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

In this paper, we present a detailed study on the magnetocaloric effect (MCE) and the critical behavior of La2NiMnO6 (LNMO) nanocrystals. Cubic-structured LNMO samples with different crystallite sizes $(d = 200$ to 26.4 nm) were prepared by a combination of solid-state reaction and mechanical milling with milling time $({t_{m}})$ changing from 0 to 30 min. The results show that all the samples undergo the second-order phase transition. Using the modified Arrott plot and critical isotherm analysis methods, we have analyzed the critical behavior for nanocrystals to obtain the optimal values of the critical parameters ( $\beta, \gamma, \delta$ , and ${T_{C}}$ ). The results indicate that short- and long-range ferromagnetic interactions coexist in the samples. In addition, we also assess the MCE of the samples via the magnetic entropy change ( ${\Delta {S_{M}}}$ ). Based on the obtained ${\Delta {S_{m}} (T, H)}$ data, we have also described the universal master curve for ${\Delta {S_{M}}}$ which as a function of ${\Delta {S_{M}} (T)/ \Delta {{S_{M}}^{P}}}$ versus rescaled temperature ${\theta = (T - {T_{C}})/({T_{r}} - {T_{C}})}$ where ${T_{r}}$ is the reference temperature. Interestingly, all the ${\Delta {S_{M}} (T, H)^{H}}$ data points are collapsed into a universal curve in the whole temperature range.

Critical Behavior and Magnetocaloric Effect in La 2 NiMnO 6 Nanocrystals

On the basis of chemical properties of the elements (Mn, Fe, Co, etc.) contained in the pinned layers of spin valves (SVs) with oxide layers (OXLs), we attempted to clarify the effect of reaction mechanism of these elements on the thermal properties of SVs. The thermal degradation of MMn-based (M=Fe, Co, Ir, Pt, etc.) specular SVs at high temperatures (> 250°C) is thought to be caused by the diffusion of the Mn component in the structure. The role of Mn oxide as a diffusion barrier of Mn migration was clarified by the secondary-ion-mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS) depth profile analyses. Mn diffusion in the free layer was inhibited at up to 300°C in a SV with an OXL, where MnO formation occurs in the OXL, which is confirmed by the MnO 2p3/2 and 2p1/2 peaks. As Mn diffused from FeMn (without an OXL), there appeared to be no significant oxide formation in the pinned layer. The details of the possible reaction mechanism concerning the chemical elements that exist in the OXLs can be understood by utilizing the standard Gibbs free energy change (ΔG°) as a function of temperature for all chemical elements (Co, Fe, Mn, etc.).

Reaction mechanism of chemical elements (Co, Fe, Mn) existing in spin valves containing oxide layers

https://ieeexplore.ieee.org/iel5/651/4488/00696321.pdf

Temperature dependence of the magnetization of nanocrystalline Fe/sub 02.5/CoNb/sub 3/Cu/sub 1/Si/sub 03.5/Be alloey

The Ca0.85Pr0.15Mn1-yRuyO3 (y = 0, 0.04, 0.08, 0.12, 0.16, and 0.20) manganate thin films were prepared by pulsed laser deposition (PLD) technique. The X-ray diffraction (XRD) analysis revealed that the samples were single-phased with orthorhombic structure. The scanning electron microscopy (SEM) images indicated that the samples were composed of homogeneous grains. The Hall-effect measurements showed that the carrier density and Hall mobility of films increased with increasing Ru-doped content. The magnetic field dependence of magnetization at various temperatures were measured by using the superconducting quantum interference device (SQUID) and showed that the increase of Ru-doping content induced the large ΔS change in broad range of temperature. This demonstrates a possible application of these materials in cooling devices as the relative cooling power (RCP) is proportial to -(ΔS)maxΔTFWHM (where ΔTFWHM is the full width at half-maximum of ΔS).

Properties of Ru-Doped Ca-Pr Manganate Thin Films Fabricated by PLD Technique

The spin wave excitation has been analyzed in a Co2MnSi solid solution compound. A Co2MnSi sample was prepared by utilizing a mechanical alloying technique in Ar atmosphere. After 72 hours of milling time, the Co2MnSi solid solution was transformed from a multiphase to a single broadening phase, which is essential in understanding the spin wave excitation in a nanocrystalline material. The magnetization was measured from 7 K to room temperature by using a superconducting quantum interference device (SQUID) magnetometer. The thermal-magnetization curve was found to obey the Bloch law, M
 S(T)=M
 S(0)(1−BT
 3/2−CT
 5/2). Based on this formula, the spin wave stiffness constant was calculated from the magnetization data at low temperatures. The values were 0.264 eV⋅A2 and 0.325 eV⋅A2 for 72 and 96 hours of milling time, respectively. The spectroscopic splitting g-factor was obtained via electron spin resonance (ESR) measurements at X-band (9.45 GHz).

Seong-Cho Yu

Papers

Critical Behavior and Magnetocaloric Effect in La 2 NiMnO 6 Nanocrystals

Reaction mechanism of chemical elements (Co, Fe, Mn) existing in spin valves containing oxide layers

Temperature dependence of the magnetization of nanocrystalline Fe/sub 02.5/CoNb/sub 3/Cu/sub 1/Si/sub 03.5/Be alloey

Properties of Ru-Doped Ca-Pr Manganate Thin Films Fabricated by PLD Technique

Spin Wave Excitation in a Mechanically Alloyed Co2MnSi Solid Solution