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Abstract
The deposition, structure, and magnetic properties of $\rm Ni\sb{x}Co\sb{1-x}O$ thin films, (NiO/CoO) and $\rm\lbrack Ni\sb{x}Co\sb{1-x}O/MgO\rbrack$ multilayers, FM/$\rm Ni\sb{x}Co\sb{1-x}O$ bilayers (FM = Ni, Co, Fe, $\rm Ni\sb{50}Co\sb{50},\ Ni\sb{80}Fe\sb{20}),$ and FM/$\rm\sp{57}Fe/Ni\sb{x}Co\sb{1-x}O$ (FM = Ni, Fe) trilayers were studied. The films were deposited by conventional and reactive sputtering. FM/$\rm Ni\sb{x}Co\sb{1-x}O$ bilayer films exhibited the characteristic magnetic behavior associated with the phenomena of exchange anisotropy. Collective magnetic ordering behavior in (NiO-CoO) multilayers and finite size reduction of the Neel temperature of thin $\rm Ni\sb{x}Co\sb{1-x}O$ layers in $\rm\lbrack Ni\sb{x}Co\sb{1-x}O/MgO\rbrack$ multilayers was observed by thin film heat capacity measurements. The uncompensated spins on the surfaces of antiferromagnetic (AFM) CoO films exhibited a stable thermoremanent magnetization after field-cooling the multilayers from T $>$ T$\rm\sb{N}.$ The thermoremanent magnetization had the same temperature dependence as the exchange field of field-cooled permalloy/CoO bilayers, which strongly suggests that these interfacial uncompensated spins are responsible for unidirectional anisotropy. A model based on the calculation of the density of these interfacial uncompensated spins predicted the correct magnitude of the exchange field, and reproduced the observed inverse dependence on the average interfacial AFM grain size. Reduced dimensions and low magnetocrystalline anisotropy of the AFM grains decreased the magnetic stability of the AFM grains, such that the exchange field disappeared at T $\ll$ T$\rm\sb{N}.$ Using FM/CoO bilayers, FM materials with a high Ni concentration exhibited a reduced unidirectional interfacial exchange energy density $\Delta\sigma.$ Mean field analysis predicted the variation in $\Delta\sigma$ and indicated that the interfacial coupling occurs via direct exchange between metal atoms and not superexchange as one might expect at a metal/oxide interface. Models of exchange anisotropy often depict the FM/AFM interface as an atomically smooth and chemically sharp interface; however, the complexity of the chemical and magnetic environment of the FM(metal)/AFM(oxide) interface was revealed by measuring NiO/$\rm\sp{57}Fe({<}14\A)$/FM (FM = Ni, Fe) trilayers using Mossbauer spectroscopy. Fitting the complicated Mossbauer spectra of the interfacial Fe atoms required multiple metal and oxide environments. Analysis indicated that both metal and oxide interfacial local magnetization directions are oriented strongly in-plane.
