The presence of a quantum-critical point (QCP) can significantly affect the thermodynamic properties of a material at finite temperatures T . This is reflected, e.g., in the entropy landscape SðT,rÞ in the vicinity of a QCP, yielding particularly strong variations for varying the tuning parameter r such as pressure or magnetic field B. Here we report on the determination of the critical enhancement of ∂S∕∂B near a B-induced QCP via absolute measurements of the magnetocaloric effect (MCE), ð∂T ∕∂BÞ S and demonstrate that the accumulation of entropy around the QCP can be used for efficient low-temperature magnetic cooling. Our proof of principle is based on measurements and theoretical calculations of the MCE and the cooling performance for a Cu 2þ -containing coordination polymer, which is a very good realization of a spin-½ antiferromagnetic Heisenberg chain-one of the simplest quantum-critical systems.quantum criticality | quantum magnetism | low-dimensional spin systems | magnetothermal effect T he magnetocaloric effect (MCE), i.e., a temperature change in response to an adiabatic change of the magnetic field, has been widely used for refrigeration. Although up until now applications have focused on cryogenic temperatures (1-3), possible extensions to room temperature have been discussed (4). The MCE is an intrinsic property of all magnetic materials in which the entropy S changes with magnetic field B. Paramagnetic salts have been the materials of choice for low-temperature refrigeration (1), including space applications (5-7), with an area of operation ranging from about one or two degrees Kelvin down to some hundredths or even thousandths degree Kelvin. Owing to their large ΔS∕ΔB values, the ease of operation, and the applicability under microgravity conditions, paramagnets have matured to a valuable alternative to 3 He-4 He dilution refrigerators, the standard cooling technology for reaching sub-Kelvin temperatures.A large MCE also characterizes a distinctly different class of materials, where the low-temperature properties are governed by pronounced quantum many-body effects. These materials exhibit a B-induced quantum-critical point (QCP)-a zero-temperature phase transition-and the MCE has been used to study their quantum criticality (8)(9)(10)(11)(12)(13)(14) or to determine their B-T phase diagrams (15)(16)(17)(18)(19). The aim of the present work is to provide an accurate determination of the enhanced MCE upon approaching a B-induced QCP both as a function of B and T and to explore the potential of this effect for magnetic cooling.Materials in the vicinity of a QCP have been of particular current interest, as their properties reflect critical behavior arising from quantum fluctuations instead of thermal fluctuations that govern classical critical points (20). Prominent examples of findings made here include the intriguing low-temperature behaviors encountered in some heavy-fermion metals, itinerant transition metal magnets (21 and references cited therein, 22), or magnetic insulators (23, 24) and the ...

We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by the Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from approximately 1 T down to approximately 1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0x10{4} ps/m at an optimum x=0.08. We find that control of the nontrivial orbital moment in the octahedral Fe sites through the Al substitution is crucial for fine-tuning the magnetic anisotropy and obtaining the conspicuously improved ME characteristics.

We present a study of the anisotropy in the superconducting energy gap in a single crystal of YNi2B2C (T(c) approximately 14.6 K) using directional point-contact spectroscopy. The superconducting energy gap at 2.7 K, when measured for I||c, is 4.5 times larger than that for I||a. The energy gaps in the two directions also have different temperature dependences. Our results support a scenario with s + g like symmetry.

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