Nanomagnetism variation with fluorine content in Co(OH)F

Journal of Alloys and Compounds 825 (2020) 153916

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Nanomagnetism variation with fluorine content in Co(OH)F Haobo Liu a, Mingjie Sun a, Y. Li a, **, Z.X. Cheng b, J.L. Wang b, W.C. Hao c, W.X. Li a, d, * a

Institute of Materials, School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, China Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW, 2500, Australia c Department of Physics and Center of Materials Physics and Chemistry, Beihang University, Beijing, 100191, China d Shanghai Key Laboratory of High Temperature Superconductors/Institute for Sustainable Energy, Shanghai, 200444, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 September 2019 Received in revised form 14 January 2020 Accepted 17 January 2020 Available online 21 January 2020

Magnetic behaviors of cobalt hydroxide fluoride, Co(OH)F2-x Fx (x ¼ 1.09 and 1.26), which are consisted with nanorods of ~5 mm length and diameter ~100 nm, obtained by a facile hydrothermal method, have been researched with the variation of fluorine content as a result of the different amount of reactant NH4F addition. Magnetic coupling performance are characterized based on the observation and analysis of magnetic susceptibility, X-ray Absorption Fine Structure (XAFS) and specific heat. Co(OH)F2-x is found to undergo an antiferromagnetic transition at 40 K. The peak temperature (T) on the transition curves shifts to lower temperature according with the increased magnetic field (H) following the rule of H2/3∞ (-T) because of the surface spin-glass behavior. The paramagnetic susceptibility can be fitted with the modified CurieeWeiss law between 100 K and 320 K. The negative Curie-Weiss temperature indicates that the antiferromagnetic coupling becomes stronger with the fluorine content. In addition, at temperatures below 5 K, the magnetic reordering was observed as spin-glass. Exchange bias behavior in Co(OH)F2-x was found after field cooling process demonstrating an exotic surface magnetic behavior generated with high fluorine contents due to the deformation of CoO6 octahedral induced large spinglass behavior. © 2020 Elsevier B.V. All rights reserved.

Keywords: Co(OH)F AC susceptibility Exchange bias Antiferromagnetism Fluorine doping Spin-glass

1. Introduction Reduction in grain size of magnetic nanoparticles has led numerous peculiar magnetic behaviors, such as spin-spiral structures [1,2], spin liquid phases [3], exchange bias phenomenon [4], negative magnetization and magnetization reversal behavior [5e8] and many of them have their practical application potentials in magnetic recording media, spin valves and magneto-electronic devices [1,9e11]. The nanosize effects on the magnetic performance are quite profound when the dimensions are less than 100 nm [12]. While the large number of ions or atoms on the exposed crystal surfaces also generate exotic magnetic performances based on the characteristic magnetic properties in a-Fe2O3 [13e15]. Structural and chemical modification of magnetic nanoparticles have been investigated by scientific and technological community to obtain desired magnetism for the particular

* Corresponding author. Institute of Materials, School of Materials Science and Engineering, Shanghai University, Shanghai, 200072, China. ** Corresponding author. E-mail addresses: [email protected] (Y. Li), [email protected] (W.X. Li). https://doi.org/10.1016/j.jallcom.2020.153916 0925-8388/© 2020 Elsevier B.V. All rights reserved.

application [16]. Element doping into transition metal oxides have been the subjected of intense investigations since it triggers a variety of magnetic phenomena [17,18]. Vishal et al. doped Co2þ into nickel ferrite to increase the coercivity for practical magnetic applications such as permanent magnet, magnetic data storage devices and magnetic tapes [19]. Imanol reported that the substitution of Co2þ by Cu2þ in the Co2(OH)PO4 framework could eliminate the magnetic frustration and decrease ferromagnetic (FM) performance [20]. The exchange bias (EB) phenomenon has been extensively studied because of its utilization in fabrication of several spintronic devices such as magnetoresistive devices [21], spin valves [10], and reading heads in magnetic hard disks [22]. EB effect was first discovered by Meiklejohn and Bean in a compact of Co nanoparticles coated with a shell of antiferromagnetic (AFM) CoO. Since then this effect has been observed in various magnetic materials such as oxidized FM particles, bilayers, multilayers of FM/AFM, FM/ spin-glass and ferrimagnet (FI)/AFM etc. [23e27] The physical origin of EB in nano particles has been generally attributed to the existence of uncompensated spins either at the interface or within the antiferromagnet. Kumar et al. [28] reported that the EB effect

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observed in core-shell CoCr2O4 nanoparticles was ascribed to the interaction between a long range ferrimagnetic core and spin glass shell. The phenomena of coercivity enhancement and bias-field shifts was also observed in a Co/CuMn bilayer system as a result of a ferromagnet in contact with a spin glass [29]. However, contradictory phenomenon has also been observed that the presence of uncompensated interfacial spins is insufficient for EB since they may strongly couple to and rotate with the ferromagnet, yielding no bias [30]. EB could be manipulated by spin-orbit torque as applying current pulses, expanding the design flexibility in spintronic devices, the reasons were not yet clear though [31]. To motivate innovative designs for future spintronics devices, microscopic mechanism of EB need further investigation. Magnetic behavior of Co(OH)2 is also found to attract attention of researchers because of its possible use in future refrigeration systems based on magnetocaloric effect [32]. b-Co(OH)2 is relatively well studied system, it has a sign of AFM state transition at
el temperature, TN ¼ 9e10 K [33]. Gupta et al. doped Al into the Ne layered a-Co(OH)2 to study magnetic behavior [34]. Herein, we have employed fluorine as dopant to modulate the magnetic behavior of b-Co(OH)2 and research the influence of fluorine content on the nanomagnetism of cobalt hydroxide fluoride Co(OH)F nanostructures. Fluorine can substitute the OH in Co(OH)2 continuously to introduce lattice distortion and change the configuration environment of Co2þ, which can be an effective indicator tracing the magnetic variation with the addition amount. Co(OH)F structures derive from the diaspore-type a-AlOOH (SG:

Pnma) [35], consist of double chains of edge-sharing Co(F,O)6 octahedra running along the b axis. These infinite chains share corners and form the channels. Yahia et al. reported that Co(OH)0.86(3)F1.14(3) with FM rutile type chains paralleling to the b axis in was AFM at ~40 K [36] indicated by the neutron powder diffraction. In this work, we present the magnetic properties of Co(OH)F with different F contents and EB behavior has been found in Co(OH)F after field cooling process due to the AFM/spin-glass core-shell structure. The magnetic behavior is sensitive to the variation of F contents and the spin-glass behavior is much stronger in the high fluorine content Co(OH)F. 2. Experimental details 2.1. Experimental matertials All the reagents were A.R. (Analytical Reagent) grade and were used in preparation without further purification: Cobaltous nitrate (Co(NO3)2$6H2O, China Medicament Co. (company)), Ammonium fluoride (NH4F, China Medicament Co.) and Urea (CO(NH2)2, China Medicament Co.). Deionized water was used throughout the experiments. Co(OH)F nanostructures were synthesized under hydrothermal conditions. In brief, Co(NO3)2$6H2O (1.2 mmol), NH4F (4 mmol) and CO(NH2)2 (6 mmol) were firstly dissolved into 15 mL deionized water, after stirring for 10 min, the solution was transferred to a 25 mL Teflon-lined autoclave, sealed and maintained at 120  C for

Fig. 1. Rietveld Refined XRD patterns for COF4M (a) and COF8M (c) collected at 300 K. The open circles show the observed counts and the continuous line passing through these counts is the calculated profile. The difference between the observed and calculated patterns is shown as a continuous line at the bottom of the two profiles. The calculated positions of the reflections are shown as vertical bars. Rp ¼ 15.58%, Rwp ¼ 11.59%, and RBragg ¼ 1.56 for COMF4, Rp ¼ 15.7%, Rwp ¼ 13.3%, and RBragg ¼ 0.9885% for COMF8. (b, d) show projection views of the crystal structures on the (010) plane for COF4M and COF8M, respectively. The legends of (F1, O1) and (F2, O2) indicate the content ratio variation of fluorine and oxygen.

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Table 1 Crystal Structural Parameters for Co(OH)F Based on the Rietveld Refinement against XRD Data Collected at 300 K [Space Group Pnma (No. 62), COF4M, Co(OH)0.90(2)F1.09(8), a ¼ 10.3149 Å, c ¼ 3.1127 Å and b ¼ 4.6646 Å, V ¼ 150.106(2) Å [3]; COF8M, Co(OH)0.73(8)F1.26(2), a ¼ 10.3055(40) Å, c ¼ 3.12115(9) Å, b ¼ 4.67118(5) Å, V ¼ 150.24(9) Å [3].

COF4M

COF8M

atom

Wyckoff

occupancy

x

y

z

Biso, Å2

Co F1/O1 H1 F2/O2 H2 Co F1/O1 H1 F2/O2 H2

4c 4c 4c 4c 4c 4c 4c 4c 4c 4c

1 0.5929(8)/0.4070(2) occ(O1) 0.5054(3)/0.4945(7) occ(O2) 1 0.5562(7)/0.4437(3) occ(O1) 0.7059(1)/0.2940(9) occ(O2)

0.3668(7) 0.5332(4) 0.5967(2) 0.2932(3) 0.6235 0.3662(9) 0.5469(3) 0.5905(8) 0.2925(9) 0.6404(1)

1

0.4781(5) 0.2876(5) 0.1471(4) 0.2285(2) 0.9126 0.4767(1) 0.2773(2) 0.147(40) 0.2369(1) 0.912

0.8159(8) 0.22952 Biso(F1/O1) 0 Biso(F2/O2) 1.0628(1) 0 Biso(F1/O1) 0.1706(9) Biso(F2/O2)

/4 /4 1 /4 3 /4 1 /4 1 /4 1 /4 1 /4 3 /4 1 /4 1

Numbers in parentheses are uncertainty in the last digit.

6 h. The autoclave, once removed from the furnace, was allowed to cool down to room temperature naturally. The precipitate was collected by centrifugation, washed alternately with deionized water and ethanol, and dried at 80  C for 24 h under vacuum. The obtained sample was marked as COF4M. In order to obtain samples of different F concentrations, we changed the NH4F to 8 mmol, the obtained sample in this condition was marked as COF8M. 2.2. Characterization X-ray powder diffraction (XRD) measurements were performed on a D8 Advance Bruker AXS diffractometer, using Cu-Ka radiation (l ¼ 1.5406 Å) at 40 kV, 40 mA, employing a scanning rate of 0.1 $s1 in the 2q range from 15 to 80 . The diffraction patterns were Rietveld refined using FULLPROF suite and structural parameters were obtained. The morphologies of the samples were observed using a scanning electron microscope (SEM, LEO 1530VP). The fine crystal structures of the particles were investigated using a transmission electron microscope (TEM, JEM-2010FEF at 200 keV) High resolution TEM (HRTEM) images and selected area electron diffraction (SAED) patterns were performed on a JEOL JEM-2100F transmission electron microscope at 200 kV. DC (Direct Current) magnetic susceptibility measurements of powdered samples were performed using a Quantum Design superconducting quantum interference device magnetometer (Quantum Design MPMS-5s).

The susceptibility was recorded in the zero-field-cooled (ZFC) and field-cooled (FC) modes over the temperature range 2 Ke200 K. Magnetization as a function of field (H) was measured using the same MPMS-5s magnetometer in the range 8 H/T  8 in the temperature range of 5 Ke60 K after cooling the sample in zero field. AC (Alternating Current) magnetic susceptibility measurements were made using a standard Quantum Design PPMS system. Data were recorded from 2.1 k to 70 K as a function of frequency. Susceptibility data at frequencies between 200 Hz and 10000 Hz were measured in the absence of an applied field. Heat capacity measurements were carried out by a relaxation method using the same PPMS system. Data were collected with zero field, under an applied field of 50 kOe and 100 kOe from 2.1 K to 50 K. X-ray absorption fine structure (XAFS) spectra were measured in transmission mode using the beamline 12C at Beijing Synchrotron Radiation Facility, BSRF. 3. Results and discussion 3.1. Structure refinement The Rietveld refined XRD patterns for two samples of Co(OH)F, i.e. COF4M and COF8M prepared at NH4F 0.4 M and 0.8 M, respectively, are presented in Fig. 1. Both samples of Co(OH)F, i.e. COF4M and COF8M prepared at NH4F 0.4 M and 0.8 M are high

Fig. 2. (a) SEM image of the hierarchical star-like COF4M micro/nanostructures assembled from nanorod bundles. (b)TEM images of an individual nanorod of COF4M. (c) HR-TEM image of the area marked with the yellow square in (b). The insert shows the corresponding electron diffraction pattern of the COF4M. (d) SEM image of the hierarchical star-like COF8M. (e)TEM images of an individual nanorod of COF8M. (f) HR-TEM image of the sample COF8M. The upper insert shows the corresponding electron diffraction pattern of the sample. The lower insert shows a schematic diagram of the single crystal nanorods. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 3. (a,c) The ZFC and FC performances of COF4M and COF8M with alternating applied fields from 500 Oe to 10000 Oe to show the influences of F introduction into Co(OH)2 lattice. (b,d) The influence of applied magnetic field on the ZFC process on the peak transition temperature in the relationship of (-T) ∞H2/3 because of the surface spin-glass behavior.

purity Co(OH)F, as indicated by the X-ray diffraction (XRD) patterns shown in Fig. 1a, c. Co(OH)F has orthorhombic structure related to diaspore-type a-AlOOH (SG: Pnma), in which all atomic positions correspond to a general 4a position. The Co(OH)0.86(3)F1.14(3) crystal structure was used as a starting model for Rietveld refinement against our product XRD data. Partial (O,F) disorder model was used considering the previously reported ZnOHF model. The refinement used constraints occ (H) ¼ occ (O) ¼ 1 - occ (F) for two nonequivalent sites in Co(OH)2-xFx, which could be used to determine x (content of F) quite precisely. The scale factor, background parameters, cell parameters, F and O site occupancies along with instrumental broadening and other parameters were refined in order to obtain a good fitting. The final Rietveld plot and crystallographic information for COF4M and COF8M at 300 K are presented in Table 1. Refinement results indicate that both COF4M and COF8M have the same crystal structure of Co(OH)F, in which Co(O, F)6 octahedral share edge and run along the b axis, forming double chains. These infinite chains share corners and give rise to channels. The protons are in the channels and form OeH$$$F bent hydrogen bonds [32,33], as shown in Fig. 1b, d. The refined lattice constants are a ¼ 10.3149 Å, c ¼ 3.1127 Å and b ¼ 4.6646 Å, for COF4M; a ¼ 10.3055 Å, c ¼ 3.12115 Å and b ¼ 4.67118 Å for COF8M respectively. The lattice parameters are similar with the reports of Yahia et al., a ¼ 10.305 Å, c ¼ 3.126 Å and b ¼ 4.677 Å [36]. The refined composition can be written as Co(OH)0.90(2)F1.09(8) for COF4M and Co(OH)0.73(8)F1.26(2) for COF8M, respectively. Fig. 2a shows a representative large area scanning electron

microscope (SEM) image of the COF4M, and reveals star-like structures of the product, assembled from bundles of nanorods with lengths between 12 and 15 mm. These nanorods gather into bundles, grow along the diagonal of the hexagon and extrude outwards along the radial direction. The detailed structure and the growth direction of the Co(OH)F nanorods are further examined by transmission electron microscopy (TEM). Fig. 2b shows a brightfield TEM image taken from a single nanorod that is about 100 nm wide of the sample COF4M. Fig. 2c is the corresponding image of the area marked by the yellow square in Fig. 2b. It shows an interplanar distance of 0.42 nm, which is comparable to that of the d-spacing of the (1 1 0) crystal planes of orthorhombic Co(OH)F. The growth direction of the nanorod is determined to be [1 1 1], which is supported by the corresponding electron diffraction pattern in the insert of Fig. 2c. Fig. 2d shows a scanning electron microscope (SEM) image of the COF8M, the sample shows a starlike structure assembled from bundles of nanorods as same as that of the COF4M. The COF8M’s TEM images and HRTEM image with SAED pattern are shown in Fig. 2e and f. From the high magnification TEM images, an interplanar distance of 0.42 nm can also be observed in COF8M, indicating the nanorod exposed (1 1 0) crystal planes. The growth direction of the nanorod is [1 1 1], which is supported by the corresponding electron diffraction pattern in the insert of Fig. 2f. The illustration on the right of Fig. 2e shows a schematic diagram of the single crystal nanorod of the COF8M. It should be noted that both samples have an obvious shape anisotropy, a star-like nanostructure assembled from bundles of

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Fig. 4. (a) Susceptibility as a function of temperature at applied fields of 1000 Oe for COF4M and COF8M between 0 and 55 K. (b) The ZFC 1/c vs. T curve fitted by the Curie-Weiss law: c(T) ¼ c0þ C/(T-q) between 100 and 320 K, as indicated by the lines. (c,d) The cp dependence on the temperature for COF4M and COF8M, respectively.

nanorods, growing along the [1 1 1] direction. We present the magnetic properties of Co(OH)F with different F contents and EB behavior has been found in Co(OH)F after field cooling process due to the AFM/spin-glass core-shell structure. But, the core-shell structure is a kind of surface state, which couldn’t be seen from SEM. However it has been supported by Godsell et al. [37] We also prove it via some measurements, such as heat capacity and EB behavior in section 3.4 and 3.6, which reveal the core-shell structure of samples, especially COF8M which contains more F content. In the end of the article (Fig. 8), we point out this phenomenon appears because of two reasons. 3.2. Magnetic susceptibility Fig. 3 shows the magnetic susceptibility c dependence on temperature (c(T)) for the two Co(OH)F samples. ZFC effects and FC effects were measured under 500, 1000, 2500, 5000, 7500, and 10000 Oe magnetic field. The magnetization exhibits significant deviation between ZFC and FC cycles below 50 K for both samples, as shown in Fig. 3aec. The peak temperature on the transition curves shifts to lower temperature according with the increased magnetic field, as shown in Fig. 3bed. The peak temperature variation follows the de Almeida-Thouless relationship given by T(H)/ T(0) ¼ 1-aH2/3 with a constant a because of the surface spin-glass behavior [38]. The similar phenomenon has been observed in NiO nanoparticle [39]. Low temperature magnetic reordering can be observed at about 5 K on all ZFC and FC curves. The substitution of F in Co(OH)2 induces cell distortion and disorders the Co-(O, F) bonds, which has influenced the magnetic ordering to introduce spin-glass behaviors. Fig. 4 compares the magnetic susceptibility c dependence on

temperature (c(T)) for the two Co(OH)F samples measured under 1000 Oe magnetic field. In case of COF4M, the magnetic moment is significantly enhanced below about 50 K both in ZFC and FC cycles and shows a peak at ~4 K, which is an indication of glassy behavior at low temperature [40]. The ZFC curve bifurcates from the FC one below about 40 K, indicating that the magnetic phase is making a transition from paramagnetism (PM) to antiferromagnetism as reported by Yahia and Shikano [36]. The high temperature susceptibility data is in good agreement with the Curie-Weiss law and therefore can be fitted to the equation c(T) ¼ c0þC/(T-q), where q is the Curie-Weiss temperature, C is the Curie-Weiss constant, and c0 comes from the diamagnetic susceptibility contribution and the temperature-independent orbital contribution. The fitting result in the range 100 Ke320 K yields q ¼ 58.4 K and an effective moment meff ¼ 5.24 mB, which is in good agreement with the value reported by Yahia et al. with q ¼ 61.4 K and meff ¼ 5.39 mB [36]. The negative q indicates the presence of strong AFM behavior of the Co(OH)F nanostructures [41]. In the case of COF8M, the ZFC magnetization shows a transition with a peak at 4 K as observed in COF4M. The FC magnetization peaks at 4K as well, it decreases continuously to 50 K and settles into a low value giving an impression of AFM to paramagnetic transition. A linear fit in the temperature region 100e320 K, to inverse susceptibility of COF8M with Curie-Weiss equation results in q ¼ 65.7 K and an effective moment 5.34 mB (see Fig. 4b). A lower magnetic moment of 2.41 mB has been deduced from the neutron powder diffraction characterization at 3 K for Co(OH)0.86(3)F1.14(3) as demonstrated by Yahia et al., [36] which can be attributed to the presence of magnetic frustration coming from distorted CoeFeCo and Coe(OH)eCo magnetic exchange paths due to the randomly F substitution of OH. The q value of COF8M is much lower than that of COF4M, implying a

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Fig. 5. (a,b) The temperature dependence of the real (c0 ) and imaginary (c”) components of the AC susceptibility of COF4M with the frequency of 100, 500, 1000, 5000, and 10000 Hz between 0 and 55 K. (c,d) The temperature dependence of the real (c0 ) and imaginary (c”) components of the AC susceptibility of COF8M with the frequency of 100, 500, 1000, 5000, and 10000 Hz between 0 and 55 K. The insets display the enlarged view of the transition at about 5 K. (e, f) log10Tp(u) against log10(u) along with linear fit for COF4M and COF8M. The c0 vs T of the AC susceptibility of COF4M and COF8M were collected with the frequency of 10, 20, 100, 500, 1000, 2000,5000, 8000 and 10000 Hz between 0 and 55 K, as insets, to Fig. out the relationship of log10Tp(u) against log10(u).

much stronger AFM coupling in COF8M. A mean field constant, ε, can be introduced to describe interactions within a sublattice, jqj= TN ¼ ðm þ εÞ=ðm  εÞ, with m the magnetic moment of Co2þ. The interaction intensity is increasing with the value of jqj= TN , The lower q value of COF8M implies a stronger AFM interaction in Co(OH)F sublattices. The TN values of COF4M and COF8M are similar, which can be understood that the major coupling of the inside atoms is AFM just like behaviors in the bulk Co(OH)F. The introduction of F atoms in Co(OH)2 lattice induces the increase of
el temperature from 10.5 K to ~ 40 K [32], the significant Ne

distinction in content of F between COF4M and COF8M makes a difference in the Curie-Weiss Temperatures. The spin-only magnetic moments of Co2þ can be estimated as
3.87 mB based on meff,Spin ¼ g[S(Sþ1)]1/2 using g ¼ 2 as the Lande factor and S ¼ 3/2 as the number of unpaired spins. The theoretical value is smaller than the observed values because the spin-only magnetic moment neglects the contribution of the 4F9/2 ground state of Co2þ induced orbital angular moment [42]. cp, the paramagnetic susceptibility of the samples, is defined as cp(T) ¼ c(T)-c0 to eliminate the influences of the diamagnetic susceptibility

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the accurate AFM transition can be figured out on the differential curves of v(cp$T)/vT as 36.45 K for COF4M and 26.51 K COF8M. The transition temperature of COF8M is proofed to be accurate as   indicated by the heat capacity characterization as ∞ discussedn later. P 1 The cp(T) can be written by a series as cp ðTÞ ¼ CT Cn 2J with kB T n¼0 kB the Boltzmann constant and J1 the exchange interaction. The first two terms can be written in the Curie-Weiss law form with is q ¼ 3J1/kB. The exchange constant can be estimated from J1/kB ¼ q/3 based on the Curie-Weiss fitting as 19.45 K for COF4M and 21.89 K for COF8M, respectively. 3.3. AC susceptibility

Fig. 6. Temperature dependence of the specific heat (per mole of the molecule) at zero field, 5 T and 10T, respectively; the dashed line represents the lattice contribution according to Debye’s law. The inset shows the specific heat difference of COF8M near 30 K between 0T and 5T (red line), 0T and 10T (dark line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 7. (a) The K-edge XANES spectrum of COF8M, (b) EXAFS spectrum of COF8M, which indicates the radial structure of Co center atoms.

contribution and the temperature-independent orbital contribution on the AFM transition temperature as shown in Fig. 4ced. The cp$T dependence on temperature are plotted in Fig. 4ced based on the c0 ¼ 4.17  104 emu$Oe1$mol1 for COF4M c0 ¼ 8.83  104 emu$Oe1$mol1 for COF8M, respectively. Then

AC susceptibility measurements of two Co(OH)F samples are carried out with different alternating excitation frequency and 10 Oe amplitude. The evolution with temperature of the real (c0 ) and imaginary (c”) component of the susceptibility is shown in Fig. 5. The steep rise below 6 K and anomalies at temperatures denoted by 5 K are observed in COF4M in both the in-phase c0 and out-phase c” components, which could be indicative of the possible existence of a spin-glass state as was observed in the Co2(OH)PO4 phase [20]. The c0 values show a slight decrease with the increasing frequency (u) (see the insert in Fig. 5a). Furthermore, the position of the maximum shifts to higher temperatures with the increase of frequency, which is usually considered as a fingerprint of the spinglass transition [43]. A parameter d (known as Mydosh Parameter) is normally used to distinguish between spin-glass, cluster-glass and superparamagnetic system: d ¼ DTp ðuÞ=½Tp ðuÞDðlnuÞ [44], where u is the applied frequency of the AC susceptibility and Tp(u) is the peak position of the transition as shown in the insets of Fig. 5a and c. To obtain accurate Mydosh parameter, more experimental results of c0 dependence on frequency are included as shown in the insets of Fig. 5e and f. The obtained value of d (0.006) for COF4M, as indicated in Fig. 5e, is comparable to those of typical canonical spin glasses such as CuMn and AuMn alloys with d z 0.005. The d value of COF8M is 0.015 as indicated in Fig. 5f, which is attributed to the cluster spin glasses behavior and comparable to the d value of CoxGa100-x (x ¼ 54e57) [45]. Both values are much lower than that of the non-interacting ideal superparamagnetic materials, in which d value is always larger than 0.1. The low d value of COF4M can be attributed to the isolated magnetic moments on the surface of the nanorods. While the magnetic clusters are formed in the surface moments with the increase F content in COF8M and a stable longrange magnetic order is established through intra- and inter-cluster interactions. The real (c0 ) and imaginary (c”) component of the susceptibility behavior of COF8M are similar to that of COF4M. The freezing temperature Tf associated with the spin-glass state in Co(OH)F is considered to be 5 K as indicated by the maximum value in c’. Generally, in antiferromagnets, a spontaneous moment emerging below AFM transition can be attributed to uncompensated or disordered spins at the surface of nanocrystals [46]. Anomalies at temperatures denoted by about 40 K are observed in COF4M and COF8M in both the in-phase c’ and out-phase c” components, which is in agreement with the TN ¼ 40 K as was observed in the Co(OH)0.86(3)F1.14(3) phase [36]. 3.4. Heat capacity The temperature dependence of the molar heat capacity (Cp) for COF8M is shown in Fig. 6. The main feature is the evolution of the magnetic ordering peak observed in COF8M, which shifts to lower temperatures with increasing applied fields. The difference of Cp between 0 and 5 T (DCp1 ¼ Cp ð0 TÞeCp ð5 TÞ) and between 0 and 10 T (DCp2 ¼ Cp ð0 TÞeCp ð10 TÞ) for the COF8M are plotted in the inset of Fig. 6. DCp1 is 0.23 J K1mol1 and represents 3% of the total

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Fig. 8. Magnetization moment as a function of field m0H for (a) ZFC and (b)1 kOe FC COF4M at different temperatures. (c,d) show the magnetization moment of COF8M in ZFC and 1 kOe FC at different temperatures. Inserts show the magnetization moment as a function of field in large-scope. (e,f) Temperature dependence of Hc1, Hc2, Hc and HEB for COF4M and COF8M, respectively.

Cp at 35 K and DCp2 is 0.52 J K1mol1 and represents 7% of the total Cp at 30 K. Although these differences are very small, they show the well-defined peak respectively, indicating that some magnetic contributions in the COF8M phase should be included. The change of the shape under magnetic field suggests that they are magnetic transitions. The double peak phenomenon is attributed to the different magnetic behavior of the core and shell of the COF8M nanorods due to their different magnetic coupling stability. 3.5. X-ray absorption fine structure We use the X-ray Absorption Fine Structure (XAFS) spectroscopy to further determine the electronic structure and orbit transitions

of COM8F. In Fig. 7a, the curves are shifted upwards for clarity. A huge peak is observed near 7724eV, which is in good consistent with the peak of the cobalt k edge [47], arising from the 1 s/4 p transition. No small peak is observed in front of this maximum, which means that the crystal of COF8M is octahedral symmetrical, agreeing with our result of Rietveld refined structure. According to the dipole selection rule, the 1 s/3 d transition is forbidden in the octahedral field, while in its counterpart tetrahedron field, this transition is permitted, resulting in a strong 1 s/3 d pre-edge features near the zero point of the threshold energy. When the zaxis coordination is vacant in the octahedral field, the shoulder at the beginning of the edge appears, which elaborate that COF8M is standard six-coordinate octahedral symmetry without vacant in

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the z-axis coordination. In order to obtain information about neighboring coordination atoms, we transform the data of the x-ray absorption coefficient in the higher energy region (50e1000 eV) by Fourier transform, resulting in the extended x-ray absorption fine structure (EXAFS) of the Co center atoms. From Fig. 7b, the adjacent shell is clearly seen and its approximate radial spacing is known. The main feature is that the position and intensity of the first peaks remain as it is with the increasing temperature, which means the crystal structures of COF8M are the same from lower temperature to room temperature, since the first peak reveals the contribution of the nearest neighbor atomic to the X-ray absorption and the short-range order state and change of the central atom in the system. All the spectra showed in Fig. 7a and b have similar chemical shifts with increasing temperature ranging from 10K to 60 K, which is usually considered as the evidence that crystal structure does not change within this temperature range change. Therefore, it is the nano-effect of the COF8M leads to the corresponding magnetic change. 3.6. Exchange bias behavior The magnetization as a function of field was measured during ZFC and FC cycles on the sample COF4M and COF8M and is presented in Fig. 8. In the case of COF4M, near above TN ~40 K, an AFM response is observed both in ZFC and FC measurements, which is in agreement with Co(OH)F AFM nature. The FM-like behavior at low temperature of COF4M is confirmed by the hysteresis loop shape in the field dependent magnetization, as residual magnetization is observed in ZFC sample from 5 K to 30 K. In the case of COF8M, AFM behavior is observed both in ZFC and FC curves below 40 K. The shift of the hysteresis loop for the COF8M after FC at 1 kOe along the negative magnetic field direction is observed at 5 K and 15 K, which is the feature of EB effect. It is interesting that the EB effect is observed for both COF4M and COF8M, which indicates the surface magnetic ordering is different from that of the core ordering due to F doping. The presence of uncompensated spins on particles surface of our COF4M and COF8M samples, resulting in an exchange-biased AFM/spin-glass core-shell structure [48], as observed in hydrothermally prepared nano a-MnO2 [49] and Co3O4 [50]. The EB field HEB and the coercive field Hc are employed to compare the magnitude of the EB effect quantitatively as Hc ¼ |Hc1 - Hc2|/2 and HEB ¼ (Hc1 þ Hc2)/2, where Hc1 and Hc2 are the left and right coercive fields, respectively. The EB effect can be observed only below 30 K as shown in Fig. 8e and f. HEB values of 270 Oe at 5 K and 72 Oe at 15 K were obtained from hysteresis loops of COF4M, while they are as high as 951 Oe and 678 Oe at 5 K and 15 K in COF8M, respectively. The stronger HEB of COF8M can be attributed to the surface spin glass behavior for intensive surface distortion induced by the higher F content. The Co(OH)F matrix is AFM ordering of FM rutile-type chains of trans-edge-sharing polyhedrons with the moments parallel to the short b axis as reported by Yahia and Shikano 29 and is illustrated in Fig. 9a. As shown in XRD analysis, we could know that we have already synthesized Co(OH)F, i.e. COF4M and COF8M, with high purity. Both COF4M and COF8M have the same crystal structure of Co(OH)F, in which Co(O, F)6 octahedral share edge and run along the b axis, forming double chains. These infinite chains share corners and give rise to channels. So there must be some corners exposed in the surface. In addition, samples have similar shape anisotropy, nanorods grow along the [111] direction and the crystal planes (110) are exposed on the surface which could be seen from Fig. 2c and (f). Considering the AFM nature of Co(OH)F, its EB behavior can be attributed to the coupling between the surface net moments with the AFM core as indicated by the observed spin glass behavior. Both samples have similar shape anisotropy, nanorods

Fig. 9. (a) Projection view of the crystal structure of COF8M on the (010) plane, chains of edge-sharing polyhedrons running along the b axis. Blue and red polyhedrons showing cobalt sites with spins up and down, respectively. (b) The side view of the (110) plane of COF8M nanorods. The highlighted parts indicate the weak linked Co(OF)6 polyhedrons on the (110) plane when they are exposed on the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

grow along the [111] direction and the crystal planes (110) are exposed on the surface. The (110) plane forms a step-type surface, which exposes uncoupled Co(OF)6 polyhedrons with magnetic moments. These polyhedrons chains share corners linked with the matrix, exposed to the surrounding environment as indicated by the highlight parts in Fig. 9. These exposed Co(OF)6 polyhedron chains have the same magnetic spin direction and are attributed to the spin glass behavior, which can couple with the AFM core inducing EB behavior. Furthermore, some Co(OF)6 polyhedrons could be absent on the rough surface of Co(OH)F nanorods during the crystal growth, which is another reason to break the AFM balance of Co(OH)F and induce spin glass behavior to enhance the EB behavior. COF8M suffers more intensive lattice distortion and crystal imperfection due to the high fluorine substitution effect. Therefore, its EB behavior is intensive than that of COF4M. 4. Conclusions F atoms have been introduced into Co(OH)2 lattice to replace eOH forming Co(OF)6 polyhedrons, which induces exotic magnetic behavior compared with pure Co(OH)2. The magnetic behavior is sensitive to the variation of F contents due to the ratio of CoO6 polyhedron and Co(O, F)6 polyhedron contents. F substitution has generated higher AFM transition at around 40 K. Simultaneously, a spin-glass magnetic transition has been observed at 5 K. The EB effects can be attributed to the surface spin glass behaviors in the nanocrystal as indicated by AC susceptibility. Author contribution section Haobo Liu：Software，Formal analysis，Investigation， Writing - Original Draft，Writing - Review & Editing. Mingjie Sun：Investigation，Writing - Original Draft，Formal analysis，Software. Y. Li：Validation，Methodology，Supervision. Z. X. Cheng：Data Curation. J. L. Wang：Data Curation.

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W. C. Hao：Data Curation. W. X. Li：Conceptualization，Methodology，Writing - Original Draft，Supervision. 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. Acknowledgment This work is financially supported by National Natural Science Foundation of China (Grand No. 51572166, 51771119), Shanghai Key Laboratory of High Temperature Superconductors (No. 14DZ2260700) and Natural Science Foundation of Shanghai (No. 17ZR1419600). The authors thank the Analysis and Research Center of Shanghai University for their technical support. W. X. Li acknowledges research supported by the Program for Professor of Special Appointment (Eastern Scholar: TP2014041) at Shanghai Institutions of Higher Learning.

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153916.

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