Driving immobilized, single-domain magnetic nanoparticles at high frequency by square wave fields instead of sinusoidal waveforms leads to qualitative and quantitative improvements in their performance both as point-like heat sources for magnetic hyperthermia and as sensing elements in frequency-resolved techniques such as magnetic particle imaging and magnetic particle spectroscopy. The time evolution and the frequency spectrum of the cyclic magnetization of magnetite nanoparticles with random easy axes are obtained by means of a rate-equation method able to describe time-dependent effects for the particle sizes and frequencies of interest in most applications to biomedicine. In the presence of a high-frequency square-wave field, the rate equations are shown to admit an analytical solution and the periodic magnetization can be therefore described with accuracy, allowing one to single out effects which take place on different timescales. Magnetic hysteresis effects arising from the specific features of the square-wave driving field results in a breakthrough improvement of both the magnetic power released as heat to an environment in magnetic hyperthermia treatments and the magnitude of the third harmonic of the frequency spectrum of the magnetization, which plays a central role in magnetic particle imaging.

通过方波场而不是正弦波形以高频驱动固定的单畴磁性纳米粒子，可以定性和定量地提高其性能，无论是作为磁热疗的点状热源，还是作为频率分辨技术（例如磁力）中的传感元件。粒子成像和磁粒子光谱。具有随机易轴的磁铁矿纳米颗粒的循环磁化强度的时间演化和频谱是通过速率方程方法获得的，该方法能够描述大多数生物医学应用中感兴趣的颗粒尺寸和频率的时间依赖性效应。在存在高频方波场的情况下，速率方程允许解析解，因此可以准确地描述周期性磁化强度，从而允许人们挑选出在不同时间尺度上发生的效应。方波驱动场的特定特征产生的磁滞效应导致磁热疗中以热量形式释放到环境中的磁功率以及磁化频谱的三次谐波的幅度得到突破性改善，它在磁粒子成像中起着核心作用。