Nuclear magnetic resonance studies of materials for spintronic applications

Abstract

Since its discovery in liquids and also in solid matter in 1946, nuclear magnetic resonance (NMR) has been widely established as a standard tool for structural analysis of a wide range of materials. This review outlines recent NMR studies on materials considered to be useful in spintronic applications. Spintronics is a new research field which combines the use of both the charge and the spin of an electron as information carriers, which promises distinct advantages over conventional electronics which makes use only of the charge of electrons. A successful application of materials in spintronic devices requires a detailed knowledge of the interplay between the structure and the magnetic and electronic properties on an atomic scale. NMR probes the local environments of the active nuclei. This local character of NMR arises from local contributions to the hyperfine field, namely, the transferred field which depends on the nearest neighbour atoms and their magnetic moments. This enables NMR to study the structural properties of bulk samples as well as of thin films of spin polarized materials. Moreover, NMR spectroscopy also provides an indirect tool to measure the density of states of spin polarized materials via a measurement of the temperature dependence of the spin–lattice relaxation time. This review starts with an introduction into the basic concepts of NMR followed by a description of the important aspects of a pulsed NMR experiment. Thereafter, information obtained by an NMR experiment is addressed. In the subsequent main part, selected recent NMR studies (published roughly after the year 2000) of materials for spintronic applications are presented including NMR studies of, for example, Co thin films, Heusler compounds, double perovskites and pyrites.

59Co nuclear magnetic resonance study of the local distribution of atoms in the Heusler compound Co2FeAl0.5Si0.5

“…The hyperfine field, regarding the 3d metals, mainly consists of contributions from the Fermicontact field. 13,14 The Fermi-contact field is related to interactions with the on-site magnetic moments and the transferred contact field from neighboring atoms. Thus, pronounced differences in resonance frequencies hint on pronounced differences in the magnetic environment of the NMR active atom.…”

Nuclear magnetic resonance reveals structural evolution upon annealing in epitaxial Co2MnSi Heusler films

“…H rest is an effective field originating from a resistance to magnetic oscillations, and is proportional to the square root of the optimal power (i.e., the power producing the maximum spin-echo intensity) of the applied rf pulses. 10,20 This provides a measure of magnetic stiffness or magnetic anisotropy on a local scale, compared with the macroscopic domain wall stiffness described by coercive fields from SQUID magnetometry. 10,20 Fig.…”

“…10,20 This provides a measure of magnetic stiffness or magnetic anisotropy on a local scale, compared with the macroscopic domain wall stiffness described by coercive fields from SQUID magnetometry. 10,20 Fig. 4 shows the T anneal -dependence of the square roots of the optimal power for the main P1 line, plotted along with the H c values for the samples as reported in Ref.…”

“…(See supplementary material for details of the NMR setup and experiments, 21 and Ref. 10 for an NMR review. )…”

“…A similar shoulder on the high frequency side is not observed. The absence of resonance lines on the high-frequency side of the main line rules out intermixing of any two (or all three) of the atoms from their nominal lattice positions, and hence the usual types of antisite disorder encountered in Heusler compounds [10][11][12] (see supplementary material for discussion on NMR signatures for other orderings). 21 Eliminating the common types of disorder, we can now look into the possibility of an element on one position replacing another on a different position one-sidedly, rather than an intermixing of both elements on both positions.…”