A systematic study of magnetic properties of ${\mathrm{Ni}}_{2+x}{\mathrm{Mn}}_{1-x}\mathrm{Ga}$ $(0⩽x⩽0.19)$ Heusler alloys undergoing structural martensite-austenite transformations while in the ferromagnetic state has been performed. From measurements of spontaneous magnetization, $M_s\left(T\right)$, jumps $\Delta M$ at structural phase transitions were determined. Virtual Curie temperatures of martensite were estimated from the comparison of magnetization in martensitic and austenitic phases. Both saturation magnetic moments in the ferromagnetic state and effective magnetic moments in paramagnetic state of Mn and Ni atoms were estimated and the influence of delocalization effects on magnetism in these alloys was discussed. The experimental results obtained show that the shift of martensitic transition temperature depends weakly on composition. The values of this shift are in good correspondence with the Clapeyron-Clausius formalism taking into account the experimental data on latent heat at martensite-austenite transformations.

We present the first data and theory for the bias magnetic field dependence of magnetoelectric coupling in the electromechanical resonance (EMR) region for ferromagnetic-piezoelectric heterostructures. Trilayers of Permendur, a $\mathrm{Co}-\mathrm{Fe}-\mathrm{V}$ alloy, and lead zirconate titanate were studied. Measurements of the magnetoelectric (ME) voltage coefficient ${\alpha }_E$ indicate a strong ME coupling in the low-frequency range and a giant ME effect due to EMR at $200–300\mathrm{kHz}$ for radial modes and at $\sim 2.7\mathrm{MHz}$ for thickness modes. Data were obtained for the bias field $H$ dependence of two key parameters, the EMR frequency $f_r$ and the ME coefficient ${\alpha }_{E,R}$ at resonance. With increasing $H$, an increase in $f_r$ and a rapid rise and fall in ${\alpha }_{E,R}$ are measured. In our model we consider two mechanisms for the magnetic field influence on ME interactions: (i) a shift in the EMR frequency due to changes in compliance coefficients ($\Delta E$ effect) and (ii) variation in the piezomagnetic coefficient that manifests as a change in ${\alpha }_{E,R}$. Theoretical profiles of ${\alpha }_E$ vs frequency and estimates of frequency shift based on the $\Delta E$ effect are in excellent agreement with the data.

The strength of magnetoelectric (ME) coupling at $10\mathrm{Hz}–3\mathrm{MHz}$ has been measured in trilayers of Fe, Co, or Ni and lead zirconate titanate (PZT). The strongest ME coupling is measured for trilayers with Ni and the weakest in Co. Data on ME voltage coefficient ${\alpha }_E$ versus bias magnetic field $H$ for Fe–PZT–Fe show unique features including zero crossing and sign reversal. Measurements of frequency dependence of ${\alpha }_E$ reveal a giant ME coupling due to the electromechanical resonance at $200–300\mathrm{kHz}$ for radial modes and at $\sim 2.7\mathrm{MHz}$ for thickness modes. Theoretical estimates of field and frequency dependence of ${\alpha }_E$ are in very good agreement with the data.

A theory of the magnetoelectric effect in ferromagnetic-piezoelectric bilayer structures is considered for platelike samples. The magnetoelectric voltage coefficient is expressed through the...

The magnetoelectric effect in multilayer ferrite-piezoelectric composites has been theoretically and experimentally studied. Using the method of effective parameters, an expression for the...