Enhancement of magnetoresistance sensitivity in Co/Cu/NiFe trilayers by softening the Co hard layer

Co/Cu/NiFe trilayers were prepared by sputtering without magnetic field applied. We have found that the Co(2 nm)Cu(1 nm)NiFe(2 nm) trilayer using Ta as buffer layer exhibits an enhanced magnetoresistance (MR) sensitivity by a factor of more than 6 and a low saturation field of 9.3 Oe. Experimental results have demonstrated that the low saturation field is attributed to the softening of the Co layer by depositing the Co(2 nm)Cu(1 nm)NiFe(2 nm) sandwich on Ta layer. The decrease of the coercivity of the Co layer also plays an important role in the enhancement of MR sensitivity by reducing the effective coercivity of the NiFe layer, which is discussed in terms of the change in interlayer coupling.     

Localized spin-wave resonance modes of ferromagnetic microstrips in the field of a magnetic probe

Some results of the micromagnetic modeling of forced magnetization oscillations in planar microstrips of NiFe with easy plane anisotropy and Co/Pt with perpendicular easy axis anisotropy in the field of a magnetic spherical probe are considered. It has been shown that the probe field provokes the appearance of a hedgehog–antivortex coupling state in the NiFe strips, due to its lateral components and a skyrmion magnetization state in the Co/Pt layer. These effects destroy spatial magnetization oscillations in the microstrips and lead to the appearance of additional resonances in the spectrum of oscillations corresponding to the modes localized in the probe field.     

Magnetic behaviour investigation on symmetric spin valves of Co／Cu／NiFe and NiFe／Cu／Co

In this paper,we have obtained and investigated the magnetic behaviours of the ferromagnetic layer in the symmetric spin valves of Co/Cu/NiFe and NiFe/Cu/Co by measuring with a vibrating sample magnetometer and analysing in terms of the multi-domain Ising models.It has been found that some magnetic layer can have quite different magnetic behaviours in different structures of spin valves,depending on the properties of the under-layer.In our investigation,we have found that the magnetic behaviour of a Co layer depends mainly on the magnetization of the under-layer,whereas this is not the case for the NiFe layer.     

Magnetoresistance in epitaxial [NiFe/Cu/Co(/Cu)] films

Epitaxial [NiFe/Cu/Co(/Cu)] films have been grown on Si(100)/Cu substrates using an ultrahigh vacuum evaporation method. Magnetoresistance (MR) and magnetization were measured at room temperature with maximum applied field, 40 kA/m. The (100) oriented [NiFe(3 nm)/Cu(6 nm)/Co(3 nm)/Cu(6 nm)] × 10 multilayers showed a sharply peaked MR curve (when the external field was applied along [011] direction) due to magnetization rotation of free NiFe layers separated from Co layers with thick Cu layers. Furthermore the interposition of a Ag layer in the Cu layer reduced the couplings between ferromagnetic layers and improved the sensitivity of the [NiFe/Cu/Co(/Cu)] film. Si(100)/Cu(5 nm)/[Co(3 nm)/Cu(2.4 nm)/Ag(0.2 nm)/Cu(2.4 nm)/NiFe(3 nm)/Cu(2.4 nm)/Ag(0.2 nm)/Cu(2.4 nm)] × 10 multilayers showed a resistivity change of about 8.2% per kA/m (12 Oe).     

Mirror domain structures induced by interlayer magnetic wall coupling

We have found that during giant magnetoresistance measurements in approximately 10 x 10 mm(2) NiFe/Cu/Co continuous film spin-valve structures, the resistance value suddenly drops to its absolute minimum during the NiFe reversal. The results reveal that the alignment of all magnetic domains in the NiFe film follow exactly that of corresponding domains in the Co film for an appropriate applied field strength. This phenomenon is caused by trapping of the NiFe domain walls through the magnetostatic interaction with the Co domain-wall stray fields. Consequently, the interlayer domain-wall coupling induces a mirror domain structure in the magnetic trilayer.     

[Co/Pd]4–Co–Pd–NiFe spring magnets with highly tunable and uniform magnetization tilt angles

By varying the Pd thickness (t_Pd) from 0 to 8 nm in [Co/Pd]₄/Co/Pd(t_Pd)/NiFe exchange springs, we demonstrate (i) continuous tailoring of the exchange coupling between a [Co/Pd]₄/Co layer with perpendicular anisotropy, and a NiFe layer with an in-plane easy axis, (ii) tuning of the NiFe out-of-plane magnetization angle from 20^○ to 80^○, and (iii) an up to two-fold increase in the NiFe damping. The partial decoupling also results in a highly uniform NiFe magnetization. These properties make [Co/Pd]₄/Co/Pd(t_Pd)/NiFe spring magnets ideal candidates for use as tilted polarizers, by combining stable and well-defined spin directions of its carriers with a high degree of angular freedom.     

Pseudo spin-valves with Al or Nb as spacer layer: GMR and search for spin switch behaviour

The magnetoresistance of several Ferromagnet/Normal metal/Ferromagnet spin-valve type structures has been investigated using Al as normal spacer layer. A magnetoresistance ratio up to 4.1% at room temperature and 5.7% at 0.3 K is found for the sandwich with both Co layers, while slightly lower signals are found for the structures involving CoFe and NiFe layers. The magnetoresistance dependence for Co/Al/Co, Co/Al/CoFe and Co/Al/NiFe on the spacer layer thickness exhibits the familiar non monotonic behaviour with second peak slightly larger than the one reported for Cu based pseudo spin valves. At cryogenic temperatures, preliminary results on the onset of spin switch effects in Co/Al/Co and the full spin switch effect in Co/Nb/Co are also reported here.     

Bi对自旋阀钉扎场的影响及机理

实验发现将Bi插入自旋阀多层膜TaNiFeCuBi(x)NiFeFeMn中可以显著地提高自旋阀的钉扎场Hex.采用XPS对Cu,Bi元素的分布情况进行了研究,发现Bi的插入明显抑制了Cu原子在自旋阀的制备过程中在NiFeFeMn界面的偏聚.进一步研究表明:自旋阀钉扎层NiFeFeMn界面中,Cu原子的存在是导致自旋阀Hex小于TaNiFeFeMn多层膜Hex的主要原因. 关键词： 自旋阀 钉扎场 交换各向异性 表面活化剂     

Perpendicular interlayer coupling in Ni80Fe20/NiO/Co trilayers

An in-plane perpendicular magnetic coupling between Ni80Fe20 and Co has been found in NiFe/NiO/Co trilayers for a NiO thickness ranging from 4 to 25 nm by magneto-optical Kerr effect and x-ray magnetic circular dichroism measurements. In the easy magnetization direction of the Co layer, the Co coercive field H(C) increases when the thickness of the NiO layer t(NiO) increases. Because of the coupling, H(C) is always larger than for NiO/Co bilayers with the same thicknesses. The saturation field of the NiFe layer H(S) decreases when t(NiO) increases, indicating a weakening of the coupling. Numerical simulations show that the presence of interface roughness combined with a small value of the NiO anisotropy can explain the observed 90 degrees coupling.     

Computer simulation of magnetization flop in magnetic tunnel junctions exchange-biased by synthetic antiferromagnets

Computer simulation in a single domain multilayer model is used to investigate magnetization flop in magnetic tunnel junctions, exchange-biased by pinned synthetic antiferromagnets with the multilayer structure NiFe/AlO_x/Co/Ru/Co/FeMn. The resistance to magnetization flop increases with decreasing cell size due to increased shape anisotropy and hence increased coercivity of the Co layers in the synthetic antiferromagnet. However, when the synthetic antiferromagnet is not or weakly pinned, the magnetization directions of the two layers sandwiching AlO_x, which mainly determine the magnetoresistance, are aligned antiparallel due to a strong magnetostatic interaction, resulting in an abnormal MR change from the high MR state to zero, irrespective of the direction of the free layer switching. This emphasizes an importance of a strong pinning of the synthetic antiferromagnet at small cell dimensions. The threshold field for magnetization flop is found to increase linearly with increasing antiferromagnetic exchange coupling between the two Co layers in the synthetic antiferromagnet. The restoring force from magnetization flop to the normal synthetic antiferromagnetic structure is roughly proportional to the resistance to magnetization flop. Irrespective of the magnetic parameters and cell sizes, the state of magnetization flop does not exist near H_a=0, indicating that magnetization flop is driven by the Zeeman energy.     

Magnetoresistance and magnetization studies through a Cu interlayer in spin valves of NiFe/Cu/NiFe/FeMn

The influence of the Cu layer thickness on the magnetic and magnetotransport properties has been investigated in Ta/NiFe/Cu/NiFe/FeMn spin valves. The magnetization and magnetoresistance measurements were carried out for magnetic field applied along the easy-axis direction. A phenomenological model, which assumes formation of a planar domain wall at the anti-ferromagnetic side of the interfaces as well as bilinear coupling between the ferromagnetic layers, was used to derive the anisotropy characteristics and orientation of each NiFe layer magnetization. The anisotropy and spin valve magnetoresistance were simulated numerically and compared with the experiment. It was found that the anisotropy magnetoresistance is negligible and that there is a poor agreement for the spin-valve one, which was attributed to the model (valid for ferromagnetic layers in single-domain state only) used for its calculation. It was found that the increase of the Cu layer thickness provokes a decrease of the interdiffusion between the NiFe and FeMn layers, and, as consequence, changes of the uniaxial anisotropy of the pinned NiFe layer, of the exchange interaction between the pinned NiFe layer and the FeMn ones, as well as of the exchange-bias field of the pinned NiFe layer.     

Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier

Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.     

Structural and magnetic properties of ion-beam deposited NiFe/Co-oxide bilayers

The structural and magnetic properties of NiFe/Co-oxide bilayers were studied. XRD investigation indicates the NiFe (Permalloy) layers (a= 3.53 Å) grow with a (111) preferred orientation. The Co-oxide layers were fabricated with oxygen content in the deposition assist beam ranging from 8% (rock-salt CoO, a= 4.27 Å) to 34% (spinel Co3O4, a= 8.21 Å). Both the coercivity Hc and exchange bias field Hex closely follow an inverse NiFe thickness relationship. A strong temperature dependence of Hc and Hex is found in these NiFe/Co-oxide bilayers. At T= 289 K, the NiFe/CoO film exhibits an enhanced Hc relative to pure NiFe/Co or NiFe/Co3O4 bilayers, an indication of exchange coupling between the NiFe and CoO phases. At T= 10 K, the NiFe/Co3O4 film exhibits an exchange-bias loop shift of Hex∼- 200 Oe that persists to temperatures greater than 30 K (the Néel temperature of bulk Co3O4). The transition temperature of the Co3O4 film component has increased above the bulk value via exchange coupling with the permalloy. Results indicate that the exchange coupling interaction between FM and AFM layers are responsible for both enhanced coercivity and cross-tie domains.     

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide(AAO) template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.     

Abnormal spin-pumping effect at Pt/NiFe interface by an oxidation of ferromagnet

The Inverse Spin Hall Effect (ISHE) voltage induced by spin-pumping was measured in Pt/[nonoxidized or oxidized NiFe] samples. The spectrum shape of the DC voltage signal was well-reproduced by simple Lorentzian functions, which were consistent with the prediction of ISHE in Pt/nonoxidized NiFe sample. On the other hand, the voltage signal from the Pt/oxidized NiFe was significantly decreased in accordance with abnormal ferromagnetic resonance (FMR) behavior. The origin of the ISHE voltage degradation was investigated by the FMR spectrum and X-ray photoelectron spectroscopy (XPS) analyses. The spin-pumping was suppressed by the oxidation of NiFe layer and deficient Pt/NiFe interface. The well-defined ferromagnet and clear interface with a normal spin mixing conductance were experimentally demonstrated to be reasonable spin-pumping.     

Giant magnetoimpedance effect in sputtered single layered NiFe film and meander NiFe/Cu/NiFe film

Giant magnetoimpedance (GMI) effect on NiFe thin film is very promising due to its application in developing the magnetic field sensors with highly sensitivity and low cost. In this paper, the single layered NiFe thin film and NiFe/Cu/NiFe thin film with a meander structure are prepared by the MEMS technology. The influences of sputtering parameters, film structure and conductor layer width on GMI effect in NiFe single layer and meander NiFe/Cu/NiFe film are investigated. Maximum of the GMI ratio in single layer and sandwich film is 5% and 64%, respectively. The results obtained are useful for developing the high-performance magnetic sensors based on NiFe thin film.     

High temperature magnetoresistance in (Co,CoFe and NiFe)/Cu/CoFe/IrMn spin-valves

The MR characteristics of temperature variations ranging from room temperature to 570 K for IrMn spin-valves which consist of free layer Co, CoFe and NiFe were studied. Co-SV had the highest MR value in all temperature ranges among them and even at 510 K the MR ratio held more than half of room temperature value. Whereas CoFe-SV had a slightly higher MR than NiFe-SV below 450 K, above 450 K CoFe-SV showed a lower MR ratio than NiFe-SV. MR loops of large coercivity such as CoFe free magnetic layer collapsed in the lower elevated temperature compared to NiFe and Co. Small coercivity of the free magnetic layer would be preferable to spin-valves.     

Growth of magnetic materials and structures on Si(0 0 1) substrates using Co2Si as a template layer

We report in this paper the use of Co₂Si silicide as a template layer for the integration of magnetic materials and structures on silicon substrate. By undertaking Co deposition on silicon at a temperature of about 300 °C, we show that it is possible to obtain a smooth and epitaxial Co₂Si layer, which can act as a template layer preventing the reaction between Co and other transition metals with silicon. Two examples of over-growth of magnetic materials and structures on this template layer will be presented: growth of ferromagnetic Co layers and of magnetic tunnel junctions (Co(Fe)/AlO_x/NiFe).     

Exchange bias studies of NiFe/FeMn/NiFe trilayer by ion beam etching

Effect of low energy ion beam etching on exchange bias in NiFe/FeMn/NiFe trilayer is investigated in multilayers prepared by rf magnetron sputtering. Stepwise etching and magnetization measurement of FeMn layer in an NiFe/FeMn bilayer show increase of bias as etching proceeds and FeMn thickness decreases. The bias show a maximum around 7 nm FeMn thickness and then fall sharply below 5 nm, broadly in line with the exchange bias variation at increasing FeMn thickness but in reverse order, particularly at low FeMn thickness. Progressive etching of top NiFe layer in the NiFe/FeMn/NiFe trilayer shows an initial gradual increase in bias followed by a sharp increase below 7 nm thickness of top NiFe layer, with a maximum at 2 nm thickness for both NiFe layers and greater bias for seed NiFe layer.     

Direct observation of unconventional topological spin structure in coupled magnetic discs

Confined magnetic thin films are known to exhibit a variety of fascinating topological spin states such as Skyrmions, vortices, and antivortices. Such topological excitations are fundamentally important to our understanding of quantum critical phenomenon and related phase transitions. Here we report on the direct observation of an unconventional topological spin state and its behavior in antiferromagnetically coupled NiFe discs at room temperature. The observed spin structure is similar to the theoretically predicted merons which have not yet been observed directly. We have used in situ Lorentz microscopy magnetizing experiments combined with micromagnetic simulations to follow the stability and the behavior of the meron state. The work presented in this paper will open new opportunities for direct experimental investigation of various topological states that can provide insights into the fundamental physics of their interactions.