J. W. Cai

Tunable interfacial properties of epitaxial graphene on metal substrates

We report on tuning interfacial properties of epitaxially-grown graphenes with different kinds of metal substrates based on scanning tunneling microscopy experiments and density functional theory calculations. Three kinds of metal substrates, Ni(111), Pt(111), and Ru(0001), show different interactions with the epitaxially grown graphene at the interfaces. The different interfacial interaction making graphene n-type and p-type doped, leads to the polarity change of the thermoelectric property of the graphene/metal systems. These findings may give further insights to the interfacial interactions in the graphene/metal systems and promote the use of graphene-based heterostructures in devices.

Low-temperature ordering of FePt thin films by a thin AuCu underlayer

We have studied the magnetic and structural properties of AuCu(0–50nm)∕FePt(2–50nm) films after annealing at various temperatures. The results indicate that, by introducing a thin equiatomic AuCu underlayer, the ordering temperature of FePt films with thickness even down to 5 nm can be significantly reduced to 350 °C, at which a large coercivity is developed. In fact, the coercivity is as high as 4.6 kOe for a 5 nm FePt film on a 10 nm AuCu underlayer after annealing at 350 °C and this value is raised up to 7.5 kOe when annealed at 400 °C, while the corresponding sample without a AuCu underlayer has to be annealed at a temperature beyond 600 °C in order to achieve a coercivity of 4 kOe. The ordering of the thin AuCu film at relatively low temperature and thus coherently inducing the ordering of FePt film led to the formation of the ordered FePt phase at a much lowered temperature.

Ultrahigh sensitivity Hall effect in magnetic multilayers

Pt-based ferromagnetic alloy thin films are known to exhibit very large extraordinary Hall effect (EHE) with maximum Hall slope around 20μΩcm∕T at room temperature for optimum Fe–Pt alloy films. The authors report features of the EHE observed in Fe∕Pt and Co0.9Fe0.1∕Pt multilayers. For Fe∕Pt multilayers, the room temperature Hall slope is comparable with that of Fe–Pt alloy thin films over a broad sublayer thickness range. For Co0.9Fe0.1∕Pt multilayers, the Hall slope increases by tens of times, reaching 545μΩcm∕T at room temperature through choosing appropriate sublayer thickness and the number of Co0.9Fe0.1∕Pt bilayers. While keeping good linearity, the EHE sensor made of Co0.9Fe0.1∕Pt multilayers has field sensitivity of up to 1200V∕AT, appreciably higher than the sensitivity of semiconductor Hall sensors commonly used. Besides, the dynamic field range can be varied in EHE sensors with compound multilayers of Co0.9Fe0.1∕Pt and Fe∕Pt. The great enhancement in Hall slope (or sensitivity) and the adjustab...

Isothermal tuning of exchange bias using pulsed fields

Exchange bias, HE, and coercivity, HC, of antiferromagnetic (AFM)/ferromagnetic bilayers can be adjusted, after deposition, at temperatures below the Neel temperature of the AFM by subjecting the samples to large pulsed fields (in excess of HPulse=550 kOe). The efficiency of the process depends on the AFM system and the direction of the applied field with respect of the unidirectional anisotropy direction. Textured (111) Fe19Ni81/Fe50Mn50 bilayers show an HE reduction and a HC increase when the pulse field is applied antiparallel to the unidirectional anisotropy, while they only exhibit a reduction in HC when the pulse is applied parallel to their unidirectional anisotropy. On the other hand, textured (111) NiO/Co bilayers exhibit a change of the angular dependence of HE when the pulse is applied away from the unidirectional anisotropy. The effects could be caused by field induced changes in the domain structure of the AFM or transitions in the AFM (spin–flop or AFM–paramagnetic).

Large enhancement of anisotropic magnetoresistance and thermal stability in Ta/NiFe/Ta trilayers with interfacial Pt addition

Ta/NiFe/Ta trilayers, extensively used for anisotropic magnetoresistance (AMR) sensors, exhibit severely reduced MR ratio at small NiFe thickness and appreciable moment loss, especially after annealing. By inserting ultrathin Pt layers at the interfaces of the trilayers, AMR can be significantly enhanced for thin NiFe films due to the strong electron spin-orbit scattering at Pt/NiFe interfaces along with suppression of interfacial magnetic dead layers. Furthermore, the Pt layers also reduce Ta and NiFe interdiffusion and result in negligible moment loss and AMR degradation after annealing at 350 °C.

Boron nanowires for flexible electronics

Flexible boron nanowires have been synthesized via thermoreduction in boron-oxygen compounds with magnesium. These as-prepared nanowires, which are structurally uniform and single crystalline, represent good semiconductor at high temperature. Tensile stress measurements demonstrate excellent mechanical property of boron nanowires as well as resistance to mechanical fracture even under a strain of 3%. Importantly, simultaneous electrical measurement reveals that the corresponding electrical conductance is very robust and remains constant under mechanical strain. Our results can be briefly explained by Mott’s variable range hopping model.

Structural and magnetic properties of NiFe/NiMn bilayers with different seed and cap layers

Abstract The structural and magnetic properties of Ni 0.81 Fe 0.19 /Ni 0.42 Mn 0.58 bilayers with Ta or (Ni 0.81 Fe 0.19 ) 0.58 Cr 0.42 as the seed and cap layers were investigated. It was found that Mn diffused into Ni 0.81 Fe 0.19 layer during the transformation of Ni 0.42 Mn 0.58 layer from nonmagnetic to antiferromagnetic phase through annealing, which causes the decrease of magnetic moment in the ferromagnetic layer. Since (Ni 0.81 Fe 0.19 ) 0.58 Cr 0.42 cap layer can accommodate Mn atoms but Ta layer cannot, the reduction of the magnetization for the films with (Ni 0.81 Fe 0.19 ) 0.58 Cr 0.42 seed and cap layers was less than that with Ta seed and cap layers. On the other hand, the grain size of the bilayers with (Ni 0.81 Fe 0.19 ) 0.58 Cr 0.42 seed layer was much larger than that of the films with Ta seed layer, which leads to a better thermal stability for the former. The present results indicate that (Ni 0.81 Fe 0.19 ) 0.58 Cr 0.42 can be promising seed and cap layers in spin valves based on Mn-alloyed antiferromagnets.

Effect of Cu surface segregation on properties of NiFe/FeMn bilayers

The films of NiFe/FeMn with Ta and Ta/Cu buffer layers were prepared by magnetron sputtering. Results show that the exchange bias field of NiFe/FeMn films with Ta/Cu buffer is lower than that of the films with Ta buffer. The crystalline texture, surface roughness and element distribution of these two sets of samples were examined, and there is no apparent difference for the texture and roughness. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta/Cu/NiFe has been observed by using the angle-resolved X-ray photoelectron spectroscopy. The decrease of the exchange bias field for NiFe/FeMn films with Ta/Cu buffer layers is mainly caused by the diffusion of Cu atoms through NiFe layer, which stayed at the interface of NiFe/FeMn film or even intruded into FeMn layer. The present results indicate that Cu segregation through NiFe layer should be suppressed in order to improve the exchange bias field in giant magnetoresistance spin valves with Cu spacer.

Enhancement of exchange-coupling field in FeMn pinned spin valve by surfactant Bi

Ta/NiFe/Cu/NiFe/FeMn/Ta spin valve multilayers with surfactant Bi introduced after deposition of Cu layer were prepared. The enhancement of the exchange bias field has been found in these spin valve multilayers. The composition and chemical states of Bi and Cu at sample surface were examined by X-ray photoelectron spectroscopy. The experiment shows that the Bi as a surfactant with very low surface energy has hindered Cu atoms from segregating to the interface of NiFe/FeMn during the fabrication of the spin valve multilayers. This mechanism would be responsible for the enhancement of the exchange bias field.

Highly sensitive linear spin valve realized by tuning 90° coupling in a NiFe/thin IrMn/biased NiFe structure through nonmagnetic spacer insertion

90° magnetic coupling between a free NiFe layer and an exchange biased NiFe layer has been realized using a thin IrMn intermediate film. This 90° coupling remains after the addition of a nonmagnetic spacer (Cu, Pt, Ru, or Ta) at the free NiFe/IrMn interface. Effective anisotropy strength of the free layer can be readily adjusted through nonmagnetic layer thickness control. Spacer layer thickness increase results in significant reduction of free layer coercivity and field offset, much faster than the 90° coupling strength drop. Linear spin valves of adjustable high field sensitivity without field offset have been demonstrated using this structure.