A. I. Abramovich

Spin-glass and reentrant spin-glass states in iron sulfospinels having dilute A and B sublattices

The magnetic and electrical properties of new compounds having spinel structure Fe1−xCr2(1−x)Sn2xS4 (0.1⩽x⩽0.33) (system 1), Fe0.67 [Fe0.165CrSn0.835]S4, and Fe0.67 [Fe0.33Cr0.67Sn]S4 has been studied. These compounds are p-type semiconductors with magnetic properties characteristic of the following magnetic-order types: ferrimagnetic (the x=0.1 composition of system 1), spin glass (the x=0.33 composition of system 1 and Fe0.67 [Fe0.165CrSn0.835]S4), and reentrant spin glass (the x=0.2 composition of system 1 and Fe0.67 [Fe0.33Cr0.67Sn]S4). For the spin-glass compositions, the dependence of the freezing temperature Tf defined as the temperature of the maximum of initial magnetic susceptibility, on temperature and magnetic field obeys the Almeida-Thouless relation, and the dependence of Tf on magnetic-field frequency is a power-law function. For the spin-glass and reentrant spin-glass compositions, a large peak in the absolute value of negative isotropic magnetoresistance was found near Tf, which becomes as high as 15% in spin glasses and 30% in reentrant spin-glass compositions. In compositions with reentrant behavior, the activation energy of conductivity in the region of Tf was found to change by about two orders of magnitude. This experimental evidence suggests that the spin-glass-paramagnet (in spin glasses) and spin-glass-long-range magnetic-order transformations are actually phase transitions, and that the spin-glass region contains ferron-type ferromagnetic clusters. These are the first spin-glasses among the chalcospinels with magnetically active ions on the tetrahedral and octahedral sublattices.

Peculiarities of the magnetic, galvanomagnetic, elastic, and magnetoelastic properties of Sm1-xSrxMnO3 manganites

Manganites of the Sm1−xSrxMnO3 system (x=0.33, 0.4, and 0.45) possess giant negative values of the magnetoresistance Δρ/ρ and the volume magnetostriction ω near the Curie temperature TC. In the compound with x=0.33, the isotherms of Δρ/ρ, ω, and magnetization σ exhibit smooth variation and do not reach saturation up to maximum magnetic field strengths (120 kOe) studied (according to the neutron diffraction data, this substance comprises a ferromagnetic (FM) matrix with distributed clusters of a layered antiferromagnetic (AFM) structure of the A type). In the compounds with x=0.4 and 0.45 containing, besides the FM matrix and A-type AFM phase, a charge-ordered AFM phase of the CE type (thermally stable to higher temperatures as compared to the A-type AFM and the FM phases), the same isotherms measured at T ≥ TC show a jumplike increase in the interval of field strengths between Hc1 and Hc2 and then reach saturation. In the interval Hc1 > H > Hc2, the σ, ω, and Δρ/ρ values exhibit a metastable behavior. At temperatures above TC, the anisotropic magnetostriction changes sign, which is indicative of rearrangements in the crystal structure. The giant values of ω and Δρ/ρ observed at T ≥ TC for all compounds, together with excess (relative to the linear) thermal expansion and a maximum on the ρ(T) curve, are explained by the phenomenon of electron phase separation caused by a strong s-d exchange. The giant values of magnetoresistance and volume magnetostriction (with ω reaching ∼10−3) are attributed to an increase in the volume of the FM phase induced by the applied magnetic field. In the compound with x=0.33, this increase proceeds smoothly as the FM phase grows through the FM layers in the A-type AFM phase. In the compounds with x=0.4 and 0.45, the FM phase volume increases at the expense of the charge-ordered CE-type AFM structure (in which spins of the neighboring manganese ions possess an AFM order). The jumps observed on the σ(H) curves, whereby the magnetization σ reaches ∼70% of the value at T=1.5 K, are indicative of a threshold character of the charge-ordered phase transition to the FM state. Thus, the giant values of ω and Δρ/ρ are inherent in the FM state, appearing as a result of the magnetic-field-induced transition of the charge-ordered phase to the FM state, rather than being caused by melting of this phase.

Colossal magnetoresistance of La0.35Nd0.35Sr0.3MnO3 epitaxial thin film on (001)ZrO2(Y2O3) substrate over a wide temperature range

Colossal negative magnetoresistance is found over a wide range of temperatures below the Curie point TC≈240 K in an epitaxial La0.35Nd0.35Sr0.3MnO3 film on a single-crystal (001)ZrO2(Y2O3) wafer substrate. 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 epitaxial films of the same composition on (001)LaAlO3, (001)SrTiO3, and (001)MgO substrates, colossal magnetoresistance only occurred near TC≈240 K and at T<TC it increased weakly, almost linearly with the field. In the film on 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 orientation, while the magnetoresistance in the region before technical saturation of the magnetization is attributable to tunnelling 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, which is 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 TC 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 mobility of the carriers (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.

Giant magnetocaloric effect near the curie temperature in the Sm0.6Sr0.4MnO3 manganite

A method is proposed for the calculation of the magnetocaloric effect from simultaneous measurements of thermal expansion and magnetostriction made in different regimes (adiabatic and isothermal). The magnitude of the magnetocaloric effect for Sm0.6Sr0.4MnO3 is estimated. It is found that near the Curie temperature TC it passes through a maximum to reach a giant value ΔT=4.6 K for ΔB=0.84 T. In addition, in the neighborhood of TC, we observed colossal magnetoresistance Δρ/ρ = [ρ(H) − ρ(0)]/ρ(0) = 72% in a weak magnetic field of 0.84 T, a giant negative volume magnetostriction ω=−5×10−4 in a field of the same strength, and a large change in the sample volume ΔV/V ≈ 0.1%.

Experimental evidences of magnetic two-phase state in Eu1−xAxMnO3 (A = Ca, Sr)

Abstract For insulating Eu1−xAxMnO3 (A = Ca, Sr; x = 0, 0.3) ceramics the magnetic properties depend on the thermomagnetic history of the sample and the maximum of initial magnetic susceptibility is located at T=TN, being independent of the AC field frequency. Magnetization isotherms are the superposition of the linear part and spontaneous magnetization. The maximum of electrical resistivity and colossal magnetoresistance are observed in the TN region. These properties are explained by the magnetic two-phase state.

Peculiarities of magnetic, galvanomagnetic, elastic, and magnetoelastic properties of EU0.55Sr0.45MnO3

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu0.55Sr0.45MnO3 ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=106 Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field HC1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and HC1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρH−ρH=0)/ρH is positive for HHC1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×105 % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10−4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu0.55Sr0.45MnO3, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ∼3% of the sample volume is occupied by the FM phase; ∼67%, by the CE-type AFM phase; and ∼30%, by the A-type AFM phase.

Colossal room-temperature magnetoresistance of La1/3Nd1/3Sr1/3MnO3 single crystals

Replacement of one half of the neodymium ions by lanthanum in Nd2/3Sr1/3MnO3 is shown to result in a considerable increase in the Curie temperature. The single-crystal La1/3Nd1/3Sr1/3MnO3, whose Curie point lies at 315 K, has been found to exhibit a record-high magnetoresistance of 27% in a weak magnetic field of 8.4 kOe in the temperature range above room temperature.

GIGANTIC MAGNETORESISTANCE IN SEMICONDUCTIVE SPIN-GLASSES CU2/3GE1/3CR2S4 AND CU0.5IN0.5CR2SE4

Gigantic negative magnetoresistance (MR) reaching 80% in a field of 30 kOe is detected in semiconductive spin glasses Cu23Ge13Cr2S4 and Cu0.5In0.5Cr2Se4. In the temperature dependence of the MR a sharp minimum is observed at the freezing temperature estimated by the maximum nonlinear magnetic susceptibility.

Correlations of the electrical and magnetic phase transitions in the Sm1-xSrxMnO3 (x=0.33, 0.45) and RE0.55Sr0.45MnO3 (RE0.55 = Eu0.397Nd0.153,Tb0.25Nd0.30) manganites at the high magnetic stationary fields

Motto The first address of Satan to the human race began most modestly with the word: why? Adam Mickiewicz (1798–1855) The resistance, magnetoresistance and magnetization measurements have been performed in the temperature range of 4.2–300 K and in the external magnetic inductions up to 14 T for the Sm1− x Sr x MnO3 (x = 0.33, 0.45) and RE0.55Sr0.45MnO3 (RE0.55 = Eu0.397Nd0.153, Tb0.25Nd0.30) manganites. In these samples the metal–semiconductor phase transitions as well as two kinds of the magnetic phase transitions the ferromagnetic–paramagnetic and metamagnetic (both spin-flip and spin-flop) have been observed. It was found that correlations between the magnetic and electrical phase transitions appear at high values of the external magnetic inductions. These correlations consist, simultaneously, of the large negative magnetoresistance and metamagnetic phase transitions. The existence and reconstruction of the ferromagnetic and antiferromagnetic clusters under the influence of the external magnetic induction is considered as a driving force of those correlations.

The giant volume magnetostriction and colossal magnetoresistance in Eu0.55Sr0.45MnO3 manganite

A doped manganite with the composition Eu0.55Sr0.45MnO3 exhibits giant negative magnetostriction and colossal negative magnetoresistance at temperatures in the vicinity of the magnetic phase transformation (T∼41 K). In the temperature interval 4.2 K≤T ≤40 K, the isotherms of magnetization, volume magnetostriction, and resistivity exhibit jumps at the critical field strength Hc1, which decreases with increasing temperature. At 70 K ≤T ≤120 K, the jumps on the isotherms are retained, but the shapes of these curves change and the Hc1 value increases with the temperature. At HHc1, the magnetoresistance becomes negative, passes through a minimum near 41 K and then reaches a colossal value. The observed behavior is explained by the existence of three phases in Eu0.55Sr0.45MnO3, including a ferromagnetic (in which the charge carriers concentrate due to a gain in the s-d exchange energy) and two antiferromagnetic phases (of the A and CE types). The volumes of these phases at low temperatures are evaluated. It is shown that the colossal magnetoresistance and the giant volume magnetostriction are related to the ferromagnetic phase formed as a result of the magnetic-field-induced transition of the CE-type antiferromagnetic phase to the ferromagnetic state.