S. U. Jen

Piezoresistance and electrical resistivity of Pd, Au, and Cu films

Electrical resistivity and piezoresistance of some metal films, such as the Pd, Au, and Cu films, were measured. Surface features of a few characteristic films, used in this study, were revealed by using an atomic force microscope. The electrical resistivity ρ is plotted as a function of the film thickness t, and the strain gauge factor γ is plotted as a function of the sheet resistivity Rsq. Two parameters are found useful: i.e. h/λ and 2h/t, where h is the amplitude of undulations of the surface roughness and λ is the electron mean free path at room temperature. By taking the Cu film as an example, the area-distribution function of island-like humps on the specimen surface is also analyzed. Two models, namely the surface roughness and the electron tunneling models, are employed to explain the electrical resistivity and piezoresistance data observed. It is found that (1) if h/λ⩽0.3 and 2h/t<0.5, the films are continuous, and the former model is effective, and (2) if 0.5<2h/t<1, the films are critically coalesced, and the latter model is important. Finally, from the area-distribution plot, we can show indirectly that if the film is at the coalescence stage, the average separation Δd between two neighboring humps is wider than if the film is at the continuous stage.

Magnetic properties of face-centered cubic Co films

We deposited face-centered cubic (FCC) Co films on glass substrates by sputtering. From Auger-depth profile analysis, we found that there is one CoO layer, about 13 /spl Aring/ thick, lying on the top surface of the Co film, and another CoO layer, about 37 /spl Aring/ thick, lying within the Co/glass interface. At room temperature, the thin CoO film is supposed to be paramagnetic. However, because of the proximity effect between CoO and Co, the CoO layer may become ferromagnetic, with saturation magnetization M/sub o/. By fitting the saturation magnetization (M/sub s/) data of the whole Co/CoO film as a function of (1/t/sub f/), where t/sub f/ is the Co thickness, we can prove that the last conjecture is correct, and the M/sub o/ of the CoO layer is indeed not zero. Both the t/sub f/ dependence of the magnetostriction constant (/spl lambda//sub s/) and the t/sub f/ dependence of the coercive field (H/sub c/) show a two-region characteristic. The dividing line for the former quantity is at t/sub f/ = 88 /spl Aring/, and for the latter it is at t/sub f/ = 120 /spl Aring/. When crossing such a dividing line, there is a discontinuous jump in /spl lambda//sub s/ (or H/sub c/). These phenomena occur because the lattice-strain and magnetoelastic effects within the CoO layer dominate the /spl lambda//sub s/ and the H/sub c/ behavior in ultra-thin (t/sub f/ < 88 /spl Aring/) Co films. In this region, the roughness-to-thickness ratio (S/sub q//t/sub f/) may also affect /spl lambda//sub s/. Finally, there seems to be no connection between the grain size (D) and H/sub c/.

Néel, cross-tie, and Bloch walls in Fe100-xNix films

Abstract Fe100-xNix magnetic thin films with nickel concentration x ranging from 70 to 90 at% and with film thickness t ranging from 100 to 2000 A have been made. Three kinds of domain walls, Neel, cross-tie and Bloch walls, are observed in these films. We have been successful in determining the Bloch to cross-tie (tR) and the Neel to cross-tie (tL) transition boundaries systematically for each FeNi alloy. In terms of the Ni concentration dependence, tR is inversely proportional to Ms, which is the saturation magnetizations of the films, and tL is proportional to (K u ) 1 2 /M s 2 , where Ku is the induced anisotropy energy of the films.

Exchange bias in sputtered FM/BiFeO3 thin films (FM = Fe and Co)

Magnetic properties of sputter-deposited ferromagnetic (FM)/BiFeO3 (BFO) films on Pt/Ti/SiO2/Si(100) substrate (FM = Co and Fe) have been investigated. Isotropic perovskite BFO single phase is obtained for 200-nm-thick BFO films deposited at 300–450 °C and BFO films at 400 °C with thickness of 50–400 nm. Large exchange bias field (HEB) of 308–400 Oe and coercivity (Hc) of 1201–3632 Oe at RT are obtained for polycrystalline Co/BFO bilayers. The roughened surface induced by high deposition temperature and increasing thickness of BFO layer enhances localized shape anisotropy of FM layer, resulting in the increase of Hc the improved crystallinity and roughened surface of BFO/Co interface might be responsible for the HEB enhancement. Additionally, comparison on the HEB in polycrystalline Co/BFO and Fe/BFO systems is also discussed.

Magnetic properties of ultra-fine iron particles

Abstract Ultra-fine iron particles of sizes ranging from 75 to 300 A were prepared by the gas-evaporation method in an argon atmosphere. By controlling the pressure of argon gas, various sizes of iron particles were made. From TEM micrographs, the mean particle size is determined. Magnetic properties of these particles were obtained by a vibrating sample magnetometer. It is considered that an oxidation layer may be formed on the surface of iron particles, when they are exposed to air. Based on this surface effect, most of the observed magnetic behavior, such as temperature dependences of magnetization and coercive force, can be explained. X-ray and differential scanning calorimetry measurements were also performed on these particles.

Sputter-prepared BiFeO3(001) films on L10 FePt(001)/glass substrates

The preparation of BFO films by sputtering at a temperature as low as 450 °C on glass and commercial Pt/Ti/SiO2/Si(001) substrates have been studied. The underlayers with different orientations were prepared on the glass substrates including strongly textured Pt(111) and L10-FePt(001) induced by rapid thermal annealing process. Isotropic perovskite BFO grains with size of about 200 nm formed on the commercial substrates, showing larger surface roughness. Pt(111) suppresses BiFeO3 phase. Single phase perovskite BFO with strong (001) texture, reduced surface roughness and fine grain size was formed on the L10-FePt(001) buffer layer. Considerable enhancement of ferroelectric properties was achieved as compared to the films grown on commercial substrate.

Frequency dependence of coercivity and initial permeability in ferromagnetic metallic glasses

Coercivity Hc( f ) and initial permeability χi( f ) of two kinds of ferromagnetic glasses, 2605S2 and VAC6030, have been measured as a function of frequency from 4 Hz to 2 kHz. Field annealing and stretching of samples have been used to define an easy axis along the longitudinal direction. Some samples were thinned to see the thickness effect. We have analyzed the f dependence of coercivity and initial permeability based on the wall pinning model.

The angular dependence of the planar Hall effect in Co-Ni films

Co100−xNix (x=70, 75, 80, 90 at. % Ni) alloy films were made by the vapor deposition method. An easy axis could be defined by a deposition field parallel to either the length (L film) or width (T film) of the sample. The anisotropic magnetoresistance (AMR) and the planar Hall effect (PHE) hysteresis loops were simultaneously measured with a slowly sweeping field H. The angle θ between the hard axis and H could be varied. Domain patterns were also recorded at each stage of the magnetization or demagnetization process. Besides the usual AMR and PHE behaviors, we have observed the forms of a voltage “mountain,” like ΔVmr and ΔVph, superimposed on the normal curves at the switching field Hsw. ΔVmr is an even function of θ and ΔVph is an odd function. The well known magneto-transport theory based on the single-domain model is modified to adopt the real situation of the multidomain structure. We have been successful in explaining all the ΔVph data observed experimentally. In addition, from the angular dependenc...

Magnetostriction of polycrystalline Co‐Pd alloys

Magnetostriction of polycrystalline Co100−xPdx alloys was measured by the strain gauge method from 4 to 300 K. The saturation magnetostriction λs=(2/3)[(ΔL/L)s∥−(ΔL/L)s⊥], where (ΔL/L)s∥ and (ΔL/L)s⊥ are the saturated values (when HE≳5 kG) of ΔL/L in fields parallel and perpendicular to sample’s length (L). λs at 4 K is found to change sign from positive to negative value as x is increased from below x0=50 at. % Pd to above. This phenomenon can be understood from the split‐band theory for Co‐Pd alloy. However, since some experimental facts indicate that unquenched orbital moments may exist in the x≳50 at. % Pd alloys it is necessary to consider λs = λsband+ λsion, where λsband is due to the split‐band theory and λsion is from the one‐ion mechanism. Under this proposal, it is possible to explain why λs(4 K) is not antisymmetric with respect to x0. Finally, two different origins of temperature dependence of λs are discussed.

Hall effect of polycrystalline Co‐Pd alloys

Hall‐effect measurements were carried out on a series of Co100−xPdx alloys from 4.2 to 255 K. The extraordinary Hall coefficient RS is analyzed. The side‐jump mechanism is dominant for Co‐Pd alloys with x≤65 at. %; however, when x≳65 at. %, both the side‐jump and skew scattering mechanisms are equally effective. The Hall conductivity γH changes its sign around xH=77 at. %. xH is shifted to the right‐hand side of the band‐gap position xG, as implied from the anisotropic magnetoresistance data and the split‐band theory.