Low-temperature synthesis of nanoscale ferromagnetic α'-MnB.

The search for tunable, size-dependent properties and unique processability has triggered the development of new synthetic routes for transition metal borides. MnB is a soft to semi-hard ferromagnetic material. This boride is now available by bottom-up, low-temperature solution chemistry. It is obtained as an unexpected metastable α'-variant that crystallises with a stacking-fault dominated CrB-type structure, as shown by transmission electron microscopy and X-ray powder diffraction (space group Cmcm, a = 300.5(8), b = 768.6(2), and c = 295.3(4) pm). The nanostructured powder consists of agglomerates of small particles (mean diameter of 85(41) nm) and transforms into well-known ß-MnB with FeB-type structure at 1523 K. The room temperature ferromagnetic behavior (TC = 545 K) is attributed to the positive exchange-correlation between the manganese atoms, that have many unpaired d electrons.

Mastering hysteresis in magnetocaloric materials.

Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La-Fe-Si-, Heusler- and Fe2P-type compounds.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.

Towards high-performance permanent magnets without rare earths.

Achieving a very strong magnetic anisotropy in a 3d material is a difficult, but not an impossible task. It is difficult because there is no general recipe (necessary condition) for a strong anisotropy in a band magnet. Several strategies can be pursued in this situation. One of them is to re-examine the less studied 3d compounds, somewhat neglected since the discovery of the Nd-Fe-B magnets 30 years ago. As an example, a single crystal of (Fe0.7Co0.3)2B has been investigated in this work.

Giant rotating magnetocaloric effect in the region of spin-reorientation transition in the NdCo5 single crystal.

We have investigated the anisotropy of the magnetocaloric effect in a NdCo5 single crystal in a wide range of temperatures, including the spin-reorientation temperature region. In the field µ(0)H =1.3 T in the spin-reorientation region 250-310 K, we discovered a giant rotating magnetocaloric effect of ~ 1.6 K, caused by rotation of the magnetization vector. The calculations of the anisotropy magnetocaloric effect for the field µ(0)H =1.3 T have been carried out.