Room temperature magnetocaloric effect of La-deficient bulk perovskite manganite La0.7MnO3−δ

Room temperature magnetocaloric effect in La-deficient bulk perovskite manganite La_0.7MnO_3−δ prepared by conventional solid-state reaction has been reported. The maximum value of the magnetic entropy change (about−1.32 J/kg K) and the refrigerant capacity (approximately close to 37 J/kg) had been obtained at 290 K corresponding to a magnetic field variation of 1 T for La_0.7MnO_3−δ. It is the strong Jahn-Teller coupling that changes Mn-O bond length and Mn-O-Mn bond angles and then the canted spin arrangement and induces the strong double-exchange coupling to a comparatively high magnetic transition temperature. This Curie temperature near room temperature with easy fabrication and higher chemical stability makes La_0.7MnO_3−δ a potential candidate as a working substance in magnetic refrigeration technology.     

Magnetocaloric effect in La0.75Sr0.25MnO3 manganite

The polycrystalline manganite La_0.75Sr_0.25MnO₃ prepared by an alternative carbonate precipitation route reveals the rhombohedral perovskite structure. Magnetization isotherms measured up to 2 T are used to determine Curie temperature of 332 K by means of Arrott plot. Maximum of magnetic entropy change is found at Curie temperature. The relative cooling power equal to 64 J/kg for 1.5 T magnetic field, is superior as compared to the manganite with the same chemical composition from the sol-gel method.     

Room temperature magnetocaloric effect in Pr1.75Sr1.25Mn2O7 double-layered perovskite manganite system

Abstract

In this work, we have studied on double-layered perovskite (Ruddlesden–Popper) manganite structure in Pr_1.75Sr_1.25Mn₂O₇ synthesised by sol–gel method. The crystal structure of the double-layered perovskite is found as tetragonal from the X-ray diffraction analysis with I4/mmm space group. A high Curie temperature, T_C = 305 K is observed from the temperature dependence of magnetisation measurement. The isothermal magnetisation curves showed that magnetic phase transition is second order due to the positive slope of the Arrott plots. Maximum magnetic entropy change (ΔS_M) and adiabatic temperature change (ΔT_ad) values are calculated as 3.99 J kg^?1 K^?1 and 2.1 K under external magnetic field of 70 kOe, respectively. Since our double-layered perovskite manganite sample has desired T_C value and relatively high ΔS_M, it can be a potential candidate as a magnetocaloric material for room temperature magnetic cooling systems.     

Preparation and magnetic properties of LaMnO3 + δ (0 ≤ δ ≤ 0.154)

The structural, electrical, and magnetic properties of ceramic perovskite manganites LaMnO_{3 + δ} (δ = 0–0.154) are investigated. It is found that, in a weak magnetic field (B = 2 G), the LaMnO_{3 + δ} manganite with δ = 0.065 at temperatures below the Curie temperature T _C of the paramagnet-ferromagnet phase transition has a mixed (spin glass + ferromagnet) phase. In LaMnO_{3 + δ} manganites with the parameter δ = 0.100–0.154, this phase transforms into a frustrated ferromagnetic phase. A similar transformation was observed previously in La_1?x Ca_xMnO₃ compounds at calcium contents in the range 0 ≤ x ≤ 0.3. This similarity is explained by the fact that, in both materials, the Mn⁴⁺ concentration and, accordingly, the hole concentration c change equally in the concentration range from ～0.13 to 0.34 with an increase in x or δ. However, the magnetic irreversibility, the concentration dependences of the Curie temperature T _C(c) and the magnetic susceptibility X(c), and the critical behavior of the temperature dependence of the susceptibility X(T) in the vicinity of the Curie temperature T _C differ substantially for these two materials. The observed differences are associated with the distortion of the cubic perovskite structure, the decrease in the degree of lattice disorder, and a more uniform distribution of holes in the LaMnO_{3 + δ} manganites as compared to the La_{1 ? x} Ca_xMnO₃ compounds.     

Magnetic and magnetocaloric properties of perovskite manganite Pr0.55Sr0.45MnO3

In this paper, we have studied the magnetic and magnetocaloric properties of the perovskite manganite Pr_0.55Sr_0.45MnO₃. It shows a sharp paramagnetic-ferromagnetic phase transition at 291 K and possesses a moderate magnetic entropy change near room temperature. In addition, a large relative cooling power (143.64 J/kg) and a wide temperature range (84 K) have been found in this material. Compare with the Landau model, we find that the itinerant electrons mainly contribute the larger magnetic entropy change at paramagnetic region.     

Phase boundary between uniaxial and planar ferromagnets in layered perovskite manganite La2−2xSr1+2xMn2O7 (0.313⩽x⩽0.350)

We have examined magnetizations as a function of temperature and magnetic field in layered perovskite manganites La_2−2xSr_1+2xMn₂O₇ single crystals (x=0.313, 0.315, 0.318, 0.320 and 0.350) in order to determine the phase boundary between two ferromagnets (one is an uniaxial ferromagnet whose easy axis is parallel to the c-axis and the other is a planar ferromagnet whose easy axis is within the ab-plane) and following results are obtained: (i) all the present manganites exhibit magnetic transitions from a ferromagnet to a paramagnet at 76, 107, 116, 120 and 125 K for x=0.313, 0.315, 0.318, 0.320 and 0.350, respectively; (ii) for x=0.318, 0.320 and 0.350, the magnetic structure is a planar ferromagnet below Curie temperature; (iii) for x=0.313 and 0.315, the magnetic structure changes from an uniaxial to a planar ferromagnet at 66 and 85 K, respectively. From the results described above we have constructed the magnetic phase diagram of layered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (0.313?x?0.350).     

Effect of a magnetic field on the thermal and kinetic properties of the Sm<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3.02</Subscript> manganite

The temperature and magnetic-field dependences of the heat capacity, thermal conductivity, thermopower, and electrical resistivity of the Sm_0.55Sr_0.45MnO_3.02 ceramic material are studied in the temperature range 77–300 K and in magnetic fields up to 26 kOe. It is revealed that the quantities under investigation exhibit anomalous behavior due to a magnetic phase transition at the Curie temperature T_C. An increase in the magnetic field strength H leads to an increase in the Curie temperature T_C and a jump in the heat capacity ΔC_p at T_C. The temperature dependences of the measured quantities are characterized by hystereses that are considerably suppressed in a magnetic field of 26 kOe and depend neither on the thermocycling range nor on the rate of change in the temperature. The thermal conductivity K at temperatures above T_C shows unusual behavior for crystalline solids (dK/dT>0) and, upon the transition to a ferromagnetic state, drastically increases as a result of a decrease in the phonon scattering by Jahn-Teller distortions. It is demonstrated that the hystereses of the studied properties of the Sm_0.55Sr_0.45MnO_3.02 manganite are caused by a jumpwise change in the critical temperature due to variations in the lattice parameters upon the magnetic phase transition.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Critical behavior of La<Subscript>0.825</Subscript>Sr<Subscript>0.175</Subscript>MnO<Subscript>2.912</Subscript> anion-deficient manganite in the magnetic phase transition region

For La _0.825 ³⁺ Sr _0.175 ² +Mn³⁺O _2.912 ^2? anion-deficient manganite, the specific magnetization, the dynamic magnetic susceptibility, and the heat capacity are investigated. This material is found to be an inhomogeneous ferromagnet below the Curie point T _C ≈ 122 K, which is much lower than the Curie point determined for the stoichiometric composition (T _C ≈ 268 K). An increase in magnetic field by two orders of magnitude leads to an increase in the Curie temperature by ΔT ≈ 12 K. The presence of oxygen vacancies leads to the frustration of a part, namely, V _fr ≈ 22%, of the indirect Mn³⁺-O-Mn³⁺ exchange interactions, but the spin glass state is not realized. The ferromagnetic matrix of the material under study is characterized by a scatter in the exchange interaction intensities. The heat capacity is found to exhibit an anomalous behavior. Based on the Banerjee magnetic criterion, it is established that the ferromagnet-paramagnet transition observed for La _0.825 ³⁺ Sr _0.175 ²⁺ Mn³⁺O _2.912 ^2? anion-deficient manganite is a second-order thermodynamic phase transition. The mechanism and origin of the critical behavior of the system under investigation are discussed.     

The nonlinear magnetic properties of the pseudocubic Nd0.77Ba0.23MnO3 single crystal in the critical paramagnetic region and phase separation

The second magnetization harmonic was studied for a moderately doped Nd_0.77Ba_0.23MnO₃ neodymium manganite single crystal in parallel constant and harmonic magnetic fields in the critical paramagnetic region. According to the neutron and X-ray diffraction data, the crystal was crystallographically single-phase and had a pseudocubic structure both at room temperature and below the Curie point T _C=124.1 K. Although the specific resistance of this compound had a singularity near T _C and exhibited giant magnetoresistance, it remained an insulator in the ferromagnetic state. Nonlinear response measurements in the T _C<T<T _*≈146.7 K paramagnetic region were indicative of the existence of two magnetic phases. Above T _*, the crystal was magnetically single-phase, and its critical behavior was well described by dynamical similarity theory for isotropic 3D ferromagnets. The unexpected appearance of a new magnetic phase in the structurally homogeneous crystal was discussed based on phase separation ideas; such a phase separation could occur in moderately doped cubic manganites experiencing orbital ordering.     

Effects of different annealing atmospheres on magnetic properties in La2CoMnO6 single crystals

We have successfully synthesized the high-quality single crystals of double perovskite La₂CoMnO₆ by flux method and investigated the effects of different annealing conditions on the magnetic properties. In an as-grown crystal, ferromagnetic order along the c axis arises below 204 K, and the spin-glass-type behavior inherent from ionic disorders is also observed. Upon annealing in different gas environments, the reconstructed magnetic states induce a metamagnetic transition (after Ar annealing), and increase Curie temperature (after O₂ annealing). After quenching, the magnetic transition temperature decreases significantly, comprising lots of defects and disorders. Our results provide adequate procedures to control magnetic properties of double-perovskite compounds.     

151Eu Mössbauer Spectroscopy in La0.38Eu0.29Ca0.33MnO3

Electrical conductivity with and without magnetic field, d.c. magnetization and ¹⁵¹Eu Mössbauer studies were carried out in La_0.38Eu_0.29Ca_0.33MnO₃ perovskite manganite system. An insulating ground state is found throughout the temperature range with charge ordered (CO) state emerging at T _CO ～ 140 K, where as an external magnetic field of 6 T induces metal-insulator transition at ～120 K. D.C. magnetization measurements show the antiferromagnetic (AFM) transition occurring at T _N ≈ 48 K. The temperature dependent ¹⁵¹Eu Mössbauer measurements showed that the substituted Eu replaces La³⁺ in the 3+ charge state and a small magnetic moment gets induced at the Eu nucleus at low temperatures. The anomalous variation of the f- factor with temperature occurring around T _N and T _CO corroborates the occurrence of antiferromagnetic (AFM) and charge ordering (CO) transition, respectively.     

Magnetic phase coexistence in Tb- and Sr-doped La0.7Ca0.3MnO3 manganite:A temperature-dependent neutron diffraction study

We report the results of the temperature-dependent neutron diffraction measurements on the nearly half-doped (La_0.325Tb_0.125)(Ca_0.3Sr_0.25)MnO₃ manganite sample. The simultaneous doping of magnetic Tb³⁺ and divalent Sr²⁺ in the La_0.7Ca_0.3MnO₃ system results into a large A-site size disorder. Rietveld refinement of neutron diffraction data reveal that the single phase sample crystallizes in a distorted orthorhombic structure. Increased 〈r_A〉 value affects the transport behavior that results into an insulating-like behavior of the sample. Under application of 1 T field sample exhibit insulating-like behavior while insulator-metal transition (T_IM) is exhibited under 5 and 8 T fields. Variable range hoping (VRH) mechanism of charge carriers is exhibited in the insulating region. Field cooled and zero field cooled magnetization measurement shows the Curie temperature (T_C)~47 K. The refinement of the ND data collected at various temperatures below 300 K shows that there is no structural phase transition in the compound. Around 100 K, a magnetic peak appears at lower angle that can be ascribed to the presence of the A-type antiferromagnetic (AFM) phase. Two more peaks are observed around 50 K at lower angles that can be fitted in CE-type antiferromagnetic phase. Splitting of the peaks at lower temperatures is the signature of orbital ordering in the presently studied nearly half-doped manganite system. Results of the detailed structural analysis of the temperature-dependent ND measurements on (LaTb)_0.45(CaSr)_0.55MnO₃ sample has been discussed in the light of coexisting A-type and CE-type antiferromagnetic phases present in the sample at low temperature.     

Spin polarized La0.7Sr0.3MnO3 thin films on silicon

La_0.7Sr_0.3MnO₃ polycrystalline manganite thin films were grown on silicon (Si) substrates covered by SiO_x amorphous native oxide. Curie temperatures of about 325 K were achieved for 70-nm-thick films. Strong room temperature XMCD signal was detected indicating high spin polarization at the surface. Cross-sectional TEM images show sharp interface between SiO_x and manganite without signature of chemical reaction at the interface. Unusual sharp splitting of the manganite film was observed: on the top of a transition layer characterized by low crystalline order, a magnetically robust layer is formed.     

Ordering of polarons in Pr0.5Ca0.5MnO3

We report detailed dielectric investigations of Pr_1?xCa_xMnO₃ (PCMO, x=0.5) ceramics, a member of the perovskite manganite family that exhibits a colossal magnetoresistance. Analysis of the electrical conductivity and dielectric constant data revealed that in the low temperature phase, below 42 K, small polarons are responsible for the charge transport in the system. This provides a new understanding of the phase diagrams in PCMO systems     

Evidence of glassy ferromagnetic phase and kinetic arrest of electronic phase in Sm0.35Pr0.15Sr0.5MnO3 manganites

The effect of doping of rare earth Pr³⁺ ion as a replacement of Sm³⁺ in Sm_0.5Sr_0.5MnO₃ is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm_0.5−xPr_x)Sr_0.5MnO₃ show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures.     

Determination of Curie temperature by Arrott plot technique in Gd5(SixGe1−x)4 for x>0.575

Determination of Curie temperature by plotting magnetic moment vs. temperature curves requires a small applied field, which influences the measurement and temporarily disturbs the temperature of the sample especially for highly magnetocaloric materials. The Arrott plot technique was therefore used in order to determine the Curie temperature for a magnetocaloric Gd₅Si_2.7Ge_1.3 (x=0.675) single crystal sample. This technique was compared with other methods such as the inflection point technique and the line projection method. The results show how applied magnetic field influences the determination of Curie temperature. Using the Arrott plot the second-order transition Curie temperature for Gd₅Si_2.7Ge_1.3 was determined to be 304 K.     

Effects of high Fe substitution for Mn on phase transition and magnetic properties of YFexMn12−x compounds

YFe_xMn_12−x (x=7-10) compounds were synthesized; their structure and magnetic properties were investigated, the results reveal that a higher Fe concentration x makes the crystal phase change from ThMn₁₂-type to Th₂Zn₁₇-type, with the phase transition point x=9; the Curie temperature increases with increasing x, which becomes 290 K for x=8.8 and 150 K for x=7; and the magnetization also increases monotonically with increasing x until the phase transition point.     

Critical behavior of the heat capacity of the manganite La<Subscript>0.87</Subscript>K<Subscript>0.13</Subscript>MnO<Subscript>3</Subscript>

The heat capacity of the manganite La_0.87K_0.13MnO₃ has been measured in the temperature range 80–350 K. The nature of the ferromagnetic phase transition and the critical properties of heat capacity near the Curie temperature have been studied. The regularities of variations in the universal critical parameters near the phase transition point have been established. The calculated critical exponent and amplitudes of the heat capacity with allowance for corrections on the scaling (α = −0.13 and A ⁺/A ⁻ = 1.178) correspond to the critical behavior of the 3D Heizenberg model.     

Pressure-induced magnetic transition in Fe2P

The weak field ac susceptibility and the resistivity of Fe₂P single crystals were measured as functions of temperature from 4.2–300 K and as functions of hydrostatic pressures up to 20 kbar, using a newly designed clamp-type pressure cell. The Curie temperature, and the first-order transition temperature, decreased rapidly with increasing pressure, and ferromagnetism vanished at about 13 kbar at 0 K. A second-order transition temperature, as well as the first-order transition, appeared in the region below 170 K and above 5 kbar (triple point) and a new pressure-induced magnetic phase was found. The phase is proposed to be antiferromagnetic for reasons discussed in the paper.