Metamagnetism Seeded by Nanostructural Features of Single‐Crystalline Gd5Si2Ge2

Abstract

Metamagnets are materials with first-order phase transition from one magnetically ordered phase to another that could provide environmentally friendly, highly efficient refrigeration near ambient temperatures using a phenomenon known as the magnetocaloric effect. Profoundly important to fundamental research and the application of room-temperature magnetic refrigeration (a solid-state cooling method that takes advantage of the entropy change associated with changes in magnetic order), is the need to understand how the metamagnetic transition evolves, and whether it can be manipulated by nanoengineering, or chemical substitutions to be sharper, occur at a lower magnetic driving fields, or be less hysteretic. Here, using Hall probe imaging, we show that inclusions of mesocopic platelets of Gd5Si1.5Ge1.5, that are inherent to all single crystals and polycrystals of this compound, seed the global metamagnetic transition to the ferromagnetic state across the Gd5Si2Ge2 crystal. We show that these inclusions, as well as well-defined mosaic boundaries, play a dramatic role on the transition properties. Strain and the associated field-driven volume changes underpin the phenomena that we have observed, and show that nanostructuring is the likely route to a more complete control of the critical properties in these materials. Themagnetic cooling field has a historic past, first conceived in the 1920’s by P. Debye and W. Giauque; the concepts have been used to reach extremely low temperatures in a process known as adiabatic demagnetization. Over the past ten years, the field has experienced a resurgence of interest after the discovery of giant magnetocaloric effects in a family of materials known as metamagnets, which are compounds that undergo a first-order transition to a magnetically ordered state, often accompanied by a coincident transformation of the crystal structure. Gd5Si2Ge2 has attracted a great deal of interest in this important area because it could play a pivotal role for energy-efficient magnetic cooling near room temperatures. It is a paramagnet (PM) above 274K with a monoclinic crystal structure; by applying a magnetic field, the material can be converted to a ferromagnetic (FM) state with a different, yet closely related, orthorhombic structure via a first-order transition. It has been shown that alloying Gd5Si2Ge2 with different elements substituting for either Si and Ge, or Gd can tune the critical temperature,Tc, but this leads to a significant shift in the magnetic character from first-order towards second-order behavior, which has an adverse effect on the magnitude of the magnetocaloric effect. Although these are interesting facts, it is true to say that a proper fundamental understanding of how to engineer the metamagnetic transition to optimize the magnetocaloric effect has yet to be established. Conventional structural characterization techniques, such as scanning and transmission electron microscopies (SEM, TEM), and X-ray diffraction have proved to be vitally important for understanding of the Gd5Si2Ge2 compound. High-resolution TEM has shown that all the Gd5Si2Ge2 crystals have a complex microstructure consisting of a dominant matrix of the singlephase parent material that is interspersed with extremely thin-plates (200 nm thick) of a second phase Gd5(SixGe1–x)3 material. In this sense the crystals are not pure, but single-crystal-like diffraction patterns are observed by backscatter Laue technique, and so for all intents and purposes these samples can indeed be considered single crystals. The platelets arrange themselves in a criss-cross pattern when viewed perpendicular or nearly perpendicular to the [010] face (see Fig. 1), with platelets extending for several hundred micrometers. When viewed perpendicular to both the [100] and [001] faces, the platelets