B. L. Ahuja

Compton profile study of bonding in BeO

The isotropic Compton profile of BeO has been measured using a γ-ray Compton spectrometer which employs a 5 Ci annular 241Am source. The measurement is compared with the theoretical HF-LCAO Compton profile published by Lichanot et al. The measurement has also been compared with our calculations based on the RFA model employing different ionic wavefunctions. It is seen that the experimental Compton profile is in very good agreement with the HF-LCAO calculations. To determine the nature of bonding, equal-valence-electron-density Compton profile of BeO has been compared with that of the isovalent compound MgO. It is observed that the bonding in BeO is partially covalent and less ionic than MgO in agreement with the conclusions of the HF-LCAO calculation.

Magnetoresistance behavior of ferromagnetic shape memory alloy Ni 1.75 Mn 1.25 Ga

A negative-positive-negative switching behavior of magnetoresistance (MR) with temperature is observed in a ferromagnetic shape memory alloy

Variation of magnetoresistance in Ni2+xMn1−xGa with composition

{\text{Ni}}_{1.75}{\text{Mn}}_{1.25}\text{Ga}

Renormalized-free-atom model and Compton profile of hcp cobalt

. In the austenitic phase between 300 and 120 K, MR is negative due to

Temperature dependent spin momentum densities in Ni-Mn-In alloys.

s\text{\ensuremath{-}}d

Investigation of orbital magnetization in inverse spinel cobalt ferrite using magnetic Compton scattering

scattering. Curiously, below 120 K MR is positive, while at still lower temperatures in the martensitic phase, MR is negative again. The positive MR cannot be explained by Lorentz contribution and is related to a magnetic transition. Evidence for this is obtained from ab initio density-functional theory, a decrease in magnetization and resistivity upturn at 120 K. Theory shows that a ferrimagnetic state with antiferromagnetic alignment between the local magnetic moments of the Mn atoms is the energetically favored ground state. In the martensitic phase, there are two competing factors that govern the MR behavior: a dominant negative trend up to the saturation field due to the decrease in electron scattering at twin and domain boundaries and a weaker positive trend due to the ferrimagnetic nature of the magnetic state. MR exhibits a hysteresis between heating and cooling that is related to the first-order nature of the martensitic phase transition.

Reversal of orbital magnetic moment on substitution of Bi in multiferroic Co2MnO4: A magnetic Compton scattering study

The magnetoresistance (MR) of Ni2+xMn1−xGa (−1≤x≤0.35) ferromagnetic shape memory alloy shows a large increase in magnitude at room temperature (RT) with increasing x. For Mn2NiGa (x=−1), MR at 8 T is −0.2%, while for Ni2.35Mn0.65Ga (x=0.35), it is −7.3%. Thus, MR of Ni2+xMn1−xGa can be varied over one order of magnitude by changing composition (x). Considering that the Curie temperature (TC) varies with x, the MR behavior in the austenitic phase is explained on the basis of the s−d scattering model. By fitting the MR at 8 T in the austenitic phase for different x and T, a (T/TC)6 power law dependence is obtained. In contrast to the monotonic MR variation with x, the magnetization at RT is highest for Ni2MnGa (x=0) and decreases for both Ni and Mn excess compositions.

Magnetic and electrical behavior of Al doped La0.7Ca0.3MnO3 manganites

Compton profile of cobalt has been calculated employing the renormalized-free-atom model for fcc as well as hcp phases choosing several 3d-4s configurations. The results have been compared with recent gamma-ray measurements on polycrystalline Co. Best agreement between theory and experiment is found for 3d74s2 configuration. Comparison with free electron model has also been made for this case.

Electronic properties of RDX and HMX: Compton scattering experiment and first-principles calculation.

The spin-dependent electron momentum densities in Ni(2)MnIn and Ni(2)Mn(1.4)In(0.6) shape memory alloy using magnetic Compton scattering with 182.2 keV circularly polarized synchrotron radiation are reported. The magnetic Compton profiles were measured at different temperatures ranging between 10 and 300 K. The profiles have been analyzed mainly in terms of Mn 3d electrons to determine their role in the formation of the total spin moment. We have also computed the spin polarized energy bands, partial and total density of states, Fermi surfaces and spin moments using full potential linearized augmented plane wave and spin polarized relativistic Korringa-Kohn-Rostoker methods. The total spin moments obtained from our magnetic Compton profile data are explained using both the band structure models. The present Compton scattering investigations are also compared with magnetization measurements.

Feasibility of magnetic Compton scattering in measurement of small spin moments: A study on LaFe1−xNixO3 (x = 0.4 and 0.5)

We present the spin momentum densities of CoFe2O4 measured at 8 and 300 K using magnetic Compton scattering. The magnetic Compton profiles were decomposed into component profiles of constituents namely Fe and Co, to determine their role in the formation of total spin moment. It is seen that the major contribution (about 80%) in the spin moment is from Co, whereas the itinerant electrons show a small reverse polarization. Moreover, it is clearly visualized that the spin moment at Co reduces from 2.55 → 2.35 μB/f.u. while going from 8 → 300 K. The magnetic Compton profiles, when compared with the magnetization data, show about 17% contribution of orbital moment to the total magnetic moment at both temperatures. The origin of the orbital moment is explained on the basis of rotation of hole on t2g orbital of Co+2 ion.