Lattice effect on magnetism in CMR perovskite La0.85Sr0.15MnOz

0.85 Sr_0.15MnO₃ system has been investigated. Enlargements of lattice parameters with grain growth have been observed. Associated with this growth are a series of magnetic changes, including magnetization, Curie temperature T_c and superparamagnetic behavior. These phenomena may provide another route to understanding the relationship between structure and magnetism in the hole-doped manganese perovskites. Received: 20 January 1997/Accepted: 28 February 1997     

First observation of ferromagnetism and ferromagnetic domains in a liquid metal

80 Pd₂₀ alloy below the Curie temperature T_C(l)=1257 K of the liquid state. The magnetization of the undercooled liquid sample has been measured as a function of temperature using a modified Faraday balance. Below T_C(l), the magnetization of the liquid metal shows a plateau in the weak external field of μ₀H_z=5.6 mT. The effect is comparable to the behaviour of the corresponding solid phase, indicating the onset of spontaneous magnetization with ferromagnetic domains. T_C(l) is about 20 K lower than the Curie temperature of the solid phase T_C(s). Received: 24 February 1997/Accepted: 16 May 1997     

Magnetocaloric effect in R2Fe17 (R=Sm,Gd, Tb,Dy, Er)

A series of R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) have been synthesized. The magnetocaloric effect (MCE) of these compounds has been investigated by means of magnetic measurements in the vicinity of their Curie temperature. The Curie temperature of Er₂Fe₁₇ is 294 K. The maximum magnetic entropy change of Er₂Fe₁₇ under 5 T magnetic field is ∼3.68 J/kg K. In the R₂Fe₁₇ (R=Sm, Gd, Tb, Dy, Er) system, the maximum magnetic entropy change under 1.5 T magnetic field is 1.72, 0.89, 1.32, 1.59, 1.68 J/kg K corresponding to their Curie temperature (400, 472, 415, 364, 294 K), respectively.     

Determination of Curie temperature by Arrott plot technique in Gd5(SixGe1−x)4 for x>0.575

Determination of Curie temperature by plotting magnetic moment vs. temperature curves requires a small applied field, which influences the measurement and temporarily disturbs the temperature of the sample especially for highly magnetocaloric materials. The Arrott plot technique was therefore used in order to determine the Curie temperature for a magnetocaloric Gd₅Si_2.7Ge_1.3 (x=0.675) single crystal sample. This technique was compared with other methods such as the inflection point technique and the line projection method. The results show how applied magnetic field influences the determination of Curie temperature. Using the Arrott plot the second-order transition Curie temperature for Gd₅Si_2.7Ge_1.3 was determined to be 304 K.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Magnetic and high-pressure magnetotransport properties of cesium-substituted lanthanum calcium manganites

The nature of the double-exchange (DE) interaction in lanthanum manganites is studied through chemical substitutions, Cs for La, and high-pressure measurements. Static and high-frequency magnetic measurements and high-pressure electrical transport studies were carried out on bulk polycrystalline and radio-frequency sputtered thin films of La_0.7-xCs_xCa_0.3MnO₃ for x=0-0.1. The samples are found to be cubic. Curie temperature T_c measurements provide evidence for bond-length-related weakening of DE as x is increased from 0 to 0.03. For higher x, the bond-angle-related changes lead to an increase in the strength of DE. High-pressure mangetoresistance data indicate both bond length and bond-angle-related increase of 10–20 K/GPa in T_c with pressure, with the largest increase measured for x=0.03. The rate of increase in the Curie temperature with pressure decreases with increasing T_c. Anomalies are observed in the magnetic parameters for x=0.03. The Cs-concentration dependence of the low-temperature saturation magnetization shows a minimum close to x=0.03. Ferromagnetic resonance studies at x-band reveal a 5% decrease in the g-value for x=0.03 relative to the end members (x=0 and 0.1). The low-field magnetostriction for x=0.03 indicates a relatively strong electron–phonon spin coupling compared to neighboring compositions. Received: 15 May 2000 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Low frequency dielectric permittivity of quasi-one-dimensional conductor (TMTTF)2Br

Conductivity and permittivity of the organic transfer salt (TMTTF)₂Br have been measured at low frequencies (10²-10⁷ Hz) between room temperature down to 4 K. The real part of the permittivity, , is shown to grow below the temperature at which the conductivity is maximum due to charge localization of Mott-Hubbard type. reaches a maximum of 10⁵-10⁶ at 35 K-50 K depending on the samples. Decreasing temperature below , sharply decreases down to helium temperature through the antiferromagnetic phase transition at T _N = 15 K. We explain the magnitude, the temperature and frequency dependence of as resulting from short range charge density wave states in the temperature range where charge localization occurs. This interpretation is supported by recent X-ray scattering measurements. Received: 10 October 1997 / Revised: 28 February 1998 / Accepted: 3 March 1998     

Chromium doped manganites: evidence for electronically phase-separated systems

X-ray absorption spectroscopy (XAS) and soft-X-ray magnetic circular dichroism (SXMCD) at the Mn L_2,3-, Cr L_2,3- and O-K edges of Sm_0.5Ca_0.5Mn_1-xCr_xO₃ () bulk polycrystalline samples have been performed at T=20 K below the ferromagnetic Curie temperature. We show the existence of a magnetic sublattice on each of the probed cations. Considering an electronically phase-separated system, results are compared with magnetization and resistivity measurements and a tentative correlation with magnetoresistance properties on such doped compounds is discussed. Received 7 January 2000     

非晶态Fe13Ni67.2P4.5B15.3合金的临界现象

本文报道非晶态Fe₁₃Ni_67.2P_4.5B_15.3合金的磁化强度与温度和磁场关系的测量结果。在居里温度附近样品的磁特性符合二级相变规律,得到临界指数β=0.39±0.02,γ=1.56±0.06,δ=5.20±0.1,样品的居里温度T_c=(180.4±0.2)K。在实验误差范围内,临界指数β,γ,δ满足γ=β(δ-1)关系,在168—192K温度范围,实验数据满足二级相变的磁状态方程。当T>270K时,样品顺磁磁化率服从居里-外斯定律,由居里-外斯常数c计算出有效顺磁磁矩P_eff=3.19 μ_B。 关键词：     

Magnetocaloric effect in polycrystalline La0.65Ba0.3M0.05MnO3 (M=Na, Ag, K) manganites

The effects of monovalent doping on the crystallographic, magnetic and magnetocaloric properties of La_0.65Ba_0.3M_0.05MnO₃ (M=Na, Ag, K) powder samples, elaborated using the solid state reaction method at high temperature, have been investigated. In our three samples the Mn⁴⁺ amount remains constant equal to 40%. The Rietveld refinement of the X-ray powder diffraction shows that all our synthesized samples are single phase and crystallize in the distorted rhombohedral system with R3¯c space group. All our studied samples undergo a paramagnetic–ferromagnetic transition with decreasing temperature. Using the Arrott plot, the second-order transition Curie temperature T_C for M=Na, Ag and K is found to be 310, 300 and 290 K, respectively. The magnetic entropy change, deduced from isothermal magnetization curves, exhibits a maximum |ΔS_M^Max| of about 2.65, 2.82 and 2.66 J/kg K for M=Na, Ag and K, respectively, in a magnetic applied field change of 5 T. Although these values are modest, the magnetocaloric effect extends over a large temperature range leading to an important value of the relative cooling power (RCP). The RCP values exhibit a nearly linear dependence with the magnetic applied field. The refrigeration capacity in a magnetic applied field of 1 T is found to be 28.8, 27.8 and 25.6 J/kg for M=Na, Ag and K compounds.     

Magnetic structure and anisotropy of YFe 6 Ga 6 and HoFe 6 Ga 6

The magnetic structure of RFe₆Ga₆ intermetallic compounds with R = Y, Ho have been determined by neutron powder diffraction, ⁵⁷Fe M?ssbauer spectroscopy, AC susceptibility, TGA (Thermo-Gravimetric Analysis) and magnetization measurements. Both compounds crystallize in the tetragonal ThMn₁₂ structure (space group I4/mmm) with the magnetic structure of YFe₆Ga₆ consisting of a simple ferromagnetic alignment of Fe moments in the basal plane with a Curie temperature of 475(5) K. Gallium atoms are found to fully occupy the 8i site, with Fe and Ga atoms equally distributed over the 8j site, whilst Fe atoms fully occupy the 8f site. The average Fe moments are 1.68(10) and 1.46(10) at 15 and 293 K, respectively. The average room temperature Fe magnetic moments determined by neutron diffraction are in overall agreement with the average Fe moment deduced from M?ssbauer spectroscopy and bulk magnetization measurements on this compound. The magnetic anisotropy of the compound HoFe₆Ga₆ is also planar in the temperature range 6-290 K, with Ho magnetic moments of 9.28(20) and 2.50(20) at 6 K and 290 K, respectively, coupled anti-ferromagnetically to the Fe sublattice and a Curie temperature of 460(10) K. The magneto-crystalline anisotropies of both compounds are comparable at low temperatures. Received 8 March 2001 and Received in final form 18 June 2001     

Hall effect plateaus and giant Shubnikov-de Haas oscillations in (BiSb)Telayered single crystals near the quantum limit

On bulk layered single crystals (Bi_0.25Sb_0.75)₂Te₃ with a hole concentration cm^-3 and a mobility cm²/Vs magnetoresistance and Hall effect investigations were performed in the temperature range T = 1.4 K ... 20 K in magnetic fields up to 18 T. For the magnetic field perpendicular to the layered structure giant Shubnikov-de Haas oscillations are measured; the positions of the maxima are triplets in the reciprocally scaled magnetic field. From the damping of the amplitudes with increasing temperature the cyclotron mass m _c = 0.12m ₀ is evaluated. Correlated with the SdH oscillations doublets of Hall effect plateaus (or kinks in low fields) are found. The weak well known Shubnikov-de Haas oscillations from the generally accepted multivallied highest valence band can be detected as a modulation on the giant oscillation. The high anisotropy of the SdH oscillations and their triplet structure in connection with the layered crystal structure lead us to suggest that the effects are caused by hole carrier pairing (mediated by the bipolaron mechanism) in quasi 2D sheets parallel to the crystal layer stacks. The measured Hall plateau resistances coincide with the quantum Hall effect values considering the number of layer stacks and the valley degeneracy of the 3D hole carrier reservoir. The ratio of spin splitting to Landau (cyclotron) splitting is observed to be . Received: 12 September 1997 / Revised: 8 January 1998 / Accepted: 22 January 1998     

Magnetic phases in Pr0.5Sr0.5Mn1 ? xCoxO3 (x ≤ 0.5) solid solutions

The magnetic and magnetotransport properties of Pr_0.5Sr_0.5Mn_{1 − x} Co _x O₃ (x ≤ 0.5) solid solutions have been investigated using neutron diffraction methods. The magnetization and electrical conductivity have been measured in magnetic fields up to 140 kOe. It has been established that, during cooling in the temperature range from 160 to 110 K, the compounds of compositions with a cobalt content x ≤ 0.07 undergo a structural phase transition from the high-temperature ferromagnetic phase to the antiferromagnetic phase. A further substitution of cobalt for manganese leads to a stabilization of the inhomogeneous dielectric ferromagnetic state, whereas a state of the cluster spin-glass type has been revealed in compositions with x = 0.15 and 0.20. At x ≥ 0.25, a new magnetic phase with a Curie temperature up to 210 K is formed as a result of the magnetic interaction between manganese and cobalt ions. A magnetic phase diagram of the system under investigation has been constructed.     

Magnetic ordering above room temperature in the sigma-phase of Fe66V34

Magnetic properties of four sigma-phase Fe_100−xV_x samples with 34.4?x?55.1 were investigated by Mössbauer spectroscopy and magnetic measurements in the temperature interval 4.2-300 K. Four magnetic quantities, viz. hyperfine field, Curie temperature, magnetic moment and susceptibility, were determined. The sample containing 34.4 at% V was revealed to exhibit the largest values found up to now for the sigma-phase for average hyperfine field, 〈B〉=12.1 T, average magnetic moment per Fe atom, 〈μ〉=0.89 μ_B, and Curie temperature, T_C=315.3 K. The quantities were shown to be strongly correlated with each other. In particular, T_C is linearly correlated with 〈μ〉 with a slope of 406.5 K/μ_B, as well as 〈B〉 is so correlated with 〈μ〉, yielding 14.3 T/μ_B for the hyperfine coupling constant.     

Phenomenological modeling of magnetic and magnetocaloric properties in rare earth doped La0.8Ca0.2MnO3

La_0.8Ca_0.2MnO₃ manganese oxides have been prepared by the conventional solid-state reaction. The samples crystallize in the orthorhombic structure with Pnma space group. A first-order magnetic phase transition from ferromagnetic to paramagnetic state is observed at the Curie temperature that is found to be around 241?K. The magnetic and magnetocaloric properties have been simulated using a phenomenological model. The maximum of magnetic entropy change is of order 8.2?J/kg?K for an applied magnetic field of 5?T, and the adiabatic temperature change is found to be of 4?K under a same magnetic field; this high value gives the sample the possibility of technologic application in the magnetic refrigeration area. The simulated results of magnetization and magnetic entropy change are compared with the experimental measurements. Finely, we found a good agreement between experimental and theoretical results.     

Pressure effects in ferromagnetic manganites and carrier scattering by disordered magnetic rare earths

The influence of hydrostatic pressure on the transport properties of (La_1-xR_x)_0.67Ca_0.33MnO₃ (,Tb) ferromagnetic manganites is investigated. The enhancement of the Curie temperature T_C under pressure agrees with previous data. In the paramagnetic range, the resistivity can be represented by a Mott localisation law, with a characteristic temperature T₀ decreasing with pressure. The variation of T_C with pressure is compared to the effect induced by replacing La by a magnetic rare earth in (La_1-xR_x)_0.67Sr_0.33MnO₃ manganites (, ..., Tm). The main effect is not related to the decrease of the mean radius of the cation, but to an additional scattering by the magnetic moment of the rare earth. Received: 15 May 1998 / Revised: 6 July 1998 / Accepted: 16 July 1998     

Anomalous ferromagnetic resonance in manganese- and aluminum-doped germanium layers deposited from the laser plasma

Ferromagnetic resonance (FMR) with an anomalous angular dependence has been observed in the Ge:(Mn, Al)/GaAs nanolayers deposited from laser plasma at a reduced temperature of 150°C. The resonance is associated with the needle-like inclusions of a high-temperature ferromagnetic phase with the Curie temperature T _C > 293 K. Such a magnetic anisotropy is confirmed by the atomicforce and magneticforce microscopy of a side chip. A low-temperature ferromagnetic phase with normal FMR and T _C < 212 K is formed between the needle-like inclusions. This phase manifests itself in the anomalous Hall effect at 77 K and probably is a solid solution of manganese in germanium.

     

Anomalous ferromagnetic resonance in manganese- and aluminum-doped germanium layers deposited from the laser plasma

Ferromagnetic resonance (FMR) with an anomalous angular dependence has been observed in the Ge:(Mn, Al)/GaAs nanolayers deposited from laser plasma at a reduced temperature of 150°C. The resonance is associated with the needle-like inclusions of a high-temperature ferromagnetic phase with the Curie temperature T _C > 293 K. Such a magnetic anisotropy is confirmed by the atomicforce and magneticforce microscopy of a side chip. A low-temperature ferromagnetic phase with normal FMR and T _C < 212 K is formed between the needle-like inclusions. This phase manifests itself in the anomalous Hall effect at 77 K and probably is a solid solution of manganese in germanium.     

The magnetic phases of Mn0.90Fe0.10P studied by Mössbauer spectroscopy

Iron/manganese substitution in MnP has been studied by ⁵⁷Mössbauer spectroscopy. The Curie temperature is 250 K, but not all iron nuclei experience a magnetic hyperfine field even at 170 K. The helical structure is developed below 170 K and show modulations of the hyperfine field. The discrepancies from pure magnetic states (ferromagnetic and circular helix) are explained on the basis of the near surroundings of the iron atoms. The differences in the values of the electric monopole and quadrupole interactions at the iron nuclei for FeP and Mn_0.90F_0.10P are interpreted as an effect of lattice expansion.     

Quantum and semiclassical dynamics of differential optical collisions

We apply quantum and semiclassical theories to differential optical collisions Na(3²S_1/2) + Kr + Na(3²P_1/2,3/2) + Kr. Our results provide a basis to analyze recent experiments in which for the first time optical collisions were investigated with angular resolution under crossed-beam conditions. A characteristic feature of the differential cross sections is the pronounced oscillatory structure due to interferences of different Condon paths. These Stueckelberg oscillations form an extremely sensitive probe of the collisional dynamics and of the molecular interactions. We demonstrate perspectives to determine geometric properties of the collision complex by excitation with polarized light. By final state analysis nonadiabatic (spin-orbit, rotational) interactions can be studied with complete control of the path. In summary it is shown that the method of differential detection of optical collisions opens a variety of new accesses to atomic and molecular subcollisions. Received: 30 July 1997 / Received in final form: 5 November 1997 / Accepted: 8 January 1998