Interface Modelling Leading to Giant Exchange Bias from CoO

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Interface Modelling Leading to Giant Exchange Bias from CoO/CoFeO Quantum Dot Heterostructure 2

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Avijit Saha, Siddhartha Sohoni, and Ranjani Viswanatha J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b11124 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019

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The Journal of Physical Chemistry

Interface Modelling Leading to Giant Exchange Bias from CoO/CoFe2O4 Quantum Dot Heterostructure Avijit Saha,a Siddhartha Sohoni,a, c and Ranjani Viswanathaa,b,*. a

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur,

Bangalore-560064, India b

International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific

Research, Jakkur, Bangalore - 560064, India c

Indian Institute of Science Education and Research, Pashan, Pune - 411008

ACS Paragon Plus Environment

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT. Research in miniaturization of devices is driven by the presence of new challenges in small sized particles. Magnetic interactions at the heterostructure interface, specifically the interface driven properties like exchange bias in core-shell magnetic quantum dots (QDs), has become one of the primary fields of interest in nano-magnetism research. The major deterrent in small sized QDs is the presence of superparamagnetic limit responsible for low or insignificant anisotropy in these materials. Formation of a sharp interface at the junction of an antiferromagnetic (AFM) and ferrimagnetic (FiM) heterostructure can improve anisotropy that can overcome the superparamagnetic limit in these small size QDs. Herein, we demonstrate a two-step synthesis of CoO/CoFe2O4 core-shell QDs and their characterization to study the effect of magnetic interaction at the interface. Formation of highly crystalline sharp interfaces, obtained as a result of interface modelling, results in a strong exchange coupling at the AFM core/FiM shell interface leading to a large exchange bias value (HE= 5.6 kOe). This value of exchange bias is comparable with the largest HE value reported for small size nanoparticles.

INTRODUCTION The recent development of QD based technology demands materials with multifunctional and superior properties for numerous applications in modern nano-electronics,1-2 sensors3-4 and bioapplications5 resulting in extensive research into the fundamental properties of QDs. In particular, tuning of magnetic properties, both from a fundamental perspective and for applications ranging from ultrahigh density magnetic recording to biomedical applications is the current interest of study in various literature reports.2,

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Amongst these studies, of particular interest is the

phenomenon of exchange bias (EB)7-9 in magnetic nanostructures resulting from the direct exchange at the interface between ferromagnetic (FM) and antiferromagnetic (AFM) layers. EB can be defined as unidirectional anisotropy that arises due to the exchange coupling at the soft


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ferro (ferri) magnetic / hard AFM interface upon field cooling below the Neel temperature (TN) of the AFM material leading to field cooled hysteresis loop shifts and/or enhancement of coercivity in FM/AFM junctions.10-11 The spins of the AFM material at the interface pin the spin orientation of the FM layer below TN which gives rise to a new anisotropy in the system that results in a hysteresis loop shift along the applied field axis and an increase in coercivity. Even though EB was first discovered 60 years back in Co nanoparticles due to the formation of a CoO layer outside the nanoparticle,12-13 due to its potential to establish reference magnetization direction, it has only recently found applications in magnetoelectronic switching devices and random access magnetic storage units.14-15 As EB is usually an interfacial phenomenon except in few examples of bulk alloy,16-17 it has been mostly studied in thin film heterostructures.18-19 Although, both thin film and core-shell heterostructures shows EB originating from the same phenomena, it is more complicated in case of the latter due to the several reasons. Firstly, small sizes induce increase in the curvature making the interface very important. Secondly, increase of canted spins due to increase of surface reduces the overall magnetization. All these effects hinder the exchange coupling in QDs and results in smaller exchange shifts compared to the thin film layered structure. However, from the application perspective, since the size of the sensitive part of a spin valve is continuously reduced to meet the requirement of high density recording, the observation of high EB in smaller particles is of primary importance leading to extensive research in the field of QD based magnetic materials. Only recently, various core-shell FM/AFM heterostructures albeit being small have been reported to show EB in bi-metallic core-shell heterostructure QDs.20-23 Hence, in order to increase the EB, understanding this phenomenon in core-shell QDs at the microscopic level and the dependence of the internal structure has been the driving force behind the research. Interestingly, it has been


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observed from recent experimental and theoretical studies that the presence of inferior EB is largely associated with the poor crystallinity of the AFM material.24-25 This is a consequence of colloidal synthesis wherein the shell material is usually forced to grow under non-ideal conditions. Hence, magnetic properties of the AFM structure are better controlled in the core than in the shell leading to an increased thrust in the study of inverted structures wherein a ferrimagnetic (FiM) or a FM material is used as a shell material while the core is composed of an AFM material. 26-27 In spite of these advances, reduction of particle size in magnetic QDs leads to thermal destabilization of magnetization due to the superparamagnetic limit,28-30 which is the bottleneck in applying these particles in magnetic recording devices. It has been shown that it is possible to tune the EB by tuning the core size and shell thickness by controlled synthesis giving more insights into this field.21, 31 For example using metal nanoparticles as seed, various core-shell heterostructures have been reported like Co/CoO,12,

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Ni/NiO,33

Mn/Mn3O434 or metal oxide with different oxidation states such as Mn3O4/MnO.35 Systems using different metal oxides such as Fe3O4/Co,36 Fe3O4/FeO,37 Fe3O4/CoO,22 Fe2O3/CoO,38 MnO/Mn3O4,27 Cr2O3/CrO2,39 BiFeO3/CoFe2O440 and even Fe/Cr41 have also been reported. Additionally, transition metal based oxides with various core-shell morphologies show tunability in magnetic response also in accordance to their internal structure.20, 35 In the specific case of AFM/FM interfaces for the observation of high EB, the idea of overcoating another magnetic layer having different anisotropy has shown higher thermal stability in small particles, due to magnetic proximity at the interface leading to EB. However, it has been known that atomic intermixing at the interface or the sharpness of the interface plays an important role in the exchange coupling42-43 between AFM/FM heterostructure. Dimitriadis et al.42 have shown theoretically the atomic scale roughness at the interface leads to


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reduction of uncompensated spins resulting suppression of HE. However, experimental characterization of the sharpness of the interface has not been extensively developed. In fact, the properties like exchange bias can be used to indirectly obtain sharpness of the interface. Transmission Electron Microscopy (TEM) is insensitive to 2-3 atomic layer thicknesses and in this report EB, observed due to the spin pinning at the interface of AFM and FM layers, has been used as one of the characterizing techniques to obtain the sharpness of the interface. Secondly, a sharp interface, that is an abrupt change in material’s composition rather than the gradual alloying over several monolayers has so far also been non-trivial to control as there will always be some inherent intermixing at the core/shell interface in QDs unless specific care is taken to avoid the gradual or smooth interface. Although, a large number of colloidal synthesis techniques claim the formation of core/shell structures, the sharpness of the interface has not been well characterized. Normally, thin films with high quality of the interface have been well studied while control over the quality of the interface in QDs has so far not been established using colloidal synthesis. It has only recently been shown that it is possible to achieve application specific properties by tuning the interface11, 44-47 as well as the material composition. Herein, we have studied the interface within a simple seed growth method to synthesize CoO cores embedded inside FiM CoFe2O4, an inverted core-shell system. Using this system, we have demonstrated the consequences of interfacial control on its magnetic properties and the EB arising in the system. Lima et al24 and Lavorato et al25 have earlier reported CoO/ CoFe2O4 inverted coreshell heterostructure showing high coercivity due to AFM/FM interaction. Although there is a strong enhancement in anisotropy and thermal stability, there was no EB observed from these systems due to the absence of control at the interface. Motivated by the research of Lottin et al 20 demonstrating high EB from a highly crystalline Co0.3Fe0.7O/Co0.6Fe2.4O4 inverted core-shell


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structure, we have used various synthesis techniques to obtain comparatively small size particles that still enhance the magnetic proximity effect resulting from the formation of a crystalline and sharp interface. Hence, our results, in contrast to earlier results show a surprisingly high exchange bias effect (5.6 kOe) along with the remarkable coercivity from CoO/CoFe2O4 heterostructures. Theoretically, high EB is observed in a heterostructure when the AFM material has a higher anisotropic energy (hard magnet) than the FM (soft magnet). However, in the current system, CoFe2O4 (FiM) has relatively higher anisotropic energy compared to AFM CoO. Hence, it is nontrivial for CoO to hold the magnetic spins (reason for loop shift) with reduction of external magnetic field and reverse its direction during M vs H measurement in field cooled condition. Herein, this meta-stable state has been stabilized due to (1) the inverted and sharp core/shell system in which CoO is highly crystalline, and (2) CoO being the major part of the system with a thin CoFe2O4 layer as shell i.e. optimized condition of core-shell thickness. These samples were characterized using X-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Magnetic EB was studied using a superconducting quantum interference device (SQUID) magnetometer, which shows that EB was observed even at temperatures as high as 200 K from these QDs. EXPERIMENTAL METHODS Materials. Cobalt (II) Acetate (Co(ac)2 99.9%), Iron (II) Acetate (Fe(ac)2 95%), oleic acid (OA, 90%), oleylamine (OlAm, 70%), were purchased from Sigma Aldrich. All these chemicals were used without further purification. Synthesis. A two-step seed growth method was employed to synthesize these QDs. In a typical synthesis, CoO QDs were first synthesized by thermal decomposition of Co(ac)2 in presence of OA and OlAm at high temperature. 0.4 mmol of Co(ac)2 along with 2 ml of OA and 5 ml of OlAm


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were taken in a three necked round bottom flask and connected with the schlenk line. The reaction mixture was degassed for 1 h with evacuation at 80 °C under vigorous stirring. The solution was then heated to 220 °C under argon flow maintained for 20 min. The reaction temperature was further increased to 300 °C with a heating rate of 5 K/min and maintained for 10 min. The resultant CoO black solution was then cooled down to room temperature and washed by centrifugation using a hexane-ethanol mixture and the powder precipitate was stored in a vial for further use. CoO/CoFe2O4 core shell heterostructure. Pre-synthesized CoO QDs were used as seed for further overcoating with CoFe2O4. 30 mg CoO powder was taken in a three necked round bottomed flask along with 2 ml OlAm and degassed at 80 °C for 1 h under evacuation. 0.05 mmol Co(ac)2, 0.1 mmol Fe(ac)2 , 1 ml of OA and 2 ml of OlAm were taken in a vial and degassed for 30 min at 80 °C. This was used further as the Co and Fe precursor for overcoating. The temperature of the reaction mixture in RB flask was raised to 200 °C under constant Ar flow. The Co and Fe precursor prepared in the vial was slowly injected into the reaction mixture and the temperature was raised slowly to 280 °C with a heating rate of 5 K/min. Sample aliquots were collected after 15 min and 30 min of annealing for different shell thickness of CoFe2O4 layer. All the samples were washed once by centrifugation using hexane-ethanol mixture and dissolved in hexane. CHARACTERIZATION Powder x-ray diffraction patterns for the nanocrystals were recorded on Bruker D8 Advance diffractometer using Cu-Kα radiation (1.5406 Å). Transmission electron microscopy (TEM) was performed on a Technai F30 UHR version electron microscope, using a field emission gun (FEG) operating at an accelerating voltage of 200 kV in bright field mode using Cu coated holey carbon TEM grids. Magnetic measurements (M vs H) at 2K and at room temperature (300 K) were carried out in SQUID VSM, Quantum Design, details. Hysteresis loops were measured in ZFC and FC


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conditions after cooling from room temperature (RT) to 2 K with 10 kOe, 3 kOe and 7 kOe applied field. The core-shell nanoparticles (0.8 mg) having maximum exchange bias were dispersed to form a colloidal suspension in 2 mL of water. The sample was placed inside the induction coil with a thermometer immersed in the sample. Change of temperature as a function of time of exposure to alternating magnetic field was measured using a high radio frequency heating machine with an alternating magnetic field at a frequency 500 kHz and strength (H0) of 37.4 kA/m using EASYHEAT induction heating system by Ameritherm Inc. The SLP value was calculated using the equation, SLP=

𝐶𝑉𝑠 𝑑𝑇 𝑚 𝑑𝑡

(1)

where C is the volumetric specific heat capacity of the sample solution, (Cwater = 4185 J/L/K ) Vs is the volume of the sample and m is the total mass (0.8 mg) of the sample in powder form. dT/dt is the slope of the change of temperature as a function of exposure time to the alternating magnetic field. RESULTS AND DISCUSSION Crystal structure and composition of the QDs were analyzed from powder XRD techniques. Figure 1 shows XRD pattern of as synthesized CoO and CoO/ CoFe2O4 core shell QDs collected after 15 min and 30 min of annealing. It is evident from the Figure that CoO QDs are formed in cubic structure similar to the bulk as observed from the Inorganic Crystal Structure Database (ICSD). Absence of any impurity phases suggests the formation of pure cubic phase CoO QDs while the broadening of peaks is indicative of their small size. The XRD patterns after different annealing time (15 min and 30 min) with Co and Fe metal complexes are also shown in Figure 1. The analysis confirms the presence of two crystalline phases, which can be indexed as cubic structure from CoO


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core (denoted by dots on top of the peaks) and cubic inverse spinel structure from CoFe2O4 (denoted by stars on top of the peaks). A comparison with the bulk data of CoO and CoFe 2O4, shows an increasing trend of CoFe2O4 feature with longer annealing time suggesting a controlled slow growth of CoFe2O4 layer on CoO QDs. (222)

(220)

.

(422) (400)

(311)

(440)

(511)

CoFe2O4 bulk

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

•

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•

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



•

Intensity (a.u.)

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CoO/CoFe2O4-30 min

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CoO/CoFe2O4-15 min

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CoO (200) (111)

(220)

CoO bulk

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30

40

50

60

70

2 ()

Figure 1. X-ray diffraction patterns of CoO core and CoO/CoFe2O4 core-shell QDs with different annealing time along with their bulk XRD patterns. (The black dots indicate the peaks corresponding to CoO while the red stars indicate peaks corresponding to CoFe2O4). The crystallite sizes (P) were calculated using Scherrer formula and the lattice parameter ‘a’ was calculated using the equation for lattice spacing (d) in cubic structure d = a ̸ (h2 + k2 + l2)1/2

(2)

Though reflection planes (111, 200, 220) for CoO phase and (220, 311, 511) for CoFe2O4 phase were considered to calculate the crystallite size, it should be noted that this is case of overlapping diffraction angles of the two materials. Hence an approximate size can only be obtained for the core only sample and is found to be about 14.7 nm. The crystallite sizes thus obtained from


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Scherrer formula is slightly larger than observed from TEM image as shown in Figure 2 as the instrumental broadening has not been considered.. Table 1: The average crystallite size and the lattice constant of the CoO and CoFe2O4 phase from core and core shell heterostructure (30 min) considering the diffraction planes (111, 200, 220) for CoO and (220, 311, 511) for CoFe2O4.. The numbers in bracket shows the sizes obtained from TEM. Sample phase

Crystallite size (nm)

Lattice parameter (Å)

CoO core QDs

14.7 (12)

4.258

CoO from core/shell QDs

16.4 (14.9)

4.246

CoFe2O4 from core/shell QDs

---

8.373

The lattice parameter of core CoO seed QDs is observed as 4.258 Å which is well agreement with previous literature report48 as well as standard bulk data (4.26 Å). However, a small shift towards higher angle in the peak position of CoO phase from core to core-shell is observed and as a result of that the calculated lattice parameter (a) has been decreased from 4.258 Å to 4.246 Å. This is due to the lattice mismatch between CoO and CoFe2O4 layer at the interface of core-shell heterostructure resulting in lattice strain. The lattice parameter value for cobalt ferrite phase is found to be 8.373 Å which agrees well with previously reported CoFe2O4 data.49 Although the XRD demonstrates the presence of CoO and an increased intensity of the CoFe2O4 phase with annealing time it cannot be used to conclusively prove the formation of CoO/ CoFe2O4 core-shell and/or independent nucleation or any other heterostructure. In order to obtain a more direct evidence of the formation of core-shell heterostructure, we have carried out TEM measurement. We have chosen CoO/ CoFe2O4 sample annealed for 30 min for the rest of the measurements and the representative TEM images are shown in Figure 2. Figure 2 (a)-(b) show the TEM images of CoO core and CoO/CoFe2O4 QDs respectively while Figure 2 (c)-(d) show their corresponding HRTEM images. It can be observed from the figure that both the particles are


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almost spherical in shape. The size distribution is broad in core shell nanoparticles, which is most likely due to the different size distribution in core nanoparticles. However, unlike optical and superparamagnetic properties of QDs, magnetic exchange bias is not strongly sensitive to the small changes in size. It is worth noting here that the superparamagnetic behavior can be influenced by nanocrystalline size (magnetic domain) and hence could give rise to a broad range of values for coercivity or the blocking temperature corresponding to the size distribution. However, the focus of our current study is the exchange bias property, which depends more strongly on the formation of interface between antiferromagnetic and ferromagnetic crystalline layer and the strong exchange coupling between them. The average sizes as obtained from the size distribution analysis of about 300-350 particles (shown in supporting information (SI) Figure S1) are found to increase in size from 12 ± 3.2 nm for CoO core to 14.9 ± 3.6 nm for the CoO/ CoFe2O4 core/shell structure, suggesting the growth of CoFe2O4 on CoO core. (b)

(a)

20 nm

20 nm

2 0 nm

(c)

(d)

(311) 2.58 Å

(111) (111)

2.43 Å

2.43 Å

5 5 nm nm

5105nmnm nm

Figure 2. TEM images of (a) CoO and (b) CoO/ CoFe2O4 QDs. HRTEM of (c) CoO and (d) CoO/ CoFe2O4 QDs.


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The formation of core-shell heterostructure was further confirmed from HRTEM images shown in Figure 2 (c) and 2(d) (more images in SI Figure S2). Figure 2 (d) clearly shows the presence of two different regions in the QDs, an inner core and an outer shell with different lattice fringe spacing. The interplanar spacing of 2.43 Å observed in the core (Figure 2 (d)) is similar to that observed in CoO core only QDs shown in Figure 2 (c) and corresponds to the (111) plane of CoO cubic crystal structure. The interplanar spacing of 2.58 Å observed in the shell of the QDs in Figure 2 (d) is consistent with (311) planes of the CoFe2O4 structure. All these findings suggest the formation of crystalline CoO/CoFe2O4 core-shell heterostructured QDs. Furthermore, the advantages of the formation of crystalline and well defined core-shell heterostructure interfaces have been investigated by studying their magnetic properties. Bulk CoO is well known as an AFM material with Neel temperature (TN) of 291 K. However, below 10 K, nano-sized particles show very weak ferromagnetism or superparamagnetism due to uncompensated spins on the surface.48 Conversely, bulk CoFe2O4 (spinel) is FiM and nano-sized particles show superparamagnetic behavior at room temperature. Herein, we have studied the magnetic properties of CoO QDs overcoated with a thin layer (~1.5 nm) of CoFe2O4. SI Figure S3 (a) shows the M vs H hysteresis loop of CoO/CoFe2O4 core shell QDs at room temperature and at 2K. Although the room temperature hysteresis shows like superparamagnetic nature, however a small coercivity exists once we zoom in low field region as depicted in inset of figure SI 2a. Though the measurement temperature is greater than the Neel temperature of CoO, the core-shell QDs show a small presence of coercivity (~95 Oe) at room temperature which might be due to the presence exchange energy between CoO and CoFe2O4 layer. M vs H measurements at 2 K shows ferromagnetic behavior with large coercivity as the spins are in blocked state at low temperature. However, it is worth noting that the low temperature hysteresis loop is not completely saturated


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even at high field of 4T. This non-saturation behavior is due to the existence of highly anisotropic AFM as well as FiM material. 30

Magnetization (emu/g)

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2K

(a)

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0 kOe -30 10 kOe

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0 kOe 30 kOe -30

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(f)

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0 kOe -30 70 kOe -40

-20

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-40

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Magnetic Field (kOe)

Figure 3. Field and temperature of magnetic exchange bias. M-H hysteresis loop of CoO/ CoFe2O4 core-shell QDs recorded at 2K using cooling field (a) 10 kOe (b) 30 kOe (c) 70 kOe. M-H hysteresis loop of CoO/ CoFe2O4 measured at (d) 10 K (e) 100 K and (f) 200 K using FC (70 kOe) and ZFC condition. DC susceptibility or the variation of magnetization as a function of temperature was measured by applying an external field of 200 Oe. SI Figure S3 (b) shows the field cooled and zero field cooled curves for core-shell QDs. ZFC curve shows maxima at temperature of 292 K above which magnetization decays monotonically and merges with FC curve. This behavior is a characteristic


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of superparamagnetic systems and the transition temperature (292 K) is known as the blocking temperature (Tb) of the material. The observed TB value (292 K) perfectly correlates with the reported Neel temperature (TN =291 K) of CoO.48 Therefore, CoO/CoFe2O4 particles stay at blocked state below 292 K, which is analogous to the reported bulk state and is not influenced by nanoscale size effect. However, the FC/ZFC curve show a shoulder between 200 K to 250 K that may be due to the very small amount of uncoated CoO phase present in the sample. In order to investigate the exchange coupling effect at the AFM/FiM interface, M vs H hysteresis loops at low temperature (2 K) are measured after field cooling from 400 K using different fields and are plotted in Figure 3 (a)-(c). Similarly, in order to find the effect of different measurement temperatures, exchange bias was measured at different temperature using the same magnetic cooling field of 70 kOe and depicted in Figure 3 (d)- (f). All these hysteresis loops show the presence of exchange bias (loop shift along field axis). It is evident that HE, the exchange field, increases with the increase in cooling field and reaches 5.6 kOe at 70 kOe (Figure 3 (c)). Such a strong exchange bias coupling between CoO core and CoFe2O4 shell is due to the highly crystalline and sharp core-shell AFM/FiM interface and the stability of AFM ordering even at very high cooling field (70 kOe). The HE values we report in this study are among the highest reported values observed in core-shell QDs and indicate the good quality of the obtained CoO/ CoFe2O4 heterostructure. In addition to the horizontal loop shift, we observed a very small amount of vertical loop shift for every case, as reported in literature in various AFM/FiM core-shell QDs.50-51 This vertical shift is attributed to the uncompensated spins that do not follow the magnetization reversal with the applied field. The variation of exchange bias (HE) with different cooling fields is shown in the Figure 4 (a). From Figure 4 (a) it is evident that the exchange coupling increases with increasing cooling field and reaching a saturation at very high field. This suggests that the antiferromagnetic


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ordering of CoO exists even at high field (above 70 kOe) due to its large anisotropic energy and exchange interaction, which are the result of crystalline nature and optimized size. (a)

HE (kOe)

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Field (kOe)

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Hc, HE (kOe)

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Variation of HE 6 4 2 0 0

100

200

300

Temperature (K)

Figure 4. (a) Variation of HE with different cooling field (0-70 kOe) measured at 2K. (b) Variation of HE (red symbols) and Hc (black symbols) as a function of measurement temperature. (Solid lines are guide to the eye) In cases of temperature variation (Figure 3 (d)- (f)), samples were cooled from 400 K (above the ordering temperature of these QDs) using a field of 70 kOe (for FC measurement). It is evident from the figure that for all different temperatures there is a significant amount of loop shift and the variation of HE and the coercivity, Hc, as a function of measurement temperature has been demonstrated in Figure 4 (b). It is interesting to note that exchange coupling between the AFM/FiM core-shell exists even above 200 K and becomes zero near the TN (291 K) of CoO. A similar trend


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was observed in the variation of coercivity with a value of 10.5 kOe at 2K while close to zero at room temperature. As the room temperature is greater than the blocking temperature of CoFe2O4 and the Neel temperature of CoO, they behave like paramagnetic materials leading to the expected superparamagnetic nature of the QDs. Following the successful observation of one of the largest EB within QDs of this size, we explore the possibility of applying these QDs in magnetic hyperthermia application.52 The formation of crystalline AFM/FiM heterostructure QDs with a sharp interface gives rise to higher value of coercivity along with high saturation magnetization that results in larger loop area. Increase of loop area can offer higher dissipation of external energy into heat in presence of alternating magnetic field, which is called specific loss power (SLP). Although the particle sizes are below superparamagnetic limit, the heterostructure provides an extra anisotropy due to exchange bias that leads to larger loop area which can result in more heat dissipation under alternating magnetic field. The plot of change of temperature as a function of exposure time to alternating magnetic field for core/shell CoO/CoFe2O4 QDs is shown in SI Figure S4 and the slope of the same dT/dt is calculated. Based on these slope values, we calculate the SLP value to be about 355 W/g. It should be noted here that though this value is smaller than the record values reported for larger core-shell heterostructures,53-54 the mass included in the calculation of the SLP value is severely overestimated as the particles are insoluble in water. So even though a suspension of the particles is created, over the course of the measurement, most of these particles settle down and hence do not contribute to the measurement. Hence making these particles water soluble can substantially increase the SLP values. Secondly, the particles sizes are much smaller compared to the previous reports53-54 and an understanding of appropriate reduction in size can benefit from a more accurate


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estimation of SLP. However, while such a study with a potential for useful applications is interesting, is beyond the scope of this work. CONCLUSIONS CoO/CoFe2O4 AFM/FiM inverted core-shell structure was designed using seed growth technique by thermal decomposition of metal complexes and controlled interface modification to enhance the magnetic properties like the EB with specific emphasis on a sharp core-shell interface. We have shown that highly crystalline and sharp core-shell heterostructure leads to a remarkable value of exchange bias with a high coercivity which was not observed earlier in CoO/CoFe 2O4 inverted system. The high coercivity and the larger value of exchange field correlates with the promising application of these materials in magnetic data storage devices. Additionally, high anisotropy due to AFM and FiM heterostructure in these particles have potential to show large value of SLP and can also be useful for magnetic hyperthermia application.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Size distribution of nanoparticles, HRTEM images of core-shell QDs, M vs H loop at 2 K and 300 K, DC susceptibility data and SLP measurement. AUTHOR INFORMATION Corresponding Author * Email: [email protected]

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT Authors thank JNCASR, Sheikh Saqr Laboratory and Department of Science and Technology, Government of India for financial support. AS thanks CSIR for a research fellowship. SS is grateful to the Indian Academy of Sciences for their summer research fellowship. We thank Somnath Ghara and Chandan De for magnetic measurement and Kannan Dhandapani for TEM images. We also thank Prof. Dhirendra Bahadur and Mr. Rohan Bahadur from IIT Bombay for SLP measurement. REFERENCES (1)

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TOC Graphic 2K CoFe2O4

30

CoO M (emu/g)

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Page 26 of 26

15

0 kOe 30 kOe 70 kOe

0

(311) 2.58 Å

-15

(111) 2.43 Å

-30

5510 nnm mnm -40

-20

0

20

40

H (kOe)


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Interface Modelling Leading to Giant Exchange Bias from CoO

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