Ni Moments Research Articles

Signatures of polarized chiral spin disproportionation in rare earth nickelates

In rare earth nickelates (RENiO3), electron-lattice coupling drives a concurrent metal-to-insulator and bond disproportionation phase transition whose microscopic origin has long been the subject of active debate. Of several proposed mechanisms, here we test the hypothesis that pairs of self-doped ligand holes spatially condense to provide local spin moments that are antiferromagnetically coupled to Ni spins. These singlet-like states provide a basis for long-range bond and spiral spin order. Using magnetic resonant X-ray scattering on NdNiO3 thin films, we observe the chiral nature of the spin-disproportionated state, with spin spirals propagating along the crystallographic (101)ortho direction. These spin spirals are found to preferentially couple to X-ray helicity, establishing the presence of a hitherto-unobserved macroscopic chirality. The presence of this chiral magnetic configuration suggests a potential multiferroic coupling between the noncollinear magnetic arrangement and improper ferroelectric behavior as observed in prior studies on NdNiO3 (101)ortho films and RENiO3 single crystals. Experimentally-constrained theoretical double-cluster calculations confirm the presence of an energetically stable spin-disproportionated state with Zhang-Rice singlet-like combinations of Ni and ligand moments.

J[formula omitted] Mott-insulating to S = 0 metal transition in Ce@Y-doped Y[formula omitted]NiIrO[formula omitted]

Double perovskite oxides (DPOs) having 3d-5d electrons provide an optimal field to investigate the entanglement between spin–orbit coupling and electron correlation, displaying unusual physical phenomena. Here, we predicted a large magnetic anisotropy energy (MAE) constant of 1.7 × 108 erg/cm3 in a pristine ferrimagnetic (FiM) Y2NiIrO6 DPO with the magnetic easy axis of the monoclinic b-axis. The estimated Curie temperature (TC) of 198 K using the Heisenberg model agrees well with the experimentally observed value of 192 K. Antiferromagnetic coupling between Ni and Ir ions is confirmed, which supports the FiM as a ground state of the system. Furthermore, it is found that the motif displays a Mott-insulating nature because of the exceptional Jeff=12 state of Ir+4 with an effective magnetic moment (meff.) of ∼2.19μB/u.c. (per unit cell), where Ni/Ir ion contains 1.68/−0.49μB. Upon substitution of Ce at Y-site (Ce@Y), few states grow at the Fermi level which belongs to Ir 5d orbitals and meff. enhanced to 3.13 μB/u.c., while FiM survives as a ground state. Along with this, Ni moment remains unchanged, while it substantially reduces to −0.10μB for Ir ion which leads to be in a ＋ 3 (5d6) state with S = 0 and almost persists in the paramagnetic phase. Due to the larger atomic radii of Ce than that of Y, the Ce@Y-doped motif exhibits a large structural distortion as compared to the pristine one, which increases the MAE and TC values simultaneously. Furthermore, calculated enthalpies of formation, distinct elastic constants, and phonon dispersion spectrums ensure the thermodynamic, mechanical, and dynamical stability of the pristine as well as the Ce@Y-doped system, respectively.

Theory of superdiffusive spin transport in noncollinear magnetic multilayers

Ultrafast demagnetization induced by femtosecond laser pulses in thin metallic layers is caused by the outflow of spin-polarized hot-electron currents describable by the superdiffusive transport model. These laser-generated spin currents can cross the interface into another magnetic layer and give rise to magnetization dynamics in magnetic spin valves with noncollinear magnetizations. To describe ultrafast transport and spin dynamics in such nanostructures, we develop here the superdiffusive theory for general noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru multilayer system with noncollinear Ni and Fe magnetic moments and analyze how the ultrafast demagnetization and spin-transfer torque depend on the noncollinearity. We employ ab initio calculations to compute the spin- and energy-dependent transmissions of hot electrons at the interfaces of the multilayer. Taking into account multiple electron scattering at interfaces and spin mixing in the spacer layer, we find that the laser-induced demagnetization of the Ni layer and the magnetization change of the Fe layer strongly depend on the angle between their magnetizations. Similarly, the spin-transfer torques on the Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer are found to vary markedly with the amount of noncollinearity. These results suggest that by changing the amount of noncollinearity in magnetic multilayers, one can efficiently control the hot-electron spin transport, which may open a way toward achieving fast, laser-driven spintronic devices.

Two-step Mott transition in Ni(S,Se)2 : μSR studies and charge-spin percolation model

A pyrite system ${\mathrm{NiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ exhibits a bandwidth controlled Mott transition via (S,Se) substitutions in a two-step process: the antiferromagnetic insulator (AFI) to antiferromagnetic metal (AFM) transition at $x\ensuremath{\sim}0.45$ followed by the AFM to paramagnetic metal (PMM) transition at $x\ensuremath{\sim}1.0$. Among a few other Mott systems which exhibit similar two-step transitions, $\mathrm{Ni}{(\mathrm{S},\mathrm{Se})}_{2}$ is of particular interest because a large intermediate AFM region in the phase diagram would provide unique opportunities to study the interplay between the spin and charge order. Muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements on ${\mathrm{NiS}}_{2\ensuremath{-}x}{\mathrm{Se}}_{x}$ have been carried out on seven different Se concentrations from $x=0$ to 1.0. The results on quantum evolution demonstrate significantly random spin correlations in the AFM region associated with a rapid reduction of the average local static Ni moment size with increasing $x$, yet without signatures of macroscopic phase separation as confirmed by nearly full volume fraction participating in the static muon relaxation process up to $x\ensuremath{\sim}$ 0.9 at low temperatures. The observed time spectra in the AFM region indicate Lorentzian distribution of static internal field expected for a spatially dilute spin structure. No signature of dynamic critical behavior was observed in thermal phase transitions. The previous neutron scattering studies found sharp magnetic Bragg peaks with a slower reduction of the average ordered moment size in the AFM region. By comparing and combining the muon and neutron results, here we propose a picture where the spin order is maintained by the percolation of ``nonmetallic'' localized and dangling Ni moments surrounded by S, while the charge transition from AFI to AFM is caused by the percolation of the conducting paths generated by the Ni-Se-Ni bonds. This model of interpenetrating charge and spin percolation captures the behavior of experimental results on (a) Se concentration for the insulator to metal transition, (b) Se concentration for the AFM to PMM transition, (c) variation of Hall effect in the AFM region due to conducting Ni charges on the backbone of the percolating charge network, (d) evolution of the neutron Bragg intensity, (e) evolution of the muon static local fields, and (f) spatial variation of the local conductance observed by STM.

Magnetism and spin transport at permalloy/Cu1−xTbx interfaces

The significance of spin transport over an interface in energy-efficient spintronic devices has stimulated interest in the spintronic society during the last few decades. Here, interfaces of $\text{permalloy}/{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Tb}}_{x}$ (Py/Cu-Tb) were investigated in depth. As the Cu-Tb thickness increases, we found that the saturation magnetization of the bilayers falls and then plateaus. Element-specific x-ray magnetic circular dichroism studies suggest that the Tb moment aligns opposite to the Fe and Ni moments, forming a self-assembled antiferromagnetic interface. As a result, the Cu-Tb adjacent layer to Py and the interface have a significant impact on spin transport. Relevant parameters, such as spin mixing conductance, spin diffusion length, and damping, can be tuned by inserting a thin Cu layer between Py and Tb or varying the compositions of Cu-Tb alloys. Using rare-earth Tb, we provide an effective method for controlling the spin transport and magnetism of ferromagnet/normal-metal interfaces. This approach is expected to have a great deal of potential in spintronic applications.

Magnetic structure of the double perovskite La2NiIrO6 investigated using neutron diffraction

A polycrystalline sample of ${\mathrm{La}}_{2}{\mathrm{NiIrO}}_{6}$ has been synthesized and characterized using magnetization and neutron powder diffraction to explore the magnetic ground state. Three anomalies are evident in the bulk magnetization measurements at 28, 50, and 74 K, of which only the 74 K transition is associated with long-range ordering of ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Ir}}^{4+}$ spins. The neutron thermodiffractogram recorded between 1.5 and 90 K and Rietveld refinements of the temperature difference data 1.5--90 K, 40--90 K, and 62--90 K confirm that both the Ni and Ir moments order at the same temperature below 74 K. No change in the magnetic peak intensities or new peaks have been found further below 50 and 28 K indicating that the spin structure remains the same down to 1.5 K. The magnetic propagation vector is $k=(\textonehalf{}, \textonehalf{}, 0)$ and the estimated magnetic moment values at 1.5 K are ${\ensuremath{\mu}}_{\mathrm{Ni}}=1.39(8){\ensuremath{\mu}}_{\mathrm{B}}$ and ${\ensuremath{\mu}}_{\mathrm{Ir}}=0.35(3){\ensuremath{\mu}}_{\mathrm{B}}$. Contrary to a recent publication by Ferreira et al. [Phys. Rev. Mater. 5, 064406 (2021)] on the same compound, the improved data quality allows now to ascertain the presence of an ordered moment on the iridium sites. The magnetic structure proposed by Ferreira et al. is discussed and it is shown that there are several magnetic structures compatible with the data but indistinguishable by neutron powder diffraction methods.

Antiferromagnetic Order Breaks Inversion Symmetry in a Metallic Double Perovskite, Pb2NiOsO6

A polycrystalline sample of Pb2NiOsO6 was synthesized under high-pressure (6 GPa) and high-temperature (1575 K) conditions. Pb2NiOsO6 crystallizes in a monoclinic double perovskite structure with a centrosymmetric space group P21/n at room temperature. Pb2NiOsO6 is metallic down to 2 K and shows a single antiferromagnetic (AFM) transition at TN = 58 K. Pb2NiOsO6 is a new example of a metallic and AFM oxide with three-dimensional connectivity. Neutron powder diffraction and first-principles calculation studies indicate that both Ni and Os moments are ordered below TN and the AFM magnetic order breaks inversion symmetry. This loss of inversion symmetry driven by AFM order is unusual in metallic systems, and the 3d–5d double-perovskite oxides represent a new class of noncentrosymmetric AFM metallic oxides.

Doping-dependent character and possible magnetic ordering of NdNiO2

The novel nickelate superconductors of infinite-layer type feature challenging electronic pecularities in the normal-state phase diagram with doping. Distinct many-body behavior and different dispersion regimes of the entangled $\{$Ni-$d_{z^2}$, Ni-$d_{x^2-y^2}$$\}$ orbital sector give rise to highly rich physics, which is here studied for the case of the NdNiO$_2$ system. An analysis based on advanced realistic dynamical mean-field theory unveils that the superconducting hole-doped region is the meeting place of a (self-)doped Mott insulator from the underdoped side, and a bad Hund metal from the overdoped side. Fermi-level crossing of the Ni-$d_{z^2}$ flat-band ties both regimes together to form a singular arena for unconventional superconductivity. We furthermore shed light on the intriguing problem of elusive magnetism in infinite-layer nickelates. Antiferromagnetic (AFM) order with small Ni moments is shown to be a vital competitor at low temperature. At stoichiometry, C-AFM order with ferromagnetic spin-alignment along the $c$-axis benefits from a conceivable coexistence with Kondo(-lattice) screening.

Defect stability and magnetic property of Ni-Mn-Sn alloys by first-principles calculations

<p indent="0mm">Ni-Mn-Sn alloy is a new type of ferromagnetic shape memory alloy with great potential for the industrial application. Chemical composition has a great influence on the martensitic transformation and magnetic properties for the non-stoichiometric Ni₂Mn₁₊_{<italic>x</italic>}Sn₁₋_{<italic>x</italic>} alloys. A large number of the point defects will be generated during the composition tuning process. In this work, the effects of point defects including antisites, vacancies, and atomic exchanges on the phase stability, magnetic property and electronic structure of the Ni-Mn-Sn alloy were investigated by using the first-principles calculations. Generally, the phase stability, magnetic and elastic properties of alloys with different compositions can be studied efficiently and economically by means of the first-principles calculations. Moreover, the ground state energy, driving force of phase transformation, and atomic magnetic moments of the parent phase and the product phase, which cannot be directly obtained by experiments, can be alternatively investigated through the first-principles calculations. The formation energy of most of the point defects is negative, indicating that these defects can form and stay stably in the parent phase of the non-stoichiometric Ni-Mn-Sn alloy. In particular, the Mn-rich and Sn-deficient Ni-Mn-Sn alloy is commonly used composition design during previous experimental studies. Martensite transition can be controlled effectively by adjusting the Mn and Sn contents. A large number of Mn_Sn antisite defects with relatively lower formation energy would be generated in the parent phase of the Mn-rich and Sn-deficient Ni-Mn-Sn alloy. The formation of the Sn vacancy dramatically reduces the stability of the parent phase. This change will promote the martensitic transformation of the alloy. When the excess Mn atom occupies Sn site, the 30% decrease of the Mn-Mn distance leads the magnetic moment of the excess Mn antiparallel to that of the normal Mn. The smaller Ni-Mn distance causes a larger Ni moment. This study may provide a better understanding of the martensitic transition and physical properties for the Ni-Mn-Sn alloy.

Antiferromagnetism of CoO-NiO bilayers studied by XMLD spectroscopy

CoO-NiO epitaxial bilayer system grown on MgO(001) substrate is investigated using x-ray magnetic linear dichroism (XMLD) spectroscopy with varying CoO overlayer thickness. An analysis of the Ni L 2 edge XMLD spectra using anisotropic XMLD formulation within a two-domain model reveals that the Ni moments undergo a spin reorientation with increasing CoO thickness. Such a spin reorientation is attributed to the competing magnetic interactions at both the NiO film interfaces, suggesting the existence of a sharp horizontal domain wall separating the in-plane and out-of-plane NiO domains. Our study also demonstrates a possible way to investigate the spin-structure along the thickness within the same chemical structure using a model-based approach, in a noninvasive manner.

Electronic structure investigation of GdNi using x-ray absorption, magnetic circular dichroism, and hard x-ray photoemission spectroscopy

GdNi is a ferrimagnetic material with a Curie temperature Tc = 69 K which exhibits a large magnetocaloric effect, making it useful for magnetic refrigerator applications. We investigate the electronic structure of GdNi by carrying out x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at T = 25 K in the ferrimagnetic phase. We analyze the Gd M$_{4,5}$-edge ($3d$ - $4f$) and Ni L$_{2,3}$-edge ($2p$ - $3d$) spectra using atomic multiplet and cluster model calculations, respectively. The atomic multiplet calculation for Gd M$_{4,5}$-edge XAS indicates that Gd is trivalent in GdNi, consistent with localized $4f$ states. On the other hand, a model cluster calculation for Ni L$_{2,3}$-edge XAS shows that Ni is effectively divalent in GdNi and strongly hybridized with nearest neighbour Gd states, resulting in a $d$-electron count of 8.57. The Gd M$_{4,5}$-edge XMCD spectrum is consistent with a ground state configuration of S = 7/2 and L=0. The Ni L$_{2,3}$-edge XMCD results indicate that the antiferromagnetically aligned Ni moments exhibit a small but finite magnetic moment ( $m_{tot}$ $\sim$ 0.12 $\mu_B$ ) with the ratio $m_{o}/m_{s}$ $\sim$ 0.11. Valence band hard x-ray photoemission spectroscopy shows Ni $3d$ features at the Fermi level, confirming a partially filled $3d$ band, while the Gd $4f$ states are at high binding energies away from the Fermi level. The results indicate that the Ni $3d$ band is not fully occupied and contradicts the charge-transfer model for rare-earth based alloys. The obtained electronic parameters indicate that GdNi is a strongly correlated charge transfer metal with the Ni on-site Coulomb energy being much larger than the effective charge-transfer energy between the Ni $3d$ and Gd $4f$ states.

Role of 4f states in infinite-layer NdNiO2

Atomic $4f$ states have been found to be essential players in the physical behavior of lanthanide compounds, at the Fermi level $E_F$ as in the proposed topological Kondo insulator SmB$_6$, or further away as in the magnetic superconductor system ${\cal R}$Ni$_2$B$_2$C (${\cal R}$=rare earth ion) and in Y$_{1-x}$Pr$_x$Ba$_2$Cu$_3$O$_7$, where the $4f$ shell of Pr has a devastating effect on superconductivity. In hole-doped ${\cal R}$NiO$_2$, the ${\cal R}$=Nd member is found to be superconducting while ${\cal R}$=La is not, in spite of the calculated electronic structures being nearly identical. We report first principles results that indicate that the Nd $4f$ moment affects states at $E_F$ in infinite-layer NdNiO$_2$, an effect that will not occur for LaNiO$_2$. Treating 20% hole-doping in the virtual crystal approach indicates that 0.15 holes empty the $\Gamma$-centered Nd-derived electron pocket while leaving the other electron pocket unchanged; hence Ni only absorbs 0.05 holes; the La counterpart would behave similarly. However, coupling of $4f$ states to the electron pockets at $E_F$ arises through the Nd intra-atomic $4f-5d$ exchange coupling $K\approx 0.5$ eV and is ferromagnetic (FM), i.e. anti-Kondo, in sign. This interaction causes spin-disorder broadening of the electron pockets and should be included in models of the normal and superconducting states of Nd$_{0.8}$Sr$_{0.2}$NiO$_2$ The Ni moments differ by 0.2$\mu_B$ for FM and antiferromagnetic alignment (the latter are larger), reflecting some itineracy and indicating that Heisenberg coupling of the moments may not provide a quantitative modeling of Ni-Ni exchange coupling.

Relation between the weak itinerant magnetism in A2Ni7 compounds (A = Y, La) and their stacked crystal structures

The weak itinerant magnetic properties of A2Ni7 compounds with A = {Y, La} have been investigated using electronic band structure calculations in the relation with their polymorphic crystal structures. These compounds crystallizes in two structures resulting from the stacking of two and three blocks of [A2Ni4 + 2 ANi5] units for hexagonal 2H-La2Ni7 (Ce2Ni7 type) and rhombohedral 3R-Y2Ni7 (Gd2Co7 type) respectively. Experimentally, 2H-La2Ni7 is a weak itinerant antiferromagnet whereas 3R-Y2Ni7 is a weak itinerant ferromagnet. From the present first principles calculation within non-spin polarized state, both compounds present an electronic density of state with a sharp and narrow peak centered at the Fermi level corresponding to flat bands from 3d-Ni. This induces a magnetic instability and both compounds are more stable in a ferromagnetic (FM) order compared to a paramagnetic state (ΔE ≈ −35 meV/f.u.). The magnetic moment of each of the five Ni sites varies with their positions relative to the [A2Ni4] and [ANi5] units: they are minimum in the [A2Ni4] unit and maximum at the interface between two [ANi5] units. For 2H-La2Ni7, an antiferromagnetic (AFM) structure has been proposed and found with an energy comparable to that of the FM state. This AFM structure is described by two FM unit blocks of opposite Ni spin sign separated by a non-magnetic layer at z = 0 and ½. The Ni (2a) atoms belonging to this intermediate layer are located in the [La2Ni4] unit and are at a center of symmetry of the hexagonal cell (P63/mmc) where the resultant molecular field is cancelled. Further non-collinear spin calculations have been performed to determine the Ni moment orientations which are found preferentially parallel to the c axis for both FM and AFM structures.

Charge-stripe order, antiferromagnetism, and spin dynamics in the cuprate-analog nickelate La4Ni3O8

We report a muon spin rotation ($\mu$SR) study of the cuprate-analog nickelate La$_4$Ni$_3$O$_8$, which undergoes a transition at 105 K to a low-temperature phase with charge-stripe and antiferromagnetic (AFM) order on square planar NiO$_2$ layers. Zero-field $\mu$SR shows that this transition is abrupt and that the AFM configuration is commensurate. Comparison of observed muon precession frequencies with Ni dipolar field calculations yields Ni moments $\lesssim 0.5\mu_B$. Dynamic muon spin relaxation above 105 K suggests critical slowing of Ni spin fluctuations, but is inconsistent with corresponding $^{139}$La NMR results. Critical slowing and an abrupt transition are also observed in the planar cuprate AFM La$_2$CuO$_{4+\delta}$, where they are evidence for weakly interplanar-coupled two-dimensional AFM spin fluctuations, but our $\mu$SR data do not agree quantitatively with theoretical predictions for this scenario.

Structural and magnetic properties of the Ni5Ti(O2BO3)2 ludwigite

This work presents a study of the oxyborate ${\mathrm{Ni}}_{5}\mathrm{Ti}{({\mathrm{O}}_{2}{\mathrm{BO}}_{3})}_{2}$ using single-crystal x-ray diffraction and magnetic and specific-heat measurements. This material has in its crystalline structure subunits in the form of three-legged ladders. Despite the magnetic ions occupying these low-dimensional structures, we find that this system has long-range magnetic order at low temperatures. The analysis of the magnetization and specific-heat data shows that the process of establishing this order is complex and occurs in several stages upon lowering the temperature. Initially at 92 K a partial magnetic ordering of the Ni moments in one of the two ladders sets in. As the system is further cooled, weakly coupled clusters of spins order, and, finally, at 78 K the whole system of spins in the two ladders are magnetically ordered. Since the Ti ions are found in the nonmagnetic valence state ${\mathrm{Ti}}^{4+}$, the magnetic order is exclusively due to the ${\mathrm{Ni}}^{2+}$ ions with spin $S=1$. We discuss the ferrimagnetic nature of the long-range order, which is consistent with our knowledge of the magnetic interactions, the atomic distances, and bond angles obtained from the x-ray data.

Experimental Observation of Monoclinic Distortion in the Insulating Regime of SmNiO3 by Synchrotron X-ray Diffraction.

RNiO3 (R = rare-earth element) perovskite materials are well-known to exhibit characteristic metal-insulator transitions. The structural distortion increases as the R member becomes smaller along the series. For SmNiO3, a high-hydrostatic-pressure preparation procedure, yielding samples with much enhanced crystalline quality, combined with the extremely high angular resolution of synchrotron X-ray diffraction (XRD) allowed us to identify a monoclinic phase in the insulating regime (below the metal-insulator transition temperature (TMI) of 127 °C), defined in the space group P21/n. This monoclinic symmetry had not been demonstrated directly using nonresonant XRD or neutron diffraction. This has important repercussions on the electronic nature of this material since the monoclinic structure contains two inequivalent Ni positions, implying a charge disproportionation phenomenon. In the metallic regime (above TMI), the standard orthorhombic Pbnm structure is observed. Therefore, there is a coupled structural and electronic transition, as happens for the very small rare-earth compounds of the RNiO3 perovskite series. Across TMI there is a dramatic rearrangement of the lattice parameters, degree of tilting, and distortion of the NiO6 octahedra, showing the convergence of the Ni-O bond lengths upon entering the metallic phase. Brown's valence analysis of the different elements agrees with other reported values in the literature, matching with bond and charge disproportionation models. By magnetization measurements a Néel temperature (TN) corresponding to the antiferromagnetic ordering of the Ni moments is identified at TN= 220 K, whereas Sm moments experience long-range ordering below 36 K.

Existence of a critical canting angle of magnetic moments to induce multiferroicity in the Haldane spin-chain system Tb2BaNiO5

We report an unusual canted magnetism due to 3d and 4f electrons, occupying two different crystallographic sites, with its consequence to electric dipole order. This is based on neutron powder diffraction measurements on Tb2BaNiO5 (orthorhombic, Immm centrosymmetric space group), exhibiting Neel order below (TN) 63 K, to understand multiferroic behavior below 25 K. The magnetic structure is made up of Ni and Tb magnetic moments, which are found to be mutually canted in the entire temperature range below TN, though collinearity is seen within each sublattice, as known in the past. First-principles density functional theory calculations (GCA plus SO and GCA plus U plus SO approximations) support such a canted ground state. The intriguing finding, being reported here, is that there is a sudden increase in this Tb-Ni relative cantingle angle at the temperature (that is, at 25 K) at which spontaneous electric polarization sets in, with bond distance and bong angle anomalies. This finding emphasizes the need for a new spin-driven polarization mechanism, that is, a critical canting angle coupled with exchangestriction, to induce multiferroicity in magnetic insulators with canted spins.

Solvent-induced high-spin transition in double-decker 3d–4f metallacrowns

Element-specific magnetic spin and orbital magnetic moments of $3d\text{\ensuremath{-}}4f$ double-decker metallacrown molecules have been investigated using x-ray magnetic circular dichroism. The double-decker metallacrowns comprise one rare-earth Gd(III) or Tb(III) ion embedded between two squared scaffolds of four Ni(II) ions. We observe a strong increase of the Ni(II) moments if the molecules are dissolved in methanol, indicating a spin crossover from a low-spin to a high-spin state. In contrast, dichloromethane does not change the spin state. This result is explained by a change of the coordination environment of nickel. The comparison of charge-transfer multiplet calculations with the experimental absorption spectra confirm the different ligand fields.

Anisotropy in the magnetic interaction and lattice-orbital coupling of single crystal Ni3TeO6

This investigation reports on anisotropy in the magnetic interaction, lattice-orbital coupling and degree of phonon softening in single crystal Ni3TeO6 (NTO) using temperature- and polarization-dependent X-ray absorption spectroscopic techniques. The magnetic field-cooled and zero-field-cooled measurements and temperature-dependent Ni L3,2-edge X-ray magnetic circular dichroism spectra of NTO reveal a weak Ni-Ni ferromagnetic interaction close to ~60 K (TSO: temperature of the onset of spin ordering) with a net alignment of Ni spins (the uncompensated components of the Ni moments) along the crystallographic c-axis, which is absent from the ab-plane. Below the Néel temperature, TN~ 52 K, NTO is stable in the antiferromagnetic state with its spin axis parallel to the c-axis. The Ni L3,2-edge X-ray linear dichroism results indicate that above TSO, the Ni 3d eg electrons preferentially occupy the out-of-plane 3d3z2−r2 orbitals and switch to the in-plane 3dx2−y2 orbitals below TSO. The inherent distortion of the NiO6 octahedra and anisotropic nearest-neighbor Ni-O bond lengths between the c-axis and the ab-plane of NTO, followed by anomalous Debye-Waller factors and orbital-lattice in conjunction with spin-phonon couplings, stabilize the occupied out-of-plane (3d3z2−r2) and in-plane (3dx2−y2) Ni eg orbitals above and below TSO, respectively.

Structural transformation, Griffiths phase and metal-insulator transition in polycrystalline Nd2−xSrxNiMnO6 (x = 0, 0.2, 0.4, 0.5 and 1) compound

Polycrystalline double perovskite Nd2−xSrxNiMnO6 (x = 0, 0.2, 0.4, 0.5 and 1) samples were synthesized using the solid state reaction method. There occurs a structural transformation from monoclinic (P21/n, for x = 0 to x = 0.5) to cubic (Fmm, for x = 1) with increasing Sr doping. Raman spectroscopy reveals the increase in static disorder with doping. The Curie temperature (TC) shows a small increase from x = 0 to 0.5 (TC ~ 200 K), but for x = 1, TC increases drastically upto ~264 K. The deviation of 1/χ(T) from Curie–Weiss behaviour for doped samples with exponent less than one, indicates a development of the Griffiths phase with doping. The systematic reduction in magnetic moment at 5 K suggests an increase in anti-site disorders with doping. Mn 3s x-ray photoemission spectra show an increase in exchange splitting, indicating a decrease in the valency of Mn. The x-ray absorption spectra at Ni and Mn 2p edges show that the formal valence remains 2+ (Ni) and 4+ (Mn) for all the samples, with changes in spectral weights. Ni 2p x-ray photoemission spectra show characteristic feature similar to Ni3+ systems, only for x = 1 sample. Our GGA-based calculations for the ordered supercell, predict half metallic character for doping (x > 0) samples due to delocalization of Ni eg orbitals. The calculations with anti-site disorders yield drastic reduction in Ni moments, with the disordered anti-ferromagnetic phase having lowest energy at maximum doping. Temperature dependent resistivity measurements exhibit a clear metallic region for x = 0.2 sample, while for higher dopings (x > 0.2), the metallicity gets suppressed due to increase in anti-site disorders in these samples.