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Magnetic behavior of $5d^1$ Re-based double perovskite Sr$_2$ZnReO$_6$

Muhammad Maikudi Isah, Biswajit Dalal, Xun Kang, Dario Fiore Mosca, Ifeanyi John Onuorah, Valerio Scagnoli, Pietro Bonfà, Roberto De Renzi, Alexei A. Belik, Cesare Franchini, Kazunari Yamaura, Samuele Sanna

Abstract

The subtle interplay between spin-orbit coupling, exchange interactions, and cation ordering can lead to exotic magnetic states in transition-metal ions. We report a comprehensive study of the Re-based (5$d^1$) ordered double perovskite oxide Sr$_2$ZnReO$_6$ combining synchrotron x-ray diffraction (XRD), magnetic susceptibility, muon spin relaxation ($μ$SR) measurements, and density functional theory (DFT) calculations. XRD reveals that Sr$_2$ZnReO$_6$ crystallizes in the monoclinic structure (space group $P2_1/n$) at low temperature. Magnetic susceptibility data indicate a transition below $\sim$13 K, with $M$--$H$ loops showing ferromagnetic-like hysteresis and an unusually high coercive field of 23 kOe at 2 K. Zero-field $μ$SR measurements detect static and spatially disordered internal fields below $T_M \simeq $ 12 K, consistent with a canted antiferromagnetic ground state determined by detailed DFT and force-theorem in Hubbard-I calculations. The reduced high-temperature effective moment ($\sim0.76~μ_B$) and very small static moment ($\lesssim 0.2~μ_B$) derived from $μ$SR analysis and local-field simulations indicate a decisive role of spin-orbit coupling. Through a combined experimental and computational approach we unambiguously determine the canted antiferromagnetic order in Sr$_2$ZnReO$_6$, showing that a very small ordered moment coexists with an exceptionally large coercivity. These results underscore the crucial role of spin-orbit coupling and orbital ordering, providing new insights into magnetism in 5$d^1$ double perovskites.

Magnetic behavior of $5d^1$ Re-based double perovskite Sr$_2$ZnReO$_6$

Abstract

The subtle interplay between spin-orbit coupling, exchange interactions, and cation ordering can lead to exotic magnetic states in transition-metal ions. We report a comprehensive study of the Re-based (5) ordered double perovskite oxide SrZnReO combining synchrotron x-ray diffraction (XRD), magnetic susceptibility, muon spin relaxation (SR) measurements, and density functional theory (DFT) calculations. XRD reveals that SrZnReO crystallizes in the monoclinic structure (space group ) at low temperature. Magnetic susceptibility data indicate a transition below 13 K, with -- loops showing ferromagnetic-like hysteresis and an unusually high coercive field of 23 kOe at 2 K. Zero-field SR measurements detect static and spatially disordered internal fields below 12 K, consistent with a canted antiferromagnetic ground state determined by detailed DFT and force-theorem in Hubbard-I calculations. The reduced high-temperature effective moment () and very small static moment () derived from SR analysis and local-field simulations indicate a decisive role of spin-orbit coupling. Through a combined experimental and computational approach we unambiguously determine the canted antiferromagnetic order in SrZnReO, showing that a very small ordered moment coexists with an exceptionally large coercivity. These results underscore the crucial role of spin-orbit coupling and orbital ordering, providing new insights into magnetism in 5 double perovskites.

Paper Structure

This paper contains 8 sections, 2 equations, 5 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experimental and Computational Methods
  3. Results and Discussion
  4. Crystal structure
  5. Magnetic susceptibility and magnetization
  6. Muon sites
  7. Magnetic structure: FT-HI and muon local field simulations
  8. Summary

Figures (5)

  • Figure 1: (a) Synchrotron XRD pattern of Sr$_2$ZnReO$_6$ at room temperature [298(2) K]. The observed pattern is indicated by red crosses, the calculated pattern by the green solid line, and the difference profile by the blue line. Vertical bars mark the Bragg reflection positions for the tetragonal ($I4/m$; first green ticks) and monoclinic ($P2_1/n$; second violet ticks) models. Inset: DSC heat-flow curves on heating (red) and cooling (blue) measured at $10\ \mathrm{K}\ \mathrm{min}^{-1}$, showing an endothermic/exothermic pair near 300 K with thermal hysteresis. (b) Crystal structure of monoclinic Sr$_2$ZnReO$_6$ showing the network of the Zn/ReO$_6$ octahedra. The three inequivalent oxygen atoms are represented with spheres of different colors: O1 (red), O2 (green) and O3 (blue).
  • Figure 2: (a) Temperature dependence of ZFC (open dot) and FC (solid red line) dc magnetic susceptibility ($\chi (T)$; main axes) and inverse of ZFC DC magnetic susceptibility with the CW fit (solid line) ($(\chi-\chi_0)^{-1} (T)$; inset axes) in an applied magnetic field ($H=10$ kOe). (b) Isothermal magnetization ($M$) loop as a function of magnetic field ($H$) ranging from -70 to +70 kOe under ZFC mode at six different temperatures. (c) Temperature dependence of coercive field ($H_C$; left axes) and saturation magnetization ($M_S$; right axes) obtained from the ZFC $M(H)$ loops at $H=70$ kOe. The solid lines are guide to the eye unless otherwise stated.
  • Figure 3: (a) ZF-$\mu$SR asymmetry time spectra at varying temperatures above and below $T_M$. The right panel is a zoom for short acquisition time window. The data are fitted with the function shown in Eq. \ref{['eqn:ZFKT_FIT_EQN']}. The temperature dependence of the fitted (b) depolarization rate (longitudinal) $\lambda_{L1}$ and (c) the internal field $B_{\mu}$ of the oscillatory component, consistent with the magnetic order transition below $T_M$ . The solid red line are fit to the phenomenological function described in the text.
  • Figure 4: Zn/ReO$_6$ (gray/cyan sphere) octahedra showing the muon stopping sites (pink spheres) bonded to the three distinct oxygen sites (red for O1, green for O2 and blue for O3). The $\mu$--$\text{O}_i$ label describes the grouping of inequivalent muons bonded to a distinct O site (see Table \ref{['tab:muonsites']}). Plots are reproduced using the VESTA program Momma:ko5060.
  • Figure 5: (a) Schematic illustration of the magnetic configuration showing the canting angle $\phi$ with spins aligned along (011) plane and the proposed magnetic structure from the FT-HI calculations with canting angle $\phi= 55^{\circ}$ between the two magnetic sublattices, labeled Re1 (blue plane) and Re2 (green plane). (b) Comparison between the experimental ZF-$\mu$SR field distribution measured at $T=1.6$ K (black line) and the calculated local field distribution $\rho(B)$. The fits are shown for the proposed pristine magnetic structure ("undistorted"; solid magenta line) and for the case including the effects of lattice distortions to the magnetic ions when evaluating the field at the muon site ('distorted"; solid yellow line). The vertical ticks of same color is the calculated local fields at the muon sites using the magnetic moment obtained from the fit.