March , Cite as. This paper discusses the electric polarization properties of ferromagnetic microwires at microwave frequencies. At the vicinity of the antenna resonance, a strong magnetic control of the wire polarization is possible owing to the magnetoimpedance MI effect. In order to realize efficient tunable properties, magnetic microwires of Co-rich compositions in amorphous state are considered. The absence of the crystalline structure is an important condition to establish well-defined magnetic anisotropies of small magnitudes, which results in the magnetization processes sensitive to the external parameters such as a magnetic field and a mechanical stress.
Here we demonstrate that the electric polarization of a microwire depends on its impedance and, hence, can be modulated changing the wire magnetization state in response to the application of a magnetic field and a mechanical stress.
12 Magnetoelectric Multiferroic Composites
The polarization problem is treated theoretically by solving the scattering problem from a cylindrical ferromagnetic wire with the impedance boundary conditions. The modelling results agree well with the available experimental data. Competitive technological developments require functional materials that combine several properties which may be tuned by external stimuli.
A large change in magnetization caused by an electric field was recently observed in hexaferrites with a conical spin structure [ 3 , 4 ] and also in M-type hexaferrites with a co-linear magnetic structure [ 5 , 6 ]. In the latter case the mechanism of spontaneous electric polarization can be still related to the magnetic order due to symmetrical exchange striction stimulating the crystal distortions.
The magnetoelectric effects are also realized in composites that combine coupled electric and magnetic dipoles [ 7 ]. The coupling mechanism typically involves piezoelectric and magnetostrictive interactions. In the present work we are dealing with the electrical dipoles induced in ferromagnetic wires by a high-frequency electric current generated by the electromagnetic field. Theoretically, this is explained by solving the scattering problem at a finite length ferromagnetic wire with the impedance boundary conditions imposed at the wire surface [ 8 , 17 ]. This requires a well-defined magnetic anisotropy of a small magnitude which can be established by various annealing treatments [ 18 ].
In the case of amorphous alloys with a near-zero magnetostriction the induced anisotropy can be tuned through the magnetoelastic interactions [ 19 , 20 , 21 , 22 ] to achieve large stress-MI. Since the wire polarization depends on a magnetic field the scattering of electromagnetic waves by such wires can be modulated with a low-frequency magnetic field. Then, the scattered signal from a single microwire may be easily detected with lock-in techniques. This was proposed for embedded wireless sensing applications [ 9 , 23 , 24 ]. At certain conditions the amplitude of the modulated signal sensitively depends on the measured parameters such as an applied magnetic field and a mechanical stress.
The other range of applications is related to composite materials containing MI-wires. The effective permittivity of composites also depends on the wire magnetic configuration. Such composites are of interest as tunable electromagnetic materials [ 25 , 26 ]. The difference is due to the retarding effects. In the case of a moderate skin effect, the radiation losses are small.
Then, the solution of Eq. The zero approximation corresponds to neglecting the radiation losses putting the imaginary parts to zero. In this case, a simplified analytical solution can be obtained which gives a reasonable approximation if the skin effect is not very strong. Magnetic structure and tuning parameters for amorphous wires with a helical anisotropy. Typically, the main contribution to the magnetic anisotropy comes from the magnetoelastic interactions.
The short-range anisotropy may be enhanced or modified by annealing in the presence of a magnetic field or stress [ 21 , 22 ]. Influence of the external tensile stress on hysteresis loops of amorphous glass-coated microwires. The external stress contributes towards the circular anisotropy. The external stress contributes towards the axial anisotropy. Impedance characteristics of Co The measurement was done with the help of Hewlett-Packard E vector network analyser.
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LUH Campus collection. There is an Open Access version for this licensed article that can be read free of charge and without license restrictions. In composite films, however, the different phases can be combined at the atomic level, and thus interface losses could be reduced significantly. By combining different phases with similar crystal lattices, epitaxial or superlattice composite films can also be designed, which facilitates the understanding of ME coupling at the atomic scale.
The renaissance of multiferroic ME films has recently been accelerated by advances in thin-film growth techniques, such as the pioneering work of Zheng et al. The new growth techniques have provided routes to novel structures and phases, and allow the properties of traditional functional materials to be modified by strain engineering. Improved theory has aided in the design of new multiferroics, and helped our understanding of the coupling between magnetic and ferroelectric orders. The number of studies devoted to research on multiferroic ME films has increased remarkably in the last five years.
Recently, Nan et al. In the present review, the focus is on the experimental and theoretical progress in multiferroic ME films and nanostructures from the perspective of the ME effect, that is, magnetic field control of electric polarization and electric control of magnetization. Finally, we discuss the applications of the nanostructured multiferroic ME composites and summarize the challenges and promising future for research and technology in this field.
The notation , , , etc.
For example, a particulate composite is composed of single-phase particles denoted by 0 embedded in a matrix of another phase denoted by 3 , as shown in Figure 2. The common connectivity schemes examined so far include particulate films, horizontal heterostructures, and vertical heterostructures of multiferroic ME composites consisting of ferroelectric and magnetic phases. Schematic illustration of three kinds of ME composite nanostructures with common connectivity schemes: a particulate nanocomposite films with magnetic particles 0 embedded in a ferroelectric film matrix 3 ; b horizontal heterostructure with alternating ferroelectric 2 and magnetic 2 layers, or simply a ferroelectric or magnetic thin film grown on a magnetic or ferroelectric substrate; and c vertical heterostructure with one-phase nanopillars 1 embedded in a matrix of another phase 3.
In composites Figure 2 a , a magnetic phase is generally distributed in a ferroelectric matrix. For example, by employing a sol—gel method, Wan et al. Zhong et al. Both films exhibited ferroelectric and ferromagnetic behaviors, and ME coefficients were measured in both studies. Liu et al.
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However, as judged from the polarization—electric field hysteresis loops of the composite films, which do not exhibit a well-saturated shape, together with the influence of large leakage current, further confirmation is needed to determine whether the magnetic field-induced electric polarization change in fact reflects ME coupling in the composite films. Murugavel et al. This might be due to lattice clamping effects by the substrate.
Pan Stanford Publishing - Magnetoelectric Composites
The sign of the ME coefficient is reversed between the two oppositely poled states, which demonstrates that the ME signals originate from ferroelectric domains with an inversion symmetry-breaking relationship. Horizontal nanostructures Figure 2 b consisting of alternating layers of a ferroelectric perovskite and magnetic spinel usually exhibit only weak ME effects due to a large in-plane constraint from the substrate 9. The ME effect could, however, be comparable to that in the bulk counterparts.
Horizontal nanostructures are easy to fabricate and the leakage problem found in and composites may not apply due to the blocking of current flow by resistive ferroelectric layers. A direct ME effect could be observed in these materials. Due to their ease of processing and integration in devices, heterostructures are the most widely investigated of the ME composite nanostructures 19 — Zhang et al. Their composite exhibited enhanced ME response, with an ME voltage coefficient of about 66 mV cm —1 Oe —1 , comparable to that for bulk ceramic composites.
To release the large clamping effect, He et al. The results showed that LNO can induce a different preferential orientation in the PZT layer, thus leading to different ferroelectric behavior. To explore the ME mechanism in the layered composite films thoroughly and quantitatively, more work on the influence of constraint stress on the ME effect is needed. Li et al. The magnetostriction of the CFO layer generates stress that is transferred mechanically to the PZT layer, resulting in Raman mode changes.
The observed magnetic-field-induced softening of the soft mode in the PZT layer bears a resemblance to direct ME output in the bilayers, providing evidence for a strain-mediated ME coupling mechanism in the multiferroic bilayers. Similar research on layered ME nanostructures has been conducted by many groups. The ME coefficients in this composite films range from 3 to 30 mV cm —1 Oe —1. Both the Pr 0. A special, simple horizontal heterostructure has recently been constructed by growing an epitaxial ferroelectric PZT film on a manganite single crystal of LSMO 28 , and conversely, by growing an epitaxial magnetic LaCa Sr MnO 3 film on a ferroelectric single crystal e.
The efficient mechanical coupling across the interface results in a remarkable ME effect in such simple horizontal heterostructures. For example, Wu et al. A strong temperature-dependent ME effect was observed in this composite as a result of the magnetostrictive properties of the manganite crystal.
Wang et al. The ME coefficient of the CFO-based composite was about 38 mV cm —1 Oe —1 , which is one order of magnitude lower than that for epitaxial films grown on carefully selected single-crystal substrates. Thus, the improvement in strain transfer efficiency across the interface between the two phases is a key issue. As a result, the constitutive equations for the coupling magnetic—mechanical electric interactions in the nanostructured films can be expressed using a direct notation for tensors, as follows 9.
In contrast, the horizontal heterostructures Figure 2 b show much weaker ME coupling assuming a complete in-plane constraint effect. The phase-field model 34 , which accounts for elastic energy in the constrained thin film, including the effect of the free film surface and the constraint of the substrate, has recently been developed for study of the ME coupling effect in nanocomposite thin films In addition to magnetic-field control of electric polarization as discussed above, the converse ME effect — electric-field control of magnetization — is also of importance.