Superparamagnetic-like Behavior Research Articles

Control of Defects and Magnetic Properties in Colloidal HfO2 Nanorods

Colloidal HfO2 nanorods with controllable defects are synthesized. An increased defect concentration is observed by high-resolution electron microscopy and by optical absorption spectroscopy. Nanocrystals with a high defect concentration exhibit ferromagnetism and superparamagnetic-like behavior (see figure). This low-temperature colloidal syntheses can produce the highly nonequilibrium defect concentrations needed to obtain ferromagnetism in closed shell oxides.

Mechanosynthesis of Fe-doped SnO 2 nanoparticles

Fe-doped nanosized SnO 2 powders were prepared by mechanosynthesis using rutile SnO 2 and hematite Fe 2O 3 as starting materials. The estimated grain size of the obtained rutile structure was between 10 and 20 nm. Mössbauer spectroscopy shows the presence of two interactions corresponding to 3+ and 2+ iron oxidation states with similar relative fractions. The magnetic measurements exhibit ferromagnetic- (FML) and superparamagnetic-like (SML) behavior. Evidence of a system of strongly interacting particles was found.

Properties of intracellular magnetite crystals produced by Desulfovibrio magneticus strain RS-1

Desulfovibrio magneticus strain RS-1 is an anaerobic sulfate-reducing bacterium. Cells form intracellular nanocrystals of magnetite but are only weakly magnetotactic. In order to understand the unusual magnetic response of this strain, we studied magnetite crystals within cells grown with fumarate and sulfate. Many cells grown under either condition did not form magnetic crystals while others contained only 1 to 18 small (∼ 40 nm) magnetite-containing magnetosomes. Bulk magnetic measurements of whole cells showed a superparamagnetic-like behavior, indicating that many of the magnetite crystals are too small to have a permanent magnetic moment at ambient temperature. The temperature of the Verwey transition is lower (∼ 86 K) than of magnetite from other magnetotactic strains, likely indicating partial oxidation of magnetite into maghemite. As a result of the small size and small number of magnetite magnetosomes, the magnetic moments of most cells grown anaerobically with fumarate or sulfate are insufficient for magnetotaxis. In addition to intracellular magnetite, in some cultures another iron oxide, hematite, formed on the surfaces of cells. The hematite grains are embedded in an extracellular polymeric material, indicating that the crystals likely resulted from a biologically-induced mineralization process. Although the hematite particles appear to consist of aggregations of many small (5 to 10 nm) grains, the grains have a consensus orientation and thus the whole particle diffracts as a single crystal. The aligned arrangement of nanoparticles within larger clusters may reflect either a templated nucleation of hematite crystallites in an extracellular organic matrix, or result from a self-assembling process during the crystallization of hematite from ferric gels or ferrihydrite.

Emission Mössbauer studies of the magnetoresistive compound, La 0.7Sr 0.3MnO 3

The magnetic behavior of the Sr 0.3 manganite is studied using a local microprobe, 57Co. In contrast with Ca substituted manganites, a much larger fraction of the material exhibits short-range order with superparamagnetic-like behavior even at 80 K. The differences in behavior are attributed to the large mismatch between the ionic radii of La +3 and the divalent substituent Sr +2, which introduces anharmonicity in local vibrations. In common with all other compounds exhibiting negative bulk magnetoresistivity, the Sr 0.3 compound also exhibits very marked softening of lattice as one approaches T c from below. Application of an external magnetic field results in coalescing of nanosized magnetic clusters to form larger ones with better alignment of spins.

Static and dynamic magnetic properties of an[Fe13]cluster

The static and dynamic magnetic properties of an $[{\mathrm{Fe}}_{13}]$ cluster were investigated using several experimental techniques. The cluster crystallizes in a cubic space group, but careful investigation of the crystallographic data revealed that the symmetry is distorted locally. dc magnetic susceptibility measurements showed the presence of competing antiferromagnetic isotropic exchange interactions leading to a high-spin ground state and many low-lying excited spin states, which was confirmed by inelastic neutron scattering measurements. From high-field electron paramagnetic resonance measurements a small zero-field splitting of the spin ground state was inferred, which supports the local symmetry distortion found in the crystallographic studies. The spin dynamics slows down at sub-kelvin temperatures, where ac susceptibility measurements indicated that part of the sample shows superparamagnetic-like behavior and the other part relaxes through magnetization tunneling. The M\"ossbauer data confirmed the slowing down of the spin dynamics, indicating that this occurs mainly in the peripheral spins.

Structural and magnetic properties of Fe and Co nanoparticles embedded in powdered Al 2O 3

The structural and magnetic properties of powdered composites consisting of nanosized Fe and Co particles embedded in alumina grains have been investigated. The composites were synthesized by a novel and simple method using co-precipitation from a hybrid gel solution containing layered double hydroxide and aluminium hydroxide. After a vacuum annealing procedure, the Fe composites have a negligible number of Fe +3 ions and a high concentration of crystalline α-Fe nanoparticles having truncated polyhedron shapes with an average diameter of 20 nm that are physically well separated from each other. Magnetization measurements as a function of temperature revealed a superparamagnetic-like behavior characteristic of an assembly of fine particles. A spurious ferromagnetism related to surface interaction between magnetic particles from different powder grains and the formation mechanism of the composites are also discussed.

Synthesis, structure, and magnetic properties of a [Mn22] wheel-like single-molecule magnet.

The synthesis and magnetic properties of the compound [Mn(22)O(6)(OMe)(14)(O(2)CMe)(16)(tmp)(8)(HIm)(2)] 1 are reported. Complex 1 was prepared by treatment of [Mn(3)O(MeCO(2))(6)(HIm)(3)](MeCO(2)) (HIm = imidazole) with 1,1,1-tris(hydroxymethyl)propane (H(3)tmp) in MeOH. Complex 1.2MeOH crystallizes in the orthorhombic space group Pbca. The molecule consists of a metallic core of 2 Mn(IV), 18 Mn(III), and 2 Mn(II) ions linked by a combination of 6 micro(3)-bridging O(2)(-) ions, 14 micro(3)- and micro(2)-bridging MeO(-) ions, 16 micro-MeCO(2)(-) ligands, and 8 tmp(3)(-) ligands, which use their alkoxide arms to bridge in a variety of ways. The metal-oxygen core is best described as a wheel made from [Mn(3)O(4)] partial cubes and [Mn(3)O] triangles. Variable-temperature direct current (dc) magnetic susceptibility data were collected for complex 1 in the 1.8-300 K temperature range in a 1 T applied field. The chi(M)T value steadily decreases from 56 cm(3) K mol(-)(1) at 300 K to 48.3 cm(3) K mol(-)(1) at 30 K and then increases slightly to reach a maximum value of 48.6 cm(3) K mol(-)(1) at 15 K before dropping rapidly to 40.3 cm(3) K mol(-)(1) at 5 K. The ground-state spin of complex 1 was established by magnetization measurements in the 0.1-2.0 T and 1.80-4.00 K ranges. Fitting of the data by a matrix-diagonalization method to a model that assumes only the ground state is populated and incorporating only axial zero-field splitting (DS(z)()(2)), gave a best fit of S = 10, g = 1.96 and D = -0.10 cm(-)(1). The ac magnetization measurements performed on complex 1 in the 1.8-8 K range in a 3.5 G ac field oscillating at 50-1000 Hz showed frequency-dependent ac susceptibility signals below 3 K. Single-crystal hysteresis loop and relaxation measurements indicate loops whose coercivities are strongly temperature and time dependent, increasing with decreasing temperature and increasing field sweep rate, as expected for the superparamagnetic-like behavior of a single-molecule magnet, with a blocking temperature (T(B)) of approximately 1.3 K.

Dynamic susceptibility of reentrant Fe-rich inhomogeneous amorphous alloys

Melt-spun amorphous alloys of Fe91Zr9, Fe91Zr7B2, Fe90Zr7B3 and Fe88Zr8B4 have been characterized by AC susceptibility at frequencies between 30 Hz and 10 KHz. The measurements reflect the existence of reentrant spin glass transitions below 40 K for all cases. The transition shift per frequency decade is large compared to the observed in conventional spin-glass alloys. The shift value increases with the boron content and becomes closer to values in fine-particle systems. We find that the relaxation in Fe91Zr9 follows a critical slowing down at the reentrant transition, with exponents \(z\nu = 7.2\) and \(\beta = 0.7\). The non-linear susceptibility displays a peak at the transition in this alloy, but wider than in canonical spin glasses. A Vogel-Fulcher (VF) activation process can explain the frequency variation in all the Fe-Zr-B alloys. The reduction of the ideal glass temperature in the VF approach, found in the higher Boron content alloys, is an indication of a superparamagnetic-like behavior. The behavior shown by these alloys is intermediate between a collective freezing and superparamagnetic-like relaxation. We propose that this is arising due to a heterogeneous spin structure. The inclusion of a reduced amount of B, affects the magnetic spin structure.

Growth and magnetism of Ni nanoparticles in Ni/Al 2O 3/Si or Si 3N 4 multilayers

We report on the fabrication of Ni/Al 2O 3/Si and textured Ni/Al 2O 3/Si 3N 4 multilayers containing Ni nanoparticles that exhibit significantly improved results. The secondary phases arising from thermal reaction between Ni and Si can be remarkably suppressed with increasing layers of Al 2O 3 and deposition of Ni/Al 2O 3 multilayers on Si 3N 4 substrates. Atomic force microscopy shows the formation of large as well as nanoclusters of Ni when grown on Si, whereas textured Ni nanoparticles are formed on Si 3N 4 substrates. The magnetization measurements on Ni/Al 2O 3/Si containing a single buffer layer of Al 2O 3 shows higher coercivity field with magnetic nanowire-like behavior, whereas with several Al 2O 3 alternate layers almost a superparamagnetic-like behavior is observed. However, significantly improved magnetic hysteresis was observed in textured Ni/Al 2O 3/Si 3N 4 multilayers due to preferred alignment of Ni nanocrystallites.

Anomalous magnetic and dynamic behavior in magnetoresistive compounds: origin of bulk colossal magnetoresistivity

We present the results of our extensive Mössbauer effect studies carried out on a wide variety of mixed valence manganites as well as other types of magnetoresistive materials, including pyrochlore Tl2Mn2O7 and the chalcospinels Fe0.5Cu0.5Cr2S4 and FeCr2S4 with absolutely different natures of the magnetism, in a search for similarities linked to their magnetoresistive behavior. The double exchange electron transfer and coupling between the electrons and Jahn–Teller lattice distortions invoked by most theories to explain the colossal magnetoresistivity and associated metal–insulator transition in manganites are not applicable to pyrochlore nor to chalcospinels. Nevertheless, we find intriguing similarities in the anomalous magnetic and dynamic behavior among these widely different systems at, above, and below the Curie temperature TC, which shed light on the origin of bulk magnetoresistivity in general. All these compounds share the following features. The long-range ferromagnetic order breaks down even below the Curie temperature, with the formation of nano-size spin clusters. Softening of the lattice was observed near TC. The short-range interactions in these spin clusters survive well above TC. When an external magnetic field is applied, the spin clusters coalesce to form large clusters, with considerable lowering of the resistivity. There is a strong evidence that the existence of nano-size spin clusters with superparamagnetic-like behavior near TC is a prerequisite for the occurrence of bulk magnetoresistivity.

The Origin of Bulk Magnetoresistivity in Manganites

On substitution of Ca2+ for La3+ in LaMnO3, the compound La1−xCaxMnO3 becomes a ferromagnetic metal for 0.18≤x<0.5. The hopping of an electron from Mn3+ to O2− is associated with simultaneous hop from the latter to Mn4+. The probability of the hopping by this double exchange is highest when both hopping electrons have the same spin orientation, requiring Mn ions to be ordered ferromagnetically. The metallic behavior is synergistically associated with the ferromagnetic order through electron hopping. The resistivity increases considerably near the Curie temperature (TC) and application of a magnetic field causes a dramatic decrease in resistivity due to regeneration of metallicity. The origin of this colossal magnetoresistivity is rather enigmatic and has attracted lot of attention from researchers. We have addressed this problem using emission Mössbauer spectroscopy where the local probe 57Co/ 57Fe substituting Mn atoms senses the microscopic behavior of the material. We find that the long-range ferromagnetic order breaks down anomalously below TC and the material degenerates into small spin clusters which fluctuate rapidly and which exhibit superparamagnetic-like behavior. On a time scale of 10−8 s, the spin clusters exhibit minimal lattice distortions. These spin clusters survive well above TC and would constitute very effective scattering centers for charge carriers due to their rapid fluctuations. On application of an external magnetic field, the small magnetic clusters coalesce to form larger ones with more ordered spins, and the material shows enhanced conductivity through percolation. We also observe superparamagnetic-like behavior below TC in the pyrochlore, Tl2Mn2(57Co)O7, which does not exhibit double exchange electron hopping. It seems that the breakup into small rapidly fluctuating ferromagnetic spin clusters near TC (exhibiting superparamagnetic-like behavior) is a prerequisite for observing bulk magnetoresistivity.

Emission Mössbauer study of the electronic phases inLa0.7Ca0.3MnO3

A well characterized homogeneous ceramic sample of La{sub 0.7}Ca{sub 0.3}Mn({sup 57}Co)O{sub 3} shows two ferromagnetic phases below T{sub c} in contrast to the 20% Ca compound where only one is observed. One of the phases exhibits stronger exchange interactions with more rapid electron transfer between Mn{sup 3+}/Mn{sup 4+}. At lower temperatures, there is a tendency for a single phase to exist, with well ordered spins. In both 20% and 30% Ca materials, the long-range ferromagnetic order breaks down above T/T{sub c}=0.7 and small spin clusters with superparamagneticlike behavior are formed. Above T{sub c}, the spin clusters persist giving predominantly a single phase (as seen by the Moessbauer probe {sup 57}Fe with a Larmor period of {approx}10{sup -8} sec). Under the influence of an external magnetic field, the small spin clusters coalesce to give larger clusters with better alignment of spins exhibiting negative magnetoresistivity by percolation. Our work raises the question whether superparamagnetic behavior is a prerequisite for observing negative magnetoresistivity.

Magnetic and magnetoresistive properties of nanostructured Au–NiFe films

Measurements of magnetization, magnetoresistance and ferromagnetic resonance were carried out at various temperatures, on as-deposited and annealed samples of granular films of Au 0.7(NiFe) 0.3. It was found that the as-deposited films exhibit a superparamagnetic-like behavior and a negative magnetoresistance up to a temperature of 120 K. Samples annealed at 673 K for 30 min exhibit a ferromagnetic behavior up to 300 K. The ferromagnetic resonance measurements show isotropic magnetic properties for the as-deposited films and a significant anisotropy in the field-depended magnetization for the annealed films.

Ferro-, Antiferro-, and Superparamagnetic Behavior of Fe-Al Alloys

In agreement with measurements of Sato and Arrott, and of Kouvel, we also found anomalous magnetic properties of Fe-Al alloys containing more than 28 at.%. Al. In order to investigate the different properties of these alloys in more detail, we have measured the temperature dependence of magnetization, susceptibility, anisotropy, rotational hysteresis, and coercive force of monocrystalline and polycrystalline specimens (containing 28 to 50 at.% Al) between 1.5° and 300°K. The following results have been found: a maximum of rotational hysteresis at about 12°K, high coercive forces and high unidirectional and uniaxial anisotropy in the temperature region below 40°K, magnetization creeping between 4° and 50°K, and superparamagnetic-like behavior at higher temperatures. These results may be interpreted by assuming the existence of two phases one of which is ferromagnetic, the second antiferromagnetic below 40°K. For T∼40°K, the domains of one phase, and for lower temperatures, the domains of both phases interact.