Title: Nanoscale magnetic phase competition throughout the Ni_50–xCo_xMn₄₀Sn₁₀ phase diagram: Insights from small-angle neutron scattering

Abstract

The Ni₂MnSn-derived Ni_50–xCo_xMn_25+ySn_25–y alloys are premier examples of a class of off-stoichiometric Heusler alloys recently discovered to exhibit attractive magnetic properties in tandem with extraordinarily reversible martensitic phase transformations. Multiferroicity, magnetic phase competition and separation, field-induced martensitic transformations, magnetic shape memory behavior, and sizable magneto-, elasto-, and barocaloric effects result, generating substantial interest and application potential. In this work we expand on a prior small-angle neutron scattering (SANS) study at a single composition (Ni₄₄Co₆Mn₄₀Sn₁₀) by exploring all three main regions of the recently established Ni_50–xCo_xMn₄₀Sn₁₀ phase diagram, i.e., at the representative y = 15 composition. Wide temperature and scattering wave-vector range (20–500K, 0.004–0.2Å^–1) SANS data on x = 2, 6, and 14 polycrystals provide a detailed picture of the evolution in magnetic order and inhomogeneity. Consistent with recent studies with a variety of techniques, phase separation into short-range coexisting ferromagnetic and antiferromagnetic regions is deduced below the martensitic transformation at x = 2 and 6, with average ferromagnetic cluster spacing of ~13 nm. Remarkably, at x = 14, where the martensitic transformation is suppressed and ferromagnetic austenite is stabilized to low temperatures, nanoscopic magnetic inhomogeneity nevertheless persists. Further, distinct ferromagnetic clusters (~36-nm average spacing) in a ferromagnetic matrix are observedmore » at intermediate temperatures, homogenizing into a uniform long-range ordered ferromagnet only at low temperatures. This unusual ferromagnet cluster/ferromagnet matrix inhomogeneity, as well as x-dependent subtleties of the superparamagnetic freezing of ferromagnetic clusters, are discussed in light of ⁵⁵Mn nuclear magnetic resonance data, and the recent observation of annealing-induced core/shell nanoprecipitates. The origins of nanoscalemagnetic inhomogeneity are discussed in terms of statistical variations in local composition and structure, tendency to chemical phase separation, and other forms of disorder.« less

Authors:

El-Khatib, S. ^[1]; Bhatti, Kanwal Preet  ^[2];

• Search DOE PAGES for author "Bhatti, Kanwal Preet"
• Search DOE PAGES for ORCID "0000-0001-6964-3379"

Srivastava, Vijay ^[2]; James, R. D. ^[2]; Leighton, C.  ^[2]

• Search DOE PAGES for author "Leighton, C."
• Search DOE PAGES for ORCID "0000-0003-2492-0816"

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+ Show Author Affiliations

1. Univ. of Minnesota, Minneapolis, MN (United States); American University of Sharjah (United Arab Emirates)
2. Univ. of Minnesota, Minneapolis, MN (United States)

Publication Date:

Thu Oct 17 00:00:00 EDT 2019

Research Org.:

Univ. of Minnesota, Minneapolis, MN (United States)

Sponsoring Org.:

USDOE Office of Science (SC); Vannevar Bush Fellowship; National Institute of Technology (NIST)

OSTI Identifier:

1802967

Alternate Identifier(s):

OSTI ID: 1575833

Grant/Contract Number:  

SC0016371; FRG-2014; FRG-2015; SC-0016371

Resource Type:

Accepted Manuscript

Journal Name:

Physical Review Materials

Additional Journal Information:

Journal Volume: 3; Journal Issue: 10; Journal ID: ISSN 2475-9953

Publisher:

American Physical Society (APS)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Materials Science