Brandon M. Howe

Voltage Tuning of Ferromagnetic Resonance with Bistable Magnetization Switching in Energy‐Efficient Magnetoelectric Composites

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.

Voltage‐Impulse‐Induced Non‐Volatile Ferroelastic Switching of Ferromagnetic Resonance for Reconfigurable Magnetoelectric Microwave Devices

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.

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

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.

Probing electric field control of magnetism using ferromagnetic resonance

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.

Comparison of spin-orbit torques and spin pumping across NiFe/Pt and NiFe/Cu/Pt interfaces

We experimentally investigate spin-orbit torques and spin pumping in NiFe/Pt bilayers with direct and interrupted interfaces. The dampinglike and fieldlike torques are simultaneously measured with spin-torque ferromagnetic resonance tuned by a dc-bias current, whereas spin pumping is measured electrically through the inverse spin-Hall effect using a microwave cavity. Insertion of an atomically thin Cu dusting layer at the interface reduces the dampinglike torque, fieldlike torque, and spin pumping by nearly the same factor of

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

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Quantifying thickness-dependent charge mediated magnetoelectric coupling in magnetic/dielectric thin film heterostructures

. This finding confirms that the observed spin-orbit torques predominantly arise from diffusive transport of spin current generated by the spin-Hall effect. We also find that spin-current scattering at the NiFe/Pt interface contributes to additional enhancement in magnetization damping that is distinct from spin pumping.

Significantly Enhanced Inductance and Quality Factor of GHz Integrated Magnetic Solenoid Inductors With FeGaB/

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.

{\rm Al}_{2}{\rm O}_{3}

Precise quantification of the magnetoelectric coupling strength in surface charge induced magnetoelectric effect was investigated in NiFe/SrTiO3 thin film heterostructures with different ultra-thin NiFe thicknesses through voltage induced ferromagnetic resonance. The voltage induced ferromagnetic resonance field shifts in these NiFe/SrTiO3 thin films heterostructures showed a maximum value of 65 Oe at an intermediate NiFe layer thickness of ∼1.2 nm, which was interpreted based on the thin film growth model at the low-thicknesses and on the charge screening effect at large thicknesses. The precise quantification and understanding of the magnetoelectric coupling in magnetic/dielectric thin films heterostructures constitute an important step toward real applications.

Multilayer Films

We report new high quality factor (Q) integrated GHz magnetic inductors based on solenoid structures with FeGaB/Al2O3 multilayer films, which show significantly enhanced inductance and quality factor at GHz frequencies over their air core counterparts. These inductors show an excellent high-frequency performance with a wide operation frequency range 0.5-2.5 GHz, in which the inductance is flat and the peak quality factor can reach ~20. The inductance of the magnetic inductor shows >100% enhancement compared with that of the same size air core inductor. These novel GHz inductors with high inductance and Q enhancement show great promise for applications in radio frequency integrated circuits.