Order Magnetic Transformation Research Articles

Mechanism of magnetostructural transformation in multifunctional Mn3GaC

Mn3GaC undergoes a ferromagnetic to antiferromagnetic, volume discontinuous cubic-cubic phase transition as a function of temperature, pressure, and magnetic field. Through a series of temperature dependent x-ray absorption fine structure spectroscopy experiments at the Mn K and Ga K edge, it is shown that the first order magnetic transformation in Mn3GaC is entirely due to distortions in the Mn sub-lattice and with a very little role for Mn-C interactions. The distortion in the Mn sub-lattice results in long and short Mn-Mn bonds with the longer Mn-Mn bonds favoring ferromagnetic interactions and the shorter Mn-Mn bonds favoring antiferromagnetic interactions. At the first order transition, the shorter Mn-Mn bonds exhibit an abrupt decrease in their length resulting in an antiferromagnetic ground state and a strained lattice.

Phase-separated magnetic ground state in Mn3Ga0.45Sn0.55C

Existence of non-ergodic ground states is considered as a precursor to a first order long range magnetostructural transformation. Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C lies compositionally between two compounds, Mn$_3$GaC and Mn$_3$SnC, undergoing first order magnetic transformation. Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C though crystallizes in single phase cubic structure, exhibits more than one long range magnetic transitions. Using a combination of magnetization, ac susceptibility, neutron diffraction and XAFS techniques it is shown that, though Mn$_3$Ga$_{0.45}$Sn$_{0.55}$C exhibits long range magnetic order, it presents a cluster glassy ground state due to formation of magnetically ordered Ga rich and Sn rich clusters. The clusters are big enough to present signatures of long range magnetic order but are distributed in such way that it limits interactions between two clusters of the same type leading to a frozen magnetic state at low temperatures. The main reason for such a cluster glass state is the difference in local structure of Mn atoms that find themselves in Ga rich and Sn rich clusters.

Effect of local structural distortions on magnetostructural transformation in Mn3SnC

In this paper we attempt to understand the different nature of first order magnetic transformation in Mn3SnC as compared to that in Mn3GaC. The transformation in Mn3SnC is close to room temperature ( K) and is associated with a large change in magnetic entropy that makes it a suitable candidate for applications in ferroic cooling. Using a combination of x-ray and neutron diffraction and x-ray absorption fine structure spectroscopy we infer that the magnetic ground state consisting of antiferromagnetic and ferromagnetic Mn atoms is due to structural distortions present in Mn6C octahedra.

CoMnSb: A magnetocaloric material with a large low-field magnetic entropy change at intermediate temperature

A large magnetocaloric effect (MCE) has been achieved in a rare-earth-free CoMnSb alloy at low magnetic field and intermediate temperature. It is interesting to note that a relatively large magnetic entropy change ∣ΔSM∣ of 2.06J∕kgK with ΔH=0.9T was obtained around 471K. Arrott plots at various temperatures reveal that a second order magnetic transformation occurs near the Curie temperature. The origin of the large ΔSM under the conditions of intermediate temperature and low field is attributed to the magnetic transition from ferromagnetic to paramagnetic. These results suggest that CoMnSb alloy is a good candidate for large MCE when being utilized in intermediate temperature refrigeration systems for some special cases. This investigation demonstrates that some transition metal alloys with large magnetic moments may exhibit large MCE, which extends the region for exploring useful MCE materials.

Influence of oxygen on the giant magnetocaloric effect of Gd 5Si 1.95Ge 2.05

The influence of oxygen on the 5:4 (i.e. 5Gd:4[Si + Ge]) monoclinic Gd 5Si 1.95Ge 2.05 was investigated. Five oxygen-containing alloys, Gd 5Si 1.95Ge 2.05O x , were prepared with x varying from 0.05 to 0.5. The addition of small amounts of oxygen, x ≤ 0.10, stabilizes the orthorhombic form at high temperature, such that both phases are present in the arc-melted alloys. At higher oxygen contents, x ≥ 0.15, oxygen tends to favor the decomposition of the Gd 5Si 1.95Ge 2.05 phase into the GdSi y Ge 1− y (1:1) and the Gd 5Si z Ge 3− z (5:3) phases. The solubility of oxygen in the two polymorphic 5:4 phases is less than x = 0.05. The presence of oxygen in the monoclinic form destroys the giant magnetocaloric effect, apparently by preventing the monoclinic phase from transforming to the orthorhombic modification at the Curie temperature giving rise to a second order magnetic transformation.

Novel Thermal Effects at the First Order Magnetic Phase Transition in Erbium, and a Comparison with Dysprosium

In low temperature studies of ultrapure erbium (and dysprosium) we have discovered unusual thermal effects at the first order magnetic transformation of erbium $(\ensuremath{\cong}19\mathrm{K})$. These include (1) superheating (i.e., the metal is colder after heat has been added to it than before the heat pulse), (2) supercooling, and (3) the existence of metastable intermediate phases during this phase transformation in erbium (four on heating and two on cooling). In comparison, dysprosium exhibits both superheating and supercooling, but no intermediate metastable phases are observed. Furthermore, none of these effects are observed in less pure metals.

Oxidation of nickel-cobalt alloys in the range of Curie temperatures

The surface oxidation kinetics of metallographically polished nickel-cobalt alloys, 10.5 and 25.1% cobalt, have been studied in the temperature range 400° to 800°C using a vacuum microbalance technique. The alloy of largest cobalt concentration oxidized most rapidly. The oxidation rates have been represented, to a first approximation, by cubic and parabolic time laws. In the temperature range of the second order magnetic transformation below the Curie temperature, the Arrhenius temperature coefficients of the rate constants demonstrate continuous variations. It is proposed, with results for nickel oxidation as datum, that these variations are caused by electronic properties of the metal substrate, which influence the rate determining reaction step of metal ion diffusion through the surface oxide layer.