High Magnetic Field Range Research Articles

Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets

Antiferromagnetic materials host pairs of spin-up and spin-down magnons which can be described in terms of a magnonic pseudospin. The close analogy between this magnonic pseudospin systems and that of electronic charge carriers led to the prediction of fascinating phenomena in antiferromagnets. Recently, the associated dynamics of antiferromagnetic pseudospin has been experimentally demonstrated and, in particular, the first observation of the magnon Hanle effect has been reported. We here expand the magnonic spin transport description by explicitly taking into account contributions of finite-spin low-energy magnons. In our experiments we realize the spin injection and detection process by two Platinum strips and investigate the influence of the Pt-strips on the generation and diffusive transport of magnons in films of the antiferromagnetic insulator hematite. For both a 15 nm and a 100 nm thick film, we find a distinct signal caused by the magnon Hanle effect. However, the magnonic spin signal exhibits clear differences in both films. In contrast to the thin film, for the thicker one, we observe an oscillating behavior in the high magnetic field range as well as an additional offset signal in the low magnetic field regime. We attribute this offset signal to the presence of finite-spin low-energy magnons.

High-field phase diagram of Ni3V2O8 studied by specific heat and magnetocaloric effect measurements

The H–T phase diagram of Ni3V2O8 is very rich and remains puzzling in a high magnetic field range. Through the state-of-the-art specific heat measurement in pulsed high field to 35 T and magnetocaloric effect measurement up to 45 T, we successfully construct the high-field phase diagram of Ni3V2O8 for fields applied along the b axis. The phase boundaries are corrected for previous results by magnetization and magneto-optical measurements. The resulting phase diagram shows that the high temperature incommensurate (HTI) phase develops well to high fields and low temperatures. In addition to the early reported C′, C, low temperature incommensurate (LTI) and HTI phases, we explore a new magnetic ordered phase called HF1 in fields of 10–30 T. A multicritical point is also observed at 6 K and 8 T. Furthermore, the specific heat data reveal enhancements of the anomalies at ∼4 K, probably associated with a strong spin–lattice coupling in this frustrated multiferroic material.

Analysis Method of High-Field Pinning Centers in NbTi Wires and MgB2 Wires

The main purpose of this article is to show an easier and more accurate method for analyzing high-field pinning centers. Previous methods did not allow for precise analysis and research on high-field pinning centers, they studied only the dominant pinning mechanism, and they required a maximum of pinning force (Fpmax) to analyze pinning centers. The main advantage of this method can be used in samples without the maximum pinning force required by other methods to analyze the pinning mechanism. In addition, this method allows us to observe even small changes in the density of individual pinning centers. This method is based on analysis of critical current density values in different ranges of reduced magnetic field. The method of analysis of high-field pinning centers allows primarily to develop methods and directions for creation of high-field pinning centers. The surface pinning centers are strong pinning centers in low magnetic fields and very weak pinning centers in middle and high magnetic fields. Additionally, the analysis of pinning centers shows that dislocations, strains, substitutions in the crystal lattice, and precipitation inside the grains create strong pinning centers in high magnetic fields and very weak pinning centers in low and middle magnetic fields. Research indicates that strains, substitutions in the crystal lattice, and precipitates inside grains form pinning centers that operate in the high magnetic field range.

Superconducting fluctuations and characteristic time scales in amorphous WSi

We study magnitudes and temperature dependences of the electron-electron and electron-phonon interaction times which play the dominant role in the formation and relaxation of photon induced hotspot in two dimensional amorphous WSi films. The time constants are obtained through magnetoconductance measurements in perpendicular magnetic field in the superconducting fluctuation regime and through time-resolved photoresponse to optical pulses. The excess magnetoconductivity is interpreted in terms of the weak-localization effect and superconducting fluctuations. Aslamazov-Larkin, and Maki-Thompson superconducting fluctuation alone fail to reproduce the magnetic field dependence in the relatively high magnetic field range when the temperature is rather close to Tc because the suppression of the electronic density of states due to the formation of short lifetime Cooper pairs needs to be considered. The time scale {\tau}_i of inelastic scattering is ascribed to a combination of electron-electron ({\tau}_(e-e)) and electron-phonon ({\tau}_(e-ph)) interaction times, and a characteristic electron-fluctuation time ({\tau}_(e-fl)), which makes it possible to extract their magnitudes and temperature dependences from the measured {\tau}_i. The ratio of phonon-electron ({\tau}_(ph-e)) and electron-phonon interaction times is obtained via measurements of the optical photoresponse of WSi microbridges. Relatively large {\tau}_(e-ph)/{\tau}_(ph-e) and {\tau}_(e-ph)/{\tau}_(e-e) ratios ensure that in WSi the photon energy is more efficiently confined in the electron subsystem than in other materials commonly used in the technology of superconducting nanowire single-photon detectors (SNSPDs). We discuss the impact of interaction times on the hotspot dynamics and compare relevant metrics of SNSPDs from different materials.

Analysis of trap-assisted triplet exciton quenching by magneto- conductance in rubrene-based organic light-emitting diodes

In this work, we have fabricated the conventional organic light-emitting diodes which are based on the promising rubrene material and their magneto-conductances (MCs) of the device have been measured at various temperatures and injection currents. The experimental results demonstrate that the current- and temperature-dependent MC curves of the device mainly show three structures as the magnetic field increases. Specifically, at room temperature (300 K), under low magnetic field (| B | 1 shows a rapid enhancement with increasing magnetic field at low currents, slowly rises and then decreases at moderate currents, and slowly decreases at high currents; in the medium magnetic field range (6 mT B |<17 mT), as magnetic field increases, MC 2 shows a slow rise in each current range; in the high magnetic field range (17 mT B | 3 rapidly reduces with increasing applied magnetic field in each current range. In order to further research the temperature effect on the MC responses, we also take the current of 50 μA as an example and measure the MC curves at different temperatures. Surprisingly, we found that temperature dependences of MC curves possess similar line-shape with current-dependent MC curves at ambient temperature. That is, with decreasing temperatures, in the low magnetic field range (| B | 1 demonstrates a slow rise at 300 K, slowly rises and then decreases at 250 and 150 K, and then slows down gradually at 100 and 20 K; in the medium magnetic field range (6 mT B | 2 shows a slow rise in each temperature range; in the high magnetic field range (17 mT B | 3 rapidly reduces with increasing applied magnetic field in each temperature range. On the whole, the line-shapes of MC 2 and MC 3 curves remain almost unchanged with decreasing temperatures (or increasing injection currents) in the medium and high magnetic field ranges (| B |>6 mT), but the tendency of MC 1 curves corresponding to low magnetic field (| B |<6 mT) will change with injection currents and temperatures. Additionally, by further analyzing the double logarithmic and temperature-dependent current density-voltage curves, we find that some traps which were usually originated from the fabrication process of devices inevitably exist in the device at room temperature and small injection currents, and the number of traps increases with decreasing temperature. Therefore, in addition to the dissociation of the triplet excitons by free charges (T+Q ® e+h+Q) and the intersystem crossing between singlet and triplet polaron pairs (PP 1 ® PP 3 ), two kinds of trap-assisted triplet exciton quenching processes (that is, trapped triplet-free polaron interaction (T t +P ® S 0 +P t ) and free triplet-trapped polaron interaction (T+P t ® S 0 +P)) are also included in the device. The combined effects of these four microscopic mechanisms lead to a complicated structure of the MC curves at small field and could be well tuned by device current density and working temperature. But the line-shapes of high-field MC responses are mainly determined by the role of TQA process. This article not only deepens the mechanism understanding of triplet-charge interactions in rubrene devices, but also enriches the manifestations of organic magneto-conductance curves.

Determination of Protein ps-ns Motions by High-Resolution Relaxometry.

Many of the functions of biomacromolecules can be rationalized by the characterization of their conformational energy landscapes: the structures of the dominant states, transitions between states and motions within states. Nuclear magnetic resonance (NMR) spectroscopy is the technique of choice to study internal motions in proteins. The determination of motions on picosecond to nanosecond timescales requires the measurement of nuclear spin relaxation rates at multiple magnetic fields. High sensitivity and resolution are obtained only at high magnetic fields, so that, until recently, site-specific relaxation rates in biomolecules were only measured over a narrow range of high magnetic fields. This limitation was particularly striking for the quantification of motions on nanosecond timescales, close to the correlation time for overall rotational diffusion. High-resolution relaxometry is an emerging technique to investigate picosecond-nanosecond motions of proteins. This approach uses a high-field NMR spectrometer equipped with a sample shuttle device, which allows for the measurement of the relaxation rate constants at low magnetic fields, while preserving the sensitivity and resolution of a high-field NMR spectrometer. The combined analysis of high-resolution relaxometry and standard high-field relaxation data provides a more accurate description of the dynamics of proteins, in particular in the nanosecond range. The purpose of this chapter is to describe how to perform high-resolution relaxometry experiments and how to analyze the rates measured with this technique.

High Magnetic Field Study on Giant Negative Magnetoresistance in the Molecular Conductor TPP[Cr(Pc)(CN)2]2

We investigated the magnetic and transport properties of the phthalocyanine molecular conductor TPP[Cr(Pc)(CN)2]2 in magnetic fields of up to 54 T. We observed giant negative magnetoresistance which hardly depends on the magnetic field direction owing to the isotropic nature confirmed by electron spin resonance measurements. The magnitude of magnetoresistance [|ρ(μ0H)/ρ(0 T) − 1|] is proportional to the square of magnetization as observed in the case of the spin scattering process, while the proportionality coefficient increases with decreasing the temperature. The magnetization does not saturate even at 53 T, indicating the existence of the large antiferromagnetic exchange interactions between the localized spins. In spite of this antiferromagnetic exchange interaction and low dimensionality, a convex magnetization curve was observed in the low temperature and high magnetic field range. To reproduce this magnetization curve, we proposed a model taking into account the antiferromagnetic exchange interaction between the neighboring π-electron spins.

The influence of HIP process on critical parameters of MgB2/Fe wires with big boron grains and without barriers

We show transport results of the critical temperature (Tc), irreversible magnetic field (Birr), upper critical field (Bc2), critical current density (Jc) and the pinning force (Fp) for undoped MgB2 wires without barriers. The results of SEM show that the hot isostatic pressure process increases the density and uniformity of MgB2 materials. Hot isostatic pressure allows for obtaining small grains and increasing the number of connections between the grains, accelerated reaction rate and limits the diffusion of Fe atoms into MgB2 material from the adjacent sheath. The pressure of 1.1 GPa increases Tc quite significantly in high magnetic field range of 6–12 T, improves the Birr and Bc2 and increase the Jc at 4.2 K and 20 K of about three times. Our results may be important for this field since many MgB2 wires are made from big grains of boron greater than 1 μm. Moreover, our research can be important for the production of MgB2 wires for the International Thermonuclear Experimental Reactor with boron 11B.

Angular dependence of giant magneto impedance and magnetic characteristic of Co-based wire in different magnetic field ranges

Angular dependence of the giant magneto impedance (GMI), hysteresis loops and magnetization curves have been investigated in amorphous wires with respect to direction and amplitude of the magnetic field in room temperature. The measurements were performed at different orientation angles of the applied magnetic field relative to the wire axis and various magnetic field strengths in moderate (0–200 Oe) and high (0–5000 Oe) ranges. The highest GMI response (500%) and magnetization (100 emu/gr) were found for angles close to the wire axis in high magnetic field range. By increasing the angle from 0°, in moderate external magnetic fields the GMI and magnetization decrease without reaching the saturation. However, in high magnetic fields (~ 5000 Oe) these parameters are nearly constant and almost saturated for all angles. In both magnetic field ranges, increasing the angle of applied field widens the impedance curves.

Analysis of the spin splitting of maxima of quantum oscillations of the resistivity of n-Bi-Sb semiconductor alloys in a magnetic field parallel to the bisector axis

In samples of semiconductor alloys n-Bi0.93Sb0.07 with different electron concentrations (n1 = 8 × 1015 cm−3, n2 = 1.2 × 1017 cm−3, and n3 = 1.9 × 1018 cm−3), dependences of the electrical resistivity on magnetic fields up to 45 T parallel to the current and the bisector axis (H ‖ C1 ‖ j) have been measured at temperatures of 1.5, 4.5, and 10 K. The obtained dependences ρ22(H) demonstrate quantum oscillations of the resistivity (Shubnikov-de Haas effect), and, in high magnetic fields, there is a resistivity maximum far away from other maxima. On assumption that this maximum is related to the spin-split Landau level N = 0− for electrons of the main ellipsoid, the spin-splitting parameters are calculated for electrons of the main ellipsoid: γ1 = 0.87, γ2 = 0.8, and γ3 = 0.73. Using these values, the oscillation maxima can be reliably related to the numbers of split Landau levels for electrons of the main and secondary ellipsoids. The dependences of the resistivity ρ11 and the Hall coefficient R31.2 on magnetic field have been measured in a transverse magnetic field at H ‖ C1 and j ‖ C2 on the sample with the electron concentration n4 = 1.4 × 1017 cm−3. Using similar analysis, the spin-splitting parameter is found to be γ4 = 0.85, which is close to the value of γ2 = 0.8 obtained for the sample with close electron concentration (n2 = 1.2 × 1017 cm−3) during the measurements in a longitudinal magnetic field. The quantum oscillation maxima of Hall coefficient R31.2 are shifted to the range of high magnetic fields as compared to the quantum oscillation maxima of resistivity ρ11.

Magnetic Field Dependent Photoinduced Superconductivity and Its Oxygen-Deficient Dependency in Undoped and BZO-doped YBCO Thin Films

Abstract We present a systematic study of photoinduced effiect illuminating the undoped and BZO-doped YBCO thin films with laser diode (1.88 eV). The pulsed laser deposited films were post-annealed in partial oxygen pressure in order to decrease the oxygen content and therefore understand the role of structural changes, oxygen ordering and hole doping in photoconductivity phenomenon. The resistivities were measured in the thermally activated flux-flow regime with wide applied magnetic field range. We find that as in undoped and optimally oxygenated YBCO films, the structural defects and dislocations formed by conventional BZO-doping does not contribute to photoinduced enhancement of superconductivity. On the contrary, in our oxygen-deficient films, the effect of photoexcitation is strongly dependent on the oxygen ordering and thereby free-carrier concentration and their mobility in the CuO2 double layers, which again has strong external magnetic field dependence. The magnetic field dependence of the increased superconducting transition shows that the illumination improves the pinning properties in high magnetic field range, indicating the existence of photosensitive pinning centers.

Pinning analyses on epitaxial YBa2Cu3O7−δ films with BaZrO3 inclusions

The introduction of artificial pinning sites in YBa2Cu3O7−δ (YBCO) epitaxial thin films has been obtained by pulsed laser deposition (PLD) technique from YBCO-BaZrO3 (BZO) composite targets with BZO concentration ranging from 2.5 to 7 mol %. The typical critical temperature, Tc, drop observed in YBCO-secondary phase films grown by PLD has been successfully recovered by increasing the deposition temperature. Transport properties analyses revealed improved pinning force values for YBCO-BZO films for BZO concentration above 5 mol %. These films exhibited an anomalous dual peak structure which has been ascribed to the presence of two pinning contributions. The crossover field, Bm, separating these two pinning regimes has been observed to be temperature independent suggesting the occurrence of the matching field effect. The measured value of Bm agreed with the matching field value, BΦ=KndΦ0, as evaluated from transmission electron microscopy investigations. The mark of this phenomenon is retained down to 10 K, making YBCO-BZO more performing than pristine YBCO films at applied magnetic fields below Bm in all the temperature range inspected. On the contrary, in the high magnetic field range (above Bm), the effectiveness of correlated pinning is progressively reduced on lowering the temperature suggesting that at low temperatures the dominant pinning contribution arises from isotropic pinning centers.

Influence of Magnetic Field on Magnetostructural Transition in Ni&lt;sub&gt;46.4&lt;/sub&gt;Mn&lt;sub&gt;32.8&lt;/sub&gt;Sn&lt;sub&gt;20.8&lt;/sub&gt; Heusler Alloy

A Mn-enriched Ni46.4Mn32.8Sn20.8 Heusler alloy has been prepared by the induction melting and casting method. The samples used were annealed at 900°C during 72 h with a subsequent water quench. The magnetostructural transformation (MST) in the vicinity of Tm=230K and Ta=250K has been found, and characterized by DSC, magnetization and resistivity measurements. Changes in the crystal structure due to the MST were observed using neutron diffraction at different temperatures. The crystal structure of martensitic phase shows a good fits to the 4-layered orthorhombic martensite with a Pmma space group. The linear shift of transformation temperatures was found in the entire high magnetic field range up 14T to be dTm/dB0 = -1.50 K/T and dTa/dB0 = -1.26 K/T, whereas in the low field range we found an increase of both characteristic temperatures. The entropy changes at the MST were evaluated using the Clausius-Clapeyron relationship. The transformation-induced magnetoresistance has been measured.

Superconducting Properties of Films Deposited From Micro-, Nanocrystalline and Optimally BZO-Doped YBCO Targets

Thin YBa2Cu3O6+x (YBCO) films are prepared by pulsed laser deposition (PLD) from pure micro- (mu) and nanocrystalline (n) targets and a n-target optimally doped with 3.9 wt% BaZrO3 (BZO). Effects of target grain size and doping on superconducting (SC) properties of the films are investigated using a SQUID magnetometer up to 5 T and transport measurements in pulsed magnetic fiefds up to 30 T. In comparison with mu-films, clearly increased values of jc are observed in n-films. Optimal doping with BZO still increases this difference in high magnetic field range and lengthens the low-field plateau of jc(B) and increases the accommodation field B* which are typically around 110 mT, 160 mT and 500 mT at 5 K in mu-, n- and BZO-doped films, respectiveiy. Transport measurements at T > 65 K show significant improvement of irreversibiiity line (IRL) and jc(B) in films prepared from n-target and BZO-doped target. These improvements of the superconducting properties in high magnetic fiefds make pure and BZO-doped n-films attractive for high field applications.

Magnetization Process and Coercivity of Fe-(Al, Ga)-(P, C, B, Si) Soft Magnetic Glassy Alloys

The density and the magnetization process of the melt-spun glassy Fe-(Al, Ga)-(P, C, B, Si) alloys are investigated to clarify the origin of low coercivity of the glassy alloys. The differences of the density (Δρ c ) between the crystalline and the amorphous phases of glassy Fe-(Al, Ga)-(P, C, B, Si) alloys are much smaller than those of the ordinary amorphous Fe-B(-Si) alloys. The H - 1 -power law behavior of ΔJ (= J s - J, where J s is the saturation magnetization) is observed for both the glassy and ordinary amorphous systems in the magnetic field (H) range of 20-25 < μ 0 H ≤ 50 mT. In the high magnetic field range of 50 ≤ μ 0 H < 70 mT, ΔJ of both the alloy systems obeys the H - 2 -power law. The length of the effective Burgers vector and width of quasi-dislocation dipole (QDD) type defects are nearly independent of the alloy system. This means that the local volume contraction is independent of the alloy system, but the density of QDDs in the glassy alloys is much smaller than the ordinary amorphous alloys because the glassy alloys exhibit much smaller Δρ c than the ordinary amorphous alloys. Since the coercivity originates from elastic stress of QDDs is proportional to (Δρ c ) 1 / 2 , the origin of low coercivity of the Fe-(Al, Ga)-(P, C, B, Si) glassy alloys is the low density of QDDs which corresponds to low density of the domain-wall pinning centers.

Effect of heavy-ion irradiation on the pinning properties of MgB2

Pinning properties of MgB2 superconductors irradiated with 5.8 GeV Pb-ions were studied. After the irradiation, enhancement of Jc was observed at high magnetic field range. We also found a suppression of magnetic flux creep by the irradiation. These results suggest that the defects introduced by the irradiation work as effective pinning centers.

E– J characteristics and n-values of a niobium–tin superconducting wire as a function of magnetic field, temperature and strain

Systematic measurements have been made of the E– J characteristics of a Nb 3Sn wire over four decades of electric field (0.1–1000 μV m −1) and five decades of current density (10 3–5×10 8 A m −2) as a function of magnetic field, temperature and strain. They were parameterised using the power law E= αJ n . At low magnetic fields n tends to a constant saturation value, which decreases at high compressive and tensile strains and increases with increasing electric field. In the high magnetic-field range, n is independent of E-field and can be approximated by a strain and temperature scaling law of the form n( B, T, ε)= G( T)( B c2 *( T, ε)− B), where B c2 *( T, ε) is an effective upper critical field.

Element specific x-ray magnetic circular dichroism magnetization curves using total electron yield

We used x-ray magnetic circular dichroism (XMCD) to obtain element specific magnetization curves. The most common way measuring XMCD in the soft x-ray region is total electron yield (TEY), because of the easy experimental setup and high signal-to-noise ratio compared to fluorescence yield. Surprisingly TEY was used only marginally for magnetization curves in the past. We show that the major problem using field dependent TEY is the complex current behavior as a function of applied magnetic fields, which directly results in corresponding normalization problems. We developed a simple but effective normalization method in combination with symmetry arguments to obtain magnetization curves using TEY as a probe for XMCD. Quick (2–6 min) and high quality data could be extracted in a wide range of high magnetic fields. In comparison to fluorescence yield hysteresis loops, those measurements could be easily done at synchrotron bending magnet beamlines and no high brilliant undulator sources and/or long measurement times are necessary. As an example we compare hysteresis loops of the colossal magnetoresistance compound La0.7Ca0.3MnO3 measured in situ by XMCD at the Mn L3 edge and conventional ex situ measurements.

Two-dimensional electron gas coupled to Mn 2+ ions: a magneto-optical study of CdMnTe/CdMgTe MDQWs

We report the results of magneto-optical experiments on two Cd 1− x Mn x Te/Cd 1− y Mg y Te modulation-doped single quantum well (QW) structures, with x=0.005, y=0.2 and electron sheet concentrations n=2.9 and 5.65×10 11 cm −2 . Experiments have been performed in magnetic fields up to 23 T and at low temperatures down to 75 mK. The results obtained in the range of low magnetic fields (B<2 T) and at temperatures 1 K<T<4.2 K are relatively well understood in terms of a standard mean-field approach adapted for a 2D electronic system coupled to the magnetic environment. Pronounced deviations from the isolated paramagnetic Mn 2+ behaviour are observed when lowering the temperature in the low-field limit as well as striking anomalies in the range of high magnetic fields in the vicinities of integer filling factors.

3D–2D crossover in melting transition on superconducting Mo/Si multilayers

Two-dimensional and three-dimensional vortex-lattice melting is observed on superconducting Mo/Si multilayers. Quasi-2D melting takes place in the range of high magnetic fields. The position of the melting transition on H-T phase diagram is in a reasonable quantitative accordance with the existing theories. Above melting line another phase transition is found connected with the decoupling in the liquid phase or with transition from hexatic to ordinary liquid. TAFF regime of the vortex motion is observed in the fluid phase governed by the plastic barrier charateristic for viscous liquid. TAFF regime with different activation energy takes place also in a vortex solid phase above the depinning line.