Niraj K. Singh

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.

Itinerant electron metamagnetism and magnetocaloric effect in Dy(Co,Si)2

Abstract Itinerant electron metamagnetism in Dy(Co1-xSix)2 compounds was studied in the light of a recent theoretical model based on magnetovolume effect and spin fluctuations. The nature of the magnetic transition in these compounds was analyzed within the framework of this model. The magnetocaloric effect in these compounds has been calculated and correlated with the strength of itinerant electron metamagnetism. The domain wall pinning effect was found to be dominant at low temperatures.

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.

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.

Effect of Si substitution on the magnetic and magnetocaloric properties of ErCo2

The magnetic and magnetocaloric properties of Er(Co1−xSix)2 compounds with 0⩽x⩽0.075 have been studied to determine their suitability as magnetic refrigerant materials. The strength of itinerant electron metamagnetism was found to decrease with Si concentration, which is responsible for the reduction of the magnetocaloric effect. Magnetization curves at low temperatures show the existence of a critical field for magnetization to increase, which is a consequence of domain wall pinning. The critical field and the coercive field were found to increase with Si concentration.

Effect of Ge substitution for Si on the anomalous magnetocaloric and magnetoresistance properties of GdMn2Si2 compounds

The effect of Ge substitution on the magnetization, heat capacity, magnetocaloric effect, and magnetoresistance of GdMn2Si2−xGex (x=0, 1, and 2) compounds has been studied. The magnetic transition associated with the Gd ordering is found to change from second order to first order on Ge substitution. Magnetic contributions to the total heat capacity and the entropy have been estimated. 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 temperature dependence of the magnetocaloric effect in all the three compounds have shown broad peaks. The maximum values of ΔSM and ΔTad for GdMn2Ge2 are found to be 5.9J∕kgK and 1.2K, respectively. The magnetoresistance is found to be very large and positive with a maximum value of about 22% in the case of GdMn2Ge2. In the other two compounds also, the magnetoresistance is predominantly positive, except in the vicinity of the Gd ordering temperature. The anomalous nature of the magnetocaloric effect and the magnetoresistance has been attributed to the canted magnetic structure of these compounds.The effect of Ge substitution on the magnetization, heat capacity, magnetocaloric effect, and magnetoresistance of GdMn2Si2−xGex (x=0, 1, and 2) compounds has been studied. The magnetic transition associated with the Gd ordering is found to change from second order to first order on Ge substitution. Magnetic contributions to the total heat capacity and the entropy have been estimated. 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 temperature dependence of the magnetocaloric effect in all the three compounds have shown broad peaks. The maximum values of ΔSM and ΔTad for GdMn2Ge2 are found to be 5.9J∕kgK and 1.2K, respectively. The magnetoresistance is found to be very large and positive with a maximum value of about 22% in the case of GdMn2Ge2. In the other two compounds also, the magnetoresistance is predominantly positive, except in the vicinity of the Gd ordering temperature....

Large reversible magnetocaloric effect in Er3Co compound

Magnetic and magnetocaloric properties of the intermetallic compound Er3Co have been studied. Temperature dependence of magnetization data shows that it exhibits ferromagneticlike bulk magnetic ordering at 14 K. For field changes (ΔH) of 20 and 50 kOe, the maximum values of isothermal magnetic entropy change are found to be 9 and 17 J/kg K, respectively. For ΔH=20 and 50 kOe, the relative cooling powers are found to be 140 and 450 J/kg, respectively. At temperatures above Tord, the spin fluctuations are found to affect the magnetocaloric properties of Er3Co.

Investigations on magnetic and magnetocaloric properties of the intermetallic compound TbAgAl

Magnetic and magnetocaloric properties of the intermetallic compound TbAgAl have been investigated. Temperature dependence of magnetization data revealed that TbAgAl exhibits magnetic ordering at 59 K and possesses competing ferromagnetic-antiferromagnetic interactions, which leads to the formation of Griffiths-like phase. The field dependence of magnetization data shows the existence of a metamagnetic-like transition at a critical field of 10 kOe. Unusually potent relaxation effects are seen in the time dependence of magnetization data in the ordered phase. The presence of spin-glass-like state is found to affect magnetocaloric properties of this compound.