Spin-filter Materials Research Articles

Covalent Organic Frameworks with Tunable Chirality for Chiral-Induced Spin Selectivity.

Chiral covalent organic frameworks (CCOFs) have attracted extensive interest for their potential applications in various enantioselective processes. However, the exploitation of chirality-induced spin selectivity (CISS) that enables a new technology for the injection of spin polarized current without the need for a permanent magnetic layer within CCOFs remains a largely untapped area of research. Here, we demonstrate that, for the first time, COFs can be an attractive platform to develop spin filter materials with efficient CISS. This facilitates the design and synthesis of a new family of Zn(salen)-based 2D CCOFs, namely, CCOFs-9-12, by imine condensation of chiral 1,2-diaminocyclohexane and tri- or tetra(salicylaldehyde) derivatives. CCOF-9, distinguished by its unique C2 symmetric "armchair" tetrasubstituted pyrene conformation, exhibits the most pronounced chirality among these materials and serves as a solid-state host, enabling the enantioselective adsorption of racemic drugs with an enantiomeric excess (ee) of up to 97%. After substituting diamagnetic zinc(II) ions for paramagnetic cobalt(II), the resulting CCOF-9-Co not only retains its high crystallinity, porosity, and exceptional chirality but also exhibits enhanced conductivity, a crucial factor for the effective observation of CISS. Magnetic conductive atomic force microscopy showed that CCOF-9-Co exhibited a remarkable CISS effect with up to an 88-94% spin polarization ratio. This phenomenon is further confirmed by the increased intensity in the magnetic circular dichroism (MCD) when CCOF-9-Co is under an external magnetic field. This work therefore shows the tremendous potential of CCOFs for controlling spin selectivity and will stimulate the creation of new types of crystalline polymers with strong CISS effects for spin filters.

First-principles characterization of structural, elastic, electronic phase transitions and magnetic properties of FeCrScZ (Z = Si, Ge) Heusler compounds under pressure

Inspired by the use of spin filter materials (SFM) as barriers in spintronics, we have studied the structural, elastic, electronic phase transitions and magnetic properties of FeCrScSi (FCSS) and FeCrScGe (FCSG) Heuslers under pressure (P) using the pseudo-potential plane wave (PP-PW) method. The obtained ground-state structural properties are in good agreement with existing theoretical counterparts. The calculated elastic properties show that the considered materials are mechanically stable and ductile. Disregarding pressure effects, electronic structure analysis reveals ferrimagnetic semiconducting properties for FCSS and FCSG materials. Under pressure, an electronic phase transition from spin filter material (SFM) to spin gapless semiconductor type II (SGS-II) is predicted at pressure values of 46.37 GPa and 32.48 GPa for FCSS and FCSG compounds, respectively. Above these critical pressures, the examined compounds exhibit a half metallic (HM) nature. Finally, the calculated total magnetic moment of FCSS/FCSG compound was found to be independent of the applied pressure.

A first-principles DFT+GW study of spin-filter and spin-gapless semiconducting Heusler compounds

Among Heusler compounds, the ones being magnetic semiconductors (also known as spin-filter materials) are widely studied as they offer novel functionalities in spintronic and magnetoelectronic devices. The spin-gapless semiconductors are a special case. They possess a zero or almost-zero energy gap in one of the two spin channels. We employ the GW approximation to simulate the electronic band structure of these materials. Our results suggest that in most cases the use of GW self energy instead of the usual density functionals is important to accurately determine the electronic properties of magnetic semiconductors.

First-principles prediction of magnetic salts: Case study of NaCl bulk and (0 0 1) surface doped with light non-metallic 2p-block elements

Based on density functional calculations, we studied the structural, electronic, and magnetic properties of sodium chloride bulk and (001) surface doped with the light non-metallic elements boron, carbon, nitrogen, and oxygen. The results indicated that doping such an isolated foreign atom can produce considerable spin polarization, mainly due to highly localized and partially filled 2p states of the dopants. From simple GGA computation, all these diluted magnetic chlorides exhibit nearly half-metallic characteristic with weak conductivity, and thereby they could be used as spin filter materials for improving spin-specific magnetic tunnel junctions. Especially, substitution onto the surface removes the degeneration of 2p impurity states, leading to anisotropic spin and electron atmosphere with specific orientation. The differences between bulk doping and surface doping were further explained by spin-resolved energy spectrums. In addition, compared with standard GGA approach, applied Ueff of 2p orbitals of C, N, and O atoms can correct the error and improve the description of the electronic structures and magnetic properties in some extent. N and O-doped NaCl bulks transform from approximate half metals to magnetic semiconductors, owning to the enhancement of electronic localization and correlation. And the total magnetic moment increases from 1 μB to 3 μB for C-doped NaCl (001), accompanying with a transformation from intermediate-spin state to high-spin state. In particular, O-substituted system is energetically more favorable than other doped systems, i.e., the negative value of the predicted formation energy, demonstrating that O-doped NaCl should be a candidate material for potential spintronic applications.

Synthesis of low-moment CrVTiAl: A potential room temperature spin filter

The efficient production of spin-polarized currents at room temperature is fundamental to the advancement of spintronics. Spin-filter materials—semiconductors with unequal band gaps for each spin channel—can generate spin-polarized current without the need for spin-polarized contacts. In addition, a spin-filter material with zero magnetic moment would have the advantage of not producing strong fringing fields that would interfere with neighboring electronic components and limit the volume density of devices. The quaternary Heusler compound CrVTiAl has been predicted to be a zero-moment spin-filter material with a Curie temperature in excess of 1000 K. In this work, CrVTiAl has been synthesized with a lattice constant of a = 6.15 Å. Magnetization measurements reveal an exceptionally low moment of μ = 2.3 × 10−3 μB/f.u. at a field of μ0H = 2 T that is independent of temperature between T = 10 K and 400 K, consistent with the predicted zero-moment ferrimagnetism. Transport measurements reveal a combination of metallic and semiconducting components to the resistivity. Combining a zero-moment spin-filter material with nonmagnetic electrodes would lead to an essentially nonmagnetic spin injector. These results suggest that CrVTiAl is a promising candidate for further research in the field of spintronics.

Spin-filter and spin-gapless semiconductors: The case of Heusler compounds

We review our recent first-principles results on the inverse Heusler compounds and the ordered quaternary (also known as LiMgPdSn-type) Heusler compounds. Among these two subfamilies of the full-Heusler compounds, several have been shown to be magnetic semiconductors. Such material can find versatile applications, e.g. as spin-filter materials in magnetic tunnel junctions. Finally, a special case are the spin-gapless semiconductors, where the energy gap at the Fermi level for the one spin-direction is almost vanishing, offering novel functionalities in spintronic/magnetoelectronic devices.

Multiferroic heterostructures for spin filter applications: Anab initiostudy

Novel imaging spin-filter techniques, which are based on low energy electron diffraction, are currently of high scientific interest. To improve the spin-detection efficiency a variety of new materials have been introduced in recent years. A new class of promising spin-filter materials are represented by multiferroic systems, as both magnetic and electric ordering exist in these materials. We have investigated Fe/BaTiO3(001), which defines a prominent candidate due to its moderate spontaneous polarization, for spin filter applications calculating diffraction patterns for spin polarized electrons incident on the Fe surface. Motivated by the fact that spin polarized low energy electron diffraction is a powerful method for the determination of the properties of surfaces we investigated the influence of switching the BaTiO3 polarization on the exchange and spin orbit scattering as well as on reflectivity and figure of merit. This system obviously offers the possibility to realize a multiferroic spin filter and manipulating the spin-orbit and exchange scattering by an external electric field. The calculations have been done for a large range of kinetic energies and polar angles of the diffracted electrons considering different numbers of Fe monolayers.

Ab-initio investigation of electronic and magnetic properties of the 18-valence-electron fully-compensated ferrimagnetic (CrV)XZ Heusler compounds: A prototype for spin-filter materials

Spin-filter materials are magnetic semiconductors susceptible of improving the performance of magnetic tunnel junction based spintronic devices. Using state-of-the-art ab initio electronic structure calculations, we exploit the electronic and magnetic properties of (CrV)XZ quaternary Heusler compounds having 18 valence electrons per formula unit. All compounds under study have been found to combine fully-compensated ferrimagnetism to magnetic semiconducting behavior and some are even gapless or spin-gapless semiconductors. Our results pave the way for incorporating these compounds in spintronic and magnetoelectronic devices.

High-T C fully compensated ferrimagnetic semiconductors as spin-filter materials: the case of CrVXAl (X = Ti, Zr, Hf) Heusler compounds

We extend our recent work on spin-filter materials (Galanakis et al 2013 Appl. Phys. Lett.103 142404) to the case of CrVXAl (X = Ti, Zr, Hf) compounds, for which, using ab initio electronic structure calculations, we show that p–d hybridization leads to the formation of a fully compensated ferrimagnetic semiconducting state with moderate exchange splitting. The magnetism is of covalent-type and the very strong antiferromagnetic Cr–V exchange interactions lead to extremely high Curie temperature, TC, values. Furthermore, all three compounds are thermodynamically and magnetically stable. The combination of very high TC values with a zero total net magnetization makes them promising materials for spintronics applications.

A proposal for an alternative class of spin filter materials: Hybridization-induced high-TC ferromagnetic semiconductors CoVXAl (X = Ti, Zr, Hf)

Using ab-initio electronic structure calculations, we propose an alternative class of spin filter materials (SFMs) based on the quaternary Heusler compounds CoVXAl (X = Ti, Zr, Hf). We show that the p-d hybridization leads to the formation of the ferromagnetic band gap with a moderate exchange splitting ΔEex and a Curie temperature TC well above the room temperature. We find that all three compounds are thermodynamically and magnetically stable. Combination of high TC value together with moderate exchange splitting, as well as crystal structures compatible to the existing semiconductors and metals, makes these compounds promising candidates to find applications as SFMs in spintronics devices.

Epitaxial strain effects in the spinel ferritesCoFe2O4andNiFe2O4from first principles

The inverse spinels CoFe2O4 and NiFe2O4, which have been of particular interest over the past few years as building blocks of artificial multiferroic heterostructures and as possible spin-filter materials, are investigated by means of density functional theory calculations. We address the effect of epitaxial strain on the magneto-crystalline anisotropy and show that, in agreement with experimental observations, tensile strain favors perpendicular anisotropy, whereas compressive strain favors in-plane orientation of the magnetization. Our calculated magnetostriction constants $\lambda_{100}$ of about -220 ppm for CoFe2O4 and -45 ppm for NiFe2O4 agree well with available experimental data. We analyze the effect of different cation arrangements used to represent the inverse spinel structure and show that both LSDA+U and GGA+U allow for a good quantitative description of these materials. Our results open the way for further computational investigations of spinel ferrites.

Theory of Josephson transport through spintronics nano-structures

We study the Josephson transport through ferromagnetic insulators (FIs) by taking into account its band structure explicitly. In the case of the fully polarized FIs (FPFIs), we found the formation of a π -junction and an atomic-scale 0 – π transition induced by increasing the FI thickness. More remarkably, in the Josephson junction through spin-filter materials such as Eu chalcogenides, the orbital hybridization between the conduction d and the localized f electron gives rise to the π -junction behavior. Such FI-based π -junctions can be used to implement highly coherent solid-state quantum bits.

Numerical evaluations on the asymmetric bias dependence of magnetoresistance in double spin filter tunnel junctions

In double spin filter tunnel junctions, the spin information is generated and analyzed purely from the spin filtering effect with nonmagnetic electrodes. In this article we numerically evaluate the bias dependence of magnetoresistance in such tunnel junctions (nonmagnetic metal/spin filter/nonmagnetic insulator/spin filter/nonmagnetic metal), particularly in cases when different spin filter materials are utilized. A large magnetoresistance with nonmonotonic and asymmetric bias dependence is expected within the framework of Wentzel–Kramers–Brillouin approximation. We illustrate the systematic influence of tunnel barrier height, tunnel barrier thickness, and exchange energy splitting on magnetoresistance, particularly focusing on the asymmetric behavior of the magnetoresistance bias dependence.