Delocalization and hybridization enhance the magnetocaloric effect in Cu-doped ${\text{Ni}}_2\text{MnGa}$

The structural and magnetic transitions in the shape-memory alloy ${\text{Ni}}_2\text{MnGa}$ can be tuned as a function of temperature by adding dopants. By altering the free energy such that the structural and magnetic transitions coincide, a giant magnetocaloric effect is created near room temperature. We show, using x-ray absorption spectroscopy and x-ray magnetic circular dichroism, how Cu, substituted for Mn, pulls the magnetic transition downward in temperature and also, counterintuitively, increases the delocalization of the Mn magnetism. At the same time, this reinforces the Ni-Ga chemical bond, raising the temperature of the martensite-austenite transition. At 25% doping, the two transitions coincide at 317 K.

Figure 1

(Color online) (a) Magnetization vs temperature curves for ${\text{Ni}}_2\text{MnGa}$ (closed red circles) and ${\text{Ni}}_2{\text{Mn}}_{0.75}{\text{Cu}}_{0.25}\text{Ga}$ (open blue triangles) for both increasing and decreasing temperatures measured in an external field of 1 kOe. (b) Neutron diffraction tracking the cubic (002) Bragg peak as a function of temperature for ${\text{Ni}}_2{\text{Mn}}_{0.75}{\text{Cu}}_{0.25}\text{Ga}$, showing the coexistence of cubic $\left(a=5.814\text{Å}\right)$ and tetragonal phases during the magnetostructural phase transition. Vertical dashed lines are guides to the eye to show peak positions.

Figure 2

(Color online) XAS curves at the (a) Ni, (b) Mn, (c) Cu, and (d) $\text{Ga}L$ edges for ${\text{Ni}}_2{\text{Mn}}_{0.75}{\text{Cu}}_{0.25}\text{Ga}$ at 80 (red line) and 330 K (black line). The XMCD curves at $T=80$, 300, and 330 K are in (e)–(h), respectively. The inset in (a) shows the Ni XAS around the peak marked B while the arrow in the inset of (b) shows the Mn XAS around the feature marked F. Green lines in (c) and (d), and (g) and (h) are guides to the eye to show the absence (presence) of XMCD at the $L_3$ peak of Cu (Ga), respectively.

Figure 3

(Color online) XAS curves at the (a) Ni and (b) $\text{Mn}L$ edges for ${\text{Ni}}_2\text{MnGa}$ (blue line) and ${\text{Ni}}_2{\text{Mn}}_{0.75}{\text{Cu}}_{0.25}\text{Ga}$ (red line) at 80 K. For clarity the XAS at the $L_3$ edge alone for Ni and Mn is replotted in (c) and (d), respectively.

Figure 4

(Color online) [(a)–(c)] XMCD curves for Ni, Mn, and Ga for ${\text{Ni}}_2\text{MnGa}$ (blue line) and ${\text{Ni}}_2{\text{Mn}}_{0.75}{\text{Cu}}_{0.25}\text{Ga}$ (red line) at 80 K. The peaks marked A and B are explained in the text. The arrow in (c) indicates increased XMCD for Ga at the $L_3$ edge for ${\text{Ni}}_2{\text{Mn}}_{0.75}{\text{Cu}}_{0.25}\text{Ga}$ as compared with ${\text{Ni}}_2\text{MnGa}$.