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Long range antiferromagnetic order in a rocksalt high entropy oxide Junjie Zhang, Jiaqiang Yan, Stuart Calder, Qiang Zheng, Michael A. McGuire, Douglas L. Abernathy, Yang Ren, Saul H. Lapidus, Katharine Page, Hong Zheng, John W Freeland, John D. Budai, and Raphael P Hermann Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00624 • Publication Date (Web): 26 Apr 2019 Downloaded from http://pubs.acs.org on April 28, 2019
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Long range antiferromagnetic order in a rocksalt high entropy oxide Junjie Zhang,†,* Jiaqiang Yan,† Stuart Calder,‡ Qiang Zheng,† Michael A. McGuire,† Douglas L. Abernathy,‡ Yang Ren,║ Saul H. Lapidus,║ Katharine Page,‡ Hong Zheng,┴ John W. Freeland,║ John D. Budai,† Raphael P. Hermann†,** †Materials
Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. ║X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA. ┴Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA ‡Neutron
ABSTRACT: We report for the first time the magnetic structure of the high entropy oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O using neutron powder diffraction. This material exhibits a sluggish magnetic transition but possesses a long-range ordered antiferromagnetic ground state, as revealed by DC and AC magnetic susceptibility, neutron diffraction and spectroscopy. The magnetic propagation wavevector is k=(½, ½, ½) based on the cubic structure Fm-3m, and the magnetic structure consists of ferromagnetic sheets in the (111) planes with spins antiparallel between two neighboring planes. Neutron spectroscopy reveals strong magnetic excitations at 100 K that survive up to room temperature. This work demonstrates that entropy-stabilized oxides represent a unique platform to study long range magnetic order with extreme chemical disorder.
INTRODUCTION High entropy oxides (HEOs),1 a class of materials based on a concept parallel to high entropy alloys,2-3 are solid solutions stabilized by high configuration entropies. Recently, these materials have attracted much attention due to fundamental issues related to phase formation and materials design as well as their potential multifunctional properties that can emerge from their high configurational entropy and spin order.4-8 (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O (hereafter MgO-HEO), the first entropy-stabilized oxide, was reported by Rost et al in 2015.4 Their choice of this system was based on the diversity in structures, coordination and cationic radii of MgO (Fm-3m), CoO (Fm-3m), NiO (Fm-3m), CuO (C2/c) and ZnO (P63mc). This material and its substitution by aliovalent elements were reported to exhibit colossal dielectric constants9 and superionic conductivity,10 making this new family of materials very promising for applications such as large-k dielectric materials and solid state electrolytes in solid-state batteries.1, 11-12 Inspired by the superionic conductivity, Sarkar et al reported the electrochemical properties of the rocksalt high entropy oxides, such as lithium storage capacity and the cycling stability.12 One of the fundamental interests in MgO-HEO is the possible Jahn-Teller distortion of the Cu2+ environment. Electron paramagnetic resonance,13 extended x-ray absorption fine structure14 and x-ray diffraction studies13 as well as theoretical calculations15-16 suggest this distortion. When combined with magnetic order, such distortions could provide a path to ferroic multifunctionality. In addition to studies of bulk samples, fundamental studies have been carried out on thin films.17-18 Recently, Braun et al reported the heat capacity and low thermal conductivity of MgO-HEOs.17 Meisenheimer et al reported antiferromagnetic order in MgO-HEO by studying the exchange anisotropy of permalloy/MgO-HEO heterostructures.18 However, up to date, there are no reports on the magnetic structure or dynamics on bulk samples. The observation of new functionality in MgO-HEO triggered the search for other types of high entropy oxides beyond the rocksalt structure. Fluorite,19-20 perovskite5-7 and spinel8 high entropy oxides have been reported. Besides structural studies, the fluorite high entropy oxides have been reported to have very low thermal conductivity.19 More recently, Witte et al studied the magnetic
properties of perovskite-structured high entropy oxides and found predominant antiferromagnetic behavior with a small ferromagnetic contribution using magnetometry and Mössbauer spectroscopy.21 In this contribution, we report the long range magnetic order in the rocksalt high entropy oxide MgO-HEO by measuring DC and AC magnetic susceptibility, heat capacity, neutron powder diffraction, and inelastic neutron scattering. The outline of the paper is as follows. First, we present the sample preparation and sample quality characterization using energy dispersive spectroscopy (EDS), high-resolution synchrotron x-ray powder diffraction, neutron pair distribution function, and x-ray absorption spectroscopy. Our results demonstrat that the cations are all 2+ and are homogeneously distributed in the rocksalt high entropy oxide. Second, we examine the physical properties. DC and AC magnetic susceptibility revealed a magnetic transition TN ~113 K, but heat capacity exhibits no anomaly around the transition, which is unexpected. Third, we performed neutron powder diffraction and inelastic neutron scattering experiments to understand the nature of the magnetic transition. Neutron powder diffraction unambiguously reveals NiO- or CoO-type long range antiferromagnetic order. Well-defined magnons observed at 100 K survive up to room temperature, suggesting magnetic fluctuations above TN. Next, we discuss the similarities and difference between the rocksalt high entropy oxide and the randomly diluted FCC systems (e.g., MgO-NiO solid solution) and find that the long range magnetic order of the rocksalt high entropy oxide cannot be well understood in the framework of diluted FCC system. Finally, we summarize our work in the concluding section. EXPERIMENTAL SECTION Synthesis. High entropy oxide MgO-HEO wwas prepared via standard solid-state reaction techniques according to Rost et al.4 Stoichiometric ratios of MgO (99.998%), Co3O4 (Puratronic), NiO (99.998%), CuO (99.995%) and ZnO (99.9995%) were weighted and ground in acetone thoroughly. The mixture were pressed into a pellet and then heated in air from room temperature to 1050 ºC at a rate of 3 ºC/min, and allowed to dwell at that temperature for 12 hours and then quenched in ice water. The solid was then reground
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and sintered twice at 1050 ºC with intermediate grinding using the same procedure and heating protocol. Energy dispersive spectroscopy (EDS). A small piece of MgOHEO pellet was polished for EDS analysis. EDS measurements were performed using a Bruker Quantax 70 EDS system on a Hitachi TM-3000 scanning electron microscope. Mg, Co, Ni, Cu and Zn K-edges were used for EDS element mapping and quantification analysis. Figure S1a shows a backscattered electrons (BSE) image for the polished MgO-HEO surface. EDS spectroscopy analysis was carried out for this region, with total acquisition time of 20 mins. EDS maps for Mg, Co, Ni, Cu, Zn using their K-edges are displayed in Figures S1b-f, respectively, showing homogenous distribution of these elements in this sample. We also quantified the atomic concentrations for the five cations, as shown in Figure S2, which are very close to the nominal composition 1:1:1:1:1. X-ray powder diffraction. Phase identification of the resultant black pellet was performed by powder X-ray diffraction (PXRD, PANalytical X’Pert PRO) using Cu Kα radiation (λ = 1.5418 Å). High resolution powder synchrotron x-ray diffraction was performed at 11-BM at the Advanced Photon Source (APS) at Argonne National Laboratory. A powder sample was prepared by attaching powders to the outside surface of a Φ0.8 mm Kapton capillary, which was previously coated with a thin layer of high temperature vacuum grease. The sample was spun continuously at 5600 rpm during data collection. Diffraction patterns were recorded at 295, 440, 400, 360, 240, 200-100 (20 K interval) and 90 K, with data in the 2θ range of 0.5-50º with a step size of 0.001º and step time of 0.2 s at 295, 440 and 90 K, and in the 2θ range of 0.5-26º with a step size of 0.0025º and step time of 0.2 s at other temperatures. An Oxford Cryostream 700 plus N2 gas blower was used for temperature control, and the cooling rate was set to be 5 K/min. The obtained diffraction data were analyzed using the TOPAS6 software. Refined parameters include background, intensity scale factor, sample displacement, unit cell, domain size, strain, atomic positions, and isotropic atomic displacement parameters (Mg, Co, Ni, Cu and Zn were grouped together and their site occupancy were fixed to be 0.2). X-ray absorption spectroscopy (XAS). To study the oxidization of cations in (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O, XAS was performed at beamline 4-ID-C at the Advanced Photon Source (APS), Argonne National Laboratory. Total scattering. X-ray and neutron total scattering experiments were performed at the 11-ID-C beamline at APS at Argonne National Laboratory, and at the Nanoscale Ordered Materials Diffractometer (NOMAD)22 beamline at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory, respectively. Polycrystalline powder was loaded into quartz capillaries. An identical empty capillary was also measured for background subtraction. The total scattering structure function S(Q)-1 was obtained experimentally, and the pair distribution function PDF G(r) is the Fourier transform of the observable total scattering function, S(Q):
∫
𝑄 = 𝑄𝑚𝑎𝑥
𝐺(𝑟) = (2/𝜋)
𝑄[𝑆(𝑄) ― 1]sin (𝑄𝑟)𝑑𝑄 𝑄 = 𝑄𝑚𝑖𝑛
where Q is the scattering vector, which is defined as Q=4πsinθ/λ, in which λ and θ are the radiation wavelength and scattering angle, respectively. For x-ray total scattering, values of 0 and 25 Å-1 were used for Qmin and Qmax, respectively, in the Fourier transform, while for neutron total scattering, values of 0.5 and 31.4 Å-1 were used for Qmin and Qmax, respectively. PDF data were analyzed using PDFgui software.23 Magnetic properties. Magnetic susceptibility measurements were performed on polycrystalline powder using a Quantum
Design MPMS SQUID Magnetometer. Zero-field cooled and field cooled data were collected between 2 and 380 K under an external field of 0.1 T. Magnetization was also measured as a function of magnetic field from -6 to 6 T at 300 K and 2 K. AC magnetic susceptibility was measured with a Quantum Design PPMS using an ac excitation of 0 Oe. Heat capacity. The heat capacity measurements were carried out in the Quantum Design PPMS using the relaxation method under a magnetic field of 0 T. Neutron powder diffraction. Neutron powder diffraction was performed on the HB-2A instrument at the High Flux Isotope Reactor (HFIR), ORNL. The monochromatic beam was selected by a stack of germanium crystals, with wavelengths of 2.41 Å and 1.54 Å from the 113 and 115 Ge reflections, respectively. The 2.41 Å wavelength was utilized for measurements to access the lowest Q available. The sample was contained in an Al cylindrical can and placed in a 4He cryostat with a temperature range of 1.5 K to 300 K. Data was collected on cooling from 300 K to base temperature. High statistics runs were obtained at 1.5 K and 160 K with 4 hour counting time. Inelastic neutron scattering (INS). Magnetic excitations of the rocksalt high entropy oxide has been measured using INS at the ARCS wide-angular-range chopper spectrometer at the Spallation Neutron Source at Oak Ridge National Laboratory.24 The sample mass was 8.8 g, which was loaded in a Al can. Inelastic spectra at 300 and 100 K were collected at two incident neutron beam energies, 140 and 30 meV. The spectra were corrected for sample environment background. The dynamic structure factor S(Q,E) was analyzed using the software Dave.25 RESULTS AND DISCUSSION Average crystal structure. We investigated the average crystal structure of MgO-HEO using high resolution synchrotron x-ray powder diffraction at 11-BM at APS, Argonne National Laboratory. The sample is single phase without any phase segregation within our resolution of ∆Q/Q~1.4x10-4 (detecting limit 0.3%wt of MgO, see Figure S3 and text in Supporting Information). The relative intensities of the Bragg peaks (111) and (200) deviates from the ideal rocksalt lattice, indicating anisotropic strain broadening. Such a phenomenon has been observed by Berardan et al.13 The Rietveld refinement on the data collected at room temperature with a single phase of Fm-3m converged to Rwp=12.5% (Figure 1a). A sketch of this MgO-HEO structure is shown in Figure 1b. In such a structure, Mg, Co, Ni, Cu, and Zn reside on the same site (000, Wyckoff position 4a) with octahedral oxygen coordination, and oxygen locates at (½,½,½) (Wyckoff position 4b). No structural phase transition was observed within our resolution down to 90 K, as no extra peaks appear (see Figure S4) or peak splitting occurs on cooling. The temperature dependence of lattice parameter from the Rietveld refinements is shown in Figure 1c, with a shrinking upon cooling. Linear thermal expansion coefficient is estimated to be 11×10-6 K-1 at room temperature, which is comparable to MgO. Local structure. The local structure of MgO-HEO obtained from total scattering of synchrotron x-rays (11-ID-C, APS) and neutrons (NOMAD, SNS), the latter of which yields better contrast for Co, Ni, Cu and Zn, can be described using the rocksalt structure with random cationic distribution. The x-ray pair distribution function (PDF) fit to the Fm-3m structure in the range of 1-49 Å (Figure 2b) converged to Rw=15.6%, demonstrating long range order of the atoms, consistent with x-ray powder diffraction analysis. The x-ray PDF fit in the 1-4 Å range converged to Rw=8.9% (Figure 2a), indicating that also the local structure can be described using Fm-3m. The neutron PDF in the 1-4 Å range was also fitted well using the cubic structure (Rw=7.6%, see Figure 2c),
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confirming similar local environments of Co, Ni, Cu and Zn. The real-space resolution attained with the present measurements is higher in the neutron case (~0.1 Å), but still precludes discrimination of small local distortions. Nonetheless, the shape of the nearest-neighbor peak is somewhat asymmetric, deviating from a single Gaussian function with extra intensity on the high-r side. This indicates that the local structure is not ideal rocksalt, which is consistent with a slightly distorted local environment resulting from the Jahn-Teller effect of Cu2+.13-16 However, the absence of a structural transition precludes a cooperative Jahn-Teller distortion. Valence states. In order to characterize the oxidation states of cations, we carried out x-ray absorption spectroscopy on MgOHEO. The oxidation states of Ni, Co, and Cu are 2+, as evidenced by the L edge x-ray absorption of Ni, Co and Cu as a function energy. Figure S5 shows the Ni, Co and Cu L edge, and Mg K edge absorption as a function of energy as well as their references. Mg, Co, Ni are 2+. For Cu, the shift in the inflection point between the MgO-HEO sample and the Cu metal is ~2 eV, which is consistent with a valence state of Cu2+ as a shift of ~1 eV per 1 e- charge was observed.26-30 Our result is in good agreement with Rost’s EXAFS report.14 Sample quality characterization summary. We combined EDS, high resolution synchrotron x-ray powder diffraction, and pair distribution function analysis, which are sensitive to different length scales from μm to Å, to evaluate sample quality. Our results provide very strong evidence that the cations are uniformly distributed, consistent with Rost et al.4 and Chellali et al.31 This is very important as we are discussing the high entropy oxide single phase, not mixed phases from phase segregation. All these measurements made sure that our sample was of high quality, providing a material platform for studying intrinsic physical properties, magnetic structure and dynamics. Magnetic susceptibility. Figure 3a shows the DC magnetic susceptibility as a function of temperature measured with a magnetic field of 0.1 T on warming after zero field cooling (ZFCW), cooling with magnetic field (FC-C) and warming with field (FC-W). An anomaly is immediately identified, which suggests an antiferromagnetic order. The transition temperature is found to be ~ 113 K by the peak position of d(χT)/dT (see Figure S6).32-33 The magnetic susceptibility continues to increase below 113 K. This suggests that not all of the local moments participate in the long range order, which is somewhat expected for such a strongly disordered system. The inverse magnetic susceptibility, 1/χ, is not linear above 113 K up to 380 K (see Figure 3b), suggesting nonCurie-Weiss behavior (see a modified Curie-Weiss fit in Figure S7). Such a deviation from the Curie-Weiss law may originate from short-range fluctuations, which is corroborated by the absence of an anomaly in heat capacity, the nonzero intensity of magnetic peaks above 113 K in neutron diffraction, and magnetic excitations at room temperature in inelastic neutron scattering, as will be discussed below. Magnetization was also measured as a function of magnetic field at 300 K and 2 K, and both show linear behavior (See Figure S8), which is consistent with paramagnetism at room temperature and an antiferromagnetic or spin glass state at low temperature. The real and imaginary part of the AC magnetic susceptibility as a function of temperature measured at various frequencies without magnetic field are presented in Figure 3c. No frequency dependence of the peak at 113 K is seen, and no peak in the imaginary part occurs near the transition. These observations are consistent with a long-range antiferromagnetic order, and inconsistent with spin-glass freezing. Anomalous heat capacity. Figure 3d shows the heat capacity as a function of temperature without applied magnetic field. Unexpectedly, no anomaly is observed around the magnetic transition. One plausible explanation is that the antiferromagnetic
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transition is not sharp and the entropy release occurs in a broad temperature range, corroborated by short range magnetic fluctuations above Néel temperature and magnetic excitations at room temperature (see Figure 4d and Figure S11). Magnetic Structure. To understand the nature of the phase transition at ~113 K observed in DC magnetic susceptibility, we performed neutron powder diffraction at HB2A at HFIR.34-35 The nuclear structure was Rietveld refined from data collected at 160 K using the cubic structure Fm-3m with the fit converging to Rwp=12.3%. Four extra peaks at Q=1.277 Å-1, 2.455 Å-1, 3.233 Å-1 and 3.856 Å-1 with similar width (see Figure S9) that do not coincide with nuclear Bragg reflections are observed at 2 K (see Figure 4a). The absence of these peaks at 160 K and the lack of a structural transition in synchrotron x-ray diffraction data confirm that they arise from antiferromagnetic order. Note that the background, ~350 counts/4 hours, comes from the spin incoherent scattering of cobalt. Indexing of these peaks leads to a commensurate propagation vector k=(½, ½, ½). The magnetic structure was determined based on representation analysis using SARAh36-37 and FullProf.38 For the space group Fm-3m with propagation vector (½, ½, ½) and magnetic ions on site 4a (0,0,0), there are two irreducible representations (IRs) (Γ3 and Γ5), following the labelling scheme of Kovalev,39 see Table S1. Both IRs were tested against the data and only Γ5 found to fit all the observed reflections (see Figure 4b). The magnetic structure consists of ferromagnetic sheets in the (111) planes, with spins antiparallel between two neighboring planes, as shown in Figure 4c. The ordered magnetic moment is 1.4(1) μB at 2 K, which is somewhat smaller than 2 μB expected for the average pure spin moment calculating from 1/3 of 3 μB from Co2+ (S=3/2), 1/3 of 2 μB from Ni2+ (S=2/2), and 1/3 of 1 μB from Cu2+ (S=1/2). The observation of reduced moment from experiments is understandable due to suppression of ordering throughout the lattice because of extreme chemical disorder and a significant amount of nonmagnetic ions, covalency (some of the moment is spread out over the oxygen), magnetic fluctuations, or isolated or poorly connected magnetic ions that simply do not order. Figure 4d shows the temperature dependence of the intensity at Q=1.277 Å-1 through the cooling and warming processes (see Figure S10 for temperature dependence of other magnetic peaks on cooling). The intensity on warming was measured on the fixed Q and counting continuous, while the intensity on cooling was extracted from diffraction data collected every 10 min at Q=1.277 Å-1, with temperature obtained by averaging the starting and ending temperatures for each scan. TN is estimated to be ~140 K at Q=1.277 Å-1, and higher Q give better agreement with TN ~113 K from magnetic susceptibility; however, the signal intensity is poorer. A plausible explanation for this observation is that neutron diffraction measurements cover a certain inelastic energy range around E=0 meV and the magnetic form factor decreases with the increasing of Q. The intensity above ~113 K suggests magnetic fluctuations, which is consistent with cluster ordering reported in Ref. 21. Such an observation strongly suggests that the magnetic order occurs in a relatively wide temperature range, in good agreement with the absence of an anomaly in heat capacity. Magnetic excitations. The antiferromagnetic origin of the phase transition at 113 K is also evidenced from collective spin dynamics, i.e. magnons, which were measured on polycrystalline powders by selecting two incident energies of neutrons (Ei=140 and 30 meV) at the Wide Angular-Range Chopper Spectrometer (ARCS) spectrometer at SNS. Figure 4e presents the dynamic structure factor S(Q,E) for the rocksalt MgO-HEO at 100 K with Ei of 30 meV. Strong magnons are observed at Q~1.277 with a steep slope, they die out quickly with the increasing of Q (see Figures S11b,d). These magnons survive up to 300 K (see Figures S11a,c). Such an observation indicates the existence of magnetic fluctuations,
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consistent with cluster ordering reported in another high entropy oxide by Witte et al. using Mössbauer spectroscopy.21 Integration of the S(Q,E) over 0-6 meV (elastic scattering) and 0-60 meV (diffraction, summing elastic and inelastic scattering) resulted in significantly different intensities (see Figure S11e) for both 100 and 300 K data, indicating the existence of strong magnetic fluctuations, even at a temperature well above 2×TN. Furthermore, significant elastic magnetic scattering at low angle is observed. These results are consistent with the observation of a non CurieWeiss behavior, a broad antiferromagnetic transition and the absence of a peak in the heat capacity. MgO-HEO vs randomly diluted FCC system. The existence of antiferromagnetic order in the rocksalt high entropy oxide is not unexpected. Both CoO and NiO have the NaCl-type face-centered cubic (FCC) crystal structure, and are antiferromagnetic with propagation wavevector of k=(½, ½, ½) and Néel temperature of 291 K for CoO and 523 K for NiO.40 The substitution with nonmagnetic ions such as Mg or Zn suppresses long range antiferromagnetic order in these FCC antiferromagnets, as was exemplified by the Co1-xMgxO solid solution which becomes a spin glass for x>0.53 and a paramagnet for x>0.87,41 and by Ni1-xMgxO which is a frustrated antiferromagnet (0.37
