W. Y. Lai

Interface reaction of NiO/NiFe and its influence on magnetic properties

Ta/NiO/NiFe/Ta multilayers were prepared by rf reactive and dc magnetron sputtering. The exchange coupling field between NiO and NiFe reached 120 Oe. The composition and chemical states at the interface region of NiO/NiFe were studied using the x-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that there are two thermodynamically favorable reactions at NiO/NiFe interface: NiO+Fe=Ni+FeO and 3NiO+2Fe=3Ni+Fe2O3. The thickness of the chemical reaction as estimated by angle-resolved XPS was about 1–1.5 nm. These interface reaction products are magnetic defects, and we believe that the exchange coupling field Hex and the coercivity Hc of NiO/NiFe are affected by these defects.

Magnetic properties and x-ray photoelectron spectroscopy study of NiO/NiFe films prepared by magnetron sputtering

Ta/NiOx/Ni81Fe19/Ta multilayers were prepared by rf reactive and dc magnetron sputtering. The exchange-coupling field (Hex) and the coercivity (Hc) of NiOx/Ni81Fe19 as a function of the ratio of Ar to O2 during the deposition process were studied. The composition and chemical states at the interface region of NiOx/NiFe were also investigated using the x-ray photoelectron spectroscopy (XPS) and peak decomposition technique. The results show that the ratio of Ar to O2 has a great effect on the nickel chemical states in NiOx film. When the ratio of Ar to O2 is equal to 7 and the argon sputtering pressure is 0.57 Pa, the x value is approximately 1 and the valence of nickel is +2. At this point, NiOx is antiferromagnetic NiO and the corresponding Hex is the largest. As the ratio of Ar/O2 deviates from 7, the exchange-coupling field (Hex) will decrease due to the presence of magnetic defects such as Ni+3 or metallic Ni at the interface region of NiOx/NiFe, while the coercivity (Hc) will increase due to the meta...

Structural and magnetic properties of NdFe12−xMoxN1−δ compounds

A series of nitrides NdFe12−xMoxNi1−δ with x=1–2.5 and δ≤0.2 have been investigated. All nitrides are of the ThMn12 tetragonal structure with the unit cell volumes about 3% larger than their parent alloys. Curie temperatures are enhanced by nitrogenation about 160 K and increase linearly with decreasing content x of Mo. NdFe12−xMox exhibits a weak uniaxial anisotropy with a value of BA less than 0.6 T when x≥1.75. After nitrogenation, all nitrides have a strong uniaxial anisotropy. Values of the anisotropy field BA increase monotonically from 6.35 T for x=2.5 up to 9.5 T for x=1 as decreasing content x of Mo. Saturation magnetizations also increase monotonically with decreasing content x.

A specular spin valve with discontinuous nano-oxide layers

Microstructures of the specular spin valve with two nano-oxide layers (NOL1 and NOL2) have been studied at the atomic level. When the NOLs are incorporated in a bottom-pinned spin valve, a significant enhancement in magnetoresistance ratio with greatly decreased sense-layer thickness is achieved. Cross-sectional high-resolution electron microscopy (HREM) studies show that the NOL1 introduced from oxidation of the original bottom-pinned CoFe layer is actually a mixture of oxides and ferromagnetic metals. No CoFe oxides but Ta2O5 is found over the oxidation-treated CoFe sense layer by HREM and x-ray photoelectron spectroscopy study. The Ta2O5 layer acting as the NOL2 can be interpreted as being formed through a solid-state oxidation reaction between the oxidized CoFe sense layer and the Ta capping layers.

Interface reaction of Ta/Ni81Fe19 or Ni81Fe19/Ta and its suppression

Ta/Ni81Fe19 and Ni81Fe19/Ta structures are commonly used in the magnetic multilayers with giant magnetoresistance. For a Ta/Ni81Fe19/Ta fundamental structure, Ta seed and Ta cap layers resulted in a loss of moment equivalent to a magnetically dead layer of thickness 1.6±0.2 nm. In order to find out the reason, the composition and chemical states at the interface regions of Ta/Ni81Fe19 and Ni81Fe19/Ta were studied using the x-ray photoelectron spectroscopy and peak decomposition technique. The results show that there are thermodynamically favorable reactions at the Ta/Ni81Fe19 and Ni81Fe19/Ta interfaces: 2Ta+Ni=NiTa2. However, the thickness of a magnetically dead layer was significantly reduced by the insertion of a small amount of Bi in the Ta/Ni81Fe19/Ta structure. This result indicates that a surfactant Bi can suppress the interface reaction in multilayers.

Interlayer segregation in magnetic multilayers and its influence on exchange coupling

Experimental results show that the exchange coupling field (H-ex) of NiFe/FeMn for Ta/NiFe/FeMn/Ta multilayers is higher than that for spin-valve multilayers Ta/NiFe/Cu/NiFe/FeMn/Ta. X-ray photoelectron spectroscopy shows that Cu atoms segregate to the NiFe/FeMn interface for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers. While studying Ta/X(X=Bi,Pb,Ag,In)/NiFe/FeMn multilayers, we also find that X atoms segregate to the NiFe/FeMn interface, which results in a decrease of the H-ex. However, a small amount of Bi, Pb, etc. deposited between Cu and pinned NiFe layer for Ta/NiFe/Cu/NiFe/FeMn/Ta multilayers can increase H-ex

Planar Hall effect in NiFe/NiMn bilayers

The exchange anisotropy in NiFe/NiMn bilayers was studied by using the planar Hall effect. The sputtered NiFe/NiMn films were patterned into strips of 1 mm in length and 200 μm in width and with six terminals for anisotropy magnetoresistance and planar Hall voltage measurements by a photolithographic process. It is shown that the planar Hall effect is an effective method to characterize the exchange anisotropy in ferromagnetic/antiferromagnetic (AF) systems. It can be used to accurately determine the exchange field and describe the magnetization reversal processes. The effective uniaxial anisotropy field HK eff, the effective unidirectional anisotropy field Hud, and AF domain wall energy Hw can be obtained by fitting the experimental results. We found that in the NiFe/NiMn bilayer system, the parameters HK eff, Hud, and Hw have the same values in reversible and irreversible measurements, and the domain wall energy in AF layer is larger than interfacial unidirectional anisotropy.

Giant magnetoresistance in Fe/Mo multilayers formed by magnetron sputtering

Fe/Mo multilayers have been prepared by magnetron sputtering. Giant magnetoresistance (GMR) has been found in the samples with an antiferromagnetic interlayer coupling. The magnetoresistance (MR) ratio exceeds 12% at 4.2 K, and it oscillates as a function of Mo spacer thickness. The oscillation period is about 10–12 A, which is consistent with the case of the reported Kerr effect. The results suggest that the presence of antiferromagnetic coupling and the absence of GMR are the result of the sharp interfaces, and that relatively rough interfaces and moderately thin Fe layer thickness are the key factors for enhancing MR in the sputtered films.

Microstructure of columnar crystallites in Ni80Fe20/Cu magnetic multilayers

We have used electron microscopy to investigate the microstructure of Ni80Fe20/Cu magnetic multilayers which were synthesized by dc magnetron sputtering. Columnar structure was found in the specimen with and without giant magnetoresistance (GMR). All the columnar crystallites (CCs) originate from the Fe buffer layer on silicon wafer or glass substrate and penetrate though all the multilayers up to the surface of the film. The lateral size of the CCs ranges from 10 to 30 nm. Cross-sectional high-resolution electron microscopy study shows that the CCs are single-crystal-like with fcc structure resulting from the epitaxial growth of NiFe and Cu sublayers. Electron diffraction contrast imaging and electron energy filtered elemental mapping confirmed that multilayer nature is maintained throughout the entire NiFe/Cu film. Grain boundaries between CCs can be the most likely place where NiFe or Cu bridging will occur. Columnar structure was also found in a Ta/NiFe/Cu/NiFe/FeMn/Ta spin valve film. The possible in...

CALCULATED ELECTRONIC AND MAGNETIC-STRUCTURES OF THE NEW TERNARY RARE-EARTH IRON NITRIDE ND2FE17N3

The electronic and magnetic structures of Nd2Fe17 and Nd2Fe17N3 have been calculated using the first-principle, spin-polarized orthogonalized linear combination of atomic orbitals method. Comparative studies of the two materials reveal important effects of the nitrogen atoms (at 9e site) on the electronic and magnetic structures. Results are presented for the total density of states, site-projected partial density of states and the spin magnetic moments on four nonequivalent Fe sites. The highest magnetic moments are found to be located on the 6c site for Nd2Fe17 and on the 9d site for Nd2Fe17N3, in agreement with the neutron and Mossbauer experiments. The variation trends of the magnetic moments on different Fe sites are discussed in terms of the separation between Fe and N atoms. Compared with Nd2Fe17, an increase in the exchange splitting of the Fe d band is found in Nd2Fe17N3, which accounts for its higher Curie temperature as observed in experiments. The calculated results show that the nitrogen atoms are charge acceptors in these compounds.