Exchange Bias Field Direction Research Articles

Field-free spintronic terahertz emitters based on IrMn/Fe/Pt exchage bias heterostructures

Spintronic terahertz (THz) emitter has more advantages such as lower cost, broader spectrum and easier operation than the commercial THz emitters, and thus has become a focus of research towards the next-generation THz source. However, in such a spintronic THz emitter, an external magnetic field is technologically required to align the orientation of the magnetization, which is detrimental for practical applications. Here, a spintronic terahertz emitter based on IrMn/Fe/Pt exchange bias structure is presented. By means of ultrafast spin injection on Fe/Pt interface followed by the spin-to-charge conversion in Pt, plus the effective magnetic field originating from the IrMn/Fe interface, the THz pulse with considerable intensity can be generated in such a structure without the assistance of external field. Besides, the remanent magnetization for thin Fe layer is enhanced by inserting an ultrathin Cu layer between the IrMn surface and the Fe surface, which is beneficial to the field-free THz emission. The range of obtained dynamic THz spectrum exceeds 60 dB and the positive saturation field can reach up to ~ –10 mT by optimizing the multilayer thickness, meeting the standard for commercial application. By rotating the sample, it is found that the polarization direction of the generated THz wave circulates simultaneously and keeps perpendicular to the direction of exchange bias field in the film plane. Moreover, we design a spin valve THz emitter based on the structure of IrMn/Fe/Pt/Fe by adding a free ferromagnetic Fe layer into the exchange bias multilayers. The emitted THz pulse amplitude is larger for the antiparallel alignment of the Fe layers at zero field than for the parallel alignment or exchange bias structure. The present work shows that the spin terahertz emitter based on IrMn/Fe/Pt exchange bias structure can produce the considerable terahertz signals without external field. Furthermore, the polarization direction of the emitted THz signal can be easily manipulated by rotating the sample. Because of this series of advantages, such exchange bias heterostructures are expected to play an important role in designing the next-generation THz source.

Complementary spin transistor using a quantum well channel

In order to utilize the spin field effect transistor in logic applications, the development of two types of complementary transistors, which play roles of the n- and p-type conventional charge transistors, is an essential prerequisite. In this research, we demonstrate complementary spin transistors consisting of two types of devices, namely parallel and antiparallel spin transistors using InAs based quantum well channels and exchange-biased ferromagnetic electrodes. In these spin transistors, the magnetization directions of the source and drain electrodes are parallel or antiparallel, respectively, depending on the exchange bias field direction. Using this scheme, we also realize a complementary logic operation purely with spin transistors controlled by the gate voltage, without any additional n- or p-channel transistor.

Improving the sensitivity of asymmetric magnetoimpedance in exchange biased NiFe/IrMn multilayers

We investigate the asymmetric magnetoimpedance effect in exchange biased NiFe/IrMn multilayers and explore the possibility of tailoring the linear region of the curves around zero magnetic field by playing with the thickness of the ferromagnetic layers, probe current frequency, and the orientation between external magnetic field and exchange bias field direction. We quantify the sensitivity by calculating the |dZ/dH|H=0 in a wide range of frequencies. The highest sensitivity is observed for the thicker multilayer, reaching ∼160mΩ/Oe at ∼2.0GHz. The results place exchange biased magnetoimpedance multilayers as attractive candidates for the development of auto-biased linear magnetic field sensors.

Reduced spin transfer torque switching current density with non-collinear polarizer layer magnetization in magnetic multilayer systems

Through micromagnetic simulations, it was found that the spin transfer torque (STT) switching current density is reduced with non-collinear polarizer layer magnetization. The dependence of the switching current density on the polarizer layer magnetization angle was investigated, and a typical magnetic tunneling junction structure with an exchange biased synthetic ferrimagnetic polarizer layers was considered. The easy axis of the polarizer layer was varied through controlling the exchange bias field direction in the ellipse cross-section nano-pillar structures. It was found that the switching current density was reduced by 39% when the exchange bias field was 10° from the long axis of the ellipse without the perpendicular STT. When the effect of the perpendicular STT was included, the switching current reductions were general for the non-collinear polarizer layer.

Experimental evidences and driving mechanisms for anisotropic misalignments in exchange coupled systems

Experimental evidence for misalignments between F anisotropy axes, AF anisotropy axes and the exchange bias field direction is shown in a CoFe/Ni 0.38 O 0.62 system. The angular dependence of the remanent magnetization, the exchange bias field and the coercive field is studied as a function of the diluted NiO thickness. The exchange coupling leads to misalignments between the applied field during growth, the exchange bias field and the coercive field directions. It shows that two different interfacial spin frustrations are present, corresponding to pinned and unpinned spins contributions of the diluted NiO.

Domain structure during magnetization reversal of PtMn∕CoFe exchange bias micropatterned lines

The magnetic domain configuration and the magnetization reversal behavior of micropatterned exchange bias elements were investigated by means of magnetic force microscopy. In addition to the unidirectional anisotropy the shape anisotropy determines the overall magnetization reversal behavior. In order to modify the ratio between both anisotropy contributions, the exchange bias field strength was reduced by means of 5keV He+ ion irradiation. For the as-prepared samples, a monodomain magnetization state with the magnetization direction aligned along the exchange bias field direction was found regardless of the element shape. After irradiation the unidirectional anisotropy contribution is reduced and hence the previously homogeneous magnetization state breaks up into small domains with 360° domain walls in between. The appearance of these domain walls, which was mainly observed for the descending branch of the magnetization reversal, is found to depend strongly on the structure width and orientation.

Magnetic relaxation of exchange biasedPt∕Comultilayers studied by time-resolved Kerr microscopy

Magnetization relaxation of exchange-biased ${(\mathrm{Pt}∕\mathrm{Co})}_{5}∕\mathrm{Pt}∕\mathrm{IrMn}$ multilayers with perpendicular anisotropy was investigated by time-resolved Kerr microscopy. Magnetization reversal occurs by nucleation and domain wall propagation for both descending and ascending applied fields, but a much larger nucleation density is observed for the descending branch, where the field is applied antiparallel to the exchange bias field direction. These results can be explained by taking into account the presence of local inhomogeneities of the exchange bias field.

Domain structure of magnetically micro-patterned PtMn/NiFe exchange bias bilayers

A PtMn/NiFe exchange bias system with an exchange bias field of 180 Oe was magnetically patterned by means of a low fluence focused ion beam. A 1 /spl mu/m wide stripe pattern with the long axis (1000 /spl mu/m) oriented parallel to the exchange bias field direction has been irradiated by 2/spl times/10/sup 14/ Ga/sup +//cm/sup 2/. In the irradiated area the coercivity is reduced and the exchange bias field vanishes, while in the nominally nonirradiated region between the irradiated stripes the exchange bias field is reduced to approximately 35 Oe. This reduction is attributed to the magnetic coupling of the irradiated stripes to their intervening nonirradiated regions. Magnetic domain imaging by means of Kerr microscopy and magnetic force microscopy support this interpretation.

Exchange anisotropy modification in NiO/NiFe bilayers by ion bombardment

The dose dependence of the exchange bias field modification by He-ion bombardment in an applied magnetic field was investigated for an oxidic antiferromagnet/ferromagnet bilayer system consisting of NiO/NiFe in continuation of previous investigations for metallic antiferromagnet/ferromagnet systems (FeMn/NiFe, PtMn/NiFe, CrMn/NiFe). For low ion doses ( <1.5×10 15 ions/ cm 2 ), in a magnetic field applied parallel to the as-grown exchange bias field direction, the exchange bias field H eb is enhanced compared to the as-grown field H eb,0, i.e. H eb/ H eb,0>1. For higher doses, the relative exchange bias field H eb/ H eb,0 is found to decrease gradually with increasing dose to roughly zero. Bombardment with low doses in a magnetic field applied antiparallel to the direction of H eb,0 results in magnitudes of | H eb/ H eb,0|>1 and reverses the sign of H eb. For higher ion doses H eb again approaches zero. These findings are similar to the findings for the bilayer system FeMn/NiFe, but differ to those for the systems PtMn/NiFe and CrMn/NiFe. The experimental results are compared with a simulation using the SRIM code.

Determination of the distribution of both exchange bias field strength and direction in ferromagnet/antiferromagnet systems

A new method for the characterizing of ferromagnet/antiferromagnet (F/AF) systems is proposed. By measuring the longitudinal Kerr effect during rotation of a constant field (ROTMOKE technique) we derive unknown magnetic features of the F/AF system. The distribution of the strength and direction of exchange bias field can be determined. Asymmetries of the ROTMOKE curve give clues to the different orientations of the uniaxial and unidirectional anisotropy. The large influence of uniaxial anisotropy on the reversal of F/AF systems is shown.

Field cooling induced changes in the antiferromagnetic structure of NiO films.

The magnetic anisotropy in antiferromagnetic 500 A thick NiO films, before and after the establishment of an exchange bias field with Co84Fe16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both <111> textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction.