Perpendicular anisotropy in Fe/Ag multilayers

Abstract

Ag(2.6 nm)/Fe(x nm)] 10 multilayers (0.2 ≤ x ≤ 1) have been prepared on Si(111) substrate by vacuum evaporation of 57 Fe enriched iron from a heated tungsten crucible in a base pressure of 10 -7 Pa. Ag was deposited by electron beam from a cold copper crucible. For ex-situ transmission Mössbauer spectroscopy measurements the samples were capped with a thick capping layer (55 nm Ag and 100 nm B). The deposited layers were removed from the substrate by using scotch tape. Field cooled (FC) and zero field cooled (ZFC) magnetization curves were measured by a superconducting quantum interference device (SQUID) magnetometer.The samples show superparamagnetic behavior in the whole Fe thickness range. This is indicated by the deviation of the FC and ZFC magnetization curves, as shown in Fig. 1 for a few representative samples. The blocking temperature, as defined by the temperature of the maximum in the ZFC magnetization, is 34 K for x = 0.2 (well reproducing the value of Ref. [2]) and rapidly increases with increasing x until x = 0.5. The broadening of the distribution of the size of the superparamagnetic grains with increasing x can also be inferred from the ZFC curves. Samples with x ≥ 0.6 are also superparamagnetic, as it is indicated by infield Mössbauer measurements not shown here, but for the majority of the Fe atoms the blocking temperature (T B ) is well above room temperature. The low temperature anomaly of the FC-ZFC curves of these samples (for x = 0.7 see Fig. 1) can be attributed to an about 10-20% of the Fe atoms forming smaller grains with T B below room temperature.The direction of the spontaneous magnetization changes from in plane to out of plane at around x = 0.6. This finding is qualitatively in agreement with results on samples prepared under different experimental conditions [1][2][3]. The magnetic anisotropy of multilayers can be affected by many different contributions [4]. For example the density and the orientation of the Ag steps of the growing superlattice were shown [5] to influence the direction of the magnetization. Our preliminary results on granular alloys prepared by co-evaporation of the elements indicate an in plane magnetization in a Fe concentration range equivalent to the present multilayer samples. In case of our multilayers the observed epitaxial growth [6] can explain the appearance of perpendicular magnetic anisotropy, since the coherent growth of the layers involves the presence of strains even in case of a small lattice mismatch. The transition from coherent to incoherent growth modes with increasing Fe layer thickness decreases the strain and can be accompanied by a change of the magnetic anisotropy direction [4].The direction of the spontaneous magnetization changes from out of plane to in plane at around x = 0.6 in [Ag(2.6 nm)/ Fe(x nm)] 10 multilayers (0.2 ≤ x ≤ 1) prepared on Si(111) substrate by vacuum evaporation. Transmission Mössbauer spectroscopy measurements of removed samples with a thick capping layer are compared to conversion electron Mössbauer spectroscopy meas...

Magnetic properties of the Fe-MgO interface studied by Mössbauer spectroscopy

“…The variation of the interatomic distances in a 300 K temperature range is an order of magnitude smaller (with bulk parameters) than the above change, however, in case of a delicate balance between anisotropies favoring in-plane and out-of-plane orientations it might play a decisive role. It is interesting to note, however, that a smaller decrease of I 2−5 was observed in the same temperature range in case of a Fe/Ag multilayer, 66 although the thermal expansion coefficient of Ag (≈19 × 10 −6 K −1 ) is larger than that of Fe (≈12 × 10 −6 K −1 ) and MgO (≈10 × 10 −6 K −1 ). The details of the layer growth can also make a difference as it is demonstrated by the close-to-random distribution of the magnetization in case of the 4-ML sample deposited onto the polished substrate.…”

Interface properties in sequence permutated Fe–B–Ag multilayers

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