Journal of Magnetism and Magnetic Materials 577 (2023) 170780 Available online 29 April 2023 0304-8853/© 2023 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Research article Effect of DC bias on dipolar response of yttrium iron garnet Najnin Bano, K. Dey, A. Tripathy, P. Garg, U. Deshpande, D.K. Shukla ∗ UGC DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452001, India ARTICLE INFO Keywords: Dielectric spectroscopy Garnet XAS XPS XRD DC bias ABSTRACT Yttrium iron garnet (YIG) is a prominent magnetic (ferrimagnetic) insulator and an excellent material for wide ranging microwave and millimeter-wave applications. In polycrystalline form it exhibits high dielectric loss due to contributions coming from grain & grain boundaries, and Fe heterovalency. We have investigated the effect of dc bias voltage on the dielectric properties of YIG in a wide range of temperature (100 - 600 K) and frequencies (100 Hz - 1 MHz). Consistent with previous observations, without bias a dominant frequency dispersion due to hopping of the Fe 2+ & Fe 3+ is observed in low temperature region and a wider frequency dispersion is observed at higher temperatures arising due to grain-grain boundary contributions. Application of a small dc bias voltage increases the dielectric constant and reduces the dielectric loss significantly at higher frequencies. The DC bias-induced improvements are much more dramatic at higher temperatures, where grain boundary contributions dominate. Our detailed analysis of dc and ac conductivity data show that the dc bias suppresses the grain boundary contributions significantly and at the same time it boosts the polarizability due to the Fe 2+ /Fe 3+ hopping. 1. Introduction Yttrium Iron Garnet (YIG), Y 3 Fe 5 O 12 , is a highly sought-after mag- netic insulator known for its exceptional properties and versatile appli- cations. With a band gap of 2.85 eV [1], YIG exhibits a robust insulating character that makes it ideal for use in high-frequency devices. Its high chemical stability, extremely small magnetization damping ( ∼3× 10 −5 )[2], and high Curie temperature (T  = 560 K) make it a top choice for microwave engineering applications. Its low dielectric losses [2] makes it perfect for use in high-frequency communication systems, and in the field of spintronics [3]. Additionally, single crystal YIG is commonly used due to its lower dielectric losses and higher quality factor (Q factor), making it an ideal choice for microwave devices such as magnetically tunable filters, resonators, and frequency-selective surfaces [4]. It is a formidable task to synthesize high-quality YIG in both single crystalline and polycrystalline forms, as it is highly sensitive to the method of preparation, sintering temperature & duration [5–8]. The use of higher sintering temperatures lead to the formation of oxygen vacan- cies, which in turn causes heterovalency of the Fe ions (Fe 3+ , Fe 2+ ). Several groups have delved into the mysterious world of dielectric properties of YIG in both single crystalline and polycrystalline forms. Hirakata et al. studied dielectric properties of single crystal YIG in tem- perature range of 77–300 K and observed dispersive behavior caused by Fe 3+ , Fe 2+ hopping [9]. Sirdeshmukh et al. pioneered synthesis of ∗ Corresponding author. E-mail address: dkshukla@csr.res.in (D.K. Shukla). single crystal YIG via the flux growth method, and reported a room temperature dielectric constant value of 12.5 at 1 MHz frequency [10]. Wu et al. studied dielectric response of polycrystalline YIG in the temperature range of 125–620 K, unearthing previously unknown re- laxations resulting from heterovalency (Fe 3+ , Fe 2+ ), heterogeneity and grain boundaries [11]. It has been established that the hetero-valency of Fe ions (Fe 3+ , Fe 2+ ) give rises to the dielectric anomaly in YIG. In polycrystalline YIG beside hetero-valency of the Fe ions heterogeneous contributions of grain & grain boundaries also contribute to dielectric anomalies and increased dielectric losses. These undesirable contributions have restricted the use of polycrystalline YIG in microwave devices and other applications. Low dielectric losses required at higher frequencies may be achieved by suppressing heterogeneities in the sample by suitably controlling the synthesis parameters such as sintering temperature & duration etc. Control of these parameters can have greater beneficial effects on the phases and properties of YIG, however, so far by any means it has been extremely challenging to control the heterogeneity in YIG. Here, we demonstrate that the dc bias can play a dramatic role in controlling the contributions of the Fe hetero-valency and that of grain & grain boundaries in dielectric response. Significantly increased dielectric constant and decrements in the dielectric losses in polycrystalline YIG is achieved with the help of small dc bias. In this letter influence of dc bias voltage on dielectric response of YIG has been evaluated in a wide temperature and frequency range. https://doi.org/10.1016/j.jmmm.2023.170780 Received 16 February 2023; Received in revised form 14 April 2023; Accepted 25 April 2023