V. Chandrasekaran

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.

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 + 10 wt% 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.

Effect of Co, Dy and Ga on the magnetic properties and the microstructure of powder metallurgically processed Nd–Fe–B magnets

Abstract The elemental additions to Nd–Fe–B for the simultaneous enhancement of the Curie temperature (TC), intrinsic coercivity (Hci), remanence (Br) appear to be the viable option for improving the temperature stability of the magnetic properties. In this work, an attempt has been made to understand the effect of individual and successive additions of Co, Dy and Ga on the degree of variation in these magnetic properties and in the microstructure of anisotropic Nd–Fe–B magnets. Co additions increase the Curie temperature in proportion to its concentration in the alloy but reduces the coercivity, Hci. Formation of new phases such as Nd(FeCo)2 and Nd(FeCo)3 in Co added alloy has been proved by X-ray diffraction and SEM-EDAX/EPMA studies. From the thermo-magnetic plots, the new phases appear to be ferromagnetic, which may be responsible for easy demagnetization and reduction in Hci. The increase in TC is attributed to the solubility of Co in the Nd2Fe14B (φ phase). With the additions of Dy and Ga, solubility of Co in φ phase increases. In 36Nd–1.2B–7Co–Fe, with successive additions of 4% Dy and 1% Ga, a significant improvement in Hci (from 300 to 970 kA/m) along with a marginal improvement in TC (from 695 to 715 K) has been observed. The changes in the composition of the φ phase as well as the improvement in the microstructural features are attributed to the enhancement in TC and Hci in Co, Dy and Ga substituted Nd–Fe–B magnets.

Structural, magnetic, and magnetotransport studies in bulk Ni55.2Mn18.1Ga26.7 alloy

Structural, magnetic, and transport properties have been investigated in off-stochiometric bulk Ni55.2Mn18.1Ga26.7 alloy. The alloy undergoes an austenite-martensite phase transition in ferromagnetic state at 264 K. The temperature and field dependence of electrical resistivity studies revealed that the electron-electron scattering is dominant in determining the resistivity below 80 K, while at higher temperatures (80–300 K), electron-magnon as well as electron-phonon scattering dominate. A negative magnetoresistance value of 7% maximum near TC (∼283 K) at 7 T field has been obtained in the alloy. Also a magnetocaloric value (ΔSM) of −1.3 J/kg K has been observed at around 264 K.

Phase coexistence, microstructure and magnetism in Ni?Mn?Sb alloys

We present here a detailed investigation on the structural and magnetic transformations in the Ni50Mn50?xSbx Heusler alloys with composition x = 12, 13 and 14. The evolution of martensitic to austenitic transformation has been investigated using temperature dependent x-ray diffraction study. A wide region of phase coexistence is observed across the martensitic transformation. The unit cell volume contracts by 0.82% and 0.74% for x = 12 and 14, respectively, at the martensitic transformation temperature. The martensitic transition temperature decreases with increase in Sb concentration. The magnetic interactions in the martensitic and austenitic phases are found to be ferromagnetic, however, with different strengths, giving rise to two separate magnetic transitions with . The magnetization studies reveal the presence of antiferromagnetic?ferromagnetic interactions in the alloys. Exchange bias blocking temperature has been observed around 100?K for the alloy x = 12. Magnetic field induced structural transition has been observed in x = 14 alloy, in which the structural transition temperature shifts by 25?K in a field of 3?T.

Effect of processing parameters on the microstructure and soft magnetic properties of Fe88Zr7B4Cu1 alloy ribbons

Structure and soft magnetic properties of melt spun Fe88Zr7B4Cu1 alloy have been investigated using x-ray diffraction, differential scanning calorimetry, transmission electron microscopy and vibrating sample magnetometer. On decreasing the cooling rate the structure of as spun ribbon changes from completely amorphous to a cellular structure of bcc solid solution along with the amorphous phase at intercellular regions. Annealing leads to the precipitation of nanocrystalline bcc-Fe phase from both amorphous phase and already existing bcc solid solution. The saturation magnetization increases sharply for all samples on annealing at 500 °C due to the precipitation of nanocrystalline bcc phase and then remains almost constant at higher annealing temperatures. The coercivity decreases initially with annealing temperature, attains a minimum value and then increases at higher temperature.

Dynamic inverse-magnetocaloric and martensite transition in Ni49Mn38Sn13 nanocrystals in low magnetic fields

Ni49Mn38Sn13 nanocrystals exhibit a sharp transition from a paramagnetic martensite MP to a strongly ferromagnetic austenite phase AF, with a well-defined peak of magnetotherm at 288.1 K (∼15 K peak-width) upon heating in a low magnetic field (a few tens of mT). The Curie point of the austenite phase, = 305 K lies well above the MF phase ( = 228 K). A kind of spin-superheating promotes fast reordering of the nanocrystals with a spin → lattice heat transfer. The low-field magnetic entropy changes as much as 6.8 mJ g−1 K−1 at 1.4 T. Cooling via shows a smaller enthalpy change (−) 8.35 J g−1 from forward transition of 9.18 J g−1 (in part with the superheating) in a calorimetric peak at 284.4 K, with a small hysteresis (−) 12.1 K. A model phase diagram is proposed to describe the transitions with intermediate spin-transport processes.

Phase relationship, microstructure and magnetocaloric effect in Gd1?x(Si0.5Ge0.5)x alloys

Microstructure and magnetocaloric effect were investigated in Gd1?x(Si0.5Ge0.5)x alloys with x = 0.38, 0.41, 0.45, 0.47 and 0.50. The phase identification and compositional analysis were carried out by a combination of x-ray powder diffraction, scanning electron microscopy and electron probe microanalysis. The Gd5(Si,Ge)4 phase was found to have an orthorhombic Gd5Si4-type structure when the coexisting phase was Gd-rich (Gd5Si3-type) and a monoclinic Gd5Si2Ge2-type structure when the coexisting phase was Gd-depleted (Gd5Ge5-type). The magnetocaloric effect was found to depend on the volume fraction and the Si/Ge ratio of the ferromagnetic 5?:?4 phase in the above series of alloys. A maximum magnetocaloric effect (?S)M of 14.3?J?kg?1?K?1 for a magnetic field change from 0 to 5?T was obtained for the x = 0.45 alloy.

Amorphization, nanocrystallization and magnetic properties of mechanically milled Sm–Co magnetic powders

Abstract The microstructure and the magnetic properties of mechanically milled Sm–Co nano powders were investigated using X-ray diffraction, transmission electron microscopy and vibrating sample magnetometry. Microstructural studies revealed that after 40 h milling the powder nearly becomes amorphous with a small fraction of nanocrystallites embedded in the amorphous matrix. With increase in milling time up to 16 h, the diffracted crystallite size decreased exponentially and thereafter it remained constant. The grain size of 40 h milled powder was found to be ∼2 – 10 nm.

Structure and magnetic properties of Nd4.5Fe77−xSnxB18.5 nanocomposite alloys

The effect of partial substitution of Sn(Fe) on the crystallization behaviour, magnetic properties and microstructure of rapidly solidified Nd4.5Fe77?xSnxB18.5 (x = 0.0, 0.5, 1.0 and 1.5) nanocomposite alloys has been studied. Sn decreases the crystallization temperature and suppresses the formation of the undesired Nd2Fe23B3 phase. Remanence (Mr), remanence ratio (Mr/Ms) and maximum energy product (BH)max increase from 10.1?kG, 0.70 and 9.1?MG?Oe to 12.6?kG, 0.82 and 12.8?MG?Oe, respectively, on addition of 1.5% Sn. The coercivity (iHc) decreases slightly from 2.6 to 2.2?kOe with an increase in Sn from x = 0.0 to x = 1.5. TC of the Nd2Fe14B increases from 588 to 602?K. The results are discussed and correlated with crystallization behaviour, microstructure and M?ssbauer studies.