A set of equations describing the motion of a free magnetic nanoparticle in an external magnetic field in a vacuum, or in a medium with negligibly small friction forces is postulated. The ...conservation of the total particle momentum, i.e. the sum of the mechanical and the total spin momentum of the nanoparticle is taken into account explicitly. It is shown that for the motion of a nanoparticle in uniform magnetic field there are three different modes of precession of the unit magnetization vector and the director that is parallel the particle easy anisotropy axis. These modes differ significantly in the precession frequency. For the high-frequency mode the director points approximately along the external magnetic field, whereas the frequency and the characteristic relaxation time of the precession of the unit magnetization vector are close to the corresponding values for conventional ferromagnetic resonance. On the other hand, for the low-frequency modes the unit magnetization vector and the director are nearly parallel and rotate in unison around the external magnetic field. The characteristic relaxation time for the low-frequency modes is remarkably long. This means that in a rare assembly of magnetic nanoparticles there is a possibility of additional resonant absorption of the energy of alternating magnetic field at a frequency that is much smaller compared to conventional ferromagnetic resonance frequency. The scattering of a beam of magnetic nanoparticles in a vacuum in a non-uniform external magnetic field is also considered taking into account the precession of the unit magnetization vector and director.
•There are three different modes of the unit magnetization vector precession for a free magnetic nanoparticle in uniform external magnetic field.•The high-frequency mode is similar to the conventional ferromagnetic resonance. The frequencies of the low-frequency modes can be two orders of magnitude lower.•The characteristic relaxation time for the low-frequency modes is remarkably long.
•An off-diagonal magnetoimpedance sensor based on amorphous ferromagnetic microwire.•Scanning GMI magnetometer for measuring of stray magnetic fields near the sample surface.•Magnetic image of ...miniature current carrying structure.•Magnetic image of a sample containing micrograms magnetic nanoparticles.•Magnetic image of Fe-rich amorphous ferromagnetic microwires.
A scanning magnetic microscope based on an off-diagonal magnetoimpedance sensor is presented. As a sensor, we use a 4 mm segment of glass covered microwire, having a metallic core diameter of 13.5 μm and CoFeCrSiB composition and a pick-up coil wound around the microwire containing 70 turns. The magneto-impedance sensor is fixed perpendicular to the surface of the sample, in such a way that the distance between the tip of the microwire and the sample is about 200 μm. The relative movement of the sample and the GMI sensor is carried out using a non-magnetic X-Y positioner. Measurements are taken inside the magnetic screen. For test measurements, samples in the form of the letters IWMW – of the conference abbreviation are used. The samples were made based on thin copper wire, Fe-rich amorphous ferromagnetic microwires and printed by laser-jet printer. Clear magnetic images were obtained on all samples, which demonstrates the high practical potential of the proposed method.
•Resistivity monitoring of two types Co-rich microwires during DC Joule heating treatment.•Microwire temperature definition during Joule heating, using a crystallized microwire.•Cr-doped microwires ...had well-marked resistivity minimum near the temperature 163 °C.•The resistivity minimum shifted to a temperature value 260 °C after annealing.•Below the temperature minimum, resistivity dependence is proportional to -ln(T).
The paper investigates the electrical resistivity of two types of the glass-coated Co-rich amorphous ferromagnetic microwires during the Joule heating by direct current. We measured the relationships between the microwire’s resistivity and the power, which applied to the samples in as-prepared and crystallized conditions. Both types of microwires were demonstrated to completely crystallize on exposure to a temperature of more than 600 °C. During heating by a power up to 2.5 W (~400 °C), the microwires with Mo-Ni content were characterized by a monotonic resistivity increase with the temperature. But the microwires with Cr content had a well-marked resistivity minimum near the temperature Tм ~ 163 °C. This minimum shifted to a temperature value of ~260 °C after annealing. The research shows that the temperature dependence of resistivity in the temperature range below Tм obeys the f(T) = -ln(T) law.
•The Co69Fe4Cr4Si12B11 microwire under heating up to 600 °C become crystallized.•Studies of microwire’s resistance when heated in a furnace and by the Joule heating.•Temperature dependence of the ...resistance of the crystallized microwire becomes linear.•Calibration dependence of the microwire’s temperature on the Joule heating power.•Control of temperature and resistance of microwire at different Joule heating modes.
The results of comparative studies of the electrically conductive properties of Co69Fe4Cr4Si12B11 glass coated amorphous microwires obtained during their heat treatment in a conventional furnace and by the Joule heating method are presented. The fully crystallized microwire dramatically changes its electrically conductive properties. We found that the crystallized microwire has a temperature coefficient of resistance, α = 315*10-6 1/ °C. The crystallized microwire was used as a reference resistance thermometer under Joule heating for determining the temperature of the microwire as a function of the applied thermal power T(P). This dependence obtained was used to determine the temperature dependence of the resistance RM(T) of other microwire samples in an amorphous or partially crystallized state of the same series. The proposed method allows to select thermal modes during Joule annealing of microwires and to compare the resistive, magnetic and structural-phase properties of microwires after thermal effects.
The present work describes the investigation of electrical and magnetic properties of the glass-coated Co-rich amorphous ferromagnetic microwires, having typical metallic core diameters d = 13–20 μm ...and tailoring by the direct current Joule heating. During this treatment, a continuous monitoring of the microwire condition was carried out, measuring its resistance by means of a DC bridge circuit. The heating of the microwire sample was provided by a direct current in the range from 20 mA to 63 mA. In this heating currents range, the microwires show a slight change in their resistivity. After annealing the resistivity of microwires may increase of about 1% with respect to as-prepared one. Depends on the heating current, the hysteresis loops of annealed microwires demonstrated the transition from a quasi linear type to a bistable one and vice versa. The maximum of the giant magnetic impedance ratio was observed in such microwires, which had a hysteresis loops describable by a near-zero anisotropy field.
•Glass-coated amorphous microwires with composition Co69Fe4Cr4Si12B11 under Joule heating treatment.•Continuous resistivity monitoring of microwires during Joule heating treatment.•Normalized resistivity of annealed microwires may increase in the range of 1%.•Changes of electrical, magnetic and magnetoimpedance properties after annealing.•Microwires having hysteresis loops with a near-zero anisotropy field had a maximum GMI-ratio.
The electrodynamic method is used to measure the hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D=25nm in the frequency range f=10–150kHz and for magnetic ...field amplitudes H0=100–300Oe. It is found that the specific loss power is determined by a demagnetizing factor of a whole sample. It diminishes approximately 4.5 times when the sample aspect ratio decreases from L/d=11.4 to L/d≈1, where L and d are the sample length and diameter, respectively. For H0≤300Oe the maximal specific loss power 120W/g is obtained for the sample with L/d=11.4 at f=120kHz. For comparison, the assembly specific absorption rate has been determined also by means of direct measurement of the temperature difference between the inner and outer surfaces of a flat cuvette containing magnetic nanoparticles. For both methods of measurement close values for the specific absorption rate are obtained for samples with similar demagnetizing factors.
► A demagnetizing field makes a significant influence on a specific absorption rate. ► Electrodynamics method has advantages in specific absorption rate measurement. ► Alternative thermal measurement confirms the electrodynamics method results.
The hysteresis losses of a dense assembly of magnetite nanoparticles with an average diameter D = 25 nm are measured in the frequency range f = 10 – 200 kHz for magnetic field amplitudes up to H
0 = ...400 Oe. The low frequency hysteresis loops of the assembly are obtained by means of integration of the electro-motive force signal arising in a small pick-up coil wrapped around a sample which contains 1 – 5 mg of a magnetite powder. It is proved experimentally that the specific absorption rate diminishes approximately 4.5 times when the sample aspect ratio decreases from 11.4 to 1. Theoretical estimate shows that experimentally measured hysteresis loops can be approximately described only by taking into account appreciable contributions of magnetic nanoparticles of both very small, D < 10 – 12 nm, and rather large, D > 30 nm, diameters. Thus the wide particle size distribution has to be assumed.
The electrodynamic method is applied to determine the specific absorption rate (SAR) of an assembly of superparamagnetic nanoparticles as a function of frequency and magnetic field amplitude. The ...home made frequency-adjustable electromagnet is used to create a nearly uniform magnetic field in a core gap of a volume 1×3×3 cm
3
in the frequency range
f
=10–150 kHz and for magnetic field amplitudes up to
H
0
=250 Oe. Two oppositely connected pick-up coils are used to record the electromotive force signal (EMF) generated by magnetic nanoparticles. By integrating the EMF signal one can determine the low-frequency hysteresis loops of the assembly and the assembly SAR. Using this method the measurement of SAR has been carried out for magnetite nanoparticles with an average diameter
D
=25 nm. The electrodynamic method is shown to be capable of measuring a small amount of magnetic nanoparticles, up to 5×10
−5
g, dispersed in a solid matrix. The maximal SAR ∼ 80 W/g has been obtained for the magnetite nanoparticle assembly investigated.
A scanning HTS SQUID microscope for magnetic imaging samples at room temperature and atmospheric pressure has been developed. A sensor for the microscope was completed by a bicrystal dc SQUID and a ...needle made of soft magnetic material, which serves as a magnetic flux guide (MFG). This sensor has allowed magnetic imaging of the warm samples with spatial resolution of the order of 100 μm. Line scans of magnetic field produced by the current-carrying wires are measured and compared with results of a numerical modeling. The influence of the MFG on the image details has been studied.
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