Gregory P. Carman

Giant electric-field-induced reversible and permanent magnetization reorientation on magnetoelectric Ni/(011) [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x heterostructure

We report here an atomic resolution study of the structure and composition of the grain boundaries in polycrystalline Sr0.6K0.4Fe2As2 superconductor. A large fraction of grain boundaries contain amorphous layers larger than the coherence length, while some others contain nanometer-scale particles sandwiched in between amorphous layers. We also find that there is significant oxygen enrichment at the grain boundaries. Such results explain the relatively low transport critical current density (Jc) of polycrystalline samples with respect to that of bicrystal films.We report giant reversible and permanent magnetic anisotropy reorientation in a magnetoelectric polycrystalline Ni thin film and (011)-oriented [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x heterostructure. The electric-field-induced magnetic anisotropy exhibits a 300 Oe anisotropy field and a 50% change in magnetic remanence. The important feature is that these changes in magnetization states are stable without the application of an electric field and can be reversibly switched by an electric field near a critical value (±Ecr). This giant reversible and permanent magnetization change is due to remanent strain originating from a non-180° ferroelectric polarization reorientation when operating the ferroelectric substrate in a specific non-linear regime below the electric coercive field.

Dielectric and piezoelectric response of lead zirconate–lead titanate at high electric and mechanical loads in terms of non-180° domain wall motion

The effect of prestress on the nonlinear dielectric (polarization) and piezoelectric (strain) response of lead zirconate–lead titanate (PZT–5H) piezoelectric ceramic is studied. The response to bipolar (−2/+2 MV/m) and unipolar (0/+2 MV/m, −0.4/+2 MV/m) electric field under constant prestress (up to 175 MPa) is experimentally evaluated. In the bipolar regime, prestress mainly influences the first non-180° process. In the unipolar regime, the dielectric and piezoelectric response achieve maximum values near 50–60 MPa because the prestress increases the number of available non-180° domains. A detailed description of the effect of the prestress on electro–mechanical response is provided in terms of non-180° domain wall motion. Based on rule of mixtures formulation, an analytical model is developed to estimate the optimum prestress value for the unipolar electric loading condition. It is found that the dielectric and piezoelectric response of the material is proportional to the volume fraction of the non-180°...

Electrical control of reversible and permanent magnetization reorientation for magnetoelectric memory devices

We report giant reversible and permanent magnetic anisotropy reorientation between two perpendicular easy axes in a magnetoelectric polycrystalline Ni thin film and (011) oriented [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT) heterostructure. The PMN-PT is partially poled prior to Ni film deposition to provide a remanent strain bias. Following Ni deposition and full poling of the sample, two giant remanent strains of equal and opposite values are used to reversibly and permanently reorient the magnetization state of the Ni film. These experimental results are integrated into micromagnetic simulation to demonstrate the usefulness of this approach for magnetoelectric based magnetic random access memory.

Electric-field-induced spin wave generation using multiferroic magnetoelectric cells

In this work, we report on the demonstration of voltage-driven spin wave excitation, where spin waves are generated by multiferroic magnetoelectric (ME) cell transducers driven by an alternating voltage, rather than an electric current. A multiferroic element consisting of a magnetostrictive Ni film and a piezoelectric [Pb(Mg1/3Nb2/3)O3](1−x)–[PbTiO3]x substrate was used for this purpose. By applying an AC voltage to the piezoelectric, an oscillating electric field is created within the piezoelectric material, which results in an alternating strain-induced magnetic anisotropy in the magnetostrictive Ni layer. The resulting anisotropy-driven magnetization oscillations propagate in the form of spin waves along a 5 μm wide Ni/NiFe waveguide. Control experiments confirm the strain-mediated origin of the spin wave excitation. The voltage-driven spin wave excitation, demonstrated in this work, can potentially be used for low-dissipation spin wave-based logic and memory elements.

Thermal energy harvesting device using ferromagnetic materials

A unique concept for harvesting electrical energy from thermal energy is presented. A thermomechanical actuator was fabricated using ferromagnetic material. The device converts thermal energy into mechanical energy, which can be converted into electrical energy using piezoelectric materials. Magnetic force and operating frequency were measured on the device. Results show that the current power density at ΔT=50K is between 1.85 and 3.61mW∕cm2. A thermal finite element analysis model is also presented to understand the influence of thermal interface, suggesting that increases of 18.5mW∕cm2 or higher are achievable.

Domain engineered switchable strain states in ferroelectric (011) [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT, x≈0.32) single crystals

The ferroelectric properties of (011) [Pb(Mg1/3Nb2/3)O3](1−x)-[PbTiO3]x (PMN-PT, x≈0.32) single crystals with focus on piezoelectric strain response were reported. Two giant reversible and stable remanent strain states and tunable remanent strain properties are achieved by properly reversing the electric field from the depolarized direction. The unique piezoelectric strain response, especially along the [100] direction, mainly stems from the non-180° ferroelectric polarization reorientation in the rhombohedral phase crystal structure. Such giant strain hysteresis with tunable remanent strain properties may be useful for magnetoelectric based memory devices as well as a potential candidate for other applications.

Sputter deposition of NiTi thin film shape memory alloy using a heated target

In this paper we present a novel method for depositing NiTi thin film by DC sputtering. The film has transformation temperatures very close to that of the target. The new process involves heating the target and does not require compositional modification of the NiTi target. Results from X-ray diffraction, differential scanning calorimeter, four-point probe, Rutherford backscattering, and transmission electron microscopy are presented. These results indicate that compositional modification can be produced by varying the target temperature. Films produced from hot targets have compositions similar to the target while films produced from cold targets were Ti deficient. Films that were produced by gradual heating of the target have compositional gradation through the film thickness. The gradated films exhibit the two-way shape memory effect.

Electric-field-induced reversible magnetic single-domain evolution in a magnetoelectric thin film

We report experimental results on a Ni-nanobar/lead zirconate titanate-film magnetoelectric device demonstrating control of a metastable magnetic single domain with an electric field due to the converse magnetoelectric effect (i.e., coupling of piezoelectric effect, mechanical coupling, and magnetostriction). The reversible single-domain evolution from an initial single-domain state to a transitional S-shape domain state with an electric field was experimentally observed with magnetic force microscopy. Upon removal of the electric field, the single domain reverts to its original domain configuration. These results confirm change of a single domain in the nanoscale magnetoelectric/multiferroic device is achievable and subsequent control of local magnetic field is possible.

Dynamic magnetomechanical behavior of terfenol-D/epoxy 1-3 particulate composites

We investigated the dynamic behavior of 1-3 type magnetostrictive particulate composites as a function of both bias field (5-140 kA/m) and frequency (1-100 kHz). The composites consist of approximately 0.5 volume-fraction Terfenol-D particles embedded and magnetically aligned in a passive epoxy matrix. The measured properties include elastic moduli (E/sub 3//sup H/ and E/sub 3//sup B/), dynamic relative permeability (/spl mu//sub r33/), dynamic strain coefficient (d/sub 33/), magnetomechanical coupling coefficient (k/sub 33/), and the ratio of the dynamic strain coefficient to the dynamic susceptibility (d/sub 33///spl chi//sub 33/). We observed the dependence of these properties on bias field and explain it here in terms of domain-wall motion followed by saturation near 40 kA/m. The spectra of /spl mu//sub r33/, d/sub 33/, and d/sub 33///spl chi//sub 33/ indicate that the magnetization process is independent of frequency and that the effect of eddy-current losses is insignificant up to 100 kHz. The observations agree with predictions made by classical eddy-current theory and suggest that the composites can be operated at significantly higher frequencies than monolithic Terfenol-D.

Reversible magnetic domain-wall motion under an electric field in a magnetoelectric thin film

We report direct microscopic measurements that confirm the magnetic stripe-domain patterns can be reversibly changed under an electric field due to the converse magnetoelectric effect in a bilayer thin film ferromagnetic-Ni/ferroelectric-lead zirconate titanate (100nm∕1.28μm) heterostructure. Electric field-induced curving, bending, branching, and elongation of magnetic stripe-domain patterns in the Ni layer are observed with the use of magnetic force microscopy. Upon removal of the electric field, the magnetic stripe-domain patterns return to their original configuration, i.e., reversible.