Maxim Tsoi

Generation and detection of phase-coherent current-driven magnons in magnetic multilayers

The magnetic state of a ferromagnet can affect the electrical transport properties of the material; for example, the relative orientation of the magnetic moments in magnetic multilayers underlies the phenomenon of giant magnetoresistance. The inverse effect—in which a large electrical current density can perturb the magnetic state of a multilayer—has been predicted and observed experimentally with point contacts and lithographically patterned samples. Some of these observations were taken as indirect evidence for current-induced excitation of spin waves, or ‘magnons’. Here we probe directly the high-frequency behaviour and partial phase coherence of such current-induced excitations, by externally irradiating a point contact with microwaves. We determine the magnon spectrum and investigate how the magnon frequency and amplitude vary with the exciting current. Our observations support the feasibility of a spin-wave maser or ‘SWASER’ (spin-wave amplification by stimulated emission of radiation).

Magnetic domain wall motion triggered by an electric current

The current-induced propagation of magnetic domain walls is studied in CoFe nanoconstrictions patterned by electron beam lithography. Propagation of the walls was confirmed by magnetic force microscopy imaging. The device geometry allows us to distinguish between various mechanisms of interaction between electric current and domain walls: a mechanism in which spin transfer associated with current traversing a domain wall dominates. As expected for such a mechanism the domain wall propagation occurs in the direction of electron current flow and has a current threshold of the order ∼1011 A/m2.

Antiferromagnetic metal spintronics

In this brief review, we explain the theoretical basis for the notion that spin-transfer torques (STTs) and giant-magnetoresistance effects can, in principle, occur in circuits containing only normal and antiferromagnetic (AFM) materials, and for the notion that antiferromagnets can play a role in STT phenomena in circuits containing both ferromagnetic and AFM elements. We review the experimental literature that provides partial evidence for these AFM spintronic effects but demonstrates that, like exchange-bias effects, they are sensitive to details of interface structure that are not always under experimental control. Finally, we speculate briefly on some strategies that might advance progress.

Changing Exchange Bias in Spin Valves with an Electric Current

An electrical current can transfer spin angular momentum to a ferromagnet. This novel physical phenomenon, called spin transfer, offers unprecedented spatial and temporal control over the magnetic state of a ferromagnet and has tremendous potential in a broad range of technologies, including magnetic memory and recording. Recently, it has been predicted that spin transfer is not limited to ferromagnets, but can also occur in antiferromagnetic materials and even be stronger under some conditions. In this paper we demonstrate transfer of spin angular momentum across an interface between ferromagnetic and antiferromagnetic metals. The spin transfer is mediated by an electrical current of high density (~10^12 A/m^2) and revealed by variation in the exchange bias at the ferromagnet/antiferromagnet interface. We find that, depending on the polarity of the electrical current flowing across the interface, the strength of the exchange bias can either increase or decrease. This finding is explained by the theoretical prediction that a spin polarized current generates a torque on magnetic moments in the antiferromagnet. Current-mediated variation of exchange bias can be used to control the magnetic state of spin-valve devices, e.g., in magnetic memory applications.

Search for point-contact giant magnetoresistance in Co/Cu multilayers

We report the first measurements of the point-contact magnetoresistance (MR) of magnetic multilayers. These measurements were intended to see if the additional local resistance associated with a small contact area between a sharpened tip and a magnetic multilayer shows a giant magnetoresistance (GMR) such as that seen in bulk multilayers. Point-contact measurements on two independent multilayers of Co/Cu revealed MRs dominated simply by that of the square resistance of the multilayer, with little or no contribution from the contact area itself. Further studies are still needed to determine if this result means that a small contact has little or no local GMR, or if the needle point-contact technique used destroyed the multilayer around the contact, thereby suppressing the GMR.

Structural, optical, and electrical properties of strained La-doped SrTiO3 films

The structural, optical, and room-temperature electrical properties of strained La-doped SrTiO3 epitaxial thin films are investigated. Conductive La-doped SrTiO3 thin films with concentration varying from 5 to 25% are grown by molecular beam epitaxy on four different substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, SrTiO3, and DyScO3, which result in lattice mismatch strain ranging from −2.9% to +1.1%. We compare the effect of La concentration and strain on the structural and optical properties, and measure their effect on the electrical resistivity and mobility at room temperature. Room temperature resistivities ranging from ∼10−2 to 10−5 Ω cm are obtained depending on strain and La concentration. The room temperature mobility decreases with increasing strain regardless of the sign of the strain. The observed Drude peak and Burstein-Moss shift from spectroscopic ellipsometry clearly confirm that the La addition creates a high density of free carriers in SrTiO3. First principles calculations were performed to help understand the effect of La-doping on the density of states effective mass as well as the conductivity and DC relaxation time.

Temperature dependence of anisotropic magnetoresistance in antiferromagnetic Sr2IrO4

Temperature-dependent magnetotransport properties of the antiferromagnetic semiconductor Sr2IrO4 are investigated with point-contact devices. The point-contact technique allows to probe very small volumes and, therefore, to look for electronic transport on a microscopic scale. Point-contact measurements with single crystals of Sr2IrO4 were intended to see whether the additional local resistance associated with a small contact area between a sharpened Cu tip and the antiferromagnet shows magnetoresistance (MR) such as that seen in bulk crystals. Point-contact measurements at liquid nitrogen temperature revealed large MRs (up to 28%) for modest magnetic fields (250 mT) applied within an IrO2 (ab) plane with angular dependence showing a crossover from four-fold to two-fold symmetry with an increasing magnetic field. Point contact measurement exhibits distinctive anisotropic magnetoresistance (AMR) in comparison to a bulk experiment, imposing intriguing questions about the mechanism of AMR in this material. T...

Diffraction of spin waves from a submicrometer-size defect in a microwaveguide

We have experimentally studied the diffraction of spin waves propagating in a Permalloy-film microwaveguide from a submicrometer-sized circular defect. For microwave excitation above the cutoff frequency of the fundamental transverse mode, the defect leads to a frequency dependent reflection of the spin wave. The efficiency of the reflection appears to be dependent on the wavelength of the incident spin wave in a nonmonotonous way. The observed two-dimensional spin wave distribution pattern after the defect can be understood based on the interference of several copropagating modes, suggesting that the defect couples the fundamental mode with higher order modes.

Minimal field requirement in precessional magnetization switching

We investigate the minimal field strength in precessional magnetization switching using the Landau-Lifshitz-Gilbert equation in undercritically damped systems. It is shown that precessional switching occurs when localized trajectories in phase space become unlocalized upon application of field pulses. By studying the evolution of the phase space, we obtain the analytical expression of the critical switching field in the limit of small damping for a magnetic object with biaxial anisotropy in both the easy and hard plane. We also calculate the switching times for the zero damping situation by numerical means. We show that applying the field along the medium axis is good for both small field and fast switching times.

Micromagnetic study of spin-transfer-torque switching of a ferromagnetic cross towards multi-state spin-transfer-torque based random access memory

We study spin-transfer-torque (STT) switching of a cross-shaped ferromagnet with unequal branches as the free layer in a magnetic tunnel junction using micromagnetic simulations. The free layer in the magnetic tunnel junction is thus designed to have four stable energy states using shape anisotropy. Switching shows distinct regions with increasing current density. Stability of the states against thermal fluctuations is considered, and the validity of the results for different dimensions and material parameters of the free layer ferromagnet is investigated. The results could be useful for a multi-bit STT-based memory.