M. Manivel Raja

Structure and soft magnetic properties of Finemet alloys

Abstract The soft magnetic properties of Fe73.5Cu1Nb3Si13.5B9 (Finemet) alloys prepared by mechanical alloying were studied using various experimental techniques including Mossbauer spectroscopy. The study includes the influence of the milling atmosphere. The results are compared with mechanically alloyed Fe–Si alloys as well as with melt-spun Finemet alloys. The coercivity of the mechanically alloyed powder is much larger than that of the melt-spun ribbons, though the saturation magnetisation is the same. This is ascribed to the absence of a grain boundary amorphous ferromagnetic phase resulting in a weakening of the exchange coupling between the nanograins. This study suggests that the interfacial component in Finemet alloys plays a crucial role in achieving good soft magnetic properties. The critical grain size for single domain particle was found to be 10 nm. For very small grain sizes, the existence of superparamagnetism was also studied using Fe-57 Mossbauer spectroscopy.

Spin gapless semiconducting behavior in equiatomic quaternary CoFeMnSi Heusler alloy

In this paper, we report the signature of spin gapless semiconductor (SGS) in CoFeMnSi that belongs to the Heusler family. SGS is a new class of magnetic semiconductors which have a band gap for one spin subband and zero band gap for the other, and thus are useful for tunable spin transport based applications. We show various experimental evidences for SGS behavior in CoFeMnSi by carefully carrying out the transport and spin-polarization measurements. SGS behavior is also confirmed by first-principles band-structure calculations. The most stable configuration obtained by the theoretical calculation is verified by experiment. The alloy is found to crystallize in the cubic Heusler structure (LiMgPdSn type) with some amount of disorder and has a saturation magnetization of

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

3.7\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}/\mathrm{f}.\mathrm{u}.

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

and Curie temperature of \ensuremath{\sim}620 K. The saturation magnetization is found to follow the Slater-Pauling behavior, one of the prerequisites for SGS. Nearly-temperature-independent carrier concentration and electrical conductivity are observed from 5 to 300 K. An anomalous Hall coefficient of 162 S/cm is obtained at 5 K. Point contact Andreev reflection data have yielded the current spin-polarization value of 0.64, which is found to be robust against the structural disorder. All these properties strongly suggest SGS nature of the alloy, which is quite promising for the spintronic applications such as spin injection as it can bridge the gap between the contrasting behaviors of half-metallic ferromagnets and semiconductors.

Origin of spin gapless semiconductor behavior in CoFeCrGa: Theory and Experiment

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 100 K 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 9 K with increasing Mn concentration. A large magnetic entropy change (ΔSM) of 32 J kg−1 K−1 has been achieved for a field change of 5 T in the x = 3 alloy.

High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

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.

Magnetostructural transformation, microstructure, and magnetocaloric effect in Ni-Mn-Ga Heusler alloys

Despite a plethora of materials suggested for spintronic applications, a new class of materials has emerged, namely spin gapless semiconductors (SGS), that offers potentially more advantageous properties than existing ones. These magnetic semiconductors exhibit a finite band gap for one spin channel and a closed gap for the other. Here, supported by the first-principles, electronic-structure calculations, we report the first experimental evidence of SGS behavior in equiatomic quaternary CoFeCrGa, having a cubic Heusler (L21) structure but exhibiting chemical disorder (DO3 structure). CoFeCrGa is found to transform from SGS to half-metallic phase under pressure, which is attributed to unique electronic-structure features. The saturation magnetization (MS) obtained at 8 K agrees with the Slater-Pauling rule and the Curie temperature (TC) is found to exceed 400 K. Carrier concentration (up to 250 K) and electrical conductivity are observed to be nearly temperature independent, prerequisites for SGS. The anomalous Hall coefficient is estimated to be 185 S/cm at 5 K. Considering the SGS properties and high TC, this material appears to be promising for spintronic applications.

Soft magnetic properties of nanostructured FINEMET alloy powder cores

We report the structure, magnetic property, and spin polarization of CoFeMnGe equiatomic quaternary Heusler alloy. The alloy was found to crystallize in the cubic Heusler structure (prototype LiMgPdSn) with considerable amount of DO3 disorder. Thermal analysis result indicated the Curie temperature is about 750 K without any other phase transformation up to melting temperature. The magnetization value was close to that predicted by the Slater-Pauling curve. Current spin polarization of P = 0.70 ± 0.01 was deduced using point contact andreev reflection measurements. The temperature dependence of electrical resistivity has been fitted in the temperature range of 5–300 K in order to check for the half metallic behavior. Considering the high spin polarization and Curie temperature, this material appears to be promising for spintronic applications.

Edge-carboxylated graphene anchoring magnetite-hydroxyapatite nanocomposite for an efficient 4-nitrophenol sensor

Magnetostructural transformation and the associated magnetic entropy change were investigated in Ni-rich ferromagnetic Heusler alloys. A direct transformation from the ferromagnetic martensite phase to the paramagnetic austenite phase was observed in selected Ni54.8Mn20.3Ga24.9 and Ni55Mn18.9Ga26.1 two-alloy compositions. The magnetic and martensitic transformations were incurred at nearly the same temperature (351 K) in the Ni54.8Mn20.3Ga24.9 alloy. Such a typical composition involves a change of the magnetic entropy ΔSM as large as −7.0 J/kg K at 332 K in an applied magnetic field of 1.2 T.

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

The soft magnetic properties of mechanically alloyed nanostructured FINEMET powder cores were studied using magnetic measurements and impedance spectroscopy. The initial permeability of the nanostructured powder core was found to be constant up to 5 MHz frequency range. The minor addition of Al for Si significantly improves the softness of the FINEMET alloys. The optimum initial permeability and minimum core losses were obtained for 1 at.% of Al addition. This study shows the possibility of using nanostructured powder cores for high frequency applications.