Interface-induced perpendicular magnetic anisotropy in Co2FeAl/NiFe2O4 superlattice: first-principles study

New insights into defects and magnetic interactions inducing lattice disordering in Co2Fe0.5Cr0.5Al

Atomic scale understanding of defect induced magnetic interactions resulting in lattice disordering has been deduced in a detailed manner for the first time in Co2Fe0.5Cr0.5Al based on Mössbauer spectroscopic studies and compared with the results obtained in Co2Fe0.8Cr0.2Al and Co2FeAl. An interesting linear correlation between valence electron concentration and the mean hyperfine fields at Fe sites in Co2FeAl based compounds has been deduced which is observed to exhibit different slopes with the substitution of Cr. This study elucidates an important role of the manifestation of the magnetic interactions especially between Fe, Co and Cr atoms leading to significant changes in the concentration and specific types of defects selectively produced in Co2Fe0.5Cr0.5Al as compared with that of Co2Fe0.8Cr0.2Al subjected to similar non-equilibrium treatments in this study. Further, for the first time this study elucidates the striking correlation of the effective value of the hyperfine field with the degree of ordering/disordering of the lattice with the Fe atoms associated with ordered sites experiencing a much higher value of the hyperfine field as compared to that of the disordered sites. This study also proposes optimal annealing treatment for the recovery of defects in Co2Fe0.5Cr0.5Al, which would be of significant importance in these spintronic materials.

High-Throughput Design of Interfacial Perpendicular Magnetic Anisotropy at Heusler/MgO Heterostructures

The perpendicular magnetic anisotropy (PMA) at ferromagnet/insulator interfaces has important technological applications, such as in the fields of magnetic recording and sensing devices. The perpendicular magnetic tunnel junctions (p-MTJs) with strong PMA have recently attracted increasing interest because they offer high stability and device performance toward low energy consumption. Heusler alloys are a large family of compounds that offer promising magnetic properties for developing p-MTJs. However, it is challenging to select appropriate combinations of Heusler ferromagnets and insulators with the desired interfacial properties. Here, we report a systematic high-throughput screening approach to search for candidate Heusler/MgO material interfaces with strong PMA and other desired material properties for spintronic technologies. On the basis of the open quantum material repositories, we developed a series of material descriptors, including formation energy, convex hull distance, magnetic ordering, lattice misfit, magnetic anisotropy constant, cleavage energy, and tunnel magnetoresistance, to filter candidate Heusler/MgO interfaces among the possible 40 000 ternary Heusler compounds. After a comprehensive screening, five full-Heusler compounds, including Co₂CrAl, Co₂FeAl, Co₂HfSn, Fe₂IrGa, and Mn₂IrGe, and two half-Heusler compounds, PtCrSb and PtMnAs, were found to be promising for designing p-MTJs. This work demonstrates a new way for the high-throughput design of functional material interfaces for spintronic applications via exploiting the open quantum material repositories and developing effective material descriptors along with the large-scale ab initio calculations for material interfaces.

Theoretical scheme of the nonvolatile strain switchable high/low resistance based on the novel strain-tunable magnetic anisotropy in Mn2.25Co0.75Ga0.5Sn0.5/MgO superlattice

It is great desirable to effectively tune electronic and magnetic properties by simple means such as applying external electrical fields or strains in spintronics research. Here, we investigate the strain...

Effects of Interfacial Termination, Oxidation, and Film Thickness on the Magnetic Anisotropy in Mn2.25Co0.75Ga0.5Sn0.5/MgO Heterostructures

Perpendicular magnetic anisotropy (PMA) is a determining factor for the realization of nonvolatile information storage devices with high efficiency and thermal stability. In this work, a new spin gapless semiconductor Mn_2.25Co_0.75Ga_0.5Sn_0.5 Heusler alloy with an inter-spin zero gap was first designed theoretically. The Mn_2.25Co_0.75Ga_0.5Sn_0.5 bulk was prepared successfully in experiment. The effects of interfacial termination, oxidation, and film thickness on the magnetic anisotropy of Mn_2.25Co_0.75Ga_0.5Sn_0.5/MgO (MCGS/MgO) heterostructures are investigated systematically by first-principles calculations. The results show that all the Mn(A)Mn(C)GaSn-, Mn(A)Mn(C)CoGaSn-, Mn(B)GaSn_I-, and Mn(B)GaSn_II-terminated MCGS/MgO heterostructures (called as AC1, AC2, BD1, and BD2 models, respectively) present PMA, which mainly derives from the interfacial and surficial MCGS layers. Furthermore, the PMA of MCGS/MgO heterostructures can be preserved in a large range of interfacial oxidization (up to ±50%). With MCGS thickness increasing from 5 to 16 monolayers, the PMA of MCGS/MgO heterostructures with an AC-type surface decreases significantly. However, the PMA of BD-type surface models is relatively robust to the thickness of the MCGS layer, and the magnetic anisotropy always points to the out-of-plane direction. Therefore, MCGS Heusler alloy is a new promising spin gapless semiconductor candidate for spintronics applications. The robust and tunable PMA in MCGS/MgO heterostructures offers the possibility for developing nonvolatile data memory devices.

The large perpendicular magnetic anisotropy induced at the Co2FeAl/MgAl2O4interface and tuned with the strain, voltage and charge doping by first principles study

The heterostructures with high perpendicular magnetic anisotropy (PMA) have advantages for the application of the nonvolatile memories with long data retention time and small size. The interface structure and magnetic anisotropy energy (MAE) of Co₂FeAl/MgAl₂O₄heterostructures were studied by first principles calculations. The stable interface atomic arrangement is the Co or FeAl layer located above the equatorial oxygen coordinate in the distorted oxygen octahedrons. The Co-O interface can induce large effective PMA up to 4.54 mJ m^-2, but this structure is a metastable structure. Meanwhile, the effective MAE decreases linearly as the thickness of the ferromagnetic layer increase. The effective MAE for the FeAl-O interface is only 1.3 mJ m^-2, while the maximum thickness of Co₂FeAl layer that maintains the PMA effect is about 1.717 nm. These values are very close to the experimental results. Throughd-orbital-resolved MAE, we confirm that the interface PMA is mainly originated from the hybridization betweendxy,dyzanddz2orbitals of interface 3datoms. In addition, the compressive strain, negative electric field and hole doping can significantly enhance the effective PMA of FeAl-O interface. At the same time, Co-O interface will become the most stable structure by tuning the Mg/Al ratio in the spinel layers. The large effective PMA makes the Co₂FeAl/MgAl₂O₄junction a perfect candidate for the next-generation of non-volatile spintronic devices.

Phase Transition and Electronic Structures of All-d-Metal Heusler-Type X2MnTi Compounds (X = Pd, Pt, Ag, Au, Cu, and Ni)

In this work, we investigated the phase transition and electronic structures of some newly designed all-d-metal Heusler compounds, X₂MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni), by means of the first principles. The competition between the XA and L2₁ structures of these materials was studied, and we found that X₂MnTi favors to feature the L2₁-type structure, which is consistent with the well-known site-preference rule (SPR). Under the L2₁ structure, we have studied the most stable magnetic state of these materials, and we found that the ferromagnetic state is the most stable due to its lower energy. Through tetragonal deformation, we found that the L2₁ structure is no longer the most stable structure, and a more stable tetragonal L1₀ structure appeared. That is, under the tetragonal strain, the material enjoys a tetragonal phase transformation (i.e., from cubic L2₁ to tetragonal L1₀ structure). This mechanism of L2₁-L1₀ structure transition is discussed in detail based on the calculated density of states. Moreover, we found that the energy difference between the most stable phases of L1₀ and L2₁, defined as ΔE _M (ΔE _M = E _Cubic-E _Tetragonal), can be adjusted by the uniform strain. Finally, the phonon spectra of all tetragonal X₂MnTi (X = Pd, Pt, Ag, Au, Cu, and Ni) phases are exhibited, which provides a powerful evidence for the stability of the tetragonal L1₀ state. We hope that our research can provide a theoretical guidance for future experimental investigations.