Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}

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Exchange bias and inhom*ogeneous spin states in ${La}_{1.5} {Sm}_{0.5} {NiMnO}_{6}$

R. Hissariya, N. Tripathi, S. K. Mishra, Vivekanand Shukla, and T. Brumme

Phys. Rev. Materials 8, 074403 – Published 8 July 2024

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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (1)$

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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (2)$

Abstract

The presence of antisite disorder in double perovskites manifests various intriguing properties like the spin-glass state, exchange bias, and memory effect. Here, we report the synthesis of a ${La}_{1.5} {Sm}_{0.5} {NiMnO}_{6}$ compound that crystallizes in a monoclinic ( $P 2_{1} / n$ ) structure. The presence of multiple oxidation states of Ni(Mn) cations induces competing (ferromagnetic and antiferromagnetic) exchange interactions that originate an inhom*ogeneous spin state, as evident from observed magnetic anomalies in temperature-dependent magnetization measurements. A spin-glass (SG) state is evolved that manifests field cooling ( $H_{CF}$ = 500 Oe) induced exchange bias ( $H_{EB} \sim 153$ Oe) below spin-glass temperature $T_{SG}$ (65 $\pm$ 1K). The strength of the exchange bias is reduced after successive magnetization reversal cycles performed at 5K. The reported magnetic training effect is explained within the frameworks of metastable magnetic disorder across frozen antiphase boundaries in the frustrated SG state. Measurements of frequency-dependent ac-susceptibility $χ (ω)$ suggest critical slowing dynamics and memory effect in the proximity of $T_{SG}$ , which is described using a critical slowing model resulting in relaxation exponent $z ν$ = 1.99 $\pm$ 0.04 and $τ_{0}$ = 8.91 x $10^{- 7} s$ . Employing first-principles calculations, we find the insulating ferromagnetic ground state of ${La}_{1.5} {Sm}_{0.5} {NiMnO}_{6}$ in the ordered phase where Ni(Mn) appears to be in the $2 + (4 +$ ) state. Further, the presence of antisite disorder eventually results in lower magnetic moments per formula unit, which is well corroborated by experimental observations. Our findings provide a pathway for designing host materials with inhom*ogeneous spin-frustrated systems and variable electronic states.

$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (3)$
$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (4)$
$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (5)$
$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (6)$
$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (7)$
$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (8)$
$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (9)$

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Received 4 March 2024
Accepted 13 June 2024

DOI:https://doi.org/10.1103/PhysRevMaterials.8.074403

Physics Subject Headings (PhySH)

Physical Systems

Magnetic nanoparticlesMagnetic systemsNanoparticlesSpin glassesStrongly correlated systems

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

R. Hissariya^*, N. Tripathi, and S. K. Mishra^†

School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi-221005, India

Vivekanand Shukla^‡

Chair of Theoretical Chemistry, Technische Universität Dresden, Bergstraße 66, 01069 Dresden, Germany and Computational materials Physics Laboratory, Department of Physics, Indian Institute of Technology Ropar, 140001- Punjab, India

T. Brumme^§

Chair of Theoretical Chemistry, Technische Universität Dresden, Bergstraße 66, 01069 Dresden, Germany

^*Contact author: ramanhissariya.rs.mst18@iitbhu.ac.in; Present address: Indian Institute of Technology, Bombay.
^†Contact author: shrawan.mst@iitbhu.ac.in
^‡Contact author: vivekanand.shukla@iitrpr.ac.in
^§Contact author: thomas.brumme@tu-dresden.de

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Vol. 8, Iss. 7 — July 2024

$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (10)$

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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (14)$
Figure 1
Room-temperature XRD pattern of LSNMO where the black open circle, red line, blue line, and green sticks represent the raw data, fitted data, difference, and Braggs position, respectively. Inset shows the polyhedral picture of the unit cell where the green and blue balls represents the La/Sm and light blue and brown octahedra denote ${NiO}_{6}$ and ${MnO}_{6}$ .
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (15)$
Figure 2
(a)ZFC/FC magnetization curves recorded under an applied external magnetic field of 100 and 500 Oe; (b) $d M / d T$ for dc magnetization in ZFC protocol at 500 Oe field value.
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (16)$
Figure 3
(a)M-H plots of LSNMO between 227K to 275K (in 4K steps); (b)Arrott plots of $M^{2}$ vs (H/M) at a different temperature around $T_{C}$ .
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (17)$
Figure 4
ZFC isothermal magnetization at 5K, 100K, 200K, and 300K. The left inset shows the variation of a coercive field with temperature, and the right inset shows pristine $M$ ( $H$ ) loops at 5K up to $+ 50$ kOe.
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (18)$
Figure 5
(a) $M$ ( $H$ ) loops measured at 5K after cooling the sample from 300K under a magnetic field of $+ 500$ Oe (red line), and $- 500$ Oe (green-line) representing the presence of exchange bias. (b) $M$ ( $H$ ) loop measured at 5K ZFC and FC mode in the presence of different cooling fields (0, 500, 1000 Oe). Inset shows the enlarged view.
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (19)$
Figure 6
(a)The zoomed view of hysteresis loop at 5K after 1000 Oe cooling field with 13 continuous cycles (magnetic training effect for EB). (b)The number of loops ( $n$ ) dependent on $H_{EB}$ extracted from training at 5K. The blue solid line represents the best fit using empirical power law and the purple solid line represents the best fit as proposed by Ref.[18].
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (20)$
Figure 7
(a)Real part of ac-susceptibility variation with the temperature at different frequencies 50, 100, 200, and 300Hz. Inset shows an enlarged view of cusp shifting, (b)ln( $τ$ ) versus ln( $T / T_{SG}$ -1) and 1/(T-T $_{VF}$ ) plots for sample LSNMO, wherein the blue and orange solid lines represent the best fits of Vogel-Fulcher and power law, respectively.
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (21)$
Figure 8
Temperature-dependent FC magnetization data during memory measurement. (b)Magnetic relaxation measurement at a different field and temperature; the black line represents the best fit using the stretched exponent function.
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (22)$
Figure 9
Upper panel shows the spin-resolved total density of the pristine LNMO in the left column and LSNMO in the right column for the chemical ordered and ferromagnetic phase. It also shows the density projections on $p$ orbital of oxygen atoms and La atoms in the right column; the left column also includes the contributions of the Sm atom. The bottom panels show the spin-polarized partial density of states for Ni- $t_{2 g}$ , Ni- $e_{g}$ and Mn- $t_{2 g}$ , Mn- $e_{g}$ , respectively. The Fermi level in the DOS is set to 0eV.
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$Exchange bias and inhom*ogeneous spin states in ${\mathrm{La}}_{1.5}{\mathrm{Sm}}_{0.5}{\mathrm{NiMnO}}_{6}$ (23)$
Figure 10
Two-dimensional slice projection of $2 \times 1 \times 2$ supercell of LSNMO shows the antisite defect ordering. In the left panel (system A), the Ni and Mn sites are exchanged cooperatively, and in the right panel (system B), Ni and Mn sites are exchanged in different unit-cell positions. In both systems, there are four Mn and four Ni atoms occupying their correct sites, and the rest of the eight octahedral; sites are occupied by an equal number of ${Mn}_{Ni}$ (Mn atoms occupying Ni) or ${Ni}_{Mn}$ (Ni atom occupy Mn sites) antisites, resulting in 50% antisite mixing in the ordered phase. System A has a mutual exchange of Mn and Ni positions in one subunit, whereas in system B, Ni and Mn atoms are exchanged from two different subunits in the supercell.
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