Measurements In Pulsed Magnetic Fields Research Articles

Field-induced magnetostructural transition inGd5Ge4studied by pulsed magnetic fields

The field-induced magnetostructural transformation in Gd{sub 5}Ge{sub 4} was examined by magnetization measurements in pulsed magnetic fields. The low-temperature irreversibility of the transition can be destroyed by the magnetocaloric effect, and depending on the heat exchange between the sample and its surroundings, the irreversibility (or kinetic arrest) can also be retained. Measurements by using various magnetic-field sweep rates were conducted to examine the dynamic response of the system in the transition region. The critical fields for the magnetostructural transition below 20 K are field sweep rate dependent--the larger the field sweep rate, the higher the critical field. However, this rate dependence is readily suppressed with increasing temperature.

Quantum tunneling of magnetization via well-defined Dy–Cu exchange coupling in a ferrimagnetic high-spin [Dy 4Cu] single-molecule magnet

A centrosymmetric rectangular complex [Dy 4Cu] was prepared from dysprosium(III) 1,1,1,5,5,5-hexafluoropentane-2,4-dionate and copper(II) dimethylglyoximate. From magnetic studies, the Dy magnetic moments are all aligned parallel at the ground state via antiferromagnetic superexchange interaction with the central Cu spin. In pulsed-field magnetization measurements at 0.5 K, complex [Dy 4Cu] exhibited magnetization steps with hysteresis, which is typical of a single-molecule magnet. This behavior is explained in terms of a simple Ising model together with well-defined Dy–Cu exchange coupling (−0.64 K).

Magnetization ofRuSr2GdCu2O8in pulsed magnetic fields up to47T

Magnetization measurements in pulsed magnetic fields up to $47\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ were utilized to estimate the average Ru moment in polycrystalline ${\mathrm{RuSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$. A ${\mathrm{NbSr}}_{2}{\mathrm{GdCu}}_{2}{\mathrm{O}}_{8}$ sample was used as reference. The extracted Ru moment of about $1.8{\ensuremath{\mu}}_{B}$ at $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $47\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ is indicative of a mixed-valence state of Ru involving ${\mathrm{Ru}}^{5+}$ and ${\mathrm{Ru}}^{4+}$ ions with magnetic moments of $2{\ensuremath{\mu}}_{B}∕{\mathrm{Ru}}^{5+}$ and $0.9{\ensuremath{\mu}}_{B}∕{\mathrm{Ru}}^{4+}$. The estimated ratio of ${\mathrm{Ru}}^{5+}:{\mathrm{Ru}}^{4+}\ensuremath{\approx}87%:13%$ corresponds to a hole concentration $p\ensuremath{\approx}0.065$ in the ${\mathrm{CuO}}_{2}$ planes suggesting an underdoped nature of the superconducting state. We propose that the magnetic structure of the Ru moments corresponds to an antiferromagnetic phase, involving only ${\mathrm{Ru}}^{5+}$ ions, interrupted by ferromagnetic stripes, where charge transfer between ${\mathrm{Ru}}^{4+}$ and ${\mathrm{Ru}}^{5+}$ ions takes place.

Magnetic-field-induced structural transformation inEr5Si4

Magnetization and magnetostriction measurements in pulsed magnetic fields reaching $600\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ reveal strong magnetoelastic effects in ${\mathrm{Er}}_{5}{\mathrm{Si}}_{4}$ at low temperature. A clear hysteretic behavior of the magnetization isotherms and sharp first-order-type anomalies of magnetostriction are observed. Neutron powder diffraction experiments carried out in steady magnetic fields ranging from $0\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}50\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ confirm that both the magnetization and magnetostriction anomalies are associated with a reversible magnetic-field-induced crystallographic transformation at low temperature. The low-temperature crystal structure of ${\mathrm{Er}}_{5}{\mathrm{Si}}_{4}$ is monoclinic in a zero field but the application of a strong magnetic field ($\mathrm{H}g80\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$ at $\mathrm{T}=5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) induces an orthorhombic phase, which is known as the ground state of numerous other compounds in the extended 5:4 family of materials. The existence of a magnetic-field-induced crystallographic transformation without the change of the magnetic state of the material is an unusual property of magnetic solids, and to our knowledge this is the first time that such an anomaly is observed in the 5:4 intermetallic materials.

Fermi surfaces of the half-Heusler compounds [formula omitted

We report on the Fermi surface in the correlated half-Heusler compounds Ce 1 - x La x BiPt . In CeBiPt, as well as in Ce 0.95 La 0.05 BiPt , we find a temperature-dependent Fermi-surface topology. In addition, we observe a field-induced change of the electronic band structure as discovered by electrical-transport measurements in pulsed magnetic fields. For magnetic fields above ∼ 25 T , in a simple one band picture, the charge-carrier concentration determined from Hall-effect measurements increases nearly 30%, whereas the Shubnikov-de Haas (SdH) signal disappears at the same field. In the non-4f compound LaBiPt the Fermi surface remains unaffected, suggesting that these features are intimately related to the Ce 4f electrons. Electronic band–structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Present status and future plan of research in high magnetic fields at KYOKUGEN in Osaka University

After brief introduction of history and facility of the High Magnetic Field Laboratory at KYOKUGEN in Osaka University, we describe our high field and multi-frequency electron spin resonance (ESR) apparatus by utilizing pulsed and superconducting magnets for the fields up to about 60 T. For the ESR measurements in pulsed magnetic fields, several Gunn and backward oscillators, and a far infrared laser are used as millimeter and submillimeter wave sources. In steady magnetic fields up to 16 T, we have utilized a vector network analyzer with extensions which covers the frequencies between 8 and 700 GHz almost continuously. The latter ESR apparatus is used not only dense magnetic materials but also weak ones, such as metalloproteins. Therefore, we have developed high sensitive multi-frequency ESR apparatus. To extend ESR studies further, we are now constructing ESR apparatus for much higher fields up to 70 T and the wide frequency range up to 7 THz. Magnetization and transport measurements are also performed in high magnetic fields up to 70 T and 60 T, respectively and magnetization measurements under high pressure up to 1 GPa. We plan to develop a 50 T wide bore pulsed magnet with the diameter of about 50 mm for the use of high sensitive ESR and high pressure measurements above 1 GPa.

High magnetic field study of RuSr2GdCu2O8

Magnetization measurements in pulsed magnetic fields up to 47 T have been performed on the magnetic (TM,Ru ≈ 133 K) superconductor (TS ≈ 46 K) RuSr2GdCu2O8 in an attempt to determine the Ru valency in this compound. The Ru ions are most probably in a mixed valence state, but the ratio of Ru5+(S=3/2)/Ru4+(S = 1) ≈ 60%/40% suggested by NMR investigations is justified only if one considers the theoretical values of 3 μB/Ru5+ and 2 μB/Ru4+. Mixed valency could lead to a competition between ferromagnetic double exchange, evolving Ru4+ and Ru5+ ions, and antiferromagnetic superexchange, evolving Ru5+ ions, leading to magnetic phase separation. We conclude that the magnetic and superconducting properties of the rutheno-cuprates critically depend on the Ru5+/Ru4+ ratio which can be affected by the preparation conditions.

A photonic band-gap resonator to facilitate GHz-frequency conductivity experiments in pulsed magnetic fields

We describe instrumentation designed to perform millimeter-wave conductivity measurements in pulsed high magnetic fields at low temperatures. The main component of this system is an entirely nonmetallic microwave resonator. The resonator utilizes periodic dielectric arrays (photonic band-gap structures) to confine the radiation, such that the resonant modes have a high Q factor, and the system possesses sufficient sensitivity to measure small samples within the duration of a magnet pulse. As well as measuring the sample conductivity to probe orbital physics in metallic systems, this technique can detect the sample permittivity and permeability allowing measurement of spin physics in insulating systems. We demonstrate the system performance in pulsed magnetic fields with both electron paramagnetic resonance experiments and conductivity measurements of correlated electron systems.

Magnetic-Field-Induced Band-Structure Change in CeBiPt

We report on a field-induced change of the electronic band structure of CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic fields. Above approximately 25 T, the charge-carrier concentration increases nearly 30% with a concomitant disappearance of the Shubnikov-de Haas signal. These features are intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt the Fermi surface remains unaffected. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Dimensionality of superconductivity and vortex dynamics in the infinite-layer cuprateSr0.9M0.1CuO2(M=La,Gd)

The high magnetic field phase diagram of the electron-doped infinite layer high-temperature superconducting (high-T_c) compound Sr_{0.9}La_{0.1}CuO_2 was probed by means of penetration depth and magnetization measurements in pulsed fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical fields with H parallel (H_{c2}^{ab}) and perpendicular (H_{c2}^c) to the CuO_2 planes, with H_{c2}^{ab} extrapolating to near the Pauli paramagnetic limit of 160 T. The longer superconducting coherence length than the lattice constant along the c-axis indicates that the orbital degrees of freedom of the pairing wavefunction are three dimensional. By contrast, low-field magnetization and specific heat measurements of Sr_{0.9}Gd_{0.1}CuO_2 indicate a coexistence of bulk s-wave superconductivity with large moment Gd paramagnetism close to the CuO_2 planes, suggesting a strong confinement of the spin degrees of freedom of the Cooper pair to the CuO_2 planes. The region between H_{c2}^{ab} and the irreversibility line in the magnetization, H_{irr}^{ab}, is anomalously large for an electron-doped high-T_c cuprate, suggesting the existence of additional quantum fluctuations perhaps due to a competing spin-density wave order.

Microminiature Hall probes based on n-InSb(Sn)/i-GaAs heterostructure for pulsed magnetic field applications up to [formula omitted

Microminiature Hall probes with sensitive area down to 33×115 μm and based on n-InSb/i-GaAs optimized Sn-doped MBE-grown heterostructures are reported. The “metallurgical” thicknesses of the n-InSb epilayers lie in the range d m =1.1– 10.5 μm giving room-temperature mobilities of (9–15) ×10 3 cm 2/ Vs with carrier densities of (0.96–2.56) ×10 18 cm −3 . Characterization of the devices was performed by magnetotransport measurements in quasi-static and pulsed magnetic fields. In the temperature range 1.1– 300 K and in magnetic fields up to 12 T (static) and up to 52 T (pulsed, τ=120 mS ), transport measurements yield remarkable linearity of the Hall voltage up to 52 T and sensitivity, as well as demonstrating the high-temperature stability of the Hall voltage, the offset voltage and the device resistivity. No significant effect of the high current up to 150 mA on either the sensitivity or the resistivity is observed.

Surface impedance measurement with use of a tunnel diode oscillator in pulsed magnetic fields

We have developed a technique of surface impedance measurement with use of a tunnel diode oscillator in pulsed magnetic fields. Compared to conventional four-terminal resistivity measurements, this is a high-sensitive and non-contact method, enabling us to measure a small sample precisely. For the purpose of fast frequency readout, we constructed a simple FM demodulator, analogous to an FM radio receiver. With this demodulator, we succeeded in resistivity measurements in pulsed magnetic fields of up to 40 T at temperatures down to 0.6 K .

Magnetization and magneto-resistance measurements of bulk YBa 2Cu 3O 7− x in pulsed magnetic fields up to 50 T

A melt-textured bulk YBa 2Cu 3O 7− x sample has been investigated by magnetization M( H) and magneto-resistance R( H) measurements in pulsed magnetic fields up to 50 T. We describe a technique which allowed us to determine the irreversibility fields H irr from M( H) and R( H) curves. The irreversibility line shows linear behavior in the fields above 20 T which cannot be explained by present models.

New Ceramic Chambers for the HLS Storage Ring

The new injection bump system of Hefei light source (HLS) storage ring is firstly introduced. The new system set a strict error limit to the rise time delay of the pulsed magnetic field, which is caused by the coating of the ceramic chamber. The error allowance is studied and the results are summarized. A new method of coating ceramic chambers was developed. The method, which employs Mo metallization techniques, is quite different from existing methods. Important issues of coating these chambers are presented. A unique, two-coil pulsed magnetic field measurement system was set up. Rise time delay of the pulsed field at different positions inside the ceramic chambers was measured. The rise time deviations inside a pair of chambers proved to be within the range of /spl plusmn/4 ns. A vacuum pressure of 2/spl times/10/sup -8/ Pa of the ceramic chambers was obtained after the installation in the storage ring. The chambers performed well. An average injection rate of 2-6 mA/s was obtained.

Magnetic susceptibility of the normal-superconducting transition in high-purity single-crystalα-uranium

We report complex ac magnetic susceptibility measurements of a superconducting transition in very high-quality single-crystal alpha-uranium using microfabricated coplanar magnetometers. We identify an onset of superconductivity at T~0.7 K in both the real and imaginary components of the susceptibility which is confirmed by resistivity data. A superconducting volume fraction argument, based on a comparison with a calibration YBCO sample, indicates that the superconductivity in these samples may be filamentary. Our data also demonstrate the sensitivity of the coplanar micro-magnetometers, which are ideally suited to measurements in pulsed magnetic fields exceeding 100 T.

Magnetovolume effect in the paramagnetic phase of Sm 1− xSr xMnO 3 ( x=0.45, 0.33) compounds

Thermal expansion and magnetostriction measurements in high pulsed magnetic fields up to 13 T, have been performed on polycrystalline samples of the mixed valence manganese oxides Sm 1− x Sr x MnO 3 ( x=0.45 and 0.33), from room temperature to 4.2 K. In the case of Sm 0.55Sr 0.45MnO 3, the large volume magnetostriction suggests that “magnetic polarons” formation is the origin of the colossal magnetoresistance observed in this compound.

Hydrostatic pressure anvil cell for electron transport measurements up to 1 GPa under high pulsed magnetic field at liquid helium temperatures

An anvil cell designed for magnetotransport measurements in pulsed high magnetic fields of long duration (∼1 s) under pressures of up to 1 GPa is described. This cell is intended for isothermal measurements at liquid helium temperatures despite heating by eddy currents generated from the cell. Experiments performed in magnetic fields of up to 33 T reveal that the temperature rise during the pulse is insignificant at liquid helium temperature and about 0.05 K only at 1.8 K.

High precision pick-up coils for pulsed field magnetization measurements

A pick-up system for precise magnetization measurements in pulsed field facilities is presented. Samples with diameters up to 5 mm can be measured (diameter of the pick-up coil is 6.8 mm, outer diameter of the pick-up system is 11.8 mm). The optimized geometry of the concentric pick-up and compensation coils reduces the influence of a sample displacement on the measured signal. The measured signal changes by less than 0.2% if the position of a spherical sample of 4.3 mm diameter is shifted by ±2 mm along the axis of the coils. An additional compensation coil is used, to correct the difference between the winding areas of pick-up and compensation coils. This correction as well as the signal of the field measuring coils depend only weakly on an axial displacement of the pick-up system relatively to the center of the magnetic field. The calibration of the field measuring coils is performed by the metamagnetic transition of a MnF 2 single crystal ( H =9.3 T at T =4.2 K ). The pick-up system is used for magnetization measurements in pulsed fields up to 50 T at temperatures between 2 and 300 K in the high magnetic field laboratory ‘Hochfeldlabor Dresden’.

Flux jumps driven by a pulsed magnetic field

The understanding of flux jumps in the high temperature superconductors is of importance since the occurrence of these jumps may limit the perspectives of the practical use of these materials. In this work, we present the experimental study of the role of heavy ion irradiation in stabilizing the HTSC against flux jumps by comparing un-irradiated and 7.5×10 10 Kr ion/cm 2 irradiated (Y x Tm 1− x )Ba 2Cu 3O 7 single crystals. Using pulsed field magnetization measurements, we have applied a broad range of field sweep rates from 0.1 to 1800 T/s to investigate the behavior of the flux jumps. The observed flux jumps, which may be attributed to thermal instabilities, are incomplete and have different amplitudes. The flux jumps strongly depend on the magnetic field, on the magneto-thermal history of the sample, on the magnetic field sweep rate, on the critical current density j c, on the temperature and on the thermal contact with the bath in which the sample is immersed.

Magnetization measurements on Ce XLa 1− XSb in static and pulsed magnetic fields

Magnetic properties of single crystals Ce X La 1− X Sb ( X=1, 0.9) have been investigated by magnetization measurements in both static and pulsed magnetic fields. For Ce 0.9La 0.1Sb, the magnetization in static fields show two or more steps below 16 K. In addition, small steps have been first observed in the FP phase by pulsed field measurements. Therefore, some complicated layer structures like the case of CeSb seem to exist. The transition from the FP phase to ferromagnetic phase in pulsed fields is sharp in the case of static field measurement as well as the case of CeSb, but is broad in the pulsed field measurement, probably due to slow relaxation.