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[en] Magnetic nanoparticles dispersed as a ferrofluid with volumetric concentrations in the range 0.4 to 10% and sizes ranging from 59-77 A have been studied via magnetic measurements at room and low temperatures. Remanence measurements have been used to determine switching and coupling effects. Particle size and concentration effects have been investigated and we find that the samples with higher concentrations have larger coercivities than expected due to coupling effects. Interactions have been found to be demagnetising overall as expected for dipolar interacting systems. Surface effects become evident below 10 K when thermal effects are weak
[en] Highlights: • Electronic & magnetic behavior of Nd0.5Ca0.5MnO3 is explored using impedance spectroscopy. • Under ac field, possible signature of suppression of robust CO/OO antiferromagnetism is studied. • We propose the existence of spin glass state at low temperature. • A novel tactic is used to estimate the existence of weak ferromagnetism at high temperature. - Abstract: Dynamics of spin ordering in the manganite Nd0.5Ca0.5MnO3 have been investigated in this paper. It was observed that the complex mixed magnetic ordering in pellets is comprised of antiferromagnetic ordering at 160 K (TN) and complete charge ordering at 250 K (TCO). Under ac field, appearance of unstable ferromagnetic correlations is observed above TCO, which is badly frustrated due to strong spin disorder induced by Jahn Teller distortions. Impedance measurements reveal the spin glass like scenario, suppressing the strong antiferromagnetic and charge ordering states below TN.
[en] We have developed a new method of the magnetization measurement using a commercial Hall sensor (Hall magnetometer), which enables us to measure the static magnetization very easily at temperatures as low as about 0.1 K and under pressure. We describe specifications of the Hall magnetometer, and show results of the magnetization measurement for UGe2 as an example
[en] To design the high-resolution magnetic force microscopy (MFM) tips suitable for high-density magnetic recording media with low bit aspect ratio (BAR), the transfer functions of tips with various sharpened tip-ends were calculated for a checkered perpendicular magnetization pattern and the resolution of these tips was estimated by considering the resolution limit due to thermal noise at room temperature. The cylindrical tip with a spheroidal tip-end having a large ellipticity is found to be a suitable candidate for high-resolution MFM imaging of magnetic recording media with low BAR. For the tips with ellipticities larger than tan 45 deg., there are no zero-signal frequencies. The sensitivity shows a maximum around an ellipticity of tan 70 deg. The spheroidal tip shows a much smaller tip thickness dependence compared to the bar shape tip with a flat tip-end or an ellipsoidal tip-end, because only the tip-end mainly contributes to signals in case of the spheroidal tip
[en] Langevin dynamics treats finite temperature effects in micromagnetics framework by adding a thermal fluctuation field to the local effective field. Several works have addressed that the numerical results depend on the cell size used to split the ferromagnetic samples on the nanoscale regime. In this short paper, we analyze a thermally perturbed micromagnetic problem by using an implicit unconditionally stable numerical scheme to integrate the Langevin equation at room temperature. The obtained micromagnetic results for several cell sizes inside the validity range of the micromagnetic formalism, indicate that the addressed cell size dependence could be associated to numerical limitations of the commonly used numerical schemes
[en] The results of a Moessbauer study of the alloy system CuAl1-xFex are presented. Spectra at room temperature, at 86 K, and in the presence of a 4 kOe external magnetic field were recorded. These spectra indicate that no long-range magnetic order exists in the alloys with x ≤ 0.3, while for x = 0.4 magnetic order develops, and superparamagnetic behavior is exhibited. The results are used to obtain information about the local chemical environment of the Fe atoms in the magnetic clusters in the alloys. (orig.)
[en] We examined the isothermal aging in spin glass AuFe fine particles so as to study the length scale of droplet in comparison with the particle size. Relaxation rates S in spin glass AuFe fine particles were obtained at 15, 28 and 32 K. A peak in S was observed at the observation time corresponding to the waiting time at 15 K, but it was observed at a time shorter than the waiting time at 28 K. The waiting time dependence of S is weakened at 32 K. This suggests that the droplet size increases up to the particle size and the growth successively stops at high temperatures
[en] The thermal stability of coupled trilayer synthetic antiferromagnets is investigated in the framework of the new model, taking into account of the magnetostatic fields. At magnetic and other parameters typical for magnetic random access memory applications, the thermal stability of magnetic cells becomes problematic as the lateral dimensions approach 150 nm. At these dimensions, the energy barrier at zero applied field is calculated to be 40 kT (at room temperature) and it decreases with increasing applied field. The effects related with the shape anisotropy, which are expected to be strong in this size range, are not properly described by the existing model. The present results show that this problem can be solved by accurately quantifying the magnetostatic fields
[en] The zero temperature equilibrium configuration of a nonuniform system made of a ferromagnetic (FM) monolayer on top of a semi-infinite FM film is calculated using a nonlinear mapping formulation of mean-field theory, where the surface is taken into account via an appropriate boundary condition. The analytical criterion for the existence of surface magnetic canting, previously obtained by Popov and Pappas, is also recovered
[en] The temperature dependence of the coercive force decrease ratio for aligned magnets was investigated from room temperature (296 K) to 473 K. The temperature properties of the angular dependence of the coercive force were also measured from room temperature to 413 K, for comparison against the coercive force decrease ratio. From the temperature dependence of the coercive force decrease ratio of magnets with different alignment, it was found that the coercive force decrease ratio decreased as temperature increased until becoming close to the calculation results that were obtained from the Gaussian distribution for Nd_2Fe_1_4B grain alignment and from the postulation that every grain follows the Kondorskii law or the 1/cos θ law. When we compared the angle of the magnetization reverse area obtained from these calculation results and the angle of the reverse magnetization area calculated from the experimental data of the coercive force decrease ratio, it was found that this latter expanded to 30° for Nd_1_3_._4_8B_5_._7_6Co_0_._5_5Fe_b_a_l_. having 0.95 alignment, at room temperature, from 14° that was the calculated angle obtained from the Gaussian distribution and the Kondorskii law. The angular dependence of coercive force of this magnet at room temperature agrees well with the calculation, when σ=31°, which is 30° for the reversed magnetization area, is applied as the standard deviation of Nd_2Fe_1_4B grain alignment distribution. For Nd_1_2_._7_5Dy_0_._8_4B_5_._8_1Co_0_._5_5Fe_b_a_l_. with 0.96 for alignment, the reverse magnetization area also expanded to 36° and agreed well with the calculation result applied σ=44°, which has 36° for the reverse magnetization area. It was also found that, as temperature increased, the angle of the reverse magnetization area obtained from the experimental data shrunk towards the calculated angle. When we apply these results to the temperature properties of the angular dependence of the coercive force, it seems that the calculated angular dependence of the coercive force can qualitatively and reasonably explain these temperature properties. These results strongly suggest that the magnetic domain walls are strongly pinned at tilted grains. It also suggests that, when such magnetic domain walls are de-pinned from their pinning sites, they leap through several grains with weak pinning force, which determines the coercive force. - Highlights: • Coercivity decrease ratio is close to calculation results, as temperature increases. • Angle of reverse magnetization area expands from the calculation results. • Angular dependence of coercivity also changes as temperature increases. • As temperature increases, angular dependence of coercive force is close to 1/cos θ. • Angular dependence of coercivity is explained by angle of reverse magnetization area.