Magnetic nanoparticles have been used in various biomedical applications. Among these, magnetic particle imaging and hyperthermia are considered clinically useful diagnostic and therapeutic methods, respectively. An alternating magnetic field is applied to the magnetic nanoparticles in these methods. The magnetization response of a magnetic nanoparticle is essential, and the dependence of its field intensity and frequency determines its diagnostic and therapeutic performance. This article reviews the fundamental issues of the magnetization process and magnetization relaxation of magnetic nanoparticles and presents our recent experimental results.

  Two types of magnetoresistance (MR), i.e., tunnel magnetoresistance (TMR) and current perpendicular to plane giant magnetoresistance (CPP-GMR), are theoretically discussed for systems with half-metallic (HFM) Co-based full Heusler compounds. In TMR, the non-collinear spin structure at the Co₂MnSi(CMS)/MgO(001) interface that results from thermal spin fluctuations significantly reduced the TMR ratio at room temperature (RT). Enhancement of the exchange stiffness of CMS/MgO was essential to suppress the reduction in TMR at RT. Furthermore, it is proposed that inserting of a B2-CoFe layer between CMS and MgO is a promising way to enhance the interface exchange stiffness constant. In CPP-GMR, the interface spin asymmetry γ in the Valet-Fert model was essential to enhance GMR at RT, because the temperature dependence of γ in experiments was much larger than that of the bulk spin asymmetry β. I showed that the experimental CPP-GMR ratio increases with the decrease in the theoretical resistance-area product R_PA, which is roughly consistent with the R_P dependence of γ. This means that matching of the conductive channel between HMF and a non-magnetic metallic spacer is a key parameter to enhance γ. These findings will be very important for room temperature spintronics devices applied in future artificial intelligence hardware.

  The effects of Al diffusion and heat treatment on the structure and magnetic properties for Sm(Fe-Co)-B thin film have been investigated. In the X-ray diffraction patterns, peaks of (002) and (004) from ThMn₁₂-type compounds were clearly observed for all the samples. The coercivity μ₀H_c was improved by introducing an Al cap layer and it was further increased by heat treatment. A high μ₀H_c of 1.78 T was obtained at 400 ºC by increasing the annealing time to 4 hours. It was considered that infiltration of Al into the Sm(Fe-Co)-B layer contributed to the improvement of magnetic properties.

  The magnetization dynamics of GdFeCo / Cu / FeCo trilayers were studied by the time-resolved magneto-optical Kerr effect (TRMOKE) using an all-optical pump-probe technique. When the pump beam was illuminated from the FeCo layer side, the magnetization precession of the GdFeCo layer was confirmed to be enhanced compared with the case without the FeCo layer. Moreover, the precession angular frequency ω and inverse of the relaxation time 1/τ of the GdFeCo layer were confirmed to be decreased by pumping the FeCo layer. The decreases in ω and 1/τ were remarkable when the external field direction approached to the film normal. We considered spin transfer torque (STT) due to the spin current injected by triggering the precession of the FeCo layer, and we obtained a positive field-like STT and negative anti-damping STT acting on the GdFeCo layer. Moreover, the spin current J_s injected from the FeCo layer can be expected to be ~ 1 MA/cm² from the magnitude of the anti-damping STT.

  We analyzed the magnetization process of magnetic nanoparticles using first-order reversal curve (FORC) analysis and verified a superparamagnetic feature and the orientation of the easy axis of the particles. The FORCs and FORC diagram of magnetic nanoparticles dispersed in water exhibited a superparamagnetic feature. The differences in the magnetizations and distributions of the coercive force of the solid-state samples were used to identify the orientation of the easy axes of magnetization.

  Ultra-low field (ULF) magnetic resonance imaging (MRI) is considered an innovative MRI technique that can be combined with magnetoencephalography (MEG). In this study, a 1-mT ULF-MRI system without a pre-polarization technique aiming at combining an MEG system is developed. The intensity of the static magnetic field B₀ is decided considered to be 1 mT because of the limitation of the magnetic field exposed to our MEG sensor made of superconducting quantum interference devices (SQUIDs). A square-shaped (750 mm × 750 mm) B₀-coil based on a Merritt-type configuration is fabricated. The inhomogeneity of B₀ is less than 0.07 % in the ±125 mm area. Further, an induction coil is employed to detect the magnetic resonance signal instead of the SQUID sensors of the MEG to confirm the feasibility of realizing MRI measurement at 1 mT without the pre-polarization technique. Relaxation time measurements and MR imaging of a phantom made of a CuSO₄ aqueous solution are performed. Reasonable longitudinal and transverse relaxation times are obtained, and clear shapes of the phantom are obtained using the gradient- and spin-echo pulse sequences with this ULF-MRI system. These results indicate the potential of the proposed 1-mT ULF-MRI to effectively integrate ULF-MRI and MEG systems without pre-polarization.