More Like this

1. Structural Stability of Magnetic Tunnel Junction Based Molecular Spintronics Devices (MTJMSD)

   https://doi.org/10.1115/IMECE2020-24134  

   Dillard, Joshua; Amir, Uzma; Tyagi, Pawan; Lamberti, Vincent (November 2020, roceedings of the ASME 2020 International Mechanical Engineering Congress and Exposition.)

   null (Ed.)

   Abstract Harnessing the exotic properties of molecular level nanostructures to produce novel sensors, metamaterials, and futuristic computer devices can be technologically transformative. In addition, connecting the molecular nanostructures to ferromagnetic electrodes bring the unprecedented opportunity of making spin property based molecular devices. We have demonstrated that magnetic tunnel junction based molecular spintronics device (MTJMSD) approach to address numerous technological hurdles that have been inhibiting this field for decades (P. Tyagi, J. Mater. Chem., Vol. 21, 4733). MTJMSD approach is based on producing a capacitor like a testbed where two metal electrodes are separated by an ultrathin insulator and subsequently bridging the molecule nanostructure across the insulator to transform a capacitor into a molecular device. Our prior work showed that MTJMSDs produced extremely intriguing phenomenon such as room temperature current suppression by six orders, spin photovoltaic effect, and evolution of new forms of magnetic metamaterials arising due to the interaction of the magnetic a molecule with two ferromagnetic thin films. However, making robust and reproducible electrical connections with exotic molecules with ferromagnetic electrodes is full of challenges and requires attention to MTJMSD structural stability. This paper focuses on MTJMSD stability by describing the overall fabrication protocol and the associated potential threat to reliability. MTJMSD is based on microfabrication methods such as (a) photolithography for patterning the ferromagnetic electrodes, (b) sputtering of metallic thin films and insulator, and (c) at the end electrochemical process for bridging the molecules between two ferromagnetic films separated by ∼ 2nm insulating gap. For the successful MTJMSD fabrication, the selection of ferromagnetic metal electrodes and thickness was found to be a deterministic factor in designing the photolithography, thin film deposition strategy, and molecular bridging process. We mainly used isotropic NiFe soft magnetic material and anisotropic Cobalt (Co) with significant magnetic hardness. We found Co was susceptible to chemical etching when directly exposed to photoresist developer and aged molecular solution. However, NiFe was very stable against the chemicals we used in the MTJMSD fabrication. As compared to NiFe, the Co films with > 10nm thickness were susceptible to mechanical stress-induced nanoscale deformities. However, cobalt was essential to produce (a) low leakage current before transforming the capacitor from the magnetic tunnel junction into molecular devices and (b) tailoring the magnetic properties of the ferromagnetic electrodes. This paper describes our overall MTJMSD fabrication scheme and process optimization to overcome various challenges to produce stable and reliable MTJMSDs. We also discuss the role of mechanical stresses arising during the sputtering of the ultrathin insulator and how to overcome that challenge by optimizing the insulator growth process. This paper will benefit researchers striving to make nanoscale spintronics devices for solving grand challenges in developing advanced sensors, magnetic metamaterials, and computer devices. 

   more » « less
2. Radical Complexes of Nickel(II)/Copper(II) and Redox Non‐innocent MB‐DIPY Ligands: Unusual Stability and Strong Near‐Infrared Absorption at λ _max ∼1300 nm

   https://doi.org/10.1002/chem.202201181  

   Zatsikha, Yuriy_V; Shamova, Liliya_I; Shepit, Michael; Berry, Steven_M; Thomas, Fabrice; Herbert, David_E; van_Lierop, Johan; Nemykin, Victor_N (June 2022, Chemistry – A European Journal)

   Abstract The preparation of radicals with intense and redox‐switchable absorption beyond 1000 nm is a long‐standing challenge in the chemistry of functional dyes. Here we report the preparation of a series of unprecedented stable neutral nickel(II) and copper(II) complexes of “Manitoba dipyrromethenes” (MB‐DIPYs) in which the organic chromophore is present in the radical‐anion state. The new stable radicals have an intense absorption atλ_max∼1300 nm and can be either oxidized to regular [M^II(MB‐DIPY)]⁺(M=Cu or Ni) or reduced to [M^II(MB‐DIPY)]⁻compounds. The radical nature of the stable [M^II(MB‐DIPY)] complexes was confirmed by EPR spectroscopy with additional insight into their electronic structure obtained by UV‐Vis spectroscopy, electro‐ and spectroelectrochemistry, magnetic measurements, and X‐ray crystallography. The electronic structures and spectroscopic properties of the radical‐based chromophores were also probed by density functional theory (DFT) and time‐dependent DFT (TDDFT) calculations. These nickel(II) and copper(II) complexes represent the first stable radical compounds with a MB‐DIPY ligand. 

   more » « less
3. Intra-molecular coupling within double-segmented molecules impacting magnetic tunnel junction-based molecular spintronics devices

   https://doi.org/10.1557/s43579-023-00508-1  

   Suh, Pius K; Grizzle, Andrew; Mutunga, Eva; Dahal, Bishnu; D’Angelo, Christopher; Tyagi, Pawan (December 2023, MRS Communications)

   The intra-molecular coupling within multiple units of paramagnetic molecules can produce various effects on molecular spintronics devices (MSD). This paper focuses on double-segmented molecules as the device element to advance understanding of the Impact of internal molecular structure on magnetic tunnel junction-based MSD (MTJMSD). We performed Monte Carlo simulations (MCS) to fill the knowledge gap about the intramolecular coupling role in the magnetic properties of the MTJMSD. This study explored a double-segmented molecule containing two atomic sections, each with a net spin state interacting via Heisenberg exchange coupling within molecules and with ferromagnetic electrodes at different thermal energies, magnetic fields, and coupling strengths. This study also investigated the effect of magnetic field on the double-segmented molecule-based cross-junction-shaped MTJMSD. We also compared the effect of the magnetic field on the mono and double-segmented molecules when connected to two ferromagnetic electrodes. In the strong coupling regime, the intramolecular coupling and molecule coupling with the two ferromagnetic electrodes dominated the MTJMSD response near the molecular junction area. This study provides insight for evaluating the Impact of molecular nanostructure internal connectedness on the integrated MSD. 

   more » « less
4. Manipulation of the Ferromagnetism in LaCoO ₃ Thin Films Through Cation‐Stoichiometric Engineering

   https://doi.org/10.1002/aelm.202201245  

   Huang, Tongtong; Lyu, Yingjie; Huyan, Huaixun; Ni, Jinyang; Saremi, Sahar; Wang, Yujia; Yan, Xingxu; Yi, Di; He, Qing; Martin, Lane_W; et al (March 2023, Advanced Electronic Materials)

   Abstract Spin‐state transitions are an important research topic in complex oxides with the diverse magnetic states involved. In particular, the low‐spin to high‐spin transition in LaCoO₃thin films has drawn a wide range of attention due to the emergent ferromagnetic state. Although various mechanisms (e.g., structural distortion, oxygen‐vacancy formation, spin‐state ordering) have been proposed, an understanding of what really underlies the emergent ferromagnetism remains elusive. Here, the ferromagnetism in LaCoO₃thin films is systematically modulated by varying the oxygen pressure during thin‐film growth. Although the samples show dramatic different magnetization, their cobalt valence state and perovskite crystalline structure remain almost unchanged, ruling out the scenarios of both oxygen‐vacancy and spin‐ordering. This work provides compelling evidence that the tetragonal distortion due to the tensile strain significantly modifies the orbital occupancy, leading to a low‐spin to high‐spin transition with emergent ferromagnetism, while samples grown at reduced pressure demonstrate a pronounced lattice expansion due to cation‐off‐stoichiometry, which suppresses the tetragonal distortion and the consequent magnetization. This result not only provides important insight for the understanding of exotic ferromagnetism in LaCoO₃thin films, but also identifies a promising strategy to design electronic states in complex oxides through cation‐stoichiometry engineering. 

   more » « less
5. Electronic and magnetic properties of nearly spin-gapless semiconducting FeVTaAl and FeCrZrAl

   https://doi.org/10.1063/9.0000855  

   Sadler, Caden; Smith, Samuel; Nguyen, Nhat_Phat; Schmidt, Brandon; Kharel, Parashu; Shand, Paul_M; Lukashev, Pavel_V (March 2025, AIP Advances)

   Here, we present results of a computational study of electronic, magnetic, and structural properties of FeVTaAl and FeCrZrAl, quaternary Heusler alloys that have been recently reported to exhibit spin-gapless semiconducting behavior. Our calculations indicate that these materials may crystallize in regular Heusler cubic structure, which has a significantly lower energy than the inverted Heusler cubic phase. Both FeVTaAl and FeCrZrAl exhibit ferromagnetic alignment, with an integer magnetic moment per unit cell at equilibrium lattice constant. Band structure analysis reveals that while both FeVTaAl and FeCrZrAl indeed exhibit nearly spin-gapless semiconducting electronic structure at their optimal lattice parameters, FeVTaAl is a 100% spin-polarized semimetal, while FeCrZrAl is a magnetic semiconductor. Our calculations indicate that expansion of the unit cell volume retains 100% spin-polarization of both compounds. In particular, both FeVTaAl and FeCrZrAl are 100% spin-polarized magnetic semiconductors at the largest considered lattice constant. At the same time, at smaller lattice parameters, both compounds exhibit a more complex electronic structure, somewhat resembling half-metallic properties. Thus, both of these alloys may be potentially useful for practical applications in spin-based electronics, but their electronic structure is very sensitive to the external pressure. We hope that these results will stimulate experimental efforts to synthesize these materials. 

   more » « less