Low-temperature specific heat of the metallic glasses ${\mathrm{Fe}}_x{\mathrm{Ni}}_{80-x}{\mathrm{P}}_{14}{\mathrm{B}}_6$ and ${\left({\mathrm{Fe}}_y{\mathrm{Ni}}_{100-y}\right)}_{79}{\mathrm{P}}_{13}{\mathrm{B}}_8$ for the spin-glass and spin-cluster-glass regimes

The specific heat of the melt-spun metallic glasses ${\mathrm{Fe}}_x{\mathrm{Ni}}_{80-x}{\mathrm{P}}_{14}{\mathrm{B}}_6$ ($x=1,3,5,7,9,11,13,20\mathrm{}$) and the piston and anvil metallic glasses ${\left({\mathrm{Fe}}_y{\mathrm{Ni}}_{100-y}\right)}_{79}{\mathrm{P}}_{13}{\mathrm{B}}_8$ ($y=1,4,6,10,16\mathrm{}$) has been measured from 0.6 to 30 K, with the results being analyzed in terms of lattice, electronic, and magnetic contributions. ${\Theta }_D\mathrm{}\left(0\right)\mathrm{}$ and $\gamma$ as functions of iron concentration are given. For $x=1\mathrm{}$ and $y=1\mathrm{}$, the complete spin-glass anomaly in the magnetic specific heat ($C_M$) is analyzed, and the associated entropy is calculated. For all $x$ and $y$, a low-temperature linear coefficient of $C_M$ is determined and found to scale with the fraction of isolated spins up to 7 at.% iron. At higher concentrations, the linear term exceeds an isolated spin contribution due to collective effects arising in the spin-cluster-glass regime. Small differences in $C_M$ between the $x$ and $y$ series are attributed to fabrication techniques.