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This paper shows that conductance quantization at room temperature is a physical and reliable observation. This is demonstrated by conductance histograms taking all ͑12 000͒ 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. ͓S0163-1829͑97͒50108-1͔Recent experimental results using scanning tunneling microscopy ͑STM͒, 1,2 mechanically controllable break junctions, 3 and quantum table-top experiments using household macroscopic wires 4,5 have shown conductance quantization ͑CQ͒ in metallic nanowires at room temperature ͑RT͒. Previous to these experiments, CQ was predicted theoretically 6,7 in these nanostructures. Metallic nanowires ͑NW͒ are obtained by breaking a contact between two metallic electrodes. The contact does not break cleanly but is stretched into many nanofilaments. 4,5 At the last stages, one thread remains, and quantum-mechanical effects take over. This happens in all the experiments mentioned above. 1-5 If, simultaneously to the formation, stretching, and breakage of the NW, its conductance is measured, a staircase dependence is found just before it breaks. Conductance histograms using up to 100-200 such curves have been reported showing CQ peaks. However, some criteria have been always used to select the conductance curves with which the histogram is built. The metallic nanowire formation has been also predicted by molecular-dynamics simulations, 8 and recent scanning electron microscopy and chemical analysis experiments have displayed it in real time as a macroscopic metallic contact breaks. 5,9 In this work histograms built using all ͑up to 12 000͒, consecutive conductance curves are presented for Au, Pt, and Ni nanowires at RT. This is ϳ100 times more than any previously reported experiments and without histogram sample selection. The conductance histogram for Au at RT and in air shows clear CQ peaks. The histogram for Ni electrodes does not show CQ peaks, even though the conductance shows a stepped behavior, most probably due to the lifting of the spin degeneracy.The experiments are performed in a home-built STM at RT in air. Two high-purity polycrystalline macroscopic electrodes with a few tens of mV potential difference between them, are brought in and out of contact. The current flowing through is measured with a current-voltage ͑I-V͒ converter working at 10 5 gain ͑100-mV/A, 3-s settling time͒. The position of one electrode is fixed, while the other is moved with a piezoelectric actuator driven with a triangular wave. The electrode speed is controlled by varying the amplitude and/or the frequency of this signal. The output of the I-V converter ͑the current signal͒ is measured with a LeCroy 9354AM oscilloscope, with a 500-MHz bandwidth and a 5-G sample/s sampling rate. The current data acquisition is trig-

We present tunneling experiments on Fe͑001͒/MgO͑20 Å͒/FeCo͑001͒ 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 ͑001͒-oriented Fe/amorphous-Al 2 O 3 /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 surfaceFe͑001͒ in this case-but depends on the actual electronic structure of the entire electrode/barrier system.

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 Mn 3 and Mn 4 via a double-exchange mechanism. The presence of the ZnO around MnO 2 modifies the kinetics of MnO 2 ! Mn 2 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.

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Magneto-ionics, understood as voltage-driven ion transport in magnetic materials, has largely relied on controlled migration of oxygen ions. Here, we demonstrate room-temperature voltage-driven nitrogen transport (i.e., nitrogen magneto-ionics) by electrolyte-gating of a CoN film. Nitrogen magneto-ionics in CoN is compared to oxygen magneto-ionics in Co3O4. Both materials are nanocrystalline (face-centered cubic structure) and show reversible voltage-driven ON-OFF ferromagnetism. In contrast to oxygen, nitrogen transport occurs uniformly creating a plane-wave-like migration front, without assistance of diffusion channels. Remarkably, nitrogen magneto-ionics requires lower threshold voltages and exhibits enhanced rates and cyclability. This is due to the lower activation energy for ion diffusion and the lower electronegativity of nitrogen compared to oxygen. These results may open new avenues in applications such as brain-inspired computing or iontronics in general.

Single phase epitaxial pure ␥Ј-Fe 4 N films are grown on MgO ͑001͒ by molecular beam epitaxy of iron in the presence of nitrogen obtained from a radio frequency atomic source. The epitaxial, single phase nature of the films is revealed by x-ray diffraction and by the local magnetic environment investigated by Mössbauer spectroscopy. The macroscopic magnetic properties of the ␥Ј-Fe 4 N films are studied in detail by means of transverse Kerr effect measurements. The hysteresis loops are consistent with the cubic atomic structure, displaying easy ͓100͔ magnetization directions. The films are single domain at remanence, and the reversal is dominated by 180°or 90°domain wall nucleation and propagation, depending on the applied field direction. When 90°domain walls are responsible for the magnetization reversal, this proceeds in two stages, and the measured coercive fields vary accordingly. Magnetic domain observations reveal the two distinct reversal -driven by 180°or 90°domain walls-modes displaying large domains, of the order of mm. From magnetometer techniques, the saturation magnetization, 0 M s , is measured to be 1.8 T. A magneto-optical torque technique is used to obtain a value of the anisotropy constant of 2.9ϫ10 4 J/m 3 .

Voltage control of magnetism through electric field‐induced oxygen motion (magneto‐ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto‐ionic rates and cyclability continue to be key challenges to turn magneto‐ionics into real applications. Here, it is demonstrated that room‐temperature magneto‐ionic effects in electrolyte‐gated paramagnetic Co3O4 films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric‐double‐layer transistor‐like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general.