A 13 monolayer Ni film was deposited on Cu(001) and followed by oxygen exposure at 300 K. It was confirmed that the surface contained Ni oxide with an amorphous atomic arrangement. The magnetization is perpendicular after surface oxidation. The coercivity is substantially enhanced without shifting of the hysteresis loop after field cooling. This could indicate that the NiO is randomly antiferromagnetic or that the Neél temperature of the NiO is above 300 K. This surface oxide leads to surface passivation so that the film is magnetically alive under ambient conditions, which could have useful applications in the future.

We estimate the thickness and ordering temperature of an antiferromagnetic and passivation surface oxide through exchange bias coupling. The surface NiO, which is generated through the exposure of a Ni/Cu(001) surface to oxygen, is taken as a model system on which to perform the estimation. Since no exchange bias is found in the surface NiO/Ni/Cu(001), we have built a sandwich structure of NiO/n ML Ni/10 ML Co/Cu(001) to measure the n dependence of exchange bias. With n ⩽ 2, a large exchange bias field is found above 300 K, which could be due to the direct contact between the oxides and the Co layer. With 3 ⩽ n ⩽ 6, a smaller exchange bias field is found with a blocking temperature of 190 K. This implies that the thickness of NiO is, at most, 3 ML. Discovering the thickness and ordering temperature of the surface NiO provides us to explore the potential applications by using surface NiO.

The magnetization of Co0.10Ni0.90/Cu(001) films before and after surface oxidization at 300 K is presented. Before the oxidization, the magnetization of the films in the thickness of 11 to 20 monolayers (ML) is in the in-plane direction at the temperature ranging from 140 K to 300 K. After the oxidization, the magnetizations of the films are in the in-plane direction at the temperature above 200 K, but transit to magnetization demolishment, in-plane-and-out-of-plane co-existence, spin reorientation transition, and coercivity enhancement, for films of 11, 12, 13, and above 15 ML, respectively. The blocking temperature of this film is also 200 K, which implies the transitions might be driven by the ordering of the antiferromagnetic surface oxides. The various magnetizations provide a model system for manipulating the magnetization direction, as well as a spin valve device by combination of the oxidized films.

Combinations of CoNi alloying and oxygen exposure are used in order to study the spin reorientation transition (SRT) in Ni/Cu(001). These modifications on the volume layers or on the surface layers may result in an increase or decrease of the critical thickness (tc) of the SRT, respectively. We present the study on the magnetic properties of the oxygen exposure on Co5Ni95/Cu(001) ultrathin films. For films with a thickness below tc, a certain amount of oxygen exposure drives the SRT, whereas a large amount of oxygen demolishes the magnetization. For films with a thickness above tc, the out-of-plane magnetization persists under a large amount of oxygen exposure. Two-domain-like state and coercivity enhancement are found in the hysteresis loops. The results imply that oxygen adsorbates may drive the SRT and a large amount of oxygen may lead to the formation of antiferromagnetic oxide on the surface layers. Coercivity enhancement may refer to the exchange bias coupling, which provides the opportunity in applications in spintronics.

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