There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4 ,M n(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material. (Some figures in this article are in colour only in the electronic version)

https://iopscience.iop.org/article/10.1088/0034-4885/68/6/R04/pdf

Recent developments in magnetocaloric materials

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Surface plasmons (SPs) are coherent oscillations of conduction electrons on a metal surface excited by electromagnetic radiation at a metal -dielectric interface. The growing field of research on such light -metal interactions is known as ‘plasmonics’. 1-3 This branch of research has attracted much attention due to its potential applications in miniaturized optical devices, sensors, and photonic circuits as well as in medical diagnostics and therapeutics. 4-8

Nanostructured plasmonic sensors.

Review of the magnetocaloric effect in manganite materials

Nanocomposite hard magnetic materials (Nd,Dy)4.5Fe77.5B18 (No. 1) and (Nd,Dy)4.5Fe76B18Nb1.2Cu0.3 (No. 2) have been prepared by crystallizing amorphous ribbons, fabricated by single roll melt-spinning. The evolution of a multiphase structure was monitored by an x-ray diffractometer and by thermomagnetic measurement. We observed that, at annealing temperatures below 670??C, there is crystallization of soft phase Fe3B and a small amount of hard phase Nd2Fe14B. At annealing temperatures above 670??C, crystallization of ?-Fe and probably Dy2Fe14B phases with large magnetocrystalline anisotropy led to a drastic enhancement in the hard magnetic properties of the materials. The maximum value of HC is found to be 4.2?kOe for sample No. 1. For sample No. 2, with co-doping of Nb and Cu, nanostructure refinement yields a strong enhancement in exchange coupling between the component phases. Thereby, we obtained high reduced-remanence of 0.78, high remanence of 1.15 and a high (BH)max value up to 16.2?MGOe.

Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets

La0.7Ca0.3MnO3 compounds with the crystallite size of D = 45-200 nm were synthesized by utilizing the solid-state reaction and mechanical ball milling methods. The reduction of the crystallite size (D) resulted in the decrease of both T
 C and magnetization, broadening of the phase transition region, and appearance of Griffiths phase features in the materials. The temperature dependence of saturation magnetization (M
 S), the variations of the coercivity (H
 C), and the deviation of magnetization from expected Bloch’s law T
 3/2 for these samples were also investigated. Based on the core-shell model, the core diameter and the nonmagnetic shell thickness for the samples were determined.

Effect of Crystallite Size on the Thickness of Nonmagnetic Shell and Magnetic Properties of La0.7Ca0.3MnO3

We have based on isothermal magnetization curves to study the magnetocaloric effect (MCE) in fine-grained perovskite manganites of La0.7Cd0.3MnO3 (LCMO), La0.7Ba0.3MnO3 (LBMO), and Nd0.7Sr0.3MnO3 (NSMO) prepared by conventional solid-state reaction. Magnetic measurements were performed on a vibrating sample magnetometer, with a temperature increment of 1.0 K, and the applied field in the range of 0–1.8 T. Under an applied field of 1.8 T, the maximum magnetic-entropy change obtained for LCMO, LBMO, and NSMO taking place at their Curie temperature are about 2.3, 2.1, and 5.1 J/(kg⋅K), respectively. The large entropy change in NSMO makes it suitable for magnetic-cooling applications.

Magnetocaloric Effect in La0.7Cd0.3MnO3, La0.7Ba0.3MnO3, and Nd0.7Sr0.3MnO3

In this paper, we investigate in detail influences of A-site substitution on the EPR parameters for La/sub 0.7/Sr/sub 0.3/MnO/sub 3/ (LSM) and La/sub 0.7/Ba/sub 0.3/MnO/sub 3/ (LBM) compounds by means of electron paramagnetic resonance (EPR) study. Magnetization and resistivity measurements were also performed. The Curie temperature T/sub C/ is 365 and 322 K for LSM and LBM, respectively. In the whole temperature range studied, metallic behavior was observed. The minimum linewidth /spl Delta/H/sub min/ is 402 and 378 Oe for LSM and LBM, respectively. Temperature dependences of the EPR linewidth /spl Delta/H/sub pp/(T) for the samples were analyzed by using the model of adiabatic hopping motion of small polarons, /spl Delta/H/sub pp/(T)=/spl Delta/H/sub 0/+(A/T)exp(-E/sub a//k/sub B/T). The activation energies E/sub a//sup (1)/ and E/sub a//sup (2)/, obtained from /spl Delta/H/sub pp/(T) and EPR intensity I(T) data, are different. At temperatures T>T/sub min/, the Lande factor of both samples is the same (g/spl ap/2), indicating that spin-spin interactions among electrons of Mn/sup 3+/ and Mn/sup 4+/ ions play a main role.

Influence of A-site substitution on the EPR parameters of La/sub 0.7/A/sub 0.3/MnO/sub 3/ (A = Sr, Ba) compounds

This work presents the magnetocaloric effect (MCE) and critical behaviors of bulk Ni0.5Mn0.5-xSnx alloys (x=0.14 and 0.15) prepared by an arc-melting method. Under an external field of 50 kOe, we have observed the coexistence of the inverse MCE with a maximum magnetic entropy change (ΔSmax) of 1.39 J·kg-1·K-1 at 218 K (associated with the martensitic-austenitic transition), and normal MCE with a minimum magnetic entropy change (ΔSmin) of -4.97 J·kg-1·K-1 at 308 K (associated with the ferromagnetic-paramagnetic transition in the austenitic phase) in x=0.14 . Meanwhile, the magnetic entropy changes for x=0.15 keeps normal negative, with ΔSmin=-4.55 J·kg-1·K-1 at 315 K. Interestingly, the ferromagnetic-paramagnetic transition in the austenitic phases for both alloys is found to be second-order type. Having used the Arrott-Noakes method, the critical exponents obtained for x=0.14 are β = 0.451 and γ = 1.217, and for x=0.15 are β = 0.501 and γ = 0.963, which are close to those expected for the mean-field model (β = 0.5 and γ = 1). This reveals that a long-range-order ferromagnetic interaction dominates in the austenitic phase. A small deviation from the theoretical values, particularly for x=0.14, is assigned to magnetic inhomogeneity induced by the martensitic-austenitic transition.

Seong-Cho Yu

Papers

Crystalline evolution and large coercivity in Dy-doped (Nd,Dy)2Fe14B/α-Fe nanocomposite magnets

Effect of Crystallite Size on the Thickness of Nonmagnetic Shell and Magnetic Properties of La0.7Ca0.3MnO3

Magnetocaloric Effect in La0.7Cd0.3MnO3, La0.7Ba0.3MnO3, and Nd0.7Sr0.3MnO3

Influence of A-site substitution on the EPR parameters of La/sub 0.7/A/sub 0.3/MnO/sub 3/ (A = Sr, Ba) compounds

Magnetocaloric and Critical Behaviors of ${\rm Ni}_{0.5}{\rm Mn}_{0.5-{\rm x}}{\rm Sn}_{\rm x}$ Heusler Alloys