Origin of Room Temperature Weak-Ferromagnetism in Antiferromagnetic Pb (Fe2/3W1/3) O3 Ceramic

We report the origin of room temperature weak ferromagnetic behavior of polycrystalline Pb(Fe2/3W1/3)O3 (PFW) powder. The structure and magnetic properties of the ceramic powder prepared by a Columbite method were characterized by X-ray and neutron diffraction, Mössbauer spectroscopy and magnetization measurements. Rietveld analysis of diffraction data confirm the formation of single phase PFW, without traces of any parasitic pyrochlore phase. PFW was found to crystallize in the cubic structure at room temperature. The Rietveld refinement of neutron diffraction data measured at room temperature confirmed the G-type antiferromagnetic structure of PFW in our sample. However, along with the antiferromagnetic (AFM) ordering of the Fe spins, we have observed the existence of weak ferromagnetism at room temperature through: (i) a clear opening of hysteresis (M–H) loop, (ii) bifurcation of the field cooled and zero-field cooled susceptibility; supported by Mössbauer spectroscopy results. The P–E loop measurements showed a non-linear slim hysteresis loop at room temperature due to the electronic conduction through the local inhomogeneities in the PFW crystallites and the inter-particle regions. By corroborating all the magnetic measurements, especially the spin glass nature of the sample, with the conduction behavior of the sample, we report here that the observed ferromagnetism originates at these local inhomogeneous regions in the sample, where the Fe-spins are not perfectly aligned antiferromagnetically due to the compositional disordering

Observation of Enhanced Magnetic Pinning in Sm3+ Substituted Nanocrystalline MnZn Ferrites Prepared by Propellant Chemistry Route

We report the effect of Sm3+ substitution on the structural and magnetic properties of nanocrystalline Mn0.5Zn0.5SmyFe2-yO4 (y = 0.00, 0.01, 0.03 and 0.05) samples prepared by propellant chemistry route using a mixture of fuels. Rietveld refinement of XRD patterns confirmed the formation of cubic spinel phase with space group View the MathML source. The lattice parameter values decreased with Sm3+ substitution up to y = 0.03, but with a noticeable increase for the sample with y = 0.05. In all the samples, entire amount of Zn2+ and Sm3+ were found to be present at the A and B sites, respectively. A distribution of Mn2+ ions at the tetrahedral (A) and the octahedral (B) sites of the spinel Mn0.5Zn0.5Fe2O4 was observed. The microstructures of the samples were observed using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). For all the samples, the average crystallite size decreased with increase in Sm3+ concentration, as determined using Williamson-Hall method. The FTIR spectra showed prominent absorption bands at ∼540 and ∼390 cm−1 corresponding to the stretching vibrations of the metal ion complexes at the tetrahedral (A) and the octahedral (B) sites, respectively. Magnetic properties such as saturation magnetization (Ms), remanence (Mr) and magneton number (ηB) were found to decrease, while the coercivity (Hc) and reduced remanence (Mr/Ms) of the samples were found to increase with increasing Sm3+ content. The increase in Hc with increase in Sm3+ concentration is interpreted as the enhanced pinning of the magnetic moments at the magnetic defects created by Sm3+ ions, which is further confirmed by Mössbauer spectroscopy through a nearly constant magnetic hyperfine field. This results in an increase in the magnetic particle size in spite of decreasing average crystallite size. Our work suggests that, Sm3+ substitution can be used to alter the magnetic hardness of Mnsingle bondZn ferrites and to enable them to be used as potential materials for various technological applications

Minutes-duration optical flares with supernova luminosities

In recent years, certain luminous extragalactic optical transients have been observed to last only a few days1. Their short observed duration implies a different powering mechanism from the most common luminous extragalactic transients (supernovae), whose timescale is weeks2. Some short-duration transients, most notably AT2018cow (ref. 3), show blue optical colours and bright radio and X-ray emission4. Several AT2018cow-like transients have shown hints of a long-lived embedded energy source5, such as X-ray variability6,7, prolonged ultraviolet emission8, a tentative X-ray quasiperiodic oscillation9,10 and large energies coupled to fast (but subrelativistic) radio-emitting ejecta11,12. Here we report observations of minutes-duration optical flares in the aftermath of an AT2018cow-like transient, AT2022tsd (the ‘Tasmanian Devil’). The flares occur over a period of months, are highly energetic and are probably nonthermal, implying that they arise from a near-relativistic outflow or jet. Our observations confirm that, in some AT2018cow-like transients, the embedded energy source is a compact object, either a magnetar or an accreting black hole

On the Room Temperature Ferromagnetic and Ferroelectric Properties of Pb(Fe1/2Nb1/2)O3

We report the origin of room temperature (RT) ferromagnetic and ferroelectric properties of Pb(Fe1/2Nb1/2)O3 (PFN) ceramic sample prepared by modified solid-state reaction synthesis by a single-step method, based on X-ray diffraction (XRD), neutron diffraction (ND), Mössbauer spectroscopy and electron paramagnetic resonance (EPR) spectroscopy results. Formation of single-phase monoclinic PFN ceramic with Cm space group was confirmed by XRD and ND at RT. The morphology studied by scanning electron microscopy (SEM) confirmed uniform microstructure of the sample with average grain size of ∼2 μm. The ND, Mössbauer spectroscopy, M–H loop and EPR studies were carried out to confirm the existence of weak ferromagnetism at RT. A clear opening of hysteresis (M–H) loop is evidenced as the existence of weak ferromagnetism at RT. EPR spectrum clearly shows the ferromagnetism through a good resonance signal. The symmetric EPR line shape with g = 1.9895 observed in PFN sample was identified to be due to Fe 3+ ions. Mössbauer spectroscopy at RT shows superparamagnetic behaviour with presence of Fe in 3 + valence state. Ferroelectric P–E loops on PFN at RT confirm the existing ferroelectric ordering. Our observation is in agreement with literature, and it supports that the origin of ferromagnetism and ferroelectricity is isolated, i.e. from different regions in the sample. Our results do not support the multiferroic nature of PFN at RT

Neutron Diffraction, Mössbauer Effect and Electron Paramagnetic Resonance Studies on Multiferroic Pb(Fe2/3W1/3) O3

Multiferroic Pb(Fe2/3W1/3)O3 ceramics were synthesized via a modified two-stage Columbite method. Single phase formation was confirmed from the analysis of x-ray and neutron diffraction patterns recorded at room temperature. Structural analysis of the diffraction data reveals cubic phase (space group Pm-3m) for the title compound. Magnetic structure of the title compound at room temperature exhibits G-type antiferromagnetic structure. The Mössbauer spectroscopy and Electron Paramagnetic Resonance (EPR) studies were carried out at 300 K. The isomer shift and quadrupole splitting of the Mössbauer spectra confirms the trivalent state of iron (Fe3+). The Mössbauer spectra also suggest that the iron and tungsten are randomly distributed at the octahedral, B site. EPR spectra show a single broad line associated with Fe3+ ions. Both spectra clearly exhibit weak ferromagnetic behaviour of Pb(Fe2/3W1/3)O3 ceramic at 300 K. Considering neutron diffraction, Mössbauer and EPR results together, it may be stated here that Pb(Fe2/3W1/3)O3 exhibits antiferromagnetic behavior along with weak ferromagnetism at room temperature

Neutron diffraction, Mossbauer effect and electron paramagnetic resonance studies on multiferroic Pb(Fe2​/3W1​/3)​O3

Multiferroic Pb(Fe2​/3W1​/3)​O3 ceramics were synthesized via a modified two-​stage Columbite method. Single phase formation was confirmed from the anal. of x-​ray and neutron diffraction patterns recorded at room temp. Structural anal. of the diffraction data reveals cubic phase (space group Pm-​3m) for the title compd. Magnetic structure of the title compd. at room temp. exhibits G-​type antiferromagnetic structure. The Mossbauer spectroscopy and ESR (EPR) studies were carried out at 300 K. The isomer shift and quadrupole splitting of the Mossbauer spectra confirms the trivalent state of iron (Fe3+)​. The Mossbauer spectra also suggest that the iron and tungsten are randomly distributed at the octahedral, B site. EPR spectra show a single broad line assocd. with Fe3+ ions. Both spectra clearly exhibit weak ferromagnetic behavior of Pb(Fe2​/3W1​/3)​O3 ceramic at 300 K. Considering neutron diffraction, Mossbauer and EPR results together, it may be stated here that Pb(Fe2​/3W1​/3)​O3 exhibits antiferromagnetic behavior along with weak ferromagnetism at room temp. (c) 2015 American Institute of Physics

57Fe internal field nuclear magnetic resonance and Mössbauer spectroscopy study of Li-Zn ferrites

We report the internal field nuclear magnetic resonance (IFNMR) and Mössbauer spectroscopy study of Li-Zn ferrites at RT. The results were supported by the IFNMR data measured at 77 K. As Zn concentration increases the IFNMR echo amplitude decreases and below certain Zn concentration no signal was detected. At RT the echo amplitude vanishes at a lower Zn concentration, whereas at 77 K, the echo amplitude does not vanish completely (except for pure Zn-ferrite). However, in Mössbauer spectroscopy at RT, we have observed magnetically ordered state of all the Li-Zn ferrite samples. This discrepancy could be related to the difference between the time scale of detection of the spins by Mössbauer spectroscopy (10−7–10−10 s) and NMR spectroscopy (10−6 s). Hence, sensitivity of zero-field NMR depends on the magnetic hyperfine field, temperature and abundance of the magnetic cations at the lattice of the spinel ferrites. We have demonstrated that the ‘two-equal-pulses’ sequence leads to higher echo signal than the spin echo pulse sequence due to the presence of distribution of internal magnetic fields throughout the material. We obtained a limiting value for the fraction of spins needed to produce an echo signal at a particular temperature and at a particular site of the Li-Zn ferrite spinels that can be sensitively detected by pulsed IFNMR technique. © 2017 Elsevier Inc

Evidence of structural damage in Sm and Gd co-​doped Mn-​Zn ferrite ceramics due to high-​energy gamma irradiation

We report the evidence of structural changes due to high energy gamma irradn. in Mn0.4Zn0.6SmxGdyFe2-​(x+y)​O4 (where x=y=0.01, 0.02, 0.03, 0.04 and 0.05) ferrites prepd. by self-​propagating high temp. synthesis method. The structural properties of the as prepd. samples and the structural modifications to the samples after gamma irradn. were studied by X-​ray diffraction (XRD)​, Mossbauer spectroscopy and Fourier Transform IR (FTIR) Spectroscopy at room temp. The XRD patterns of the as prepd. samples reveal the formation of mixed cubic spinel phase along with small amts. of α-​Fe2O3, SmFeO3 and​/or GdFeO3 impurity phases. All samples were exposed to gamma radiation dose of 50 kGy originating from a 60Co source. Our results demonstrate that after gamma irradn. the structure and compn. of the various phases in the sample were changed and more stable new phases like ZnFe2O4, α-​Fe2O3 and MnO2 evolved. As a result the (magnetic) properties of the materials were also altered. Our observation of changes in structural and magnetic properties of the reported ferrites are important for understanding the stability and performance of the materials used in various devices, which are exposed to high energy gamma-​rays