N. I. Solin

Charge segregation and a nonuniform magnetic state in donor-and acceptor-doped LaMnO3

A coordinated study of the magnetic, electrical, optical, and EPR properties of LaMnO3 single crystals doped by donors (7 at. % Ce) and acceptors (7 at. % Sr) revealed that in all cases, except undoped LaMnO3, charge segregation associated with large-scale crystal-field fluctuations occurs and the magnetic properties originate from the existence of ferromagnetic phase inclusions and localized ferrons in the matrix with a canted magnetic structure.

Intrinsic inhomogeneities of low-doped lanthanum manganites in the paramagnetic temperature range

The nature of the electrical resistivity for low-doped lanthanum manganites is elucidated. The electrical resistivity is described by the Efros-Shklovskii law (lnρ √ (T0/T)−1/2, where T0 √ 1/Rls) in the temperature range from T* ≈ 300 K ≈ TC (TC is the Curie temperature for conducting manganites) to their TC and is explained by the tunneling of carriers between localized states. The magnetoresistance is explained by a change in the size of localized states Rls in a magnetic field. The patterns of change in Rls with temperature and magnetic field strength determined from magnetotransport properties are satisfactorily described in the model of phase separation into small-radius metallic droplets in a paramagnetic matrix. The sizes Rls and their temperature dependence have been estimated through magnetic measurements. The results confirm the existence of a Griffith phase. The intrinsic inhomogeneities produced by thermodynamic phase separation determine the electrical resistivity and magnetoresistance of lanthanum manganites.

Erratum: “Efros-shklovskii law and localized states in weakly doped lanthanum manganites”

The existence of clusters in weakly doped lanthanum manganites at temperatures about twice as high as their Curie temperature TC has been shown. Electrical resistance in weakly doped lanthanum manganites obeys the Efros-Shklovskii law. The temperature and magnetic-field dependences of a cluster size determined from the magnetotransport properties have been described using the model of phase separation into small metallic droplets within the dielectric paramagnetic and antiferromagnetic matrices. The results agree with the existence of the Griffiths phase.

Intercluster conduction in lightly doped La1 − xCaxMnO3 manganites in the paramagnetic temperature range

The magnetotransport and magnetic properties of La 1 − xCaxMnO3 polycrystalline samples (x = 0–0.3) annealed under vacuum and in the oxygen environment are investigated in the temperature range from 77 to 400 K. The magnetic studies of lightly doped manganites reveal persistence of short-range magnetic order up to a temperature T* ≈ 300 K, which is about 2–3 times higher than their Curie temperature TC. The temperature dependence of the electrical resistivity measured from T* down to nearly T ≈ TC is fitted by the relation logρ ∼ T−1/2, which is characteristic of granular metals with electrons tunneling among nanoclusters of magnetic metals embedded in a dielectric host. The magnetoresistance of polycrystalline samples annealed in the oxygen environment has been observed to increase. The electrical, magnetic, and magnetotransport properties of the manganites can be accounted for by the formation of magnetic nanoclusters below T*, tunneling (or hopping) of carriers among the nanoclusters, variation in the magnetic cluster size, and tunneling barrier thickness with variations in temperature and magnetic field strength, as well as by the effect of annealing in different media on the cluster properties.

Magnetoresistance and Hall effect of the magnetic semiconductor HgCr2Se4 in strong magnetic fields

The giant decrease of the electrical resistance of HgCr2Se4 (more than by a factor of 200) caused by magnetic field-induced changes in the carrier mobility and concentration, the quadratic dependences of magnetoresistance and normal Hall constant on magnetic induction in the paramagnetic region, as well as the deviations from these dependences observed to occur as one approaches the Curie temperature, are discussed within a model involving carriers of several types (holes in the valence band, electrons localized at ferron-type impurity centers, and electrons hybridized in the impurity and conduction bands).

Colossal magnetoresistance of the inhomogeneous ferromagnetic semiconductor HgCr2Se4

A new method is described for attaining high magnetoresistance in inhomogeneous magnetic materials, which makes use of the formation of a depleted layer and a contact potential difference at the interface separating two semiconductors with different Fermi levels and the magnetic-field-induced variation in the contact potential difference and thickness of the interface layer. The proposed model of the magnetoresistive structure is realized on the basis of the HgCr2Se4 magnetic semiconductor. Layers of n-HgCr2Se4 up to a few tens of microns thick were prepared on the surface of p-HgCr2Se4 bulk single crystals by the diffusion technique. Application of a magnetic field stimulated in the structures a strong (by a factor of more than 200) increase of the current flowing through the n-layer.

Phase Separation and Anisotropy in the Electrical Properties of Weakly Doped Lanthanum Manganites

Variations in the thermopower, electrical resistivity, magnetoresistance, thermal expansion coefficients, and their anisotropy with temperature were detected near room temperature in single crystals of weakly doped lanthanum manganites La1−xAxMnO3 (A = Ca, Sr; x = 0.07–0.125) with orthorhombic structure. The results obtained are discussed in terms of a model of phase separation related to polaron anisotropy. Due to a gain in exchange and elastic energies in the lattice, small-radius magnetic polarons can merge to form polarons of a larger size, which would contain now not one but rather a few electrons (equal in number to the polarons in the cluster). As a result, short-range order in a cluster and phase separation set in at a temperature Tps ≈ 250–300 K, which is approximately equal to the Curie temperature TC of conducting manganites with x ≈ 0.2–0.3.

Room-temperature phase separation in weakly doped lanthanum manganites

The properties of single crystals of weakly doped lanthanum manganites La1−xAxMnO3 (A = Ca, Ce, Sr; x = 0, 0.07−0.1) have been studied in the temperature range from 77 to 400 K. It is established that these lanthanum manganites exhibit (in addition to the well-known characteristic features observed in the region of the temperature of magnetic ordering) changes in the electrical and magnetic properties in the region of room temperature (T ≈ 270–300 K), which is about two times the Curie temperature (T ≈ 120–140 K) and is far from the temperature of structural transitions in the samples studied. The results are explained in terms of phase separation related to the formation of magnetic clusters in the nonconducting medium. The phase separation is caused by a gain in the exchange energy and by the development of elastic stresses in the crystal lattice and proceeds via combination of small-radius magnetic polarons into a large-size magnetic cluster containing several charge carriers. The short-range order in the cluster appears and the phase separation begins at a temperature Tps, which is close to TC ≈ 300 K, typical of doped conducting manganites. The results of magnetic measurements show that, as the temperature decreases from 300 to 190 K, the size of superparamagnetic droplets increases from about 8 to 15 Å.

Role of surface phenomena in the magnetoresistivity of polycrystalline manganites La1−xCaxMnO3

The dc electrical resistivity and magnetoresistivity of polycrystalline manganites La1−xCaxMnO3 (x=0–0.3) are investigated as functions of the temperature, magnetic field and electric field, along with the microwave surface resistance. The investigations show that the dc electrical resistivity and magnetoresistivity are governed by the surface properties of the intergranular boundaries. The dc electrical resistivity is observed to decrease substantially (tenfold) for a comparatively small electric field (E⋟100 V/cm). Estimates are obtained for the internal electrical resistivity of the granules, the thickness of the contact layer (which depends on the temperature and the magnetic field), and the height of the potential barrier between the interfaces separating the surface layer and inner layer of a granule.

Magnetic polarons, clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La1−xAxMnO3(A≡Ca,Sr;x=0.07–0.1) single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy ΔEρ, its temperature coefficient γ, the thermopower activation energy ΔES, the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of 275±25 K, which is approximately twice as high as their Curie point TC and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10–12 Å in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to TC of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.