Session J15: Focus Session: Spins in Metals - Spin Transport in Novel Devices and Structures

An Alternative Approach for Fabricating Ring-shaped Spin-transfer Torque RAM Devices

Several techniques have been investigated to fabricate spin-torque transfer RAM (STT RAM) devices with ring-shaped geometries. In-plane STT RAM devices with ring-shaped geometries are advantageous in that the magnetization is confined to the ring structure, eliminating the demagnetization field. This, in turn, reduces the critical current needed to switch the magnetization. To date, the most popular fabrication technique of ring-shaped STT RAM involves a combination of electron beam lithography and ion beam milling. A significant issue with ion beam milling, however, involves the redeposition of the MTJ material on the ring-shaped surface, which leads to shorting of the devices. In our study, we seek an alternative method for forming the ring-shaped devices which eliminates the shorting problem. As a result, we are using a nanopillar and cap structure template to define the ring-shaped geometry of the devices. The nanopillar and cap structure template is fabricated using a combination of a bilayer resist and electron beam lithography to either create a mold for the deposition of a nanopillar and cap structure via ion beam deposition or a resist-based nanopillar and cap structure. This study will report on the results of this process and its prospects as an alternative to ion beam milling.   

Spin dependent tunneling and possible spin torque effects in magnetic nanoparticle tunnel junctions

It has been predicted that electron tunneling between two magnetic electrodes via a magnetic nanoparticle can exhibit enhanced magnetoresistance at low temperature due to a coherent tunneling mechanism in the Coulomb blockade regime. This has been qualitatively demonstrated for large ensembles of nanoparticles. It has also been predicted that the tunneling current in such a system might exert spin transfer torque with quantum behaviors associated with the discrete energy level of the nanoparticles. Here we report the fabrication and measurement of sub-100 nm nano-pillar tunnel junctions with the layer structure CoFeB/MgO/(CoFeB nanoparticles)/MgO/CoFeB with the dual goal of 1) probing the spin-dependent coherent tunneling associated with the magnetization of a single or very few nanoparticles and 2) exploring the potential for magnetization switching and current-driven dynamics in the nanoparticles induced by spin transfer torque.   

What determines the sign of magnetoresistance in a molecular tunnel junction?

The recent observations of both positive and negative sign of tunneling magnetoresistance (TMR) in the same organic spin-valve structure has mystified researchers working in organic spintronics. In this article, we resolve this puzzle by exploring the role of interfacial metal-molecule coupling on TMR in a single molecular spin-valve junction. A planar organic molecule sandwiched between two nickel electrodes is used to build a prototypical spin-valve junction. A parameter-free, single particle Green's function approach in conjunction with a posteriori density functional theory involving a hybrid orbital dependent functional is used to calculate the spin-polarized current. The effect of external bias is explicitly included to investigate the spin-valve behavior. Our calculations show that only a 3\% change in the interfacial distance at the metal-molecule junction can alter the sign of the TMR from a positive to a negative value.   

Spin-dependent inertial force and spin current in accelerating systems

In the frontier of spintronics, much attention is paid on the control of spin currents. Due to the recent progress of nanoelectromechanics, mechanical manipulation of spins will increase in importance. We discuss theoretically the generation of spin currents in both rotationally and linearly accelerating systems. The explicit form of the spin-dependent inertial force acting on electrons in accelerating systems in the presence of electromagnetic fields is derived from the generally covariant Dirac equation. It is shown that the force is responsible for the generation of spin currents by mechanical rotation and vibration in the first order of the spin-orbit coupling. We also investigate SU(2) x U(1) gauge theory in accelerating systems, which allows us to extend the spintronic theory in inertial frame to non-inertial frame.   

Anomalous Magneto Transport in amorphous TbFeCo thin films

TbFeCo has attracted some interests because of its high perpendicular anisotropy and tunable magnetic properties for nanomagnetic and spintronics application. In this study, we report a strong size dependence of the coercive field in 30 nm-thick Tb$_{30}$Fe$_{63.5}$Co$_{6.5 }$films with MgO capping. Magneto Optical Kerr effect (MOKE) and Vibrating Sample Magnetometer are performed on unpatterned films. The films exhibited strong PMA characteristics with M$_{S} \quad \sim $200 emu/cc, H$_{C}\sim $6000 Oe, and K$_{U} \quad \sim $ 5 x 10$^{6 }$erg/cc. The films were then fabricated into Hall bars with 500 nm, 50 $\mu $m and 500 $\mu $m in width. From anomalous Hall effect (AHE), H$_{C}$ was determined for these patterned films. For Hall bars with the width less than 50 $\mu $m, an increase in the coercivity field ($\sim $ 1.4 Tesla) was observed at room temperature. The temperature dependent of AHE was characterized from 50K to 300K. The thickness and composition dependent will also be studied and discussed.   

Anisotropic Magnetoresistance in single-crystalline Ag/NiO/Fe$_{3}$O$_{4}$/MgO(001) sample

Anisotropic Magnetoresistance (AMR) is a well-known phenomenon in ferromagnetic (FM) materials that the resistivity exhibit different values as the electric current flows parallel and perpendicular to the magnetization direction, respectively. Recognizing that the AMR depends on the spin axis rather than spin direction, we propose that AMR effect should also exist in antiferromagnetic (AFM) materials. In this presentation, we will report the AMR effect in single crystalline Ag/Fe$_{3}$O$_{4}$/NiO/MgO(001) films in which the electrical current is mainly carried by the nonmagnetic Ag film. By changing the FM Fe$_{3}$O$_{4}$ magnetization direction with an external magnetic field, the AFM NiO spin axis direction can be changed through the Fe$_{3}$O$_{4}$/NiO coupling. We observe a non-zero AMR effect in Ag and that the AMR value depends sensitively on the Ag thickness, suggesting that the observed AMR comes from the spin-dependent NiO/Ag interfacial scattering. Moreover, the magnitude of the AMR effect at 1nm thick Ag in Ag/Fe$_{3}$O$_{4}$/NiO/MgO(001) is comparable to the AMR value from single Fe film.   

AIP Prize for Industrial Applications of Physics Lecture: Controlled exchange in recording media and spintronic devices

Controlled exchange in recording media and spintronic devices Eric Fullerton University of California, San Diego New functionality and performance advantages have been achieved in magnetic recording media by the formation of complex heterostructures and have helped delay predicted recording limits that arise from thermal instabilities in the media [1]. Example media structures include antiferromagnetically coupled (AFC) media [2] and exchange-spring media structures [3, 4] which emerged in longitudinal and perpendicular recording products and have helped to extend recording densities [1]. In this presentation I will review the implementation of exchange-coupled media, the surprising physics that emerged and its impact on hard-drive products. I will further discuss how similar approaches can be used in emerging recording technologies such as heat assisted magnetic recording [4] and bit patterned media [5, 6] as well as many spintronic devices [7,8]. \newline \newline [1]. A. Moser, et al., J. Phys. D: Appl. Phys. \bf 35\rm, R157 (2002). \newline [2]. E. E. Fullerton, et al., Appl. Phys. Lett. \bf 77\rm, 3806 (2000). \newline [3]. A. Berger, et al., Appl. Phys. Lett. \bf 93\rm, 122502 (2008). \newline [4]. J.-U. Thiele, S. Maat and E. E. Fullerton, Appl. Phys. Lett. \bf 82\rm, 2859 (2003). \newline [5]. S. Li, et al., Appl. Phys. Lett. \bf 94\rm, 202509 (2009). \newline [6]. M. V. Lubarda et al., IEEE Trans. Magn. \bf 47\rm, 18 (2011). \newline [7]. S. Mangin, et al., Nature Materials \bf 5\rm, 210 (2006). \newline [8]. I. Yulaev et al., Appl. Phys. Lett. \bf 99\rm, 132502 (2011).   

Influence of the exchange bias on the magnetic losses in CoFeB/MgO/CoFeB tunnel junctions

We report the influence of the exchange bias on the low-frequency magnetic losses of the reference layer in CoFeB/MgO/CoFeB tunnel junctions near maximum resistance susceptibility. The phase lag associated with the magnetic losses in the reference layer, $\varepsilon $, is field-dependent during its magnetic reversal, being largest around the antiparallel state and slowly decreasing with higher applied fields. Such behavior would indicate a direct influence of the exchange bias strength. Its strength is determined by the magnitude of the reference layer switching field, H$_{ref}$. This is defined as the field at which the magnetoresistance-sensitivity product exhibits its maximum. Devices with the strongest exchange bias tend to have the thickest seed layers and exhibit elevated values for H$_{ref}$ and $\varepsilon $. However, ones with weakened exchange bias due to prolonged annealing show a reduction in H$_{ref}$ and $\varepsilon $ with increased annealing time. A comparison between top and bottom pinning configurations is also discussed along with its impact on double-barrier magnetic tunnel junctions.   

Magneto-optical properties of Fe thin films in an external electric field

Controlling magnetic properties by an external electric field ($E$-field) is a key challenge in magnetic physics. Previously, from first-principles calculations,\footnote{Nakamura, Shimabukuro, Fujiwara, Akiyama, Ito, Freeman, PRL{\bf 102}, 187201(2009); Nakamura, Akiyama, Ito, Weinert, Freeman, PRB{\bf 81}, 220409R(2010)} we demonstrated the $E$-field-driven magnetocrystalline anisotropy modification in Fe thin films and at the Fe/MgO interface. Here, we extend our investigations to treat the magneto-optical properties of Fe thin films in an $E$-field. Calculations were carried out using the film-FLAPW method\footnote{Wimmer, Krakauer, Weinert, Freeman, PRB{\bf 24}, 864(1981); Weinert, Wimmer, Freeman, PRB{\bf 26}, 4571(1982)},in which an $E$-field is incorporated and the conductivity tensor is obtained by applying the Kubo formula of linear response theory. Results predict that for an Fe monolayer, when the $E$-field is introduced over 1V/{\AA}, the calculated interband conductivity in the low energy region (less than about 2eV from $E_{\rm F}$) are modified compared to that in zero field, due to a magnetization reorientation from out-of-plane to in-plane. The calculated plasma frequency is also found to be reduced by 7\%, which suggests an $E$-field-driven magnetoresistance.   

Spin transport in metal-oxide switching devices

Metal-oxide-based devices in which resistive switching occurs, often referred to as memristors or resistive-RAM (random oxide memory), show promise for use in technologically exciting applications such as high-density non-volatile memories, electronically reconfigurable logic, and neural networks. We report on electron spin transport through electrochemically precipitated copper filaments formed in TaO$_{x}$ memristive devices consisting of Co(60 nm)/TaO$_{x}$ (16nm)/Cu(5 nm)/Py(60 nm) with crossbar-type electrode geometry. These metal-oxide switching devices with ferromagnetic electrodes show memristive behavior having a typical OFF/ON resistance ratio of 10$^{5}$. Magnetoresistance measurements performed by sweeping an external magnetic field display evidence of spin transport in the low-resistance ON-state at 77 K. Spin transport vanishes in the OFF-state. These data are strong evidence that the fundamental switching mechanism in these metal-oxide devices is the creation of Cu filaments in the ON-state that completely span the 16 nm thick TaO$_{x}$ and form a continuous metallic conduction path. In addition to helping elucidate the conduction pathway in these intriguing structures, our findings can advance electronics combining spintronic and electronic functions.