Spintronik: eine Herausforderung für Materialwissenschaften und Festkörperchemie

Orbital Contribution to Paramagnetism and Noninnocent Thiophosphate Anions in Layered Li2MP2S6 Where M = Fe and Co

Transition-metal thiophosphates and selenophosphates are layered systems with the potential for displaying two-dimensional (2D) magnetic phenomena. We present the crystal structures and magnetic properties of two lithium transition-metal thiophosphates, Li_1.56Co_0.71P₂S₆ and Li_2.26Fe_0.94P₂S₆. The previously unreported Li_1.56Co_0.71P₂S₆ crystallizes in the trigonal space group P31m with lattice parameters a = 6.0193(6) Å and c = 6.5675(9) Å. The CoS₆ octahedra are arranged in a honeycomb lattice and form 2D layers separated by lithium cations. The previously solved Li_2.26Fe_0.94P₂S₆ is isostructural to Li_1.56Co_0.71P₂S₆ but displays site mixing between the Li⁺ and Fe²⁺ cations within the thiophosphate layer. Unusually, Li_1.56Co_0.71P₂S₆ appears to have P₂S₆^3- and not P₂S₆^4- anions. We therefore term it a "noninnocent" anion because of the ambiguous nature of its oxidation state. Combined neutron diffraction and magnetization measurements reveal that both Li_1.56Co_0.71P₂S₆ and Li_2.26Fe_0.94P₂S₆ display magnetic anisotropy as well as no long-range magnetic order down to 5 K. In the iron thiophosphate, susceptibility indicates an effective moment of 5.44(3) μ_B, which may be best described by an S + L model, where S = 2 and L = 2, or close to the free ion limit. In the cobalt thiophosphate, we found the effective moment to be 4.35(2) μ_B, which would point to an S = ³/₂ and L = 1 model due to octahedral crystal-field splitting.

Unique Superparamagnetic-like Behavior Observed in Non-π-delocalized Nitroxide Diradical Compounds Showing Discotic Liquid Crystalline Phase

A unique superparamagnetic-like behavior and a large "positive magneto-LC effect" were observed in the solid phases and the hexagonal columnar (Col_h ) liquid crystalline (LC) phase, respectively, of novel achiral non-π-delocalized nitroxide diradical compounds (R,S)-1, which showed polymorphism in the solid phases (solids I and II). The SQUID magnetization measurement revealed that (1) (R,S)-1 containing a small amount of racemic diastereomers (R*,R*)-1 possessed an unusual and large temperature-independent magnetic susceptibility (χ_TIM >0) component in the original nanocrystalline solid I that was responsible for the observed superparamagnetic-like behavior under low magnetic fields and did not arise from the contamination by extrinsic magnetic metal or metal ion impurities, besides ordinary temperature-dependent paramagnetic susceptibility (χ_para >0) and temperature-independent diamagnetic susceptibility (χ_dia <0) components, (2) a large increase in molar magnetic susceptibility (χ_M ) (positive magneto-LC effect) that occurred at the solid I-to-liquid crystal transition upon heating was preserved as an additional χ_TIM increase in the resulting polymorphic nanocrystalline solid II by cooling, and (3) such unique magnetic phenomena were induced by thermal processing for (R,S)-1 or by adding a small amount of (R*,R*)-1 to (R,S)-1 as the impurity.

Ferromagnetism and Half-Metallicity in a High-Band-Gap Hexagonal Boron Nitride System

Metal-free half-metallicity is the subject of intense research in the field of spintronics devices. Using density functional theoretical calculations, atom-thin hexagonal boron nitride (h-BN)-based systems are studied for possible spintronics applications. Ferromagnetism is observed in patterned C-doped h-BN systems. Interestingly, such a patterned C-doped h-BN exhibits half-metallicity with a Curie temperature of approximately 324 K at a particular C-doping concentration. It shows half-metallicity more than metal-free systems studied to date. Thus, such a BN-based system can be used to achieve a 100 % spin-polarised current at the Fermi level. Furthermore, this C-doped system shows excellent dynamical, thermal, and mechanical properties. Therefore, a stable metal-free planar ferromagnetic half-metallic h-BN-based system is proposed for use in room-temperature spintronics devices.

A25Mg,89Y and115In solid state MAS NMR study of YT2X and Y(T0.5T′0.5)2X (T/T′ = Pd, Ag, Au; X = Mg, In) Heusler phases

Yttrium–transition metal–magnesium (indium) Heusler phases were synthesized from the elements in sealed niobium ampoules in a high-frequency furnace or by arc-melting, respectively.

The Effect of Cobalt-Sublattice Disorder on Spin Polarisation in Co₂FexMn1-xSi Heusler Alloys

In this work we present a theoretical study of the effect of disorder on spin polarisation at the Fermi level, and the disorder formation energies for Co₂Fe_xMn_1-xSi (CFMS) alloys. The electronic calculations are based on density functional theory with a Hubbard U term. Chemical disorders studied consist of swapping Co with Fe/Mn and Co with Si; in all cases we found these are detrimental for spin polarisation, i.e., the spin polarisation not only decreases in magnitude, but also can change sign depending on the particular disorder. Formation energy calculation shows that Co-Si disorder has higher energies of formation in CFMS compared to Co₂MnSi and Co₂FeSi, with maximum values occurring for x in the range 0.5-0.75. Cross-sectional structural studies of reference Co₂MnSi, Co₂Fe_0.5Mn_0.5Si, and Co₂FeSi by Z-contrast scanning transmission electron microscopy are in qualitative agreement with total energy calculations of the disordered structures.

Design scheme of new tetragonal Heusler compounds for spin-transfer torque applications and its experimental realization

Band Jahn-Teller type structural instabilities of cubic Mn(2)YZ Heusler compounds causing tetragonal distortions can be predicted by ab initio band-structure calculations. This allows for identification of new Heusler materials with tunable magnetic and structural properties that can satisfy the demands for spintronic applications, such as in spin-transfer torque-based devices.

High-Tc ferromagnetic semiconductor-like behavior and unusual electrical properties in compounds with a 2×2×2 superstructure of the half-Heusler phase

Heusler phases, including the full- and half-Heusler families, represent an outstanding class of multifunctional materials on account of their great tunability in compositions, valence electron counts (VEC), and properties. Here we demonstrate a systematic design of a series of new compounds with a 2×2×2 superstructure of the half-Heusler unit cell in X-Y-Z (X=Fe, Ru, Co, Rh, Ir; Y=Zn, Mn; Z=Sn, Sb) systems. Their structures were solved by using both powder and single-crystal X-ray diffraction, and also directly observed by using high-angle annular dark-field imaging in a scanning transmission electron microscope (HAADF-STEM). The VEC values of these new compounds span a wide and continuous range comparable to those for the full- and half-Heusler families, thereby implying tunability in compositions and physical properties in the superstructure. In fact, we observed abnormal electrical properties and a ferromagnetic semiconductor-like behavior with a high and tunable Curie temperature in these superstructures.