The temperature T and magnetic field H dependence of anisotropic in-plane ρ ab and out-of-plane ρc resistivities have been investigated in single crystals of the bilayer manganite La1.2Sr1.8Mn2O7. Below the Curie transition temperature Tc = 125 K, ρ ab and ρc display almost the same temperature dependence with an up-turn around 50 K. In the metallic regime (50 K ≤ T ≤ 110 K), both ρ ab (T ) and ρc(T ) follow a T 9/2 dependence, consistent with the two-magnon scattering. We found that the value of the proportionality coefficient B f it ab and the ratio of the exchange interaction J ab /Jc obtained by fitting the data are in excellent agreement with the calculated B ab based on the twomagnon model and J ab /Jc deduced from neutron scattering, respectively. This provides further support for this scattering mechanism. At even lower T , in the non-metallic regime (T < 50 K), both the in-plane σ ab and out-of-plane σc conductivities obey a T 1/2 dependence, consistent with weak localization effects. Hence, this demonstrates the three-dimensional metallic nature of the bilayer manganite La1.2Sr1.8Mn2O7 at T < Tc.

The temperature T and magnetic field H dependence of the resistivity ρ has been measured for La0.8−ySr0.2MnO3 (y=0 and 0.128) films grown on (100) SrTiO3 substrates. The low-temperature ρ in the ferromagnetic metallic region follows well ρ(H, T ) = ρ0(H) + A(H)ωs/ sinh(hωs/2kBT ) + B(H)T 7/2 with ρ0 being the residual resistivity. We attribute the second and third term to smallpolaron and spin-wave scattering, respectively. Our analysis based on these scattering mechanisms also gives the observed difference between the metal-insulator transition temperatures of the films studied. Transport measurements in applied magnetic field further indicate that spin-wave scattering is a key transport mechanism at low temperatures. The observation of colossal magnetoresistance CMR effect in manganite films [1] has produced a resurgence of interest in these materials for both fundamental physics and their possible application in recording media and magnetic switching devices. The microscopic transport mechanism in these materials has long been thought to be double exchange DE [2,3,4]. However, it has been realized [5] that the effective carrier-spin interaction in the DE model is too weak to lead to a significant reduction of the electronic bandwidth, which would justify the observed several orders of magnitude increase in conductivity just below the Curie temperature T C . Indeed, a large number of experiments have shown that the DE scenario alone cannot account for the properties of the manganites and that CMR is not purely electronic in origin [6,7].Low-temperature charge transport measurements of manganites in the ferromagnetic metallic state are essential in clarifying the specific mechanisms responsible for the CMR effect. At low temperatures, a dominant T 2 term in resistivity has generally been observed [8,9,10]. Although the T 2 behavior is consistent with electronelectron interaction [11], the coefficient of the T 2 term is about 60 to 70 times larger than the one expected for electron-electron scattering [12]. Moreover, a careful check of the low-temperature resistivity [13,14] has shown substantial deviation from the T 2 −like behavior in the very low temperature region. Other power-law temperature dependences of the resistivity have also been reported [10,14,15,16,17,18,19]. At present, there is no agreement on the actual scattering mechanism below the Curie temperature.Here, we address the low-temperature scattering mechanism in manganites through resistivity measurements of La 0.8−y Sr 0.2 MnO 3 (y = 0 and 0.128) films grown on SrTiO 3 substrates, measured in zero field as well as applied magnetic fields up to 14 T. Our data indicate that spin-wave scattering, which gives a T 7/2 dependence in the low-temperature resistivity, is a dominant dissipation mechanism in the ferromagnetic state of these manganites, besides scattering of small polarons by a soft optical phonon mode. Our analysis of the resistivity data in terms of small-polaron and spin-wave scattering mechanisms, and the spin fluctuation model also g...

We report measurements of the in-plane ( ab ) and out-of-plane ( c ) resistivities on a single crystal of the half-doped bilayer manganite LaSr 2 Mn 2 O 7 . In the temperature T range 220 to 300 K, the resistive anisotropy c / ab ϭAϩB/T (A and B constants͒, which provides evidence for the variable-range-hopping conduction in the presence of a Coulomb gap. This hopping mechanism also accounts for the quadratic magnetic field H and sin 2 dependences of the negative magnetoresistivity ln͓ i (T,H,)/ i (T,Hϭ0)͔ (iϭab,c), where is the in-plane angle between the magnetic field and the current.

We report anisotropic resistivity measurements on a La 1.2 Sr 1.8 Mn 2 O 7 single crystal over a temperature T range from 2 to 400 K and in magnetic-fields H up to 14 T. For Tу218 K, the temperature dependence of the zero-field in-plane resistivity ab (T) obeys the adiabatic small polaron hopping mechanism, while the out-ofplane resistivity c (T) can be ascribed by an Arrhenius law with the same activation energy. Considering the magnetic character of the polarons and the close correlation between resistivity and magnetization, we developed a model which allows the determination of ab,c (H,T). The excellent agreement of the calculations with the measurements indicates that small polarons play an essential role in the electrical transport properties in the paramagnetic phase of bilayer manganites.Elucidating the nature of the paramagnetic-insulating state is crucial to understand the correlation between the electrical transport and magnetic properties of 3d transition-metal manganese oxides. Most previous studies of the manganite perovskites R 1Ϫx A x MnO 3 films (Rϭrare-earth ion and Aϭdivalent ion͒ reveal that the high-temperature resistivity follows the adiabatic small polaron transport. 1,2 The effect of an applied magnetic-field H on the resistivity and thermal expansion above the Curie temperature T C indicates that the polarons have magnetic character. 3 The existence of polarons in the paramagnetic phase of bilayer manganites La 2Ϫ2x Sr 1ϩ2x Mn 2 O 7 (xϭ0.4) has been supported by measurements of Raman spectra, 4 x ray, and neutron scattering, 5 optical conductivity spectra, 6,7 and thermoelectric power. 8 However, there are no magnetotransport measurements that support the presence of polarons in the paramagnetic state of these materials.Recently, bilayer manganites La 2Ϫ2x Sr 1ϩ2x Mn 2 O 7 have attracted considerable attention since: ͑i͒ the physical properties along the ab plane and c axis are strongly anisotropic, which should yield important insight into the colossal magnetoresistance ͑CMR͒ effect, ͑ii͒ they can be viewed as an infinite array of ferromagnetic metal ͑FM͒ -insulator ͑I͒ -FM junctions, 9 ͑iii͒ both the in-plane and out-of-plane magnetoresistivities are sensitive to even small magnetic fields, 10 pointing to their possible device applications, ͑iv͒ they display a rich magnetic phase diagram that depends strongly on the doping level x, 11 and ͑v͒ they are good candidates for systematic investigations of the electrical resistivity in the paramagnetic regime over a broad temperature T range due to their relative low T C compared to the manganite perovskites.The understanding of electrical transport in the paramagnetic state and in the presence of an applied magnetic field, and of the enhanced CMR effect in bilayer manganites is still incomplete and challenging. The resistivity is semiconducting-like in the high-T paramagnetic state. On cooling, it reaches a maximum followed by a metallic behavior. When an external magnetic field is applied, this metalinsulator transition shifts to higher temp...

The large (10 2 − 10 5 ) and strongly temperature dependent resistive anisotropy η = (σ ab /σc) 1/2 of cuprates perhaps holds the key to understanding their normal state inplane σ ab and out-of-plane σc conductivities. It can be shown that η is determined by the ratio of the phase coherence lengths ℓ i in the respective directions: σ ab /σc = ℓ 2 ab /ℓ 2 c . In layered crystals in which the out-of-plane transport is incoherent, ℓc is fixed, equal to the interlayer spacing. As a result, the T-dependence of η is determined by that of ℓ ab , and vice versa, the in-plane phase coherence length can be obtained directly by measuring the resistive anisotropy. We present data for hole-doped Y Ba 2 Cu 3 Oy (6.3 < y < 6.9) and Y 1−x P rxBa 2 Cu 3 O 7−δ (0 < x ≤ 0.55) and show that σ ab of crystals with different doping levels can be well described by a two parameter universal function of the in-plane phase coherence length. In the electron-doped N d 2−x CexCuO 4−y , the dependence σ ab (η) indicates a crossover from incoherent to coherent transport in the c-direction.We present data on the normal state in-plane σ ab and out-of-plane σ c conductivities of anisotropic layered crystals as diverse as hole-doped Y Ba 2 Cu 3 O y (6.35 < y < 6.93) and Y 1−x P r x Ba 2 Cu 3 O 7−δ (0 < x ≤ 0.55), and electron-doped N d 2−x Ce x CuO 4−y . The conductivities of oxygen deficient Y Ba 2 Cu 3 O y single crystals were measured using the four-point method as well as a multiterminal technique 1 , in zero field and in a field of 14 T in order to suppress superconductivity and reveal the normal state down to lower temperatures. The conductivities of Y 1−x P r x Ba 2 Cu 3 O 7−δ and N d 2−x Ce x CuO 4−y single crystals were measured using the multiterminal method.Recently, it has been shown 2,3 that the ratio of the conductivities is given by 1