S. N. Mishra

Enhanced magnetization in cubic ferrimagnetic CuFe2O4 nanoparticles synthesized from a citrate precursor: the role of Fe2+

We report a simple liquid dehydration route involving citrate precursors to synthesize single-phase ferrimagnetic copper ferrite (CuFe2O4) nanoparticles with a cubic crystal structure. Superparamagnetism was exhibited by 5 nm CuFe2O4 particles while ferrimagnetism prevailed at 11 nm and above. The cubic CuFe2O4 samples grown under specific conditions show a systematic enhancement in the saturation magnetization. Both quantitative chemical analysis and the unit cell volume data indicate that samples with larger magnetization contain a higher fraction of Fe2+ ions in place of Fe3+. We provide a plausible model for the cation distribution among the available tetrahedral and octahedral sites that quantitatively explains the observed enhancement in the saturation magnetization. This appears to be the first attempt to increase the magnetization by controlling the Fe2+ concentration.

Do ruthenium atoms possess local moment in the magnetically ordered system Fe3−xRuxSi (?)

Abstract Magnetization and the 57 Fe hyperfine field values show a very small variation on substitution of Fe with Ru atoms in the ferromagnetic alloys Fe 3−x Ru x Si for x values as large as 1.35. The data are strongly suggestive of the presence of moment of ∼ 1 μ B at Ru atoms.

Magnetic behaviour of the heusler alloy Ru2FeSi

Abstract Magnetization measurements and 57 Fe Mossbauer effect studies in a new Heusler alloy Ru 2 FeSi indicate that the moments of Fe in this alloy have antiferromagnetic orientation. The values obtained at 4.2 K for μ/f.u. and the average value of the hyperfine field at Fe are 4X10 -2 μ B (in a field of 8 kOe) and 225 kOe, respectively. The alloy exhibits a complex magnetic behaviour due to the presence of inherent atomic disorder in the sample depicting two ordering temperatures as seen from X ac ( T ) at 280 and 35 K. The latter temperature is conjectured to be the temperature at which the reorientation of Fe moments occur. The Ru substitution of Fe in Fe 3 Si thus brings about the transformation from a ferromagnetic ordering of Fe 3 Si to an antiferromagnetic ordering in Ru 2 FeSi.

Mössbauer and magnetization studies in Fe3−xRuxSi

Magnetization and 57Fe Mossbauer studies have been made in Fe3−xRuxSi for 0≤x≤2. Up to x=1.35, the alloys behave like collinear ferromagnets and μ/f.u. and HF (Fe) values do not show any appreciable change, thus suggesting that Ru replacing magnetic Fe (A,C) atoms also possess a large moment of ∼1 μB. For x>1.35, the approach of magnetization to saturation becomes progressively slower and the system acquires an inhomogeneous magnetic structure. In the region 1.45 ≤x 1.8, majority of the Fe atoms are on B site and they presumably undergo AF/random freezing. Even though Tm and magnetization behavior show considerable variation across the region 1≤x≤2, the character of Mossbauer spectra does not change and 〈HF (Fe)〉av measured at 4.2 K remain ∼250 kOe.

Quadrupole interactions at 111Cd nuclei in light rare earths

Abstract Quadrupole interaction at 111 Cd nuclei introduced as dilute impurity in light rare-earth hosts have been studied using the TDPAC technique. The electronic enhancement of the electric field gradient is found to be almost independent of the rare earth in the first half of the series contrary to a steep decrease in the second half with a sharp change at Gd.

Electronic structure and magnetic properties of 3d impurities in antiferromagnetic Cr

Using the full-potential linearized augmented plane wave (FLAPW) method based on the density function theory (DFT) we have performed ab initio calculations of magnetic properties for dilute Cr–X alloys with 3d transition metals. We present here our results for the density of states, local magnetic moments and hyperfine fields. The paramagnetic local density of states (LDOS) analyzed with the Stoner model suggests the occurrence of local moments for Mn, Fe, Co and Ni consistent with results obtained from spin polarized calculations. In addition, we find a large magnetic moment for the non-magnetic V atom in Cr. The impurity moments of Sc, Ti, V and Mn are found to be antiparallel with respect to the near neighbor Cr moment while, they show ferromagnetic coupling for Fe, Co, and Ni. The observed sign change for the exchange coupling is also reflected in our calculated hyperfine field results. The trend observed for the variation of the impurity moment and hyperfine fields of 3d impurities in antiferromagnetic Cr show remarkable similarity with the results reported for ferromagnetic Fe, Co and Ni hosts. The results presented here would be helpful for a common understanding of local magnetic properties of transition metal impurities in ferro and antiferromagnetic hosts.

Magnetic interaction in NdScGe: a local investigation by perturbed angular correlation spectroscopy

The magnetic and electric hyperfine interactions for the 111Cd probe nucleus in the equi-atomic ferromagnetic compound NdScGe ( K) have been investigated by the time differential perturbed angular correlation (TDPAC) technique. The Cd probe occupying the Sc site experiences a large magnetic hyperfine field with saturation value Bhf(0) = −8.5 T. By comparing the results with the hyperfine field data in Nd metal and estimates made with the RKKY interaction, we find an indication for sizeable spin polarization of the conduction electrons in NdScGe. In addition, we find evidence of lattice softening near the Curie temperature reflected by an abrupt decrease in the quadrupole interaction frequency νQ(T).

Dynamic lattice distortions in Sr2RuO4: microscopic studies by perturbed angular correlation spectroscopy and ab initio calculations.

Applying time differential perturbed angular correlation (TDPAC) spectroscopy and ab initio calculations, we have investigated possible lattice instabilities in Sr(2)RuO(4) by studying the electric quadrupole interaction of a (111)Cd probe at the Ru site. We find evidence for a dynamic lattice distortion, revealed from the observations of: (i) a rapidly fluctuating electric-field gradient (EFG) tensor showing non-Arrhenius relaxation, (ii) an anomalous temperature dependence of the quadrupole interaction frequency, and (iii) a monotonic increase of the EFG asymmetry (η) below 300 K. We argue that the observed dynamic lattice distortion is caused by strong spin fluctuations associated with the inherent magnetic instability in Sr(2)RuO(4).

Spin fluctuations of isolated Fe impurities in Pd-based dilute alloys: effect of ferromagnetic host spin polarization

The magnetic moment and spin fluctuation temperature TSF of isolated Fe atoms in a number of Pd-based binary alloys, namely Pd0.95M0.05 (M = Ni, Rh, Mo, Ag, Cd, In, Sn, Th and U), have been determined from the local susceptibility χloc(T) of 54Fe probe nuclei measured by means of the time differential perturbed angular distribution (TDPAD) technique. Depending on the element M added to the Pd matrix, the results derived from Curie–Weiss analysis, χloc(T) = C/(T+TSF), reveal strong enhancement or suppression of the ferromagnetic host spin polarization Sh associated with the giant moment of Fe. Concurrent with the variation in host spin polarization, we have observed a large change in the spin fluctuation temperature TSF, showing an exponential rise with diminishing value of Sh. The results, analysed on the basis of the Kondo model, indicate that spin fluctuations caused by the antiferromagnetic interaction associated with the negative conduction electron spin polarization are suppressed progressively by an incremental addition of a positive contribution to the effective exchange interaction.

Local magnetism of isolated Mo atoms at substitutional and interstitial sites in Yb metal: experiment and theory

We have observed large 4d moments on isolated Mo atoms at substitutional and octahedral interstitial lattice sites in Yb metal, showing Curie-Weiss local susceptibility and a Korringa-like spin relaxation rate. The interstitial Mo atoms, despite strong hybridization with the Yb neighbors, show rather small Kondo temperature ( T(K)</=60 K), implying high moment stability. While magnetism of Mo at the substitutional site is consistent with Kondo-type d-sp interaction, we suggest that moment stability at the interstitial site is strongly influenced by ferromagnetic polarization of Yb-4f5d band electrons.