Giant magnetoresistive structures based on CrO2 with epitaxial RuO2 as the spacer layer

Abstract

Epitaxial ruthenium dioxide (RuO2)∕chromiumdioxide(CrO2) thin film heterostructures have been grown on (100)-TiO2 substrates by chemical vapor deposition. Both current-in-plane (CIP) and current-perpendicular-to-plane (CPP) giant magnetoresistive stacks were fabricated with either Co or another epitaxial CrO2 layer as the top electrode. The Cr2O3 barrier, which forms naturally on CrO2 surfaces, is no longer present after the RuO2 deposition, resulting in a highly conductive interface that has a resistance at least four orders of magnitude lower. However, only very limited magnetoresistance (MR) was observed. Such low MR is due to the appearance of a chemically and magnetically disordered layer at the CrO2 and RuO2 interfaces when Cr2O3 is transformed into rutile structures during its intermixing with RuO2.

Electronic and magnetic structure of CrO2-RuO2interfaces

“…In CrO 2 -RuO 2 multilayered structures grown in the (100) direction by chemical vapor deposition, no significant GMR effects were observed 9 for either CPP or CIP (current in the plane). 9 It was observed that the CrO 2 layer intermixed with the RuO 2 layer at the interface. 9 First-principles calculations of CrO 2 and CrO 2 -RuO 2 interfaces in the (100) and (110) directions were conducted in order to determine whether non-collinear spins at the interface could form, thereby eliminating the GMR effect.…”

“…This is an undesirable effect for a GMR structure that may lead to increased spin-flip scattering and lower MR. We speculate that this effect may contribute to the low MR value observed in the (100)-oriented CrO 2 /RuO 2 /CrO 2 GMR structure. 9 The calculated energy differences shown in Figure 5 correspond to a difference in…”

“…In particular, CrO 2 , which is the only material for which there exists definitive experimental evidence for a half-metallic electronic structure, 1,2,3,4,5,6 and which (among known half-metals) has the largest gap in the non-metallic spin channel, has shown only minimal spintronic effects. 7,8,9 CrO 2 also appears to be unique among the half metals currently known in that its surface is also predicted to be half-metallic. 10 One could also argue that in addition to being the "best" half metal (for the reasons mentioned above) it is also the "simplest" in the sense that it consists of only two elements and has a robust structure, e.g.…”

Interface effects in spin-polarized metal/insulator layered structures

“…Charge transfer in LSMO/SrTiO 3 interfaces reduces spin polarization [215]. At CrO 2 /RuO 2 interfaces, evidence of non-collinear magnetization was found [218]. Such spin polarization reduction diminishes TMR in these systems with postulated half-metallic electrodes from the predicted infinity to much more modest values (Sec.…”

“…For example, CrO 2 has been predicted [168] and shown experimentally, using both Andreev reflection [178] and MeserveyTedrow type measurements [179], to be a half metal (although it may not be metallic but rather a semi-metal with high polarization [217]). CrO 2 was used as electrodes in CrO 2 /RuO 2 /CrO 2 spin valves, where RuO 2 is a metal which has the same rutile structure and very closely matched lattice constant [218]. No appreciable GMR effect was obtained due to a large chemically and magnetically disordered layer at the CrO 2 /RuO 2 interface.…”