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Point contacts of magnetite exhibit large, reversible increases of conductance by a factor of up to 5 in the presence of a small magnetic field ͑Ͻ10 mT͒, provided the conductance is less than the quantum of conductance G 0 ͑12 900 ⍀͒ −1 . Smaller effects are found in ͑La 0.7 Sr 0.3 ͒MnO 3 and Co 2 Cr 0.6 Fe 0.4 Al. Comparable effects observed in magnetite nanoconstrictions milled using a focused-ion beam in thin films deposited on oxide substrates are only reversible on a time scale of hours. High-resistance nanogaps show evidence of field emission beyond a voltage threshold, which is itself field dependent. The results are discussed in terms of spin-polarized transport, magnetostriction, magnetic dipole strain, and Fowler-Nordheim tunneling.

A versatile technique for performing spin polarization measurements via point contact Andreev reflection has been developed. This technique involves depositing a superconductor ͑lead͒ onto a thin film of the material to be studied through a nanohole formed in a layer of photoresist, using an atomic force microscope as a nanoindenter. Copper and nickel were used to demonstrate the method. The polarizations of CrO 2 and Co 2 MnSi were also measured, the former giving a value of 95%, as expected, and the latter giving 20%, which was surprisingly low for a candidate half metal.

Due to its negligible spontaneous magnetization, high spin polarization and giant perpendicular magnetic anisotropy, Mn2RuxGa (MRG) is an ideal candidate as an oscillating layer in THz spin-transfer-torque nano-oscillators. Here, the effect of ultrathin Al and Ta diffusion barriers between MRG and MgO in perpendicular magnetic tunnel junctions is investigated and compared to devices with a bare MRG/MgO interface. Both the compensation temperature, Tcomp, of the electrode and the tunneling magnetoresistance (TMR) of the device are highly sensitive to the choice and thickness of the insertion layer used. High-resolution transmission electron microscopy, as well as analysis of the TMR, its bias dependence, and the resistance-area product allow us to compare the devices from a structural and electrical point of view. Al insertion leads to the formation of thicker effective barriers and gives the highest TMR, at the cost of a reduced Tcomp. Ta is the superior diffusion barrier which retains Tcomp, however, it also leads to a much lower TMR on account of the short spin diffusion length which reduces the tunneling spin polarization. The study shows that fine engineering of the Mn2RuxGa/barrier interface to improve the TMR amplitude is feasible.

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