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Register a new userMagnetic resonance in nanoparticles: between ferro- and paramagnetism
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Magnetic nanoparticles of gamma-Fe2O3 coated by organic molecules and suspended in liquid and solid matrices, as well as a non-diluted magnetic fluid have been studied by electron magnetic resonance (EMR) at 77-380 K. Slightly asymmetric spectra observed at room temperature become much broader, symmetric, and shift to lower fields upon cooling. An additional narrow spectral component (with the line-width of 30 G) is found in the diluted samples, its magnitude obeying the Arrhenius law with the activation temperature of about 850 K. The longitudinal spin-relaxation time, T1 >> 10 ns, was determined by the specially developed modulation method. Angular dependence of the EMR signal position in field-freezing samples unambiguously points to the domination of the uniaxial magnetic anisotropy. Substantial alignment is achieved in moderate freezing fields of 4-5 kG, suggesting formation of dipolar-coupled chains consisting from several particles separated by organic nanolayers. The shift and broadening of the spectrum upon cooling are ascribed to the role of the surface layer, which is considered with taking into acount the strong surface-related anisotropy. To describe the overall spectrum shape, a quantization model is used which includes summation of the resonances corresponding to varios orientations of the particle magnetic moment. This approach, supplemented with some phenomenological assumptions, provides satisfactory agreement with the experimental data.
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In order to better understand the transition from quantum to classical behavior in spin system, electron magnetic resonance (EMR) is studied in suspensions of superparamagnetic magnetite nanoparticles with an average diameter of ~ 9 nm and analyzed in comparison with the results obtained in the maghemite particles of smaller size (~ 5 nm). It is shown that both types of particles demonstrate common EMR behavior, including special features such as the temperature-dependent narrow spectral component and multiple-quantum transitions. These features are common for small quantum systems and not expected in classical case. The relative intensity of these signals rapidly decreases with cooling or increase of particle size, marking gradual transition to the classical FMR behavior.
The anisotropic paramagnetism and specific heat in Nd2Ti2O7 single crystals are investigated. Angular dependence of the magnetization and Weiss temperatures show the dominant role of the crystal field effect in the magnetization. By incorporating the results from the diluted samples, contributions to Weiss temperature from exchange interactions and crystal field interactions are isolated. The exchange interactions are found to be ferromagnetic, while the crystal field contributes a large negative part to the Weiss temperature, along all three crystallographic directions. The specific heat under magnetic field reveals a two-level Schottky ground state scheme, due to the Zeeman splitting of the ground state doublet, and the g-factors are thus determined. These observations provide solid foundations for further investigations of Nd2Ti2O7.
The interaction between strain and spin has received intensive attention in the scientific community due to its abundant physical phenomena and huge technological impact. Until now, there is no experimental report on ultra-high frequency magnetic resonance through the strain-spin coupling for any technologically relevant perpendicular magnetic material. Here we report the experimental detection of the acoustic strain waves that have a response time on the order of 10 picoseconds in perpendicular magnetic [Co/Pd]n multilayers via a femtosecond laser pulse excitation. Through direct measurements of acoustic strain waves, we observe an ultra-high frequency magnetic resonance up to 60 GHz in [Co/Pd]n multilayers. We further report a theoretical model of the strain-spin interaction. Our model reveals that the energy could be transferred efficiently from the strain to the spins and well explains the existence of a steady resonance state through exciting the spin system. The physical origins of the resonance between strain waves and magnetic precession and the requested conditions for obtaining magnetic resonance within thin magnetic films have also been discussed after thorough analysis. These combined results point out a potential pathway to enable an extremely high frequency (EHF) magnetic resonance through the strain-spin coupling.
The ZFC and FC magnetization dependence on temperature was measured for BiFeO3 ceramics at the applied magnetic field up to H=10T in 2K-1000K range. The antiferromagnetic order was detected from the hysteresis loops below the Neel temperature TN=646K. In the low magnetic field range there is an anomaly in M(H), probably due to the field-induced transition from circular cycloid to the anharmonic cycloid. At high field limit we observe the field-induced transition to the homogeneous spin order. From the M(H) dependence we deduce that above the field Ha the spin cycloid becomes anharmonic which causes nonlinear magnetization, and above the field Hc the cycloid vanishes and the system again exhibits linear magnetization M(H). The anomalies in the electric properties, which are manifested within the 640K-680K range, coincide to the anomaly in the magnetization M(T) dependence, which occurs in the vicinity of TN. We propose to ascribe this coincidence to the critical behaviour of the chemical potential, related to the magnetic phase transition.
We address the theory of the coupled lattice and magnetization dynamics of freely suspended single-domain nanoparticles. Magnetic anisotropy generates low-frequency satellite peaks in the microwave absorption spectrum and a blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency resonances are very sharp with maxima exceeding that of the FMR, because their magnetic and mechanical precessions are locked, thereby suppressing Gilbert damping. Magnetic nanoparticles can operate as nearly ideal motors that convert electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is significant even in the absence of a net rotation.
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