Session H15: Focus Session: Spins in Semiconductors - Ferromagnetic Semiconductors

Interplay of confinement and spin-orbit interaction in ferromagnetic semiconductors

We review experimental and theoretical works which show a surprising influence of confinement on properties of III-V ferromagnetic semiconductors related to the spin-orbit interaction. In particular, according to SQUID studies, magnetization of (Ga,Mn)As thin films show two distinct components of orthogonal in-plane easy axes, whose relative strength can be controlled by the gate voltage [M. Sawicki et al. Nature~Phys.~6\textbf{, }22 (2010)]. Furthermore, in high $T_{C}$ structures a confinement leads to an unanticipated collapse of the anomalous Hall effect [D. Chiba et al. Phys.~Rev.~Lett.~104\textbf{, }106601 (2010)]. A possibility of a non-trivial interplay of confinement and spin orbit interaction is further highlighted by the theoretical prediction of a non-collinear spin arrangement in thin films of (Ga,Mn)As [A. Werpachowska and T. Dietl, Phys.~Rev.~B 82\textbf{, }085204 (2010)]. Finally, we show how a large energy separation between the heavy and light hole subbands in compressively strained gated InAs:Mn quantum wells leads to hysteretic behavior even in the single Mn impurity limit [U. Wurstbauer et al. Nature Phys. (2010) doi:10.1038/nphys1782].   

Room Temperature Ferromagnetism in GaN-AlN Quantum Confined Heterostructures

GaN and AlN two dimensional electron gas heterostructures were grown by plasma assisted molecular beam epitaxy. During growth, a $\delta$-doped layer of Gd was introduced at a controlled distance from the interface of the GaN and AlN layers. Resulting magnetic and structural properties were characterized by a variety of complementary methods including X-ray diffractometry, atomic force microscopy, transmission electron microscopy, superconducting quantum interference device magnetometry, photoluminescence spectroscopy and measurement of the anomalous Hall effect. Doping with Gd was observed to give rise to a colossal magnetic moment of 200$\mu_{\rm{B}}$/Gd with a Curie point in excess of 300K. To elucidate the mechanism by which introduction of Gd results in such a large moment, a wide variety of heterostructure geometries and growth conditions have been explored.   

Digital magnetic heterostructures based on GaN

Delta doped heterostructures based on GaN are possible wide band gap spintronic material with high temperature ferromagnetic transition [Hory et al, Physica B 324, 142 (2002); Reed et al, Appl. Phys. Lett. 79, 3473 (2001)]. They are formed by a thin magnetic layer or submonolayer, embedded into thick semiconductor layers [Marques et al, Phys. Rev. B 73, 224409 (2006)]. In this work we studied digital magnetic heterostructures with isolated magnetic monolayers of V, Cr, Mn, Fe, Co, Ni, or Cu embedded in GaN. We performed first-principles calculations within the density functional theory with self-energy, to account for excitation energies [Ferreira et al, Phys. Rev. B 78, 125116 (2008)]. The technique allowed us to calculate the GaN band gap as $E^{calculated}_{gap}=3.53eV$ with no adjustable parameters. GaN:V and GaN:Cr do have ferromagnetic ground states with high Curie temperatures, as predicted by a mean-field approximation. GaN:Cr presents a 2D half-metallic ground state with 100\% spin polarization. GaN:V has an unusual ground state, with a 2.0$\mu_B$ magnetic moment, no states at the Fermi energy, a minority spin gap of 3.0eV, and a majority spin gap of only 0.3eV.   

Intrinsic ferromagnetism at AlN-MgB$_2$ interfaces

Spin polarization without magnetic elements is possible utilizing dangling bonds. In particular, nitrogen-dangling bonds of cation vacancies are responsible for spin polarization in nitride semiconductors. Enhancement of ferromagnetism due to cation vacancies by Gd dopants has also been found [1], which is consistent with colossal magnetic moments per Gd observed in experiments. However, randomness of point-defect distribution is not feasible to control magnetisation. In this situation, ordered structures of spin sites are highly desirable for the control of magnetization. In this study, we demonstrate by means of first-principles calculations that nitride-boride interfaces could be a candidate for such ordered spin sites. Partially occupied N $p$ states at AlN-MgB$_2$(0001) interfaces exhibits two-dimensional spin polarization, which cannot be anticipated from the atomic structure because the N dangling bonds are apparently saturated by Mg. Hund's coupling of the two N $p_{||}$ orbitals as well as high density of spin-unpolarized states at the Fermi energy contribute to strong itinerant ferromagnetism. Roles of metal-induced gap states [2] will also be discussed. \\[4pt] [1] Y. Gohda and A. Oshiyama, Phys. Rev. B 78, 161201(R) (2008). \\[0pt] [2] Y. Gohda, S. Watanabe, and A. Gro\ss, Phys. Rev. Lett. 101, 166801 (2008).   

Room temperature ferromagnetism in cluster free, Co doped Y$_{2}$O$_{3}$ dilute magnetic oxide

Diluted magnetic oxides (DMO) displaying the ferromagnetic behavior far above room temperature has attracted much attention for potential spintronic applications. Our study using low temperature co-deposition has produced uniformly doping of transitional metal (TM) Co (2-10 at.{\%}) in Y$_{2}$O$_{3}$ films without formation of clusters or second phases, stable up to 450\r{ }C anneals under most ambient. This was confirmed by EXAFS local structural analysis, XANES, and XMCD measurement. Ferromagnetic behavior of magnetic moment was observed at 300K, and the Co saturation magnetization was modulated by the concentration of oxygen vacancies under various post treatments. The observation is consistent with the impurity band exchange model to account for apparent ferromagnetism in these nearly insulating DMO films. One surprising implication from this model is the occurrence of ferromagnetic insulator behavior in TM doped HfO$_{2}$ is more likely than the widely studied TM doped ZnO, TiO$_{2}$, and SnO$_{2}$ systems of smaller band gaps for doping concentrations kept under cation percolation threshold.   

No role of magnetic impurities in observed ferromagnetism in Ti$_{1-x}$Ta$_{x}$O$_{2}$ thin film

Recently, the idea of cationic-vacancy-induced FM in wide band-gap semiconducting oxides was proposed on theoretical grounds. Experimentally, we observed ferromagnetism in thin films of anatase Ti$_{1-x}$Ta$_{x}$O$_{2}$ grown by PLD. Ta incorporation gives rise to cationic defects, acting as magnetic centers and free charge carriers as detected by various spectroscopic and transport measurements. To confirm that the ferromagnetism is an intrinsic property of Ti$_{1-x}$Ta$_{x}$O$_{2}$ and to rule out any impurity issues such as presence of magnetic elements and clustering, we carried out in-depth analysis based on Rutherford backscattering spectroscopy (RBS), Proton induced X-Ray Emission spectroscopy (PIXE) and Secondary Ion Mass Spectroscopy (SIMS). Form these results we concluded that the observed FM was not due to magnetic impurities. Rather it is an intrinsic property of Ti$_{1-x}$Ta$_{x}$O$_{2}$ thin film.   

Magnetism of DMS SnO$_{2}$:Co Thin Films Grown by RF Sputtering

SnO$_{2}$:Co thin films with dopant concentrations ranging from 2-15at{\%} were deposited on r-cut sapphire substrates via RF sputtering, to examine the origin of the room temperature ferromagnetism (RTFM) observed in such materials. Films deposited with 9:1 Ar/O$_{2}$ partial pressure ratio have a saturation moment of $\sim $0.34$\mu _{B}$/Co. Utilizing Coey's generalized grain boundary model [1], this implies that only $\sim $(2 $\pm $ 0.5){\%} of the sample is FM. Furthermore, XPS studies reveal that the cobalt valance is 2+, suggesting it exists in the form of an oxide, either Co substitutional or as CoO clusters. Furthermore, angle dependent measurements indicate no sign of phase segregation. We speculate that the FM is due to the spontaneous magnetization of uncompensated spins at the surface of the CoO antiferromagnetic nanocrystals [2] in the host SnO$_{2}$. This model may also explain the large anisotropy observed in some of our films.\\[4pt] [1] Coey et. al IEEE Trans. Magn. (2010)\\[0pt] [2] Dietl et. al PRB \textbf{76}, 155312 (2007)   

Observation of room-temperature ferromagnetism in Cu:ZnO films part I; soft X-ray Magnetic Circular Dichroism

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient Cu:ZnO films by using soft x-ray magnetic circular dichroism and x-ray absorption [1]. 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 Cu$^{2+}$ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order.\\[4pt] [1] T.S. Herng et al, Phys. Rev. Lett. {\bf 105}, 207201 (2010)   

Observation of room-temperature ferromagnetism in Cu:ZnO films part II; a theoretical study

To better understand the observation of room-temperature ferromagnetic ordering in O-deficient Cu:ZnO films [1], we calculated the configuration-averaged spectral function $\langle A(k,\omega)\rangle$ of ZnO with 2\% Cu impurities and 1\% O vacancies within the ``LDA+U'' approximation, solving the Hamiltonian only within the low energy Hilbert space, defined via the first-principles Wannier functions [2]. Based on these first principles results we proposed a microscopic ``indirect double-exchange'' model for the FM in Cu:ZnO that explains our main experimental findings.\\[4pt] [1] T.S. Herng et al, Phys. Rev. Lett. {\bf 105}, 207201 (2010)\\[0pt] [2] T. Berlijn et al, arXiv:1004.1156 (2010)   

Electrodeposition of Co-doped Cu$_{2}$O layers with high Curie temperature

In this work, we have studied the magnetic properties of room temperature electrodeposited Cu$_{2}$O layers doped with Co. These layers were growth from electrolytes containing lactic acid and copper sulfate, with the addition of cobalt sulfate for the doping process. The layers are considered as a diluted magnetic semiconductor, showing ferromagnetic behavior above room temperature and saturation magnetization proportional to the concentration of cobalt sulfate. The decrease of lattice parameter and resistivity with the increase of the band gap for doped samples were results that pointed out to the Co incorporation to the growing layers as individual atoms. In addition, no evidences for the existence of superparamagnetic particles were observed from ZFC and FC curves, hysteresis loops and HRTEM images. The magnetic behavior is associated to Co atoms diluted in the Cu$_{2}$O lattice and a promising Curie temperature for spintronic application of 550 K was determined.   

The spin polarized electronic structures and magnetic properties of Co doped and Ga codoped ZnO

The understanding of the magnetic property of Co doped ZnO (ZnO:Co) has been inconclusive with confusing experimental observations. Here, spin-polarized first-principles calculations have been performed for ZnO:Co to better understand its magnetic property. Without O and Zn vacancies, the total energy per Co ion in ZnO:Co in the ferromagnetic (FM) state was found to be only 6meV lower than that in the antiferromagnetic (AFM) state, which suggests that at room or higher temperature ZnO:Co is in the spin glass state. O vacancies and co-doping with Ga ions were found to enhance Co-Co FM coupling by induced delocalized states in the vicinity of Co 3d bands. The O vacancy was found to have a greater effect of FM enhancement than the co-doped Ga ion. While Zn vacancies were found to lower the Fermi level and enhance hybridization between Co 3d and valence-band O 2p states, which enhances super-exchange coupling and renders ZnO:Co to be in the AFM state. The opposite effects of O and Zn vacancies imply that the magnetic property of ZnO:Co depends strongly on the relative concentrations of O and Zn vacancies.   

Magnetotransport in the amorphous Ge(1-x)Mn(x) with self-assembled nanostructures

Mn ions have been reported to segregate into intermetallic precipitates or form coherent clusters in crystalline Ge(1-x)Mn(x) thin films. In this study, we investigated the microstructure of amorphous Ge(1-x)Mn(x) synthesized using low temperature molecular beam epitaxy, and observed the self-assembly of Mn rich nanostructures in Ge matrix with both cluster and column shapes by varying the Mn concentration. The magnetotransport properties were found to closely correlate with the magnetism. Negative magnetoresistance (MR) showed a dominant effect in as-grown materials, and required a very high magnetic field to saturate, whereas only positive MR was observed in post-annealed Ge(1-x)Mn(x). The anisotropic behavior in magnetism and magnetoresistance will also be discussed.