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Magnetocaloric properties of TbCrO3 and TmCrO3 are reported and compared with those of the previously reported rare-earth chromites RCrO3 (R = Gd, Dy, Ho, and Er) and other perovskite-type oxides. The samples of TbCrO3 and TmCrO3 in this work were synthesized using a citrate gel combustion technique, and their magnetic properties were investigated and compared with those reported previously on RCrO3 (R = Gd, Dy, Ho, and Er). The Cr3+–Cr3+ ordering temperatures were found to strongly depend on the ionic radii of the rare-earth. By fitting the dc magnetization data with modified Curie–Weiss law including the Dzyaloshinsky–Moriya antisymmetric exchange interaction (D) and the symmetric exchange constant Je, spin canting angles (α) were obtained. In general, α was found to increase with the decreasing ionic radii of R3+ in RCrO3. The magnetocaloric properties investigated included the magnetic entropy change (−ΔS) for a given change in magnetic field (ΔH), the corresponding adiabatic temperature change (ΔTad), and their relative variations (ΔTad/ΔH) and (−ΔS/ΔH). It is observed that for RCrO3, (−ΔS) measured in the vicinity of the ordering temperature of R3+–R3+, varies almost as G2/3 where G is the de Gennes factor. Among RCrO3, GdCrO3 shows the largest value of (−ΔS/ΔH), because of its largest G factor and its magnitudes of (ΔTad/ΔH) and (−ΔS/ΔH) compare well with the reported values for the perovskites GdFeO3 and EuTiO3. These comparisons presented here provide useful information on the potential use of these materials in magneto-refrigeration technology. 

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.