V. Siruguri

High-pressure X-ray diffraction study of UMn2Ge2

Abstract Uranium manganese germanide, UMn2Ge2, crystallizes in body-centered tetragonal ThCr2Si2 structure with space group I4/mmm, a=3.993 A and c=10.809 A under ambient conditions. Energy dispersive X-ray diffraction was used to study the compression behavior of UMn2Ge2 in a diamond anvil cell. The sample was studied up to static pressure of 26 GPa and a reversible structural phase transition was observed at a pressure of 1 6.1 GPa . Unit cell parameters were determined up to 12.4 GPa and the calculated cell volumes were found to be well reproduced by a Murnaghan equation of state with K 0 =73.5 GPa and K′=11.4. The structure of the high-pressure phase above 16.0 GPa is quite complicated with very broad lines and could not be unambiguously determined with the available instrument resolution.

Correlation of exchange bias with magneto-structural effects across the compensation temperature of Co(Cr1–xFex)2O4 (x = 0.05 and 0.075)

A small amount of Fe (5% and 7.5%) substitution in the Cr-site of the multiferroic compound CoCr2O4 leads to a magnetization reversal. In these compounds, we report a sign change in the exchange bias across the compensation temperature, accompanied by a non-monotonic change in the local moments across the compensation temperature. Such non-monotonic change in the magnetic moments is triggered by a similar change in the lattice structure. We relate here the sign change of exchange bias with that of the crystalline energy of the lattice and the Zeeman energy term arising from the anti-site disorder.

Giant magnetocaloric effect from reverse martensitic transformation in Ni–Mn–Ga–Cu ferromagnetic shape memory alloys

Abstract In an effort to produce Giant Magnetocaloric effect (GMCE) near room temperature, in a first ever such study, the austenite transformation temperature (A s ) was fine tuned to ferromagnetic Curie temperature (T C ) in Ferromagnetic Shape Memory Alloys (FSMA) and a large GMCE of ΔSMxa0=xa0−81.8xa0J/Kg-K was achieved in Ni 50 Mn 18.5 Cu 6.5 Ga 25 alloy during reverse martensitic transformation (heating cycle) for a magnetic field change of 9xa0Txa0at 303xa0K. Fine tuning of A s with T C was achieved by Cu substitution in Ni 50 Mn 25−x Cu x Ga 25 (0xa0≤xa0xxa0≤xa07.0)-based FSMAs. Characterizations of these alloys were carried out using Optical and Scanning Electron Microscopy, X-ray Diffraction (XRD), Differential Scanning Calorimetry (DSC) and DC magnetization measurements. Addition of Cu to stoichiometric Heusler type Ni 2 MnGa increases the martensitic transformation temperatures and decreases T C . Concurrently, Δ SM increases with Cu addition and peaks at 6.5xa0at% Cu for which there is a virtual overlap between T C and A s . Maximum Refrigerant Capacity (RCP) of 327.0xa0J/Kg was also achieved in the heating cycle for 9xa0T field change at 303xa0K. Corresponding values for the cooling cycle measurements (measured during forward transformation) were 30.4xa0J/Kg-K and 123.5xa0J/Kg respectively for the same 6.5xa0at% Cu sample under the same thermo-magnetic conditions.

Studies on the magnetoelastic and magnetocaloric properties of Yb1−xMgxMnO3 using neutron diffraction and magnetization measurements

We report the magnetic ordering and magnetoelastic coupling of polycrystalline hexagonal Yb1−xMgxMnO3 (x = 0.00 and 0.05) compounds by using neutron diffraction measurements. The magnetocaloric properties of these Yb1−xMgxMnO3 compounds are also studied using magnetization measurements. The temperature dependence of the lattice parameters (a and c/a ratio) and unit cell volume V show anomalous behavior near TN1 ∼ 85 K (the Mn ordering temperature) due to the magnetoelastic effect. Also all the Mn–O bond distances display considerable variation at TN1. Isothermal magnetization curves measured near the Yb long range ordering temperatures indicate a field induced magnetic transition with applied field. The isothermal magnetic entropy change (−ΔSM) is calculated from the magnetization curves measured for different temperatures. Values of maximum entropy change (−ΔSmaxM), the adiabatic temperature change (ΔTad) and the relative cooling power (RCP) for these compounds are found to be 3.02 ± 0.37 J mol−1 K−1, 8.6 ± 0.95 K and 41 ± 9 J mol−1 for x = 0.00, and 2.63 ± 0.36 J mol−1 K−1, 9.06 ± 0.96 K and 40.0 ± 10 J mol−1 for x = 0.05, respectively, for ΔH = 100 kOe. Rescaling of the −ΔSM vs. T curves for various fields fit into a single curve, implying the second-order phase transition.

Stability of Sr3Ti2O7structure in La1.2(Sr1 − xCax)1.8Mn2O7and Ca3 − yLayMn2O7

Structural studies on the samples of nominal compositions of nthe series, La1.2(Sr1xa0−xa0xCax)1.8Mn2O7 and Ca3xa0−xa0yLayMn2O7, nhave been carried out using neutron and X-ray diffraction techniques. These nstudies show that, unlike the case of the (La,Sr)3Mn2O7 nsystem, the samples belonging to the (La,Ca)3Mn2O7 nsystem form layered Sr3Ti2O7-type structures nin a very restricted composition range. Through a systematic study on the neffect of substitution of La for Ca in the layered compound Ca3Mn2O7, nthe solubility of La in Ca3Mn2O7 has been ndetermined. The results go against the literature reports, which claim that nthe samples of nominal compositions Ca3xa0−xa0yLayMn2O7 n(1.0xa0≤xa0yxa0≤xa02.3) form with layered Sr3Ti2O7 nstructures. The usefulness of the neutron diffraction technique in unambiguous nstructural characterization of the layered compounds has also been discussed.

Low temperature neutron diffraction studies on Co(Cr1−xFex)2O4 (x = 0.05 and 0.075)

We report the magnetic structures of an Fe substituted cobalt chromite system, Co(Cr1−xFex)2O4 (x = 0.05 and 0.075), determined from the analysis of temperature dependent neutron diffraction measurements. The ferrimagnetic transition temperature (Tc) was found to increase from ∼110 K to ∼118 K with the increase of Fe substitution. Neutron diffraction studies reveal magnetic peaks below the transition temperature, Tc. In addition, we observed a broad hump below the Bragg reflection (111), indicative of diffuse scattering from short-range magnetic interactions coexisting with long range magnetic order below Tc. The length scale of this short range magnetic order shows unusual non-monotonic temperature dependence. The bulk magnetization data reveals that the magneto-structural transition (Ts) remains unaffected with temperature in both the compounds. This magneto-structural transition at Ts gives rise to the magnetic satellite peaks of the conical spin-spiral structure which can be indexed as (220)* and (002)* with a magnetic structure using an incommensurate propagation vector. With a further lowering of the temperature around the lock-in transition, TL (∼10 K), the incommensurate peak of the propagation vector (0.62, 0.62, 0) splits into two peaks with a deviation of 0.02 in Q-value from the centre.

ErFe2–H2 system I. The interstitial-site occupancy by hydrogen atoms and model predictions

Abstract The validity of various models used for predicting the interstitial-site occupancy by H or D atoms in metal hydride systems is reviewed. The geometrical parameters of the various types of tetrahedral interstitial site have been calculated using the Goldschmidt radii as well as compressed radii for the constituent metal atoms. Explicit expressions for the interstitial-site coordinates, the hole radii and the intersite separations are presented. Neutron diffraction studies have been carried out to find the D atom coordinates and the site occupancy in the ErFe2Dx, system. These findings are compared, vis-a-vis, with the predictions based on the semiempirical models, with a particular emphasis on the composition range with 0 < x ⩽ 2. It is concluded that for the system under investigation the choice of Goldschmidt radii is more appropriate and, in view of the comparable hole radii of the relevant interstitial sites, the relative affinity of H with the metal atoms (forming the interstitial hole) decid...

Magnetic behaviour of TbMnFe

Neutron diffraction and Mössbauer measurements have been carried out on the cubic Laves phase intermetallic TbMnFe. The magnetic moment on the transition metal atom is found to be low, 0.2µB, at room temperature. This moment is temperature independent down to 10 K. Magnetic moment on the rare earth atom varies from 2.5µB at 296 K to 7.27µB at 10 K. Mössbauer spectra recorded at 298 K and 78 K have magnetic character but there is a large distribution of hyperfine field values. Both these features arise due to magnetic frustration created in the sample due to the competing ferro and antiferromagnetic interactions between the transition metal atoms.

Onsite magnetic moment through cation distribution and magnetocrystalline anisotropy studies in NiFe2−xRxO4 (R = Y and Lu; x = 0, 0.05, and 0.075)

Onsite magnetic moments through cation distribution and magnetocrystalline anisotropy studies of NiFe2−xRxO4 (Ru2009=u2009Y and Lu; xu2009=u20090, 0.05, and 0.075) compounds were investigated, and the results are discussed and presented in this paper. All the compounds were prepared by solid state reaction, and the compounds formed in the cubic inverse spinel phase with the space group Fd 3 ¯ m. The cation distribution, bond lengths, u-parameter, etc. were estimated through the Rietveld refinement of XRD patterns. Increment in the lattice constant was observed upon partial substitution of Fe3+ by Y3+/Lu3+. The presence of all elements and their ionic states were confirmed from X-ray photoelectron spectroscopy studies. Analyses of Mossbauer spectra revealed that the hyperfine fields and the magnetic moments at the B-site (and hence net moment) decreased with increasing Y3+/Lu3+ occupancy and that the compounds exhibited a Neel-type, collinear ferrimagnetic structure. Magnetization measurements revealed that the magnetic m...

Sign reversal of magnetoresistance in TbMn2Si2

Temperature and field dependent magnetization and resistivity of TbMn2Si2 have been studied. Two first order magnetic transitions at Tc1 = 52u2005K and Tc2 = 64u2005K are observed in both the measurements. The rise in resistivity at Tc2 with decrease in temperature is attributed to large difference in the periodicity of magnetic and crystallographic lattice. Resistivity fits at low and relatively higher temperatures give good account of the data. These fits further suggest the role of spin fluctuations in the antiferromagnetic region. Magnetoresistance calculated from in-field resistivity data, up to 8 T, shows switching from positive to negative to positive values with temperature.