Room-Temperature Ferromagnetism of Cu-Doped ZnO Films Probed by Soft X-Ray Magnetic Circular Dichroism

Abstract

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu2+ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V(O)) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic "indirect double-exchange" model, in which alignments of localized large moments of Cu in the vicinity of the V(O) are mediated by the large-sized vacancy orbitals.

Cationic-vacancy-induced room-temperature ferromagnetism in transparent, conducting anatase Ti 1− x Ta x O 2 ( x ∼0.05) thin films

“…Despite these developments, magnetic-impurity-based DMS oxides are the subject of intense debate as a result of the many inconsistent results related to secondary phase formation, unknown impurities or substitutionality, solubility, clustering and/or segregation of the magnetic impurities in the host oxide matrix [16][17][18]. More recently, there have been reports of FM in ZnO and other oxides (attributed to anionic vacancies) by doping with non-magnetic ions, but besides evidence from superconducting quantum interference device (SQUID) data [13][14][15], a necessary but not sufficient criterion [17,18], there has been no reliable element-specific evidence for FM in these cases, with the exception of recent publications [19,20]. To establish true FM in the DMS oxides and to differentiate from impurity artefacts, various magnetic measurements-ranging from extremely sensitive SQUID and optical magnetic circular dichroism (OMCD) measurements, which directly probe the spin-polarized bands, to element-specific soft X-ray magnetic circular dichroism (SXMCD) measurements-are required.…”

Exploring nanoscale magnetism in advanced materials with polarized X-rays

“…It is worth noting that it is the unambiguous fingerprint characteristics of X-ray absorption spectroscopy and XMCD as its magnetic counterpart obtained at both the O K-edge and the Cu L edges, which has led to this conclusion [176].…”

Transition from diamagnetic to ferromagnetic state in laser ablated nitrogen doped ZnO thin films

“…In this context, various non-magnetic dopants such as Cu, Al, K have been extensively studied for possible RT ferromagnetism in ZnO. [4][5][6][7][8][9][10][11] Ma et al observed RT FM in Al doped ZnO after vacuum annealing and ascribed it to the charge transfer between Al and ZnO, which can cause the electronic structure alternation of Zn and Al. 5 Ferromagnetism was reported in Cu doped ZnO by Tiwari et al and led to the conclusion that concentration of n-type carriers should not exceed a critical value after which the ferromagnetism completely vanishes.…”

“…10 Herng et al reported the existence of ferromagnetism in Cu doped ZnO thin films prepared by pulsed laser deposition (PLD) in oxygen deficient environment and identified that both oxygen vacancies (V O ) and Cu impurities are essential for the observed ferromagnetism. 4 Despite the numerous reports on the study of magnetism in non-magnetic dopants based ZnO, the origin of magnetism is highly controversial and it is still questionable that whether point defects are essential to explain magnetism in ZnO. Defects such as oxygen vacancies are deep donors that tend to form readily and have low formation enthalpies.…”