Hall resistivity in ferromagnetic manganese-oxide compounds

The transport properties of manganese-oxides are studied using the spin correlation fluctuation scattering mechanism. It is shown that the Hall resistivity in a small magnetic field exhibits a maximum near the Curie point, and a strong field shifts the peak position to high temperature and suppresses the peak value; the dependence of the Hall resistivity on the magnetic field above T_c and below T_c is different. These results agree with the experimental curves qualitatively, but disagree quantitatively, which indicates that the spin correlation fluctuation scattering might not be the dominant mechanism of the colossal magnetoresistance. The double polaron mechanism due to strong electron-phonon and electron-spin coupling is proposed to be responsible for the colossal magnetoresistance in manganese-oxides.     

Colossal Magnetoresistance of Layered Manganite La<Subscript>1.2</Subscript>Sr<Subscript>1.8</Subscript>Mn<Subscript>2</Subscript>O<Subscript>7</Subscript> and Its Description by a “Spin–Polaron” Conduction Mechanism

The resistance of a La_1.2Sr_1.8Mn_2(1–z)O₇ single crystal has been studied in magnetic fields from 0 to 90 kOe. The magnetoresistance at temperature T = 75 K, near which a colossal magnetoresistance maximum is observed, has been successfully described in terms of the “spin–polaron” electric conduction mechanism. This value of the colossal magnetoresistance is due to a three-fold increase in the polaron size. The method of separating contributions of various conduction mechanisms to the magnetoresistance developed for materials with activation type of conduction is generalized to compounds in which a metal–insulator transition is observed. It is found that, at a temperature of 75 K, the contribution of the “orientation” mechanism is maximum (≈20%) in a magnetic field of 5 kOe and almost disappears in fields higher than 50 kOe.     

Connection between colossal magnetoresistance in La0.85Sr0.15MnO3 and magnetic ordering inhomogeneities of a sample

Resistive and magnetic measurements are made for La_0.85Sr_0.15MnO₃. The dependence of resistivity on the applied magnetic field (10, 20, 30, and 50 kOe) and temperature (200–310 K) is analyzed using the s-d model and the obtained experimental data. The physical features that should be contained in models proposed to explain the colossal magnetoresistance of manganites with activation-type conductivity are determined. It is shown that the proposed mechanism associating the colossal magnetoresistance effect with phase separation into ferromagnetic and paramagnetic microregions near the Curie temperature has the necessary features.     

Effect of oxygen non-stoichiometry on magnetic and electrical transport properties of La0.67Sr0.33MnO3

The effect of oxygen deficiency and oxygen excess on the magnetic and electrical transport properties of La_0.67Sr_0.33MnO₃ has been investigated. The thermal and isothermal magnetization measurement results show that the Curie temperature and saturation magnetization of oxygen deficient sample (defined as A) are higher than those oxygen excess sample (defined as B). The electrical resistivity of A is lower than that of B in studied temperature range. The magnetoresistance (MR) of B is larger than that of A in the temperature range from 280 to 360 K, which agrees with the magnetic field needed full spin polarization at room temperature. The colossal MR (CMR) around transition temperature from ferromagnetic metal to paramagnetic insulator (T_MI) for A is larger than that for B, which arises from assistance of stronger lattice deformation for A.     

La1－xPrxMnO3(x=0.1，0.2)薄膜庞磁电阻性质的研究

一种新的庞磁电阻氧化物薄膜La_1-xPr_xMnO₃(x=0.1，0 .2)薄膜用脉冲激光沉积(PLD)方法生长在(100)SrTiO₃单晶基底上.XRD结果显示 薄膜具有很好的外延单晶取向.电输运和磁性质的研究表明薄膜具有显著的庞磁电阻效应(CM R)效应，其中磁电阻比率达95％(在5T的磁场下).X射线光电子能谱(XPS)的结果表明薄膜体 系中Pr离子的价态为+4价，因此该薄膜很可能是电子掺杂的庞磁电阻体系. 关键词： 脉冲激光沉积 1-xPr_xMnO₃')" href="#">La_1-xPr_xMnO₃ 电子 掺杂 庞磁电阻     

Positive magnetoresistance peaked at a ferromagnetic transition in Mn-doped GaAs/AlGaAs quantum wells

A large positive magnetoresistance peaked at the Curie temperature has been observed in quantum well structures GaAs/AlGaAs doped by Mn. We suggest a new mechanism of magnetoresistance within low T _c ferromagnets resulting from a pronounced dependence of spin polarization at the vicinity of T _c on the external magnetic field. As a result, any contribution to resistance dependent on the Zeeman splitting of the spin subbands is amplified with respect to the direct effect of the external field. In our case we believe that the corresponding contribution is related to the upper Hubbard band. We propose that the mechanism considered here can be exploited as the mark of ferromagnetic transition.     

Nature of the low-temperature colossal magnetoresistance of La0.35Nd0.35Sr0.3MnO3 epitaxial films

The colossal negative magnetoresistance (approximately 12%) in a field of 8.4 kOe over a wide range of temperatures below the Curie point T _C ≈240 K in a single-crystal La_0.35Nd_0.35Sr_0.3MnO₃ film on a single-crystal (001)ZrO₂(Y₂O₃) wafer substrate is discussed. Isotherms of the magnetoresistance of this film reveal that its absolute value increases with the field, abruptly in the technical magnetization range and almost linearly in stronger fields. For three single-crystal films of the same composition on (001)LaAlO₃, (001)SrTiO₃, and (001)MgO substrates, colossal magnetoresistance only occurred near T _C ≈240 K and at T<T _C it increased weakly, almost linearly, with the field. In the film on a ZrO₂(Y₂O₃) substrate the electrical resistivity was almost 1.5 orders of magnitude higher than that in the other three films. It is shown that this increase is attributable to the electrical resistance of the interfaces between microregions having four types of crystallographic orientations, while the magnetoresistance in the region before technical saturation of the magnetization is attributable to tunneling of polarized carriers across these interfaces which coincide with the domain walls (in the other three films there is one type of crystallographic orientation). The reduced magnetic moment observed for all four samples, being only 46% of the pure spin value, can be attributed to the existence of magnetically disordered microregions which originate from the large thickness of the domain walls which is greater than the size of the crystallographic microregions and is of the same order as the film thickness. The colossal magnetoresistance near T _C and the low-temperature magnetoresistance in fields exceeding the technical saturation level can be attributed to the existence of strong s-d exchange which is responsible for a steep drop in the carrier mobility (holes) and their partial localization at levels near the top of the valence band. Under the action of the magnetic field the carrier mobility increases and they become delocalized from these levels.     

Electrical transport and magnetic properties of perovskite-type electron-doped La–Pr–Mn–O epitaxial films

La_1−xPr_xMnO₃ (LPrMO) thin films have been epitaxially grown on (1 0 0)SrTiO₃ single-crystal substrates by pulsed-laser deposition. The films have a perovskite structure and give rise to the colossal magnetoresistance effect with the maximum magnetoresistance ratio of 10³% (at 240 K and 5 T). The electrical transport and magnetic properties have been investigated for the La_0.8Pr_0.2MnO₃ film with thickness 3000 Å. The results indicate that the films have quite a distinctive magnetotransport behavior compared to the bulk. The analysis of X-ray photoemission spectroscopy suggests that the valence state of Pr is 4+ in LPrMO film. Therefore, the epitaxial film is most likely an electron-doped colossal magnetoresistance system.     

Magnetoresistance of porous polycrystalline HTSC: Effect of the transport current on magnetic flux compression in intergranular medium

The hysteretic dependences of the magnetoresistance of porous (38% of the theoretical density) granular high-temperature superconductor (HTSC) Bi_1.8Pb_0.3Sr_1.9Ca₂Cu₃O _x have been analyzed in the model of the effective intergranular field. This effective field has been defined by the superposition of the external field and the field induced by magnetic moments of superconducting grains. The magnetic flux compression in an intergranular medium, characterized by the effective field, controls the hysteretic behavior of the magnetoresistance. It has been found that the magnetoresistance hysteresis width for the studied porous HTSC depends on the transport current, in contrast to the superconductor of the same composition with high physical density (more than 90% of the theoretical value). For a porous superconductor, a significant current concentration occurs in the region of the grain boundaries, which is caused by features of its microstructure. A current-induced increase in the effective boundary length results in a decrease in the flux compression, a decrease in the effective field in the intergranular medium, and a magnetoresistance hysteresis narrowing with increasing current.     

Temperature and field dependent electronic structure and magnetic properties of LaCoO3 and GdCoO3

The transformation of the band structure of LaCoO₃ in the applied magnetic field has been theoretically studied. If the field is below its critical value B_C≈65 T, the dielectric band gap decreases with the field, thus giving rise to negative magnetoresistance that is highest at T≈300÷500 K. The critical field is related to the crossover between the low- and high-spin terms of Co³⁺ ions. The spin crossover results in an insulator–metal transition induced by an increase in the magnetic field. Similar calculations have been done for GdCoO₃ which is characterized by large spin gap∼2000 K.     

Theoretical study of the magnetoresistance under electric field in III–V diluted magnetic semiconductor

In this paper we study the magnetoresistance and the coupling energy in heterostructures formed by two magnetic layers Ga_1−xMn_xAs separated by a nonmagnetic spacer GaAs under an electric field and develop a mean-field theory of carrier in diluted magnetic semiconductor. Our main result indicates that magnetoresistance can be dramatically suppressed by an external electric field.     

Anisotropic low-temperature in-plane magnetoresistance in electron doped Nd<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Ce<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CuO<Subscript>4+δ</Subscript>

Nominally electron doped antiferromagnetic tetragonal nonsuperconducting Nd_2?xCe _x CuO_4+δ(x=0.12) has been shown to manifest strong angular dependence of the in-plane magnetoresistance on the orientation of the external magnetic field within the ab plane in many aspects similar to that observed in hole doped YBa₂Cu₃O_7?δ and La_2?xSr_xCuO₄. Specific fourfold angular magnetoresistance anisotropy amounting to several percents was observed in oxygen annealed films at low temperatures and in an external magnetic field up to 5.5 T. The strong temperature dependence and fourfold symmetry observed in our sample points to a specific role of rare-earth (Nd) ions in magnetoresistance anisotropy. At low temperature T = 1.4 K, we observed the unusual transformation of magnetoresistance response with increasing the external magnetic field, which seems to be a manifestation of a combined effect of a crossover between first and second order spin-flop transitions and a field-dependent rare-earth contribution to quasiparticle magnetotransport.     

Effect of inhomogeneity on magnetic, magnetocaloric, and magnetotransport properties of La0.6Pr0.1Ca0.3MnO3 single crystal

The effect of magnetic inhomogeneity on magnetic, magnetocaloric, and transport properties of the colossal magnetoresistance manganites with first order ferromagnetic-to-paramagnetic phase transition is studied. The experiments were performed on the single-crystalline samples of La_0.6Pr_0.1Ca_0.3MnO₃. The inhomogeneity is described by the Curie temperature distribution function, which is found from the magnetization data. The temperature dependence of the magnetic field induced change in the entropy is shown to be determined by the distribution function and the shift of the transition temperature in a magnetic field. Similarly, magnetoresistance in the transition region is determined by the resistivity at H=0 and the shift of the transition temperature. The maximum entropy change as well as maximum magnetoresistance can be achieved in the magnetic field of order δT_C/B_M where δT_C is the transition width and B_M is the rate of change of the Curie temperature with magnetic field.Our approach to analysis of the effects of inhomogeneity is general and therefore can be used for all compounds with the first order magnetic phase transition.     

Pulsed magnetic field study of the spin gap in intermediate valence compound SmB6

In this work, we report the behavior of electrical resistivity of SmB₆ at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at T_C~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB₆ proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.     

Effect of an in-plane magnetic field on magnetoresistance hysteresis of the two-dimensional electron gas in the integer quantum Hall effect regime

Effect of an in-plane magnetic field on the features of the magnetoresistance of a narrow conducting channel placed in the bath of a macroscopic two-dimensional electron gas has been studied. These features are manifested in the hysteretic behavior of the magnetoresistance in the quantum Hall effect regime. It has been found that the hysteresis loops observed in different ranges of the filling factor may be separated into two groups that differ in both the response to the in-plane magnetic field and the temperature dependence. The basic features observed near the integer filling factors ν = 1 and 2 are almost independent of the in-plane magnetic field. Therefore, their origin is not associated with spin effects. At the same time, additional features that appear at ν ≈ 1.8 and 2.2 are suppressed by the in-plane magnetic field B _‖ ≈ 6 T and almost temperature-independent from 45 mK to 1 K.     

Effect of thickness on magnetic phase coexistence and electrical transport in Nd0.51Sr0.49MnO3 films

We present the impact of the film thickness on the coexistence of various magnetic phases and its link to the magnetoresistance of Nd_0.51Sr_0.49MnO₃ thin films. These epitaxial films are deposited on LaAlO₃ (001) substrates by DC magnetron sputtering. Films with thicknesses of approximately 30 nm are found to be under full compressive strain while those with thicknesses ∼100 nm and beyond exhibit the presence of both strained and relaxed phases, as evidenced from X-ray diffraction studies. Both films exhibit multiple magnetic transitions controlled by strong electron correlations and phase coexistence. These films also display insulator–metal transitions (IMT) and colossal magnetoresistance (CMR) under moderate magnetic fields. Among the two set of films, only the 30-nm films show a weak signature of charge ordering at T≈50 K. Even at temperatures much lower than the IMT, the 30-nm films show huge magnetoresistance (MR) ∼80%. This suggests presence of softened charge-ordered insulating (COI) clusters that are transformed into ferromagnetic metallic (FMM) ones by the external magnetic field. In the 100-nm films, the corresponding MR is suppressed to less than 20%. Our study demonstrates that the softening of the COI phase is induced by the combined effect of the in-plane compressive strain and a slight reduction in Sr concentration.     

Two-phase paramagnetic-ferromagnetic state of La<Subscript>0.7</Subscript>Pb<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> single-crystal lanthanum manganite

Two phases, paramagnetic and ferromagnetic, were shown by the magnetic resonance method to coexist below the temperature T _C in La_0.7Pb_0.3MnO₃ single crystals exhibiting colossal magnetoresistance. The magnetic resonance spectra were studied in the frequency range 10–78 GHz. The specific features in the behavior of the spectral parameters were observed to be the strongest at the temperatures corresponding to the maximum magnetoresistance in the crystals. The concentration ratios of the paramagnetic and ferromagnetic phases in the samples were found to be sensitive to variations in temperature and external magnetic field. This behavior suggests realization of the electronic phase separation mechanism in the system under study.     

Crystallographic,magnetic, and electrical properties of thin <Emphasis Type="Italic">Re</Emphasis><Subscript>0.6</Subscript>Ba<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript> epitaxial films (<Emphasis Type="Italic">Re</Emphasis> = La,Pr, Nd,Gd)

Thin Re_0.6Ba_0.4MnO₃ epitaxial films (Re = La, Pr, Nd, Gd) grown on (001)SrTiO₃ and (001)ZrO₂(Y₂O₃) single crystal substrates have been prepared and studied. All the films were found to have a cubic perovskite structure, with the exception of the film with Re = La, which revealed rhombohedral distortion of the perovskite cell. The temperature dependences of the electrical resistivity and magnetoresistance pass through a maximum near the Curie point T_C, where the magnetoresistance reaches a colossal value. The magnetization isotherms M(H) are superpositions of a magnetization that is linear in field (like that of an antiferromagnet) and a weak spontaneous magnetization. The magnetic moment per formula unit is substantially smaller than that expected under complete ferro-or ferrimagnetic ordering. The magnetizations of samples cooled in a magnetic field (FC samples) and with no field applied (ZFC samples) differ by an amount that persists up to the highest measurement fields (50 kOe). The M(T) dependence obtained in strong magnetic fields is close to linear. Hysteresis loops of the FC samples are shifted along the field axis. The above magnetic and electric properties of thin films are explained in terms of two coexisting magnetic phases, which are due to strong s-d exchange coupling.