SQUID Measurements Research Articles

Magnetic behavior of electrodeposited cobalt nanowires using different electrolytic bath acidities

Co nanowires were produced by deposition in porous alumina templates using different electrolytic bath acidities, in order to study the effect on their magnetic properties. Room temperature FMR measurements and SQUID measurements as a function of temperature showed that the properties of the nanowires are dependent of the bath pH. The results show that the easy magnetization axes change with both temperature and bath acidity.

MBE growth of dilute magnetic (Zn,Mn)Se on Si substrates

AbstractWe have demonstrated growth of the dilute magnetic II–VI semiconductor (Zn,Mn)Se on As‐passivated (100) oriented Si substrates. Low temperature epitaxy has been used to optimize the growth start. The morphology has been assessed by Nomarski and scanning electron microscopy (SEM). SQUID measurements show the full magnetic moment of the Mn ions and optical investigations show a clear Zeeman splitting of up to 30 meV. All observations indicate a high crystalline quality of the epitaxial layers. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photomagnetism of A-DNAs Intercalated with Photoresponsive Molecules

We have reported that the natural DNA and DNAs intercalated with organic radicals were able to show the noble magnetic properties in dry state. Extending our study on the Magnetic properties of DNAs, we intercalated the photoresponsive proflavine (PF) molecules into DNAs when the intercalated DNA complexes were illuminated by a laser beam of 488 nm. Electron magnetic resonance (EMR) new signals appeared. We succeeded in measurement of photomagnetic properties of modified DNAs through electron magnetic resonance (EMR) spectroscopy and SQUID measurement. A significant increase in magnetization was observed for the photo-illuminated DNA/proflavine samples.

Magnetization of 3He spin filter cells

A number of valved quartz glass 3He spin filter cells have been repeatedly exposed to various external magnetic fields in order to determine the influence of induced wall magnetization on the relaxation time in the cells. The procedures of magnetizing and degaussing of cells are described. A comparison of T1 measurements performed in the same cell by different methods attest the good reliability of the measurements as well as the time stability of T1 in Cs-coated quartz glass cells. No orientation dependence of the relaxation in fields of 8 G was observed. A strong dependence of T1 on the strengths of external magnetic fields, applied perpendicular to the direction of the guide field used during the measurements, was observed at low and moderate fields. T1 decreases rapidly by increasing the field strength up to values of about 1 kG both in Cs-coated and bare-wall cells. Further increase in the field to 7 kG has no significant influence on the relaxation value, suggesting the saturation of the cell magnetization. Macroscopic magnetization produced in the bulk of the glass as well as Cs-caused magnetization on the surface are proposed to be two different relaxation mechanisms present in the cells. The magnetization induced in cells has been determined independently of the 3He relaxation by SQUID measurements. The influence of Cs coating and of heating on T1 in magnetized cells is discussed.

Evidence of ferromagnetism in Zn1−xMxO (M = Ni,Cu) nanocrystals for spintronics

We present the conditions for room-temperature synthesis of bare, Ni-and Cu-doped ZnOnanocrystals in ethanol and their characterization. The formation of a ZnO host structurewas delayed when dopant ions co-existed in the starting solutions. After ageing in motherliquors, well-crystallized doped nanocrystals were produced. Monodispersity of thenanocrystals (5–8 nm) was shown by high resolution transmission electron microscopy(HRTEM). Ultraviolet–visible (UV–vis) and photoluminescence (PL) measurements alsoconfirmed the nanocrystalline nature and the continuous growth of the crystals when agedat room temperature. Room-temperature SQUID measurements showed weakbut noticeable ferromagnetism, which was dependent on synthesis conditions.

Preparation of Graphite-Coated Iron Nanoparticles Using Pulsed Laser Decomposition of Fe3(CO)12 and PPh3 in Hexane

Graphite-coated iron nanoparticles with high magnetization have been synthesized by 355 nm pulsed laser photodecomposition of triiron dodecacarbonyl Fe3(CO)12 in the presence of triphenylphosphine PPh3. The iron particles have been found to be stable toward oxygen and strong mineral acids. The graphite shell surrounding the iron core has been shown to possess defects on its outer layers, rendering them suitable for functionalization. These superparamagnetic spherical nanoparticles show a high magnetization value of 103.9 emu/g, as indicated by SQUID measurements. We believe that the as-prepared graphite-coated iron nanoparticles have great potential in magnetic resonance imaging and other biomagnetic applications.

Ultrafine magnetic particles dispersed in silica: Characterization of cobalt iron citrate precursor and magnetic properties

Ultrafine magnetic particles dispersed in a silica matrix were successfully obtained by treatment of a cross-linked cobalt iron citrate precursor, synthesized by a modified Pechini route, with 0.001 M K 2Cr 2O 7 at 130 °C. The IR and NMR spectroscopic characterization of the precursor gel containing Co 2+ and Fe 3+ shows that the citric acid reacts with the metallic ions by coordination, the ethylene glycol by esterification and the tetraethylorthosilicate by substitution. SQUID measurements of the composite indicate superparamagnetic behavior. The blocking temperature, from the peak of the zero-field-cooled measurements, was 3 K at 1000 Oe. The magnetic diameter calculated using Langevin's equation was 4 nm.

Complete 'Melting' of Charge Order in Hydrothermally Grown Pr0.57Ca0.41Ba0.02MnO3 Nanowires

Nanowires of Pr0.57Ca0.41Ba0.02MnO3 (PCBM) (diameter approximately 80-90 nm and length approximately 3.5 microm) were synthesized by a low reaction temperature hydrothermal method. Single-phase nature of the sample was confirmed by XRD experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. While the bulk PCBM is known to exhibit charge order (CO) below 230 K along with a ferromagnetic transition at 110 K, SQUID measurements on the nanowires of PCBM show that the charge order is completely absent and a ferromagnetic transition occurs at 115 K. However, the magnetization in the nanowires is observed to be less compared to that in the bulk. This observation of the complete 'melting' of the charge order in the PCBM nanowires is particularly significant in view of the observation of only a weakening of the CO in the nanowires of Pr0.5Ca0.5MnO3. Electron paramagnetic resonance experiments were also carried out on the PCBM nanowires using an X-band EPR spectrometer. Characteristic differences were observed in the line width of nanowires when compared with that of the bulk.

HTS SQUID measurements of evoked magnetic fields from transverse hippocampal slices

We have developed a high-transition-temperature superconducting quantum interference device system aimed at neuromagnetic measurements of evoked fields from in vitro brain slices. Recordings from transverse hippocampal slices from rat have been performed and they showed neuromagnetic fields of ∼ 5 pT

SQUID Systems Adapted to Record Nuclear Magnetism in Low Magnetic Fields

We modified two SQUID measurement systems, originally developed and optimized for biomagnetic applications, with the aim to employ them for observing the precession of nuclear magnetism in low fields of a microtesla or less. To this end, coil systems for the polarization and for the observation of the nuclear magnetization were added to the set-up. In addition, SQUID readout electronics had to be fast enough to allow for short dead times after switching off the polarizing field. One system was a single channel gradiometer that is routinely used for measuring the relaxing magnetism of magnetic nanoparticles. The second system was a 304 SQUID system that was designed for multichannel vector-magnetocardiography and vector-magneto-encephalography. Both systems had in common that they operated in a magnetically shielded environment provided by multi-layer mu-metal walk-in rooms. With the single channel system we determined the natural proton NMR line width of benzene in the fast motional limit to be (112 plusmn 3) mHz. The accuracy of this value was improved by excluding experimentally the presence of line broadening by the inhomogeneity of the residual environmental field in the magnetically shielded room. To this end, the transversal relaxation of hyperpolarized 129Xe gas was measured demonstrating that the spectrometer has a spectral resolution of about 6 mHz for 1H. By using a 304 SQUID vector magnetometer system the precession of 1H in multiple distilled water samples was recorded. From the spatially resolved data of the multi-SQUID system, positions and moments of the multiple samples were estimated.

Magnetic characterization of ZnO doped with vanadium

The magnetic properties of vanadium doped ZnO nanorods formed by different growth methods were examined. The samples under investigation were either grown by metallorganic vapour-phase epitaxy (MOVPE) with in situ incorporation of the V atoms or by aqueous chemical growth (ACG) in combination with an ion-implantation process. The V concentration for all samples was less than 2 at.%. Field-cooled SQUID measurements only reveal a weak ferromagnetic signal still in a very sensitive measuring range. However, the MFM micrographs obtained at room temperature show a clear magnetic contrast and complex structures, the images are in good agreement with the phase images expected for vertically aligned magnetic dipoles. This is a strong indication for a ferromagnetic behaviour at room temperature.

Bis(α-diimine)nickel Complexes: Molecular and Electronic Structure of Three Members of the Electron-Transfer Series [Ni(L)2]z (z = 0, 1+, 2+) (L = 2-Phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene). A Combined Experimental and Theoretical Study

From the reaction of Ni(COD)(2) (COD = cyclooctadiene) in dry diethylether with 2 equiv of 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene (L(Ox))(0) under an Ar atmosphere, dark red, diamagnetic microcrystals of [Ni(II)(L*)(2)] (1) were obtained where (L*)(1-) represents the pi radical anion of neutral (L(Ox))(0) and (L(Red))(2-) is the closed shell, doubly reduced form of (L(Ox))(0). Oxidation of 1 with 1 equiv of ferrocenium hexafluorophosphate in CH(2)Cl(2) yields a paramagnetic (S = 1/2), dark violet precipitate of [Ni(I)(L(Ox))(2)](PF(6)) (2) which represents an oxidatively induced reduction of the central nickel ion. From the same reaction but with 2 equiv of [Fc](PF(6)) in CH(2)Cl(2), light green crystals of [Ni(II)(L(Ox))(2)(FPF(5))](PF(6)) (3) (S = 1) were obtained. If the same reaction was carried out in tetrahydrofuran, crystals of [Ni(II)(L(Ox))(2)(THF)(FPF(5))](PF(6)) x THF (4) (S = 1) were obtained. Compounds 1, 2, 3, and 4 were structurally characterized by X-ray crystallography: 1 and 2 contain a tetrahedral neutral complex and a tetrahedral monocation, respectively, whereas 3 contains the five-coordinate cation [Ni(II)(L(Ox))(2)(FPF(5))](+) with a weakly coordinated PF(6)(-) anion and in 4 the six-coordinate monocation [Ni(II)(L(Ox))(2)(THF)(FPF(5))](+) is present. The electro- and magnetochemistry of 1-4 has been investigated by cyclic voltammetry and SQUID measurements. UV-vis and EPR spectroscopic data for all compounds are reported. The experimental results have been confirmed by broken symmetry DFT calculations of [Ni(II)(L*)(2)](0), [Ni(I)(L(Ox))(2)](+), and [Ni(II)(L(Ox))(2)](2+) in comparison with calculations of the corresponding Zn complexes: [Zn(II)((t)L(Ox))(2)](2+), [Zn(II)((t)L(Ox))((t)L*)](+), [Zn(II)((t)L*)(2)](0), and [Zn(II)((t)L*)((t)L(Red))](-) where ((t)L(Ox))(0) represents the neutral ligand 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene and ((t)L*)(1-) and ((t)L(Red))(2-) are the corresponding one- and two-electron reduced forms. It is clearly established that the electronic structures of both paramagnetic monocations [Ni(I)(L(Ox))(2)](+) (S = 1/2) and [Zn(II)((t)L(Ox))((t)(L*)](+) (S = 1/2) are different.

Magnetism in V-/Mn-doped ZnO layers fabricated on sapphire

Doping ZnO with transition metals (TM) is an obvious approach to produce diluted magnetic semiconductors for magnetoelectronic and spintronic applications. We have carried out experimental studies on the fabrication and characterisation of Mn-doped ZnO layers and V-doped ZnO layers and nanorods, the results of which are reviewed in this paper. From SQUID measurements, both epitaxial and implanted ZnMnO layers show paramagnetic behaviour. Epitaxial ZnVO layers show ferromagnetic SQUID signals, but the presence of any secondary phases in the ZnVO layers may not be ruled out. We also show that the used Al2O3 substrates produce a ferromagnetic SQUID signal, that complicates the analysis of magnetisation data and hence the confirmation of ferromagnetism only from SQUID results.

Room temperature ferromagnetic (Zn,Co)O epitaxial films obtained by low-temperature MOCVD process

High quality epitaxial films of Zn 1 − x Co x O solid solutions were obtained by low-temperature MOCVD process using water vapor up to x = 0.33. Depositions were carried out at 300 °C on r-sapphire substrates. Films structure and composition were investigated by XRD, XPS and EDX measurements. Magnetic properties were examined using SQUID and Kerr effect measurements. All samples showed ferromagnetic behavior at room temperature. NEXAFS and PES were employed to investigate electronic structure of the films.

Squid measurement of the Verwey transition on epitaxial (1 0 0) magnetite thin films

We report results on epitaxial magnetite (Fe 3O 4) thin films grown by electron beam ablation on (1 0 0) MgAl 2O 4 substrates. At 120 K magnetite undergoes a structural and electronic transition, the so-called Verwey transition, at which magnetic and conducting properties of the material change. We observed the Verwey transition on epitaxial films with a thickness of 50 nm by comparing zero-field cooling (ZFC) and field cooling (FC) curves measured with a superconducting quantum interference device (SQUID) magnetometer. Observation of the Verwey transition by SQUID measurements in the films is sign of their high crystalline quality. Room temperature ferromagnetism has also been found by magneto-optical Kerr rotation (MOKE) and confirmed by SQUID measurements, with a hysteresis loop showing a coercive field of hundreds of Oe.

SQUID measurements of remanent magnetisation in refillable 3He spin-filter cells (SFC)

A strong influence of external magnetic fields on the relaxation time constant T 1 of glass cells serving as reservoirs for polarised 3He, observed for various alkali metal-coated cells made of different glass types, was initially associated with the presence of a large number of ferromagnetic clusters on the glass surface. Later experiments showed the presence of the so-called “ T 1 hysteresis” phenomenon with a similar distinctiveness also in uncoated cells made of pure synthetic quartz glass. It suggests that the origin of such a relaxation is a macroscopic magnetisation in the bulk of the cell. We present the results of a multi-SQUID system investigation on magnetised and non-magnetised quartz glass cells, Cs coated as well as bare wall, to be used as neutron spin filters at HMI Berlin. The presence of a macroscopic remanent magnetic moment in the cells after their exposition to external magnetic fields has been experimentally shown. More than 80% of the remanent magnetic moment of the magnetised cells was found to be concentrated in the region of the glass valves. SQUID measurements reveal the existence of some remanent magnetisation in all valve parts and also in the vacuum grease, but most magnetic are the plastic parts and the O-ring. Different valve and sealing types have been compared in order to find the less magnetisable one.

Nonaqueous Synthesis of Manganese Oxide Nanoparticles, Structural Characterization, and Magnetic Properties

The synthesis, structural characterization, and magnetic properties of crystalline manganese oxide nanoparticles are presented. The procedure is based on the reaction of benzyl alcohol with the two precursors: potassium permanganate KMnO4 and manganese(II) acetylacetonate Mn(acac)2. Depending on the precursor used, the composition of the final product can be varied in such a way that in the case of KMnO4 mainly Mn3O4 is formed, whereas Mn(acac)2 leads predominantly to MnO. Rietveld refinement of the XRD powder patterns, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) analysis, as well as electron energy loss spectroscopy (EELS) were employed for the structural characterization of the as-synthesized compounds. Especially the MnO manganosite nanocrystals exhibit some interesting features. HRTEM investigations point to the formation of a superstructure, which can be described as an ordered Mn vacancy cubic superstructure with the general formula of Mn0.875Ox and a lattice parameter of 8.888 A. The SQUID measurement proves a superparamagnetic behavior of the MnO nanoparticles.

Synthesis, Redox, and Magnetic Properties of a Neutral, Mixed-Valent Heptanuclear Manganese Wheel withS= 27/2 High-Spin Ground State,

Reaction of lithium tetrachloromanganate(II) with N-n-butyldiethanolamine H2L3 (3) in the presence of LiH leads to the formation of wheel-shaped, mixed-valent heptanuclear, neutral complex {MnII subset[MnII2MnIII4Cl6(L3)6]} (4). The manganese wheel crystallizes in the triclinic space group P as 4.2CHCl3 or 4.3THF when either diethyl ether or n-pentane was allowed to diffuse into solutions of 4 in chloroform or tetrahydrofuran. The oxidation states of each manganese ion in 4.2CHCl3 or 4.3THF were assigned on the basis of detailed symmetry, bond length, and charge considerations, as well as by the Jahn-Teller axial elongation observed for the manganese(III) ions, and were further supported by cyclic voltammetry. The analysis of the SQUID magnetic susceptibility data for complex 4.2CHCl3 showed that the intramolecular magnetic coupling of the manganese(II,III) ions is dominated by ferromagnetic exchange interactions. This results in an S = 27/2 ground-state multiplet at low magnetic field. At fields higher than 0.68 T, the energetically lowest state is given by the mS = 31/2 component of the S = 31/2 multiplet due to the Zeeman effect. The ligand-field-splitting parameters were determined by anisotropy SQUID measurements on single crystalline samples along the crystallographic x, y, and z axes (D = -0.055 K, E = 6.6 mK) and by high-frequency electron spin resonance measurements on a polycrystalline powder of 4.2CHCl3 (D = -0.068 K, E = 9.7 mK). The resulting barrier height for magnetization reversal amounts to U approximately 10 K. Finally, 2DEG Hall magnetization measurements revealed that 4.2CHCl3 shows single-molecule magnet behavior up to the blocking temperature of about 0.6 K with closely spaced steps in the hysteresis because of the quantum tunneling of the magnetization.

Synthesis and characterization of monodisperse colloidal spheres of Pb containing superparamagnetic Fe 3O 4 nanoparticles

Monodisperse colloidal spheres of Pb embedded with superparamagnetic Fe 3O 4 nanoparticles have been synthesized by refluxing lead acetate in the presence of a magnetite-based ferrofluid in tetra(ethylene glycol), followed by quenching with cold ethanol. Depending on the amounts of ferrofluid and lead acetate fed to the reaction mixture, the molar ratio of encapsulated Fe 3O 4 to Pb could be varied in the range of 0.001–0.06. Both superparamagnetic and superconducting properties were characterized by SQUID measurements. These results demonstrate a simple, solution-phase approach to the controlled synthesis of composite colloidal spheres displaying a combination of superconducting and superparamagnetic properties.

Study on phase relations, crystal structure and magnetic properties of Ti1－xCrxO2±δ system

A series of samples of Ti1-xCrxO2±δ were prepared by Sol-Gel process. The particle size, morphology, valence, solid solution range, phase relations and magnetic properties of Ti1-xCrxO2±δ samples were studied by XRD, SEM, XPS and SQUID measurements. Meanwhile, the Rietveld refinement method was employed to study the effect of Cr doping on the crystal structure ofTiO2. The results show that the solid solution range of both Ti1-xCrxO2±δ systems annealed at 1000℃ are x＝0—0.03. Their lattice parameters decrease with increasing Cr concentration. The results of both XRD analysis and magnetic measurements show that the solid solution range of both Ti1-xCrxO2±δ systems annealed at 1000℃ are x＝0—0.02. Cr ions of samples sintered at 500℃ and 1000℃ have two chemical states, Cr+3 and Cr+6， respectively, while the samples sintered at 1000℃ have much greater Cr+6 content than the ones at 500℃. According to the results of M-H and M-T measurements, magnetic properties of Ti1-xCrxO2±δ samples sintered at 500 ℃ with x=0—0.02 are ferromagnetic. When x≥0.04, they consisted of both ferromagnetic phase and paramagnetic phase. They have stronger ferromagnetic properties at low temperature (50K). The paramagnetic phase is the main constituent and the ferromagnetic phase has less volume fraction at room temperature.