Guo-Xing Miao

A spin triplet supercurrent through the half-metallic ferromagnet CrO2

In general, conventional superconductivity should not occur in a ferromagnet, though it has been seen in iron under pressure. Moreover, theory predicts that the current is always carried by pairs of electrons in a spin singlet state, so conventional superconductivity decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. It has been predicted that this rapid spatial decay would not occur if spin triplet superconductivity could be induced in the ferromagnet. Here we report a Josephson supercurrent through the strong ferromagnet CrO2, from which we infer that it is a spin triplet supercurrent. Our experimental set-up is different from those envisaged in the earlier predictions, but we conclude that the underlying physical explanation for our result is a conversion from spin singlet pairs to spin triplets at the interface. The supercurrent can be switched with the direction of the magnetization, analogous to spin valve transistors, and therefore could enable magnetization-controlled Josephson junctions.

Tunneling magnetoresistance observed in La0.67Sr0.33MnO3/organic molecule/Co junctions

Tunneling magnetoresistance has been observed in organic based spintronic devices using the organic semiconductors tetraphenyl porphyrin (TPP) and aluminum tris(8-hyroxyquinoline) (Alq3) as the spacer layer between La0.67Sr0.33MnO3 (LSMO) and Co films. The evidence for tunneling is twofold: (1) nonlinear current and conductance versus voltage curves and (2) an increasing junction resistance with decreasing temperature. In general, the magnetoresistance is found to decrease with increasing bias voltage and increasing temperature in both Alq3 and TPP junctions. These results demonstrate that organic molecules can form tunnel barriers that perform as well as most inorganic barrier materials on LSMO.

Tunneling path toward spintronics

The phenomenon of quantum tunneling, which was discovered almost a century ago, has led to many subsequent discoveries. One such discovery, spin polarized tunneling, was made 40 years ago by Robert Meservey and Paul Tedrow (Tedrow and Meservey 1971 Phys. Rev. Lett. 26 192), and it has resulted in many fundamental observations and opened up an entirely new field of study. Until the mid-1990s, this field developed at a steady, low rate, after which a huge increase in activity suddenly occurred as a result of the unraveling of successful spin tunneling between two ferromagnets. In the past 15 years, several thousands of papers related to spin polarized tunneling and transport have been published, making this topic one of the hottest areas in condensed matter physics from both fundamental science and applications viewpoints. Many review papers and book chapters have been written in the past decade on this subject. This paper is not exhaustive by any means; rather, the emphases are on recent progress, technological developments and informing the reader about the current direction in which this topic is moving.

Inelastic tunneling spectroscopy of magnetic tunnel junctions based on CoFeB∕MgO∕CoFeB with Mg insertion layer

Magnetic tunnel junctions (MTJs) based on textured MgO barriers have thus far shown the highest tunneling magnetoresistance (TMR) at room temperature. In contrast to traditional magnetic tunnel junctions, it appears that the large TMR observed in these systems arises from a type of coherent tunneling in which the symmetry of the Bloch state wave functions plays a critical role. We have fabricated MTJs with artificial asymmetric barriers by depositing a thin layer of Mg of varying thickness (0–10 A) prior to the growth of the MgO barrier into otherwise identical CoFeB∕MgO∕CoFeB MTJs. The inelastic tunnel spectrum shows magnon and phonon excitation peaks similar to traditional Al2O3 barriers, and an additional peak at about 300 meV. The conventional interpretation that this peak corresponds to density of states of the s electrons in the ferromagnetic electrodes, however, does not apply in the MgO system.

Frontiers in spin-polarized tunneling

Thanks to recent advances in materials research, magnetic tunnel junctions that control the flow of polarized electrons are poised to revolutionize information technology.

Epitaxial growth of MgO and Fe∕MgO∕Fe magnetic tunnel junctions on (100)-Si by molecular beam epitaxy

Epitaxial growth of MgO barrier on Si is of technological importance due to the symmetry filtering effect of the MgO barrier in conjunction with bcc-ferromagnets. We study the epitaxial growth of MgO on (100)-Si by molecular beam epitaxy. MgO matches Si with 4:3 cell ratio, which renders Fe to be 45° rotated relative to Si, in sharp contrast to the direct epitaxial growth of Fe on Si. The compressive strains from Si lead to the formation of small angle grain boundaries in MgO below 5nm, and also affect the transport characteristics of Fe∕MgO∕Fe magnetic tunnel junctions formed on top.

In-situ characterization of rapid crystallization of amorphous CoFeB electrodes in CoFeB/MgO/CoFeB junctions during thermal annealing

We report the crystallization study of CoFeB/MgO/CoFeB magnetic tunnel junctions using in-situ, time-resolved synchrotron-based x-ray diffraction and transmission electron microscopy. It was found that the crystallization of amorphous CoFeB electrodes occurs on a time scale of seconds during the postgrowth high temperature annealing. The crystallization can be well fit by the Johnson–Mehl–Avrami model and the effective activation energy of the process was determined to be 150 kJ/mol. The solid-state epitaxy mode of CoFeB was found to involve separate crystallization at different locations followed by subsequent merging of small grains, instead of layer-by-layer growth of CoFeB film along the MgO template.

Exchange splitting and bias-dependent transport in EuO spin filter tunnel barriers

We report on transport measurements in EuO-based tunnel junctions, studying the characteristic features of spin filter tunneling in magnetic insulators. Current-voltage (I-V) curves show a unique voltage dependence that reveals the interplay between direct and spin-selective Fowler-Nordheim (FN) tunneling. Successive onsets of FN tunneling give direct evidence for a spin filtering effect, without relying on the use of external magnetic fields for spin detection. The variation of the effective tunnel barrier height below the Curie temperature was extracted from temperature-dependent I-V characteristics, and systematically correlates with the spontaneous magnetization of EuO. The magnitude of the exchange splitting was deduced and allows the evaluation of spin filter efficiency of EuO spin filter tunnel barriers by fully electrical means.

Giant Hall resistance in Pt-based ferromagnetic alloys

We report on the observation of a dramatically increased extraordinary Hall Effect in Pt-based ferromagnetic alloy thin films with varying composition and thickness that were deposited using magnetron sputtering. Hall slope as high as 76.8 μΩ cm/T has been obtained at 110 K and 22.6 μΩ cm/T at 300 K. Excellent sensitivity, linearity, and a small temperature coefficient have been achieved in a particular composition, Fe35Pt65, for a film thickness of 10 nm. The optimized Fe–Pt thin films compare favorably with the commonly used semiconductor Hall sensors.

Superconducting microstrip resonator for pulsed ESR of thin films.

This article describes a superconducting microstrip resonator operating at 9.5 GHz (X-band) that is specially designed for pulsed ESR on thin films. A novel configuration consisting of an array of half-wave length microstrip transmission lines generates a uniform magnetic field over a 2-D region of 100×1000 μm(2) with field homogeneity better than 5×10(-2). Using the device, we demonstrate strong coupling of the resonator to an electron spin ensemble and pulsed ESR on Si:P.