Dependence of Interlayer AF Coupling on Ferromagnetic Layer Thickness in [Pt/Co]5/Ru/[Co/Pt]5 Multilayers

We study magnetization reversal in the interlayer coupled [Pt/Co]5/Ru/[Co/Pt]5 multilayers （MLs） by means of the measurement of extraordinary Hall effect （EHE）. Fitting experimental data to a simple model, we determine the interlayer coupling strength for various thicknesses of the ferromagnetic layers at a fixed Ru spacer thickness. It is found that the dependence of interlayer coupling strength on the Pt layer thickness is much stronger than the previous report in the ferromagnetic/nonmagnetic/ferromagnetic multilayers.     

Influence of Pt thickness on magnetization reversal processes in (Pt/Co)3 multilayers with perpendicular anisotropy

We present a detailed study of the magnetization reversal in perpendicularly magnetized (Pt/Co)₃ multilayers with different values of the platinum interlayer thickness t_Pt. To study the magnetization reversal in our samples we combined measurements of relaxation curves with the direct visualization of domain structures. Magnetization reversal was dominated by domain wall propagation for t_Pt=1 nm and by domain nucleation for t_Pt=0.2 nm, while a mixed process was observed for t_Pt=0.8 nm. We interpret our results within the framework of a model of thermally activated reversal where a distribution of activation energy barriers is taken into account. The reversal process was correlated with the energy barrier distribution.     

Large spontaneous Hall angle in [Co,CoFe/Pt] multilayer

We have quantitatively investigated the Hall effect in [Co, CoFe/Pt] multilayer films. The [Co, CoFe/Pt] multilayers exhibit large spontaneous Hall resistivity (ρ_H) and Hall angle (ρ_H/ρ). Even though the Hall resistivity in [Co, CoFe/Pt] multilayer films (2.7–4 × 10⁻⁷ Ω cm) is smaller than that of amorphous RE–TM alloy films which show large spontaneous Hall resistivity (<2 × 10⁻⁶ Ω cm), the Hall angle of multilayer (6–8%) is almost twice than that in amorphous rare earth–transition metal alloy films (∼3%). The Hall angle provides evidence of the effects of the exchange interaction of the Hall scattering. The exchange is between conduction electron spins and the localized spins of the transition metal. The large Hall angle of [Co, CoFe/Pt] multilayer can be considered due to the high spin polarization and high Curie temperature of Co and CoFe transition metal layers. Even though the role of interfaces and surfaces in the magnetic properties of multilayer films may dominate that of the bulk, the Hall effects in [Co, CoFe/Pt] multilayer may be mainly dominated by the bulk effect.     

Alloy formation of Co/Ni/Pt(1 1 1)

The initial growth and alloy formation of ultrathin Co films deposited on 1 ML Ni/Pt(1 1 1) were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), and ultraviolet photoelectron spectroscopy (UPS). A sequence of samples of d_Co Co/1 ML Ni/Pt(1 1 1) (d_Co = 1, 2, and 3 ML) were prepared at room temperature, and then heated up to investigate the diffusion process. The Co and Ni atoms intermix at lower annealing temperature, and Co-Ni intermixing layer diffuses into the Pt substrate to form Ni-Co-Pt alloys at higher annealing temperature. The diffusion temperatures are Co coverage dependent. The evolution of UPS with annealing temperatures also shows the formation of surface alloys. Some interesting LEED patterns of 1 ML Co/1 ML Ni/Pt(1 1 1) show the formation of ordered alloys at different annealing temperature ranges. Further studies in the Curie temperature and concentration analysis, show that the ordered alloys corresponding to different LEED patterns are Ni_xCo_1−xPt and Ni_xCo_1−xPt₃. The relationship between the interface structure and magnetic properties was investigated.     

Exchange interactions and magnetization reversal of nanocrystalline (NdDyTb)12.3(FeZrNbCu)81.7B6.0 ribbons with Co substitution

Alloys of composition Nd_10.8Dy_0.75Tb_0.75Fe_79.7−xCo_xZr_0.8Nb_0.8Cu_0.4B_6.0 (x=0, 3, 6, 9, 12, 15) were prepared by melt spinning at 22 m/s and subsequent annealing. Phase analysis revealed single-phase materials. Magnetic structure and remanence analysis indicated strong exchange coupling between neighboring grains in all samples. The remanence polarization J_r and maximum energy product (BH)_max increased first and then decreased with further increasing Co content x although the intrinsic coercivity H_ci decreased with increasing x. The increase in remanence polarization J_r by the substitution of Co for Fe is mainly caused by the increase in the saturation polarization J_s rather than by the improvement of exchange-coupling interactions. Optimum magnetic properties with J_r=1.041 T, H_ci=944.9 kA/m and (BH)_max=155.1 kJ/m³ were achieved for x=12 ribbons. The mechanism of magnetic hardening in all samples was of pinning type by analyzing initial magnetization and the dependence on applied magnetizing field of the coercivity and remanence.     

Ultrathin (CoFe/Pt)n multilayers with tuned magnetic properties

Perpendicular magnetic anisotropy (PMA) has been investigated in ultrathin (CoFe [0.2] nm/Pt [0.2] nm)_n multilayers. The Pt layers show an fcc crystal structure with a preferred [111] orientation. The multilayers with n=3, 4 show PMA in the as-grown state, which can be enhanced by thermal annealing. However, no PMA is observed in the as-grown state with higher repetitions (n>&=5), although it is observed after thermal annealing. For 1=&<n=&<8, the anisotropy energy is around 10⁵ J/m³ for all (CoFe [0.2]/Pt [0.2])_n stacks. The perpendicular anisotropy is related to layer thickness and interface roughness.     

Electrical resistivity and magnetic investigations of the orthorhombic Tb(Ni,Cu)2 system

The orthorhombic Tb(Ni, Cu)₂ and Gd(Ni, Cu)₂ systems (CeCu₂ structure) are closely similar according to electrical resistivity and magnetic results. The Tb(Ni_xCu_1?x)₂ system presents a transition from antiferromagnetism (AF) for x ? 8% Ni to ferromagnetism (FM) for x &#62; 8% Ni. The CeCu₂ structure becomes unstable for x &#62; 45% Ni. The AF samples show metamagnetism at 4.2 K with critical fields. Hysteresis, which occurs for all samples at 4.2 K, is attributed to intrinsic pinning due to large anisotropy where the mechanism for the AF range is analogous to intrinsic pinning of narrow domain walls in FM samples.Spin disorder resistivity measurements show a discontinuity at the AF-FM transition composition for both the Tb and Gd systems. This is due to a step up of the residual resistivity at 4.2 K as a result of AF ordering. This interpretation is confirmed by applying a magnetic field to destroy the AF ordering.     

Thin film Co[TCNE]2 and VyCo1−y[TCNE]2 magnetic materials

A chemical vapor deposition method has been developed for the synthesis of both solvent-free Co[TCNE]₂ and V_yCo_1−y[TCNE]₂ thin films. Both materials have been previously synthesized by solution methods, but contain solvent. The Co[TCNE]₂ thin films were characterized by infrared spectroscopy and magnetic studies, and albeit solvent-free were determined to be similar to the analogous solution-prepared samples as they are paramagnetic with slight antiferromagnetic coupling. In contrast to the solution-based synthesis, V_yCo_1−y-[TCNE]₂ showed no dependence of coercive field based on the composition of the films, even though infrared spectroscopic data indicates formation of a solid-solution thin film, and not a physical mixture.     

Magnetization reversal and domains structures in (Co/Ni/Co/Pt) multilayers

Multilayers of [Co/Ni(t_Ni)/Co/Pt]_×4 are investigated for different Ni insertion layer thicknesses. The resulting magnetic properties and magnetic domain structures are compared with [Co/Ni]_×8 multilayers. As determined by magneto-optical Kerr effect microscopy and a vibrating sample magnetometer measurements, all multilayers exhibited a perpendicular magnetic anisotropy. It is found that the nucleation field and magnetic coercivity of [Co/Ni(t)/Co/Pt]_×4 multilayers are lower than (Co/Ni)_×8 and decreased with Ni thickness. Magnetization decay measurements reveal that these multilayers did not show an exponential decay behavior as was observed in rare earth transition metal alloys. Very narrow wires will remain stables for several hours even with an applied magnetic field closer to the coercivity. Insertion of very thin Ni in (Co/Pt) multilayers offers a good way to optimize the magnetic properties of the material and adjust the domain size for nanowire-based devices.     

Exchange bias in thin-film (Co/Pt)3/Cr2O3 multilayers

We have fabricated exchange-biased Co/Pt layers ((0.3 nm/1.5 nm)×3) on (0 0 1)-oriented Cr₂O₃ thin films. The multilayered films showed extremely smooth surfaces and interfaces with root mean square roughness of ≈0.3 nm for 10 μm×10 μm area. The Cr₂O₃ films display sufficient insulation with a relative low leakage current (1.17×10⁻² A/cm² at 380 MV/m) at room temperature which allowed us to apply electric field as high as 77 MV/m. We find that the sign of the exchange bias and the shape of the hysteresis loops of the out-of-plane magnetized Co/Pt layers can be delicately controlled by adjusting the magnetic field cooling process through the Néel temperature of Cr₂O₃. No clear evidence of the effect of electric field and the electric field cooling was detected on the exchange bias for fields as high as 77 MV/m. We place the upper bound of the shift in exchange bias field due to electric field cooling to be 5 Oe at 250 K.     

Structural and electrical properties of (Na0.85K0.15)0.5Bi0.5TiO3 thin films deposited on LaNiO3 and Pt bottom electrodes

(Na_0.85K_0.15)_0.5Bi_0.5TiO₃ thin films were deposited on LaNiO₃(LNO)/SiO₂/Si(1 0 0) and Pt/Ti/SiO₂/Si(1 0 0) substrates by metal-organic decomposition, and the effects of bottom electrodes LNO and Pt on the ferroelectric, dielectric and piezoelectric properties were investigated by ferroelectric tester, impedance analyzer and scanning probe microscopy, respectively. For the thin films deposited on LNO and Pt electrodes, the remnant polarization 2P_r are about 22.6 and 8.8 μC/cm² under 375 kV/cm, the dielectric constants 238 and 579 at 10 kHz, the dielectric losses 0.06 and 0.30 at 10 kHz, the statistic d_33eff values 95 and 81 pm/V. The improved piezoelectric properties could make (Na_1−xK_x)_0.5Bi_0.5TiO₃ thin film as a promising candidate for piezoelectric thin film devices.     

Microscopic origin of asymmetric magnetization reversal of Co/Pt multilayers with perpendicular magnetic anisotropy

We have comprehensively investigated asymmetric magnetization reversal behaviors of (x-Å Co/7.7 Å Pt)₅ multilayers (x = 3.1 and 4.7) with perpendicular magnetic anisotropy. Our direct observation of magnetic domain structures by means of magneto-optical microscopy reveals that the asymmetry arises both from nucleation and wall-motion processes. An asymmetric nucleation behavior is observed, which could be originated from the preexisting non-reversed domains which might have a reproducible or random spatial distribution, controllable by tuning the field profile. An asymmetric wall-motion behavior stemming from asymmetric stripe domain evolution is also observed.     

Effect of 3d metal ion doping on the structure and superconductivity of (Tl0.5Pb0.5)Sr2CaCu2O7

(Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ (M=Co, Ni and Zn) have been synthesized and investigated by means of X-ray diffraction, scanning electron microscope, electrical resistivity and magnetic susceptibility measurements. X-ray diffraction patterns show that all studied samples contain the nearly single ‘1212’ phase. They crystallize in a tetragonal unit cell with a=3.8028-3.8040 Å and c=12.0748-12.1558 Å. In (Tl_0.5Pb_0.5)Sr₂Ca(Cu_2−xM_x)O₇ system (M=Co or Ni), the superconducting critical temperature T_c decreases linearly with both Co and Ni concentrations and the rate of T_c decrease is around −6.5 and −7.0 K/at%, respectively. For (Tl_0.5Pb_0.5)Sr₂Ca (Cu_2−xZn_x)O₇ system, the dependence of T_c on the Zn dopant concentration deviates from a linear behavior and the Zn substitution suppresses T_c much less (−2.5 K/at%) than the Co and Ni substitutions. The suppression in T_c in Co and Ni doped samples are attributed to the magnetic pair-breaking mechanism and the reduction in the carrier concentration. The suppression of T_c in Zn doped samples is not caused by the reduction in carrier concentration which should remain constant, but rather due to nonmagnetic pair-breaking mechanism induced by disorder as well as the filling of the local Cu d_x²−y² state due to the full d band of Zn ions.     

Spin reversal and magnetization jumps in ErMexMn1−xO3 perovskites (Me=Ni,Co)

The erbium-based manganite ErMnO₃ has been partially substituted at the manganese site by the transition-metal elements Ni and Co. The perovskite orthorhombic structure is found from x(Ni)=0.2–0.5 in the nickel-based solid solution ErNi_xMn_1−xO₃, while it can be extended up to x(Co)=0.7 in the case of cobalt, provided that the synthesis is performed under oxygenation conditions to favor the presence of Co³⁺. Presence of different magnetic entities (i.e., Er³⁺, Ni²⁺, Co²⁺, Co³⁺, Mn³⁺, and Mn⁴⁺) leads to quite unusual magnetic properties, characterized by the coexistence of antiferromagnetic and ferromagnetic interactions. In ErNi_xMn_1−xO₃, a critical concentration x_crit(Ni)=1/3 separates two regimes: spin-canted AF interactions predominate at x<x_crit, while the ferromagnetic behavior is enhanced for x>x_crit. Spin reversal phenomena are present both in the nickel- and cobalt-based compounds. A phenomenological model based on two interacting sublattices, coupled by an antiferromagnetic exchange interaction, explains the inversion of the overall magnetic moment at low temperatures. In this model, the ferromagnetic transition-metal lattice, which orders at T_c, creates a strong local field at the erbium site, polarizing the Er moments in a direction opposite to the applied field. At low temperatures, when the contribution of the paramagnetic erbium sublattice, which varies as T⁻¹, gets larger than the ferromagnetic contribution, the total magnetic moment changes its sign, leading to an overall ferrimagnetic state. The half-substituted compound ErCo_0.50Mn_0.50O₃ was studied in detail, since the magnetization loops present two well-identified anomalies: an intersection of the magnetization branches at low fields, and magnetization jumps at high fields. The influence of the oxidizing conditions was studied in other compositions close to the 50/50=Mn/Co substitution rate. These anomalies are clearly connected to the spin inversion phenomena and to the simultaneous presence of Co²⁺ and Co³⁺ magnetic moments. Dynamical aspects should be considered to well identify the high-field anomaly, since it depends on the magnetic field sweep rate.     

Investigation of ferromagnetic coupling between Co layers in a Co/Pt multilayer with perpendicular anisotropy

The coercivity of a Co/Pt multilayer with out-of-plane anisotropy can be lowered greatly if it is grown onto an ultrathin NiO underlayer . By making use of this characteristic, a series of samples glass/NiO(10 Å)/[Co(4 Å)/Pt(5 Å)]₃/Pt(x Å)/[Co(4 Å)/Pt(5 Å)]₃ with different Pt spacer thickness have been prepared to determine the ferromagnetic (FM) coupling between Co layers across the Pt layer. The measurements of major and minor hysteresis loops have shown that the FM coupling between the top and bottom Co/Pt multilayers decreases monotonically with the Pt layer thickness and disappears above the Pt layer thickness of 40 Å. This thickness of 40 Å is much larger than that in the literature. In addition to the FM coupling between the top and bottom Co/Pt multilayers across the Pt spacer, there exists a weak biquadratic coupling, which induces the broad transition of the bottom Co/Pt multilayer.     

Synthesis,structure and preliminary results on electrical and magnetic properties of (Perylene)2 [Pt(mnt)2]

In this paper we report the preparation, crystal structure and some physical properties of the conducting molecular (Perylene)₂ [Pt(S₄C₄(CN)₄)].The crystal structure consists of segregated stacks of perylene and Pt S₄C₄ (CN)₄ units, with a spacing of 3.3 Å between the carbon atoms in the perylenes. The temperature dependence of the electrical conductivity is metallic with a maximum at approximately 15 K. At this temperature a sharp transition to an insulating state occurs. Magnetic suceptibility measurements confirm the existence of a transition in this temperature range.     

Size-dependent microstructure and europium site preference influence fluorescent properties of Eu-doped Ca10(PO4)6(OH)2 nanocrystal

In this study, Eu³⁺-doped nanocrystalline Ca₁₀(PO₄)₆(OH)₂ (Ca_10−xEu_x(PO₄)₆(OH)₂) with different particle sizes have been prepared by the thermal decomposition of precursors. Size-dependent microstructure could be observed in nanocrystalline Ca_10−xEu_x(PO₄)₆(OH)₂. The lattices of Ca_10−xEu_x(PO₄)₆(OH)₂ nanocrystals were more distorted in comparison with the bulk, and the smaller the particle size, the more distorted the lattices. Room temperature photoluminescence showed europium site preference was also size-dependent, with the majority of Eu³⁺ ions occupying Ca(II) sites in the bulk, but more and more Eu³⁺ ions occupying Ca(I) sites in Ca_10−xEu_x(PO₄)₆(OH)₂ with decreasing particle size. Fluorescent properties of Ca_10−xEu_x(PO₄)₆(OH)₂ were considered to be influenced by both microstructure and site preference of Eu³⁺ ions. An abnormal strong intensity of ⁵D₀-⁷F₀ transition was observed in bulk and larger Ca_10−xEu_x(PO₄)₆(OH)₂ nanocrystals, but the relative intensities of ⁵D₀-⁷F₀ transition to ⁵D₀-⁷F_1,2,3,4 transition of Eu³⁺ became weaker as the particle sizes decreased. As the particle sizes became smaller, the ratios of the red emission transition (⁵D₀-⁷F₂) to the orange emission transition (⁵D₀-⁷F₁) (R/O values) first increased by comparing the bulk sample with 96 nm sample, and then decreased by comparing 96 nm sample to 57 nm sample. The quenching concentrations of Ca_10−xEu_x(PO₄)₆(OH)₂ samples increased with decreasing particle size. Possible mechanisms responsible for these phenomena were proposed. Since nanosized Ca_10−xEu_x(PO₄)₆(OH)₂ showed higher fluorescent intensities, higher R/O values and higher quenching concentrations, this material is considered to be a promising phosphor.     

The magnetocaloric effect in Nd(Co1−xFex)12B6 alloys

The first-order phase transitions in NdFe₁₂B₆ and PrFe₁₂B₆ alloys give rise to giant values of magnetic entropy changes in relatively low field. However, the metastable nature of these alloys associates with a special procedure of preparation and considerable amount of impurities inevitably. By alloying NdFe₁₂B₆ with the iso-structural compound of NdCo₁₂B₆ appropriately, a Nd(Co_1−xFe_x)₁₂B₆ system which possesses the stable SrNi₁₂B₆-type structure can be obtained directly via the standard casting-and-annealing method. Remarkably improved thermal and magnetic reversibility are observed in the present system. The second-order phase transitions in NdCo₁₂B₆ alloy give rise to the relative cooling power, which is comparable with that of NdFe₁₂B₆ alloy around the ordering temperature.     

Thermoelectric power of TTF[Ni(dmit)2]2

The 1D organic salt TTF[Ni(dmit)₂]₂ becomes superconductor with T_c=1.6 K under an applied hydrostatic pressure of 7 kbar. Structural determinations in this system lead us to suspect that superconductivity (SC) coexists with a charge density wave (CDW) instability at low pressure. In order to better understand how SC emerge from a CDW and to revisit the pressure–temperature phase diagram of the TTF[Ni(dmit)₂]₂ we performed transport and thermoelectric power measurements under pressure.     

超高泵浦功率下的Xe(6p[1/2]0, 6p[3/2]2, 和6p[5/2]2)原子的能量转移过程

本文研究了Xe(6p[1/2]₀, 6p[3/2]₂, and 6p[5/2]₂)原子在聚焦条件下的动力学过程. 激发能级的原子密度在聚焦条件下会显著地增加,因此两个高激发态原子之间的energy-pooling碰撞的概率也会增加. 这种energy-pooling碰撞主要有三种类型. 第一种类型为energy-pooling碰撞导致的电离. 一旦将激发激光聚焦,就可以从侧面的窗口观察到非常明显的电离现象,不论激发能级是6p[1/2]₀、6p[3/2]₂或6p[5/2]₂能级. 这种电离的产生机理是energy-pooling电离或者一个Xe^*原子再吸收一个光子产生电离. 第二种类型为跨越较大能极差的energy-pooling碰撞. 当激发能级为6p[1/2]₀能级的情况下,两个6p[1/2]₀原子碰撞会产生一个5d[3/2]₁原子和一个6s''[1/2]₀原子. 第三种类型为跨越较小能级差的energy-pooling碰撞. 以5个二次产生的6p能级为上能级的荧光强度都变得更强,并且这些荧光的上升沿都变得更陡峭. 产生这些6p原子的主要机理是energy-pooling碰撞并非简单的碰撞弛豫. 基于理想气体原子之间的碰撞概率公式,推导出两个6p[1/2]₀原子的energy-pooling碰撞速率为6.39x10⁸s^-1. 此外,6s原子在聚焦条件下的密度也会增加. 因此所有的荧光曲线会因为辐射俘获效应而出现非常严重的拖尾.