Low-field negative magnetization and coercive-field magnetization reversal in transition metal chalcogenides; Cr2FeSe4 magnetic structure from neutron diffraction

Low-field negative magnetization, of the order of −10⁻¹ emu/g-Oe, from 4.2 K up to room temperature and higher (350 K), and coercive-field magnetization reversal are both present in Cr_(3−x)Fe_xX₄ for X=S, Se, Te and x=0 to 3, and for Cr₅Te₈ and Cr₇Te₈. For Cr₂FeSe₄ the zero-field-cooled (ZFC) magnetization is negative for 5 Oe and below. To obtain a more detailed knowledge of the magnetic phases involved in the observed magnetization versus temperature M(T) curves, we obtained and studied neutron diffraction (n.d.) scans on the compound Cr₂FeSe₄, taken at 14 temperatures from 4.2 to 300 K. For this same n.d. sample, the temperature for magnetization reversal of value −3×10⁻⁴ emu/g-Oe is 80 K in 40 Oe applied field, then the reversal disappears for 65 Oe applied field. The complex magnetic interactions responsible for this reversal are revealed in the hysteresis curves.     

Search for GUT magnetic monopoles and nuclearites with the MACRO experiment

We present the final results obtained by the MACRO experiment in the search for GUT magnetic monopoles and nuclearites. Several searches were performed with different subdetectors, i.e. scintillation counters, limited streamer tubes and nuclear track detectors. No magnetic monopole or nuclearite candidates were found. The MACRO upper limit to the local flux of GUT magnetic monopoles is at the level of 1.4×10⁻¹⁶ cm⁻² s⁻¹ sr⁻¹.     

交变力磁力显微镜动态成像技术的研究

近年来磁力显微镜(magnetic force microscopy,MFM)对动态磁场信号的测量与分析由于其特殊的工业要求和重要用途而受到广泛关注,本文旨在利用交变磁力对磁性探针的周期性调制发展一种交变力磁力显微镜技术,为磁信息存储工业等重要领域关键技术的发展提供新型的有力的工具.与目前标准MFM采用的设计思路不同,本文的关键在于合理利用MFM频率调制机理,优化设计MFM磁性探针,并且引入动态信号处理模块,实现对交变磁场信号的MFM成像.为达到这些目的,需要从理论上研究MFM探针的频率调制机理,并由实验上设计出动态信号提取模块,二者相辅结合优化设计出具有动态信号测试和分析能力的交变力磁力显微镜技术,由此来测量和解释纳米尺度磁畴结构.     

Magnetic dipole moments near Sn: Measurement on isomeric 11/2 states in odd-A Te and Te by NMR/ON

On-line low temperature nuclear orientation (OLNO) experiments have been performed on the odd-A Te isotopes ¹³¹Te and ¹³³Te using the technique of nuclear magnetic resonance on oriented nuclei (NMR/ON). The magnetic moments of the isomeric 11/2⁻ states have been measured extending the known data on these states in the Te isotopes up to the neutron shell closure at N = 82. The contribution to the 11/2⁻ magnetic moment in ¹³³Te due to core polarisation is calculated using an RPA shell model as well as corrections to the magnetic dipole operator caused by mesonic exchange currents. The neutron number dependence of the magnetic moments of the 11/2⁻ isomers in heavy Te isotopes is discussed in terms of particle-core coupling model (PCM) calculations.     

Magnetic elements for switching magnetization magnetic force microscopy tips

Using combination of micromagnetic calculations and magnetic force microscopy (MFM) imaging we find optimal parameters for novel magnetic tips suitable for switching magnetization MFM. Switching magnetization MFM is based on two-pass scanning atomic force microscopy with reversed tip magnetization between the scans. Within the technique the sum of the scanned data with reversed tip magnetization depicts local atomic forces, while their difference maps the local magnetic forces. Here we propose the design and calculate the magnetic properties of tips suitable for this scanning probe technique. We find that for best performance the spin-polarized tips must exhibit low magnetic moment, low switching fields, and single-domain state at remanence. The switching field of such tips is calculated and optimum shape of the Permalloy elements for the tips is found. We show excellent correspondence between calculated and experimental results for Py elements.     

Ion transport studies in calcium alginate gels by magnetic resonance microscopy

Polyelectrolyte biopolymers such as calcium alginate are becoming increasingly important for the recovery of heavy metals from aqueous solutions. To understand the mechanism of ion transport in these biopolymer systems, the transport of copper ions into calcium alginate gels was investigated using proton nuclear magnetic resonance (NMR) microscopy. Copper ion transport was imaged using an inversion recovery technique which utilizes the paramagnetic effect of copper on water proton relaxation times. Diffusion experiments were performed in a diffusion cell designed to approximate a semi-infinite slab geometry at temperatures between 278 and 313 K using copper reservoir concentrations between 10 and 60 mM. The diffusion coefficient of copper in these gels was calculated from the NMR data to fit a combined diffusion-reaction model involving a diffusion term (D) and a kinetic binding term (k). At 23 °C, the diffusion coefficients in 1, 2, and 3% (w/v) gels were 3.1 · 10⁻¹⁰, 2.0 · 10⁻¹⁰, and 1.4 · 10⁻¹⁰ m²/s, respectively. The activation energy for diffusion in the 2% (w/v) gel was 28 kJ/mol.     

Electrical and magnetic properties of polyaniline/Fe3O4 nanostructures

We report on electrical and magnetic properties of polyaniline (PANI) nanotubes (150 nm in diameter) and PANI/Fe₃O₄ nanowires (140 nm in diameter) containing Fe₃O₄ nanoparticles with a typical size of 12 nm. These systems were prepared by a template-free method. The conductivity of the nanostructures is 10⁻¹–10⁻² S/cm; and the temperature dependent resistivity follows a ln ρT^−1/2 law. The composites (6 and 20 wt% of Fe₃O₄) show a large negative magnetoresistance compared with that of pure PANI nanotubes and a considerably lower saturated magnetization (M_s=3.45 emu/g at 300 K and 4.21 emu/g at 4 K) compared with the values measured from bulk magnetite (M_s=84 emu/g) and pure Fe₃O₄ nanoparticles (M_s=65 emu/g). AC magnetic susceptibility was also measured. It is found that the peak position of the AC susceptibility of the nanocomposites shifts to a higher temperature (>245 K) compared with that of pure Fe₃O₄ nanoparticles (190–200 K). These results suggest that interactions between the polymer matrix and nanoparticles take place in these nanocomposites.     

The search for magnetic monopoles in magnetite from North China

Using the 27 kG magnetic field produced by a superconducting solenoid we have searched for trapped magnetic monopoles in a magnetite sample from North China. We hvve scanned the nuclear emulsion both for low ionizing tracks and heavily ionizing tracks, and polycarbonate track detectors for heavily ionizing tracks of magnetic monopoles. We obtained a 90% confidence level upper limit of 1.8×10⁻²⁶ monopoles per nucleon in the magnetite sample.     

The magnetic moment of the 344 keV level of Gd

The magnetic moment of the 344 keV level of ¹⁵²Gd was determined from measurements of the rotations of the angular correlations of γ-γ cascades involving this level in the magnetic hyperfine field of Gd metal. The value found for gτ is (2.43±0.22)·10⁻¹¹ sec.. Taking τ = (7.6±1.3)·10⁻¹¹ sec, a value of 0.32±0.06 was derived for the gyromagnetic ratio of this level.     

Characterization of high‐density bit‐patterned media using ultra‐high resolution magnetic force microscopy

Bit‐patterned media at one terabit‐per‐square‐inch (Tb/in²) recording density require a feature size of about 12 nm. The fabrication and characterization of such magnetic nanostructures is still a challenge. In this Letter, we show that magnetic dots can be resolved at 10 nm spacing using magnetic force microscopy (MFM) tips coated with a magnetic film possessing a perpendicular magnetic anisotropy (PMA). Compared to MFM tips with no special magnetic anisotropy, MFM tips with PMA can resolve the bits clearly, because of a smaller magnetic interaction volume, enabling a simple technique for characterizing fine magnetic nanostructures. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

氧化物隔离对Si基片上生长L1_0相FePt薄膜磁性的影响

用MgO和SiO_2两种氧化物将FePt薄膜与Si(100)基片隔离,分析隔离层在FePt层发生A1→L1_0转变过程中的作用,寻找用Si母材涂敷L1_0-FePt磁性层来提高磁力显微镜针尖矫顽力的合理方案.采用磁控溅射法在400?C沉积Fe Pt薄膜,在不同温度进行2 h的真空热处理,分析晶体结构和磁性的变化.结果表明:没有隔离层,Si基片表层容易发生扩散,50 nm厚FePt薄膜的矫顽力最大只有5kOe(1 Oe=10~3/(4π)A·m~(-1));而插入隔离层,矫顽力可以超过10 kOe;MgO在Si基片上容易碎裂,热处理温度不能高于600?C,用作隔离层,FePt的最大矫顽力为12.4 kOe;SiO_2与Si基片的晶格匹配更好,热膨胀系数差较小,能承受的最高热处理温度可以超过800?C,使得Fe Pt的矫顽力可以在5 kOe到15 kOe范围内调控,更适合用于制作矫顽力高并可控的磁力显微镜针尖.     

Localized canting of spins structure in the spinel oxide system: ZnzTizFe2−x−zCrx−zCoO4

The spinel oxide system Zn_zTi_zFe_2−x−zCr_x−zCoO₄; z=x²; x=0.60, 0.65, 0.70 and 0.80, was studied using neutron diffraction technique, low field DC magnetization measurements (ZFC–FC measurements), magnetic hysterisis, Mössbauer spectroscopy and low field AC susceptibility measurements. All the compositions show significantly less B-site magnetic moments at 10 K temperature derived from neutron diffraction data than the free ions site moments deduced assuming collinear arrangement of spins. This combined with some other features seen in the low temperature neutron diffraction patterns suggest localized canting of spins (LCS) type of magnetic ordering in the present system where a long range order of longitudinal component of moments co-exists along with totally disordered transverse component of moments. The conclusion is also supported by the features seen in the other measurements. The magnetic moments derived from 10 K neutron diffraction data are explained using the LCS approach for different exchange integrals ratios.     

MFM probe control of magnetic vortex chirality in elliptical Co nanoparticles

Magnetic force microscopy (MFM) methods were applied to investigate the peculiarities of magnetization distribution in elliptical 400×600×27 nm Co particles. Reversible transitions between the uniform and vortex states under inhomogeneous magnetic field of MFM probe were observed. Possibility to control the chirality of a magnetic vortex in these particles by MFM probe manipulation was shown.     

Magnetic microstructures of a high coercivity Nd-Fe-B sintered magnet in remanent and incomplete thermal demagnetization states

Magnetic microstructures of a high coercivity Nd-Fe-B sintered magnet in remanent and incomplete thermal demagnetization states have been revealed by using magnetic force microscopy (MFM) with high coercivity tips. MFM results indicate that specimens in a remanent state are single domain and their magnetizations align with the direction of the magnetizing field. The evolution of the magnetic domains with annealing temperatures shows that the thermal demagnetization process consists of four stages. Nd-Fe-B should be heat-treated at about 120-170 °C to make its magnetic state stable before practical applications.     

Domain wall pinning in polycrystalline perovskite La0.7Sr0.3MnO3

Reversible and irreversible domain wall (DW) motions have been investigated in La_0.7Sr_0.3MnO₃ ceramic samples using frequency-response complex permeability with various amplitudes of AC field. We also examine the effects of temperature in the range from 293 to 368 K and transverse DC magnetic field with a maximum of 4.40×10⁵ A/m on the real part of permeability (μ′). Two relaxations corresponding to reversible wall motions and domain rotations occur in low and high frequency regions, respectively. The irreversible DW displacements can be activated as the amplitude larger than the pinning field of 3 A/m, leading to an increase in μ′. The μ′ obeys a Rayleigh law at the temperature below 343 K or under DC field of less than 4.22×10⁴ A/m. The Rayleigh constant η increases from 5.45×10⁻² to 1.54×10⁻¹ (A/m)⁻¹ as the temperature rises from 293 to 343 K, and η decreases from 5.58×10⁻² to 3.67×10⁻² (A/m)⁻¹ with increasing DC field from 1.99×10³ to 4.22×10⁴ A/m.     

A new measurement of the weak magnetism term in the betaspectra of B and N

The betaspectra of ¹²B and ¹²N have been measured with a NaI crystal as spectrometer. Assuming a shape correction factor 1 + aW + bW² and b₋ = 1.106 × 10⁻⁴ MeV⁻², b₊ = −1.397 × 10⁻⁴ MeV⁻², the spectra yield a₋ = (+0.91 ± 0.11) × 10⁻² MeV⁻¹ and a₊ = (−0.07 ± 0.09) × 10⁻² MeV⁻. the a₋ − a₊ = (+0.98 ± 0.09) × 10⁻² MeV⁻¹ is in agreement with the weak magnetism prediction.     

Magnetorheological suspension electromagnetic brake

The magnetorheological suspension (MRS) brake is of the monoblock type. The main part of the electromagnetic brake is an electromagnet, between whose poles two MRS disks are placed. For distances between disks of 0.65×10⁻³ m±10%, revolutions of the electric motor, coupled to the electromagnetic brake, ranging between 200 and 1600 rev/min and braking powers of up to 85 W, there are no differences in revolutions between the disks of the electromagnetic brake. For fixed revolutions of the electric motor, the revolution of the parallel disk can be modified continuously by means of the intensity of the magnetic field. In all cases, the quantity of MRS is of 0.35×10⁻³ kg.     

Thermal-expansion anomalies and spontaneous magnetostriction of Lu2Fe17−xSix intermetallic compounds

The thermal expansion of Lu₂Fe_17−xSi_x solid solutions has been measured by X-ray powder diffraction. The magnetic ordering in all compounds within the homogeneity range (x3.4) is accompanied by a large spontaneous volume magnetostriction, distributed anisotropically over the principal axes of the hexagonal crystal structure. The volume effect ω_s in the ground state reaches 14.7×10⁻³ in Lu₂Fe₁₇ and decreases monotonously to 8.9×10⁻³ for x=3.4, following the reduction of magnetic moment. Despite a still large ω_s, the Invar behavior observed in Lu₂Fe₁₇ changes to a positive thermal expansion for x>1 due to an increasing Curie temperature.     

Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin film tips. For example, with a Cobalt film of the thickness of 100 nm and 30 nm on a tip, we found a vertical force of 4*10^–10 N and 1.5*10^–10 N, respectively. The lateral force exerted on a tip by the flux line was also calculated. The lateral force must be small enough to prevent the flux line from becoming depinned.     

Magnetic moments of the nucleon octet in a relativistic light-cone quark model

The magnetic moments of the baryon octet are calculated in a relativistic constituent quark model formulated in the light-cone Fock approach. Invoking a natural idea of strangeness-dependent hadron size we find very good agreement (to an accuracy of 1σ) for the recent precision hyperon magnetic-moment data and reveal 10–15% discrepancy for the Σ⁻ and nucleon. It suggests pions as a missing ingredient in the baryon magnetic moment calculations.