On switching behavior of ferrite substrate with magnetostatic and spin wave exchange term

Due to the generation of magnetostatic and spin waves, switching phenomena of ferrite substrate with a normal magnetic biasing field is presented. Generally below the X-band of microwave range, the Pozar’s quasi TEM waves (extraordinary waves) were studied, but for the study of X-band there should be an inclusion of spin wave exchange term (ωr) in the magnetostatic wave analysis which depends upon the static internal field (Hex). This term is included in analysis because the wavelength of microwave approach is the inter-atomic distance of ferrite material. In this work we synthesize LiTi ferrite through Solid State Reaction Technique (SSRT) and obtained electric and magnetic properties for the analysis. Absorbing and transmission power coefficients have been calculated to obtain the power loss and transmitted power through the substrate, respectively. The absorbing power coefficient verifies the switching behavior of substrate for certain range of applied external magnetic field (H _o) which depends on the resonance line width parameter (ΔH) of ferrite material.     

Switching field dependence on heating pulse duration in thermally assisted magnetic random access memories

The minimum applied field H_SW required to reverse the magnetic moment of the ferromagnetic/antiferromagnetic storage layer of a thermally assisted magnetic random access memory (TA-MRAM) device during the application of a heating electric pulse is investigated as a function of pulse power P_HP and duration δ. For the same power of the heating pulse P_HP (or, equivalently, for the same temperature of the storage layer), H_SW increases with decreasing heating time δ. This behavior is consistently interpreted by a thermally activated propagating domain-wall switching model, corroborated by a real-time study of switching. The increase of H_SW with decreasing pulse width introduces a constraint for the minimum power consumption of a TA-MRAM where writing combines heating and magnetic field application.     

FDTD Analysis of EW Wave Circulating by a Magnetized Ferrite Body in Free Space

An efficient numerical method has been devised for the study of wave circulating by a magnetised ferrite body of an arbitrary shape. It is a finite-difference time-domain formulation that incorporates Mur's absorbing boundary conditions and a perfectly matched layer. Several shapes of interest have been studied, including spheres, circular cylinders. The electromagnetic fields inside ferrite and the power-density distribution on the ferrite's surface normal to the bias external magnetic field are obtained. It is observed that an incident plane wave would circulate around the magnetised ferrite body in an open space as if the ferrite were placed inside a waveguide junction circulator.     

磁化铁氧体电磁散射的移位算子FDTD分析

将移位算子（shift operator,SO）-时域有限差分（fintie difference time domain,FDTD）方法推广到外磁场沿任意方向时磁化铁氧体的情形.从磁化铁氧体的相对磁导系数张量和各向异性情形下Maxwell旋度方程出发,推导出外磁场沿任意方向情形下磁化铁氧体电磁散射的SO-FDTD迭代公式.应用该方法计算了磁化铁氧体层、磁化铁氧体球及含磁化铁氧体涂层的Von Karman形雷达罩电磁散射,数值计算结果表明了该算法的正确有效性. 关键词： 磁化铁氧体 FDTD 移位算子法     

Classical dependence of the domain wall velocity on the magnetic field in garnet ferrite films with orthorhombic magnetic anisotropy

The dependence of the domain wall velocity V on the acting magnetic field H is investigated for bismuth-containing single-crystal garnet ferrite films with orthorhombic magnetic anisotropy. It is shown that this dependence includes both the initial linear portion and a saturation portion and exhibits a complex behavior. This behavior is explained within the model of domain wall motion with spin wave radiation.     

Time-Domain Analysis of Millimeter Wave Circulation Around a Magnetised Ferrite Sphere

An efficient numerical method has been devised for the study of wave circulated by a magnetised ferrite sphere. It is a finite-difference time-domain formulation that incorporates Mur's absorbing boundary conditions and a perfectly matched layer. The electromagnetic fields inside the ferrite body are calculated using special updating equations derived from the equation of motion of the magnetization vector and Maxwell's curl equations in consistency. The electromagnetic fields inside ferrite and the power-density distribution on the ferrite's surface normal to the bias external magnetic field are obtained in a wide frequency band with a single time domain run. It is observed that an incident plane wave would circulate around the magnetised ferrite body in an open space as if the ferrite were placed inside a waveguide / microstrip junction circulators.     

Angular dependences of the switching field and coercivity for magnetic multilayer films with strain caused by lattice mismatch

The angular dependences of the switching field and coercivity for magnetic multilayer films with strain caused by lattice mismatch are studied in detail with the help of the Stoner-Wohlfarth model. The expressions or equations for determining the switching field and the coercivity are obtained analytically, taking the strain effect into consideration. With these analytical expressions, the roles of the strain and the unidirectional anisotropy (exchange coupling) are displayed clearly. Compared to the standard Stoner-Wohlfarth model (without strain), the centers of the curves of the switching field and coercivity are found to shift from π/2 to π/2+θ^∗ due to the in-plane strain caused by the lattice mismatch between the film and substrate. For the ferromagnetic/antiferromagnetic exchange bias systems, the angularly dependent amplitude of the switching field or coercivity exhibits an obvious asymmetry. The staggering or separation of the peaks of the two branches of the coercivity results in a jump phenomenon in the curves for the exchange bias field and the coercivity near θ^∗.     

X波段左手材料高功率微波天线罩实验研究

为改善高功率微波天线的性能,设计了一种新型X波段左手材料高功率微波天线罩,并对其进行了实验冷测研究。结果表明,加载一到四层左手材料天线罩均可在一定频率范围内汇聚波束和提高增益,其中加载一层左手材料天线罩时,天线增益提高最大,在频率为9.32 GHz时增益提高最大幅度约为9.64 dB。将实验结果与理论仿真进行对比发现,两者并没有吻合一致。分析了天线罩的功率容量,可以实现高功率微波的应用。     

Individual bit island reversal and switching field distribution in perpendicular magnetic bit patterned media

The switching of single bit magnetic islands in bit patterned media (BPM) for two cases with 10 times difference in coercivity, as well as the switching field distribution (SFD) of the islands, has been studied using magnetic force microscopy (MFM) measurements. The intrinsic SFD is measured to be ∼9-11% of the remanence coercivity (H_cr), which contributes only ∼20-50% of the total SFD broadening (∼23-41% of H_cr). High resolution MFM observations clearly showed the influence of surrounding islands on the switching behaviour and switching fields of individual bit islands, resulting in significant contributions in SFD broadening due to non-intrinsic dipolar interactions. It was further observed that single magnetic islands could be switched within a very narrow switching field range as small as 4 Oe, which indicates very sharp and uniform switching for each individual island of BPM.     

隧穿磁电阻效应磁场传感器中低频噪声的测量与研究

基于隧穿磁电阻效应(TMR)的磁场传感器具有很高的磁场灵敏度, 但同时噪声也较大,有效抑制TMR磁场传感器的噪声, 尤其是低频噪声的抑制对于其在高灵敏度要求场合的应用具有重要的意义. 本文采用高精度数据采集卡搭建了噪声测量系统, 测量了全桥结构TMR磁场传感器的噪声频谱图, 发现TMR传感器的噪声在低频段表现为1/f特性, 同时噪声功率谱密度与工作电流平方成正比关系; 低频噪声在自由层翻转区间内噪声急剧增大, 证明了1/f噪声主要来源于磁噪声, 这一结果为TMR磁场传感器的噪声特性优化指明了方向.     

Generation of an external magnetic field with the spin orientation effect in a single layer Ising nanographene

In this work, the magnetic properties of the single layer Ising nanogaphene (SLING) are investigated by using Kaneyoshi approach (KA) within the effective field theory for different spin orientations of its magnetic atoms. We find that the magnetizations of the SLING has no phase transition, certain Curie temperature and distinct peak of susceptibility at T_c for the some spin orientations at the zero external magnetic field (H=0.0). Because these behaviors occur at H≠0.0, we suggest that the SLING generates an external magnetic field and behaves as an external magnetic field generator for these spin orientations. However, the SLING exhibits ferromagnetic behaviors for only one spin orientations. But, it exhibits antiferromagnetic behaviors for the others. For the AFM cases, diamagnetic susceptibility behaviors and type II superconductivity hysteresis behaviors are obtained. We hope that these results can open a door to obtain new class of single layer graphene and graphene-based magnetic field generator devices with the spin orientation effect.     

Compensation of nonlinear phase noise in an in-plane-magnetized anisotropic spin-torque oscillator

A theory of generation linewidth of a spin-torque oscillator (STO) based on an in-plane-magnetized nano-pillar with an anisotropic “free” magnetic layer has been developed. It is predicted that by choosing the direction of the in-plane bias magnetic field H₀ along the “hard” anisotropy axis of the STO “free” layer and the magnitude of this field to be four times larger than the anisotropy field H_A (H₀=4H_A) it would be possible to compensate the nonlinear phase noise and to achieve the minimum value of the generation linewidth, characteristic for an auto-oscillator without a nonlinear frequency shift.     

The effects of Co-metal clusters on exchange bias for Co-doped NiO/FeNi bilayers

Co-doped NiO inhomogeneous films were synthesized by sputtering metallic Co chips and NiO together and the exchange bias of bilayers Co-doped NiO/FeNi was investigated. When Co content was up to 25.2%, the exchange bias field H_E at the room temperature increased to the maximum which was about three times compared to the undoped-bilayers. With further increase of Co content, the exchange bias field H_E and blocking temperature T_B decreased. Analysis suggests that the configuration of nanometer-sized Co-metal clusters enchased into NiO matrix played an important role in the change of magnetic behavior for the bilayers.     

Manipulating the magnetoimpedance by dc bias current in amorphous microwire

We have studied the effect of the internal circumferential magnetic field H_B created by the dc bias current I_B on longitudinal and off-diagonal magnetoimpedance (MI) in amorphous microwire with helical anisotropy and experimentally demonstrated that by changing the dc current I_B it is possible to considerably change the MI dependencies. The bias current applied to such microwire transforms the symmetric and hysteretic MI curve to asymmetrical and anhysteretic. The minimum of longitudinal MI curve shifts from the zero-field point. Reversing the bias current causes reversal of the bias field direction and results in a mirroring of the MI curves. It is proposed to apply a cross-checking of two MI curves with I_B of different polarity for magnetic field sensing. In particular, this method allows to overcome the drawbacks usually associated with longitudinal MI—namely the impossibility to determine the direction of an external axial magnetic field H_E and the low sensitivity near the zero-field point. Moreover, the operating range of the longitudinal MI sensor, in contrast to the off-diagonal one, can be much extended as it exhibits a quite high sensitivity in the field range up to one order of magnitude higher than the anisotropy field.     

Temperature dependent exchange bias effect in polycrystalline BiFeO3/FM (FM = NiFe, Co) bilayers

Extensive studies on the temperature (T) dependent exchange bias effect were carried out in polycrystalline BiFeO₃(BFO)/NiFe and BFO/Co bilayers. In contrast to single-crystalline BFO/ferromagnet (FM) bilayers, sharp increase of the exchange bias field (H _E ) below 50 K were clearly observed in both of these two bilayers. However, when T is higher than 50 K, H _E increases with T and decreases further when T is larger than 230 K (for BFO/NiFe) or 200 K (for BFO/Co), which is similar to those reported in single-crystalline BFO/FM bilayers. After the exploration of magnetic field cooling, the temperature dependent exchange bias can be explained considering two contributions from both the interfacial spin-glass-like frustrated spins and the polycrystalline grains in the BFO layer. Moreover, obvious exchange bias training effect can be observed at both 5 K and room temperature and the corresponding results can be well fitted based on a recently proposed theoretical model taking into account the energy dissipation of the AFM layer.     

外场作用下铁电/铁磁双层膜的极化磁化性质

建立了铁电/铁磁双层膜模型,铁电层的电矩用连续标量描述,而铁磁层的自旋应用经典矢量描述.利用蒙特卡罗方法模拟了体系的热力学性质和极化、磁化行为.给出了零场下体系的内能、比热、极化和磁化随温度变化的关系,并分别研究了体系在外磁场和外电场下的极化和磁化行为.模拟结果表明,双层膜体系的内能、比热、极化和磁化性质因层间耦合系数的不同而明显不同,当界面耦合较弱时,双层膜表现出各自的热力学性质,当层间耦合增强到一定程度时,双层膜耦合为一个整体,表现出统一的热力学性质.该双层膜在外场中形成电滞回线和磁滞回线,并表现出偏置特性,界面耦合强度和温度影响滞后回线和偏置现象.     

Thermodynamic functions of a two-sublattice ferrite in a magnetic field

This paper considers the effect of the magnetic field on the specific heat, entropy, and temperature behavior of the magnetization in a two-sublattice ferrite in the collinear phase. It is shown that the specific heat and entropy vary nonmonotonically with the magnetic field, while at low temperatures the magnetization increases with the temperature when the magnetic field takes values in the range H₁/2 < H h₁, where H₁ is the value of the magnetic field for which deviation from the collinear structure begins [1, 2].Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 33–36, January, 1977.In conclusion, the author wishes to express his indebtedness to G. M. Nedlin and to thank him for his valuable appraisal of the work.     

Ferromagnetic behavior of high-purity ZnO nanoparticles

ZnO nanoparticles with the wurtzite structure were prepared by chemical methods at low temperature in aqueous solution. The size of the nanoparticles is in the range from about 10 to 30 nm. Ferromagnetic properties were observed from 2 K to room temperature and above. Magnetization versus temperature, M(T), and isothermal M(H) measurements were obtained. The coercive field clearly shows ferromagnetism above room temperature. An exchange bias was observed, and we related this behavior to the core-shell structure present in the samples. The chemical synthesis, structure, and defects in the bulk related to oxygen vacancies are the main factors for the observed magnetic behavior.     

Magnetic interactions and media noise in rigid disks

This paper discusses the relationship between media-noise and magnetic properties. Two topics in longitudinal barium ferrite (Ba-ferrite) particulate rigid disks are mainly investigated; the relationship between media-noise characteristics and interparticle magnetic interactions, and the particle-size effect in read/write properties. For the first topic, a series of longitudinal Ba-ferrite disks with a variation in volumetric particle-packing densities are investigated. The magnetomotive force dependences or reverse DC-erase noise are measured and compared with interaction field factor. For the second topic, the particle-size effect in read/write properties of Ba-ferrite particulate disks are described compared with the those of Co–γFe₂O₃ particulate disks. The magnetic interactions in Ba-ferrite disks consist of two mechanisms: the interparticle positive interactions in stacks of Ba-ferrite particles, and the negative interactions between the stacks in which the magnetic moments of the Ba-ferrite platelet particles are aligned in the stack direction. H_c and switching field distribution values depend on interparticle interactions and extra stack-like particle interactions. Media noise depends on particle size. These ideas are effective for thin film media without exchange of magnetic interactions.     

Microwave properties of composites with chromium dioxide

The permittivity and permeability of composites filled with CrO₂ powder are measured within the frequency range from 0.05 to 12 GHz. A sharp line of magnetic absorption is detected near 8 GHz. The effects of magnetic bias and remanence on the permittivity and permeability spectra are analyzed. The hysteretic behavior of dynamic permeability is observed for both parallel and perpendicular bias orientations relative to the microwave magnetic field. The effect is due to switching of the magnetic texture under bias equal to coercive field. At 50 MHz the parallel bias close to coercive field affects permeability much stronger than the perpendicular one. At 10 GHz the effect of perpendicular bias is higher than that of the parallel one. The effect of remanence on the microwave permeability is negligible. The 3 kOe parallel bias suppresses the line of magnetic absorption and decreases the conductivity of the composite and its microwave permittivity. This can be attributed to the magnetostatic interaction of inclusions in the vicinity of the percolation threshold.