Bulk-like first-order magnetoelastic transition in FeRh particles
Introduction
Discontinuous first-order magnetic phase transitions (FOMPTs) are commonly observed when magnetic and crystallographic phase transformations in solids occur in tandem due to the strong interplay between the spins and orbitals of the magnetic moment-carrying species and the underlying crystal lattice [1]. Over the years, first-order magnetic phase transitions have been connected to a number of potentially functional phenomena, such as giant magnetoresistance and giant magnetocaloric effects, unusually strong magnetostriction, and magnetic shape-memory effects, leading to a steady rise of interest in discovery, synthesis, processing, and characterization of materials that exhibit FOMPTs [2], [3], [4], [5], [6], [7], [8]. Hence, a fundamental understanding of the mechanisms of such transitions in various compounds and how materials respond to external stimuli, including temperature, pressure, and magnetic field, applied individually or in concert, was and remains the focus of numerous studies.
It is typical for first-order magnetic (dis)order-order transformations to coincide with changes in chemical bonding as well as the symmetry of crystal lattice, and those cases are known as magnetostructural transitions (MSTs). Among various classes of materials that exhibit first-order MSTs, notable examples include Gd5(GexSi1−x)4 when x ≅ 2 [9], [10], stoichiometric and off-stoichiometric Ni2MnX (Heusler) compounds, where X = p-block metal or metalloid [11], and MnTX, where T = Ni, Co, and Fe [12], [13], [14]. On the other hand, FOMPTs may also occur without changes in crystallographic symmetry across the corresponding magnetic transitions despite discontinuous changes in phase volume. Those kinds of FOMPTs, commonly known as magnetoelastic transitions (METs), are observed in a handful of materials, mostly transition metal-based, such as MnFeP1−xAsx [15], La(Fe1−xSix)13 and their hydrides [16], and FeRh [17] at or close to room temperature, although they have also recently been discovered in certain other lanthanide-based compounds, such as R2In, where R is light lanthanide [18], [19]. In general, first-order MSTs and METs are associated with thermomagnetic hysteresis, which becomes an impediment for applications where complete reversibility of an FOMPT is desired [20].
There is a general consensus that the hysteresis is considerably lower and easier to tailor in materials exhibiting first-order METs when compared to those that exhibit MSTs, fueling efforts focused on design and manipulation of the former by chemical substitutions, hydrostatic pressure and uniaxial strain, magnetic field, and temperature [20]. Further, effects of processing, more specifically effects of particle size on FOMPTs is a topic of both fundamental and applied interest because exploitation of magneto-functional properties in certain applications requires particles of active materials with sizes ranging from nanometers to micrometers, biomedical applications being a prime example [17], [21], [22], [23]. Compounds demonstrating FOMPTs are attractive for theranostic applications that involve magnetic resonance imaging [21], in vivo magnetic resonance thermometry [23], controlled delivery and release of drugs [22], [24], and cancer therapy via magnetic hyperthermia [22]. Particle size reduction, however, commonly affects both MSTs and METs, broadening the transitions substantially and, in some cases, completely hindering them [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. Fabrication of materials in the form of particles that retain sharp FOMPTs present in their bulk parents is indeed a known materials science challenge. Thus one of the objectives of this work is to demonstrate that carefully designed synthetic routes can address this long-standing issue by focusing on the nearly equiatomic FeRh particles and assessment of their structural and magnetic properties.
As summarized by Swartzendruber [35], a total of five different phases exist in the binary Fe – Rh system. They are listed in Table 1 together with their basic crystallographic information and ground-state magnetic properties; the corresponding phase diagram is reproduced in Fig. S1, Supplemental Information. The phase of interest is a nearly equiatomic α"–FeRh that contains between 47 and 53 at% Rh and crystallizes in an ordered CsC1-type structure, also known as the B2 structure. The α" phase has an antiferromagnetic (AFM) ground state and it displays a reversible, hysteretic FOMPT between the AFM and FM states near room temperature [36]. The AFM ↔ FM transition on heating (cooling) is accompanied by an isotropic expansion (contraction) of the unit cell volume by ~1% without changing crystal symmetry [37] and with a significant, 5.1 J/g, latent heat[38]. The temperatures of the AFM ↔ FM magnetic transition during heating and cooling match the corresponding elastic structural transition temperatures, hence the AFM ↔ FM phase transformation in α"–FeRh is first-order MET; hereafter, the temperature at which the MET occurs will be referred as Tt.
The magnetoelastic transition in α"–FeRh is sensitive both to hydrostatic pressure (P) and magnetic field (H): Tt increases with the application of hydrostatic pressure with = 5 K/kbar, and it decreases with the application of magnetic field at a rather high rate of = −8 K/T [39], [40]. The magnetoelastic response of the system is sensitive to the presence of impurities [41]. Among important functional properties that continue to draw research interest to α"–FeRh are giant caloric effects that can be actuated near room temperature by application and removal of magnetic field and/or pressure [41], [42], [43], [44]. Direct measurements of the inverse magnetocaloric effect in Fe51Rh49 alloy reveal adiabatic temperature change as high as ΔTad = –12 K upon first application of the magnetic field μ0ΔH = 1.9 T [45] and ΔTad = ± 6.5 K during repeated cycling between 0 and 1.9 T [46]. Those adiabatic temperature changes are among the highest reported near room temperature for any magnetocaloric material in the same magnetic field.
Although considerable work has been performed using α"–FeRh in bulk, thin film and composite forms [47], research on micro- or nanoparticles of the same remains in its infancy. As reported in [27], [28], [31] nearly equiatomic, ~20–50 nm and 100–200 nm FeRh nanoparticles synthesized, respectively, via polyol co-reduction and solid phase reduction methods, undergo hysteretic magnetic transitions at composition-dependent Tts ranging from 320 to 400 K. The transformations are broad, spanning 50–100 K, and the reported magnetic property data reveal that substantial ferromagnetic contributions – features that may be attributed to chemical inhomogeneity, wide particle size distribution, or/and surface oxidation – persist far below Tt. Further, Hillion et al. [48] and Dupuis et al. [49] found that 3–5 nm FeRh nanocrystals remain FM down to temperatures as low as 3 K, hence the known Fe – Rh phase diagram does not apply to alloy particles reduced to this length scale. Considering published data, preserving the sharpness of the MET characteristic to bulk α"–FeRh in fine particles remains an unresolved challenge. Here we demonstrate a solid-state mechanochemical redox synthesis of fine alloy powders with a nominal composition of Fe49Rh51 that exhibit a nearly discontinuous first-order MET similar to its bulk counterpart.
Section snippets
Experimental methods
In a mechanochemical redox syntheses, a mixture of metal salts is milled together with suitable reductants enabling synthesis of nano to sub-micron particles of otherwise difficult to synthesize materials, particularly ductile alloys like FeRh. This bottom-up powder preparation route provides good compositional and particle size control, and it affords unique microstructures, which significantly influence physical properties of the product [50]. As implemented here, during the first step of the
Structural properties of as-milled and annealed FeRh powders
The Rietveld refinements of structure models using room-temperature powder XRD patterns of all samples are shown in Fig. 1, with the results of Rietveld analyses listed in Table 2. The Bragg peaks observed in the AM-FeRh powder, Fig. 1(a), reflect a mixture of four phases – B2-FeRh, A1-FeRh, A1-Rh, and iron oxide, Fe3O4, with the first two being the major phases. Since the milling was performed in oxygen-free atmosphere, a minor amount of Fe3O4 likely formed when reduced Fe nanoparticles
Conclusions
Successful synthesis of B2-ordered α ´ /α′′–FeRh powders with fine particles that exhibit a sharp first-order magnetoelastic transition near room temperature is demonstrated via solid-state mechanochemical redox synthesis and subsequent heat treatments. Overall, this study presents the ability to control phase composition, microstructure, and physical behavior of FeRh particles using the mechanochemical synthesis. The relative stabilities of three different phases, α′, α′′, and γ, in the powder
CRediT authorship contribution statement
Anis Biswas: Investigation, Formal analysis, Writing – original draft. Shalabh Gupta: Conceptualization, Investigation, Formal analysis, Writing – original draft. Dustin Clifford: Investigation, Formal analysis. Yarosalv Mudryk: Formal analysis, Writing – review & editing. Ravi Hadimani: Writing – review & editing. Radhika Barua: Conceptualization, Formal analysis, Resources, Supervision, Writing – original draft, Writing – review & editing. Vitalij K. Pecharsky: Resources, Supervision, Writing
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
Research at Ames Laboratory is supported by the Materials Sciences and Engineering Division, Office of Basic Energy Sciences of the US Department of Energy (DOE). Ames Laboratory is operated for the US DOE by Iowa State University under Contract No. DE-AC02-07CH11358. Research at VCU was supported by VCU College of Engineering start-up funds and NSF-MRI Grant (1726617).
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