G. Markandeyulu

Effect of Al, Cu, Ga, and Nb additions on the magnetic properties and microstructural features of sintered NdFeB

This study describes the relative effect on the permanent magnet characteristics viz. remanence (Br), intrinsic coercivity (Hci), Curie temperature (TC), and rectangularity of the intrinsic demagnetization curve, when Al, Cu, Ga, and Nb are added individually to NdFeB. Each elemental addition causes significant improvement in Hci but the level of improvement differs from one additive element to the other. The addition of Nb is favored over other elements for realizing maximum enhancement in Hci and rectangularity of the demagnetization curve. The microstructural features of the sintered samples of NdFeB with elemental addition show the formation of a new phase, in addition to the phases (φ,η, and Nd-rich) generally found in the ternary sample. The factors influencing the permanent magnet characteristics of sintered samples are the distribution of the Nd-rich phase in the intergranular region, the size and distribution of the minor phases at the grain junctions, the formation and distribution of new phases...

Giant magnetostriction in Sm1−xNdxFe1.93 compounds

The polycrystalline Sm1−xNdxFe1.93 (x=0, 0.08, 0.12, 0.16, 0.2, 0.24, 0.32, and 0.36) compounds are found to stabilize in MgCu2-type C15 cubic Laves phase structure. The compound x=0.12 is found to exhibit giant magnetostriction value of −1572×10−6 due to the anisotropy compensation between Sm3+ and Nd3+ ions. The easy direction of magnetization (EMD) is observed towards ⟨111⟩ for the 0⩽x⩽0.20 compounds, accompanied by a rhombohedral distortion. For the x=0.32 compound, an orthorhombic distortion, with a support that the EMD is towards ⟨110⟩, is observed.

Impedance Spectroscopic Characterization of Sm and Ho Doped Ni Ferrites

We report on the impedance spectroscopic characterization of Sm and Ho doped Ni ferrite materials, namely NiO.Fe 1.925 Sm 0.075 O 3 and NiO.Fe 1.925 Ho 0.075 O 3 , to demonstrate their improved electrical properties compared to pure NiO.Fe 2 O 3 . Sm and Ho doped Ni ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO 3 and HoFeO 3 as the additional phase, respectively. Atomic force microscopy measurements indicate that the bulk grains are approximately 2―5 μm in size while the grain boundaries are thin compared to bulk grains. Frequency variation of the dielectric constant shows the dispersion that can be modeled with a modified Debyes function, which considers the possibility of more than one ion, contributing to the relaxation. The resistivity values (at 3.5 KHz) of NiO . Fe 2 O 3 , NiO . Fe 1.925 Sm 0.075 O 3 , and NiO . Fe 1.925 Ho 0.075 O 3 compounds are found to be 0.1 × 10 4 Ω m, 0.5 × 10 4 , Ω m and 0.8 × 10 4 Ω m, respectively. Impedance spectroscopic analysis indicates the different relaxation mechanisms and their variation with temperature, bulk grain and grain-boundary contributions to the electrical conductivity (Rg), and capacitance (C g ) of these materials. While the conductivity in pure NiFeO 4 is predominantly due to intrinsic bulk contribution (R g = 213 kΩ and C g = 4.5 × 10 ―8 F), NiO.Fe 1.925 R 0.075 O 3 (R = Sm,Ho) exhibits distinct grain and grain-boundary contributions to the conductivity.

Ferroelectric and ferromagnetic properties of Gd substituted nickel ferrite

Ferromagnetic and ferroelectric characteristics of Gd substituted nickel ferrite (NiO⋅Fe1.925Gd0.075O3) were investigated. The material formed in the cubic inverse spinel phase and in addition, a small amount of GdFeO3 phase was identified. A small distortion of the cubic lattice was observed upon the substitution of Fe by Gd in the B site. Substitution of Gd for Fe lowered the saturation magnetization. However, the saturation magnetostriction is seen not to change significantly by the substitution of Gd. From the temperature variation of dielectric constant measurement, the ferroelectric transition temperature was found to be 512K. The existence of the ferroelectricity was confirmed from the ferroelectric loop. The (high) dielectric constant with frequency is seen to reveal a dispersion of relaxation times.

Effect of hydrogen on the magnetic properties of Ho0.85Tb0.15Fe2 and Dy0.73Tb0.27Fe2

X‐ray powder photographs on Ho0.85Tb0.15Fe2Hx (x=0–3.1) and Dy0.73Tb0.27Fe2Hx (x=0–3.3) revealed that there is an expansion of the lattice upon hydrogenation. Magnetization measurements were performed using a PAR vibrating sample magnetometer in the temperature range 100–700 K. The total magnetic moment is found to decrease with increase of hydrogen content. The Curie temperature of the hydrides is considerably lowered, though the exact TC in hydrides could not be determined due to desorption. Both Ho0.85Tb0.15Fe2 and Dy0.73Tb0.27Fe2 do not exhibit any compensation, whereas Tcomp is observed and found to decreasse with x in the case of the Ho0.85Tb0.15Fe2Hx system, x=0.18–3.1. No compensation was observed even in the hydrides of the Dy0.73Tb0.27Fe2 system. The temperature variation of magnetization of Ho0.85Tb0.15Fe2Hx and Dy0.73Tb0.27Fe2Hx suggests the occurrence of a spin‐reorientation transition.

Magnetostriction and anisotropy compensation in TbxDy0.9−xNd0.1Fe1.93 [0.2≤x≤0.4]

Structure and magnetostriction of TbxDy0.9−xNd0.1Fe1.93 (0.2≤x≤0.4) compounds are investigated. All the compounds are found to stabilize in MgCu2-type C15 cubic Laves phase structure. The easy magnetization direction changes from ⟨100⟩(x=0.2) to ⟨111⟩(x=0.3) through an intermediate state at x=0.25. Anisotropy compensation is realized near x=0.25 for the TbxDy0.9−xNd0.1Fe1.93 compounds. The Laves phase compound Tb0.4Dy0.5Nd0.1Fe1.93 has large spontaneous magnetostriction (1670×10−6) and a low anisotropy at room temperature which could make it a good candidate material for magnetostriction applications.

Magnetoelectric effect of (100-x)BaTiO3-(x)NiFe1.98O4 (x=20-80 wt %) particulate nanocomposites

The magnetoelectric (100−x)BaTiO3–(x)NiFe1.98O4 (x=20, 40, 60, and 80 wt %) particulate composites have been prepared and the effects of size and interface are studied through microscopy measurements. Large magnetoelectric voltage coefficient (αE) values accompanied by large piezoelastic coefficient, large magnetostrictive strain coefficient, and an adequate interface contact between the magnetic and electric phases were observed in these nanocomposites. The nanocomposite with x=40 has an αE value of 252 mV cm−1 Oe−1, which is the largest value for any particulate magnetoelectric composite, based on the available open literature.

EFFECTS OF CO SUBSTITUTION ON MAGNETIC PROPERTIES OF PR3(FE1-XCOX)27.5TI1.5 (X=0-0.3)

X-ray diffraction carried out on random and oriented samples of Pr3(Fe1−xCox)27.5Ti1.5 (x=0, 0.1, 0.2, 0.3) showed that the easy magnetization direction is near the b axis for x=0, 0.2, and 0.3 and is almost along the b axis for x=0.1. Magnetic hysteresis data taken on oriented samples showed that the anisotropy field (HA) varies from 12 kOe for x=0 to 25 kOe for x=0.3 at 300 K. At 10 K, HA increases from 55 kOe for x=0.1 to 70 kOe for x=0.3. An indication of spin reorientation transition has been observed at ∼250 K in x=0.3.

Enhanced Dielectric Property of Ni Ferrite by Sm and Ho Substitution

The enhancement in the dielectric constant of NiO·Fe 1.925 Sm 0.075 O 3 and NiO·Fe 1.925 Ho 0.075 O 3 when compared to pure NiO·Fe 2 O 3 is reported. Sm- and Ho-substituted Ni ferrites crystallize in the cubic inverse spinel phase. Frequency variation in the dielectric constant shows the dispersion that can be modeled with a modified Debyes function, which considers the possibility of more than one ion, contributing to the relaxation. Temperature-dependent electrical conductivity curves exhibit two distinct regions indicative of two different types of conduction mechanisms, the small polaron and variable range hopping, at 220-300 and 160-220 K regions, respectively.

Microstructure and magnetostriction of Tb0.3Dy0.7Fe1.95 prepared under different solidification conditions by zoning and modified Bridgman techniques

An intermetallic compound of nominal composition Tb0.3Dy0.7Fe1.95 was conventionally cast in the form of cylindrical rods of 8mm diameter and directionally solidified by zoning at three different growth rates. Compounds of the same nominal composition were also directionally solidified in the form of cylindrical rods of 20mm diameter by modified Bridgman technique, at two different growth rates. The microstructure of the directionally solidified compounds has been investigated as a function of solidification rate and compared with that of conventionally cast compound. The observed microstructural features of these samples have been correlated with the magnetostriction measured on the corresponding samples. Further, by examining the longitudinal sections cut along its cylindrical axis, a correlation of microstructure with magnetostriction has been brought out for each directionally solidified sample as a function of distance from the initially solidified end to the other. It has been observed that a large ...