EPR and Magnetization Studies of Polymer-Derived Fe-Doped SiCN Nanoceramics Annealed at Various Temperatures: Blocking Temperature,Superparamagnetism and Size Distributions

X-band EPR spectra on SiCN ceramics, doped with Fe(III) ions, annealed at 800 °C, 1000 °C, 1100 °C, 1285 °C, and 1400 °C have been simulated to understand better their magnetic properties, accompanied by new magnetization measurements in the temperature range of 5–400 K for zero-field cooling (ZFC) and field cooling (FC) at 100C. The EPR spectra reveal the presence of several kinds of Fe-containing nanoparticles with different magnetic properties. The maxima of the temperature variation of ZFC magnetization were exploited to estimate (i) the blocking temperature, which decreased with annealing temperature of the samples and (ii) the distribution of the size of Fe-containing nanoparticles in the various samples, which was found to become more uniform with increasing annealing temperature, implying that more homogenous magnetic SiCN/Fe composites can be fabricated by annealing at even higher temperatures than 1400 °C to be used as sensors. The hysteresis curves showed different behaviors above (superparamagnetic), below (ferromagnetic), and about (butterfly shape) the respective average blocking temperatures, 〈T_B〉. An analysis of the coercive field dependence upon temperature reveals that it follows Stoner–Wohlfarth model for the SiCN/Fe samples annealed above 1100 °C, from which the blocking temperatures was also deduced.     

Sol-gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis

Nanoparticles of Zn substituted lithium ferrite (Li_0.32Zn_0.36Fe_2.32O₄) have been prepared by a sol-gel method where the ultra-sonication technique has been adopted to reduce the agglomeration effect among the nanoparticles. The samples were heat-treated at three different temperatures and the formation of the nanocrystalline phase was confirmed by X-ray diffractograms (XRD). The average particle size of each sample has been estimated from the (311) peak of the XRD pattern using the Debye-Scherrer formula and the average sizes are in the range of 10-21 nm. The average particle size, crystallographic phase, etc. of some selected samples obtained from the high-resolution transmission electron microscopy are in agreement with those estimated from the XRD patterns. Static magnetic measurements viz., hysteresis loops, field cooled and zero field cooled magnetization versus temperature curves of some samples carried out by SQUID in the temperature range of 300 to 5 K clearly indicate the presence of superparamagnetic (SPM) relaxation of the nanoparticles in the samples. The maximum magnetization of the SPM sample annealed at 500 °C is quite high (68 Am²/Kg) and the hysteresis loops are almost square shaped with very low value of coercive field at room temperature (827.8 A/m). The particle size, magneto-crystalline anisotropy, etc. have been estimated from the detailed theoretical analysis of the static magnetic data. The dynamic magnetic behavior of the samples was also investigated by observing the ac hysteresis loops and magnetization versus field curves with different time windows at room temperatures. The different soft magnetic quantities viz., coercive field, magnetization, remanance, hysteresis losses, etc. were extracted from dynamic measurements. Dynamic measurements confirmed that the samples are in their mixed state of SPM and ordered ferrimagnetic particles, which is in good agreement with the results of static magnetic measurements. Mössbauer spectra of the samples recorded at room temperature (300 K) and at different temperatures down to 20 K confirmed the presence of the SPM relaxation of the nanoparticles of the samples.     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Die Temperaturabhängigkeit des magnetischen Barkhausen-Effekts. I

The apparatus described allows to measure the size distribution of Barkhausen discontinuities with a magnetic moment greater than 10^?6 e.m.u. in a temperatur range from liquid air up to the Curie point. We used the counting method, developed byTebble and his co-workers, in connection with a multichannel-analyzer. Nickel samples of different purity and of different thermal treatment have been investigated. The results are as follows: 1. The size distribution of two samples, having in practice the same hysteresis loop can show a completely different behaviour as a function of the magnetic field. 2. The number of great discontinuities is reduced more rapid with rising temperature than that of the smaller ones; thus leading to the conclusion, that the Barkhausen component of total magnetization is vanishing much stronger with increasing temperature than that of differential susceptibility. 3. The critical field strength, being characterized by a maximum in the number of discontinuities per unit field, shows the same temperature dependence as the coercive force. This critical field is only slightly dependent on the size of jumps. 4. The “average” magnetic moment of the discontinuities in the measured range of size appears to vary less with temperature than the spontaneous magnetization. 5. Above a temperature of about 270°C spontaneous jumps will be produced, even in a magnetic hard sample, by diminutive vibrations, such as speaking loud or coughing. 6. The Barkhausen part of total magnetization varies with temperature like the coercive force and therefore seems to be a structure-dependent quantity. The results of our measurements are discussed in connection with those of other authors.     

Training effect in an exchange bias system: the role of interfacial domain walls

Polarized neutron reflectivity (PNR) is used to obtain the magnetic depth profile of an antiferromagnetically coupled ferrimagnetic/ferrimagnetic bilayer, Gd40Fe60/Tb12Fe88. This system shows a transition from positive to negative exchange bias field H(E) as the cooling field H(cf) is increased from small to large positive value. It also exhibits training behavior upon field cycling which affects H(E) and the coercive field H(C). From the PNR measurements at room temperature and at 15 K, we confirm that the magnetic configuration inside the TbFe layer is frozen when the sample is cooled in various H(cf). The thickness and pitch of the magnetic twist inside the TbFe layer depend on H(cf) and give rise to the observed differences in the bias field. Irreversible reorganization of the TbFe magnetization at the interface occurs upon GdFe magnetization reversal and is found to explain the training effect as well as the overall reduction in coercivity.     

GdFeCo磁光薄膜中RE-TM反铁磁耦合与激光感应超快磁化翻转动力学研究

使用飞秒时间分辨抽运-探测磁光克尔光谱技术,研究了激光加热GdFeCo磁光薄膜跨越铁磁补偿温度时稀土-过渡金属(RE-TM)反铁磁交换耦合行为和超快磁化翻转动力学. 实验观察到由于跨越铁磁补偿温度、净磁矩携带者交换而引起的磁化翻转反常克尔磁滞回线以及在同向外磁场下,反常回线上大于和小于矫顽力部分的饱和磁化强度不同,显示出GdFeCo中RE与TM之间的非完全刚性反铁磁耦合. 在含有Al导热底层的GdFeCo薄膜上观测到饱和磁场下激光感应磁化态翻转及再恢复的完整超快动力学过程. 与剩磁态的激光感应超快退磁化过 关键词： 补偿温度 磁化翻转 反铁磁耦合 GdFeCo     

一级相变磁制冷材料的基础问题探究

由于一级相变磁制冷材料发生磁相变时有晶胞体积的突变,相变过程中有相变潜热存在,其磁化过程中有许多磁学问题有待于进一步探究.本文以LaFe13-xSix合金为研究对象,在现有对磁一级相变基础问题的分析基础上,对一级相变材料中系统熵变、等温熵变、绝热温变、热滞、磁滞、铁磁与顺磁态两相共存的温度区间和磁场区间、制冷能力的计算等磁学基础问题进行了较为细致的探究.分析表明,在忽略完全铁磁态和顺磁态对磁热效应的贡献时,Maxwell方程和Clausius-Clapeyron方程计算熵变的值具有等效性.等温磁化过程中升温和降温曲线包围的面积SABCE(磁滞的大小),实际上是升温过程和降温过程中磁场做的净功,等于相变潜热之差.磁滞和热滞的大小与磁化过程数据测量的时间有关,测量时间越长则滞后越小,当相变是平衡相变则滞后为零.另外,对温度和磁场诱导磁相变过程进行了分析,提出了一级相变磁制冷材料制冷能力的不同计算模型.本文对一级相变磁制冷材料的磁学基础问题研究有一定的参考价值.     

Coercive field vs. temperature in Fe/Ag nanogranular films

We investigate the temperature dependence of the coercive field of a nanogranular co-sputtered Fe/Ag film. The sample is superparamagnetic at room temperature and displays a 3% giant magnetoresistance. As temperature decreases from 40 to 9 K, hysteresis appears in magnetization loops. The coercive field dependence on temperature turns out to be different from the power law expected for single-domain monodispersed ferromagnetic particle ensembles. The results are analyzed in terms of a model taking into account the effect of magnetic grain size distribution on coercive field. The agreement between experiment and the model is excellent. The possibility to gain information concerning width and symmetry of the particles volume distribution from the study of coercivity's scaling with temperature is discussed.     

Observation of reversed hysteresis loops and negative coercivity in CoFeAlO magnetic thin films

The magnetic and magneto-optical properties of CoFeAlO thin films fabricated by the RF magnetron reactive sputtering technique have been studied via vibrating sample magnetometer and the magneto-optical Kerr effect. It is found that, along the hard magnetization direction, the magnetic hysteresis loop is reversed and the coercive force is negative. This phenomenon can qualitatively be interpreted within the framework of the proposed two-layers model. It was also found that when as-deposited CoFeAlO thin films were annealed in vacuum (2×10⁻⁷ Torr) at a temperature of 180 °C for 2 h, the negative coercivity disappeared. The relationship between the magnetic and microstructural behaviors of the thin films is discussed.     

Magnetic domain and local atomic structures of the Fe<Subscript>0.94</Subscript>Si<Subscript>0.06</Subscript> alloy before and after thermomagnetic treatment in an alternating-current magnetic field

The changes observed in the local atomic structure of an iron alloy with 6 at % Si as a result of thermomagnetic treatment in an alternating-current magnetic field and their correlation with magnetic characteristics, such as the domain structure and magnetic properties, have been investigated using Mössbauer spectroscopy. It has been shown that the destabilization of the domain structure is caused by the destruction of uniaxial magnetic anisotropy in domains due to a disordered distribution of pairs of silicon atoms along the 〈100〉 easy magnetization axes. Annealing and cooling without an external magnetic field and in an alternating-current magnetic field lead to a change in the magnetic texture of single-crystal samples of the siliconiron alloy. When the samples are annealed in an alternating-current magnetic field, the magnitude of this effect depends on the orientation of the magnetic field with respect to the crystallographic axes of the sample.     

Room-temperature ferromagnetism in Zn1−xMnxO

In view of recent controversies on above room-temperature ferromagnetism (RTFM) in transition-metal-doped ZnO, the present paper aims to shed some light on the origin of ferromagnetism by investigating annealing effects on structure and magnetism for polycrystalline Zn_1−xMn_xO powder samples prepared by solid-state reaction method and annealed in air at different temperatures. Magnetic measurements indicate that the samples are ferromagnetic at room temperature (RTFM). Room temperature ferromagnetism has been observed in the sample annealed at a low temperature of 500 °C with a saturated magnetization (M_s) of 0.159 emu/g and a coercive force of 89 Oe. A reduction in RTFM is clearly observed in the sample annealed at 600 °C. Furthermore, the saturation magnetic moment decreases with an increase in grain size, suggesting that ferromagnetism is due to defects and/or oxygen vacancy confined to the surface of the grains. The experimental results indicate that the ferromagnetism observed in Zn_1−xMn_xO samples is intrinsic rather than associated with secondary phases.     

Observation of Persistent Currents in Finely Dispersed Pyrolytic Graphite

The trapped magnetic flux in the finely ground pyrolytic graphite sample annealed at 670 K in air has been observed. Flux trapping occurs on cooling of the sample from room temperature to 10 K in a magnetic field of 1 T. The magnitude and sign of the induced trapped moment remain unchanged when the applied magnetic field is varied within ±1 T at T K. The trapped magnetic flux is manifested in the displacement of the magnetization curve relative to that of the sample cooled in zero field. Displacement magnitude gradually decreases with the temperature increase up to 350 K, not reaching zero. The set of experimental observations probably reflects the presence in the sample of a granular high-temperature superconducting phase.     

Effects of the trimodal random field on the magnetic properties of a spin-1 Ising nanotube

In this work, the hysteresis behavior of a nanotube, consisting of a ferromagnetic core of spin-1 atoms surrounded by a ferromagnetic shell of spin-1 atoms with ferro-or anti-ferromagnetic interracial coupling is studied in the presence of a random magnetic field. Based on a probability distribution method, the effective-field theory has been used to investigate the effects of the random magnetic field, the interfacial coupling constant, and the temperature on the hysteresis loops of the nanotube. Some characteristic behaviors have been found, such as the existence of double or triple hysteresis loops for appropriate values of the system parameters. The remanent magnetization and the coercive field, as functions of the temperature, are examined.     

Magnetic properties of CoFeB alloys doped with Dy and Pr

Powders of the ferrimagnetic alloy CoFeBDyPr have been synthesized by the gas atomization method. Annealing of the samples at 900°C leads to the recrystallization of the amorphous phase into the crystalline phase (Pr,Dy)₂Fe₇Co₇B, which changes the magnetic properties of the alloy (the coercive force decreases, the shape of the magnetic hysteresis loop changes, etc.). The temperature dependences of the remanent magnetization and Mössbauer spectra depend on the size of particles. This indicates the contribution from subsurface magnetic states of the phase of different chemical composition, which is identified by X-ray photoelectron spectroscopy in particles with sizes of less than 10 μm. A weakening of the dipole-dipole interaction between particles in the sample, where they are diluted in the solid organic matrix (eicosane), leads to a change in the shape of the hysteresis loop. The elimination of the mechanical mobility of particles in eicosane significantly increases (to 2.5 times) the coercive force.     

Characteristics of magnetism in nanoporous carbon with palladium clusters

The magnetic properties of samples of nanoporous carbon with palladium clusters prepared from polycrystalline SiC have been studied over a wide range of temperatures. The specific magnetic moment rapidly decreases with increasing temperature. The obtained experimental magnetization curves have a nonlinear character but do not reach saturation even in a field of 10 kOe. Being constructed as functions of H/T, they lie on a universal curve, which suggests superparamagnetism of the system. The experimental curves have a hysteresis. The coercive force, first, decreases with increasing temperature and, then (in the range of 5–20 K), increases, thus forming a notch singularity. On the contrary, the residual magnetization rapidly decreases with increasing temperature. A conclusion on the prevailing contribution of superparamagnetic clusters to the magnetic subsystem is made.     

Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.     

Kerr-effect magnetometry of the single monolayer Co/Cu(001)

Experimental evidence is given for a four-fold in-plane magnetic anisotropy of the p(1×1) Co monolayer epitaxially grown on a Cu(001) substrate. Temperature dependent hysteresis curves show the magnetization remaining almost constant up to 400 K and the coercive field drastically increasing as the temperature is decreased.     

Magnetic properties of field-annealed FeCo thin films

Fe₅₀Co₅₀ thin films with thickness of 30 and 4 nm have been produced by rf sputtering on glass substrates, and their surface has been observed with atomic force microscopy (AFM) and magnetic force microscopy (MFM); MFM images reveal a non-null component of the magnetization perpendicular to the film plane. Selected samples have been annealed in vacuum at temperatures of 300 and 350 °C for times between 20 and 120 min, under a static magnetic field of 100 Oe. DC hysteresis loops have been measured with an alternating gradient force magnetometer (AGFM) along the direction of the field applied during annealing and orthogonally to it. Samples with a thickness of 4 nm display lower coercive fields with respect to the 30 nm thick ones. Longer annealing times affect the development of a harder magnetic phase more oriented off the film plane. The field applied during annealing induces a moderate magnetic anisotropy only on 30 nm thick films.     

MnZnFe nanoparticles for self-controlled magnetic hyperthermia

Manganese zinc iron magnetic nanoparticles were synthesized by a co-precipitation method for application as hyperthermia inducing agents. The structure, morphology and magnetic properties of the nanoparticles are characterized using scanning electron microscopy, X-ray diffraction, and a superconducting quantum interference device. The magnetic properties being investigated include Curie temperature, saturation magnetization, remnant magnetization, coercive field, and hysteresis. The study showed that adjusting the Mn contribution to the particles contributed to the adjustment of all magnetic properties of the complex.     

Remanence of the interparticle interactions and its influence on the microwave absorption in Co-ferrite

Cobalt ferrite nanoparticles were obtained by the sol–gel method at several annealing temperatures: 400, 450, 500, 550 and 600 °C. X-ray diffraction (XRD) and transmission electron microscopy (TEM) showed the formation of the spinel phase with a nanoparticle size in the 17–26 nm range as function of the annealing temperature. The Mössbauer spectra at room temperature showed the presence of a partial inverse spinel structure. Saturation magnetization and the coercive field are strongly dependent on the annealing temperature and they can be associated with variations of the nanoparticles size. Microwave power absorption (MPA) (dP/dH) measurements were carried out as a function of DC field (H_DC) in asymmetric sweeps in the 0 kOeH_DC9 kOe range, at X-band (9.4 GHz), for all annealing temperatures. The large hysteresis in the MPA is due to interparticle interaction associated with its demagnetizing-like nature.