Senoy Thomas

Colossal thermoelectric power in charge ordered lanthanum calcium manganites (La0.5Ca0.5MnO3)

Lanthanum calcium manganites (La0.5Ca0.5MnO3) with a composition close to charge ordering, synthesized by high energy ball milling, was found to exhibit colossal thermoelectric power. Thermoelectric power (TEP) data was systematically analyzed by dividing the entire temperature range (5 K–300 K) into three different regimes to explore different scattering mechanisms involved. Mandals model has been applied to explain TEP data in the region below the Curie temperature (TC). It has been found that the variation of thermoelectric power with temperature is pronounced when the system enters the charge ordered region at T < 200 K. For temperatures lower than 120 K, due to the co-existence of charge ordered state with a spin-glass state, the variation of thermoelectric power is maximum and exhibited a peak value of −80 mV/K at 58 K. This has been explained by incorporating Kondo properties of the spin-glass along with magnon scattering. FC-ZFC magnetization measurements indicate the existence of a glassy state ...

Magnetocaloric properties, exchange bias, and critical behavior of Ge substituted Ni50Mn36Sn14 Heusler alloys

The effect of Ge substitution on the magnetic, magnetocaloric, and exchange bias properties of Heusler alloy system Ni50Mn36Sn14-xGex (x = 1, 2) has been investigated. With the increase of Ge content, the cell volume decreases due to the smaller Ge radius and the martensitic transition temperature increases, while the Curie temperature of the austenite phase shows a small decrease. Metamagnetic behavior is observed in the low temperature magnetization isotherms for x = 1, whereas it is less pronounced in x = 2. The maximum magnetic entropy changes associated with the martensitic transition are 7.8 J/kg K and 2.3 J/kg K for x = 1 and 2, respectively, for a field change of 5 T. Relative cooling power is found to be more in the vicinity of the Curie temperature of the austenite phase, compared to that at the martensitic transition temperature in both x = 1 and 2. At low temperatures, both the samples exhibit exchange bias effect, with x = 2 showing higher value of exchange bias field. This is ascribed to the...

Structural properties, magnetic interactions, magnetocaloric effect and critical behaviour of cobalt doped La0.7Te0.3MnO3

The effect of cobalt doping on the structural, magnetic and magnetocaloric properties of electron-doped manganite La0.7Te0.3Mn1−xCoxO3 (x = 0, 0.1, 0.2, 0.25, 0.3 and 0.5) has been investigated. The parent compound La0.7Te0.3MnO3 crystallizes in a rhombohedral structure with Rc space group. With the increase in Co concentration to x = 0.2, a structural transition from rhombohedral (Rc space group) to orthorhombic (Pbnm space group) is observed. X-ray photoelectron spectroscopy (XPS) indicates that the structural transition is due to the disordered distribution of Mn2+/Mn3+ and Co2+/Co3+ ions. All the samples undergo a paramagnetic–ferromagnetic (PM–FM) phase transition. With the increase in Co content to x = 0.1, the unit cell volume increases with a decrease in both Mn–O–Mn bond angle and Tc indicating a weakening of the double exchange interaction. However, with further increase in Co concentration, Tc increases. The presence of competing ferromagnetic and antiferromagnetic interactions leads to a glassy behaviour at low temperatures for low Co doping concentrations. However, for higher Co concentrations, no such behaviour is observed. Arrott plots reveal a second order nature of magnetic transition for all the samples. The magnetic exchange interactions for x = 0.3 and 0.5 follow the mean-field model. Magnetization results show that the magnetocaloric property of the electron-doped manganite is affected by the substitution of Co at Mn sites. Relatively large values of relative cooling power and broad temperature interval of the magnetocaloric effect make the present compounds promising for sub-room temperature magnetic refrigeration applications.

Observation of high-temperature magnetic transition and existence of ferromagnetic short-range correlations above transition in double perovskite La2FeMnO6

We present the observation of a high-temperature magnetic transition along with ferromagnetic short-range correlations (FSCs) in La2FeMnO6 perovskite system. XPS analysis confirmed the presence of Fe3+ and Mn3+ cations. Magnetization vs. temperature curves show two distinct transitions at TC1 ∼ 60 K and TC ∼ 425 K. Coercivity values of ∼1140 Oe and ∼35 Oe are observed at 2 K and 300 K respectively. Thermomagnetic analysis reveals the presence of FSCs in La2FeMnO6 up to T* = 570 K, well above the transition point, similar to a Griffiths-like phase (GP). The presence of ferromagnetic clusters in the paramagnetic region might be due to the intrinsic inhomogeneities associated with the structure, the quenched disorder related to the B-site cations and the antisite boundaries. The coefficient of lower temperature electronic specific heat is as high as 59.5 mJ mol−1 K−2. The electron spin resonance spectra show ferromagnetic resonance signals that point to the possibility of the presence of FSCs at room temperature. The material seems to be quite a promising candidate for some room temperature applications due to the possibility of the coexistence of functionalities like ferromagnetism, ferrimagnetism, GP, magnetotransport coupling, etc. in a single material.

Martensitic transition, spin glass behavior and enhanced exchange bias in Si substituted Ni50Mn36Sn14 Heusler alloys

The martensitic transition and exchange bias properties of the Heusler alloy system Ni50Mn36Sn14−xSix (x = 0, 1, 2 and 3) have been investigated. As the Si concentration increases, the martensitic transition temperature decreases while the Curie temperature of the austenite phases shows a slight increase. The temperature dependence of AC susceptibility with different frequencies at a low temperature regime was analyzed and the frequency dependence of the spin freezing temperature (Tf) showing a conventional critical slowing down relation confirms the spin glass behavior at low temperatures. Furthermore, the sample was recognized as a reentrant spin glass with both ferromagnetic and glassy states coexisting at least in the field cooled state. Due to these competing magnetic interactions, all the studied alloys exhibit exchange bias at low temperatures. For x = 2 alloy, an exchange bias field of 235 Oe is observed. However, on further increasing x, the exchange bias decreases. Therefore, it is quite evident that the unidirectional anisotropy decreases as a function of Si concentration. The double shifted hysteresis loop after zero field cooling and training effect strongly support the presence of exchange bias in these alloys. The dependencies of EB on various parameters like temperature and cooling field have also been investigated.

Tuning the structural and magnetocaloric properties of Gd5Si2Ge2 with Nd substitution

Structural and magnetocaloric properties of Nd substituted Gd5−xNdxSi2Ge2 with x = 0.05, 0.1, and 0.2 have been investigated. The composition with x = 0.05 crystallizes in monoclinic Gd5Si2Ge2 structure with P1121/a space group and undergoes a first order phase transition with a Curie temperature of 275 K. With increase in Nd content to x = 0.1, the compound is found to stabilize in orthorhombic Gd5Si4 phase with Pnma space group. The compounds with x = 0.1 and 0.2 undergoes a second order magnetic transition at 300 K and 293 K, respectively. Temperature variation of maximum entropy change for Gd5−xNdxSi2Ge2 alloys with x = 0.05, 0.1, and 0.2 are 7.9, 3.7, and 3.2 J/kg K, respectively, for a field change of 20 kOe, and 12.8, 7.6, and 7.2 J/kg K, respectively, for 50 kOe. A large relative cooling power of 295, 205, and 188 J/kg are obtained for x = 0.05, 0.1, and 0.2, respectively, fulfilling the required criteria for a potential magnetic refrigerant in the room temperature regime.

Size effect on the colossal thermoelectric power in charge ordered small band width manganites based on Gd-Sr

Earlier we observed colossal thermoelectric power in charge ordered intermediate band width manganite La0.5Ca0.5MnO3 and was explained based on charge ordering and occurrence of spin glass states. With a view to extending such a study on small band width Gd-Sr manganites, both unmilled and milled forms of Gd1−xSrxMnO3 (x = 0.3, 0.5, and 0.6) samples were prepared. Nano forms of Gd1−xSrxMnO3 were prepared by high energy ball milling. All compositions, both milled and unmilled forms, exhibited colossal thermoelectric power. The absolute value of thermoelectric power almost doubled in the case of milled samples and attained a maximum value of −69 mV K−1 at 42 K, where a spin glass transition takes place. In order to explain the occurrence of transition at ~42 K, field cooling and zero-field cooling magnetic measurements were conducted and we found that the peak value of thermoelectric power is observed at the same magnetic ordering temperature. The results are further modelled using Mandals model by incorporating Kondo properties of spin glass along with magnon scattering.