High pressure laboratory

The research of High pressure laboratory was focused both on experimental studies (magnetic, transport and magnetotransport properties) of intermetallic compounds at extreme conditions and the investigation of novel magnetocaloric materials and corresponding magnetocaloric effects. The scope of the laboratory research included, among all, the design and construction of diamond anvil cell (DAC) for direct magnetization measurements under very high pressures.

In the field of magnetocaloric materials we followed a most recent trend where, due to their remarkable properties, Ni₂MnGa based Heusler alloys are often considered as promising prototypical materials for magnetic refrigeration. It has been previously shown that the magnetocaloric effect (MCE) can be substantially improved when the structural martensitic transformation and the Curie temperature of the austenitic phase are driven to coincidence by proper stoichiometric control. We have employed this coincidence by the synthesis off-stoichiometric variation of the alloy composition around the Mn-rich composition - Ni_50-xCo_xMn₃₀Ga₂₀. Here the saturation magnetization of austenite is strongly enhanced by Co with respect to the martensitic one (contrary to the Co-free alloys) and thus MCE turns from direct into inverse, i.e. a cooling is induced by a putting of the alloy into external magnetic field and vice versa. By varying the composition, it is possible to tune the critical temperatures of martensitic transitions and the saturation magnetization jumps accompanied structural transitions. The remarkable values of the magnetic properties related to MCE, i.e., the high saturation magnetization jumps, the strong field dependence of the transformation temperature, dT_A,M/dH, were discovered determined and presented, together with high positive values of the isothermal magnetic entropy change, ΔS.

Fig. 1: Evolution of physical properties of Ni-Mn-Sn compound with respect to external condition – high pressure, high magnetic field and temperature.

This possibility to switch the direct MCE into the inverse one just by off-stoichiometric variation of composition of refrigerants gives new application ideas and opens new ways of a design of refrigeration devices [2–6].

Fig. 2: The Multifunctional Measurement Instrument in configuration for direct measurement of magnetocaloric effect [1].

IDiamond anvil cell (DAC) for direct magnetization measurements under very high pressures in the commercial SQUID magnetometer (Quantum Design) has been thus designed and built considering: (i) very limited available space (9 mm diameter), (ii) the need to support large forces (~ 10 kN) necessary for generating the high pressures and the (iii) specific geometric constrains needed to handle the background signal of the pressure cell. The result of this engineering challenge is shown in Fig. 1 (let us note that there is very few high pressure facilities in the world).

Fig. 3: DAC (Diamond Anvil Pressure Cell) for the SQUID magnetometer: 1) body of the cell, 2) bottom locking nut, 3) springs, 4) piston-guide for the diamond anvils, 5) gasket, 6) diamond anvils, 7) highly polished zirconia ceramic washer with a MoS lubricant, 8) position lock for the upper spring.

Literature

1. J. Kamarád, J. Kaštil, and Z. Arnold
   Practical system for the direct measurement of magneto-caloric effect by micro-thermocouples
   Rev. Sci. Instrum. 83 (2012) 083902(1) - 083902(7).
2. S. Fabbrici, J. Kamarád, Z. Arnold, F. Casoli, A. Paoluzi, F. Bolzoni, R. Cabassi, M. Solzi, G. Porcari, C. Pernechele, F. Albertini
   From direct to inverse giant magnetocaloric effect in Co-doped NiMnGa multifunctional alloys
   Acta Mater. 59 (2011) 412 - 419.
3. G. Porcari, S. Fabbrici, C. Pernechele, F. Albertini, M. Buzzi, A. Paoluzi, J. Kamarad, Z. Arnold, and M. Solzi
   Reverse magnetostructural transformation and adiabatic temperature change in Co- and In-substituted Ni-Mn-Ga alloys
   Phys. Rev. B 85 (2012) 024414(1) - 024414(7).
4. J. Kaštil, J. Kamarád, K. Knížek, Z. Arnold, P. Javorský
   Peculiar magnetic properties of Er conditioned Ni₄₃Co₇Mn₃₁Ga₁₉ at ambient and hydrostatic pressures
   J. Alloy. Compd. 565 (2013) 134 - 138
5. J. Kamarád, F. Albertini, Z. Arnold, S. Fabbrici, J. Kaštil
   Extraordinary magnetic and structural properties of the off-stoichiometric and the Co-doped Ni₂MnGa Heusler alloys under high pressure
   Acta Mater. 77 (2014) 60 - 67
6. S. Fabbrici, G. Porcari, F. Cugini, M. Solzi, J. Kamarad, Z. Arnold, R. Cabassi, F. Albertini
   Co and In Doped Ni-Mn-Ga Magnetic Shape Memory Alloys: A Thorough Structural, Magnetic and Magnetocaloric Study
   Entropy 16 (2014) 2204 - 2222.