V.I. Zverev

The maximum possible magnetocaloric ΔT effect

The current boom of research activity in magnetocaloric materials science is fuelled by the expectation that new advanced refrigerants may be found whose ΔT will significantly surpass that of gadolinium (Gd) metal (2.6–2.9 K/T). Because of this expectation, the main effort in the field has been diverted from the important issues of refrigerator design to the routine characterization of magnetic materials. Estimating the maximum adiabatic temperature change that can be achieved in principle by applying a certain magnetic field, say 1 T, is a matter of priority. In this work the problem of maximum ΔT is approached from general principles. According to the most optimistic estimates, ΔT can never exceed ∼18 K/T, the more realistic upper limit lying somewhere in high single figures. We therefore deem it most unlikely that a refrigerant much better than Gd, in respect of the ΔT value, will ever be found.

Field dependence of the adiabatic temperature change in second order phase transition materials: Application to Gd

The field dependence of the adiabatic temperature change ΔTad of second order phase transition materials is studied, both theoretically and experimentally. Using scaling laws, it is demonstrated that, at the Curie temperature, the field dependence of ΔTad is characterized by H1/Δ. Therefore, as the magnetic entropy change ΔSM follows a H(1−α)/Δ power law, these two dependencies coincide only in the case of a mean field model. A phenomenological construction of a universal curve for ΔTad is presented, and its theoretical justification is also given. This universal curve can be used to predict the response of materials in different conditions not available in the laboratory (extrapolations in field or temperature), for enhancing the resolution of the data and as a simple screening procedure for the characterization of materials.

Influence of structural defects on the magnetocaloric effect in the vicinity of the first order magnetic transition in Fe50.4Rh49.6

The large magnetocaloric effect (MCE), which accompanies the first order ferromagnetic/anti-ferromagnetic transition in CsCl-ordered Fe-Rh alloys, has been investigated by measurements in slowly cycled magnetic fields of up to 2 T in magnitude for a range of temperatures, 300 K < T < 350 K. A bulk sample with composition Fe50.4Rh49.6 was used and the results were compared with those produced by the ab-initio density functional theory-based disordered local moment theory of the MCE. The measurements revealed an irreversibility effect in which the temperature of the material did not return to its initial value following several cycles of the magnetic field. These observations were explained in the framework of the ab-initio theory for the first order transition in which the consequences of the incomplete long range compositional order and small compositional inhomogeneities of the sample were included. The mean value of the long range order parameter S used in the theoretical work was 0.985, close to the va...

Magnetic and magnetothermal properties and the magnetic phase diagram of high purity single crystalline terbium along the easy magnetization direction

The magnetic and magnetothermal properties of a high purity terbium single crystal have been re-investigated from 1.5 to 350 K in magnetic fields ranging from 0 to 75 kOe using magnetization, ac magnetic susceptibility and heat capacity measurements. The magnetic phase diagram has been refined by establishing a region of the fan-like phase broader than reported in the past, by locating a tricritical point at 226 K, and by a more accurate definition of the critical fields and temperatures associated with the magnetic phases observed in Tb.

Magnetic and magnetothermal properties, and the magnetic phase diagram of single-crystal holmium along the easy magnetization direction.

The magnetic and magnetothermal properties of holmium single crystal have been investigated from 4.2 to 300 K in magnetic fields up to 100 kOe using magnetization and heat capacity data measured along the easy magnetization direction, which is the crystallographic b-axis, i.e. [112¯0] direction. The magnetic phase diagram of Ho has been refined by examining data measured using a high purity single crystal.

Magnetically guided capsule endoscopy

Wireless capsule endoscopy (WCE) is a powerful tool for medical screening and diagnosis, where a small capsule is swallowed and moved by means of natural peristalsis and gravity through the human gastrointestinal (GI) tract. The camera-integrated capsule allows for visualization of the small intestine, a region which was previously inaccessible to classical flexible endoscopy. As a diagnostic tool, it allows to localize the sources of bleedings in the middle part of the gastrointestinal tract and to identify diseases, such as inflammatory bowel disease (Crohns disease), polyposis syndrome, and tumors. The screening and diagnostic efficacy of the WCE, especially in the stomach region, is hampered by a variety of technical challenges like the lack of active capsular position and orientation control. Therapeutic functionality is absent in most commercial capsules, due to constraints in capsular volume and energy storage. The possibility of using body-exogenous magnetic fields to guide, orient, power, and operate the capsule and its mechanisms has led to increasing research in Magnetically Guided Capsule Endoscopy (MGCE). This work shortly reviews the history and state-of-art in WCE technology. It highlights the magnetic technologies for advancing diagnostic and therapeutic functionalities of WCE. Not restricting itself to the GI tract, the review further investigates the technological developments in magnetically guided microrobots that can navigate through the various air- and fluid-filled lumina and cavities in the body for minimally invasive medicine.

Magnetocaloric Effect: From Theory to Practice

This article reviews the recent progress in magnetocaloric effect (MCE) studies. One of the most important aspects in design of MCE-based equipment is the correct choice of the magnetocaloric material. The last decades have shown the real ‘boom’ in research activity and the attempts to find the ‘best’ MCE material which reveals the highest value of the effect. Here we point out the importance of theoretical models? development which demonstrate the interactions between magnetic and structural subsystems of the magnetic material in the vicinity of magnetic phase transitions.