Collinear Antiferromagnetic Structure Research Articles

Origin of enhanced multiferroic properties in Dy and Co co-doped BiFeO3 ceramics

Structural, magnetic and ferroelectric properties of polycrystalline BiFeO3, Bi0.9Dy0.1FeO3, BiFe0.97Co0.03O3 and Bi0.9Dy0.1Fe0.97Co0.03O3, which were prepared by solid-state method, have been investigated. The X-ray diffraction (XRD) patterns reveal that all the samples are in single phase and show rhombohedrally distorted perovskite structure with R3c space group. Both XRD and Raman-scattering studies show that Dy and Co co-doped sample has a compressive lattice distortion induced by co-substitution of Dy and Co ions at the A and the B sites, respectively. Dy and Co co-doping favors weak ferromagnetism ordering with evident magnetic hysteresis loop and enhances magnetization values at room temperature. Ferroelectric hysteresis loop for Dy and Co co-doped sample shows the nearly saturated polarization at 40 kV/cm and large remnant polarization. Dy dopant is prominent in the reduced leakage current density, while Co dopant is remarkable in the improved remnant magnetization in Bi0.9Dy0.1Fe0.97Co0.03O3 ceramic. By using a simple model, it was found that the anharmonicity of canted spiral cycloidal spin structure was responsible for the weak ferromagnetism of pure BiFeO3, and the enhanced magnetization in Co-doped sample was attributed to the transition from the incommensurate cycloidal spiral spin structure towards the G-type canted collinear antiferromagnetic structure.

Giant gigahertz optical activity in multiferroic ferroborate

In contrast to well studied multiferroic manganites with a spiral structure, the electric polarization in multiferroic borates is induced within collinear antiferromagnetic structure and can easily be switched by small static fields. Because of specific symmetry conditions, static and dynamic properties in borates are directly connected, which leads to giant magnetoelectric and magnetodielectric effects. Here we prove experimentally that the giant magnetodielectric effect in samarium ferroborate SmFe3(BO3)4 is of intrinsic origin and is caused by an unusually large electromagnon situated in the microwave range. This electromagnon reveals strong optical activity exceeding 120 degrees of polarization rotation in a millimeter thick sample.

A multifunctional magnetic material under pressure.

Fe(II)(Metz)6](Fe(III)Br4)2 (Metz = 1-methyltetrazole) is one of the rare systems combining spin-crossover and long-range magnetic ordering. A joint neutron and X-ray diffraction and magnetometry study allows determining its collinear antiferromagnetic structure, and shows an increase of the Néel temperature from 2.4 K at ambient pressure, to 3.9 K at 0.95 GPa. Applied pressure also enables a full high-spin to low-spin switch at ambient temperature.

Mössbauer Study of the Ferroelectric State in Ga-Substituted CuFeO2

57Fe Mossbauer spectroscopic measurements have been conducted on CuFe1−xGaxO2 single crystals, in order to investigate the magnetic structures and hyperfine parameters in ferroelectric CuFe0.965Ga0.035O2 and paraelectric CuFeO2. We confirmed that the Mossbauer spectrum at 4.2 K in CuFeO2 is explained by the collinear antiferromagnetic structure with spins along the hexagonal c-axis. In the ferroelectric phase in CuFe0.965Ga0.035O2, the spectrum consists of three magnetic components, where magnetic hyperfine fields distribute in three different directions within the (110) plane. This configuration is not completely consistent with a simple proper screw structure with a uniform spin distribution. The quadrupole coupling constants e2qQ/2 are unexpectedly large for both specimens. In the ferroelectric state in CuFe0.965Ga0.035O2, e2qQ/2 becomes noticeably smaller than that in the paraelectric phase in CuFeO2. We discuss the difference in quadrupole coupling constant between the ferroelectric and paraelectric ...

Magnetic correlations of the quasi-one-dimensional half-integer spin-chain antiferromagnets SrM2V2O8(M = Co, Mn)

Magnetic correlations of two isostructural quasi-one-dimensional (1D) antiferromagnetic spin-chain compounds Sr${M}_{2}$V${}_{2}$O${}_{8}$ (M = Co, Mn) have been investigated by magnetization and powder neutron diffraction. Two different collinear antiferromagnetic (AFM) structures, characterized by the propagation vectors, $k=$ (0 0 1) and $k$ = (0 0 0), have been found below $\ensuremath{\sim}$5.2 and $\ensuremath{\sim}$42.2 K for the Co and Mn compounds, respectively. For the Mn compound, AFM chains (along the $c$ axis) order ferromagnetically within the $ab$ plane, whereas, for the Co compound, AFM chains order ferromagnetically/antiferromagnetically along the a/b direction. The critical exponent study confirms that the Co and Mn compounds belong to the Ising and Heisenberg universality classes, respectively. For both compounds, short-range spin-spin correlations are present over a wide temperature range above ${T}_{N}$. The reduced ordered moments at base temperature (1.5 K) indicate the presence of quantum fluctuations in both compounds due to the quasi-1D magnetic interactions.

Enhanced dielectric and ferroelectric properties of Ba and Ti co-doped BiFeO3 multiferroic ceramics

Multiferroic ceramics Bi0.8Ba0.2Fe1−xTixO3 with x = 0, 0.1 and 0.2 were prepared by using the conventional solid state reaction method. The structural, dielectric, ferroelectric and magnetic properties were investigated. The structure phase transition from rhombohedral to tetragonal with increasing the Ti substitution concentration was confirmed using X-ray diffraction. The reduction of low frequency dispersion in the dielectric constant and loss with an increase in the Ti content was observed. The ferroelectric measurements revealed that the leakage current is significantly suppressed with Ti substitution. Magnetic hysteresis loops showed a continuous decrease in magnetization with Ti substitution, which can be attributed to the collinear antiferromagnetic spin structure in tetragonal structure. In addition, a remarkable change in the polarization and ferroelectric loop area after poling the samples in the dc magnetic field indicates the presence of magnetoelectric coupling at room temperature.

The structural, elastic, phonon, thermal and electronic properties of MnX (X=Ni, Pd and Pt) alloys: First-principles calculations

We present first-principles studies of structural, elastic, phonon, thermal and electronic properties of MnX. According to the calculated total energies, the predicted phase stability trends are consistent with previous experiments. For the L10 lattice, the calculated Mn–Mn exchange couplings JMn–Mns, depending on the valence electron concentration Cv, have been investigated. The dominant inter-layer nearest-neighbors coupling J1L and in-plane next-nearest-neighbor coupling J2 are responsible for the differences in energy between the two structural phases (T=0K). The negative shear modulus C′ of the paramagnetic B2 (phase) triggers the cubic to tetragonal distortion. The calculated phonon dispersions indicate that the primary driving force for the paramagnetic L10 to antiferromagnetic L10 transition is the magnetic ordering energy, rather than some entropic contribution from thermal vibrations. The Debye temperatures, anisotropy and Poisson's ratio are also assessed. The calculated results show that the collinear antiferromagnetic CuAu-I type structures are mechanically and dynamically stable, and agree with the empirical ground states.

Transport and magnetic properties of Fe doped CaMnO3

The structural, transport, and magnetic properties of CaMn1−xFexO3−δ (0.0 ≤ x ≤ 0.3) have been studied by using resistivity, magnetization, and neutron powder diffraction techniques. The compounds are found to be isostructural and crystallize in GdFeO3-type orthorhombic structure (space group Pnma). With Fe doping, no structural change is observed. Mössbauer and paramagnetic susceptibility measurements show that Fe substitutes in 4+ valence state, and XANES measurements indicate the presence of mixed valence state of Mn. The compounds exhibit insulating behavior in the studied temperature range. The temperature dependence of resistivity is found to be described by small polaron model for x = 0 and variable range hopping model for x = 0.1. For higher x values, it follows a parallel combination resistance model. A small reduction in TN from 120 K to 100 K with increase in x is found. The magnetic structure changes from Gz-type collinear antiferromagnetic (AFM) structure for x = 0.0 to canted AFM structure GZFY-type for Fe doped compounds. The AFM component of the moment progressively decreases with x while FM component exhibits a maximum at x = 0.2.

Exchange Bias in Polycrystalline BiFe1−xMnxO3/Ni81Fe19 Bilayers

Polycrystalline BiFe1−xMnxO3 films with x up to 0.50 are prepared on LaNiO3 buffered surface oxidized Si substrates. The doped Mn is confirmed to be partially in a +4 valence state. A clear exchange bias effect is observed with a 3.6 nm Ni81Fe19 layer deposited on the top BiFe1−xMnxO3 layer, which decreases drastically with increasing Mn doping concentration and finally to zero when x is above 0.20. These results clearly demonstrate that the exchange bias field comes from the net spins due to the canted antiferromagnetic spin structure in polycrystalline BiFe1−xMnxO3 films, which transforms to a collinear antiferromagnetic spin structure when the Mn doping concentration is larger than 0.20.

Magnetic properties of theS=12quasisquare lattice antiferromagnet CuF2(H2O)2(pyz) (pyz=pyrazine) investigated by neutron scattering

We have performed elastic and inelastic neutron scattering experiments on single crystal samples of the coordination polymer compound CuF${}_{2}$(H${}_{2}$O)${}_{2}$(pyz) (pyz = pyrazine) to study the magnetic structure and excitations. The elastic neutron diffraction measurements indicate a collinear antiferromagnetic structure with moments oriented along the [0.7 0 1] real-space direction and an ordered moment of 0.60 $\ifmmode\pm\else\textpm\fi{}$ 0.03 ${\ensuremath{\mu}}_{B}/$Cu. This value is significantly smaller than the single-ion magnetic moment, reflecting the presence of strong quantum fluctuations. The spin wave dispersion from magnetic zone center to the zone boundary points (0.5 1.5 0) and (0.5 0 1.5) can be described by a two-dimensional Heisenberg model with a nearest-neighbor magnetic exchange constant ${J}_{2D}=0.934$ $\ifmmode\pm\else\textpm\fi{}$ 0.0025 meV. The interlayer interaction ${J}_{\mathrm{perp}}$ in this compound is less than 1.5$%$ of ${J}_{2D}$. The spin excitation energy at the (0.5 0.5 0.5) zone boundary point is reduced when compared to the (0.5 1 0.5) zone boundary point by $\ensuremath{\sim}$10.3$%$ $\ifmmode\pm\else\textpm\fi{}$ 1.4$%$. This zone boundary dispersion is consistent with quantum Monte Carlo and series expansion calculations for the $S=\frac{1}{2}$ Heisenberg square lattice antiferromagnet, which include corrections for quantum fluctuations to linear spin wave theory.

Dimensionality Driven Spin-Flop Transition in Layered Iridates

Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO2 layers is due to strong competition among intra- and interlayer bond-directional pseudodipolar interactions of the spin-orbit entangled J(eff)=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.

Effect of electron doping on the magnetic correlations in the bilayered brownmillerite compound Ca2.5−xLaxSr0.5GaMn2O8: a neutron diffraction study

The effect of electron doping on the magnetic properties of the brownmillerite type bilayered compounds has been investigated by neutron powder diffraction in La substituted Ca2.5−xLaxSr0.5GaMn2O8 compounds (x = 0.05 and 0.1), in comparison with the undoped compound (x = 0). In all compounds, a long-range three-dimensional collinear antiferromagnetic (AFM) structure is found below the Néel temperature TN of the respective compound, whereas, well above TN, three-dimensional short-range magnetic ordering is observed. In the intermediate temperature range just above TN, a strong effect of electron doping (La substitution) on the magnetic correlations has been observed. Here, a short-range AFM correlation with a possible dimensionality of three has been found for substituted compounds (x = 0.05 and 0.1) as compared to the reported two-dimensional long-range AFM ordering in the parent compound. With increasing electron doping, a decrease in TN is also observed. The short-range magnetic correlations set in over a large temperature range above TN. A magnetic phase diagram in the x–T plane is proposed from these results.

Pressure- and temperature-induced structural phase transitions of CaFe2As2and BaFe2As2studied in the Hund’s rule correlation picture

With the proposed Hund's rule correlation picture, i.e., the fluctuating Fe local moments with the As-bridged antiferromagnetic superexchange interactions, the exceptional collapsed tetragonal phase and related phase transitions observed in CaFe${}_{2}$As${}_{2}$ are well understood. Within the same framework, a pressure-temperature phase diagram is predicted for BaFe${}_{2}$As${}_{2}$ as well, in which a paramagnetic tetragonal structure and a collinear antiferromagnetic orthorhombic structure to nonmagnetic tetragonal structure transitions take place around 4--8 GPa and 10--15 GPa, respectively, and a nonmagnetic tetragonal to a nonmagnetic collapsed tetragonal structure transition takes place over 26 GPa. Our study helps to better understand the complex correlation among crystal structure, magnetism, and electronic structure in pnictides, a precondition to understanding the superconductivity in pnictides.

Giant magnetoresistance in IrMn based antiferromagnetic spin valve

According to the parameter-free first principles calculations, we investigate the spin polarized transport in antiferromagnetic spin valve (AFSV) based on noncollinear IrMn. The giant magnetoresistance (GMR) in Co/IrMn/Cu(111) with L12-type IrMn shows three-fold rotational symmetry, which is easy to be distinguished from the GMR of normal ferromagnetic spin valves. Moreover, GMR based on -phase IrMn with current-in-plane (CIP) structure shows that GMR is 7.7%, around two time larger than that in current-pendicular-to-plane (CPP) structure (3.4%). Our study demonstrates that the AFSV CIP structure possesses a larger GMR effect than the CPP structure, and the GMR effect in AFSV based on noncollinear antiferromagnetic structure is larger than that based on collinear antiferromagnetic structure such as FeMn.

Neutron diffraction investigation of the crystal and magnetic structures inKCrF3perovskite

${\text{KCrF}}_{3}$ represents another prototypical orbital-ordered perovskite, where ${\text{Cr}}^{2+}$ possesses the same electronic configuration of $3{d}^{4}$ as that of strongly Jahn-Teller distorted ${\text{Mn}}^{3+}$ in many colossal magnetoresistance manganites. The crystal and magnetic structures of ${\text{KCrF}}_{3}$ compound are investigated by using polarized and unpolarized neutron powder-diffraction methods. The results show that the ${\text{KCrF}}_{3}$ compound crystallizes in tetragonal structure at room temperature and undergoes a monoclinic distortion with the decrease in temperature. The distortion of the crystal structure indicates the presence of cooperative Jahn-Teller distortion which is driven by orbital ordering. With decreasing temperature, four magnetic phase transitions are observed at 79.5, 45.8, 9.5, and 3.2 K, which suggests a rich magnetic phase diagram. Below ${T}_{N}=79.5\text{ }\text{K}$, the ${\text{Cr}}^{2+}$ moment orders in an incommensurate antiferromagnetic arrangement, which can be defined by the magnetic propagation vector $(\frac{1}{2}\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},\frac{1}{2}\ifmmode\pm\else\textpm\fi{}\ensuremath{\delta},0)$. The incommensurate-commensurate magnetic transition occurs at 45.8 K and the magnetic propagation vector locks into $(\frac{1}{2},\frac{1}{2},0)$ with the Cr moment of $3.34(5)\text{ }{\ensuremath{\mu}}_{B}$, aligned ferromagnetically in (220) plane, but antiferromagnetically along [110] direction. Below 9.5 K, the canted antiferromagnetic ordering and weak ferromagnetism arise from the collinear antiferromagnetic structure while the Dzyaloshinskii-Moriya interaction and tilted character of the single-ion anisotropy might give rise to the complex magnetic behaviors below 9.5 K.

Nature of magnetic phase transitions in TbCu 2X 2 (X = Si, Ge) and HoCu 2Si 2 compounds

Physical properties of TbCu 2X 2 (X = Si, Ge) and HoCu 2Si 2, crystallizing with the tetragonal ThCr 2Si 2-type crystal structure, were investigated by means of neutron diffraction as well as by magnetic and calorimetric measurements. All compounds exhibit antiferromagnetic ordering below T N equal to 12 K for TbCu 2Si 2, 13 K for TbCu 2Ge 2 and 6.4 K for HoCu 2Si 2. Neutron diffraction data indicate collinear antiferromagnetic structure described by the propagation vector k → = [ ( 1 / 2 ) , 0 , ( 1 / 2 ) ] at low temperatures. While increasing temperature the direction of the Tb magnetic moment changes from [1 1 0] to [0 1 0] at T t = 8.5 K for TbCu 2Si 2 and 9.7 K for TbCu 2Ge 2. In HoCu 2Si 2 a change of the magnetic structure from the collinear to a modulated one described by the propagation vector k → = [ k x , 0 , k z ] was found at T t = 4.7 K.

Theoretical Analysis of the Spin Exchange and Magnetic Dipole−Dipole Interactions Leading to the Magnetic Structure of Ni3TeO6

The origin of the collinear antiferromagnetic magnetic structure of Ni(3)TeO(6) below 52 K was analyzed by calculating its spin exchanges on the basis of density functional calculations, and the cause for the parallelc-spin orientation found for this magnetic structure by calculating the spin-orbit coupling and magnetic dipole-dipole interaction energies. The calculated exchanges correctly predict the observed magnetic structure below 52 K, and lead practically to no spin frustration. The perpendicularc- and parallelc-spin orientations are predicted by the spin-orbit coupling and the magnetic dipole-dipole interactions, respectively. However, the magnetic dipole-dipole interactions are stronger than the spin-orbit coupling interactions, and hence are responsible for the spin orientation observed for Ni(3)TeO(6).

Magnetic structures of orthorhombic rare earth stannides LSn 2 (L = Tb–Tm)

The magnetic structures of the orthorhombic compounds LSn 2 (S.G.: Cmcm; L = Tb–Tm) have been determined from neutron diffraction studies. The compounds LSn 2 (L = Tb, Dy, Ho Er) order below 26.7, 17.2, 7.8 and 4.5 K, respectively, in a sine-modulated structure characterized by the propagation vector k = (≈0.39, 0, ≈0.41). Below 12.7, 5, 4.8 and 2 K, respectively, these compounds undergo a transition towards a collinear antiferromagnetic structure characterized by the propagation vector k = (0, 0, 1/2) for Tb and Er, k = (1/2, 1/2, 0) for Ho and k = (1/2, 1/2, 1/2) for Dy. The compound TmSn 2 orders below 4.0 K in a sine-modulated structure characterized by the propagation vector k = (0, 0.667, 0.380). The moment direction is close to [0 0 1] for L = Tb, Dy, Ho and aligned along [1 0 0] for L = Er, Tm. The results are discussed and a relative scale of the stability of the moment configuration in the (0 1 0) plane is proposed.

Mössbauer study of superconductingNdFeAsO0.88F0.12 and its parent compound NdFeAsO

57Fe Mössbauer spectroscopy has been used to investigate the magnetic order of non-superconducting NdFeAsO(TN = 140 K) andsuperconducting NdFeAsO0.88F0.12 (Tc = 45 K). A magnetichyperfine field Bhf was observed at the 57Fe nucleus below TN∼140 K forNdFeAsO. Below ∼2 K an increase of Bhf relative to the saturation value was attributed to the transferredBhf at the Fe site resulting from the collinear antiferromagnetic (AF) spin structure of the Ndmoments. The analysis of the spectra is consistent with a commensurate AF order of Fe spins. No Bhf is observed insuperconducting NdFeAsO0.88F0.12 down to 1.5 K.

Magnetic phase transitions in [formula omitted][formula omitted] compounds

The magnetic phase transitions of the ternary compounds TbIrGe 2 and HoIrGe 2 have been studied by magnetic, specific heat and neutron diffraction measurements. Both compounds crystallize in the orthorhombic YIrGe 2 -type structure (space group Immm ) in which the rare earth atoms occupy two non-equivalent crystallographic sites. The rare earth magnetic moments at different crystallographic sites order independently with two different types of magnetic ordering. The Tb and Ho moments at 4 i site form a collinear commensurate antiferromagnetic structure (Tb moments lie in bc -plane while those of Ho in ab -plane ). The magnetic moments at 4 h site also form a collinear antiferromagnetic structure at low temperature but the orientation of magnetic moments is different: Tb moments are aligned along the c-axis while Ho moments lie in ac -plane (in case of TbIrGe 2 the magnetic unit cell is four times larger than the crystallographic one). In TbIrGe 2 the magnetic order of moments at 4 h site at T t = 9 K turns into a sine modulated one with k ⇒ = [ 0.45 , 0 , 0 ] . The Néel temperatures for the different sublattices are: for 4 h site ∼ 12 K (Tb) and 5 K (Ho) and for 4 i site: 10 K (Tb) and 2.3 K (Ho). The above results suggest that interactions between the moments at 4 h site are stronger than those for 4 i site. Each sublattice has an uniaxial antiferromagnetic moment arrangement but different reorientation processes and different ordering temperatures reflect the presence of two distinct order parameters.