Current-Induced Magnetic Switching in an L1_(0) FePt Single Layer with Large Perpendicular Anisotropy Through Spin–Orbit Torque

In this study,current-induced partial magnetization-based switching was realized through the spin–orbit torque(SOT)in single-layer L1_(0) FePt with a perpendicular anisotropy(K_(u⊥))of 1.19×10^(7) erg·cm^(-3)(1 erg·cm^(-3)=0.1 J·m^(-3)),and its corresponding SOT efficiency(βDL)was 8×10^(-6) Oe·(A·cm^(-2))^(-1)(1 Oe=79.57747 A·m^(-1)),which is several times higher than that of the traditional Ta/CoFeB/MgO structure reported in past work.The SOT in the FePt films originated from the structural inversion asymmetry in the FePt films since the dislocations and defects were inhomogeneously distributed within the samples.Furthermore,the FePt grown on MgO with a granular structure had a larger effective SOT field and effi-ciency than that grown on SrTiO_(3)(STO)with a continuous structure.The SOT efficiency was found to be considerably dependent on not only the sputtering temperature-induced chemical ordering but also the lattice mismatch-induced evolution of the microstructure.Our findings can provide a useful means of efficiently electrically controlling a magnetic bit that is highly thermally stable via SOT.

Formation of L1_(0)-FeNi hard magnetic material from FeNi-based amorphous alloys

L1_(0)-FeNi hard magnetic alloy with coercivity reaching 861 Oe was synthesized through annealing Fe_(42)Ni_(41.3)Si_8 B_(4)P_(4)Cu_(0.7)amorphous alloy,and the L1_(0)-FeNi formation mechanism has been studied.It is found the L1_(0)-FeNi in annealed samples at 400℃mainly originated from the residual amorphous phase during the second stage of crystallization which could take place over 600 C lower than the measured onset temperature of the second stage with a50 C/min heating rate.Annealing at 4000 C after fully crystallization still caused a slight increase of coercivity,which was probably contributed by the limited transformation from other high temperature crystalline phases towards L1_(0)phase,or the removal of B from L1_(0)lattice and improvement of the ordering quality of L1_(0)phase due to the reduced temperature from520℃to 400℃.The first stage of crystallization has hardly direct contribution to L1_(0)-FeNi formation.Ab initio simulations show that the addition of Si or Co in L1_(0)-FeNi has the effect of enhancing the thermal stability of L1_(0)phase without seriously deteriorating its magnetic hardness.The non-monotonic feature of direction dependent coercivity in ribbon segments resulted from the combination of domain wall pinning and demagnetization effects.The approaches of synthesizing L1_(0)-FeNi magnets by adding Si or Co and decreasing the onset crystallization temperature have been discussed in detail.

Microstructure and magnetic properties of granularFePt/(FePt)_(27)Ti_(73) films for ultrahigh-density recording media

FePt granular films were prepared by direct current facing-target magnetron sputtering system onto glass substrates and subsequently in-situ annealed in vacuum. Vibrating sample magnetometer, X-ray diffraction and scanning probe microscope were applied to study the magnetic properties, microstructures, morphologies and domain structures of the samples. （FePt）27Ti73 bilayer films were fabricated at various conditions to investigate the effect of Ti on FePt grains. The results show that without Ti matrix layer, FePt films deposited onto the glass substrates are fcc disordered; with addition of Ti matrix layer, FePt/Ti films form a ternary （FePt）27Ti73 alloy possessing fcc and L10 （111） mixed texture. FePt/（FePt）27Ti73 films with perfectly ordered L10（111） structure and unique magnetic properties can be obtained at Ti thickness of 35nm and substrate temperature of 250℃. The maximum coercivity is more than 240kA/m and the squareness ratio is more than 0.9. The obtained results suggest that the granular FePt/（FePt）27Ti73 films can be applicable to ultrahigh-density magnetic recording media.

Bi元素诱导高有序度L1_(0)-FePt纳米粒子的一步合成

利用第三金属元素Bi诱导,以十六胺为溶剂,一步合成出分散性好、高有序的L1_(0)-FePt纳米粒子(NPs),探究了反应温度对L1_(0)-FePt纳米粒子有序结构和磁性能的影响。结果表明,随着反应温度的升高,L1_(0)-FePt纳米粒子的有序度(s)和矫顽力(H_(c))均逐渐增加。当合成温度为400℃时,L1_(0)-FePt纳米粒子的粒子尺寸约为24 nm,H_(c)达到0.724 kA·m^(-1),s约为0.8978。由于使用了十六胺作为溶剂,可以促进较多晶核的形成,使得溶剂中的阳离子浓度较低,从而降低了纳米粒子的生长速度,获得了分散性较好、尺寸较小的L1_(0)-FePt纳米粒子。反应过程中,低表面能的Bi原子从核心区域迁移到表面区域,促使晶格中出现大量空位,可大幅度降低Fe原子和Pt原子有序化扩散难度,促进了FePt纳米粒子的有序化转变。因此,通过合理的合成工艺和第三元素诱导,可直接获得高有序和良好分散性的L1_(0)-FePt纳米粒子。通过优化合成工艺有望实现高有序度L1_(0)-FePt纳米粒子的可控制备。

Pb元素诱导高有序度L1_(0)-FePt纳米粒子的一步合成

面心四方相(L1_(0))结构的FePt纳米粒子具有较大的磁晶各向异性,生物相容性好,催化活性高的特点,在信息储存、军事、催化剂、永磁体和生物医学等领域具有广阔的应用前景。研究表明,在制备L1_(0)-FePt纳米粒子的过程中,添加Au、Ag、Cu等贵金属或过渡族金属元素,可以简化制备过程,同时显著降低L1_(0)-FePt纳米粒子的无序-有序转变温度,诱导高有序度L1-FePt纳米粒子的一步合成。掺杂第三金属元素Pb,通过湿化学法直接合成出成分均匀、无团聚还具有高有序度和良好磁性能的L1_(0)-FePt纳米粒子。结果表明,随着反应温度的升高,L1_(0)-FePt纳米粒子的有序度s和矫顽力H。均逐渐增加。实验最佳制备合成温度为360℃,可制得磁性能最佳的L1_(0)-FePt纳米粒子。此时,纳米粒子的原子组分为Fe_(62)Pt_(15)Pb_(23),酸洗后矫顽力,H_(c)=8622.4Oe。因此,合理的合成工艺和第三元素诱导可直接合成出高有序和良好分散性的L1_(0)-FePt纳米粒子。Pb元素诱导高有序度L1_(0)-FePt纳米粒子的一步合成工艺有望实现高有序度L1-FePt纳米粒子的可控制备。

Highly dispersed L1_(0)-PtZn intermetallic catalyst for efficient oxygen reduction

Highly active and durable electrocatalysts with minimal Pt usage are desired for commercial fuel cell applications.Herein,we present a highly dispersed L1_(0)-PtZn intermetallic catalyst for the oxygen reduction reaction(ORR),in which a Zn-rich metal–organic framework(MOF)is used as an in situ generated support to confine the growth of PtZn particles.Despite requiring high-temperature treatment,the intermetallic L1_(0)-PtZn particles exhibit a small mean size of3.95 nm,which confers the catalysts with high electrochemical active surface area(81.9 m^(2)g_(Pt)^(-1))and atomic utilization.The Pt electron structure and binding strength between Pt and oxygen intermediates are optimized through ligand effect and compressive strain.These advantages result in ORR mass activity and specific activity of 0.926 A mg_(Pt)^(-1) and 1.13 mA cm^(-2),respectively,which are 5.4 and 4.0 times those of commercial Pt/C.The stable L10structure provides the catalysts with superb durability;only a halfwave potential loss of 11 mV is observed after 30,000 cycles of accelerated stress tests,through which the structure evolves into a more stable PtZn-Pt core-shell structure.Therefore,the development of a Zn-based MOF as a catalyst support is demonstrated,providing a synergy strategy to prepare highly dispersed intermetallic alloys with high activity and durability.

Mn掺杂直接合成高有序L1_(0)-FePt纳米粒子及其合成工艺

通过掺杂Mn成功直接合成了高有序度、小尺寸L1_(0)-FePt纳米粒子。研究表明:随着合成温度的增加,FePt纳米粒子的分散性,有序度(s)和矫顽力逐渐改善。在合成温度为360℃,获得了尺寸约为8 nm,s为0.903的L1_(0)-FePt纳米粒子。掺杂Mn元素合成FePt纳米粒子中会引起FePt相的晶格畸变,加速Fe和Pt原子的迁移,促进有序化扩散,从而在较低温度下直接合成出L1_(0)-FePt纳米粒子。升温速率对L1_(0)-FePt纳米粒子的有序度和磁性能起到了重要作用,升温速率较快和较慢分别会导致纳米粒子有序化扩散不完全和有序相的晶核较少,进而导致产物有序度不高且矫顽力较低。通过Mn掺杂和合理工艺调控,可以获得小尺寸、分散性好和高有序度的L1_(0)-FePt纳米粒子。

湿化学法一步合成高有序L1_(0)-AuCu纳米粒子

利用湿化学法成功一步合成出高品质L1_(0)-AuCu纳米粒子。研究了合成工艺对AuCu纳米粒子的形貌、成分和有序度的影响。研究表明:随着温度的升高,AuCu纳米粒子结构将发生有序转变。当温度高于320℃时,AuCu纳米粒子开始转变为L1_(0)结构。AuCu纳米粒子中前驱体的添加比例也会影响其形貌和有序度。当金和铜的摩尔比x由小增大时,AuCu纳米粒子的分散度变差,L1_(0)结构的特征峰强度呈现出先增强后减弱的趋势。在合成温度为340℃,金铜摩尔比x为1.11时,成功获得了粒径小、有序度高、分散性好的AuCu纳米粒子。该制备方法工艺简单,可控性强,为直接合成高品质AuCu有序化纳米粒子提供了新思路。

Synthesis of super-fine L1_(0)-FePt nanoparticles with high ordering degree by two-step sintering under high magnetic field

Super-fine L1_(0)-Fe Pt nanoparticles(NPs)with high ordering degree were successfully prepared by a modified two-step sintering method,which includes low-temperature pre-sintering,and the high magnetic field(HMF)assisted post-sintering processes.The particle size of the L1_(0)-FePt NPs was obviously refined by lowering the sintering temperature.By applying the HMF during the post-sintering process,the fine size characteristics of L1_(0)-Fe Pt NPs were retained,and the ordering degree was significantly improved.The L1_(0)-Fe Pt NPs with sizes of about 4.5 nm,ordering degree of 0.940,and coercivity of 22.01 k Oe were obtained by this two-step sintering under a magnetic field of 12 T.The mechanism investigation of HMF enhancing the ordering degree indicates that the HMF enhances lattice distortion and magnetization energy(Zeeman energy).The enhanced lattice distortions cause high stress existing in the lattice,which can effectively promote the disordered-order transition.When the magnetic field reaches to 3 T,the Zeeman energy of the NPs is higher than the thermal disturbing energy of the NPs,and the magnetization effect is stronger.Therefore,the HMF(higher than 3 T)can obviously improve the disorder-order transition by lowering the energy barrier and accelerating the orderly diffusions of atoms.The HMF is a promising assistant to synthesize the L1_(0)-phase NPs with both of high ordering degree and super-fine size.

Micromagnetic studies of perpendicular recording FePt media with controllable grain size distributions

A 3-dimensional(3D)micromagnetic model combined with Fast Fourier Transform(FFT)method was built up to study the writability in the L1_(0)FePt perpendicular medium.The effects of controllable grain size distributions were studied by grain growth simulation.It is found that the cross-track-averaged magnetization changes little between the L1_(0)FePt medium with uniform or non-uniform grain size distribution.

隔离介质对Fe(Co)Pt薄膜结构和性能的影响研究进展

L1_(0)相FePt和CoPt薄膜因具有较高的各向异性、高的饱和磁化强度以及良好的抗氧化化学稳定性,已经成为高密度磁存储介质材料最佳候选者。隔离介质在L1_(0)相Fe(Co)Pt薄膜中应用,可以起到细化晶粒、提高存储密度、促进柱状粒子生长、增强磁晶各向异性、降低晶粒之间磁偶极相互作用及提高信噪比等作用。概括了非金属、金属和氧化物三类不同隔离介质对Fe(Co)Pt薄膜的结构和磁性能的影响概况和研究进展,总结了各种隔离介质作用的优缺点,展望了隔离介质研究和多层设计在未来的发展趋势。

A facile solution phase synthesis of directly ordering monodisperse FePt nanoparticles

The ordered Pt-based intermetallic nanoparticles(NPs)with small size show superior magnetic or catalytic properties,but the synthesis of these NPs still remains a great challenge due to the requirement of high temperature annealing for the formation of the ordered phase,which usually leads to sintering of the NPs.Here,we report a simple approach to directly synthesize monodisperse ordered L1_(0)-FePt NPs with average size 10.7 nm without further annealing or doping the third metal atoms,in which hexadecyltrimethylammonium chloride(CTAC)was found to be the key inducing agent for the thermodynamic growth of the Fe and Pt atoms into the ordered intermetallic structure in the synthetic process.In particular,10.7 nm L1_(0)-FePt NPs synthesized by the proper amount of CTAC show a coercivity of 3.15 kOe and saturation magnetization of 45 emu/g at room temperature.The current CTAC-assisted synthetic strategy makes it possible to deeply understand the formation of the ordered Pt-based intermetallic NP in solution phase synthesis.