Neil Heiman

Exchange induced unidirectional anisotropy at FeMn‐Ni80Fe20 interfaces

We have systematically investigated exchange coupled films of sputter deposited Ni80Fe20/FeMn and FeMn/Ni80Fe20 and obtained films with high exchange bias and low coercive forces. The variation of film properties with deposition conditions as well as with the permalloy and FeMn thicknesses have also been studied. The results demonstrated a strong dependence of exchange bias effect (HUA) on the amount of γ‐phase FeMn, the amount of impurities, as well as the abruptness of the transition profile at the interface. In addition, our results have also raised a number of new questions concerning the magnetic state of the interface and the origin of the interface coupling phenomena.

Modified mean‐field model for rare‐earth–iron amorphous alloys

A mean‐field model of the magnetic properties of amorphous rare‐earth–iron alloys has been developed which incorporates an Fe spin whose dependence on both the concentration and the species of rare earth is based on Mossbauer spectra. Using this model with a single fixed set of exchange constants, we have been able to calculate Curie temperatures which are in reasonable agreement with the data for amorphous rare‐earth (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu–iron alloys containing between 50 and 100 at.% iron. Better agreement with the measured Curie temperatures is achieved if the magnitudes of the Fe‐Fe and rare‐earth–Fe exchange constants are allowed to increase linearly with rare‐earth concentration. The latter case has the additional advantage of reproducing the observed shape of the magnetization‐vs‐temperature curves for wide range of composition and rare‐earth species.

Magnetism in rare earth‐transition metal amorphous alloy films

A number of rare earth (RE) transition metal (TM) amorphous alloy films (RE=Gd, Ho, Tb, Y; TM=Co, Fe, Ni, Mn) have been prepared across a broad compositional range by thermal evaporation. The samples were examined by polar Faraday rotation, polar Kerr rotation and magnetometer techniques. Results show that the major differences between amorphous and crystalline alloys are due to the altered state of the TM. Specifically the ordering temperature of amorphous RE‐Ni and RE‐Co alloys is higher than in the corresponding crystalline alloys, whereas the reverse is true for RE‐Fe alloys; at the same time, the TM ’’sublattice’’ magnetization is larger in the amorphous state for both Fe and Co compounds. Furthermore amorphous RE‐Mn alloys are not ordered. These facts tend to support the general concepts of the charge transfer model. Other properties such as coercivity and anisotropy are strongly dependent on the RE constituent.

Effects of substrate bias and annealing on the properties of amorphous alloy films of Gd‐Co, Gd‐Fe, and Gd‐Co‐X (X=Mo,Cu,Au)

We have examined the dependence of various properties of sputter‐deposited amorphous GdCo, GdFe, and GdCoX (X=Mo,Cu,Au) films upon substrate bias and annealing. Particular interest was directed to changes in magnetization, anisotropy, composition, x‐ray diffraction patterns, and Ar content of the films. In the case of the binary films, we applied substrate bias voltages to 400 V. We found that for the GdCo system, Ku rises with increasingly negative bias voltage to about Vb=−200 V. Further increases in Vb cause Ku to decrease, so that Ku?0 for Vb?−400 V. Furthermore, it was found that the Ar content of the films tends to correlate with Vb, reaching a maximum for Vb?−200 V. X‐ray diffraction patterns of these films also appear to correlate with Ku but in a more subtle manner than has been suggested by other workers. The results for amorphous GdFe films were considerably different. In particular, it was found that Ku was large for Vb?0 but decreased rapidly with negative bias. The behavior of ternary GdCoX ...

Uniaxial anisotropy in rare earth (Gd, Ho, Tb) ‐transition metal (Fe, Co) amorphous films

We present evidence that magnetic uniaxial anisotropy in amorphous RE‐TM films is a more common feature than had been presumed. The incident atomic beam direction appears to define the anisotropy axis in thermally evaporated films while the electric field direction at the surface is the dominant influence in bias sputter deposition. The nature of the coupling of the magnetization to the structural anisotropy is also found to vary.

Low‐coercivity amorphous magnetic alloy films

It is well known that rapidly quenched bulk samples of amorphous transition‐metal–metalloid alloys can be made with high permeability and low coercivity; however, little is known of the magnetic properties of these alloys when prepared by thin‐film‐deposition techniques. Sputter‐deposited films of iron‐carbon, iron‐silicon‐boron, iron‐chromium‐phosphorus‐carbon, cobalt‐iron‐boron, and cobalt‐iron‐chromium‐phosphorus‐boron have been prepared, and it is shown that these films can be made with low coercivity and high saturation magnetization by proper choice of deposition conditions. The magnetic properties of iron‐carbon films were found to depend on the sputtering conditions, in particular on the argon‐gas pressure. At an argon‐gas pressure of 15 μm, films with 70 at.% iron and 30 at.% carbon were obtained with a 4πMs of 12 000 G and coercivities in the range 0.1–0.9 Oe. Films made with higher and lower argon‐gas pressure had significantly higher coercivity. Other alloys were prepared to explore corrosion ...

Magnetization of amorphous REx‐Cu1−x alloys (RE=Gd, Tb, Dy, Ho)

Films of amorphous RExCu1−x (RE=Gd, Tb, Dy, Ho: 0.30⩽x⩽0.60) were prepared by two source evaporation of the elements. Magnetization measurements were made at 4.2 K with a vibrating sample magnetometer in applied fields up to 70 kOe. In the case of the Gd alloys, the magnetization is consistent with a fully aligned moment of 7μB. The dependence of the Curie temperatures on x can be understood in terms of a nearest neighbor statistical model with a percolation limit of x=0.33. In the case of Tb, Dy, Ho alloys, the magnetization is considerably less than that expected for fully aligned moments. The degree to which the magnetization is reduced is proportional to the Stevens factor of the RE confirming that the reduction is due to local anisotropy effects. The field dependence of the magnetization is consistent with existing models but the dependence of Hc on D/J does not agree with the theory.

Amorphous transition metal oxide films

We have been able to prepare amorphous oxides of Fe and Cr by sputter deposition. Magnetization measurements were made between 4.2 K and 300 K. Mossbauer spectra were obtained for the iron oxide samples in the same temperature range. Amorphous CrO2 films showed no evidence of magnetic order. For the amorphous Fe oxide films, the 4.2 K Mossbauer spectrum showed a broad symmetric six line pattern which collapsed to a doublet at T?100 K. The susceptibility of amorphous iron oxide shows a broad maximum near T=100 K and a paramagnetic moment of only 3μβ. We present arguments that this behavior cannot be described by superparamagnetism and therefore represents the intrinsic magnetic behavior of amorphous iron oxide.

Magnetic properties of amorphous FeC thin films

Thin films of amorphous FexC1−x (0.75⩽x⩽0.50) were prepared by sputter deposition. Room temperature values for 4πMs ranged from 13000 gauss (x=0.75) to 6500 gauss (x=0.50). Thus the room temperature magnetization is much higher than that for amorphous alloys of Fe with other group IV elements. The increased magnetization appears to be due to a higher Curie temperature as well as a larger atomic moment. It is well known that bulk amorphous samples of rapidly quenched alloys of transition metals with group III, IV and V elements can be prepared with low Hc; however, little is known of the properties of these alloys when prepared with thin films. We show that amorphous films of FeC can be prepared as Hc≈0.10 Oe without the need for post deposition annealing by proper selection of deposition parameters, in particular the argon gas pressure. The anisotropy energy is also dependent upon deposition gas presssure, going to zero for the same pressure that produces low Hc. Magnetostriction measurements have been ma...

Magnetic and recording properties of noncrystalline FeN films

We have prepared noncrystalline films of FeXN1−x in the range of 0.40⩽X⩽0.75. These films had large coercivities, some in excess of 600 Oe. The films were corrosion resistant and stable against annealing below 200 °C. Films with large Hc could not be produced with thicknesses less than ∼5000 A or with 4πMS ≲ 8000 G. The usefullness of these films as recording media is thus limited by the large MRt product.