Temperature dependence of anisotropy in exchange biased Fe/KCoF3 system

Abstract

We used molecular beam epitaxy to deposit a novel ferro-/antiferromagnet (Fe/KCoF3) system on gallium terminated GaAs (100) substrates. We varied the thicknesses of single crystal Fe (001) layers from 1.05 to 3 nm. The antiferromagnetic fluoride, with a thickness of 30 nm, was deposited either in a single-crystal or a polycrystalline form, depending on the deposition conditions. KCoF3 is an antiferromagnet with a Néel temperature of 114 K. Its cubic structure almost perfectly matches the Fe film structure. The growth was monitored by reflection high energy electro diffraction. The magnetic properties of the system were studied using ferromagnetic resonance. Uniaxial and unidirectional anisotropy fields, due to exchange bias, were measured at low temperatures in the field-cooled samples and were smaller than 45 and 72 Oe, respectively. Both anisotropy fields, unidirectional and uniaxial, decreased with increasing thickness of the Fe film or with increases in temperature. The temperature dependence of the fourfold anisotropy was studied for the samples with different thicknesses of Fe films and different crystalline states of the fluoride layer. Room temperature values of the fourfold anisotropy also increased with increasing thickness of the Fe layer and were in a fairly good agreement with previously reported values for single crystal Fe films. Also, the temperature characteristics of the fourfold anisotropy for the structures with single crystal fluoride seemed to reproduce low-temperature literature data for Fe (001) films. In contrast, the structures with polycrystalline KCoF3 demonstrated a significant enhancement of the fourfold anisotropy at low temperatures. For the thinnest sample (tFe=1.05 nm) the low temperature anisotropy is more than tripled compared to room temperature values. It is believed that the surface effects can be responsible for this enhancement.

The Anisotropy Induced by a Magnetostriction in Exchange-Biased Two-Layer Films

“…Simultaneous observation of the FM and AF spins in Co/LaFeO 3 [16] and Co/NiO [17] systems revealed that FM magnetisation is aligned parallel or antiparallel to the in-plane projection of the AF axis in contrast to the usually observed perpendicular coupling consistent with the Koon's model [15]. Uniaxial anisotropy is also detected after deposition of Fe on top of KCoF 3 [18,19]. All these AFs are known to have rather large magnetostriction of the exchange nature (see Table 1).…”

“…Nevertheless, this value will last to choose preferable axis of magnetisation as was clearly observed in the experiments [18,19]. [19] More pronounced effect is expected in Co films, which have rather high magnetostriction and small bulk magnetic anisotropy.…”

Microstructure and magnetic properties of Fe/KCoF3 thin films

“…Exchange coupling between the uncompensated spins of the antiferromagnet and the ferromagnetically ordered spins introduces a unidirectional anisotropy and a pinning of the ferromagnet, which result in modification of overall magnetic anisotropy, increases coercivity and shifts the hysteresis loop when the structure is cooled in the presence of magnetic field [1][2][3]. All the effects discussed above were observed in a Fe=KCoF 3 system consisting of an ultra-thin ferromagnetic layer and a thicker antiferromagnetic fluoride [4,5]. An interesting feature of this system is that by modification of the growth condition we could produce samples with single crystal or polycrystalline fluorides having different interface structures and consequently different degrees of spin compensation.…”

“…Further X-ray studies indicated congruent growth of KCoF 3 fluoride with a cubic perovskite structure [4]. The j-scans (in plane rotation) of single crystal fluorides showed distinct [1 1 0] peaks with four-fold symmetry [5].…”

“…Additional details can be found in Ref. [4,5]. An important fact is that during deposition at room temperature with the deposition rate above 1 ( A=s the antiferromagnetic fluoride grew in the polycrystalline form, whereas lower rates and an elevated temperature of the substrate gave rise to epitaxial growth of the fluoride.…”