Long- and short-range magnetism in the frustrated double perovskite $\mathrm{B}{\mathrm{a}}_{2}\mathrm{MnW}{\mathrm{O}}_{6}$

Long- and short-range magnetism in the frustrated double perovskite $B a_{2} MnW O_{6}$

Heather Mutch, Otto Mustonen, Helen C. Walker, Peter J. Baker, Gavin B. G. Stenning, Fiona C. Coomer, and Edmund J. Cussen

Phys. Rev. Materials 4, 014408 – Published 27 January 2020

Abstract

The structural and magnetic properties of the face-centered-cubic double perovskite $B a_{2} MnW O_{6}$ were investigated using neutron powder diffraction, DC magnetometry, muon spin relaxation, and inelastic neutron scattering. $B a_{2} MnW O_{6}$ undergoes type II long-range antiferromagnetic ordering at a Néel temperature of 8(1) K with a frustration index, $f \approx 8$ . Inelastic neutron scattering was used to identify the magnetic coupling constants $J_{1}$ and $J_{2}$ , which were found to equal −0.080 and −0.076 meV, respectively. This indicates that both of the magnetic coupling constants are antiferromagnetic with similar magnitudes, which is in contrast to other known $3 d$ metal double perovskites of the form $B a_{2} M W O_{6}$ . Above the Néel temperature, muon spin-relaxation measurements and inelastic neutron-scattering techniques identify a short-range correlated magnetic state that is similar to that observed in the archetypical face-centered-cubic lattice antiferromagnet MnO.

Received 9 October 2019

DOI:https://doi.org/10.1103/PhysRevMaterials.4.014408

Physics Subject Headings (PhySH)

Exchange interaction Frustrated magnetism Magnetic interactions Magnetism Solid-state chemistry

Condensed Matter, Materials & Applied PhysicsInterdisciplinary Physics

Authors & Affiliations

Heather Mutch¹, Otto Mustonen ¹, Helen C. Walker², Peter J. Baker ², Gavin B. G. Stenning², Fiona C. Coomer ³, and Edmund J. Cussen ^1,*

¹Department of Material Science and Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, United Kingdom
²ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory, Harwell Campus, Didcot OX11 0QX, United Kingdom
³Johnson Matthey Battery Materials, Blount's Court, Sonning Common, Reading RG4 9NH United Kingdom

^*e.j.cussen@sheffield.ac.uk

Article Text (Subscription Required)

Click to Expand

References (Subscription Required)

Click to Expand

Issue

Vol. 4, Iss. 1 — January 2020

Reuse & Permissions

Access Options

Author publication services for translation and copyediting assistance advertisement

Images

Figure 1
Crystallographic structure of $BaMnW O_{6}$ with the magnetic exchange interactions $J_{1}$ and $J_{2}$ displayed.
Reuse & Permissions
Figure 2
DC magnetic susceptibility of $B a_{2} MnW O_{6}$ measured in an applied field of 1000 G between 2 and 300 K. The ZFC and FC susceptibility $(χ)$ are plotted on the main graph, while the inverse susceptibility $(χ^{- 1})$ can be seen in the inset. No divergence is observed between the ZFC and FC responses. The susceptibility curve indicates that $B a_{2} MnW O_{6}$ goes through a transition to a long-range antiferromagnetic state at 8(1) K.
Reuse & Permissions
Figure 3
(a) Neutron powder-diffraction pattern of $B a_{2} MnW O_{6}$ at 2 K. This contains both the nuclear phase (upper Bragg reflections) and the magnetic phase (lower Bragg reflections). The nuclear phase is characterized using the cubic space group $F m \bar{3} m$ , with lattice parameter $a = 8.187 30 (2) Å$ . The magnetic phase has the propagation vector $k = (1 / 2, 1 / 2, 1 / 2)$ . The solely magnetic peaks are outlined by an asterisk. (b) Thermal evolution of lattice parameter. A general increase is observed in the lattice parameter as the temperature increases, showing conventional thermal expansion behavior.
Reuse & Permissions
Figure 4
(a) Schematic of the magnetic structure of $B a_{2} MnW O_{6}$ . Only magnetic $M n^{2 +}$ ions are depicted within the crystallographic structure, with antiparallel magnetic moments outlined in red or blue. The nearest-neighbor (NN) interactions are depicted by the dashed line denoted by $J_{1}$ , while the next-nearest neighbor (NNN) interactions are signified by $J_{2}$ . (b) The thermal evolution of the ordered magnetic moment of $B a_{2} MnW O_{6}$ . This reaches a maximum value of $4.5 (1) μ_{B}$ at 2 K and decreases as the temperature tends to $T_{N}$ . (c) Thermal evolution of the magnetic Bragg peaks from neutron diffraction data collected on GEM. The magnetic Bragg peaks are absent at 9 K and increase in magnitude as the temperature decreases.
Reuse & Permissions
Figure 5
(a), (b) TF-µSR measurement carried out at 20 and 1.5 K, respectively. At 20 K, oscillations are observed as muons couple to the perpendicular field, indicating that the muons observe paramagnetic behavior. At 1.5 K, $B a_{2} MnW O_{6}$ , is magnetically ordered and therefore these oscillations are reduced in amplitude as only muons stopping in the silver sample holder do not experience the large magnetic fields inside the sample. (c) The asymmetry of the oscillating relaxation was then normalized and has been plotted against temperature. It is initially low between 2 and ∼8 K before a sharp rise at the transition temperature; this is followed by a more gradual increase between 8.5 and 30 K, which is attributed to short-range correlations. (d) ZF measurements carried out at a range of temperatures; above the transition the relaxation has a high asymmetry, while below $T_{N}$ the initial asymmetry is damped. (e) ZF measurement carried out at 20 K, fit using $A (t) = A_{1} e^{- λ_{1} t} + A_{2} e^{- λ_{2} t} + A_{3}$ . This fit is indicative of potential short-range correlations as a second exponential $A_{2}$ , corresponding to slowly fluctuating dynamic fields, is necessary to fit the data. (f) The longitudinal-field datasets collected at 30 K, which is significantly above $T_{N} = 8 (1) K$ . Full decoupling of the muon spin is not achieved even at a field of 4000 G, whereas in a paramagnetic state this should occur in low magnetic fields of up to 50-G strength. The high fields required could suggest short-range correlations between the $M n^{2 +}$ cations.
Reuse & Permissions
Figure 6
(a) Inelastic neutron-scattering data collected on LET at 2 K. Distinct spin waves can be observed due to long-range order within the system. (b) A graphical output of the SpinW simulation carried out to obtain the coupling constants $J_{1}$ and $J_{2}$ ; these were determined to be −0.080 and −0.076 meV, respectively, indicating antiferromagnetic interactions for both the NN and NNN interactions. Both the experimentally observed data and the simulation tally to a reasonable degree, indicating that the Heisenberg model used was consistent with the data. (c) A vertical cut through the dataset for $| Q | = 0.9 - 1.1 Å^{- 1}$ ; the left-hand solid line shows the simulation output using $J_{1} = - 0.080 meV$ and $J_{2} = - 0.076 meV$ , where it is clear that the peaks in the data intensity are well reproduced, while the right-hand solid line depicts only one peak in intensity which would be observed if $J_{2} = - 0.010 meV$ and $J_{2} = - 0.146 meV$ . It is clear from this that it is necessary for $J_{1}$ and $J_{2}$ to have similar magnitudes, where the peak splitting determines the ratio between $J_{1}$ and $J_{2}$ . (d), (e), (f) The observed inelastic neutron-scattering data collected at 12, 40, and 100 K, respectively. These are indicative of short-range magnetic excitations being present within the system above $T_{N}$ , which diminish as the temperature is increased.
Reuse & Permissions

Physical Review Materials

Long- and short-range magnetism in the frustrated double perovskite Ba2MnWO6

Heather Mutch, Otto Mustonen, Helen C. Walker, Peter J. Baker, Gavin B. G. Stenning, Fiona C. Coomer, and Edmund J. Cussen

Phys. Rev. Materials 4, 014408 – Published 27 January 2020

Abstract

Physics Subject Headings (PhySH)

Authors & Affiliations

Article Text (Subscription Required)

References (Subscription Required)

Issue

Access Options

Authorization Required

Other Options

Download & Share

Images

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Log In

Search

Article Lookup

Paste a citation or DOI

Enter a citation

Long- and short-range magnetism in the frustrated double perovskite $B a_{2} MnW O_{6}$