Magnetic Relaxation Studies Research Articles

The "Recognition Helix" of the Type II Acyl Carrier Protein (ACP) Utilizes a "Ubiquitin Interacting Motif (UIM)"-like Surface To Bind Its Partners.

Interaction interfaces comprise a finite number of sequence/structural motifs, which are often repeated in nature. Here we show how a helical motif present in the acyl carrier protein (ACP), involved in multiprotein interactions, displays a binding interface similar to that of the "ubiquitin interacting motif (UIM)". Analysis of the crystal structures of the ACP-enzyme complexes gave the first hint that helix II of the ACP ("universal recognition helix") utilizes UIM-like noncovalent interactions to associate with the type II fatty acid biosynthesis (FAS) pathway enzymes. The ACP interacting functional surface of the FAS enzymes comprises positively charged residues, flanking a central hydrophobic patch, akin to ubiquitin. Our nuclear magnetic resonance chemical shift perturbation studies, relaxation studies, and surface plasmon resonance measurements unequivocally show that helix II of ACP behaves like a UIM motif in its interactions with ubiquitin, by binding the Ile 44 functional surface of the latter. A synthetic peptide with the ACP helix II sequence showed equivalent binding to ubiquitin. The evolution of similar interaction motifs in the two systems has probably been driven by functional constraints, as both ACP and UIM participate in multiprotein interactions.

Coexistence of long-range cycloidal order and spin-cluster glass state in the multiferroic BaYFeO4

We report the presence of spin glass state below the cycloidal spin ordering in the multiferroic BaYFeO4. This compound is known to crystallize in an orthorhombic structure with a centrosymmetric space group Pnma and exhibits two successive antiferromagnetic phase transitions. Upon cooling, it undergoes a spin density wave (SDW)-like antiferromagnetic ordering at TN1 ~ 48 K and a cycloidal ordering at TN2 ~ 35 K. Using dc magnetic memory effect and magnetization relaxation studies, we have shown that this oxide undergoes a reentrant spin glass transition below T* ~ 17 K. Our analysis suggests the presence of spin clusters in the glassy state. The coexistence of spin-cluster glass and long-range cycloidal ordered states results in an exchange bias effect at 2 K. The origin of the glassy state has been attributed to freezing of some Fe3+ moments, which do not participate in the long-range ordering.

Static magnetism and thermal switching in randomly oriented L1 FePt thin films

Static magnetism and thermally activated magnetic relaxation were investigated in granular FePt films (20 nm-200 nm thick) with random magnetic anisotropy through hysteresis loop, torque curve and magnetization time dependence measurements. While the magnetism of thicker film (200 nm thick) is dominated by a single switching of the ordered L10 phase, thinner film (20 nm) displays a double switching, which is indicative of the presence of the disordered cubic phase. The pronounced behavior of double switching in thinner film suggests that the film grain boundary is composed of soft cubic magnetic phase. The magnetic relaxation study reveals that magnetic viscosity S of the films is strongly dependent on the external applied field and exhibits a maximum value (12 kAm) around the switching field and a vanishing behavior at low (1 kOe) and large (12 kOe) fields. The activation volume of the thermal switching was found to be much smaller than the physical volume of the granular structure due to the incoherent rotation mode of the magnetization reversal mechanism, which is established to be domain wall nucleation.

Isolation and characterization of a high-spin mixed-valent iron dinitrogen complex.

We report a rare example of a mixed-valence iron compound with an FeNNFe core, which gives insight into the structural, spectroscopic, and magnetic influences of single-electron reductions and oxidations. In the new compound, the odd electron is localized as judged from Mössbauer spectra at 80 K and infrared spectra at room temperature, and the backbonding into the N2 unit is intermediate between diiron(i) and diiron(0) congeners. Magnetic susceptibility and relaxation studies on the series of FeNNFe compounds show significant magnetic anisotropy, but through-barrier pathways enable fairly rapid magnetic relaxation.

Magnetization reversal, exchange interaction, and switching behavior studies on Ru doped GdCrO3

A systematic study on the crystal structure and magnetic properties of GdCr1−xRuxO3 (where 0 ≤x≤ 0.15) was carried out by powder x-ray diffraction along with low temperature magnetization measurements. Lattice parameters and cell volume continuously increase with doping due to the incorporation and formation of higher ionic radius Ru4+ and Cr2+ ion, respectively. Pure GdCrO3 shows the antiferromagnetic (AFM) ordering at 170 K and a spin-reorientation transition around 8 K. Surprisingly, the AFM ordering temperature does not show any variation with the tetravalent Ru doping; while, spin-reorientation completely disappears with 5% Ru doping within the investigated temperature region. All these compounds exhibit magnetization reversal under low applied fields in field-cooled (FC) protocol. Highly delocalized 4d orbitals and occupation number of Ru4+ result in a decrease of antisymmetric exchange constant (D) with doping. Additionally, the formation of high-spin Cr2+ in weak octahedral field increases the effective magnetic moment for doped samples. Magnetic relaxation study in FC mode reveals the memory effect of pure GdCrO3. Magnetic field-assisted switching behavior study corroborates that magnetic state (positive or, negative) can easily be switched by the application of magnetic field.

NMR paramagnetic probing of polymer solutions using manganese(II) ions

Nuclear magnetic relaxation studies show that manganese(II) ions can serve as paramagnetic probes for investigation of aqueous solutions of polyelectrolytes (polystyrene sulfonate and polyethylenimine) and their mixtures. The analysis of the measured rates of spin–lattice (R 1) and spin–spin (R 2) relaxations reveals the differences in the binding character of manganese(II) ions with polystyrene sulfonate and polyethylenimine. In a mixture of the polymers in acidic and neutral media, manganese(II) forms two types of ternary complexes. Using the suggested method for definition of the hydration degree of a coordination sphere of the bound probe ions, the number of water molecules in the first sphere of the polymer-bound manganese(II) ions is found on the basis of the value of the R 2/R 1 ratio.

Magnetocaloric effect and slow magnetic relaxation in CsGd(MoO4)2 induced by crystal-field anisotropy

The experimental and theoretical study of magnetocaloric effect and magnetic relaxation of the powder sample of CsGd(MoO4)2 were performed. The large conventional magnetocaloric effect was found around 2K with −ΔSmax ≈ 26.5J/(kgK) for B = 7T. AC susceptibility measurement revealed multiple-time scale magnetic relaxation effects on different time scales. Slowest relaxation effect was attributed to the direct process with a bottleneck effect and two faster relaxation processes are effectively temperature independent, probably as a result of averaging in the powder sample.

Plastic pinning replaces collective pinning as the second magnetization peak disappears in the pnictide superconductor Ba0.75K0.25Fe2As2

We report a detailed study of isofield magnetic relaxation and isothermal magnetization measurements with $H$$\parallel$c on an underdoped Ba$_{0.75}$K$_{0.25}$Fe$_2$As$_2$ pnictide single crystal, with superconducting transition temperature $T_c$ = 28 K. The second magnetization peak (SMP) has been observed at temperatures below $T_c$/2 and vanished at higher temperatures. The observed behaviour of the SMP has been studied by measuring the magnetic field dependence of relaxation rate, $R(H)$ and by performing the Maley's analysis. The results suggest that the crossover from collective to plastic pinning observed in the SMP disappears above 12 K with plastic pinning replacing collective pinning. An interesting $H$-$T$ phase diagram is obtained. The critical current density ($J_c$) was estimated using Bean's model and found to be $\sim$ $3.4 \times 10^9$ A/m$^2$ at 10 K in the SMP region, which is comparable to an optimally doped Ba-KFe$_2$As$_2$ superconductor and may be exploited for potential technological applications. The pinning mechanism is found to be unconventional and does not follow the usual $\delta l$ and $\delta T_c$ pinning models, which suggest the intrinsic nature of pinning in the compound.

Heat capacity and nuclear magnetic relaxation times of non-freezing water restrained by polysaccharides, revisited

Calorimetric and nuclear magnetic relaxation studies on non-freezing water restrained by polysaccharide are introduced together with the historical background of this research field. Non-freezing water restrained by the hydrophilic group of polysaccharide shows no first order thermodynamic phase transition. The amount of non-freezing water calculated from the melting enthalpy of water restrained by various kinds of polysaccharides was collected. Molecular motion of polysaccharides is markedly enhanced by the introduction of non-freezing water and glass transition shifts to the ca. 200 K low temperature side. At the same time, molecular chains rearrange to a more stabilized state, which can be observed as the decrease in heat capacities. 1H nuclear magnetic relaxation studies at a temperature lower than glass transition indicate that a part of the water molecules is closely bound to the backbone proton and is not sufficiently isolated. Calorimetric and nuclear magnetic relaxation studies suggest that non-freezing water inevitably cooperates with matrix polysaccharide molecules.

Nuclear magnetic resonance and nuclear spin relaxation in AlAs quantum well probed by ESR

The study of nuclear magnetic resonance and nuclear spin-lattice relaxation was conducted in an asymmetrically doped to $n\sim1.8\times10^{11}$ cm$^{-2}$ 16 nm AlAs quantum well grown in the $[001]$-direction. Dynamic polarization of nuclear spins due to the hyperfine interaction resulted in the so-called Overhauser shift of the two-dimensional conduction electron spin resonance. The maximum shifts achieved in the experiments are several orders of magnitude smaller than in GaAs-based heterostructures indicating that hyperfine interaction is weak. The nuclear spin-lattice relaxation time extracted from the decay of Overhauser shift over time turned out to depend on the filling factor of the two-dimensional electron system. This observation indicates that nuclear spin-lattice relaxation is mostly due to the interaction between electron and nuclear spins. Overhauser shift diminishes resonantly when the RF-radiation of certain frequencies was applied to the sample. This effect served as an indirect, yet powerful method for nuclear magnetic resonance detection: NMR quadrupole splitting of $^{75}$As nuclei was clearly resolved. Theoretical calculations performed describe well these experimental findings.

Proton magnetic relaxation study of the thermodynamic characteristics of water adsorbed by cellulose fibers

The possibility of determining the thermodynamic parameters that characterize the sorption properties of cellulose and the state of water associated with its fibers is demonstrated using modern concepts of the structure of this vegetable polymer and methods based on theories of adsorption and NMR relaxation in heterogeneous systems.

Nuclear Magnetic Resonance Relaxation Studies in Shale

The present work involves a comprehensive experimental study of proton (1 H) nuclear magnetic resonance (NMR) transversal relaxation in shale. The sample studied belongs to the “Vaca Muerta” formation of the Neuquen basin, Argentina. The results show that the relaxation process is governed by anomalous diffusion processes in micro-pores. These processes result from the combination of interactions between the liquid molecules and the pore walls, whose structure is characterized by both large tortuosity and abundance of paramagnetic impurities, giving rise to local strong time-dependent magnetic field gradients. In addition, a simultaneous experimental method of data fitting is presented which allows processing all the relaxation profiles within a single time domain. Thus, yielding results univocally related to the complete set of relaxation data.

Kinetic arrest of first-order transition between charge-ordered and ferromagnetic phases in Gd0.5Sr0.5MnO3 single crystals: magnetization relaxation studies

We have studied the span and nature of first-order phase transition (FOPT) between charge-ordered insulating and ferromagnetic metallic phases in oriented single crystals of Gd0.5Sr0.5MnO3. Magnetic field—temperature phase diagram was formulated from magnetization data for different crystallographic axes and non-monotonic variation of supercooling limit was observed at low temperature. A peculiar nature of magnetization was observed as irreversible open hysteresis loops during thermal cycling. We perceive that the nature of metastable states responsible for open hysteresis loops is different from that of supercooled ones. Further, thermal cycling magnetization data reveal that magnetic phases formed at 8 K after zero-field or field-cooled protocols (89 kOe) are not in equilibrium. Relaxation time constant is found to increase below 30 K in magnetization relaxation measurements made across the FOPT. The non-monotonic variation of relaxation time constant is a manifestation of kinetic arrest of the FOPT. We propose that the non-equilibrium, glass-like magnetic phase (at 8 K and 89 kOe) is a consequence of kinetic arrest.

Kinetics of glass transition, negative magnetization and exchange bias effects in Sm1−xBixCrO3

DC magnetization and magnetic relaxation studies of polycrystalline Sm1−xBixCrO3 (x = 0, 0.1) demonstrated the kinetics of magnetic glass behaviour in SmCrO3: the frozen antiferromagnetic state was dominant to the glassy transition state.

Proton NMR of water colloidal solutions of nanosized crystalline LaF3and LaF3:Gd3+particles

A study of the nuclear magnetic relaxation of water protons in a series of colloidal solutions of nanosized lanthanum trifluoride and lanthanum fluoride particles, doped with gadolinium (0.5%). The proton NMR signal was recorded by a “Proton-20 M (Chromatech)” spectrometer. It is established that the rate of longitudinal relaxation depends on the nanoparticle concentration in the water solution, and on their dimension. It is demonstrated that the proton relaxation rate in the LaF3:Gd3+ solution increases with paramagnet concentration in the nanoparticles, and decreases with increasing particle size.

Features of the structural organization and sorption properties of cellulose

Modern concepts about the structural organization of cellulose and the character of formation of its inter- and intermolecular hydrogen bonds are analyzed. A scheme for the structure of the cellulose microfibril that supposes the presence of slit-shaped micropores in its structure is proposed. The proton magnetic relaxation study of sorption processes reveals that, at a moisture content of 7–10%, the filling of cellulose micropores occurs with a concomitant increase in their lateral sizes and decreases in the degrees of crystallinity of samples.

High resolution NMR study of T1 magnetic relaxation dispersion. IV. Proton relaxation in amino acids and Met-enkephalin pentapeptide

Nuclear Magnetic Relaxation Dispersion (NMRD) of protons was studied in the pentapeptide Met-enkephalin and the amino acids, which constitute it. Experiments were run by using high-resolution Nuclear Magnetic Resonance (NMR) in combination with fast field-cycling, thus enabling measuring NMRD curves for all individual protons. As in earlier works, Papers I-III, pronounced effects of intramolecular scalar spin-spin interactions, J-couplings, on spin relaxation were found. Notably, at low fields J-couplings tend to equalize the apparent relaxation rates within networks of coupled protons. In Met-enkephalin, in contrast to the free amino acids, there is a sharp increase in the proton T1-relaxation times at high fields due to the changes in the regime of molecular motion. The experimental data are in good agreement with theory. From modelling the relaxation experiments we were able to determine motional correlation times of different residues in Met-enkephalin with atomic resolution. This allows us to draw conclusions about preferential conformation of the pentapeptide in solution, which is also in agreement with data from two-dimensional NMR experiments (rotating frame Overhauser effect spectroscopy). Altogether, our study demonstrates that high-resolution NMR studies of magnetic field-dependent relaxation allow one to probe molecular mobility in biomolecules with atomic resolution.

Reentrant spin-glass state and magnetodielectric effect in the spiral magnetBiMnFe2O6

The complex oxide $\mathrm{Bi}\mathrm{Mn}{\mathrm{Fe}}_{2}{\mathrm{O}}_{6}$ crystallizing in a centrosymmetric orthorhombic structure ($Pbcm$) is known to exhibit a spiral antiferromagnetic ordering at ${T}_{N}\ensuremath{\sim}212$ K and an anomaly in magnetization at low temperature (${T}_{P}\ensuremath{\sim}34$ K). To understand the origin of the low temperature anomaly and to verify experimentally whether or not the spiral magnetism induces ferroelectricity, we have carried out detailed magnetization and electrical measurements on a polycrystalline sample. Frequency-dependent ac susceptibility, dc magnetic memory effect, and dc magnetization relaxation studies show that the low temperature anomaly at ${T}_{P}$ is associated with a reentrant spin-glass transition. From dc magnetization, we found that there exists another distinct temperature (${T}_{\text{irr}}$), which corresponds to a strong thermomagnetic irreversibility. The field dependence of these two temperatures, ${T}_{P}(H)$ and ${T}_{\text{irr}}(H)$, follow Gabay-Toulouse and de Almeida--Thouless lines, respectively, which are indicative of the Heisenberg spin-glass state. We also observe another magnetic anomaly at ${T}^{*}\ensuremath{\sim}170$ K, where a dielectric anomaly with a significant magnetocapacitance effect is observed. However, pyroelectric current and positive-up-negative-down measurements reveal the absence of ferroelectricity in this compound.

AC Susceptibility Studies of Phase Transitions and Magnetic Relaxation: Conventional, Molecular and Low-Dimensional Magnets

The paper presents the review of AC susceptibility measurements in study of magnetic phase transitions and spin dynamics. Examples of several types of phase transitions in conventional and molecular magnets are shown. The phenomenon of magnetic relaxation is brie y ascribed and illustrated with results on relaxation in low-dimensional molecular magnets, such as single molecule magnets, single chain magnets and quasi 1-dimensional systems showing magnetic phase transition to the long-range ordered phase. The following experimental possibilities of the AC technique, which allow to get insight into properties of novel magnetic materials are mentioned: (i) change of the AC eld amplitude (in order to distinguish ferro-(ferri) and antiferromagnets); (ii) use of a static eld HDC (to conclude on the type of magnetic order and to study the phase diagram); (iii) measurement of nonlinear susceptibilities (to conclude on the type of phase transition); (iv) probing the dependence of both components of susceptibility on frequency of the AC eld (to study magnetic relaxation); (v) tests of the DC eld e ect on the frequency dependence of χAC (to analyze the possibility of blocking or reentrant spin-glass transition).

Correspondence between neutron depolarization and higher order magnetic susceptibility to investigate ferromagnetic clusters in phase separated systems

It is a tough task to distinguish a short-range ferromagnetically correlated cluster-glass phase from a canonical spin-glass-like phase in many magnetic oxide systems using conventional magnetometry measurements. As a case study, we investigate the magnetic ground state of La0.85Sr0.15CoO3, which is often debated based on phase separation issues. We report the results of two samples of La0.85Sr0.15CoO3 (S-1 and S-2) prepared under different conditions. Neutron depolarization, higher harmonic ac susceptibility and magnetic relaxation studies were carried out along with conventional magnetometry measurements to differentiate subtle changes at the microscopic level. There is no evidence of ferromagnetic correlation in the sample S-2 attributed to a spin-glass phase, and this is compounded by the lack of existence of a second order component of higher harmonic ac susceptibility and neutron depolarization. A magnetic relaxation experiment at different temperatures complements the spin glass characteristic in S-2. All these signal a sharp variance when we consider the cluster-glass-like phase (phase separated) in S-1, especially when prepared from an improper chemical synthesis process. This shows that the nonlinear ac susceptibility is a viable tool to detect ferromagnetic clusters such as those the neutron depolarization study can reveal.