On the Origin of d⁰ Magnetism in Transparent Metal Oxide Nanocrystals

Abstract

Controlling magnetic and magneto-optical properties of transparent metal oxide semiconductors has a significant potential for spintronics and photonics. Although ferromagnetism has been reported for several nanostructured transparent metal oxides in the absence of magnetic dopants, its origin and the nature of the exchange interactions remain controversial. Here, we report a variable-temperature−variable-field magnetic circular dichroism study of ZnO and SnO 2 nanocrystals prepared under oxidizing and reducing conditions. We observe the band splitting in ZnO and SnO 2 nanocrystals induced by localized doublet (S = 1/2) and triplet (S = 1) ground states, respectively. Photoluminescence measurements suggest that these states are associated with oxygen vacancies, either as isolated paramagnetic sites in ZnO or as local vacancy-based complexes in SnO 2 nanocrystals. The results of this work demonstrate the ability to tune carrier polarization in metal oxide nanocrystals by in situ control of the native defect formation and attest to the anomalous Zeeman splitting of the band states, which may play an important role in generating ferromagnetism in this class of materials.

Size Control of the Mechanism of Exciton Polarization in Metal Oxide Nanocrystals through Fermi Level Pinning

“…The MCD intensity increases rapidly at low magnetic fields and then shows the effect of saturation as the field increases. This type of magnetic field dependence is characteristic for paramagnetic samples. , In the absence of any appreciable free electron density, the MCD intensity for nonmagnetic metal oxide NCs may only originate from anomalous (spin-induced) Zeeman splitting involving localized electrons associated with lattice defects, such as oxygen vacancies . To confirm this conclusion, we fitted the integrated intensity of the positive MCD band as a function of the magnetic field (black circles in Figure b) with the Brillouin function (eq ), using the spin state S = 1/2 (single unpaired electron) and Landé g factor g S = 2.002, leaving the localized electron concentration, N , as the only floating parameter M s = 1 2 N g S μ B true[ ( 2 S + 1 ) .25em normalcoth ( false( 2 S + 1 false) g S μ B B 2 k B T ) − normalcoth ( g S μ B B 2 k B T ) true] where M s is the saturation magnetization, B is the external magnetic field, μ B is the Bohr magneton, k B is the Boltzmann constant, and T is the sample temperature.…”

“…This type of magnetic field dependence is characteristic for paramagnetic samples. 33,39 In the absence of any appreciable free electron density, the MCD intensity for nonmagnetic metal oxide NCs may only originate from anomalous (spininduced) Zeeman splitting involving localized electrons associated with lattice defects, such as oxygen vacancies. 39 To confirm this conclusion, we fitted the integrated intensity of the positive MCD band as a function of the magnetic field (black circles in Figure 2b) with the Brillouin function (eq 3), using the spin state S = 1/2 (single unpaired electron) and Landég factor g S = 2.002, 60 leaving the localized electron concentration, N, as the only floating parameter…”

“…The LSPR in intentionally undoped metal oxide NCs is, therefore, mostly determined by the degree of delocalization of electrons in the donor-defect states. On the other hand, localized defects containing unpaired electrons can also lead to spin splitting of the NC band states, similar to magnetic dopants in diluted magnetic semiconductors (DMSs) . Consequently, a control over the electron (de)­localization in metal oxide NCs will not only affect the nature of carrier polarization and the exciton splitting but also offer insights into the plasmon–exciton correlation in semiconductor NCs.…”

Properties of Free Charge Carriers Govern Exciton Polarization in Plasmonic Semiconductor Nanocrystals

“…The magnetic field and temperature dependences in Figures S4 and S5 also clearly rule out a possibility that MCD signal arises from anomalous Zeeman splitting associated with the presence of paramagnetic Cu 2+ in Cu 2– x Se NCs. Such spin-induced Zeeman splitting would result in the MCD signal saturation with increasing magnetic field and the Curie-type decay with increasing temperature. ,− As reported previously, stoichiometric Cu 2 Se NCs are less stable and tend to convert to Cu 1.8 Se NCs at ambient conditions, which makes it difficult to perform control MCD measurement on nonplasmonic Cu 2 Se NCs. However, we note that the intensity of the excitonic MCD signal increases with increasing LSPR absorption intensity (Figure S6), as expected on the basis of the correlation between the two phenomena.…”

First-principles study of $d^0$ magnetism in group-IV-doped monolayer GaN

“…Besides that, bulk oxides of nonmagnetic (NM) cations such as ZnO and SnO 2 can also exhibit room-temperature ferromagnetism, namely "d 0 ferromagnetism". The origin of d 0 magnetism was explained by native lattice defect sites [4,5] or spin-split defect impurity bands of surface defects [6]. In 2017, 2D magnets were demonstrated in atomically thin CrI 3 [7] and Cr 2 Ge 2 Te 6 [8].…”