Nonmagnetic Sites Research Articles

Investigations of mechanical, electronic, and magnetic properties of non-magnetic MgTe and ferro-magnetic Mg0.75TM0.25Te (TM = Fe, Co, Ni): An ab-initio calculation

The mechanical, electronic and magnetic properties of non-magnetic MgTe and ferro-magnetic (FM) Mg0.75TM0.25Te (TM = Fe, Co, Ni) in the zinc-blende phase are studied by ab-initio calculations for the first time. We use the generalized gradient approximation functional for computing the structural stability, and mechanical properties, while the modified Becke and Johnson local (spin) density approximation (mBJLDA) is utilized for determining the electronic and magnetic properties. By comparing the energies of non-magnetic and FM calculations, we find that the compounds are stable in the FM phase, which is confirmed by their structural stabilities in terms of enthalpy of formation. Detailed descriptions of elastic properties of Mg0.75TM0.25Te alloys in the FM phase are also presented. For electronic properties, the spin-polarized electronic band structures and density of states are computed, showing that these compounds are direct bandgap materials with strong hybridizations of TM 3d states and Te p states. Further, the ferromagnetism is discussed in terms of the Zener free electron model, RKKY model and double exchange model. The charge density contours in the (110) plane are calculated to study bonding properties. The spin exchange splitting and crystal field splitting energies are also calculated. The distribution of electron spin density is employed in computing the magnetic moments appearing at the magnetic sites (Fe, Co, Ni), as well as at the non-magnetic sites (Mg, Te). It is found that the p–d hybridization causes not only magnetic moments on the magnetic sites but also induces negligibly small magnetic moments at the non-magnetic sites.

Study of the Zn0.75M0.25Te (M = Fe, Co, Ni) diluted magnetic semiconductor system by first principles approach

Structure, mechanical and electronic properties of ternary Zn0.75M0.25Te (M = Fe, Co and Ni) diluted magnetic semiconductor system has been studied by first principles approach. Theoretical results revealed that the structure of all three alloys was stable in ferromagnetic phase. The enthalpies of formation were negative which also confirmed the structural stability of all three alloys in ferromagnetic phase. For Zn0.75M0.25Te (M = Fe, Co, Ni) alloys, the equilibrium lattice constants decreased from Fe to Ni. The band structure calculations showed that the Zn0.75M0.25Te (M = Fe, Co, Ni) alloys exhibit ferromagnetic semiconductor behavior. The energy band gap (Eg) increased from 2.19 eV to 2.33 eV with the addition of 25%M (M = Fe, Co, Ni) in Zn0.75M0.25Te alloys. For Zn0.75M0.25Te (M = Fe, Co, Ni) alloys, the magnetic moments of magnetic ions (Fe, Co, Ni) decreased from Fe to Ni, whereas small local magnetic moments were also found to appear at the non-magnetic (Zn and Te) atomic sites due to p-d hybridization.

On the dodecahedral site substituted Lu3Fe5O12 ceramics for room temperature magneto-dielectric applications

Materials exhibiting low-field magneto-electric and magneto-dielectric (MD) response at room temperature are essential for utilization in multifunctional devices. Magneto-dielectric properties of Lu3Fe5O12 substituted with a magnetic ion (Terbium) at its dodecahedral (non-magnetic) site are investigated. The measured dielectric permittivity at various magnetic fields exhibits a giant MD effect (~65%) at remarkably low fields (H~1kOe) at room temperature. The MD response of Lu3Fe5O12 is enhanced due to the coupling effect of substituted magnetic Terbium ions in the non-magnetic Lu-site interacting with the neighboring Fe-sites. Studies suggest a route towards future applications of the MD effect in advanced devices.

Intrinsic versus extrinsic ferromagnetism inHfO2−xandNi:HfO2−xthin films

We have investigated the possible evolution of an intrinsic stable ferromagnetic moment in oxygen deficient undoped and magnetically doped ${\mathrm{HfO}}_{2\ensuremath{-}x}$ thin films grown by reactive molecular beam epitaxy. Neither oxygen vacancies nor substituted Ni in combination with such vacancies results in an observable magnetic moment for a broad range of oxygen vacancy concentrations. By combining integral and element specific magnetization measurements, we show that a fluctuating deposition rate of the magnetic dopant induces extrinsic ferromagnetism by promoting the formation of metallic clusters. We suggest the element specific measurement of an induced magnetic moment at the nonmagnetic site as a proof of intrinsic ferromagnetism in diluted magnetic semiconductors.

Dzyaloshinskii-Moriya interaction and chiral magnetism in3d−5dzigzag chains: Tight-binding model andab initiocalculations

We investigate the chiral magnetic order in free-standing planar 3$d$-5$d$ bi-atomic metallic chains (3$d$: Fe, Co; 5$d$: Ir, Pt, Au) using first-principles calculations based on density functional theory. We found that the antisymmetric exchange interaction, commonly known as Dzyaloshinskii-Moriya interaction (DMI), contributes significantly to the energetics of the magnetic structure. We used the full-potential linearized augmented plane wave method and performed self-consistent calculations of homogeneous spin spirals, calculating the DMI by treating the effect of spin-orbit interaction (SOI) in the basis of the spin-spiral states in first-order perturbation theory. To gain insight into the DMI results of our ab initio calculations, we develop a minimal tight-binding model of three atoms and 4 orbitals that contains all essential features: the spin-canting between the magnetic $3d$ atoms, the spin-orbit interaction at the $5d$ atoms, and the structure inversion asymmetry facilitated by the triangular geometry. We found that spin-canting can lead to spin-orbit active eigenstates that split in energy due to the spin-orbit interaction at the $5d$ atom. We show that, the sign and strength of the hybridization, the bonding or antibonding character between $d$-orbitals of the magnetic and non-magnetic sites, the bandwidth and the energy difference between states occupied and unoccupied states of different spin projection determine the sign and strength of the DMI. The key features observed in the trimer model are also found in the first-principles results.

Structural ground states of(A,A′)Cr2O4(A=Mg,Zn; A′=Co,Cu)spinel solid solutions: Spin-Jahn-Teller and Jahn-Teller effects

We examine the effect of small amounts of magnetic substituents in the $A$ sites of the frustrated spinels MgCr$_2$O$_4$ and ZnCr$_2$O$_4$. Specifically we look for the effects of spin and lattice disorder on structural changes accompanying magnetic ordering in these compounds. Substitution of Co$^{2+}$ on the non-magnetic Zn$^{2+}$ site in Zn$_{1-x}$Co$_{x}$Cr$_2$O$_4$ where 0\,$<$\,$x$\,$\leq$\,0.2 completely suppresses the spin-Jahn-Teller distortion of ZnCr$_2$O$_4$ although these systems remain frustrated, and magnetic ordering occurs at very low temperatures of $T$\,$<$\,20\,K. On the other hand, the substitution of Jahn-Teller active Cu$^{2+}$ for Mg$^{2+}$ and Zn$^{2+}$ in Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ where 0\,$<$\,$x$\,$\leq$\,0.2 induce Jahn-Teller ordering at temperatures well above the N\'eel temperatures of these solid solutions, and yet spin interactions remain frustrated with long-range magnetic ordering occurring below 20\,K without any further lattice distortion. The Jahn-Teller distorted solid solutions Mg$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ and Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ adopt the orthorhombic $Fddd$ structure of ferrimagnetic CuCr$_2$O$_4$. Total neutron scattering studies of Zn$_{1-x}$Cu$_{x}$Cr$_2$O$_4$ suggest that there are local $A$O$_4$ distortions in these Cu$^{2+}$-containing solid solutions at room temperature and that these distortions become cooperative when average structure distortions occur. Magnetism evolves from compensated antiferromagnetism in MgCr$_2$O$_4$ and ZnCr$_2$O$_4$ to uncompensated antiferromagnetism with substitution of magnetic cations on the non-magnetic cation sites of these frustrated compounds.

13C NMR study of commensurate antiferromagnetism in (TMTTF)2Br

Quasi-one-dimensional organic conductors (TMT$C$F)${}_{2}$$X$ have various electric and magnetic properties. Although theoretical and experimental studies have suggested that (TMTTF)${}_{2}$Br has the properties of commensurate antiferromagnetism, details of the magnetic structure of this compound are unclear. Two types of antiferromagnetism are expected, one due to a localized electron and the other to the nesting of the Fermi surface. We therefore assessed the antiferromagnetic structure of (TMTTF)${}_{2}$Br using ${}^{13}$C NMR. Site assignment of the observed antiferromagnetic spectrum confirmed that there were two magnetic molecular sites, with staggered moments and amplitude of 0.11${\ensuremath{\mu}}_{B}$/molecule, as well as nonmagnetic molecular sites. A commensurate structure with antiferromagnetic ordering of ($\ensuremath{\uparrow}\ensuremath{\circ}\ensuremath{\downarrow}\ensuremath{\circ}$) along a one-dimensional chain would be expected as the freezing of antiferromagnetic fluctuations in the charge-ordered phase of the (TMTTF)${}_{2}$AsF${}_{6}$ salt. The presence of a nodal site is strongly suggestive of the nesting type antiferromagnetism. The fine structure of the antiferromagnetic spectrum suggests superlattice along the interchain direction. We could not observe line broadening due to the charge order above the antiferromagnetic transition.

Effect of substitution of magnetic rare earth Nd at non-magnetic La site on structure and properties of LaSrFeO4

Abstract K 2 NiF 4 phases La 1− x Nd x SrFeO 4 ( x =0.0, 0.3, 0.6, and 0.9) have been successfully prepared by solid-state reactions. Rietveld refinement shows that all the phases crystallize in tetragonal K 2 NiF 4 structure (space group I4 / mmm ). The change in the lattice constants was interpreted in terms of the ionic radius of the substituted ion. The variation of inverse molar susceptibility with temperature follows the Curie–Weiss law. The dominant magnetic interactions in the materials are antiferromagnetic, which could be due to Fe 3+ –O–Fe 3+ superexchange coupling. All the samples were semiconducting over the temperature range from 150 to 350 K and the resistivity data has been best fitted using the Arrhenius equation, ρ = ρ 0 exp(– E a / k B T ).

Inhomogeneous magnetism in the doped kagome lattice of LaCuO2.66

The hole-doped kagome lattice of Cu${}^{2+}$ ions in LaCuO${}_{2.66}$ was investigated by nuclear quadrupole resonance, electron spin resonance, electrical resistivity, bulk magnetization, and specific-heat measurements. For temperatures above $\ensuremath{\sim}180$ K, the spin and charge properties show an activated behavior suggestive of a narrow-gap semiconductor. At lower temperatures, the results indicate an insulating ground state which may or may not be charge ordered. While the frustrated spins in remaining patches of the original kagome lattice might not be directly detected here, the observation of coexisting nonmagnetic sites, free spins, and frozen moments reveals an intrinsically inhomogeneous magnetism. Numerical simulations of a $\frac{1}{3}$-diluted kagome lattice rationalize this magnetic state in terms of a heterogeneous distribution of cluster sizes and morphologies near the site-percolation threshold.

Study of half-metallic ferromagnetism in V-doped CdTe alloys by using first-principles calculations

For investigating the structural, electronic and magnetic properties of zincblende (ZB) Cd1−xVxTe (0≤x≤1), we have employed the Wu–Cohen generalized gradient approximation (WC-GGA) and the modified Becke and Johnson local density approximation (mBJLDA) functionals within the frame-work of spin polarized density functional theory (DFT). The former exchange-correlation parameterization scheme has been used for optimizing the equilibrium structural properties, while the electronic band structures, electron density of states (DOS) and charge densities are computed using both functionals and their performances are compared. Our results show that the electron spin polarization in unfilled V-3d orbitals gives rise to spin exchange splittings (Δx(d) and Δx(pd)) which is the cause of half metallic (HM) ferromagnetism in the V-doped CdTe as revealed by the computed DOS. Consequently, the nature of effective potential is more attractive in spin-down case rather than that in spin-up case. The total magnetic moment for each of the compounds under study is 3μB where the main contribution comes from the V atom, while the nonmagnetic sites Cd and Te are also bestowed with minor atomic magnetic moments due to Te-5p–V-3d hybridization. Furthermore, we calculated the exchange constants N0α and N0β to determine the conduction and valence band contributions in exchange splitting process. A comparison of the two functionals, considered in this work, shows that the mBJLDA provides a better description of the electronic structure especially of the ferromagnetic Cd1−xVxTe (0.75≤x≤1). Moreover, as compared to WC-GGA, the mBJLDA predicts high Curie temperatures in V-doped CdTe by providing significantly larger values of Δx(d), Δx(pd), N0α, N0β and, importantly, a quite wide HM gap.

Low-temperature spin dynamics of a valence bond glass in Ba2YMoO6

We carried out ac magnetic susceptibility measurements and muon spin relaxation spectroscopy on the cubic double perovskite Ba2YMoO6, down to 50 mK. Below ∼1 K the muon relaxation is typical of a magnetic insulator with a spin-liquid type ground state, i.e. without broken symmetries or frozen moments. However, the ac susceptibility revealed a dilute-spin-glass-like transition below ∼1 K. Antiferromagnetically coupled Mo5+ 4d1 electrons in triply degenerate t2g orbitals are in this material arranged in a geometrically frustrated fcc lattice. Bulk magnetic susceptibility data has previously been interpreted in terms of a freezing to a heterogeneous state with non-magnetic sites where 4d1 electrons have paired in spin-singlets dimers, and residual unpaired Mo5+ 4d1 electron spins. Based on the magnetic heat capacity data it has been suggested that this heterogeneity is the result of kinetic constraints intrinsic to the physics of the pure system (possibly due to topological overprotection) leading to a self-induced glass of valence bonds between neighbouring 4d1 electrons. The muon spin relaxation (μSR) unambiguously points to a heterogeneous state with a static arrangement of unpaired electrons in a background of (valence bond) dimers between the majority of Mo5+ 4d electrons. The ac susceptibility data indicate that the residual magnetic moments freeze into a dilute-spin-glass-like state. This is in apparent contradiction with the muon-spin decoupling at 50 mK in fields up to 200 mT, which indicates that, remarkably, the time scale of the field fluctuations from the residual moments is ∼5 ns. Comparable behaviour has been observed in other geometrically frustrated magnets with spin-liquid-like behaviour and the implications of our observations on Ba2YMoO6 are discussed in this context.

Partial Disorder and Metal–Insulator Transition in the Periodic Anderson Model on a Triangular Lattice

Ground state of the periodic Anderson model on a triangular lattice is systematically investigated by the mean-field approximation. We found that the model exhibits two different types of partially disordered states: one is at half filling and the other is at other commensurate fillings. In the latter case, the kinetic energy is lowered by forming an extensive network involving both magnetic and nonmagnetic sites, in sharp contrast to the former case in which the nonmagnetic sites are rather isolated. This spatially extended nature of nonmagnetic sites yields a metallic partially-disordered state by hole doping. We discuss the mechanism of the metal-insulator transition by the change of electronic structure.

Disorder instability of the magnon condensate in a frustrated spin ladder

The effect of disorder is studied on the field-induced quantum phase transition in the frustrated spin-ladder compound H${}_{8}$C${}_{4}$SO${}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}$Cu${}_{2}$(Cl${}_{1\ensuremath{-}x}$Br${}_{x}$)${}_{4}$ using bulk magnetic and thermodynamic measurements. The parent material ($x=0$) is a quantum spin liquid, which in applied fields is known to form a magnon condensate with long-range helimagnetic order. We show that bond randomness introduced by a chemical substitution on the nonmagnetic halogen site destroys this phase transition at very low concentrations, already for $x=0.01$. The extreme fragility of the magnon condensate is attributed to random frustration in the incommensurate state.

Partial Disorder in the Periodic Anderson Model on a Triangular Lattice

We report our theoretical results on the emergence of a partially-disordered state at zero temperature and its detailed nature in the periodic Anderson model on a triangular lattice at half filling. The partially-disordered state is characterized by coexistence of a collinear antiferromagnetic order on an unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites. This state appears with opening a charge gap between a noncollinear antiferromagnetic metal and Kondo insulator while changing the hybridization and Coulomb repulsion. We also find a characteristic crossover in the low-energy excitation spectrum as a result of coexistence of magnetic order and nonmagnetic sites. The result demonstrates that the partially-disordered state is observed distinctly even in the absence of spin anisotropy, in marked contrast to the partial Kondo screening state found in the previous study for the Kondo lattice model.

Magnetic quantum wire as a spin filter: An exact study

We propose that a magnetic quantum wire composed of magnetic and non-magnetic atomic sites can be used as a spin filter for a wide range of applied bias voltage. We adopt a simple tight-binding Hamiltonian to describe the model where the quantum wire is attached to two semi-infinite one-dimensional non-magnetic electrodes. Based on single particle Green's function formalism all the calculations which describe two-terminal conductance and current through the wire are performed numerically. Our exact results may be helpful in fabricating mesoscopic or nano-scale spin filter.

Magnetic properties of strongly frustrated and correlated systems

While consequences of frustration of magnetic interactions are much studied in localized spin systems, much less studies have been performed on frustrated metallic systems. However, several effects of strong geometrical frustration in metallic correlated system have also been experimentally observed in rare-earth or transition metal compounds: coexistence of magnetic and non-magnetic sites in ordered magnetic structure, heavy fermion behaviour and anomalous Hall effect due to spin chirality are consequences of frustration. An overview of the experimental observations and of the proposed models is given. Other interesting effects due to magnetic frustration in metallic systems, which have been predicted theoretically, are also reviewed.

Magnetic Hysteresis Properties in Dilute Ising Ultra-Thin-Film: Monte Carlo Investigation

Monte Carlo simulation was used to observe the dynamic magnetic behavior of dilute Ising ultra-thin-film. The hysteresis properties were investigated as varying the non-magnetic concentration, field frequency and field amplitude where the Metropolis algorithm was used. From the results, at fixed temperature and field amplitude, the hysteresis loop increases in size with increasing frequency at low frequency region but reduces at high frequency region due to the increase of the phase-lag between magnetization and external field signals. With the inclusion of non-magnetic sites, the phase-lag shifts to higher frequency as a result of weaker magnetic interaction. In addition, the scaling relation among the hysteresis area, the field parameters and non-magnetic concentration was proposed to provide general information of how the dilution affects hysteresis properties of dilute magnetic in ultra-thin-film structure.

Charge disproportionation and dynamics inθ−(BEDT−TTF)2CsZn(SCN)4

The electronic state of two-dimensional organic conductor, $\ensuremath{\theta}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}\mathrm{Cs}\mathrm{Zn}{(\mathrm{S}\mathrm{C}\mathrm{N})}_{4}$, has been investigated by means of $^{13}\mathrm{C}\text{\ensuremath{-}}\mathrm{NMR}$ line shape analyses on selectively $^{13}\mathrm{C}$-enriched single crystal sample. Strong charge disproportionation was found to develop in the ``metallic'' state well above metal-insulator transition temperature of $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The charge disproportionation becomes almost static below $\ensuremath{\sim}140\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, forming a short-range charge ordering, but no long-range ordering is stabilized at low temperatures down to $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. About half of the molecular sites become nonmagnetic, while the other half carries finite magnetic moments. Charge disproportionation becomes very much reduced at least at the nonmagnetic molecular sites, implying that charge rearrangement occurs at $\ensuremath{\sim}20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.

Profound changes on the geometrically frustrated magnetism of Ca 3CoRhO 6 by the disturbance of the non-magnetic site

The compound, Ca 3CoRhO 6 (K 4CdCl 6-type rhombohedral structure) containing magnetic chains of Co–Rh running along c-axis arranged in a triangular fashion in the basal plane has been known to exhibit a ‘partially disordered antiferromagnetic structure (PDA)’ in the range 40–90 K with a spin-glass phase below 40 K. Here we report the influence of partial replacement of Ca by Y (that is, without disrupting magnetic chain) on its magnetization behavior by investigating the solid solution, Ca 3− x Y x CoRhO 6. We find that the solid solution forms up to x=0.5 among the compositions studied and that the feature in the magnetization data attributable to spin-chain magnetism and PDA structure get depressed dramatically for a very small substitution of Ca by Y ( x=0.15), however, with the persistence of only low-temperature spin-glass phase.

NMR properties of 51V in a complex high-temperature phase of Fe 2VSi

NMR properties of 51V in the high-temperature phase of Fe 2VSi with the α-Mn-type structure (A12) are reported. A free-decay signal with a temperature-independent shift and broad-spin echo signal with a temperature-dependent shift and hyperfine-coupling constant of −3.3 kOe/ μ B have been observed. Both come from nonmagnetic vanadium sites, and paramagnetic moment of 1.6 μ B/f.u. and is suggested to be due to magnetic iron atoms.