Large energy product enhancement in perpendicularly coupled MnBi/CoFe magnetic bilayers

We demonstrate substantial enhancement in the energy product of MnBi-based magnets by forming robust ferromagnetic exchange coupling between a MnBi layer and a thin CoFe layer in a unique perpendicular coupling configuration, which provides increased resistance to magnetization reversal. The measured nominal energy product of $172\mathrm{kJ}/{\mathrm{m}}^3$ at room temperature is the largest value experimentally attained for permanent magnets free of expensive raw materials. Our finding shows that exchange-coupled MnBi/CoFe magnets are a viable option for pursuing rare-earth-free magnets with energy products approaching those containing rare-earth elements.

Figure 1

Density functional theory modeling of MnBi/$\mathrm{C}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_{1-x}$ interface. (a) Unit cell for periodically repeated MnBi(0001)/bcc-CoFe(111), shown with alternating layers of Co (five layers) and Fe (four layers) with Co at the interfaces (corresponding to a $\mathrm{C}{\mathrm{o}}_{0.56}\mathrm{F}{\mathrm{e}}_{0.44}$ soft layer). Mn (gray), Bi (purple), Co (green), and Fe (gold) atoms and cell boundaries (lines) are shown. Arrows indicate (anti) parallel alignment of magnetic moments across the interface. (b) Interfacial energy difference Δγ as a function of atomic layers for multilayer cell (a) having pure bcc Co and Fe, and $\mathrm{C}{\mathrm{o}}_{0.56}\mathrm{F}{\mathrm{e}}_{0.44}$ $\left(\mathrm{C}{\mathrm{o}}_{0.44}\mathrm{F}{\mathrm{e}}_{0.56}\right)$ with Co (Fe) at the interface.

Figure 2

Large energy product enhancement in perpendicularly coupled MnBi/$\mathrm{C}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_{1-x}$ bilayers: (a) micromagnetic schematic of the bilayer spin structure in a moderately high reverse field. The arrows indicate the spin directions in the MnBi and $\mathrm{C}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_{1-x}$ layers. $t_0$ is the critical thickness above which the magnetization of the soft layer has a negative contribution to the total magnetization; (b) experimental hysteresis loops of a pure MnBi, MnBi/Co (3 nm), and of MnBi/Co (5 nm) bilayers with the magnetic field perpendicular to the film plane, and simulated hysteresis loop of MnBi/Co (3 nm) bilayer (green line). We note that magnetization values in the experimental curves have been adjusted to take into account a small amount of voids formed in the film (≈5% of the total film volume); (c) ${\left(BH\right)}_{\max }$ of MnBi/$\mathrm{C}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_{1-x}$ bilayers for different soft-layer compositions and thicknesses; (d) experimental ${\left(BH\right)}_{\max }$ as a function of Co concentration $x$ of $\mathrm{C}{\mathrm{o}}_x\mathrm{F}{\mathrm{e}}_{1-x}$ for bilayers with 3 nm of soft layer. The uncertainty of 24 kA/m for the saturation magnetization of MnBi film in the text arises from the uncertainty in the thickness measurement.

Figure 3

X-ray magnetic circular dichroism (XMCD): Out-of-plane element-sensitive XMCD hysteresis loops for MnBi (20 nm)/$\mathrm{C}{\mathrm{o}}_{0.7}\mathrm{F}{\mathrm{e}}_{0.3}$ (5 nm) Mn (a), Co (b), and Fe (c).

Figure 4

Reversal mode in MnBi/$\mathrm{C}{\mathrm{o}}_{0.7}\mathrm{F}{\mathrm{e}}_{0.3}$ is probed by polarized neutron reflectometry: (a) depth profiles of the structure. $\mathrm{C}{\mathrm{o}}_{0.7}\mathrm{F}{\mathrm{e}}_{0.3}$ and MnBi layer regions are indicated by green and light blue, respectively. (b), (c) In-plane component of the magnetization (red lines) obtained from fits to polarized neutron reflectometry measurements where the bilayer was first magnetized in the out-of-plane directions (+2000 mT), and reversed out-of-plane field was applied in the order of −5 mT (b), and −200 mT (c). Yellow and brown denote the $\mathrm{C}{\mathrm{o}}_{0.7}\mathrm{F}{\mathrm{e}}_{0.3}$ and MnBi in the OOMMF simulation insets. In (b) and (c), the blue arrows describe the magnetic field direction and the short black arrows depict the corresponding successive evolution of the magnetization profile.