Out-of-plane coercive field of Ni80Fe20 antidot arrays

The out-of-plane magnetic anisotropy and out-of-plane magnetization reversal process of nanoscale Ni₈₀Fe₂₀ antidot arrays deposited by magnetron sputtering technique on an anodic aluminum oxide (AAO) membrane are investigated. The angular dependence of out-of-plane remanent magnetization of Ni₈₀Fe₂₀ antidot arrays shows that the maximum remanence is in-plane and the squareness of the out-of-plane hysteresis loop follow a |cos θ| dependence. The angular dependence of out-of-plane coercivity of Ni₈₀Fe₂₀ antidot arrays shows that the maximum coercivity lies on the surface of a cone with its symmetric axis normal to the sample plane, which indicates a transition of magnetic reversal from curling to coherent rotation when changing the angle between the applied magnetic field and the sample plane.     

Coercivity and squareness enhancement in ball-milled FeCo–MnO nanocomposites

FeCo–MnO nanocomposites were prepared by ball milling mixtures of Fe, Co and Mn, where MnO was obtained from the oxidation of Mn after or during the milling process. The coercivity and squareness of FeCo–MnO nanocomposites increase with increasing milling time. After annealing treatment, the coercivity and squareness increase and exhibit a maximum at 120 h milling time. The temperature dependence of the magnetic properties of FeCo–MnO was also studied and there is a distinct boundary at 120 K. The exchange-bias field and the coercivity decrease quickly with increasing temperature from 30 to 120 K. However, there are no shifted hysteresis loops and the coercivity decreases slightly with increasing temperature from 120 K to room temperature. The enhancement of the coercivity and squareness is mainly attributed to the exchange-bias effects and the reduced magnetic interactions between the FeCo particles by the efficient isolation in MnO matrix.     

Magnetic behaviour of arrays of nickel nanowires with small diameter

Arrays of nickel nanowires have been fabricated within a template of porous alumina by electrochemical deposition. Measurements of magnetic hysteresis loops were performed at room temperature with a vibrating sample magnetometer. Coercivity and squareness of the arrays are closely related to l/D and diameter of the nanowires, and the angle θ between the normal line of the alumina surface and the applied magnetic field. For the same diameter of 10 nm, the coercivity and squareness increase remarkably with l/D when the l/D is less than 100. The diameter and angle θ dependences of coercivity do not follow the relationships of curling, fanning or coherent rotation mode of magnetization while thermal activation for magnetization reversal becomes remarkable for the arrays of Ni nanowires with the diameter less than 18 nm but the same l/D of 50. The coercivity of the arrays with the magnetic field perpendicular to the film surface is linear with D^−3/2 of Ni nanowires. From the fitting line, the critical diameter for superparamagnetism at room temperature and pure coercivity for such Ni nanowire arrays are found to be 6 nm and 1200 Oe respectively.     

The influence of crystallinity enhancement on the magnetic properties of ac electrodeposited Fe nanowires

Fe nanowire arrays with different degrees of crystallinity were fabricated by ac electrodeposition into anodic aluminium oxide templates. A (110) preferential crystal orientation was observed for all the samples. An extensive investigation of X-ray patterns revealed that the variation of the magnetic properties is a direct consequence of the crystallinity degree of Fe nanowires. The results indicate that the degree of crystallinity varies with electrolyte acidity, waveform and frequency of the ac electrodeposition voltage. Regardless of the waveform and electrolyte acidity, higher deposition frequency induces higher coercivity. The effect of waveform and pH value on the magnetic properties is seen to be more pronounced in the low deposition frequency. Improving the ac electrodeposition conditions increased the coercivity and squareness from almost 1010 Oe and 0.49 to 1810 Oe and 0.99, respectively. It was seen that annealing improved the crystallinity, thereby increasing the coercivity and squareness.     

Co-Ni合金纳米线有序阵列的制备与磁性研究

关键词：     

Dimensional effects in the properties of maghemite

Single-phase γ-Fe₂O₃ powders with a coercivity of 161–263 Oe are obtained from iron(III) hydroxide by hydrothermal synthesis followed by heat treatment. Particle size distributions are plotted, based on the TEM data. Magnetic measurements show that a significant decrease in the squareness ratio and coercivity occurs at a mean particle volume of 60000 nm³.     

Comparative studies on Zn0.95Co0.05O thin films on C- and R-sapphire substrates

Zn0.95Co0.05 O precipitate-free single crystal thin films were synthesized by a dual beam pulsed laser deposition method.The films form a wurtzite structure whose hexagonal axis is perpendicular or parallel to the plane of the surface depending on the C-plane (0001) or R-plane (11 ˉ 20) sapphire substrate.Based on the results of high-resolution transmission electron microscopy and x-ray diffraction,C-plane films show larger lattice mismatch.The films exhibit magnetic and semiconductor properties at room temperature.The coercivity of the film is about 8000 A/m at room temperature.They are soft magnetic materials with small remanent squareness S for both crystal orientations.There is no evidence to show that the anisotropy is fixed to the hexagonal axis (C-axis) for the wurtzite structure.     

Fabrication of CoFe_2O_4 ferrite nanowire arrays in porous silicon template and their local magnetic properties

CoFe_2O_4 ferrite nanowire arrays are fabricated in porous silicon templates. The porous silicon templates are prepared via metal-assisted chemical etching with gold(Au) nanoparticles as the catalyst. Subsequently, CoFe_2O_4 ferrite nanowires are successfully synthesized into porous silicon templates by the sol–gel method. The magnetic hysteresis loop of nanowire array shows an isotropic feature of magnetic properties. The coercivity and squareness ratio(M_r/M_s) of ensemble nanowires are found to be 630 Oe(1 Oe = 79.5775 A·m~(-1) and 0.4 respectively. However, the first-order reversal curve(FORC) is adopted to reveal the probability density function of local magnetostatic properties(i.e., interwire interaction field and coercivity). The FORC diagram shows an obvious distribution feature for interaction field and coercivity. The local coercivity with a value of about 1000 Oe is found to have the highest probability.     

c轴垂直取向FePt薄膜的磁和磁光性能研究

采用直流磁控溅射的方法在氧化镁(100)单晶基板上生长了一系列c轴垂直取向的FePt薄膜，通过改变沉积时的基板温度，薄膜从Fe，Pt原子无序排列的面心立方结构逐渐变化到有序排列的L1₀相面心四方结构.在此基础上，系统研究了FePt薄膜的化学有序度对磁和磁光性能的影响.随着有序度的增加，FePt薄膜的磁晶各向异性能，以及沿垂直方向的矫顽力、剩磁比均增加，在基板温度高于530 ℃时制备的薄膜中的磁晶各向异性能超过1 J/cm³.同时，还观察到有序FePt合金薄膜的磁光克尔光谱(克尔转角的大小和极值所对应的跃迁光子能量)随化学有序度的显著变化. 关键词： FePt薄膜 化学有序度参数 磁光克尔光谱     

Magnetic properties of oblique-evaporated Co-Ni thin films

Co_{1 - x}Ni_x (x = 0, 0.2 and 0.3) thin films of thickness about 1500 Å were electron-beam evaporated onto silicon and polymide substrates at various oblique angles . In-plane coercivities and squareness ratios both along and transverse to the incidence plane were examined. Also, the angular variations of coercivity of films prepared at = 0 ° to 85 ° were investigated. The magnetic anisotropy changes from an in-plane anisotropy with the easy axis perpendicular to the incidence plane to an out-of-plane anisotropy parallel to the incidence plane, the transition occuring at about 60 °. Also discussed is the effect of the substrate temperature on the magnetic properties and columnar microstructure of the oblique-evaporated films. At room temperature, there is a small drop in coercivity at = 60 ° before a sharp rise in coercivities to 1400 Oe as the oblique angle increases.     

Magnetic properties of Fe/Pt films by symmetrical insertion of Ag

Ultrathin Ag (0.5 nm) pinning layers (APLs) were symmetrically inserted into [Fe/Pt] bilayers to introduce controllable defects on the interfaces between Ag and Fe/Pt multilayers. The highest coercivity 7700 Oe and remanent squareness 0.95 were obtained with five APLs. The large enhancement in coercivity (75% increment compared with that without APL) is due to the relative uniform defects that introduced pinning effects on the interfaces between the APLs and Fe/Pt multilayers. According to the distribution of angule- dependent coercivity of Fe/Pt multilayers without and with APLs, a tendency is suggested of weakened domain-wall motion while enhanced rotation of reverse domain mode.     

Fabrication and magnetic properties of Fe-Pd nanowire arrays

Ordered 20 nm Fe-Pd nanowire arrays with different compositions have been fabricated by alternating current electrodeposition into nanoporous anodic alumina. The structural and magnetic properties of the arrays were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and vibrating sample magnetometer (VSM). When Fe content is lower than 46 at.%, Fe-Pd alloy phase with fcc structure forms for the as-deposited. After annealing the alloy structure remains unchanged, but the coercivity (H_C) and squareness (M_r/M_s) increase. When Fe content is up to 60 at.%, α-Fe and Fe-Pd phases with fcc structure coexist for the as-deposited. After annealing the nanowires consist of a uniform Fe-Pd phase with fcc structure and the coercivity and squareness decrease. The change of the structure and magnetic properties with the alloy composition and annealing are explained reasonably.     

Fe_3O_4单晶薄膜磁性电场调控的微磁学仿真研究

磁性薄膜磁学特性电场调控的相关研究对开发新型低功耗磁信息器件具有突出意义.本文基于电场调控磁性的基本理论,以OOMMF(Object Oriented Micro-Magnetic Frame)微磁学仿真软件为工具,研究了电场对生长于PZN-PT单晶衬底上Fe_3O_4单晶薄膜磁学特性的调控.研究结果显示:无外加电场时,薄膜表现出典型的软磁特性;沿衬底[001]方向施加的外加电场可以使得薄膜矫顽力、矩形比等磁学特性发生显著改变:当外加磁场沿[100]([010])时,施加正值(负值)电场强度可以显著增大薄膜的矫顽力与矩形比,当电场强度不小于0.6 MV/m时薄膜矩形比达到1.这是因为外加电场导致薄膜产生单轴应力各向异性,使得薄膜的等效磁各向异性发生了从无外电场下的面内四重磁晶各向异性向高电场下的近似单轴磁各向异性的过渡.外加1 MV/m与-1 MV/m的电场时等效易磁化轴分别沿[100]与[010]方向.另外,外加1 MV/m(-1 MV/m)的电场强度可以使得铁磁共振的频率增大(减小)接近1 GHz.     

不同易轴取向下对Nd_2Fe_(14)B/Fe_(65)Co_(35)磁性双层膜的微磁学模拟

运用三维数值模拟计算方法,计算了膜面外不同易轴取向下Nd2Fe14B/Fe65Co35磁性双层膜的磁滞回线、角度分布、成核场、矫顽力和磁能积等,并与实验结果进行了细致比较.计算结果表明:只有当易轴与外场之间的夹角β=0?时,才有明显的成核现象,其成核场和矫顽力均随着软磁相厚度Ls的增加而降低;随着易轴偏角β的增大,剩磁逐渐减小,磁滞回线的方形度降低,从而磁能积减小,在Ls=1 nm,β=0?时磁能积(561.61 kJ/m3)最大.理论计算所得的磁滞回线与实验磁滞回线符合得很好,剩磁和矫顽力的理论值与实验值相差很小.     

Effects of annealing temperature on structure and magnetic properties of amorphous Fe61Co27P12 nanowire arrays

Arrays of Fe₆₁Co₂₇P₁₂ nanowire with an aspect ratio about 70 were prepared in anodic aluminum oxide templates by electrodeposition. The influences of annealing temperature on structure and magnetic properties of Fe₆₁Co₂₇P₁₂ nanowires were studied. When the specimens were annealed below 400 °C, there are no obvious changes in structure except relaxation. With the annealing temperature increasing from 400 to 600 °C, the Fe-Co phase is detected by X-ray diffraction and Mössbauer spectra. The crystalline fraction and hyperfine field can be derived from Mössbauer spectra. The room temperature magnetic hysteresis loops show that the coercivity and squareness of the nanowire arrays in parallel to the wire axis increase with the increasing of annealing temperature, which mainly attributes to the strengthening of anisotropy.     

Sm–Co and Nd–Fe–B thin films with perpendicular anisotropy for high-density magnetic recording media

Sm–Co and Nd–Fe-B thin films have been prepared by sputtering. Particular efforts are concentrated on the perpendicular texture growth of the films. The Sm–Co and Nd–Fe–B thin films with perpendicular magnetic anisotropy can be prepared on Cu and W underlayer, respectively. Those underlayers play an important role to prevent oxidation and improve crystal lattice orientation. Perpendicular coercivity and remanent squareness ratio are higher than 10 kOe and almost 1.0, respectively, in both films prepared under optimum conditions.     

Depth dependence of Néel wall pinning on amorphous CoxSi1−x films with diluted arrays of elliptical antidots

Diluted arrays of elliptical antidots have been fabricated by optical lithography, electron beam lithography and plasma etching on amorphous Co₇₄Si₂₆ magnetic films with a well-defined uniaxial anisotropy. The magnetic behavior of two identical antidot arrays but with different hole depth in comparison with film thickness has been studied by transverse magneto-optical Kerr effect. Significant differences appear in the coercivity depending on whether the magnetic film is completely perforated or not, indicating a much more effective domain wall pinning process when the depth of the holes is smaller than the magnetic film thickness.     

真空退火对L1_0-CoPt薄膜结构、磁性及表面形貌的影响

本文利用磁控溅射的方法制备了L1_0-CoPt薄膜,研究了不同退火条件对薄膜结构、磁性以及表面形貌的影响.通过优化退火温度、保温时间以及升温速率,制备出了具有大矫顽力、高矩形比、粗糙度小的垂直各向异性L1_0-CoPt薄膜.实验发现,较高的退火温度有利于克服CoPt薄膜有序化转变所需要的能量,同时适当延长退火时间可以提高CoPt薄膜的扩散系数,从而促使无序相fcc转化为L1_0有序相fct结构.此外,退火过程中减缓升温速率可以有效减小薄膜粗糙度,从而形成平整连续膜.     

Magnetic properties of YCo5 (70%wt)+Y2Co17 (30%wt) nanocomposite powders at low temperatures

Nanostructured YCo₅ (70%wt)+Y₂Co₁₇ (30%wt) composite powders were prepared by mechanical milling and subsequent annealing at 1073 K for 1.5 min. The average grain size D of the YCo₅ and Y₂Co₁₇ phases, obtained from XRD data, was 14 and 12 nm, respectively. The temperature dependence of the magnetic properties was studied by DC magnetization measurements at temperatures T ranging from 3 to 300 K. Hysteresis loops (H_max=70 kOe) show that both the coercivity H_C and the squareness σ_r/σ_max are temperature-dependent. The coercivity increases from 12 kOe at room temperature to 18 kOe at T=3 K. The observed enhanced remanence (σ_r/σ_max>0.5) indicates that a strong exchange coupling is present at all temperatures used in this study. The maximum magnetization σ_max changes little with temperature and has a value of about 70% of the effective saturation magnetization of the title compound.     

Microstructure and magnetic properties of Co-Cu nanowire arrays fabricated by galvanic displacement deposition

CoCu nanowires were fabricated in anodic alumina templates by a simple metal displacement deposition method and the as-deposited samples were subsequently annealed at 400 °C in vacuum. The CoCu nanowires are 80 nm in diameter and 50 μm in length. The aspect ratio (ratio of length to diameter) is larger than 600, which results in distinctive magnetic anisotropy. Enhanced coercivity (about 2245 Oe) and large squareness of 92% have been observed in the annealed samples.