Spin wave measurements over the full Brillouin zone of multiferroic BiFeO3

Using the inelastic neutron scattering technique, we measured the spin wave dispersion over the entire Brillouin zone of room temperature multiferroic BiFeO(3) single crystals with magnetic excitations extending to as high as 72.5 meV. The full spin waves can be explained by a simple Heisenberg Hamiltonian with a nearest-neighbor exchange interaction (J=4.38 meV), a next-nearest-neighbor exchange interaction (J'=0.15 meV), and a Dzyaloshinskii-Moriya-like term (D=0.107 meV). This simple Hamiltonian determined, for the first time, for BiFeO(3) provides a fundamental ingredient for understanding the novel magnetic properties of BiFeO(3).     

Tuning exchange coupling by replacing CoFe with amorphous CoFeB in the CoFe/Ru/CoFe synthetic antiferromagnetic structure

We studied exchange coupling in the CoFe/Ru/CoFe synthetic antiferromagnetic structure with systematical replacement of the crystalline CoFe with amorphous CoFeB. Antiferromagnetic exchange coupling intensity decreases with an increase in the replacement in the bottom magnetic layer, which indicates that exchange coupling intensity could be tuned by the replacement. The origin of weakening antiferromagnetic exchange coupling is attributed to the amorphous CoFeB replacement inducing incomplete crystallization and disordered orientation in the Ru layer.     

Exchange-coupling-induced fourfold magnetic anisotropy in CoFeB/FeRh bilayer grown on SrTiO3(001)

Exchange coupling across the interface between a ferromagnetic (FM) layer and an antiferromagnetic (AFM) or another FM layer may induce a unidirectional magnetic anisotropy and/or a uniaxial magnetic anisotropy, which has been extensively studied due to the important application in magnetic materials and devices. In this work, we observed a fourfold magnetic anisotropy in amorphous CoFeB layer when exchange coupling to an adjacent FeRh layer which is epitaxially grown on an SrTiO₃(001) substrate. As the temperature rises from 300 K to 400 K, FeRh film undergoes a phase transition from AFM to FM phase, the induced fourfold magnetic anisotropy in the CoFeB layer switches the orientation from the FeRh$\langle 110\rangle $ to FeRh$\langle 100\rangle $ directions and the strength is obviously reduced. In addition, the effective magnetic damping as well as the two-magnon scattering of the CoFeB/FeRh bilayer also remarkably increase with the occurrence of magnetic phase transition of FeRh. No exchange bias is observed in the bilayer even when FeRh is in the nominal AFM state, which is probably because the residual FM FeRh moments located at the interface can well separate the exchange coupling between the below pinned FeRh moments and the CoFeB moments.     

Local weak ferromagnetism in single-crystalline ferroelectric BiFeO3

Polarized small-angle neutron scattering studies of single-crystalline multiferroic BiFeO(3) reveal a long-wavelength spin density wave generated by ～1° spin canting of the spins out of the rotation plane of the antiferromagnetic cycloidal order. This signifies weak ferromagnetism within mesoscopic regions of dimension 0.03 microns along [110], to several microns along [111], confirming a long-standing theoretical prediction. The average local magnetization is 0.06 μ(B)/Fe. Our results provide an indication of the intrinsic macroscopic magnetization to be expected in ferroelectric BiFeO(3) thin films under strain, where the magnetic cycloid is suppressed.     

不同磁场下退火的纳米BiFeO3颗粒的拉曼散射研究

对不同外磁场强度(B = 0T, 5T和10T)下退火的纳米BiFeO3颗粒进行了室温和液氮温度下的拉曼光谱研究。实验发现, 与0T磁场下退火样品的Raman光谱相比, 同一温度下, 纳米BiFeO3颗粒中的Raman振动模式峰的线宽随着磁场的增加而减小, 表明在外磁场的作用下, 纳米BiFeO3颗粒内部的缺陷在减少,晶格完整性得到改善。经外磁场处理的纳米BiFeO3颗粒位于139 cm-1处的A-1模式峰明显向紫端移动, 说明磁有序的程度可以影响到声子-自旋耦合。5T和10T磁场下退火的样品相比, 该峰峰位没有明显改变, 说明影响样品中磁有序排列的磁场存在一个临界值。三块样品的低温拉曼光谱证实了磁场的作用确实影响样品内磁有序的排列。     

Magnetic annealing of the ion-beam sputtered IrMn/CoFeB bilayers – positive exchange bias and coercivity behaviour

The effect of optimum dilution of antiferromagnetic (AF)/ferromagnetic (FM) interface necessary for observance of positive exchange bias in ion-beam sputtered Si/Ir₂₂Mn₇₈ (t _AF = 12, 18, 24 nm)/Co₂₀Fe₆₀B₂₀(t _FM = 6,9,15 nm) exchange coupled bilayers is investigated by magnetic annealing at 380, 420 and 460 °C for 1 h at 5 × 10^-6 Torr in presence of 500 Oe magnetic field. While the coercivity of the exchange coupled FM layer decreases with the increase in annealing temperature irrespective of the value of t _AF or t _FM, the hysteresis loops however shift by ≈+ 10 Oe whenever the coercivity drops in the 10–15 Oe range. This is consistent with the phase diagram of exchange bias field and coercivity derived from Meiklejohn and Bean model. The X-ray diffraction and X-ray reflectivity measurements confirmed that the texture, grain size and interface roughness of IrMn/CoFeB bilayers are thickness dependent and are correlated to the observed magnetic response of the bilayers. The results establish that optimum dilution of the IrMn/CoFeB interface by thermally diffused Mn-spins is necessary in inducing the effective coupling between the IrMn domains and diluted CoFeB layer. It is further shown that the annealing temperature required for the optimum dilution of the CoFeB interface critically depends on the thickness of the layers.     

Magnetoelectric Effect in CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

JETP Letters - The possibility of electrical control of the interlayer exchange interaction in CoFeB/MgO/CoFeB tunnel junctions exhibiting magnetoresistance of ~200% is studied. It is shown that...     

Magnetic properties and spin dynamics of CoFeB–SiO<Subscript>2</Subscript> multilayer granular heterostructures

It has been found that CoFeB–SiO₂/C and CoFeB–SiO₂/Bi₂Te₃ multilayer heterostructures with a cluster structure of CoFeB layers feature a long-range magnetic order in the entire temperature range from 2 to 300 K. At high temperatures (T = 300 K), CoFeB clusters exhibit magnetic properties characteristic of superparamagnets. At low temperatures (T = 5 K), clusters are ferromagnetic, and the easy magnetization axis is in the film plane. The temperature of the ferromagnetic-to-superparamagnetic state transition of clusters depends on a dielectric interlayer material: the use of Bi₂Te₃ instead of C as a spacer layer leads to an increase in the transition temperature by a factor of 4 and an increase in the magnetization blocking temperature of CoFeB clusters in a field of 100 Oe by a factor of 3.     

Anomalous Hall effect in perpendicular CoFeB thin films

Our recent research achievements in the perpendicular magnetic anisotropy （PMA） properties of the CoFeB sand- wiched by MgO and tantalum layers are summarized. We found that the PMA behaviors of Ta/CoFeB/MgO and MgO/CoFeB/Ta thin films are different. The larger PMA in the latter film is related to the lower magnetization of CoFeB deposited on MgO. Furthermore, we have demonstrated a large anomalous Hall effect in perpendicular CoFeB thin fihn. Our results show large anomalous Hall resistivity, large longitudinal resistivity, and low switching field can be achieved, all at the same time, in the perpendicular CoFeB thin film. Anomalous Hall effect with high and linear sensitivity is also found in an MgO/CoFeBFFa thin film with a thick MgO layer, which opens a door tbr future device applications of perpendicular ferromagnetic thin films.     

不同生长条件下BiFeO_3薄膜的显微拉曼光谱研究

使用显微拉曼散射技术观察了用脉冲激光沉积方法(PLD)在不同的生长条件下制备的BiFeO3薄膜的拉曼光谱,并且研究了薄膜的结构以及不同的生长条件对薄膜的结构和微结构的影响。与BiFeO3粉末的拉曼光谱比较,我们发现在N2气氛中,在LaNiO3缓冲层上沉积的BiFeO3薄膜具有单一的三方相和最完整的晶格结构。     

Terahertz radiation by an ultrafast spontaneous polarization modulation of multiferroic BiFeO3 thin films

Terahertz (THz) radiation has been observed from multiferroic BiFeO3 thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO3 thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe.     

Large planar Hall effect in perpendicularly magnetized W/CoFeB/MgO structures

The planar Hall effect (PHE) in W/CoFeB/MgO structure with perpendicular magnetic anisotropy was investigated as a function of CoFeB thickness (t_CoFeB). The PHE is measured by sweeping the in-plane magnetic field at various azimuthal angles as well as by rotating strong magnetic field which is enough to saturate the magnetization. We observed a huge PHE in the W/CoFeB/MgO sample, which is even larger than anomalous Hall effect (AHE). This is distinct from the results in Ta/CoFeB/MgO samples showing a much smaller PHE than AHE. Since the PHE is insensitive to the t_CoFeB while the AHE is proportional to the t_CoFeB, the unprecedented PHE can be attributed to the W layer with a large spin-orbit coupling.     

Mo覆盖层对MgO/CoFeB/Mo结构磁各向异性的影响

研究了Mo覆盖层厚度对MgO/CoFeB结构磁各向异性的影响. 研究发现, 加平行磁场生长出来的MgO/CoFeB/Mo样品表现为面内各向异性, 并且随着CoFeB的厚度减小, 面内各向异性逐渐减弱; 在CoFeB厚度减小到1.1 nm时, 仍可以保持面内各向异性, 垂直方向的外加饱和场逐渐减少; 厚度在0.9 nm及以下的情况下, 面内各向异性消失. 改变Mo覆盖层厚度, 当t_Mo= 1.6 nm时, 垂直方向的饱和场最小. 当生长过程的磁场变为垂直磁场时, 不同厚度的Mo覆盖层对MgO/CoFeB 的磁各向异性影响不同. Mo厚度在1 nm及以下时MgO/CoFeB/Mo样品表现为面内各向异性, Mo覆盖层厚度在1.2和5 nm之间时样品出现了垂直磁各向异性; 并且垂直方向的矫顽力也发生了变化, Mo覆盖层厚度为1.4 nm时样品的磁滞损耗会大一些.     

Electronic structure and magnetic and optical properties of double perovskite Bi2 FeCrO6 from first-principles investigation

Double perovskite Bi2 FeCrO6 , related with multiferroic BiFeO3 , is very interesting because of its strong ferroelectricity and high magnetic Curie temperature beyond room temperature. We investigate its electronic structure and magnetic and optical properties by using a full-potential density-functional method. Our optimization shows that it is a robust ferrimagnetic semiconductor. This nonmetallic phase is formed due to crystal field splitting and spin exchange splitting, in contrast to previous studies. Spin exchange constants and optical properties are calculated. Our Monte Carlo magnetic Curie temperature is 450 K, much higher than any previously calculated value and consistent with experimental results. Our study and analysis reveal that the main magnetic mechanism is an antiferromagnetic superexchange between Fe and Cr over the intermediate O atom. These results are useful in understanding such perovskite materials and exploring their potential applications.     

Effects of Fe-Oxide and Mg Layer Insertion on Tunneling Magnetoresistance Properties of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions

To study the influence of CoFeB/MgO interface on tunneling magnetoresistance(TMR),different structures of magnetic tunnel junctions(MTJs) are successfully prepared by the magnetron sputtering technique and characterized by atomic force microscopy,a physical property measurement system,x-ray photoelectron spectroscopy,and transmission electron microscopy.The experimental results show that TMR of the CoFeB/Mg/MgO/CoFeB structure is evidently improved in comparison with the CoFeB/MgO/CoFeB structure because the inserted Mg layer prevents Fe-oxide formation at the CoFeB/MgO interface,which occurs in CoFeB/MgO/CoFeB MTJs.The inherent properties of the CoFeB/MgO/CoFeB,CoFeB/Fe-oxide/MgO/CoFeB and CoFeB/Mg/MgO/CoFeB MTJs are simulated by using the theories of density functions and non-equilibrium Green functions.The simulated results demonstrate that TMR of CoFeB/Fe-oxide/MgO/CoFeB MTJs is severely decreased and is only half the value of the CoFeB/Mg/MgO/CoFeB MTJs.Based on the experimental results and theoretical analysis,it is believed that in CoFeB/MgO/CoFeB MTJs,the interface oxidation of the CoFeB layer is the main reason to cause a remarkable reduction of TMR,and the inserted Mg layer may play an important role in protecting Fe atoms from oxidation,and then increasing TMR.     

Magnetic and microstructural characterizations of CoFe and CoFeB nanowires

Magnetic and microstructural characterizations of cobalt-rich CoFe and CoFeB nanowires fabricated in commercially available alumite templates were investigated. The modifications in magnetic and microstructural behaviors of crystalline CoFe and amorphous CoFeB nanowires were observed after magnetic field annealing and reannealing of the samples.     

过量B的Ta/CoFeB/MgO薄膜垂直各向异性和温度稳定性的增强

以CoFeB/MgO为核心单元的垂直各向异性薄膜体系和相关的垂直磁隧道结已获得广泛研究,其中CoFeB的B含量基本都保持为原子比20%.本文采用磁控溅射制备了Ta/(Co0.5Fe0.5)1-xBx/MgO三明治结构及生长顺序相反的系列薄膜,并在573—623K进行真空退火,研究了样品垂直各向异性随B成分的变化.结果显示,当B含量减小到10%时,Ta/CoFeB/MgO体系的垂直各向异性明显降低;相反,当B含量增加至30%时,该体系的垂直各向异性明显增强;发现在高B含量的情形下,样品的垂直各向异性大小与温度稳定性均与三明治结构的生长顺序密切相关;获得了具有优异温度稳定性的垂直磁化MgO/CoFeB/Ta样品.结果表明适当增加B含量是增强CoFeB/MgO体系垂直各向异性和温度稳定性的有效途径之一.     

Investigation of ferromagnetic resonance and damping properties of CoFeB

In this study, the influences of thin film thickness and post-annealing process on the magnetic properties of CoFeB thin films were investigated. The angular dependency and linewidth of the ferromagnetic resonance signal were used to explore the magnetic behavior of sputtered single-layer and trilayer thin film stacks of CoFeB. A micromagnetic simulation model was employed based on the metropolis algorithm comprising the demagnetizing field and in-plane induced uniaxial anisotropy terms with all relevant contributions. Our results reveal that the direction of magnetization changes from in-plane to out-of-plane as a result of the annealing process and induces a perpendicular magnetic anisotropy in the 1-nm thick CoFeB thin film. The ferromagnetic resonance (FMR) linewidth can be defined well by the intrinsic Gilbert damping effect and the magnetic inhomogeneity contribution in both as-grown and annealed samples. The difference between the linewidths of the single and trilayer film is mainly caused by the spin pumping effect on damping which is associated with the interface layers.     

Electric-field-induced spin flop in BiFeO3 single crystals at room temperature

Bismuth ferrite, BiFeO3, is the only known room-temperature magnetic ferroelectric material. We demonstrate here, using neutron scattering measurements in high quality single crystals, that the antiferromagnetic and ferroelectric order parameters are intimately coupled. Initially in a single ferroelectric state, our crystals have a canted antiferromagnetic structure describing a unique cycloid. Under electrical poling, polarization reorientation induces a spin flop. We argue here that the coupling between the two orders may be stronger in the bulk than in thin films where the cycloid is absent.     

Microwave response to the magnetization switching of CoFeB/Ta/CoFeB spin valves and CoFeB films

Negative magnetoresistance modifying the quality factor of a microwave cavity under the magnetization switching of ferromagnetic layers has been discovered in a MgO/CoFeB/MgO/Ta film with a single ferromagnetic layer and a MgO/CoFeB/Ta/CoFeB/MgO/Ta spin valve consisting of two ferromagnetic CoFeB layers. The dependence of the first derivative dP/dH of the microwave absorption signal on the dc magnetic field of the spectrometer exactly reproduce the magnetic hysteresis loops of the sample. The slope of these dependences and the amplitude of dP/dH jumps under remagnetization of the layers are determined by the interplay of a negative magnetoresistance of individual layers and a positive giant magnetoresistance of the entire multilayer structure. The discovered phenomenon allows using microwave absorption for making a high-sensitivity contact-free indicator of the basic magnetization states of a spin valve.