Magnetocaloric effect: From materials research to refrigeration devices

Magnetic Heusler alloys which undergo a martensitic transition display interesting functional properties. In the present review, we survey the magnetocaloric effects of Ni-Mn-based Heusler alloys and discuss their relation with the magnetic shape-memory and magnetic superelasticity reported in these materials. We show that all these effects are a consequence of a strong coupling between structure and magnetism which enables a magnetic field to rearrange martensitic variants as well as to provide the possibility to induce the martensitic transition. These two features are respectively controlled by the magnetic anisotropy of the martensitic phase and by the difference in magnetic moments between the structural phases. The relevance of each of these contributions to the magnetocaloric properties is analysed.

https://iopscience.iop.org/article/10.1088/0953-8984/21/23/233201/pdf

Magnetocaloric effect and its relation to shape-memory properties in ferromagnetic Heusler alloys

The phenomenology of exchange bias effects observed in structurally single-phase alloys and compounds but composed of a variety of coexisting magnetic phases such as ferromagnetic, antiferromagnetic, ferrimagnetic, spin-glass, cluster-glass and disordered magnetic states are reviewed. The investigations on exchange bias effects are discussed in diverse types of alloys and compounds where qualitative and quantitative aspects of magnetism are focused based on macroscopic experimental tools such as magnetization and magnetoresistance measurements. Here, we focus on improvement of fundamental issues of the exchange bias effects rather than on their technological importance.

https://iopscience.iop.org/article/10.1088/0953-8984/23/7/073201/pdf

Exchange bias effect in alloys and compounds

https://www.cell.com/trends/microbiology/pdf/S0966-842X(09)00238-8.pdf

Evolution of new variants of Vibrio cholerae O1.

Many notions regarding the function, structure and regulation of cholera toxin expression have remained essentially unaltered in the last 15 years. At the same time, recent findings have generated additional perspectives. For example, the cholera toxin genes are now known to be carried by a non-lytic bacteriophage, a previously unsuspected condition. Understanding of how the expression of cholera toxin genes is controlled by the bacterium at the molecular level has advanced significantly and relationships with cell-density-associated (quorum-sensing) responses have recently been discovered. Regarding the cell intoxication process, the mode of entry and intracellular transport of cholera toxin are becoming clearer. In the immunological field, the strong oral immunogenicity of the non-toxic B subunit of cholera toxin (CTB) has been exploited in the development of a now widely licensed oral cholera vaccine. Additionally, CTB has been shown to induce tolerance against co-administered (linked) foreign antigens in some autoimmune and allergic diseases.

/pdf/cholera-toxin-structure-gene-regulation-and-6u0wggscp6.pdf

Cholera toxin structure, gene regulation and pathophysiological and immunological aspects

The ground-state properties of the ferromagnetic shape-memory alloy of nominal composition ${\text{Ni}}_{2}{\text{Mn}}_{1.36}{\text{Sn}}_{0.64}$ have been studied by dc magnetization and ac susceptibility measurements. Like few other Ni-Mn based alloys, this sample exhibits exchange bias phenomenon. The observed exchange pinning was found to originate right from the temperature where a steplike anomaly is present in the zero-field-cooled magnetization data. The ac susceptibility study indicates the onset of spin-glass freezing near this steplike anomaly with clear frequency shift. The sample can be identified as a reentrant spin glass with both ferromagnetic and glassy phases coexisting together at low temperature at least in the field-cooled state. The result provides us with a comprehensive view to identify the magnetic character of various Ni-Mn-based shape-memory alloys with competing magnetic interactions.

Reentrant-spin-glass state in Ni 2 Mn 1.36 Sn 0.64 shape-memory alloy

The magnetoresistance (MR) behaviour and the magnetocaloric effect (MCE) of the ferromagnetic shape memory alloy of nominal composition Ni2Mn1.36Sn0.64 have been investigated. We observe giant negative MR (~ − 48% at 90 kOe) around the martensitic transition (MT) temperature. The observed MR was found to be irreversible with respect to the applied magnetic field, particularly in the vicinity of MT. The sample exhibits a large but positive magnetocaloric effect (a peak value of 5.1 J kg−1 K−1 for the magnetic field changing from 0 to 20 kOe) around MT. A comparison with the other Ni–Mn-based shape memory alloys indicates that the observed MR scales with the magnitude of the MCE.

https://iopscience.iop.org/article/10.1088/0022-3727/42/6/065001/pdf

Giant magnetoresistance and large inverse magnetocaloric effect in Ni2Mn1.36Sn0.64 alloy

Magnetostructural instability in the ferromagnetic shape memory alloy of composition ${\mathrm{Ni}}_{2}{\mathrm{Mn}}_{1.4}{\mathrm{Sn}}_{0.6}$ is investigated by transport and magnetic measurements. Large negative magnetoresistance is observed around the martensitic transition temperature $(90--210\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Both magnetization and magnetoresistance data indicate that upon the application of an external magnetic field at a constant temperature, the sample attains a field-induced arrested state which persists even when the field is withdrawn. We observe an intriguing behavior of the arrested state that it can remember the last highest field it has experienced. The field-induced structural transition plays the key role for the observed anomaly and the observed irreversibility can be accounted for by the Landau-type free energy model for the first-order phase transition.

Metastability and magnetic memory effect in Ni 2 Mn 1.4 Sn 0.6

Summary

The ctxAB genes encoding cholera toxin, reside in the genome of a filamentous bacteriophage CTXφ. The presence of CTX prophage in non-epidemic environmental Vibrio cholerae strains is rare. The CTX prophage, the lysogenic form of CTXφ in V. cholerae, is comprised of the ‘RS2’ and the ‘Core’. Analysis of the rstR gene present in the RS2 region of the CTX prophage revealed the presence of new alleles of the prophages in four environmental non-O1, non-O139 strains VCE22 (O36), VCE228 (O27), VCE232 (O4) and VCE233 (O27), and the CTX prophages are located in the small chromosomes. Phylogenetic analysis based on the nucleotide sequences of the rstR and orfU (present in the core) genes of these prophages placed them in a single unique cluster, which is distally located compared with that of epidemic V. cholerae O1 strains. Further analysis indicated that the genome of the prophage present in the strain VCE22 is devoid of the ctxAB genes, called pre-CTX prophage and the strain also possess the toxin-coregulated pilus protein coding gene tcpA of classical type, another important pathogenicity determining locus of the epidemic V. cholerae strains. Comparative analysis of the nucleotide sequences of the rstR and orfU genes indicated that the pre-CTX prophage of VCE22 might be the progenitor of new alleles of the CTX prophages present in these environmental strains.

Molecular analysis of the rstR and orfU genes of the CTX prophages integrated in the small chromosomes of environmental Vibrio cholerae non-O1, non-O139 strains

Souvik Chatterjee

Papers

Reentrant-spin-glass state in Ni 2 Mn 1.36 Sn 0.64 shape-memory alloy

Giant magnetoresistance and large inverse magnetocaloric effect in Ni2Mn1.36Sn0.64 alloy

Metastability and magnetic memory effect in Ni 2 Mn 1.4 Sn 0.6

Molecular analysis of the rstR and orfU genes of the CTX prophages integrated in the small chromosomes of environmental Vibrio cholerae non-O1, non-O139 strains

Thermomagnetic irreversibility in Ni 2 Mn 1.36 Sn 0.64 shape-memory alloy