Surface ferromagnetism and exchange bias in vacuum annealed Co3−yZnyO4 films

We report on the structural and the magnetic properties of spin coated Co3−yZnyO4 thin films. The microstructure of high vacuum annealed films exhibit a crystalline Zn:CoO core with a disordered oxygen deficient amorphous Co:ZnO surface. Weak ferromagnetism (m∼15mμB∕Co) and an exchange bias (HE∼500Oe) are observed at 10K despite the absence of any Co metal clusters. HE is smaller for samples with smaller Zn fractions (for Co3O4-HV, HE∼250Oe). Adding Zn significantly enhances HE. The exchange anisotropy in an antiferromagnetic material due to surface disordered weak ferromagnetism and its overall implications to dilute magnetic semiconducting oxides are discussed.

[1]  P. Kharel,et al.  Raman spectroscopic studies of oxygen defects in Co-doped ZnO films exhibiting room-temperature ferromagnetism , 2007 .

[2]  D. Fiorani,et al.  Surface effect in the magnetic order of antiferromagnetic nanoparticles , 2006 .

[3]  G. Hadjipanayis,et al.  Weak ferromagnetism and exchange biasing in cobalt oxide nanoparticle systems , 2006 .

[4]  J. Coey,et al.  Dilute magnetic oxides , 2006 .

[5]  David J. Smith,et al.  Enhancing exchange bias with diluted antiferromagnets. , 2006, Physical review letters.

[6]  Jordi Sort,et al.  Exchange bias in nanostructures , 2005 .

[7]  K. Temst,et al.  Reversing the training effect in exchange biased CoO/Co bilayers. , 2005, Physical review letters.

[8]  J. Stöhr,et al.  Correlation between exchange bias and pinned interfacial spins. , 2003, Physical review letters.

[9]  F. Hellman,et al.  Finite size effects on the moment and ordering temperature in antiferromagnetic CoO layers , 2003 .

[10]  T. Schmitte,et al.  Interfacial domain formation during magnetization reversal in exchange-biased CoO/Co bilayers , 2002, cond-mat/0204607.

[11]  V. Puntes,et al.  Synthesis, structural order and magnetic behavior of self-assembled /spl epsi/-Co nanocrystal arrays , 2001 .

[12]  Peter Grunberg,et al.  Layered magnetic structures: History, highlights, applications , 2001 .

[13]  Keller,et al.  Diluted antiferromagnets in exchange bias: proof of the domain state model , 2000, Physical review letters.

[14]  D. Peng,et al.  Magnetic properties of monodispersed Co/CoO clusters , 2000 .

[15]  Kentaro Takano,et al.  Exchange anisotropy — a review , 1999 .

[16]  A. Berkowitz,et al.  Magnetic anomalies in NiO nanoparticles , 1997 .

[17]  Ambrose,et al.  Finite-size effects and uncompensated magnetization in thin antiferromagnetic CoO layers. , 1996, Physical review letters.

[18]  W. Meiklejohn,et al.  New Magnetic Anisotropy , 1956 .

[19]  D. Gonbeau,et al.  Systematic XPS studies of metal oxides, hydroxides and peroxides , 2000 .