A universal relationship between magnetization and changes in the local structure below T_c in La_(1-x)Ca_(x)MnO_(3)
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Samples of polycrystalline (La 1- x Ca x )MnO 3 , where x takes the values ranging from 0.005 to 0.3, were studied experimentally. Stoichiometric specimens were made by carefully controlling the oxygen partial pressure. For specimens with x less than 0.1, the Néel temperature T N is almost constant at 141°K, while the paramagnetic Curie temperature \(\varTheta_{\text{P}}\) increases with x . For specimens with x greater than 0.1, both T N (or T C ) and \(\varTheta_{\text{P}}\) both increase almost linearly with x . The usual Mn 55 NMR spectra associated with Mn 3+ and Mn 4+ ions are observed for x ≤0.175. Motionally narrowed spectra are observed for x =0.2 and x =0.3. These results can be qualitatively explained by assuming that the d -holes at the sites around Ca 2+ ions contribute both to the electrical conduction and to the ferromagnetic interaction.
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Temperature- and field-dependent magnetization $M(T,H)$ measurements were carried out for the series of ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{B}}_{6}$ compounds $(x=0.0,$ 0.13, 0.35, 0.54, 0.74, and 0.87). The small Ca doping in ${\mathrm{Eu}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{B}}_{6}$ induced a suppression of a ferromagnetic transition temperature from ${T}_{c}\ensuremath{\approx}12\mathrm{K}$ for ${\mathrm{EuB}}_{6}$ to ${T}_{c}\ensuremath{\approx}5.5\mathrm{K}$ for ${\mathrm{Eu}}_{0.87}{\mathrm{Ca}}_{0.13}{\mathrm{B}}_{6}.$ Interestingly, an antiferromagnetic transition, rather than ferromagnetic transition, was observed in $M(T)$ with $H=100\mathrm{Oe}$ at ${T}_{N}\ensuremath{\approx}4.5\mathrm{K}$ for ${\mathrm{Eu}}_{0.65}{\mathrm{Ca}}_{0.35}{\mathrm{B}}_{6}$ and at ${T}_{N}\ensuremath{\approx}3\mathrm{K}$ for ${\mathrm{Eu}}_{0.46}{\mathrm{Ca}}_{0.54}{\mathrm{B}}_{6}.$ Temperature-dependent resistivity and specific heat data at low temperatures were presented and discussed in terms of the antiferromagnetic transition. With further increase of Ca doping, paramagnetic state was observed down to $T=2\mathrm{K}.$
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Spontaneous magnetization
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Nearly a 50% decrease of resistivity \rho (T,x) due to just 4% Cu doping on the Mn site of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) is observed. Attributing the observed phenomenon to the substitution induced decrease of the spin polaron energy E_s(x) below the Curie point T_C(x)=T_C(0)(1-x), all data are found to be well fitted by the nonthermal coherent tunneling expression \rho (T,x) = \rho_0*exp(-\gamma M^2(T,x)) assuming M(T,x)=M_R(x)+M_0(x)*tanh{\sqrt{[T_C(x)/T]^2-1}} for the magnetization in the ferromagnetic state. The best fits through all the data points yield M_0(x)= \sqrt(1-x)M_0(0), M_R(x)=\sqrt(x)M_0(0), and E_s(x)=E_s(0)(1-x)^4 for the Cu induced modifications of the Mn spins dominated zero-temperature spontaneous magnetization, the residual paramagnetic contribution, and spin-polaron energy, respectively, with E_s(0)=0.12 eV.
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The sound velocities dependencies on temperature V(T) were measured by means of resonance method have on the thin plates of quasimonocrystalline manganites La(1-x)Sr(x)MnO3. Results are presented for x=0.1, 0.125, 0.15, 0.175 and 0.2 in the temperature range from 70 to 350K. The V(T) behavior correlates with those of susceptibilities and magnetic moment M(T). For x=0.175 and 0.2 the transition to ferromagnetic state is accompanied by appearance of large but smooth rise in V(T) whereas structural phase transition between the rhomboedral (R) and orthorhombic (O) structures is accompanied by sharp increase up to 15-20% in V(T). For x=0.1, 0.125 and 0.15 the structural phase transition between Jahn-Teller distorted orthorhombic structure (O') and pseudocubic one (O'') is accompanied by deep minimum (~20-30%) in V(T).
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The magnetic properties of perovskite (La 1-x Y x) 2/3 Ca 1/3 MnO 3 (0≤x≤0.3) compounds are systematically investigated and it is given that the transition temperature T c is related to average ion size of La site. It is found that with the increasing of Y 3+ doped content, the metal-insulator transition temperature T MI (M-I) shifts to lower temperature. While the relevant resistance peak ρ p sharply increases, for the sample of x= 0.3 , it has enhanced eight orders of magnitudes larger than the non-doped samples (x=0.0). The decrease of average ion size of La site and the local distortion of lattice constant due to the introduction of Y can explain the experimental results. At high concentration area, say when x0.1, magnetic studies show a gradual increase of antiferromagnetic interaction with the increase of x, ultimately leading to a spin-glass-like state for x=0.2 and x=0.3 compounds at about 35K.
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Hall measurements on La(2/3)Ca(1/3)MnO(3-delta) and Nd(2/3)Sr(1/3)MnO(3-delta) - a comparative study
The Hall resistivity of epitaxial La(2/3)Ca(1/3)MnO(3-delta 1) and bulk Nd(2/3)Sr(1/3)MnO(3-delta 2) separates into a normal and an anomalous part. The anomalous Hall resistivity correlates with magnetisation data while the normal part suggests the use of a two-band model for these perovskites. Single carrier concentrations and mobilities are p less than or equal to 0.75 x 10(21) cm(-3) (4.2 K, Nd-Sr) and p less than or equal to 6.4 x 10(22) cm(-3) (4.2 K, La-Ca) and then mobilities at 4.2 K, B = 0 are mu(p) greater than or equal to 1.03 cm(2)/(V s) (Nd-Sr) or greater than or equal to 0.44 cm(2)/(V s) (La-Ca). (C) 1998 Elsevier Science B.V. All rights reserved.
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