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1.
The helicoidal magnetic structure of a MnGe compound doped with 25% Fe is studied by means of small-angle neutron scattering in a wide temperature range of 10–300 K. Analysis of the scattering-function profile demonstrates that magnetic structures inherent to both pure MnGe and its doped compounds are unstable. The doping of manganese monogermanide is revealed to lead to higher destabilization of the magnetic system. In passing from MnGe to Mn0.75Fe0.25Ge, the magnetic-ordering temperature T N decreases from 130 to 95 K, respectively. It is demonstrated that, at temperatures close to 0 K, the intensity of the contribution to scattering from stable spin helices decreases and the intensity of scattering by spin helix fluctuations increases with increasing impurity-metal concentration. An increased intensity of anomalous scattering caused by spin excitations existing in the system is observed. Helicoidal fluctuations and spin excitations corresponding to low temperatures indicate the quantum nature of the instability in the doped compound. However, MnGe doping with Fe atoms has no influence on the compound’s magnetic properties at temperatures of higher than T N. The temperature range of short-range ferromagnetic correlations is independent of concentrations and is restricted by temperatures T ranging from 175 to 300 K. 相似文献
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3.
A. I. Golovashkin N. D. Kuz’michev V. V. Slavkin 《Journal of Experimental and Theoretical Physics》2008,107(4):581-586
Temperature dependences of the magnetization harmonics of YBa2Cu3O7 ? x monodomain samples are studied experimentally at temperatures of 77–120 K. It is found that nonlinearity of magnetization of YBa2Cu3O7 ? x (higher harmonics generation) is observed up to temperatures T = 103–112 K, which are much higher than the superconducting transition temperature of this compound. At the same temperatures, the temperature dependence of resistivity begins to deviate from linearity. The observed singularity of the magnetization of YBa2Cu3O7 ? x is associated with the emergence of a pseudogap state in this compound. 相似文献
4.
《Journal of Physics and Chemistry of Solids》2004,65(2-3):321-326
The existence of two temperatures: Tp-Cooper pairing and Tc-Bose–Einstein condensation in high temperature superconductors has been stipulated in a lightly potassium-doped C60 by Magnetically Modulated Microwave Absorption. This doping level corresponds to the carrier density greater than the critical one: x>x1. In case of rubidium lightly doped C60, where the carrier density x was smaller than the critical one: x<x1, anomalous EPR temperature dependence was observed. The characteristic temperature of bound electron pair formation Tp≈65 K and the energy gap 2Δ/k=30 K were estimated from the temperature dependence of the EPR signal intensity in non-superconducting state. These results suggest that the liquid fermions–liquid bosons transition can be observed as the opening of the spin gap at temperature Tp postulated in Micnas–Ranninger–Robaszkiewicz theory. 相似文献
5.
Soma Das 《Solid State Communications》2005,134(12):837-842
Electrical conductivity and magnetoresistance of a series of monovalent (K) doped La1−xKxMnO3 polycrystalline pellets prepared by pyrophoric method have been reported. K doping increases the conductivity as well as the Curie temperature (TC) of the system. Curie temperature increases from 260 to 309 K with increasing K content. Above the metal-insulator transition temperature (T>TMI), the electrical resistivity is dominated by adiabatic polaronic model, while in the ferromagnetic region (50<T<TMI), the resistivity is governed by several electron scattering processes. Based on a scenario that the doped manganites consist of phase separated ferromagnetic metallic and paramagnetic insulating regions, all the features of the temperature variation of the resistivity between ∼50 and 300 K are described very well by a single expression. All the K doped samples clearly display the existence of strongly field dependent resistivity minimum close to ∼30 K. Charge carrier tunneling between antiferromagnetically coupled grains explains fairly well the resistivity minimum in monovalent (K) doped lanthanum manganites. Field dependence of magnetoresistance at various temperatures below TC is accounted fairly well by a phenomenological model based on spin polarized tunneling at the grain boundaries. The contributions from the intrinsic part arising from DE mechanism, as well as, the part originating from intergrannular spin polarized tunneling are also estimated. 相似文献
6.
The mechanism of hole carrier generation is considered in the framework of a model assuming the formation of negative U centers (NUCs) in HTSC materials under doping. The calculated dependences of carrier concentration on the doping level and temperature are in quantitative agreement with experiment. An explanation is proposed for the pseudogap and 60 K phases in YBa2Cu3O6+δ. It is assumed that a pseudogap is of superconducting origin and arises at temperature T* > Tc∞ > Tc in small nonpercolating clusters as a result of strong fluctuations in the occupancy of NUCs (Tc∞ and Tc are the superconducting transition temperatures of an infinitely large and finite NUC clusters, respectively). The T*(δ) and Tc(δ) dependences calculated for YBa2Cu3O6+δ correlate with experimental dependences. In accordance with the model, the region between T*(δ) and Tc(δ) is the range of fluctuations in which finite nonpercolation clusters fluctuate between the superconducting and normal states due to NUC occupancy fluctuations. 相似文献
7.
A. Bhattacharjee P.J. van Koningsbruggen Joel S. Miller 《Journal of Physics and Chemistry of Solids》2008,69(11):2713-2718
The 57Fe Mössbauer spectroscopy of mononuclear [Fe(II)(isoxazole)6](ClO4)2 has been studied to reveal the thermal spin crossover of Fe(II) between low-spin (S=0) and high-spin (S=2) states. Temperature-dependent spin transition curves have been constructed with the least-square fitted data obtained from the Mössbauer spectra measured at various temperatures between 84 and 270 K during a cooling and heating cycle. This compound exhibits an unusual temperature-dependent spin transition behaviour with TC(↓)=223 and TC(↑)=213 K occurring in the reverse order in comparison to those observed in SQUID observation and many other spin transition compounds. The compound has three high-spin Fe(II) sites at the highest temperature of study of which two undergo spin transitions. The compound seems to undergo a structural phase transition around the spin transition temperature, which plays a significant role in the spin crossover behaviour as well as the magnetic properties of the compound at temperatures below TC. The present study reveals an increase in high-spin fraction upon heating in the temperature range below TC, and an explanation is provided. 相似文献
8.
The temperature dependences of the third and other higher magnetization harmonics for YBa2Cu3O7 ? x textured polycrystals are experimentally investigated in the temperature range 77–120 K. It is revealed that the magnetization of the YBa2Cu3O7 ? x textured polycrystals exhibits a nonlinear behavior up to temperatures considerably higher than the superconducting transition temperature. The observed nonlinearity of the magnetization of the YBa2Cu3O7 ? x textured polycrystals is attributed to the pseudogap state that appears for this compound at a temperature T ~ 102 K. 相似文献
9.
M. Battabyal 《Journal of Physics and Chemistry of Solids》2004,65(11):1895-1900
Thermal conductivity (λ) of nanocrystalline La1−xAgxMnO3 (x=0.05, 0.15, 0.25, 0.3) pellets prepared by pyrophoric method is reported between 10 and 300 K. Magnitude of thermal conductivity has been found to be strongly influenced by monovalent (Ag) substitution at the La site. Silver doping in LaMnO3 enhances TC of the system to ∼299 K. Qualitative nature of the temperature variation of thermal conductivity of the silver substituted lanthanum manganites remains closely similar to that for divalent doped systems. Our analysis demonstrates that in La1−xAgxMnO3 also, the mechanism of heat conduction is predominantly by phonons. The contribution of the electronic part is only ∼1% of the total λ. The spin wave contribution is also estimated close to TC, which for all the samples lies within ∼2%. At temperatures below ∼100 K, the measured data have been analyzed using phonon relaxation time method and the strengths of the various phonon scattering processes have been estimated. Our analysis further suggests strong influence of phonon scattering by 2D like defects in the thermal conductivity of monovalent doped lanthanum manganites at low temperatures (<70 K) in the ferromagnetic region. 相似文献
10.
The superconducting state of doped semiconductors, in which a superconducting gap opens in the spectrum of one-particle states formed (upon doping) in the region of the initial energy gap, is studied within the framework of the Holstein-Anderson model. The wavefunction of the relative motion of a pair of particles in the bound state is determined. The dependence of the size of this wavefunction on the level of doping in the overdoping region of the phase diagram is consistent with the published experimental data for La2 ? x SrxCuO4. Variation of the temperature in the range of T < T c only influences the natural normalization of the boson wavefunction (normalized to the boson density), while the size of the wavefunction is independent of the temperature. Theoretical values of the condensation energy and the heat capacity jump upon the superconductor-metal transition in the region of optimal doping agree with the experimental data for La2 ? x SrxCuO4. 相似文献
11.
The temperature and angular dependence of the X-band electron spin resonance (ESR) and51V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi2-xMgxV2O8 (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the51V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT N≈ 3.5K) in doped compositions. 相似文献
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AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x thr, 1, (2) lead to a superconducting state for x thr, 1 < x < x thr, 2, and (3) decay as x > x thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level. 相似文献
14.
M. Fähnle 《Solid State Communications》1985,55(8):743-746
The temperature dependence of the non-linear susceptibility ≈2(T) of random anisotropy magnets in the Ising limit (speromagnets) is calculated for temperatures above the freezing temperature Tf within the framework of the correlated molecular field theory. For the effective susceptibility exponent λs(T) = (T?Tf)≈2d-1≈2/dT a non-monotonic temperature dependence is found as for the case of spin glasses. This must be taken into account in order to obtain reliable values for the critical susceptibility exponent from experimental data. 相似文献
15.
I. K. Kamilov A. G. Gamzatov A. M. Aliev A. B. Batdalov Sh. B. Abdulvagidov O. V. Mel’nikov O. Yu. Gorbenko A. R. Kaul’ 《Journal of Experimental and Theoretical Physics》2007,105(4):774-781
We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La1 ? x Ag y MnO3 (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρFM(T) = ρ0 + AT 2 + BT 4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons. 相似文献
16.
Polycrystalline manganites Pr0.8Na0.2MnO3 doped by ruthenium (0.0≤xRu≤0.2) were prepared by the sol-gel process. The magnetic field induced metamagnetism was observed to occur with a large resistivity drop at 3 K for xRu≤0.02 samples, which can be sorted into the kind of CMR phenomenon. It was found that the 0.01 Ru doping increased TC and decreased the metamagnetic critical field than that of the undoped sample. The doped manganites show a quick increase in their magnetic moments as xRu increases from 0.01 to 0.04, but the larger fraction of Ru doping (0.04≤xRu≤0.2) reduces their M and TC. All the five doped samples have larger magnetic moments than that of the host sample. Ru doping of xRu>0.01 results in a rapid disappearance for the observation of long range spin and charge ordering in the samples’ M-T curves, which is characteristic of the undoped sample. It was found that larger low-temperature MR favored decrease in the metamagnetic critical field. Finally, the phase diagram of Ru doping vs. transition temperatures is presented to summarize the experiments. 相似文献
17.
The effect of doping of rare earth Pr3+ ion as a replacement of Sm3+ in Sm0.5Sr0.5MnO3 is investigated. Temperature dependent dc and ac magnetic susceptibility, resistivity, magnetoresistance measurements on chemically synthesized (Sm0.5−xPrx)Sr0.5MnO3 show various unusual features with doping level x=0.15. The frequency independent ferromagnetic to paramagnetic transition at higher temperature (∼191 K) followed by a frequency dependent reentrant magnetic transition at lower temperature (∼31 K) has been observed. The nature of this frequency dependent reentrant magnetic transition is described by a critical slowing down model of spin glasses. From non-linear ac susceptibility measurements it has been confirmed that the finite size ferromagnetic clusters are formed as a consequence of intrinsic phase separation, and undergo spin glass-like freezing below a certain temperature. There is an unusual observation of a 2nd harmonic peak in the non-linear ac susceptibility around this reentrant magnetic transition at low temperature (∼31 K). Arrott plots at 10 and 30 K confirm the existence of glassy ferromagnetism below this low temperature reentrant transition. Electronic- and magneto-transport measurements show a strong magnetic field—temperature history dependence and strong irreversibility with respect to the sweeping of magnetic field. These results are attributed to the effect of phase separation and kinetic arrest of the electronic phase in this phase separated manganite at low temperatures. 相似文献
18.
M. Brando N. Büttgen V. Fritsch J. Hemberger H. Kaps H.-A. Krug von Nidda M. Nicklas K. Pucher W. Trinkl A. Loidl E.W. Scheidt M. Klemm S. Horn 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,25(3):289-298
We present susceptibility, microwave resistivity, NMR and heat-capacity results for Li1-xZnx(V1-yTiy)2O4 with 0 ?
x
? 0.3 and 0 ?
y
? 0.3. For all doping levels the susceptibility curves can be fitted with a Curie-Weiss law. The paramagnetic Curie-Weiss temperatures
remain negative with an average value close to that of the pure compound Θ≈ - 36 K. Spin-glass anomalies are observed in the susceptibility, heat-capacity and NMR measurements for both type of dopants.
From the temperature dependence of the spin-lattice relaxation rate we found critical-dynamic behavior in the Zn doped compounds
at the freezing temperatures. For the Ti-doped samples two successive freezing transitions into disordered low-temperature
states can be detected. The temperature dependence of the heat capacity for Zn-doped compounds does not resemble that of canonical
spin glasses and only a small fraction of the total vanadium entropy is frozen at the spin-glass transitions. For pure LiV2O4 the spin-glass transition is completely suppressed. The temperature dependence of the heat capacity for LiV2O4 can be described using a nuclear Schottky contribution and the non-Fermi liquid model, appropriate for a system close to
a spin-glass quantum critical point. Finally an (
x
/
y
,
T
)-phase diagram for the low-doping regime is presented.
Received 16 March 2001 and Received in final form 30 October 2001 相似文献
19.
L.J. Chang Y. SuY.-J. Kao Y.Z. ChouK. Kakurai R. Mittal H. SchneiderTh. Brückel G. BalakrishnanM.R. Lees 《Physica B: Condensed Matter》2011,406(12):2393-2396
Diffuse polarized neutron scattering studies have been carried out on single crystals of pyrochlore spin ice Ho2−xYxTi2O7 (x=0, 0.3, and 1) to investigate the effects of doping and anisotropy on spin correlations in the system. The crystals were aligned with the (1 −1 0) orientation coincident with the direction of neutron polarization. For all the samples studied the spin flip (SF) diffuse scattering (i.e. the in-plane component) reveals that the spin correlations can be described using a nearest-neighbour spin ice model (NNSM) at higher temperatures (T=3.6 K) and a dipolar spin ice model (DSM) as the temperature is reduced (T=30 mK). In the non-spin flip (NSF) channel (i.e. the out-of-plane component), the signature of strong antiferromagnetic correlations is observed for all the samples at the same temperature as the dipolar spin ice behaviour appears in the SF channel. Our studies show that the non-magnetic dopant Y does not significantly alter SF or NSF scattering for the spin ice state, even when Y doping is as high as 50%. In this paper, we focus on the experimental results of the highly doped spin ice HoYTi2O7 and compare our results with pure spin ice Ho2Ti2O7. The crossover from a dipolar to a nearest-neighbour spin ice behaviour and the doping insensitivity in spin ices are briefly discussed. 相似文献
20.
Single crystal growth and characterizations of iron arsenide superconductor BaFe2-xNixAs2(0.0≤x≤0.12) 下载免费PDF全文
A series of big single crystals of BaFeFe2-xNixAs2 have been prepared by the FeAs self-flux method,
with nominal nickel doping x = 0--0.12. The dimensions of the
cleaved crystals are over 10~mm along ab plane and ~ 2~mm
in maximum along the c direction. The measurements of x-ray
diffraction, electrical resistance and magnetic property are carried
out on the crystals. For the undoped parent compound
BaFe2As2, both resistance and magnetization data display
an anomaly associated with spin density wave and/or structural phase
transition, with the transition temperatures at ~ 138~K. For
Ni-doped BaFe2-xNixAs2 crystals, the
superconducting critical temperature Tc ranges from 4.3~K
for x=0.06 sample to 20~K for the optimally doped x=0.10
crystal. 相似文献