Muon-spin-rotation studies of organic magnets

Abstract

A muon is an unstable spin-1 2 particle with a lifetime of 2.2 µs. Beams of spinpolarized positive muons can be prepared at accelerator facilities and then subsequently implanted in various types of condensed matter. Both the time and direction dependence of the subsequent positron emission can be monitored. This allows the precession and relaxation of the average muon-spin polarization to be measured and the local magnetic field in the sample to be directly inferred. The muon thus behaves essentially as a 'microscopic magnetometer' and is used to follow the magnetic order at a local level and to investigate both static and dynamic effects. This article outlines the principles of various experimental techniques that involve implanted muons, and reviews some recent experimental data on organic and molecular magnets.

The Internal Field in a Ferromagnetic Crystal with Chiral Molecular Packing of Achiral Organic Radicals

“…These fits allow the extraction of the precession frequencies as a function of temperature, and these are plotted in Figure 3a. Their magnitudes, both reaching ≈ 1 MHz as T → 0, are typical for precession signals that are measured in nitronyl nitroxide magnets [8][9][10][11][12][13], which result from a relatively dilute array of spin-1 2 moments, one per radical molecule. Both of the precession frequencies follow a typical temperature dependence for a magnetic order parameter and that can be fitted using the phenomenological function ν(T) = ν(0)(1 − (T/T C ) α ) β (the best fit parameters given in Table 1) and they provide a well constrained estimate of the critical temperature as T C = 1.105(1) K, which is consistent with the earlier measurements using magnetic susceptibility and heat capacity [21].…”

“…µSR experiments on p-NPNN show the development of coherent spin precession oscillations below T C [7,8]. A number of other nitronyl nitroxide systems were studied while using this technique [9][10][11][12][13], but the transition temperatures are all below 1 K.…”

Muon–Nitrogen Quadrupolar Level Crossing Resonance in a Charge Transfer Salt

“…One area where μSR experiments can provide some unique insight is in the field of magnetism and magnetic materials, in particular, when one is studying materials based on organic molecules. ,, The relatively weak and delocalized electronic magnetic moment on the organic molecule can prove difficult to measure using other techniques. Often, these organic-based materials take the form of charge transfer compounds , and they can also display other interesting physical properties such as electrical conductivity, ferroelectricity, and multiferroic behavior. , A prominent organic molecule used within these types of compounds is TCNQ (7,7,8,8-tetracyanoquinodimethane), a planar-conjugated molecule that is capable of stabilizing a radical anion leading to an S = 1/2 state, as shown in Figure .…”