Magnetic studies of spin wave excitations in Fe/Pt multilayers

The magnetization of Fe/Pt multilayers has been measured as a function of temperature. For Fe/Pt multilayers with fixed iron layer thickness of 2.5 Å, the magnetization decreases faster with temperature as the platinum layers are made thicker. A simple theoretical model has been used to explain the temperature dependence of the magnetization. From the model the approximate values for the bulk exchange interaction J_b and the interlayer coupling strength J_I for various Fe/Pt multilayers have been obtained.     

Subloop Magnetic Moment Reversal-Induced Recovery Effect in Exchange-Biased IrMn/Pt/Co/Pt Multilayers

For perpendicular exchange-biased IrMn/Pt/ Co/Pt multilayers with bidomain state, double hysteresis loops (DHLs) are achieved. It is found that exchange bias field and coercivity in full loop and two subloops’ training show different dependencies on the cycle number. Further studies show that the negative-biased (positive-biased) subloop measurement will influence training effect of the positive-biased (negative-biased) subloop, acting as a recovery process. This recovery procedure is different from the conventional ways, i.e., by applying a magnetic field perpendicular to the initial pinning direction or by leaving the sample alone for several days without any applied magnetic fields. This recovery effect can be explained by the re-rotation of antiferromagnet (AFM) spins after corresponding subloop magnetic moment reversal during the full loop measurement, which cannot happen during the subloop measurement.     

Perpendicular exchange bias in IrMn/Pt/[Co/Pt]3 multilayers with cone magnetization

The perpendicular exchange bias and magnetic anisotropy were investigated in IrMn/Pt/[Co/Pt]₃ multilayers through the analysis of in-plane and out-of-plane magnetization hysteresis loops. A phenomenological model was used to simulate the in-plane curves and the effective perpendicular anisotropies were obtained employing the area method. The canted state anisotropy was introduced by taking into account the first and second uniaxial anisotropy terms of the ferromagnet with the corresponding uniaxial anisotropy direction allowed to make a nonzero angle with the film's normal. This angle, obtained from the fittings, was of approximately 15° for IrMn/[Co/Pt]₃ film and decreases with the introduction of Pt in the IrMn/Pt/ [Co/Pt]₃ system, indicating that the Pt interlayer leads to a predominant perpendicular anisotropy. A maximum of the out-of-plane anisotropy was found between 0.5 and 0.6 nm of Pt, whereas a maximum of the perpendicular exchange bias was found at 0.3 nm. These results are very similar to those obtained for IrMn/Cu/[Co/Pt]₃ system; however, the decrease of the exchange bias with the spacer thickness is more abrupt and the enhacement of the perpendicular anisotropy is higher for the case of Cu spacer as compared with that of Pt spacer. The existence of a maximum in the perpendicular exchange bias as a function of the Pt layer thickness was attributed to the predominance of the enhancement of exchange bias due to more perpendicular Co moment orientation over the exponential decrease of the ferromagnetic/antiferromagnetic exchange coupling and, consequently, of the exchange-bias field.     

Ion beam mixing of Co/Fe multilayers: Magnetic and structural properties

[Co(7 nm)/Fe(7 nm)]₆ multilayers were electron-beam evaporated onto Si(100) substrates in ultrahigh vacuum and irradiated at room temperature with 200-keV Xe ions, leading to ion beam mixing within the Co/Fe multilayer, but not with the Si substrate. Irradiation-induced changes in structural and magnetic properties were characterized by means of Rutherford backscattering spectroscopy, X-ray diffraction and in-plane magneto-optical Kerr effect. Irradiation with 1 × 10¹⁶ Xe ions/cm² induced Co/Fe intermixing to a 1 : 1 atomic concentration ratio (RBS) and the formation of the Fe₅₀Co₅₀ permendur phase in the intermixed zone (XRD). For lower ion fluences, the coercivity decreased strongly, but then increased slowly for higher fluences. The angular pattern of the relative remanence showed a perfect uniaxial anisotropy. The magnetic energy density was parametrized with the expression E_s / M_s ∝ (K_u1 / M_s) sin²(φ − φ₀) + (K_u2 / M_s) sin⁴(φ − φ₀), M_s being the saturation magnetization and φ₀ the symmetry angle. The second-order term K_u2 / M_s was found to decrease strongly with increasing Xe fluence.     

Investigations on the interlayer coupling in Co/Pt multilayers with perpendicular anisotropy via the extraordinary Hall effect

The interlayer coupling in Co/Pt multilayers has been investigated via the measurements of extraordinary Hall effect. In the conventional [Co/Pt]_n multilayer, the coercivity H_C has been observed to increase exponentially for n < 6 and almost linearly for n > 6 with the decrease of temperature. Surprisingly, H_C for n = 1 shows the fastest increase at low temperatures, and becomes the largest one at T < 40 K. As a function of the repetition number n, the almost temperature-independent oscillation of H_C has been observed, being indicative of the Ruderman-Kittel-Kasuya-Yosida type ferromagnetic interlayer coupling in the [Co/Pt]_n multilayer. However, the antiferromagnetic interlayer coupling can be realized via the spin-valve configuration of the Co/Pt multilayer. In the Co/Pt/[Co/Pt]_n multilayer, the antiferromagnetic-to-ferromagnetic transition of the interlayer coupling has been observed at low temperatures. These observed phenomena are strongly related to the temperature-dependent polarization in the Co/Pt multilayers.     

Strength and aging behavior of Mo/Pt multilayers

The strength of Mo/Pt multilayers as a function of layer thickness has been investigated using nanoindentation. Structures with layer thicknesses varying from 20 to 100 nm were sputter deposited and the hardness of the films was measured using nanoindentation. It is shown that at this length scale the strengthening behavior of the films can be described by the confined layer slip model which has been proposed for other multilayered systems. Of specific interest in this study was the increase in the strength of the films after aging in ambient conditions with no apparent change in modulus. X-ray photoelectron spectroscopy revealed partial oxidation of the Mo layers in the aged samples which was associated with an increase in the residual compressive stress of the films measured by the bulge testing technique. The formation of molybdenum oxides in the Mo layer could account for the increase in strength through a second phase precipitation strengthening mechanism.     

Perpendicular magnetic anisotropy in amorphous ferromagnetic CoSiB/Pt multilayers

Magnetic anisotropy properties of amorphous ferromagnetic CoSiB/Pt multilayers with perpendicular magnetic anisotropy (PMA, K(u)) were systematically investigated as a function of CoSiB layer thickness (t(coSiB)) and Pt layer thickness (t(Pt)). In two series of [CoSiB t(coSiB)Pt t(P1)]5 multilayers, the perpendicular coercivity (H(c)) increased to reach a maximum and then decreased with further increase in both t(coSiB) and t(Pt), due to intermixing of CoSiB/Pt interfaces. Particularly, using the amorphous soft magnetic CoSiB, the coercivity became very sensitive to the CoSiB thickness. These multilayer films exhibited a high K(u) of 2 x 10(6) erg/cc and a high H(c) of 360 Oe with marked squareness. It was found that even after annealing at 350 degrees C, the CoSiB/Pt multilayers had a high PMA and their H(c) increased.     

流动式单电解槽电结晶制[Co/Pt]n(n≥200)金属多层膜的研究

采用电化学结晶的方法，在硼酸系流动式单电解槽中成功制得[Co／Pt]200多层膜。通过低角度X射线衍射(LXRD)证实了多层膜结构的存在．中角度X射线衍射(MXRD)则证明了Co—Pt界面上有CoPt3化合物的存在．首次确认了用电结晶法所得的[Co／Pt]n多层膜中存在CoPt3化合物的事实。     

CoSiB/Pt多层膜垂直磁各向异性及热稳定性

采用磁控溅射方法在单面附有300 nm SiO2的单晶硅基片上制备了以Pt为底层的CoSiB/Pt多层膜样品.CoSiB/Pt层周期数确定为2,对样品底层厚度及周期层厚度进行调制,根据反常霍尔效应系统地研究了CoSiB/Pt多层膜垂直磁各向异性(perpendicular magnetic anisotropy,PMA)及薄膜的热稳定性.通过对这些参数的调节获得了具有良好垂直磁各向异性的最佳多层膜样品结构Pt(1)/[CoSiB(0.5)/Pt(1)]2,底层Pt和周期层中CoSiB,Pt的最佳厚度分别为1,0.5 nm和1 nm.对最佳样品进行XRD图谱分析,磁滞回线测量以及一系列退火处理.结果表明,样品具有明显的(111)CoPt衍射峰,形成了较好的(111)织构,界面耦合增强,结晶度较好,计算出样品的有效磁各向异性常数Keff达到5.11×104 J·m-3,样品具有良好的PMA;当退火温度为200℃时,样品的CoPt(111)峰强度显著增强,界面形成了较强的(111)织构,Keff达到最大值1.0×105J·m-3,当退火温度不超过400℃时,样品仍能保持良好的PMA.多层膜样品结构Pt(1)/[CoSiB(0.5)/Pt(1)]2具有良好的PMA和热稳定性,且合适的退火温度有利于提高样品的PMA.     

反铁磁层厚度对垂直磁各向异性钴/铂/铁锰多层膜磁性质的影响

在室温下用超高真空磁控溅射系统制备了一系列的Pt(4.0nm)/[Co(0.5nm)/Pt(0.3nm)]3-FeMn(tAFnm)多层膜样品,研究了反铁磁层厚度对于易轴垂直于样品表面的Co/Pt/FeMn多层膜磁性质的影响。在室温下利用样品的剩磁进行了X射线磁圆二色测量(XMCD),结果表明在铁磁/反铁磁界面有反铁磁层铁锰的净磁矩,这些净磁矩仅来自于铁元素。铁的磁矩倾向于垂直于膜面排列。磁测量结果表明,随着铁锰层厚度的增加,交换偏置场HEB增加直到饱和,而HC先增加,然后轻微减少,在tAF7.5nm以后,HEB和HC都几乎不变了。没有观察到磁锻炼效应。     

Pt/(Pt/Co)n/FeMn/Pt多层膜中Pt插层对交换偏置场的影响

在具有垂直磁各向异性Pt／（Pt／Co）n／FeMn／Pt多层膜中的Co／FeMn界面插入极薄的Pt层时，其交换偏置场有明显提高。研究结果表明：由于在Co／FeMn界面存在界面反应，破坏了（Pt／Co）n多层膜中靠近FeMn层的Co层的垂直磁各向异性，导致垂直交换偏置场Hex减弱。当在（Pt/Co）n与FeMn界面之间插入Pt层时可以有效地阻止这一反应发生，从而提高了多层膜的垂直交换偏置场Hex。     

Co/Cu多层膜层间耦合作用的调节

利用在Fe（5nm）/[Co/Cu]30多层膜的隔离层（Cu）中掺入磁性元素Co，引入与相邻Co层之间反铁磁的RKKY作用相竞争的直接的铁磁耦合作用，降低了相邻Co层间的交换耦合作用。在Cu层中掺入不同原子数比例的磁性元素时，饱和磁场有所降低，从而提高了多层膜对外磁场变化的灵敏度。     

Spin-Wave Excitations in Evaporated Co/Pt Multilayers

The magnetic properties of evaporated Co/Pt multilayers have been studied by magnetic measurements and ferromagnetic resonance (FMR). The spin-wave resonances were observed in some multilayers in FMR experiments, which implied that spin waves were sustained by the whole and propagated through Pt layers. The relation of the resonance field H _res with the mode number n obeys the so-called n ² law and the interlayer coupling strength J _I has been determined. The temperature dependence of the spontaneous magnetization can be well described by Bloch’s law, in all multilayers. The increase of the spin-wave parameter B with decreasing cobalt thickness has been discussed. A spin-wave theory has been used to explain the temperature dependence of the magnetization and the approximate values for the bulk exchange interaction J _b and surface exchange interaction J _S for various Co/Pt multilayers have been obtained.     

Magnetic properties of sputtered multilayers of Mo/Ni

We have studied the d.c. magnetization of Mo/Ni artificial superlattices in the temperature range 5–300 K and in magnetic fields of up to 10⁷/4πAm^-1 (10kG). The saturation magnetization and the Curie temperature behavior are consistent with expectations based on thin film effects. However, there are indications of ferromagnetic coupling across the normal metal and the coercive field shows an unexpected peak at a layer thickness of 25 Å.     

真空退火Co/Pt多层膜的结构、磁及磁光特性分析

本文用直流磁控溅射方法制备了Co／Pt多层膜,并对其进行了较为细致的真空退火处理,结果表明,适度的低温退火可增加Co／Pt多层膜的矫顽力,而并不削弱其磁滞克尔回线的矩形特征．当退火温度达到300℃以后,Co／Pt多层膜的矫顽力、垂直各向异性和克尔角将强烈下降,晶体结构的改善及应力的释放、晶粒的增长、层间原子扩散引起的Co层有效厚度减薄,进而居里温度的下降,分别是Co／Pt多层膜在低温、中温、高温退火过程中,磁及磁光性能变化的主要机制．     

Pt/Co改性石墨电极的制备及其电催化性能

本文采用化学镀在块状石墨表面镀Co,然后用真空离子镀沉积Pt制备Pt/Co石墨电极。通过扫描电子显微镜(SEM)对改性后的石墨电极试样微观组织进行观察;用X射线荧光光谱分析(XRF)确定石墨电极表面成分;用循环伏安法研究了石墨电极的电催化性能,并探讨了峰值电流与化学镀Co的时间、甲醇浓度等的关系。结果表明,在真空离子镀Pt工艺参数不变的情况下,镀Co时间在120s范围内,电极的甲醇电催化性随着化学镀时间延长和甲醇浓度增大而增强。     

DC magnetization studies in Nb/Fe superconducting multilayers

The behavior of superconducting transition temperature T_C in superconducting/ferromagnetic (S/F) multilayers as a function of different layer thicknesses and for varying magnetic moment μ_B of the F layer atoms is studied. Multilayer structures consist of five bilayers of constant superconducting Nb layer thickness of 400 Å and Fe of 6 and 24 Å each. The analysis of the magnetization data revealed that for t_Fe=6 Å, the Fe layer is non-magnetic. The interpretation of the observed T_C behavior is attributed to the change of the interaction of the cooper pairs with this layer at the onset of ferromagnetism for t_Fe=6 Å. The hysteresis curve recorded under isothermal conditions at 4.5 K for [Nb (400 Å)/Fe (6 Å)]₅ multilayers shows the usual M–H hysteresis behavior which is typical of a hard type-II superconductor exhibiting an irreversibility field H_irr of 3.5 kOe with substantial pinning at lower field. In addition, [Nb(400 Å)/Fe(6 Å)]₅ multilayer displays anomalous behavior in the form of paramagnetic peak in the superconducting state just below the transition temperature T_C=6.25 K.     

Effect of Ga irradiation on the magneto-optic spectra of Pt/Co/Pt sandwiches

Interfacial changes in rf sputtered Pt/Co(2.6 nm)/Pt sandwiches grown onto sapphire (Al₂O₃) substrates induced by irradiation of 30 keV Ga⁺ ions at low dose (10¹⁴ ions/cm²) have been investigated by magneto-optic polar Kerr rotation (PKR) spectroscopy between 1 and 5 eV. The irradiation resulted in an increase of PKR over the whole spectral range. The measured PKR spectra were compared with those computed from the transfer matrix formalism using known polar Kerr rotation and ellipticity spectra for Co and five Co_xPt_1 − x alloys. The comparison between measured and computed PKR spectra provided an in-depth profile of Co and Pt ion distributions across the sandwich and confirmed that irradiation favors alloying in the vicinity of the two interfaces. These results are in a good agreement with the profile evaluated independently by TRIDYN simulations. Our results evidence an asymmetry in the irradiation effect due to an excess of Pt-Co alloying at the upper interface. Moreover, the observation of a negative PKR peak around 3.2 eV states definitively the presence of a chemically ordered Co_0.75Pt_0.25 alloy phase inside the irradiated film structure.