| Electrons have both charge and spin properties. Microelectronics which based on the charge properties have played an important role in promoting the progress of human society.However, with the miniaturization and integration of devices, the problem of high power consumption is becoming more serious,making the development of microelectronics difficult. Moore's Law is no longer applicable. In recent years,in order to overcome this obstacle, the spin property has been widely concerned. In the history of the development of spintronics, a landmark work is that Albert Fert and Peter Griinberg found giant magnetoresistive effect (GMR) in 1988. Since then, the field of spintronics has developed rapidly. The tunnel magnetoresistance effect (TMR) based on AlOx and MgO, spin transfer torque effect (STT) and spin Hall effect (SHE) have been found. Nowadays, a large number of spintronic devices have been developed,therefore,spintronics has played an important role in promoting both the national economic development and human social progress.There is much work focused on the generation, detection, and regulation of spin current, especially because of its applications in the future spintronic devices. It has been found that the pure spin current can transport electron spin angular momentum without charge current, and its material application field can be broadened into magnetic insulator,which can greatly reduce the power consumption of the device. In recent years, many methods of generating pure spin current have been found, such as spin Seebeck effect based on temperature gradient, non-local electrical injection, circularly polarized light injection,and spin pumping effect based on ferromagnetic resonance (FMR). In ferromagnetic /paramagnetic metal heterostructures, when the ferromagnetic resonance occurs, the magnetization precesses and continues to transfer the angular momentum into paramagnetic metal.The sample consists of ferromagneic material and normal metal bilayers is widely used in the study of spin current. Both permalloy Ni81Fe19 (Py) and magnetic insulator, such as yttrium iron garnet Y3Fe5O12 (YIG) are used in the spin pumping experiment, the contacted paramagnetic metal is usually with a relatively strong spin orbit coupling such as: Ta, Pt, W, etc. The inverse spin Hall effect (ISHE) voltage is one of the important signs of the spin pumping effect. Until now, the spin Hall angle of such paramagnetic is between 0.07?0.2. It is not enough large to be applied in the device. Looking for a material with large spin Hall angle is a vital problem.Recent years,due to the unique physical properties of antiferromagnetic materials,such as no net magnetization, very large spin-orbit coupling and non-collinear spin, they have attracted much attention in the study of pure spin current. Very large spin Hall effect(SHE) is theoretically predicted that may be found in y-FeMn, IrMn3 and Cr. Previously,the anisotropic magnetoresistance (AMR) of antiferromagnetic materials have been reported. Antiferromagnetic materials began to emerge in the fields related to spin Hall effect (SHE), but there is a lot of problems to be solved.Another research hotspot in the spintronics is the magnetic proximity effect in YIG/Pt system. In general, when Pt and magnetic materials are in contact, the atomic layer of Pt at interface will be spin polarization,this effect is known as the magnetic proximity effect. X-ray magnetic circular dichroism (XMCD) has been used to confirm the existence of the magnetic proximity effect of Pt. In the YIG/Pt system, the argument about the magnetic proximity effect has been debated and not unified until now. The debated field is in the spin Seebeck effect and magnetoresistance measurement of Pt. Spin pumping is also an effect connected with the interface, however, there is still no reported research on the influence of magnetic proximity effect of Pt in the spin pumping experiment of YIG/Pt.Based on the current research of spin current, we choose to study the spin Hall effect of FeMn with or without antiferromagnetic order; the origin of spin current in YIG/Pt system and the amplification of spin rectification voltage with antenna. The main results are as follows:I. Compared to paramagnetic (PM) FeMn film, the antiferromagnetic (AF) FeMn has larger and opposite spin Hall angle; the spin diffusion length of antiferromagnetic FeMn is 6.8 nm.In order to discover the effect of antiferromagnetic order on the spin Hall effect of FeMn,we choose Cu as the buffer layer. YIG/FeMn/Ni81Fe19 and YIG/Cu/FeMn/Ni8iFei9 samples were prepared and their magnetoresistance measurements at room temperature and low temperature were performed respectively. According to the results of exchange bias effect, it shows that FeMn in YIG/FeMn/Ni81Fe19 system is paramagnetic, and FeMn in YIG/Cu/FeMn/Ni81Fe19 system is antiferromagnetic. We have studied the spin pumping and spin Seebeck in YIG/FeMn/Cu and YIG/Cu/FeMn systems, respectively. The spin pumping voltages are 1 ?V and 2 ?V and the spin Seebeck voltages are 0.2 ?V and 0.4 ?V respectively. Furthermore,the inverse spin Hall voltages of both samples are with opposite polarity.It is also found that the difference of voltage is due to the contribution of the FeMn layer rather than the Cu layer. In addition, in order to characterize the polarity of spin Hall angle of the paramagnetic FeMn and the antiferromagnetic FeMn, we fabricated a YIG/W sample, and measured the inverse spin Hall voltage. As reported, W is a material having a negative spin Hall angle. The results show that the paramagnetic FeMn has a negative spin Hall angle and the antiferromagnetic FeMn has a positive spin Hall angle. Furthermore, we fabricated a series of sample with different FeMn thickness, calculated the spin diffusion length of antiferromagnetic FeMn by fitting the ?H of voltage with the thickness of FeMn,and got the spin diffusion length of antiferromagnetic FeMn is 6.8 nm.We first clarified the antiferromagnetic FeMn has larger spin Hall angle than paramagnetic FeMn, and their spin Hall angle has different polarity by experiment. The sign of ISHE voltage could depend on a subtle interplay between the orientations of orbital and spin momenta as well as on the character (repulsive vs. attractive) of scattering potentials, which might be strong related to the large exchange filed in AF-FeMn system.In other words, the influence of the non-collinear AF order on the spin-orbit coupling and/or on Berry phase curvature can be significant. Therefore, the intrinsic SHE of FeMn alloy critically depends on its AF ordering.II. In contrast to the general belief, in the spin pumping measurement of YIG/Pt, the origin of spin current is the interface between YIG and Pt.The home-made FMR and spin-pumping systems were used to characterize the inverse spin Hall voltage of YIG/Pt sample with nanometer thickness, in situ. With compared experiments of the samples with Pt partially covered YIG and fully covered YIG, we found that the voltage corresponds to a weak microwave absorption signal with smaller resonance field than YIG. Especially, the weak microwave absorption signal could only be found with the sample connected with Cu patch antenna. In addition, the ISHE voltage and the spin-rectification voltage were measured simultaneously in the YIG/Pt system. We assume that the origin of spin current in YIG/Pt system with nanometer thickness is the interface between YIG and Pt. The interface may be the magnetic Pt magnetized by YIG or the Fe ion diffused from YIG.In order to prove this assumption, we do the first principles calculation of YIG/Cu and YIG/Al interfacial magnetic moment. The results show that the magnetic Cu has opposite magnetic moment with magnetic Pt, and the value is one fifth of Pt's magnetization. On the x other hand, there is no magnetic Al at the interface. As a compared experiment, the samples of YIG/Cu/Pt and YIG/Cu/Ta were prepared. The ISHE voltage shows that the peak of voltage is not corresponded to the FMR signal of YIG, which corresponds to another peak with magnetic field 25 Oe smaller that YIG. A sample of YIG/Al/Pt was also prepared, and no inverse spin Hall voltage was measured in the magnetic sample system. This is consistent with our first-principles calculation that the Al at the interface is not magnetized.We assume that the spin current comes from YIG and estimate the upper-limited value of ISHE voltage. The results show that the ISHE voltage may be in the order of nV which is three orders smaller than the value we measured. In other words,the voltage we measured in the experiment is not from the spin current pumped by YIG.Our experiment results show that, due to the large difference in the electronic structure between YIG and Pt, in YIG/Pt system with nanometer thickness, the spin current may not be effectively diffused into the Pt,which provides a new thought of the interfacial mixed conductivity between the different materials.III. We found that the amplification effect of magnetization dynamics in the magnetic metal with patch antenna.We combined 10 nm Ni81Fe19 film with a patch antenna to effectively control the spin rectification voltage, and got the amplication order by fitting the spin rectification voltage with angle. Furthermore, we combined 10 nm Co film with the patch antenna and effectively amplied the low-filed microwave absorption and spin rectification voltage at non-resonance condition. The microwave absorption signal during the magnetization reverse process of ferromagnetic metal film under 10 nm was firstly detected by us with the patch antenna. Finally, we combined the results of OOMMF micro magnetic simulation and found that the low-field microwave absorption peak corresponds to the process of that the reversal magnetic moment tending to saturation, magnetic domain wall disappearing.Our experiment provides a new method of controlling the high frequency dynamics process of ferromagnetic metal and detecting the magnetization inversion.In shorts, combined with the ferromagnetic resonance, spin pumping and spin Seebeck measurements, we investigated the spin Hall effect of antiferromagnetic FeMn, the origin of spin current in YIG/normal metal system, and the control of spin rectification voltage with antenna. Our results offered some theoretical and experimental guide in the spin pumping and spin Hall effect. |
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