Large In-plane Magnetic Field Research Articles

Giant Electric Field Tuning of Magnetism in Novel Multiferroic FeGaB/Lead Zinc Niobate–Lead Titanate (PZN‐PT) Heterostructures

Multiferroic composite materials consisting of both a magnetic phase and a ferroelectric phase are of great current interest, as they offer the possibility ofmagnetoelectric (ME) coupling, that is, electric field manipulation of magnetic properties (converse ME effect) or vice versa (direct ME effect), and have led to many novel multiferroic devices. One important series of such multiferroic devices is constituted by electrostatically tunable microwave multiferroic signal processing devices, including tunable resonators, phase shifters, and tunable filters. Compared to conventional tunable microwave magnetic devices, which are tuned by magnetic fields, these electrostatically tunable microwave multiferroic devices are much more energy efficient, less noisy, compact, and lightweight. ME effects can be realized in multiferroic composites through a strain/stress-mediated interaction, which enables effective energy transfer between electric and magnetic fields and leads to important new functionalities and devices. Strong ME coupling is critical for multiferroic devices; however, it has been difficult to achieve at microwave frequencies, leading to a very limited tunability in electrostatically tunable microwave multiferroic devices. The demonstrated tunable range of most of these devices has been very limited, with a frequency tunability of Df< 150MHz and a low tunable magnetic field of DH< 50Oe (1 Oe 79.6 A m ). This is mainly due to the large loss tangents at microwave frequencies of the two constituent phases, that is, the ferroelectric phase and, particularly, the magnetic phase, which is less resistive. The ME coupling strength in multiferroic composites is determined by many factors, such as the properties of the two constituent phases, the interface between them, the mode of ME coupling, and the orientation of the magnetic and electric fields. As a result, layered multiferroic heterostructures with magnetic thin films provide great opportunities for achieving strong ME coupling at microwave frequencies, owing to minimized charge leakage paths and low loss tangents associated with magnetic thin films. It is also desirable for the magnetic phase in the multiferroic composites to have a narrow ferromagnetic resonance (FMR) linewidth and a large piezomagnetic coefficient (dl/dH), that is, a large saturation magnetostriction constant (ls) and a low saturation magnetic field (Hs). However, suchmagnetic materials have not been readily available. Very recently, we have reported a new class of metallic magnetic FeGaB films that has a high ls of ca. 70 ppm, a lowHs of ca. 20Oe, and a narrow FMR linewidth of ca. 16Oe at X-band (ca. 9.6GHz). The maximum piezomagnetic coefficient of the FeGaB films is about 7 ppm Oe , which is much higher than those of other well-known magnetostrictive materials used in multiferroic composites, such as Terfenol-D (Tb-Dy-Fe), Galfenol (Fe-Ga), and Metglas (FeBSiC), as shown in Figure 1. The combination of narrow FMR linewidth and high piezomagnetic coefficient makes these FeGaB films excellent candidates for the magnetic material in microwave multiferroic composites. Single-crystal ferroelectrics such as lead magnesium niobate–lead titanate (PMN-PT) and lead zinc niobate–lead titanate (PZN-PT) having giant piezoelectric coefficients and low loss tangents are desired for microwave multiferroic composites as well. In particular, (011)-cut PMN-PT and PZN-PT single-crystal slabs have anisotropic piezoelectric coefficients d31 and d32 when poled along their [011] crystalline direction. For example, (011)-cut PZN-PT single crystals with 6% lead titanate have high anisotropic piezoelectric coefficients d311⁄4 3000 pC N 1 and d321⁄4 1100 pC N . The giant anisotropic piezoelectric coefficients of the PZN-PTsingle crystal provide great opportunities for generating a large in-plane magnetic anisotropic field and

Magnetotransport study in the layer imbalanced v = 1 bilayer quantum Hall state

We carried out magnetotransport measurements of the charge imbalanced bilayer quantum Hall state (QHS) at the total Landau level filling factor ν = 1 under the titled magnetic fields. Changing the total electron densities nT and the layer imbalance parameter σ = (nf - nb)/(nf + nb), where nf(nb) indicates the density of the front (back) layer, we investigated magnetoresistance near the phase transition point between the charge transferable bilayer QHS and the 1/3+2/3 compound QHS at large in-plane magnetic fields B||. We measured the thermal activation energy Δ for various nT and θ, and constructed detailed phase diagram in the nT-θ plane at σ = 0.33, including commensurate(C)-like, incommensurate (IC)-like and the compound state. Results indicate that each transition between the compound QHS and the C-like or IC-like phases reflects the pseudospin symmetries.

Understanding the current-induced magnetization dynamics in the presence of large magnetic fields

We have theoretically studied the magnetization behavior in the magnetic trilayer nanostructures under the influence of spin-transfer torque using the Landau–Lifshitz–Gilbert equation. The focus is the coherent transition of the magnetization from a static state to another static state induced by the spin current even in the presence of a large applied field. Based on the study, the experimental observations, which were thought to be due to spin-wave excitations previously, can be directly and quantitatively explained by the out-of-plane static states of the magnetization induced by the spin-current in the presence of large in-plane magnetic fields in the range of several Teslas.

Semiconductor few-electron quantum dot operated as a bipolar spin filter

We study the spin states of a few-electron quantum dot defined in a two-dimensional electron gas, by applying a large in-plane magnetic field. We observe the Zeeman splitting of the two-electron spin triplet states. Also, the one-electron Zeeman splitting is clearly resolved at both the zero-to-one and the one-to-two electron transition. Since the spin of the electrons transmitted through the dot is opposite at these two transitions, this device can be employed as an electrically tunable, bipolar spin filter. Calculations and measurements show that higher-order tunnel processes and spin-orbit interaction have a negligible effect on the polarization.

Drastic changes of the domain size in an ultrathin magnetic film

A general framework for the domain size in any ultrathin film with perpendicular magnetic anisotropy is here discussed. The domain structure is analyzed by using the classical theory taking into consideration the demagnetization field contribution to the domain wall energy. A sinusoidal model is considered to describe the domain structure while approaching, in two different cases, the monodomain state with in-plane magnetization. The first case is realized applying a large enough in-plane magnetic field. The second one is obtained by decreasing the perpendicular magnetic anisotropy, which is connected in many ultrathin systems with the increase of film thickness. A change in the domain size of several orders of magnitude is obtained while approaching the magnetization reorientation region. The minimal stripe domain period p=8πlex2/d is calculated from the sinusoidal model, where lex is the exchange length and d is the thickness of the film. The range of possible domain size changes in ultrathin films is predicted. The domain size has been experimentally studied in a 1 nm Co film characterized by a square hysteresis loop. The investigations have been performed by polar Kerr based microscopy and magnetic force microscopy. The domain structure of two remnant states generated by applying an in-plane and a perpendicular magnetic field has been compared. Drastically, the smallest domain size has been observed for the former.

Detecting spin-polarized currents in ballistic nanostructures.

We demonstrate a mesoscopic spin polarizer/analyzer system that allows the spin polarization of current from a quantum point contact in a large in-plane magnetic field to be measured. A transverse electron focusing geometry is used to couple current from an emitter point contact into a collector point contact. At large in-plane fields, with the point contacts biased to transmit only a single spin (g<e(2)/h), the voltage across the collector depends on the spin polarization of the current incident on it. Spin polarizations of >70% are found for both emitter and collector at 300 mK and 7 T in-plane field.

Onset of anisotropic transport of two-dimensional electrons in high Landau levels: Possible isotropic-to-nematic liquid-crystal phase transition

The recently discovered anisotropy of the longitudinal resistance of two-dimensional electrons near half filling of high Landau levels is found to persist to much higher temperatures T when a large in-plane magnetic field B|| is applied. Under these conditions we find that the longitudinal resistivity scales quasi-linearly with B||/T. These observations support the notion that the onset of anisotropy at B||=0 does not reflect the spontaneous development of charge density modulations but may instead signal an isotropic-to-nematic liquid crystal phase transition.

Evidence for spin–orbit effects in an open ballistic quantum dot

We report on magnetoconductance measurements of an open two-dimensional ballistic cavity in a tilted magnetic field. In agreement with Folk et al. (Phys. Rev. Lett. 86 (2001) 2102), we show that, due to an increasing spin–orbit interaction, the variance of conductance fluctuations decreases when a large in-plane magnetic field is applied. When the magnetic field is applied parallel to the 2DEG, we observe a negative magnetoconductance peak around zero total field, which is reminiscent of the weak antilocalization effect induced by spin–orbit interaction.

Effect of large in-plane magnetic field on the negative Hall states of (TMTSF) 2ClO 4

Orientation dependence of the negative Hall phase in (TMTSF) 2 ClO 4 was studied in presence of a large in-plane magnetic fields. Rotations within the b - c crystallographic plane reveal a highly anisotropic behavior in the magnitude of the negative Hall state. A discernible oscillation in the magnitude of the negative Hall states is observed as a function of angle. We identify the Lebed resonance associated with the commensurate electronic motion in the negative Hall state.

Magnetization reversal processes in ultrathin cobalt films starting from demagnetized states

Remagnetization processes were investigated in ultrathin cobalt films with perpendicular anisotropy, after interrupting the magnetization reversal of a saturated sample and applying a large in-plane magnetic field. This study was performed by magnetooptical magnetometry and microscopy. Drastic differences were observed in magnetic properties depending upon the magnetic sample history.