Magnetic Inhomogeneity Research Articles

Oxygen defect engineered magnetism of La2NiMnO6 thin films

The double perovskite La2NiMnO6 (LNMO) exhibits complex magnetism due to the competition of magnetic interactions that are strongly affected by structural and magnetic inhomogeneities. In this work, we study the effect of oxygen annealing on the structure and magnetism of epitaxial thin films grown by pulsed laser deposition. The key observations are that a longer annealing time leads to a reduction of saturation magnetization and an enhancement in the ferromagnetic transition temperature. We explain these results based upon epitaxial strain and oxygen defect engineering. The oxygen enrichment by annealing caused a decrease in the volume of the perovskite lattice. This increased the epitaxial strain of the films that are in-plane locked to the SrTiO3 substrate. The enhanced strain caused a reduction in the saturation magnetization due to randomly distributed anti-site defects. The reduced oxygen defects concentration in the films due to the annealing in oxygen improved the ferromagnetic long-range interaction and caused an increase in the magnetic transition temperature.

Berry-phase interpretation of thin-film micromagnetism

Magnetic flux densities (B-fields) and field intensities (H-fields) in thin films are investigated from the viewpoints of Berry phase and topological Hall effect. The well-known origin of the topological Hall effect is an emergent B-field originating from the Berry phase of conduction electrons, but Maxwell’s equations predict the relevant perpendicular component Bz to be zero. This paradox is solved by treating the electrons as point-like objects in Lorentz cavities. These cavities can also be used to interpret magnetization measurements in the present and other contexts, but structural and magnetic inhomogeneities lead to major modifications of the Lorentz-hole picture.

Quantitative magnetization transfer imaging in relapsing-remitting multiple sclerosis: a systematic review and meta-analysis.

Myelin-sensitive MRI such as magnetization transfer imaging has been widely used in multiple sclerosis. The influence of methodology and differences in disease subtype on imaging findings is, however, not well established. Here, we systematically review magnetization transfer brain imaging findings in relapsing-remitting multiple sclerosis. We examine how methodological differences, disease effects and their interaction influence magnetization transfer imaging measures. Articles published before 06/01/2021 were retrieved from online databases (PubMed, EMBASE and Web of Science) with search terms including ‘magnetization transfer’ and ‘brain’ for systematic review, according to a pre-defined protocol. Only studies that used human in vivo quantitative magnetization transfer imaging in adults with relapsing-remitting multiple sclerosis (with or without healthy controls) were included. Additional data from relapsing-remitting multiple sclerosis subjects acquired in other studies comprising mixed disease subtypes were included in meta-analyses.Data including sample size, MRI acquisition protocol parameters, treatments and clinical findings were extracted and qualitatively synthesized. Where possible, effect sizes were calculated for meta-analyses to determine magnetization transfer (i) differences between patients and healthy controls; (ii) longitudinal change and (iii) relationships with clinical disability in relapsing-remitting multiple sclerosis. Eighty-six studies met inclusion criteria. MRI acquisition parameters varied widely, and were also underreported. The majority of studies examined the magnetization transfer ratio in white matter, but magnetization transfer metrics, brain regions examined and results were heterogeneous. The analysis demonstrated a risk of bias due to selective reporting and small sample sizes. The pooled random-effects meta-analysis across all brain compartments revealed magnetization transfer ratio was 1.17 per cent units (95% CI −1.42 to −0.91) lower in relapsing-remitting multiple sclerosis than healthy controls (z-value: −8.99, P < 0.001, 46 studies). Linear mixed-model analysis did not show a significant longitudinal change in magnetization transfer ratio across all brain regions [β = 0.12 (−0.56 to 0.80), t-value = 0.35, P = 0.724, 14 studies] or normal-appearing white matter alone [β = 0.037 (−0.14 to 0.22), t-value = 0.41, P = 0.68, eight studies]. There was a significant negative association between the magnetization transfer ratio and clinical disability, as assessed by the Expanded Disability Status Scale [r = −0.32 (95% CI −0.46 to −0.17); z-value = −4.33, P < 0.001, 13 studies]. Evidence suggests that magnetization transfer imaging metrics are sensitive to pathological brain changes in relapsing-remitting multiple sclerosis, although effect sizes were small in comparison to inter-study variability. Recommendations include: better harmonized magnetization transfer acquisition protocols with detailed methodological reporting standards; larger, well-phenotyped cohorts, including healthy controls; and, further exploration of techniques such as magnetization transfer saturation or inhomogeneous magnetization transfer ratio.

Driven nonlinear damping and mode coupling in a superconducting levitated magnet

The magnetic flux pinning phenomenon enables high-$Q$ trapping of a magnet above a superconductor. In practice, the trapped magnet can undergo nonlinear mode coupling and dissipation as its vibration can induce variations of the magnetic field and vortex displacement on the superconductor surface. Here we study nonlinear interactions between vibrational modes of a levitated magnetic mirror above a superconducting disk in vacuum conditions. We observe that by exciting one vibrational mode of the levitated mirror, the vibrational noise of another mode can be suppressed by more than 17 dB. We attribute this unique noise suppression mechanism to the mode coupling and nonlinear dissipation caused by the driven magnetic inhomogeneity of the levitated object. The results suggest that noise reduction of a vibrational mode is possible by driving another orthogonal mode.

Transient Anomalous Diffusion MRI in Excised Mouse Spinal Cord: Comparison Among Different Diffusion Metrics and Validation With Histology.

Neural tissue is a hierarchical multiscale system with intracellular and extracellular diffusion compartments at different length scales. The normal diffusion of bulk water in tissues is not able to detect the specific features of a complex system, providing nonlocal, diffusion measurement averaged on a 10-20 μm length scale. Being able to probe tissues with sub-micrometric diffusion length and quantify new local parameters, transient anomalous diffusion (tAD) would dramatically increase the diagnostic potential of diffusion MRI (DMRI) in detecting collective and sub-micro architectural changes of human tissues due to pathological damage. In DMRI, the use of tAD parameters quantified using specific DMRI acquisition protocols and their interpretation has often aroused skepticism. Although the derived formulas may accurately fit experimental diffusion-weighted data, the relationships between the postulated dynamical feature and the underlying geometrical structure remains elusive, or at most only suggestive. This work aimed to elucidate and validate the image contrast and information that can be obtained using the tAD model in white matter (WM) through a direct comparison between different diffusion metrics and histology. Towards this goal, we compared tAD metrics extracted from pure subdiffusion (α-imaging) and super-pseudodiffusion (γ-imaging) in excised mouse spinal cord WM, together with T2 and T2* relaxometry, conventional (normal diffusion-based) diffusion tensor imaging (DTI) and q-space imaging (QSI), with morphologic measures obtained by optical microscopy, to determine which structural and topological characteristics of myelinated axons influenced tAD contrast. Axon diameter (AxDiam), the standard deviation of diameters (SDax.diam), axonal density (AxDens) and effective local density (ELD) were extracted from optical images in several WM tracts. Among all the diffusion parameters obtained at 9.4 T, γ-metrics confirmed a strong dependence on magnetic in-homogeneities quantified by R2* = 1/T2* and showed the strongest associations with AxDiam and ELD. On the other hand, α-metrics showed strong associations with SDax.diam and was significantly related to AxDens, suggesting its ability to quantify local heterogeneity degree in neural tissue. These results elucidate the biophysical mechanism underpinning tAD parameters and show the clinical potential of tAD-imaging, considering that both physiologic and pathologic neurodegeneration translate into alterations of WM morphometry and topology.

Quantum dynamics of Gaudin magnets

Abstract Quantum dynamics of many-body systems is a fascinating and significant subject for both theory and experiment. The question of how an isolated many-body system evolves to its steady state after a sudden perturbation or quench still remains challenging. In this paper, using the Bethe ansatz wave function, we study the quantum dynamics of an inhomogeneous Gaudin magnet. We derive explicit analytical expressions for various local dynamic quantities with an arbitrary number of flipped bath spins, such as: the spin distribution function, the spin-spin correlation function, and the Loschmidt echo. We also numerically study the relaxation behavior of these dynamic properties, gaining considerable insight into coherence and entanglement between the central spin and the bath. In particular, we find that the spin-spin correlations relax to their steady value via a nearly logarithmic scaling, whereas the Loschmidt echo shows an exponential relaxation to its steady value. Our results advance the understanding of relaxation dynamics and quantum correlations of long-range interacting models of Gaudin type.

Dynamo instabilities in plasmas with inhomogeneous chiral chemical potential

We study the dynamics of magnetic fields in chiral magnetohydrodynamics, which takes into account the effects of an additional electric current related to the chiral magnetic effect in high-energy plasmas. We perform direct numerical simulations, considering weak seed magnetic fields and inhomogeneities of the chiral chemical potential ${\ensuremath{\mu}}_{5}$ with a zero mean. We demonstrate that a small-scale chiral dynamo can occur in such plasmas if fluctuations of ${\ensuremath{\mu}}_{5}$ are correlated on length scales that are much larger than the scale on which the dynamo growth rate reaches its maximum. Magnetic fluctuations grow by many orders of magnitude due to the small-scale chiral dynamo instability. Once the nonlinear backreaction of the generated magnetic field on fluctuations of ${\ensuremath{\mu}}_{5}$ sets in, the ratio of these scales decreases and the dynamo saturates. When magnetic fluctuations grow sufficiently to drive turbulence via the Lorentz force before reaching maximum field strength, an additional mean-field dynamo phase is identified. The mean magnetic field grows on a scale that is larger than the integral scale of turbulence after the amplification of the fluctuating component saturates. The growth rate of the mean magnetic field is caused by a magnetic $\ensuremath{\alpha}$ effect that is proportional to the current helicity. With the onset of turbulence, the power spectrum of ${\ensuremath{\mu}}_{5}$ develops a universal ${k}^{\ensuremath{-}1}$ scaling independently of its initial shape, while the magnetic energy spectrum approaches a ${k}^{\ensuremath{-}3}$ scaling.

Interaction of Ferromagnetically Ordered Clusters with Dislocations in Austenite and Twinning

The relationship between the interaction of ferromagnetically ordered clusters in austenite with dislocations, twinning and nucleation of the martensite phase is considered. It is shown that the regions with short-range order existing in austenite affect the dislocation structure. In turn, dislocations are involved in the formation of twins and martensite nuclei. The imposition of an external magnetic field enhances the magnetic inhomogeneity of austenite and the effects of magnetoelastic interaction between clusters and dislocations.

T1D -weighted ihMT imaging - Part I. Isolation of long- and short-T1D components by T1D -filtering.

To identify T1D -filtering methods, which can specifically isolate various ranges of T1D components as they may be sensitive to different microstructural properties. Modified Bloch-Provotorov equations describing a bi-T1D component biophysical model were used to simulate the inhomogeneous magnetization transfer (ihMT) signal from ihMTRAGE sequences at high RF power and low duty-cycle with different switching time values for the dual saturation experiment: Δt = 0.0, 0.8, 1.6, and 3.2 ms. Simulations were compared with experimental signals on the brain gray and white matter tissues of healthy mice at 7T. The lengthening of Δt created ihMT high-pass T1D -filters, which efficiently eliminated the signal from T1D components shorter than 1 ms, while partially attenuating that of longer components (≥ 1 ms). Subtraction of ihMTR images obtained with Δt = 0.0 ms and Δt = 0.8 ms generated a new ihMT band-pass T1D -filter isolating short-T1D components in the 100-µs to 1-ms range. Simulated ihMTR values in central nervous system tissues were confirmed experimentally. Long- and short-T1D components were successfully isolated with high RF power and low duty-cycle ihMT filters in the healthy mouse brain. Future studies should investigate the various T1D -range microstructural correlations in in vivo tissues.

Magnetocaloric effect in La0.70Ag0.25MnO[formula omitted] magnetic nanoparticles

Experimental study of magnetocaloric effect (MCE) was performed by direct and indirect methods on La0.70Ag0.25MnO3+δ nanoparticles with rhombohedral LaAlO3 – type crystal structure (R3¯c space group – No. 167, hP30). The content of oxygen was modified by annealing at 800 °C/48h in different atmosphere (air, O2 and Ar). The Curie temperature TC increases from 250 K to 319.5 K and to 322.9 K with the content of oxygen. The adiabatic temperature change ΔT(4T) = 1.52 K, the change of magnetic entropy −ΔSmax(4T) = 3.96 Jkg−1K−1 and the relative cooling power RCT(4T) = 172 J/kg are maximal for sample treated in Ar and decrease with oxygen content. The frequency dependence of ΔT (f) and the shape of maximum in ΔT (T) and −ΔSmax (T) indicate relaxation processes and presence of magnetic inhomogeneities which are present mostly in sample treated in Ar. The samples demonstrate the stability of the MCE up to 1000 cycles of switching on and off the magnetic field without any signs of degradation.

T1D -weighted ihMT imaging - Part II. Investigating the long- and short-T1D components correlation with myelin content. Comparison with R1 and the macromolecular proton fraction.

To investigate the long- and short-T1D components correlation with myelin content using inhomogeneous magnetization transfer (ihMT) high-pass and band-pass T1D -filters and to compare ihMT, R1 , and the macromolecular proton fraction (MPF) for myelin specific imaging. The 3D ihMT rapid gradient echo (ihMTRAGE) sequences with increasing switching times (Δt) were used to derive ihMT high-pass T1D -filters with increasing T1D cutoff values and an ihMT band-pass T1D -filter for components in the 100 µs to 1 ms range. 3D spoiled gradient echo quantitative MT (SPGR-qMT) protocols were used to derive R1 and MPF maps. The specificity of R1 , MPF, and ihMT T1D -filters was evaluated by comparison with two histological reference techniques for myelin imaging. The higher contribution of long-T1D s as compared to the short components as Δt got longer led to an increase in the specificity to myelination. In contrast, focusing on the signal originating from a narrow range of short-T1D s (< 1 ms) as isolated by the band-pass T1D -filter led to lower specificity. In addition, the significantly lower r2 correlation coefficient of the band-pass T1D -filter suggests that the origin of short-T1D components is mostly associated with non-myelin protons. Also, the important contribution of short-T1D s to the estimated MPF, explains its low specificity to myelination as compared to the ihMT high-pass T1D -filters. Long-T1D components imaging by means of ihMT high-pass T1D -filters is proposed as an MRI biomarker for myelin content. Future studies should enable the investigation of the sensitivity of ihMT T1D -filters for demyelinating processes.

Features of absorption of thin films La-=SUB=-0.69-=/SUB=-Ba-=SUB=-0.31-=/SUB=-MnO-=SUB=-3-delta-=/SUB=- obtained using the method of pulsed laser deposition

The electrical, optical, and magneto-optical properties of La0.69Ba0.31MnO3-delta thin films with a Curie temperature near room temperature are studied. It is shown that in the region of the phase transition there is a sharp change in the behavior of the temperature dependences of the electrical resistance and optical transparency of the films, which is associated with the insulator-metal transition. In a narrow temperature region near the transition, a change in the external magnetic field leads to the appearance of negative magnetoresistance and magnetotransmission of unpolarized light in the spectral range from 1 to 12 μm. It is shown that the effect of magnetic transmission (magnetoabsorption) in films is mainly due to the contribution of free charge carriers. The magnetic transmission spectra are sensitive to the magnetic and electronic inhomogeneities of the films. Keywords: Thin films, manganites, magnetoabsorption, colossal magnetoresistance, IR spectroscopy, metal-insulator transition, magnetic inhomogeneities.

Особенности поглощения тонких пленок La-=SUB=-0.69-=/SUB=-Ba-=SUB=-0.31-=/SUB=-MnO-=SUB=-3-delta-=/SUB=-, полученных методом импульсного лазерного осаждения

The electrical, optical, and magneto-optical properties of La0.69Ba0.31MnO3 – δ thin films with a Curie temperature near room temperature are studied. It is shown that in the region of the phase transition there is a sharp change in the behavior of the temperature dependences of the electrical resistance and optical transparency of the films, which is associated with the insulator-metal transition. In a narrow temperature region near the transition, a change in the external magnetic field leads to the appearance of negative magnetoresistance and magnetotransmission of unpolarized light in the spectral range from 1 to 12 μm. It is shown that the effect of magnetic transmission (magnetoabsorption) in films is mainly due to the contribution of free charge carriers. The magnetic transmission spectra are sensitive to the magnetic and electronic inhomogeneities of the films.

Spin-wave resonances in a diffusion-sensitive epitaxial film YIG

It is shown that in a tangentially magnetized film of iron yttrium garnet exist two types of resonances --- a homogeneous ferrimagnetic resonance in the region of the film's homogeneous magnetization (outside the diffusion layer), and inhomogeneous spin-wave resonance within the thickness of the diffusion layer. A method of calculating the magnetization distribution over the film thickness using measured frequencies of the spin-wave and ferrimagnetic resonances. The method of mathematical modeling has established that the magnetic inhomogeneity of the transition layer layer introduces significant distortions of the oscillation epure at the frequency of the spin-wave resonance, and the nature of the distortions significantly depends on the magnitude of the magnetizing field. The results of the conducted can be useful for nondestructive control of the layered structure of YIG epitaxial films. Keywords: yttrium iron garnet epitaxial films, exchange spin waves, spinwave electronics, magnonics.

Structure of Magnetic Inhomogeneities in Films with Topological Features

Magnetic films with easy plane uniaxial anisotropy are studied. It is shown that in these films structural defects such as artificially created holes or nonmagnetic inclusions can give rise to vortex-like inhomogeneities with a topological charge. Analysis of their stability reveals parameters (the magnitude and direction of electric currents, the number of holes, etc.) at which they form stable configurations suitable for information encoding.

Спин-волновые резонансы в диффузионном слое эпитаксиальной пленки ЖИГ

It is shown that in a tangentially magnetized film of iron yttrium garnet exist two types of resonances - a homogeneous ferrimagnetic resonance in the region of the film's homogeneous magnetization (outside the diffusion layer), and inhomogeneous spin-wave resonance within the thickness of the diffusion layer. A method of calculating the magnetization distribution over the film thickness using measured frequencies of the spin-wave and ferrimagnetic resonances. The method of mathematical modeling has established that the magnetic inhomogeneity of the transition layer layer introduces significant distortions of the oscillation epure at the frequency of the spin-wave resonance, and the nature of the distortions significantly depends on the magnitude of the magnetizing field. The results of the conducted can be useful for nondestructive control of the layered structure of YIG epitaxial films.

Changing the Dynamic Parameters of Localized Breather and Soliton Waves in the Sine-Gordon Model with Extended Impurity, External Force, and Decay in the Autoresonance Mode

Possibility of changing the dynamic parameters of localized breather and soliton waves for the sine-Gordon equation in the model with extended impurity, variable external force and dissipation was investigated using the autoresonance method. The model of ferromagnetic structure consisting of two wide identical layers separated by a thin layer with modified values of magnetic anisotropy parameter was taken as a basis. Frequency of external field is a linear function of time. The sine-Gordon equation (SGE) was solved numerically using the finite differences method with explicit scheme of integration. For certain values of the extended impurity parameters a magnetic inhomogeneity in the form of magnetic breather is formed when domain wall passes through it with constant velocity. The numerical simulation showed that using special variable force and small amplitude it is possible to resonantly increase the amplitude of breather. For each case of the impurity parameters values, there is a threshold value of the magnetic field amplitude leading to resonance. Geometric parameters of thin layer also have influence on the resonance effect — for decreasing layer width the breather amplitude grows more slowly. For large layer width the translation mode of breather oscillations is also excited. For certain parameters of extended impurity, a soliton can form. For a special type of variable field with frequency linearly dependent on time, soliton is switched to antisoliton and vice versa.

Mixing of acoustic and optical excitations in Ru/Co based multilayers

Mixing of Acoustic and Optical mode in Ru/Co based multilayers as a result of the inhomogeneous magnetization profile along their thickness.

Reduced myelin density in unmedicated major depressive disorder: An inhomogeneous magnetization transfer MRI study

ObjectivesTo detect the whole-brain reduced myelin density in unmedicated patients with major depressive disorder (MDD) using the inhomogeneous magnetization transfer (ihMT) imaging technology. Compared to other technologies, the ihMT provides high specificity and sensitivity to detect myelin. MethodIn this prospective study, fifty unmedicated patients (mean age 25.36 years, 40% men) with MDD and 57 age- and sex-matched healthy controls (HCs) (mean age 25.02 years, 53% men) were recruited between January 2019 and December 2019. All participants underwent ihMT imaging, and pseudo-quantitative ihMT (qihMT) and ihMT ratio (ihMTR) were obtained. The mean values of qihMT and ihMTR extracted from the 50 WM masks (extracted from the International Consortium for Brain Mapping, ICBM-152) in each participant were compared between participants in the MDD and HCs groups. The symptoms of patients were evaluated using the 24-item Hamilton Depression Rating scale (HDRS). ResultsCompared with the HC group, the MDD group showed significantly decreased qihMT and ihMTR values in the left inferior fronto-occipital fasciculus (IFOF) (t = -4.057, p < 0.001; t = -3.662, p < 0.001) and the left uncinate fasciculus (UF) (t = -4.776, p < 0.001; t = -3.800, p < 0.001) after Bonferroni correction. The correlation analysis displayed a significant negative correlation between qihMT values of the left IFOF and HDRS total scores in patients with MDD (r = -0.390, p = 0.012). LimitationsThis was a cross-sectional study with a relative small sample. ConclusionsThese findings suggest the reduced myelin density in the IFOF and UF in patients with MDD, which might be associated with the pathophysiology of MDD.

Probing Magnetic Defects in Ultra-Scaled Nanowires with Optically Detected Spin Resonance in Nitrogen-Vacancy Center in Diamond.

Magnetic nanowires (NWs) are essential building blocks of spintronics devices as they offer tunable magnetic properties and anisotropy through their geometry. While the synthesis and compositional control of NWs have seen major improvements, considerable challenges remain for the characterization of local magnetic features at the nanoscale. Here, we demonstrate nonperturbative field distribution mapping in ultrascaled magnetic nanowires with diameters down to 6 nm by scanning nitrogen-vacancy magnetometry. This enables localized, minimally invasive magnetic imaging with sensitivity down to 3 μT Hz-1/2. The imaging reveals the presence of weak magnetic inhomogeneities inside in-plane magnetized nanowires that are largely undetectable with standard metrology and can be related to local fluctuations of the NWs' saturation magnetization. In addition, the strong magnetic field confinement in the nanowires allows for the study of the interaction between the stray magnetic field and the nitrogen-vacancy sensor, thus clarifying the contrasting formation mechanisms for technologically relevant magnetic nanostructures.