N.V. Rama Rao

Influence of Ni/Mn concentration on the structural, magnetic and magnetocaloric properties in Ni50−xMn37+xSn13 Heusler alloys

We report the structure, magnetism and magnetic entropy change in a Mn-rich Ni50−xMn37+xSn13 Heusler alloy system in the composition range 0 ≤ x ≤ 4. An excess Mn content stabilizes the cubic austenite phase at room temperature. Martensitic transition decreases from 305 to 100u2009K with increasing Mn concentration (x: 0 → 4) and also it was found to shift to a lower temperature with the application of a higher magnetic field. The exchange bias blocking temperature was found to decrease drastically from 149 to 9u2009K with increasing Mn concentration. A large magnetic entropy change (ΔSM) of 32u2009Ju2009kg−1u2009K−1 has been achieved for a field change of 5u2009T in the x = 3 alloy.

Effect of Ni/Mn concentration on exchange bias properties in bulk Ni50−xMn37+xSn13 Heusler alloys

We report the effect of Ni/Mn variation on the exchange bias properties in the bulk Mn-rich Ni50−xMn37+xSn13 (0 ≤ x ≤ 4) Heusler alloys. The excess Mn content was found to increase the exchange bias field while it decreases the exchange bias blocking temperature (TEB) from 149 to 9 K. A maximum shift in the hysteresis loop of 377 Oe is observed for the Ni46Mn41Sn13 alloy. As compared to Mn/Sn variation, Ni/Mn variation strongly influences the exchange bias properties in Ni-Mn-Sn alloys. We observed that if the Mn content is above 37 at. % in Ni-Mn-Sn alloys, the TEB value would show a decreasing trend either by varying the Ni or Sn content.

Effect of Ni/Mn ratio on phase transformation and magnetic properties in Ni-Mn-In alloys

The effect of variation in Ni/Mn ratio on structure, phase transformation, and magnetic properties was investigated in the Ni50−xMn37+xIn13 alloys. Small change in the Ni/Mn ratio drives the structure from martensite of tetragonal L10 to austenite of cubic L21 at room temperature. With decrease in Ni/Mn ratio or increase in Mn content the martensitic transformation temperature was found to decrease and the alloys do not undergo phase transformation below a critical value (7.86) of valence electron concentration (e/a). Temperature and field dependence of magnetization data reveals the complex magnetic nature arising from the coexistence of ferromagnetic and antiferromagnetic interactions in the system. It was found that the effect of Ni/Mn and Mn/In ratios on phase transformation and magnetic properties in Ni–Mn–In alloys is similar if the e/a value of the alloy system remains unchanged.

Effect of initial particle size on phase transformation temperature of surfactant capped Fe3O4 nanoparticles

We investigate the effect of particle size on reduction temperatures in surfactant capped fine iron oxide (Fe3O4) nanoparticles in size ranging from 7 to 3 nm using in situ high temperature X-ray diffraction (HTXRD). The 7 nm size particles are reduced to metallic α-Fe and FeO phase at 400°C and remains stable up to 600°C. On further heating, α-Fe phase grows at the expense of FeO and the growth process completes at 800°C. Above 900°C, α-Fe is converted to γ-Fe phase and at 1000°C, a part of γ-Fe phase is converted to α-Fe2O3. As the size is decreased from 7 to 3 nm, the onset of reduction to metallic Fe and FeO is enhanced by 100 to 200°C, due to the increased surface spin disorder. Irrespective of the initial particle size, the final phase obtained after annealing at 1000°C and cooled back to room temperature was a mixed phase of α-Fe and α-Fe2O3. Thermo Gravimetric Analysis coupled Mass Spectra (TGA-MS) confirm that the evolved carbon from the oleic acid assist the removal of oxygen atom from Fe3O4 lat...

SmCo5/Fe nanocomposite magnetic powders processed by magnetic field-assisted ball milling with and without surfactant

A magnetic field-assisted ball milling has been employed for the preparation of SmCo5 + 10u2009wt% Fe nanocomposite powders in the presence of oleic acid as surfactant. Milling experiments were also carried out without using surfactant and the nanocomposite powders so obtained, with and without surfactant, were investigated for their structural and magnetic properties using SEM, XRD, VSM and Mossbauer spectrometry. The field-milled SmCo5/Fe nanopowders in the presence of surfactant display a possible grain orientation and possess relatively high coercivity as compared with that of SmCo5/Fe powders obtained with field-milling or conventional milling. Mossbauer studies revealed that the formation of α-Fe(Co) (soft magnetic phase) is more pronounced for the powders milled without surfactant.

Hydrostatic pressure effect on the martensitic transition, magnetic, and magnetocaloric properties in Ni50-xMn37+xSn13 Heusler alloys

We report upon the effect of hydrostatic pressure on the martensitic transition, magnetic properties, and magnetic entropy change in Ni50-xMn37u2009+u2009xSn13 (xu2009=u20092, 3) Heusler alloys. The application of pressure has significantly shifted the martensitic transition temperature to higher values. A large rate of change of the martensitic transition with a pressure of ∼3.1 K/kbar has been obtained for the xu2009=u20092 alloy, whereas the Curie temperature changes marginally with pressure (∼0.3 K/kbar). Magnetization of both the austenite and martensite phases decreases with an increase of pressure. The maximum magnetic entropy change of 34 J kg−1K−1 at ambient pressure and 22.5 J kg−1K−1 at 8 kbar was observed around the martensitic transition temperature for the xu2009=u20093 alloy.

Influence of Si substitution on the structure, magnetism, exchange bias and negative magnetoresistance in Ni48Mn39Sn13 Heusler alloys

The effect of partial Si substitution for Sn on the structure, magnetism, magnetic entropy change, exchange bias and negative magnetoresistance in Ni48Mn39Sn13?xSix(1???x???4) Heusler alloy systems is investigated. A small amount of Si in the Sn position influences the characteristic transition temperatures and the magnetocaloric effect. A maximum positive entropy change of 5.13?J?kg?1?K?1 is observed for a field change of 9?T in the x?=?1 alloy. An increase in Si content increases the exchange bias field. The exchange bias field and the coercive field strongly depend on the Si content. For the x?=?4 alloy, an exchange bias field of 354?Oe is observed. In addition, a negative magnetoresistance is observed in this alloy system, which decreases with Si content. The increase in resistivity at the martensitic transition could be attributed to the formation of superzone boundary gaps that changes the density of states near the Fermi surface.

Microstructure, magnetic and Mössbauer studies on spark-plasma sintered Sm–Co–Fe/Fe(Co) nanocomposite magnets

Nanocomposite powders comprising Sm–Co–Fe intermetallic phases and Fe(Co) were synthesized by high-energy ball milling and were consolidated into bulk magnets by the spark-plasma sintering (SPS) technique. While the microstructure of the SPS samples was characterized by transmission electron microscopy (TEM), the solubility of Fe in different phases was investigated using Mossbauer spectroscopy. TEM studies revealed that the spark-plasma sintered sample has Sm(Co,Fe)5 as a major phase with Sm2(Co,Fe)17, Sm(Co,Fe)2 and Fe(Co) as secondary phases. The size of the nanocrystalline grains of all these phases was found to be in the range 50–100u2009nm. The Mossbauer spectra of the as-milled powders exhibited two different subspectra: a sextet corresponding to the Fe phase and a broad sextet associated with the Fe(Co) phase; while that of the SPS sample showed four different subspectra: a sextet corresponding to Fe and other three sextets corresponding to the Fe(Co), Sm(Co,Fe)5 and Sm2(Co,Fe)17 phases; these results are in accordance with the TEM observation. Recoil magnetization and reversible susceptibility measurements revealed magnetically single phase behaviour of the SPS magnets.

Neutron diffraction studies on the Heusler alloy Ni50Mn37Sb13

The evolution of martensitic to austenitic transformation in Ni50Mn37Sb13 has been studied usingtemperature dependent neutron diffraction, thermal property, and magnetization studies. Differential scanning calorimetric studies reveal a martensitic transformation TM around 291 K. The magnetization data yield a ferromagnetic ordering temperature of 329 K in the austenitic phase and 230 K in the martensitic phase. The analysis of the powder neutron diffraction data in the temperature range of 325–12 K indicates a structural transition from a high temperature cubic L21 type structure to an orthorhombic structure. At 270 K, both cubic and orthorhombic phases coexist. Anisotropic unit cell changes are observed at the martensitic transformation: The unit cell expands by about 1.5% along the a axis, by about 2.5% along the c axis, and compresses by about 4.28% along the b axis. Both cubic and orthorhombic phases show commensurate collinear ferromagnetic ordering with a magnetic moment of ∼3.67 μB/Mn in Mn (2a and...