Spin-carrier coupling induced ferromagnetism and giant resistivity peak in ${\mathrm{EuCd}}_{2}{\mathrm{P}}_{2}$

Spin-carrier coupling induced ferromagnetism and giant resistivity peak in ${EuCd}_{2} P_{2}$

V. Sunko, Y. Sun, M. Vranas, C. C. Homes, C. Lee, E. Donoway, Z.-C. Wang, S. Balguri, M. B. Mahendru, A. Ruiz, B. Gunn, R. Basak, S. Blanco-Canosa, E. Schierle, E. Weschke, F. Tafti, A. Frano, and J. Orenstein

Phys. Rev. B 107, 144404 – Published 4 April 2023

Abstract

$Eu {Cd}_{2} P_{2}$ is notable for its unconventional transport: upon cooling the metallic resistivity changes slope and begins to increase, ultimately 100-fold, before returning to its metallic value. Surprisingly, this giant peak occurs at 18 K, well above the Néel temperature ( $T_{N}$ ) of 11.5 K. Using a suite of sensitive probes of magnetism, including resonant x-ray scattering and magneto-optical polarimetry, we have discovered that ferromagnetic order onsets above $T_{N}$ in the temperature range of the resistivity peak. The observation of inverted hysteresis in this regime shows that ferromagnetism is promoted by coupling of localized spins and itinerant carriers. The resulting carrier localization is confirmed by optical conductivity measurements.

Received 9 August 2022
Revised 4 November 2022
Accepted 17 March 2023

DOI:https://doi.org/10.1103/PhysRevB.107.144404

Physics Subject Headings (PhySH)

Antiferromagnetism Birefringence Electrical conductivity Electronic phase separation Ferromagnetism Magnetic phase transitions Magneto-optical Kerr effect Magneto-optical effect

Kerr effect Polarized optical microscopy Reflectivity Resonant elastic x-ray scattering Symmetries in condensed matter

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

V. Sunko ^1,2,*, Y. Sun ^1,2,*, M. Vranas ³, C. C. Homes ⁴, C. Lee ², E. Donoway ^1,2, Z.-C. Wang ⁵, S. Balguri ⁵, M. B. Mahendru⁵, A. Ruiz³, B. Gunn³, R. Basak³, S. Blanco-Canosa^6,7, E. Schierle⁸, E. Weschke⁸, F. Tafti ⁵, A. Frano ³, and J. Orenstein ^1,2

¹Department of Physics, University of California, Berkeley, California 94720, USA
²Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
³Department of Physics, University of California, San Diego, California 92093, USA
⁴NSLS II, Brookhaven National Laboratory, Upton, New York 11973, USA
⁵Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA
⁶Donostia International Physics Center, DIPC, 20018 Donostia-San Sebastian, Basque Country, Spain
⁷IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain
⁸Helmholtz-Zentrum Berlin für Materialien und Energie, Albert-Einstein-Straße 15, 12489 Berlin, Germany

^*Theses authors contributed equally to this work.

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Vol. 107, Iss. 14 — 1 April 2023

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Figure 1
(a) and (b) The resistivity of $Eu {Cd}_{2} P_{2}$ (structure in the inset) shows a pronounced peak at $18 K$ . (c) Above $75 K$ it exhibits metallic behavior. Data are from Ref. [5].
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Figure 2
(a) Thermally modulated polarization rotation as a function of incident polarization and corresponding fits [Eq. (1)], measured at three sample locations (probe: $20 μ W$ , 633 nm; pump: $50 μ W$ , 780 nm, modulated at 2345 Hz). (b) and (c) Maps of (b) principal axis orientation and (c) $d M_{z}$ , extracted from the phase and offset of curves, such as those in (a) (step size: $25 μ m$ , light spot diameter: $5 μ m$ ). (d) Thermally modulated birefringence amplitude (teal) and $d M_{z}$ (red) as a function of temperature ( $T_{N} = 11.5 K$ ). Both signals onset at $\sim 2 T_{N}$ (the inset). (e) Normalized antiferromagnetic [blue; $(h, k, l) = (0, 0, 0.5)$ ] and ferromagnetic [orange; $(h, k, l) = (0, 0, 0.9)$ with structural scattering subtracted] resonant elastic x-ray scattering (REXS) as a function of temperature. AFM scattering onsets sharply at $T_{N}$ , in contrast to the gradual onset of ferromagnetism around $2 T_{N}$ , emphasized in the logarithmic plot in the inset.
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Figure 3
(a) The phase difference between the $H_{z}$ -linear birefringence (diamonds) and the $H = 0$ birefringence (circles) proves $γ \neq 0$ Eq. (3)], and, therefore, $M_{y} \neq 0$ [the inset of (c)]. For clarity the curves are normalized by the respective amplitudes, and constant offsets are subtracted. (b) Diagonal ( $β$ ) and off-diagonal ( $γ$ ) LMB: the orientation and length of the ellipse axes represent the orientation of the optic principal axes and the reflectivity along them, respectively. Magnetic point groups (MPGs) and order parameters compatible with the two effects are listed. (c) The amplitude of the LMB as a function of temperature ( $T_{N} = 11.5 K$ ) reveals the onset of TR—and $C_{3}$ —broken phase at $\sim 2 T_{N}$ . Probe beam: $20 μ W$ , 633 nm; field amplitude: 0.2 mT, modulated at 500 Hz.
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Figure 4
$d M_{z}$ as a function of $H_{z}$ for increasing (red) and decreasing (blue) field at (a) 3 K and (b) 14 K, revealing the opposite sense of hysteresis at the two temperatures. Removing the linear background in (b) shows the inverted hysteresis (the inset). (c) and (d) The real solutions of Eq. (6) for (c) $J < 2 α$ and (d) $J > 2 α$ . The full and dotted lines correspond to local minima and saddle points of the free energy, respectively. (e) The temperature dependence of the magnetization at zero field, defined as half the difference between the $d M_{z} (H = 0)$ , measured whereas increasing and decreasing the field.
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Figure 5
(a) Optical conductivity at 50, 18, and 5 K, showing electron localization at 18 K. The inset is the difference between the 5- and the 18-K data, showing the conduction electron spectral weight. The dashed lines are interpolations across a region where the measurement is dominated by the spectral weight of an optical phonon. (b) Schematic temperature evolution of the coupled Eu-electron system.
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Physical Review B

covering condensed matter and materials physics

Spin-carrier coupling induced ferromagnetism and giant resistivity peak in ${EuCd}_{2} P_{2}$

V. Sunko, Y. Sun, M. Vranas, C. C. Homes, C. Lee, E. Donoway, Z.-C. Wang, S. Balguri, M. B. Mahendru, A. Ruiz, B. Gunn, R. Basak, S. Blanco-Canosa, E. Schierle, E. Weschke, F. Tafti, A. Frano, and J. Orenstein

Phys. Rev. B 107, 144404 – Published 4 April 2023

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Spin-carrier coupling induced ferromagnetism and giant resistivity peak in EuCd2P2

V. Sunko, Y. Sun, M. Vranas, C. C. Homes, C. Lee, E. Donoway, Z.-C. Wang, S. Balguri, M. B. Mahendru, A. Ruiz, B. Gunn, R. Basak, S. Blanco-Canosa, E. Schierle, E. Weschke, F. Tafti, A. Frano, and J. Orenstein

Phys. Rev. B 107, 144404 – Published 4 April 2023

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Spin-carrier coupling induced ferromagnetism and giant resistivity peak in ${EuCd}_{2} P_{2}$