Session B19: Focus Session: Transition Metal Oxide Ferromagnetic Semiconductors

Oxide Ferromagnetic Semiconductors

The field of oxide ferromagnetic semiconductors has witnessed tremendous interest and activity over the past few years, especially in the context of realization of intrinsic oxide based diluted magnetic semiconductors (O-DMS). In this talk I will review the important developments in this field, highlighting apparent successes, concerns and questions. Research results obtained by various groups on a number of systems such as transition element (especially Co, Mn, Cr) doped TiO2, ZnO, La1-xSrxTiO3, HfO2 will be discussed including some related device efforts involving spin transport and field induced modulation of magnetization. Some cases of undoped or dual doped oxide films will also be addressed. In the light of the potentially serious possibility of extrinsic effects in most systems the significance of the choice and implementation of an appropriate characterization scheme will also be highlighted. Collaborators : T. Venkatesan, Nigel Browning, Y. V. Idzerda, R. Ramesh, D. K. Kundaliya, S. X. Zhang, L. F. Fu, S. Dhar, A. Lussier, S. R. Shinde, Y. Zhao, M. S. R. Rao, T. Zhao.   

Model for vacancy-induced ferromagnetism in ``d0" magnetic oxides

J. Coey has coined the term ``d0'' ferromagnetism to describe exciting but puzzling effects seen in HfO2, ZrO2 and other oxides. He has also postulated a connection to the occurrence of cation or anion vacancies, which, recent theories indicate, may reasonably lead to the formation of local moments. What has lacked has been a theory which explains why, and at what temperatures, one could expect ferromagnetism. Here we present a simple model for vacancy-based ferromagnetism which permits quantitative estimates of the Curie temperature. The model is based on a random Hubbard model for anions with potentials that represents the occurrence of cation vacancies. By an extension of techniques successfully used to model ferromagnetism in host substituted III-V diluted magnetic semiconductors, we calculate the Curie temperature and dynamical magnetic correlations as a function of the effective strength of the vacancy potential and doping properties, the density of vacancies, as well as the correlation and bandwidth of the host. For physically reasonable parameters this explains large Curie temperatures for rather small concentrations of vacancies.   

Anisotropic magnetic properties of Co-doped SnO$_{2}$ thin films

There is strong interest in oxide magnetic semiconductors, which show not only room-temperature ferromagnetism (FM), but also other interesting magnetic properties. Large magnetic moments and anisotropic FM have been observed in Co-doped SnO$_{2}$ [1] and Co-doped ZnO [2], respectively. Here we report the preparation and magnetic properties of Co-doped SnO$_{2}$ (Sn$_{1-x}$Co$_{x}$O$_{2}$, x=0.05) thin films. Sn$_{1-x}$Co$_{x}$O$_{2}$ thin films were grown on different substrates by pulsed-laser deposition and characterized by X-ray diffraction and SQUID magnetometry, and their magnetic properties depend on the substrates on which the films are grown. Interestingly, the films grown on R-plane-cut Al$_{2}$O$_{3}$ show strongly anisotropic magnetic properties, with [101] being the easy axis. The anisotropy also depends on the substrate on which the films are grown. The results will be discussed in terms of spin-orbit coupling and crystalline defects. [1]. S. B. Ogale, et al., Phys. Rev. Lett. 91, 077205 (2003). [2]. M. Venkatesan, et al., Phys. Rev. Lett. 93, 77206 (2004).   

Defect compensation determined magnetism in wide gap dilute magnetic semiconductors (DMS)

Ferromagnetism (FM) in wide gap DMS is thought to be closely related to intrinsic defects; the F-center Bound Magnetic Polaron (FC/BMP) model proposes that it is mediated by shallow donor defects. Our first principles FLAPW calculations show that %on Cr doped %TiO$_2$ where high $T_c$ FM was initially %reported\footnote{T.Droubay {\it et al.} J.Appl.Phys. {\bf 97}, %046103 (2005)}. We find Cr:TiO$_2$ is a charge transfer insulator with strong defect compensation - which explains the insulating behavior and the Cr chemical valence. We predict different magnetic phases can form at different compensation levels, consistent with the $p-d$ hopping interaction model\footnote{Mahadevan, Zunger, Sarma, Phys.Rev.Lett. {\bf 93}, 177201 (2004)}. FM only exists with uncompensated Cr$^{4+}$, which explains the high $T_c$ FM initially reported\footnote{T.Droubay {\it et al.} J.Appl.Phys. {\bf 97}, 046103 (2005)}. Fully-compensated Cr$^{3+}$ is predicted NOT to be FM which has been verified recently\footnote {T.C.Kaspar {\it et al.}, Phys.Rev.Lett. {\bf 95}, 217203 (2005)}. Due to insufficient hybridization between dopant and impurity states, the FC/BMP mechanism does not apply, and oxygen vacancies destroy the FM instead of mediating it. Our theory extended to Mn:GaN is found to work equally well; it provides a new understanding of the correlation between defects and magnetism. This may explain why magnetism in wide gap DMS seems to be extremely sensitive to fabrication processes. Supported by NSF through the NU MRC.   

Room temperature ferromagnetism in highly-resistive Ni-doped TiO$_2$ films prepared by sol-gel method

We investigated the magnetic and transport properties of Ni-doped TiO$_2$ films grown by a sol-gel method with Ni concentrations from 1\% to 8\%. All the samples exhibited ferromagnetism at room temperature identified by optical magnetic circular dichroism (MCD)along with SQUID measurement. For the samples with low Ni concentrations, both results argee each other, however, MCD results of 8\% Ni-doped sample showed a noticeable difference in the temperature dependence of the coercive field and the saturation magnetization from the SQUID results. The Hall effect study showed that the carriers are electrons with densities lower than $10^{18} cm^3$ at room temperature, but no anomalous Hall effect has been observed due to the high resistivity of our samples. The observation of ferromagnetism in highly-resistive Ni-doped TiO$_2$ films questions the carrier-mediated exchange interaction as an origin of ferromagnetism in this material.   

Niobium doping effects on the ferromagnetism and microstructure of anatase Co: TiO$_{2}$ films

Niobium doping is shown to have a significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO$_{2}$ films. Epitaxial films of anatase TiO$_{2}$ with 3{\%} Co, with and without 1{\%} niobium doping were grown by pulsed-laser deposition at 875 $^{0}$C at different oxygen pressures. For growth at 10$^{-5 }$Torr Niobium doping suppresses ferromagnetism, while it enhances the same in films grown at 10$^{-4 }$Torr. The results of High-resolution Z-contrast Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy study reveal uniform surface enrichment of cobalt in the form of Ti$_{1-x-y}$Co$_{x}$Nb$_{y}$O$_{2-\delta }{\rm g}$ phase, without cobalt metal clusters. The transport and Hall effect results will also be presented and discussed.   

Correlation between ferromagnetism and film structure in Co-doped ZnO films

An experimental study on reactively sputtered ZnO films with a fixed Co doping concentration of 6 percent is presented. The magnetic moment of these films is largest (about 0.7 Bohr magneton per Co) for deposition temperatures between 650 K and 800 K. The evolution of the magnetic moment with temperature correlates with an increase of the lattice parameter perpendicular to the film plane. For films on C-plane (0001) and R-cut (1-102) sapphire substrates the maximum elongation amounts to 2.5 percent and 1.3 percent, respectively. Annealing the films at 720 K for 6 hours in vacuum results in a relaxation of the lattice parameters towards the bulk values. In addition, the Co-doped films become electrically conducting during the annealing procedure. Both effects, however, do not drastically alter the magnetic moment of the Co-doped films: For films on C-plane sapphire the moment remains practically unchanged, while the magnetic moment increases only slightly for films on R-cut sapphire. These results and their implications for the understanding of ferromagnetism in Co-doped ZnO will be discussed.   

Room Temperature Ferromagnetism in Transition Metal Doped CVD-Grown ZnO Films and Nanostructures

We have characterized the chemical, compositional, and magnetic properties of Mn- and Fe-ion implanted epitaxial ZnO films and single crystal nanostructures grown by MOCVD as candidate room temperature diluted magnetic semiconductors. X-ray absorption spectroscopy (SXAS) shows that Mn-implanted films contain Mn$^{2+}$ ions which convert to a mixture of Mn$^{3+}$ and Mn$^{4+}$ upon annealing. Fe-implanted films contain a mixture of Fe$^{2+}$ and Fe$^{3+}$ which converts to a higher concentration of Fe$^{3+}$ upon annealing. XAS and preliminary analysis of EXAFS data indicate that the TM ions are substitutional for Zn. SQUID magnetometry shows that as-implanted films are ferromagnetic at 5K and the annealed films are ferromagnetic at room temperature. X-ray diffraction shows that the annealed films remain epitaxial with excellent long range order. Rutherford backscattering spectrometry indicates a substantial recovery of local order upon annealing as well. The properties of in-situ Fe-doped MOCVD-grown ZnO epitaxial films and nanostructures will also be discussed.   

Defect activated room temperature ferromagnetism in Co:ZnO films - micro-Raman studies

Intense research for room temperature (RT) ferromagnetism (FM) in semiconductors doped with both magnetic and non-magnetic dopants is ongoing because of their potential applications in spin- {\&} photo-electronics.$^{ }$We successfully employ a simple non vacuum based spin-coating technique to prepare Zn$_{1-x}$Co$_{x}$O films (0.5 to 1$\mu $m) on Al$_{2}$O$_{3}$ (001) substrate using precursor solutions of Zn and Co-ethylhexanoate. The films were annealed in air at 700$^{0}$C/1h followed by annealing at 550$^{0}$C/1h at 10$^{-5}$ -- 10$^{-6}$ Torr. X-ray and Raman spectrum indicate no change of wurtzite structure and no cluster formation due to the incorporation of Co in ZnO. Magnetic measurements reveal lack of RTFM in air annealed films, whereas on vacuum annealing films acquire FM ordering. Raman spectra gives a direct evidence on the influence of defect activated FM. The observed FM varies non-monotonically with cobalt concentration. Finally, the effect of annealing, role of oxygen defects, and concentration of Co$^{2+}$ ion on the magnetization properties will be discussed.   

Annealing Temperature Dependence of the Magnetic Moment Observed in ZnO Nanotips Implanted with Mn

Magnetic properties of Mn ion-implanted ZnO nanotips grown on quartz substrates have been investigated as a function of anneal temperature. This work tracks the strength of the coercive (H$_C$) and remanent (B$_R$) fields of these samples as a function of anneal temperature over a range of 675 to 800~$^o$C for a period of 10 or 20~min in flowing O$_2$. The results show that the magnetic properties are highly dependent on anneal temperature. It has been found that 750~$^o$C is the optimal anneal temperature for producing the greatest values of H$_C$ and B$_R$, and a noticeable drop is observed when this annealing temperature is exceeded. This observation is true for magnetic moment measurements at both 5 and 300~K. The presence of an optimal temperature above which H$_C$ and B$_R$ decrease seems to indicate that post annealing causes magnetic activation and ordering in this material. These observations would not be expected if the ferromagnetic effects observed in this material were due purely to clusters, precipitates, or secondary phase formation.   

Characterization of Mn and Fe diffusion into ZnO Films and single crystals for diluted magnetic semiconductor applications.

There is growing interesting in the use of transition metal (TM) doped ZnO films as a diluted magnetic semiconductor for room temperature spintronics applications. Various methods of doping have been investigated including ion implantation and co-deposition during the growth process. In essentially all approaches, doping either accompanies, or is followed by, an annealing step intended to achieve a uniform distribution of the TM dopant, and/or encourage substitution of TM ions for Zn in the Wurtzite lattice. We have studied the diffusion of Mn and Fe into epitaxial and highly-oriented ZnO films, and into ZnO single crystals. The epitaxial films were grown on r-sapphire, exposing the (11-20) surface, while the highly oriented films were grown on a-sapphire exposing the (0001) surface of ZnO. The single crystal substrates were cut to the (0001) and (1100) orientations. Metallic films of $\sim $ 20nm thickness were deposited onto the ZnO substrates, which were subsequently annealed to 700C for between 10 and 40 minutes. We find that the diffusion rate for Mn is approximately an order of magnitude larger than for Fe under the same annealing conditions. Differences in diffusion rates associated with crystallographic orientation and film morphology will be discussed.   

Magnetic Properties of Co-doped HfO2

Dilutely doped Co in HfO$_{2}$ films were recently reported to show strong enhancement of Co magnetic moments. We have systematically studied the structural, chemical, and magnetic properties of HfO$_{2}$ epitaxial films $\sim $1000{\AA} thick grown on YSZ with Co doping concentrations of 1, 2, 4, and 20 atomic percent. The crystal structures of these samples are predominantly monoclinic, and the XPS Co spectra also indicate the co-existence of cobalt metals and cobalt oxides. Tendency to form Co metal clusters in high doping concentrations was also observed by EXAFS. M-H loops at 10K and 300K by SQUID magnetometer were clearly seen for samples with Co doping of 4{\%} and higher. But no evidence for strong moment enhancement was found. However, our magnetic measurement do showed the unusual decay of magnetic moment with time, which was also seen in Co-doped SnO$_{2}$ films.   

Local Environment Surrounding Co in MBE-grown HfO$_{2}$:Co Thin Films Probed by EXAFS and XMCD

Local structures in MBE-grown HfO$_{2}$:Co films with different Co concentration has been investigated using the EXAFS technique. The average local environment surrounding Co exhibited by our EXAFS data consists of two major near shells attributed to O and Co neighboring atoms at distances of 2.04 \AA and 2.49 \AA from the central Co atom, respectively. As the Co concentration increases, the average coordination number of the Co shell systematically increases while that of the O shell decreases. Our experimental results indicate that while chemically bonded with O at a most-likely interstitial location, Co impurity atoms may also form Co clusters even at a relatively low concentration of $\sim $1\%. The progressive formation of Co clusters is also consistent with our XMCD results that demonstrate increasing magnetic moment with increasing Co concentration.