Reversible strain control of magnetic anisotropy in magnetoelectric heterostructures at room temperature

Rare earth trace element doping of extrinsic multiferroics for an energy efficient remote control of magnetic properties

Developing functional materials for optical remote control of magnetism can lead to faster, more efficient wireless data storage and sensing devices. In terms of desired material properties, this development requires the combined optimization of elastic interactions, low magnetic coercivity, and a narrow linewidth of ferromagnetic resonance to establish low-loss dynamic functionalities. A general pathway to achieve these requirements is still lacking. Here, we demonstrate that rare-earth trace element doping of an extrinsic multiferroic promotes strain mediated energy efficient remote control of static and dynamic magnetic properties induced by non-pulsed visible light. The strain under illumination arises from the photostrictive property of the ferroelectric substrate whereas the magnetism control originates from the enhanced magnetostrictive property of a rare-earth trace element doped ferromagnetic thin film. Combining the light-strain-magnetic interaction in the rare-earth doped extrinsic multiferroic provides a general approach for enhanced photo-magnetic elastic control extendable to optically tunable magnetic devices.

Enhancing Converse Magnetoelectric Coupling Through Strain Engineering in Artificial Multiferroic Heterostructures

Magnetoelectric materials present a unique opportunity for electric field-controlled magnetism. Even though strain-mediated multiferroic heterostructures have shown unprecedented increase in magnetoelectric coupling compared to single-phase materials, further improvements must be made before ultra-low power memory, logic, magnetic sensors, and wide spectrum antennas can be realized. This work presents how magnetoelectric coupling can be enhanced by simultaneously exploiting multiple strain engineering approaches in heterostructures composed of Fe_0.5Co_0.5/Ag multilayers on (011) Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ piezoelectric crystals. When grown and measured under strain, these heterostructures exhibit an effective converse magnetoelectric coefficient in the order of 10^-5 s m^-1: the highest directly measured, non-resonant value to-date. This response occurred at room temperature and at low electric fields (<2 kV cm^-1). This large effect is enabled by magnetization reorientation caused by changing the magnetic anisotropy with strain from the substrate and the use of multilayered magnetic materials to minimize the internal stress from deposition. Additionally, the coercive field dependence of the magnetoelectric response under strain suggests contributions from domain-mediated magnetization switching modified by voltage-induced magnetoelastic anisotropy. This work highlights how multicomponent strain engineering enables enhanced magnetoelectric coupling in heterostructures and provides an approach to realize energy-efficient magnetoelectric applications.

Perpendicular magnetic anisotropy in half-metallic thin-film Co2CrAl

Magnetocrystalline anisotropy (MCA) is one of the key parameters investigated in spin-based electronics (spintronics), e.g. for memory applications. Here, we employ first-principles calculations to study MCA in thin film full Heusler alloy Co₂CrAl. This material was studied in the past, and has been reported to exhibit half-metallic electronic structure in bulk geometry. In our recent work, we showed that it retains a 100% spin-polarization in thin-film geometry, at CrAl atomic surface termination. Here, we show that the same termination results in a perpendicular magnetic anisotropy, while Co surface termination not only destroys the half-metallicity, but also results in in-plane magnetization orientation. In addition, for films thicker than around 20 nm the contribution from magnetic shape anisotropy may become decisive, resulting in in-plane magnetization orientation. To the best of our knowledge, this is one of the first reports of half-metallic thin-film surfaces with perpendicular magnetic anisotropy. This result may be of interest for potential nano-device applications, and may stimulate a further experimental study of this and similar materials.

Photoinduced spin dynamics in a uniaxial intermetallic heterostructure TbCo 2 / FeCo

Intermetallic heterostructures of rare-earth and transition metals exhibit physical properties prospective for various applications. These structures combine giant magnetostriction, controllable magnetic anisotropy, magneto-optical activity and allow spin reorientation transitions (SRT) induced by magnetic field at room temperature. Here, we present the results of a study of spin dynamics induced by ultrafast optical excitation in theTbCo 2 /FeCo heterostructure. The time dependence of the light polarization rotation excited by a pump optical pulse with a duration of 35 fs was measured in the total range of the SRT created by external DC magnetic field. We found hysteretic dependence of the polarization rotation on magnetizing field that is specific for spin dynamics near SRT. Enhancement of the rotation is observed in the critical points of the SRT and near the points of magnetization switch from metastable to stable spin states. In the time-domain, two characteristic delays of 20 ps and 200 ps were found, corresponding to the maximum deviation of the light polarization after excitation. The first is explained by the precession motion of spins out of the plane of the structure. The latter is accounted for the spin in-plane deviation from its initial position and thermal relaxation of the anisotropy.

Demonstration of a pseudo-magnetization based simultaneous write and read operation in a Co60Fe20B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructure

Taking advantage of the magnetoelectric and its inverse effect, this article demonstrates strain-mediated magnetoelectric write and read operations simultaneously in Co60Fe20B20/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures based on a pseudo-magnetization µ ≡ mx2 - my2. By applying an external DC-voltage across a (011)-cut PMN-PT substrate, the ferroelectric polarization is re-oriented, which results in an anisotropic in-plane strain that transfers to the CoFeB thin film and changes its magnetic anisotropy Hk. The change in Hk in-turn results in a 90° rotation of the magnetic easy axis for sufficiently high voltages. Simultaneously, the inverse effect is employed to read changes of the magnetic properties. The change of magnetization in ferromagnetic (FM) layer induces an elastic stress in the piezoelectric (PE) layer, which generates a PE potential that can be used to readout the magnetic state of the FM layer. The experimental results are in excellent qualitative agreement with an equivalent circuit model that considers how magnetic properties are electrically controlled in such a PE/FM heterostructure and how a back-voltage is generated due to changing magnetic properties in a self-consistent model. We demonstrated that a change of easy axis of magnetization due to an applied voltage can be directly used for information processing, which is essential for future ME based devices.

Magnetoelastic Effects in Doubly Clamped Electroplated Co77Fe23 Microbeam Resonators

Magnetostrictive Co77Fe23 films are fully suspended to produce free-standing, clamped-clamped, microbeam resonators. A negative or positive shift in the resonant frequency is observed for magnetic fields applied parallel or perpendicular to the length of the beam, respectively, confirming the magnetoelastic nature of the shift. Notably, the resonance shifts linearly with higher-bias fields oriented perpendicular to the beam's length. Domain imaging elucidates the distinction in the reversal processes along the easy and hard axes. Together, these results suggest that through modification of the magnetic anisotropy, the frequency shift and angular dependence can be tuned, producing highly magnetic-field-sensitive resonators.

Large Interfacial Magnetostriction in (Co/Ni)4/Pb(Mg1/3Nb2/3)O3-PbTiO3 Multiferroic Heterostructures

The magnetoelastic behavior of multiferroic heterostructures-coupling of magnetic anisotropy or domain dynamics to structural deformations-has been intensively studied for developing materials for energy-efficient, spin-based applications. Here, we report on a large, interface-dominated magnetostriction in (Co/Ni)₄/Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ multiferroic heterostructures. Ferromagnetic resonance spectroscopy under voltage-induced strains enabled estimation of the saturation magnetostriction as a function of Ni thickness. The volume and the interface components to the saturation magnetostriction are (6.6 ± 0.9) × 10^-6 and (-2.2 ± 0.2) × 10^-14 m, respectively. Similar to perpendicular magnetic anisotropy in Co/Ni, the large, negative magnetostriction originates from the Co/Ni interfaces. This interfacial functionality delivers an effect over 300% larger than the bulk contribution and can enable low-energy, nanoelectronic devices that combine the tunable magnetic and magnetostrictive properties of Co/Ni multilayers with the ferroelectric properties of Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃.

Giant magneto-electric coupling in 100 nm thick Co capped by ZnO nanorods

Here we report a giant, completely reversible magneto-electric coupling of 100 nm polycrystalline Co layer in contact with ZnO nanorods. When the sample is under an applied bias of ±2 V, the Co magnetic coercivity is reduced by a factor 5 from the un-poled case, with additionally a reduction of total magnetic moment in Co. Taking into account the chemical properties of ZnO nanorods measured by X-rays absorption near edge spectroscopy under bias, we conclude that these macroscopic effects on the magnetic response of the Co layer are due to the microstructure and the strong strain-driven magneto-electric coupling induced by the ZnO nanorods, whose nanostructuration maximizes the piezoelectric response under bias.