Unexpected magnetism in a dielectric oxide

Abstract

It is generally accepted that magnetic order in an insulator requires the cation to have partially filled shells of d or f electrons. Here we show that thin films of hafnium dioxide (HfO2), an insulating oxide better known as a dielectric layer for nanoscale electronic devices, can be ferromagnetic even without doping. This discovery challenges our understanding of magnetism in insulators, because neither Hf4+ nor O2- are magnetic ions and the d and f shells of the Hf4+ ion are either empty or full.

Magnetic Properties of ZnO Nanoparticles

“…For thiol capped thin films the magnetization curves are fairly different upon applying field parallel or perpendicular to the surface (hence confirming the huge anisotropy) and the magnetic moments are giant [14,15]. Similar features (giant magnetic moments and huge anisotropy) have been observed for DMS thin films [10,11,17] suggesting a possible common origin of both effects. In this work we experimentally show that capping ZnO NPs with a variety of organic molecules modifies its electronic structure arising ferromagnetic-like behaviour up to 300 K. Therefore, this method opens a new way to obtain the desired magnetic semiconductors without using any type of magnetic atoms and avoiding the superparamagnetic limit.…”

“…Rubi et al [9] found similar results for Co and Mn doped ZnO powder samples. Moreover, Coey et al [10] demonstrated that doping ZnO thin films with 3d non magnetic ions (as Ti or V) also leads to RT ferromagnetic behaviour and for insulating HfO2 the effect appear even without doping [11]. Those results point on the alteration of the electronic structure of the semiconductor (induced by both, the presence of the magnetic impurity and the defects) as responsible of the origin of the observed magnetism.…”

“…For most of the experimental results doubts arose about the real origin of magnetism [4,5,6]. The most recent and outstanding works on this field showed that the magnetic properties are not exclusively related to the presence of the magnetic ions but strongly determined by the defects in the host matrix [7,8,9,10,11,12]. For instance, Kittisltved et al [7,8] showed that Mn:ZnO nanoparticles and thin films show room temperature (RT) ferromagnetism when doped p-type defects, while other capping that introduce n-type defects leads to no RT ferromagnetism.…”

Room-temperature ferromagnetism in Li-dopedp-type luminescent ZnO nanorods

“…Dilute ferromagnetism in oxide semiconductors generally has been reported with 3d dopants in ZnO particularly with Mn, Co, Ni, Fe, Cu, and V dopings. [1][2][3] Unexpected magnetization without transition metal has been reported in dielectric oxides such as HfO 2 , ZrO 2 , 4 and nonmagnetic oxides, e.g., ZnO, 5 hexaborides, 6 carbon-doped ZnO, 7 and in irradiated graphite. 8 The theory of Dietl et al 9 suggests that carrier-induced ferromagnetism in Mn-doped p-type material may be observed at higher temperature.…”

Magnetic Resonance Study of MnO/ZnO Nanopowders

“…Moreover, doping of ZnO thin films with non-magnetic ions from the 3d group (e.g. titanium or vanadium) can cause emergence of ferromagnetic properties [9]. All these results indicate on a critical role of electronic structure of a semiconductor (modified by the presence of magnetic ions) in appearance of ferromagnetic properties.…”