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Effects of oxygen deficit, Sr, Ti and Cr substitution on the antiferro–ferromagnet transition in Nd 0.6Ca 0.4MnO 3
Abstract
We have investigated phase transitions in Nd0.6Ca0.4−xSrx(Mn1−yMy)O3−γ (M=Cr, Ti) by measurement of magnetization and electrical transport in magnetic fields up to 12 T. It is shown that oxygen deficit up to γ=0.03, Sr doping up to x=0.05 and Cr doping up to y=0.03 lead to strong decrease of magnetic field which induces antiferro–ferromagnet (A–F) transition. Samples with chromium content above y=0.03 and Sr-content above x=0.1 are ferromagnets whereas in the samples with large oxygen deficit an antiferromagnetic structure is stabilized. The magnetization measurements indicate the ferromagnetic coupling between manganese and chromium ions.
The La0.44Ho0.11Sr0.45MnO3-delta a nd Nd0.44Ho0.11Sr0.45MnO3-delta compounds were obtained by sol-gel method. The structure, magnetic and transport characteristics were investigated between 77 and 600 K. The lattice parameters of the sintered in air samples are smaller then those of the sintered in O-2 samples. An important increase of the chemical disorder, in the same time with the decrease of the tolerance factor, was observed at the substitution of the La with Nd. The substitution of La with Nd leads to a decrease of the specific magnetization and of the Curie temperatures of the samples. The transition temperature from the metallic to the semiconductor state decreases for the samples sintered in air as compared with those sintered in O-2. A decrease of the activation energy of the charge transport at temperatures higher than the transition temperature takes place also when the La is substituted with Nd.
The La 0.44Ho 0.11Sr 0.45MnO 3-δ and Nd 0.44Ho 0.11Sr 0.45MnO 3-δ compounds were obtained by sol-gel method. The structure, magnetic and transport characteristics were investigated between 77 and 600 K. The lattice parameters of the sintered in air samples are smaller then those of the sintered in O 2 samples. An important increase of the chemical disorder, in the same time with the decrease of the tolerance factor, was observed at the substitution of the La with Nd. The substitution of La with Nd leads to a decrease of the specific magnetization and of the Curie temperatures of the samples. The transition temperature from the metallic to the semiconductor state decreases for the samples sintered in air as compared with those sintered in O 2. A decrease of the activation energy of the charge transport at temperatures higher than the transition temperature takes place also when the La is substituted with Nd.
With a view to understanding the structural, electrical and magnetic, elastic and anelastic behaviour of charge ordered Nd0.6Ca0.4MnO3, systematic investigations were undertaken. The sample was synthesized by a sol–gel route. The sample was characterized by X-ray diffraction (XRD) using a Rietveld refinement technique and was found to have orthorhombic structure with space group Pnma. A study on the variation of electrical resistivity with temperature has been carried out in the range 100–300 K and it was found to exhibit a field induced transition. The ac susceptibility studies show two transition temperatures, which are attributed to charge ordering and a Néel transition. Internal friction and longitudinal modulus studies were carried out using the composite oscillator technique. An effort has been made to explain the observed anomalous behaviour by a qualitative model.
It is shown that oxidized La0.5Sr0.5 (Mn3+0.5Mn4+0.5)O3 and reduced Pr0.5Ba0.5Mn3+O2.75 samples show similar magnetic and magnetoresistive properties. Both compounds, characterized by TCgamma 320 K, exhibit a ferromagnetic-antiferromagnetic transition around 200 K, where a large magnetoresistance develops. However, the reduced sample has a much larger resistivity than the oxidized one. These data suggest that some of the intriguing properties of the manganites can also be realized without mixed manganese valences.
It is shown that oxidized La0.5Sr0.5 (Mn3+0.5Mn4+0.5)O3 and reduced Pr0.5Ba0.5Mn3+O2.75 samples show similar magnetic and magnetoresistive properties. Both compounds, characterized by TCγ 320 K, exhibit a ferromagnetic-antiferromagnetic transition around 200 K, where a large magnetoresistance develops. However, the reduced sample has a much larger resistivity than the oxidized one. These data suggest that some of the intriguing properties of the manganites can also be realized without mixed manganese valences.
NdMn1−x
Cr
x
O3 and Nd0.6Ca0.4Mn1−x
Cr
x
O3 solid solutions have been studied by neutron diffraction and magnetic measurements. NdMn0.5Cr0.5O3 is found to have a magnetic structure consisting of an antiferromagnetic G-type component and a ferromagnetic component, which are caused by 3d ions. The magnetic moments of the neodymium ions are parallel to the ferromagnetic component. Nd0.6Ca0.4Mn0.5Cr0.5O3 mainly has a G-type magnetic structure, and the magnetic moments of the neodymium ions are normal to the antiferromagnetism vector. Magnetic
phase diagrams are plotted for both systems. They are interpreted on the assumption that the Mn3+-O-Cr3+ superexchange interactions are positive and the Mn4+-O-Cr3+ interactions are negative; the fact that manganese and chromium ions are not ordered in a crystal lattice is taken into account.
Concentration magnetic phase transformations proceed through a two-phase state because of the internal chemical inhomogeneity
of the solid solutions.
We have investigated the magnetic-field-induced phase transitions of R1-xCaxMnO3 (R = Pr and Nd, x = 0.50, 0.45 and 0.50, 0.45, 0.40) by measurements of magnetization, magnetoresistance, and magnetostriction utilizing a nondestructive long-pulse magnet (generating up to 40 T). We observed processes where magnetic fields destroy the real-space ordering of the charge carriers and cause insulator-to-metal phase transitions over the whole temperature region below about 250 K. We found that the destruction of the charge ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect. The different profiles of the temperature vs transition field curve depending on the carrier concentration x may be ascribed to the difference in the entropy between the commensurate and the discommensurate charge-ordered state. It turned out that the stability of the charge-ordered state is strongly correlated with the colinear antiferromagnetic ordering of the localized Mn moments.
We present the results of a neutron-powder-diffraction study in which we
have examined the magnetic and structural properties of
Pr0.6(Ca1-xSrx)0.4MnO3
(0<=x<=1) as a function of doping concentration x and temperature.
For x<=0.15 charge ordering and a Jahn-Teller distortion below 250 K
are evident from rapid changes in the lattice parameters and bond
lengths as a function of temperature and a transition from a
high-temperature orthorhombic to a low-temperature monoclinic structure.
For x=0.0 the system orders antiferromagnetically at TN=170
K. At low temperature the system has a canted CE structure. As the
temperature approaches TN there is a transition to a
collinear magnetic arrangement. For Sr doping of up to x=0.15 the
magnetic transition temperature remains almost constant but the magnetic
arrangement now has a ferromagnetic component which persists up to
TN. For x>0.15 the charge ordering is removed and the
lattice parameters show a more normal temperature dependence. The
materials are now simple ferromagnets and the ferromagnetic transition
temperature TC increases rapidly with x.
A study has been made of the magnetic properties of the series of perovskite-type compounds [(1-x)La, xCa]MnO3. The investigations have been made primarily by neutron diffraction methods, but x-ray diffraction measurements of lattice distortions and ferromagnetic saturation data are also included. This series of compounds exhibits ferromagnetic and antiferromagnetic properties which depend upon the relative trivalent and tetravalent manganese ion content. The samples are purely ferromagnetic over a relatively narrow range of composition (x∼0.35) and show simultaneous occurrence of ferromagnetic and antiferromagnetic phases in the ranges (0<x<0.25) and (0.40<x<0.5). Several types of antiferromagnetic structures at x=0 and x>0.5 have also been determined. The growth and mixing of the various phases have been followed over the whole composition range, the ferromagnetic and antiferromagnetic moment contributions to the coherent reflections have been determined, and Curie and Néel temperatures have been measured. The results have been organized into a scheme of structures and structure transitions which is in remarkable accord with Goodenough's predictions based on a theory of semicovalent exchange.
A reentrant transition from an incipient charge-ordered (CO) state to a charge-delocalized ferromagnetic (CDFM) state has been established in the manganate Nd{sub 0.25}La{sub 0.25}Ca{sub 0.5}MnO{sub 3}, in which the average A-site ionic radius is 1.19 {Angstrom}. The reentrant CDFM phase is associated with a first-order phase transition that reduces the orthorhombic distortion of the lattice, in contrast to the CO transition in other manganates where the orthorhombically distorted CO state is stabilized at low temperatures. At the CO-CDFM transition, there is a collapse of the charge-ordering gap as measured by vacuum tunneling spectroscopy. {copyright} {ital 1998} {ital The American Physical Society}
We have studied resistivity, magnetization, and magnetoresistance in polycrystalline La0.67Ba0.33MnOz by reducing the oxygen stoichiometry from z=2.99 to 2.80. As the oxygen content decreases, the resistivity of La0.67Ba0.33 MnOz increases and the magnetic transition temperature shifts to lower temperature. A large magnetoresistance effect was observed over a wide temperature range for all samples except the insulating z=2.80 sample. The similarity between our results on oxygen-deficient polycrystalline La0.67 Ba0.33MnOz and films previously reported to have a very large intrinsic magnetoresistance is discussed. At low temperature the magnetoresistance was observed to be strongly dependent on the magnetization. A possible mechanism for this effect is discussed.
We have investigated the magnetic-field-induced phase transitions of R1-xCaxMnO3 (R=Pr and Nd, x=0.50, 0.45 and 0.50, 0.45, 0.40) by measurements of magnetization, magnetoresistance, and magnetostriction utilizing a nondestructive long-pulse magnet (generating up to 40 T). We observed processes where magnetic fields destroy the real-space ordering of the charge carriers and cause insulator-to-metal phase transitions over the whole temperature region below about 250 K. We found that the destruction of the charge ordering is accompanied with a structural phase transition as well as with the magnetic phase transition and the colossal magnetoresistance effect. The different profiles of the temperature vs transition field curve depending on the carrier concentration x may be ascribed to the difference in the entropy between the commensurate and the discommensurate charge-ordered state. It turned out that the stability of the charge-ordered state is strongly correlated with the colinear antiferromagnetic ordering of the localized Mn moments.
Both internal chemical pressure by ion substitution and external
hydrostatic pressure induce an insulator-to-metal (I-M) transition in
the Pr1-xCaxMnO3 system. The behavior
of spin and charge ordering through the I-M transition has been studied
with use of neutron diffraction techniques. As a function of effective
pressure, we identify three different pressure regions for the
low-temperature state.
Transport and magnetic properties of have been studied to construct the magnetic phase diagram. is found to be a weak ferromagnet with K. Anomalous magnetic behaviour of x = 0 and x = 0.1 compositions is attributed to the Nd-sublattice magnetic contribution. Materials with and are suggested to be mixtures of antiferromagnetic and ferromagnetic states. The inhomogeneous ferromagnets exhibit the giant magnetoresistance effect without the temperature-induced metal-insulator transition. The results are discussed in terms of the superexchange interaction model proposed by Goodenough.
The doping of a manganese site with various M elements M=Fe <sup> 3+ </sup>, Al <sup> 3+ </sup>, Ga <sup> 3+ </sup>, Ti <sup> 4+ </sup>, Sn <sup> 4+ </sup>, and Mg <sup> 2+ </sup> was studied in insulating charge ordered manganite Pr <sub> 0.6 </sub> Ca <sub> 0.4 </sub> MnO <sub> 3 </sub> that exhibits a canted CE-type magnetic structure. Whatever M, the charge ordering disappears for low x levels (x≥0.02) in doped manganite Pr <sub> 0.6 </sub> Ca <sub> 0.4 </sub> Mn <sub>1-x</sub> M <sub>x</sub> O <sub> 3 </sub>. But the most important feature deals with the fact that a transition to a ferromagnetic metallic state is induced for M=Fe <sup> 3+ </sup>, Al <sup> 3+ </sup>, Ga <sup> 3+ </sup>, and Mg <sup> 2+ </sup> in spite of the small size of the interpolated cation (Pr,Ca). In contrast, doping with a tetravalent cation does not drastically modify the magnetic and transport properties of the phase. © 1997 American Institute of Physics.