Use of environmental magnetic measurements to characterize and correlate tephra-A case study in Iceland

A set of environmental magnetic parameters(i.e. magnetic susceptibility, xARM, IRMs, hysteresis loops and thermomagnetic curves) has been applied to two soil sections from SE Iceland. Results demonstrate that the main magnetic minerals in the tephras are ferrimagnetic minerals (e.g.magnetite) and canted antiferromagnetic minerals (e.g.haematite), with abundant paramagnetic material also present. Cross plots of Mrs/Ms vs. (B0)cr/(B0)c and xfd% vs.xARM/SIRM indicate that the main magnetic grain sizes in tephras are pseudo single domain (PSD) and multidomain(MD). Initial correlation of tephra layers was achieved, usingall the measured magnetic parameters, by use of the multi-variate statistical measures of Similarity Coefficient (SC) andEuclidean Distance (ED). This demonstrates that magnetictechniques can potentially assist in the identification andcorrelation of distal tephra.     

磁性参数的环境指示意义

介绍了岩石磁学研究中常用的能够指示环境变迁和气候变化的几种磁性参数及其组合。研究表明：磁化率是气候变化的代用指标之一，其值的高低反映气候的暖冷和沉积颗粒的细粗，用参数HSIRM，Bcr,HSIRM/k可以识别磁性矿物类型，较多磁铁矿的存在指示暖湿的沉积环境，反映沉积物颗粒细；而较多赤铁矿的存在则指示干冷的沉积环境，反映沉积物颗粒粗。HARM，Ber,HSIRM/k，S比(HSIRM-100mT/HSIRM)在环境监测中有重要的意义。     

Mineral magnetic properties of surface sediments at Bei’anhe, Beijing, and its environmental significance

A multiparameter investigation of surface sediments from the Bei＇anhe section （Beijing） has been conducted using environmental magnetic approaches. The studied sedimentary sequence consists of pluvial and lacustrine silts and clay of the Holocene age. Magnetite, hematite and pyrrhotite were identiffed as the dominant magnetic minerals. The mineral magnetic characteristics are considered to reflect the Holocene paleoenvironmental processes of the studied area, which are different from the mineral magnetic properties of typical polluted sections reported in the Beijing area. The low-field magnetic susceptibility and saturated isothermal remanence magnetization of the section decrease gradually with increasing depth. This indicates that the concentration of magnetic minerals in the strata decreases down the section. Values of interparametric ratios, such as ARM/SIRM, XARM/Xand SIRM/X, gradually increase with increasing depth, which indicates a down-section decreasing trend in the magnetic grain size of the sediments. Pyrrhotite occurs below the depth of 22 cm, suggesting the existence of redox interface. The magnetic variations of the Bei＇anhe surface sediments might be controlled by two geological processes： reductive diagenesis and pedogenesis. Our findings may have provided a reference point for magnetic investigation of polluted surface sediments in the Beijing area.     

Effects of the grain size distribution on magnetic properties of magnetite: constraints from micromagnetic modeling

Magnetite is the most important magnetic mineral in paleomagnetism. Its magnetic properties are controlled by many factors, such as grain size distribution, shape and interactions. Traditional rock magnetic experiments, however, have great difficulty in decoupling the effects of these parameters. In this study, we attempted to investigate the effects of grain size distribution on magnetic properties of magnetite powders by using a micromagnetic method. The particle geometries used in the micromagnetic model were based on the grain size distribution observed in a synthetic magnetite powder. The simulated hysteresis parameters agree well with the experimental measurements and provide clear microstructures of the magnetic remanence. Our results show that grain size plays a more important role in affecting hysteresis parameters of magnetite assemblages than shape under effects of interactions. Uniform or vortex superstates formed by two or more particles are found and display different stabilities of magnetic recording in assemblages. Some domain structures of single-domain （SD） particles are reversed as the applied field decreases to zero. Small pseudo-single-domain particles behave as SD structures and may dominate the magnetic recordings. In all, micromagnetic modeling of grain size distributions provide a better understanding of magnetic assemblages consisting of nanoscale particles.     

Magnetic properties of surface soils across the southern Tarim Basin and their relationship with climate and source materials

The magnetic susceptibility (χ) of surface soils on the Chinese Loess Plateau (CLP) has strong positive correlation with mean annual precipitation.It is widely accepted that ultrafine magnetite/maghemite grains produced during pedogenesis are responsible for the enhancement of χ,and loess χ on the CLP has been widely used as a proxy for the intensity of the East Asian summer monsoon.However,few works have investigated mechanisms for the enhancement of χ in the case of surface aeolian sediments in westerly-dominated inland China,north of the Tibetan Plateau.Here detailed rock magnetism and grain size studies of 49 surface samples taken across the southern Tarim Basin for different rainfall/temperature/altitude conditions are presented.The results show that samples taken from desert have the lowest χ values and that χ of loessic sand and loess samples decreases with increasing altitude.In addition,the rock magnetism studies suggest that the magnetic properties of surface soils across the southern Tarim Basin are mainly controlled by the concentration of coarse aeolian ferrimagnetic minerals.The contribution of ultrafine pedogenically produced magnetic grains to χ is very limited.     

Contrasting magnetic properties from two loess/paleosol sections in Gansu Province, west China

We made comparison of magnetic property between two loess/paleosol sediments from the Beiyuan section, Linxia and the Shajinping section, Lanzhou. The two sections apart only 100 km in NE-SW direction. At the Beiyuan section, enhancement of the low-field magnetic susceptibility, ARM susceptibility and SIRM are observed in paleosols. The positive correlations among these three parameters suggest the presence of two magnetic components, i.e. the background component and the enhanced component. The unblocking curves of low-temperature IRM give strong evidence for the idea of the two components. On the contrary, the magnetic parameters are almost constant throughout the Shajinping section. The enhanced component is not found in the Shajinping section. The magnetic properties of the background component are indistinguishable between the two sections. Contrasting magnetic properties imply the difference of soil formation process between the two sections.     

Susceptibility variations of multiple origins of loess from the Ily Basin (NW China)

The magnetic susceptibility of loess from the Ily Basin,northwestern China shows maximum values in S0 paleosols but minimum values in other paleosols,the mechanism of which has been well debated.In this work,systematic magnetic measurements were made on a representative section from Neleke county.The results show that the loess horizons(L1,L2 and L3) have multi-domain magnetite grains of aeolian origin,S0 is characterized by production of pedogenetic ultrafine-grained ferrimagnetic minerals,and the other paleosols(S1,S2,and S3) are characterized by the formation of nonferrimagnetic minerals associated with waterlogging.The correlation between the low concentration of ferrimagnets,high paramagnetic content,high magnetic coercivity remanence,fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols.Pedogenetic depletion dominates and is responsible for the low susceptibility.Changes in magnetic grain size distribution occur during pedogenetic depletion.The susceptibility variations are of multiple origins in the loess of the Ily Basin.Pedogenetic enhancement,pedogenetic depletion,and allochthonous input of magnetic minerals should all be taken into account to explain the variations of magnetic parameters.     

长江口外CX21柱样的磁性特征及其影响因素

以长江口外、长340 cm的CX21柱样为对象,在磁性特征、地球化学和粒度分析的基础上,结合前期对Y7柱样的研究,探讨粒度、早期成岩作用和物源变化对沉积物磁性特征的影响.研究结果表明,该柱样磁性特征和粒度垂向变化显著.磁性参数χ、SIRM、χ_fd和χ_fd%与 2~4 μm、4~8 μm粒级含量呈相对较弱的正相关, 而χ_ARM、χ_ARM/SIRM和χ_ARM/χ与16 μm以下的各细颗粒组分有较强的正相关,表明CX21柱样沉积物磁性特征受到粒度变化的影响.利用BCR方法进行的铁的形态分析结果表明,CX21柱样170 cm深度以下具有较强的还原环境,磁性矿物受到较强的早期成岩作用影响.氧化还原不敏感元素Ti的垂向变化指示了物源变化,Ti向表层的增加与χ和SIRM向表层的增强具有同步性,说明物源变化也是CX21沉积物磁性特征影响的重要因素.尽管粒度组成和沉积速率不同,CX21与Y7柱样的磁性特征在垂向上具有相似性,表明磁性特征可作为长江口外沉积物地层对比的工具之一.     

Geomagnetic anomalies recorded in L9 of the Songjiadian loess section in southeastern Chinese Loess Plateau

Detailed magnetostratigraphic and rock magnetic investigations on L8-S12 of the Songjiadian loess section in the Sanmenxia area,southernmost margin of the Chinese Loess Plateau were conducted in this study.Matuyama/Brunhes (M/B) boundary occurred at the bottom of the loess unit L8.The top and bottom boundaries of the Jaramillo polarity subchron are found in the middle of L10 and the bottom of L12,respectively.Magnetic fabric of the loess layers maintains the original depositional features and the recorded r...     

Magnetic properties of highroad-side pine tree leaves in Beijing and their environmental significance

This is a report on magnetic properties of highroad-side tree (Pinus pumila Regel) leaves collected along an expressway linking Beijing City and the Capital International Airport and further focus on their environmental contributions. A series of rock magnetic experiments show that the primary mag-netic mineral of leaf samples was identified to be magnetite,in the pseudo-single domain (PSD) grain size range 0.2―5.0 μm. On the other hand,magnet-ite concentration and grain size in leaves are ascer-tained to decrease with increasing of sampling dis-tance to highroad asphalt surface,suggesting that high magnetic response to traffic pollution is localized within a distance of some two meters away from highroad asphalt surface. Although magnetic suscep-tibility is ordinarily regarded as a simple,rapid and low-cost method for monitoring traffic pollution,saturation isothermal remanent magnetization (SIRM) can be treated as a valid proxy for monitoring air par-ticulate matter (PM) when samples are magnetically weak. It is believed that a synthetic rock magnetic study is an effective method for determining concen-tration and grain size of ferromagnets in the atmos-pheric PM,and then it should be a rapid and feasible technique for monitoring atmospheric pollution.     

Fe/SiO2纳米颗粒的相互作用和磁滞行为

文章研究了用机械合金（MA）法制备的Fe／SiO2纳米颗粒系统，应用退磁曲线分析方法，由直流退磁剩磁曲线（DCD）和等温剩余磁化曲线（IRM）得到的△M图和转变场分布（sFD）显示增加Fe含量将导致更高偶极场和大偶极耦合区域。在不考虑Fe含量的影响或在逾渗以下时,转换机制是基于个别颗粒翻转而激发的雪崩现象。     

Effect of Dy substitution on the microstructure and magnetic properties of nanograin Nd-Fe-B single-phase alloys

Nanocrystalline single-phase alloys with the nominal compositions (at%) of Nd_12.3-xDy_xFe_79.7Zr_0.8Nb_0.8Cu_0.4B_6.0 (x=0, 0.5, 1.5, and 2.5) were prepared by melt-spinning and subsequent annealing. X-ray diffraction analysis shows that the as-spun ribbons were mainly composed of the amorphous phase. A slight content of Dy stabilizes the amorphous phase during annealing treatment. The grain size becomes smaller and the coercivity of the annealed ribbon is gradually improved with the increase of Dy content. Excessive Dy is harmful to the remanence. It is found that no intergranular phase exists between the grains by high-resolution transmission electron microscopy, and the grain boundaries are crystallographically coherent in the optimally annealed sample. The optimum magnetic properties of remanence (J_r=1.09 T), coercivity (H_ci=1048 kA/m), and maximum magnetic energy product ((BH)_m=169.5 kJ/m3) are obtained from the x=0.5 ribbon in a post heat-treated state (700℃, 10 min).     

磁场环境下工作电压对线板式静电除尘器中同种颗粒除尘效率的影响

为了提升线板式静电除尘器(electrostatic precipitator,ESP)的除尘性能,利用Gambit软件构建了ESP网格模型,结合磁场能降低颗粒逃逸率的机理,将磁场引入到ESP中,模拟了同种颗粒的运动轨迹,计算并对比了不同工作电压下的除尘效率。结果表明:磁场的引入可有效提升线板式ESP对同种颗粒的捕集性能,且在低工作电压时更为显著;同一磁场环境下,工作电压对颗粒除尘效率的提升能力由强变弱;随着磁感应强度的增大,颗粒逃逸的数量不断减少,但磁场的相对贡献逐步减小。     

Fe54Pt46纳米薄膜的结构与磁性

采用磁控溅射法在SiO₂〈0001〉基片上制备Fe₅₄Pt₄₆薄膜, 并经过650 ℃原位真空退火1 h. 利用X射线衍射和振动样品磁强计对样品的结构和磁性进行研究. 结果表明, 经过原位真空退火后, 样品发生了无序\|有序相转变, 得到了面心四方结构的L1₀-FePt纳米薄膜. 样品的有序度为0.92, 晶粒尺寸约为10 nm, 样品平行膜面的矫顽力为0.8 MA/m. 当磁场与膜面的夹角为50°时, 矫顽力达到最大值, 这与(111)面为能量最低面相符.      

超快冷下X65管线钢微观组织演变及强化机制分析

基于热连轧生产线开发了X65管线钢超快冷新工艺，系统表征了该工艺下实验钢的微观组织特征，并进一步讨论了其强化机制.结果表明，超快冷下X65管线钢微观组织为细小针状铁素体(AF)+准多边形铁素体(QPF)+M/A岛+弱化珠光体(DP)混合组织，有效晶粒尺寸为2.93μm，大角晶界百分比为31.5%;实验钢组织亚结构为细小的块状铁素体，铁素体尺寸分布在200~1000nm;在铁素体基体上析出了大量尺寸＜10nm的Nb(C，N)粒子;实验钢各项力学性能均满足API SPEC 5L标准要求.超快冷工艺下X65管线钢的主要强化机制为细晶强化、固溶强化、位错强化及纳米析出强化的耦合强化，其中纳米析出强化强度贡献值为96.1MPa.     

Effects of the grain alignment on the coercivity and its angular dependence for NdCoFeB permanent magnets

The dependence of the macroscopic magnetism for NdCoFeB permanent magnet on the degree of grain alignment has been studied. With an increase in grain alignment degree, the coercivity of the permanent magnet decreases and the remanence increases. The decrease rate of the coercivity is smaller than the increase rate of the remanence. The coercivity H c(θ) increases with increasing angle θ between the applied field and the texture axis, and for the permanent magnets with good grain alignment the increase rate of H c(θ) is large. These experimental results can be interpreted by using the starting field theory of coercivity.     

Lamellar magnetism in the haematite-ilmenite series as an explanation for strong remanent magnetization

Magnetic anomalies associated with slowly cooled igneous and metamorphic rocks are commonly attributed to the presence of the mineral magnetite. Although the intermediate members of the ilmenite-haematite mineral series can also carry a strong ferrimagnetic remanence, it is preserved only in rapidly cooled volcanic rocks, where formation of intergrowths of weakly magnetic haematite and paramagnetic ilmenite is suppressed. But the occurrence of unusually large and stable magnetic remanence in rocks containing such intergrowths has been known for decades, and has recently been the subject of intense investigation. These unmixed oxide phases have been shown to contain pervasive exsolution lamellae with thickness from 100 microm down to about 1 nm (one unit cell). These rocks, many of which contain only a few per cent of such oxides, show natural remanent magnetizations up to 30 A m(-1) --too strong to be explained even by pure haematite in an unsaturated state. Here we propose a new ferrimagnetic substructure created by ferrous-ferric 'contact layers' that reduce charge imbalance along lamellar contacts between antiferromagnetic haematite and paramagnetic ilmenite. We estimate that such a lamellar magnetic material can have a saturation magnetization up to 55 kA m(-1) --22 times stronger than pure haematite-- while retaining the high coercivity and thermal properties of single-domain haematite.     

放电等离子烧结热变形Nd-Fe-B磁体的磁性能研究

采用放电等离子烧结技术,利用不同速率的快淬薄带制备出各向异性的热变形Nd-Fe-B磁体,运用振动样品磁强计和扫描电子显微镜对热变形磁体的磁性能和微观结构进行研究.结果表明：随着快淬薄带速率的增加,获得最佳磁性能的热变形温度也逐渐增加,三类热变形Nd-Fe-B磁体获得最佳磁性能的热变形温度分别为650,680和700°C;磁体最佳磁性能中的剩磁和最大磁能积随着快淬薄带速率的增加而降低,而内禀矫顽力却略有增加.磁体的晶粒尺寸随着热变形温度的增加而增大;相同热变形温度下,磁体的晶粒尺寸随快淬速率的增加而减小.     

Magnetic properties of different grain-sized particles of sediments from the Okinawa Trough and their relationships to sedimentary environment

Multiple magnetic parameters were measured for nine different grain-sized fractions separated from the sediment samples that are representatives of four different sedimentary environments of the Okinawa Trough. Based on the measured results, the contributions of different grain-sized particles to total magnetic susceptibility of bulk sediments, the magnetic mineral assemblage and magnetic domain state as well as their relationships to sedimentary environment were discussed. Our research shows that the magnetic mineral is dominated by magnetite with a small amount of hematite and is primarily in pseudo-single-domain state. That indicates that the different sedimentary environments in the Okinawa Trough have certain correlation in material provenance. The magnetic minerals enrich in different grain-sized particles in response to different sedimentary environments. The contribution of the grain sizes from coarse to fine to coarse and fine to the magnetic susceptibility from the west to the east is in accordance with terrigenous material transportation from continental shelf of the East China Sea to the Okinawa Trough. It also shows difference in magnetic properties as a result of some environmental factors.     

Pedogenesis affecting the Matuyama-Brunhes polarity transition recorded in Chinese loess?

A detailed record of the Matuyama-Brunhes (M-B) transition has been obtained from the loess unit 8 (L8) at Duanjiapo (34.2°N, 109.2°E),Shannxi Province of China. An investigation of the rock magnetic properties using hysteresis loops, thermomagnetic analyses identifies pseudo- single domain magnetite as the main carrier of the remanence, with a small contribution from maghemite and hematite. The paleo-direction records obtained reveal: (ⅰ) The M-B transition was recorded in the middle and lower part of L8, and comprises of five fast reversals. (ⅱ) The duration of the M-B polarity transition related to the directional change is about 4800 a. (ⅲ) The virtual geomagnetic pole (VGP) path during the transition is confined over Africa, peaked 90° away from the sampling site, in contrast with the results obtained from the Weinan loess section. The different VGPs are probably attributed to the pedogenesis.