Weak Ferromagnetic Ordering Research Articles

Weak itinerant ferromagnetism and clean superconductivity in Y9Co7

We present low-temperature magnetic properties for a high-quality polycrystalline sample of the first ferromagnetic superconductor Y9Co7. The results of susceptibility and magnetization measurements show the coexistence of a weak itinerant ferromagnetic order with a Curie temperature of TC ≅ 4.5 K and superconductivity below TS = 3 K. Several electronic quantities and parameters characterizing the superconducting and normal state are calculated within the Ginzburg–Landau–Abrikosov–Gorkov theory. The data reveal clean limit superconductivity in Y9Co7 due to the good chemical purity and structural order of the specimen.

Structural, magnetic and optical properties of Ce substituted BiFeO3 nanoparticles

Ce substituted Bi1−xCexFeO3 (x=0.03, 0.05, 0.07 and 0.10) nanoparticles were prepared by a tartaric acid based sol–gel route. X-ray diffraction patterns and Raman spectra revealed a structural phase transition from rhombohedral to orthorhombic phase for x=0.10 sample. Room temperature magnetic measurements showed weak ferromagnetic ordering and enhancement in magnetization with increasing Ce concentration. The improved magnetic properties due to the breaking of spin cycloid with Ce substitution have been observed from electron spin resonance (ESR) study. The measured g-values for all samples are greater than 2 and the ESR lines shift towards higher g-value with increasing Ce concentration, indicating ferromagnetic nature of these samples. UV–visible diffuse reflectance spectra showed a sharp absorption in the visible region with two d–d and three charge transfer (C-T) transitions. Prominent red shift in the band gap indicates a significant change in the band structure of the doped nanoparticles.

Giant magnetocaloric effect in ferromagnetic superconductor RuSr2GdCu2O8

We have investigated the magnetic and magnetocaloric properties of ferromagnetic superconductor Ru(Sr1–xLax)2GdCu2O8 (x = 0–0.10) by magnetization and heat capacity measurements. RuSr2GdCu2O8 shows onset of superconductivity at around 46 K, which coexists with the weak ferromagnetic ordering of the RuO2 moments and the antiferromagnetic ordering of the Gd moments. Both superconducting RuSr2GdCu2O8 and non-superconducting Ru(Sr0.9La0.1)2GdCu2O8 compounds exhibit a large magnetic entropy change due to the field-induced metamagnetic transition. The maximum values of entropy change, adiabatic temperature change, and refrigerant capacity are ∼15.5 J kg−1 K−1, ∼14 K, and ∼125 J kg−1, respectively, for a field change of 0–7 T for both the compounds. These magnetocaloric parameters also have reasonably good values for a moderate field change (2–3 T), therefore, fulfilling the necessary conditions for refrigeration in the low-temperature region.

Synthesis and characterization of La(Cr,Fe,Mn)O3 nanoparticles obtained by co-precipitation method

Abstract Magnetic and structural properties have been investigated in La(Cr,Fe,Mn)O3 nanoparticles obtained by co-precipitation method. The X-ray diffraction measurements allied to Rietveld method confirm the formation of LaCrO3, LaFeO3 and LaMnO3 nanoparticles with crystal structure orthorhombic (Pbnm), orthorhombic (Pnma) and rhombohedral (R-3c), respectively. We also verified an decreasing in the average crystallite size from 73 to 26 nm, depending of the transition metal. The magnetic measurements reveal an antiferromagnetic behavior for the LaCrO3 sample with TN~289 K, and a weak ferromagnetic ordering for the LaMnO3 sample with Tc~200 K.

Structural, vibrational and magnetic properties of Ti substituted bulk hematite: α-Fe2−xTi x O3

Ti doped hematite α-Fe2−xTi x O3 (x = 0.0, 0.0206 and 0.0344) samples are synthesized using solid-state ceramic route technique. Single phase and corundum (Al2O3) type structure is revealed from the X-ray diffraction (XRD) pattern. On substitution of Ti at Fe site, all Raman active modes are shifted to higher wave numbers. An additional feature of E u (LO) mode at about 660 cm-1 is observed. The E u mode frequency is decreased and pronounced systematically as a function of Ti doping, and it reaches a value of 658 cm−1 for x = 0.0344. The coercivity Hc (remanence Mr) for x = 0.0, 0.0206 and 0.0344 are determined to be 995 Oe (0.44 emu/mg), 1404 Oe (0.00019 emu/mg) and 2023 Oe (0.00016 emu/mg), respectively. The larger coercivity for Ti doped samples can be attributed to their enhanced shape and magneto-crystalline anisotropy. The observed isomer shift (δ) from room temperature Mossbauer data clearly shows the presence of ferric (Fe3+) and Ti4+ ions illustrating strong ferromagnetic ordering up to x = 0.0206 in α-Fe2−xTi x O3 hematite and weak ferromagnetic ordering of α-Fe2−xTi x O3 for x = 0.0344.

1D Heterometallic Oxalate Compounds as Precursors for Mixed Ca–Cr Oxides – Synthesis, Structures, and Magnetic Studies

AbstractThe heterodimetallic compounds {[CaCr2(L)2(C2O4)4]·xH2O}n [L = 2,2′‐bipyridine (bpy), x = 0.83 (1); L = 1,10‐phenanthroline (phen), x = 0.33 (2)] were synthesized and characterized by IR spectroscopy, X‐ray diffraction and magnetic measurements. The crystal structure of 2 is reported: the chelation of a [Cr(phen)(C2O4)2]– units towards Ca cations results in 1D double zigzag chains. The ability of 1 and 2 to act as single‐source precursors for the formation of Ca–Cr oxides is explored by thermal analysis, powder X‐ray diffraction and IR spectroscopy. The thermal processing of 1 and 2 at 1100 °C in air led to the appearance of the β‐CaCr2O4 phase as the major crystalline oxide. By heating the samples up to 1100 °C in nitrogen, only the β‐CaCr2O4 phase formed, whereas heating at 1400 °C caused the crystallization of two polymorphs, α‐ and β‐CaCr2O4. The magnetic behaviour of β‐CaCr2O4 pointed to the freezing of magnetic moments in coexistence with a weak ferromagnetic order.

Influence of divalent metal cation Zinc doping on the structural and magnetic characterization of hemataite: α-Fe2O3

Divalent metal cation Zn2+ doped hemataite [α-Fe2−xZnxO3 (x=0, 0.01, 0.02 and 0.03)] have been synthesized by ceramic route technique aimed at investigating structural and magnetic properties with corundum structure shaped α-Fe2O3. Divalent non-magnetic Zn (1%, 2%, and 3%) doping at Fe site allows all seven Raman active modes to shift to higher wave numbers. An additional feature with Eu mode (0.01⩽x 0.03) appears at about 660cm−1. Eu mode of parent hemataite is centered at about 660cm−1 grows systematically as a function of Zn doping and it reaches a value of 662cm−1 for (x=0.03). The oxidation state of Fe was analyzed in terms of Mössbauer spectroscopy and observed isomer shift δ values from room temperature Mössbauer data clearly shows oxidation sate of iron (Fe) is Fe3+ (Ferric) and Zn is Zn2+ ions. A strong Ferro magnetic ordering persists up to (x=0.02) in α-Fe2−xZnxO3 hemataite and weak Ferro magnetic ordering for α-Fe2−xZnxO3 (x=0.03) hemataite.

Structural and Magnetic Properties of Bismuth Ferrite Nanopowders Prepared via Sol-Gel Method

Bismuth ferrite (BiFeO3, short for BFO) powders were prepared at low temperature using sol-gel process. Single-phase BiFeO3 powders with distorted perovskite structure were obtained after calcining at 600°C. According to the Scherrer formula the calculated crystallite size (D XRD) is about 39 nm. The value of the average particle size (D BET) is larger than that of D XRD. A typical magnetic hysteresis loop was observed at room temperature, indicating that the BiFeO3 nanoparticles show a weak ferromagnetic order. The remnant magnetization (2M r) and coercive field (2H C) are 0.033 emu/g and 395 Oe, respectively.

Magnetic properties and electronic structures of (YTiO3)2/(BaTiO3)n superlattices

The magnetic properties and electronic structures of (YTiO3)2/(BaTiO3)n superlattices are investigated using the first-principles calculations. It is revealed that the in-plane compressive strain results in the A-type antiferromagnetic order in the YTiO3 component. Surprisingly, the Ti ions in BaTiO3 layers exhibit a weak ferromagnetic order for n = 4. The ferromagnetism in the BaTiO3 layers near the interface is related to the polar discontinuity of YTiO3 and ferroelectric polarization of BaTiO3. The electronic structures indicate that the n = 4 superlattice shows the two-dimensional electron gas at the interface.

Inelastic neutron scattering studies ofYFeO3

Spin waves in the the rare earth orthorferrite YFeO$_3$ have been studied by inelastic neutron scattering and analyzed with a full four-sublattice model including contributions from both the weak ferromagnetic and hidden antiferromagnetic orders. Antiferromagnetic (AFM) exchange interactions of $J_1 = -4.23 \pm 0.08$ (nearest-neighbors only) or $J_1 = -4.77 \pm 0.08$ meV and $J_2 = -0.21 \pm 0.04$ meV lead to excellent fits for most branches at both low and high energies. An additional branch associated with the hidden antiferromagnetic order was observed. This work paves the way for studies of other materials in this class containing spin reorientation transitions and magnetic rare earth ions.

Magnetic and structural properties of pure and Cr-doped haematite: α-Fe2−xCr x O3 (0 ≤ x ≤ 1)

Solid-state ceramic technique route is used for synthesizing Cr-doped haematite α-Fe2−xCr x O3 (x = 0, 0.125, 0.50 and 1) samples. Single phase and corundum (Al2O3) type structure is revealed from the X-ray diffraction (XRD) patterns. The Raman spectra of α-Fe2−xCr x O3 illustrate seven phonon modes. On substitution of Cr (x = 0, 0.125, 0.50 and 1) at Fe site, all Raman active modes are shifted to higher wave numbers. The coercivity and remanence of Cr-doped haematites increase as x increases. The increased coercivity and remanence for Cr-doped samples can be attributed to their enhanced shape and magneto-crystalline anisotropy. The observed isomer shift δ values from room-temperature Mössbauer data clearly show the presence of ferric (Fe3+) and Cr3+ ions illustrating strong ferromagnetic ordering up to x = 0.125 in α-Fe2−xCr x O3 haematite and weak ferromagnetic ordering for α-Fe2−xCr x O3 (x > 0.125) haematites.

Invar Like Behavior of the Cr2AlC MAX Phase at Low Temperature

The structure of Cr2AlC has been studied using neutron scattering experiments performed in a temperature range running from room temperature down to 1.8 K. It is shown that the unit cell volume does not vary significantly below 80 K. This anomalous behavior, correlated with the existence of a weak ferromagnetic ordering revealed from superconducting quantum interference device experiments, is similar to that characteristic of Invar alloys. Just above the Curie temperature TC ~ 73 ± 5 K, the Cr2AlC undergoes significant distortions of the octahedron and triangular prisms units. Above 100 K, Cr2AlC follows an usual thermal expansion.

Coherent magnon and acoustic phonon dynamics in tetragonal and rare-earth-doped BiFeO3multiferroic thin films

Coherent magnons and acoustic phonons were impulsively excited and probed in thin films of the room temperature multiferroic Bi${}_{1\ensuremath{-}x\ensuremath{-}y}$Dy${}_{x}$La${}_{y}$FeO${}_{3}$ using femtosecond laser pulses. The elastic moduli of rhombohedral, tetragonal, and rare-earth doped BiFeO${}_{3}$ were determined from acoustic-mode frequencies in conjunction with spectroscopic ellipsometry. A weak ferromagnetic order, induced alternately by magnetization in the growth direction or by tetragonality, created a magnon oscillation at 75 GHz, indicative of a Dzyaloshinskii-Moriya interaction energy of 0.31 meV.

Complex structural-ferroelectric domain walls in thin films of hexagonal orthoferrites RFeO3 (R = Lu, Er)

Hexagonal orthoferrites have recently attracted much attention as possible high-temperature ferromagnetic ferroelectrics. The ferroelectric domain structure of hexagonal RMnO3, their antiferromagnetic structural analogies, has recently shown an atypical and complicated behavior. Hexagonal RFeO3 are expected to exhibit similar domain structure that should coexist with weak ferromagnetic order and may represent a material with a unusual magnetoelectric interaction. In this report, we discuss microscopic ferroelectric domain structure of hexagonal orthoferrites in a thin-film state and demonstrate a distinct and unusual improper ferroelectric behavior of these oxide materials.

Hydrothermal synthesis of DyMn2O5 nanorods and their magnetic properties

Abstract Highly pure DyMn2O5 nanorods have been synthesized via hydrothermal route. FT-IR and Raman patterns were employed to determine DyMn2O5. PLE and PL spectra of DyMn2O5 showed UV excitation and near band edge (NBE) emission. Magnetic measurement indicated the crystalline DyMn2O5 showed a weak ferromagnetic ordering due to incomplete spin compensation on the surface. The ferromagnetic transition could be explained by an antiferromagnetic core and a ferromagnetic surface.

Influence of Cr and Mn substitution on the structural and spectroscopic properties of doped haematite: α-Fe2−xMxO3 (0.0 ⩽ x < 0.50)

Solid-state ceramic route technique is used for synthesizing Cr and Mn doped haematite α-Fe2−xMxO3. The X-ray diffraction pattern identifies the single phase and corundum (Al2O3) type structure of the polycrystalline samples α-Fe2−x MxO3 (M=Cr and Mn; x=0, 0.10, and 0.50). The Raman measurements of α-Fe2O3 samples shows seven Raman active modes at A1g(1)≅225cm−1, Eg(1)≅249cm−1, Eg(2)≅292cm−1, Eg(3)≅297cm−1, Eg(4)≅409cm−1, Eg(5)≅496cm−1, and A1g(2)≅609cm−1.On substitution of Cr and Mn (x=0.1, and 0.50) at Fe site, all Raman active modes are shifted to higher wavenumber. The observed isomer shift values from room temperature Mössbauer data clearly show the presence of ferric (Fe3+) state and strong magnetic ordering in all samples. The observed isomer shift δ values from room temperature Mössbauer data clearly show the Fe is in Ferric (Fe+3) nature. In doped haematite Cr+3 and Mn2+ ions illustrating strong Ferro magnetic ordering up to (x=0.10) and weak Ferro magnetic ordering for α-Fe2−xMxO3 (x>0.10) haematite.

Evidence of new magnetic ordering at high temperatures in Pb-based multiferroics perovskites

Presented here are the results of magnetic data that reveal that both Pb-based perovskites samples, Pb(Fe1/2Nb1/2)O3 (lead iron niobate – PFN) and Pb(Fe2/3W1/3)O3 (lead iron tungstate – PFW), exhibit a change in magnetic ordering from paramagnetic (PM) to weak ferromagnetic state at high temperature: a weak ferromagnetic order occurs in the samples around 530 K. The experimental results suggest a possible involvement of the Pb ions into the superexchange of antiferromagnetic (AFM) phase.

Room-temperature multiferroic hexagonal LuFeO3 films.

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

Spinel-Type Cobalt Oxide (Co3O4) Nanoparticles from the mer- Co(NH3)3(NO2)3 Complex: Preparation, Characterization, and Study of Optical and Magnetic Properties

In this paper, the mer-Co(NH3)3(NO2)3 complex was used as a new precursor for synthesizing spinel-type cobalt oxide nanoparticles (Co3O4NPs).Thermal decomposition of the complex at low temperature (175 °C) resulted in the Co3O4NPs without using expensive and toxic solvents or complicated equipment. XRD, FT-IR, SEM, EDX, and TEM were employed to characterize the product, and its optical and magnetic properties were studied by UV-visible spectroscopy and a VSM, respectively. FT-IR, XRD and EDS analyses confirmed the formation of single-phase Co3O4 with cubic structure. The lattice constant calculated from XRD peaks is 8.0650 A. SEM and TEM images showed that Co3O4NPs have a sphere-like morphology with an average size of 19 nm. Optical spectrum of Co3O4NPs revealed the presence of two band gaps at 2.20 and 3.45 eV values, which in turn confirmed the semiconducting properties. The magnetic measurement of Co3O4NPs showed a weak ferromagnetic order at room temperature.

Weak magnetic order in high-Tc superconductors produced by spontaneous Josephson currents

We develop a model for high-Tc superconductors based on an electronic phase separation where low-and high-density domains are formed. At low temperatures this system may act as a granular superconductor forming an array of Josephson junctions. Cuprates are also known to have low superfluid densities and strong correlation effects. Both characteristics activate a negative Josephson coupling due to frustration that leads to spontaneous currents responsible for the weak ferromagnetic order. This original approach reproduces the observed onset of spontaneous magnetic signal and its dependence on the doping level.