Session L29: Focus Session: Spin Transport and Voltage Control

Voltage controlled magnetism in 3d transitional metals

Despite having attracted much attention in multiferroic materials and diluted magnetic semiconductors, the impact of an electric field on the magnetic properties remains largely unknown in 3d transitional ferromagnets (FMs) until recent years. A great deal of effort has been focused on the voltage-controlled magnetic anisotropy (VCMA) effect where the modulation of anisotropy field is understood by the change of electron density among different d orbitals of FMs in the presence of an electric field. Here we demonstrate another approach to alter the magnetism by electrically controlling the oxidation state of the 3d FM at the FM/oxide interface. The thin FM film sandwiched between a heavy metal layer and a gate oxide can be reversibly changed from an optimally-oxidized state with a strong perpendicular magnetic anisotropy to a metallic state with an in-plane magnetic anisotropy, or to a fully-oxidized state with nearly zero magnetization, depending on the polarity and time duration of the applied electric fields. This is a voltage controlled magnetism (VCM) effect, where both the saturation magnetization and anisotropy field of the 3d FM layer can be simultaneously controlled by voltage in a non-volatile fashion. We will also discuss the impact of this VCM effect on magnetic tunnel junctions and spin Hall switching experiments. This work, in collaboration with C. Bi, Y.H. Liu, T. Newhouse-Illige, M. Xu, M. Rosales, J.W. Freeland, O. Mryasov, S. Zhang, and S.G.E. te Velthuis, was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.   

Field-dependent perpendicular magnetic anisotropy and interfacial metal-insulator transition in CoFeB/MgO systems

The CoFeB/MgO systems play a central role in magnetic tunnel junction devices due to the high tunneling magnetoresistance ratio. A strong perpendicular anisotropy (PMA) and voltage-controlled anisotropy are beneficial for spintronics application. We study PMA in thin films of Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO in the thickness range of 0.9-2.5 nm and find that it can be best described by the first two order terms. Surprisingly, we find PMA to be strongly field-dependent [1]. Our results show that the field dependence has significant implications for determining and customizing magnetic anisotropy in spintronic applications. Our data suggest that it can be caused by an inhomogeneous interfacial spin pinning with a possibly ferrimagnetic phase at the CoFeB/MgO interface. We perform magnetometry and transport measurements and find a magnetization peak and resistance transitions at 160K, which are consistent with the presence of an interfacial oxide phase undergoing a Morin-like [2] transition. [1] I. Barsukov et al., 105, 152403 (2014) [2] F. J. Morin, Phys. Rev. Lett. 3, 34 (1959)   

Electric Polarization Controllable Magnetoresistance of Magnetic Ferroelectric Tunnel Junctions

The tunneling of electrons through ferroelectric material sandwiched by ferromagnetic electrodes, dubbed magnetic ferroelectric tunnel junctions (MFTJs), can be affected by not only the magnetic alignments between the two ferromagnetic electrodes, but also the electric polarization of the ferroelectric layer, which is right for multi-functional device applications. With additional degree of freedom to control carrier propagation through the multi layers in MFTJs, the effects of electric polarization on tunneling magnetoresistance (TMR) need to be clarified. In this work, we investigate the TMR response during the switching process of electric polarization of the ferroelectric layer. Using a parallel connection mode for polarized up and polarized down domains of the PZT layer, the percentage of switched domain and its corresponding TMR are determined. The calculation results agree well with the experiments data.   

Voltage controlled magnetocrystalline anisotropy at the Fe/MgO (001) interface

The effect of electric fields on magnetocrystalline anisotropy energy (MAE) is a promising way to control the magnetization orientation purely by voltage (rather than by current required for a spin transfer torque), which can potentially alleviate the energy dissipation bottleneck of the existing magnetic memory technology. Here we perform density-functional calculations to explore the voltage controlled magnetic anisotropy (VCMA) of a thin film Fe stacked along the [001] direction when an external electric field is applied across an adjacent epitaxial MgO layer. The results are analyzed by evaluating layer and orbital resolved contributions to MAE. We find that MAE is confined to a depth of few atomic layers near the interface, as determined by the metal screening length, indicating that the VCMA is an interface effect. The applied electric field leads to a nearly linear change in the interface MAE due to a change in the 3d-orbital occupancy of the interfacial Fe atoms and is enhanced, as compared to the clean Fe (001) surface, due a relatively large dielectric constant of MgO. In addition to the electric field screening, there is a notable effect of atomic displacements driven by an applied electric field, when atomic relaxations are taken into account.   

First-principles study of electric field and structural strain impact on perpendicular magnetic anisotropy of Fe/MgO interfaces

Electric-field (EF) control of magnetic anisotropy is promising in the context of establishing low-energy consumption memory devices [1] since it allows EF-assisted switching of magnetization in magnetic tunnel junctions with perpendicular magnetic anisotropy (PMA) [2]. Using first-principles calculations, we demonstrate that both the EF and structural strain induce changes of the PMA in Fe/MgO interfaces which originally exhibit strong PMA [3]. Namely, we find that the PMA change in response to strain is much larger than that induced by applied EF. This suggests that the EF control of PMA is caused not only by charge accumulation and depletion mechanism but rather mediated by structural modifications occurring at the interface in agreement with recent experimental reports [4,5]. In addition, using atomic and orbital-resolved analysis of PMA, we elucidate the effect of both the EF and structural strain on PMA showing in particular that it extends beyond the interfacial layer.\\[4pt] [1] Y. Shiota et al., Nat. Mater. 11, 39 (2011).\\[0pt] [2] W.-G. Wang et al., Nat. Mater. 11, 64 (2012).\\[0pt] [3] H. X. Yang et al., Phys. Rev. B 84, 054401 (2011).\\[0pt] [4] V. B. Naik et al., Appl. Phys. Lett. 105, 052403 (2014).\\[0pt] [5] F. Bonell et al., Appl. Phys. Lett. 102, 152401 (2013).   

Electric-field manipulation of magnetization rotation and tunneling magnetoresistance of magnetic tunnel junctions at room temperature

Recent studies on the electric-field control of tunneling magnetoresistance (TMR) have attracted considerable attention for low power consumption. So far two methods have been demonstrated for electric-field control of TMR. One method uses ferroelectric or multiferroic barriers, which is limited by low temperature. The other is nanoscale thin film magnetic tunnel junction (MTJ), but the assistance of a magnetic field is required. Therefore, electric-field control of TMR at room temperature without a magnetic field is highly desired. One promising way is to employ strain-mediated coupling in ferromagnetic/piezoelectric structure. Though MTJs/piezoelectric has been predicted by theory, experiment work is still lacking. We deposited CoFeB/AlOx/CoFeB on Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) ferroelectric single crystal. Under external electric fields, PMN-PT will produce a piezostrain due to piezoelectric effect, and the piezostrain transfers to ferromagnetic film to change the magnetic anisotropy. We demonstrate a reversible, continuous magnetization rotation and manipulation of TMR at room temperature by electric fields without the assistance of a magnetic field.   

Voltage-controlled magnetic tunnel junctions with Gd$_{2}$O$_{3}$ barriers

It is of great importance to investigate magnetic tunnel junction (MTJ) with high-k barriers, with the premise that a large voltage-controlled magnetic anisotropy (VCMA) can be achieved due to the increased charge transfer effect. Gd$_{2}$O$_{3}$ has a dielectric constant of 22, which is substantially larger than that of MgO ($\sim$ 9). It is critical to achieve crystalline barrier with cubic phase in order to obtain symmetry-conserved tunneling as in MgO-based MTJs. We have demonstrated that Cubic Gd2O3 can be grown on amorphous CoFeB by reactive sputtering under proper conditions. In exchanged-biased MTJs with in-plane anisotropy, tunneling magnetoresistance (TMR) up to 12{\%} has been obtained. The sharp switching at near zero field and exchange-bias field higher than 800 Oe indicate the magnetic properties of the CoFeB in these junctions are nearly as good as in MgO-based MTJs. MTJs with interfacial perpendicular magnetic anisotropy (PMA) has been created with TMR $\sim$ 10{\%}. A very interesting VCMA effect in these Gd$_{2}$O$_{3}$-based MTJs has been observed and will be discussed. This work was supported in part by NSF (ECCS-1310338) and by C-SPIN, one of six centers of STARnet, a Semiconductor Research Corporation program, sponsored by MARCO and DARPA.   

A transition in the magneto-transport in the L1$_{0}$ MnAl thin films

In this talk we will report on L1$_{0}$ MnAl thin films with perpendicular magnetic anisotropy prepared on single crystal MgO substrates by co-sputtering Mn and Al targets. A Cr seeding layer enabled the epitaxial growth of the MnAl films. The magneto-resistance (MR) of these films was measured using a Hall bar structure. When the external magnetic field was applied perpendicular to the thin film surface, a change of the sign of MR was observed as will be discussed below. Above 175K, a negative magnetoresistance was observed with two maxima occuring at the coercivity fields of the MnAl thin films. Below 175K, the MR became positive, and the MR ratio increased with decreasing temperature. The possible mechanisms for the transition in the MR will be discussed in detail in this talk. They include the effects of inhomogeneity, chemical ordering and the underlying domain structure.   

Near Room Temperature Kondo-Suppression of Spin Accumulation in Cu-Based Non-Local Spin Valves

Recent studies on metallic non-local spin valves have focused on the anomalous temperature dependence of the spin accumulation signal, $\Delta R_{NL}$, which unexpectedly decreases at low temperatures. O'Brien \textit{et al.} (Nat. Commun. \textbf{5}, 3927, 2014) advanced an explanation, based on interdiffusion-induced local moments suppressing injected spin polarization via a manifestation of the Kondo effect. Here we extend this work to devices based on Co/Cu, a combination for which the Kondo temperature can exceed 300 K. Non-magnetic channel thicknesses, $t_{N}$, from 50 to 200 nm have been explored, along with annealing temperatures up to 500 \textordmasculine C. The decrease in spin diffusion length in Cu from 300 nm for $t_{N}=$ 200 nm to 90 nm for $ t_{N}=$ 50 nm, and its change with annealing, will be discussed in detail. Most importantly we find that, despite the limited miscibility of Co in Cu, a significant decrease in $\Delta R_{NL}$ occurs with decreasing temperature as the Cu channel thickness is reduced. In the thinnest channels we find the maximum in $\Delta R_{NL}$ occurs near room temperature. This result implies that local moment formation and the associated Kondo physics can impact the performance of spin transport devices at ambient temperature in a very common and technologically important materials system.   

Asymmetric spin absorption into a nonlocal spin detector

Nonlocal spin detection measures a spin-dependent chemical potential difference associated with a spin accumulation in a nonmagnetic channel. Typically a ferromagnetic spin detector is in contact with the nonmagnetic channel. A spin-dependent voltage is developed between the detector and the channel when a fraction of the spin current in the channel is absorbed into the detector. We explore an unconventional approach for nonlocal spin detection by probing the voltage between the two ends of the ferromagnetic spin detector. The nonlocal spin valves with 150 nm wide Cu channels are fabricated by e-beam lithography. The ferromagnetic Py spin injectors are 250 nm wide and Py spin detectors are 120 nm wide. Low-resistance AlOx barriers are placed between the Py and the Cu. Since the spin absorption across the Cu/AlOx/Py detector interface is not spatially uniform, a net emf is formed near the junction and a net voltage develops between the two ends of the spin detector. This spin-dependent voltage is clearly detectable at room temperature and suggests an unconventional method of detecting nonlocal spin accumulation.   

Transport properties of $R$PtBi ($R = $ Gd, Dy, Tm, and Lu) under applied magnetic fields

It has been suggested that the combination of strong spin-orbit coupling and noncentrosymmetric crystal structure make ternary Heusler compounds a strong candidate for 3D topological materials. The crystal structure of rare-earth platinum bismuth ($R$PtBi) half-Heusler compounds lacks an inversion symmetry, hence the material is a noncentrosymmeteric. The earlier electrical resistivity data of $R$PtBi revealed a systematic change from a small gap semiconductor for lighter rare-earth to metallic for heavier rare-earth compounds. The angle resolved photoemission spectroscopy showed a clear spin-orbit splitting of the surface bands that cross the Fermi surface. Here we present very large magnetic field dependences of transport properties in single crystals of $R$PtBi ($R = $ Gd, Dy, Tm, and Lu). Successfully grown the high quality $R$PtBi single crystals reveal that a large non-saturating magnetoresistance (MR) of as high as 800 {\%} at 2 K and over 300 {\%} at 300 K under a moderate magnetic field of 14 T. In addition to the large MR, the samples exhibit pronounced temperature and magnetic field dependences of Hall coefficient and thermoelectric power. Obtained transport data suggest that the high hole and electron mobility dominate the magnetotransport.   

Effect of thickness and strain on the metamagnetic transition temperature of ultra-thin epitaxial FeRh films

The antiferromagnetic to ferromagnetic transition in ultra-thin epitaxial FeRh films was studied as a function of film thickness and substrate-induced strain. The lattice mismatch from MgO, STO and KTO was used to provide different strain states on FeRh films with thicknesses spanning 5 to 22 nm. The interplay of these parameters was studied using magnetometry, diffractometry, atomic force microscopy and energy dispersive spectroscopy. Our results provide insight into the growth mechanisms of FeRh and how the onset of the magnetic transition can be controlled via strain engineering.   

Temperature-dependent spin scattering in Pt and at its interfaces

Pt is an important material for spintronic devices, as it exhibits a significant spin Hall effect, enabling its applications as an efficient source of spin currents. Among key parameters describing spin-transport properties are the spin diffusion length (SDL) and the interfacial spin-scattering $\delta $. The reported values of SDL in Pt range from 0.5 to 15nm, likely due to the differences in the measurement approaches and material purity. Little is presently known about $\delta $. We utilized current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) and magnetic nanopillar structures to determine the dependence of $\delta $ and SDL in Pt on temperature T. Both $\delta $ and SDL increase by almost a factor of two between 300K and 7K, implying that the bulk spin scattering decreases while the interfacial spin scattering increases with decreasing T. These opposite trends result in a nonmonotonic dependence of GMR on T for thin Pt layers. We discuss the possible mechanisms for the unexpected dependence of $\delta $ on T. We also show that the SDL is within a factor of 2 of the mean free path, implying that almost every scattering event is spin flipping. This result provides a simple approach to estimate SDL in Pt and other materials with strong spin-orbit interaction.