Erratum: “Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces” [Rev. Sci. Instrum. 85, 073901 (2014)]
No abstract prepared.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
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Long-range triplet supercurrents induced by singlet supercurrents parallel to magnetic interfaces
Employing a spin-parameterized Keldysh-Usadel technique for the diffusive regime, we demonstrate that even in the low proximity limit, considerable long-ranged triplet supercurrents can be effectively generated by spin-singlet supercurrents flowing parallel to the interfaces of uniform double ferromagnet interlayers with noncollinear exchange fields independent of actual junction geometry. The triplet supercurrents are found to be most pronounced when the thicknesses of the ferromagnet strips are unequal. To experimentally verify this generic phenomenon, we propose an accessible and controllable structure that can fully isolate the long-range triplet effects.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Nanoparticles of spinel and perovskite ferromagnets and prospects for their application in medicine
In this work, nanoparticles of La{sub 0.75}Sr{sub 0.25}MnO{sub 3} compounds with perovskite structure and AFe{sub 2}O{sub 4} (A = Mn, Fe, Co, Ni, Zn) with spinel structure have been synthesized by precipitation from diethylene glycol and microemulsion using Triton X-100 surfactant. Comparative X-ray diffraction and magnetic studies of the synthesized nanoparticles have been carried out. Magnetic fluids prepared from synthesized nanopowders have been characterized by calorimetric measurements of specific loss power (SLP)
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Ultrafast magnetization switching by spin-orbit torques
Spin-orbit torques induced by spin Hall and interfacial effects in heavy metal/ferromagnetic bilayers allow for a switching geometry based on in-plane current injection. Using this geometry, we demonstrate deterministic magnetization reversal by current pulses ranging from 180 ps to ms in Pt/Co/AlO{sub x} dots with lateral dimensions of 90 nm. We characterize the switching probability and critical current I{sub c} as a function of pulse length, amplitude, and external field. Our data evidence two distinct regimes: a short-time intrinsic regime, where I{sub c} scales linearly with the inverse of the pulse length, and a long-time thermally assisted regime, where I{sub c} varies weakly. Both regimes are consistent with magnetization reversal proceeding by nucleation and fast propagation of domains. We find that I{sub c} is a factor 3–4 smaller compared to a single domain model and that the incubation time is negligibly small, which is a hallmark feature of spin-orbit torques.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Peculiar long-range supercurrent in superconductor-ferromagnet-superconductor junction containing a noncollinear magnetic domain in the ferromagnetic region
We study the supercurrent in clean superconductor-ferromagnet-superconductor heterostructure containing a noncollinear magnetic domain in the ferromagnetic region. It is demonstrated that the magnetic domain can lead to a spin-flip scattering process, which reverses the spin orientations of the singlet Cooper pair and simultaneously changes the sign of the corresponding electronic momentum. If the ferromagnetic layers on both sides of magnetic domain have the same features, the long-range proximity effect will take place. That is because the singlet Cooper pair will create an exact phase-cancellation effect and gets an additional π phase shift as it passes through the entire ferromagnetic region. Then, the equal spin triplet pair only exists in the magnetic domain region and can not diffuse into the other two ferromagnetic layers. So, the supercurrent mostly arises from the singlet Cooper pairs, and the equal spin triplet pairs are not involved. This result can provide a approach for generating the long-range supercurrent.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Effect of the oxide layer on current-induced spin-orbit torques in Hf|CoFeB|MgO and Hf|CoFeB|TaO{sub x} structures
We study the effect of the oxide layer on the current-induced spin-orbit torques (SOTs) in perpendicularly magnetized Hf|CoFeB|MgO (MgO-capped) or Hf|CoFeB|TaO{sub x} (TaO{sub x}-capped) structures. The effective fields corresponding to both the field-like and damping-like current-induced SOTs are characterized using electric transport measurements. Both torques are found to be significantly stronger in MgO-capped structures than those in TaO{sub x}-capped structures. The difference in field-like and damping-like SOTs in the different structures may be attributed to the different Rashba-like Hamiltonian, arising from the difference in the electric potential profiles across the oxide|ferromagnet interfaces in the two cases, as well as possible structural and oxidation differences in the underlying CoFeB and Hf layers. Our results show that the oxide layer in heavy-metal|ferromagnet|oxide trilayer structures has a very significant effect on the generated SOTs for manipulation of ferromagnetic layers. These findings could potentially be used to engineer SOT devices with enhanced current-induced switching efficiency.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
A comprehensive study of the magnetic, structural, and transport properties of the III-V ferromagnetic semiconductor InMnP
The manganese induced magnetic, electrical, and structural modification in InMnP epilayers, prepared by Mn ion implantation and pulsed laser annealing, are investigated in the following work. All samples exhibit clear hysteresis loops and strong spin polarization at the Fermi level. The degree of magnetization, the Curie temperature, and the spin polarization depend on the Mn concentration. The bright-field transmission electron micrographs show that InP samples become almost amorphous after Mn implantation but recrystallize after pulsed laser annealing. We did not observe an insulator-metal transition in InMnP up to a Mn concentration of 5 at. %. Instead all InMnP samples show insulating characteristics up to the lowest measured temperature. Magnetoresistance results obtained at low temperatures support the hopping conduction mechanism in InMnP. We find that the Mn impurity band remains detached from the valence band in InMnP up to 5 at. % Mn doping. Our findings indicate that the local environment of Mn ions in InP is similar to GaMnAs, GaMnP, and InMnAs; however, the electrical properties of these Mn implanted III-V compounds are different. This is one of the consequences of the different Mn binding energy in these compounds.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Tunnel magnetoresistance and linear conductance of double quantum dots strongly coupled to ferromagnetic leads
We analyze the spin-dependent linear-response transport properties of double quantum dots strongly coupled to external ferromagnetic leads. By using the numerical renormalization group method, we determine the dependence of the linear conductance and tunnel magnetoresistance on the degree of spin polarization of the leads and the position of the double dot levels. We focus on the transport regime where the system exhibits the SU(4) Kondo effect. It is shown that the presence of ferromagnets generally leads the suppression of the linear conductance due to the presence of an exchange field. Moreover, the exchange field gives rise to a transition from the SU(4) to the orbital SU(2) Kondo effect. We also analyze the dependence of the tunnel magnetoresistance on the double dot levels' positions and show that it exhibits a very nontrivial behavior.
77 NANOSCIENCE AND NANOTECHNOLOGY↗
Laser-induced ultrafast demagnetization time and spin moment in ferromagnets: First-principles calculation
When a laser pulse excites a ferromagnet, its spin undergoes a dramatic change. The initial demagnetization process is very fast. Experimentally, it is found that the demagnetization time is related to the spin moment in the sample. In this study, we employ the first-principles method to directly simulate such a process. We use the fixed spin moment method to change the spin moment in ferromagnetic nickel, and then we employ the Liouville equation to couple the laser pulse to the system. We find that in general the dependence of demagnetization time on the spin moment is nonlinear: It decreases with the spin moment up to a point, after which an increase with the spin moment is observed, followed by a second decrease. To understand this, we employ an extended Heisenberg model, which includes both the exchange interaction and spin-orbit coupling. The model directly links the demagnetization rate to the spin moment itself and demonstrates analytically that the spin relaxes more slowly with a small spin moment. A future experimental test of our predictions is needed.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Spin-orbit torque induced spike-timing dependent plasticity
Nanoelectronic devices that mimic the functionality of synapses are a crucial requirement for performing cortical simulations of the brain. In this work, we propose a ferromagnet-heavy metal heterostructure that employs spin-orbit torque to implement spike-timing dependent plasticity. The proposed device offers the advantage of decoupled spike transmission and programming current paths, thereby leading to reliable operation during online learning. Possible arrangement of such devices in a crosspoint architecture can pave the way for ultra-dense neural networks. Simulation studies indicate that the device has the potential of achieving pico-Joule level energy consumption (maximum 2 pJ per synaptic event) which is comparable to the energy consumption for synaptic events in biological synapses.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Systematic study of the spin stiffness dependence on phosphorus alloying in the ferromagnetic semiconductor (Ga,Mn)As
We study the dependence of the spin stiffness constant on the phosphorus concentration in the ferromagnetic semiconductor (Ga,Mn)(As,P) with the aim of determining whether alloying with phosphorus is detrimental, neutral, or advantageous to the spin stiffness. Time-resolved magneto-optical experiments are carried out in thin epilayers. Laser pulses excite two perpendicular standing spin wave modes, which are exchange related. We show that the first mode is spatially uniform across the layer corresponding to a k≈0 wavevector. From the two frequencies and k-vector spacings we obtain the spin stiffness constant for different phosphorus concentrations using weak surface pinning conditions. The mode assessment is checked by comparison to the spin stiffness obtained from domain pattern analysis for samples with out-of-plane magnetization. The spin stiffness is found to exhibit little variation with phosphorus concentration in contradiction with ab-initio predictions.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Spreading of a ferrofluid core in three-stream micromixer channels
Spreading of a water based ferrofluid core, cladded by a diamagnetic fluid, in three-stream micromixer channels was studied. This spreading, induced by an external magnetic field, is known as magnetofluidic spreading (MFS). MFS is useful for various novel applications where control of fluid-fluid interface is desired, such as micromixers or micro-chemical reactors. However, fundamental aspects of MFS are still unclear, and a model without correction factors is lacking. Hence, in this work, both experimental and numerical analyses were undertaken to study MFS. We show that MFS increased for higher applied magnetic fields, slower flow speed of both fluids, smaller flow rate of ferrofluid relative to cladding, and higher initial magnetic particle concentration. Spreading, mainly due to connective diffusion, was observed mostly near the channel walls. Our multi-physics model, which combines magnetic and fluidic analyses, showed, for the first time, excellent agreement between theory and experiment. These results can be useful for lab-on-a-chip devices.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Spin-current diode with a ferromagnetic semiconductor
Diode is a key device in electronics: the charge current can flow through the device under a forward bias, while almost no current flows under a reverse bias. Here, we propose a corresponding device in spintronics: the spin-current diode, in which the forward spin current is large but the reversed one is negligible. We show that the lead/ferromagnetic quantum dot/lead system and the lead/ferromagnetic semiconductor/lead junction can work as spin-current diodes. The spin-current diode, a low dissipation device, may have important applications in spintronics, as the conventional charge-current diode does in electronics.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Spin filter and spin valve in ferromagnetic graphene
We propose and demonstrate that a EuO-induced and top-gated graphene ferromagnetic junction can be simultaneously operated as a spin filter and a spin valve. We attribute such a remarkable result to a coexistence of a half-metal band and a common energy gap for opposite spins in ferromagnetic graphene. We show that both the spin filter and the spin valve can be effectively controlled by a back gate voltage, and they survive for practical metal contacts and finite temperature. Specifically, larger single spin currents and on-state currents can be reached with contacts with work functions similar to graphene, and the spin filter can operate at higher temperature than the spin valve.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Separation of spin Seebeck effect and anomalous Nernst effect in Co/Cu/YIG
The spin Seebeck effect (SSE) and Anomalous Nernst effect (ANE) have been observed in Co/Cu/YIG (yttrium iron garnet) multi-layer structure, where the ferromagnetic insulator YIG acts as the pure spin injector and the ferromagnetic metal Co layer acts as the spin current detector. Finally, with the insertion of 5 nm Cu layer, the two ferromagnetic layers are decoupled, thus allowing unambiguous separation of the SSE and ANE contributions under the same experimental conditions in the same sample.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Enhancement of the anti-damping spin torque efficacy of platinum by interface modification
We report a strong enhancement of the efficacy of the spin Hall effect (SHE) of Pt for exerting anti-damping spin torque on an adjacent ferromagnetic layer by the insertion of ≈0.5 nm layer of Hf between a Pt film and a thin, ≤2 nm, Fe{sub 60}Co{sub 20}B{sub 20} ferromagnetic layer. This enhancement is quantified by measurement of the switching current density when the ferromagnetic layer is the free electrode in a magnetic tunnel junction. The results are explained as the suppression of spin pumping through a substantial decrease in the effective spin-mixing conductance of the interface, but without a concomitant reduction of the ferromagnet's absorption of the SHE generated spin current.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Experimental observation of the interaction of propagating spin waves with Néel domain walls in a Landau domain structure
The interaction of propagating dipolar spin waves with magnetic domain walls is investigated in square-shaped microstructures patterned from the Heusler compound Co{sub 2}Mn{sub 0.6}Fe{sub 0.4}Si. Using magnetic force microscopy, the reversible preparation of a Landau state with four magnetic domains separated by Néel domain walls is confirmed. A local spin-wave excitation using a microstructured antenna is realized in one of the domains. It is shown by Brillouin light scattering microscopy that the domain structure in the remanence state has a strong influence on the spin-wave excitation and propagation. The domain walls strongly reflect the spin waves and can be used as spin-wave reflectors. A comparison with micromagnetic simulations shows that the strong reflection is due to the long-range dipolar interaction which has important implications for the use of these spin waves for exerting an all-magnonic spin-transfer torque.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Noise signatures of metastable resistivity states in ferromagnetic insulating manganite
Pronounced noise signatures enabling one to discriminate metastable resistivity states in La{sub 0.86}Ca{sub 0.14}MnO{sub 3} single crystals have been observed. The normalized noise spectra for metastable resisitivity differ both in shape and magnitude, indicating that the metastable state is associated with transition of the electronic system into another local minimum of the potential landscape. Such scenario is consistent with freezing of the electronic system into a Coulomb glass state.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗