Phase transitions and magnetism in layer-structured(C18H37NH

Abstract

We have studied lattice dynamics and magnetism in the perovskite-type layered structure compound (C 18 H 37 NH 3 ) 2 MnCl 4 by means of nuclear magnetic resonance, electron paramagnetic resonance, and superconducting quantum interference device measurements. As a result, three structural phase transitions attributed to the hydrocarbon chains and the magnetic inorganic planes, as well as the spin relaxation behaviors, were identified and characterized.

Nuclear magnetic relaxation by regularly distributed dense paramagnetic ions in a quasi-two-dimensional system

“…partial chain melting [7][8][9]. Therefore, as observed in C18Mn [12], the abrupt drop in T 21 1S can be attributed to the reduction of the proton NMR second moment, and the transitions reflected in T 21 1S may be ascribed to the conformational transitions. The fraction of the T 1L in C10Mn was too small, just about 1%, to measure the temperature dependence.…”

“…Apparently, the fraction of the T 1L component increases with increasing chain length. As observed in C18Mn [12], the initial decay with a time constant of order of 10 ms can be attributed to the nuclear spin diffusion toward the electron spin system. On the other hand, the long-time decay with a time constant of order of 1 s can be assigned to the proton dipole -dipole interaction modulated by molecular motions, which will be discussed below in detail.…”

“…The (C n H 2nþ1 NH 3 ) 2 MnCl 4 powder samples were made according to previous works [12]. They were twice recrystallized and then vacuum dried, and DSC (differential scanning calorimetry) measurements were made to identify the phase transition temperatures, which were well consistent with the literature [3].…”

“…The 1 H NMR spin-lattice relaxation in C2Mn was shown to be dominated by the two-dimensional electron spin diffusion [1]. In our recent work for a long-chain compound, C18Mn, the 1 H NMR spin-lattice relaxation rate showed a logarithmic dependence on the Larmor frequency at room temperature, which indicates that the two-dimensional electron spin diffusion dictates the spin-lattice relaxation [12].…”

Apparent diamagnetism in quasi-two-dimensional Heisenberg antiferromagnets

“…The antiferromagnetic ordering temperature of C 12 Mn is T N = 43 ± 1 K and the intralayer exchange energy is about 6 K [22]. The humps around 160 and 230 K seem to be due to structural changes [24]. All the measurements described in this work were carried out for C 12 Mn.…”