Magnetic order and anisotropy in hexagonal MnPtGa: Noncollinear magnetism contra frustration

MnPtGa is a hexagonal intermetallic compound with a rich variety of magnetic orders. Its magnetic state is reported to range from collinear ferromagnetism, to noncollinear skyrmion-type order. MnPtGa is a system with strongly localized magnetic moments at the Mn atoms, as was demonstrated using calculations for disordered local moments. The magnetic moments at the Mn sites stay at <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo>≈</a:mo> <a:mn>3.9</a:mn> <a:mspace width="0.222222em"/> <a:msub> <a:mi>μ</a:mi> <a:mi>B</a:mi> </a:msub> </a:mrow> </a:math> even above the calculated magnetic transition temperatures ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:mrow> <c:msub> <c:mi>T</c:mi> <c:mi>N</c:mi> </c:msub> <c:mo>=</c:mo> <c:mn>220</c:mn> <c:mspace width="0.16em"/> <c:mi mathvariant="normal">K</c:mi> </c:mrow> </c:math> or <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:mrow> <f:msub> <f:mi>T</f:mi> <f:mi>C</f:mi> </f:msub> <f:mo>=</f:mo> <f:mn>285</f:mn> <f:mspace width="0.16em"/> <f:mi mathvariant="normal">K</f:mi> </f:mrow> </f:math> ). In the present work, a special emphasis was placed on the possible noncollinear magnetic order using first-principles calculations. The investigations included magnetic anisotropy, static noncollinear orders in the form of spin canting, and dynamic noncollinearity in spin spirals. It is found that the energy differences between ferromagnetic, antiferromagnetic, canted, or spiral magnetic order are in the order of not more than 30 meV, which is in the order of thermal energies at ambient temperature. This hints that a particular magnetic state—including skyrmions, antiskyrmions, or spin glass transitions—may be forced when an external field is applied at finite temperature.