Hydrostatic pressure to trigger and assist magnetic transitions: baromagnetic refrigeration

The possible application of the barocaloric effect to produce solid state refrigerators is a topic of interest in the field of applied physics. In this work, we present experimental data about the influence of external pressure on the magnetic properties of a manganite with phase separation. Using the Jahn Teller effect associated with the presence of the charge ordering we were able to follow the transition to the ferromagnetic state induced by pressure. We also demonstrated that external pressure can assist the ferromagnetic state, decreasing the magnetic field necessary to generate the magnetic transition.

The possible application of the barocaloric effect to produce solid state refrigerators is a topic of interest in the field of applied physics. In this work, we present experimental data about the influence of external pressure on the magnetic properties of a manganite with phase separation. Using the Jahn Teller effect associated with the presence of the charge ordering we were able to follow the transition to the ferromagnetic state induced by pressure. We also demonstrated that external pressure can assist the ferromagnetic state, decreasing the magnetic field necessary to generate the magnetic transition.
Conventional vapor-cycle refrigerators are based on the barocaloric effect, heating and cooling a fluid by the application of an external pressure. The main drawback of this technology is the necessity of a fluid with large temperature change and with good heat transfer properties, simultaneously 1 . These limitations reduce the available compounds to a small number of gases that are harmful to the environment.
The ever growing demand for refrigeration has motivated intense research into the development of new alternatives to replace existing technology. Magnetic refrigeration has appeared as one of the environmental friendly possibilities.
Magnetic refrigeration is based in the magnetocaloric effect 2 3 and its temperature change is produced by the application of an external magnetic field on a magnetic refrigerant material. The independence between the refrigerant material and the heat exchanger allows the possibility of using nonhazardous fluids such as water or noble gases.
Despite of the great advances observed in the development of prototypes of room temperature magnetic refrigerators 4 , the massive use of this technology in the near future remains uncertain. The challenges that must be overcome are related both to the design of the prototypes and to the selection of the magnetic material 5 . From the point of view of the design, the aspect requiring improvement is the characteristic time related to the heat exchange, which acts as a limitation on the minimum duration of a cycle.
Another limitation is the necessity of a high magnetic field to induce a temperature change in the magnetic material 6 . As a consequence, the efficiency of these prototypes is still not high enough to allow general deployment of the technology.
The idea of cooling by adiabatic application of pressure has already been proposed 7 and reported for the Pbnm to R3c structural transition observed in Pr 1-x La x NiO 3 . A few years ago, Mañosa and co-workers 8 reported a large barocaloric effect in a Ni-Mn-In alloy at room temperature, obtaining values that compare with those obtained in giant magnetocaloric materials. This discovery triggered the idea to use the barocaloric effect in solid state refrigeration combined with the application of a fixed magnetic field to generate a magnetic moment that could be controlled by the application of external pressure 9 10 11 . The thermodynamic cycles require a strong coupling between the magnetic and structural degrees of freedom to ensure that a large magnetization change is produced when an external pressure is applied.
The number of systems that exhibit a strong coupling between the magnetic and structural degrees of freedom is quite large, but the complexity of a system tends to increase as the efficiency grows. Manganites represent one of the most studied systems because of the rich variety of degrees of freedom coupled between them.
In this work, we present a structural and magnetic characterization of the manganite La 0.625-y Pr y Ca 0,375 MnO 3 with a Pr concentration y = 0.3. For this particular concentration, the manganite exhibits a coexistence of regions with different magnetic ordering, the so called phase separation phenomena. The coupling between the magnetic and structural degrees of freedom will be exposed and discussed as a function of the applied external pressure. We will also describe the magnetocaloric properties enhancement by the application of external pressure.
Polycristalline samples of La 0.325 Pr 0,3 Ca 0,375 MnO 3 were obtained by a liquid mix method that has been described elsewhere 12 . Magnetic measurements were performed in a VSM magnetometer model Versalab manufactured by Quantum Design. Hydrostatic pressure was applied using a high pressure cell manufactured by HMD model CC-SPr- X ray diffraction (XRD) data at different temperatures and ambient pressure were obtained using a Rigaku diffractometer. High pressure XRD measurements at room and low temperature were conducted at the beamline ID27 of the European Synchrotron Radiation Facility (ESRF). Powder samples were loaded in a large opening membrane-driven diamond anvil cell (DAC) using stainless-steel gaskets and filled with helium as the pressure-transmitting medium. The monochromatic X-ray beam at 33 keV was focused down to 3×3 μm 2 and a MARCCD 165 detector was used to collect the diffraction data. For the low-temperature experiment, the DAC was installed in a home made continuous He-flow cryostat. The pressure was increased in situ at constant T and measured using the fluorescence of a ruby sphere placed next to the sample. A fullpattern Rietveld refinement of the XRD data was done using the gsas package 13 The compound used in this work belongs to the widely studied system La 0. With these considerations in mind, we can now analyze the system behavior with temperature, magnetic field and pressure.
At ambient pressure and low magnetic field the system presents a room temperature paramagnetic insulator (PI) state followed by a CO phase around T CO = 220 K. The It is clear that the application of a magnetic field or external pressure affects the formation and stability of the CO phase. Considering that the thermodynamic cycles used for refrigeration usually require a stable temperature at the beginning of the cycle (the temperature change is produced by the pressure and not by an external temperature controller), we wanted to know whether it would be possible to suppress the CO phase once it has developed in the sample. To answer this question, we studied the pressure evolution of the structure at a fixed temperature of 150 K, in the phase separated region (see figure 2b).
As the pressure is increased we can clearly observe a jump of the lattice parameters at 0.5 GPa. The a and c axis are reduced by ~0.1% and ~0.2%, respectively, while the b axis expands by ~0.05% . At higher pressure, up to 5 GPa, the lattice parameters do not show any other modification.
To highlight the anomaly in the volume compression and to clarify the order of the transition, the cell volume is plotted in the inset Fig. 2 that can be correlated with the disappearance of the JT distortion and the CO phase.
This is consistent with the results obtained from the magnetization measurements at 150 K with different pressures presented in figure 3, where the evolution from the phase separated state to the FM state is evident in the magnetization curves.
For pressure above 0.5 GPa, the linear compressibilities along the a and b axes, k a = 0.0021(2) GPa -1 and k b =0.0022(3) GPa -1 are slightly larger than those corresponding to the c axes, k c =0.0016(1) GPa -1 , in good agreement with those reported for other manganite systems 16 17 18 .
Another interesting feature is the possibility of assisting the magnetic transition with hydrostatic pressure. In figure 3, we also present magnetization as function of magnetic field with different applied pressure at T = 220 K. We can see how the magnetic field necessary to reach an ordered FM state decreases with pressure. To quantify this phenomena, we calculated the entropy change related with the magnetocaloric effect (MCE) using a well know Maxwell relation 2 for different pressures at 220K (figure 4).
If we consider the magnetic field necessary to obtain some arbitrary entropy change (i.e.