共查询到20条相似文献,搜索用时 31 毫秒
1.
M. Opel K. -W. Nielsen S. Bauer S. T. B. Goennenwein J. C. Cezar D. Schmeisser J. Simon W. Mader R. Gross 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(4):437-444
The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One
particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc. These are often
tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially
on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic
behavior was investigated by SQUID magnetometry, X-ray magnetic circular dichroism, and AC susceptibility measurements. The
results were correlated to a detailed microstructural analysis based on high resolution X-ray diffraction, transmission electron
microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt
moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like
behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates. 相似文献
2.
W. Westerburg F. Martin P.J.M. van Bentum J.A.A.J. Perenboom G. Jakob 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):509-513
We measured the temperature dependence of the linear high field Hall resistivity of ( K) and ( K) thin films in the temperature range from 4 K up to 360 K in magnetic fields up to 20 T. At low temperatures we find a
charge-carrier density of 1.3 and 1.4 holes per unit cell for the Ca- and Sr-doped compound, respectively. In this temperature
range electron-magnon scattering contributes to the longitudinal resistivity. At the ferromagnetic transition temperature
a dramatic drop in the number of charge-carriers n down to 0.6 holes per unit cell, accompanied by an increase in unit cell volume, is observed. Corrections of the Hall data
due to a non saturated magnetic state will lead a more pronounced charge-carrier density collapse.
Received 22 July 1999 and Received in final form 7 October 1999 相似文献
3.
N.J. Poirot P. Simon P. Odier J.M. Bassat 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):469-474
We report an investigation of magnetic resonance by ESR in oxygen-doped La2NiO emphasizing extensively for the first time the role of oxygen stoichiometry. The work is performed at room temperature and
on powders with . At low field an hysteresis is detected between increasing and decreasing fields, it depends upon . The resonance lines have characteristic features of ferromagnetic resonance. The intensity () is used as an experimental parameter for investigating the effect of . It allows to built a diagram closely connected to the phase diagram . It suggests a ferromagnetic coupling depending upon . When the (average) structure is tetragonal the low intensity is due to a magnetic polaron of low resistivity. In ranges
where a phase separation is detected, the ferromagnetic coupling has a structural origin (DM or local anisotropy) with apparently
a strong influence on the electrical resistivity.
Received: 3 November 1997 / Accepted: 23 December 1997 相似文献
4.
E. Barborini C. Lenardi P. Piseri P. Milani R. G. Agostino T. Caruso E. Colavita S. La Rosa M. Bertolo C. Ducati 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2003,24(1-3):273-276
Inclusions of metals in the growth process of carbon
cluster assembled materials (ns-C) induce modifications in the
structural and electronic properties of the material. A novel
pulsed microplasma cluster source (PMCS) is able to deliver
highly intense, collimated and stable beams suitable for
producing bulk quantities of cluster-assembled nanocomposite
films. Loading of metal nanoparticles into carbon cluster based
films is obtained either by mixing a gas phase metallorganic
compound with the carrier gas (He) before entering into the
source (for example molybdenum (V) isopropoxide), or by using a
double component sputtering target (metal (Ti, Ni)/graphite). The study of film
morphology on nanometer scale, carried out by transmission
electron microscopy (TEM), reveals the dispersion in a ns-C
matrix of metallic particles and, in the case of molybdenum
containing films, also of carbide particles. Spatially resolved
ultraviolet photoemission spectroscopy confirms the segregation
of metal particles and exhibits evident anisotropy in the
Mo:ns-C films, mainly ascribable to the formation of carbide
nanoparticles. 相似文献
5.
N. Shannon 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,27(4):527-540
Above 40 K, the magnetic susceptibility of the heavy Fermion spinel LiV2O4 has many features in common with those of geometrically frustrated magnetic insulators, while its room temperature resistivity
comfortably exceeds the Mott-Regel limit. This suggests that local magnetic moments, and the underlying geometry of the pyrochlore
lattice, play an important role in determining its magnetic properties. We extend a recently introduced tetragonal mean field
theory for insulating pyrochlore antiferromagnets to the case where individual tetrahedra contain spins of different lengths,
and use this as a starting point to discuss three different scenarios for magnetic and electronic transitions in LiV2O4.
Received 12 March 2002 and Received in final form 3 May 2002 Published online 25 June 2002 相似文献
6.
N.A. Babushkina L.M. Belova E.A. Chistotina O.Yu. Gorbenko A.R. Kaul B. Güttler K.I. Kugel 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(3):409-415
The effect of
16
O
→
18
O isotope substitution on electrical resistivity, magnetoresistance, and ac magnetic susceptibility was studied for La0.35Pr0.35Ca0.3MnO3 epitaxial thin films deposited onto LaAlO3 and SrTiO3 substrates. For the films on LaAlO3, the isotope substitution resulted in the reversible transition from a metal-like to insulating state. The applied magnetic
field ( H
≥ 2
T) transformed the sample with 18O back to the metallic state. The films on SrTiO3 remained metallic at low temperatures for both 16O and 18O, but the shift of the resistivity peak corresponding to onset of metallic state exceeded 63 K after
16
O
→
18
O substitution. The temperature dependence of both resistivity and magnetic susceptibility was characterized by hysteresis,
especially pronounced in the case of the films on LaAlO3. Such a behavior gives certain indications of the phase separation characteristic of interplay between ferromagnetism and
charge ordering.
Received 11 February 2000 and Received in final form 13 September 2000 相似文献
7.
C.C. Tsuei T. Doderer 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(2):257-262
The fact that the stripe phase and pseudogap in the cuprate superconductors occur in the same doping regime is emphasized.
A model based on charge confinement in self-organized nanometer-scale stripe fragments is proposed to understand various generic
features of the normal-state energy gap including the magnitude of the gap, its anti-correlation with the superconducting
gap, and the d-wave symmetry in its -dependence. This model also provides a basis for understanding other anomalous normal-state properties such as the linear
temperature dependence of electrical resistivity.
Received 7 December 1998 相似文献
8.
A. T. Zheleznyak V.M. Yakovenko 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):385-399
We present a heuristic, semiphenomenological model of the anomalous temperature (T) dependence of resistivity recently observed experimentally in the quasi-one-dimensional (Q1D) organic conductors of the family in moderately strong magnetic fields. We suggest that a Q1D conductor behaves like an insulator (), when its effective dimensionality is one, and like a metal (), when its effective dimensionality is greater than one. Applying a magnetic field reduces the effective dimensionality of
the system and switches the temperature dependence of resistivity between the insulating and metallic laws depending on the
magnitude and orientation of the magnetic field. We critically analyze whether various microscopic models suggested in literature
can produce such a behavior and find that none of the models is fully satisfactory. In particular, we perform detailed analytical
and numerical calculations within the scenario of magnetic-field-induced spin-density-wave precursor effect suggested by Gor'kov
and find that the theoretical results do not agree with the experimental observations.
Received 20 October 1998 相似文献
9.
C. Hordequin D. Ristoiu L. Ranno J. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(2):287-293
Magnetism and transport properties of the semi-Heusler compound NiMnSb are re-examined in great details. A wide set of experiments
(elastic and inelastic neutron scattering, static magnetic measurements, magnetoresistance, Hall effect, thermopower, FMR)
have been performed on polycrystals, single crystals or single-crystalline thin films, and the results are analysed. Special
emphasis is given to the magnetic excitations and to the relaxation mechanisms in this metallic ferromagnet. At low temperatures,
all experimental results hint at the existence of a fully spin-polarized conduction band (half metallic state). At higher
temperature (T > 80 K), but well below the Curie temperature (730 K), a cross-over to a usual metallic ferromagnetic state is evidenced
and discussed.
Received 10 January 2000 相似文献
10.
M.-C. Chung M. Kaulke I. Peschel M. Pleimling W. Selke 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(4):655-661
The influence of surface defects on the critical properties of magnetic films is studied for Ising models with nearest-neighbour
ferromagnetic couplings. The defects include one or two adjacent lines of additional atoms and a step on the surface. For
the calculations, both density-matrix renormalization group and Monte Carlo techniques are used. By changing the local couplings
at the defects and the film thickness, non-universal features as well as interesting crossover phenomena in the magnetic exponents
are observed.
Received 27 July 2000 and Received in final form 5 October 2000 相似文献
11.
C. Stockinger G. Heine W. Markowitsch W. Lang R. Adam R. Sobolewski R. Rössler J.D. Pedarnig D. Bäuerle 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,18(2):227-231
() thin films were photodoped with white light at various temperatures from 70 K to 290 K. Before and after the excitation,
the magnetoconductivity was measured in a magnetic field B
= 0.5 T, and the experimental results were fitted to the Aslamazov-Larkin theory of superconducting order-parameter fluctuations
to determine the superconducting coherence lengths, and . We observed that the photodoping process enhanced and and that the amount increased with the photodoping temperature increase. On the other hand, the superconducting anisotropy
/ decreased with increasing temperature. The photodoping effect enhances superconducting properties of partially oxygen-deficient
samples and is considerably increased by high doping temperatures.
Received 15 December 1999 and Received in final form 24 May 2000 相似文献
12.
The current standard of electronic devices and data storage media has reached a level such that magnetic materials have to be fabricated on a nanometer scale. In particular, the emerging concept of spintronics, which is based on fact that current carriers have not only charge but also spin, requires the assembling of nanometer-sized magnetic structures with desired magnetic properties. It is this background that motivates scientists and engineers to attempt to grow and characterize magnetic objects at smaller and smaller length scales, from 2D films and multilayers to 1D wires and eventually to 0D dots. In this article, some of the most significant progress in recent years in the effort of growing artificially structured magnetic materials are reviewed. The new structural and magnetic properties of these materials are discussed, with an emphasis on the correlation between structure and magnetism, which also serves as guidance for improving their magnetic properties. The emerging emphasis is on converting the existing knowledge into growing and studying low-dimensional complex materials, which promise to have considerably higher “tuning” ability for desired properties. 相似文献
13.
14.
15.
S. Paschen E. Felder H.R. Ott 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):169-176
We present measurements of the specific heat, the electrical resistivity, the Hall effect, and the magnetic susceptibility
of CeAuAl3, a new heavy-electron compound that crystallizes in an ordered derivative of the tetragonal BaAl4-type structure. For comparison we have also done some of these measurements on the isostructural non-magnetic reference compound
LaAuAl3, which appears to be a simple metal. Below K, CeAuAl3 orders antiferromagnetically and below 1K, we encounter Fermi liquid behaviour with considerably enhanced effective masses,
i.e., a quadratic temperature dependence of the resistivity with a large prefactor and a sizable linear-in-T contribution to the specific heat. This linear-in-T contribution increases by more than a factor 50 from its value at to its value at . Consequently CeAuAl3 develops a heavy-electron ground state, coexisting with antiferromagnetic order. The small energy scales involved in the
problem make CeAuAl3 a good candidate for tuning it, by varying external parameters, towards a quantum critical point. At high temperatures we
observe local moment behaviour. From the temperature dependence of the magnetic susceptibility and the specific heat we have
derived the crystalline-electric-field-split level scheme of the J
=5/2 multiplet. Distinct features in the electrical resistivity provide additional evidence for this level splitting.
Received: 6 November 1997 / Accepted: 17 November 1997 相似文献
16.
On nanoscale, thermal conduction
is affected by system size. The reasons are
increased phonon scattering and changes in phonon group
velocity. In this paper, the in-plane thermal resistivity
of nanoscale silicon thin films is analyzed
by molecular dynamics (MD) techniques. Modifications
to the dispersion relation are calculated directly with
MD methods at high temperature. The results indicate that
the dispersion relation starts to change for very thin
films, at around two nanometers. The reasons are band folding
and phonon confinement.
Thermal resistivity is analyzed by the direct
non-equilibrium method, and the results are compared
to kinetic theory with modified dispersion relations. Thermal
resistivity is affected by both surface scattering and dispersion.
Moreover, in thin films, the characteristic vibrational frequency
decreases, which in standard anharmonic scattering models indicates
a longer relaxation time and affects the resistivity.
The results indicate that in very thin films, the resistivity becomes
highly anisotropic due to differences in surface scattering.
In two cases, surface scattering was found to be the most important mechanism
for increasing thermal resistivity, while in one case, phonon
confinement was found to increase resistivity more than surface
scattering. 相似文献
17.
P. Ruszczynski L. Schimansky-Geier I. Dikshtein 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):569-577
A model for the motion of a single ferromagnetic domain is studied numerically and analytically. A single strip in two dimensions
and pinned at two inhomogeneities is considered. We suppose two stable configurations (positively or negatively curved with
pinned ends) due to the action of a bistable potential. Further, it is assumed that the domain is driven externally by periodic
and noisy magnetic fields. The noise makes the domain able to flip between the two configurations. The small temporally periodic
fields synchronize these flippings and the phenomenon of stochastic resonance is observed. The signal to noise ratio of the
output is investigated and shows a maximum for a nonvanishing intensity of the applied noise. Its dependency on the stiffness
of the domain is studied.
Received 14 May 1999 and Received in final form 14 October 1999 相似文献
18.
X.L. Lei S.Y. Liu 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(2):271-277
We investigate the magnetotransport in semiconductors under the influence of a dc or slowly-varying electric field, an intense
polarized radiation field of terahertz frequency, and a uniform magnetic field, being in arbitrary directions and having arbitrary
strengths. Effective force- and energy-balance equations are derived by using a gauge that the magnetic field and the high-frequency
radiation field are described by a vector potential and the dc or slowly-varying field by a scalar potential, and by distinguishing
the slowly-varying velocity from the rapidly-oscillating velocity related to the high-frequency field. These equations, which
include the elastic photon process and all orders of multiphoton absorption and emission processes, are applied to the examination
of the effect of a terahertz radiation on the magnetophonon resonance of the longitudinal resistivity in the transverse configuration
in nonpolar and polar semiconductors. We find that the previous zero-photon resonance peaks are suppressed by the irradiation
of the terahertz field, while many new peaks, which may be related to multiphoton absorption and emission processes, emerge
and can become quite distinct, at moderately strong radiation field.
Received 17 May 1999 相似文献
19.
We have investigated the influence of composition and annealing conditions on the magnetic properties and microstructural features of SmCox films that were prepared by sputtering and subsequent annealing. A huge in-plane coercivity of 5.6 T was obtained from an optimally annealed Sm–Co film, which was attributed to the nanometer sized polycrystalline microstructure of the highly anisotropic SmCo5 phase. Although a high density of planar defects were observed in the films that were annealed at high temperatures, they did not act as strong pinning sites for domain wall motion. The effect of Cu on [SmCo4.5(9 nm)/Cu(xnm)]10 multilayer thin films was also studied. An appropriate Cu content increased the coercivity. 相似文献
20.
L. Ramos P. Fabre R. Ober 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):319-326
We describe and study by small-angle X-ray scattering (SAXS) a new type of hybrid system. It is composed of a swollen lyotropic
hexagonal phase into the cylinders of which solid magnetic particles of nanometric size have been incorporated. It has been
found to be stable for volume fractions of particles up to 2% provided the cylinders are sufficiently large. A unidimensionnal
magnetic liquid is thus realized. The structural properties of this colloidal assembly have been investigated by SAXS, and
the specific features of the scattering spectra are analysed and interpreted. One of the remarkable results is the evidence
of depletion interactions between the particles and the inner walls of the cylinders inside which particles are located.
Received: 4 September 1997 / Revised: 14 October 1997 / Accepted: 19 November 1997 相似文献