R. Meservey

Geometrically enhanced magnetoresistance in ferromagnet–insulator–ferromagnet tunnel junctions

Ferromagnetic–insulator–ferromagnetic trilayer tunnel junctions show magnetoresistance (JMR) effects of about 14% or greater at room temperature. Much larger values of the JMR (100% or more) are observed when the actual tunneling resistance (RT) is comparable to electrode film resistance (RL) over the junction area. This latter apparent JMR is an artifact of the nonuniform current flow over the junction in the cross geometry of the electrodes. The ferromagnetic films were CoFe and Co or Ni0.8Fe0.2, and the tunnel barrier was AlN or Al2O3. These junctions show nonvolatile memory effects. The geometrically enhanced large JMR in junctions can be effectively used as magnetic sensors and memory elements.

MEASUREMENTS OF THE KINETIC INDUCTANCE OF SUPERCONDUCTING LINEAR STRUCTURES.

The kinetic inductance Lk is associated with the inertial mass of the current carriers. For a long superconductor of length l and very small cross‐sectional area σ, Lk = (m* / nse*2) (l/σ) and is the main contribution to the temperature‐dependent inductance LT. We have measured LT in superconducting tin wires and thin film meander lines by a technique which uses a counter to determine frequency changes of a 15 MHz tunnel‐diode oscillator whose tank circuit contains LT. The measured frequency changes are proportional to z = [λ(t)/λ(0)]BCS for wires and thick films; for thin films the frequency changes are proportional to z2. This result agrees with calculations of LT from the London theory and the values of λ(0) agree well with the values expected for these samples. This technique allows measurement of the carrier concentration or penetration depth in thin films over the whole H, T plane of the superconducting state and can be independent of the penetration law assumed. Frequency shifts observed with thin film alumimum meander lines have been used to detect changes of temperature of 5∼10−7 °K, changes of magnetic field of 10−5 G, and changes of current associated with one quantum in a flux quantum magnetometer.

Current distribution effects in magnetoresistive tunnel junctions

The influence of an inhomogeneous current density on the (magneto)resistance of a ferromagnet–insulator–ferromagnet tunnel junction in the cross-strip geometry is analyzed using a finite element approach. The four-probe resistance is smaller than the actual resistance for electrode resistances (in the junction area) comparable to or higher than the junction resistance. Even negative four-probe resistances can be obtained. The apparent resistance change due to the junction magnetoresistive effect also decreases, but always remains positive. This results in unrealistically large apparent magnetoresistance ratios which can even approach infinity, which explains some recent experiments.

Tunneling characteristics of amorphous Si barriers

Tunnel junctions using a barrier of amorphous silicon (a‐Si) between normal and superconducting metals were studied at temperatures from 300 to 0.45 K. These junctions were reliably made by depositing on a 77 K glass substrate Ni or Au, a‐Si 60 to 100 A thick, and then Al. It was demonstrated that the dominant conduction process was elastic tunneling by the presence of structure caused by the superconducting energy gap of Al, and by comparing measurements of the voltage, temperature, and barrier thickness dependence of the conductance with theory. The effective barrier heights were grouped close to 2×10−2 eV. A semiquantitative argument suggests that the barriers controlling the elastic tunneling are much the same as those controlling the phonon‐activated variable range tunneling at higher temperatures. Although a‐Si barriers can be formed reliably and have low leakage, the low barrier height leads to large nonlinearity even at low voltages.

Properties of Very Thin Aluminum Films

Thin films of aluminum have been produced in the thickness range of 1000–30 A in small area samples with photoetched edges. The superconducting transition temperature Tc, the critical magnetic field Hc, and the room‐temperature conductivity have been measured as a function of thickness d. The results indicate that films as thin as 30 A act essentially as uniform layers in which the crystal size is approximately equal to the film thickness. The transition temperature was found to vary linearly with d−1. Hc (T) was measured from Tc to 0.4°K. For thickness from 1000 to 200 A, Hc∼d−3/2, as expected from the Ginzburg‐Landau theory. For d <200 A, Hc is paramagnetically limited to about 49 kOe = 19.6Tc, slightly above the Clogston limit.

Ferromagnetic‐ferromagnetic tunneling and the spin filter effect

Tunneling characteristics of a ferromagnetic‐antiferromagnetic‐ferromagnetic (FM‐AFM‐FM) thin film tunnel junction were studied in high magnetic fields with a view to investigate magnetic coupling by the tunneling process. Gd2O3, a stable oxide which undergoes antiferromagnetic ordering below about 3.9 K, was chosen as the tunnel barrier between the ferromagnetic electrodes Gd and permalloy. Tunnel characteristics showed as much as 32% decrease in junction resistance in an applied field of 20 T, below 4.2 K. The resistance behavior as a function of H can be explained by two different effects: firstly, the change in tunnel conductance due to change in the relative magnetization of the two FM electrodes in low H; secondly, the spin filter effect in high fields, due to the exchange splitting of the Gd2O3 conduction band.

Properties of amorphous germanium tunnel barriers

The properties of tunnel barriers made with amorphous Ge (a‐Ge) deposited at approximately 80 K were studied in Al/a‐Ge/Al tunnel junctions and also in junctions where one electrode was Ni or Fe. The conduction process was shown to be tunneling for barriers less than about 100 A at liquid He temperature and consistent with Mott variable‐range hopping for higher temperatures and thicknesses. Measurements were made of current density J and dynamic conductance dJ/dV as a function of voltage V, thickness s, and temperature T. The measurements were compared with available theoretical expressions for rectangular tunnel barriers based on the WKB approximation. The applicability of these expressions for barrier heights less than 100 meV was examined and a modified equation for J(V) was derived which eliminated assumptions which are inaccurate for such low barriers. The measurements were also compared to this modified equation and to numerical solutions. Values for the effective tunnel barrier height ranging from ...

Artificial tunnel barriers produced by cryogenically deposited Al2O3

Electron beam evaporated sapphire has been used to form tunnel barriers on substrates cooled to near 77 K. Ni/Al2O3/Al junctions show tunneling characteristics comparable with those obtained by plasma oxidation of Al and the subsequent deposition of Ni. The deposited Al2O3 barriers are stable and usually have insignificant leakage currents. The barrier heights are about 2.5 eV, and Al2O3 does not degrade the spin polarization of tunnel currents (as does oxidized amorphous Si). Au/Al2O3/Al junctions were made with good superconducting tunneling characteristics and with leakage less than 1%. Even Au/Al2O3/Au junctions apparently exhibit good tunneling characteristics.

Electron spin polarized tunneling study of ferromagnetic thin films

Abstract The development of spin-polarized electron tunneling is reviewed beginning with the spin properties of superconductors. Previous results on ferromagnetic films of the transition metals and the heavy rare earth metals are briefly summarized. Spin polarization results of the proximity effect in Ni and Fe films are given. Recent work on artificial tunnel barriers of amorphous Si and Al 2 O 3 is described and high quality spin-polarized tunneling curves are presented for Ni using an artificial Al 2 O 3 barrier. Application of the technique to resolving the spin densities of states of superconductors is presented. Ferromagnetic-ferromagnetic tunneling is discussed.

Spin polarization of tunneling electrons from films of Fe, Co, Ni, and Gd

Abstract Tunneling measurements of spin-polarized electrons from superconducting aluminum into films of Fe, Co, Ni, and Gd give, for the electron-spin polarization, values of 44, 34, 11 and 4.3 per cent respectively. The agreement of these results with photoemission measurements makes explanations of the polarization based on the Stoner-Wohlfarth-Slater band model implausible.