Reciprocating thermal behavior in the family of single ion magnets

Among the mononuclear transition metal and lanthanide complexes showing the slow magnetic relaxation, as a prerequisite of the single ion magnetism, there are examples of a non-traditional thermal behavior when at the low temperature the relaxation time, referring to the high-frequency relaxation channel, is shortened on further cooling instead of an expected prolongation. This reciprocating thermal behavior has been identified in a number of examples: for the Mn(II), Co(II), Ni(II), Cu(II), {Co(II)Co(III)}, {Gd(III)Ni(II)} and Dy(III) systems. The relaxation time in the low-temperature range can be recovered by applying equation τ–1 = CpbTl + FT–k where the phonon bottleneck process (rather than Raman process) and the “strange” process participate in the inverse relaxation time. The exponents of the temperature variable typically vary as l = 2–4 and k = 0.5–1.