Magnetic Field-induced Changes Research Articles

Extreme magnetoresistance at high-mobility oxide heterointerfaces with dynamic defect tunability

Magnetic field-induced changes in the electrical resistance of materials reveal insights into the fundamental properties governing their electronic and magnetic behavior. Various classes of magnetoresistance have been realized, including giant, colossal, and extraordinary magnetoresistance, each with distinct physical origins. In recent years, extreme magnetoresistance (XMR) has been observed in topological and non-topological materials displaying a non-saturating magnetoresistance reaching 103−108% in magnetic fields up to 60 T. XMR is often intimately linked to a gapless band structure with steep bands and charge compensation. Here, we show that a linear XMR of 80,000% at 15 T and 2 K emerges at the high-mobility interface between the large band-gap oxides γ-Al2O3 and SrTiO3. Despite the chemically and electronically very dissimilar environment, the temperature/field phase diagrams of γ-Al2O3/SrTiO3 bear a striking resemblance to XMR semimetals. By comparing magnetotransport, microscopic current imaging, and momentum-resolved band structures, we conclude that the XMR in γ-Al2O3/SrTiO3 is not strongly linked to the band structure, but arises from weak disorder enforcing a squeezed guiding center motion of electrons. We also present a dynamic XMR self-enhancement through an autonomous redistribution of quasi-mobile oxygen vacancies. Our findings shed new light on XMR and introduce tunability using dynamic defect engineering.

Temperature- and magnetic field-induced magnetic structural changes in the Fe3Si/FeSi2 superlattice

Artificial lattices with semiconductor spacers are expected to exhibit changes in their magnetic structure owing to the control of their electronic states. The temperature (T) and magnetic-field (H ext) dependence of the in-plane magnetic structure of an [Fe3Si/FeSi2]20 superlattice with a nonmagnetic and semiconducting FeSi2 spacer layer is investigated using magnetization and polarized neutron reflectivity measurements. When H ext = 5 mT, nearly collinear antiferromagnetic (AF) structures are observed from 4 to 298 K. When H ext = 1 T, field-induced fan-like, noncollinear AF structures showing ferromagnetic components along H ext and transverse AF components are observed at low T.

Magnetostructural Transformation and Magnetocaloric Properties of (Ni37.5Co12.5Mn35Ti15)100−xBx (x = 0.0 and 0.4) Melt-Spun Ribbons

The effect of B-doping on the martensitic transformation (MT), microstructure, room temperature (RT) crystal structure, and magnetocaloric properties of a typical all-d-metal Ni37.5Co12.5Mn35Ti15 quaternary alloy was studied by synthesizing melt-spun ribbon samples of nominal composition (Ni37.5Co12.5Mn35Ti15)100−xBx with x = 0.0 and 0.4. For B-free samples, SEM images show a grain-oriented microstructure formed by the columnar in shape-elongated grains with their major axis oriented along the thermal gradient during solidification. By contrast, the B-doped samples show smaller grains whose orientation tends to be perpendicular to the contact surface with the copper wheel. For all samples, austenite (AST) and martensite (MST) phases exhibited a cubic B2-type and 5M monoclinic crystal structure, respectively. The martensitic transition temperature (TM) and the Curie temperature of the austenite phase (TCA) were reduced from 295 K to 253 K and 333 K to 276 K, respectively, with the addition of B. The effect of thermal annealing for different times (from 30 min to 4 h) at 1073 K was studied. Thermal annealing increases the martensitic transformation temperature, whereas TCA remains unchanged. The maximum magnetic field-induced entropy changes |ΔST|max for B-doped samples were around 4.5 J kg−1 K−1 and 4.7 J kg−1 K−1 for as-solidified and annealed samples (1073 K–4 h), respectively, compared to that found for the undoped samples (i.e., ΔST = 16 J kg−1 K−1). However, the entropy reduction is accompanied by an increase in the full width at half-maximum of the ΔST(T) curve.

Tunable contact angle hysteresis on compliant magnetoactive elastomers

It is shown that the advancing (ACA) and receding (RCA) contact angles of water on extremely soft (shear modulus of the order of 10kPa) magnetoactive elastomer (MAE) films significantly depend on the applied magnetic field. The difference between these angles, known as the contact angle hysteresis, is examined. The roles of the filler concentration and material softness are elaborated. The highest change in the contact angle hysteresis (CAH) from 34∘ in the absence of magnetic field to 76∘ in a magnetic field of 0.4T is achieved for the softest sample with the lowest mass fraction of iron particles (70wt%). The dependence of the CAH on magnetization history (“magnetic hysteresis”) is observed. This magnetic hysteresis is clearly pronounced for the ACA and has little effect on the RCA. Magnetic field-induced changes of the surface roughness exhibit qualitatively the same hysteresis behavior with regard to the external magnetic field as the ACA. The results are promising for the development of smart surfaces for applications where the dynamic wetting has to be controlled.

Post-acquisition upsampling method for scanning x-ray microscopy.

A method of post-acquisition upsampling for scanning x-ray microscopy is developed in this study to improve the spatial resolution beyond the Nyquist frequency as determined by the intervals of a raster scan grid. The proposed method is applicable only when the probe beam size is not negligibly small compared with the pixels that constitute a raster micrograph-the Voronoi cells of a scan grid. The unconvoluted spatial variation in a photoresponse is estimated by solving a stochastic inverse problem at a higher resolution than that at which the data are acquired. This is followed by a rise in the spatial cutoff frequency due to a reduction in the noise floor. The practicability of the proposed method was verified by applying it to raster micrographs of x-ray absorption in Nd-Fe-B sintered magnets. The improvement thus achieved in spatial resolution was numerically demonstrated via spectral analysis by using the discrete Fourier transform. The authors also argue for a reasonable decimation scheme for the spatial sampling interval in relation to an ill-posed inverse problem and aliasing. The computer-assisted enhancement in the viability of scanning x-ray magnetic circular dichroism microscopy was illustrated by visualizing magnetic field-induced changes in domain patterns of the Nd2Fe14B main-phase.

Magnetic Control of Soft Chiral Phonons in PbTe.

PbTe crystals have a soft transverse optical phonon mode in the terahertz frequency range, which is known to efficiently decay into heat-carrying acoustic phonons, resulting in anomalously low thermal conductivity. Here, we studied this phonon via polarization-dependent terahertz spectroscopy. We observed softening of this mode with decreasing temperature, indicative of incipient ferroelectricity, which we explain through a model including strong anharmonicity with a quartic displacement term. In magnetic fields up to 25T, the phonon mode splits into two modes with opposite handedness, exhibiting circular dichroism. Their frequencies display Zeeman splitting together with an overall diamagnetic shift with increasing magnetic field. Using a group-theoretical approach, we demonstrate that these observations are the result of magnetic field-induced morphic changes in the crystal symmetries through the Lorentz force exerted on the lattice ions. Thus, our Letter reveals a novel process of controlling phonon properties in a soft ionic lattice by a strong magnetic field.

Hidden first-order phase transitions and large magnetocaloric effects in GdNi1−xCox

We report a large magnetocaloric effect tunable between 70 and 115 K in a series of rare earth intermetallic compounds GdNi1−xCox with x varying from 0 to 0.15 that manifest rather unconventional second-order phase transitions with concurrent changes in both magnetic and crystallographic sublattices. While the Curie temperature, TC, linearly increases with x(Co), the maximum isothermal entropy change induced by magnetic field varying between 0 and 50 kOe slightly decreases from 14 J/kg K at 71 K when x(Co) = 0 to10 J/kg K at 115 K when x(Co) = 0.15. The temperature-averaged entropy change figures of merit calculated at temperature spans of 10 K for all examined compositions are comparable to those of the best known magnetocaloric materials that exhibit second-order phase transitions, including elemental Gd and La(Fe,Co,Si)13 alloys. Through a detailed analysis of the magnetic field-induced entropy changes along with assessment of critical exponents we explore the role magneto-elastic coupling plays in controlling magnetocaloric properties of GdNi1−xCox compounds. We also formulate a model based on the mean field theory approximation to describe both magnetic and magnetocaloric properties of the title materials in qualitative agreement with experimental results.

Magnetic field tuning of mechanical properties of ultrasoft PDMS-based magnetorheological elastomers for biological applications

We report tuning of the moduli and surface roughness of magnetorheological elastomers (MREs) by varying applied magnetic field. Ultrasoft MREs are fabricated using a physiologically relevant commercial polymer, SylgardTM 527, and carbonyl iron powder (CIP). We found that the shear storage modulus, Young’s modulus, and root-mean-square surface roughness are increased by ∼41×, ∼11×, and ∼11×, respectively, when subjected to a magnetic field strength of 95.5 kA m−1. Single fit parameter equations are presented that capture the tunability of the moduli and surface roughness as a function of CIP volume fraction and magnetic field strength. These magnetic field-induced changes in the mechanical moduli and surface roughness of MREs are key parameters for biological applications.

Nanostructured multiferroic Pb(Zr,Ti)O[formula omitted]–NiFe[formula omitted]O[formula omitted] thin-film composites

Pb(Zr,Ti)O3–NiFe2O4 thin-film composites were fabricated by spin coating a nickel ferrite precursor solution onto self-assembled porous lead zirconate titanate thin films. The morphology investigations revealed a very extensive interface area between both constitutents. The X-ray diffraction shows that NiFe2O4 coatings induce large compressive in-plane stresses in Pb(Zr,Ti)O3, which are generated during synthesis due to the significant difference in the thermal expansion coefficients of both phases. Diffraction peaks, moreover, corroborate a two phase pure system, without any other detectable secondary phase. The multiferroicity of the composites is evidenced by both ferroelectric and ferromagnetic hysteresis loops, while magnetic field-induced changes of the dielectric constant imply the potential utility of the developed material in magnetocapacitive applications.

On the instability of the giant direct magnetocaloric effect in CoMn0.915Fe0.085Ge at. % metamagnetic compounds

The giant magnetocaloric effect was quantified in CoMn1-xFexGe (x = 0.085–0.12) nom. at. % polycrystals across the high temperature hexagonal (P63/mmc) to low temperature orthorhombic (Pnma) phase transition via differential scanning calorimetry (DSC) and multiple (thermo) magnetization measurements. It was found that increasing Fe content led to the decrease of both the martensitic transformation temperature and entropy change (Delta S) at the point of the phase transition. Moreover, first-time magnetocaloric measurements resulted in irreproducible entropy change versus temperature diagrams, which was attributed to the release of internal pressure in bulk samples that disintegrated into powder upon transformation. CoMn1-xFexGe demonstrated larger magnetic field-induced entropy changes and giant magnetocaloric effect (MCE) compared to other CoMnGe alloys doped with Si, Sn, Ti, and Ga. However, the observed brittleness and apparent change in volume at the magnetic transition was posited to influence the material’s potential for regenerative applications.

Room-temperature multiferroic behavior in layer-structured Aurivillius phase ceramics

Multiferroics that simultaneously exhibit ferroelectricity and ferromagnetism have recently attracted great attention due to their potential application in next generation electronic devices. However, only a few single-phase multiferroic materials exhibit ferroelectric and ferromagnetic orders at room temperature. Recently, some bismuth layer-structured Aurivillius compounds were reported as multiferroics at room temperature, but the origin of their magnetic property is still under debate because the net magnetization may originate from the presence of secondary phases that are not easily detected by laboratory XRD diffractometers. Here, textured Aurivillius phase Bi5.25La0.75FeCoTi3O18 ceramics were prepared by Spark Plasma Sintering. The ferromagnetic character of the ceramics was indicated by the magnetic field-induced reversible intensity changes of a certain set of crystalline planes belonging to the Aurivillius phase, as measured by in situ neutron diffraction under the applied magnetic field. The first principles calculations indicate that the ferromagnetism originates from double exchange interactions Fe3+–O–Fe3+, Co3+–O–Co3+, and Fe3+–O–Co3+ in the ferro-toroidal main phase. The magnetic-controlled ferroelectric domain switching was observed by piezoelectric force microscopy at room temperature. The prepared Aurivillius phase ceramics, with Co/Fe contributing to magnetization and polarization at the same time, can be considered an intrinsic room-temperature multiferroic.

Effects of static magnetic fields on eukaryotic cytoskeleton

The cytoskeleton includes microtubules, actin filaments (also called microfilaments) and intermediate filaments, which play vital roles in maintaining cell morphology, intracellular transport and cell migration. They are involved in several physiological and pathological processes, such as the development and oncogenesis. Here in this review, we focus on summarizing the known effects of static magnetic fields on eukaryotic cytoskeleton, including microtubules, actin filaments and intermediate filaments. Since 1970s and 1980s, a series of progresses about the effects of static magnetic fields on eukaryotic cytoskeleton have been made both theoretically and experimentally. Theoretically, researchers have calculated the diamagnetic anisotropy of peptide bonds, which is relatively weak but could be amplified by highly ordered and organized structures, such as alpha helix and beta sheet in proteins. The diamagnetic anisotropy can be further amplified by highly ordered polymer structures such as microtubules. Experimentally, the orientation of purified microtubule and microfilament, as well as the cellular microtubule and microfilament changes that were induced by static magnetic fields has all been reported by multiple studies. For microtubules, it was shown that a 0.02 T static magnetic field was able to align the purified microtubule polymers in parallel with the magnetic field direction. The degree of purified microtubule polymer orientation changes is directly correlated with the magnetic field intensity. Moreover, there were also studies about the microtubule related cellular structures, such as mitotic spindles, sperm tails and Paramecium cilia. The mitotic spindle orientation of human nasopharyngeal carcinoma cell CNE-2Z, human retinal pigment epithelial cell RPE1 and frog eggs could all be affected by high static magnetic fields in a magnetic field intensity dependent manner. However, the magnetic field-induced orientation changes in these cells are also determined by chromosome alignment as well as the spindle morphology. Moreover, the orientation and swimming behaviors of Paramecium were both affected by high static magnetic fields. For actin and actin polymers, it has been shown that 10 T static magnetic field could affect the self-assembly process of G-actin in vitro . Moreover, static magnetic fields of various intensities could also cause the microfilament distribution changes or actin alteration in some cells. For example, 80 mT static magnetic field increased the accumulation of actin and myosin and promoted the formation of large multinucleated myotubes. There were also a few studies about static magnetic fields and intermediate filaments, but the evidences are much less than that of microtubules or microfilaments. Overall, the progresses about magnetic field effects on human and animal cytoskeleton are promising but still at an initial stage. The direct effect of magnetic fields on cytoskeleton dynamics has not been investigated due to the microscopy technology limitations under magnetic conditions. More importantly, the organism complexity and the different magnetic field parameters (such as homogeneous or gradient magnetic fields, magnetic fields with different intensities, etc.) make the investigation complicated. It is very important to further explore the static magnetic fields of different parameters for their effects on various cytoskeleton, which will be critical to understand the static magnetic field effects on some physiological and pathological conditions, as well as to explore the potentials of static magnetic fields in applications such as cancer therapy.

Magnetoelectric magnetic field sensors

Abstract

Pulsed magnetic field-induced changes in the meso- and nanostructure of Co49Ni21Ga30 martensite

A near single-variant martensitic state of Co[Formula: see text]Ni[Formula: see text]Ga[Formula: see text] alloy was induced by a static axial stress of 66[Formula: see text]MPa. Upon application of a magnetic field pulse, the single-variant state of stressed specimen was transformed into a hierarchic martensitic structure at micron, submicron and nanometer scales. The martensitic structures, which were induced during the magnetic field pulse, did not disappear after the field was switched off. This stability of the field-induced martensitic structure is attributed to the appearance of additional twinning systems with hardly mobile twin boundaries.

Foolproof Method for Fast and Reversible Switching of Water-Droplet Adhesion by Magnetic Gradients.

Reversible switching of water-droplet adhesion on solid surfaces is of great significance for smart devices, such as microfluidics. In this work, we designed a foolproof method for fast and reversible magnet-controlled switching of water-droplet adhesion surfaces by doping iron powders in soft poly(dimethylsiloxane). The water adhesion is adjusted by magnetic field-induced structure changes, avoiding complex chemical or physical surface design. The regulation process is so convenient that only tens of milliseconds are needed. The on-site responsive mechanism extends its use to unusual curved surfaces. Moreover, the excellent reversibility and stability make the film an ideal candidate for real-time applications.

Interlayer electronic transport inCaMnBi2antiferromagnet

We report interlayer electronic transport in CaMnBi$_{2}$ single crystals. Quantum oscillations and angular magnetoresistance suggest coherent electronic conduction and valley polarized conduction of Dirac states. Small cyclotron mass, large mobility of carriers and nontrivial Berry's phase are consistent with the presence of Dirac fermions on the side wall of the warped cylindrical Fermi surface. Similar to SrMnBi$_{2}$ that features an anisotropic Dirac cone, our results suggest that magnetic field-induced changes in the interlayer conduction are also present in layered bismuth-based materials with zero-energy line in momentum space created by the staggered alkaline earth atoms.

Electric polarization observed in single crystals of multiferroicLu2MnCoO6

We report electric polarization and magnetization measurements in single crystals of double perovskite Lu2MnCoO6 using pulsed magnetic fields and optical second harmonic generation (SHG) in DC magnetic fields. we observe well-resolved magnetic field-induced changes in the electric polarization in single crystals and thereby resolve the question about whether multiferroic behavior is intrinsic to these materials or an extrinsic feature of polycrystals. We find electric polarization along the crystalline b-axis, that is suppressed by applying a magnetic fields along c-axis and advance a model for the origin of magnetoelectric coupling. We furthermore map the phase diagram using both capacitance and electric polarization to identify regions of ordering and regions of magnetoelectric hysteresis. This compound is a rare example of coupled hysteretic behavior in the magnetic and electric properties. The ferromagnetic-like magnetic hysteresis loop that couples to hysteretic polarization can be attributed not to ordinary ferromagnetic domains, but to the rich physics of magnetic frustration of Ising-like spins in the axial next-nearest neighbor interaction model.

Interplanar coupling-dependent magnetoresistivity in high-purity layered metals.

The magnetic field-induced changes in the conductivity of metals are the subject of intense interest, both for revealing new phenomena and as a valuable tool for determining their Fermi surface. Here we report a hitherto unobserved magnetoresistive effect in ultra-clean layered metals, namely a negative longitudinal magnetoresistance that is capable of overcoming their very pronounced orbital one. This effect is correlated with the interlayer coupling disappearing for fields applied along the so-called Yamaji angles where the interlayer coupling vanishes. Therefore, it is intrinsically associated with the Fermi points in the field-induced quasi-one-dimensional electronic dispersion, implying that it results from the axial anomaly among these Fermi points. In its original formulation, the anomaly is predicted to violate separate number conservation laws for left- and right-handed chiral (for example, Weyl) fermions. Its observation in PdCoO2, PtCoO2 and Sr2RuO4 suggests that the anomaly affects the transport of clean conductors, in particular near the quantum limit.

Magnetic field-induced changes of lattice parameters and thermal expansion behavior of the CoMnSi compound

The crystal structure of the CoMnSi compound during zero-field cooling and field cooling from room temperature down to 200 K was studied using the synchrotron radiation X-ray diffraction technique. The results show that the lattice parameters and thermal expansion behavior of the sample are changed by the applied magnetic fields. The lattice contracts along the a axis, but expands along the b and c axes. Due to enlarged and anisotropic changes under a magnetic field of 6 T, the lattice shows an invar-like behavior along all three axes. Critical interatomic distances and bond angles also show large changes under the influence of such a high magnetic field. These magnetic field-induced changes of the lattice are discussed with respect to their contributions to the large magnetocaloric effect of the CoMnSi compound.

Effect of Weak Magnetic Field Treatment on Optical Properties of Epitaxial GaN Films

The results of optical studies of the effect of pulsed magnetic field (B = 60 mT, τ = 1.2 ms, t = 5 min) on the structural perfection and recombination properties of epitaxial GaN films grown on sapphire are presented. Nonmonotonic changes in the spectral dependence of transmittance and photoluminescence were detected for GaN films after exposure to magnetic field. They occurred during several days, with a maximum deviation from the initial values on the 12th day. The observed effect is presumably related to the magnetic field-induced changes in parameters of the surfaces and interfaces of epitaxial films.