Publications by authors named "Brandon M Howe"

11 Publications

  • Page 1 of 1

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals.

Nat Commun 2020 Nov 27;11(1):6039. Epub 2020 Nov 27.

Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, 91125, USA.

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.
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http://dx.doi.org/10.1038/s41467-020-19872-wDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7699621PMC
November 2020

Ultralow Damping in Nanometer-Thick Epitaxial Spinel Ferrite Thin Films.

Nano Lett 2018 07 8;18(7):4273-4278. Epub 2018 Jun 8.

Advanced Light Source, Lawrence Berkeley National Laboratory , Berkeley , California 94720 , United States.

Pure spin currents, unaccompanied by dissipative charge flow, are essential for realizing energy-efficient nanomagnetic information and communications devices. Thin-film magnetic insulators have been identified as promising materials for spin-current technology because they are thought to exhibit lower damping compared with their metallic counterparts. However, insulating behavior is not a sufficient requirement for low damping, as evidenced by the very limited options for low-damping insulators. Here, we demonstrate a new class of nanometer-thick ultralow-damping insulating thin films based on design criteria that minimize orbital angular momentum and structural disorder. Specifically, we show ultralow damping in <20 nm thick spinel-structure magnesium aluminum ferrite (MAFO), in which magnetization arises from Fe ions with zero orbital angular momentum. These epitaxial MAFO thin films exhibit a Gilbert damping parameter of ∼0.0015 and negligible inhomogeneous linewidth broadening, resulting in narrow half width at half-maximum linewidths of ∼0.6 mT around 10 GHz. Our findings offer an attractive thin-film platform for enabling integrated insulating spintronics.
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http://dx.doi.org/10.1021/acs.nanolett.8b01261DOI Listing
July 2018

Tuning Perpendicular Magnetic Anisotropy by Oxygen Octahedral Rotations in (La_{1-x}Sr_{x}MnO_{3})/(SrIrO_{3}) Superlattices.

Phys Rev Lett 2017 Aug 14;119(7):077201. Epub 2017 Aug 14.

Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305, USA.

Perpendicular magnetic anisotropy (PMA) plays a critical role in the development of spintronics, thereby demanding new strategies to control PMA. Here we demonstrate a conceptually new type of interface induced PMA that is controlled by oxygen octahedral rotation. In superlattices comprised of La_{1-x}Sr_{x}MnO_{3} and SrIrO_{3}, we find that all superlattices (0≤x≤1) exhibit ferromagnetism despite the fact that La_{1-x}Sr_{x}MnO_{3} is antiferromagnetic for x>0.5. PMA as high as 4×10^{6}  erg/cm^{3} is observed by increasing x and attributed to a decrease of oxygen octahedral rotation at interfaces. We also demonstrate that oxygen octahedral deformation cannot explain the trend in PMA. These results reveal a new degree of freedom to control PMA, enabling discovery of emergent magnetic textures and topological phenomena.
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http://dx.doi.org/10.1103/PhysRevLett.119.077201DOI Listing
August 2017

Acoustically actuated ultra-compact NEMS magnetoelectric antennas.

Nat Commun 2017 08 22;8(1):296. Epub 2017 Aug 22.

W.M. Keck Laboratory for Integrated Ferroics, and Department of Electrical and Computer Engineering, Northeastern University, Boston, MA, 02115, USA.

State-of-the-art compact antennas rely on electromagnetic wave resonance, which leads to antenna sizes that are comparable to the electromagnetic wavelength. As a result, antennas typically have a size greater than one-tenth of the wavelength, and further miniaturization of antennas has been an open challenge for decades. Here we report on acoustically actuated nanomechanical magnetoelectric (ME) antennas with a suspended ferromagnetic/piezoelectric thin-film heterostructure. These ME antennas receive and transmit electromagnetic waves through the ME effect at their acoustic resonance frequencies. The bulk acoustic waves in ME antennas stimulate magnetization oscillations of the ferromagnetic thin film, which results in the radiation of electromagnetic waves. Vice versa, these antennas sense the magnetic fields of electromagnetic waves, giving a piezoelectric voltage output. The ME antennas (with sizes as small as one-thousandth of a wavelength) demonstrates 1-2 orders of magnitude miniaturization over state-of-the-art compact antennas without performance degradation. These ME antennas have potential implications for portable wireless communication systems.The miniaturization of antennas beyond a wavelength is limited by designs which rely on electromagnetic resonances. Here, Nan et al. have developed acoustically actuated antennas that couple the acoustic resonance of the antenna with the electromagnetic wave, reducing the antenna footprint by up to 100.
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http://dx.doi.org/10.1038/s41467-017-00343-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5567369PMC
August 2017

Coexistence of Low Damping and Strong Magnetoelastic Coupling in Epitaxial Spinel Ferrite Thin Films.

Adv Mater 2017 Sep 10;29(34). Epub 2017 Jul 10.

Materials and Manufacturing Directorate, Air Force Research Laboratory, WPAFB, OH, 45433, USA.

Low-loss magnetization dynamics and strong magnetoelastic coupling are generally mutually exclusive properties due to opposing dependencies on spin-orbit interactions. So far, the lack of low-damping, magnetostrictive ferrite films has hindered the development of power-efficient magnetoelectric and acoustic spintronic devices. Here, magnetically soft epitaxial spinel NiZnAl-ferrite thin films with an unusually low Gilbert damping parameter (<3 × 10 ), as well as strong magnetoelastic coupling evidenced by a giant strain-induced anisotropy field (≈1 T) and a sizable magnetostriction coefficient (≈10 ppm), are reported. This exceptional combination of low intrinsic damping and substantial magnetostriction arises from the cation chemistry of NiZnAl-ferrite. At the same time, the coherently strained film structure suppresses extrinsic damping, enables soft magnetic behavior, and generates large easy-plane magnetoelastic anisotropy. These findings provide a foundation for a new class of low-loss, magnetoelastic thin film materials that are promising for spin-mechanical devices.
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http://dx.doi.org/10.1002/adma.201701130DOI Listing
September 2017

Non-Volatile Ferroelectric Switching of Ferromagnetic Resonance in NiFe/PLZT Multiferroic Thin Film Heterostructures.

Sci Rep 2016 09 1;6:32408. Epub 2016 Sep 1.

Department of Electrical and Computer Engineering, Northeastern University, Boston, Massachusetts 02115, USA.

Magnetoelectric effect, arising from the interfacial coupling between magnetic and electrical order parameters, has recently emerged as a robust means to electrically manipulate the magnetic properties in multiferroic heterostructures. Challenge remains as finding an energy efficient way to modify the distinct magnetic states in a reliable, reversible, and non-volatile manner. Here we report ferroelectric switching of ferromagnetic resonance in multiferroic bilayers consisting of ultrathin ferromagnetic NiFe and ferroelectric Pb0.92La0.08Zr0.52Ti0.48O3 (PLZT) films, where the magnetic anisotropy of NiFe can be electrically modified by low voltages. Ferromagnetic resonance measurements confirm that the interfacial charge-mediated magnetoelectric effect is dominant in NiFe/PLZT heterostructures. Non-volatile modification of ferromagnetic resonance field is demonstrated by applying voltage pulses. The ferroelectric switching of magnetic anisotropy exhibits extensive applications in energy-efficient electronic devices such as magnetoelectric random access memories, magnetic field sensors, and tunable radio frequency (RF)/microwave devices.
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http://dx.doi.org/10.1038/srep32408DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5007664PMC
September 2016

Probing electric field control of magnetism using ferromagnetic resonance.

Nat Commun 2015 Jan 29;6:6082. Epub 2015 Jan 29.

Electric and Computer Engineering Department, Northeastern University, Boston, Massachusetts 02115, USA.

Exchange coupled CoFe/BiFeO3 thin-film heterostructures show great promise for power-efficient electric field-induced 180° magnetization switching. However, the coupling mechanism and precise qualification of the exchange coupling in CoFe/BiFeO3 heterostructures have been elusive. Here we show direct evidence for electric field control of the magnetic state in exchange coupled CoFe/BiFeO3 through electric field-dependent ferromagnetic resonance spectroscopy and nanoscale spatially resolved magnetic imaging. Scanning electron microscopy with polarization analysis images reveal the coupling of the magnetization in the CoFe layer to the canted moment in the BiFeO3 layer. Electric field-dependent ferromagnetic resonance measurements quantify the exchange coupling strength and reveal that the CoFe magnetization is directly and reversibly modulated by the applied electric field through a ~180° switching of the canted moment in BiFeO3. This constitutes an important step towards robust repeatable and non-volatile voltage-induced 180° magnetization switching in thin-film multiferroic heterostructures and tunable RF/microwave devices.
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http://dx.doi.org/10.1038/ncomms7082DOI Listing
January 2015

Interfacial charge-mediated non-volatile magnetoelectric coupling in Co₀.₃Fe₀.₇/Ba₀.₆Sr₀.₄TiO₃/Nb:SrTiO₃ multiferroic heterostructures.

Sci Rep 2015 Jan 13;5:7740. Epub 2015 Jan 13.

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, OH, USA 45433-7707.

The central challenge in realizing non-volatile, E-field manipulation of magnetism lies in finding an energy efficient means to switch between the distinct magnetic states in a stable and reversible manner. In this work, we demonstrate using electrical polarization-induced charge screening to change the ground state of magnetic ordering in order to non-volatilely tune magnetic properties in ultra-thin Co₀.₃Fe₀.₇/Ba₀.₆Sr₀.₄TiO₃/Nb:SrTiO₃ (001) multiferroic heterostructures. A robust, voltage-induced, non-volatile manipulation of out-of-plane magnetic anisotropy up to 40 Oe is demonstrated and confirmed by ferromagnetic resonance measurements. This discovery provides a framework for realizing charge-sensitive order parameter tuning in ultra-thin multiferroic heterostructures, demonstrating great potential for delivering compact, lightweight, reconfigurable, and energy-efficient electronic devices.
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http://dx.doi.org/10.1038/srep07740DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4291561PMC
January 2015

Quantification of strain and charge co-mediated magnetoelectric coupling on ultra-thin Permalloy/PMN-PT interface.

Sci Rep 2014 Jan 14;4:3688. Epub 2014 Jan 14.

Department of Electrical and Computer Engineering, Northeastern University, Boston, MA, USA.

Strain and charge co-mediated magnetoelectric coupling are expected in ultra-thin ferromagnetic/ferroelectric multiferroic heterostructures, which could lead to significantly enhanced magnetoelectric coupling. It is however challenging to observe the combined strain charge mediated magnetoelectric coupling, and difficult in quantitatively distinguish these two magnetoelectric coupling mechanisms. We demonstrated in this work, the quantification of the coexistence of strain and surface charge mediated magnetoelectric coupling on ultra-thin Ni0.79Fe0.21/PMN-PT interface by using a Ni0.79Fe0.21/Cu/PMN-PT heterostructure with only strain-mediated magnetoelectric coupling as a control. The NiFe/PMN-PT heterostructure exhibited a high voltage induced effective magnetic field change of 375 Oe enhanced by the surface charge at the PMN-PT interface. Without the enhancement of the charge-mediated magnetoelectric effect by inserting a Cu layer at the PMN-PT interface, the electric field modification of effective magnetic field was 202 Oe. By distinguishing the magnetoelectric coupling mechanisms, a pure surface charge modification of magnetism shows a strong correlation to polarization of PMN-PT. A non-volatile effective magnetic field change of 104 Oe was observed at zero electric field originates from the different remnant polarization state of PMN-PT. The strain and charge co-mediated magnetoelectric coupling in ultra-thin magnetic/ferroelectric heterostructures could lead to power efficient and non-volatile magnetoelectric devices with enhanced magnetoelectric coupling.
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http://dx.doi.org/10.1038/srep03688DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3891213PMC
January 2014

Voltage-impulse-induced non-volatile ferroelastic switching of ferromagnetic resonance for reconfigurable magnetoelectric microwave devices.

Adv Mater 2013 Sep 15;25(35):4886-92. Epub 2013 Jul 15.

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433-7707, USA.

A critical challenge in realizing magnetoelectrics based on reconfigurable microwave devices, which is the ability to switch between distinct ferromagnetic resonances (FMR) in a stable, reversible and energy efficient manner, has been addressed. In particular, a voltage-impulse-induced two-step ferroelastic switching pathway can be used to in situ manipulate the magnetic anisotropy and enable non-volatile FMR tuning in FeCoB/PMN-PT (011) multiferroic heterostructures.
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http://dx.doi.org/10.1002/adma.201301989DOI Listing
September 2013

Voltage tuning of ferromagnetic resonance with bistable magnetization switching in energy-efficient magnetoelectric composites.

Adv Mater 2013 Mar 10;25(10):1435-9. Epub 2013 Jan 10.

Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson Air Force Base, OH 45433-7707, USA.

Dual E- and H-field control of microwave performance with enhanced ferromagnetic resonance (FMR) tunability has been demonstrated in microwave composites FeGaB/PZN-PT(011). A voltage-impulse-induced non-volatile magnetization switching was also realized in this work, resulting from the hysteretic type of phase transition in PZN-PT(011) at high electric fields. The results provide a framework for developing lightweight, energy efficient, voltage-tunable RF/microwave devices.
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http://dx.doi.org/10.1002/adma.201203792DOI Listing
March 2013