应力各向异性对铁磁/反铁磁双层薄膜磁性质的影响

 采用铁磁共振方法,研究了交换各向异性和应力各向异性对铁磁/反铁磁双层薄膜性质的影响。结果表明：界面交换作用导致单向各向异性,应力各向异性对材料的磁化难易程度有较大影响。在外磁场方向接近应力场方向时,共振频率向高值方向移动,其它区域共振频率则向低值方向移动。对频率线宽而言,接近应力场方向,频率线宽加宽,其它区域频率线宽则变窄。此外,当磁场变化时,应力的存在使得共振频率向低值方向移动,尤其在β＝π方向情况较为复杂,在弱场范围出现了两个区域：即在某磁场范围内,共振频率向高值方向移动,且频率线宽加宽;而其它范围的共振频率（线宽）是向高值方向移动（加宽）还是向低值方向移动（变窄）,取决于外磁场的相对强弱。     

EXPERIMENTAL STUDY OF AC ZEEMAN EFFECT IN 87Rb ATOMIC FREQUENCY STANDARD

We have proved experimentally that the frequency shift formula of AC Zeeman effect presented by us is correct. In a ⁸⁷Rb atomic frequency standard we add a new microwave field which causes the frequency shift of O-O transition frequency of ⁸⁷Rb atomic ground state. This frequency shift as a function of frequency and power of the added microwave field is measured. The experimental results agree with theory.     

双层磁性薄膜系统中铁磁共振性质

利用能量极小原理研究了双层磁性薄膜系统中的铁磁共振特性,给出共振频率与线宽随外加磁场的变化关系.以及考虑应力各向异性、交换各向异性和单轴各向异性后,系统共振频率与线宽的变化情况.数值计算结果表明:外应力场和交换各向异性场对铁磁共振频率、频谱宽度以及磁化行为均有影响.     

Shape Anisotropy and Resonance Mode Guided Reliable Interconnect Design for In-plane Magnetic Logic

Dipole coupled nanomagnets controlled by the static Zeeman field can form various magnetic logic interconnects.However, the corner wire interconnect is often unreliable and error-prone at room temperature. In this study, we address this problem by making it into a reliable type with trapezoid-shaped nanomagnets, the shape anisotropy of which helps to offer the robustness. The building method of the proposed corner wire interconnect is discussed,and both its static and dynamic magnetization properties are investigated. Static micromagnetic simulation demonstrates that it can work correctly and reliably. Dynamic response results are reached by imposing an ac microwave field on the proposed corner wire. It is found that strong ferromagnetic resonance absorption appears at a low frequency. With the help of a very small ac field with the peak resonance frequency, the required static Zeeman field to switch the corner wire is significantly decreased by ~21 m T. This novel interconnect would pave the way for the realization of reliable and low power nanomagnetic logic circuits.     

EXPERIMENTAL STUDY OF AC ZEEMAN EFFECT IN 87Rb ATOMIC FREQUENCY STANDARD

We have proved experimentally that the frequency shift formula of AC Zeeman effect presented by us is correct. In a ⁸⁷Rb atomic frequency standard we add a new microwave field which causes the frequency shift of O-O transition frequency of ⁸⁷Rb atomic ground state. This frequency shift as a function of frequency and power of the added microwave field is measured. The experimental results agree with theory.     

Magnetic resonance in amorphous FexNi80-xP14B6: I. Ferromagnetic and reentrant alloys

We present a study of ferromagnetic resonance in amorphous Fe_xNi_80-xP₁₄B₆ for Fe concentrations down to that required for ferromagnetism. The resonance was studied at microwave frequencies between 10 and 35 GHz and at temperatures between 2 and 300 K. We find i) in accord with previous data on amorphous ferromagnets, each alloy investigated is magnetically inhomogeneous even in its ferromagnetic state, ii) the intrinsic relaxation parameter λ / Mγ for each alloy falls between the value for pure Fe and the value for pure Ni, iii) a low temperature linewidth rise which is frequency independent and follows an empirical form suggested earlier, iv) frequency dependent linewidth maxima at low T which do not correspond to low field spin freezing temperatures, and v) anisotropy fields intruding at low temperatures. The corresponding anisotropy energy is similar to that proposed for spin glasses as are the temperature and frequency dependences of the anisotropy constant. With part II of this paper, this represents the most complete resonance study to date of the evolution of spin glass behavior in Fe based alloys.     

Theoretical investigation of ferromagnetic resonance in a ferromagnetic thin film with external stress anisotropy

We use the ferromagnetic resonance(FMR)method to study the properties of ferromagnetic thin film,in which external stress anisotropy,fourfold anisotropy and uniaxial anisotropy are considered.The analytical expressions of FMR frequency,linewidth and the imaginary part of magnetic susceptibility are obtained.Our results reveal that the FMR frequency and the imaginary part of magnetic susceptibility are distinctly enhanced,and the frequency linewidth or field linewidth are broadened due to a strong external stress anisotropy field.The hard-axis and easy-axis components of magnetization can be tuned significantly by controlling the intensity and direction of stress and the in-plane uniaxial anisotropy field.     

Dynamic magnetization reversal and bistable states in antiferromagnetic multilayer structures

The dynamic behavior of the magnetization under a transverse microwave field is investigated in a system of magnetic layers with cubic crystallographic anisotropy coupled through interlayer antiferromagnetic exchange interaction. An orientational phase transition is found to occur as the microwave field frequency and amplitude are varied. It is established that there is a frequency range in which several steady-state regimes of precession of magnetic moments exist. The limits of this range can be efficiently controlled both by varying the strength of the bias magnetic field and the amplitude of the microwave field.     

Electron spin resonance and magnetocrystalline anisotropy in YCo3

We present the electron spin resonance measurements in powdered samples of the intermetallic compound YCo₃ in Q, K and X microwave bands. The grains of the powdered samples were magnetically aligned in a static magnetic field. The resonant spectra consist of the lines connected with localized and unlocalized magnetic moments. We were able to describe only the part of the spectrum associated with localized moments and to estimate the anisotropy field at the room temperature. Next we present the magnetization data by the pulse magnetometer. The anisotropy field which was determined from these data is much larger than that from ferromagnetic resonance. We suggested a possible reason for such discrepancy.     

Broad-Band FMR Linewidth of Co_2MnSi Thin Films with Low Damping Factor:The Role of Two-Magnon Scattering

The low Gilbert damping factor,which is usually measured by ferromagnetic resonance,is crucial in spintronic applications.Two-magnon scattering occurs when the orthogonality of the ferromagnetic resonance mode and other degenerate spin wave modes was broken by magnetic anisotropy,voids,second phase,surface defects,etc.,which is important in analysis of ferromagnetic resonance linewidth.Direct fitting to linewidth with Gilbert damping is advisable only when the measured linewidth is a linear function of measuring frequency in a broad band measurement.We observe the nonlinear ferromagnetic resonance linewidth of Co_2MnSi thin films with respect to measuring frequency in broad band measurement.Experimental data could be well fitted with the model including two-magnon scattering with no fixed parameters.The fitting results show that two-magnon scattering results in the nonlinear linewidth behavior,and the Gilbert damping factor is much smaller than reported ones,indicating that our Co_2 MnSi films are more suitable for the applications of spin transfer torque.     

Ferromagnetic resonance of nonstoichiometric zinc ferrite and cobalt-doped zinc ferrite nanoparticles

Ferromagnetic resonance spectra of zinc ferrite and cobalt doped zinc ferrite nanoparticles, measured at various temperatures, exhibit an invariant point at a given field. This makes it possible to determine the equation relating the resonance field shift to the peak-to-peak linewidth. When particles are frozen in a matrix in a magnetic field, the anisotropy constant of the material can be derived from the angular variation of the resonance field. This procedure is useful to determine the thermal dependence of the anisotropy constant, but is shown to require various freezing temperatures experiments to estimate the accuracy of the deduced anisotropy constant values. It is also shown that the angular dependence of the resonance field is similar for a uniaxial (zinc ferrite) and cubic (zinc ferrite containing 40% cobalt ions) anisotropy. This unexpected result is explained by the weakness of the texturation, leading to a distribution in easy axes directions.     

Ferromagnetische Resonanz von polykristallinem Nickel in Abhängigkeit von Temperatur und Frequenz

Resonant magnetic field and linewidth of bulk nickel were determined between ?160 °C and room temperature at the frequencies of 9.2, 19.7 and 26.2 Gc/s. At 19.7 and 26.2 Gc/s the resonance-field shift is found to be proportional to the anisotropy field. Below ?90 °C the linewidth can be described by the independent-grain theory. For temperatures above ?90 °C the validity of the dipole-narrowing model is discussed. At 9.2 Gc/s resonance field and linewidth are strongly influenced by additional absorption peaks caused by domain structure.     

Magnetic resonance in amorphous FexNi80-xP14B6: II. Spin glass alloys

We report EPR measurements on amorphous Fe_xNi_80-xP₁₄B₆ alloys for concentrations less than that required for ferromagnetism. Measurements were made at microwave frequencies between 10 and 35 GHz and at temperatures between 2 and 150 K. We find i) at high temperatures the alloys are simple paramagnets with ? ≈ 2.14 and a relaxation rate (linewidth) increasing linearly with temperature, ii) the linewidth increases at low T and follows an empirical form proposed earlier, iii) as the temperature is decreased, the susceptibility (measured by reference to the line intensities) increases, iv) in order to account for the frequency dependence of the resonance field we must introduce an anisotropy energy with uniaxial symmetry; the “hard axis” being normal to the sample plane. The associated anisotropy constant K' appears at several times the spin glass transition temperature T_SG, v) at still lower temperatures ( < 2T_SG) another type of anisotropy field appears. The corresponding anisotropy energy is similar to that introduced to explain data on the reentrant alloys of this system and the archetypal spin glasses CuMn and AgMn and the anisotropy constant K has the same type of temperature dependence. However, the frequency dependence of K is different.     

Temperature dependence of superparamagnetic resonance of iron oxide nanoparticles

Nanoparticles of maghemite (γ-Fe₂O₃) are formed in a sol–gel silicate glass with a molar ratio Fe/Si of 2% by a treatment at 1000°C for 6 h. Electron paramagnetic resonance spectrum at 300 K shows a relatively narrow sharp line at g_eff≈2. As the temperature lowers to 5 K, the apparent resonance field decreases and the linewidth considerably increases. We develop a theoretical formalism based on a distribution of diameters or volumes of the nanoparticles following a lognormal. The nanoparticles are considered as single magnetic domains with random orientations of magnetic moments and thermal fluctuations of anisotropic axes. The individual line shape function is derived from the damped precession equation of Landau–Lifshitz. An appropriate linewidth expression is put forward, which account for the averaging of the fluctuations of orientations of the magnetic moments with respect to the magnetic field and to the magnetic anisotropy axes. A single set of parameters provides good fits to the spectra recorded at the different temperatures. The low-temperature blocking of the nanoparticle magnetic moments has been clearly evidenced in the resonance absorption intensity and the blocking temperature of the assembly of nanoparticles (averaged over the distribution in the nanoparticle volume) has been evaluated as 90 K.     

铷原子频标中的微波功率频移

根据我们的测量，在铷原子频标中微波功率频移系数总是正的。已有的关于微波功率频移的解释无法解释此现象。本文从微波场引起交流Ｚｅｅｍａｎ效应的观点出发，解释了这一现象。     

铯原子束频标中的Majorana跃迁频移

本文对铯原子频标中的Majorana跃迁频移进行理论上的探讨。通过经受低频和微波多重共振的多能级系统的铯原子量子态变化的分析,认为产生这种频移的物理机制在于C_s¹³³原子穿过空间变化磁场时感应了塞曼子能级间的跃迁,扰动了频标赖以工作的两个能级相应态的相位,这个过程又相干于Ramsey共振过程中,最后导致Ramsey共振频移。基于此机制,解释一些有关实验现象并提出消除和减少Majorana跃迁的判断方法。     

Ferromagnetic resonance in polycrystalline plates subjected to a planar strain: The spin wave approach

New formulas of the ferromagnetic resonance are presented for polycrystalline plates subjected to an isotropic planar strain. The width and the shift of the resonance line is calculated using the spin wave model for the case when the applied field is perpendicular to the plane of the plate. Crystalline and strain induced anisotropy are both considered. It is shown that the contribution of the crystalline anisotropy to the linewidth can be cancelled out for a certain value of the strain.     

Angular and NiFe thickness dependence of exchange bias in IrMn/NiFe/IrMn thin film

Exchange biased IrMn/NiFe/IrMn thin films were studied as a function of NiFe thickness. In plane angular dependence of a resonance field distribution which is measured by FMR was analyzed as a combined effect of an unidirectional anisotropy and an uniaxial anisotropy. The unidirectional anisotropic field and the uniaxial anisotropic field were linearly varied with NiFe thickness while the films with a thicker NiFe layer do not follow the linear variation. Resonance field and linewidth variations were also analysed with NiFe thickness.     

Magnetic field measurement based on a stimulated two-photon Raman transition

The magnetic field in the microwave interaction zone of the fountain atomic clock was measured by stimulated Raman transitions.By measuring the two-photon transition frequency between the Zeeman levels of the two ground states,we achieved a magnetic field measurement accuracy of the order of 0.28 nT.This method is immune to the Doppler shift and the AC Stark shift.The second order Zeeman shift of the fountain clock is 170.7×10-15,with the uncertainty of 7.2×10-16.