We present tunneling experiments on Fe(001)/MgO(20 Å)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-${\mathrm{Al}}_{\mathrm{2}}{\mathrm{O}}_3/\mathrm{FeCo}$ tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

It has been recently reported that some nonmagnetic materials in bulk state exhibit magnetic behavior at the nanoscale due to surface and size effects. The experimental observation of these effects is based on the measurement of very small magnetic signals. Thus, some spurious effects that are not critical for bulk materials with large magnetic signals may become important when measuring small signals (typically below 10−4emu). Here, we summarize some sources of these small magnetic signals that should be considered when studying this new nanomagnetism.

In this Letter, we experimentally show that the room temperature ferromagnetism in the Mn-Zn-O system recently observed is associated with the coexistence of ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ via a double-exchange mechanism. The presence of the ZnO around ${\mathrm{MnO}}_2$ modifies the kinetics of ${\mathrm{MnO}}_2\to {\mathrm{Mn}}_2{\mathrm{O}}_3$ reduction and favors the coexistence of both Mn oxidation states. The ferromagnetic phase is associated with the interface formed at the Zn diffusion front into Mn oxide, corroborated by preparing thin film multilayers that exhibit saturation magnetization 2 orders of magnitude higher than bulk samples.

In this work we present experimental results concerning conductance quantization at room temperature in metallic nanowires. The experiments are performed both at ambient presure and in ultrahigh vacuum using scanning tunneling microscopy (STM)-like devices. Both microscopic and macroscopic electrodes are used. The obtained results are independent of the electrode size, indicating that nanowires form at the last stages of the contact-breakage process, independently of the initial size and shape of the contact. In addition, we study (a) the stability of the nanowires, finding their lifetimes remarkably long, on the order of minutes; (b) the I-V curves for different quantum channels; (c) nanowires formed and visualized using a STM inside a scanning electron microscope (SEM); (d) the statistical behavior of the conductance, sampling tens of thousands of consecutive conductance experiments, showing clear quantized peaks up to five quanta of conductance; and (e) the length of the conductance plateaus, that might indicate the chaotic behavior of the quantum states formed at the nanowire. The statistical study of the conductance plateau lengths shows a broad Wigner-like distribution with an average value of 0.13 nm for the first quantum channel. This is at variance with a constant length, predicted and experimentally observed, in the plateaus between force jumps.

This paper shows that conductance quantization at room temperature is a physical and reliable observation. This is demonstrated by conductance histograms taking all (12 000) consecutive nanowire conductance experiments in Au at room temperature. On the other hand, conductance curves in Ni, a room-temperature ferromagnet, show staircase-quantized behavior, but the histograms do not show quantized peaks, most probably due to the lifting of the spin degeneracy.