V N Stamov

Variable-range hopping conductivity in La1-xCaxMnO3

In ceramic La1-xCaxMnO3 samples with 0≤x≤0.15 and the concentration of holes c = 0.18-0.22 between T = 77 and 340 K the resistivity, ρ, is shown to have an activated character both above and below the ferromagnetic Curie temperature, TC. The temperature dependence of the local activation energy, Eloc (T) = dln ρ(T)/d(kT)-1 gives evidence for a variable-range hopping conductivity over states of the Coulomb gap. The width of the gap Δ≈0.24 eV, the effective dielectric permeability κ≈3.6 and the carrier localization radius ξ≈5.1 A in the paramagnetic phase are determined. The value of ξ suggests hopping of small lattice polarons at T>TC. With decreasing temperature two successive transitions, at temperatures TC and TC, associated with divergence of the correlation length, are observed in the vicinity of TC. They can be interpreted in terms of critical increase (as T approaches TC from higher temperatures) and decrease (when T shifts from TC towards lower temperatures) of the radius of magnetic polarons, generated by a small fraction of the lattice polarons as T→TC. The difference between TC and the temperature of the first divergence of ξ, TC

Coexistence of ferromagnetic and spin-glass phenomena in La1-xCaxMnO3 (0 ≤ x ≤ 0.4)

Low-field magnetic properties of ceramic La1-xCaxMnO3 (0≤x≤0.4) are investigated between T = 5 and 310xa0K. The paramagnetic-ferromagnetic transition is observed in all the samples. The dependence of the Curie temperature, TC, on x is described within a model of spin polarons associated with electronic localization. Critical behaviour of the susceptibility χ-1(T ) ~ (T-TC)γ is observed for T>TC, with the critical exponents γ = 1.20±0.05 and γ* = 1.64±0.06 below and above the composition x≈0.18 corresponding to the Mn4+ ion concentration c ≈ 0.23, respectively. For the compound with x = 0.3 no temperature hysteresis of the resistivity is observed in magnetic fields between 0 and 8xa0T. In all the samples the field-cooled and zero-field-cooled magnetizations deviate below TC, the difference being approximately equal to the thermoremanent magnetization (TRM). Long-time relaxation of TRM in LCMO is observed for time scales up to 104xa0s. The relaxation rate reaches a maximum near a wait time tW ~ 103xa0s. The time dependence of TRM can be described with a stretched exponential law, as in spin or cluster glasses in conditions where the observation time is comparable with tW .

Asymmetry of a complex gap near the Fermi level, determined from measurements of the thermopower in La1−xCaxMn1−yFeyO3

The thermopower, S, of La0.7Ca0.3Mn1−yFeyO3 (y = 0–0.09), measured for T = 25–310 K and magnetic fields of B = 0–10 T, exhibits strong sensitivity to doping with Fe. The weak increase of S observed in the undoped material with lowering temperature is enhanced considerably in the sample with y = 0.03 until a steep decrease takes place in the vicinity of the paramagnetic–ferromagnetic transition temperature, TC. S increases with further growth of y and its maximum follows the decrease of TC from 228 K at y = 0.03 to 119 K at y = 0.09. Application of a magnetic field reduces S strongly. The experimental data are analysed quantitatively with a percolation model using results of our recent investigations of the Shklovskii–Efros-like variable-range hopping conductivity in the same set of La1−xCaxMn1−yFeyO3 samples with evidence for a soft Coulomb gap and a rigid gap in the density of states near the Fermi level (Laiho et al 2002 J. Phys.: Condens. Matter 14 8043). In the paramagnetic insulating phase the temperature dependence of S is determined by competing contributions from two types of asymmetry of the gap: a small shift of its centre with respect to the Fermi level and cubic non-parabolicity. Numerical values of the shift and the cubic asymmetry terms are determined.

Low-field magnetic properties of LaMnO3+δ with 0.065≤δ≤0.154

In a weak ðB ¼ 2G Þ magnetic field LaMnO3þd exhibits at d ¼ 0:065 below the paramagnetic-to-ferromagnetic (FM) Curie temperature, TC; a mixed (spin-glass and FM) phase followed by a frustrated FM phase at d between 0.100 and 0.154. The same behavior is observed in La12xCaxMnO3 with x between 0 and 0.3. This can be understood by the similar variation of the Mn 4þ concentration, c between < 0.13 and 0.34, in both materials when x or d is increased. On the other hand, considerable differences are found between these compounds in the values of the magnetic irreversibility, in the dependencies of TCðcÞ and the magnetic susceptibility, xðcÞ; as well as in the critical behavior of xðTÞ near TC: These differences can be explained by distortions of the cubic perovskite structure, by the reduced lattice disorder and by the more homogeneous hole distribution in LaMnO3þd than in La12xCaxMnO3.

Critical behavior of magnetoresistance near the metal–insulator transition of La0.7Ca0.3MnO3

Abstract The resistivity, ρ(T), of thin film and bulk samples of La0.7Ca0.3MnO3 is investigated in magnetic fields between 0–10xa0T and temperatures between 10–340xa0K. Metallic resistivity, ρlt(T)=ρr+ρ2T2+ρ4.5T4.5, is observed well below the metal–insulator transition (MIT) temperature TMI. For T>TMI, the resistivity is governed by the Shklovskii-Efros variable-range hopping mechanism, giving ρ ht (T)=ρ 0 exp [(T 0 /T) 1/2 ] . Combining the contributions of itinerant and localized carriers, that means ρlt(T) and ρht(T), respectively, a good fit of ρ(T) is obtained over the whole temperature region investigated. Below MIT, for T→TMI−0, a critical behavior obeying the law ρ(T)∼(TMI−T)−ν is observed. For T→TMI+0, the critical behavior of the resistivity is determined by divergence of the correlation length ξ∼(T−TMI)−ν, due to the relation T0∼1/ξ. In both cases, ν≈1. The temperature dependence of ξ, when approaching TMI from the high-temperature side, is attributed to generation of critical clusters of the metallic phase in the semiconductor matrix. Rapid increase of the metallic component is observed when B is increased.

Low-field magnetic properties of La1-xCaxMnO3 (0 ≤ x ≤ 0.4)

Abstract Magnetic properties of cubic La 1− x Ca x MnO 3 (0⩽ x ⩽0.4) are investigated between T =5 and 310xa0K in the external field of 2xa0G. The dependence of the ferromagnetic transition temperature on x is analyzed with the model of spin polarons associated with electronic localization and electron–electron interaction. The bandwidth of the localized electrons is found to be W =1.90±0.05xa0eV. In the samples with x 4+ concentration c T 1 , on c can be well described by a model including the Mn 3+ –Mn 3+ superexchange and the Mn 3+ –Mn 4+ double-exchange interactions. In the samples with x ⩽0.2 and c ⩽0.22 the value of the critical exponent, γ =1.21±0.05, is obtained for the temperature dependence of the zero-field-cooled susceptibility function χ −1 ( T )∼( T − T C ) γ , for T > T C .

Shubnikov-de Haas effect in (Cd1−x−yZnxMny)3As2 far from the zero-gap state

Abstract The Shubnikov-de Haas effect in the type II 3 V 2 diluted magnetic semiconductor alloy ( Cd 1− x − y Zn x Mn y ) 3 As 2 was investigated in the region of 0⩽ y ⩽0.08, x + y = 0.3. Single period oscillations of the conductivity were observed in the temperature interval of 4.2–31 K and magnetic fields up to 11 T. In one of the samples ( y = 0.02) a well-resolved spin-splitting was seen. Values of the cyclotron effective mass and the g -factor were determined. A strong dependence of the cyclotron mass on the magnetic field was observed in crystals with a high Mn concentration.

Variable-range hopping conductivity of La1 − xSrxMn1 − yFeyO3

The temperature dependence of the resistivity, ρ, of ceramic La(1 - x)Sr(x)Mn(1 - y)Fe(y)O(3) (LSMFO) samples with x = 0.3 and y = 0.03, 0.15, 0.20 and 0.25 (or simply #03, #15, #20 and #25, respectively) is investigated between temperatures T ∼ 5 and 310 K in magnetic fields B up to 8 T. Metallic conductivity in #03 is changed eventually to activated in #25. In #15 and #20 the behavior of ρ(T) is more complicated, comprising of two extremes, divided by an interval of metallic behavior in #15, and two inflections of ρ(T) in #20 within similar intervals ΔT below approximately 100 K. Mott variable-range hopping (VRH) conductivity is observed in #15 above the ferromagnetic Curie temperature, T(C). In #20 the Mott VRH conductivity takes place in three different temperature intervals at T > T(C), T close to T(C) and T < T(C). In #25, the Mott VRH conductivity is observed in two different intervals, above and below T(C), divided by an intermediate interval of the Shklovskii-Efros VRH conduction regime. Analysis of the VRH conductivity yielded the values of the localization radius, α, and the dependence of α and of the density of the localized states, g, near the Fermi level, on B. Above T(C) the localization radius in all samples at B = 0 has similar values, α approximately 1.0-1.2 Å, which is enhanced to α approximately 3.3 Å (#20) and 2.0 Å (#25) below T(C). The sensitivity of α and g to B depend on y and T. The complicated behavior of the mechanisms of the hopping charge transfer, as well as of the microscopic parameters α and g, is attributable to different electronic and magnetic phases of LSMFO varying with temperature and Fe doping.

Mechanisms of hopping conductivity in weakly doped La1−xBaxMnO3

The resistivity, ?, of ceramic La1?xBaxMnO3 with x = 0.02?0.10 corresponding to the concentrations of holes c?0.15?0.17 displays an activated behaviour both above and below the paramagnetic to ferromagnetic transition temperature TC = 175?209?K, obtained from measurements of the magnetization. Above T~310?390?K ?(T,x) is determined by nearest-neighbour hopping of small polarons with activation energy Ea = 0.20?0.22?eV. Below the onset temperature Tv = 250?280?K, depending on x, a Shklovskii?Efros-like variable-range hopping conductivity mechanism, governed by a soft temperature independent Coulomb gap, ??0.44?0.46?eV, and a rigid gap, ?(T), is found. For the range T~50?120?K, ?(T) is connected to the formation of small lattice polarons in conditions of strong electron?phonon interaction and lattice disorder. The rigid gap obeys a law ?(T)~T1/2 within two temperature intervals above and below TC, exhibits an inflection at TC and reaches at Tv a value of ?v?0.14?0.18?eV. Such behaviour suggests a spin dependent contribution to ?(T). The localization radius of the charge carriers, a, has different constant values within the temperature intervals where ?(T)~T1/2. With further decrease of T, a increases according to the law expected for small lattice polarons.

Changes in donor and acceptor states in Au-doped ZnSe samples induced by ageing at room temperature

Photoluminescence (PL) spectra of n-ZnSe single crystals doped with Au are investigated at temperatures between 4.4 and 300 K immediately after doping and after storage for four years at room temperature in darkness in air. The formation of donor-type Aui interstitial defects stimulated by time is found in ZnSe for the first time. Changes in the structure of the edge and long-wavelength PL spectra with the course of time are observed. The long-wavelength band attributed to (AuZn-DIII,VII) acceptors is not found in the PL spectra of the stored samples. At the same time, a green PL band with maximum at 527 nm (2.35 eV) appears. This band is ascribed to (VZn–Aui) acceptors formed as a result of displacement of the AuZn atoms into interstitial sites due to deformation forces. When the Au concentration in the Zn + Au melt exceeds 1 at.%, an Aui-donor impurity band is formed and the intensities of all the PL bands are reduced.