Flat-band tuning and emergent itinerant magnetism in Sr(Co1−xPdx)2As2

Santanu Pakhira; Yongbin Lee; Asish K. Kundu; Farhan Islam; Zhenhua Ning; Volodymyr Smetana; Anja Verena Mudring; Thomas Heitmann; Elio Vescovo; Liqin Ke; David Vaknin; David C. Johnston

doi:10.1073/pnas.2519523122

Flat-band tuning and emergent itinerant magnetism in Sr(Co₁_−xPd_x)₂As₂

Santanu Pakhira
, Yongbin Lee
, Asish K. Kundu
, Farhan Islam
, Zhenhua Ning
, Volodymyr Smetana
, Anja Verena Mudring
, Thomas Heitmann
, Elio Vescovo
, Liqin Ke
, David Vaknin
, David C. Johnston

Research output: Contribution to journal › Article › peer-review

Abstract

The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi energy (E_F). In this study, we investigate this phenomenon in the Pauli paramagnet SrCo₂As₂, where an FB associated with Co e_g orbitals lies close to E_F. Remarkably, a minute substitution of the nonmagnetic element Pd onto the Co site (∼2%) induces antiferromagnetic order with a transition temperature as high as T_N = 25 K. Temperature- and magnetic-field-dependent magnetic and transport measurements, complemented by zero-field neutron diffraction, reveal a helical magnetic order for x ≤ 0.10 in Sr(Co_1−xPd_x)₂As₂, transitioning to a complex ferromagnetic state at higher Pd concentrations. Spectroscopic evidence and theoretical band structure calculations demonstrate that electron doping shifts the flat band closer to E_F, significantly enhancing the Stoner parameter. This enhancement drives a strong ferromagnetic instability, leading to helical magnetic ordering dominated by in-plane ferromagnetic interactions. The emergence of robust magnetic ordering through substitution with nonmagnetic elements is a unique phenomenon that underscores the pivotal role of flat-band instability in tuning magnetism in itinerant systems.

Original language	English
Article number	e2519523122
Journal	Proceedings of the National Academy of Sciences of the United States of America
Volume	122
Issue number	51
DOIs	https://doi.org/10.1073/pnas.2519523122
State	Published - Dec 2025
Externally published	Yes

Funding

ACKNOWLEDGMENTS. This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering. Ames National Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. DE-AC02-07CH11358. The research at Brookhaven National Laboratory was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704.

Keywords

Stoner mechanism
flat-band physics
helical magnetic order
itinerant magnetism

Access to Document

10.1073/pnas.2519523122

Cite this

Pakhira, S., Lee, Y., Kundu, A. K., Islam, F., Ning, Z., Smetana, V., Mudring, A. V., Heitmann, T., Vescovo, E., Ke, L., Vaknin, D., & Johnston, D. C. (2025). Flat-band tuning and emergent itinerant magnetism in Sr(Co₁_−xPd_x)₂As₂. Proceedings of the National Academy of Sciences of the United States of America, 122(51), Article e2519523122. https://doi.org/10.1073/pnas.2519523122

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abstract = "The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi energy (EF). In this study, we investigate this phenomenon in the Pauli paramagnet SrCo2As2, where an FB associated with Co eg orbitals lies close to EF. Remarkably, a minute substitution of the nonmagnetic element Pd onto the Co site (∼2\%) induces antiferromagnetic order with a transition temperature as high as TN = 25 K. Temperature- and magnetic-field-dependent magnetic and transport measurements, complemented by zero-field neutron diffraction, reveal a helical magnetic order for x ≤ 0.10 in Sr(Co1−xPdx)2As2, transitioning to a complex ferromagnetic state at higher Pd concentrations. Spectroscopic evidence and theoretical band structure calculations demonstrate that electron doping shifts the flat band closer to EF, significantly enhancing the Stoner parameter. This enhancement drives a strong ferromagnetic instability, leading to helical magnetic ordering dominated by in-plane ferromagnetic interactions. The emergence of robust magnetic ordering through substitution with nonmagnetic elements is a unique phenomenon that underscores the pivotal role of flat-band instability in tuning magnetism in itinerant systems.",

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author = "Santanu Pakhira and Yongbin Lee and Kundu, \{Asish K.\} and Farhan Islam and Zhenhua Ning and Volodymyr Smetana and Mudring, \{Anja Verena\} and Thomas Heitmann and Elio Vescovo and Liqin Ke and David Vaknin and Johnston, \{David C.\}",

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AU - Ning, Zhenhua

AU - Smetana, Volodymyr

AU - Mudring, Anja Verena

AU - Heitmann, Thomas

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AU - Johnston, David C.

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AB - The interplay between magnetism and flat-band (FB) instability is a central theme in quantum materials research. A striking example is the emergence of magnetic order in a nominally nonmagnetic compound when a flat band is tuned near the Fermi energy (EF). In this study, we investigate this phenomenon in the Pauli paramagnet SrCo2As2, where an FB associated with Co eg orbitals lies close to EF. Remarkably, a minute substitution of the nonmagnetic element Pd onto the Co site (∼2%) induces antiferromagnetic order with a transition temperature as high as TN = 25 K. Temperature- and magnetic-field-dependent magnetic and transport measurements, complemented by zero-field neutron diffraction, reveal a helical magnetic order for x ≤ 0.10 in Sr(Co1−xPdx)2As2, transitioning to a complex ferromagnetic state at higher Pd concentrations. Spectroscopic evidence and theoretical band structure calculations demonstrate that electron doping shifts the flat band closer to EF, significantly enhancing the Stoner parameter. This enhancement drives a strong ferromagnetic instability, leading to helical magnetic ordering dominated by in-plane ferromagnetic interactions. The emergence of robust magnetic ordering through substitution with nonmagnetic elements is a unique phenomenon that underscores the pivotal role of flat-band instability in tuning magnetism in itinerant systems.

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