Spin glass dynamics of the short range ising system Fe0.5Mn0.5TiO3

The short range Ising spin glass Fe_0.5Mn_0.5TiO₃ has been investtigated in a SQUID magnetometer. It is shown that the time dependence of the zero-field-cooled magnetization is dependent on wait-time at constant temperature prior to the field application and that the field-cooled magnetization is dependent on the cooling rate. The extracted dynamic exponent, zv = 10.4±1.0, is larger than values obtained for Heisenberg-like spin glasses.     

Magnetization process related to the singular point detection signal observed at high fields on Pr2Fe14B and Nd2Fe14B

The singular point detection signal, d²M/dt², observed on Pr₂Fe₁₄B and Nd₂Fe₁₄B at high fields and low temperatures is explained on the basis of one sublattice crystal field model. The signal is attributed to a steeper reversible magnetization process and a first order magnetization process for Pr₂Fe₁₄B and Nd₂Fe₁₄B respectively.     

Anisotropy of the magnetization and of the iron hyperfine field in R2Fe17 compounds

A single crystal of the hexagonal Y₂Fe₁₇ compound has been prepared. The exact composition, Y₂Fe_18.9 has been refined through X-rays measurements. A large anisotropy of the magnetization is associated with the large magnetocrystalline anisotropy. Mössbauer experiments have been performed at 4.2 K under high magnetic fields. A large anisotropy of the orbital contribution to the hyperfine field is reduced. This can explain the anomalies of the hyperfine field observed in Tm₂Fe₁₇ and ErFe₃ when magnetization reorientations occur with temperature.     

Conversion-electron Mössbauer study of sputtered Bi3Fe5O12

Temperature dependence of the magnetization measurements and Conversion Electron Mössbauer Spectroscopy (CEMS) have been performed on sputtered Bi₃Fe₅O₁₂ (BiIG), Y₃Fe₅O₁₂ (YIG) and (Bi, Y)IG films. Under the effect of the diamagnetic dilution of the diamagnetic yttrium sublattice the magnetization of BiIG is decreased with respect to YIG. The hyperfine field at the tetrahedral iron sites is increased, indicating a decrease in the intra-sublattice exchange interaction rather than the decrease of the tetrahedral iron moment.     

Deposition of ZnO Nanocrystals on Fe3O4 Nanocubes and Their Special Luminescent and Magnetic Properties

With increasing miniaturization, it is extremely important to maintain the magnetization stability at small scale. Herein, more efforts and interests focus on the interface of magnetic core and semiconductor shell to obtain desired magnetic and/or luminescent properties. Here, Fe₃O₄ nanocubes are synthesized via a thermal decomposition followed by coating ZnO nanocrystals. To create a large interface, large Fe₃O₄ nanocubes with 78 ± 3 nm average side‐length are synthesized through adjusting the ratio of iron precursor to stabilizer. The average diameter of the particular ZnO nanostructures coated on the nanocubic Fe₃O₄ is around 10 ± 2 nm. In addition to the photoluminescent properties of the ZnO‐coated nanostructures, core‐shell Fe₃O₄@ZnO nanostructures demonstrate enhanced UV absorption at 360 nm, which has a 20 nm blueshift compared to bulk ZnO. The superparamagnetic properties of Fe₃O₄@ZnO core–shell hybrid nanocrystals at room temperature are dominated by the ferromagnetic properties when the temperature is lower than the Blocking temperature, 235.7 K. The observed exchange bias and temperature‐dependent magnetization can result from the interfacial interphase between ZnO and Fe₃O₄. The anisotropy contributed by the interfacial interphase allows the nanostructures to maintain stable magnetization in miniaturized devices.     

Magnetocrystalline anisotropy of Er2Fe14B

A rotation-magnetic-alignment method was used to align fine-powdered (< 20 μm) Er₂Fe₁₄B at room temperature while the easy magnetization direction of Er₂Fe₁₄B lies in the basal plane. X-ray diffraction was used to check the magnetic alignment. For the first time, the temperature dependence of the anisotropy field of Er₂Fe₁₄B was measured in a wide temperature range from about 170 to 530 K. The anisotropy field was determined using the SPD technique in a pulsed-field magnetometer from 170 to 320 K (T_SI = 323 K) on a magnetically aligned sample and from 330 to 530 K (T_C = 550 K) on a bulk polycrystal.     

Positive muon spectroscopy of R2Fe14B

Muon spin rotation (μSR) experiments were performed on polycrystalline samples of R₂Fe₁₄B in zero applied field. In all samples a single spin precession frequency was observed. In Nd₂Fe₁₄B and Ho₂Fe₁₄B pronounced anomalies showed up in the temperature dependence of the μSR frequencies at the spin reorientation temperatures of 150 K and 60 K, respectively. Our data can be explained with the assumption that only thec-axis component of the magnetization is sampled by the muon. We find a strong dependence of the local field on the magnetic moment of the rare earth ion. This is in accordance with calculations of the dipolar fields at the assumed muon stopping site.     

Thickness-dependent magnetic properties of Ni81Fe19, Co90Fe10 and Ni65Fe15Co20 thin films

We present results of magnetization and magnetic anisotropy measurements in thin magnetic films of the alloys Ni₈₁Fe₁₉, Co₉₀Fe₁₀ and Ni₆₅Fe₁₅Co₂₀ that are commonly used in magnetoelectronic devices. The films were sandwiched between layers of Ta. At room temperature the critical thickness for all the films to become ferromagnetic is in the range 11–13 Å. In Co₉₀Fe₁₀ the coercivity and the anisotropy field both depend strongly on layer thickness.     

Magnetic properties of lithographically defined lateral Co/Ni80Fe20 wires

A novel micro-fabrication technique has been used to create an array of lateral magnetic multilayers consisting of micron-sized sputtered Co and Ni₈₀Fe₂₀ wires. The structures were fabricated using conventional optical lithography and a combination of hard and soft lift-off methods. For the field applied parallel to the wires intrinsic easy axis, we observed two switching fields corresponding to the distinct coercive field of the Ni₈₀Fe₂₀ wires (H_c1) and Co wires (H_c2) constituting the lateral multilayer wire array. A state of anti-parallel relative alignment of magnetization was observed when the applied field is greater than the switching field of Ni₈₀Fe₂₀ wires but less than the switching field of Co wires. We found the region of anti-parallel alignment of magnetization between the Co and Ni₈₀Fe₂₀ wires to be very sensitive to the relative orientation of the applied magnetic field.     

Positive muon spectroscopy of Nd2Fe14B and Pr2Fe14B

We have performed positive muon spin rotation measurements on polycrystalline samples of Nd₂Fe₁₄B and Pr₂Fe₁₄B in zero applied field. In both samples a single sharp μSR line was observed which was unexpected in this complicated structure. The temperature dependence of the muon frequency for Nd₂Fe₁₄B clearly reflects the spin reorientation below 150 K and can be explained qualitatively by assuming that only the c-axis component of a magnetization is sampled by the muon. A smooth decrease of the muon frequency with increasing temperature is observed for Pr₂Fe₁₄B.     

Magnetic properties of fcc (Co95Fe5)1-xAlx ribbons

Rapidly quenched (Co₉₅Fe₅)_1-xAl_x ribbons are investigated by X‐ray diffraction, magnetization, and Mössbauer effect measurements. A single fcc phase is obtained for all ribbons x ? 10 at.%. The lattice constant increases linearly with x and is discussed in connection with magnetic moment. The influence of Al substitution on both magnetization and Fe‐atom hyperfine field (H) is studied. At 296 K, the magnetization decreases linearly while H drops nonlinearly as x increases. Al substitution leads to substantial differences in iron hyperfine fields in bcc and fcc systems. Fe moment is perturbed differently by Al substitution in fcc (Co₉₅Fe₅)_1-xAl_x and bcc Fe–Al systems.     

Magnetic properties and Fe NMR studies of U-type hexaferrites

U-type hexaferrites with compositions Ba₄Me₂Fe₃₆O₆₀ (Me=Cu, Fe, Co, Mn and Mg) have been synthesized and characterized by magnetization measurements and zero field ⁵⁷Fe nuclear magnetic resonance (NMR) spectra. The NMR spectra show resonance peaks corresponding to different crystallographic sites of iron (tetrahedral, octahedral and trigonal bipyramidal) with integral intensities according to their site multiplicity. In comparison with the NMR spectrum of M-type barium hexaferrite the intensities of some of the peaks arising due to iron sublattices (f_IV, a and b sites) are different, though there is no difference in peak positions indicating the same local hyperfine field strength at Fe nuclei. The maximum saturation magnetization at room temperature was found in Cu₂U (70 emu/g) and Fe₂U (67 emu/g). The Curie temperatures, saturation magnetizations and coercitivities for the U-type hexaferrites are given.     

Study on magnetic property of Fe14Ni86 alloy nanowire array

Ordered Fe₁₄Ni₈₆ alloy nanowire arrays implanted in anodic alumite template (AAT) have been fabricated by electrodepositing the corresponding material into the nanochannels. The wires are 43 nm in diameter and 50 μm in length. Their aspect ratio (ratio of length to diameter) is more than 1000, which results in distinctive magnetic anisotropy. The easy magnetization axis of this system is perpendicular to the membrane. Enhanced coercivity (about 769 Oe) and remanent magnetization up to 70% of the saturation magnetization have been observed. We also studied angular dependent coercivities of the Fe₁₄Ni₈₆ alloy nanowire arrays and found that they fit well with the chain-of-sphere combined model of uniform rotation and non-symmetric fanning.     

Hyperfine measurement of magnetic compensation in ferrimagnetic (GdCe)Fe2 and (GdY)Fe2 compounds

Magnetic compensation in the ferrimagnetic series of compounds (Gd_XY_1?X)Fe₂ and (Gd_XCe_1?X)Fe₂ has been observed by Mössbauer measurements of the hyperfine field at Gd and Fe nuclei in external fields (B_app) up to 6 T. The hyperfine field shows a sharp change of 2B_app at the compensation composition x_C due to the alignment of the magnetization of the compound with the external field. Strong compositional dependence of this change allows an accurate determination of x_C in the two systems.     

A note on exchange and crystal field interactions in R2Fe14B compounds: Yb2Fe14B

The R₂Fe₁₄B phase has been found to exist for R=Yb. The magnetic properties presented in this paper complete the characterization of the compounds in this series for which the Stevens α_J coefficient of the R³⁺ ion is positive. ⁵⁷Fe Mössbauer spectroscopy establishes the existence of a magnetization reorientation at 115 K of the type observed in Er and Tm compounds associated with a small Fe magnetization anisotropy. From the neutron diffraction measurements obtained at 4.2 K with and without an applied magnetic field, the easy direction of magnetization was found to be along the [100] direction, in the basal plane of the tetragonal structure. These results show that in all compounds where α_J>0 for the R³⁺ ion, the easy direction of magnetization in the plane is determined by the second order crystal field terms and rare earth-Fe exchange interactions and is independent of the sign of the 4th order crystal field terms.     

Direction of magnetization and domain wall mobility in (Dy,Y)Fe2

Magnetization measurements in the (Dy _x Y_1−x )Fe₂ intermetallic compounds show evidence of blocking of domain wall motion. This effect is a function of concentrationx. The direction of magnetization is also dependent on the Dy concentration. We have used the Mossbauer spectroscopy of⁵⁷Fe to verify the direction of magnetization in the series as a function ofx and temperature. The results show that this change in easy direction occurs at higher temperatures than those where the blocking is detected through DC magnetization measurements. supported by RHAE/SCT.     

Damage processes in Fe 3 O 4 magnetic insulator irradiated by swift heavy ions. Experimental results and modelisation

The behavior of the magnetic properties of magnetite Fe₃O₄ irradiated by swift heavy ions is investigated by magnetization measurements. Although there is no induced structural phase transformation, both coercive field and saturation magnetization are sensitive to ion irradiation and exhibit different behaviors depending on the ion fluence range. In the low fluence regime, the coercive field increases, which is evidence for a strong pinning of magnetic domain boundaries by the induced defects. The magnetization shows a decrease in the saturation value and tends to reorient perpendicularly to the ion track axis. At high fluence, the initial magnetic properties of the sample are nearly restored. The changes in the magnitude and the direction of magnetization are interpreted by magnetostrictive effects related to the stress induced by irradiation. A phenomenological model is applied to reproduce the fluence evolution of the saturation magnetization, assuming relaxation of the stress induced around the core of defects of the tracks by overlapping effects at high fluence. The results are compared to those obtained in the case of yttrium iron garnet Y₃Fe₅O₁₂. Received 18 April 2001 and Received in final form 24 July 2001     

Computer simulation of spin reorientations in polycrystalline materials - application to R2Fe14Ba)

The easy direction of magnetization (DOM) is an extremely important property of a magnetic material, in regards to both permanent magnet and recording applications. In many magnetic materials, most notably Nd₂Fe₁₄B, the DOM is temperature dependent. When utilizing methods which permit the orientation of either a single crystal or pseudo-single crystal to be rigidly constrained (e.g., VSM and extraction), it is possible to determine the temperature dependence of the DOM directly. However, the determination of spin-reorientation temperatures with powder samples is not always straighforward. In some cases, a peak is observed in the magnetization versus temperature curve, while in other either a drop or an increase is observed. Using a two-dimensional, two-crystallite grain as a model and assumed temperature dependencies of the anisottropy constants, K₁ and K₂, it is shown that these peaks and other features can be explained by polycrystalline particles (or, equivalently, imperfect alignment of the powder). It is further shown that spin-reorientation temperatures are invariably characterized by the inflection points in the low field magnetization versus temperature curve. Examples of such curves obtained from a Faraday balance for Nd₂Fe₁₂Si₂B and Pr₂Co₁₄B are given.     

First order magnetization process in Pr2Fe14B single crystal

High-field magnetization process in Pr₂Fe₁₄B single crystal has been studied in static magnetic fields up to 230 kOe. The spontaneous magnetization is along the [001] direction of the tetragonal structure down to 1.5 K. When the field is applied along the [100] and [110] directions at low temperatures, magnetization jumps are observed at about 130 kOe and 160 kOe, respectively, which are considered to correspond to the first order magnetization process (FOMP). Observed features of the magnetization curves including FOMP and their temperature dependence are well reproduced by the calculation based on a simplified Hamiltonian including the crystalline electric fields and the FePr     

Magnetic and magnetotransport properties of nanocrystalline Ag0.85Fe0.15 and Ag0.70Fe0.30 alloys prepared by mechanical alloying

The magnetic and magnetotransport properties of nanocrystalline Ag_0.85Fe_0.15 and Ag_0.70Fe_0.30 alloys have been studied by Mössbauer spectroscopy, magnetization and resistivity measurements. The samples were prepared by mechanical alloying of Fe and Ag powders in a high-energy ball mill. Mössbauer spectroscopy and magnetic measurements of the final milled samples indicate the presence of single-domain ‘Fe’ particles. The magnetoresistance values, at 4.2 K and for a magnetic field of 8 T, are 2.5% and 5.7% for samples Ag_0.85Fe_0.15 and Ag_0.70Fe_0.30, respectively. The magnetoresistance behavior indicates the cluster-glass-like features in both the final milled samples.