[Paper Review] Negative-pressure-induced helimagnetism in ferromagnetic cubic perovskites Sr$_{1-x}$Ba$_{x}$CoO$_{3}$

Hideaki Sakai, Shigeyuki Yokoyama+10 more|arXiv (Cornell University)|Aug 28, 2018

Magnetic and transport properties of perovskites and related materials4 citations

TL;DR

This study demonstrates that isotropic lattice expansion in ferromagnetic cubic perovskite Sr1-xBaxCoO3 induces a transition to incommensurate helimagnetic order near x = 0.4, driven by competition between ferromagnetic double exchange and antiferromagnetic superexchange as the lattice expands. Neutron diffraction reveals incommensurate magnetic scattering with a propagation vector Q ≈ (0.079, 0.079, 0.079), while first-principles calculations confirm a first-order transition to a helimagnetic ground state with [δδδ] symmetry, highlighting the role of bandwidth control in stabilizing topologically nontrivial magnetic phases.

ABSTRACT

Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries, as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite SrCoO$_3$ with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content $x$ in Sr$_{1-x}$Ba$_x$CoO$_3$ continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near $x$=0.4, where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron diffraction measurements. The emergence of helimagnetism is semi-quantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong $d$-$p$ hybridisation can exhibit a variety of novel magnetic phases originating from competing exchange interactions.

Motivation & Objective

• To investigate the emergence of helimagnetic order in ferromagnetic SrCoO3 under isotropic lattice expansion via solid solution with Ba.
• To determine the role of lattice size and bandwidth in stabilizing competing magnetic phases in cubic perovskite cobaltates.
• To explore the potential of Sr1-xBaxCoO3 thin films for strain-engineered spintronic devices based on helimagnetic and ferromagnetic phases.
• To clarify the microscopic origin of the helimagnetic instability using first-principles calculations.

Proposed method

• Synthesis of single-crystalline Sr1-xBaxCoO3 (0 ≤ x ≤ 0.4) using high-pressure techniques to achieve solid solutions with controlled lattice expansion.
• Measurement of magnetic and electronic properties via SQUID magnetometry, specific heat, and neutron diffraction to probe magnetic order and phase transitions.
• Neutron diffraction in reciprocal space to detect incommensurate magnetic scattering, with line profiles and Gaussian fitting used to extract propagation vector Q = (δ, δ, δ) with δ = 0.079(5).
• First-principles density functional theory (DFT) calculations to compute total energy as a function of magnetic propagation vector and lattice constant, identifying energy minima for helimagnetic vs. ferromagnetic states.
• Comparison of calculated energy landscapes for [δδδ] and [00δ] propagation vectors to determine the most stable helimagnetic structure.
• Analysis of magnetic correlation length from half-width at half-maximum (HWHM) of magnetic peaks, yielding ~12 Å for x = 0.4.

Experimental results

Research questions

• RQ1 How does isotropic lattice expansion in Sr1-xBaxCoO3 affect the stability of its ferromagnetic ground state?
• RQ2 What is the nature and symmetry of the emergent magnetic order near x = 0.4, and how does it differ from the parent SrCoO3 ferromagnet?
• RQ3 Can first-principles calculations reproduce the observed incommensurate helimagnetic correlations and predict the preferred propagation vector?
• RQ4 What is the microscopic origin of the helimagnetic instability—specifically, the competition between double exchange and superexchange mechanisms?
• RQ5 How does the bandwidth, controlled by lattice expansion, govern the transition between ferromagnetic and helimagnetic phases?

Key findings

• The cubic lattice constant of Sr1-xBaxCoO3 increases monotonically with Ba content, reaching a = 3.85 Å at x = 0.4, indicating isotropic lattice expansion.
• The ferromagnetic transition temperature TC decreases continuously with increasing x and vanishes near x = 0.4, where helimagnetic correlations emerge.
• Neutron diffraction detects incommensurate magnetic scattering with a propagation vector Q = (0 0 1) ± (δ δ δ) at δ = 0.079(5), corresponding to a periodicity of ~28 Å.
• The magnetic correlation length is estimated to be ~12 Å, indicating short-range helimagnetic order likely due to strong quantum fluctuations near the FM-HM phase boundary.
• First-principles calculations show that increasing lattice constant stabilizes a global energy minimum at δ ≈ 0.1 for the [δδδ] propagation vector, confirming the experimental observation.
• The calculated energy landscape favors the [δδδ] helimagnetic state over [00δ], with the former being more stable by ~10 meV per formula unit, consistent with the observed incommensurate order.