Session V22: Focus Session: Magnetic Tunneling II

From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions

We fabricate ferromagnetic constrictions whose cross section can be reduced gradually from 100 $\times $ 30 nm$^{2}$ to the atomic scale and eventually to the tunneling regime by means of electromigration. The contacts are attached to non-magnetic substrates and are measured at 4.2 K, so they are much more mechanically stable than previous room-temperature studies. We measure magnetoresistances (MR) $<$ 3{\%} for contacts $<$ 400 $\Omega $, consistent with previous experiments. As the contact diameter is reduced in the range 400 $\Omega $ - 25 k$\Omega $, we observe reproducible non-monotonic changes in the MR. We find first a minimum in the MR and sometimes a change in sign to small negative values, and then a strongly increasing positive MR as the contact approaches the atomic scale ($\sim $ 25 k$\Omega )$. For near-atomic-sized constrictions the maximum MR is 80{\%}. Measurements as a function of the direction of applied magnetic field allow us to separate the contribution of anisotropic magnetoresistance from the MR due to a domain wall. In the tunneling regime the MR fluctuates over a wide range, -10{\%} to 85{\%}, even for small changes in the atomic structure in a single device.   

Magnetoresistance in oxidized Ni nanocontacts

Whether or not ferromagnetic nanocontacts display large magnetoresistance (MR) is still a matter of debate [1,2]. From the theory side it has been shown[3] that MR in pure Ni nanocontacts is certainly not large in good agreement with recent experiments [2]. Here we explore the effect of oxygen atoms in the electronic structure and transport of nickel nanocontacts. Since bulk nickel oxide is an insulating antiferromagnets, nano-oxidized nickel is an interesting system on its own. Here we present ab-initio quantum transport calculations of Ni nanocontacts in the presence of oxygen adsorbates in the contact region. We show that the presence of a single oxygen atom leads to strongly spin-polarized transport for parallel alignment of electrodes magnetizations while for antiparallel alignment the conduction is strongly suppressed resulting in large MR. \\ (1) H. D. Chopra, Nature Materials 4, 832 (2005) \\ (2) K. I. Bolotin et al., cond-mat/0510410; W. F. Egelhoff et al., J. Appl. Phys. 95, 7554 (2004) \\ (3) D. Jacob et al., Phys. Rev. B 71, 220403(R) (2005)   

Bias dependent oscillations in spin polarized tunneling

We investigated the bias dependence of spin polarized tunnelling in (pinned)CoFeB/ MgO/(free)CoFeB and (pinned)CoFeB/MgO/(free)NiFe tunnel junctions as a function of temperature and applied field angle. The differential magnetoresistance (MR) exhibits oscillations about zero MR when the free layer of the asymmetric devices is above +0.7 V; no oscillations were observed for negative bias. Oscillations were not observed for any bias in the symmetric devices. The zero-crossing voltages were independent of temperature and relative magnetization angle between the two ferromagnetic layers. A model using spin-split free electron energy bands in the ferromagnets and a trapezoidal tunnel barrier demonstrates qualitative agreement with the experimental data.   

Negative spin polarization in Co$\vert $SrTiO$_{3}$$\vert $Co magnetic tunnel junctions$^{1}$

We perform an \textit{ab-initio }study of spin-polarized tunneling in epitaxial Co\textit{$\vert $}SrTiO$_{3}$\textit{$\vert $}Co magnetic tunnel junctions with bcc Co(001) electrodes. We predict a large tunneling magnetoresistance in these junctions, originating from a mismatch in the majority- and minority-spin bands both in bulk bcc Co and at the Co\textit{$\vert $}SrTiO$_{3}$ interface. The intricate complex band structure of SrTiO$_{3}$ enables effcient tunneling of the minority $d$-electrons which causes the spin polarization of the Co\textit{$\vert $}SrTiO$_{3}$ interface to be negative in agreement with experimental data$^{2}$. Our results indicate that epitaxial Co\textit{$\vert $}SrTiO$_{3}$\textit{$\vert $}Co MTJs with bcc Co(001) electrodes is a viable alternative for device applications. [1] J. Velev et al., Phys. Rev. Lett. \textbf{95}, 216601(2005). [2] J. M. De Teresa \textit{et al.}, Phys. Rev. Lett. \textbf{82}, 4288 (1999); J. M. De Teresa, \textit{et al.}, Science \textbf{286}, 507 (1999).   

Probing spin-dependent tunneling and transmission below the Fermi level with a p-type magnetic tunnel transistor.

A magnetic tunnel transistor (MTT) can be used to probe spin-polarized tunneling involving states away from the Fermi level, as well as spin-dependent transmission of non-equilibrium carriers in a ferromagnet. Here we have used a p-type MTT, in which the magnetocurrent (MC) is determined by the tunnel spin polarization of the states below the Fermi level, and the spin-dependent scattering of hot holes in the magnetic base. For p-type MTT's with the structure p-Si/Au/Co/Al$_{2}$O$_{3}$/NiFe and large Co base thickness (8nm) and/or large emitter bias, the MC has the usual positive sign. Thus, the transmission of holes in the majority spin band of Co is larger than that of minority spin holes. Surprisingly, for smaller Co thickness and bias near the collection threshold (0.3 eV), the MC reverses sign and becomes negative. This unusual result shows that the Co interfaces preferentially transmit carriers of minority spin. With help of specifically designed MTT's and ab-initio calculations we discuss possible contributions of spin-dependent transmission across Au/Co interfaces in the base, and of a negative tunneling spin-polarization of the Co/Al$_{2}$O$_{3}$ interface for states below the Fermi level.   

Spin-polarized tunneling time for non-centrosymmetric photonic barriers with out-of-plane magnetization.

Tunneling barriers that lack space-inversion and time-reversal symmetries along the tunneling axis are found to display fundamentally different tunneling dynamics properties than normally expected. In a model 1D photonic crystal barrier, we show that the two symmetry constraints lead to indirect photonic band gaps which contain eigenmodes that are complex, with nonzero and frequency dispersive real components of their wavevectors. These nonpropagating modes are circularly polarized, and appear as complex conjugate pairs for opposite decay directions. We show that the Hartman effect does not apply, and that the tunneling phase time becomes dependent on barrier length, with, remarkably, the sign of the group delay changing with spin. The tunneling phase time for finite barriers of varying widths is found to agree with the group-like velocity, $1/v_{g}$ = dRe{\{}$k_{gap}${\}}/d\textit{$\omega $}, of the gap eigenmodes of the photonic crystal. The implications of these results for the case of spin-polarized electronic tunneling in noncentrosymmetric barriers (e.g. GaAs-like semiconductors, or chiral carbon nanotubes) with magnetization along the tunneling axis will be discussed.   

Spin-Polarized Electron Transport through Nanometer-Scale Al Grains

We had investigated the spin-polarized electron tunnelling through ensembles of nanometer scale Al grains embedded between two Co-reservoirs at 4.2K, and observed tunnelling-magnetoresistance (TMR) and the Hanle effect. The Spin-coherence time ($T$2 ), measured from the Hanle effect, is around nanoseconds. Fast dephasing is attributed to electron spin-precession in the local fringing fields. Dephasing does not destroy \textit{TMR}, in contrast to spin-relaxation. \textit{TMR }is strongly asymmetric with bias voltage, which we explain by spin-relaxation.   

Electron-magnon coupling and non-linear tunneling transport in magnetic nanoparticles

We present a theory of single-electron tunneling transport through a ferromagnetic nanoparticle in which particle-hole excitations are coupled to spin collective modes. The model employed to describe the interaction between quasiparticles and collective excitations captures the salient features of a recent microscopic study. Our analysis of nonlinear quantum transport in the regime of weak coupling to the external electrodes is based on a rate-equation formalism for the nonequilibrium occupation probability of the nanoparticle many- body states. For strong electron-boson coupling, we find that the tunneling conductance as a function of bias voltage is characterized by a large and dense set of resonances. Their magnetic field dependence in the large-field regime is linear, with slopes of the same sign. Both features are in agreement with recent tunneling experiments.   

Point Contact Spin Spectroscopy of Ferromagnetic Fe3Si epitaxial Films

We report transport study of ferromagnetic Fe$_{3}$Si epitaxial films using point contact Andreev reflection technique. Fe$_{3}$Si is a ferromagnetic metal with a high Curie temperature of 840K and nearly lattice-matched to GaAs. The Heusler-like structure makes it a good candidate of half metals for spintronics. The observed spectra of Nb and NbTi / Fe$_{3}$Si point contacts using a Nb or NbTi etched tip are often broad, along with the presence of sharp dips at the superconducting Fermi energy possibly attributed to the proximity effect occurring at the interface of the contact. Using a modified Blonder-Tinkham-Klapwijk theory, the data analysis gave a spin polarization ranging from 27{\%} to 47{\%} under various contact conditions. The best-fit gave P= 41{\%}, Z= 0.05, =1.49 meV at 2K. Since the thickness of our Fe$_{3}$Si samples are often small, current distribution effects in the probed layer are specially considered by varying the sample thickness.   

Magneto--Optical Ellipsometry on Ni$_{2}$MnIn and NiMnIn Heusler Alloys

We use generalized magneto-optical ellipsometry [1,2] for measurements of the complete dielectric tensor of Ni$_{2}$MnIn [3] and NiMnIn Heusler alloys in the energy range from 1.6 eV to 5.5 eV and in the temperature range from 50 K to 450 K. Generalized magneto-optical ellipsometry allows the investigation of spin-polarized states and to understand the coupling between spin and charge degrees of freedom. We show differences in the metallic behavior of the semi-Heusler alloy NiMnIn and the full-Heusler alloy Ni$_{2}$MnIn related to the half-metallic ferromagnetism of the latter one. The polycrystalline Ni$_{2}$MnIn and NiMnIn films were co-evaporated from two independent sources of Ni and MnIn on a Si(100) substrate under UHV conditions. The Ni$_{2}$MnIn alloy exhibits the ordered L2$_{1}$ crystalline structure and the NiMnIn alloy has a C1$_{b}$ structure. [1] A. Berger and M. Pufall, Appl. Phys. Lett. \textbf{71}, 965 (1997) [2] R. Rauer, G. Neuber, J. Kunze, J. B\"{a}ckstr\"{o}m, and M. R\"{u}bhausen, Rev. Sci. Instr. \textbf{76}, 023910 (2005){\}} [3] S. von Oehsen, J.M. Scholtyssek, C. Pels, G. Neuber, R. Rauer, M. R\"{u}bhausen, and G. Meier et al., JMMM \textbf{290}, 1371 (2005){\}}   

Electronic and structural properties of TBrPP-Fe-Cl molecular self-assembly

Self-assembled molecular (SAM) layers of TBrPP-Fe-Cl [5, 10, 15, 20 --Tetrakis -(4-bromophenyl)-Porphyrin-iron-chloride] are formed on a Cu(111) surface. The electronic and structural properties of these SAM layers are investigated by using scanning tunneling microscopy (STM) imaging and tunneling spectroscopy at 4.6 K. The STM images reveal three distinct molecular conformations inside the SAM layers. Local tunneling spectroscopy data are acquired on individual molecules with different conformations. These spectroscopic investigations provide the conformation dependent HOMO-LUMO orbital alignments of the molecules. Further more, the conductance tunneling spectroscopy measured in a small voltage range near the surface. Fermi level show a Kondo resonance originated by interactions between the spin of iron atom inside the molecule, and the surface state free electrons of Cu(111). This work is supported by a US-DOE grant, DE-FG02-02ER46012, and a NSF-NIRT grant, DMR-0304314.   

Single molecule Kondo switch

We present manipulation of Kondo resonance originated from the interaction between the spin of a Co atom inside a TBrPP-Co molecule and free electrons from a copper surface by switching the conformation of isolated single molecules with a scanning tunneling microscope (STM) tip. The STM studies of isolated TBrPP-Co molecules [5, 10, 15, 20 --Tetrakis -(4-bromophenyl)-porphyrin-Co] deposited on a Cu(111) are performed at 4.6K in an ultra-high-vacuum environment. The molecules anchor on Cu(111) with two molecular conformations, planar and saddle. In the saddle conformation, the metal atom is lifted away from the surface as compared to the planar. We are able to switch from a saddle conformation of TBrPP-Co to a planar conformation by applying +2.2V voltage pulses from the STM-tip, thereby varying the vertical distance of Co atom from the Cu(111) surface. This conformational switching increases molecule-substrate interaction resulting in an enhanced spin-electron coupling and changes the associated Kondo temperature from 130K to 170K. This work is supported by a US-DOE grant, DE-FG02-02ER46012, and a NSF-NIRT grant, DMR-0304314.