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When taking into account the electronic correlations such as the onsite Coulomb repulsion and coupling between electrons, spins and orbitals, many fascinating novel quantum states beyond the free-electron framework can emerge, e.g., unconventional superconductivity and quantum spin liquids. The understanding of these new states not only will expand the existing territory of our knowledge, but also likely lead to revolution in quantum science and technology. Therefore, studying the strongly correlated physics is a cutting-edge theme in condensed matter physics. The parent state of cuprate high-temperature superconductors is a Mott insulator, an insulating state due to the strong electronic correlation, whereas the band theory predicts it to be metallic. Due to the Coulomb gap in Mott insulators, the charge degree of freedom is often frozen, which makes electrical transport measurements inapplicable. As a probe sensitive to the elementary excitations of quasiparticles not limited to electrons, but also including magnons, spinons, as well as phonons, thermal transport measurements play an important role in the study of strongly correlated electronic systems. In this paper, we review some of the recent interesting results on unconventional superconductors, heavy fermions and quantum spin liquids utilizing the longitudinal thermal transport measurements, complimentary to our recent review article on the progress of the transverse thermal conductivity measurements on the thermal Hall effect. [ABSTRACT FROM AUTHOR]

Thermal Hall effect (THE) is to describe the phenomenon where heat carriers are deflected by an external magnetic field applied perpendicular to the heat flow, and thus the carriers gain transverse velocity, leading to a finite temperature gradient on the two sides orthogonal to the heat flow and field. THE is predicted to occur in systems with nontrivial Berry curvatures and thus can reveal topological properties, similar to the electrical Hall effect. However, THE is not limited to charge excitations as in the electrical Hall effect, but rather, to all kinds of excitations that are able to conduct heat, making it possible to explore the exotic properties in strongly correlated electronic systems, which are typically insulators. Therefore, THE is more universal than the electrical form and has become a powerful probe in detecting charge-neutral excitations, such as phonons and magnons. Moreover, there are some sources such as chiral phonons, which are beyond a simple nontrivial-Berry-curvature scenario, that can also give rise to THE; examining THE wherein will shed light on the complex microscopic mechanism hidden in materials. Despite these, heat signals are much weaker than electrical ones. Especially for measurements of the thermal Hall conductivity, it is often needed to collect weak signals on top of a large background. This makes measuring the THE challenging一but thanks to the sustained efforts of the community, this field is developing rapidly in recent years, with many interesting results on the measurements of the thermal Hall conductivity. In this review article, we try to summarize some of these exciting accomplishments, point out remaining outstanding issues, and suggest possible future directions. [ABSTRACT FROM AUTHOR]

We report the dc and ac magnetic field dependence of the low-frequency noise in a microelectromechanical system (MEMS) flux concentrator device containing a giant magnetoresistance spin valve (SV). The noise is dominated by resistance fluctuations having a magnetic origin. Under nominally zero magnetic-field biasing conditions, the noise power is large and varies rapidly with small changes in magnetic field. Metastability between distinct resistive states is observed and can be suppressed with the application of a moderate longitudinal field. Stationary flux concentrators do not contribute excess noise, rather the dominant source of noise is the SVs themselves. This result indicates that the device is likely to increase the sensitivity of many magnetic sensors at low frequencies by orders of magnitude. Index Terms--Field sensor, flux concentrator, magnetometer, magnetoresistance, noise.

A study was conducted to analyze structures supporting enhanced magnetic properties for giant magnetoresistance applications. Atomic force microscopy was utilized to monitor roughness and grain size. Experimental results indicated that grain size increased with thickness and correlated with poor switching for cobalt rich alloys. thin iron, chromium and tantalum interlayers were effective in suppressing grain growth and maintaining (111) texture.

A study was conducted to analyze the ferromagnetic resonance spectra of thin alloy films. The films were deposited by magnetron sputtering. Experimental results indicated that linewidths and coercivity increased as a function of the total (Cu+magnet) layer thickness at about 20 nm. Findings also showed that the linewidth was sensitive to the damping parameter for the thinner films and to magnetocrystalline anisotropy.

With the microelectromechanical system (MEMS) flux concentrator, we have been able to increase the operating frequency of small magnetic sensors above the region where 1/f noise dominates. The device accomplished this by modulating the field via the oscillatory motion of flux concentrators on MEMS flaps. Electrostatic comb drives were used to drive the MEMS flaps. We have demonstrated an increase in the signal to noise ratio at 1 Hz, that the power signal correctly depends on V4 where V is the amplitude of the voltage energizing the comb drives, and that the signal increases dramatically with vacuum packaging. [ABSTRACT FROM AUTHOR]

To take advantage of the potential offered by the recent increase in the magnetoresistance of magnetic tunnel junctions, it is necessary to address the problem of 1/f noise. ARL has been working on a device, the microelectromechanical system (MEMS) flux concentrator, that mitigates this problem by modulating the sensed magnetic field so that the sensor operates at a higher frequency region where the 1/f noise is smaller. Initial results of testing the device are reported. Though the final step in the fabrication of the device appears to have adversely affected the MEMS motion, two important results were obtained. They are (1) applying the large amplitude voltage at kilohertz frequencies necessary to drive the MEMS structure does not increase the background noise and (2) even though the width of the drive voltage is several hertz, one can demodulate the signal using a lock-in amplifier in order to extract 1 Hz modulation signals. This last result shows that the sensor can be used to detect slow moving or varying magnetic anomalies. [ABSTRACT FROM AUTHOR]

The ferromagnetic resonance properties of thin polycrystalline layers in the sequence Cu/Fe/Cu/Fe/Cu, grown on Si wafers, were studied. Fe grown on Cu can have a very narrow ferromagnetic resonance (FMR) linewidth. Similar structures are of interest for spin transport studies and for giant magnetoresonance applications. The thinner Fe underlayer ranged from 2 to 5 nm, the intermediate Cu spacer thickness from 2 to 10 nm, and the Fe outer layer was 20 nm thick. The increased damping of the thinner Fe layer in this structure, as reflected in the FMR linewidths observed at 9.46 and 33.5 GHz, is similar to that predicted by Berger and recently observed experimentally using epitaxial single crystal Fe films. In addition to confirming the 1/(Fe layer thickness) dependence of the additional damping, the present measurements show a decrease of damping with increasing Cu spacer thickness, implying a short spin transport decay length in our Cu of about 3.5 nm. The linewidth in isolated Cu/Fe/Cu layers also increased with decreasing Fe thickness, as predicted and observed in some other cases. Magnetization data indicate negligible magnetic coupling of the Fe layers except for the smallest interlayer Cu thicknesses used. The linewidths studied increased moderately with cooling to 77 K. [ABSTRACT FROM AUTHOR]

The effects of grain/particle size and morphology on the coercivity of Sm[SUB2](Co[SUB1-x]Fe[SUBx])[SUB17] based powders and spin cast ribbons are investigated in an attempt to make pure Sm-Co 2:17 magnets with a higher energy product than that currently available. It is observed that the coercive force is not only affected by the composition, but also significantly by the grain/particle size. The coercivity of the as-cast SmCo[SUB6] powder was increased from 0.5 to ∼12 kOe when the grain/particle size was decreased from 14 to 1.5 μm. A twofold increase in coercivity was also observed for some Sm(CO[SUB0.85]Fe[SUB0.15])[SUB7.6] spin cast ribbons when the wheel speed was increased from 6 to 25 m/s and the grain size of the ribbons was decreased from 2-15 to 0.5-5 μm. The initial magnetization curves show a high degree of texture that exists in the free side of ribbons, with the c axis parallel to the longitudinal direction of the ribbons. The presence of a trace of boron in the ribbons changes the grain morphology and eliminates the texture. Some results show promise of making a high coercivity higher energy product Sm[SUB2](Co[SUB1-x]Fe[SUBx])[SUB17] based magnets without adding nonmagnetic elements such as Cu and Zr. [ABSTRACT FROM AUTHOR]

Spin-transfer torque film stacks and devices having in-plane magnetization were irradiated using a cobalt-60 gamma source. Samples were also exposed to 2 MeV and 220 MeV protons. Measurements of magnetization vs. field, ferromagnetic resonance, and tunnel magnetoresistance were performed on the film stacks before and after exposure to these sources and no changes were observed in the associated material properties. Spin-transfer torque devices were exposed to the same sources and show no changes in bit-state or write performance. [ABSTRACT FROM AUTHOR]

Magnetic and structural properties of as-cast and melt-spun alloys of the RxFeyTiz system with R≡Sm, Nd and 3≤x≤15, 78≤y≤87, and 3≤z≤13 are presented. Two major anisotropic phases with approximate compositions (RTi0.9)15–17Fe83–85 and (R3Ti0.9)15–17Fe83–85 were found to have anisotropy fields greater than 18 kOe and Tc in the range of 150–320 °C. Room-temperature coercivities in crystallized ribbons, powders, and sintered magnets were found to be in the range 0.2–2.5 kOe. [ABSTRACT FROM AUTHOR]

Epitaxial VO 2 /TiO 2 thin film heterostructures were grown on (100) (m-cut) Al 2 O 3 substrates via pulsed laser deposition. We have demonstrated the ability to reduce the semiconductor-metal transition (SMT) temperature of VO 2 to ∼44 °C while retaining a 4 order of magnitude SMT using the TiO 2 buffer layer. A combination of electrical transport and X-ray diffraction reciprocal space mapping studies help examine the specific strain states of VO 2 /TiO 2 /Al 2 O 3 heterostructures as a function of TiO 2 film growth temperatures. Atomic force microscopy and transmission electron microscopy analyses show that the columnar microstructure present in TiO 2 buffer films is responsible for the partially strained VO 2 film behavior and subsequently favorable transport characteristics with a lower SMT temperature. Such findings are of crucial importance for both the technological implementation of the VO 2 system, where reduction of its SMT temperature is widely sought, as well as the broader complex oxide community, where greater understanding of the evolution of microstructure, strain, and functional properties is a high priority.