Hiroaki Mamiya

Recent Advances in Understanding Magnetic Nanoparticles in AC Magnetic Fields and Optimal Design for Targeted Hyperthermia

Targeted hyperthermia treatment using magnetic nanoparticles is a promising cancer therapy that enables selective heating of hidden microcancer tissues. In this review, I outline the present status of chemical synthesis of such magnetic nanoparticles. Then, the latest progress in understanding their heat dissipation mechanisms under large magnetic fields is overviewed. This review covers the recently predicted novel phenomena: magnetic hysteresis loops of superparamagnetic states and steady orientations of easy axes at the directions parallel, perpendicular, or oblique to the AC magnetic field. Finally, this review ends with future prospects from the viewpoint of optimal design for efficacy with a low side-effect profile.

STRUCTURAL AND MAGNETIC STUDIES ON VANADIUM SPINEL MGV2O4

The magnetic properties and the crystal structure of MgV2O4 and Mg(V0.85Al0.15)2O4 have been studied. Both compounds are the normal cubic spinels with highly frustrated magnetic lattice. Around T2=65 K, MgV2O4 has magnetic orders accompanied with the cubic-tetragonal transition. Below T2, the susceptibility shows complex behavior. In Mg(V0.85Al0.15)2O4, the spin-glasslike state appears. The 51V-Knight shift of MgV2O4 has an anomalous temperature dependence, which is not simply related by that of the susceptibility.

Absence of strong rejuvenation in a superspin glass

Effects of temperature changes on the nonequilibrium spin-glass dynamics of a strongly interacting ferromagnetic nanoparticle system (superspin glass) are studied. In contrary to atomic spin glasses, strong cooling rate effects are observed, and no evidence for temperature-chaos is found. The flip time of a magnetic moment is much longer than that of an atomic spin and hence much shorter time scales are probed within the experimental time window for a superspin glass than for an atomic spin glass. Within a real space picture the cumulative aging observed for the superspin glass can be explained considering that all investigated length scales are shorter than the temperature-chaos overlap length. The transient relaxation, observed in experiments after temperature changes, can be understood as the adjustment of thermally active droplets, which is mutatis mutandis the Kovacs effect observed in most glassy systems.

Magnetic Hysteresis Loop in a Superparamagnetic State

Magnetization curves of superparamagnetic nanoparticles in ac magnetic field are numerically studied by Brownian dynamics simulation with considering a more precisely estimated magnetic torque. Consequently, we confirmed that rotatable nanoparticles with magnetic easy axes show magnetic hysteresis loops without remanence, even when the thermal fluctuations of the magnetizations are much faster than the oscillation of the magnetic fields. The origin is that magnetic alignments of the easy axes by the magnetic torque in the peak period of the oscillations of field alternate with their randomizations by Brownian relaxation in the period when the field becomes almost zero. Because the equilibrium magnetization curve of the aligned nanoparticles is steeper than that of randomly oriented ones, a new kind of hysteresis appears when both the Brownian relaxation time and the characteristic time of the field-driven rotation are comparable to the alternation period of magnetic field. This finding sheds a new light on the widely accepted association between superparamagnetism and the anhysteretic magnetization curves. Furthermore, this knowledge will help reconsideration of the design of biomedical application using magnetic nanoparticles, since it has been optimized assuming magnetic response described by Langevin function.

Formation of Nonequilibrium Magnetic Nanoparticle Structures in a Large Alternating Magnetic Field and Their Influence on Magnetic Hyperthermia Treatment

We simultaneously simulated thermally activated magnetization reversal and magnetic torque-driven rotation of mono- and polydisperse magnetite nanoparticles exposed to a large alternating magnetic field. We found that if the nanoparticles are rotatable, their easy axes are stationarily oriented to the directions parallel, perpendicular, or oblique to the magnetic field, depending on the relative conditions between the size of the nanoparticles and the amplitude and frequency of the magnetic field. The formations of such nonequilibrium oriented structures lead to significant variations of dynamic hysteresis loops. This knowledge would help reconcile good efficacy with a low side-effect profile in targeted magnetic thermotherapy using magnetite nanoparticles.

Sample dependence of giant magnetocaloric effect in a cluster-glass system Ho5Pd2

In order to investigate the effect of vacancy on the magnetocaloric effect in Ho5Pd2, we have carried out X-ray diffraction, magnetization, and specific heat measurements in the rare-earth intermetallic compound Ho5+xPd2(−0.4 ≤ x ≤ 0.4). The maximum magnetic entropy change −ΔSmmax, the maximum adiabatic temperature change ΔTadmax, and the relative cooling power of Ho5+xPd2 take large values at x = 0−0.4 for the field change of 5 T. The paramagnetic Curie temperature θp increases with an increase of x. This fact suggests that the enhancement of ferromagnetic coupling among the correlated spins leads to the increase of magnetocaloric effect.

Reexamination of macroscopic quantum tunneling in ferritin—temperature dependence of magnetic relaxation

Abstract The existence of pure quantum tunneling of macroscopic magnetic moment in the Kelvin regime is reexamined for antiferromagnetic nanoparticles in ferritin. In the present study, we discuss effects of temporary temperature change on magnetic relaxation. The results show that the relaxation slows down when the temperature is decreased, as expected for thermal activation process.

Magnetic Bragg dip and Bragg edge in neutron transmission spectra of typical spin superstructures

Neutron diffractometry has been a critical tool for clarifying spin structures. In contrast, little attention has been paid to neutron transmission spectroscopy, even though they are different types of the same phenomenon. Soon, it will be possible to measure the wavelength dependence of transmissions easily using accelerator-driven neutron facilities. Therefore, we have started studying the potential of spectroscopy in magnetism, and in this paper, we report the first observation of a magnetic Bragg dip and Bragg edge in the neutron transmission spectra of a typical spin superstructure; clear antiferromagnetic Bragg dips and Bragg edges are found for a single crystal and powder of nickel oxide, respectively. The obtained results show that transmission spectroscopy is a promising tool for measurements under multi-extreme conditions and for the precise analyses of spin structures, not only in MW-class pulsed spallation source facilities but also in compact neutron source facilities.

Aging in the ferromagnetic phase of terbium

We report on aging, rejuvenation and memory effects in the ferromagnetic phase of pure terbium. We have applied an experimental method specifically for investigating slow dynamics of spin glasses, because these effects cannot be interpreted as conventional diffusion after-effects. Results show that relaxation times of the magnetic response are widely distributed, and isothermal aging shifted the distribution towards longer durations. If the sample was heated/cooled after such isothermal aging, the relaxation times shortened as if aging was starting anew; the behavior resembles that in spin glasses. Uniform magnetization experiments indicate that, unlike rejuvenation in spin glasses, ferromagnetic correlations are not returned to disorder by thermal perturbations. In contrast with memory effects in spin glasses, the effects of isothermal aging cannot be recovered once these disappear, even if the system is returned to its initial temperature. The observed results can be explained as collective pinning of the domain walls for which the potential is given by a rugged temperature-sensitive energy landscape.

Reexamination of macroscopic quantum tunnelling observed in ferritin: magnetic field dependence of magnetic relaxation

Field-dependence of magnetic relaxations in the Kelvin regime is reexamined for nanomagnets in ferritin protein, in order to clarify whether their magnetization vectors relax owing to thermally assisted resonant quantum tunnelling. This paper describes that unknown variations of static properties due to an applied field seriously affect both the field-dependence of magnetic viscosity and that of complex susceptibility. In this study, the effects cancel out by comparing thermoremanent magnetization with the sum of zero-field-cooled magnetization and reversed-thermoremanent magnetization. The results obtained in the range from 2.0 to 11.0 K can be qualitatively explained by using classical models for superparamagnetic fluctuations.