Exchange bias effects in Fe nanoparticles embedded in an antiferromagnetic Cr₂O₃matrix

Abstract

Powders consisting of ferromagnetic (FM) Fe nanoparticles, of about 7 nm in size, embedded in an antiferromagnetic (AFM) Cr2O3 matrix have been obtained by high-temperature reduction under a hydrogen atmosphere of a mixed Cr–Fe oxide. This FM–AFM system exhibits exchange bias effects, i.e. a loop shift (HE) and coercivity enhancement (ΔHC), when field-cooled through the Néel temperature, TN, of Cr2O3. The exchange bias properties were measured as a function of temperature. HE and ΔHC are found to vanish at about TN(Cr2O3), indicating a good quality AFM matrix. Hence, high-temperature reduction of mixed oxides is demonstrated to be a suitable technique to develop new types of FM–AFM exchange-biased nanoparticles, from which novel applications of this phenomenon may be developed.

Exchange bias in Co-Cr2O3 nanocomposites

“…The amount of shift in hysteresis loop termed as exchange bias field (H EB ) is determined and presented in Table.1 along with the coercivity field H C . Maximum H EB of 427 Oe is observed in case of sample C. However, the value of H EB in the present case is less compared with the corresponding values usually observed in multi-layer systems but in accordance with the values observed in powder samples [17]. At 50K, the enhancement of H C value is observed in FC case (except in case of Sample B) compared to that for ZFC samples.…”

“…However, a small exchange bias field is observed in ZFC measurements also, which is attributed to the effect of FM magnetic moment in aligning the AFM moments [16,17]. This requires a considerable particle moment, which is possible in samples containing larger particles of Co.…”

Aerosol nanoparticles in the Fe1−xCrx system: Room-temperature stabilization of the σ phase and σ→α-phase transformation

“…Exchange bias has previously been reported for 7 nm pure Fe nanoparticles embedded in a Cr 2 O 3 matrix. 32 In our case, the absence of coupling can be explained on the basis of the low anisotropy constant of Cr 2 O 3 and the small thickness ͑Ͻ5 nm͒ of this oxide shell. 33 …”

High Temperature Magnetic Stabilization of Cobalt Nanoparticles by an Antiferromagnetic Proximity Effect

“…In fact, it has been demonstrated [4] that ferromagnetic-antiferromagnetic (FM-AFM) interfacial exchangecoupling is an effective method, later patented by Seagate [12], to increase the effective K of FM nanoparticles. However, a T B enhancement beyond RT using this approach has been rarely reported [22][23][24][25][26] (where often broad particle size distribution can partly account for the "apparent" T B increase [22][23][24][25]). The reason for this scarcity is that high Néel temperature (T N ) AFMs tend to have a low anisotropy constant (e.g., NiO), and vice versa (e.g., CoO), while substantial values of both properties are required for high-temperature stabilization.…”

“…Despite the foreseeable change of recording paradigm from continuous to patterned media, where each bit is recorded in an individual nanostructure [7], the key for sustained storage density increase will remain the introduction of progressively more anisotropic (high K) materials [8], which allow for magnetic stability at very small volumes, V (i.e., blocking temperature, T B ∝ KV, above room temperature, RT). Two main strategies are largely investigated to achieve high K (both of them with implications in other active technologies beyond information storage, such as permanent magnets, magnetic hyperthermia or even sensors [5,[9][10][11]): (i) the use of compounds with intrinsically high magnetocrystalline anisotropy (such as FePt [3,8]) and (ii) the design of exchange-coupled nanocomposites [4,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].Unfortunately, most high-K materials require high-temperature annealing processes to obtain the desired phase, which could hamper their implementation in certain structures. Thus, FM-AFM exchange coupling alternatives may be an appealing option.…”