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1.
The NMR spin echo of Dy nuclei in ferromagnetic DyAl2 yields for the Dy161 hyperfine field constant, a=?845.2±0.3 MHz and quadrupolar splitting, 2P=420.0±0.3 MHz compared to a=?830.0±0.5 MHz and 2P= 387.8±0.5 MHz in ferromagnetic Dy metal. The different contributions to these parameters are discussed. A line, observed at 1273 MHz Dy metal, is attributed to ΔmI = 2 transition.  相似文献   

2.
We report the first 17O NMR studies of a triangular-lattice superconductor Na(1/3)CoO2 x 4/3H(2)O and the host material Na(x)CoO2 (x=0.35 and 0.72). Knight shift measurements reveal that p-d hybridization induces sizable spin polarization in the O triangular-lattice layers. Water intercalation makes CoO2 planes homogeneous and enhances low frequency spin fluctuations near T(c)=4.5 K at some finite wave vectors different from both the ferromagnetic and "120 degree" modes.  相似文献   

3.
为了解释Au1-xNix合金(x=0.30—0.42)低温出现电阻率极小值的实验结果,本文提出一个低浓度自旋集团顺磁态合金的模型,得到自旋集团孤立近似下和自旋集团耦合作用下的电阻率公式,并和晶格散射的贡献(由Au80Ni20合金或Au-Cu合金的ρi(T)实验数据代替)联合起来,得到ρ(T)的计算曲线,和实验结果符合得很好。当Tmin,电阻率随温度增高而下降,主要是自旋集团孤立近似下的Kondo效应引起的。自旋集团之间的RKKY耦合作用对电阻率的贡献在低温时大,随着温度增高按1/T规律迅速减小,所以ρ(T)-ρi(T)实验值在相当宽温度范围出现logT关系。随着温度增高,晶格散射对电阻率的贡献将变得重要,当T=Tmin,电阻率出现极小值。 关键词:  相似文献   

4.
The specific heats of both a two-layer ferromagnetic superlattice and atwo-layer ferrimagnetic one are studied. It is found that the spin quantumnumbers, the interlayer and intralayer exchange couplings, the anisotropy,the applied magnetic field, and the temperature all affect the specificheat of these superlattices. For both the ferromagnetic and ferrimagneticsuperlattices, the specific heat decreases with increasing the spin quantumnumber, the absolute value of interlayer exchange coupling, intralayerexchange coupling, and anisotropy, while it increases with increasingtemperature at low temperatures. When an applied magnetic field is enhanced, the specific heat decreases in the two-layer ferromagnetic superlattice, while it is almost unchanged in the two-layer ferrimagnetic superlattice at low field range at low temperatures.  相似文献   

5.
We have performed ac susceptibility and dc magnetization measurements in Ni(50-x)Co(x)Mn(38)Sb(12) Heusler alloys. From the ac susceptibility measurements, the existence of reentrant spin glass (RSG) state is observed for x=0-5. It is found that the signature of RSG behavior diminishes with increase in x. This behavior is in contrast to the fact that the exchange bias field increases with x, which reveals that the origins of RSG and exchange bias are different in the present system. It is found that the system enters a frustrated ferromagnetic state just below the Curie temperature of the martensite phase (T(M)(C)) and then the RSG state at low temperature. The strength of the RSG state is critically dependent on the sharpness of the magnetic transition at (T(M)(C)). This proposition is further supported by the thermo-remanent magnetization and low field thermomagnetic measurements.  相似文献   

6.
A 55Mn and 139La NMR study of La0.35Ca0.65MnO3 is reported. The zero field 55Mn spectra consist of two lines centered at 290 and 375 MHz. Their behavior under an applied magnetic field makes it possible to attribute them to regions of antiferromagnetically and ferromagnetically coupled Mn spins, respectively. This gives evidence for the existence of electronic phase segregation of microscopic ferromagnetic regions of double exchange coupled Mn spins over a charge ordered antiferromagnetic background. The behavior of these ferromagnetic regions in the applied magnetic field is related to the magnetoresistive properties of the manganites.  相似文献   

7.
The first-order spin-reorientation transition in the Mn-substituted yttrium orthoferrites, YFe(1-x)Mn(x)O(3) (x = 0.1, 0.15 and 0.2), has been investigated using (57)Fe M?ssbauer spectroscopy. Owing to its large anisotropy, substitution of Mn(3+) ions in YFeO(3) induces a spin-reorientation transition from the low-temperature antiferromagnetic state to a high-temperature weak ferromagnetic state. With increasing x, the spin-reorientation transition temperature (T(SR)) increases whereas the Néel temperature (T(N)) decreases. Analysis of the M?ssbauer spectra unambiguously confirms the occurrence of spin reorientation relative to crystal axes. At a given temperature, the mean hyperfine field decreases with the increasing Mn concentration. The variation of canting angle with temperature for YFe(0.85)Mn(0.15)O(3) has been estimated.  相似文献   

8.
We have discovered spontaneous ordering of nuclear spins in lithium metal by NMR measurements at very low temperatures. In low magnetic fields, B<0.2 mT, the NMR spectra show a pronounced low-frequency anomaly. Also, nonadiabatic response to a slowly varying magnetic field was observed. A rich phase diagram with three different nonparamagnetic regions is proposed. We estimate a critical spin temperature T(c) approximately 350 nK at B=0. We also report the absence of superconductivity in lithium at normal pressure down to T(e) approximately 100 microK (B<10 nT).  相似文献   

9.
In this work, we study the magnetic properties of itinerant electron systems using the Hubbard-like tight binding Hamiltonian along with inter-site exchange and hybrid interactions. We have used the mean-field approximation to deal with the exchange and hybrid interactions. It is found that hybrid interaction is more effective than exchange interaction for the on-set of ferromagnetic state. We have studied the effect of hybrid interaction on various physical quantities at different temperatures. The effective mass (m*/m) of up spin electrons increases slowly as the temperature decreases but below the critical temperature (Tc), it decreases rapidly. For down spin electrons effective mass increases slowly as the temperature decreases and below Tc, it increases more rapidly. Spectral weight (n/m*) for up spin electrons decreases slowly upto Tc and below Tc, it increases rapidly. For down spin electrons spectral weight decreases slowly upto Tc and below Tc, it decreases rapidly. Our results for both the effective mass and spectral weight are in good agreement with recently observed experimental behaviour in itinerant ferromagnet Ga1−xMnxAs [Phys. Rev. Lett. 89 (2002) 097203]. We have also studied variation of the spectral weight and optical absorption with temperature in presence of magnetic field. We found that these two quantities for up spin electrons increase as applied magnetic field increases at all temperatures (∼4Tc). For down spin electrons these two quantities decrease as applied magnetic field increases.  相似文献   

10.
We report a study of spin waves in ferromagnetic La1-xCaxMnO3, at concentrations x=0.17 and x=0.2 very close to the metallic transition (x=0.225). Below T(C), in the quasimetallic state (T=150 K), nearly q-independent energy levels are observed. They are characteristic of standing spin waves confined into finite-size ferromagnetic domains, defined only in the (a,b) plane for x=0.17, and in all directions for x=0.2. They allow an estimation of the domain's size, a few lattice spacings, and of the magnetic coupling constants inside the domains. These constants, anisotropic, are typical of an orbital-ordered state, allowing one to characterize the domains as "hole poor." The precursor state of the collossal magnetoresistance metallic phase appears, therefore, as an assembly of small orbital-ordered domains.  相似文献   

11.
We report new zero-field muon spin relaxation and neutron spin echo measurements in ferromagnetic (FM) (La,Ca)MnO3 which suggest at least two spatially separated regions possessing very different Mn-ion spin dynamics. One region displays diffusive relaxation, "critical slowing down" near T(C) and an increasing volume fraction below T(C), suggesting overdamped FM spin waves below T(C). The second region possesses more slowly fluctuating spins, a linewidth independent of q, and a decreasing volume fraction below T(C). The estimated length scale for the inhomogeneity is 相似文献   

12.
Thin epitaxial films of the diluted magnetic semiconductor (DMS) GaMnAs have been studied by low energy muon spin rotation and relaxation (LE-microSR) as well as by transport and magnetization measurement techniques. LE-microSR allows measurements of the distribution of magnetic field on the nanometer scale inaccessible to traditional macroscopic techniques. The spatial inhomogeneity of the magnetic field is resolved: although homogeneous above Tc, below Tc the DMS consists of ferromagnetic and paramagnetic regions of comparable volumes. In the ferromagnetic regions the local field inhomogeneity amounts to 0.03 T.  相似文献   

13.
Hyperpolarized (hp) (131)Xe with up to 2.2% spin polarization (i.e., 5000-fold signal enhancement at 9.4 T) was obtained after separation from the rubidium vapor of the spin-exchange optical pumping (SEOP) process. The SEOP was applied for several minutes in a stopped-flow mode, and the fast, quadrupolar-driven T(1) relaxation of this spin I = 3/2 noble gas isotope required a rapid subsequent rubidium removal and swift transfer into the high magnetic field region for NMR detection. Because of the xenon density dependent (131)Xe quadrupolar relaxation in the gas phase, the SEOP polarization build-up exhibits an even more pronounced dependence on xenon partial pressure than that observed in (129)Xe SEOP. (131)Xe is the only stable noble gas isotope with a positive gyromagnetic ratio and shows therefore a different relative phase between hp signal and thermal signal compared to all other noble gases. The gas phase (131)Xe NMR spectrum displays a surface and magnetic field dependent quadrupolar splitting that was found to have additional gas pressure and gas composition dependence. The splitting was reduced by the presence of water vapor that presumably influences xenon-surface interactions. The hp (131)Xe spectrum shows differential line broadening, suggesting the presence of strong adsorption sites. Beyond hp (131)Xe NMR spectroscopy studies, a general equation for the high temperature, thermal spin polarization, P, for spin I ≥ 1/2 nuclei is presented.  相似文献   

14.
15.
Systematic theoretical results for the effects of a dilute concentration of magnetic impurities on the thermodynamic and transport properties in the region around the quantum critical point of a ferromagnetic transition are obtained. In the quasiclassical regime, the dynamical spin fluctuations enhance the Kondo temperature. This energy scale decreases rapidly in the quantum fluctuation regime, where the properties are those of a line of critical points of the multichannel Kondo problem with the number of channels increasing as the critical point is approached, except at unattainably low temperatures where a single channel wins out.  相似文献   

16.
We measured the optical decoherence times T2, or, equivalently, the homogeneous line width, in an Er-doped optical fiber at low temperature as a function of external magnetic field and temperature using two-pulse photon echoes. The decoherence times were up to 230 ns at fields above 3 T. The magnitude of the line narrowing induced by a magnetic field of 3 T is 2.5 MHz, which is anomalously large compared to that typical for oxide crystals with similar Er3+ concentration. This is interpreted as evidence for dephasing by coupled spin-elastic tunneling modes where the normal glass tunneling modes acquire a magnetic character by coupling to the Er3+ spin.  相似文献   

17.
The perovskite compounds of have been studied by means of NMR spectroscopy in powder samples prepared by the solid state reaction and sol–gel methods. The NMR signals were observed in the frequency range 250–650 MHz at 4.2 K in zero external magnetic field. A relatively narrow spectral line assigned to 55Mn in Mn4+ is situated around 270–320 MHz in all spectra. Additional spectral lines are observed at higher frequencies. Line intensities and forms are dependent on the cobalt concentration and preparation techniques.  相似文献   

18.
Structure, magnetic and transport properties of YMn6Sn6-xGax (0≤x≤0.6) compounds with a HfFe6Ge6-type structure were investigated. It was found that the Ga substitution leads to a contraction of the unit-cell volume. A transition from an antiferromagnetic to a ferromagnetic (or ferrimagnetic) state can be observed for samples (0.1≤x≤0.2) with increasing temperature. The antiferro-ferromagnetic transition for samples with x≤0.2 can also be induced by an external field. The required field is very low, and decreases with increasing Ga concentration. More Ga concentration (x≥0.3) leads to the samples being ferromagnetic in the whole temperature range below the Curie temperature. The Ga substitution weakens the interlayer magnetic coupling between the Mn spins. Corresponding to the metamagnetic transition, a magnetoresistance as large as 32% under a field of 5 T was observed at 5 K for the sample with x=0.2.  相似文献   

19.
We have investigated the spin dynamics using 17O-NMR in the bilayered perovskite Sr3Ru2O7, which sits close to a metamagnetic quantum critical point. The nuclear spin-lattice relaxation rate divided by temperature 1/T1T is enhanced on approaching the metamagnetic critical field of approximately 7.9 T, and at the critical field 1/T1T continues to increase and does not show Fermi-liquid behavior down to 0.3 K. The temperature dependence of T1T in this region suggests the critical temperature Theta to be approximately 0 K, which is strong evidence that the spin dynamics possesses a quantum critical character. Comparison between uniform susceptibility and 1/T1T reveals that antiferromagnetic fluctuations instead of two-dimensional ferromagnetic fluctuations dominate the spin fluctuation spectrum at the critical field, which is unexpected for itinerant metamagnetism.  相似文献   

20.
The spin glass magnetic enhancement factor has been measured at T=1.3 K with63/65Cu NMR spin echo technique. Alloying iron to CuMn, (Ho) shows a large range of constancy against an external field Ho, applied in and opposite to the direction of the freezing field Hc. It is supposed that the stiffness of the frozen spin glass is stabilized in this range by a subsystem of ferromagnetic iron clusters. Bound by its own crystalline anisotropy energy, the subsystem has its own stiffness. It probably behaves as a powder system consisting of ferromagnetic single domain particle spheres, having also an isotropic distribution of domain magnetization directions and giving rise to a mean ferromagnetic NMR enhancement factor.  相似文献   

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