J. J. Wang

Full 180° Magnetization Reversal with Electric Fields

Achieving 180° magnetization reversal with an electric field rather than a current or magnetic field is a fundamental challenge and represents a technological breakthrough towards new memory cell designs. Here we propose a mesoscale morphological engineering approach to accomplishing full 180° magnetization reversals with electric fields by utilizing both the in-plane piezostrains and magnetic shape anisotropy of a multiferroic heterostructure. Using phase-field simulations, we examined a patterned single-domain nanomagnet with four-fold magnetic axis on a ferroelectric layer with electric-field-induced uniaxial strains. We demonstrated that the uniaxial piezostrains, if non-collinear to the magnetic easy axis of the nanomagnet at certain angles, induce two successive, deterministic 90° magnetization rotations, thereby leading to full 180° magnetization reversals.

Effect of strain on voltage-controlled magnetism in BiFeO3-based heterostructures

Voltage-modulated magnetism in magnetic/BiFeO3 heterostructures can be driven by a combination of the intrinsic ferroelectric-antiferromagnetic coupling in BiFeO3 and the antiferromagnetic-ferromagnetic exchange interaction across the heterointerface. However, ferroelectric BiFeO3 film is also ferroelastic, thus it is possible to generate voltage-induced strain in BiFeO3 that could be applied onto the magnetic layer across the heterointerface and modulate magnetism through magnetoelastic coupling. Here, we investigated, using phase-field simulations, the role of strain in voltage-controlled magnetism for these BiFeO3-based heterostructures. It is predicted, under certain condition, coexistence of strain and exchange interaction will result in a pure voltage-driven 180° magnetization reversal in BiFeO3-based heterostructures.

Electric-field-driven magnetization reversal in square-shaped nanomagnet-based multiferroic heterostructure

Based on phase field modeling and thermodynamic analysis, purely electric-field-driven magnetization reversal was shown to be possible in a multiferroic heterostructure of a square-shaped amorphous Co40Fe40B20 nanomagnet on top of a ferroelectric layer through electrostrain. The reversal is made possible by engineering the mutual interactions among the built-in uniaxial magnetic anisotropy, the geometry-dependent magnetic configuration anisotropy, and the magnetoelastic anisotropy. Particularly, the incorporation of the built-in uniaxial anisotropy made it possible to reverse magnetization with one single unipolar electrostrain pulse, which is simpler than previous designs involving the use of bipolar electrostrains and may alleviate ferroelectric fatigue. Critical conditions for triggering the magnetization reversal are identified.

Strain-domain structure and stability diagrams for single-domain magnetic thin films

Strain effects on domain structures and thermal stability in single-domain magnetic thin films were studied using thermodynamic analysis. The strain-domain structure and stability diagrams were established and compared to several existing experimental results. The structure diagram displays various stable single-domain states under in-plane normal and/or in-plane shear strains by minimizing the free energy density whereas the stability diagram takes into account possible thermal excitations and hence illustrate the thermally stable magnetic single-domain states. The results improve the understanding of strain-magnetization correlation in magnetic thin films and provide useful insight for the development of strain-engineered magnetic nanostructures with novel functionalities.

Static magnetic solution in magnetic composites with arbitrary susceptibility inhomogeneity and anisotropy

The static magnetic solutions in magnetic composites with arbitrary susceptibility inhomogeneity and anisotropy are accurately computed using an efficient numerical algorithm based on a proposed Fourier spectral iterative perturbation method for 3-dimensional systems. An advantage of this method is that the interphase boundary conditions are automatically considered without explicitly tracking interphase interfaces in the composites. This method can be conveniently implemented in phase-field modeling of microstructure evolution in systems with inhomogeneous susceptibility as well as inhomogeneous spontaneous magnetization distributions. Based on the proposed method, the effects of microstructures including the susceptibility mismatch between the inclusions and matrix, inclusions volume fraction, and inclusions arrangement on the effective susceptibility and local static magnetic field distribution of the composite are investigated. It is found that the interactions among the inclusions embedded in the matrix play critical roles in determining the composite properties.

A thermodynamic potential for Ni45Co5Mn36.7In13.3 single crystal

A thermodynamic potential as a function of the ferromagnetic, antiferromagnetic, and martensite order parameters is developed using existing experimental data. It successfully reproduces the single domain properties of Ni45Co5Mn36.7In13.3 bulk crystal, including the Curie temperature, the transition temperature from ferromagnetic austenite phase to antiferromagnetic martensite phase as well as its response to an externally applied magnetic field, the saturated magnetization, the martensite strain, the entropy change, and the magnetic permeability. It can be applied to explore the stress-strain and magnetic field-strain behaviors and implemented in phase field simulations to study the ferromagnetic, antiferromagnetic, and martensite domain stability and evolution.

High-temperature diffusion behavior of ZrC in C matrix and its promotion on graphitization

Abstract C/C composite material is widely used in aerospace field and others, however, it is easy to be oxidized at high temperature. In order to improve the oxidation resistance, ZrC is introduced as an oxidation inhibitor used in matrix modification of C/C composite material. Flat plate samples of ZrC/C composite materials were prepared by hot-pressing sintering. The degree of graphitization increases with rising sintering temperature, and layer structure of carbon matrix is observed clearly in the sample treated at 2273 K. Diffusion behavior of Zr in C matrix at high temperature is studied, which can be generally expressed as D =3.382×10 −11 exp[2.029×10 5 /( RT )]. The diffusion of Zr in C matrix leads to the over-saturation of C in the micro area and the oversaturated C precipitates as graphite. This continuous process promotes the transformation of carbon to graphite.

Corrigendum: Full 180° Magnetization Reversal with Electric Fields

Scientific Reports 4: Article number: 7507; published online: 16 December 2014; updated: 25 February 2016. This Article contains a typographical error in a grant number in the Acknowledgements section. “This work was supported by the NSF of China (Grant Nos. 51332001, 11234005, 51472140 and 51221291), and the NSF (Grant No: DMR-1410714, DMR-0820404, and DMR-1210588).

A thermodynamic potential, energy storage performances, and electrocaloric effects of Ba1-xSrxTiO3 single crystals

A thermodynamic potential for Ba1-xSrxTiO3 solid solutions is developed, and the corresponding thermodynamic properties of Ba1-xSrxTiO3 single crystals are calculated. The predicted temperature-composition phase diagram from the thermodynamic potential agrees well with the experimental measurements. Based on this potential, the energy storage performances and electrocaloric effects of Ba1-xSrxTiO3 single crystals are obtained using the phase-field method. It is found that there is an optimal Sr concentration which maximizes the discharged energy density of a Ba1-xSrxTiO3 single crystal under an applied electric field. The electrocaloric effects of Ba0.8Sr0.2TiO3, Ba0.7Sr0.3TiO3, Ba0.6Sr0.4TiO3, and Ba0.5Sr0.5TiO3 single crystals are also predicted, from which the corresponding optimal temperatures are identified.

Influence of ZrC on the microstructure of its surrounding resin-based carbon at high temperature

Abstract The microstructures of a refractory ZrC particle and its surrounding resin-based carbon treated above 2500 °C were investigated by SEM, TEM, HRTEM and selected area electron diffraction. Results show that the ZrC particle is enclosed by a transition carbon layer that has a d 002 of 0.3375 nm, close to that of ideal graphite, indicating the formation of a well graphitized structure. The carbon far away from the ZrC particle remains isotropic and amorphous. These results are verified by selected area electron diffraction, and can be attributed to catalytic graphitization and stress-induced graphitization.