Giant Magnetocaloric Effect by Low Magnetic Field Metamagnetic Transition in Eu4Ga8Ge16 Compound

The Eu
4
Ga
8
Ge
16
is a homologous compound comparable with a cage-structured type-I clathrate Eu
8
Ga
16
Ge
30
. The crystal structure of the Eu
4
Ga
8
Ge
16
compound has an orthorhombic structure (Cmcm) with a unique one-dimensional arrangement of Eu atoms along the a-axis. The charge-balancing Ga/Ge framework encompasses the Eu atomic chains, resulting in the one-dimensional caged spin chain structure. The quasi-one-dimensional antiferromagnetic spin alignment along the a-axis shows low magnetic field metamagnetic transition at H
M
= 3.3 kOe at T = 2 K. The magnetic hysteresis curve at T = 2 K presents an unconventional inverse magnetic hysteresis (rapid spin saturation for increasing magnetic field and spin disordering for decreasing magnetic field) at intermediate magnetic field ranges, which may be related to the quasi-one-dimensional spin chain structure. The magnetocaloric properties of the compound exhibit high spin entropy change and adiabatic temperature difference, such as -ΔS
M
= 14.56 J kg
^-1
K
^-1
and ΔT
ad
= 7.77 K at T= 7.9 K, which are superior values than those of previously reported type-I clathrate Eu
8
Ga
16
Ge
30
compound. The exceptionally high values of magnetocaloric properties in the antiferromagnetic ground state of the Eu
4
Ga
8
Ge
16
may originate from the low magnetic field metamagnetic transition. Therefore, the high magnetocaloric materials do not restrict to materials showing the first-order ferromagnetic ordering but can be extendable to an antiferromagnetic ground state with low-magnetic field metamagnetic transition.