Separating the ferromagnetic and glassy behavior within the metal-organic magnetNi(TCNQ)2

Abstract

Publisher's copyright statement:Reprinted with permission from the American Physical Society: Physical Review B 92, 184431 c 2015 by the American Physical Society. Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modied, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society.Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. An in-depth study of the metal-organic magnet Ni(TCNQ) 2 was conducted where the deuterated form was synthesised both to attempt to alter the magnetic properties of the material and to be advantageous in techniques such as neutron scattering and muon spectroscopy. Deuteration saw a 3 K increase in T C with magnetization and heat capacity measurements demonstrating a spin wave contribution at low temperatures confirming the 3D nature of the ferromagnetic state shown by Ni(TCNQ − D 4 ) 2 . AC susceptibility results suggest there is a glassy component associated with the magnetically ordered state, though muon spectroscopy measurements did not support the presence of a spin glass state. Instead muon spectroscopy at zero magnetic field indicated the presence of two magnetic transitions, one at 20 K and another below 6 K; the latter is likely due to the system entering a quasistatic regime, similar to what one might expect of a superspin or cluster glass. Neutron diffraction measurements further supported this by revealing very weak magnetic Bragg peaks suggesting that the magnetism may have a short coherence length and be confined to small grains or clusters. The separation of the ferromagnetic and glassy magnetic components of the material's properties suggest that this system may show promise as a metal-organic magnet which is easily modified to change its magnetic properties, providing larger grain sizes can be synthesized.

A collapse of ferromagnetism in an organic based magnet under pressure

“…However, the transition is similar to that previously reported [13] at ambient conditions, with a T C of approximately 25 K, as can be seen in Figure 1, it should be noted that the high applied field may broaden the transition. The low temperature fall in the magnetisation is due to the onset of superconductivity within the small amount of Sn used to calibrate the applied pressure.…”

“…At ambient conditions, there is a clear increase in the relaxation rate below 20 K. This has previously been observed for NiTCNQ 2 , where the take of λ is related to the critical slowing down of magnetic fluctuations [13]. On application of pressure, this transition is destroyed and there is no dramatic rise present within the relaxation rate, except at the lowest temperatures where it is likely that the ferromagnetism has collapsed and the sample behaves more like a paramagnet, where the paramagnetic fluctuations slow down as T→0.…”

“…For Bloch's law to apply, β must equal 3/2. Previous results on Ni(TCNQ) 2 have shown that Bloch's law does fit at low temperatures well below T C [13]. The results are summarised in Table 1 The two set of results are strikingly different, where the saturation magnetisation is greatly reduced by the application of pressure.…”

“…Deuterated Ni(TCNQ) 2 was synthesised using TCNQ-D 4 , where the method was devised by Dolphin et al [14] Preparation of the Ni(TCNQ-D 4 ) 2 was done following the route of Clerac et al [10] and other details can be found in our previous works [11,13]. The deuteration was convenient for the muon spin relaxation measurements as it decreased the interactions of the muon with nuclear moments on the TCNQ, i.e.…”

“…Nickel Bis-7,7,8,8-tetracyanoquinodimethane (Ni(TCNQ) 2 ) is a ferromagnetic charge-transfer compound with a T C of approximately 20 K. The material is one in the series of M(TCNQ) 2 salts [10] and has previously been studied in detail by us [11,12,13], where we showed that the glassy nature of the magnetism was from a separate mechanism to the bulk 3D ferromagentic transition. Due to the low broad diffraction peaks it is hard to elucidate much information on the structure or indeed the level of crystallinity.…”