Quasi-linear Temperature Dependence Research Articles

Comparison of surface spin wave modes at Fe/MnF2 and Fe/Mn interfaces

We have studied the thermal demagnetization in semi-infinite ferromagnets in Fe/MnF2 and Fe/Mn bilayers using Mössbauer spectroscopy. We find that the hyperfine field at the Fe/MnF2 interface follows a quasilinear temperature dependence, which reverts to a T3/2 dependence further into the bulk. The region in which linear temperature dependence was observed also showed significantly higher spin canting than in the film’s bulk layers. The interface in the Fe/Mn system immediately showed a T3/2 dependence which persisted deeper into the bulk. We attribute the linear temperature behavior to surface spin wave modes created by a perpendicular surface anisotropy at the interface. This behavior diminishes farther away from the interface, until the hyperfine field goes like T3/2 as expected for bulk, 3D spin waves. We conclude that the perpendicular surface anisotropy is much stronger at the Fe/MnF2 than the Fe/Mn interface.

Magnetic studies of Fe/Pt multilayers

The magnetization and anisotropy of Fe/Pt multilayers prepared by RF sputtering are presented. The samples with individual Fe thickness of 2.5 Å show a dependence of magnetisation on Pt thickness. We found that as the Pt layer became thick enough (>18 Å) to magnetically isolate the Fe layers, a quasi-linear temperature dependence of the magnetisation results due to the 2D spin-wave excitations. As the Pt layer is reduced, a dimensional crossover in the excitations is induced by the magnetic interaction between Fe layers which makes M( T) change from quasi-linear behaviour to more bulk-like behaviour. Magneto-optical polar Kerr effect spectra between 1.5 and 5.7 eV measured on a series of magnetic multilayers Fe/Pt with a varied thickness of ultrathin Fe are reported.

Temperature dependence of magnetization at Fe/MnFe2 interface

Mössbauer spectroscopy is used to extract depth sensitive information near the surface of an Fe film in Fe/MnF2 bilayers. We find that the hyperfine field at the surface has a quasilinear temperature dependence. For layers deeper into the film, the quasilinear temperature dependence diminishes and deep enough into the films, the hyperfine field goes like T3/2 as expected for bulk. We interpret this behavior as evidence of surface spin wave modes created by a perpendicular surface anisotropy, as previously predicted.

Electrical transport properties of YPd 5B 3C 0.3

We have measured the electrical resistivity and the thermoelectric power (TEP) of a superconducting intermetallic compound, YPd 5B 3C 0.3, as a function of temperature between 4K and 300K. The bulk sample was prepared by arc-melting. The electrical resistivity shows quasi linear temperature dependence and drops sharply to zero showing the superconductivity at 23K. TEP at room temperature is positive (2.1μV/K) and shows zero-crossing behavior at 80K. At T>100K, TEP decreases linearly as the temperature is lowered. Unlike the cuprate superconductors, TEP does not drop sharply to zero at the superconducting transition temperature, 23K,

Transport in polyaniline near the critical regime of the metal-insulator transition.

Critical behavior of the electrical conductivity near the disorder-induced metal-insulator (MI) transition has been observed in polyaniline (PANI) doped with camphor sulfonic acid (CSA). The temperature dependence of the resistivity (\ensuremath{\rho}) depends on the degree of disorder present in the PANI-CSA films. In the most metallic samples the resistivity is nearly temperature independent; whereas in the critical region of MI transition, \ensuremath{\rho}(T) is characterized by a power-law temperature dependence, \ensuremath{\rho}(T)\ensuremath{\propto}${\mathit{T}}^{\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\beta}}}$. Resistivity ratios \ensuremath{\rho}(1.4 K)/\ensuremath{\rho}(300 K) as low as 1.6 have been observed in the most metallic samples of PANI-CSA. In the metallic regime, the conductivity is characterized by \ensuremath{\sigma}(T)=\ensuremath{\sigma}(0)+${\mathit{mT}}^{1/2}$ at low temperatures; the dependence of m(H) on magnetic field provides information on the role of electron-electron interactions in the transport near the MI transition. The ${\mathit{T}}^{\mathrm{\ensuremath{-}}1}$ dependence found for inelastic-scattering time (${\mathrm{\ensuremath{\tau}}}_{\mathrm{in}}$) is in agreement with that predicted for metallic systems near the MI transition. For the samples initially in the critical regime, a magnetic field of 8--10 T induces the transition to variable-range hopping. The typical localization length in PANI-CSA just on the insulating side of the MI transition is about 80--130 \AA{}. The magnitude of the positive magnetoresistance increases considerably as the system moves from the metallic to the insulating regime. The magnitude and quasilinear temperature dependence of the thermopower near the critical regime of the MI transition is typical of that expected for a metal.

Electron-phonon interaction and quasiparticle damping in a layered metal

It is shown that in layered metal (i) there exists a “band” of phonon excitations, (ii) for large wave vectors the longitudinal phonon spectrum coincides with that of the two-dimensional high frequency ion plasmon, and (iii) the inelastic electron-high-frequency-phonon interaction can produce the quasi-linear temperature dependence of the resistivity ϱ∼Tln(2T/Ω c) with a low value of the energy threshold Ω c, thereby providing an explanation of the observed quasi-linearity of the resistivity of Cu-O superconductors in the normal state.

Spin Waves and Temperature Dependence of Magnetization in Nickel Thin Films Covered with Iron and Copper

Energy spectrum and amplitudes of spin waves, as well as the temperature dependence of the local magnetization in Ni thin films covered with Fe and Cu overlayers are investigated within the framework of the itinerant electron approach. Two acoustic surface modes strongly localized in Fe overlayers are found. Energies of these modes are very close to each other but they are considerably lower than in Ni films with free surfaces. The presence of the strongly localized low-energy modes leads to fast decrease in the surface magnetization as compared to the central layer one. The quasi-linear temperature dependence of the surface magnetization is found whereas the central layer magnetization follows a quasi T312 law. On the other hand, in films covered with Cu the temperature dependence of the magnetization can be fitted to the Bloch law for all layers. Inside the film the Bloch coefficient weakly depends on the layer index but it rapidly decreases at the surface in the Cu layers. PACS numbers: 75.10.Lp, 75.30.Ds, 75.70.—i

Anomalous thermoelectric power of the alkali metal doped and the pyrolyzed fullerene

We report the temperature dependent thermoelectric power (TEP) and conductivity of the alkali-metal doped bulk fullerene, Na x C 60, K x C 60 and the pyrolyzed fullerene. For K x C 60, their TEP data can be classified into two kinds according to their signs and temperature dependences. In the case of the samples with negative TEP, their TEP data show quasi-linear temperature dependence with S RT ≅ −110 μV/K. However, for the samples with positive TEP, the TEP data show big anomalies. The TEP value of the pyrolyzed C 60 is about S RT ≅ +3.8 μV/K and upon cooling, the TEP decreases quasi linearly with the change of slope at T = 180K. Our preliminary analysis indicates that the positive TEP data of doped C 60 are originated from the collectively bridged C 60 molecules while the negative TEP data are due to the electrons donated from the alkali metals in doped C 60 fullerene.

Mössbauer studies of spin wave excitations in Fe/Ag multilayers

The magnetic properties of molecular beam epitaxially (MBE) grown FE(110)/Ag(111) heterostructures were investigated with Mossbauer spectroscopy. The Fe bilayers were fixed at 3 ML (monolayer) thickness and the Ag bilayer thickness varied from 4 ML to 20 ML. We found that as the Ag layer became thick enough (>17 ML) to magnetically isolate the Fe layers, a quasi-linear temperature dependence of the hyperfine field results due to the 2-D spin wave excitations. As the Ag layer is reduced, a dimensional crossover in the excitations is induced by the magnetic interaction between Fe layers which makesM(T) change from a two-dimensionalT relation to a three-dimensionalT 3/2 dependence. We constructed a simple theoretical model to motivate the explanation for the experimental results and obtained approximate values for the interlayer coupling strength for various Ag bilayer thicknesses.

Dimensional crossover in the magnetism of MBE-grown Fe(110)/Ag(111) multilayers

Fe(110)/Ag(111) heterostructures, composed of fixed 3 monolayer (ML) Fe bilayers and variable-thickness Ag bilayers, were grown by molecular-beam epitaxy and investigated by transmission Mössbauer spectroscopy in the temperature range from 4.2 to 300 K. We found that as the Ag layer is thick enough (≳17 ML) to magnetically isolate the neighboring Fe layers, a quasilinear temperature dependence of the Mössbauer hyperfine field results due to the two-dimensional spin-wave excitations. As the Ag layer is reduced to 4 ML, a dimensional crossover in the spin-wave excitations is induced by the magnetic interaction between neighboring Fe layers which makes the magnetic temperature dependence change from a two-dimensional T-linear relation to a three-dimensional T3/2 dependence.

Observability of a quasilinear temperature dependence of the magnetization in a semi-infinite ferromagnet (abstract)

Current theory [G. T. Rado, Phys. Rev. B 40, 407 (1989)] shows that the existence of magnetic surface anisotropy leads to surface spin waves which can provide a mechanism for a quasilinear temperature dependence of the magnetization (MT) near the surface of a semi-infinite ferromagnet. Since volume spin waves as well as surface spin waves are excited thermally, we now generalize this theory in two ways. First, we include in the calculation the effects of both surface and volume spin waves, and second, we include the effects of both of the surface anisotropy constants Kss and Ks. The fractional magnetization deviation (M0−MT)/M0 caused by the volume spin waves competes, of course, with that caused by the surface spin waves. We find that, for all allowed values of Ks and Kss, it is mostly at very low temperatures that the contribution to (M0−MT)/M0 arising from surface spin waves is comparable to the contribution to (M0−MT)/M0 arising from volume spin waves. This result depends on the assumption that the material parameters are independent of position throughout the sample volume. A full account of this work will be published elsewhere.

Inelastic Coulomb scattering between extended and localized electrons: A quasilinear temperature dependence.

The inelastic Coulomb scattering rate ${\mathrm{\ensuremath{\tau}}}_{\mathrm{in}}^{\mathrm{\ensuremath{-}}1}$ of extended electrons by localized ones is considered and shown to be proportional to ${\mathit{T}}^{2}$(lnT${)}^{\mathit{d}+1}$, where d is the dimensionality. This nonlinear dependence appears surprisingly linear over significant temperature regions, and its magnitude is large enough to account for ${\mathrm{\ensuremath{\tau}}}_{\mathrm{in}}$ of, e.g. (normal- and super-)conducting oxides.

Effects of magnetic surface anisotropy near the (110)Fe/MnF2 and (110)Fe/(111)Ag/MnF2 interfaces (abstract)

Two series of samples, Ag/56Fe/57Fe/56Fex/MnF2 and Ag/56Fe/57Fe/Agx/MnF2 with Ag and Fe in (111) and (110) orientations, respectively, have been grown epitaxially in ultrahigh vacuum (∼1×10−9 τ). Several thicknesses are chosen for the 56Fe and Ag intervening layers separating 57Fe and MnF2. The magnetic hyperfine field for the 57Fe probe layer about 3 monoatomic layers (ML) thick is measured with Mössbauer spectroscopy from room temperature down to 4.2 K. For the first series of samples, a quasi-linear temperature dependence of the hyperfine field is observed for the probe layer up to 10 ML away from the (110)Fe/MnF2 interface. The magnitude of the linear slope is smaller for the 10-ML sample than for the 5-ML sample. An ordinary T3/2 dependence is recovered when the 57Fe probe layer is placed 25 ML away from the interface. This result can be qualitatively understood within the framework of a semiclassical spin-wave calculation involving a large positive surface anisotropy constant Kss by Rado. For the second series of samples, significant amount of FeF2 (about 40% of the 57Fe) is found to be present in the 57Fe probe layer. The temperature dependence of the hyperfine field exhibits an initial weak dependence at low temperatures (<76 K) changing over to a fast quasi-linear falloff at higher temperatures for both samples with x(Ag)=9 and 19 ML. A possible explanation for this behavior is that below ∼76 K, the Néel temperature for antiferromagnetic FeF2, the 57Fe probe behaves as a three-dimensional antiferromagnet and thus exhibits a T2 dependence, while for temperatures above ∼76 K it exhibits a quasi-linear temperature dependence presumably due to the same mechanism responsible for the quasi-linear temperature dependence in the first series of samples.

Mechanism of quasilinear temperature dependence of the surface magnetization in a semi-infinite ferromagnet (abstract)

A semiclassical method involving surface spin waves is used to calculate the spontaneous magnetization M near the surface of a semi-infinite ferromagnet. It is assumed as before1 that the magnitude of the surface anisotropy constant Ks introduced by Néel is negligible compared to the surface anisotropy constant Kss introduced by the present author. We now generalize our calculations in two respects. First, we include dipolar interactions to keep the calculated M from diverging even when the value of Kss is relatively large. We find, as in our previous work,1 that under the above conditions any positive value of Kss causes the dependence of M on the temperature T to be quasilinear rather than proportional to T3/2 and the dependence of M on position to be exponential. Alternative predictions2 of a quasilinear T dependence of M apply only to T values near the Curie temperature. Second, we explore the consequences of assuming that the exchange stiffness constant A is weakened at the surface. We show that such a weakening is equivalent to the use of a spatially uniform value of A and an increased value of Kss. This mechanism provides a possible interpretation of recent experimental results3 on MnF2-covered Fe(110) which require a relatively large value of Kss. The full text of the present paper recently appeared4 in Physical Review B.

11B spin-lattice relaxation and disorder modes in ionic glassy conductors (AgI)x(Ag2O

The temperature and frequency dependence of the $^{11}\mathrm{B}$ nuclear spin-lattice-relaxation rate has been investigated in a number of ionic glassy conductors of general composition (AgI${)}_{\mathit{x}}$(${\mathrm{Ag}}_{2}$O\ensuremath{\cdot}n${\mathrm{B}}_{2}$${\mathrm{O}}_{3}$${)}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, with n=1, 2 and variable x. At T>100 K a BPP-type maximum in ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ is found that is consistent with a thermally activated reorientational motion of ${\mathrm{BO}}_{4}$ groups. [BPP indicates Bloembergen, Percell, and Pound, Phys. Rev. 73, 679 (1948).] It is found that the sum of the activation energy for ${\mathrm{BO}}_{4}$ motion with the activation energy recently measured for ${\mathrm{Ag}}^{+}$ local motion is equal to the activation energy derived from conductivity measurements. At low temperature one observes the usual quasilinear temperature dependence of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ and an unusual frequency dependence not described by a power law. An interpretative model of nuclear relaxation due to disorder modes is developed to explain the frequency dependence. The analysis of the data indicates the presence of a relatively large number of slowly fluctuating two-level systems, formed by coupled ${\mathrm{BO}}_{4}$ units, responsible for the frequency dependence of the experimental relaxation rate.

Heat capacity of glasslike materials: The free volume approach

The experimental quasi-linear temperature dependence of the heat capacity of glasses is explained in terms of the free volume concept. The state number density n(0) is calculated as a function of microscopic system parameters. It is noted that the proposed model is also suitable for investigation of the dissipation properties of glasses and proteins.

Mechanism of quasilinear temperature dependence of the surface magnetization in a semi-infinite ferromagnet.

Mechanism of quasilinear temperature dependence of the surface magnetization in a semi-infinite ferromagnet.

Persistence of magnetic surface anomalies in (110) Fe near an interface with MnF2 (abstract)

An anomalous quasilinear temperature dependence has been previously observed1 in the surface layers of Fe at an interface with MnF2. The (110) Fe was epitaxially grown to a thickness of about 50 layers on a (111) Ag substrate. The magnetic hyperfine field in the surface layers was probed by making the Fe sample from isotopically pure 56Fe and then growing the layer(s) of interest from 57Fe. Mössbauer spectroscopy is used to determine the magnetic hyperfine field on a layer-by-layer basis. For other interfaces besides MnF2 (for example, Ag, MgO) the surface layers of (110) Fe show a T3/2 dependence somewhat different from bulk. To better understand the unusual quasilinear behavior at the MnF2 interface, we have examined the temperature dependence of the magnetic hyperfine fields at layers somewhat deeper in the (110) Fe sample than the actual surface/interface. We report here measurements showing a linear temperature dependence for 57Fe layers at least five layers into the sample from the surface/interface. The slope of the Heff vs T curve is much smaller than for the surface region. Still deeper layers at least 10 layers into the sample from the surface/interface begin to show a T3/2 behavior which differs little from bulk values. We offer an explanation of this persistence of the surface effects into the deeper layers of the (110) Fe sample based on a surface anisotropy model. This explanation does not depend on the actual mechanism of the surface anisotropy and can remain valid if the source of the anisotropy is an exchange interaction between the surface Fe spins and the spins associated with the Mn ions in the MnF2.

Electronic structure of the polyacetylene film oriented by a liquid crystal solvent under magnetic field

The electrical transport properties of the liquid crystal and magnetic field oriented polyacetylene film are studied. FeCl 3, SO 3, iodine and AsF 5 are the dopants. The observed maximum room temperature conductivity is around 6000 Ω −1 cm −1 with SO 3 doped polyacetylene. High anisotropies ( σ 11(RT)/ σ ⊥(RT) ≳ 10) of room temperature conductivity are observed for FeCl 3, AsF 5 and iodine doped polyacetylenes indicating that the charge transfer is one dimensional in these polyacetylene derivatives. However, in the case of SO 3 doped polyacetylene, the anisotropy is only around three suggesting that the interchain coupling is relatively strong. In thermopower data, no anisotropy is observed confirming that thermopower measures the intrinsic property of the polymer. The temperature dependence of conductivity data of the FeCl 3 doped polyacetylene indicates that there might be an electric field effect and/or a structural rearrangement (which could be the result of the one dimensional instability) below 200 K. Quasi-linear temperature dependence of thermopower is observed for all dopants listed above at the high concentration regime showing the metallic nature of these heavily doped polyacetylenes. After careful examination of AsF 5 and SO 3 doped polyacetylene thermopower data in comparison with the data of unoriented-polyacetylene films, we believe that the AsF 5 doped oriented-polyacetylene shows slightly better crystalinity and the SO 3 doped oriented-polyacetylene shows a coupling between the chain and the dopant.

Thermal properties ofa−Si:Hat low temperatures

The specific heat and thermal conductivity of bulk samples of $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ have been measured in the temperature range 0.1-5 K. A quasilinear temperature dependence of the specific heat and a ${T}^{2}$ thermal conductivity indicates a density of two-level tunneling systems very similar to that in fused silica. A flow of heat from the sample has been observed at low temperatures, with a decay extending over a period of days.