Joshua L. Hockel

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.

A method to control magnetism in individual strain-mediated magnetoelectric islands

Patterned electrodes on a piezoelectric substrate are demonstrated to produce a localized strain of sufficient magnitude to control the magnetic anisotropy of a Ni island. Strain-induced magnetic anisotropy was measured using the magneto-optical Kerr effect, and the measured shifts in magnetic anisotropy were consistent with strain predicted using linear finite element analysis. This approach overcomes the effect of the substrate clamping the in-plane strain and should be scalable to thin films. This approach represents a key step toward realizing the next generation of strain mediated magneto-electric magnetic random access memory devices with low writing energy and high writing speed.

Electric field induced magnetization rotation in patterned Ni ring/Pb(Mg1/3Nb2/3)O3](1−0.32)-[PbTiO3]0.32 heterostructures

Electric field induced magnetoelastic anisotropy is shown to rotate the magnetization of a ring-shaped magnet by 90° in a Ni ring/(011) Pb(Mg1/3Nb2/3)O3](1−0.32)-[PbTiO3]0.32 heterostructure. The 2000 nm diameter ring is initially field annealed forming the “onion” magnetization state. A 0.8 MV/m electric field is applied to the substrate creating anisotropic piezostrain and a perpendicular in-plane easy axis. Magnetic force microscopy confirms the 90° rotation of the vortex-type domain walls from the field annealing direction. Rotations are stable without electric field due to remnant strains induced during the poling process, supporting the viability of strain-based magnetic recording methods.

Electric-poling-induced magnetic anisotropy and electric-field-induced magnetization reorientation in magnetoelectric Ni/(011) [Pb(Mg1/3Nb2/3)O3](1-x)-[PbTiO3]x heterostructure

This study reports the influence of poling a PMN-PT single crystal laminated structure on the magnetic properties of a 35 nm polycrystalline Ni thin film. During the poling process, a large anisotropic remanent strain is developed in the PMN-PT that is transferred to the ferromagnetic film creating a large predefined magnetic anisotropy. Test results show that operating the PMN-PT substrate in the linear regime following poling produces sufficient anisotropic strain to reversibly reorient the magnetization toward an easy axis oriented 90° to the magnetic easy axis induced during poling. The influence of poling prestress on the magnetic anisotropy field, coercive field and magnetic remanence is discussed.

Electrically Driven Magnetic Domain Wall Rotation in Multiferroic Heterostructures to Manipulate Suspended On-Chip Magnetic Particles

In this work, we experimentally demonstrate deterministic electrically driven, strain-mediated domain wall (DW) rotation in ferromagnetic Ni rings fabricated on piezoelectric [Pb(Mg1/3Nb2/3)O3]0.66-[PbTiO3]0.34 (PMN-PT) substrates. While simultaneously imaging the Ni rings with X-ray magnetic circular dichroism photoemission electron microscopy, an electric field is applied across the PMN-PT substrate that induces strain in the ring structures, driving DW rotation around the ring toward the dominant PMN-PT strain axis by the inverse magnetostriction effect. The DW rotation we observe is analytically predicted using a fully coupled micromagnetic/elastodynamic multiphysics simulation, which verifies that the experimental behavior is caused by the electrically generated strain in this multiferroic system. Finally, this DW rotation is used to capture and manipulate micrometer-scale magnetic beads in a fluidic environment to demonstrate a proof-of-concept energy-efficient pathway for multiferroic-based lab-on-a-chip applications.

Magnetoelectric manipulation of domain wall configuration in thin film Ni/[Pb(Mn1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (001) heterostructure

This paper reports experimental observations of partial and reversible out-of-plane magnetization change in a thin film Ni/[Pb(Mn1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (001) heterostructure. Electric-field-induced isotropic in-plane compressive strain (∼1000 ppm) eliminates the stripe domain pattern in a 60-nm-thick Ni thin film. When the electric field is removed, the stripe domains are returned to their original configurations with some domain wall pinning perturbations due to ferroelectric domain texturing. The observed domain structure change is attributed to the transition from Bloch wall to Neel wall and the broadening of the Bloch wall. This out-of-plane magnetization change does not occur in thicker (100-nm-thick) Ni thin film.This paper reports experimental observations of partial and reversible out-of-plane magnetization change in a thin film Ni/[Pb(Mn1/3Nb2/3)O3]0.68-[PbTiO3]0.32 (001) heterostructure. Electric-field-induced isotropic in-plane compressive strain (∼1000 ppm) eliminates the stripe domain pattern in a 60-nm-thick Ni thin film. When the electric field is removed, the stripe domains are returned to their original configurations with some domain wall pinning perturbations due to ferroelectric domain texturing. The observed domain structure change is attributed to the transition from Bloch wall to Neel wall and the broadening of the Bloch wall. This out-of-plane magnetization change does not occur in thicker (100-nm-thick) Ni thin film.

Voltage bias influence on the converse magnetoelectric effect of PZT/terfenol-D/PZT laminates

The converse magnetoelectric effect (CME) of a 2-mm-thick Pb(Zr0.52Ti0.48)O3 (PZT)/Tb0.30Dy0.7Fe2 (Terfenol-D)/PZT laminate subjected to an applied dc voltage bias has been investigated. Experimental data demonstrate that the CME coefficient αCME (B/Vac)is highly dependent on the applied voltage bias. In the present work, the voltage bias is shown to increase the magnitude of magnetostriction in the Terfenol-D, which results in an increase in the magnitude and range of piezomagnetic coefficient and αCME values. It is shown that αCME can range from a minimum of 0.4 G/V at a voltage bias of −200 V to a value of 1.65 G/V at a voltage bias of 700 V. This range represents a greater than 400% change in the αCME at a fixed magnetic field bias, the largest change in αCME due to voltage bias yet reported. The expanded range is primarily caused by giant shifts in the piezomagnetic coefficient and not by nonlinearities in the piezoelectric coefficient.

Magneto-electric tuning of the phase of propagating spin waves

The utilization of a magnetoelectric film composite to control, by an electric field, the phase of magnetostatic surface spin waves propagating along thin films is reported. Laminates of ferromagnetic films of Ni and NiFe are deposited on a ferroelectric substrate, lead magnesium niobate-lead titanate. The phase of propagating spinwaves is shown to be modulated by an electric field while traveling a finite distance along the surface. The observed phase change in the spinwaves is in agreement with the anisotropy field changes measured with magneto optical Kerr effect hysteresis loops. A quantitative agreement is demonstrated.

Electrical and Mechanical Manipulation of Ferromagnetic Properties in Polycrystalline Nickel Thin Film

The ferromagnetic properties of a 35 nm polycrystalline nickel thin film deposited on a single-crystal, optical, ferroelectric lithium niobate substrate are magnetoelectrically tunable. The coercive field of the nickel film, which has low magnetocrystalline anisotropy and appropriate magnetoelasticity, changes by about 80% owing to the anisotropic strain produced by the substrate. In addition, mechanical strain changes the coercive field by about 260% and increases the normalized remanence \bm M\bmr/\bm M\bms from 0.3 to 1.0. This giant tunability would be achievable by combining polycrystalline Ni thin film with a ferroelectric substrate having large anisotropic piezoelectric coefficients.

Strain-induced magnetization change in patterned ferromagnetic nickel nanostructures

We report strain-induced coercive field changes in patterned 300 × 100 × 35 nm3 Ni nanostructures deposited on Si/SiO2 substrate using the magnetoelastic effect. The coercive field values change as a function of the applied anisotropy strain (∼1000 ppm) between 390 and 500 Oe, demonstrating that it is possible to gradually change the coercive field elastically. While the measured changes in coercive field cannot be accurately predicted with simple analytical predictions, fairly good agreement is obtained by using a micromagnetic simulation taking into account the influence of nonuniform strain distribution in the Ni nanostructures. The micromagnetic simulation includes a position dependant strain-induced magnetic anisotropy term that is computed from a finite element mechanical analysis. Therefore, this study experimentally corroborates the requirement to incorporate mechanical analysis into micromagnetic simulation for accurately predicting magnetoelastic effects in patterned ferromagnetic nanostructures.