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Cecilia Zapata and Silvia P. Heluani Laboratorio de Física del Sólido, Dpto. de Física, FCEyT, Universidad Nacional de Tucumán, 4000 Tucumán, Argentina (Received 23 May 2013; accepted 2 July 2013)

We have studied the magnetoresistance (MR) of hydrogen plasma-treated pure ZnO wires of tens of micrometer diameter at different temperatures. A negative MR of 1% at 8 T applied field is measured for all wires at 4 K, independent of the temperature (300 K . . . 773 K) used during the hydrogen treatment. However, a positive MR develops, the higher the treatment temperature. The MR can be explained with a semiempirical model taking into account local magnetic moments and the s–d exchange interaction. These results together with field anisotropy in the MR indicate the appearance of magnetic order due to the hydrogen treatment in agreement with recently published reports on the influence of hydrogen in bulk ZnO single crystals. Hydrogen doping may provide a way to trigger defect-induced magnetism in small oxide structures.

I. INTRODUCTION

Magnetic order in intrinsically nonmagnetic solids due to the influence of hydrogen has been theoretically shown to be possible1–5 and experimentally found in graphite6,7 as well as in undoped8 and doped9,10 ZnO samples. Although the detailed reason for triggering or enhancement of the magnetic order depends on the compound, the observed effect can be included in the nowadays general phenomenon in magnetism known as defect-induced magnetism (DIM).11–14 A recently published work showed that hydrogen plasma treatment at low energies of implantation (;300 eV) and relatively low temperatures (T , 700 K) can trigger magnetic order at room temperature in ZnO bulk single crystals after implantation of a few atomic % H.15,16 The observed hysteresis loops in magnetization, the anomalous Hall effect (AHE) as well as the observation of an anisotropic magnetoresistance (AMR) in the same crystalline samples left no doubt on the existence of magnetic order,16 whose origin is related to the implantation of hydrogen in the near surface region.15 Although the DIM phenomenon can be observed in a large number of compounds, the obtained magnetization values are in general much smaller than that theoretically expected if the defects or ad atoms were uniformly distributed in the appropriate places of the corresponding atomic lattices. Due to the lack of appropriate defect engineering, the usual samples with DIM are not magnetically homogeneous. One possibility to increase the a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.219 78

J. Mater. Res., Vol. 29, No. 1, Jan 14, 2014

http://journals.cambridge.org

Downloaded: 21 Mar 2015

amount of magnetic mass would be to decrease the total size of the sample. In this way, one may expect to get micrometer-large regions magnetically homogeneous and with higher magnetization values. On the other hand, upon the size of magnetic domains and the lack or not of magnetic anisotropy, decreasing the sample size might affect