首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
The longitudinal and transverse relaxation rates of 19F nuclei in Pr2CuO4−x Fx (x=0.20) samples are measured at high temperatures (150 K<T<600 K). A feature is found in the temperature dependence of the relaxation rates at temperature T′≅300 K. The magnetic properties of the electronic superconductor Pr2CuO4−x Fx as a possible system with a stripe ordering of carriers and spins are discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 328–332 (25 February 1997)  相似文献   

2.
We have studied magnetic correlations in the low-carrier-doping regime of Y1−xCaxBa2Cu3O6. Samples with carrier densities psh=0.015–0.035 (x=0.03–0.07) were studied in the temperature range 2.6–200 K using the zero-field time differential muon spin rotation (ZF TD μSR) technique. An oscillating signal, of 4.1 MHz (∼300 G) in the low-temperature limit (T→0 K), indicative of antiferromagnetism, was found in each sample. This frequency diminished slightly with increasing Ca concentration. Two distinct regimes of frequency as a function of temperature are separated by a kink, which is interpreted as an independent ordering of the spins of the doped holes. The doped-hole spin-freezing temperature is doping dependent and occurs in the range 20.8–33 K. Even in the low-carrier-density regime, the long-range antiferromagnetic ordering is still strongly frustrated by the spin-glass mechanism through calcium doping. Apparent transitions between the 3D frozen-hole antiferromagnetic state and the spin-glassy state with diffusing holes were observed. Clearly the same magnetic interaction mechanisms are operating in both low-carrier-density (Y,Ca)-123 and (La,Sr)2CuO4.  相似文献   

3.
The crystal structure of (TMTSF)2ClO4 has been determined at (7 K, 1 bar) and at (7 K, 5 kbar) with a high accuracy. For the latter, low temperature and pressure were applied simultaneously using a X-ray diffraction instrumentation designed in our laboratory, these results are the first for molecular compounds. The effects of lowering the temperature are not the same as those produced by increasing the pressure. At (7 K, 1 bar) the anion ordering which occurs in this compound, and which is characterised by the appearance of b * /2 superlattice reflections, is well observed. This anion ordering leads to the presence of two independent stacks of TMTSF cations which is the only case found in the Bechgaard salts family. The comparison of the low temperature crystal structures under atmospheric pressure and at 5 kbar shows that the centres of mass are nearly the same, independent of the pressure: the interchain interactions do not depend on the doubling of the unit cell. Under pressure, the ordering (0, 1/2, 0) does not occur at any temperature. These structural data are confirmed by the quantum chemical calculations which show that the difference in the site energy of the two independent cations is 100 meV. Received 10 April 2000 and Received in final form 27 September 2000  相似文献   

4.
In the UxLa1-xS system there is an abrupt loss of the long-range ferromagnetic ordering found in pure US at a critical concentration x c ∼ 0.57, which is far above the percolation limit. As the magnetic ground state in such a system can be strongly affected by small variations of the 5f localization, we have investigated a set of samples with different x by polarized neutron diffraction and X-ray magnetic circular dichroism (XMCD). The neutron results are consistent with early measurements performed on pure US. Even at the lowest U content (x = 0.15, below x c ) the shape of the induced form factor (f ( Q )) is comparable with that found for x = 1 and is well reproduced by either a U4+ or a U3+ state. The ratio between the orbital and the effective spin moments in the XMCD measurements confirms this result, but the evolution of the shape at the M5 edge suggests an abrupt change in the distribution of the electrons (holes) in the 5 f density of states around x c . Received 31 January 2001  相似文献   

5.
As most previous work on non-stoichiometric magnetite Fe3−xO4 has been carried out with fine powders prepared in the metastable domain, with the inherent uncertainties, we have prepared small crystals by quenching from the high temperature stability range.With increasing x, the cell parameter decreases, the Verwey temperature falls below 4.2 K for x = 0.09, the thermal stability decreases (disproportionation occurs at ~550K for x = 0.09), the saturation magnetization at 125 K varies as (4.1 – 2x) suggesting that the vacancies are mainly octahedral; this is confirmed by the Mössbauer effect at 150 K and 5 Teslas. At 4.2 K with 8 T, the Mössbauer spectrum is very complex but displays a mixed-valence signal involving a fraction (~ 10%) of the octahedral sites. This signal is evidence for electron tunnelling. The conductivity, at 295 K, decreases more rapidly than (1 − 3x); the conductivity of stoichiometric magnetite is described by the dynamic correlation model applied to short-range ordering; for x = 0.09 an activation energy of 0.077 eV is deduced between 39 and 120K.These results are interpreted as evidence that the tunnel effect at low temperature is limited to domains separated by vacancies, associated with 5 times more electron holes, and by regions where short-range order prevails.  相似文献   

6.
We have studied the resistivity and thermoelectromotive force (thermo emf) in a temperature range of T = 80–1000 K, the magnetic susceptibility and magnetization in a temperature range of T = 4.2–300 K at an external magnetic field of up to 70 kOe, and the structural characteristics of Co x Mn1?x S sulfides (0 ≤ x ≤ 0.4). Anomalies in the transport properties of these compounds have been found in the temperature intervals ΔT 1 = 200–270 K and ΔT 2 = 530–670 K and at T 3T N. The temperature dependences of the magnetic susceptibility, magnetization, and resistivity, as well as the current-voltage characteristics, exhibit hysteresis. In the domain of magnetic ordering at temperatures below the Néel temperature (T N), the antiferromagnetic Co x Mn1?x S sulfides possess a spontaneous magnetic moment that is explained using a model of the orbital ordering of electrons in the t 2g bands. The influence of the cobalt-ion-induced charge ordering on the transport and magnetic properties of sulfides has been studied. The calculated values of the temperatures corresponding to the maxima of charge susceptibility, which are related to a competition between the on-site Coulomb interaction of holes in various subbands and their weak hybridization, agree well with the experimental data.  相似文献   

7.
We investigate the transport properties of La1.8−xEu0.2SrxCuO4 (x = 0.04, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For x = 0.125 and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except x = 0.2 exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La1.8−xEu0.2SrxCuO4 and superconducting La2−xSrxCuO4 at the doping levels x = 0.125 and x = 0.15 reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.  相似文献   

8.
We have investigated the magnetic and electrical transport properties of Si1−xMnx single crystals grown by the vertical Bridgman method. The alloys with Mn concentrations up to x=0.64 have weak ferromagnetic ordering around TC∼30 K. However, Si0.25Mn0.75 alloys show weak ferromagnetic ordering at 70 K and antiferromagnetic ordering at 104 K, which is confirmed by magnetization and electrical transport studies.  相似文献   

9.
57Fe M?ssbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4. Received: 23 June 1998 / Accepted: 14 July 1998  相似文献   

10.
The magnetic structures of Mn1-xFexWO4 with x = 0.0, 0.16, 0.21, 0.225, 0.232, 0.24, 0.27, 0.29, and 1.0 were refined from neutron powder diffraction data. The magnetic phase diagram could be completed in the coexistence range of different magnetic structures up to x = 0.29. For the magnetic state at 1.5 K a commensurate antiferromagnetic structure with a propagation vector = (±1/4, 1/2, 1/2) was found for x ⩽ 0.22 while the magnetic spins order with = (1/2, 0, 0) for x ≥ 0.22. In the latter phase, additionally, weak magnetic reflections indexed to an incommensurate ordering with = (- 0.214, 1/2, 0.457) occur in the diffraction pattern up to x = 0.29 indicating the occurence of a reentrant phase. For 0.12 ⩽ x ⩽ 0.29 the low temperature phases are separated from a magnetic high temperature phase showing only magnetic reflections indexed to a spin arrangement with = (1/2, 0, 0). The magnetic phase diagram is discussed qualitatively considering random superexchange between the statistically distributed Mn2+- and Fe2+-ions in the coexistence range 0.12 ⩽ x ⩽ 0.29 of different magnetic structures related to those of pure MnWO4 and FeWO4. Received 9 October 2002 Published online 14 March 2003  相似文献   

11.
We have used low temperature (90?K) transmission electron microscopy to investigate the ‘charge ordering’ modulation in the mixed valent manganite, La1? x Ca x MnO3. It has been stated that Mn3+ and Mn4+ ions order at low temperature to produce a structural modulation composed of supercells whose size is an integer multiple of the unmodulated unit cell. Here, we use convergent beam electron diffraction to show that the periodicity of the modulation need not be an integer multiple of the undistorted cell, even on the smallest scales. We therefore suggest that this modulation is a charge density wave with a uniform periodicity. We show that the modulation wavevector lies close to the a* axis of the crystal but need not be exactly collinear. A typical grain of size 0.5?µm in La0.48Ca0.52MnO3 had a wavevector which varied on a scale of tens of nanometres with an average of ?q??=?0.450a * and a standard deviation Δq?=?0.004?a* in its magnitude and Δθ?=?0.56° in its direction at 90?K. The magnitude of the wavevector in this composition fell by 20% as the temperature was increased from 90?K to room temperature. This change occurred by nucleation and growth. Although weak, the modulation was still present at room temperature, some 30?K above the ‘charge ordering temperature’.  相似文献   

12.
Ultrasonic velocity measurements have been performed on polycrystalline Tl0.9Bi0.1Sr2?xCexCa0.9Y0.1Cu1.99Fe0.01O7?δ superconductor to study the influence of Ce substitution on elastic properties of the samples. Ce was observed to influence elastic moduli at 80 K which showed the largest value obtained at x = 0.10 where, coincidentally, the highest superconducting temperature Tc among samples was also shown. A longitudinal velocity anomaly was observed at around 260 K for the unsubstituted sample (x = 0). Ce substitution caused the temperature of the elastic anomaly to shift to 250 K (x = 0.1) and 262 K (x = 0.2). The existence of the step-like elastic anomaly was suggested to be due to oxygen ordering taking place in Tl–O planes. The analysis of elastic behavior in the vicinity of the elastic anomalies using the Landau free-energy model suggests that the anomaly is due to a phase transition which involves oxygen ordering.  相似文献   

13.
NaNiO2 has been studied by neutron-powder diffraction, magnetic susceptibility and submillimeter wave ESR. The monoclinic structure at room temperature is characterised by a ferrodistorsive orbital ordering due to the Jahn-Teller (JT) effect of the Ni3+ ions in the low spin state. NaNiO2 undergoes a structural transition at around 480 K, above which the orbital ordering disappears. The high temperature phase is rhombohedral with the layered -NaFeO2 structure ( space group). The magnetic susceptibility exhibits hysteresis and we observe a change of the Curie-Weiss law parameters above the JT transition. The anisotropy of the g-factor at 200 K can be attributed to the JT effect which favours the orbital occupation. Finally, the interplay between the magnetic and structural properties of NaNiO2 and Li1-xNi1+xO2 is discussed. Received 29 May 2000  相似文献   

14.
Abstract

The highly polarizable perovskite-type oxide, KTaO3 doped simultaneously with Li+ and Nb5+ (K1?xLixTa1?yNbyO3, KLTN), reveals unexpected properties and ordering effects. Studies of the dielectric permittivity ?'(T, f) (10—300K, 100Hz-1 MHz) for x = 0.0014 and y = 0.024 show collective dipolar ordering effects with a transition from paraelectric into a mixed phase (coexisting dipole-glass-like and long-range ordered ferroelectric phases) taking place near 39 K. At 15 K another phase transition into a reentrant dipolar glass-like state is observed. Such a sequence of transitions and the existence of a reentrant glass state are unknown for electrical dipolar systems.  相似文献   

15.
Neutron diffraction study on samples MnxCr3?xO4 with x=1.0, 1.2, 1.4 and 1.6 has been performed at temperatures below ~40 K where magnetic ordering is present. At 5 K the structure of the MnCr2O4 compound was found to be a general LKDM magnetic spiral with the propagation vector deviating by 13.3″ from the [110] axis. This results differs from previous findings. At about 16 K the structure becomes collinear. In samples higher content of manganese the magnetic structure is collinear over the whole temperature region (5–40) K, i.e. in the sense of long range ordering. However, a short range ordering of the transverse components of the spins is present both in these samples and in MnCr2O4 above 16 K.  相似文献   

16.
The transport properties of GaAs/Mn/GaAs/In x Ga1 ? x As/GaAs structures with a layer that is separated from the quantum well and contains Mn impurities in the concentration range 4–10 at % corresponding to the reentrant metal-insulator transition observed in the bulk GaMnAs material [17] have been investigated. The hole mobility in the objects under investigation is more than two orders of magnitude higher than the known values for the GaMnAs semiconductor and GaMnAs-based magnetic heterostructures. This makes it possible to observe Shubnikov-de Haas oscillations, which confirm a two-dimensional character of the hole energy spectrum. The calculated Curie temperature for heterostructures with indirect exchange interaction through a two-dimensional hole channel is in good agreement with the position of the maximum (at 25–40 K) in the temperature dependences of the electrical resistance of the channel. This suggests that two-dimensional holes play an important role in ferromagnetic ordering of the Mn layer under these conditions. The observations of a negative spin-dependent magnetoresistance and an anomalous Hall effect, whose magnitude correlates well with the results of theoretical calculations for two-dimensional ferromagnetic systems based on III-Mn-V, also indicate a significant role of the two-dimensional channel in ferromagnetic ordering.  相似文献   

17.
Anelastic relaxation and 139LaNQR relaxation rates in La2–xSrxCuO4 for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La2–xSrxCuO4 at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the 139LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La1.98Sr0.02CuO4, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000  相似文献   

18.
The temperature dependence of the magnetic susceptibility of La2NiO4+δ single crystals with δ = 0.095 and 0.105 has been analyzed. The anomalous dependence of χ(T) characteristic of stripe structures previously found in La1.8Sr0.2NiO4 has been found. Investigation of the dependence χ(T) on the impurity and charge disorder degrees makes it possible to separate the contribution associated with the ordering of holes and impurity oxygen. It has been shown that the contribution from the ordering of holes is well described by the model of ferromagnetic Heisenberg chains of spins S = 1/2. The length of the chains and intrachain interaction J N have been determined from the temperature dependence χ(T).  相似文献   

19.
The compound Fe0.34TiSe2 crystallizes in space group D33d?P3&#x0304;ml of the trigonal system with 8 formula units in a cell of dimensions a = 7.148(4), c = 11.880(7) Å at 117 K. The structure of Fe0.34TiSe2 can be derived from that of TiSe2 by insertion of Fe atoms into some of the octahedral holes between the layers. No two adjacent octahedral holes along c contain Fe atoms, that is no two Fe atoms are related by a translation of c2. The compound FexTiSe2, x = 0.34, exhibits physical properties remarkably different from those found in either the host material, TiSe2, or in low iron concentration (x < 0.2) intercalated materials. The electrical resistivity of this compound exhibits a sharp decrease below 120 K which is coincident with a well defined peak in the Hall coefficient. These data have been interpreted as the onset of antiferromagnetic ordering as suggested by results of magnetic susceptibility measurements. Optical reflectance measurements indicate a simple Drude-like free carrier behavior and are consistent with the interpretation of iron as an electron donor in this material.  相似文献   

20.
The low temperature magnetic and transport properties of the Pr0.5Ca0.5Mn1-xNixO3 manganites ( 0≤ x ≤0.1) have been investigated. The presence of Ni hinders the charge and orbital ordering observed in Pr0.5Ca0.5MnO3 and favors the creation of ferromagnetic regions, leading to phase separation. The ferromagnetic fractions induced by the Ni substitution have been estimated from magnetization measurements, they are large and reach 40% for 4% of Ni. Steps are observed in the M ( H ) and ρ( H ) curves of all the samples at T < 5 K. They are similar to the steps observed in Pr0.5Ca0.5Mn1-xMxO3, where M is a non magnetic cation (Mg2+, Ga3+,...), and for which the ferromagnetic fractions are very small (less than 2%), however, their appearance is restricted to lower temperatures (T < 5 K) with Ni dopant than with non magnetic cations. This study shows that steps can be observed in a wide range of phase-separated systems, even when the ferromagnetic fraction is very large. Received 5 April 2002 / Received in final form 8 July 2002 Published online 14 October 2002 RID="a" ID="a"e-mail: antoine.maignan@ismra.fr  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号