05 décembre 2010
Contact: Vincent Baltz

Fig. 1: Blocking temperature distributions

The interaction between ferromagnetic and anti-ferromagnetic materials is known as exchange anisotropy, and it constitutes one of the foundations of spin electronic devices such as hard disk read reads, magnetic random access memories, and magnetic oscillators for use in telecom devices. Spintec has recently identified and shed some light on the possible stumbling blocks to the reproduction of exchange properties of nano-scale spintronic devices. This is the first step in the optimisation process.


Exchange anisotropy makes it possible for the device to have a reference state. This is established by setting the direction of magnetization in a ferromagnetic layer. Above the blocking temperature TB the reference state is lost (inset). One of the challenges for defining device specifications is that TB must be higher than the operating temperature of the device. The difficulty arises from the fact that the blocking temperature is not fixed; rather it is characterised by a distribution.


Fig. 2: Scanning electron microscopy images of nanoscale dot arrays

We have recently measured this distribution (Fig. 1) and to our surprise it has two peaks. At high temperature, as expected for a granular media, the peak reproduces the grain size distribution of the polycrystalline layers (typical centre at 20-30 nm). The peak at low temperature results from an interface effect and poses a problem to devices. Frustration appears in some areas due to roughness, structural defects or the presence of grain boundaries that prevent coupling between the ferromagnet and anti-ferromagnet, already at low temperature. Our work is presently focused on reducing the area affected by magnetic frustration in thin film layers.


We must not forget that the final device size will be less than 100nm and, while one of the points for exchange bias optimization lies in the thin film itself, lateral size reduction can also influence the properties of the device. Arrays of dots were fabricated and studied for their blocking temperature distribution (Fig. 2). Regarding the high temperature peak, we had previously observed that the reduction of the dot’s lateral size results in a shift of the peak towards lower temperatures. Since dots are patterned from a continuous film, certain grains are cut and reduced in size at the edges of the dot. This amounts to shifting the grain size distribution towards smaller sizes, and therefore also blocking temperature distribution towards lower temperatures, as expected for a polycrystalline granular material. The low temperature blocking temperature distribution peak increases in amplitude as the dot size is reduced. The reason is thought to be that the lateral size reduction adds to the total area where frustration occurs. At the edges of the dot, the layer is no longer continuous and symmetry is lost in the spin structure.


Through this study we have shown the double effect of patterning the dots to nanometer dimensions: the reduction of the grain size distribution because of grains being cut at the edges, and the increase of the total area where magnetic frustration occurs. These two effects can result in poor device performance, especially since the device size is so small. By clearly identifying the problem, new approaches are being taken to solve it.


Further reading: V. Baltz et al., Appl. Phys. Lett. 96 (2010) 262505


Exchange interaction


The exchange energy is present in the short range interaction between magnetic moments. These interactions contribute to the final magnetic state of the macroscopic film. In a ferromagnetic material, the exchange interaction tends to align all magnetic moments in the same sense and direction.  Above the Curie temperature the thermal fluctuations become dominant and the magnetic moment alignment is lost. In anti-ferromagnetic materials, the exchange interaction tends to align neighbouring magnetic moments in the same direction but with opposite senses. This order is lost above the Néel temperature. At the interface between the ferromagnet and the anti-ferromagnet, the magnetic moments of each layer influence each other’s orientation through the exchange interaction. Similarly, the blocking temperature is defined as the temperature above which the thermal energy dominates over the ferromagnet/anti-ferromagnet exchange energy.


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