Superexchange Angle Research Articles

On the effects of dislocations on the magnetism of BiFeO3 nanoparticles

A Strain-related defects, such as dislocations, and their effects on physical properties are well known for metallic materials as well as for oxide thin films. However, little to no research regards these effects on oxide nanoparticles. Herein, we deeply explore the effects of dislocations on the magnetic properties of high-strained BiFeO3 nanoparticles obtained by high-energy cryo-milling. Several structural changes such as superexchange angle, Fe-O-Fe bond length and FeO6 octahedral distortions are confirmed. Besides, the introduction of high dislocation density and reduced crystallite size gives rise to an unusual magnetic behavior changing the magnetic hysteresis from an antiferromagnetic-like to a weak-ferromagnetic-like loop, suggesting that high concentration of dislocations in BiFeO3 nanoparticles can be used to directly control its magnetic properties.

Spin–phonon interaction and magnetoelastic coupling associated with unusual cluster-glass states in YFe0.9Cr0.1O3

The magnetic, optical phonon behavior of the polycrystalline YFe0.9Cr0.1O3 (YFC-10), prepared by solid-state reaction route, had been studied via static and ac magnetization measurements and Raman spectroscopic techniques respectively. The results indicated that YFe0.9Cr0.1O3 undergoes unusual cluster-glass transitions at low temperatures viz. Tf1 (SG1) = 108 K (Freez.1) and Tf2 (SG2) = 30 K (Freez.2) coexisting with the long-range weak ferromagnetic (WFM) state. Raman spectroscopic (RS) measurements revealed 12 distinct modes of vibrations at all temperatures with 83–433 K. Interestingly, the freezing transition at Tf1 is found to be accompanied by significant phonon renormalization. From the detailed analysis of the line-shape function of the Raman modes, the anomalous T-variation of the peak position and the linewidths suggests the existence of spin-phonon interaction in the present solid solution. Moreover the temperature dependent synchrotron X-ray diffraction studies between 10 and 300 K revealed the anomalous thermal variation of the lattice parameters and microscopic structural parameters such as super-exchange angles and (Fe/Cr)O6 octahedral bond lengths and bond angles at both the freezing transitions. Overall, the system exhibits the strong magnetoelastic and spin phonon coupling over a broad thermal regime viz. from Liquid. N2 to temperatures above room temperature (RT = 300 K) which makes YFe0.9Cr0.1O3 reliable for application purpose.

Magnetoelastic interaction in the two-dimensional magnetic material MnPS3 studied by first principles calculations and Raman experiments

We report experimental and theoretical studies on the magnetoelastic interactions in MnPS3. Raman scattering response measured as a function of temperature shows a blue shift of the Raman active modes at 120.2 and 155.1 cm−1, when the temperature is raised across the antiferromagnetic-paramagnetic transition. Density functional theory (DFT) calculations have been performed to estimate the effective exchange interactions and calculate the Raman active phonon modes. The calculations lead to the conclusion that the peculiar behavior with temperature of the two low energy phonon modes can be explained by the symmetry of their corresponding normal coordinates which involve the virtual modification of the super-exchange angles associated with the leading antiferromagnetic (AFM) interactions.

Enhanced magnetic and room temperature intrinsic magnetodielectric effect in Mn modified Ba2Mg2Fe12O22 Y-type hexaferrite

We have reported a systematic investigation on structural, magnetic, magnetodielectric and magnetoimpedance characteristics of Y-type Ba2Mg2(Fe1−xMnx)12O22 (0 ⩽ x ⩽ 0.12) hexaferrite synthesized by solid-state reaction route. Rietveld refinement of x-ray diffraction pattern confirms the phase purity of all the samples with rhombohedral crystal structure. The Mn dopant modulates not only superexchange angle near to the boundary of magnetic blocks but also magnetic transition temperature. Temperature-dependent magnetization data suggests that due to Mn doping at Fe sites, ferrimagnetic to proper screw transition temperature (TII) increases from 190 K to 208 K, while there is a decrease in proper screw to longitudinal conical spin transition temperature (TI) from 35 K to 25 K. We observe remarkable decrease in the magnetic field from 20 kOe to 12 kOe to produce intermediate spin ordering from ferrimagnetic ordering which can be understood because of modification of superexchange angle due to Mn doping. The value of loss tangent decreases with increasing doping concentration at 300K, i.e. ~60% and 180% in BMFM4 (x = 0.04) and BMFM8 (x = 0.08) respectively as compared to BMF, suggesting the evolution of intrinsic feature in the doped samples. Magnetodielectric (MD) effect shows that in the low-frequency regime, the robust MD effect is because of Maxwell–Wagner interfacial polarization, whereas in the high-frequency regime intrinsic effect dominates. Further, magnetoimpedance measurement confirms the presence of substantial intrinsic MD% (~6%) at 1.3 T applied field at 300 K for 4% Mn-doped sample. Finally, the nature and strength of magnetoelectric coupling in BMFM4 and BMFM8 samples at 300 K is found to be biquadratic (P2M2) and maximum strength of coupling is 3.09 × 10−4 emu2 g−2 and 2.34 × 10−4 emu2 g−2, respectively.

Magnetic and anomalous dielectric behavior of Mn modified Ba2Mg2Fe12O22 hexaferrite

We have investigated structural, dielectric, magnetic and room temperature magnetodielectric properties of Mn-doped Y-type hexaferrite Ba2Mg2(Fe1-xMnx)12O22 (0 ≤ x ≤ 0.08). Rietveld refined of X-ray pattern of all samples confirms the phase purity with the rhombohedral crystal structure (space group R-3 m). Magnetization measurements suggest that ferrimagnetic to paramagnetic transition temperature (Tc) decreased from 647 K (x = 0.0, BMF) to 621 K (x = 0.08, BMFM8) for Mn-doped sample. Our results confirm the decrease in magnetocrystalline anisotropy constant (K) by ∼49% and ∼117% in BMFM4 (x = 0.04) and BMFM8 samples respectively in comparison to BMF due to substantial increase in superexchange angle of Fe/Mg ions in octahedral and tetrahedral sites. Substantial decrease in dielectric constant and inverse distorted butterfly loop like behavior in magnetodielectric (MD) effect is observed at room temperature.

XAFS investigation of the correlation of Bi-sublattice disorder with ferromagnetism of multiferroic BiFeO3 nanoparticle

Practical utilization of room temperature multiferroic BiFeO3 is intrinsically limited by the absence of ferromagnetism. In this backdrop, development of weak ferromagnetism in 20 nm-sized BiFeO3 nanoparticles is very optimistic. The origin of ferromagnetism is curious and paradoxical from long-range-order perspective since average superexchange angle Fe–O–Fe of the nanoparticle is in antiferromagnetic configuration. In this work, we resolve this paradox by establishing the beneficial role of local disorder with x-ray absorption spectroscopy. We distinguish between the natures of (Bi, Fe)-sublattice disorder and establish their correlation that eventually leads to ferromagnetism. Our results reveal intrinsic large Bi positional disorder, which may be attributed to 6s2 lone pair activity of Bi atom and which leads to greater susceptibility of Bi-sublattice to modification during size reduction. Thus, local (BiO6, FeO6) units are observed to undergo large distortion and rotation respectively. We demonstrate with calculations that FeO6 rotation is geometric consequence of BiO6 distortion. In the case of our BiFeO3 nanoparticles, experimental BiO6 disorder induces FeO6 rotation that drives Fe–O–Fe into ferromagnetic configuration. These local ferromagnetic units give rise to weak magnetism. This structural route to magnetism in BiFeO3 can be generalized to encourage A-site disorder controlled magnetism or any functional octahedral rotation in ABO3 perovsites. The results additionally propagate the effectiveness of particle size-dependence in generating A-site strain rather than chemical doping or external pressure.

High-pressure spectroscopic investigation of multiferroic Ni3TeO6

Here, a combined experimental and theoretical investigation of the properties of Ni${}_{3}$TeO${}_{6}$ under pressure reveals reduced compressibility near 4 GPa due to the unique behavior of one bond angle. This trend arises from a nearby cavity in the crystal structure that enhances tunability in this and other corrundumlike materials. Analysis of superexchange angles reveals enhanced antiferromagnetic interactions, in line with an increase in ${T}_{N}$ observed in magnetization. Magnetoelectric coupling likely enhances polarization under pressure. This work suggests that incorporating empty cavities into crystal structures provides a new route for control of multiferroic properties.

Temperature and high-pressure dependent x-ray absorption of SmNiO3 at the Ni K and Sm L3 edges

We report on x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) measurements of SmNiO3 from 20 K to 600 K and up to 38 GPa at the Ni K and Sm L3 edges. A multiple component pre-Ni K edge tail is understood, originating from 1 s transitions to 3d-4p states while a post-edge shoulder increases distinctively smoothly, at about the insulator to metal phase transition (TIM), due to the reduction of electron–phonon interactions as the Ni 3d and O 2p band overlap triggers the metallic phase. This effect is concomitant with pressure-induced Ni-O-Ni angle increments toward more symmetric Ni3+ octahedra of the rhombohedral R¯3c space group. Room temperature pressure-dependent Ni white line peak energies have an abrupt ∼3.10 ± 0.04 GPa valence discontinuity from non-equivalent Ni3+δ + Ni3–δ charge disproportionate net unresolved absorber turning at ∼TIM into Ni3+ of the orthorhombic Pbnm metal oxide phase. At 20 K the overall white line response, still distinctive at TIM ∼8.1 ± 0.6 GPa is much smoother due to localization. Octahedral bond contraction up to 38 GPa and at 300 K and 20 K show breaks in its monotonic increase at the different structural changes. The Sm L3 edge does not show distinctive behaviors either at 300 K or 20 K up to about 35 GPa but the perovskite Sm cage, coordinated to eight oxygen atoms, undergoes strong uneven bond contractions at intermediate pressures where we found the coexistence of octahedral and rhombohedral superexchange angle distortions. We found that the white line pressure-dependent anomaly may be used as an accurate alternative for delineating pressure–temperature phase diagrams.

Magnetic nanopantograph in the SrCu2(BO3)2 Shastry–Sutherland lattice

Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)2 (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu superexchange angle that, due to the orthogonal Cu(2+) dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼ 10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

Strategies for increasing the Néel temperature of magnetoelectric Fe2TeO6

Ways to increase the Néel temperature TN in the magnetoelectric Fe2TeO6 antiferromagnet are explored with the help of first-principles calculations. Substitution of larger ions like Zr or Hf for tellurium increases the superexchange angles. The compensating O vacancies tend to form bound complexes with Zr dopants, which do not degrade the electronic band gap. TN is estimated to increase by 15% at 12.5% Te → Zr substitution with such compensation. Substitution of N for O is favorable due to the decreased charge-transfer gap. The overall effect for N3− substitution compensated by O vacancies is estimated at 3–4% TN enhancement per 1% O → N substitution. A 1% compressive (0 0 1) epitaxial strain enhances TN by about 6%.

Coupled structural and magnetic properties of ferric fluoride nanostructures: Part II, a Monte Carlo–Heisenberg study

We present a numerical study of the magnetic structure of nanostructured iron fluoride, using the Monte Carlo Metropolis simulated annealing technique and a classical Heisenberg Hamiltonian with superexchange angle dependent interactions. The parameters are adjusted on experimental results, and the atomic structure and topology taken from a previous atomistic model of grain boundaries in the same system. We find perfect antiferromagnetic crystalline grains and a disordered magnetic configuration (speromagnetic) at the grain boundary, in agreement with experimental features. Both the lowest magnetic energy and the rate of magnetic frustration are found to be dependent on the relative disorientation of crystalline grains, i.e. on the cationic topology. We conclude on possible extensions of the model.

Entanglement of charge transfer, hole doping, exchange interaction, and octahedron tilting angle and their influence on the conductivity of La1−xSrxFe0.75Ni0.25O3−δ: A combination of x-ray spectroscopy and diffraction

Substitution of La by Sr in the 25% Ni doped charge transfer insulator LaFeO3 leads to structural changes that inflect the electrical conductivity, which is caused by small polaron hopping via charge transfer and exchange interactions. The substitution forms electron holes and causes a structural transition from orthorhombic to rhombohedral symmetry, and then to cubic symmetry. The structural crossover is accompanied by a crossover from the Fe3+–O2−–Fe3+ superexchange interaction to the Fe3+–O2−–Fe4+ double exchange interaction in the course of substitution, as evidenced by a considerable increase in the conductivity at ambient temperature. The charge transfer and exchange interactions depend on the superexchange angle, which approaches 180° upon increasing Sr concentration. An increase in superexchange angle leads to an increase in overlapping between the O 2p and the Fe/Ni 3d orbitals.

Antiferromagnetic dimers of Ni(II) in theS=1spin-ladderNa2Ni2(C2O4)3(H2O)2

We report the synthesis, crystal structure, and magnetic properties of the $S=1$ 2-leg spin-ladder compound ${\mathrm{Na}}_{2}{\mathrm{Ni}}_{2}{({\mathrm{C}}_{2}{\mathrm{O}}_{4})}_{3}{({\mathrm{H}}_{2}\mathrm{O})}_{2}$. The magnetic properties were examined by magnetic susceptibility and pulsed high field magnetization measurements. The magnetic excitations have been measured in high field high frequency ESR. Although the Ni(II) ions form structurally a 2-leg ladder, an isolated dimer model consistently describes the observations very well. The analysis of the temperature dependent magnetization data leads to a magnetic exchange constant of $J=43\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ along the rungs of the ladder and an average value of the $g$-factor of 2.25. From the ESR measurements, we determined the single ion anisotropy to $D=11.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The validity of the isolated dimer model is supported by quantum Monte Carlo calculations, performed for several ratios of interdimer versus intradimer magnetic exchange and taking into account the experimentally determined single ion anisotropy. The results can be understood in terms of the different coordination and superexchange angles of the oxalate ligands along the rungs and legs of the 2-leg spin ladder.

Local atomic and electronic structure inLaMnO3across the orbital ordering transition

The local atomic disorder and the electronic structure in the environment of manganese atoms in LaMnO_3 has been studied by x-ray absorption spectroscopy, over a temperature range (300K to 870K) covering the orbital ordering transition (710K). The Mn-O distances splitting into short and long bonds (1.95 and 2.15A) is kept across the transition temperature, so that the MnO_6 octahedra remain locally Jahn-Teller distorted. Discontinuities in the Mn local structure are identified in the extended x-ray fine structure spectra at this temperature, associated to a reduction of the disorder in the super-exchange angle and to the removal of the anisotropy in the radial disorder within the coordination shell. Subtle changes in the electronic local structure also take place at the Mn site at the transition temperature. The near edge spectra show a small drop of the Mn 4p hole count and a small enhancement in the pre-edge structures at the transition temperature. These features are associated to an increase of the covalence of the Mn-O bonds. Our results shed light on the local electronic and structural phenomena in a model of order-disorder transition, where the cooperative distortion is overcome by the thermal disorder.

Geometry optimization and electronic structure of BaCoO 3

Geometry optimization and electronic band structure calculations were performed on pseudo-perovskite BaCoO 3. A drastic change in electronic structure is obtained by varying the geometry (changing the superexchange angle Co–O–Co and Co–O distances). Calculations show the key importance of the conditions of anion–cation interactions in electrical conduction properties of the material.

Spin-glass behavior in a three-dimensional antiferromagnet ordered phase: Magnetic structure ofCo2(OH)(PO4)

${\mathrm{Co}}_{2}(\mathrm{OH})({\mathrm{PO}}_{4})$ has been prepared from hydrothermal synthesis and characterized from powder x-ray diffraction. The nuclear and magnetic structures have been determined by neutron (D2B and D1B) diffraction data. The structure consists of a three-dimensional framework in which $\mathrm{Co}(1){\mathrm{O}}_{5}$-trigonal bipyramid dimers and $\mathrm{Co}(2){\mathrm{O}}_{6}$-octahedra chains are simultaneously present. The EPR spectrum of ${\mathrm{Zn}}_{2}(\mathrm{OH})({\mathrm{PO}}_{4}):0.1%\mathrm{Co}$ at 4.2 K shows a strong anisotropy of the g factor. The values obtained for the g tensor and the hyperfine coupling constants for the octahedral symmetry were ${g}_{1}=5.890,$ ${g}_{2}=4.550,$ and ${g}_{3}=2.021$ and ${A}_{1}=240\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{cm}}^{\ensuremath{-}1},$ ${A}_{2}=155\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{cm}}^{\ensuremath{-}1},$ and ${A}_{3}=85\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}{\mathrm{cm}}^{\ensuremath{-}1}.$ Signals corresponding to the five-coordinated Co(II) ions were also observed. Magnetization measurements show the presence of two maxima at circa 75 and 15 K, respectively. The first peak was attributed to a three-dimensional antiferromagnetic ordering and the second one reveals the existence of a spin-glass-like state. This state with a cooperative freezing was also confirmed by both ac susceptibility measurements and magnetic irreversibility observed in the zero-field-cooled--field-cooled signals. From low-temperature neutron-diffraction data, antiferromagnetic ordering is established with an ordering temperature of 71 K. The propagation vector of the magnetic structure is $k=[0,0,0].$ The magnetic moments at 1.7 K are ferromagnetically coupled between ${\mathrm{CoO}}_{6}$-octahedra chains and the ${\mathrm{Co}}_{2}{\mathrm{O}}_{10}$ dimers in the z direction. The values obtained for the magnetic moments are: $3.39(7){\ensuremath{\mu}}_{B}$ [Co(1)] and $3.84(5){\ensuremath{\mu}}_{B}$ [Co(2)]. The absence of any anomaly in both the specific heat and thermal evolution of the magnetic moments below \ensuremath{\sim}20 K confirms the blocking process of a spin glass behavior. The crystal-field splitting of the ${\mathrm{Co}}^{2+}$ ions causes a single ion anisotropy along the z (c-axis) direction, giving an Ising character in which the local spins from the Co(1) dimers are frozen. A magnetic frustration in the Co(1) magnetic moments is observed as due to the presence of antiferromagnetic interactions between Co(2) neighbor chains. It is to note the existence of a $\mathrm{Co}(1)\ensuremath{-}\mathrm{O}(3)({\mathrm{PO}}_{3})\ensuremath{-}\mathrm{Co}(2)$ superexchange angle with a value of 107\ifmmode^\circ\else\textdegree\fi{} that involves ferromagnetic couplings between chain and dimer neighbors ferromagnetically coupled. This exchange pathway together with the anisotropy and frustration could be the responsible of the spin glass behavior observed in the three-dimensional antiferromagnetic ${\mathrm{Co}}_{2}(\mathrm{OH})({\mathrm{PO}}_{4})$ ordered phase.

Structural Distortion and Chemical Bonding in TlFeO3: Comparison with AFeO3 (A=Rare Earth)

The crystal structure and the magnetic properties for thallium orthoferrite, TlFeO3, were characterized and compared with those of rare earth orthoferrites, AFeO3 (A=rare earth). TlFeO3 has a GdFeO3-type perovskite structure (a=5.3172(2) Å, b=5.4465(2) Å, and c=7.7927(3) Å; space group, Pbnm). Its cell parameters and unit-cell volume considerably deviate from a trend observed in the rare earth orthoferrites, which are attributed to a specific coordination of Tl3+ ion. According to magnetization measurements, TlFeO3 shows an antiferromagnetic behavior accompanied with weak ferromagnetism. The magnetic ordering temperature, TN (560 K), for TlFeO3 is lower than that for any other orthoferrite while the Fe–O–Fe superexchange angle (144.0°) in TlFeO3 is comparable to that in ErFeO3. The Mössbauer spectrum reveals a single Fe3+ quadrupole doublet with an isomer shift δ=0.18±0.02 mm/s and a quadrupole splitting value Δ=0.47±0.02 mm/s at 623 K (>TN), whereas it shows a magnetic hyperfine sextet with δ=0.47mm/s and a hyperfine field Hhf(Fe3+)=535 kOe at 80 K (<TN). The magnetism and the Mössbauer results underline a significant competing effect of the Tl–O bond with the covalency of the Fe–O one. Despite the anomalous character of TlFeO3 in structure and magnetism, it is found that the TN values for all orthoferrites, including TlFeO3, are strongly correlated with their crystallographic axial ratio, c/√2a. Through this type of analysis, the electronic effects of A cation on structural distortion and magnetic behavior in perovskite are discussed.

Manganese reactivity in the synthesis of magnetoresisting complex oxides

To study the superexchange interactions in ‘giant magnetoresistors’–manganites with perovskite structure, diluted solid solutions of La0.67Ca0.33MnO3–LaAlO3 were prepared on the basis of MnO2 and Mn2O3. Examination of temperature and concentration dependencies of paramagnetic susceptibility and effective magnetic moment showed a strong tendency of manganese atoms to clustering in the solutions. The clusters are linked by ferromagnetic superexchange with an exchange parameter varying with temperature, which correlates with the unusual electrophysical properties of pure oxides and can be accounted for by varying the superexchange angles Mn–O–Mn with temperature.

Noncollinear antiferromagnetic structure of the molecule-based magnetMn[N(CN)2]2

The crystallographic and magnetic properties of the $\mathrm{Mn}[\mathrm{N}(\mathrm{CN}{)}_{2}{]}_{2}$ compound have been investigated by dc magnetization, ac susceptibility, specific heat, and zero-field neutron diffraction on polycrystalline samples. The magnetic structure consists of two sublattices which are antiferromagnetically coupled and spontaneously canted. The spin orientation is mainly along the a axis with a small uncompensated moment along the b axis. The ground state is a crystal-field sextet with large magnetic anisotropy. The crystal structure consists of discrete octahedra which are axially elongated and successively tilted in the $\mathrm{ab}$ plane. Comparisons of the magnetic structures for the isostructural $M{[\mathrm{N}(\mathrm{CN})}_{2}$]${}_{2}$ $(M=\mathrm{Mn},$ Fe, Co, Ni) series suggest that the spin direction is stabilized by crystal fields and the spin canting is induced by the successive tilting of the octahedra. We propose that the superexchange interaction is the mechanism responsible for the magnetic ordering in these compounds and we find that a crossover from noncollinear antiferromagnetism to collinear ferromagnetism occurs for a superexchange angle of ${\ensuremath{\alpha}}_{c}=142.0(5)\ifmmode^\circ\else\textdegree\fi{}.$

Neutron-diffraction study of the magnetic and orbital ordering in154SmNiO3and153EuNiO3

Neutron-powder-diffraction experiments on ${}^{154}{\mathrm{SmNiO}}_{3}$ and ${}^{153}{\mathrm{EuNiO}}_{3}$ at different temperatures have been performed. At variance with the case of Pr and Nd nickelates, the metal-insulator transition temperature in these compounds are different from the N\'eel temperature (${T}_{M\ensuremath{-}I}\ensuremath{\approx}400\mathrm{K},$ ${T}_{N}\ensuremath{\approx}230\mathrm{K}$ for Sm and ${T}_{M\ensuremath{-}I}\ensuremath{\approx}480\mathrm{K},$ ${T}_{N}\ensuremath{\approx}220\mathrm{K}$ for Eu) so that the magnetic ordering develops upon cooling in an insulating matrix. With decreasing temperature the crystal structure undergoes a 0.15% lattice expansion at ${T}_{M\ensuremath{-}I}.$ The crystal symmetry $\mathrm{Pbnm}$ does not change within the experimental error, but the Ni-O average distance increases and the superexchange angle Ni-O-Ni decreases slightly in the insulating regime. The magnetic structure of both compounds has the same propagation vector $\mathbf{k}=(\frac{1}{2},0,\frac{1}{2})$ as observed in Pr and Nd nickelates. This feature leads us to speculate that the orbital ordering responsible for the magnetic structure is a characteristic of the insulating state of the $R{\mathrm{NiO}}_{3}$ series (at least for light rare earths $R$). A search for small nuclear superstructure peaks characterizing the orbital ordering has been carried out without success. The consequences of our experimental results are discussed in the context of current ideas about superexchange in oxides.