A Novel Spinel Ferrite-Hexagonal Ferrite Composite for Enhanced Magneto-Electric Coupling in a Bilayer with PZT.

The magnetoelectric effect (ME) is an important strain mediated-phenomenon in a ferromagnetic-piezoelectric composite for a variety of sensors and signal processing devices. A bias magnetic field, in general, is essential to realize a strong ME coupling in most composites. Magnetic phases with (i) high magnetostriction for strong piezomagnetic coupling and (ii) large anisotropy field that acts as a built-in bias field are preferred so that miniature, ME composite-based devices can operate without the need for an external magnetic field. We are able to realize such a magnetic phase with a composite of (i) barium hexaferrite (BaM) with high magnetocrystalline anisotropy field and (ii) nickel ferrite (NFO) with high magnetostriction. The BNx composites, with (100 - x) wt.% of BaM and x wt.% NFO, for x = 0-100, were prepared. X-ray diffraction analysis shows that the composites did not contain any impurity phases. Scanning electron microscopy images revealed that, with an increase in NFO content, hexagonal BaM grains become prominent, leading to a large anisotropy field. The room temperature saturation magnetization showed a general increase with increasing BaM content in the composites. NFO rich composites with x ≥ 60 were found to have a large magnetostriction value of around -23 ppm, comparable to pure NFO. The anisotropy field HA of the composites, determined from magnetization and ferromagnetic resonance (FMR) measurements, increased with increasing NFO content and reached a maximum of 7.77 kOe for x = 75. The BNx composite was cut into rectangular platelets and bonded with PZT to form the bilayers. ME voltage coefficient (MEVC) measurements at low frequencies and at mechanical resonance showed strong coupling at zero bias for samples with x ≥ 33. This large in-plane HA acted as a built-in field for strong ME effects under zero external bias in the bilayers. The highest zero-bias MEVC of ~22 mV/cm Oe was obtained for BN75-PZT bilayers wherein BN75 also has the highest HA. The Bilayer of BN95-PZT showed a maximum MEVC ~992 mV/cm Oe at electromechanical resonance at 59 kHz. The use of hexaferrite-spinel ferrite composite to achieve strong zero-bias ME coupling in bilayers with PZT is significant for applications related to energy harvesting, sensors, and high frequency devices.

Structure-property correlations and scaling in the magnetic and magnetocaloric properties of GdCrO3particles.

The structure, magnetic, and magnetocaloric (MC) properties of orthorhombic nanocrystalline GdCrO3with six particle sizes: ⟨d⟩ = 87, 103, 145, 224, 318, and 352 nm are reported. The particle size was tailored by annealing under different temperatures and estimated by scanning electron microscopy. With increase in ⟨d⟩, Goldschmidt tolerance factort, orthorhombic strains, and out-of-plane Cr-O1-Cr bond angle first decrease, reaching minimum values for ⟨d⟩ = 224 nm, and then increase for sample with ⟨d⟩ = 318 nm and 352 nm, thus showing a V-shaped variation. Temperature dependence of the magnetization (M) reveals an antiferromagnetic transition atTNCr∼168K for ⟨d⟩ ⩾ 224 nm andTNCr∼167K for ⟨d⟩ < 224 nm and an essentiallyd-independent spin-reorientation atTSR= 9 K.Mmeasured at 5 K and 7 T first increases with increase in ⟨d⟩, reaching maximum value for sample with ⟨d⟩ = 224 nm, and then decreases for samples with ⟨d⟩ = 318 nm and 352 nm, showing an inverted-V variation with ⟨d⟩. Similar ⟨d⟩-dependence is observed for the magnetic entropy change (MEC) and relative cooling power (RCP) showing a close relationship between the structural and magnetic properties of GdCrO3nanoparticles investigated here. The 224 nm sample with the minimum values oft,s, and Cr-O1-Cr bond angle exhibits the maximum value of MEC (-ΔS) = 37.8 J kg-1 K-1at 5 K under a field variation (ΔH) of 7 T and its large estimated RCP of 623.6 J Kg-1is comparable with those of typical MC materials. Both (-ΔS) and RCP are shown to scale with the saturation magnetizationMS, suggesting thatMSis the crucial factor controlling their magnitudes. Assuming (-ΔS) ∼ (ΔH)n, the temperature dependence ofnfor the six samples are determined,nvarying between 1.3 at 5 K ton= 2.2 at 130 K in line with its expected magnitudes based on mean-field theory. These results on structure-property correlations and scaling in GdCrO3suggest that its MC properties are tunable for potential low-temperature magnetic refrigeration applications.