Viewpoint: Charge Hopping in Glassy Magnets

Gavin Lawes, Department of Physics and Astronomy, Wayne State University, Detroit, MI 48201, USA
Published March 19, 2012  |  Physics 5, 35 (2012)  |  DOI: 10.1103/Physics.5.35

In some transition metal oxides, disorder makes  the dielectric constant highly sensitive to an applied magnetic field over a broad range of temperatures.

Near-Room-Temperature Colossal Magnetodielectricity and Multiglass Properties in Partially Disordered La2NiMnO6

D. Choudhury, P. Mandal, R. Mathieu, A. Hazarika, S. Rajan, A. Sundaresan, U. V. Waghmare, R. Knut, O. Karis, P. Nordblad, and D. D. Sarma

Published March 19, 2012 | PDF (free)
+Enlarge image Figure 1

Figure 1 Schematic illustration of the mechanism for magnetodielectric coupling in partially antisite-disordered La2NiMnO6. Green: La; Blue: Ni; Red: Mn; Gray: Bridging O. The gray arrows on the O ions represent the magnetic interactions between neighboring magnetic ions. Charge transfer between the Ni2+ and Mn4+ produces a large dipole moment in the unit cell, which is suppressed by the ferromagnetic spin alignment in an applied magnetic field.

Magnetodielectric materials, which have a dielectric constant that is modulated by an applied magnetic field, provide rich insight into the physics of spin-charge coupling. The coupling between the dielectric constant and magnetic field—the magnetodieletric coupling—is often mediated by lattice dynamics [1], typically leading to only very small shifts in the dielectric constant. Some special classes of materials do exhibit exceptionally large shifts—a relative change in the dielectric constant of 500% in high magnetic fields [2]—but these effects are generally present only in a very narrow range of temperatures near a phase transition and are not suitable for robust device applications. Materials having a substantial magnetodielectric coupling at high temperatures are expected to lead to the development of novel devices, including capacitive magnetic field sensors and tunable high-frequency filters [3, 4]. One of the outstanding challenges in the study of magnetodiectrics is to identify mechanisms that can lead to strong spin-charge coupling, with the goal of producing large magnetically induced shifts in the dielectric response over a wide temperature range encompassing room temperature.

In a paper appearing in Physical Review Letters [5], Debraj Choudhury at the Indian Institute of Science, Bangalore, and coauthors propose a design for materials with strong magnetodielectric coupling based on tuning the spin-dependent electron hopping in site-disordered crystals. Their experimental and theoretical investigation on the magnetic and electrical properties of partially disordered La2NiMnO6 has produced the unexpected result that the spatial arrangement of the transition metal ions leads to large magnetodielectric coupling over a broad temperature range.

The authors considered the effects of chemical disorder—central to the work—on magnetodielectric coupling in the double perovskite La2NiMnO6. X-ray absorption spectroscopy measurements indicate that the nickel (Ni) and manganese (Mn) ions are present in the 2+ and 4+ valence states, respectively. Previous work on disorder-free La2NiMnO6 had established that the Ni2+-O-Mn4+ magnetic interactions are ferromagnetic [6]. When the Ni2+ and Mn4+ ions switch positions, referred to as antisite disorder, Mn4+-O-Mn4+ and Ni2+-O-Ni2+ additional magnetic interactions are formed in the system. These exchange interactions are antiferromagnetic and therefore reduce the saturation moment per unit cell of the sample; the measured moment indicates the presence of approximately 20% antisite disorder.

The antisite disorder modifies the temperature-dependent magnetic properties of La2NiMnO6. Both disorder-free and disordered La2NiMnO6 exhibit a ferromagnetic transition near 270K [6, 7]. In addition, the authors find strong evidence for a spin-glass-like phase developing at lower temperatures in the disordered sample, which they attribute to frustration arising from the competition between the ferromagnetic and disorder-induced antiferromagnetic interactions. The phase has a dramatic effect on the magnetodielectric coupling.

The antisite disorder also has important ramifications for the electrical properties of the system. Because dielectric measurements are often dominated by effects arising from charge accumulation near electrodes or grain boundaries [7], the authors first carefully separated these extrinsic contributions from the intrinsic behavior of disordered La2NiMnO6. Detailed analysis of the frequency-dependent dielectric measurements established that the capacitive response arose from two contributions, a relaxation term that dominates at low frequencies and one suitable for noninteracting dipoles (the Debye term) that becomes relevant at higher frequencies. The Debye component may be associated with the hopping of charge carriers between Ni2+ and Mn4+ sites, and can lead to the large dielectric constant of disordered La2NiMnO6, which is found to be significantly larger than that measured in disorder-free La2NiMnO6 [6].

Most significantly, the authors observed that the dielectric constant of antisite-disordered La2NiMnO6 has a strong dependence on magnetic field over a wide temperature range. Large magnetodielectric shifts are observed from temperatures as low as 100K, associated with the development of the glassy state, to room temperature, where the magnitude of the shift reaches 16%. This work complements an earlier study on disorder-free La2NiMnO6, which also found similar magnetodielectric shifts but over a smaller range of temperatures [6]. The large magnetic-field-induced shift in the capacitance of the disordered La2NiMnO6 cannot be readily attributed to extrinsic effects. While charge accumulation at grain boundaries or electrodes combined with magnetoresistive effects can cause a similar magnetocapacitive shift [8], the La2NiMnO6 sample in this study shows only a small magnetoresistance (less than 0.3%).

To explain their observations, the authors propose a model in which the large magnetodielectric coupling is produced by asymmetric charge hopping between the transition metal sites. This model is schematically illustrated in Fig. 1. In the presence of an electric field, electrons will hop between the Ni2+ and Mn4+ ions to produce a charge-transferred state with Ni2+δ and Mn4-δ. This introduces a large dipole moment through the unit cell, yielding the relaxation observed in the dielectric measurements. This hopping is enhanced by the antisite disorder in the system, which introduces midgap states associated with local deviations from the ferromagnetic order. The application of a large magnetic field reduces the concentration of these states, which is argued to produce the experimentally observed reduction in the dielectric constant. The authors make a case for a strong interplay between the magnetic properties and disorder in La2NiMnO6 with first-principles calculations, which show that a ferrimagnetic state is stabilized, relative to the ferromagnetic structure, by antisite disorder.

This work, taken together with earlier studies on disorder-free La2NiMnO6 [6], demonstrates the important role played by disorder in the coupling between spin and charge, and offers a mechanism for introducing large magnetodielectric shifts in insulating magnetic systems. While the study focuses on La2NiMnO6, the conceptual framework for this behavior may be translated to other antisite-disordered dielectric magnets. Experimental specifics, however, may prove challenging. The dielectric response is sensitive to extrinsic effects and charge accumulation. The introduction of disorder in other systems may increase the leakage current, limiting the suitability for dielectric applications. More fundamentally, dielectric ferromagnets are relatively rare, and it is not clear whether the very dramatic magnetodielectric effects observed in La2NiMnO6 associated with charge transfer will persist in other types of magnetic materials that have different spin arrangements. Despite these challenges, the mechanism for charge-hopping-mediated magnetodielectric effects presented in this Letter offers a new angle for exploring a fascinating and important instance of spin-charge coupling in insulating magnets.

References

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  5. D. Choudhury, P. Mandal, R. Mathieu, A. Hazarika, S. Rajan, A. Sundaresan, U. V. Waghmare, R. Knut, O. Karis, P. Nordblad, and D. D. Sarma, Phys. Rev. Lett. 108, 127201 (2012).
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About the Author: Gavin Lawes




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