Using first-principles calculations, we studied Mn$_2$RuZ (Z=Al, Ga, Si, Ge) and their heterojunctions with MgO along (001) direction. All these alloys possess Hg$_2$CuTi-type inverse Heusler alloy structure and ferrimagnetic ground state. Our study reveals the half-metallic electronic structure with highly spin-polarized $\Delta_1$ band, which is robust against atomic disorder. Next we studied the electronic structure of Mn$_2$RuAl/MgO and Mn$_2$RuGe/MgO heterojunctions. We found that the MnAl- or MnGe-terminated interface is energetically more favorable compared to the MnRu-terminated interface. Interfacial states appear at the Fermi level in the minority-spin gap for the Mn$_2$RuGe/MgO junction. We discuss the origin of these interfacial states in terms of local environment around each constituent atom. On the other hand, in the Mn$_2$RuAl/MgO junction, high spin polarization of bulk Mn$_2$RuAl is preserved independent of its termination.
Voltage control of spin enables both a zero standby power and ultralow active power consumption in spintronic devices, such as magnetoresistive random-access memory devices. A practical approach to achieve voltage control is the electrical modulation of the spin–orbit interaction at the interface between 3d-transition-ferromagnetic-metal and dielectric layers in a magnetic tunnel junction (MTJ). However, we need to initiate a new guideline for materials design to improve both the voltage-controlled magnetic anisotropy (VCMA) and perpendicular magnetic anisotropy (PMA). Here we report that atomic-scale doping of iridium in an ultrathin Fe layer is highly effective to improving these properties in Fe/MgO-based MTJs. A large interfacial PMA energy, Ki,0, of up to 3.7 mJ m−2 was obtained, which was 1.8 times greater than that of the pure Fe/MgO interface. Moreover, iridium doping yielded a huge VCMA coefficient (up to 320 fJ Vm−1) as well as high-speed response. First-principles calculations revealed that Ir atoms dispersed within the Fe layer play a considerable role in enhancing Ki,0 and the VCMA coefficient. These results demonstrate the efficacy of heavy-metal doping in ferromagnetic layers as an advanced approach to develop high-density voltage-driven spintronic devices. Researchers from Japan's AIST demonstrated a new approach to reduce the energy consumption of spintronic devices. Magnetic random-access memory requires approximately 10,000 times more energy to record data than to safely maintain it — a discrepancy that arises due to the wastefulness of electric-current-based switching of magnetic bits. Takayuki Nozaki and colleagues now report a device that enables us to write magnetic memory using electric fields, a more energy-efficient control mechanism. The team introduced an iridium-doped ultrathin iron film in magnesium oxide-based magnetic tunnel junctions, and found that the heavy-metal dopants provoked a strong voltage-controlled magnetic anisotropy change with high-speed response. Physical role of heavy metal dopants was unveiled by first-principles calculations. The developed technique can lead to a new type of non-volatile memory with ultra-low energy consumption. Highly efficient voltage control of magnetic anisotropy has been demonstrated utlizing an ultrathin Ir-doped Fe layer in MgO-based magnetic tunnel junctions. Ir adoms are dispersed inside the ultrathin Fe layer through the interdiffusion process. Large spin–orbit interaction of Ir atoms having proximity-induced magnetism is attributed to the enhancement of the voltage-controlled magnetic anisotropy (VCMA) effect. High speed response of the VCMA effect was also confirmed by voltage-induced ferromagnetic resonance. The achieved properties first satisfy the required specification for the new type of magnetoresistive random access memory (MRAM) driven by voltage.
We investigated the magnetic anisotropy of an iron layer on a Pt(001) surface and some related systems by employing the local spin density approximation in a theoretical ab initio approach. We found that the surface system $\mathrm{Pt}∕\mathrm{Fe}∕\mathrm{Pt}(001)$ showed a perpendicular magnetic anisotropy and its anisotropy energy per iron atom amounted to a value which is 2 times larger than the value of bulk FePt. The surface relaxation much enhances the anisotropy energies, related to a large attractive force between the iron and platinum layers. A remarkable cap effect---that the covering platinum layer changes anisotropy energy---was also found to exist. We investigated the microscopic origin of the perpendicular anisotropy in relation to the local densities of states of the Fe atom. These quantities were discussed as a fingerprint of magnetic anisotropy in comparison with the results of the Fe chain at the step edge on a vicinal surface Pt(664). The atomic orbital magnetic moments were enhanced at the respective surface atoms.
Anisotropic magnetoresistance (AMR) effects in Cox(Mn0.44Ga0.56)100−x epitaxial thin films were investigated in the temperature range of 5–300 K by changing the current (I) direction to the crystal axis and the Co content (x). The AMR ratios were mostly positive for I // Co2MnGa[100] and negative for I // Co2MnGa[110] and showed peak values at x = 49.7 at% for both current directions. AMR ratios calculated from s–d scattering theory, including crystal field effects with density of states information from first-principles calculations, well reproduced experimental features.
Structures and magnetic properties were systematically investigated for the monolayer (ML)-controlled Co | Ni multilayers with various Ni layer thicknesses, where the Co and Ni layers were epitaxially grown on a Al2O3 (\(11\bar{2}0\)) single crystal substrate or deposited on a thermally oxidized Si substrate. The epitaxial Co | Ni multilayers exhibited high uniaxial magnetic anisotropy energies (Ku) and low effective magnetic damping constants (αeff), resulting in the perpendicular magnetization with low damping even for the 1 ML-Ni | 1 ML-Co multilayer. By comparing the experimental results with the first principles calculation, we found that the Ni atoms largely contributed to the perpendicular anisotropy for the Co | Ni multilayers. We also discussed the possible mechanism dominating the magnitude of αeff for the Co | Ni multilayers based on the experiments and the calculations. The present results suggest that the epitaxial Co | Ni multilayer is a promising material to achieve high Ku and low magnetic damping simultaneously.
Co-rich Co$_{1-x}$Mn$_x$ alloys have hcp or fcc disordered phases and those ferromagnetic orderings are significantly deteriorated with increasing Mn concentration $x$ in bulk. On the other hand, those metastable bcc phases show properties attractive to spintronics, e.g., high tunnel magnetoresistance (TMR) ratio of more than 200% (600%) at 300 K (10 K) in magnetic tunnel junctions (MTJs) with the $x$ = 0.25 bcc alloy electrodes [Kunimatsu et al., Appl. Phys. Express 13, 083007 (2020)]. Here, we report systematic study of structure and magnetism for epitaxial thin films as well as the TMR effect in MgO(001)-barrier MTJs with electrodes comprising those bcc films. The single phase bcc Co$_{1-x}$Mn$_x$(001) films were pseudomorphically grown on Cr(001) for 0.14 < $x$ < 0.50 with a sputtering technique. The magnetization was larger than that of pure Co for $x$ = 0.14-0.25 and deceased with further increasing $x$. This behavior mainly stemmed from the composition dependence of magnetic moment of Mn that exceeded 2 $μ_B$ at the maximum, unveiled by X-ray magnetic circular dichroism. Correspondingly, within the range of 0.25 < $x$ < 0.37, the TMR ratio decreased from 620% (229%) to 450% (194%) at 10 K (300 K) as $x$ increased. We discussed the relationship between the magnetism and high TMR ratio with different $x$ with the aid of the ab-initio band structure calculations.
