David Bono

Empirical mapping of Ni–Mn–Ga properties with composition and valence electron concentration

A range of Ni–Mn–Ga alloy compositions close to the stoichiometric Heusler composition, Ni2MnGa, has been reported to show field-induced strains of several percent. Such observations, and the magnitude of the strain observed, depend on the values of several critical material parameters, most importantly the martensitic transformation temperature (Tmart), Curie temperature (TC), saturation magnetization (Ms), strength of the magnetocrystalline anisotropy, and the details of the martensite structure. Here, data collected from a variety of sources are plotted and their variations are fit with empirical formulas to afford a better overall picture of the behavior of this system. It is found that the martensitic transformation temperature is the parameter most sensitive to the composition; saturation magnetization appears to peak sharply at 7.5 valence electrons/atom, finally the composition field over which the saturation magnetization exceeds 60 emu/g, and 300 K

Spin Hall torque magnetometry of Dzyaloshinskii domain walls

Current-induced domain wall motion in the presence of the Dzyaloshinskii-Moriya interaction (DMI) is experimentally and theoretically investigated in heavy-metal/ferromagnet bilayers. The angular dependence of the current-induced torque and the magnetization structure of Dzyaloshinskii domain walls are described and quantified simultaneously in the presence of in-plane fields. We show that the DMI strength depends strongly on the heavy metal, varying by a factor of 20 between Ta and Pa, and that strong DMI leads to wall distortions not seen in conventional materials. These findings provide essential insights for understanding and exploiting chiral magnetism for emerging spintronics applications.

High efficiency vibration energy harvester

In a vibrational energy harvester, an external vibration causes relative motion between a permanent magnet and a magnetic field sensing element composed of a magnetostrictive material bonded to an electroactive material. The changing magnetic field causes a rotation of magnetization in the magnetostrictive material and the rotating magnetization generates a stress in the magnetostrictive material. The stress is transmitted to the electroactive material, which responds by generating electrical power.

ac field-induced actuation of single crystal Ni–Mn–Ga

Single crystals of Ni–Mn–Ga have shown 6% field induced strain in quasistatic actuation against an applied load. Here we report the ac actuation response and a magnetostrictivity constant of an off-stoichiometry single crystal of Ni–Mn–Ga measuring approximately 14×5.6×1 mm. This sample shows a maximum 3% strain under a bias stress of 1.1–1.5 MPa. The deficit between the low-frequency ac field-induced strain and the 6% dc results are discussed. Additionally, the e–H indicate a peak differential value for d13 of 1%/kOe (13×10−8 m/A) which is about two times that of Terfenol-D.

Energy absorption in Ni-Mn-Ga/ polymer composites

Martensitic Ni-Mn-Ga single crystals are easily twinned and the twin boundaries can be displaced by applying a mechanical stress or magnetic field. Twin boundary motion is a highly dissipative process. Composites of aligned Ni-Mn-Ga particles in a polymer matrix show evidence of stress-induced twin boundary motion that results in stress-strain curves with significantly greater hysteresis than for Fe-polymer composites or polymer samples.

Faraday rotation, ferromagnetism, and optical properties in Fe-doped BaTiO3

The structural, magneto-optical, optical, and magnetic properties of BaTi1−xFexO3 films grown by pulsed laser deposition onto MgO (100) have been investigated for Fe content, x=0.15, 0.2, 0.3, and 0.5. Saturation Faraday rotation varies from 0.033deg∕μm for x=0.2 to 0.109deg∕μm for x=0.5 at a 1.55-μm wavelength, with corresponding saturation magnetizations of 11.3emu∕cm3 and 18.3emu∕cm3. The optical absorption drops with a reduction in iron content and the BaTi0.85Fe0.15O3 sample has an optical absorption of 1.9dB∕mm. The magneto-optic figure of merit, which is the ratio of Faraday rotation to absorption, peaks at x=0.2 among the samples considered. The magnetic properties are interpreted in terms of the electronic structure of Fe4+ ions, and their exchange coupling with Fe3+ ions associated with oxygen vacancies.

Noninvasive Deep Brain Stimulation via Temporally Interfering Electric Fields

Summary We report a noninvasive strategy for electrically stimulating neurons at depth. By delivering to the brain multiple electric fields at frequencies too high to recruit neural firing, but which differ by a frequency within the dynamic range of neural firing, we can electrically stimulate neurons throughout a region where interference between the multiple fields results in a prominent electric field envelope modulated at the difference frequency. We validated this temporal interference (TI) concept via modeling and physics experiments, and verified that neurons in the living mouse brain could follow the electric field envelope. We demonstrate the utility of TI stimulation by stimulating neurons in the hippocampus of living mice without recruiting neurons of the overlying cortex. Finally, we show that by altering the currents delivered to a set of immobile electrodes, we can steerably evoke different motor patterns in living mice.

New high-sensitivity hybrid magnetostrictive/electroactive magnetic field sensors

A recently developed class of magnetic field sensors is based on the action of a magnetostrictive material on a piezoelectric material. Described here is a significant improvement on this class, a passive magnetic field sensor made of layers of Terfenol-D {Fe2(Dy0.7Tb0.3)} magnetostrictive material and ceramic PZT-5. These devices show a large magnetic field sensitivity of order 6 kV/T. Application of this device to harvesting vibration energy indicates that more than 10 mW of electrical power can be harvested using a Terfenol-D/PZT/ Terfenol-D sandwich (volume = 1 cm3) from 30 Hz vibrations having an acceleration of 0.5 g.

Improved Wireless, Transcutaneous Power Transmission for In Vivo Applications

Electric power, sufficient for many in vivo applications, can be transmitted wirelessly from a small external solenoid (filled with a soft magnetic core), to a novel, magnetoelectric (ME) receiver a few centimeter (cm) inside the body. The ME receiver is a sandwich of electroactive (e.g., piezoelectric) material bonded between two magnetostrictive layers. The electroactive layer may be poled in its plane so that it can function in the stronger g33 mode (induced voltage parallel to the direction of principal magnetostrictive stress). Preliminary experimental results indicate that a 7 cm long ferrite-filled solenoid (NI ap 122 Amp-turns) producing an RMS magnetic field of order 1600 A/m (20 Oe) at the ME receiver (of volume 0.1 cm3) 3 cm from the field source, generates in the ME receiver a power of 200 mW (2 W/cm3). The receiver, in turn, generates a power of 160 mW.

Structural, magnetic, and magneto-optical properties of Co-doped CeO2−δ films

Magnetically doped CeO2 is a promising dilute magnetic semiconductor and may also be useful in magneto-optical applications. Ce1−xCoxO2−δ (x=0, 0.02, 0.06, 0.15, and 0.25) films were deposited by pulsed laser deposition on MgO(100) substrates and their structural, magnetic, and magneto-optical properties were characterized. The films show a textured ceria single phase with (111) preferred orientation. All the Co-doped samples show room temperature ferromagnetism and large magnetic anisotropy with an out-of-plane easy axis. Magneto-optical measurements indicate that the Co-doped films also have high saturation Faraday rotation ranging from 230to6000deg∕cm depending on the Co concentration, and their refractive index and extinction coefficient also increase with Co concentration.