Wuyan Lai

Nanoparticle morphology in a granular Cu–Co alloy with giant magnetoresistance

The morphology of nanometer-sized cobalt granules in a granular Cu88Co12 alloy was directly determined utilizing an atom probe-field ion microscope. The granules are spherical in shape, and exhibit a size distribution. Giant magnetoresistance (GMR) was observed in alloys with an average granule size ranging from 1.5 to 6 nm in diameter. A well-known theoretical model of general GMR behavior in magnetic granular systems was confirmed based on measurement of size distribution of the granules.

Relation between microstructures and magnetic properties upon annealing in Fe50Mn50/Ni80Fe20 films

The annealing-temperature-dependent change in the exchange bias and coercivity is investigated in Fe50Mn50/Ni80Fe20 films. It is interesting to note that, as the annealing temperature increases, the exchange bias first decreases, and then increases, and finally decreases for the case of annealing at the higher temperature. The coercivity will increase upon annealing at moderate temperature, but decrease upon higher-temperature annealing. We can qualitatively interpret the change of the magnetic properties with annealing temperature in connection to the microstructures by x-ray scattering technologies. The results show that both the large exchange bias field and low coercivity of Fe50Mn50/Ni80Fe20 films are dependent of not only the interfacial roughness but also the antiferromagnetic structure.

Microstructure, magnetic properties and giant magnetoresistance of granular Cu-Co alloy

The microstructure in a granular Cu88Co12 alloy was investigated by means of x-ray diffraction (XRD), a field ion microscope with atom probe (AP-FIM) and a field emission gun-transmission electron microscope (FEG-TEM) with energy dispersive x-ray spectroscopy (EDS). Giant magnetoresistance (GMR) and magnetic properties were studied to clarify the relationship between the GMR, magnetic properties and microstructures. The morphology of nanometre-sized cobalt granules within the grains were directly determined utilizing AP-FIM and FEG-TEM with EDS. Most cobalt granules are spherical and some others are plate-like in shape. GMR was observed in alloys with an average granule size ranging from 1.5 to 6 nm in diameter. TEM investigations showed that larger Co-rich precipitates formed at grain boundaries. The magnetic properties of the granular alloy were interpreted using the microstructure information and the dependence of GMR on magnetization was discussed.

Pseudo-Hall effect in spin-valve multilayers

Magnetoresistance and the pseudo-Hall effect in NiFe/Cu/NiFe/FeMn spin-valve multilayers were measured simultaneously in fields rotating in the film plane. Large pseudo-Hall voltages have been observed when the magnetization of the free layer was perpendicular to the sensing current, which was applied along the magnetization of the pinned layer. The pseudo-Hall voltages cannot be explained by treating the anisotropic magnetoresistance of the two permalloy layers independently. Such a cross effect of the free and pinned layers on the anisotropic magnetoresistance is dependent upon the angle between their magnetization.

The effect of microstructure and interface conditions on the exchange coupling fields of NiFe/FeMn

The effect of microstructure and interface conditions on the exchange coupling field H-ex and coercivity H-c of NiFe/FeMn systems was investigated. The results show that the exchange coupling was dependent on [111] crystal orientation and interface roughness as well as the grain size of FeMn. The high H-ex (approximate to 120 Oe) and low H-c (approximate to 5 Oe) of the pinned NiFe layer was achieved by a two-step deposition procedure. The down sublayers were deposited at a lower argon pressure, and the upper sublayers were deposited at a higher argon pressure. The former promoted the formation of the strong (111) textures and smooth interfaces. The latter promoted small domains and sharp interfaces

Exchange coupling and thermostability in NiFe/NiMn bilayers

The effects of annealing time and temperature on the exchange coupling between ferromagnetic NiFe and antiferromagnetic NiMn layers prepared by magnetron sputtering were systematically studied. With both increasing annealing temperature and annealing time, the exchange coupling energy Jex, derived from exchange coupling field Hex and saturation magnetization MS, showed an initial increase followed by a decrease. Due to the reduction of MS resulting from the interdiffusion across the interfaces during annealing, there is a slight difference between the way Jex varies with annealing temperature and annealing time and the way Hex varies. From the evolution of Jex, it can be deduced that over annealing, including both annealing for too long time and at too high a temperature, is detrimental to the interfacial exchange coupling. The exchange coupling between NiFe and NiMn layers was very stable within an environmental temperature ranging from room temperature to about 210 °C. The exchange path was checked and the distribution of blocking temperature Tb was estimated.

The dependence of giant magnetoresistance in an Fe - Mo multilayer on the thickness of the Fe layers

The magnetoresistance in Fe-Mo multilayers increases with decreasing thickness of the Fe layers. By comparing magnetoresistance and magnetization results, evidence for interlayer coupling variation was found with decreasing Fe layer thickness. Moreover, the behaviour of discontinuous or granular layers was observed in a sample with ultrathin Fe layers. The specific field dependence of the magnetoresistance, which obeys a Langevin-like function, suggests that scattering from an assemblage of superparamagnetic spins could be responsible for the observed magnetoresistance behaviour.

Interlayer coupling in NiFe/Mo multilayers and effect of Fe underlayer

We report our studies of exchange coupling in NiFe/Mo multilayers with and without Fe underlayer. The interlayer coupling of NiFe layers across the Mo spacer as well as the magnetic exchange coupling between Fe underlayer and the NiFe/Mo multilayers were observed: The interlayer coupling in NiFe/Mo multilayers exhibits the typical oscillations as a function of the Mo spacer with a period of about 11 A, and the magnetic exchange coupling between Fe underlayer and the NiFe/Mo multilayers is strongly dependent on the thickness of the Mo spacer tMo and the bilayer number N of NiFe/Mo multilayers. For the [NiFe(20u2002A)/Mo(tMo)]30 samples with a 100 A Fe underlayer, the hysteresis loops with thinner Mo spacer (tMo<4 A) are single loops indicating that the Fe underlayer is strongly magnetic exchange coupled with the NiFe/Mo multilayers; whereas hysteresis loops with thicker Mo spacer (tMo⩾5 A) show a two-magnetic-phase feature. For the [NiFe(20u2002A)/Mo(10u2002A)]30 sample with 100 A Fe underlayer, the hysteresis loops a...

Enhancement of exchange bias in Ir–Mn/Co0.9Fe0.1 bilayers by inserting an ultra-thin Co0.6Fe0.4 layer

The exchange biasing of Co0.9Fe0.1 film by the antiferromagnet Ir?Mn via a thin intermediate Co0.6Fe0.4 layer was investigated. It has been found that the exchange bias is drastically enhanced with the insertion of an ultra-thin Co0.6Fe0.4 layer at the interface between Co0.9Fe0.1 and Ir?Mn layers. For the Ir0.25Mn0.75(100??)/Co0.9Fe0.1(150??) bilayer, the pinning field at room temperature is raised from 92 to 206?Oe when a 4?? Co0.6Fe0.4 layer is introduced at the interface, and further enhanced to 262?Oe with a 10?? Co0.6Fe0.4 insertion layer. The corresponding unidirectional anisotropy increases from 0.18 to 0.4?erg?cm?2 and reaches a value of 0.54?erg?cm?2. The extra large unidirectional anisotropy is maintained as the Co0.6Fe0.4 thickness exceeds 10??. The high blocking temperature of the Ir?Mn/Co0.9Fe0.1 bilayer is unaffected by the insertion of the Co0.6Fe0.4 layer. The present results indicate that exchange bias is essentially determined by interfacial spins and, most importantly, the performance of the exchange bias system Ir?Mn/Co0.9Fe0.1, which is commonly used in spin valve giant magnetoresistance devices, can be greatly promoted by the insertion of an ultra-thin interfacial Co0.6Fe0.4 layer.

A study of exchange coupling in NIFE/NIMN bilayers

The effects of annealing time and temperature on the exchange coupling between ferromagnetic (FM) NiFe and antiferromagnetic (AFM) NiMn layers prepared by magnetron sputtering were systematically studied. With both increasing annealing temperature and annealing time, the interfacial exchange coupling energy J(ex) showed an initial increase followed by a decrease. Magnetic measurement and X-ray diffraction (XRD) analysis revealed that the evolution of J(ex) should be attributed to the competing of paramagnetic (PM)-AFM phase transition and the interdiffusion across the interfaces caused by annealing. It can be deduced that overannealing, including both too long annealing time and too high annealing temperature, is detrimental to the interfacial exchange coupling. It was also shown that the exchange coupling between NiFe and NiMn layers was thermally very stable. PACS: 75.50 (C) 2000 Elsevier Science S.A. All rights reserved.