and Fe3+ or higher is found in the as-grown structure, which is reduced by annealing to Fe2+. These results indicate that the stoichiometry of as-deposited MgO barrier layers plays an important role in controlling the microstructure and optimizing the performance of magnetic tunnel junctions.

Perpendicular magnetic anisotropy properties of Co2FeSi/Pt multilayers deposited on amorphous dielectric Ta2O5

Perpendicular magnetic anisotropy (PMA) is the mostsignificant characteristic for the development ofnonvolatile magnetic storage systems with superior performance. Hence, we investigate PMA of Co2FeSi (CFS)/Pt structures deposited on amorphous Ta2O5 of high dielectric constant. This design has the potential to be beneficial for low-power voltage-controlled spintronic devices. Strong PMA around ∼ 106 erg/cm3 is noticed in Ta2O5/[CFS/Pt]3 stacks with CFS thickness ranging from 0.6 ∼ 1.0 nm. The interfacial PMA energy density is found to be ∼ 0.63 erg/cm2. The PMA rises in the beginning with increase in number of repetitions (n), reaches a maximum of1.4x106 erg/cm3 at n = 3, and then falls. Pronounced thickness effects of Ta2O5 buffer on magnetic properties are observed. We clearly demonstrate the process of engineering PMA in the CFS/Pt bilayer structures deposited on Ta2O5 dielectric. Our findings might be useful for designing CFS/Pt multilayers on high-K dielectric for low power consumption spintronic devices.

Twist pz Orbital and Spin Moment of the Wavy-Graphene/L10-FePd Moiré Interface

A crystallographically heterogeneous moiré interface of hexagonal graphene (Gr) and a tetragonal L10-FePd alloy is bonded via van der Waals (vdW) forces. Robust interfacial perpendicular magnetic anisotropy was discovered at the L10-FePd side of the Gr/L10-FePd heterogeneous interface (H. Naganuma et al. ACS Nano, 2022, 16, 4139). This study focuses on the Gr side of the Gr/L10-FePd interface. X-ray absorption spectroscopy measurements of Gr from two different angles demonstrated that in addition to the π* orbital peak being observed at a glance incident angle (θΑ = 30°), it was also observed at a nominal incident (NI) angle (θΑ = 90°). The appearance of the π* peak at NI is attributed to the wavy Gr and strong bonding of the chemisorption-type vdW force. The densities of states of px, py, and pz from first-principles calculations indicate another reason for the π* peak at NI. The pz orbital of C twists into the x–y plane owing to its chemical bond with Fe or perturbation of the Fe ion to the C orbital. This twisted pz orbital appears near the Fermi level. Thus, the π* peak at NI can be interpreted to appear owing to three reasons: (i) the wavy Gr, (ii) the twisted pz orbital near the Fermi level, and (iii) the chemisorption-type vdW force. The X-ray magnetic circular dichroism of the C K-edge and first-principles calculations revealed that wavy Gr has a spin magnetic moment of 0.018 μB/C atoms but no orbital magnetic moment.

First-principles study on the magnetic anisotropy of the CoFe/ MgAl2O4 heterostructures

Practical applications of novel spintronic devices require heterostructures with high magnetic anisotropy (MA). Here, the MA of the CoFe/MgAl2O4 heterostructures was studied by using first-principles calculations. It was found that all the CoFe/MgAl2O4 models exhibit perpendicular magnetic anisotropy (PMA) and that the Co/MgAl2O4 interface is generally more favorable than the Fe/MgAl2O4 interface for inducing large PMA. Importantly, the CoFe/MgAl2O4 heterostructure with the Co/MgAl2O4 interface shows an interfacial PMA density of up to ∼ 0.91 mJ/m2. Moreover, the PMA of the CoFe/MgAl2O4 heterostructure is greatly enhanced compared to the corresponding bare CoFe film. Through density of state analysis combined with the layer- and orbital-hybridization-resolved MA contributions, it is found that the positive contribution from spin-orbit coupling interaction between opposite spin dz2 and dyz orbitals of the interfacial Co for CoFe/MgAl2O4 heterostructure is significantly higher than that of the surface Co for the bare CoFe film, which can well explain the enhancement of the PMA. Our calculated results extend the application of perpendicular magnetic tunnel junctions based on MgAl2O4.

Perpendicular Magnetic Properties on hcp-Co<sub>80</sub>Pt<sub>20</sub> Stacked Films with Oxide Seed Layers

The influence of oxide seed layers on the magnetic properties and crystalline structures of hcp-Co80Pt20 perpendicular magnetic thin films were investigated for magneto-optical chemical and biological sensing system. Cr-O, Al-doped ZnO (AZO) and Al2O3 films were used as the oxide seed layers. The Cr-O layer was most effective to improve the perpendicular magnetic properties of Co-Pt. The Cr-O seed layer produced the largest values of the interfacial perpendicular magnetic anisotropy (KS) and perpendicular magnetocrystalline anisotropy (KV). The investigation in the crystalline and chemical bonding conditions suggested that the both perpendicular magnetic anisotropies were originated from the interaction of Co and oxide at the Co-Pt/Cr-O interface. The improvements of KS and KV on the Cr-O seed layer are attributed to the hybridization of Co-3d and O-2p orbits and the hcp(001)-CoPt crystal growth with self-orientation, respectively. We also demonstrated the magneto-optical cavity effect on the Co-Pt perpendicular magnetic stacked films with the Cr-O intermediate layer. The stacked films indicated a large magneto-optical response by optical interference and good perpendicular magnetic properties with an ideal square shaped hysteresis loop.

Effect of MgO Grain Boundaries on the Interfacial Perpendicular Magnetic Anisotropy in Spin-Transfer Torque Magnetic Random Access Memory: A First-Principles Study

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.

Magnetic anisotropy of Fe/MgO interfaces inserted with alkali halide layers

Perpendicular magnetic anisotropy (PMA) at the Fe/MgO interface has been widely studied for memory applications, and further improving PMA has been one of the central challenges. In this study we fabricate epitaxial Fe/alkali halide (LiF, NaCl, or CsI)/MgO multilayers using molecular beam epitaxy and investigate how the alkali halide layer insertion can enhance PMA at the Fe/MgO interface. We find that insertion of an ultrathin LiF layer (0.1--0.4 nm) enhances interfacial PMA, but thicker LiF layer insertion (0.6--1 nm) weakens it. For the CsI and NaCl cases, interfacial PMA energy decreases monotonically with CsI or NaCl thickness. The present results indicate that well-defined Fe-anion orbital hybridization achieved by good lattice matching between the Fe and insulator layers is essential to have strong PMA. It is also suggested that anion atoms with high electronegativities, such as F, are beneficial for interfacial PMA, probably because of the weaker Fe-F hybridization and stronger electron localization at the interface. Our study shall serve as a guiding principle for designing a new dielectric layer to achieve stronger PMA in ultrathin Fe films.

Perpendicular magnetic anisotropy, tunneling magnetoresistance and spin-transfer torque effect in magnetic tunnel junctions with Nb layers

Nb and its compounds are widely used in quantum computing due to their high superconducting transition temperatures and high critical fields. Devices that combine superconducting performance and spintronic non-volatility could deliver unique functionality. Here we report the study of magnetic tunnel junctions with Nb as the heavy metal layers. An interfacial perpendicular magnetic anisotropy energy density of 1.85 mJ/m2 was obtained in Nb/CoFeB/MgO heterostructures. The tunneling magnetoresistance was evaluated in junctions with different thickness combinations and different annealing conditions. An optimized magnetoresistance of 120% was obtained at room temperature, with a damping parameter of 0.011 determined by ferromagnetic resonance. In addition, spin-transfer torque switching has also been successfully observed in these junctions with a quasistatic switching current density of 7.3 times ;10^{5} A/cm2.

Sputtered L10-FePd and its Synthetic Antiferromagnet on Si/SiO2 Wafers for Scalable Spintronics.

As a promising alternative to the mainstream CoFeB/MgO system with interfacial perpendicular magnetic anisotropy (PMA), L10-FePd and its synthetic antiferromagnet (SAF) structure with large crystalline PMA can support spintronic devices with sufficient thermal stability at sub-5 nm sizes. However, the compatibility requirement of preparing L10-FePd thin films on Si/SiO2 wafers is still unmet. In this paper, we prepare high-quality L10-FePd and its SAF on Si/SiO2 wafers by coating the amorphous SiO2 surface with an MgO(001) seed layer. The prepared L10-FePd single layer and SAF stack are highly (001)-textured, showing strong PMA, low damping, and sizeable interlayer exchange coupling, respectively. Systematic characterizations, including advanced X-ray diffraction measurement and atomic resolution-scanning transmission electron microscopy, are conducted to explain the outstanding performance of L10-FePd layers. A fully-epitaxial growth that starts from MgO seed layer, induces the (001) texture of L10-FePd, and extends through the SAF spacer is observed. This study makes the vision of scalable spintronics more practical.

Influence of sidewall damage on thermal stability in quad-CoFeB/MgO interfaces by micromagnetic simulation

The influence of the sidewall damage on the thermal stability factor (Δ) of quad-interface magnetic tunnel junctions (quad-MTJs) was investigated through a string method-based micromagnetic simulation. The quad-MTJs consist of a reference layer/MgO-barrier/CoFeB/middle-MgO/CoFeB/MgO-cap, which has four CoFeB/MgO interfaces to enhance the interfacial perpendicular magnetic anisotropy for large Δ. Experimentally obtained magnetic parameters at room temperature [e.g., saturation magnetization (Ms), stiffness constant (As), interfacial perpendicular magnetic anisotropy constants (Ki), and exchange coupling (Jex)] in blanket multilayer films of the quad-MTJs were used in micromagnetic simulation. The influence of the sidewall damage on the quad-MTJs, which is difficult to be analyzed in the experimental way, was investigated. The quad-MTJs without damaged layers having relatively higher Ki show the split of the energy barrier into two, resulting in a decrease in Δ. When the decrease in magnetic anisotropy energy (Eani) is more than the increase in the static magnetic energy (Esta), the antiferromagnetically (AF) coupled state of two free layers is formed at the midpoint to minimize the total energy (Eall). This causes the split of the energy barrier. The sidewall damage plays a role in lowering Ki in each layer, consequently avoiding the formation of the AF state. Note that the value of Δ with the sidewall damage, which shows the unified energy barrier, is comparable to non-damaged Δ, which shows the split of the energy barrier; these quad-MTJs have the same volume of free layers.

Voltage-controlled magnetic anisotropy effect through a LiF/MgO hybrid tunneling barrier

Improving the perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy (VCMA) properties are fundamentally important for the development of voltage-controlled magnetoresistive random access memories (VC-MRAM). Recently, we reported on a large increase in PMA at an Fe/MgO interface brought about by inserting an ultrathin LiF layer at the interface. In this paper, we investigate the PMA, VCMA, and TMR properties in MTJs with an Ir-doped ultrathin ferromagnetic layer and a LiF/MgO hybrid tunneling barrier. We observed a clear increase in the interfacial PMA by a factor of 2.5 when an ultrathin 0.25 nm LiF layer was inserted. A large VCMA coefficient, exceeding −300 fJ/Vm, was also achieved while maintaining the high TMR ratio and high interfacial PMA. These results demonstrate the high potential of interface engineering using ultrathin LiF layers for spintronic devices.

Correlation between interface perpendicular magnetic anisotropy and interfacial Dzyaloshinskii–Moriya interactions in Pt/Pd(t Pd)/Co(t Co)/Au

The interfacial Dzyaloshinskii–Moriya interaction (iDMI) and perpendicular magnetic anisotropy (PMA) can be manipulated via interface engineering. Their strength determines the generation and the size of skyrmions, and the correlation between the iDMI and PMA allows them to be controlled simultaneously, thus choosing the material parameters for which skyrmions can be formed. This opens up the possibility of synthesizing suitable magnetic multilayers that are needed for low-power high-density memory and in logic applications. Vibrating sample magnetometry (VSM), Brillouin light scattering (BLS), and microstrip ferromagnetic resonance (MS-FMR) were used to investigate the correlation between the iDMI and the interface PMA. For this, Pt/Pd(t Pd)Co(t Co)/Au thin film structures with 0 ⩽ t Pd⩽1.2 nm and 1.2 nm ⩽ t Co⩽3 nm were grown by combined sputtering and e-beam evaporation on Si/SiO2 substrates. VSM measurements revealed a negligible magnetic dead layer and a Pd thickness-independent magnetization at saturation of around 1200 emu cm−3. MS-FMR and BLS allowed us to conclue that PMA results from weak volume and interfacial contributions induced by Pd/Co or Pt/Co and Co/Au interfaces. The interface anisotropy constants are found to be 0.65 mJ m−2 and 0.85 mJ m−2 for Pd/Co and for Pt/Co interfaces, respectively. The Pd thickness-dependence of surface iDMI (D s) and PMA (K s) constants follow an exponential decay with a characteristic thickness higher for PMA than the iDMI. The slower decrease of K s vs Pd thickness suggests a shorter range of the iDMI with respect to PMA and points out a strongly localized origin for the iDMI. This difference between the iDMI and PMA is most likely responsible for the nonlinear correlation between PMA and iDMI constants. The investigation of the Co thickness-dependence of the iDMI and PMA in Pd/Co/MgO/Ta systems allows us to conclude a zero iDMI constant of Co/Au and to determine the iDMI constants of Pd/Co and Co/MgO, estimated to be D s Pd/Co = −0.096 pJ m−1 and D s Co/MgO = −0.15 pJ m−1, respectively. The criterion of skyrmions stability, applied for our samples, revealed the possibility of the existence of stable skyrmions in some samples.

Strong perpendicular magnetic anisotropy of Pt/Co/AlN structure

In this study, the perpendicular magnetic anisotropy (PMA) and microstructure of Pt/Co/AlN stacks were investigated. At a Co thickness of 0.5 nm, the magnetic moment was not observed in the as-deposited state, but after annealing, it increased significantly. Furthermore, a strong PMA was obtained in the annealed samples; after annealing at 300 °C and 400 °C, the effective PMA energy densities were 2.28 and 1.96 × 107 erg/cm3, respectively. Similar behavior was observed for samples with thicker Co layers. The magnetic dead layer and interfacial PMA energy density were also investigated to determine the interfacial effects on PMA. The occurrence of strong PMA was supported by high-resolution cross-sectional transmission electron microscopy results, demonstrating a well-defined layered structure for the annealed Pt/Co/AlN samples.

Perpendicularly Magnetized MnxGa‐Based Magnetic Tunnel Junctions: Materials, Mechanisms, Performances, and Potential Applications

AbstractMagnetic tunnel junctions (MTJs) exhibit a great profusion of unique functional properties, such as nonvolatility, scalability, high endurance, and low power consumption. For this reason, they have been widely investigated in spin‐based logic and memory devices. Compared with conventional in‐plane MTJs, perpendicular MTJs (pMTJs) enable lower critical switching current and higher integration density. So far, CoFeB/MgO structures with interface‐induced perpendicular magnetic anisotropy (PMA) are the most common ferromagnetic (FM) electrodes in pMTJs, whereas their relatively weak interfacial PMA restricts further development of device nodes. In the past decade, perpendicularly magnetized MnxGa binary alloys, including FM L10‐MnGa and ferrimagnetic D022‐Mn3Ga, have drawn intense attention for serving as a promising magnetic electrode in various MTJ stacking structures because of their large PMA, high spin polarization, and small magnetic damping constant. From an application perspective, MTJs with ultrathin MnxGa electrode show merits in developing high‐density magnetic memory, high‐frequency spin‐torque oscillators, and tunneling magnetoresistance effect‐based high‐field sensors. Herein, the aim is to provide a comprehensive review of recent progress in perpendicularly magnetized MnxGa‐based MTJs. The conclusions and future prospects are also given to inspire more in‐depth research and advance the practical applications.

Enhanced magnetoresistance in perpendicular magnetic tunneling junctions with MgAl2O4 barrier

Perpendicular magnetic tunnel junction with MgAl2O4 barrier is investigated. It is found that reactive RF sputtering with O2 is essential to obtain strong perpendicular magnetic anisotropy and large tunneling magnetoresistance in MgAl2O4-based junctions. An interfacial perpendicular magnetic anisotropy energy density of 2.25 mJ/m2 is obtained for the samples annealed at 400 °C. An enhanced magnetoresistance of 60% has also been achieved. The Vhalf, bias voltage at which tunneling magnetoresistance drops to half of the zero-bias value, is found to be about 1 V, which is substantially higher than that of MgO-based junctions.

Enhancement of perpendicular magnetic anisotropy in MgAl2O4/CoFeMnSi/MgAl2O4/W multilayer films

The effect of annealing temperature and film thickness on the enhancement in interfacial perpendicular magnetic anisotropy (PMA) is investigated in MgAl2O4(MAO)/CoFeMnSi(CFMS)/MAO/W stacks. A maximum effective PMA energy density (Keff) of ≈ 2.48 × 106 erg/cc is observed when the thickness of the MAO layers (tMAO) is 1.5 nm. The development of the PMA and saturation magnetization (Ms) is significantly stimulated by the structural changes at the MAO/CFMS and CFMS/MAO interfaces owing to the adequate interfacial oxidation state while tuning CFMS and MAO thickness. The study demonstrates that controlling the oxygen diffusion across the adjoining interface could be an easier means of achieving specific magnetic properties. These results open a new path towards the realization of novel spintronic devices employing PMA.

Investigation of perpendicular magnetic anisotropy in CoFeMnSi based heterostructures

In this work, perpendicular magnetic anisotropy (PMA) is realized in MgAl2O4(MAO)/CoFeMnSi(CFMS)/MgAl2O4/Ti heterostructures annealed at 300 °C and 400 °C. We show that interfacial PMA is very responsive to the annealing temperature and the CFMS layer thickness (tCFMS). A large PMA is achieved for tCFMS = 2.0 nm. An improvement in the out-of-plane saturation magnetization (Ms⊥) with PMA is found to get remarkably stimulated by the bottom MgAl2O4/CFMS interface. Besides, a uniaxial PMA is observed for an as-deposited stack with tCFMS ≤ 1.5 nm. Annealing at 400 °C significantly promoted adequate interfacial oxidation with the improved thermal stability of the heterostructure. Through X-ray photoelectron spectroscopy (XPS), the formation of Co–O bonds occurring as a result of the hybridization of Co-3dZ2 and O-2pZ orbitals at the interface shared by CFMS with MAO is identified as the microscopic origin of the observed PMA in these stacks. These findings indicate that the CFMS and MAO-based structures have immense potential for the next generation spintronic devices.

Interfacial perpendicular magnetic anisotropy in Co2FeSi alloy films sandwiched by Pt and MgAl2O4

Co2FeSi (CFS) alloy film is one of promising ferromagnetic electrodes and MgAl2O4 (MAO) spinel is an attractive oxide barrier. In this work, we fabricate a series of Pt/CFS/MAO/Pt stacks using sputtering method. For the as-deposited stacks the easy axis lies in the in sample plane. After annealing at 350 °C perpendicular magnetic anisotropy is shown to build in the stacks with CFS thickness ranged from 3.5 to 5.5 nm. A high saturation magnetization of 908 emu/cm3, close to the bulk CFS, is achieved. The perpendicular magnetic anisotropy constant of 105 erg/cm3 in the order of magnitude is witnessed. A large interfacial perpendicular magnetic anisotropy of 1.32 ± 0.18 erg/cm2 is obtained in the annealed stacks. Positive contributions to the PMA from the Pt/CFS and CFS/MAO/Pt interfaces are experimentally confirmed. Significantly improved crystallinity of Pt layers and (111)-oriented CoPt alloy were revealed by X-ray diffractometry measurements on the annealed stacks, which may be additionally responsible for the PMA.

Enhancement of spin-orbit torque efficiency by tailoring interfacial spin-orbit coupling in Pt-based magnetic multilayers

We study inserting Co layer thickness-dependent spin transport and spin–orbit torques (SOTs) in the Pt/Co/Py trilayers by spin-torque ferromagnetic resonance. The interfacial perpendicular magnetic anisotropy (IPMA) energy density (K s = 2.7 erg/cm2, 1 erg = 10−7 J), which is dominated by interfacial spin–orbit coupling (ISOC) in the Pt/Co interface, total effective spin-mixing conductance ) and two-magnon scattering (β TMS = 0.46 nm2) are first characterized, and the damping-like torque (ξ DL = 0.103) and field-like torque (ξ FL = –0.017) efficiencies are also calculated quantitatively by varying the thickness of the inserting Co layer. The significant enhancement of ξ DL and ξ FL in Pt/Co/Py than Pt/Py bilayer system originates from the interfacial Rashba–Edelstein effect due to the strong ISOC between Co-3d and Pt-5d orbitals at the Pt/Co interface. Additionally, we find a considerable out-of-plane spin polarization SOT, which is ascribed to the spin anomalous Hall effect and possible spin precession effect due to IPMA-induced perpendicular magnetization at the Pt/Co interface. Our results demonstrate that the ISOC of the Pt/Co interface plays a vital role in spin transport and SOTs-generation. Our finds offer an alternative approach to improve the conventional SOTs efficiencies and generate unconventional SOTs with out-of-plane spin polarization to develop low power Pt-based spintronic via tailoring the Pt/FM interface.