Charge ordering and exchange bias behaviors in Co3O4 porous nanoplatelets and nanorings

https://doi.org/10.1016/j.jmmm.2016.08.012Get rights and content

Highlights

  • Charge-ordered state appears for the Co3O4 nanoplatelets but absent for the nanorings.

  • Quite significant exchange bias is only observed for Co3O4 nanorings.

  • Exchange bias behavior of Co3O4 nanorings is consistent with spin-glass like shell.

  • Potential for ultrahigh-density magnetic recording and spin valve devices.

Abstract

We present the synthesis of α-Co3O4 porous nanoplatelets and hexagonal nanorings using microwave-assisted hydrothermal and conventional chemical reaction methods. The x-ray diffraction (XRD) and refinement analyses indicate the α-Co3O4 crystal structure, and the x-ray photoelectron spectrum (XPS) indicates the high purity of the samples. The M–T (including 1/χT) curves indicate an antiferromagnetic transition at about 35 K in both kind of samples but the interesting finding was made that a charge-ordered (CO) state appears at 250 K for the nanoplatelets sample whereas it is inattentive for the nanorings. The antiferromagnetic transition temperature TN is lower than that of the bulk α-Co3O4 single crystal due to the nanosized structures. We observed quite significant exchange bias for nanorings. The exchange bias behavior of the α-Co3O4 hexagonal nanorings is consistent with an antiferromagnetic (AFM) Co3O4 core and spin-glass like shell.

Introduction

The key to the successful design of magnetic structures for application is the ability to manipulate and control magnetic properties. The basic energies involved are exchange and anisotropy, where the former controls magnetic ordering and the later controls the preferred orientation. Both are phenomenological descriptions of fundamental correlations and energies associated with the electronic and crystalline structure of a material. A powerful technique for modifying and controlling magnetic characteristics is based on the use of magnetic heterostructures with properties governed by the interface region. One of the most interesting interfaces for basic study and application is the interface between a ferromagnet and an antiferromagnet. The exchange coupling at the interface between a ferromagnetic layer and an antiferromagnetic layer often results in an interesting phenomenon called “exchange bias” [1].

When a system consisting of ferromagnetic (FM)-antiferromagnetic (AFM) [1], FM spin glass (SG) [2], AFM-ferrimagnetic (FI) [3], FM–FI [4], AFM–SG [5] and FI–FI [6] interface is cooled with field through the Néel temperature (TN) of the AFM or glass temperature (TSG) of the SG, exchange bias (HEB) is induced showing a shift of hysteresis loop [M(H)] along the magnetic field axis. Since its discovery by Meiklejohn and Bean in 1956 [1], HEB has been extensively studied during the past fifty years, partly because of its applications in ultrahigh-density magnetic recording, giant magnetoresistance and spin valve devices [7], [8]. The HEB effect is attributed to an FM unidirectional anisotropy formed at the interface between different magnetic phases [7]. Generally, the process of field cooling (FC) from higher temperature is used to obtain FM unidirectional anisotropy in different HEB systems [1], [2], [3], [4]. The FM unidirectional anisotropy can also be realized by depositing the AFM layer onto a saturated FM layer [7], by ion irradiation in an external magnetic field [9], or by zero-field cooling (ZFC) with remnant magnetization [10], [11]. Application of sufficient large fields may also induce exchange bias [12], [13]. In summary, the FM unidirectional anisotropy in these HEB systems is formed by reconfiguring the FM spins at the interface between different magnetic phases. This phenomenon is fascinating not only for technological applications in memories and spin electronics but also for exceeding the super paramagnetic limit in ultrahigh density media [14], [15].

The first evidence of exchange bias was reported in the charge ordered (CO) manganite Pr1/3Ca2/3MnO3 among mixed valent perovskites, where ferromagnetic (FM) droplets are spontaneously embedded in the antiferromagnetic (AFM) background [16]. The exchange bias effect has recently been reported in another CO manganite, where the strong cooling field dependence of the exchange bias effect is ascribed to the spontaneous lamellar FM/AFM phase separation in Y0.2Ca0.8MnO3 [17]. The exchange bias effect is also reported for cluster-glass (CG) compounds La(Ba, Sr)CoO3, which is attributed to the cluster-glass state consisting of FM and spin-glass (SG) phases [18], [19], [20]. Recently the exchange bias phenomenon has been detected in another CG compound, where the HEB effect is observed in the case of 30% Fe substitution in LaMnO3 [21], [22].

Cobalt (II, III) oxide (Co3O4) is described by a formula unit AB2O4 (A → Co2+, B → Co3+) and exhibits a normal spinel crystal structure (Fd3m) with occupation of tetrahedral A sites by Co2+ and octahedral B sites by Co3+. The magnetic moment arises largely due to Co2+ ions because of spins, with a small contribution from spin–orbit coupling [23]. Conversely, Co3+ ions have no permanent magnetic moment as a consequence of the splitting of 3d levels by the octahedral crystal field and complete filling of t2 g levels. Two paths for the super exchange interaction between Co2+ ions have been suggested: A–O–A with z1=4 neighbors and A–O–B–O–A with z2=12 neighbors (O stands for the oxygen O2− ion) but without specifying their relative strengths [23]. Cobalt (II, III) oxide (Co3O4) has many functional applications, which include magnetic materials, solar energy absorbers, electrochromic devices, solid-state sensors, and heterogeneous catalysts [24], [25], [26], [27], [28]. In particular, Co3O4 nanoplatelets, nanorings, nanowires and nanorods have attracted wide interest.

A number of exchange bias studies have been reported recently for Co3O4: (a) Zeng et al. [29] reported the effects of size and orientation on magnetic properties and exchange bias in Co3O4 mesoporous nanowires; (b) Salabas et at [30] studied exchange anisotropy in nanocasted Co3O4 nanowires; and (c) Benitez et al. [31] stated the evidence for core–shell magnetic behavior in antiferromagnetic Co3O4 nanowires. Mesoporous Co3O4 nanostructures have been extensively studied for exchange bias [32], [33], [34], [35]. The exchange bias has also been observed in pure Co3O4 nanoparticles [36], [37]. As per our knowledge no study has been done on Co3O4 nanoplatelets and nanorings.

In this letter we report the charge ordering and exchange bias behaviors in Co3O4 porous nanoplatelets and nanorings where the antiferromagnetic transition temperature TN is lower than that of the bulk α-Co3O4 single crystal. The magnetic measurements include exchange bias (HEB), training effects, and the thermoremanent magnetization (TRM) have been performed. It was found that, the core–shell structure with the shells existing spin-glass-like behaviors, and charge-ordering significantly influence the exchange bias behaviors.

Section snippets

Experimental procedures

All chemicals were analytical grade reagents purchased from Sigma-Aldrich. The α-Co3O4 porous nanoplatlets and hexagonal nanorings were synthesised by using microwave-assisted hydrothermal and conventional chemical reaction methods [38]. The phase identity of the prepared products were characterized by X-ray diffraction measurements were conducted at room temperature using a mini materials analyzer x-ray diffractometer made by GBC Scientific Equipment, Inc. The morphology of the as-prepared

Results and discussion

The phase identity of the as-prepared Co3O4 nanoplatelets and nanorings was determined by X-ray diffraction (XRD). Fig. 1a shows the XRD pattern of the Co3O4 nanoplatelets and nanorings. It shows a high degree of crystallinity. All the peaks match well with the Bragg reflections of the standard spinel Co3O4 structure (SG: Fd3m) [39] and also in good agreement with Ref. [23]. The derived lattice constants from le bail fitting method at room temperature for nanoplatelets and nanorings are a

Conclusions

The magnetometry studies on α-Co3O4 porous nanoplatlets and hexagonal nanorings have been presented. The x-ray diffraction (XRD) and refinement analysis indicate the α-Co3O4 crystal structure, and the x-ray photoelectron spectrum (XPS) indicates the high purity of the samples. The magnetization and susceptibility curves indicate an antiferromagnetic transition at about 35 K in both samples. A charge-ordered (CO) state is appeared at 250 K for the nanoplatelets sample but it is inattentive for the

Acknowledgment

J.C. Debnath acknowledges the Alfred Deakin Postdoctoral Research Fellowship, Deakin University (Grant number RM0000029546), Australia.

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