Ferromagnetic-paramagnetic Transition Temperature Research Articles

Evolution of Magnetotransport Properties and Spin-Glass Behavior of the La0.7-xNdxPb0.3MnO3 System

The effect of neodymium substitution on the lanthanum site in La0.7-xNdxPb0.3MnO3 perovskite polycrystalline has been systematically studied. The substitution of neodymium for lanthanum causes structure change from rhombohedral to orthorhombic. Ferromagnetism is suppressed as neodymium content increases. The magnetic order changes from a ferromagnetic long-range order with x=0.0 to a spin-glass nature with x=0.7. Besides, the ferromagnetic-paramagnetic transition temperature values are lower for the neodymium phases than for the lanthanum ones. Therefore, the introduction of neodymium degrades symmetry of the structure, diminishes the spin-coupling exchange interaction, and results in changes of magnetic properties. This fact is in good agreement with increasing the B value in Bloch's T3/2 law and decreasing the spin-wave stiffness parameter D in quadratic dispersion relation.

Possible role of phase segregation in the disagreement between the metal-insulator and ferromagnetic transition temperatures in some colossal magnetoresistance compounds

The isovalent substitution of Gd for La in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ lowers the paramagnetic-ferromagnetic transition temperature ${T}_{c}$ and metal-insulator transition temperature ${T}_{\mathrm{MI}}$ with a substantial increase in the electrical resistivity \ensuremath{\rho}. Reduction of the ferromagnetic saturation moment M (5 K) below that expected for complete ferromagnetic alignment of the Mn magnetic moments occurs and ${T}_{\mathrm{MI}}$ is significantly lower than ${T}_{c}$ for $x>0.2.$ The observations are consistent with the existence of a granular mixture of ferromagnetic/conducting La-Ca-Mn-O and ferrimagnetic/insulating Gd-Ca-Mn-O. The ionic radius of the substituted rare-earth ion is found to be an important factor in determining whether or not granular behavior occurs. The results provide insight into why ${T}_{c}\ensuremath{\ne}{T}_{\mathrm{MI}}$ in some colossal magnetoresistance oxides.

Picosecond dynamics of the spin-lattice relaxation inLa0.7Ca0.2MnO3:Magnetic-field dependence

We have employed ultrafast optical pump-probe measurements at 1.5 eV to study the picosecond dynamics of the magnetic-field-induced metal-insulator (MI) transition in ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_{3}$ thin films in fields up to 7 T. We show that field induced modification of the photoinduced dynamic spectral weight transfer (DSWT) is related to changes in the magnetic entropy. The magnetic-field-induced increase of the ferromagnetic-paramagnetic transition temperature is evident in the spin-lattice relaxation time and amplitude scaling of the DSWT.

Spin–lattice interaction in colossal magnetoresistance manganites

The metal–insulator transition and underlying spin dynamics in La0.7D0.3MnO3 (D=Ca, Sr) are investigated using optical pump–probe spectroscopy at 1.5 eV. Our measurements, which span the ferromagnetic–paramagnetic transition temperature, reveal that the dynamics of the optically induced spectral weight transfer follow the temperature dependence of the magnetic specific heat. This dependence reflects the intrinsic interdependence between the optical conductivity and magnetism in the manganites allowing for the determination of the spin-lattice coupling magnitude.

Pressure study of the magnetic and electrical properties of the Eu-Yb alloy system.

Measurements of the ac magnetic susceptibility, magnetization, and electrical resistivity at hydrostatic pressures up to 15 kbar and of the thermal expansion were made for the ${\mathrm{Eu}}_{\mathrm{x}}$${\mathrm{Yb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloy system (x\ensuremath{\le}0.4) in temperatures from 1.6 to 400 K. The paramagnetic-ferromagnetic transition temperature ${T}_{c}$ versus pressure p curve for x=0.025 has a maximum near p=1 kbar. For x=0.25, ${T}_{c}$ decreases and the ferromagnetic--spin-glass-like transition temperature ${T}_{t}$ increases at rates of \ensuremath{\Delta}${T}_{c}$/\ensuremath{\Delta}p=-9.5 K/kbar and \ensuremath{\Delta}${T}_{t}$/\ensuremath{\Delta}p=+1.7 K/kbar, respectively. The ferromagnetic region disappears near p=2.5 kbar, and above it the paramagnetic--spin-glass-like transition temperature ${T}_{g}$, which is pressure insensitive, appears. The ${T}_{g}$ value for x=0.35 decreases initially and increases slowly with increasing p above p=2 kbar at a rate of \ensuremath{\Delta}${T}_{g}$/\ensuremath{\Delta}p=+0.15 K/kbar. Both the magnetization in the ferromagnetic concentration range and the value of the ac magnetic susceptibility near ${T}_{g}$ in the spin-glass-like state decrease with increasing p. From electrical-resistivity measurements under various pressures, semiconducting character was observed at the ambient pressure for x=0.35 and the transition from semimetallic to semiconducting states appears with increasing p for x=0.025. Thermal expansion shows the large positive volume magnetostriction. These results of the measurements under pressure are discussed on the basis of the Ruderman-Kittel-Kasuya-Yosida interaction by considering that the dominant variable is the number of carriers (electrons or holes) which is strongly controlled by the pressure.

The Ettingshausen-Nernst coefficient and transport properties of alumel from 200 to 473 K

The adiabatic Ettingshausen-Nernst coefficient, Q a , and the Righi-Leduc coefficient, R a , have been measured on Alumel and Nickel. The results show that Q a of Alumel increases with temperature and has a precipitous drop near its ferromagnetic-paramagnetic transition temperature ( T c =425.6 K). The experimental Q a relates well with temperature measurements errors experienced using Chromel-P vs Alumel thermocouples in crossed thermal and magnetic fields.