A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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Caloric materials near ferroic phase transitions

Magnetocaloric effect: From materials research to refrigeration devices

Theoretical aspects of the magnetocaloric effect

www.MaterialsViews.com C O M M Novel Design of La(Fe,Si) 13 Alloys Towards High Magnetic Refrigeration Performance U N IC By Julia Lyubina , * Rudolf Schäfer , Norbert Martin , Ludwig Schultz , and Oliver Gutfl eisch A IO N Magnetic refrigeration near room temperature has great potential to become an energy-effi cient alternative to the conventional vapor-compression technology. [ 1–3 ] Attractive candidates for use as magnetic refrigerants are materials that undergo a fi rstorder magnetic phase transition, [ 4–7 ] which is accompanied by a change of crystal symmetry and/or volume. The latter leads inevitably to fracture during thermal and magnetic fi eld cycling and is a signifi cant challenge for the integration of these materials in the emerging magnetic cooling technology. [ 8 ] Here, we present a novel approach to controlling the magnetocaloric effect (MCE) in these materials by introducing porosity. In particular, we show that LaFe 13 − x Si x magnetic refrigerants with porous architecture do not degrade mechanically during cycling, and thus maintain their excellent cooling performance. This very signifi cant improvement is attributed to partial removal of grain boundaries that restrain volume expansion in the bulk material. We demonstrate that the fi rst-order magnetic phase transition is retained in porous LaFe 13– x Si x and the volume effects lead to particle size–dependent transition parameters. These results provide insight into how size-tuning can enhance the MCE and have exciting technological implications for the development of high-performance magnetic refrigerants. Magnetic cooling relies on the reversible magnetization and demagnetization of a magnetic material (magnetic refrigerant) and the resultant change in temperature in the refrigerant material under adiabatic conditions, Δ T ad . [ 1 , 9 ] The latter phenomenon is known as the magnetocaloric effect (MCE) and is maximized at temperatures corresponding to a magnetic phase transition, for example, the Curie point in ferromagnets. Usually the Curie transition is a phase transition of second order, but there exist magnetic materials in which the transition from one magnetic (disordered or ordered) state to another is a fi rst-order phase transition. [ 10–13 ] The fi rst-order magnetic phase transition gives rise to an abrupt change of the magnetization near the transition point and this, in turn, results in a large (giant) MCE. [ 2–7 ]

Novel Design of La(Fe,Si)13 Alloys Towards High Magnetic Refrigeration Performance

We report a dysprosium(III) bis(methanediide) single molecule magnet (SMM) where stabilisation of the highly magnetic states and suppression of mixing of opposite magnetic projections is imposed by a linear arrangement of negatively-charged donor atoms supported by weak neutral donors. Treatment of [Ln(BIPMTMS)(BIPMTMSH)] [Ln = Dy, 1Dy; Y, 1Y; BIPMTMS = {C(PPh2NSiMe3)2}2−; BIPMTMSH = {HC(PPh2NSiMe3)2}−] with benzyl potassium/18-crown-6 ether (18C6) in THF afforded [Ln(BIPMTMS)2][K(18C6)(THF)2] [Ln = Dy, 2Dy; Y, 2Y]. AC magnetic measurements of 2Dy in zero DC field show temperature- and frequency-dependent SMM behaviour. Orbach relaxation dominates at high temperature, but at lower temperatures a second-order Raman process dominates. Complex 2Dy exhibits two thermally activated energy barriers (Ueff) of 721 and 813 K, the largest Ueff values for any monometallic dysprosium(III) complex. Dilution experiments confirm the molecular origin of this phenomenon. Complex 2Dy has rich magnetic dynamics; field-cooled (FC)/zero-field cooled (ZFC) susceptibility measurements show a clear divergence at 16 K, meaning the magnetic observables are out-of-equilibrium below this temperature, however the maximum in ZFC, which conventionally defines the blocking temperature, TB, is found at 10 K. Magnetic hysteresis is also observed in 10% 2Dy@2Y at these temperatures. Ab initio calculations suggest the lowest three Kramers doublets of the ground 6H15/2 multiplet of 2Dy are essentially pure, well-isolated |±15/2〉, |±13/2〉 and |±11/2〉 states quantised along the CDyC axis. Thermal relaxation occurs via the 4th and 5th doublets, verified experimentally for the first time, and calculated Ueff values of 742 and 810 K compare very well to experimental magnetism and luminescence data. This work validates a design strategy towards realising high-temperature SMMs and produces unusual spin relaxation behaviour where the magnetic observables are out-of-equilibrium some 6 K above the formal blocking temperature.

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A monometallic lanthanide bis(methanediide) single molecule magnet with a large energy barrier and complex spin relaxation behaviour

We studied polycrystalline samples of the prototype magnetocaloric system Gd5Ge4 using scanning Hall imaging and global magnetometry. The magnetic field required to complete the phase transition that is critical for magnetocaloric application is reduced by up to 20% when small fragments, each consisting of several randomly oriented crystallites, are removed from the bulk. The removal of competing strain fields from neighboring grains when the fragment is embedded in the bulk underlies the observation. We support the results by showing local Hall imaging of phase separation in the bulk. When the bulk is ground into a fine powder, the effect vanishes.

Reducing the operational magnetic field in the prototype magnetocaloric system Gd5Ge4 by approaching the single cluster size limit

We have prepared a Ni50(Mn,2%Cr)34In16 alloy by substituting Mn by Cr in a Ni50Mn34In16 alloy. A large isothermal magnetic entropy change is observed across the austenite–martensite phase transition in this new alloy for a moderate field change of 50 kOe. In comparison with the parent Ni50Mn34In16 alloy where the peak in isothermal entropy change amounts to nearly 19 J kg−1 K−1, in this Ni50(Mn,2%Cr)34In16 alloy the peak value of the isothermal entropy change is much larger, 24.4 J kg−1 K−1. Moreover, this large magnetocaloric effect in the Ni50(Mn,2%Cr)34In16 alloy occurs at ambient temperature near 294 K as compared with the temperature regime of the magnetocaloric effect being near 240 K in the parent Ni50Mn34In16 alloy. Thus the partial substitution of Mn by Cr in the Ni50Mn34In16 alloy is shown to be an effective method for enhancing the potential of the alloy system for near-room temperature applications.

Elevating the temperature regime of the large magnetocaloric effect in a Ni-Mn-In alloy towards room temperature

The magnetocaloric effect (MCE) is studied in CeFe2 and Ru-doped CeFe2 alloys with dc magnetization measurements. The pure CeFe2 compound shows a distinct peak in MCE around the paramagnetic to ferromagnetic transition. In Ru-doped CeFe2 alloys there is a further transition from the ferromagnetic to antiferromagnetic state at low temperatures. This latter magnetic transition gives rise to a relatively large inverse MCE. A comparative study of the MCE associated with two different magnetic transitions is made and the possible role of a structural transition in the inverse MCE is discussed.

https://iopscience.iop.org/article/10.1088/0022-3727/39/6/002/pdf

Magnetocaloric effect in CeFe2 and Ru-doped CeFe2 alloys

We report a detailed study of magnetoresistance in the intermetallic compounds ${\mathrm{UCu}}_{2}$${\mathrm{Ge}}_{2}$ and (${\mathrm{U}}_{0.95}$${\mathrm{Ce}}_{0.05}$)${\mathrm{Cu}}_{2}$${\mathrm{Ge}}_{2}$ in the temperature range of 4.5 to 140 K and in magnetic fields up to 50 kOe. The present measurements support the earlier conjecture drawn from the magnetic studies that the ferromagnetic to antiferromagnetic transition takes place gradually over a wide temperature range. The magnetic-field dependence of magnetoresistance shows that the low-temperature antiferromagnetic state is more fragile in ${\mathrm{UCu}}_{2}$${\mathrm{Ge}}_{2}$ than in (${\mathrm{U}}_{0.95}$${\mathrm{Ce}}_{0.05}$)${\mathrm{Cu}}_{2}$${\mathrm{Ge}}_{2}$.

Ferromagnetic-to-antiferromagnetic transition in UCu2Ge2: Magnetoresistance study.

We present a study of critical current density estimated through dc magnetization measurements in the superconducting alloys Ti60V40 and Ti70V30. The magnetization is irreversible below the irreversibility field (BIrr), which is different from the upper critical field for the alloys. Additionally, the alloys are found to exhibit a peak effect in magnetization below the upper critical field. The critical current densities of the alloys estimated from the magnetization results decrease strongly with increasing magnetic field. The pinning force density follows a universal scaling relation with respect to the magnetic field divided by the BIrr. The field dependence of the pinning force density is analyzed in terms of the size of the grains of the main β phase, the possible presence of dislocation arrays within the grains of the main phase, the presence of additional metallurgical phases, and the configuration of the grain boundaries in the system. The temperature dependence of critical current density is also analyzed within the framework of existing theories.

S. B. Roy

Papers

Reducing the operational magnetic field in the prototype magnetocaloric system Gd5Ge4 by approaching the single cluster size limit

Elevating the temperature regime of the large magnetocaloric effect in a Ni-Mn-In alloy towards room temperature

Magnetocaloric effect in CeFe2 and Ru-doped CeFe2 alloys

Ferromagnetic-to-antiferromagnetic transition in UCu2Ge2: Magnetoresistance study.

Magnetic irreversibility and pinning force density in the Ti-V alloys