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

Magnetocaloric effect: From materials research to refrigeration devices

Modern society relies on readily available refrigeration. Magnetic refrigeration has three prominent advantages compared with compressor-based refrigeration. First, there are no harmful gases involved; second, it may be built more compactly as the working material is a solid; and third, magnetic refrigerators generate much less noise. Recently a new class of magnetic refrigerant-materials for room-temperature applications was discovered. These new materials have important advantages over existing magnetic coolants: they exhibit a large magnetocaloric effect (MCE) in conjunction with a magnetic phase-transition of first order. This MCE is larger than that of Gd metal, which is used in the demonstration refrigerators built to explore the potential of this evolving technology. In the present review we compare the different materials considering both scientific aspects and industrial applicability. Because fundamental aspects of MCE are not so widely discussed, we also give some theoretical considerations.

https://iopscience.iop.org/article/10.1088/0022-3727/38/23/R01/pdf

Developments in magnetocaloric refrigeration

This review encompasses the story of the Verwey transition in magnetite over a period of about 90 years, from its discovery up to the present. Despite this long period of thorough investigation, the intricate multi-particle system Fe3O4 with its various magneto-electronic interactions is not completely understood, as yet - although considerable progress has been achieved, especially during the last two decades. It therefore appeared appropriate to subdivide this retrospect into three eras: (I) from the detection of the effect to the Verwey model (1913-1947), being followed by a period of: (II) checking, questioning and modification of Verwey's original concepts (1947-1979). Owing to prevailing under-estimation of the role of crystal preparation and qualitiy control, this period is also characterized by a series of uncertainties and erroneous statements concerning the reaction order (one or two) and type of the transition (multi-stage or single stage). These latter problems, beyond others, could definitely be solved within era (III) (1979 to the present) - in favour of a first-order, single-stage transition near 125 K - on the basis of experimental and theoretical standards established in the course of a most inspiring conference organized in 1979 by Sir Nevill Mott in Cambridge and solely devoted to the present topic. Regarding the experimental field of further research, the remarkable efficiency of magnetic after-effect (MAE) spectroscopy as a sensitive probe for quality control and investigation of low-temperature (4 K Tv) into a Wigner crystal (T<Tv), describes most adequately the various low-temperature mechanisms in Fe3O4 in terms of tunnelling and variable range hopping of small polarons. On the other hand, the well-elaborated Ihle-Lorenz model, assuming a superposition of polaron-band and -hopping conductivity, is in better agreement with the high-temperature data (Tv<T<600 K).

https://iopscience.iop.org/article/10.1088/0953-8984/14/12/203/pdf

The Verwey transition - a topical review

Ferroelectrics are electro-active materials that can store and switch their polarity (ferroelectricity), sense temperature changes (pyroelectricity), interchange electric and mechanical functions (piezoelectricity), and manipulate light (through optical nonlinearities and the electro-optic effect): all of these functions have practical applications. Topological switching of pi-conjugation in organic molecules, such as the keto-enol transformation, has long been anticipated as a means of realizing these phenomena in molecular assemblies and crystals. Croconic acid, an ingredient of black dyes, was recently found to have a hydrogen-bonded polar structure in a crystalline state. Here we demonstrate that application of an electric field can coherently align the molecular polarities in crystalline croconic acid, as indicated by an increase of optical second harmonic generation, and produce a well-defined polarization hysteresis at room temperature. To make this simple pentagonal molecule ferroelectric, we switched the pi-bond topology using synchronized proton transfer instead of rigid-body rotation. Of the organic ferroelectrics, this molecular crystal exhibits the highest spontaneous polarization ( approximately 20 muC cm(-2)) in spite of its small molecular size, which is in accord with first-principles electronic-structure calculations. Such high polarization, which persists up to 400 K, may find application in active capacitor and nonlinear optics elements in future organic electronics.

Above-room-temperature ferroelectricity in a single-component molecular crystal

Single-phase multiferroic BiFeO3 powder was prepared by improved solid-state synthesis, and dense BiFeO3 ceramics were fabricated by a spark plasma sintering (SPS) process. The sintering temperature of SPS ranged from 650 to 800 °C under 50 MPa. It is found that the sample is not only single phased but also of high density when sintered at 650 °C. The dielectric constant and loss of the samples were tested between 100 Hz and 20 MHz at several temperatures. Oxygen and vacuum annealing conditions impact the dielectric behaviour of the BiFeO3 ceramics prepared by the SPS technique obviously. The piezoelectric coefficient d33 of the sample annealed in vacuum is 4.2 pC N−1 at room temperature.

https://iopscience.iop.org/article/10.1088/0022-3727/43/44/445403/pdf

Electrical properties of multiferroic BiFeO3 ceramics synthesized by spark plasma sintering

The magnetocaloric effect (MCE) of Mn AS 1− x Sb x has been examined for 0⩽ x ⩽0.3. The magnetic entropy change caused by a magnetic field of Mn AS 1− x Sb x reaches 25–30 J/K kg, which is about twice as large as those of conventional materials near room temperature. The origin of the large MCE of Mn AS 1− x Sb x is unusual magnetic behavior in the vicinity of the first-order magnetic transition.

Giant magnetocaloric effect of MnAs1-xSbx in the vicinity of first-order magnetic transition

A needle-shaped BaTi2O5 crystal was grown from a Ti-rich solution. The chemical composition of the crystal was analyzed by an electron probe micro-analyzer (EPMA). X-ray Weissenberg photographs were taken at room temperature (RT): the crystal is monoclinic with a=16.91 A, b=3.94 A, c=9.49 A, β=103.0° and Z=6. A broad and large dielectric anomaly reaching 30,000 was observed at 430°C for the dielectric constant along the b-axis. Also, spontaneous polarization was estimated to be approximately 7 µC/cm2 from the D-E loops at RT. Thus the crystal is a new ferroelectric substance with a high Curie temperature of 430°C and the space group C2 at RT.

https://iopscience.iop.org/article/10.1143/JJAP.42.L946/pdf

New Ferroelectric BaTi2O5

Single crystals of the ferroelectric BaTi2O5 and BaTiO3 were prepared from a solution of 33-mol% BaO and 67-mol% TiO2 by a rapid cooling method. The dielectric constant (e′) and dielectric loss tangent (tanδ) were measured in a wide temperature range of 10–860 K and in a frequency range of 0.1–3,000 kHz. The e′ along the b-axis of the BaTi2O5 crystal, prepared in air, shows a sharp dielectric anomaly reaching 30,000 at the ferroelectric Curie temperature of TC = 752 K. By contrast, the crystal prepared in a reducing atmosphere shows a diffuse phase transition near TC = 703 K. The values of e′ and tanδ are compared between these three crystals consisting of two kinds of BaTi2O5 and one BaTiO3.

Crystal Growth and Dielectric Properties of New Ferroelectric Barium Titanate: BaTi2O5

The longitudinal acoustic (LA) phonon of a ${\text{Ba}}_{088}{\text{K}}_{0.12}{\text{TiO}}_{2.88}{\text{F}}_{0.12}$ perovskite crystal was investigated through micro-Brillouin scattering measurements to clarify the effect of KF substitution in ${\text{BaTiO}}_{3}$ on the dynamical properties related to the ferroelectric phase transition. It was found that the LA phonon correlates with the relaxation of dipoles via the piezoelectric coupling inside the polar nanoclusters and that the relaxation has a time scale of ${10}^{\ensuremath{-}13}\text{ }\text{s}$; this relaxation is attributed to the hopping of Ti ions in the off-centered sites, even in the high-symmetry cubic phase. Upon KF substitution, the anomalies in the LA phonon become large, indicating that the fluctuation of the order parameter originating from the competition between dipolar forces and thermal agitation is enhanced. We consider that the fluctuation is enhanced by the random electric field induced by the KF substitution.

/pdf/enhanced-polarization-fluctuation-in-kf-substituted-batio-3-29jeb9t7d7.pdf

Yukikuni Akishige

Papers

Electrical properties of multiferroic BiFeO3 ceramics synthesized by spark plasma sintering

Giant magnetocaloric effect of MnAs1-xSbx in the vicinity of first-order magnetic transition

New Ferroelectric BaTi2O5

Crystal Growth and Dielectric Properties of New Ferroelectric Barium Titanate: BaTi2O5

Enhanced polarization fluctuation in KF-substituted BaTiO 3 single crystals