When high resolution matters: Cu Kα1 radiation data reveal subtle structural changes in MnBi
Permanent magnets are everywhere in our daily life: on our fridge, in our televisions, telephones, computers, audio- systems or automobiles. The technology is already very well developed and many materials including SmCo and Nd2Fe14B have been investigated. So why try to find new materials?
Most of these permanent magnets are using rare earth elements (e.g. Sm and Nd). Rare-earth elements are actually quite abundant and some of these elements are as common as copper. However, the deposits of minerals containing these elements are not very concentrated giving rise to high extraction costs. The number of viable ore deposits is small and they are concentrated only in few places globally. Consequently, it is of importance to find alternative rare earth-free materials which could replace materials such as SmCo or Nd2Fe14B. Here we present a detailed investigation of the crystal structure of MnBi, one of the most promising future rare earth-free materials for applications as a permanent magnet. Full details of the investigation are published by McGuire
et al. . Following is a summary of their findings plus some additional results demonstrating the powerfulness of the use of Kα1 radiation.
The X-ray diffraction measurements shown here were conducted on a PANalytical diffractometer, equipped with an incident-beam monochromator (Ge crystal for Cu Kα1 radiation) and an Oxford PheniX closed-cycle helium cryostat. A fixed 0.5° divergence slit and primary and secondary side 0.04 rad Soller slits were used with an X’Celerator detector.
Figure 1. Best resolution available for Cu Kα1 radiation using a unique symmetric Johansson monochromator, providing data with unrivaled resolution, excellent peak symmetry, and ultra-low backgrounds - perfect for structural analysis.
MnBi is ferromagnetic with a high Curie temperature of 630 K . This material crystallizes at room temperature in a NiAs-type structure with P63/mmc symmetry. At room temperature, the spins are aligned along the c-axis . Upon cooling, a spin reversal starts around 130 K and is completed at 90 K. Below 130 K the spins start to rotate away from the c-axis and gradually settle in the ab-plane at 90K . In such materials, it is expected that the magnetism is coupled to the crystal structure. When the spins are along the c-axis, this magnetic arrangement does not break the hexagonal symmetry of the crystal structure. However as soon as the spins starts to get away from the c-axis, hexagonal symmetry is no longer valid.
McGuire et al. started to investigate the low-temperature structure of this material during the spin reversal . Below about 90 K, the authors could identify for the first time the splitting of a reflection. We show in Figure 2 the (300) reflection below and above the complete spin reversal ( T = 90 K). Thanks to the high resolution available, they could show a structural change from hexagonal to orthorhombic (Cmcm) symmetry. This explained the change in coercivity below 90 K which is also bound to the crystal structure.
However, the magnetic spin reversal starts at Tp ~ 130 K. Below this, the hexagonal symmetry is supposed to be already breaking. While no splitting is visible between 130 K and 90 K, one would expect some structural signature that confirms a loss of hexagonal symmetry. For this purpose, we had a look at the full width at half maximum (FWHM) as function of temperature and in particular below Tp. As illustrated in Figure 3, the FWHM shows a clear increase below Tp demonstrating the structural signature of the pre-transition towards the orthorhombic symmetry at 90 K. Those results are the confirmation that the symmetry starts to be lowered below Tp as a result of the gradual spin rotation. This study was made possible thanks to the high resolution available with the use of PANalytical‘s Kα1 solution and in particular of the unique symmetric Johansson monochromator. We thank Dr. M. A. McGuire from Oak Ridge National Laboratory for sharing those data with us, giving us the opportunity to further examine the data and demonstrate the pre-transitional signature of the hexagonal to orthorhombic phase transition.
Figure 2. Splitting of the reflection (300) revealing the structural phase transition between P63/mmc and Cmcm symmetries in MnBi. To be noted: the splitting of the (300) reflection gives rise to a splitting of Δ2θ = 0.077° which can be resolved only by using Kα1 radiation (best resolution with Kα data at this angle: FWHM=0.063°).
Figure 3. Full width half maximum (FWHM) measured as function of temperature using Cu Kα1 radiation and an Oxford PheniX closed-cycle helium cryostat. Tp determines the temperature below which the FWHM increases significantly. This temperature corresponds to the structural signature of the beginning of the spin reversal in MnBi.
The Johansson monochromator in combination with a PANalytical diffractometer makes a powder diffractometer with exceptional high-resolution performance. Here, subtle crystallographic changes have been revealed in state-of-the-art magnetic materials, illustrating the importance of this unique product in cutting-edge materials’ research.