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Register a new userThermodynamics around the First-Order Ferromagnetic Phase Transition of ${rm Fe}_2{rm P}$ Single Crystals
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The specific heat and thermodynamics of ${rm Fe}_2{rm P}$ single-crystals around the first order paramagnetic (PM) to ferromagnetic (FM) phase transition at $T_{rm C} = 217 ,{rm K}$ are empirically investigated. The magnitude and direction of the magnetic field relative to the crystal axes govern the derived H-T phase diagram. Strikingly different phase contours are obtained for fields applied parallel and perpendicular to the $c$-axis of the crystal. In parallel fields, the FM state is stabilized, while in perpendicular fields, the phase transition is split into two, with an intermediate FM phase where there is no spontaneous magnetization along the $c$-axis. The zero-field transition displays a text-book example of a first order transition with different phase stability limits on heating and cooling. The results have special significance since ${rm Fe}_2{rm P}$ is the parent material to a family of compounds with outstanding magnetocaloric properties.
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The antiferromagnetic (AFM) to ferromagnetic (FM) first order phase transition of an epitaxial FeRh thin-film has been studied with x-ray magnetic circular dichroism using photoemission electron microscopy. The FM phase is magnetized in-plane due to shape anisotropy, but the magnetocrystalline anisotropy is negligible and there is no preferred in-plane magnetization direction. When heating through the AFM to FM phase transition the nucleation of the FM phase occurs at many independent nucleation sites with random domain orientation. The domains subsequently align to form the final FM domain structure. We observe no pinning of the FM domain structure.
We report the synthesis and physical properties of single crystals of stoichiometric BaNi2As2 that crystalizes in the ThCr2Si2 structure with lattice parameters a = 4.112(4) AA and c = 11.54(2) AA. Resistivity and heat capacity show a first order phase transition at T_0 = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4x10^{-10} Omega-cm/Oe. Resistivity, ac-susceptibility, and heat capacity find evidence for bulk superconductivity at T_c = 0.7 K. The Sommerfeld coefficient at T_c is 11.6 pm 0.9 mJ/molK^2. The upper critical field is anisotropic with initial slopes of dH_{c2}^{c}/dT = -0.19 T/K and dH_{c2}^{ab}/dT = -0.40 T/K, as determined by resistivity.
Single crystals of the three-dimensional frustrated magnet and spin liquid candidate compound PbCuTe$_2$O$_6$, were grown using both the Travelling Solvent Floating Zone (TSFZ) and the Top-Seeded Solution Growth (TSSG) techniques. The growth conditions were optimized by investigating the thermal properties. The quality of the crystals was checked by polarized optical microscopy, X-ray Laue and X-ray powder diffraction, and compared to the polycrystalline samples. Excellent quality crystals were obtained by the TSSG method. Magnetic measurements of these crystals revealed a small anisotropy for different crystallographic directions in comparison with the previously reported data. The heat capacity of both single crystal and powder samples reveal a transition anomaly around 1~K. Curiously the position and magnitude of the transition are strongly dependent on the crystallite size and it is almost entirely absent for the smallest crystallites. A structural transition is suggested which accompanies the reported ferroelectric transition, and a scenario whereby it becomes energetically unfavourable in small crystallites is proposed.
We used temperature dependent high-resolution x-ray powder diffraction and magnetization measurements to investigate structural, magnetic and electronic degrees of freedom across the ferromagnetic magneto-elastic phase transition in Mn1Fe1P0.6-wSi0.4Bw (w = 0, 0.02, 0.04, 0.06, 0.08). The magnetic transition was gradually tuned from a strong first-order (w = 0) towards a second-order magnetic transition by substituting P by B. Increasing the B content leads to a systematic increase in the magnetic transition temperature and a decrease in thermal hysteresis, which completely vanishes for w = 0.08. Furthermore, the largest changes in lattice parameter across the magnetic transition occur for w = 0, which systematically becomes smaller approaching the samples with w = 0.08. Electron density plots show a strong directional preference of the electronic distribution on the Fe site, which indicates the forming of bonds between Fe atoms and Fe and P/Si in the paramagnetic phase. On the other hand, the Mn-site shows no preferred directions resembling the behaviour of a free electron gas. Due to the low B concentrations (w = 0 - 0.08), distortions of the lattice are limited. However, even small amounts of B strongly disturb the overall topology of the electron density across the unit cell. Samples containing B show a strongly reduced variation in the electron density compared to the parent compound without B.
Molecular spin qubits with long spin coherence time as well as non-invasive operation methods on such qubits are in high demand. It was shown that both molecular electronic and nuclear spin levels can be used as qubits. In solid state systems with dopants, an electric field was shown to effectively change the spacing between the nuclear spin qubit levels when the electron spin density is high at the nucleus of the dopant. Inspired by such solid-state systems, we propose that divalent lanthanide (Ln) complexes with an unusual electronic configuration of Ln$^{2+}$ have a strong interaction between the Ln nuclear spin and the electronic degrees of freedom, which renders electrical tuning of the interaction. As an example, we study electronic structure and hyperfine interaction of the $^{159}$Tb nucleus in a neutral Tb(II)(Cp$^{rm{iPr5}}$)$_2$ single-molecule magnet (SMM) using the complete active space self-consistent field method with spin-orbit interaction included within the restricted active space state interaction. Our calculations show that the low-energy states arise from $4f^8(6s,5d_{z^2})^1$, 4$f^8$(5$d_{x^2-y^2}$)$^1$, and $4f^8(5d_{xy})^1$ configurations. We compute the hyperfine interaction parameters and the electronic-nuclear spectrum within our multiconfigurational approach. We find that the hyperfine interaction is about one order of magnitude greater than that for Tb(III)Pc$_2$ SMMs. This stems from the strong Fermi contact interaction between the Tb nuclear spin and the electron spin density at the nucleus that originates from the occupation of the $(6s,5d)$ orbitals. We also uncover that the response of the Fermi contact term to electric field results in electrical tuning of the electronic-nuclear level separations. This hyperfine Stark effect may be useful for applications of molecular nuclear spins for quantum computing.
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