Molecular nanomagnets are considered valid candidates for magnetic refrigeration at low temperatures. Designing these materials for enhanced cooling requires the control and optimization of the quantum properties at the molecular level, in particular: spin ground state, magnetic anisotropy, and presence of low-lying excited spin states. Herein, we present the theoretical framework together with a critical review of recent results, and perspectives for future developments.

No abstract

We report ac susceptibility and continuous wave and pulsed EPR experiments performed on GdW10 and GdW30 polyoxometalate clusters, in which a Gd3+ ion is coordinated to different polyoxometalate moieties. Despite the isotropic character of gadolinium as a free ion, these molecules show slow magnetic relaxation at very low temperatures, characteristic of single molecule magnets. For T≲200  mK, the spin-lattice relaxation becomes dominated by pure quantum tunneling events, with rates that agree quantitatively with those predicted by the Prokof'ev and Stamp model [Phys. Rev. Lett. 80, 5794 (1998)]. The sign of the magnetic anisotropy, the energy level splittings, and the tunneling rates strongly depend on the molecular structure. We argue that GdW30 molecules are also promising spin qubits with a coherence figure of merit Q(M)≳50.

We critically review recent results obtained by studying the low-temperature specific heat of some of the most popular molecular magnets. Perspectives of this field are discussed as well.

Over the last few years, great interest has emerged in the synthesis and magnetothermal studies of polymetallic molecular clusters based on paramagnetic ions, often referred to as molecular nanomagnets, in view of their potential application as lowtemperature magnetic refrigerants. [1,2] What makes them promising is that their cryogenic magnetocaloric effect (MCE) can be considerably larger than that of any other magnetic refrigerant, e.g. lanthanide alloys and magnetic nanoparticles.[3] The MCE is the change of magnetic entropy (∆S m ) and related adiabatic temperature (∆T ad ) following the change of applied magnetic field and it can be exploited for cooling applications via a field removal process called adiabatic demagnetization. Although the MCE is intrinsic to any magnetic material, in only a few cases are the changes sufficiently large to make them suitable for applications. The ideal molecular refrigerant comprises the following key characteristics:[1] (i) a large spin ground state S, since the magnetic entropy amounts to Rln(2S+1); (ii) a negligible magnetic anisotropy, which permits easy polarization of the net molecular spins in magnetic fields of weak or moderate strength; (iii) the presence of low-lying excited spin states, which enhances the field dependence of the MCE due to the increased number of populated spin states; (iv) dominant ferromagnetic exchange, [3(c)] favouring a large S and hence a large field dependence of the MCE; (v) a relatively low molecular mass (or a large metal:ligand mass ratio) since the non-magnetic ligands contribute passively to the MCE. Although this last point is crucial for obtaining an enhanced effect, it has been mostly ignored to date. Molecular cluster compounds tend to have a very low magnetic density because of the large complex structural frameworks required to encase the multi-metallic core.In this communication we propose a drastically different approach by focusing on the simple and well-known ferromagnetic molecular dimer gadolinium acetate tetrahydrate, [4] [{Gd(OAc) 3 (H 2 O) 2 } 2 ]•4H 2 O (1). The structure of 1 is depicted in Figure 1 and comprises a dimer of Gd 3+ ions bridged through two of the six carboxylate groups which bond in a η 2 :η 1 :µ 2 -fashion. The remaining acetates are chelating with the nine-coordinate [capped square anti-prismatic] geometry of the metal centres being completed by the presence of two terminally bound H 2 O molecules. These partake in intra-molecular H-bonding to the neighbouring chelating acetate ligands, and are responsible for both the direct inter-molecular H-bonds in the a-b plane and the inter-plane Hbonds mediated by the lattice H 2 O molecules ( Fig. S1 and Table S1). Our theoretical and experimental investigations (see Supporting Information for details) of the magnetothermal properties of 1 down to millikelvin temperatures reveal a truly enormous MCE. In addition to magnetization and heat capacity experiments, which we employ to indirectly estimate the MCE, we make use of a homemade experimental set-up th...

The synthesis, structures, and magnetic properties of six families of cobalt-lanthanide mixed-metal phosphonate complexes are reported in this Article. These six families can be divided into two structural types: grids, where the metal centers lie in a single plane, and cages. The grids include [4 × 3] {Co(8)Ln(4)}, [3 × 3] {Co(4)Ln(6)}, and [2 × 2] {Co(4)Ln(2)} families and a [4 × 4] {Co(8)Ln(8)} family where the central 2 × 2 square is rotated with respect to the external square. The cages include {Co(6)Ln(8)} and {Co(8)Ln(2)} families. Magnetic studies have been performed for these compounds, and for each family, the maximum magnetocaloric effect (MCE) has been observed for the Ln = Gd derivative, with a smaller MCE for the compounds containing magnetically anisotropic 4f-ions. The resulting entropy changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J kg(-1) K(-1) for {Co(8)Gd(2)}; 20.0 J kg(-1) K(-1) for {Co(4)Gd(2)}; 21.1 J kg(-1) K(-1) for {Co(8)Gd(4)}; 21.4 J kg(-1) K(-1) for {Co(8)Gd(8)}; 23.6 J kg(-1) K(-1) for {Co(4)Gd(6)}; and 28.6 J kg(-1) K(-1) for {Co(6)Gd(8)}, from which we can see these values are proportional to the percentage of the gadolinium in the core.

We show that a chemically engineered structural asymmetry in ½Tb 2 molecular clusters renders the two weakly coupled Tb 3þ spin qubits magnetically inequivalent. The magnetic energy level spectrum of these molecules meets then all conditions needed to realize a universal CNOT quantum gate. A proposal to realize a SWAP gate within the same molecule is also discussed. Electronic paramagnetic resonance experiments confirm that CNOT and SWAP transitions are not forbidden. DOI: 10.1103/PhysRevLett.107.117203 PACS numbers: 75.50.Xx, 03.67.Lx, 75.40.Gb, 85.65.+h Quantum computation [1,2] relies on the physical realization of quantum bits and quantum gates. The former can be in any of two distinguishable states, denoted here as spinup j *i and spin-down j +i, and also, as opposed to classical bits, in any arbitrary linear superposition of these. The latter involve controlled operations on two coupled qubits [1]. The universal controlled-NOT (CNOT) gate is the archetype of such a controlled operation. It flips the target qubit depending on the state of the control qubit [see Fig. 1(a)]. This definition implies that each of the two qubits should respond inequivalently to some external stimulus, e.g., electric or magnetic fields. A SWAP gate exchanges the states of both qubits; i.e., it takes j*i 1 j+i 2 to j+i 1 j*i 2 and vice versa.Solid-state candidates for these elements include superconducting circuits [3][4][5], spins in semiconductors [6][7][8], and molecular nanomagnets [9][10][11][12][13][14]. The last ones are attractive for scalability, since arrays of identical magnetic molecules can be prepared and grafted to solid substrates or devices via simple chemical methods [15,16]. The recent development of devices able to induce and readout the spin reversal of individual atoms [17][18][19] might also make feasible the coherent manipulation of one of these molecular qubits. State-of-the-art achievements with molecular nanomagnets include the measurement and minimization of single qubit decoherence rates [20][21][22] and the synthesis of mutually interacting qubit pairs [23,24]. However, the realization of a two-qubit quantum gate inside a molecular cluster remains an outstanding challenge [12,14]. Here, we show that ½Tb 2 molecular clusters display a magnetic asymmetry that should enable the realization of CNOT and SWAP gates.Lanthanide ions are promising candidates for encoding quantum information [25]. For the realization of a quantum gate, it seems therefore natural to look for molecules made of just two weakly coupled lanthanide qubits. However, the synthesis of asymmetric molecular dimers is not straightforward, as nature tends to make them symmetric. We propose a solution, sketched in Fig. 1(b), that exploits the ability of chemical design to finely tune the internal molecular structure. We synthesized a dinuclear complex of Tb 3þ ions, hereafter briefly referred to as ½Tb 2 , in which the metallic dimer is wrapped by three asymmetric organic ligands [26]. Each metal ion is in a different coordination enviro...

A robust, stable and processable family of mononuclear lanthanoid complexes based on polyoxometalates (POMs) that exhibit single-molecule magnetic behavior is described here. Preyssler polyanions of general formula [LnP(5)W(30)O(110)](12-) (Ln(3+) = Tb, Dy, Ho, Er, Tm, and Yb) have been characterized with static and dynamic magnetic measurements and heat capacity experiments. For the Dy and Ho derivatives, slow relaxation of the magnetization has been found. A simple interpretation of these properties is achieved by using crystal field theory.