Cr Sublattices Research Articles

The B-site ordering in RFe0.5Cr0.5O3 ceramics and its effect on magnetic properties

Abstract To insight into the B-site ordering in RFe0.5Cr0.5O3 ceramics, a series of RFe0.5Cr0.5O3 ceramics (R = La, Y, Lu) were synthesized by the sol–gel method, and the structural and magnetic properties were systemically investigated. By using the Rietveld refinement of all samples, it is found that the structural distortion is increased as the R ionic radius decreases, leading to the weakened interactions between Fe/Cr ions. Moreover, the Fe and Cr are arranged in disorder in LaFe0.5Cr0.5O3, but partially ordered in YFe0.5Cr0.5O3 and LuFe0.5Cr0.5O3, showing an increasing trend of the proportion of ordered domains with the decrease of R ionic radius. Through fitting the temperature-dependent magnetizations, it is identified that the magnetization reversal (MR) in disorder LaFe0.5Cr0.5O3 is resulted from the competition between the moments of Cr and Fe sublattices. In the partially ordered YFe0.5Cr0.5O3 and LuFe0.5Cr0.5O3 ceramics, because of the presence of Fe–O–Cr networks in the ordered domains whose moment is antiparallel to that of Fe–O–Fe and Cr–O–Cr in the disordered domains, the compensation temperature T comp of MR is increased by nearly 50 K. These results suggest that the changing of R-site ions could be used very effectively to modify the Fe–O–Cr ordering, apart from the structural distortion, which has a direct effect on the magnetic exchange interactions in RFe0.5Cr0.5O3 ceramics. Then at values of composition where ordered domains are expected to be larger in number as compared to disordered domains and with a weaker structural distortion, one can expect a higher transition temperature T comp, providing a different view for adjustment of the magnetic properties of RFe0.5Cr0.5O3 ceramics for practical applications.

Temperature assisted size dependent synthesis and magnetic properties of rare-earth chromium oxide nanoparticles

We report the synthesis of homogeneous single-phase nanoparticles of RCrO4 and RCrO3 (R = Sm, Gd, Dy, and Er) compounds using a modified sol–gel hydrothermal method followed by heat treatment. Annealing of the as-prepared powder sample at ambient pressure enables chromium to remain in the metastable Cr5+ (4s03d1) state by crystallizing into a zircon-type tetragonal RCrO4 (S.G. I41/amd, D4h19 symmetry) structure at 773 K, while it chooses to form orthorhombic RCrO3 (S.G. Pbnm, D4h19 symmetry) structure with a stable Cr3+ (4s03d3) state at 973 K. The analysis of DC magnetization (M) versus temperature (T) in an external magnetic field, H = 100 Oe for polycrystalline RCrO4 indicates the magnetic transition temperatures, TC ∼ 20 K. The derivative of zero-field cooled magnetic susceptibility with respect to T, dχZFCdT, versus T, indicates the presence of competing magnetic interactions due to the ferromagnetic (FM) and antiferromagnetic (AFM) ordering of Cr5+ and R3+ in RCrO4 respectively near magnetic transition. This feature specifies magnetic frustration that leads to metamagnetism in RCrO4. The fit of non-linear magnetic susceptibility, χ=MH, to χ=χ0+CT-θ, with a non-zero χ0 determined from high-T data gives effective magnetic moment, μeff, which is in accordance with the theoretical value. The analysis of M vs. T indicates that RCrO3 undergoes a paramagnetic (PM) to canted-antiferromagnetic (CAFM) transition of Cr sublattices at Néel temperature (TNCr) because of antisymmetric Dzyaloshinskii-Moriya (DM) interaction due to Cr–O–Cr superexchange with a possible antiferromagnetic (AFM) ordering of the rare-earth at low temperature (TNR < 20 K). The linear fit of χ-1 vs. T with a zero χ0 shows Curie-Weiss law is more appropriate at T>TC resulting expected μeff values for RCrO3.

Proximate Quantum Spin Liquid on Designer Lattice.

Complementary to bulk synthesis, here we propose a designer lattice with extremely high magnetic frustration and demonstrate the possible realization of a quantum spin liquid state from both experiments and theoretical calculations. In an ultrathin (111) CoCr2O4 slice composed of three triangular and one kagome cation planes, the absence of a spin ordering or freezing transition is demonstrated down to 0.03 K, in the presence of strong antiferromagnetic correlations in the energy scale of 30 K between Co and Cr sublattices, leading to the frustration factor of ∼1000. Persisting spin fluctuations are observed at low temperatures via low-energy muon spin relaxation. Our calculations further demonstrate the emergence of highly degenerate magnetic ground states at the 0 K limit, due to the competition among multiply altered exchange interactions. These results collectively indicate the realization of a proximate quantum spin liquid state on the synthetic lattice.

Crystal and magnetic structure of polar oxide HoCrWO6

Polar magnetic oxide HoCrWO6 is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO6 forms the polar crystal structure (space group Pna21 (#33)) due to the cation ordering of W6+ and Cr3+. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.−1K−1 at 70 kOe). Neutron diffraction measurement indicates that both Cr and Ho sublattices are ordered with the moment of 2.32(5)μB and 8.7(4)μB at 2 K, respectively. While Cr forms A-type collinear antiferromagnetic (AFM) structure with magnetic moment along the b axis, Ho sublattice orders in a non-coplanar AFM arrangement. A comparison with isostructural DyFeWO6 and DyCrWO6 indicates that the magnetic structure of this family of compounds is controlled by the presence or absence of eg electrons in the transition metal sublattice.

Decoupling of Gd–Cr magnetism and giant magnetocaloric effect in layered honeycomb tellurate GdCrTeO6

We report the decoupling of Gd–Cr magnetism and the resulting giant magnetocaloric effect in honeycomb-lattice tellurate GdCrTeO6. This compound adopts the hexagonal structure with the space group P-3. The susceptibility curve presents a paramagnetic behavior from room temperature to 2 K. Very intriguingly, however, the heat capacity measurement probes a λ-like peak at TN = 10 K, characterizing the long-range antiferromagnetic order. This suggests that the Gd and Cr sublattices are magnetically independent, i.e., the Gd ions are paramagnetic and the strong paramagnetism nearly masks the antiferromagnetic order of Cr ions, which is further evidenced by our ESR data. The maximum magnetic entropy change at 2 K is 41.8 J kg−1 K−1 in a field range of 0–7 T and 30.3 J kg−1 K−1 for 0–3 T, larger than those of most of the Gd oxides reported. The observed giant magnetocaloric effect is due to strong paramagnetism of Gd ions.

Non-Heisenberg magnetism in a quaternary spin-gapless semiconductor

Understanding of spin-gapless semiconductors with fully spin-polarized charge carriers is critically important because of their promise for spintronic applications. Here, we report non-collinear spin structures, magnetic ground state, and effective exchange interactions of the spin-gapless semiconductor CoFeCrAl investigated with noncollinear density functional calculations. The ground state of CoFeCrAl is ferrimagnetic and has a spin configuration with ↓ Fe, ↑ Co and ↑ Cr spins. In our constrained calculations, the magnetizations of the Fe, Co, and Cr sublattices are rotated by various angles θ, which give rise to three sets of noncollinear spin structures. For all three elements, the magnetic energy increases with the angle, which reconfirms the ferrimagnetic spin structure. During rotation, the magnitudes of the Co and Cr spins remain almost unchanged, whereas that of Fe strongly decreases as a function of the angle θ. This indicates that the finite-temperature behavior of CoFeCrAl is characterized by a pronounced non-Heisenberg behavior of the ↓ Fe moments, whereas the ↑ Co and ↑ Cr moments are Heisenberg-like. We discuss how this feature affects the finite-temperature behavior of the alloy beyond the commonly considered intersublattice Heisenberg exchange.

Emergent Magnetic State in (111)-Oriented Quasi-Two-Dimensional Spinel Oxides.

We report on the emergent magnetic state of (111)-oriented CoCr2O4 ultrathin films sandwiched between Al2O3 spacer layers in a quantum confined geometry. At the two-dimensional crossover, polarized neutron reflectometry reveals an anomalous enhancement of the total magnetization compared to the bulk value. Synchrotron X-ray magnetic circular dichroism measurements demonstrate the appearance of a long-range ferromagnetic ordering of spins on both Co and Cr sublattices. Brillouin function analyses and ab-initio density functional theory calculations further corroborate that the observed phenomena are due to the strongly altered magnetic frustration invoked by quantum confinement effects, manifested by the onset of a Yafet-Kittel-type ordering as the magnetic ground state in the ultrathin limit, which is unattainable in the bulk.

Spin switching and unusual exchange bias in the single-crystalline GdCrO3 compensated ferrimagnet

An unusual exchange bias (EB) effect in single-crystalline $\mathrm{GdCr}{\mathrm{O}}_{3}$ compensated ferrimagnet (fM), composed of two antiferromagnetically coupled Gd and Cr sublattices, presenting two opposite ferromagnetic (FM) moments, is reported. It is shown that the temperature- or field-induced fast reversal of the net FM moment in $\mathrm{GdCr}{\mathrm{O}}_{3}$ may be identified as a switching between two opposite spin configurations, attendant with abrupt drop in Zeeman energy $\ensuremath{\approx}{k}_{\mathrm{B}}(7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ per formula unit. It was found that the FM moment reversal is exchange biased in the narrow temperature interval above the compensation temperature ${T}_{\mathrm{comp}}=144\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Namely, the EB field emerges and diverges upon approaching ${T}_{\mathrm{comp}}$ at temperatures $Tg{T}_{\mathrm{comp}}$ while unexpectedly it collapses to zero below ${T}_{\mathrm{comp}}$. This is in contrast to experimental results obtained for compensated fMs $R\mathrm{Fe}{\mathrm{O}}_{3}\phantom{\rule{4pt}{0ex}}(R=\mathrm{Nd}$, Sm, Er) orthoferrites showing EB in a narrow range, both above and below ${T}_{\mathrm{comp}}$. We suppose that breakdown of EB in $\mathrm{GdCr}{\mathrm{O}}_{3}$ may be linked to the lack of anisotropy of the spin-only Gd $S$-ion magnetic moment dominating below ${T}_{\mathrm{comp}}$.

Room-Temperature Ferromagnetism in Mn-Doped CuCrO&lt;sub&gt;2&lt;/sub&gt; Nanopowders

(Cu1-xMnx)CrO2 (0≤ x ≤6 at%) and Cu (Cr1-yMny)O2 (0≤ y ≤6 at%) nanopowders were prepared by combining solid-state reaction and ball milling. The Mn concentration dependences of microstructure, morphology and magnetic properties were investigated. It is found that all the samples have a pure 3R-CuCrO2 delafossite structure. The lattice expansion supports the Mn entrance into the Cu and Cr sublattices, respectively, in (Cu1-xMnx)CrO2 and Cu (Cr1-yMny)O2, which is further proved by X-ray photoelectron spectroscopy to some degree. A-site Mn substitution brings about paramagnetic behavior. However, room-temperature ferromagnetism is achieved in B-site Mn-doped samples, originating from the hole-mediated Cr3+–Mn3+ double-exchange interaction. The saturation magnetization of this CuMO2 delafossite (M=Cr, Mn) is about an order of magnitude higher than literature values, and gradually decreased with the Mn addition due to the combined influence of the number of the M–M pairs, the M–M distances and the hole density.

Ferroelectricity driven by soft phonon and spin order in multiferroic BiMn3Cr4O12

AbstractBiMn3Cr4O12 shows an unusual joint multiferroicity, which facilitates the coexistence of considerable ferroelectric polarization and remarkable magnetoelectric coupling in a single‐phase multiferroic material. Based on first‐principles calculations, we investigate the two different types of ferroelectric phase transitions in the BiMn3Cr4O12 material. Our results show that the first ferroelectric phase transition is driven by soft mode and leads BiMn3Cr4O12 into the Cm space group. The predicted ferroelectric polarization in single crystal is about ~9.8 μC/cm2. With the emergence of spin order on both Mn and Cr sublattices, it is the polar Cm structure that triggers the exchange striction mechanism and therefore results in a large type‐II multiferroicity (~1.1 μC/cm2). In addition, the intrinsic direction of the spin‐driven ferroelectric polarization is always opposite to that of the existing Cm phase structure. Our results imply a feasible strategy in searching/designing novel type‐II multiferroics with large ferroelectric polarization.

Magnetically ordered and disordered sublattices in geometrically frustrated Ni chromite

The magnetic order/disorder of the Cr moments in the antiferromagnetic state of the ferrimagnet $\mathrm{NiC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ is reinvestigated by neutron powder diffraction to clarify the observation of reduced Cr moments located on a pyrochlore-like lattice [M. Reehuis et al., Phys. Rev. B 91, 024407 (2015)]. The change of the spin structure in the slightly Cu-doped chromite $\mathrm{N}{\mathrm{i}}_{0.98}\mathrm{C}{\mathrm{u}}_{0.02}\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ has been studied as well, where orthorhombic lattice distortions become stronger. For both chromites we observe at 2 K two magnetic phases: one with two propagation vectors $\mathbit{k}=$ (0,0,1) and (0,0,${k}_{z}$), and a second one with $\mathbit{k}=$ ($1/2,1/2,1/2$). The latter phase disappears at 21 K for $\mathrm{NiC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ and at 24 K for $\mathrm{N}{\mathrm{i}}_{0.98}\mathrm{C}{\mathrm{u}}_{0.02}\mathrm{C}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$. Data analysis shows that the two phases reside on two disparate Cr sublattices. These are identified by different chain directions of strongly bonded spin pairs along diagonals in the pseudotetragonal ${a}_{1}{a}_{2}$ plane. The less distorted $\mathrm{NiC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ shows the same subdivision of the lattice still with a saturated Cr moment for the (0,0,1)/(0,0,${k}_{z}$) phase, however, with a strongly reduced moment for the ($1/2,1/2,1/2$) phase, whereas an almost saturated moment shows up in the Cu-doped chromite. The reduced moment in $\mathrm{NiC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ indicates disorder on one Cr sublattice down to low temperature due to stronger frustration on the less distorted tetragonal pyrochlore lattice. Magnetoelastic effects and further neighbor spin interactions are mainly considered in the discussion of the observed peculiar ordering processes. Notable is the transition temperature of the ($1/2,1/2,1/2$) phase of $\mathrm{NiC}{\mathrm{r}}_{2}{\mathrm{O}}_{4}$ around 21 K, where anomalies for the specific heat and for the magnetodielectric behavior have recently been reported [T. D. Sparks et al., Phys. Rev. B 89, 024405 (2014)].

Magnetic ordering in an (Fe0.2Cr0.8)1.5[Cr(CN)6] Prussian blue analogue studied with synchrotron radiation based spectroscopies

CrII HS–CrIII pairs and misfit between the Fe and Cr sublattices control ferromagnetic order in the FeCrCr Prussian blue analogue.

Exchange couplings calculation in A2CrMoO6(A= Sr, Ca) Double Perovskites

Exchange couplings in A2CrMoO6 (A = Sr, Ca) Double Perovskites have been calculated for the first time by using a Monte Carlo Simulation and a relation between internal energy per site and magnetization per site. This work shows how effects of A-site substitution could be captured by an overall scaling factor 'alpha', which allows to deduce a numerical estimation of exchange couplings in question. Thermal dependence of magnetization per site, magnetic susceptibility, internal energy per site and specific heat have been investigated. Super Exchange coupling has been found equal to −7,488meV in the presence of Sr and −1,977meV in the presence of Ca. Double Exchange couplings corresponding respectively to Cr and Mo sublattices have been found equal to 17.473meV and 5.611meV in the presence of Sr and 4.614meV and 1.481meV in the presence of Ca. Internal energy per site for both materials at low temperatures has been given.

Competing exchange couplings in Sr2CrReO6 Double Perovskite

Super Exchange (SE) coupling and Double Exchange (DE) couplings in the Double Perovskite (DP) Sr2CrReO6 have been calculated by using a new approximation method, proposed in this paper, based on a relation between internal energy per site and magnetization per site. Monte Carlo simulation has been performed, as computational method in the framework of Ising model, to find the exchange couplings that verify the experimental Curie temperature measured under an external field h=1 T. SE exchange coupling has been found equal to −19.992 meV and the DE couplings corresponding respectively to Cr and Re sublattices have been found equal to 18.334 meV and 18.741 meV. Internal energy per site has been calculated.

Physical properties and electronic structure ofSr2Cr3As2O2containingCrO2andCr2As2square-planar lattices

We report the physical properties and electronic structure calculations of a layered chromium oxypnictide, ${\mathrm{Sr}}_{2}{\mathrm{Cr}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{2}$, which crystallizes in a ${\mathrm{Sr}}_{2}{\mathrm{Mn}}_{3}{\mathrm{As}}_{2}{\mathrm{O}}_{2}$-type structure containing both ${\mathrm{CrO}}_{2}$ planes and ${\mathrm{Cr}}_{2}{\mathrm{As}}_{2}$ layers. The newly synthesized material exhibits a metallic conduction with a dominant electron-magnon scattering. Magnetic and specific-heat measurements indicate at least two intrinsic magnetic transitions below room temperature. One is an antiferromagnetic transition at 291 K, probably associated with a spin ordering in the ${\mathrm{Cr}}_{2}{\mathrm{As}}_{2}$ layers. Another transition is broad, occurring at around 38 K, and possibly due to a short-range spin order in the ${\mathrm{CrO}}_{2}$ planes. Our first-principles calculations indicate predominant two-dimensional antiferromagnetic exchange couplings, and suggest a KG-type [i.e., ${\mathrm{K}}_{2}{\mathrm{NiF}}_{4}$ (abbreviated as ``K'') type in the ${\mathrm{CrO}}_{2}$ sublattice and N\'eel (usually denoted by ``G'') type in the ${\mathrm{Cr}}_{2}{\mathrm{As}}_{2}$ sublattice] magnetic structure, with reduced moments for both Cr sublattices. The corresponding electronic states near the Fermi energy are mostly contributed from $\mathrm{Cr}\ensuremath{-}3d$ orbitals which weakly (modestly) hybridize with the $\mathrm{O}\ensuremath{-}2p$ ($\mathrm{As}\ensuremath{-}4p$) orbitals in the ${\mathrm{CrO}}_{2}$ (${\mathrm{Cr}}_{2}{\mathrm{As}}_{2}$) layers. The bare band structure density of states at the Fermi level is only $\ensuremath{\sim}1/4$ of the experimental value derived from the low-temperature specific-heat data, consistent with the remarkable electron-magnon coupling. The title compound is argued to be a possible candidate to host superconductivity.

Prediction of the site ordering behaviours of elements in C15 NbCr2-based intermetallics by combining thermodynamic model with ab-initio calculation

The ordering behaviours of the transition metal elements in NbCr2-based alloys were predicted by combining thermodynamic model with ab-initio calculation. The predicted results show that for the ternary alloying elements in C15-type NbCr2-based alloy 64Cr–32Nb–4M (M = Cr, Hf, Mn, Mo, Nb, Ta, Ti, V, W or Zr), Mn atoms occupy only Cr sublattices (16d) and Hf, Ta, Ti and Zr atoms always prefer to occupy the Nb sublattices (8a). It is also found that the site preference of alloying elements Hf, Ta, Ti, Zr, or Mn are independent of the xCr/xNb (atomic ratio) and heat treatment temperature, while the site preferences of V, Mo or W depend on those. Present predicted results agree considerably with the available literatures, and some disagreements were also argued in this paper, i.e., in some literatures, alloying element V atoms always prefer to 16d sublattices and Ti atoms vary strongly with the alloy composition (xCr/xNb) and heat treatment temperature.

Neutron diffraction study of NdCrGe3

The hexagonal perovskite (BaNiO3-type) structure of NdCrGe3 and its ferromagnetic order at TC=122K have been confirmed by neutron powder diffraction measurements. The magnetic structure is characterized by an axial ferromagnetic arrangement of both the Cr and Nd sublattices below TC. The relatively high Curie temperature of this compound is mainly driven by the magnetic ordering of the Cr sublattice with Nd–Cr magnetic interactions also playing a role. The magnetic ordering of the Nd sublattice, deduced from the thermal variation of the magnetic moments, occurs at around T≈70K. The absence of a spin reorientation of the Nd and Cr magnetic order allows us to conclude that the complex magnetic behaviour previously observed at low temperatures in macroscopic magnetic measurements is a consequence of strong magnetocrystalline anisotropy effects.

Optical bandgap modulation and magnetic characterization of Fe-doped CuCrO2 nanopowders

Using p-type transparent conducting CuCrO2 as a host material, (Cu1−xFex)CrO2 (0≤x≤6at.%) and Cu(Cr1−yFey)O2 (0≤y≤6at.%) nanocrystals were prepared by combining solid-state reaction and ball milling. The microstructure, morphology, optical and magnetic properties were characterized by X-ray diffraction, atomic force microscopy, diffuse reflectance spectroscopy and vibrating sample magnetometer. It is found that all the samples have a single CuCrO2 delafossite structure. The variations in lattice constants support the Fe entrance into the Cu and Cr sublattices, respectively, in (Cu1−xFex)CrO2 and Cu(Cr1−yFey)O2. The gradual increase of optical bandgap with the Fe content and the low-temperature ferromagnetism are observed in two series of samples. The present work indicates that Fe-doped CuCrO2 is a dilute magnetic semiconductor and the Curie temperature can be expected to enhance by the optical bandgap modulation.

Photovoltaic properties of Bi2FeCrO6 epitaxial thin films

We report a large photovoltaic (PV) effect in multiferroic Bi2FeCrO6 (BFCO) films under monochromatic illumination at 635 nm with an intensity of 1.5 mW cm−2. These multiferroic films exhibit a large photocurrent at zero bias voltage and an open-circuit voltage of about 0.6 V. A high PV power conversion efficiency of about 6% for red light is achieved and attributed to a high degree of B-site cationic ordering between Fe and Cr sublattices, the tuning of which is likely to play a key role in further improvements of the PV properties in BFCO.

Magnetic properties and neutron diffraction study of (NixMnl-x)3[Cr(CN)6]2 molecule-based magnets

Neutron diffraction measurements were performed on (NixMn1-x)3[Cr(CN)6]2·zD2O (x = 0, 0.38 and 1) powders above and below the Curie temperature TC. The crystal structure, including D-sites, was fully described. Neutron diffraction experiments revealed magnetic contribution for all compounds. Magnetization measurements and elastic neutron diffraction of Mn3[Cr(CN)6]2 indicate magnetic structure consisting of Mn and Cr sublattices with antiparallel magnetic moments μMN = 3.790 μB and μCr = -1.375 μB leading to overall ferromagnetic ordering below the Curie temperature TC = 64.5 K. Very weak magnetic contribution was observed on investigated Ni3[Cr(CN)6]2, and (Ni0.38Mn0.62)3[Cr(CN)6]2 powders at 1.6 K and at 60 K respectively. Our results are compared and discussed with magnetization measurements of M(T) and heat capacity measurements C(T) in zero magnetic field and magnetic fields up to μ0H = 5 T.