Rabindra Nath Mahato

Colossal magnetoresistance in the double perovskite oxide La2CoMnO6

Polycrystalline sample of La2CoMnO6 has been synthesized by sol-gel technique. The powder x-ray diffraction data confirm the single phase nature of the sample. This compound has monoclinic crystal structure (space group P21/n) at room temperature. The temperature dependence of magnetization in low field shows considerable variation between zero-field-cooled and field-cooled magnetization curve below ∼210 K (TC) and it follows Curie–Weiss law in the paramagnetic region. The hysteresis loop at 5 K indicates a coercive field of ∼6 kOe and remnant magnetization of ∼2.32 μB/f.u. The temperature dependence of the resistivity data shows semiconductorlike behavior in the temperature range of 5–350 K and follows variable range hopping conduction mechanism in the temperature range 215–350 K. A colossal magnetoresistance of ∼80% is observed at 5 K in an applied field of 80 kOe and MR has a negative sign.

Large magnetic entropy change in nanocrystalline Pr0.7Sr0.3MnO3

Nanocrystalline Pr0.7Sr0.3MnO3 sample has been prepared by sol-gel method. The room temperature powder x-ray diffraction data show single phase nature of the sample and confirm the cubic crystal structure with Fm3¯m space group. The average crystallite size is calculated using Scherrer formula, and it is found to be ∼25 nm. Transmission electron microscopy image shows that the particles are spherical in shape and the average particle size is ∼35 nm. The sample undergoes ferromagnetic ordering at 235 K (TC) and obeys the Curie–Weiss law in the paramagnetic region. The maximum value of the magnetic entropy change |ΔSM|max is ∼6.3 J kg−1 K−1, and the relative cooling power is ∼385 J kg−1 for a field change of 50 kOe. The Arrott plot confirms that the magnetic ordering is of second order nature. The experimentally observed magnetic entropy change of the sample obeys Landau theory of phase transition well.

Magnetic, transport, and magnetocaloric properties of double perovskite oxide LaCaMnCoO6

Magnetic, magnetoresistive, and magnetocaloric properties of a novel double perovskite oxide, namely, LaCaMnCoO6 have been studied. Polycrystalline sample of LaCaMnCoO6 has been synthesized by sol-gel technique. It has cubic crystal structure (space group Fm3¯m) at room temperature. The temperature variation in magnetization reveals a steep increase in magnetization around 168K (TC). The magnetization does not even saturate at 5K and a magnetic moment of 0.7μB∕f.u. is obtained at 5K in an applied field of 50kOe. The electrical resistivity measurement indicates that the material is semiconducting-like in the temperature range of ∼300–50K and below ∼50K the sample becomes insulating. A maximum magnetoresistance (MR) of about 8% is found at 200K in an applied field of 7T and MR has a negative sign. The magnetocaloric effect is calculated from the magnetization versus temperature data and a maximum magnetic entropy change of 3.1J∕kgK for a field change of 11kOe is obtained near TC. Thus a moderate magnetocalo...

Giant magnetoresistance and table-like magnetocaloric effect in double perovskite oxide PrSrMnCoO6

Magnetoresistance and magnetocaloric effect of a double perovskite oxide PrSrMnCoO6 (cubic, Fm3¯m) has been studied in fields up to 7 T. This compound is semiconductor-like and its electrical resistivity increases by 5 orders while going from 300 to 50 K. Giant magnetoresistance of ∼40% is observed at 200 K in 7 T field. PrSrMnCoO6 orders ferromagnetically at ∼150 K and shows a maximum magnetic entropy change of ∼4.6 J/kg/K for 5 T field change in the temperature range of 110–190 K. This nearly constant magnetocaloric effect over a broad temperature span is highly suitable for Ericsson-cycle magnetic refrigeration.

Magnetic, electrical transport and structural investigations of orthorhombic perovskite Pr2MnFeO6

Substituting Mn ions in PrMnO3 by 3d-transition metal Fe ions, the structural, magnetic and magnetotransport properties are modified due to change in tolerance factor. The compound has an orthorhombic structure with the space group Pbnm. The field cooled magnetization data with temperature indicates that the compound is ferromagnetic below ∼200 K. The variation in magnetization with applied magnetic field show a hysteresis behavior, which confirms the ferromagnetic nature. The remanent magnetization and coercive field of the compound, at 2 K, were found to be 0.6169μB/f.u. and 0.4291 T respectively. Mossbauer spectrum indicates that the Fe3+ ions are in high spin state. The temperature variation in electrical resistivity measurement shows the sample to be insulating in nature.

Size effect on the structural, magnetic, and magnetotransport properties of electron doped manganite La0.15Ca0.85MnO3

Nanocrystalline La0.15Ca0.85MnO3 samples of various grain sizes ranging from ∼17 to 42 nm have been prepared by sol-gel technique. Phase purity and composition were verified by room temperature x-ray diffraction and SEM-EDAX analysis. The bulk La0.15Ca0.85MnO3 is known to order antiferromagnetically around 170 K and to undergo a simultaneous crystal structural transition. DC magnetization measurements on 17 nm size La0.15Ca0.85MnO3 show a peak at ∼130 K (TN) in zero-field-cooled (ZFC) state. Field-cooled magnetization bifurcates from ZFC data around 200 K hinting a weak ferromagnetic component near room temperature due to surface moments of the nanoparticle sample. Low temperature powder neutron diffraction experiments reveal that the incomplete structural transition from room temperature orthorhombic to low temperature orthorhombic-monoclinic state also occurs in the nanoparticle sample as in the bulk. Magnetization in the ordered state decreases as particle size increases, thus indicating the reduction ...

Magnetic and Magnetoresistive Properties of Double Perovskite Oxides RE-SrMnCoO

Polycrystalline samples of NdSrMnCoO6 and LaSrMnCoO6 have been synthesized by sol-gel technique and were characterized by means of X-ray diffraction (XRD). The Rietveld refinement of the XRD data confirms the single phase nature of the samples. These oxides have cubic crystal structure (space group Fm3macrm) at room temperature. The magnetization data measured in a field of 100 Oe, under zero field cooled and field cooled conditions, show thermomagnetic irreversibility below 210 K. The electrical resistivity of these samples increase with decreasing temperature from 300 K down to 80 K. The conduction above 115 K can be described by variable range hopping mechanism. A maximum magnetoresistance (MR) of about 5% and 11% are achieved at 90 K in an applied field of 7 T for NdSrMnCoO6 and LaSrMnCoO6 samples, respectively, and MR has a negative sign.

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Magnetic and magnetocaloric properties of polycrystalline DyCoAl compound have been studied in fields up to 9 T. This compound orders ferromagnetically at ∼37 K (TC). A maximum magnetic entropy change of ∼−18 J/kg/K is obtained for a field change of 9 T near TC which is reasonably large. Low temperature neutron powder diffraction data indicate collinear ferromagnetic structure where Dy magnetic moments lie in the ab plane. At 10 K, the magnetic moment at Dy site is only ∼4.8μB.

(RE = Nd, La)

We have investigated the structural and electrical transport properties of nanocrystalline SrCo0.95Mn0.05O3-δ sample synthesized by the sol-gel method. The powder X-ray diffraction patterns show that sample has hexagonal crystal structure at room temperature. The temperature dependence of electrical resistivity data shows the insulating behavior below 100 K. Above 100 K, the sample shows the semiconducting-like behavior. At the high-temperature region 180 K - 300 K, the electrical transport behavior is dominated by the Mott’s variable range hoping (VRH) conduction mechanism and the VRH fitted data shows the density of states in the vicinity of Fermi energy, N (EF) ∼ 15.624 × 1018 ev−1cm−3 which is comparable to other semiconducting oxide materials. Field dependence magnetoresistance (MR) data shows the negative MR of 2.7% at room temperature for the field change of 3 T. The experimental result reveals the suitability of the magnetic compound for the memory devices near room temperature.

Magnetic structure and magnetic entropy change in the intermetallic compound DyCoAl

The nanocrystalline La0.8Te0.2MnO3 samples are prepared by sol-gel method and show rhombohedral crystal structure with R3c space group at room temperature. The calculated crystallite sizes are ~55 nm, 40 nm and 25 nm for calcined at 700°C, 800°C and 900°C temperatures. The SEM images show the grain size increases as the calcination temperature increases and the values are in good agreement with that obtained from X-ray diffraction analysis. The samples undergo paramagnetic to ferromagnetic transition and follow Curie–Weiss law in the paramagnetic region. The maximum entropy change are ~3.2 J kg-1 K-1, 3 J kg-1 K-1 and 2 J kg-1 K-1 for a field change of 20 kOe for 55 nm, 40 nm and 25 nm samples respectively. In the framework of Landau theory of phase transition, the experimentally observed magnetic entropy change and theoretical predicted model fits well for all the nanoparticles.