Structural and proximity-induced ferromagnetic properties of topological insulator-magnetic insulator heterostructures
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36 MATERIALS SCIENCE↗
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Exquisite growth control and magnetic properties of yttrium iron garnet thin films
A layer-by-layer epitaxial growth up to 227 atomic layers of ferrimagnetic insulator yttrium iron garnet (YIG) thin films is achieved on (110)-oriented gadolinium gallium garnet substrates using pulsed laser deposition. Atomically smooth terraces are observed on YIG films up to 100 nm in thickness. The root-mean-square roughness is as low as 0.067 nm. The easy-axis lies in the film plane, indicating the dominance of shape anisotropy. For (110)-YIG films, there is well-defined two-fold in-plane anisotropy, with the easiest axis directed along [001]. The Gilbert damping constant is determined to be 1.0 × 10{sup −4} for 100 nm thick films.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Continuous control of spin polarization using a magnetic field
The giant magnetoresistance (GMR) of a point contact between a Co/Cu multilayer and a superconductor tip varies for different bias voltage. Direct measurement of spin polarization by Andreev reflection spectroscopy reveals that the GMR change is due to a change in spin polarization. This work demonstrates that the GMR structure can be utilized as a spin source and that the spin polarization can be continuously controlled by using an external magnetic field.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
In-plane current-driven spin-orbit torque switching in perpendicularly magnetized films with enhanced thermal tolerance
Here, we study spin-orbit-torque (SOT)-driven magnetization switching in perpendicularly magnetized Ta/Mo/Co 40 Fe 40 B 20 (CoFeB)/MgO films. The thermal tolerance of the perpendicular magnetic anisotropy (PMA) is enhanced, and the films sustain the PMA at annealing temperatures of up to 430 C, due to the ultra-thin Mo layer inserted between the Ta and CoFeB layers. More importantly, the Mo insertion layer also allows for the transmission of the spin current generated in the Ta layer due to spin Hall effect, which generates a damping-like SOT and is able to switch the perpendicular magnetization. When the Ta layer is replaced by a Pt layer, i.e., in a Pt/Mo/CoFeB/MgO multilayer, the direction of the SOT-induced damping-like effective field becomes opposite because of the opposite sign of spin Hall angle in Pt, which indicates that the SOT-driven switching is dominated by the spin current generated in the Ta or Pt layer rather than the Mo layer. Quantitative characterization through harmonic measurements reveals that the large SOT effective field is preserved for high annealing temperatures. This work provides a route to applying SOT in devices requiring high temperature processing steps during the back-end-of-line processes.
42 ENGINEERING↗
Platinum/yttrium iron garnet inverted structures for spin current transport
30-80 nm thick yttrium iron garnet (YIG) films are grown by pulsed laser deposition on a 5 nm thick sputtered Pt atop gadolinium gallium garnet substrate (GGG) (110). Upon post-growth rapid thermal annealing, single crystal YIG(110) emerges as if it were epitaxially grown on GGG(110) despite the presence of the intermediate Pt film. The YIG surface shows atomic steps with the root-mean-square roughness of 0.12 nm on flat terraces. Both Pt/YIG and GGG/Pt interfaces are atomically sharp. The resulting YIG(110) films show clear in-plane uniaxial magnetic anisotropy with a well-defined easy axis along 〈001〉 and a peak-to-peak ferromagnetic resonance linewidth of 7.5 Oe at 9.32 GHz, similar to YIG epitaxially grown on GGG. Both spin Hall magnetoresistance and longitudinal spin Seebeck effects in the inverted bilayers indicate excellent Pt/YIG interface quality.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Wireless current sensing by near field induction from a spin transfer torque nano-oscillator
We demonstrate that spin transfer torque nano-oscillators (STNO) can act as wireless sensors for local current. The STNO acts as a transducer that converts weak direct currents into microwave field oscillations that we detect using an inductive coil. We detect direct currents in the range of 300–700 μA and report them wirelessly to a receiving induction coil at distances exceeding 6.5 mm. This current sensor could find application in chemical and biological sensing and industrial inspection.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Ultraviolet electroluminescence from Au-ZnO nanowire Schottky type light-emitting diodes
Here, ultraviolet electroluminescence from Schottky type LED device is demonstrated. The device prototype is based on Schottky junctions formed between Au and the top ends of ZnO nanowire arrays. Rectifying current-voltage characteristics are observed, and three different charge transport mechanisms are discussed in detail. Excitonic electroluminescence at around 380 nm is detected at high forward bias and the linear relationship between intensity and current suggests a LED device performance. The observation of LED signals from the simple Schottky structure provides a potential supplement to the category of ultraviolet LED devices.
36 MATERIALS SCIENCE↗
Prime factorization using magnonic holographic devices
Determining the prime factors of a given number $N$ is a problem that requires super-polynomial time for conventional digital computers. A polynomial-time algorithm was invented by Shor for quantum computers. In this paper, we present experimental data that demonstrate prime factorization using spin-wave interference but without quantum entanglement. Prime factorization includes three major steps. First, a general-type computer calculates the sequence of numbers m k mod($N$), where $N$ is the number to be factorized, m is a randomly chosen positive integer, and k = 1, 2, 3, 4, 5, 6…. Next, the period of the calculated sequence $r$ is determined by exploiting spin-wave interference. Finally, the general-type computer determines the primes based on the obtained $r$. The experiment for period finding was conducted on a six-terminal Y 3 Fe 2 (FeO 4 ) 3 device. We chose number 15 for testing and determined its primes using a sequence of measurements. The obtained experimental data for a micrometer-sized prototype aimed to demonstrate the benefits of using spin-wave devices to solve complex computational problems. Scalability is one of the major strengths inherent in this type of wave-based device, which may provide a route to nanometer-sized logic circuits. We discuss the physical and technological limitations of this approach, which define the maximum size of N and the computational speed. Although this classical approach cannot compete with the quantum algorithm in terms of efficiency, magnonic holographic devices can potentially be used as complementary logic units aimed at speeding up prime factorization for classical computers.
97 MATHEMATICS AND COMPUTING↗
Nd:AlN polycrystalline ceramics: A candidate media for tunable, high energy, near IR lasers
We present processing and characterization of Nd-doped aluminum nitride (Nd:AlN) polycrystalline ceramics. We compare ceramics with significant segregation of Nd to those exhibiting minimal segregation. Spatially resolved photoluminescence maps reveal a strong correlation between homogeneous Nd doping and spatially homogeneous light emission. The spectroscopically resolved light emission lines show excellent agreement with the expected Nd electronic transitions. Notably, the lines are significantly broadened, producing near IR emission (~1077 nm) with a remarkable ~100 nm bandwidth at room temperature. We attribute the broadened lines to a combination of effects: multiple Nd-sites, anisotropy of AlN and phonon broadening. These broadened, overlapping lines in a media with excellent thermal conductivity have potential for Nd-based, tunable lasers with high average power.
36 MATERIALS SCIENCE↗
Zero bias anomaly in Andreev reflection spectroscopy
Zero bias anomalies have been observed in point contact Andreev reflection spectroscopy using conventional superconductors, and they cannot be suppressed by a highly spin-polarized current. By systematically varying the size of the point contact, the anomalies are found to be evolved from conductance dips outside the superconducting gap. Furthermore, these anomalies, which depend on the size of the interface, are not intrinsic to the superconductors but due to a large resistance change when the contact region is driven to normal by an Oersted field.
47 OTHER INSTRUMENTATION↗
Spin-orbit torques in perpendicularly magnetized Ir22Mn78/Co20Fe60B20/MgO multilayer
The current-induced spin-orbit torques (SOTs) in the perpendicularly magnetized Ir 22 Mn 78 / Co 20 Fe 60 B 20 /MgO structures are investigated. The damping- and field-like torques are characterized using a harmonic technique. The spin Hall angle of Ir 22 Mn 78 is determined to be $θ$ SHE $=+0.057 ± 60.002$. The SOT-driven magnetization switching is also demonstrated with the assistance of an external in-plane field. Furthermore, the magneto-optical Kerr effect imaging experiments show that the magnetization switching is realized through domain nucleation and domain wall motion. These results may promise potential practical applications in high-performance SOT devices based on the antiferromagnetic materials.
42 ENGINEERING↗
Enhancement of voltage-controlled magnetic anisotropy through precise control of Mg insertion thickness at CoFeB|MgO interface
We studied the impact of different insertion layers (Ta, Pt, and Mg) at the CoFeB|MgO interface on voltage-controlled magnetic anisotropy (VCMA) effect and other magnetic properties. Inserting a very thin Mg layer of 0.1–0.3 nm yielded a VCMA coefficient of 100 fJ/V-m, more than 3 times higher than the average values of around 30 fJ/V-m reported in Ta|CoFeB|MgO-based structures. Ta and Pt insertion layers also showed a small improvement, yielding VCMA coefficients around 40 fJ/V-m. Electrical, magnetic, and X-ray diffraction results reveal that a Mg insertion layer of around 1.2 nm gives rise to the highest perpendicular magnetic anisotropy, saturation magnetization, as well as the best CoFe and MgO crystallinity. Other Mg insertion thicknesses give rise to either under- or over-oxidation of the CoFe|MgO interface; a strong over-oxidation of the CoFe layer leads to the maximum VCMA effect. These results show that precise control over the Mg insertion thickness and CoFe oxidation level at the CoFeB|MgO interface is crucial for the development of electric-field-controlled perpendicular magnetic tunnel junctions with low write voltage.
36 MATERIALS SCIENCE↗
Tuning the magnetoresistance symmetry of Pt on magnetic insulators with temperature and magnetic doping
We present a comparison study of the temperature dependence of the intriguing magnetoresistance (MR) in Pt/YIG (yttrium iron garnet), Pt/YIG BB (the YIG substrate has been bombarded with Ar + ), and Pt/SiO 2 (with different Fe doping levels). With decreasing temperature, the MRs in Pt/YIG and Pt/YIG BB change symmetry from R z = R x > R y at room temperature to R x > R z > R y at low temperature. A similar behavior in both Pt/YIG and Pt/YIG BB implies that the underlying physics is due to magnetic scattering, instead of the pure spin current across the interface. By changing the Fe doping level in the SiO 2 substrate, we can further systematically modulate the symmetry of MR in Pt/SiO 2 (Fe doped). The doping level dependent symmetry can also qualitatively explain the controversy over the MRs of Pt/YIG and similar structures at low temperature.
36 MATERIALS SCIENCE↗
Effective g factor of low-density two-dimensional holes in a Ge quantum well
Here we report the measurements of the effective g factor of low-density two-dimensional holes in a Ge quantum well. Using the temperature dependence of the Shubnikov-de Haas oscillations, we extract the effective g factor in a magnetic field perpendicular to the sample surface. Very large values of the effective g factor, ranging from ~13 to ~28, are observed in the density range of 1.4×10 10 cm -2 – 1.4×10 11 cm -2 . When the magnetic field is oriented parallel to the sample surface, the effective g factor is obtained from a protrusion in the magneto-resistance data that signify full spin polarization. In the latter orientation, a small effective g factor, ~1.3-1.4, is measured in the density range of 1.5×10 10 cm -2 –2×10 10 cm -2 . Finally, this very strong anisotropy is consistent with theoretical predictions and previous measurements in other 2D hole systems, such as InGaAs and GaSb.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Quantification of intergranular exchange coupling in CoPtCr-based perpendicular recording media via ferromagnetic resonance measurements
In this study, intergranular exchange fields in CoPtCr granular media materials were quantified through ferromagnetic resonance measurements in various magnetic states. The data indicate that the exchange field in CoPtCr granular films with no oxide segregant is comparable to the saturation magnetization of the films. With an introduction of a SiO 2 segregant, however, the exchange field decreases. A 30% volume fraction of the segregant reduces the strength of the intergranular exchange coupling to zero.
36 MATERIALS SCIENCE↗
Influence of MgO Barrier Quality on Spin-Transfer Torque in Magnetic Tunnel Junctions
Here, we studied the bias dependence of spin transfer torque in the MgO-based magnetic tunnel junction using a field-modulated spin torque ferromagnetic resonance measurement technique for three devices with tunneling magnetoresistances (MRs) of 60%, 67%, and 73%, respectively. The devices with a lower MR ratio showed the presence of multiple modes, while the device with higher MR (73%) showed a single resonance mode. We found a lower out-of-plane torkance in our devices compared to the in-plane torkance. The out-of-plane torque is linear with applied bias, while the bias dependence of in-plane torque shows a strong dependence on the MR ratio and hence the barrier quality.
42 ENGINEERING↗
Effects of the magnetic field variation on the spin wave interference in a magnetic cross junction
This article reports results of the investigation of the effect of the external magnetic field variation on the spin wave interference in a magnetic cross junction. The experiments were performed using a micrometer scale Y 3 Fe 5 O 12 cross structure with a set of micro-antennas fabricated on the edges of the cross arms. Two of the antennas were used for the spin wave excitation while a third antenna was used for detecting the inductive voltage produced by the interfering spin waves. It was found that a small variation of the bias magnetic field may result in a significant change of the output inductive voltage. The effect is most prominent under the destructive interference condition. The maximum response exceeds 30 dB per 0.1 Oe at room temperature. It takes a relatively small bias magnetic field variation of about 1 Oe to drive the system from the destructive to the constructive interference conditions. The switching is accompanied by a significant, up to 50 dB, change in the output voltage. The obtained results demonstrate a feasibility of the efficient spin wave interference control by an external magnetic field, which may be utilized for engineering novel type of magnetometers and magnonic logic devices.
36 MATERIALS SCIENCE↗
Brillouin-Mandelstam spectroscopy of standing spin waves in a ferrite waveguide
This article reports results of experimental investigation of the spin wave interference over large distances in the Y 3 Fe 2 (FeO 4 ) 3 waveguide using Brillouin-Mandelstam spectroscopy. Two coherent spin waves are excited by the micro-antennas fabricated at the edges of the waveguide. The amplitudes of the input spin waves are adjusted to provide approximately the same intensity in the central region of the waveguide. The relative phase between the excited spin waves is controlled by the phase shifter. The change of the local intensity distribution in the standing spin wave is monitored using Brillouin-Mandelstam light scattering spectroscopy. Experimental data demonstrate the oscillation of the scattered light intensity depending on the relative phase of the interfering spin waves. The oscillations of the intensity, tunable via the relative phase shift, are observed as far as 7.5 mm away from the spin-wave generating antennas at room temperature. The obtained results are important for developing techniques for remote control of spin currents, with potential applications in spin-based memory and logic devices.
36 MATERIALS SCIENCE↗