Ferrimagnetic Alignment Research Articles

Electronic, magnetic, and structural properties of CoVMnSb: Ab initio study

We present computational results on electronic, magnetic, and structural properties of CoVMnSb, a quaternary Heusler alloy. Our calculations indicate that this material may crystallize in two energetically close structural phases: inverted and regular cubic. The inverted cubic phase is the ground state, with ferrimagnetic alignment, and around 80% spin polarization. Despite having a relatively large bandgap in the minority-spin channel close to the Fermi level, this phase does not undergo a half-metallic transition under pressure. This is explained by the “pinning” of the Fermi level at the minority-spin states at the Γ point. At the same time, the regular cubic phase is half-metallic and retains its perfect spin polarization under a wide range of mechanical strain. Transition to a regular cubic phase may be attained by applying uniform pressure (but not biaxial strain). In practice, this pressure may be realized by an atomic substitution of non-magnetic atoms (Sb) with another non-magnetic atom (Si) of a smaller radius. Our calculations indicate that 25% substitution of Sb with Si results in a half-metallic regular cubic phase being the ground state. In addition, CoVMnSb0.5Si0.5 retains its half-metallic properties under a considerable range of mechanical pressure, as well as exhibits thermodynamic stability, thus making this alloy attractive for potential spintronic applications. We hope that the presented results will stimulate experimental efforts to synthesize this compound.

Electronic, magnetic, and structural properties of CrMnSb0.5Si0.5

Half-metallic Heusler compounds are among the most actively studied materials for applications in spin-based devices. Largely, this is due to their higher Curie temperature compared with the other half-metallic compounds, and relative ease of fabrication. Here we theoretically study one such Heusler alloy CrMnSb0.5Si0.5. In particular, we demonstrate a potential stability of this compound by estimating its formation energy, its half-metallic electronic structure (stable under a considered range of biaxial strain), and ferrimagnetic alignment. The calculated Curie temperature of this material is 787 K, much higher than room temperature. At the same time, we have shown that in thin-film geometry the spin-polarization of this material is strongly reduced due to the emergence of surface states in the minority-spin energy gap. In addition, one of the considered termination surfaces is thermodynamically unstable, while the other is stable. The presented results may be useful for researchers working on practical applications in the field of spintronics.

Stability and electronic properties of the graphene-supported FeO nanostructures including clusters and monolayer

Integration of graphene with subnano clusters or monolayer of FeO can facilitate the formation of nanostructures with applications in magnetic storage or health-related areas. In this paper, first-principles calculations are performed to investigate the stability and electronic properties of such supported nanostructures. The results show that a noticeable hybridization occurs between Fe and C atoms at the interface that provide stability to both the clusters and monolayer on graphene. The substrate-induced changes in the electronic properties of the (FeO)n clusters are small since the clusters appeared to be weakly adsorbed on the surface. However, this is not the case with FeO(111) monolayer for which a buckled configuration is predicted to be energetically preferred on graphene. Subsequently, graphene-supported FeO(111) monolayer exhibits half-metallicity with ferrimagnetic alignment of the magnetic moments in the lattice with finite total magnetic moment. The interface bonding, therefore, appears to define the characteristics of graphene-supported FeO(111) monolayer, though it makes small but noticeable changes in the graphene-supported (FeO)n clusters.

Electronic, magnetic, and optical properties of bulk and (1 1 1)-surfaces of CoMnZnSi quaternary Heusler alloy

The structural stability, electronic structure, and magnetic and optical properties of CoMnZnSi quaternary Heusler alloy (QHA) in the bulk and (111)-slab forms have been investigated by performing density functional theory (DFT) calculations. Formation and cohesive energies, and elastic constants confirm that the bulk CoMnZnSi is chemically and mechanically stable at an equilibrium lattice parameter of 5.81 Å. The calculated elastic constants also indicate that this QHA has ductile and anisotropic features. We investigate and discuss the bonding behavior from charge density distribution and density of states. These calculations in the bulk phase show a perfect half-metallic behavior with an integer value of magnetic moment (4 µB) and a large spin-flip gap of 0.39 eV. On the other hand, the Co (111) and Si (111)-slabs exhibit semi-metallic nature at the Fermi level. These results indicate that CoMnZnSi qualifies for spintronic applications. In the bulk and Co (111), Zn (111), and Si (111)-surfaces, the magnetic moment of Co and Mn atoms are ferromagnetically aligned, while the ferrimagnetic alignment of Co with Mn has been found in Mn (111)-surface. We also investigate the optimal conductivity, dielectric functions, reflectivity, absorption coefficient, refractive index, and loss function to understand the underlying optical properties of this alloy.

Crucial Role of the Co Cations on the Destabilization of the Ferrimagnetic Alignment in Co-Ferrite Nanoparticles with Tunable Structural Defects

The key role of the structural defects on the magnetic properties of cobalt ferrite nanoparticles (NPs) is investigated by complementary local probes: element- and site-specific X-ray magnetic circ...

Chemical substitution induced half-metallicity in CrMnSb(1−x)Px

Half-metallic Heusler alloys have been intensively studied in recent years due to their potential applications in spin-based devices, e.g., in magnetic tunnel junctions. Yet, their properties may be very sensitive to the choice of the substrates, i.e., to the epitaxial strain and interface properties. Here, we report the results of our computational work on the half-Heusler compound CrMnSb(1−x)Px. In particular, we demonstrate that the parent compound CrMnSb is close to a half-metallic material at the optimized lattice parameter, with the onset of the half-metallic bandgap a few meV above the Fermi energy. Moreover, although it undergoes a half-metallic transition under a uniform compression of ∼1.5%, such a transition is absent under epitaxial strain. At the same time, we show that a half-metallic transition could be induced by a chemical substitution of Sb with P, which results in a volume reduction of the unit cell. In particular, 50% substitution of Sb with P leads to a robust half-metallicity in CrMnSb(1−x)Px, with 100% spin polarization being retained at a large range of epitaxial strain. Thus, our results indicate that CrMnSb0.5P0.5 could be grown on different types of substrates, e.g., GaAs, without its electronic properties being detrimentally affected by biaxial strain. In addition, CrMnSb0.5P0.5 exhibits a fully compensated ferrimagnetic alignment, which could be potentially useful in applications where stray magnetic fields are undesirable.

Perpendicularly magnetized CoFeB multilayers with tunable interlayer exchange for synthetic ferrimagnets

A study of the multilayer system MgO/CoFeB(1.1nm)/Ta(t)/CoFeB(0.8nm)/MgO is presented, where the two CoFeB layers are separated by a Ta interlayer of varying thickness t. The magnetization properties deduced from complementary techniques such as superconducting quantum interference magnetometry, ferromagnetic resonance frequency measurements and Brillouin light scattering spectroscopy can be tuned by changing the Ta thickness between t=0.25nm, 0.5nm and 0.75nm. For t=0.5nm, a ferromagnetic coupling is observed, whereas for t=0.75nm, the antiferromagnetic coupling needed to construct a synthetic ferrimagnet is realized. In the latter case, the shape of magnetic domain walls between two ferrimagnetic alignments or between a ferro- and a ferrimagnetic alignment is very different. This behavior can be interpreted as a result of the change in dipolar as well as interlayer exchange energy and domain wall pinning, which is an important conclusion for the realization of data storage devices based on synthetic ferri- and antiferromagnets.

Realizing topological stability of magnetic helices in exchange-coupled multilayers for all-spin-based system.

Topologically stabilized spin configurations like helices in the form of planar domain walls (DWs) or vortex-like structures with magnetic functionalities are more often a theoretical prediction rather than experimental realization. In this paper we report on the exchange coupling and helical phase characteristics within Dy-Fe multilayers. The magnetic hysteresis loops with temperature show an exchange bias field of around 1.0 kOe at 10 K. Polarized neutron reflectivity reveal (i) ferrimagnetic alignment of the layers at low fields forming twisted magnetic helices and a more complicated but stable continuous helical arrangement at higher fields (ii) direct evidence of helices in the form of planar 2π-DWs within both layers of Fe and Dy. The helices within the Fe layers are topologically stabilized by the reasonably strong induced in-plane magnetocrystalline anisotropy of Dy and the exchange coupling at the Fe-Dy interfaces. The helices in Dy are plausibly reminiscent of the helical ordering at higher temperatures induced by the field history and interfacial strain. Stability of the helical order even at large fields have resulted in an effective modulation of the periodicity of the spin-density like waves and subsequent increase in storage energy. This opens broad perspectives for future scientific and technological applications in increasing the energy density for systems in the field of all-spin-based engineering which has the potential for energy-storing elements on nanometer length scales.

Competing Jahn-Teller distortions and ferrimagnetic ordering in the geometrically frustrated systemNi1−xCuxCr2O4

Competing Jahn-Teller distortions combined with geometrical frustration give rise to a rich phase diagram as a function of x(Cu) and temperature in the spinel system Ni1-xCuxCr2O4. The Jahn-Teller distortion of the end members acts in opposite ways, with an elongation of the NiO4 tetrahedra resulting in a structural transition at T-S1 = 317K in NiCr2O4, but a flattening in the CuO4 tetrahedra at T-S1 = 846K in CuCr2O4. In both cases the symmetry is lowered from cubic (Fd (3) over barm) to tetragonal (I4(1)/amd) on cooling. In order to follow the influence of Jahn-Teller active Ni2+ and Cu2+ ions on the structural and magnetic properties of chromium spinels, we have investigated a series of samples of Ni1-xCuxCr2O4 by x-ray and neutron powder diffraction. In the critical range 0.10 < x(Cu) < 0.20, strong orthorhombic distortions were observed, where competing Jahn-Teller activities between the Cu2+ and Ni2+ ions result in distortions along both the a and c axes. For Ni0.85Cu0.15Cr2O4, the orthorhombic structure (Fddd) is stabilized up to T-S2 = 368(2) K, close to the first structural phase transition at T-S1 = 374(2) K. A ferrimagnetic spin alignment of the Ni/Cu and chromium atoms sets in at much lower temperature TC = 95K in this compound. The end members NiCr2O4 and CuCr2O4 undergo this ferrimagnetic transition at TC = 74 and 135 K, respectively. These transitions are accompanied by the structural change to the orthorhombic symmetry which relieves the frustration. NiCr2O4 and Ni0.85Cu0.15Cr2O4 undergo a second magnetic transition at TM2 = 24 and 67K due to a superimposed antiferromagnetic ordering of the Cr moments resulting in a noncollinear magnetic structure. In the system Ni1-xCuxCr2O4, the magnetic transitions TC and TM2 merge with increasing copper content up to x(Cu) similar to 0.5. For the Ni-rich chromites, geometrical frustration causes a strong reduction of the chromium moments, where magnetic long-range order coexists with a disordered spin-liquid-like or a reentrant-spin-glass-like state. This paper provides insight into the interplay between the Jahn-Teller effect, geometrical frustration, and long-range magnetic order in these complex systems.

From chromium-chromium quintuple bonds to molecular squares and porous coordination polymers.

Reaction of the quintuply bonded chromium(I) dimer [ApCrCrAp] (Ap = sterically demanding 2-aminopyridinate) with pyrazine yields a chromium(II) complex with a η(4):η(4) face-on coordinated pyrazine dianion. Reaction with 4,4'-bipyridine, on the other hand, completely cleaves the metal-metal bond, leading to a chromium(II)-based molecular square. XRD and magnetic measurements show ligand radical anions and a ferrimagnetic alignment of alternating metal and ligand magnetic moments. Controlled polymerization of the molecular square with pyrazine yields a porous coordination polymer featuring both reduced and nonreduced linkers.

CoxCu1–xCr2S4 Nanocrystals: Synthesis, Magnetism, and Band Structure Calculations

Spin-based transport in semiconductor systems has been proposed as the foundation of a new class of spintronic devices. For the practical realization of such devices, it is important to identify new magnetic systems operating at room temperature that can be readily integrated with standard semiconductors. A promising class of materials for this purpose is magnetic chromium-based chalcogenides that have the spinel structure. Nanocrystals of CoxCu1–xCr2S4 have been synthesized over the entire composition range by a facile solution-based method. While CuCr2S4 is a ferromagnetic metal, CoCr2S4 is known to be a ferrimagnetic semiconductor. Systematic changes in the lattice parameter, size, and magnetic properties of the nanocrystals are observed with composition. The nanocrystals are magnetic over the entire range, with a decrease in the magnetic transition temperature with increasing Co content. Band structure calculations have been carried out to determine the electronic and magnetic structure as a function of composition. The results suggest that ferrimagnetic alignment of the Co and Cr moments results in a decrease in magnetization with increasing Co concentration.

On the Reentrant Transitions and Magnetization Plateaus in the Spin-1/2 Ising–Heisenberg Model on Diamond-Like Decorated Bethe Lattices

A critical behavior and magnetization process of the frustrated spin-1/2 Ising–Heisenberg model on diamond-like decorated Bethe lattices is examined within the framework of the decoration–iteration transformation and exact recursion relations. It is demonstrated that the investigated spin model with a sufficiently high coordination number of the Bethe lattice may exhibit reentrant phase transitions. Three different magnetization scenarios with up to two consecutive fractional plateaus can be detected in the low-temperature magnetization curve, whereas the intermediate magnetization plateau either corresponds to the classical ferrimagnetic spin alignment and/or the more striking quantum monomer–dimer spin arrangement.

Ca2MnRuO6: Magnetic Order Arising from Chemical Chaos

We predict the existence of half metallicity and thus highly spin-polarized conduction in the completely disordered double-perovskite Ca2MnRuO6, based on first principles calculations. For ordered structures we find the predominantly Ru t2g electrons to be itinerant whereas the Mn eg spins are nearly localized, thus favoring a ferrimagnetic alignment of the Ru and Mn spins. For disordered structures, we find the Mn ions to favor ferromagnetic coupling to other Mn ions in both Mn and Ru sublattices. This explains the overall ferrimagnetic alignment of Mn and Ru ions, irrespective of chemical disorder, that has been found experimentally. The existence of ferrimagnetism and half metallicity in the presence of high levels of chemical disorder is an unexpected combination not seen in other double perovskites.

Ferrimagnetism of New Filled Skutterudite SmFe4As12Synthesized at High Pressure

A new filled skutterudite SmFe 4 As 12 has been synthesized at high temperature and high pressure. The magnetic properties of this compound was studied in terms of electrical resistivity, magnetic susceptibility, magnetization, and specific heat measurments. SmFe 4 As 12 shows a magnetic transition at T A = 39 K. Temperature dependence of the magnetic susceptibility and field dependence of T A observed in the specific heat measurement suggest ferrimagnetic alignment of the magnetic moments of Fe 3 d band and Sm 4 f electrons below T A .

Structural and magnetic properties of bimetallic Co n − 1Cr clusters with density functional theory

We have investigated the structural and magnetic properties of small bimetallic Con − 1Cr (n = 2 − 9) clusters by means of spin-polarized density functional theory approach. We found the ground state structures of Con − 1Cr were very similar to those of pure Con except n = 3,6. The clusters were of high stability at n = 6. Magnetic moments showed an interesting size-dependent variation: the magnetic moments of Con − 1Cr can be obtained by minus 7µB for n = 2, 4, 6–9 or by plus 1µB for n = 5 from those of the Con counterparts. The different magnetic behavior stems from ferromagnetic alignment or ferrimagnetic alignment of Co and Cr atoms in Con − 1Cr, associated with their longer or shorter interatomic distances.

Neutron diffraction study of a layered cobalt oxide SrCo 6O 11

The magnetic structure of a hexagonal layered cobalt oxide SrCo 6O 11, which exhibits a 1/3 magnetization plateau and a concomitant large negative megnetoresistance below 20 K, was studied by means of neutron diffraction measurements and Rietveld analysis. It has been found that only the trigonal bipyramidal site carries a localized moment, which corresponds to S=2, and that the moment has an Ising-like anisotropy along the c-axis. These moments are aligned ferromagnetically in the c-plane, while the interlayer coupling is field dependent. The 1/3 magnetization plateau reflects a field-induced ferrimagnetic alignment along the c-axis like “up–up–down–up–up–down”. We interpret this complex spin structure as resulting from competition of an antiferromagnetic superexchange interaction between the nearest ferromagnetic layers and carrier-mediated, ferromagnetic interactions.

Benzyl-phenyl ether derivatives of nitronyl nitroxide triradicals as a model for single-component organic molecule-based ferrimagnetics

As a novel molecular design for genuinely organic molecule-based ferrimagnets, we have proposed a strategy of “single-component organic molecule-based ferrimagnetics”. In this study, we have designed and synthesized a triradical p - trisNN ( 1) as a building block for single-component ferrimagnetics, in which a m-phenylene-based π-biradical and a π-monoradical are connected in a benzyl-phenyl ether framework. The triradical retains the magnetic degrees of freedom for both S = 1 and S = 1/2 spins in the single molecule. X-ray crystallographic analyses show that 1 has a head-to-tail packing with an approximately isosceles triangular geometry, which is favorable for a ferrimagnetic spin alignment. From the analysis of crystal and magnetic susceptibility measurements, the triradical is found to form exchange-coupled systems composed of six S = 1/2 spins in the repeating unit of the crystalline solid state.

Studies on the electronic structure of Ag 2NiO 2, an intercalated delafossite containing subvalent silver

Structural and electronic properties of Ag 2NiO 2 and AgNiO 2 were investigated and compared. Single crystal X-ray diffraction experiments on Ag 2NiO 2 at 100 K provide evidence for a ferrodistortive phase below 260 K. Ni K-edge and Ag L III-edge XANES spectra, both measured and simulated, as well as core level and valence band photoelectron spectra are analysed. They agree well with the results of bandstructure calculations, where pure DFT and mixed Hartree–Fock/DFT (hybrid) functionals were applied and spin-polarisation was considered. The electronic structure of the [NiO 2] − layers with Ni 3+ in a low spin state, forming a spin-1/2 triangular lattice, is very similar in both compounds. A ferrimagnetic alignment of the spins within the [NiO 2] − layers and their antiferromagnetic coupling via the intermediate silver layers is suggested.

Experimental Evidence for the Triplet-Like Spin State Appearing in Ground-State Singlet Biradicals as a Key Feature for Generalized Ferrimagnetic Spin Alignment

The authors have previously proposed a theoretical model for exotic spin alignment in organic molecular assemblages: The alternating chain of organic biradicals in a singlet (Sb=0) ground state and monoradicals with S=1/2 has a ferrimagnetic ground state for the whole chain, which has been termed generalized ferrimagnetism. An important feature of the generalized ferrimagnetic spin alignment has been found in the deviation of the expectation value Sb2 of the biradical spin from zero. Even a triplet-like spin state Sb2=2 (Sb=1) has been predicted in the theoretical calculations. In this study, we have found experimental evidence for the pseudo-triplet state appearing in the ground-state singlet biradical of a real open-shell compound. At first, we have demonstrated from theoretical calculations that the singlet biradical has Sb2=2 (Sb=1) in a molecular pair with an S=1 metal ion as well as with the S=1/2 monoradical. The pseudo-triplet state of the biradical affords a singlet state of the whole system of the biradical-metal ion pair, which is readily detectable in experiments for verifying the theoretical prediction. As a model compound for the biradical-metal ion pair, a transition metal complex, [(bnn)(Ni(hfac)2)1.5(H2O)] (1), has been synthesized from a nitronyl nitroxide-based ground-state singlet biradical bnn and Ni(hfac)2. From X-ray crystallographic analyses, the compound contains a molecular pair of bnn and Ni(hfac)2, which serves as a model system under the above theoretical studies. It has been found from the analysis of the temperature dependence of magnetic susceptibility that the bnn-Ni(hfac)2 pair has the singlet (S=0) ground state. The singlet ground state of the pair results from an antiparallel coupling of the pseudo-triplet of the biradical and the S=1 spin on the Ni ion. The pseudo-triplet state in the ground-state singlet biradical has thus been verified experimentally, which is crucially important to realize the generalized ferrimagnetic spin alignment.

Magnetism of novel organic heterospin composite systems in supramolecular complexes of nitronyl nitroxide

The authors have shown from theoretical calculations that a one-dimensional molecular assemblage composed of ground-state singlet biradicals and S = 1/2 monoradicals has three kinds of ground states; two with seemingly ferrimagnetic spin alignments and one with a spinless (diamagnetic) state. The energy preference of the three states depends on the relative ratios of the intra- and intermolecular magnetic interactions. This gives a quantum switching device, in which a conversion of the spin state is controlled by external perturbation such as pressure and magnetic field. As a model system for the magnetic switching device, we have designed and synthesized organic coordination compound of an anionic biradical ( 1 −) with a singlet ( S = 0) ground state and a Na +-15-crown-5-ether derivative of nitronylnitroxide monoradical ( 2), 2,5-Bis(1 -oxyl-3-oxido-dihydroxy-4,4,5,5-tetramethyl imidazolin-2-yl)phenol-Sodium(4-(1 -oxyl-3-oxido-dihydroxy-4,4,5,5-tetramethyl imidazolin-2-yl)-15-crown-5) Salt, [Na2] + 1 − . Possible occurrence of the exotic ferrimagnetic spin alignment in the complex salt [Na2] + 1 − is discussed from magnetic susceptibility measurements. A novel 18-crown-6-ether substituted nitronyl nitroxide monoradical 3 has also been synthesized as a building block for organic ferrimagnet. The intermolecular interaction of 3 was found to be antiferromagnetic from magnetic susceptibility measurements.