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1.
Erik B. Karlsson 《Hyperfine Interactions》1991,64(1-4):331-349
Some recent result of muon spin relaxation measurements in rare earth metals and intermetallic compounds are reviewed. Special
emphasis is put on measurements that relate to the properties of correlated regions of spins existing relatively far above
the ordering temperature in the rare earth ions. As far as comparable data from paramagnetic neutron scattering exist, they
will be discussed in the same framework. For each temperature the correlated regions (or short-lived magnetic clusters) are
characterized by their size, possible anisotropy with respect to the crystalline axes and their lifetime. The actual form
of the interaction between the rare earth spins themselves and with the crystal fields determine the temperature dependence
of these properties; a strong dipole interaction can, for instance, be expected to change the critical behaviour nearT
c
. Much of the time will be devoted to experiments on Gd-metal where there are experimental indications that several interesting
phenomena occur: (1) a strong effect of a cross-over from a non-conserved dynamics (dipolar) regime to a conserved (exchange
dominated) regime some 10 K aboveT
c
, (2) an anisotropy of the magnetic clusters with respect to the hexagonalc-axis, and (3), a persistence of spin correlations far aboveT
c
. Some attempts to correlate the rare earth spin relaxation times measured in this region with cluster lifetimes deduced from
neutron scattering will be reviewed, as well as a model for understanding these lifetimes in terms of temperature dependent
cluster wall motion, which is determined by exchange and magnetic anisotropy parameters. Effects of possible quantum correlations
originating from the “spin system+bath” interaction will be mentioned. 相似文献
2.
Alternating-current electrical resistance measurements between 17 Hz and 100 kHz were made on high purity Fe, Co, and Ni wires in the vicinity of their Curie temperatures (T c). The electrical resistance was independent of frequency for temperatures (T) aboveT c. As the temperature was lowered, however, there was an abrupt jump in the electrical resistance atT c followed by a gradual decrease toward its dc value. The magnitude of the electrical resistance jump atT c increased as the square root of the frequency. The enhancement of the electrical resistance forT≦T c is produced by an abrupt decrease of the skin depth atT c which, in turn, is due to the sudden increase in the initial magnetic permeability atT c. Measurements of the ac electrical resistance in the vicinity of the Curie temperature of certain ferromagnetic metals can be utilized to 1) accurately determine the Curie temperature using frequencies as low as 17 Hz, and 2) quantitatively determine the initial magnetic permeability as a function of temperature and heat treatment. 相似文献
3.
K. R. A. Ziebeck H. Capellmann P. J. Brown J. G. Booth 《Zeitschrift für Physik B Condensed Matter》1982,48(3):241-250
Polarised neutrons and polarisation analysis have been used to determine the diffuse magnetic scattering from MnSi as a function of temperature. Measurements at 50 K (1.72T
c) and 100 K (3.45T
c) in the paramagnetic phase confirmed the presence of strong spatial correlations previously reported at 300 K (10T
c) and 580 K (20T
c). The spatial correlations between regions of spin density separated by 12 Å are of a ferromagnetic nature and determine the static susceptibility. Integration of the paramagnetic response over an inverse atomic volume yields a moment per manganese atom of 1.2
B
, a value considerably larger than the 0.4
B
observed in Bragg scattering and magnetisation measurements at 4.2 K. Measurements made at 11 K in the ordered magnetic phase reveal strong diffuse scattering corresponding to 0.8
B
per Mn atom. The increase in the diffuse scattering observed in the paramagnetic phase corresponds to the passage of the Bragg component into the background.The presence of significant scattering in the ordered phase and the very unusual wave vector dependence of the diffuse scattering observed at 11 K has been interpreted assuming MnSi to be a heavy Fermi liquid. Furthermore we believe these measurements give the first direct observation of the exchange hole. 相似文献
4.
D. Stauffer 《Journal of statistical physics》1978,18(2):125-136
A scaling assumption for the numberg
ns of different cluster configurations with perimeters and sizen leads to the desired cluster numbers near the percolation threshold. The perimeter distribution function has a mean square width proportional ton for largen. The relation between the average perimeter and the cluster sizen for percolation has three different forms atp
c, belowp
c, and abovep
c and is closely related to the shape of the cluster size distribution. 相似文献
5.
Starting from the structure of the layered antiferroelectric squaric acid and assuming an ice rule type interaction mechanism of the protons it is shown that aboveT
c
the molecular layers may be considered as a system of parallel and orthogonal nearly independent linear Ising spin chains with strong intrachain and comparatively weak interchain interactions. Whereas the former are treated exactly as nearest neighbour interactions the interchain interactions are taken into account either by the molecular field approximation or, in computer simulations, by nearest neighbour interactions. The parameters of the model are related to the unusually small transition entropy, to the high and nearly temperature independent dielectric constant aboveT
c
,and to neutron scattering data. 相似文献
6.
N. M. Chubova V. A. Dyadkin E. V. Moskvin S. V. Grigoriev 《Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques》2014,8(5):1020-1026
A new way of analyzing the polarization of thermal and cold neutron beams, based on the dependence of the neutron-scattering cross section on the neutron polarization upon diffraction from a magnetic spiral, is proposed. In this method, the working element of the neutron-polarization analyzer is a single-crystal noncentrosymmetric cubic helicoidal MnSi magnet, the spin spiral in which is formed at T < T c (T c = 29 K) in the magnetic-field range H < H C2 ~ 500 mT. Since the spiral period d in MnSi is 180 Å, thermal and cold neutrons with wavelengths λ ≤ 2d diffract from this structure. It is established that the efficiency of neutron-polarization analysis is as high as 100% with the experimental geometry when the polarization vector P is parallel to the scattering vector Q. 相似文献
7.
Neutron (Bragg and small angle) scattering and susceptibility measurements are used to study magnetic ordering in Eu
x
Sr1–x
S with ferromagnetic nearest neighbor exchangeJ
1 and antiferromagnetic next-nearest neighbor exchangeJ
2. We present data for 0.50x0.70 which cannot be analyzed within the merely geometrical treatments of percolation theory. Breakdown of ferromagnetism occurs atx
c
=0.51, far above the percolation thresholdx
p
=0.136, and a spin-glass phase is observed in the intermediate concentration regime. Close tox
c
, the ferromagnetic state is also displaced by the spinglass phase at lower temperatures. Both properties are a general characteristic of diluted systems with competing interactions. An effective decoupling of finite magnetic clusters from the ferromagnetic net arises from frustration, which enhances the ground-state entropy. Anomalous properties below the Curie temperatureT
c
as well as atT
c
support this microscopic picture. 相似文献
8.
The phase transition behavior of a self-avoiding linear polymer model on a three-dimensional lattice is studied. An approximate procedure is proposed which gives the exact divergence of the specific heat aboveT
c for the two-dimensional case. In three dimensions our method predicts a finite value forC atT
c and demonstrates clearly what neglected contributions to the partition function lead to such a behavior of the model. 相似文献
9.
O. Hartmann E. Karlsson R. Wäppling J. Chappert A. Yaouanc L. Asch G. M. Kalvius 《Hyperfine Interactions》1984,18(1-4):491-496
Transverse fieldSR measurements in the paramagnetic region on polycrystalline samples of CeAl2, NdAl2, PrAl2, GdAl2, ErAl2, HoAl2, DyAl2 and TmAl2 are reported. We have observed increased damping rate and frequency shifts of the muon precession signal asT
c is approached from above. The shifts are linear in applied magnetic field, while the damping rates are more complicated functions of magnetic field strength and temperature.We interpret the damping as being partly due to inhomogeneous broadening proportional to the magnetic susceptibility, and partly due to apparent slowing down of RE spin fluctuations. These slow fluctuations are ascribed to the existence of correlations between the RE spins also at temperatures well aboveT
c. The variations of shifts and damping rates within the series of REAl2 compounds will be discussed. 相似文献
10.
M. A. Weilert M. E. Msall J. P. Wolfe A. C. Anderson 《Zeitschrift für Physik B Condensed Matter》1993,91(2):179-188
Thermal conductivity and ballistic phonon imaging measurements in KH2PO4 (KDP) at low temperature (T<3K) indicate that scattering from domain walls has a large effect on phonon transport. kDP has a ferroelectric phase transition from tetragonal to orthorhombic structure atT
c
=122 K. BelowT
c
domains of opposite electric polarization and crystal orientation form unless the sample is colled in an electric field. Thermal conductivity measured along the [100] (tetragonal) axis drops 30% when domain walls are present, which is independent of sample size and temperature. We attribute this decrease to phonon polarization-dependent scattering at the domain boundaries. This is verified by measurements of ballistic transport, using phonon imaging techniques, which reveal the phonon polarization and mode dependence of the scattering. The scattering is successfully modelled using continuum acoustics with simple acoustic mismatch at the domainwall. The interface scattering is found to be mode dependent: Caustic structures in the phonon images due to slow transverse phonons are most affected by the domain wall scattering, which channels these phonons along parallel planes by multiple reflections without mode conversion. Mode conversion scattering, though possible for a number of phonons, has little effect on the overall phonon transmission. 相似文献
11.
A new ternary metallic glass system, La75Si25–x
M
x
(where M is a 3d or a IIIA element) has been prepared by melt spinning. The superconducting transition temperature,T
c
, is depressed by the magnetic 3d elements. For low Fe concentrations (less than 5 at.%) the depression is about 0.3 K/at.%. Part of the depression may be explained by a decreased density of states atE
F
as determined by UPS measurements. The strongest depression is associated with the magnetic pair breaking due to spin scattering. In the IIIA additions we correlateT
c
with the atomic radius of the additive. 相似文献
12.
Detailed magnetization measurements up toH=6T have been performed on a powdered specimen of the weak ferromagnet NiCO3 in the temperature rangeT
c
<T<4T
c
. Starting atT=1.15T
c
, an excellent agreement is found at all temperatures between the experimental values of the magnetization and those calculated up toH
3 terms in an extension ofDzyaloshinsky's theory of weak ferromagnetism. Both isotropic and antisymmetrical exchange constants are determined and found in good agreement with previous low temperature magnetization results. These results indicate that at allT>1.15T
c
, the molecular field approximation satisfactorily describes the magnetic properties of NiCO3. 相似文献
13.
Y. J. Uemura A. Keren L. P. Le G. M. Luke B. J. Sternlieb W. D. Wu 《Hyperfine Interactions》1994,85(1):133-144
An overview is given on muon spin relaxation (SR) measurements in frustrated and/or low dimensional spin systems. In the frustrated Kagomé lattice system SrCr8Ga4O19, we observed dynamic spin fluctuations of 30 GHz, without any static frozen component even atT=0.1 K, much below the susceptibility-cusp temperatureT
g=3.5 K. This is in clear contrast with the case in dilute-alloy spin glassesCuMn andAuFe, where static order develops belowT
g. We also present the dimensionality dependence of the sub-lattice magnetization curves in 2-d Heisenberg systems, the remarkable suppression of the ordering temperature in a 1-d system Sr2CuO3, the observation of activation type spin dynamics in a 1-d Ising ferromagnet (DMeFc)(TCNE) aboveT
c, slow spin fluctuations ( 60 MHz) in Haldane-gap systems at low temperatures, and some results from organic 1-d and 2-d magnetic systems. 相似文献
14.
A. P. Gerashenko K. N. Mikhalev S. V. Verkhovskii T. V. D’yachkova A. P. Tyutyunnik V. G. Zubkov 《Applied magnetic resonance》2001,21(2):157-163
Nuclear magnetic resonance (NMR) spectra and nuclear spin-lattice relaxation rateT
1−1 of11B have been measured in superconducting polycrystalline MgB2 with 7cons = 39.5 K. It is shown that (T
1T−1 and the Knight shiftK
s are independent of temperature and nearly isotropic aboveT
c. Both of these quantities are decreased gradually in going to the superconducting state. According to NMR data the density
of states near the Fermi level is flat at the scale of about 500 K. Some conclusions on the orbital content of the density
of states at the Fermi level were drawn and compared with the results of the band structure calculations. 相似文献
15.
Pavel Svoboda Pavel Javorský Fuminori Honda Vladimír Sechovský Alois A. Menovsky 《Central European Journal of Physics》2004,2(2):397-418
The tetragonal compound UNi2Si2 exhibits in zero magnetic field three different antiferromagnetic phases belowT
N
=124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q
z
). Two additional order-order magnetic phase transitions are observed below T
N
, namely atT
1=108 K and T
2=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel
to the c-axis. UNi2Si2 single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity,
susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5
T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured
in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties
of UNi2Si2. 相似文献
16.
K. Lal S. K. Chattopadhyay A. K. Meikap S. K. Chatterjee M. Ghosh 《Czechoslovak Journal of Physics》2001,51(9):897-908
A detailed study on the weak localization phenomenon vis-a-vis electron-electron interaction effects in magnetic metallic
glasses has been carried out. We measured the electrical conductivity and magnetoconductivity within the temperature range
1.8≤T≤300K. A maximum on the conductivity versus temperature curve exists atT=T
m. The conductivity was observed to follow aT
1/2 law forT<T
m andT
2 law forT>T
m. Magnetoconductivity data of these alloys indicate the prominence of electron-electron interaction at low temperatures. The
authors have determined the inelastic scattering field and spin-orbit scattering field from the magnetoconductivity data.
The inelastic scattering field obeys aT
p law (p=2) at low temperatures. 相似文献
17.
A. D. Bianchi E. Felder A. Schilling M. A. Chernikov F. Hulliger H. R. Ott 《Zeitschrift für Physik B Condensed Matter》1995,99(1):69-76
We report measurements of the specific heatC
p(T), electrical resistivity ϱ(T) and magnetic susceptibility ξ(T) of hexagonal CePd2In, at low temperatures. Anomalies inC
p(T), χ(T) and ϱ(T) atT=1.23 K, indicate a phase transition, most likely to an antiferromagnetically-ordered phase. The electronic entropy reachesR ln2 per mole Ce at 9.2K, suggesting that the phase transition involves a doublet state. The ordered phase coexists with moderately
correlated itinerant electrons. 相似文献
18.
Wolf -Dietrich von Szalghary 《Zeitschrift für Physik A Hadrons and Nuclei》1970,235(2):130-139
The results of the three-dimensional liquid droplet model are compared with experiment and other phenomenological theories. The homogeneity assumption of the scaling laws holds both above and belowT c , and various series expansions can be derived. But there is a nonanalyticity near the critical isotherm aboveT c for fixed “field” μ?μ(p c ,T); and liquid and gas are not symmetric about the critical isochore aboveT c . Both results contradict the usual scaling assumptions and experiment. The equation of state is fixed if the density on the coexistence curve and the critical pressure are known. Therefore we can derive various relations between critical quantities. They are compared with experiment and the corresponding relations in the Vicentini-Missoni ansatz, the parametric representation, and the generalized Landau ansatz. The disagreement ranges from about 20% to one order of magnitude. 相似文献
19.
J. Engels J. Fingberg K. Redlich H. Satz M. Weber 《Zeitschrift fur Physik C Particles and Fields》1989,42(2):341-347
InSU(2) lattice gauge theory, we study deviations from ideal gas behaviour near the deconfinement point. On lattices of sizeN σ 3 ×4,N σ=8, 12, 18 and 26, we calculate the quantityΔ≡(ε?3P)/T 4. It increases sharply just aboveT c , peaks atT/T c =1.15 ±0.05 and then drops quickly. This form of behaviour is shown to be the consequence of a second order phase transition. Dynamically it could arise because just aboveT c , the low momentum states of the system are remnant massive modes rather than deconfined massless gluons. 相似文献
20.
G. M. Luke L. P. Le B. J. Sternlieb Y. J. Uemura J. H. Brewer R. Kadono R. F. Kiefl S. R. Kreitzman T. M. Riseman C. L. Seaman Y. Dalichaouch M. B. Maple J. D. Garrett 《Hyperfine Interactions》1991,64(1-4):517-522
We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe13, URu2Si2 and U6Fe. The magnetic field penetration depth in the superconducting state of UBe13 is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT c . A sharp increase in the precession frequency is seen, starting atT c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu2Si2 show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U6Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor. 相似文献