Anjali Krishnamurthy

Re-entrant spin glass like behaviour of (Fe0.90Cr0.05Ni0.05)2P

Abstract Di-iron phosphide with small substitutions of iron with Cr and Ni, (Fe 0.90 Cr 0.05 Ni 0.05 ) 2 P, has been investigated by SQUID-magnetometry. The sample exhibits a first order ferromagnetic transition at T c ≈ 180 K and a re-entrant spin glass transition at T g ≈ 125 K. The ferromagnetic transition is severely smeared by weak applied magnetic fields. The re-entrant transition is evidenced from both dc-magnetisation and ac-susceptibility measurements. An aging phenomenon is observed at temperatures below T g and the freezing temperatures, T f , and their corresponding relaxation times, τ, scale according to conventional critical slowing down: τ/τ 0 = (( T f - T g )/ T g ) - zv , with an exponent zv ≈ 9, a value that is found also at ordinary paramagnetic to spin glass transitions.

Magnetic behaviour of Cr-substituted Fe2P

Mossbauer measurements are reported in the temperature range 80-300 K in the ferromagnetic Fe2P-based Cr-substituted pseudo-binary (Fe1-xCrx)2P for x=0.01, 0.03 and 0.05. It was found that only 1% Cr reduces the magnetic ordering temperature to 150 K from 216 K in pure Fe2P. Further substitution of Cr reduces the transition point at a much slower rate. Susceptibility variation as a function of temperature shows that for a 1% Cr substitution the system is no longer ferromagnetic. It could be of either an antiferromagnetic or a spin-glass nature. Analysis of the Mossbauer spectra shows that the presence of Cr in the second coordination sphere of Fe drastically reduces the internal magnetic field at both crystalline sites.

Magnetic structure of (Fe0.97Cr0.03)2P

Magnetic behaviour of di-metal iron phosphide with a small substitution of iron by chromium, (Fe0.97Cr0.03)2P, has been studied using SQUID magnetometry and powder neutron diffraction. It is paramagnetic at temperatures above ∼180 K with persisting short range ferromagnetic (FM) order. At lower temperatures three different regions of magnetic behaviour are identified. FM order evolves in the region 180 K-120 K but much more slowly and with much less magnetic moments than in Fe2P. In the region 120 K-50 K negative exchange interactions gain some importance leading to a loss of FM order. Below 50 K FM interactions again dominate. Pinning centres influence the behaviour at low temperature up to ∼100 K.

Magnetic behaviour of Ni-substituted Fe2P

Magnetization and Mossbauer measurements are reported on two alloys, namely (Fe0.99Ni0.01)2P and (Fe0.95Ni0.05)2P. Small substitutions of Ni in the ferromagnetic compound Fe2P strengthen the ferromagnetic order, raising the Curie temperature from 216 K for Fe2P (value in the literature) to 235 (+or-10) K and 295 (+or-5) K in the system substituted with 1 and 5 at.% Ni, respectively. The magnetic anisotropy decreases. The inverse susceptibility does not follow a strictly linear temperature dependence even up to 800 K Mossbauer results suggest that Ni has a nearly total preference for the tetrahedral sites. The presence of Ni in the second coordination sphere of iron reduces the 57Fe hyperfine fields at both the crystalline sites.

Magnetic behaviour of nano-particles of Fe2.9Zn0.1O4

DC magnetization measurements are reported in the temperature range 20–100 K on a poly-disperse nano-particle sample of the spinel ferrite Fe2.9Zn0.1O4 with a log-normal size distribution of median diameter 43.6 Å and standard deviation 0.58. Outside a core of ordered spins, moments in surface layer are disordered. Results also show some similarities with conventional spin glasses. Blocking temperature exhibits a near linear variation with two-third power of the applied magnetic field and magnetizationM evolves nearly linearly with logarithm of timet. Magnetic anisotropy has been estimated by analysing theM-logt curve. Anisotropy values show a large increase over that of bulk particle samples. Major contribution to this enhancement comes from the disordered surface spins.

Electric transport behaviour of sodium-substituted perovskites La1−xNaxMnO3 (for x = 0.1 and 0.2) and the effect of magnetic fields

Sodium-substituted perovskites La0.9Na0.1MnO3 and La0.8Na0.2MnO3, which crystallize in pseudo-cubic symmetry, and are respectively a disordered and an ordered ferromagnet, have been studied for their electrical behaviour. The two samples show large negative magnetoresistance. The lesser sodium-containing system shows a metal?insulator transition at ~210?K, which is well below the temperature where it completely goes to the paramagnetic state. In the higher sodium-containing sample both the magnetic and electrical transitions would fall only above 300?K. In the conducting state in both samples, resistivity has contributions from scattering from grain boundaries, electron?electron scattering and electron?magnon (spin wave) scattering. However, in the insulating (semiconducting) state even the lesser sodium-containing system (in which the insulating state has been observed in the temperature range of study) follows small polaron hopping under an adiabatic approximation and the estimated activation energy values at different fields are also identical to those reported in magnetically ordered perovskites. La0.9Na0.1MnO3 shows an upturn in resistivity with decreasing temperature at ~30?K which is attributed to spin-polarized intergranular tunnelling.

Magnetic behavior of (Fe0.90Cr0.03Ni0.07)2P and (Fe0.90Cr0.05Ni0.05)2P

(Fe0.90Cr0.03Ni0.07)2P and (Fe0.90Cr0.05Ni0.05)2P are found to crystallize in Fe2P‐like hexagonal symmetry (hereafter referred to as FCN37 and FCN55, respectively). dc magnetization measurements have been made in external fields upto 8 kOe and in the temperature range 80–800 K. ac susceptibility measurements have been performed in the temperature range 80–300 K. dc measurements show that the two alloys are ferromagnetic with Tc∼250 K for FCN37 and ∼180 K for FCN55. Magnetic moments of the polycrystalline samples at 7 kOe and ∼90 K are 67 and 47 emu/g for FCN37 and FCN55, respectively. Both of them also exhibit small but finite remanence and coercivity. The temperature dependent magnetization measurements made on FCN37 and FCN55 in zero field cooling and field cooling modes branch off at ∼230 and ∼125 K, respectively. Remanent magnetization, measured in the remanent field of the magnet (∼20 Oe), preceded by isothermal cooling of the samples down to 85 K and applications of different fields for fixed interv...

On nature of magnetism in ferromagnetic alloys (Fe1−xCox)2P

Inverse magnetic susceptibility χ−1 has been measured as a function of temperature for five alloys in the series (Fe1−xCox)2P. Short range magnetic order above the Curie temperature in the studied alloys was observed. Paramagnetic moments have been obtained using χ−1–T curves. These are much larger than saturation moments showing dominance of the itinerant nature of magnetism. The degree of itinerancy is smaller in the orthorhombic phase than in the hexagonal phase. Substitution of Co causes magnetization to monotonically decrease. Anisotropy drops in the hexagonal phase with increasing Co but in the orthorhombic phase it again progressively increases.

Structural and magnetic properties of (Fe0.93Ni0.07)2P

Structural and magnetic properties of (Fe0.93Ni0.07)2P have been investigated by means of powder x-ray and neutron diffraction, magnetization and paramagnetic susceptibility measurements over a temperature range of 10–500 K. The system crystallizes in the Fe2P type hexagonal structure ( space group, Z = 3) in which the Ni atoms occupy the tetrahedral MI sites with total preference. Refined values of the cell parameters and bond distances are found slightly higher than those for Fe2P and the atomic positional parameters are found quite close to those reported for the parent compound Fe2P. The temperature dependence of the magnetization shows a sharp magnetic phase transition around 298(5) K. However there is difference of the zero-field cooled and field cooled modes of the magnetization, which is indicative of the formation of ferromagnetic clusters. The ferromagnetic to paramagnetic transition shifts towards the higher temperature side with increase in the applied magnetic field, which indicates that the phase transition is a field induced type first-order magnetic phase transition. There is no crystallographic structural transition associated with the magnetic phase transition. The transition is caused by the change in the c/a ratio. The alloy retains the ferromagnetic order of Fe2P with the moments orienting along the [001] direction. The Rhodes–Wohlfarth ratio (µp/µs) of (Fe0.93Ni0.07)2P is found to be 1.53 (>1), showing itinerant magnetism in this compound. At 297 K the magnetic moment at the MI site is found negligible but at the MII site it is ~0.51 µB. The observed non-linearity above Tc in the χ−1–T curve gives clear evidence of the presence of short range magnetic order above Tc. The moments at the MII sites in the paramagnetic state and the exchange interactions are responsible for the short range one-dimensional ferromagnetic chains along [001] well above the Tc in (Fe0.93Ni0.07)2P.

Lattice sum calculations and a Mössbauer study on electric field gradient in phlogopite

Lattice sum contributions have been calculated at the two octahedral sites in a phlogopite mica assuming a systematic distribution of octahedral cationic charges. This, unlike the case of a random distribution of charges, is able to reproduce broad features of quadrupole doublet spectra in Mössbauer experiments suggesting that the doublet assignments in terms of the two structural sites,M1 (trans) andM2(cis), are quite valid. Angle dependence of Mössbauer spectra has also been studied for a ferric-rich phlogopite mica sheet to determine the orientation of the EFG principal axis component and the sign of the quadrupole coupling constant. Quadrupole splitting values and the positive sign of coupling constants match the theoretical predictions quite well but the model fails to predict the observed EFG orientation.