Temperature Dependence Of Anisotropy Research Articles

Temperature dependent magnetic anisotropy of epitaxial Co2FeAl films grown on GaAs

Co2FeAl films with different thickness were prepared at different temperature by molecular beam epitaxy. Their magnetic anisotropy was studied by rotating magneto-optical Kerr rotation measurements under different temperature. It is found that the cubic anisotropy depends only on the temperature-dependent fourth order magneto-elastic coefficients. However, the results of growth and measurement temperature-dependent uniaxial anisotropy suggest that the uniaxial anisotropy of Co2FeAl films may be attributed to contributions from both shear strain and anisotropic interfacial bonding. Our experimental findings proposed a new point of view to understand the origin of magnetic anisotropy in ferromagnet/GaAs(001) heterostructures.

Zero-temperature 2D stochastic Ising model and anisotropic curve-shortening flow

Let \mathcal D be a simply connected, smooth enough domain of \mathbb R^2 . For L>0 consider the continuous time, zero-temperature heat bath dynamics for the nearest-neighbor Ising model on \mathbb Z^2 with initial condition such that \sigma_x=-1 if x\in L\mathcal D and \sigma_x=+1 otherwise. It is conjectured [23] that, in the diffusive limit where space is rescaled by L , time by L^2 and L\to\infty , the boundary of the droplet of " - " spins follows a deterministic anisotropic curve-shortening flow, where the normal velocity at a point of its boundary is given by the local curvature times an explicit function of the local slope. The behavior should be similar at finite temperature T<T_c , with a different temperature-dependent anisotropy function. We prove this conjecture (at zero temperature) when \mathcal D is convex. Existence and regularity of the solution of the deterministic curve-shortening flow is not obvious a priori and is part of our result. To our knowledge, this is the first proof of mean curvature-type droplet shrinking for a model with genuine microscopic dynamics.

Rotational dynamics of imidazolium-based ionic liquids: do the nature of the anion and the length of the alkyl chain influence the dynamics?

The rotational dynamics of 1-alkyl-3-methylimidazolium-based ionic liquids has been investigated by monitoring their inherent fluorescence with the intent to unravel the characteristics of the emitting species. For this purpose, temperature-dependent fluorescence anisotropies of 1-alkyl-3-methylimidazolium (alkyl = ethyl and hexyl) ionic liquids with anions such as tris(pentafluoroethyl)trifluorophosphate ([FAP]), bis(trifluoromethylsulfonyl)imide ([Tf2N]), tetrafluoroborate ([BF4]), and hexafluorophosphate ([PF6]) have been measured. It has been observed that the reorientation times (τr) of the ionic liquids with an ethyl chain scale linearly with viscosity and were found to be independent of the nature of the anion. The experimentally measured τr values are a factor of 3 longer than the ones calculated for 1-ethyl-3-methylimidazolium cation using the Stokes-Einstein-Debye (SED) hydrodynamic theory with stick boundary condition, which suggests that the emitting species is not the imidazolium moiety but some kind of associated species. The reorientation times of ionic liquids with a hexyl chain, in contrast, follow the trend τr([FAP]) > τr([Tf2N]) = τr([BF4]) > τr([PF6]) at a given viscosity (η) and temperature (T). The ability of the ionic liquids with longer alkyl chains to form the organized structure appears to be responsible for the observed behavior considering the fact that significant deviations from linearity have been noticed in the τr versus η/T plots for strongly associating anions [BF4] and [PF6], especially at ambient temperatures.

Temperature-dependent elastic anisotropy and mesoscale deformation in a nanostructured ferritic alloy.

Nanostructured ferritic alloys are a new class of ultrafine-grained oxide dispersion-strengthened steels that have promising properties for service in extreme environments in future nuclear reactors. This is due to the remarkable stability of their complex microstructures containing numerous Y-Ti-O nanoclusters within grains and along grain boundaries. Although nanoclusters account primarily for the exceptional resistance to irradiation damage and high-temperature creep, little is known about the mechanical roles of the polycrystalline grains that constitute the ferritic matrix. Here we report an in situ mesoscale characterization of anisotropic responses of ultrafine ferrite grains to stresses using state-of-the-art neutron diffraction. We show the experimental determination of single-crystal elastic constants for a 14YWT alloy, and reveal a strong temperature-dependent elastic anisotropy that leads to elastic softening and instability of the ferrite. We also demonstrate, from anisotropy-induced intergranular strains, that a deformation crossover exists from low-temperature lattice hardening to high-temperature lattice softening in response to extensive plastic deformation.

Complementary terahertz absorption and inelastic neutron study of the dynamic anisotropy contribution to zone-center spin waves in a canted antiferromagnetNdFeO3

We employ a combination of pulsed- and continuous-wave polarized terahertz spectroscopy techniques to probe temperature-dependent spin waves in the antiferromagnet $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_{3}$. Our optical data span 1.6--467 K and reveal a conspicuous spin reorientation between 110 and 170 K, during which the lower-energy mode softens completely. Complementary inelastic neutron scattering reveals that the frequencies of the optically excited spin waves are consistent with a temperature-variable spin gap in the low-energy spin-wave dispersion of $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_{3}$. The result links the temperature dependence of the spin waves to a dynamic in-plane anisotropy. The magnetic anisotropy is calculated based on the results of the optical measurements. The change observed in the anisotropy energy along the $a$ and $c$ crystal axes suggests that the spin reorientation evident in $\mathrm{Nd}\mathrm{Fe}{\mathrm{O}}_{3}$ is driven by temperature-dependent in-plane anisotropy.

Crossover of uniaxial magnetic anisotropy direction mediated by interfacial strain of CoFe2O4 films

This study examined the deposition temperature-dependent magnetic anisotropy of CoFe2O4 films grown on Al2O3(0001) substrates using pulsed-laser deposition. X-ray diffraction revealed all films to have a 〈111〉 orientation except for the films grown at room temperature, which exhibited amorphous characteristics. Furthermore, the films deposited between 350°C and 550°C exhibited out-of-plane tensile strain even though, which was relieved as the deposition temperature increased. On the other hand, film deposited at 650°C showed out-of-plane compressive strain. The in-plane and out-of-plane magnetic hysteresis loops, which were measured at room temperature, showed a decreased out-of-plane anisotropy when the deposition temperature was increased. Simple uniaxial magnetic anisotropy energy calculations based on the experimental data showed a direct correlation between the uniaxial magnetic anisotropy direction and stress of the films. X-ray photoelectron spectroscopy revealed variations in the cation distribution according to the deposition temperature.

Rotational Diffusion of Nonpolar and Charged Solutes in Propylammonium Nitrate–Propylene Glycol Mixtures: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?

Rotational diffusion of two structurally similar nonpolar and charged solutes has been examined in mixtures of an ionic liquid and an organic solvent of comparable size and viscosity with an intent to find out whether the organized structure of the former influences solute rotation. To this effect, temperature-dependent fluorescence anisotropies of 9-phenylanthracene (9-PA) and rhodamine 110 (R110) have been measured in n-propylammonium nitrate (PAN), propylene glycol (PG), and also four different compositions of PAN-PG mixtures. Analysis of the data carried out with the aid of Stokes-Einstein-Debye (SED) hydrodynamic theory indicates that the reorientation times of 9-PA and R110 scale more or less linearly with the ratio of viscosity to temperature and are found to be independent of the mole fraction of PAN. In other words, apart from the viscosity and temperature, rotational diffusion of both the solutes is not affected by the composition of PAN-PG mixtures. It has also been observed that the reorientation times of R110 are significantly longer compared to those of 9-PA due to the specific interactions prevailing between the cationic solute and PAN-PG mixtures. However, the important finding of this work is that, even though PAN forms an organized structure, rotational diffusion of the solute molecules is similar in both the ionic liquid and the organic solvent. The disordered lamellar structure present in PAN probably does not offer compact organized domains unlike ionic liquids with long alkyl chains wherein solute rotation is influenced significantly.

Low field magnetoresistance of gadolinium nanowire

We report low field (μ0H < 0.2 T) magnetoresistance (MR) studies on a single Gd nanowire patterned from a nano-structured film (average grain size ∼ 35 nm) by focused ion beam. For comparison, we did similar MR measurements on a polycrystalline sample with large crystallographic grains (∼4 μm). It is observed that in the low field region where the MR is due to motion of magnetic domains, the MR in the large grained sample shows a close relation to the characteristic temperature dependent magnetocrystalline anisotropy including a sharp rise in MR at the spin reorientation transition at 235 K. In stark contrast, in the nanowire, the MR shows complete suppression of the above behaviours and it shows predominance of the grain boundary and spin disorder controlling the domain response.

Temperature-dependent anisotropic material modeling of the sheet metal component within the polymer injection forming process

In the course of this work, an extended material model for a carbon steel sheet metal has been developed based on Hill’48 yield criterion, considering temperature-dependent plastic anisotropy coefficients. This material model is applied on a polymer injection forming process in which the sheet metal heats up to a critical forming temperature through the contact with the plastic melt. At this temperature range blue brittleness occurs. The elastic properties, the yield stress as well as the plastic anisotropy coefficients of the sheet material become significantly different compared to those at room temperature. It should be emphasized that especially temperature-dependent anisotropy coefficients are not yet considered in most common material models. With the help of the presented modelling approach a more precise modelling of the temperature-dependent carbon steel material behavior can be realised.

Temperature dependence of the switching field in all-perpendicular spin-valve nanopillars

We present temperature dependent switching measurements of the Co/Ni multilayered free element of 75-nm-diameter spin-valve nanopillars. Angular dependent hysteresis measurements as well as switching field measurements taken at low temperature are in agreement with a model of thermal activation over a perpendicular anisotropy barrier. However, the statistics of switching (i.e. the mean switching field and the variance of the switching field distribution) from 20 up to 400 K are in disagreement with a N\'eel-Brown model that assumes a temperature independent barrier height and anisotropy field. We introduce a modified N\'eel-Brown model that fits the experimental data in which we attribute a ${T}^{3/2}$ dependence to the barrier height and the anisotropy field due to the temperature dependent magnetization and anisotropy energy.

Roles of Viscosity, Polarity, and Hydrogen-Bonding Ability of a Pyrrolidinium Ionic Liquid and Its Binary Mixtures in the Photophysics and Rotational Dynamics of the Potent Excited-State Intramolecular Proton-Transfer Probe 2,2′-Bipyridine-3,3′-diol

The room-temperature ionic liquid [C3mpyr][Tf2N] and its binary mixtures with methanol and acetonitrile provide microenvironments of varying viscosity, polarity, and hydrogen-bonding ability. The present work highlights their effects on the photophysics and rotational dynamics of a potent excited-state intramolecular double-proton-transfer (ESIDPT) probe, 2,2'-bipyridine-3,3'-diol [BP(OH)2]. The rotational diffusion of the proton-transferred diketo (DK) tautomer in [C3mpyr][Tf2N] ionic liquid was analyzed for the first time from the experimentally obtained temperature-dependent fluorescence anisotropy data using Stokes-Einstein-Debye (SED) hydrodynamic theory and Gierer-Wirtz quasihydrodynamic theory (GW-QHT). It was found that the rotation of the DK tautomer in neat ionic liquid is governed solely by the viscosity of the medium, as the experimentally observed boundary-condition parameter, Cobs, was very close to the GW boundary-condition parameter (CGW). On the basis of photophysical studies of BP(OH)2 in IL-cosolvent binary mixtures, we suggest that methanol molecules form hydrogen bonds with the cationic counterpart of the DK tautomers, as evidenced by the greater extent of the decrease in the fluorescence lifetime of BP(OH)2 upon addition of methanol compared to acetonitrile. It is also possible for the methanol molecules to form hydrogen bonds with the constituents of the RTIL, which is supported by the lesser extent of the decrease in the viscosity of the medium upon addition of methanol, leading to a less effective decrease in the rotational relaxation time compared to that observed upon acetonitrile addition.

Fluorescence Anisotropy of a Nonpolar Solute in 1-Alkyl-3-Methylimidazolium-Based Ionic Liquids: Does the Organized Structure of the Ionic Liquid Influence Solute Rotation?

Temperature-dependent fluorescence anisotropies of a nonpolar solute 9-phenylanthracene (9-PA) have been measured in 1-alkyl-3-methylimidazolium-based ionic liquids with anions such as bis(trifluoromethylsulfonyl)imide ([Tf2N–]), tris(pentafluoroethyl)trifluorophosphate ([FAP–]), tetrafluoroborate ([BF4(–)]), and hexafluorophosphate ([PF6(–)]) to find out if the organized structure of the ionic liquid has a bearing on solute rotation. Analysis of the experimental data using the Stokes–Einstein–Debye hydrodynamic theory indicates that there is no significant variation in the solute–solvent coupling constants (Cobs) with an increase in the length of the alkyl chain on the imidazolium cation for the ionic liquids with [Tf2N–] and [FAP–] anions. However, in the case of ionic liquids with [BF4(–)] and [PF6(–)] anions, the rotation of 9-PA for a given viscosity at constant temperature becomes progressively faster and C(obs) decreases by a factor of 2.4 from ethyl to octyl derivatives. Quasihydrodynamic theories of Gierer–Wirtz and Dote–Kivelson–Schwartz could not account for the significant decrease in the C(obs) values. The observed behavior has been rationalized in terms of the organized structure of the ionic liquids having [BF4(–)] and [PF6(–)] anions, which results as a consequence of the high charge-to-size ratio of these anions compared to [Tf2N–] and [FAP–].

Superconductivity with extremely large upper critical fields in Nb2Pd0.81S5.

Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb2Pd0.81S5, with a transition temperature Tc ≅ 6.6 K. Despite its relatively low Tc, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. μ0Hc2 (T → 0 K) > 37 T for fields applied along the crystallographic b-axis. For a field applied perpendicularly to the b-axis, μ0Hc2 shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb2Pd0.81S5 is a multi-band superconductor. This is consistent with band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively. The static spin susceptibility as calculated through the random phase approximation, reveals strong peaks suggesting proximity to a magnetic state and therefore the possibility of unconventional superconductivity.

Upper critical field of isoelectron substituted SrFe2(As1−xPx)2

The upper critical field ${H}_{c2}$ of optimally doped, iron-based superconductor SrFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ ($x$ $=$ 0.35, ${T}_{c}$ $=$ 25 K) was measured as a function of temperature down to 1.6 K for two principal directions of magnetic field $H\ensuremath{\parallel}c$ and $H\ensuremath{\parallel}a$. Measurements were performed in pulsed magnetic fields up to 65 T using a tunnel-diode resonator technique on as-grown and heavy-ion-irradiated single crystals, with columnar defect density corresponding to a matching field ${B}_{\ensuremath{\phi}}$ $=$ 25 T. The ${H}_{c2,c}(T)$ is close to $T$ linear, while clear saturation is observed for ${H}_{c2,a}(T)$, leading to a strongly temperature-dependent anisotropy parameter $\ensuremath{\gamma}$. The linear shape of ${H}_{c2,c}(T)$ is very similar to that observed in nodal KFe${}_{2}$As${}_{2}$, but very different from full-gap LiFeAs. Irradiation does not introduce any additional features on the ${H}_{c2}(T)$ line corresponding to the matching field. Instead, it suppresses uniformly both ${T}_{c}$ and ${H}_{c2}$, keeping their ratio constant.

The generalized method of quantitative description of the uniaxial anisotropy in weak ferromagnet rhombohedral calcite type structure crystals with S-state ions

The method for investigation of the uniaxial and hexagonal anisotropies in weakly ferromagnetic rhombohedral FeBO3 crystals with the Fe2+ magnetic impurity with the use of the single-ion and pair approximation has been developed. In the framework of this method, a quantitative discrepancy between theoretical and experimental data on a uniaxial field has been eliminated. The energy contributions to the hexagonal anisotropy that are caused by the Dzyaloshinsky–Moriya interaction, the crystal field of Fe3+ ions and the temperature dependence of this anisotropy were analytically treated. The experimental temperature dependence of the hexagonal anisotropy of FeBO3 crystals is explained. It is shown that the compensation point and the typical temperature dependence of the hexagonal anisotropy result from the competition of the contributions of Fe3+ and Fe2+ ions. It is shown that respective considerable contributions to the anisotropy of Fe3+ and Fe2+ ions are made by the cubic symmetry and the uniaxial symmetry of the exchange “single-ion” type. The anisotropy constants for FeBO3 crystal was obtained by using the described results. It was shown, that for the calcite type structure crystals (iron borate and manganese carbonate) with S-state ions, the single ion and pair models (which previously raised doubts in their validity and accuracy) describe the experimental data good enough.

A study on the interaction of nanoparticles with lipid membranes and their influence on membrane fluidity

In recent years, liposomes encapsulated with nanoparticles have found enormous scopes in various biomedical fields such as drug design, transport, imaging, targeted delivery and therapy. These applications require a clear understanding about the interaction of nanoparticles with cell membranes. The present work aims to investigate the effect of encapsulation of uncharged and positively charged nanoparticles in three different types of lipids such as1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC),1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine and1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine(SOPC-POPS) mixture and archaeal lipids. Through the temperature dependent fluorescence anisotropy measurements, we have found that the entrapment of nanoparticles in the bilayer has decreased the lipid transition temperature and increased the membrane fluidity of all three types of lipid vesicles. The results were more predominant in SOPC-POPS mixture because of high density encapsulation of nanoparticles in the vesicles due to electrostatic interaction between negatively charged membrane and positively charged iron oxide nanoparticles.

Time Dependence of Magnetic Anisotropy at Finite Temperature for Homogeneous and Composite Media

A simple torque-based method for calculating the transient behavior of temperature-dependent magnetic anisotropy is introduced. Using this method we obtain several physical quantities at finite temperature, including effective anisotropy, anisotropy field, and their fluctuations, that are crucial to heat-assisted magnetic recording (HAMR). We show that use of a composite grain that includes a high Curie temperature soft layer reduces the anisotropy fluctuations.

Rotational Diffusion of Nonpolar and Ionic Solutes in 1-Alkyl-3-Methylimidazolium Bis(trifluoromethylsulfonyl)imides: Is Solute Rotation Always Influenced by the Length of the Alkyl Chain on the Imidazolium Cation?

In an attempt to find out whether the length of the alkyl chain on the imidazolium cation has a bearing on solute rotation, temperature-dependent fluorescence anisotropies of three structurally similar solutes have been measured in a series of 1-alkyl-3-methylimidazolium (alkyl = methyl, ethyl, propyl, butyl, and hexyl) bis(trifluoromethylsulfonyl)imides. Solute-solvent coupling constants obtained from the experimentally measured reorientation times with the aid of Stokes-Einstein-Debye hydrodynamic theory indicate that there is no influence of the length of the alkyl chain on the rotation of nonpolar, anionic, and cationic solutes 9-phenylanthracene (9-PA), fluorescein (FL), and rhodamine 110 (R110), respectively. It has also been noticed that the rotational diffusion of 9-PA is closer to the predictions of slip hydrodynamics, whereas the rotation of negatively charged FL and positively charged R110 is almost identical and follows stick hydrodynamics in these ionic liquids. Despite having similar shape and size, ionic solutes rotate slower by a factor of 3-4 compared to the nonpolar solute. Interplay of specific and electrostatic interactions between FL and the imidazolium cation of the ionic liquids, and between R110 and the bis(trifluoromethylsulfonyl)imide anion, appear to be responsible for the observed behavior. These results are an indication that the length of the alkyl chain on the imidazolium cation does not alter their physical properties in a manner that has an effect on solute rotation.

Differential Temperature-Dependent Multimeric Assemblies of Replication and Repair Polymerases on DNA Increase Processivity

Differentiation of binding accurate DNA replication polymerases over error prone DNA lesion bypass polymerases is essential for the proper maintenance of the genome. The hyperthermophilic archaeal organism Sulfolobus solfataricus (Sso) contains both a B-family replication (Dpo1) and a Y-family repair (Dpo4) polymerase and serves as a model system for understanding molecular mechanisms and assemblies for DNA replication and repair protein complexes. Protein cross-linking, isothermal titration calorimetry, and analytical ultracentrifugation have confirmed a previously unrecognized dimeric Dpo4 complex bound to DNA. Binding discrimination between these polymerases on model DNA templates is complicated by the fact that multiple oligomeric species are influenced by concentration and temperature. Temperature-dependent fluorescence anisotropy equilibrium binding experiments were used to separate discrete binding events for the formation of trimeric Dpo1 and dimeric Dpo4 complexes on DNA. The associated equilibria are found to be temperature-dependent, generally leading to improved binding at higher temperatures for both polymerases. At high temperatures, DNA binding of Dpo1 monomer is favored over binding of Dpo4 monomer, but binding of Dpo1 trimer is even more strongly favored over binding of Dpo4 dimer, thus providing thermodynamic selection. Greater processivities of nucleotide incorporation for trimeric Dpo1 and dimeric Dpo4 are also observed at higher temperatures, providing biochemical validation for the influence of tightly bound oligomeric polymerases. These results separate, quantify, and confirm individual and sequential processes leading to the formation of oligomeric Dpo1 and Dpo4 assemblies on DNA and provide for a concentration- and temperature-dependent discrimination of binding undamaged DNA templates at physiological temperatures.

Layering and temperature-dependent magnetization and anisotropy of naturally produced Ni/NiO multilayers

Ni/NiO multilayers were grown by magnetron sputtering at room temperature, with the aid of the natural oxidation procedure. That is, at the end of the deposition of each single Ni layer, air is let to flow into the vacuum chamber through a leak valve. Then, a very thin NiO layer (∼1.2 nm) is formed. Simulated x-ray reflectivity patterns reveal that layering is excellent for individual Ni-layer thickness larger than 2.5 nm, which is attributed to the intercalation of amorphous NiO between the polycrystalline Ni layers. The magnetization of the films, measured at temperatures 5–300 K, has almost bulk-like value, whereas the films exhibit a trend to perpendicular magnetic anisotropy (PMA) with an unusual significant positive interface anisotropy contribution, which presents a weak temperature dependence. The power-law behavior of the multilayers indicates a non-negligible contribution of higher order anisotropies in the uniaxial anisotropy. Bloch-law fittings for the temperature dependence of the magnetization in the spin-wave regime show that the magnetization in the multilayers decreases faster as a function of temperature than the one of bulk Ni. Finally, when the individual Ni-layer thickness decreases below 2 nm, the multilayer stacking vanishes, resulting in a dramatic decrease of the interface magnetic anisotropy and consequently in a decrease of the perpendicular magnetic anisotropy.