We highlight our experimental studies and micromagnetic simulations of the rotational hysteresis in exchange-spring magnets. Magneto-optical imaging and torque magnetometry measurements for Sm–Co/Fe exchange-spring films with uniaxial in-plane anisotropy show that the magnetization rotation created in the magnetically soft Fe layer by a rotating magnetic field is hysteretic. The rotational hysteresis is due to the reversal of the chirality of the spin spiral structure. Micromagnetic simulations reveal two reversal modes of the chirality, one at low fields due to an in-plane untwisting of the spiral, and the other, at high fields, due to an out-of-plane fanning of the spiral.

Rotational Hysteresis of Exchange-Spring Magnets

A general model of a hybrid interfacial domain wall (HIDW) in ferromagnetic/antiferromagnetic exchange biased bilayers is proposed, where an interfacial domain wall is allowed to extend into either the ferromagnetic or antiferromagnetic layer or across both. The proposition is based on our theoretical investigation on thickness and field dependences of ferromagnetic domain wall (FMDW) and antiferromagnetic domain wall (AFDW), respectively. Good match of the simulation to the hysteresis loops of a series of NiFe/FeMn exchange-biased bilayers confirms the existence of the HIDW, where the AFDW part is found to preferentially occupy the entire antiferromagnetic layer while the FMDW shrinks with the increased magnetic field as expected. The observed asymmetry between the ascending and descending branches of the hysteresis loop is explained naturally as a consequence of different partition ratios between AFDW and FMDW.

Model of hybrid interfacial domain wall in ferromagnetic/antiferromagnetic bilayers*

YCo5 alloy is processed by surfactant-assisted high-energy ball milling. Oleic acid is used as the surfactant and is 20% of the starting powder. The resultant particles are flakes of several microns in length and width, and 20–200 nm in thickness. The flakes have significant crystallographic anisotropy and the c-axis (also easy magnetization axes) is perpendicular to the flake's surface. Maximum coercivity of 192 kA/m is obtained in the sample milled for 100 min. Excess milling results in the appearance of Fe and deteriorates the permanent magnetic properties seriously.

http://iopscience.iop.org/article/10.1088/0256-307X/29/5/057501/pdf

Processing YCo 5 Permanent Magnetic Submicron Flakes by Surfactant-Assisted High-Energy Ball Milling

We develop an element-specific x-ray microscopy method by using Zernike phase contrast imaging near absorption edges, where a real part of refractive index changes abruptly. In this method two phase contrast images are subtracted to obtain the target element: one is at the absorption edge of the target element and the other is near the absorption edge. The x-ray exposure required by this method is expected to be significantly lower than that of conventional absorption-based x-ray elemental imaging methods. Numerical calculations confirm the advantages of this highly efficient imaging method.

https://iopscience.iop.org/article/10.1088/1674-1056/25/10/108702/pdf

Elemental x-ray imaging using Zernike phase contrast*

Research on exchange-spring magnets has focused on the microstructures of the materials. However, research has seldom been concerned with the effect of magnetic properties of soft magnetic phase on the energy product of an exchange-spring magnet. In this paper, a simple one-dimensional numerical simulation is used to investigate this effect in a Nd2Fe14B-based exchange-spring magnet. The results reveal that the larger the anisotropy constant, the stronger the exchange coupling, and the higher the magnetization of the soft magnetic phase, the larger the energy product of an exchange-spring magnet. This provides evidence for choosing a soft magnetic phase in an exchange-spring magnet.

Effect of magnetic properties of soft magnetic phase on the energy product of an exchange-spring magnet

A hard/soft SmCo5/Fe nanocomposite magnetic bilayer system is fabricated on x-ray transparent 100–200 nm thin Si3N4 films by magnetron sputtering. The microscopic magnetic domain pattern and its behaviours during magnetization reversal in the hard and the soft magnetic phases are studied separately by element specific magnetic soft x-ray microscopy at a spatial resolution of better than 25 nm. We observe that the domain patterns for the soft and hard phases show coherent behaviours in varying magnetic fields. We derive local M(H) curves from the images of Fe and SmCo5 separately and find the switches for hard and soft phases to be the same.

https://iopscience.iop.org/article/10.1088/1674-1056/19/3/037504/pdf

Layer resolved magnetization reversal study in SmCo 5 /Fe nanocomposite bilayers

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Layer resolved magnetization reversal study in SmCo 5 /Fe nanocomposite bilayers