K. G. Suresh

Itinerant electron metamagnetism and magnetocaloric effect in RCo2-based Laves phase compounds

Abstract By virtue of the itinerant electron metamagnetism (IEM), the RCo 2 compounds with R=Er, Ho and Dy are found to show first-order magnetic transition at their ordering temperatures. The inherent instability of Co sublattice magnetism is responsible for the occurrence of IEM, which leads to interesting magnetic and related physical properties. The systematic studies of the variations in the magnetic and magnetocaloric properties of the RCo 2 -based compounds show that the magnetovolume effect plays a decisive role in determining the nature of magnetic transitions and hence the magnetocaloric effect (MCE) in these compounds. It is found that the spin fluctuations arising from the magnetovolume effect reduce the strength of IEM, which subsequently lead to a reduction in the MCE. Most of the substitutions at the Co site are found to result in a positive magnetovolume effect, leading to an initial increase in the ordering temperature. Application of pressure, on the other hand, causes a reduction in the ordering temperature due to the negative magnetovolume effect. A comparative study of the magnetic and magnetocaloric properties of RCo 2 compounds under various substitutions and applied pressure is presented. Analysis of the magnetization data using the Landau theory of magnetic phase transitions has shown that there is a strong correlation between the Landau coefficients and the MCE. The variations seen in the order of magnetic transition and the MCE values seem to support the recent model proposed by Khmelevskyi and Mohn for the occurrence of IEM in RCo 2 compounds. Metastable nature of the transition metal sublattice in RCo 2 -based compounds and its role in determining the magnetic and magnetocaloric properties is explained.

Effect of magnetic polarons on the magnetic, magnetocaloric, and magnetoresistance properties of the intermetallic compound HoNiAl

The magnetic, magnetocaloric, and magnetoresistive properties of the polycrystalline compound HoNiAl have been studied. The temperature variations of magnetization and heat capacity show that the compound undergoes two magnetic transitions, one at 14K and the other at 5K. The former is due to the paramagnetic-ferromagnetic transition, while the latter is attributed to the onset of an antiferromagnetic ordering, as the temperature is lowered. The M-H isotherm obtained at 2K shows a metamagnetic transition with a critical filed of about 13kOe. The maximum values of isothermal magnetic entropy change and adiabatic temperature change, for a field change of 50kOe, are estimated to be 23.6J∕kgK and 8.7K, respectively. The relative cooling power is found to be about 500J∕kg for a field change of 50kOe. A large magnetoresistance of about 16%, near the ordering temperature of 14K, is observed for a field of 50kOe. The magnetic, magnetocaloric, and magnetoresistance data seem to suggest the presence of magnetic pol...

Measurement of pressure effects on the magnetic and the magnetocaloric properties of the intermetallic compounds DyCo2 and Er(Co1−xSix)2

The effect of external pressure on the magnetic properties and magnetocaloric effect of polycrystalline compounds DyCo2 and Er(Co1−xSix)2 (x = 0,0.025 and 0.05) has been studied. The ordering temperatures of both the parent and the Si-substituted compounds are found to decrease with pressure. In all the compounds, the critical field for metamagnetic transition increases with pressure. It is seen that the magnetocaloric effect in the parent compounds is almost insensitive to pressure, while there is considerable enhancement in the case of Si-substituted compounds. Spin fluctuations arising from the magnetovolume effect play a crucial role in determining the pressure dependence of the magnetocaloric effect in these compounds. The variation of the magnetocaloric effect is explained on the basis of the Landau theory of magnetic phase transitions.

High spin polarization in CoFeMnGe equiatomic quaternary Heusler alloy

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 750u2009K 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 Pu2009=u20090.70u2009±u20090.01 was deduced using point contact andreev reflection measurements. The temperature dependence of electrical resistivity has been fitted in the temperature range of 5–300u2009K 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.

Correlation between magnetism and magnetocaloric effect in the intermetallic compound DyNiAl

Magnetization studies carried out in polycrystalline sample of DyNiAl show the presence of two magnetic transitions, one at 15 K and the other at 30 K. The low-temperature transition is attributed to the onset of antiferromagnetic ordering, while the other one corresponds to the ferro-para transition. Thermomagnetic irreversibility found in the temperature dependence of magnetization data is attributed to the domain-wall pinning effect and also to the magnetic frustration. Magnetocaloric effect is found to be negative in the antiferromagnetic phase and positive above the Neel temperature.

Heat capacity and magnetoresistance in Dy(Co,Si)2 compounds

Magnetocaloric effect and magnetoresistance have been studied in Dy(Co1−x,Six)2 (x=0, 0.075, and 0.15) compounds. Magnetocaloric effect has been calculated in terms of adiabatic temperature change (ΔTad) as well as isothermal magnetic entropy change (ΔSM) using the heat-capacity data. The maximum values of ΔSM and ΔTad for DyCo2 are found to be 11.4JKg−1K−1 and 5.4 K, respectively. Both ΔSM and ΔTad decrease with Si concentration, reaching values of 5.4JKg−1K−1 and 3 K, respectively for x=0.15. The maximum magnetoresistance is found to be about 32% in DyCo2, which decreases with increase in Si. These variations are explained on the basis of itinerant electron metamagnetism occurring in these compounds.

Role of Fe substitution on the anomalous magnetocaloric and magnetoresistance behaviour in Tb(Ni1?xFex)2 compounds

We report the magnetic, magnetocaloric and magnetoresistance results obtained from Tb(Ni1?xFex)2 compounds with x = 0, 0.025 and 0.05. Fe substitution leads to an increase in the ordering temperature from 36?K for x = 0 to 124?K for x = 0.05. In contrast to the single sharp magnetocaloric effect (MCE) peak seen in TbNi2, the MCE peaks of the Fe-substituted compounds are quite broad. We attribute the anomalous MCE behaviour to the randomization of the Tb moments brought about by the Fe substitution. Magnetic and magnetoresistance results seem to corroborate this proposition. The present study also shows that the anomalous magnetocaloric and magnetoresistance behaviour seen in the present compounds is similar to that of Ho(Ni,Fe)2 compounds.

Multiple magnetic transitions and the magnetocaloric effect in Gd1−xSmxMn2Ge2 compounds

The magnetic and magnetocaloric properties of polycrystalline samples of Gd1−xSmxMn2Ge2 have been studied. All the compounds except GdMn2Ge2 show re-entrant ferromagnetic behavior. Multiple magnetic transitions observed in these compounds are explained on the basis of the temperature dependences of the exchange strengths of the rare earth and Mn sublattices. The magnetocaloric effect is found to be positive at the re-entrant ferromagnetic transition, whereas it is negative at the antiferro–ferromagnetic transition. The Sm compounds are found to show a negative magnetocaloric effect at temperatures below the re-entrant ferromagnetic transition temperature. In SmMn2Ge2, the magnetic entropy change associated with the re-entrant transition is found to decrease with field, which may be attributed to the crystal-field effect as well as the increase in the intralayer antiferromagnetic component of the Mn sublattice. The maximum value of the isothermal magnetic entropy change in this series of compounds is found to decrease with increase in Sm concentration.

Magnetic and electrical properties of RCo2Mn (R=Ho, Er) compounds

Abstract The RCo 2 Mn (R=Ho and Er) alloys, crystallizing in the cubic MgCu 2 -type structure, are isostructural to RCo 2 compounds. The excess Mn occupies both the R and the Co atomic positions. Magnetic, electrical and heat capacity measurements have been done in these compounds. The Curie temperature is found to be 248 and 222xa0K for HoCo 2 Mn and ErCo 2 Mn, respectively, which is considerably higher than that of the corresponding RCo 2 compounds. Saturation magnetization values in these samples are less compared to that of the respective RCo 2 compounds.

Structure and magnetocaloric properties of the Fe-doped HoTiGe alloy

The structure and magnetocaloric properties of the Fe-doped HoTiGe compound were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy, x-ray diffraction, magnetometry, and calorimetry. As with the early studies on the undoped compound, the Fe-containing alloy exhibited an antiferromagnetic-to-paramagnetic transition and a magnetocaloric effect peak at 90K. The magnetization (M) versus temperature (T) data showed peaks at 10 and 90K, while M versus field (H) curves showed the presence of a field-induced transition for all T<120K; additionally, for all T<60K, open hysteresis loops at the magnetic transitions were observed. XRD measurements between 10 and 60K under various magnetic fields up to 3184kA∕m (40kOe) showed that the hysteresis was not accompanied by any change in crystallography. The magnetization derived entropy change −ΔSm vs T plot also showed the presence of two peaks, at 20 and 90K; but below 15K, −ΔSm increased steeply with decreasing temperature. It is...