Session Z20: Focus Session: Novel Ferromagnetic Semiconductors II

Defect-induced Ferromagnetism in Insulators without Magnetic Ions: The Case of Cation Vacancy in CaO

We have investigated by means of first-principle supercell calculations the posibility of ferromagnetism being induced by cation vacancies in non-magnetic oxides in four steps: ({\em i}) A single neutral Ca vacancy $\mathrm{V_{Ca}^0}$ is found to have a magnetic moment of 1.9 $\mu_{B}$ due to its electronic configuration: $(a_1^2\, t_{1+}^3\, e_+^2)t_{1-}^p\, e_-^q$ where, the $e_-$ state is partially occupied ($q \approx 0.5)$, leading to a transfer of some hole density to the $t_{1-}$ valence band states ($p \approx 2.5)$. ({\em ii}) The ferromagnetic interaction between two vacancies is found to extend only to four neighbors or less. ({\em iii}) To achieve magnetic percolation on a fcc lattice with such an interaction range one needs a minimum vacancy concentration of $1.8 \times 10^{21}$ cm$^{-3}$ (4.9 \%). However, ({\em iv}) due to the high vacancy formation energy even under the most favorable growth conditions one can not obtain at equilibrium more than $10^{18}$ cm$^{-3}$ vacancies. Thus, a non-equilibrium vacancy-enhancement factor of 10$^{3}$ is needed to achieve ferromagnetism in such systems. Comparison with other non-magnetic oxides will be also discussed.   

Ferromagnetism in Transparent ZnO:xCu Sputter Deposited Thin Films

Currently, detailed understanding on the intrinsic or extrinsic nature of the diluted magnetic oxides when doped with cations (magnetic or nonmagnetic) is an intricate issue. We report a systematic study of ferromagnetism (FM) above room temperature in transparent ZnO:xCu (x in at.{\%}) films deposited by reactive magnetron sputtering. Cu$^{2+}$ ions are found to be in wurtzite structure for x$<$3, whereas CuO phase is deciphered for x$>$3, and located within the ZnO lattice with the associated stacking faults or at grain boundary regions. An anomalous large magnetic moment (\textbf{\textit{M}}) of $\approx $1.76$\pm $0.2 $\mu _{B}$/Cu is observed for x$\approx $0.6. \textbf{\textit{M}} decreases drastically ($<<$ 0.4 $\mu _{B}$/Cu) for x$>$1 due to increased Cu-O-Cu anti-FM interactions. Micro-Raman spectral studies reveal plausible Cu-O clusters of few {\AA} in ZnO lattice giving rise to anomalous high \textbf{\textit{M}} even at low concentrations (x$<$1) in ZnO lattice. Detailed discussion will be presented on related results.   

Measurement of the spin polarization of the magnetic semiconductor EuS with zero-field and Zeeman-split Andreev reflection spectroscopy

Measurements of the spin polarization (\textbf{\textit{P}}) of doped EuS using zero-field and Zeeman-split Andreev reflection spectroscopy (ARS) on EuS/Al planar junctions are reported. EuS films (100nm thick) of different conductivities were grown via UHV electron-beam deposition at various substrate temperatures. A thin (7nm) Al film was used as the counterelectrode. The zero-field ARS spectra can be fit straightforwardly, with \textit{zero} spectral broadening and expected gap values, to the spin-polarized BTK model. The fits consistently yield \textbf{\textit{P}} on the order of 80{\%} regardless of the barrier strength. Moreover, we performed ARS in the presence of a Zeeman-splitting of the quasiparticle density of states in Al. The Zeeman-split ARS spectra are well described theoretically by combining the solution to the Maki-Fulde equations with the spin-polarized BTK analysis. The results have provided an independent verification of the validity of the zero-field ARS, and demonstrated the utility of field-split superconducting spectroscopy on Andreev junctions of arbitrary barrier strengths.   

Observation of the Temperature driven Insulator to Semiconductor (Ferromagnetic) Phase

We investigated the changes of magnetic properties (para to ferromagnetic) and phase transitions (insulator to semiconductor) in films of EuO grown on a MgO(100) single crystal as a function of temperature. In between 70 K, we found a clear phase transition and magnetic transition using the spin-summed and spin-resolved photoemission spectra, as well as the low energy electron diffraction. We will elucidate the temperature driven electronic and magnetic property in EuO on MgO(100) substrate.   

Critical behavior and spin polarization of a single crystal Mn$_{5}$Ge$_{3}$.

Despite difficulties of injecting spin into semiconductor using ferromagnetic metals, spin injection into semiconductor is essential for spintronics in order to take advantage of the silicon-based electronics. The intermetallic compound Mn$_{5}$Ge$_{3}$ is a promising candidate as a spin injector for semiconductor because of its relatively high Curie temperature and good lattice match with semiconductors. Recent theoretical calculations show that Mn$_{5}$Ge$_{3}$ has a spin polarization of as much as 70{\%} in the purely diffusive region. In this work, we have determined the critical exponents of a single crystal Mn$_{5}$Ge$_{3}$ using magnetometry. The critical temperature has been determined to be T$_{C}$ = 283.68$\pm $0.02 K from spontaneous magnetization with the critical exponents of $\beta $ = 0.358$\pm $0.005 and $\gamma $ = 1.367$\pm $0.005. The spin polarization of the crystal determined using point contact Andreev reflection (PCAR) is 54$\pm $2{\%}, indicating that it is a good spin injector with a substantial spin polarization compared with ordinary ferromagnetic metals such as Fe, Co and Ni. Work supported by NSF grant No. DMR05-20491 and DMR04-03849.   

Epitaxial Ferromagnet on Ge(111) : Mn$_{5}$\textbf{Ge}$_{3}$

Magnetic materials used for recording and reading information involve the use of the electron spin, while semiconducting devices normally take advantage of the electron charge. Recently, a big effort has been made in adding the spin degree of freedom to conventional semiconductors. A good potential candidate for spin injection in a silicon- compatible semiconductor are Mn$_{5}$Ge$_{3}$ thin films on Ge(111) [1]. To further gain information on the usefulness of possible spintronics applications of Mn$_{5}$Ge$_{3}$/Ge(111) multilayers angle resolved photoemission spectroscopy (ARPES) was employed to map the band dispersion of Mn$_{5}$Ge$_{3}$ films on Ge(111) above and below the Curie temperature. Furthermore, we performed temperature dependent x-ray magnetic dichroism measurements of Mn$_{5}$Ge$_{3}$ films on Ge(111) to probe the magnetic properties of Mn$_{5}$Ge$_{3}$. [1] C. Zeng \textit{et al}., \textit{Appl. Phys. Lett}. \textbf {83}, 5002 (2003).   

Growth and magnetization dynamics of thin film tetradymite-type diluted magnetic semiconductors based on Sb2-xTMxTe3 (M = V, Cr)

Recently we reported on a new MBE-grown thin film ferromagnetic semiconductor based on Sb2-xVxTe3 with a Curie temperature of 177 K when x = 0.35 [1]. Ferromagnetism was confirmed by magnetization measurements and anomalous Hall effect. We have now extended our investigations to thin films of Sb2-xCrxTe3 and we observed ferromagnetism in this system through magnetic measurements. The Curie temperature increases with the increasing concentration of Cr. Ferromagnetic resonance (FMR) was detected in both Sb2-xVxTe3 and Sb2-xCrxTe3 thin films at low temperatures using an electron paramagnetic resonance (EPR) spectrometer. Optical ultrafast spin dynamics investigations in both Sb2-xVxTe3 and Sb2-xCrxTe3 thin films reveal the existence of spin waves in these diluted magnetic semiconductors. The spin wave parameters are obtained based on the FMR and optical methods. [1] Z. Zhou, Y.-J. Chien and C. Uher, “Thin film ferromagnetic semiconductors Sb2-xVxTe3 with TC of 177 K”, Applied Physics Letters, vol.87, 112503 (2005).   

A Study of Magnetoresistance of Yb$_{14-x}$La$_{x}$MnSb$_{11}$: an Ordered Dilute Magnetic Semiconductor

Yb$_{14-x}$La$_{x}$MnSb$_{11 }$is an ordered dilute magnetic semiconductor, showing evidence for partial screening of the Mn magnetic moments and mass renormalization from 2 m$_{e}$ near room temperature to 20 m$_{e}$ at 5 K. We have measured both the transverse (I$\bot $H) and longitudinal (I//H) magnetoresistance (MR) of Yb$_{14-x}$La$_{x}$MnSb$_{11 }$(x=0, 0.7) single crystals above and below the ferromagnetic (FM) transition temperature T$_{c}$ = 53 K (x=0) and 39 K (x=0.7). While it is negative in a wide temperature range, the MR becomes positive below T$_{c}$/2 and increases with decreasing temperature. Strikingly, the positive MR increases linearly with applied field, showing no sign of saturation up to 32 tesla. Quantitative analysis and implications of the data will be reported.   

Observation of a new magnetic anomaly in low field ac and dc magnetic measurements in Yb$_{14}$MnSb$_{11}$

Yb$_{14}$MnSb$_{11}$ is the first known example of a ferromagnetic Kondo lattice compound in the underscreened limit. Recent optical, Hall, magnetic, and thermodynamic measurements indicate that Yb$_{14}$MnSb$_{11}$ may be a rare example of an underscreened Kondo lattice. This heavily doped magnetic semiconductor is ferromagnetic below 52 $\pm $1 K. We report the first experimental observation of a new magnetic anomaly in this system at around 47 K, a few degrees below the Curie temperature. Systematic investigations of the AC and DC susceptibilities of Yb$_{14}$MnSb$_{11}$ single crystals reveal features associated with possible spin re-orientation at this temperature. This new anomaly is extremely sensitive to the applied field and is absent in DC magnetization measurements for fields above 50 Oe. The origin of this could be related to a change in magnetic anisotropy caused by the decoupling of energetically close FM and AFM sub-lattices.   

Transport and magnetic properties of rare-earth nitrides

There is controversy about the conducting character of the rare-earth nitrides, with reports existing from metallic to moderately wide band-gap semiconductors. In a programme intended to clarify that issue we have grown thin films of the rare-earth mononitrides GdN, SmN, DyN and ErN by ion assisted deposition (IAD). Their stoichiometry and nanocrystalline structure have been characterised by RBS, SIMS, XRD, TEM, and EXAFS. The as-prepared materials are very reactive in the atmosphere, but they are effectively passivated by capping layers of either MgF$_{2}$ or IAD GaN. Their magnetic properties have been studied from ambient temperature to 5 K and found to be in agreement with reported behaviour in the literature. The conductivity is typical of semiconductors, as regards both its magnitude and its temperature dependence.