Magnetic and Chemical Order in Pd2MnAl in Relation to Order in the Heusler Alloys Pd2MnIn, Pd2MnSn, and Pd2MnSb

The chemical and magnetic structures of the alloy ${\mathrm{Pd}}_2\mathrm{MnAl}$ , determined from magnetic and neutron diffraction measurements, are described, and compared with results recently obtained for the alloys ${\mathrm{Pd}}_2\mathrm{MnIn}$ , ${\mathrm{Pd}}_2\mathrm{MnSn}$ , and ${\mathrm{Pd}}_2\mathrm{MnSb}$ . ${\mathrm{Pd}}_2\mathrm{MnSn}$ and ${\mathrm{Pd}}_2\mathrm{MnSb}$ have the Heusler ${\text{(L2}}_1)$ structure, and are ferromagnetic with Curie temperatures of 189°K and 247°K, and magnetic moments, confined to the Mn atoms, of ${\text{4.23μ}}_B$ and ${\text{4.40μ}}_B$ , respectively. ${\mathrm{Pd}}_2\mathrm{MnIn}$ may be obtained in ordered ${\text{(L2}}_1)$ and partially disordered $\text{(B2)}$ forms by slow‐cooling or quenching, respectively. The ordered alloy is antiferromagnetic with a Néel temperature of 142°K and a magnetic moment of ${\text{4.3μ}}_B$ per Mn atom. The magnetic moments are aligned in ferromagnetic (111) planes, with neighbouring planes oriented antiparallel as in $\mathrm{NiO}$ . In the partially disordered regions the Pd atoms remain ordered, but disorder occurs between the Mn and In atoms resulting in a $\text{B2}$ , $\mathrm{CsCl}$ type, structure. The Mn atoms are still antiferromagnetically aligned, but on a magnetic unit cell of reduced size consistent with the change in chemical order. ${\mathrm{Pd}}_2\mathrm{MnAl}$ has the $\text{B2}$ chemical structure, and a magnetic structure identical to that of the $\text{B2}$ regions of ${\mathrm{Pd}}_2\mathrm{MnIn}$ . The alloy has a Néel temperature of 240°K, and a magnetic moment of ${\text{4.4}}_{\mu \mathrm{B}}/\mathrm{Mn}$ atom.