Characterization of HoCo3 nanoflakes synthesized via high energy ball – milling

Materials Chemistry and Physics 194 (2017) 105e117

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Characterization of HoCo3 nanoflakes synthesized via high energy ball e milling Anna Bajorek a, b, *, Krystian Prusik b, c, Maciej Zubko b, c, Marcin Wojtyniak a, b, _ Grazyna Chełkowska a, b a b c

A. Chełkowski Institute of Physics, University of Silesia in Katowice, Uniwersytecka 4, 40e007 Katowice, Poland w, Poland Silesian Center for Education and Interdisciplinary Research, University of Silesia in Katowice, 75 Pułku Piechoty 1A, 41-500 Chorzo w, Poland Institute of Materials Science, University of Silesia in Katowice, 75 Pułku Piechoty 1A, 41-500 Chorzo

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


HEBM method was used for synthesis HoCo3 nanopowders based on its parent compound.
The morphology of as e milled specimens is dependent on the pulverization duration (t).
The fabrication of nanoflakes-like particles with thickness dependent on t were evidenced.
The decrease in crystallites/particles size over t is confirmed by XRD, TEM and AFM.
HEBM results in the non-linear variation of magnetic parameters.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 November 2016 Received in revised form 25 February 2017 Accepted 16 March 2017

The microstructure and magnetic properties of bulk crystalline and ball e milled binary HoCo3 intermetallic compounds were studied. The presence of PuNi3 type of crystal phase was confirmed in all investigated specimens. The emergence of a partly amorphous phase was demonstrated at the end of grinding process. The influence of the applied high energy ball e milling parameters on the microstructure was investigated. The influence of the increased milling time on the final size of particles and crystallites was characterized by a variety of complementary measurement methods. The fabrication of nanoflakes and agglomerates for long pulverization times was demonstrated. Furthermore, the nonlinear dependence of the saturation magnetization, coercivity and remanence driven by high energy ball e milling was evidenced. The enhancement of the coercivity for a low milling time was shown. A quantitative analysis of the XPS spectra over ion etching indicates the formation of Co oxides on the nanoflakes surface. © 2017 Elsevier B.V. All rights reserved.

Keywords: Intermetallics Mechanical milling Nanostructured materials Magnetic properties X-ray photoelectron spectroscopy (XPS)

1. Introduction * Corresponding author. A. Chełkowski Institute of Physics, University of Silesia in Katowice, Poland. Tel.: þ48 32 3497582. E-mail addresses: [email protected] (A. Bajorek), [email protected]. pl (K. Prusik), [email protected] (M. Zubko), [email protected] (M. Wojtyniak), [email protected] (G. Chełkowska). http://dx.doi.org/10.1016/j.matchemphys.2017.03.034 0254-0584/© 2017 Elsevier B.V. All rights reserved.

The application of the high energy ball-milling (HEBM) method for the rare earth-based intermetallics is driven by the development of an innovative nanocomposite permanent magnets with

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enhanced properties [1,2,4e6]. The HEBM allows for the production of nanopowders with the particles size and shape controlled by selection of various parameters [1,2,6e25]. Among others, the milling time is a major parameter that greatly influences the morphology and the crystal structure of investigated materials [5,26,27]. This leads to significant changes of the magnetic properties, such as the enhancement of coercivity [7e54], deformation of the hysteresis loop, or the exchange spring phenomenon [1,2,52e54]. Among all intensively studied R e T (R e rare earths, T e transition metals) nanomaterials obtained via HEBM from the bulk parent compounds the presence of RT3 e type of intermetallics may be revealed [25,28e30,46e54,73e75]. The magnetic properties of the bulk RT3 compounds are dependent on the relation between localized (R-4f) and itinerant (T-3d) magnetism. This is a consequence of the 3d band filling and its polarization by the R atoms via conduction electrons [55e66]. As the result, the Curie temperatures (Tc) of RCo3 series, are dependent on the R atoms [55,59,67e72] and varies from 78 K (CeCo3) to 612 K (GdCo3) [55]. Simultaneously, the saturation magnetic moments (MS) changes from 0.2 [mB/mol] (CeCo3) to 5.45 [mB/mol] (HoCo3) indicating the ferrimagnetic arrangement of R and Co magnetic moments [55]. Additionally, one may distinguish the compensation temperature (Tcomp), where the magnetic moment between R and Co sublattice in the RCo3 group is compensated [55,67,68]; like in the case of DyCo3 (Tcomp z 400 K) [70] and HoCo3 (Tcomp z 328 K) [55,67,68]. The magnetic unstable state in RCo3 system can be induced via application of external temperature, field or pressure [69e72], similar to ErCo3 and HoCo3 compounds. It was shown that the itinerant e electron metamagnetic state (IEM) was induced and evidenced in thermal expansion, specific heat and electrical resistivity measurements [69e72]. It is worth mentioning, that the HoCo3 compound exhibited an abrupt change/peak at Tm z 55 K besides the magnetic transition from ordered magnetic state into paramagnetic (TC z 418 K). This peak is attributed to the temperature of spin reorientation, where the easy axis rotates out of the basal plane leading to the formation of non-collinear magnetic structure [69e72]. Thus, the application of external parameters may easily change the magnetism in RCo3 type of intermetallics. Similarly, the magnetism of HoCo3 compound may be changed via HEBM process. Therefore, the main aim of presented work was to fabricate HoCo3 nanopowders via ball e milling from its bulk parent compound, and to study the impact of HEBM parameters on its morphology and magnetism. First of all, we were focused on the enhancement of magnetic parameters via HEBM, and therefore the entire synthesis process and all measurements were performed through various time intervals. We are convinced, that the procedure applied by us can have a significant potential for the future design and development of anisotropic nanocomposites magnets having enhanced magnetic parameters. 2. Materials and methods The HoCo3 compound was prepared by arc melting from high purity elements (Ho e Rare Earth Product Limited e 99.99 wt% and Co Johnson Matthey Chemicals e spectrographically standardized) under argon atmosphere. The ingot was melted several times in order to obtain homogeneity. Afterwards, the as-cast sample was wrapped in tantalum foil, placed in quartz tubes, sealed and annealed at 900  C for one week. The crystal structure was checked by X-ray Powder Diffraction (XRD) using Empyrean PANalytical diffractometer equipped with Cu X-ray source (Ka1 of 1.54056 Å). In order to obtain the HoCo3 compound as nanopowders, first the bulk compound was crushed and pre-milled with a standard agate mortar for about 10 min. Afterwards, the mixtures were ground for

1.5 h, 7 h, 10 h, 13 h, 27 h, 38 h, 53 h, 67 h and 80 h at 30 Hz (1800 rpm) using Mixer Mill 400 (Retsch). Similar to the case of other Ho e based nanopowders [49e54], an innovative wet milling method was carried out in dimethylformamide (DMF) using the Eppendorf vials and 2 mm ZrO2 balls. The balls to powder ratio was set to 10:1 by weight. The structure and morphology of the as e milled specimens were investigated by: Dynamic Light Scattering (DLS) Zetasizer Nano ZS Malvern Instruments, X-ray Powder Diffraction (XRD) (Empyrean PANalytical, Cu X-ray source Ka1 of 1.54056 Å and for t  13 h with the use of Co X-ray source Ka1 of 1.789007 Å) and Scanning Electron Microscopy (SEM) Jeol JSM 6480. DLS measurements were carried out after selected grinding times. The as-milled samples were suspended in 10 ml of DMF in a glass cell. The Zetasizer instrument was set into 173 configuration and the appropriate standard operating procedure (SOP) was applied. The XRD measurements for all HoCo3 nanopowders were carried out at the room temperature. Before each measurement the mixture of the HoCo3 powders and DMF was dried on the circular quartz plate. The SEM microscope was operated at 20 kV. All samples in the powdered form were fixed to the sample holder by the carbon double coated conductive tape. The SEM analysis was performed after 7 h, 38 h, 80 h of grinding. The images were taken in the SE mode in the magnification range of 2000e50000 times. After the last stage of grinding (after 80 h), the optical microscopy (OM) Olympus BX51, the Transmission Electron Microscopy (TEM) Jeol JEM 3010, the Atomic Force Microscopy (AFM) Omicron STM/AFM VT-50/500 UHV and the X-ray Photoelectron Spectroscopy (XPS) PHI 5700/660 Physical Electronics measurements were performed. For the OM measurements the sample was suspended in ethanol and placed onto a glass plate. The magnifications was set to 10 and two setups were used: with and without external magnetic field from neodymium magnet. For the TEM studies the as-milled specimen was dried and purified in ethanol in the ultrasonic washer for 2 h in order to remove the excess of DMF and dissolve possible agglomerates. Next, a few drops of the HoCo3 powder suspended in ethanol were placed on the carbon coated Cu grid (400 mesh). As usual, TEM images were recorded for more than one region of interests. The AFM measurements were performed at room temperature using tapping mode and standard silicon tip. The suspension of the HoCo3 specimen milled for 80 h dispersed in ethanol was placed onto freshly cleaved mica substrate and dried up. All XPS spectra were obtained at room temperature by using monochromatized X-ray source Al Ka (1486.6 eV). First, the bulk HoCo3 sample was cleaved and measured in a vacuum of 1010 Torr. Afterwards, the sample milled for 80 h was fixed into the sample holder by double coated conductive carbon tape. Such prepared specimen was stored under ultra-high vacuum for one weak. Next, the XPS measurement was performed in three consecutive steps. In the first step the as e milled material was studied directly. Next, the same specimen was sputtered by Arþ ion beam for 1 h and then measured. During the last step the sputtering time was extended to 2 h and then such sample was measured. All the obtained XPS spectra were calibrated using C1s peak (BE ¼ 284.8 eV), since in our samples the carbon peak originates from the adsorbates, and thus can be used as a reference for charge correction. The magnetic properties were determined based on wide-range Superconducting Quantum Interference Device (SQUID) MPMS XL7 Quantum Design magnetometer, from 2 K to 400 K temperature range and magnetic field up to m0H ¼ 7 T.

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3. Results and discussion 3.1. DLS results In order to evaluate as-obtained particles average size, its evolution and the size distribution over milling process, the dynamic light scattering (DLS) method was applied. In the case of nanopowders obtained via HEBM such distribution is usually broad due to the heterogeneity of as e milled samples [42,49e54]. On the other hand, the inhomogeneity caused by irregular shape or variation in particle dimensions is reflected in a rather high index of polydispersity (PDI). Thus, for non-ideal i.e. polydisperse samples, the particle size distribution (PSD) is broad or/and exhibits more than one peak. Fig. 1 presents the intensity PSD histograms for HoCo3 powders after 1.5 h, 7 h, 10 h, 13 h, 27 h, 38 h, 53 h, 67 h and 80 h of pulverization. The obtained results showed a large polydispersity of the milled compounds. The PDI index was close to 1 (t < 27 h) and the inhomogeneous distribution of particles was especially visible for specimens ground for 1.5 h, 7 h, 10 h and 13 h. The maxima visible in PSD histograms correspond to the different fractions in the particles size distribution in percentage (see Table 1). For higher milling time histograms become narrower, which is accompanied by a decrease of PDI index. Nevertheless, at the last milling stage (80 h) one may notice additional peak in PSD, which is probably caused by a mixture of finer particles and conglomerates. Based on PSD the relation of the average (mean) size of particles (dp) versus milling time (t) may be estimated. The Fig. 2 shows the dp(t) dependence for the dominant fraction of particles (higher than 50% for all measured powders) for the HoCo3 sample. One may observe a change in dp value from 291.7 nm (t ¼ 1.5 h) to 623.6 nm (t ¼ 80 h). For the moderate milling time (t ¼ 13 h) a distinct minimum in dp(t) is clearly perceivable. The observed increase within dp value can be attributed to the flakes aggregation for extended pulverization (t  13 h). The similar behavior was previously notice for other Ho e based powders [49e54]. Furthermore, it is well known that during DLS measurements, due to Brownian motion of particles suspended in the liquid solution, the intensity autocorrelation function of the light scattered by particles is measured. Subsequently, such function is fitted in order to obtain particles size. The fitting procedure is realized via two different fitting algorithms. The first one, named cumulant method, gives an overall average size (Z e average) and polydispersity (PDI index) values (Table 1). The second, named distribution method, gives a distribution of particle size with an average (mean) size and width for each separate size peak in the PSD histogram. For a perfect monodiperse samples, mostly with spherical particles, these

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two approaches should give the same results with one peak in PSD histogram. For heterogeneous and polydisperse samples, where particles are not spherical, those two values are significantly different. In the measured samples the Z e average values are higher than the mean ones, which may be caused by particle agglomeration. Nonetheless, the trend observed in Z e average variation versus milling time is opposite to the mean particle size. So, first we observe an increase up to 13 h of milling and then, a drastic drop in Z e average value is visible (see Fig. 2). The observed curvature is a consequence of diminishing of PDI values due to extended milling process. For the polydispersed systems the estimation of mean particles size gives more information about the nature of applied HEBM. Using the distribution approach, one may follow the evolution of particles size through pulverization process in regard to the presence of several fractions of particles with different sizes. The agglomeration process in the HoCo3 powder milled by 80 h is apparent and it was proven by using the OM. Fig. 3 shows two different images, both were recorded for 10 magnification. The first one (see Fig. 3a) shows typical image, while the second was acquired after applying the external magnetic field in a direction marked by an arrow (see Fig. 3b). One can easily recognize a nonhomogeneous mixture composed of large agglomerates as well as individual particles of various sizes. Under the influence of magnetic field, one may notice the stronger agglomeration process due to an enhancement of magnetic interaction between finer particles. Similar behavior was previously observed for other Ho e based nanopowders [49e54].

3.2. XRD results The obtained XRD patterns for the crystalline bulk compound and all HoCo3 ball-milled powders are presented in Fig. 4a. The crystal phase is recognized as rhombohedral PuNi3 e type (1:3 phase) and is usually presented as equivalent to hexagonal one where a ¼ b s c and a ¼ b ¼ 90 , g ¼ 120 [55e58]. The lattice parameters for the bulk crystalline HoCo3 compounds are equal to: a ¼ 4.9802 ± 0.0004 Å and c ¼ 24.4644 ± 0.0004 Å respectively. Simultaneously, the estimated volume of the unit cell is about V ¼ 525.46 ± 0.1313 Å. For as-milled powders all lattice parameters slightly varies upon grinding and for the maximum applied milling time (t ¼ 80 h) they are equal to a ¼ 4.9178 ± 0.0071 Å and c ¼ 24.2982 ± 0.0268 Å, whereas V ¼ 508.92 ± 2.0208 Å. The observed contraction of the lattice parameters over milling may be an effect of the possible reduction of the interplanar spacings caused by the atomic displacements within crystallites/grains due to compressive long-range stress fields and larger lattice strain

Fig. 1. The intensity particle size distribution (PSD) histograms for various milling times of HoCo3 samples.

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Table 1 The Z e average (DLS cumulant method) and mean (DLS distribution method) particles size, the average crystallites size and microstrain (XRD) for the crystalline and ball-milled HoCo3 compounds. t [h] Z e average [nm] bulk 1.5 7 10 13 27 38 53 67 80

e 3176 ± 797.6 3465 ± 1325 4009 ± 1419 5385 ± 740.7 2957 ± 489.7 2117 ± 325.5 2132 ± 168.2 1661 ± 240.1 729.1 ± 54.5

PDI

Mean particle size dp [nm] Mean particle size dp [nm] Mean particle size dp [nm] Average crystallites size dcryst [nm]

Microstrain [%]

e 1.00 1.00 1.00 1.00 0.828 0.859 0.591 0.785 0.543

e 291.7 ± 19.2 (60%) 278.8 ± 15.7 (56.8%) 228.4 ± 24.3 (59.1%) 113.1 ± 9.5 (60%) 637 ± 72.9 (60.7%) 705.7 ± 86.7 (59.1%) 786 ± 116.2 (54.2%) 551.8 ± 78.1 (100%) 623.6 ± 163.4 (91.2%)

0.46 0.58 0.53 0.65 0.54 0.53 0.55 0.57 0.62 0.83

± ± ± ± ±

0.201 0.143 0.124 0.069 0.038

e 190 ± 0.0 (40%) 229.3 ± 15.3 (40.3%) 338.5 ± 12.2 (21.3%) 285.4 ± 17.4 (20%) 1007 ± 118 (39.3%) 430.8 ± 34.9 (40.6%) 118.9 ± 20.9 (45.8%) e 118.6 ± 28.5 (8.8%)

e e 15.6 ± 0.5 (2.9%) 58.8 ± 0.0 (19.7%) 45.2 ± 2.8 (20%) e e e e e

34.4 ± 13.2 18.6 ± 9.9 12.5 ± 4.5 8.8 ± 4.9 8.5 ± 3.3 8.8 ± 1.7 10.5 ± 3.6 10.3 ± 2.6 9.8 ± 2.7 8.4 ± 3.3

± ± ± ± ± ± ± ± ± ±

0.09 0.22 0.24 0.18 0.19 0.09 0.23 0.24 0.24 0.24

the value of 8.4 nm for 80 h of milling. Simultaneously, a gradual increase of microstrain from 0.46 (bulk compound) to 0.83 (t ¼ 80 h) was observed due to substantial lattice disorder produced during HEBM by various deformation mechanisms (see Fig. 4b and Table 1). As one may notice during first stages of milling process (t  10 h) the crystallites size decreases rapidly but further milling lead only to a slight variation of dcryst values. Such behavior, which is typical for almost all ball e milled materials [77e79], is related to the change within deformation mechanism from plastic deformation to grains boundaries sliding. The first one dominates at the beginning of milling process up to about t ¼ 10 h, when HEBM is mostly based on creation and movement of dislocations localized in shear bands. After this process the second mechanism starts to dominates, which is reflected in a slight variation of dcryst values. 3.3. SEM

Fig. 2. The average mean particle diameter (dp) (distribution method) and Z e average (cumulant method) estimated from intensity PSD histograms.

generated by extended milling process. In addition, the XRD analysis detected a small amount of regular 1:2 phase (HoCo2), which not exceed 5% and is indexed with asterisks in Fig. 4a. It is worth mentioning, that XRD analysis of all asmilled patterns did not reveal peaks characteristic for ZrO2, so there was no contamination from the grinding balls. Thus, the application of wet HEBM method in DMF allowed not only to synthesize fine powder, but also prevent contamination by ZrO2 and any excessive oxidation, which was confirmed by XRD studies. The diffraction lines broaden along the pulverization time, and their intensity vary, which becomes more apparent for longer milling times. It is known, that the pulverization process may lead not only to production of finer particles/crystallites but also to gradual amorphization upon grinding. The crystallites size and the lattice microstrains were estimated using the Williamson e Hall method after taking into account the Ka2 contribution as well as the instrumental broadening. The data was analyzed and processed by Powder Cell software using the Pseudo e Voigt 2 function and Bragg e Brentano geometry [76,77]. As it was previously revealed for various R e based intermetallic nanopowders [4,8,49e54,78] and pure elements [4,77,79], the grinding time has a significant influence of the microstructure and crystal structure of as-milled powders. In the case of the HoCo3 specimen it was evidenced, that the average crystallite size (dcryst) decreases significantly from 34.4 nm (bulk compound) and reaches

SEM images acquired after 7 h, 38 h, 80 h of grinding (see Fig. 5) revealed the presence of irregularly shaped flaked particles. Their dimensions vary upon applied milling time. After the last stage of grinding the thickness of as e obtained flakes is even lower than 100 nm. Nonetheless, the prolonged milling up to 80 h favors the creation of agglomerates of the size of microns, due to tendency to reduction of particles surface via coalescence of finer particles. Therefore, SEM measurements indicate the mixture composed of polycrystalline HoCo3 agglomerates and randomly oriented single finer particles. Such morphology is typical for powders fabricated by HEBM under different conditions [1e10,14,15,22e24,31e37,49e54]. 3.4. TEM Fig. 6 presents the TEM micrographs of HoCo3 nanopowder after 80 h of mechanical grinding. The first analyzed TEM image displays a part of irregular shaped flake denoted as I, which is composed of well-defined grained structure (see Fig. 6aeb). One may notice, that crystallites are irregularly distributed in the studied flake and they exhibit various sizes and shapes. The crystallites statistical distribution (CSD) of about 100 grains was estimated based on Fig. 6b and results in an average size of crystallites of about dcryst ~ 4.5 ± 2.6 nm. The selected area electron diffraction (SAED) pattern is depicted in Fig. 6c and was indexed according to the HoCo3 (PuNi3) type of crystal structure having rhombohedral symmetry. The recorded SAED pattern is typical for polycrystalline specimen. The morphology of the second studied flake denoted as II (see Fig. 6deg) is a bit different than the previous one. It is about 180 mm long and 105 ÷ 40 nm in wide and possess ragged edge. Furthermore, it has a denser distribution of polycrystalline grains

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Fig. 3. Optical microscopy images obtained for the as-milled HoCo3 compound after 80 h of pulverization using reflected light with a magnification 10 (a) without and (b) with an application of an external magnetic field.

Fig. 4. (a) The evolution of the XRD diffraction patterns upon milling collected by using Cu X-ray source (t  10 h) and Co X-ray source (t  13 h); (b) the average crystallites size (dcryst) and microstrain versus grinding time t for the HoCo3 compound in the crystalline as well as the as-milled form.

compared to flake I. The CSD gives an average size of nonhomogeneous crystallites of about dcryst ~3.0 ± 1.5 nm. The SAED pattern depicted in Fig. 6f is typical for polycrystalline HoCo3 (PuNi3) type of structure. The high resolution transmission electron microscopy (HRTEM) image for a part of II flake is presented in Fig. 6g. Here, one may distinguish several irregular crystallites having well defined crystal structure, with the visible arrangement of atoms, while the material in between is amorphous. For the best visible crystallite, marked in the image by a doted line, the FFT filtering was carried out. The obtained FFT pattern is placed as

insert into Fig. 6g in the upper left corner and is characteristic for rhombohedral symmetry. The magnification of the marked crystallite is shown in bottom right corner. Here, one can clearly see the crystalline arrangement of the rows of atoms having an estimated average interplanar distance d close to 2.5 Å. Thus, the performed TEM analysis shows that applying of HEBM method for production Ho e based nanopowder lead to formation of non-uniform flakes with irregular nanocrystallites randomly distributed in an amorphous matrix. However, in contrast to previously obtained Ho e based powders with two various T elements

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Fig. 5. SEM micrographs for the as-milled HoCo3 powders after (aeb) 7 h; (ced) 38 h; (eef) 80 h of mechanical grinding.

[50,51,54], as-milled HoCo3 powder indicates larger amounts of finer grains. Similar morphology was also evidenced for HoFe3 nanopowder [53], but the nature of HoNi3 flakes is rather similar to doped specimens [49]. Such remarks may be useful in the design and fabrication of other intermetallic nanomaterials via HEBM.

3.5. AFM Fig. 7 shows a topography of HoCo3 nanoflakes (t ¼ 80 h) placed on mica substrate. One may notice heterogeneously distributed flakes having irregular shapes and sizes (see Fig. 7a e left panel). The investigation of the individual flakes by several profiles is provided in the graph in Fig. 7a e right panel. The length is in the range of 50 ÷ 210 nm, while the average height (thickness) was found to be less than 1 nm. Each nanoflake is composed of nonhomogenously distributed grains, which are directly related to the distribution of crystallites. Fig. 7b shows the enlarged central region of previous image (left panel), which was subsequently analyzed. The analysis was started from marking the grains by the grain segmentation method implemented in Gwyddion software [80]. The marking parameters were adjusted for the correct marking of the visible grains. Due to its semi-automatic character this method provides a satisfactory estimation, rather than absolute value, of a grain sizes. As a

consequence, a histogram with ‘equivalent radius’ of a grain (req) is provided. The req value is obtained by taking the area of each grain and calculating the radius of a circle with the same area as the grain. This allows for the reliable comparison of different grains aside from their shape. The obtained distribution of req values in the range of about 1.4 nm ÷ 7.5 nm presented in right upper charts panel in Fig. 7b was further fitted using Gaussian function, which gives the center of req distribution at about 2.5 nm. Similar analysis provides the distribution of approximated projected boundary lengths (Lb0), i.e. the length of the grain boundary projected to the horizontal plane. The application of Gaussian fitting into the distribution of Lb0 values in the interval of about 9.1 ÷ 72.2 nm gives a distinct maximum at about 18.2 nm (right bottom chart). It is worth mentioning, that the studied specimen is composed of fine flakes/ particles, which was previously confirmed by TEM studies. Such fine and grained structure was already evidenced for HoFe3 powder apart from samples doped with others transition metals [49e54].

3.6. Magnetic properties The hysteresis loops M(H) for the crystalline as well as ballmilled HoCo3 specimens are depicted in Fig. 8aeb. For the bulk crystalline compound the magnetization at the maximum applied magnetic field (Happl ¼ 7 T) almost reaches saturation. For the ball e

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Fig. 6. TEM micrographs for the part of HoCo3 specimen denoted as I, (a) bright field image, (b) dark field image, (c) SAED pattern. TEM micrographs for the part of the HoCo3 sample milled for 80 h denoted as II (d) bright field images; (e) dark field image (f) SEAD pattern registered from DF image; (g) HRTEM image where upper insert represents FFT image taken from a marked crystallite. Bottom right insert shows the rows of atoms from the marked crystallite.

milled powders the magnetization at 7 T changes non-linearly (see Table 2) and the magnetization does not saturate, which may be related to the various particles size and shape. In order to estimate the values of saturation magnetization (MS) an extrapolation to the infinite field was used. The obtained results show, that the value of MS generally decreases starting from 5.35 mB/f.u (t ¼ 0) to 3.54 mB/f.u (t ¼ 80 h). The observed non-linearity in MS(t) dependence is associated with the variation within morphology across the pulverization process. Nonetheless, the visible enhancement of MS value for extended milling time (t  38 h) may be caused

by possible aggregation of finer particles, especially in the presence of external magnetic field, as it was demonstrated in optical microscopy studies. The aggregation process is an effect of reduction of surface energy by finer particles, which under external applied magnetic field are usually aligned along an easy axis of magnetization. This can lead to the enhanced value of MS as it was already evidenced for many as e milled intermetallics [1,13,17,19,21,25,31,32,34,41,52]. The values of the cobalt magnetic moments (MS/Co atom) were calculated from the total saturation magnetization by

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Fig. 7. (a) AFM topography of HoCo3 nanoflakes (t ¼ 80 h) on mica substrate (left panel) with marked 1e4 profiles (right panel). (b) Magnification of the HoCo3 nanoflake depicted in previous image with applied mask (left panel) with distribution of equivalent disc radius (right upper chart) and projected boundary length (right bottom chart).

Fig. 8. (aeb) Hysteresis loops for bulk as well as ball-milled HoCo3 compounds. (c) The variation of the magnetic parameters versus milling time.

Table 2 The magnetic parameters for the crystalline and as-milled HoCo3 compounds. t [h]

MS [mB/f.u] at 7 [T]

MS [mB/f.u] extrapolated to 0 [T]

MS/Co atom [mB/f.u]

MR [mB/f.u]

HC [T]

HEB [T]

MR/MS

0 7 27 38 80

5.60 2.85 2.03 4.45 3.54

5.35 2.26 1.52 3.38 2.99

1.55 2.58 2.83 2.21 2.34

0.26 1.03 0.36 0.74 0.75

0.008 0.139 0.077 0.058 0.056

e 0.021 0.015 0.012 0.002

0.05 0.45 0.24 0.22 0.25

subtraction of pure holmium magnetic moment. We assumed the stability of Ho moment (MHo3þ ¼ 10 mB) and the ferrimagnetic arrangement between Ho and Co moments (see Table 2). We have to be aware, that this estimation is very rough because it assumes a constant value of the holmium magnetic moment upon milling. Of course one cannot exclude changes in the magnetic moment in holmium magnetic sublattice with an increase of grinding time due to random magnetic anisotropy. On the other hand holmium Ho4f magnetic moment is rather localized, while the cobalt magnetic moment is mainly

associated with Co3d band and is more susceptible to changes induced by milling. By analyzing the MS/Co atom dependence one may notice it sensitivity on the milling duration (see Fig. 8c and Table 2). The visible non-linearity may be connected to the different particles and crystallites size, their possible aggregation but also to their conceivable oxidation during extended milling. What is more, the magnetic moment of metallic cobalt depends on its form and differs from the bulk to nanocompounds [81e83]. Certainly, the use of HEBM generates lattice disorder in the crystal structure, which may

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lead to a change in the Co local environment and subsequent modification of cobalt magnetic moment. Obviously, we cannot exclude a similar change in the Ho sublattice. Apart from that, the subsequent application of HEBM into the bulk compound is reflected as an appearance of slight exchange bias (EB) phenomenon for as e milled specimens as it is evidenced from the analysis of hysteresis loops M(H). The EB effect associated with the shift of hysteresis loops along H axis was already demonstrated in various nanomaterials [52,53,84e92] and even for some bulk compounds [93e99]. The EB phenomenon is observed as an effect of the exchange coupling caused by the pinning of magnetic moments at the interface of two magnetic phases. The properties of such interface (roughness, thickness, magnetic behavior) decide about the magnitude of EB phenomenon. Thus, in all materials, which exhibit the shift of hysteresis loops, the exchange bias field (HEB) can be estimated as HEB ¼ |HCþ þ HC|/2 and simultaneously the average coercivity as HC ¼ |HCþHC|/2. Additionally, the EB effect often occurs together with the deformation of the hysteresis loop [52,53]. For the as e milled HoCo3 powders all measured hysteresis loops are shifted towards negative Happl. The largest value of HEB was found in the sample milled for t ¼ 7 h (see Table 2). Additionally, for the same specimen the deformation of M(H) curve is observed. It is worth mentioning, that the magnitude of EB effect in our compounds is similar to previously studied Ho e based powders [52,53], but considerably smaller than evidenced in other nanomaterials [85e88]. In the case of powdered samples with core-shell structure the EB phenomenon is strictly dependent on the thickness of shell, its structure and chemical composition, which is extremely important especially for particles covered mostly by oxides layer [84e88]. Nonetheless, usually EB magnitude increases with the subsequent reduction in particles size. In our case, the HEB(t) variation is non-linear probably due to changes of surface-to-volume ratio during HEBM process. Those changes originate from the variation within microstructure, partial amorphization or chemical changes (oxidation) of as e obtained nanoflakes. The analysis of the hysteresis loops indicates the increase of coercivity upon pulverization from 0.008 T (bulk compound) to the maximum value of 0.139 T (t ¼ 7 h). However, these values are relatively low in comparison to other milled intermetallics containing Co and Fe [7e25,52] or Ho e based nanopowders [50,51,53,54]. This excludes pure HoNi3 compound, where coercivity even for powdered samples is very low [49]. Further applying of HEBM leads to drop in HC(t) values, which at the end of the process is almost half of the maximum value (see Table 2). As it is depicted in Fig. 8b for the t ¼ 7 h one may notice the largest deformation of hysteresis loop. Such phenomenon was demonstrated in a variety of other intermetallic nanomaterials [15,36,41,51e54,100e103]. Usually it is the effect of formation of an additional (soft) magnetic phase during HEBM, but also it could be ascribed as the influence of microstructural non-uniformities. In our case the second factor seems to play the main role. It is worth noticing, that the extension of milling process typically leads to the refinement of crystallites size, however it also promotes the creation of agglomerates composed of finer particles which tend to reduce their surface. Additionally, the ball e milling duration favors the creation of defects and larger shape anisotropy. This leads to the change within magnetic anisotropy and possible decay of the long-range magnetic order, which can result in the formation of partly amorphous (magnetic) phase. All above factors may have a significant impact on the HC(t) dependence. The analysis of MR(t) dependence demonstrates the maximum value of 1.03 mB/f.u. for nanopowder obtained after t ¼ 7 h milling (see Table 2). The slight non-linearity visible for extended milling

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may be caused by the large inhomogeneity of specimens due to modification of the microstructure. This effect can be accompanied by variation of interactions between neighbouring nanograins, change in the shape anisotropy and a noticeable agglomeration of particles confirmed by various techniques. The remanence-to-saturation (MR/MS) ratio described by the Stoner e Wohlfarth (S-W) model [17,35,104e112] in the HoCo3 powders is significantly lower than for other R-T intermetallics obtained via HEBM [16,17,35,50e52,105,107,108]. As it was demonstrated for Ho e based nanopowders, the MR/MS ratio is usually larger for specimens containing two different transition metals rather than for binary alloys [49e54]. Nevertheless, its magnitude is still sensitive to the variation of particles/crystallites size versus grinding time, which can be associated with the anisotropy induced by external magnetic field. Thus, the MR/MS ratio exhibits the maximum value of 0.45 for the sample ground by t ¼ 7 h. For other powders, such ratio decreases almost two times up to 0.25 at the last stage of milling process. It can be the effect of the decrease of particles/crystallites size, enhancement of the random anisotropy or destruction of the long-range magnetic order and its transformation into short-range one. The observed low values of MR/MS may be associated with the so e called pseudosingle domains (PSD) particles characterized by small multidomains grains, with higher remanence but low coercivity. So, the MR/MS variation related to PSD structure can be caused by the particles surface associated to the surface domains structure imperfections [81]. 3.7. Electronic structure (XPS) The survey XPS spectra for the crystalline bulk compound and as e milled HoCo3 powders are presented in Fig. 9a. The elemental composition of all samples was calculated based such spectra. It was evidenced, that even for the bulk compound with the fresh fractured surface, despite several cleaving trials, there is certain amount of contaminants like carbon (5.3 at.%) and oxygen (10.4 at.%). Samples produced via arc-melting and subsequent annealing are usually brittle. Therefore during their synthesis process all adsorbents are most probably accumulated between grain boundaries. In the cleaving procedure under UHV conditions the sample is fractured along such boundaries, and thereby such contamination is visible on the cleaved surface. The as e milled sample after 80 h of pulverization shows large increase of impurities (O1s e 42.7 at. %, C1s e 37.4 at.%), even though the sample was kept under UHV for quite a long time. However, a 2 h long ion etching revealed significant drop in carbon content to about 22.2 at.% and oxygen to about 37.5%. The 1s core level of both elements are composed of several peaks (see Fig. 9ced). The C1s line (see Fig. 9c) is dominated by the component present in all measurement steps and typical for surface adsorbed carbon (BE ¼ 284.8 eV). Other peaks with a lower intensities can be assigned to various carbonates. The significantly large amount of C impurities may suggest, that the parameters used for ion etching procedure, such as sputtering rate and sputtering time, are not sufficient to remove thoroughly the carbon contaminants from nanoflakes surface. For the O1s line the main peak around 531.5 ± 0.8 eV can be assigned to the O2 states, which are typical for oxygen adsorbed on the surface and dominates for as-milled sample. The second line around 529.8 ± 0.2 eV may be linked to cobalt oxides. Such states are visible during all applied measurement procedures, but dominates after two sputtering stages. The third weak line around 527 ± 1 eV may be associated with holmium oxides. The variation of oxides components may be caused by the partial oxidation of nanopowder. It may also suggest the formation of Co oxides shell e

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Fig. 9. (a) XPS survey spectra; (b) valence band (VB) in the broad energy range; (c) the C1s core level; (d) line the O1s core level line for the bulk and as-milled (t ¼ 80 h) HoCo3 compound. (e) The XPS Co2p core level line for the bulk and as-milled (t ¼ 80 h) HoCo3 compound.

like layer covering flaked particles, similar to the previously studied Co e based nanopowders [114]. The valence band (VB) in the broader binding energy range (0.5 eV ÷ 40 eV) shows Ho5p lines, which are partially overlapped with the O2s (BE z 24 eV) states, even for bulk sample (see Fig. 9b). For the crystalline bulk compound one may notice Ho4f multiplet structure composed of six overlapping narrow lines [49,113]. However, the visible peak around 9.8 ± 0.5 eV can be assigned to Ho oxides, as it intensity increase for powdered sample. Thus, the Ho4f fine structure becomes very broad, which may confirm the partial oxidation of nanopowder after last milling step. Furthermore, for the bulk crystalline sample the VB region close to the Fermi level (EF) is dominated by clearly visible Co3d states. Such states broadened and merged with Ho4f states for powdered sample probably due to the presence of a significant amount of surface impurities. The evolution of the Co2p core level line is depicted in Fig. 9e. We may infer, that for the bulk compound 2p lines are narrow and demonstrates the L-S splitting of DE z 15.3 eV (Co2p3/2 e BE z 777.9 eV, Co2p1/2 e BE z 793.2 eV). For the powdered sample the cobalt line is broadened and moved into higher binding energies, but also shows the presence of satellites around 787.1 ± 1.4 eV and 803.1 ± 1.3 eV, typical for contaminated surface. After two steps of ion etching such line is slightly modified. Nevertheless, broad Co2p3/2 and Co2p1/2 photoemission peaks exhibit their complex nature, which is typical for cobalt hydroxides Co(OH)2 (BE z 781 eV) and oxides CoO (BE z 780 eV), Co2O3 (BE z 779.9 eV) but also Co3O4 (BE z 779.6 eV) [115,116]. The two first mentioned elements probably dominates for as e milled sample, but after sputtering the latter emerges. 4. Conclusions In this paper, we have presented the novel results for the HoCo3 nanopowders synthesized from its bulk crystalline parent

compound. Based on presented results the following conclusions can be drawn: C The high energy ball e milling method can be effectively applied for the production of the HoCo3 nanopowder from its bulk parent compound. However, the as e milled specimens are rather inhomogeneous and contain finer particles and agglomerates. The particles shape and size are sensitive to the milling duration. Thus, during milling process the presence of nanoflakes with thickness dependent on the applied pulverization time t was demonstrated. The as e obtained polycrystalline nanoflakes are composed of irregular nanocrystallites having a size varied over milling time. C The magnetic properties of the HoCo3 nanopowders are sensitive to the pulverization duration. The non e linear variation of the saturation magnetic moment, the coercivity and the remanent magnetization versus t parameter is caused by the particles/crystallites refinement, the increase of nanograin boundaries over milling process, the change of the anisotropy and the random orientation of particles/ crystallites. For extended milling times additional effects, caused by of nanoflakes agglomeration, can be present. In addition, the formation of oxides layer or possible amorphization process can be noted. The slight exchange bias phenomenon and simultaneous deformation of hysteresis loops, especially observed for t ¼ 7 h, may be an indication of microstructural non-uniformities, dependent on the size and shape effects. Such phenomenon can be also related to the presence of core/shell e like structure due to formation of oxides layer and/or partial oxidation of HoCo3 flakes. C The X-ray photoemission spectroscopy studies for the HoCo3 sample after the last milling stage (t ¼ 80 h) indicate a presence of Co oxides, which are probably placed on the nanoflakes surface. The analysis of the core level lines during applied measurement procedure may lead to the conclusion,

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that the as e milled material demonstrates the Co oxides e rich shell. Despite the application of ion etching for up to 2 h, the photoemission results do not provide reliable information on whether the sample is oxidized only on the surface or in deeper regions. Summarizing, our approach showed a significant potential for fabrication binary HoCo3 nanopowders via ball e milling method. The applied high energy ball e milling parameters had a strong influence on the final structure, size, shape and magnetic behavior of the synthesized nanomaterials. Especially, the milling duration has to be carefully chosen in order not to “overmill” the specimen. Thus, from the application point of view, the optimization of high energy ball e milling preparation details is extremely important for design of as e obtained powders, which can be afterwards used in various applications e.g. as a part of nanocomposites. Acknowledgements The authors want to thank prof. dr hab. R. Wrzalik and dr M. Dulski for the opportunity to perform optical microscopy measurements. We are grateful to mgr Paweł Skornia for his assistance in the preparation of samples for measurements and for his help in the preparation of some images. References [1] O. Gultfleisch, M.A. Willard, E. Brück, Ch. H. Chen, S.G. Sankar, J. Ping Liu, Magnetic materials and devices for the 21st century: stronger, lighter, and more energy efficient, Adv. Mater 23 (2011) 821e842. [2] N. Poudyal, J. Ping Liu, Advanced in nanostructured permanent magnets, J. Phys. D. Appl. Phys. 46 (2013) 043001. €r, P. Delocroix, S. Be gin e Colin, T. Ziller, High e energy ball-milling [3] G. le Cae of alloys and compounds, Hyperfine Interact. 141/142 (2002) 63e72. [4] C.C. Koch, Synthesis of nanostructured materials by mechanical milling: problems and opportunities, NanoStruct. Mater. 9 (1997) 13e22. [5] C.C. Koch, Review: mechanical milling/alloying of intermetallics, Intermetallics 4 (1996) 339e355. [6] M. Ullach, Md. Eaqub Ali, S. Bee Abd Hamid, Surfactant-assisted ball milling: a novel route to novel materials with controlled nanostructure e a review, Rev. Adv. Mater. Sci. 37 (2014) 1e14. [7] S.K. Pal, L. Schultz, O. Gutfleisch, Effect of milling parameters on SmCo5 nanoflakes prepared by surfactant-assisted high energy ball milling, J. Appl. Phys. 113 (2013) 013913. [8] A.M. Gabay, N.G. Akdogan, M. Marinescu, J.F. Liu, G.C. Hadjipanayis, Rare earthecobalt hard magnetic nanoparticles and nanoflakes by high-energy milling, J. Phys. Condens. Matter 22 (2010) 164213e164216. [9] B.Z. Cui, G.C. Hadjipanayis, Formation of SmCo5 single-crystal submicron flakes and textured polycrystalline nanoflakes, Acta Mater. 59 (2011) 563e571. [10] L. Zheng, B. Cui, L. Zhao, W. Li, G.C. Hadjipanayis, A novel route for the synthesis of CaF2-coated SmCo5 flakes, J. Alloys. Compd. 549 (2013) 22e25. [11] W.F. Li, H. Sepehri e Amin, L.Y. Zheng, B.Z. Cui, A.M. Gabay, K. Hono, W.J. Huang, C. Ni, G.C. Hadjipanayis, Effect of ball-milling surfactants on the interface chemistry in hot-compacted SmCo5 magnets, Acta Mater. 60 (2012) 6685e6691. [12] L. Zheng, B. Cui, G.C. Hadjipanayis, Effect of different surfactants on the formation and morphology of SmCo5 nanoflakes, Acta Mater. 59 (2011) 6772e6782. [13] L. Zheng, A.M. Gabay, W. Li, B. Cui, G.C. Hadjipanayis, Influence of the type of surfactants and hot compaction on the magnetic properties of SmCo5 nanoflakes, J. Appl. Phys. 109 (2011), 07A721. [14] J. Nie, X. Han, J. Du, W. Xia, J. Zhang, Z. Guo, A. Yan, W. Li, J. Ping Liu, Structure and magnetism of SmCo5 nanoflakes prepared by surfactant-assisted ball milling with different ball sizes, J. Magn. Magn. Mater 347 (2013) 116e123. [15] N. Yu, M. Pan, P. Zhang, H. Ge, Q. Wu, Effect of milling time on the morphology and magnetic properties of SmCo5 nanoflakes fabricated by surfactant-assisted high-energy ball milling, J. Magn. Magn. Mater 378 (2015) 107e111. [16] P. Saravanan, M. Prekumar, A.K. Singh, R. Gopalan, V. Chandrasekaran, Study on morphology and magnetic behavior of SmCo5 and SmCo5/Fe nanoparticles synthesized by surfactant-assisted ball milling, J. Alloys. Compd. 480 (2009) 645e649. [17] N. Poudyal, B. Altuncevahir, V. Chakka, K. Chen, T.D. Black, J.P. Liu, Y. Ding, Z.L. Wang, Field-ball milling induced anisotropy in magnetic particles, J. Phys. D Appl. Phys. 37 (2004) L45eL48.

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