Direct measurements of the magnetocaloric effect in pulsed magnetic fields: The example of the Heusler alloy Ni50Mn35In15

Abstract

We have studied the magnetocaloric effect (MCE) in the shape-memory Heusler alloy Ni 50 Mn 35 In 15 by direct measurements in pulsed magnetic fields up to 6 and 20 T. The results in 6 T are compared with data obtained from heat-capacity experiments. We find a saturation of the inverse MCE, related to the first-order martensitic transition, with a maximum adiabatic temperature change of ∆T ad = −7 K at 250 K and a conventional field-dependent MCE near the second-order ferromagnetic transition in the austenitic phase. The pulsed magnetic field data allow for an analysis of the temperature response of the sample to the magnetic field on a time scale of ∼ 10 to 100 ms which is on the order of typical operation frequencies (10 to 100 Hz) of magnetocaloric cooling devices. Our results disclose that in shape-memory alloys the different contributions to the MCE and hysteresis effects around the martensitic transition have to be carefully considered for future cooling applications.PACS numbers: 75.30. Sg, 75.40.Cx, 75.60.Ej Recently, tremendous efforts have been made to find alternative technologies to replace the conventional gas compression/expansion technique for cooling applications. 1,2Higher efficiency cooling and environmental-friendly magnetic refrigeration based on the magnetocaloric effect (MCE) have initiated intensive research activity. Besides its practical application, MCE studies can give an extra insight in the properties of the magnetic phase transitions. Among the most promising materials, Heusler-type Ni-Mn-In magnetic shape-memory alloys are attractive candidates for both fundamental research and application. These alloys have been shown to exhibit diverse functional properties such as shape memory, 3,4 magnetic superelasticity, 5 magnetocaloric, 6 and barocaloric effect, 7 which originate from magnetoelastic couplings. Ni 50 Mn 35 In 15 exhibits on cooling a paramagnetic to ferromagnetic transition around 315 K, followed by a first-order structural transition from a cubic high-temperature phase to a low-temperature monoclinic phase around 246 K, the so-called martensitic transition. A conventional MCE is observed around the ferromagnetic transition in the austenitic phase. Here, an increase in the sample temperature is observed upon application of a magnetic field in adiabatic conditions. Additionally, an inverse MCE is present around the martensitic first-order transition leading to a decrease in the sample temperature with increasing field. Due to the first-order character of the latter transition, hysteresis losses and fatigue are critical issues on the search of suitable materials for magnetocaloric cooling devices. So far, most MCE studa) Electronic mail: Catalina.Salazar@cpfs.mpg.de ies have been done by calculating the isothermal entropy change, ∆S iso , based on indirect methods, however, for first-order phase transitions additional considerations should be applied.8 Direct measurements, besides giving the adiabatic temperature change, ∆T ad , which is one of the important parameters for m...

Hysteresis Design of Magnetocaloric Materials—From Basic Mechanisms to Applications

“…A rate of about 20 T s −1 can be realized by a pneumatic sample holder moving the magnetocaloric material in a static magnetic field . Fast field‐sweep rates of up to 1 kT s −1 are achieved in pulsed magnetic fields …”

“…In the following, DT ad measurements with field-sweep rates in the range between 10 mT s À1 and 1kTs À1 are compared to study the effects of different rates on the MCE in Ni 50 Mn 35 In 15 , [43,222] La(Fe,Si,Co) 13 , [223] and Mn 3 GaC. [171] The generation of magnetic-field-sweep rates in these ranges requires the use of different devices and magnets.Alow fieldsweep rateo fa pproximately 10 mT s À1 can be generated by using as uperconducting magnet.…”

“…[226] Fast field-sweep rates of up to 1kTs À1 are achieved in pulsed magnetic fields. [222] TheM CE at the martensitic magnetostructural transition in Ni 50 Mn 35 In 15 Heusler alloys is studied by direct DT ad measurements at different field-sweep-rates. [227] Figure 30 shows the comparison of DT ad as a function of the applied magnetic field.…”

Strain behavior and lattice dynamics in Ni50Mn35In15

“…Recently, we showed that Ni 50 Mn 35 In 15 exhibits an inverse magnetocaloric effect of −7 K, in a field change of 6 T, that arises from a change in the entropy due to the magneto-structural transition [5]. At 4 K the application of a 20 T field still induces the martensitic transition and, besides the change in magnetic moment, leads to a relative change in length of 0.8% [15].…”

“…Properties such as shape memory [1,2], magnetic superelasticity [3], magnetocaloric [4,5] and barocaloric effects [6] have attracted considerable attention due to their potential use in applications. However, the lattice dynamics and its coupling to magnetic properties have been studied relatively little in these alloys.…”