High Pressure Magnetic Measurements Research Articles

High-pressure structural transition and magnetic phase diagram of CuB2O4

Noncentrosymmetric $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$ is a prototypical system with interacting spin sublattices of different dimensionality, exhibiting diverse magnetic structures and magnetic phase transitions. Here we report pressure-induced structural evolution of $\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4}$ and its effect on the magnetic phase diagram. Powder x-ray diffraction (XRD), Raman scattering, and infrared phonon measurements have been performed under high pressure at room temperature. Although XRD shows lattice stability up to $\ensuremath{\sim}25\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, Raman and IR phonon measurements reveal structural anomaly near 3 GPa, followed by a structural phase transition at $\ensuremath{\sim}12\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. Pressure evolution of the zero-phonon lines in the low-temperature optical absorption studies helps identifying the symmetry change of the Cu sites modifying the crystal-field energy levels at the structural transition. High-pressure magnetization measurements have been performed with magnetic field applied perpendicular to the tetragonal $c$ axis. With increasing pressure the saturation magnetization of the commensurate weak ferromagnetic phase increases with systematic reduction of its turnup critical field. Pressure-induced systematic suppression of the dc susceptibility in this phase is attributed to the reduced critical field, indicating pressure-driven enhanced stability of the weak ferromagnetic phase at high pressure. Whereas, below 10 K the critical field of the incommensurate-commensurate transition increases with pressure, indicating enhanced stability of the incommensurate helical order at high pressure.

Using methodical compositional tuning to optimize CoxTb1−x structural and magnetic properties

The Co-rich end of the Co–Tb binary phase diagram (CoxTb1−x, x = 0.66–0.82) has been investigated to understand the phases which form in the bulk and how they interact to yield magnetic behavior which has been reported to be ideal for use in spintronic devices. This work shows that the phases and phase fractions present across this composition range follow those predicted by the binary phase diagram, and all compounds in this composition range are multiphase. Magnetic measurements show similar behavior in this composition range to related thin film work, and we attribute the observed behavior to the respective binary phases present in each compound. Ideal magnetic behavior of minimized magnetic saturation and maximized coercivity is observed in the range of x=0.78−0.80 related to the majority phase Co7Tb2 in these two compounds. High pressure magnetic measurements show magnetic saturation and coercivity at 300 K change little with respect to external pressure. The extension of the synthesis of these binaries into the bulk allows for specific binary phases to be targeted and analyzed for consideration in future devices.

Metamagnetic transition, magnetocaloric effect and electronic structure of the rare-earth anti-perovskite SnOEu3

Rare-earth anti-perovskites with oxygen are an interesting magnetic materials family at the boundary between intermetallics and oxides, they however remain largely unexplored. Here, magnetic and heat capacity investigations, as well as density functional theory (DFT) calculations, were carried out on SnOEu3. At low magnetic field (B ≤ 0.5 T), a Néel temperature separates antiferromagnetic and paramagnetic phases at 31 K. When applying higher magnetic field below the Néel temperature, successive transformations toward a ferromagnetic state via a number of intermediate canted magnetic structures are observed and are associated with only modest latent heat and transition entropy. High-pressure magnetic measurements confirm the stable divalent state of Eu up to 1.05 GPa. A direct magnetocaloric effect progressively increases with applied magnetic field above the Néel temperature, reaching −16 J kg–1 K−1 for ΔB = 7 T. On the other hand, the inverse magnetocaloric effect of the field-induced transition below TN saturates at ~+5 J kg−1 K−1. DFT calculations support magnetic instabilities observed experimentally in SnOEu3 and reveal an unusual exchange mechanism and band topology near the Fermi level.

Structural and electronic properties of the overexpanded quaternary superconducting fulleride K0.25Rb0.25Cs2.5C60

The structural and magnetic properties of the overexpanded quaternary K0.25Rb0.25Cs2.5C60 fulleride are investigated. Rietveld analysis of the high-resolution synchrotron X-ray powder diffraction data shows that K0.25Rb0.25Cs2.5C60 adopts a cubic structure with face-centered (fcc) symmetry. Magnetization measurements at ambient pressure reveal that the material is a superconductor with a transition temperature, Tc, of 29.4 K. Temperature dependence of the magnetic susceptibility does not show a simple temperature-independent Pauli susceptibility term – instead, it shows a cusp at a certain temperature implying an insulator-to-metal crossover on cooling. Complementary high-pressure magnetization measurements to a pressure of 10.2 kbar show a non-monotonic response of Tc with a dome-shaped scaling with pressure and a maximum value of 32.4 K at 3.9 kbar. The experimental results are consistent with quaternary K0.25Rb0.25Cs2.5C60 following the established unconventional behavior of overexpanded fullerides. However, our results also unveil well-defined detrimental cation specific effects on the electronic properties associated with the introduction of increased disorder in the tetrahedral interstices of the fcc fulleride structure.

High pressure magnetic measurements on strongly correlated electron systems with a miniature ceramic anvil high pressure cell

We have designed a miniature ceramic anvil high pressure cell (mCAC) for magnetic measurements at pressures up to 12.6 GPa in a commercial superconducting quantum interference (SQUID) magnetometer [1-3]. The simplified mCAC without anvil alignment mechanism is easy-to-use for researchers who are not familiar with high-pressure technology and magnetic measurements can be done above 10 GPa. Here, we show one additional modification in the mCAC. This modification enables more precise magnetic measurements on samples with smaller magnetization. We confirm the spontaneous dc magnetization in the pressure-induced ferromagnetic phase in YbCu2Si2 by high pressure magnetic measurements with the mCAC. The pressure-induced phase has the strong uniaxial Ising-type anisotropy.

Hydrothermally synthesized oxalate and phenanthroline based ferrimagnetic one-dimensional spin chain molecular magnets [{Fe(Δ)Fe(Λ)}1−x{Cr(Δ)Cr(Λ)}x(ox)2(phen)2]n (x = 0, 0.1 and 0.5) with giant coercivity of 3.2 Tesla

Oxalate (ox) and phenanthroline (phen) ligands based one dimensional spin chain molecular magnets, [{Fe(Δ)Fe(Λ)}1−x{Cr(Δ)Cr(Λ)}x(ox)2(phen)2]n (x = 0, 0.1, and 0.5) have been designed, and synthesized using a hydrothermal synthesis method. The Rietveld refinement of the powder X-ray and neutron diffraction patterns at room temperature confirms the single-phase formation of the compounds in the monoclinic structure with a space group P21. The compounds consist of two ligands, the oxalate (C2O42−) as a coordination acceptor building block and the neutral phen (C12H8N2) as a coordination donor building block. Both ligands are connected to Fe ions of different symmetry {Fe(Δ) and Fe(Λ)}, thus forming an alternating zigzag chain like crystal structure having the repeating unit of [phen-Fe(Δ)-C2O4-Fe(Λ)-phen]n. The chain is infinite in length and lies in the crystallographic ac plane. The interchain is well separated with an intermetallic distance of ∼8.8 Å and the absence of an interchain π–π overlap between the organic ligands, resulting in a magnetic isolation between the interchains. The Mössbauer spectroscopy reveals the presence of high spin states of the Fe2+ ions of the compound for x = 0 whereas, both high-spin Fe2+ (t2g4eg2, S = 2) as well as low spin Fe2+ (t2g6eg0, S = 0) states are present for the compounds x = 0.1 and 0.5. The dc magnetization measurements show that the compounds exhibit spontaneous magnetization below ∼9 K. The transition temperature is found to be ∼8.7, 8.2 and 4.0 K for x = 0, 0.1 and 0.5 compounds, respectively. Moreover, a short range antiferromagnetic spin–spin correlation around 18–45 K has been observed for the compounds x = 0 and 0.1. An application of the Ising chain model to the dc magnetization data reveals the presence of a one-dimensional magnetic nature of all compounds with alternately spaced magnetic Fe sites. It is observed that the different Landé g factors (3.4 and 2.8) and exchange coupling constant values (−86 and −54 K) for x = 0 at two alternating Fe sites give rise to a ferrimagnetic-like behavior of the chains. The ferrimagnetic chain like structure transforms toward antiferromagnetic with Cr doping i.e. for x = 0.1 and 0.5. A hysteresis loop with a giant coercivity (3.2 T for x = 0) has been observed at 1.6 K, indicating a hard magnet-type behavior. The frequency dependence of the peak temperature in ac susceptibility vs. temperature curves for the x = 0 compound has been fitted and analyzed using the Arrhenius law as well as the power law, which exclude the possibility of a spin glass behavior. The fitted parameters (Δ/kB = 208 K and τ0 = 2.9 × 10−14 s obtained from the Arrhenius law, and τ0 = 6.1 × 10−8 s, and zν = 2.6 from the power law) show that the compound obeys the Glauber dynamics and is a real ferrimagnetic one-dimensional single chain magnet. In addition, the high pressure magnetization measurements for the x = 0 compound show an enhancement in the transition temperature from ∼8.7 to 10.7 K with increasing pressure. The observation of both a one-dimensional spin chain nature and giant coercivity (3.2 Tesla) in such compounds opens up new opportunities to design and develop low dimensional molecular chain magnets through the appropriate choice of ligands using the hydrothermal synthesis method, because the observation of magnetic hysteresis of molecular origin in single-molecule magnets is considered one of the most relevant achievements in molecular magnetism.

Pressure dependence of Meissner effect in films of ropes of boron-doped carbon nanotubes

Superconductivity in carbon nanotubes (CNTs) is attracting considerable attention. Recently, we have reported successful boron doping into single-walled CNTs (SWNTs) and also revealed its correlation with superconductivity. In the present study, we report results of high-pressure magnetization measurements in films consisting of ropes of boron-doped SWNTs. It reveals that T c and magnitude for Meissner effect are mostly independent of applied pressure, while the magnitude of graphite diamagnetism drastically increases as pressure increases. We also report the observation of resistance drop and correlation of boron doping with multi-walled CNTs, in which we reported superconductivity previously.

Quantum criticality and partial order in itinerant magnets

High pressure magnetization measurements on ZrZn 2 contradict the expectation of a genuine ferromagnetic quantum critical point previously believed to exist at p c=2.1 GPa. This quantum criticality puzzle is discussed in the light of recent neutron diffraction studies of the non-Fermi liquid puzzle of MnSi. Both studies may identify stable new phases of itinerant-electron magnets.

High-field X-ray diffraction and high-pressure magnetization measurements of the pseudobinary compound Ce(Fe 0.9Co 0.1) 2

The magnetic and structural properties of Ce(Fe 0.9Co 0.1) 2 with the C15 cubic Laves-phase structure have been studied by magnetization measurement in high magnetic fields under high pressures, and powder X-ray diffraction measurement in fields up to 5 T. The C15 cubic structure is distorted to rhombohedral symmetry accompanied with the magnetic phase transition from the ferromagnetic to the antiferromagnetic state, which recovered the C15 structure by applying the magnetic field. The antiferromagnetic phase is induced by the lattice contraction due to hydrostatic pressure. Obtained results suggest that the AFM correlation due to the Ce 4f–Fe 3d hybridization plays an important role on the structural and magnetic properties.

Novel high pressure magnetic measurements with application to magnetite

We report a novel system designed to measure reversible magnetic susceptibility of micron‐sized samples under high pressures in a diamond anvil cell. We find that magnetite reversible hysteresis parameters vary <15% below 0.6 to 1.0 GPa, while at higher pressures significant increases occur in (1) bulk coercivity (Hc) and (2) the ratio of saturation remanent magnetization (Mrs) to saturation magnetization (Ms). The net effect of pressure is to displace magnetite toward a truer single domain state with both higher Mrs/Ms and Hc. Our data, together with the fact that magnetite Curie temperature increases with pressure, suggest that magnetite can account for geomagnetic anomalies related to some subduction zones and potentially to meteorite impact sites on Earth, as well as magnetic signatures observed on some planetary bodies like Mars.

High Pressure Magnetization Measurements on URu2Si2

We have made high pressure magnetization measurements on a single crystalline URu 2 Si 2 up to P = 1.61 GPa, in magnetic fields up to 15 T and in the temperature range between 4.2 and 100 K. A characteristic temperature T 0 which corresponds to an antiferromagnetic phase transition shows a slight increase with pressure, while T max , at which the temperature dependence of the magnetization exhibits a maximum, remains nearly constant with pressure. We have also observed that the pressure dependence of the high field magnetization at low temperatures shows a shallow minimum at around 1 GPa, in contrast to a strong and monotonic increase in a staggered magnetic moment revealed by recent elastic neutron diffraction experiments under pressures. To reconcile this discrepancy, we suggest, in conjunction with other experimental results, that the observed large staggered moment in the neutron diffraction experiments is not static but fluctuating in time.

Pressure effects on the metamagnetic transition of ThCo 5

We present high pressure magnetization measurements performed at low temperature on a single crystal of ThCo 5. This compound exhibits under an applied magnetic field a sharp itinerant electron metamagnetic transition. A large increase of the critical field is observed with the applied pressure. Results are well described within the itinerant electron model

Magnetic properties of Fe xCo 1−xTi and Fe xCo 1−xSi alloys

We discuss the weak ferromagnetic properties of the two series of alloys Fe x Co 1− x Ti and Fe x Co 1− x Si in the light of new specific heat and resistivity studies as well as high field and high pressure magnetic measurements.