Zero—field—cooled and field—cooled magnetizations and magnetic susceptibility of itinerant ferromagnet SrRuO3

Zero-field-cooled(ZFC) magnetization,field-cooled(FC) magnetization,ac magnetic susceptibility and major hysteresis loops of itinerant ferromagnet SrRuO3 have been measured at magnetic ordering temperatures ranging from 5 to 160K.An empirical model is proposed to calculate the measured ZFC magnetization.The result indicates that the calculated ZFC magnetization compares well with the measured one.Based on the generalized Preisach model.both the ZFC and FC curves are reproduced by numerical simulations.The critical temperature and critical exponents are determined by measuring the ac magnetic susceptibility in different bias magnetic fields at temperatures in the vicinity of the point of phase transition.     

Nonlinear response of a thin metamaterial film containing Josephson junctions

An interaction of electromagnetic field with metamaterial thin film containing split-ring resonators with Josephson junctions is considered. It is shown that dynamical self-inductance in a split-rings results in reduction of magnetic flux through a ring and this reduction is proportional to a time derivative of split-ring magnetization. Evolution of thin film magnetization taking into account dynamical self-inductance is studied. New mechanism for excitation of waves in one dimensional array of split-ring resonators with Josephson junctions is proposed. Nonlinear magnetic susceptibility of such thin films is obtained in the weak amplitude approximation.     

Spin excitations and thermodynamics of the <Emphasis Type="Italic">t</Emphasis>-<Emphasis Type="Italic">J</Emphasis> model on the honeycomb lattice

We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic t-J Heisenberg model on the honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures and hole dopings, the electronic spectrum of excitations, the spin-excitation spectrum and thermodynamic quantities (two-spin correlation functions, staggered magnetization, magnetic susceptibility, correlation length) are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature and doping dependence of the magnetic (uniform static) susceptibility is ascribed to antiferromagnetic short-range order. Our results on the doping dependencies of the magnetization and susceptibility are analyzed in comparison with previous results for the t-J model on the square lattice.     

“Aharonov-Bohm antiferromagnetism” and compensation points in the lattice of quantum rings

We investigate the magnetic properties of the lattice of non-interacting quantum rings using the 2D rotator model. The exact analytic expressions for the free energy as well as for the magnetization and magnetic susceptibility are found and analyzed. It is shown that such a system can be considered as a system with antiferromagnetic-like properties. We have shown also that all observable quantities in this case (free energy, entropy, magnetization) are periodic functions of the magnetic flux through the ring's area (as well known, such a behavior is typical for the Aharonov-Bohm effect). For the lattice of quantum rings with two different geometric parameters we investigate the ordinary compensation points (“temperature compensation points”, i.e. points at which the magnetization vanishes at fixed values of the magnetic field strength). It is shown that the positions of compensation points in the temperature scale are very sensitive to small changes in the magnetic field strength.     

直流磁化测量中样品长度修正因数的分析

本文基于磁化电流在空间中磁场分布的计算,对不同长度的超导样品在测量线圈中真正排磁通作了分析,导出简单、明瞭的由检测线圈参数和样品长度所决定的修正因数。并将BiPbSrCaCuO高温单相样品截成不同长度,进行实验验证,实验结果与本文中的分析符合得较好。为小截面超导样品绝对抗磁矩的测量和计算提供了一种准确的分析方法。 关键词：     

基于时空变换恒定磁化的起始磁化曲线推算方法

起始磁化曲线作为铁磁材料磁学特性的重要表征, 研究其获取方法具有重要意义. 现有方法采用随时间变化磁场作为激励磁场, 通过改变激励磁场大小, 逐步改变试件内的磁场及磁感应强度并进行测量以得到起始磁化曲线, 效率较低, 准备繁琐. 为此, 本文从基本的磁学物理定理出发, 提出一种基于时空变换的起始磁化曲线推算方法. 该方法以细长棒状试件或环形试件作为被测试件; 采用恒定磁化在被测试件内产生空间变化磁场作为激励磁场; 通过测量试件表面的磁场值来推算试件内磁场值, 从而获取铁磁材料起始磁化曲线. 直流线圈恒定磁化环形和棒状试件仿真实验验证了该方法的理论正确性. 进一步地, 考虑实际应用限制因素的推算结果表明了该方法在实际应用中是可行的, 可为探索新的起始磁化曲线测量方法提供理论指导.     

One-dimensional Ising chain with generalized molecular field

The behavior of a spin-$\text{1}\text{2}$ one-dimensional nearest-neighbor Ising chain coupled to a generalized internal molecular field is considered. The internal field is shown to be equivalent to long-range spin-spin interactions. An assumed expansion of the internal field in powers of the magnetization results in the possibility of first- or second-order phase transitions and softening in the spontaneous magnetization and specific heat near the transition temperature. The internal energy and magnetic susceptibility are also discussed.     

基于永磁恒定磁场激励的起始磁化曲线测量

现有起始磁化曲线测量系统需绕制励磁线圈和感应线圈,在线应用受限.为此,本文提出了一种基于永磁恒定磁场激励的起始磁化曲线测量原理并搭建了相应测量系统.该系统采用永磁磁化器作为激励磁源,以对称磁化方法在圆柱棒状构件上激励出随轴向位置变化的恒定磁场作为激励磁场;采用阵列霍尔探头测量构件表面不同提离下的轴向和法向磁感应强度;并基于多项式外推法和磁场高斯定理外推法,推算构件与空气分界面上的轴向和法向磁感应强度;进一步地,根据分界面上的磁感应强度获取构件的起始磁化曲线.系统测量结果表明,在永磁恒定磁场激励下,无须励磁线圈和感应线圈即可方便地获取棒状构件的起始磁化曲线,测量误差小于10%,测量误差标准差小于0.01,重复性较好.该系统可为便捷地在线测量棒状构件起始磁化曲线提供新途径.     

Out-of-Plane Torque Influence on Magnetization Switching and Susceptibility in Magnetic Multilayers

Based on both the spin diffusion equation and the Landau-LlTshitz-Gilbert （LLG） equation, we demonstrate the influence of out-of-plane spin torque on magnetization switching and susceptibility in a magnetic multilayer system. The variation of spin accumulation and local magnetization with respect to time are studied in the magnetization reversal induced by spin torque. We also research the susceptibility subject to a microwave magnetic field, which is compared with the results obtained without out-of-plane torque.     

The influence of measurement and relaxation time on flux jumps in high temperature superconductors

The influence of the magnetization and relaxation time on flux jumps in high temperature superconductors (HTSC) under varying magnetic field is studied using the fundamental electromagnetic field equations and the thermal diffusion equation; temperature variety corresponding to flux jump is also discussed. We find that for a low sweep rate of the applied magnetic field, the measurement and relaxation times can reduce flux jump and to constrain the number of flux jumps, even stabilizing the HTSC, since much heat produced by the motion of magnetic flux can transfer into coolant during the measurement and relaxation times. As high temperature superconductors are subjected to a high sweep rate or a strong pulsed magnetic field, magnetization undergoes from stability or oscillation to jump for different pause times. And the period of temperature oscillation is equal to the measurement and relaxation time.     

Magnetic field penetration into half-space for type-II superconductors of the second kind

Equations that simulate the magnetic induction and current density distributions in half-space in view of the power I-V characteristic are derived. The magnetization front velocity is determined for a given mean rate of external magnetic field variation at the boundary of the sample. An integral condition for the electrical resistance (nonlinearly depending on the magnetic field) under which the magnetic flux penetrates into the sample with a finite rate is found. An analytical solution that simulates the power variation of the magnetic field at the boundary is given. The Bean generalized model describing the current density distribution near the critical current is considered. It is shown that solutions like shock waves may arise beyond the applicability domain of the Bean model.     

Quantification of entanglement from magnetic susceptibility for a Heisenberg spin 1/2 system

We report temperature and magnetic field dependent magnetization and quantification of entanglement from the experimental data for dichloro (thiazole) copper (II), a Heisenberg spin chain system. The plot of magnetic susceptibility vs. temperature indicates an infinite spin chain. Isothermal magnetization measurements (as functions of magnetic field) were performed at various temperatures below the antiferromagnetic (AFM) ordering, where the AFM correlations persist significantly. These magnetization curves are fitted to the Bonner–Fisher model. Magnetic susceptibility is used as an entanglement witness to quantify the amount of entanglement in the system.     

Magnetism of a relativistic degenerate electron gas in a strong magnetic field

The magnetization and magnetic susceptibility of a degenerate electron gas in a strong magnetic field in which electrons are located on the ground Landau level and the electron gas has the properties of a nonlinear paramagnet have been calculated. The paradoxical properties of the electron gas under these conditions??a decrease in the magnetization with the field and an increase in the magnetization with the temperature??have been revealed. It has been shown that matter under the corresponding conditions of neutron stars is a paramagnet with a magnetic susceptibility of ?? ?? 0.001.     

Wilson ratio of a Tomonaga-Luttinger liquid in a spin-1/2 Heisenberg ladder

Using micromechanical force magnetometry, we have measured the magnetization of the strong-leg spin-1/2 ladder compound (C(7)H(10)N)(2)CuBr(2) at temperatures down to 45 mK. Low-temperature magnetic susceptibility as a function of field exhibits a maximum near the critical field H(c) at which the magnon gap vanishes, as expected for a gapped one-dimensional antiferromagnet. Above H(c) a clear minimum appears in the magnetization as a function of temperature, as predicted by theory. In this field region, the susceptibility in conjunction with our specific-heat data yields the Wilson ratio R(W). The result supports the relation R(W)=4K, where K is the Tomonaga-Luttinger-liquid parameter.     

Features of magnetization reversal in LCMO/YBCO heterostructures

The change in the magnetic domain structure due to the proximity of a superconductor has been experimentally investigated for the first time. The complex character of magnetization reversal at temperatures below critical, caused by the mutual long-range effect of a superconductor and a magnet, has been shown. In particular, it is found that even magnetization reversal of the heterostructure by an in-plane field leads to the formation of Abrikosov vortices in the superconductor, carrying a flux perpendicularly to the film plane. It is shown that this is a consequence of the transformation of narrow domain walls into wide stripes due to the interaction with scattering fields from the superconductor. In turn, after penetration of the magnetic flux into the superconductor at some depth, the scattering fields cause backward magnetization reversal of the external film edge, as a result of which vortices with oppositely directed fluxes enter the crystal and propagate in the superconductor bulk in the form of chains along twins, as in the case of magnetization by a perpendicular magnetic field. Thus, at longitudinal magnetization, the flux enters the superconducting film in the form of wide stripes with alternating perpendicular induction, which is explained by the long-range interaction of the scattering fields of the superconductor with the manganite magnetization.     

Experimental observation of the 1/3 magnetization plateau in the diamond-chain compound Cu3(CO3)2(OH)2

The magnetic susceptibility, high field magnetization, and specific heat measurements of Cu3(CO3)2(OH)2, which is a model substance for the frustrating diamond spin chain model, have been performed using single crystals. Two broad peaks are observed at around 20 and 5 K in both magnetic susceptibility and specific heat results. The magnetization curve has a clear plateau at one third of the saturation magnetization. The experimental results are examined in terms of theoretical expectations based on exact diagonalization and density matrix renormalization group methods. An origin of magnetic anisotropy is also discussed.     

Spin excitations and thermodynamics of the antiferromagnetic Heisenberg model on the layered honeycomb lattice

We present a spin-rotation-invariant Green-function theory for the dynamic spin susceptibility in the spin-1/2 antiferromagnetic Heisenberg model on a stacked honeycomb lattice. Employing a generalized mean-field approximation for arbitrary temperatures, the thermodynamic quantities (two-spin correlation functions, internal energy, magnetic susceptibility, staggered magnetization, Néel temperature, correlation length) and the spin-excitation spectrum are calculated by solving a coupled system of self-consistency equations for the correlation functions. The temperature dependence of the magnetic (uniform static) susceptibility is ascribed to antiferromagnetic short-range order. The Néel temperature is calculated for arbitrary interlayer couplings. Our results are in a good agreement with numerical computations for finite clusters and with available experimental data on the β-Cu₂V₂O₂ compound.     

Commensurability effects in magnetic properties of superconducting Nb thin films with periodic submicrometric pores

Pinning properties in 100 nm thick continuous and porous superconducting Nb films are examined by ac susceptibility and dc magnetization measurements. The Nb film was deposited on a smooth Si substrate, while the porous film, Nb_P, was deposited on an anodized Al oxide substrate. Pores or “antidots” 40 nm in diameter, 100 nm apart, form a triangular array. The porous film presents commensurate or matching field effects for applied magnetic fields where the magnetic flux threading each unit cell is an integer number of the flux quantum, where ac shielding capability and dc diamagnetic magnetization show an abrupt increase. The response to ac fields as a function of temperature and dc field provided a way to determine that Nb_P sample has higher pinning than the continuous one, and that T_C suppression due to fluxoid quantization is not relevant for the investigated temperature range.     

Magnetization of two-dimensional superconductors with defects

A new method is developed for numerical simulation of the magnetization of layered superconductors with defects that is based on the Monte Carlo algorithm. The minimization of the free energy functional of a two-dimensional vortex system enables one to obtain equilibrium configurations of vortex density and calculate the magnetization of a superconductor with arbitrary distribution of defects in a wide temperature range. Magnetization curves are obtained for the first time for a defective superconductor under conditions of cyclic variation of the external magnetic field for different temperatures. The magnetic induction profiles and the magnetic flux distribution inside a superconductor are calculated, which support the validity of Bean’s model. It is demonstrated that the process of magnetization reversal is accompanied by the emergence of an annihilation wave, i.e., the motion of a zone with zero magnetic induction at the leading front of the incoming magnetic flux.