Magnetocalorics of singlet ground state induced moment magnets

In <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mi>f</a:mi></a:math>-electron materials like Pr or U compounds with non-Kramers states of the <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>f</b:mi></b:math> shell the ground state may be a nonmagnetic singlet due to the action of the crystalline electric field. Nevertheless, these compounds can order magnetically. They develop a ground state moment and long-range order due to the spontaneous admixture of the excited state into the ground state caused by intersite exchange. This mechanism can establish magnetic order if a control parameter exceeds a critical value defining the quantum critical point between disordered and magnetic phases. Here we investigate the magnetocaloric properties of such quantum magnets where the entropy release at low temperature is due to a competition between thermal depopulation and spontaneous order effects. We determine field and temperature dependence of specific heat for ferromagnetic and antiferromagnetic order and also calculate the adiabatic magnetocaloric and barocaloric cooling rates. As a model application we discuss the magnetic specific heat of the new induced ferromagnet <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>PrIr</c:mi><c:mn>3</c:mn></c:msub></c:math>. Furthermore, we analyze the excitation spectrum of the singlet induced moment magnets in an external field with particular emphasis on field and temperature dependence of the critical soft mode. We find that the latter is fragile and exists only at specific points in the phase diagram.