Features of magnetization reversal in trilayer nanostructures

The process of magnetization reversal in an ultrathin magnetic trilayer is analyzed. It is shown that the shape of magnetization hysteresis loops and the giant magnetoresistance essentially depend on the relative magnitudes of magnetic parameters of the top and bottom layers. Hysteresis loops are found for characteristic relative magnitudes of the parameters. Analysis is performed of the dependence of the shape of hysteresis loops on the magnitude of interlayer exchange. A phase diagram is constructed, which determines the regions of existence of characteristic hysteresis loops for different relative magnitudes of the uniaxial anisotropy constant and exchange constant J ₁.     

Study of the critical current density and the thermodynamic critical field in deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor

ABSTRACT

We have studied the reversible and irreversible part of the hysteresis loops as a function of slow cooling rate through the order–disorder transformation near 80?K for the deuterated (κ-D₈-Br) κ-(BEDT-TTF)₂Cu[N(CN)₂]Br organic superconductor. We estimated the critical current density J_C and the thermodynamic critical field H_C from the magnetic hysteresis loops. Temperature dependence of the critical current density derived from the irreversible part using Bean’s model. The thermodynamic critical field H_C has been obtained from the reversible part of the hysteresis loops.     

Inverted near-surface hysteresis loops in heterogeneous (nanocrystalline/amorphous) Fe<Subscript>81</Subscript>Nb<Subscript>7</Subscript>B<Subscript>12</Subscript> alloys

Inverted hysteresis loops were observed for the first time in the near-surface layers of heterogeneous (nanocrystalline/amorphous) Fe₈₁Nb₇B₁₂ alloys. In particular, a negative residual magnetization is observed when a positive magnetic field applied in the sample plane is decreased to zero. The inverted hysteresis is qualitatively explained within the framework of a two-phase model, according to which the heterogeneous alloys contain two dissimilar phases exhibiting uniaxial magnetic anisotropy and featuring antiferromagnetic exchange interaction.     

晶粒易轴取向度对纳米晶永磁Pr2Fe14B磁性的影响

构造了立方和不规则形状晶粒的各向异性纳米晶单相Pr₂Fe₁₄B磁体 .利用微磁学的有限元法，模拟计算了样品的磁滞回线.计算结果表明，随着磁体晶粒易轴取向度的变差， 磁体的剩磁、矫顽力均随之下降.不同晶粒尺寸的纳米晶单相Pr₂Fe_{14B磁体，其磁 性能随取向度的变化快慢不同，原因在于磁体中的晶间交换作用 (IGEC) 的强弱不同.随着 晶粒取向度的提高，纳米晶单相磁体的矫顽力逐渐增加，这完全不同于烧结磁体. 关键词： 纳米晶磁体 矫顽力 剩磁}     

Refractive indices of K<Subscript>2</Subscript>ZnCl<Subscript>4</Subscript> crystals in an incommensurate phase under uniaxial stresses

The influence of uniaxial mechanical stresses directed along the principal crystallophysical axes on refractiveindex temperature dependences in K₂ZnCl₄ crystals was studied. It is established that the refractive indices ni are quite sensitive to uniaxial stresses. Significant baric shifts of the paraphase–incommensurate–commensurate phase transition points to different temperature regions were observed, which is due to the effect of the uniaxial stress on the K₂ZnCl₄ crystal structure. It is found that applying uniaxial pressure increases the value of the temperature hysteresis of the commensurate–incommensurate phase transition.     

Permanent magnet applications

Rare-earth permanent magnets are ideally suited to generate magnetic fields comparable to their spontaneous polarization J_S. Near-square hysteresis loops and large values of the coercivity and anisotropy fields greatly simplify magnet design, as each magnet block is effectively transparent to the magnetic fields produced elsewhere in the magnet assembly. The fields generated by compact and efficient magnet structures requiring no continuous expenditure of energy can be static or variable, uniform or nonuniform. Permanent magnets are fully competitive with electromagnets for fields up to 2 T, and fields as high as to 5 T can be produced in a small volume. When a field with a rapid spatial variation is required, permanent magnets may offer the only practicable solution. Both permanent magnet structures and the uses to which they are put are reviewed, classifying the magnet applications in terms of the nature of the field, the effect on the magnet and the physical effect exploited.     

Effects of size and surface anisotropy on thermal magnetization and hysteresis in the magnetic clusters

Based on Monte Carlo simulation, the spin configurations, thermal magnetization and hysteresis loops of the clusters coated by the surface shell with radial anisotropy are studied. Interestingly, a new multidomain containing a few of subdomains whose easy directions are along those of the configurational anisotropy, a magnetization curve in steps and a first order phase transition from the single domain to the multidomain in the thermal and field magnetization processes, are found, which is as a result of the interplay of the configurational anisotropy, the size effect, the surface anisotropy, the applied field and the thermal fluctuation. In this first order transition, we find a critical temperature, a critical surface anisotropy and a critical size. The simulated temperature dependence of the coercivity of the cluster with the surface anisotropy can be fitted by H_c (T)=H_c (0)(1-C_αT^α) with low value of α, which explains well the experimental results of the nanoparticles. Moreover, it is found that the hysteresis loops and coercivity are strongly affected by the cluster size and the thickness of the surface layer.     

Ferroelastoelectric phenomena in a uniaxial ferroelectric TGS crystal

Possibility of the ferroelastoelectric behavior manifestations in a uniaxial ferroelectric TGS crystal was investigated. Analysis of the spontaneous piezoelectric moduli tensors for each of the 180°-domains formed as a result of the phase transition in TGS revealed possible directions for simultaneous application of the electric field and mechanical stresses to induce the switching effect in TGS. Influence of the uniaxial mechanical stresses σ₁₁, σ₂₂, σ₃₃ on the parameters of saturated and unsaturated dielectric hysteresis loops in TGS is considered. It was found that a nontraditional E ₃σ₁₂ combination of fields may lead to the domain switching in TGS crystals.     

Low temperature magnetic properties of Ti-rich Fe-Ti spinels

Low temperature hysteresis loops have been measured on compositions of the spinel series Fe_3-xTi_xO₄, 0?x?1, with x=0.4, 0.5, 0.6 and 0.8. All compounds studied exhibit an anomaly of the hysteresis parameters in the temperature range 40–80 K below which a marked effect of field cooling on the hysteresis loops is observed.     

Hysteretic behavior of quadrupolar ordering in a 2D magnetic spin˗1 Ising nanoparticle

Pair approximation technique has been used to study the quadrupolar ordering properties for the 2D magnetic spin-1 Ising nanoparticles, consisting of core and surface parts with an interface coupling. Similar to the magnetic hysteresis, it has been shown that the quadrupole order parameter (Q) as a function of single-ion anisotropy (D) has a ‘hysteresis’ character which is generally called quadrupole (or QD) hysteresis. The observed QD loops strongly depend on temperature (T), external magnetic field (H) and biquadratic exchange interaction (K). Shifted QD loops with an asymmetry are found when H and K values are increased. These behaviors are also discussed in relation to other theoretical findings.     

Theory of first order magnetization processes: Uniaxial anisotropy

The first order magnetization process (FOMP) is associated with the irreversible rotation of the magnetization vector M_s between inequivalent states. The critical field H_cr and the amplitude ΔM of the jump observed in the direction parallel to the applied magnetic field are functions of the anisotropy constants of the crystal. A complete phenomenological analysis is given for the case of uniaxial anisotropy in terms of the first three anisotropy constants K₁, K₂ and K₃, as well as in terms of the anisotropy coefficients χ_2,0, χ_4,0, χ_6,0. Computer plots of the critical parameters and the magnetic phase diagram are given together with an analytical treatment of the problem. The results give a unified view of the FOMP in uniaxial crystals and provide a method for the accurate determination of the anisotropy constants at the temperatures where the phenomenon is present.     

Anisotropy of quasistatic magnetization-reversal processes in (210)-oriented iron-garnet films

An experimental study is made of the effect of an in-plane field H _p of various orientations on the domain structure and shape of the hysteresis loops of epitaxial iron garnet films with the (210) orientation. The characteristic of the magnetization reversal process (in fields somewhat lower than the anisotropy field) is taken to be the critical fields H _p1, H _p2, and H _p3, for which the magnetization reversal processes is interrupted at distinct stages. A method is proposed for constructing the phase diagram of the magnetic states of films, using measurements of the critical fields H _p for different amplitudes of the magnetization reversal field H _z . Two directions in the plane of the film are determined with an accuracy of a fraction of a degree from the hysteresis loops, where in the corresponding field Hp the transition from a single-domain state to a multidomain state occurs as a second-order phase transition. The characteristic changes in the shape of the hysteresis loop are consistent with the features in the reorganization of the domain structure of the (210) film. The preferential orientations of the stripe domain structure of the samples are determined relative to the crystal axes as determined by x-ray methods. Zh. Tekh. Fiz. 67, 32–35 (June 1997) Deceased     

Study of the magnetic interaction in nanocrystalline Pr-Fe-Co-Nb-B permanent magnets

The magnetic properties of an isotropic, epoxy resin bonded magnets made from Pr-Fe-Co-Nb-B powder were investigated. The magnetization reversal process and magnetic parameters were examined by measurements of the initial magnetization curve, major and minor hysteresis loops and sets of recoil curves. From the initial magnetization curve and the field dependencies of the reversible and irreversible magnetization components derived from the recoil loops it was found that the magnetization reversal process is the combination of the nucleation of reversed domains and pinning of domain walls at the grain boundaries and the reversible rotation of magnetization vector in single domain grains. The interactions between grains were studied by means of δM plots. The nonlinear behavior of δM curve approve that the short range intergrain exchange coupling interactions are dominant in a field up to the sample coercivity.The interaction domains and fine magnetic structure were revealed as the evidence of exchange coupling between soft α-Fe and hard magnetic Nd₂Fe₁₄B grains.     

Growth-induced anisotropy in Ba0.6K0.4BiO3 superconducting single crystal

0.6 K_0.4BiO₃ single crystal have been performed to study the nature of the growth-induced anisotropy revealed using chemical etching technique. It was found that this anisotropy is responsible for differences in low-field susceptibility and in hysteresis loops observed for different magnetic field orientations. Received: 9 December 1997/Accepted: 28 May 1998     

Uniaxial stress dependence of ferroelectric properties of xPMN-(1–x)PZT ceramic systems

Effects of uniaxial compressive pre-stress on the ferroelectric properties of ceramics in the PMN-PZT system are investigated. The ceramics with a formula xPb(Mg_1/3Nb_2/3)O₃-(1-x)Pb(Zr_0.52Ti_0.48)O₃ or xPMN-(1-x)PZT when x=0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 are prepared by a conventional mixed-oxide method. The ferroelectric properties under the uniaxial compressive pre-stress of the PMN-PZT ceramics are observed at the stress levels up to 13 MPa using a compressometer in conjunction with a modified Sawyer–Tower circuit. It is found that with increasing compressive pre-stress levels the area of the ferroelectric hysteresis (P–E) loops, the saturation polarization (P_sat), the remanent polarization (P_r), and the coercive field (E_c) decrease. These results are interpreted through the non-180° ferroelastic domain-switching processes. Finally, the calculated differential permittivity for all the xPMN-(1-x)PZT ceramics, except for 0.5 PMN-0.5 PZT, decreases with increasing applied stress. PACS 77.22.Ch; 77.84.Dy     

Polarization switching process of soft lead zirconate titanate bulk ceramics

The domain switching dynamics was investigated in Nb-doped Pb(Zr_0.52Ti_0.48)O₃ bulk ceramics through observing systematically the evolution of the hysteresis loops at frequencies ranging from 0.01 to 100 Hz and in a field of 7.5–35 kV/cm. The experimental results indicate that the hysteresis loops are remarkably dependent on the fields and the frequencies. The data are analyzed by the Merz equation. The analysis results reveal that the requirement to satisfy the Merz equation is saturated loops. Based on these results, we divided the polarization switching process into three regions.     

Field-induced uniaxial anisotropy in partially ordered Ni3Mn at low temperatures

A uniaxial anisotropy is induced in partially ordered MnNi alloys in the composition range near Ni₃Mn, by applying a magnetic field during cooling from room temperature to 77 K. The uniaxial anisotropy constant K_u depends on both the magnitude of the torque measuring field and that of the cooling field. The easy axis direction tends to be between the 〈100〉 and the direction of the cooling field. The value of K_u shows a maximum when the cooling field is applied along 〈100〉, amounting to 2 × 10⁴ crg/cm³. On the other hand, the crystalline anisotropy is not affected by field cooling. When the temperature is raised from 77 K to 300 K, neither K_u nor the rotational hysterisis loss is observed to have any critical temperature, for a torque measuring field of 20 kOe. As atomic ordering proceeds, these effects become smaller and appear to vanish in the perfectly ordered state.     

Magnetostrictive properties of Ni-ferrite

Initial curve, major and minor magnetostriction hysteresis loops of the NiFe₂O₄ ferrite as the functions of static magnetic field were investgated. Magnetostriction hysteresis loops as the functions of static induction are also presented. The experimental phenomenon of “liftoff” value of the magnetostriction loop is detected.     

Antiferromagnetic coupling between Co/Ni multilayers with perpendicular magnetization across MBE-grown Cu(111) layers

A cleat oscillatory exchange coupling has been observed for an epitaxial (111) M₁/Cu/M₂ structure, grown by Molecular Beam Epitaxy (MBE) on a single crystal Cu(111) substrate. The magnetic layers M₁ and M₂ were designed to have perpendicular easy axis by using Co/Ni multilayers. The Cu layer was deposited in the form of a wedge. The magneto-optically measured hysteresis loops show, in the case of antiferromagnetic (AF) coupling, clear first-order spin-flip transitions between the saturated and antiparallel states. Signatures of a ferromagnetic or biquadratic component, observed earlier in loops of an AF coupled Co/Cu/Co(111) sandwich, were absent in the present loops. The absence of these signatures is believed to be a consequence of the relatively strong (perpendicular) uniaxial anisotropy of the present samples.     

Comparative study of type-II superconducting properties in polycrystalline NdFeAsO0.88F0.12 prepared by different methods

Polycrystalline NdFeAsO_0.88F_0.12 superconductors prepared by high pressure (HP) and ambient pressure (AP) methods were comparatively studied by magnetization and transport measurements. Upper critical field H_c2, irreversibility field H_irr and the anisotropy parameter Γ were estimated from resistance transition curves. The broadening of transition width was observed, and was ascribed to both H_c2 anisotropy and superconductivity inhomogeneity of samples. Magnetic hysteresis loops (MHLs) in low fields were measured to detect the trace of weak-link behavior. The reclosed hysteresis loops in low fields indicate that there are weak links in both samples. Magnetization critical current density J_cm were derived from MHLs. Sample HP shows higher J_cm than sample AP. Direct transport I–V measurements show that the transport critical current density J_ct are very low but persist up to 9 T, suggesting intrinsic strong-link existing in both samples.