T. Mazet

Mn3Sn2: A promising material for magnetic refrigeration

Mn3Sn2 presents two second-order magnetic transitions of ferromagnetic origin at TC1∼262K and TC2∼227K. Both transitions give peaks of similar magnitude (ΔSMmax∼27mJcm−3K−1 for ΔH=5T) in the temperature dependence of the magnetic entropy change yielding an anomalous magnetocaloric response approaching that of a two-component hybrid material. Its refrigerant capacity of ∼1.1Jcm−3 (ΔH=5T) for an optimal reversible cycle with cold and hot ends at ∼220 and ∼280K is about half that of the best known magnetic refrigerants working around room temperature. However, Mn3Sn2 possess several advantages making it a promising candidate for magnetic refrigeration applications: (i) it has a large temperature span with a roughly constant ΔSM, (ii) it is not subjected to hysteresis losses, and (iii) it is made from low-cost and nontoxic elements.

Evidence of spin reorientation in YbFe6Ge6 from neutron diffraction and 57Fe Mössbauer experiments

High-resolution neutron diffraction and 57 Fe Mossbauer experiments have been performed on powder samples of the ternary intermetallic YbFe6 Ge6 . This compound crystallizes with a hexagonal structure (P 6/mmm ) which can be described as an ordered state intermediate between the HfFe6 Ge6 - and YCo6 Ge6 -type structures, with cell parameters suggesting that the Yb ion is in (or close to) a trivalent state. At room temperature, the Fe-sublattice magnetic arrangement consists of an antiferromagnetic stacking along the c -axis of the ferromagnetic (001) Fe planes with the easy direction of magnetization along [001]. Below about 85 K a spin-reorientation process occurs; a fraction (T ) of the iron moments rotate abruptly from the c -axis to a given direction in or close to the basal plane. This phenomenon allows the observation of anisotropic contributions to the total hyperfine field at the Fe site. At 4.2 K, approximately 18% of the iron moments remain along the c -axis. No long-range magnetic order of the Yb sublattice is observed, at least above 1.5 K. We also report 57 Fe Mossbauer investigations of the HfFe6 Ge6 -type LuFe6 Ge6 compound.

Macroscopic magnetic properties of the HfFe6Ge6-type RFe6X6 (X=Ge or Sn) compounds involving a non-magnetic R metal

Abstract The magnetic properties of the HfFe 6 Ge 6 -type RFe 6 Ge 6 (R=Sc, Lu, Ti, Zr, Hf and Nb) and RFe 6 Sn 6 (R=Sc, Lu and Zr) compounds have been investigated by susceptibility measurements in the 4.2–700 K temperature range and by magnetization measurements at 300 K and 4.2 K in fields up to 1.6 T. The germanides order antiferromagnetically with Neel points situated above 450 K and show deviation from simple uniaxial antiferromagnetic behavior at low temperature. The stannides order below about 570 K and are characterized by a small spontaneous magnetization. In both series the ordering temperature is found to slightly increase when increasing the R metal valence. The results are discussed and related to previously published powder neutron diffraction and 57 Fe Mossbauer spectroscopy data.

A magnetic study of Mg1−xCaxMn6Sn6 compounds (0.0≤x≤0.7).: First example of ferromagnetic HfFe6Ge6-type structure compounds

Abstract Investigations in the (Mg,Ca)–Mn–Sn system lead us to obtain new HfFe 6 Ge 6 -type Mg 1− x Ca x Mn 6 Sn 6 compounds (0≤ x ≤0.7). They order ferromagnetically between 290 K for x =0.0 to 253 K for x =0.6. The maximum magnetization value at 4.2 K ranges between 11.3 and 12.6 μ B mol −1 . The changes of magnetic properties between ferromagnetic alkaline earth compounds and antiferromagnetic rare earth compounds are discussed.

Valence change and magnetic order in YbMn6Ge6 ? xSnx

The YbMn(6)Ge(6-x)Sn(x) compounds (0 < x < 6) have been investigated using x-ray diffraction, magnetic measurements, neutron diffraction and (170)Yb Mössbauer spectroscopy. The YbMn(6)Ge(6-x)Sn(x) system comprises three solid solutions: (i) 0 < x ≤ 1.1, (ii) 3.2 ≤ x ≤ 4.6 and (iii) 5.3 ≤ x < 6, all of which crystallize in the hexagonal (P6/mmm) HfFe(6)Ge(6)-type structure. The substitution of Sn for Ge yields changes in the type of magnetic order (antiferromagnetic, helimagnetic, ferromagnetic, conical and ferrimagnetic), in the easy magnetization direction (from easy axis to easy plane) as well as in the valence state of Yb (from trivalent to divalent). The Mn moments order at or above room temperature, while magnetic ordering of the Yb sublattice is observed at temperatures up to 110 K. While Yb is trivalent for x ≤ 1.1 and divalent for x ≥ 5.3, both magnetic and (170)Yb Mössbauer spectroscopy data suggest that there is a gradual reduction in the average ytterbium valence through the intermediate solid solution (3.2 ≤ x ≤ 4.6), and that intermediate valence Yb orders magnetically, a very unusual phenomenon. Analysis of the (170)Yb Mössbauer spectroscopy data suggests that the departure from trivalency starts as early as x = 3.2 and the loss of ytterbium moment is estimated to occur at an average valence of ∼2.5+.

Magnetic structures and (x,T) magnetic phase diagram of ZrMn6Sn6−xGax

ZrMn6Sn6−xGax () compounds of HfFe6Ge6-type structure (P6/mmm) have been studied using powder neutron diffraction experiments in the 2–600 K temperature range. Below their magnetic ordering temperatures, these phases adopt commensurate easy-plane magnetic arrangements: +−− + antiferromagnetic AF2 () or ferromagnetic (). At low temperature, the Ga-poor compounds () exhibit a transition to concentration-dependent incommensurate configurations. Upon increasing the Ga content, this incommensurate state evolves from helimagnetic to fan-like through arrangements we termed antifan-like. The Mn moment value is close to for the Ga-rich and Ga-poor compounds and reaches a maximum amplitude of in the (anti)fan-like structures of intermediate concentrations. A tentative (x,T) magnetic phase diagram is presented. The occurrence of Lifshitz point(s) is forecast for 0.60