Due to their reduced symmetry, surfaces often host physical phenomena that are absent in the
corresponding bulk material. Of particular interest for spintronics applications is the case of
surfaces showing a non-vanishing magnetic dipole per unit area, named “surface magnetization”
[1-2], which can occur despite the corresponding bulk being a perfectly compensated
antiferromagnet with zero net magnetization. Such surface magnetization underlies intriguing
physical phenomena like interfacial magnetic coupling, and can be used as a readout method of
antiferromagnetic domains [3-4].
In this talk, I will first introduce a classification scheme based on whether the surface of interest is
magnetically compensated or uncompensated if the bulk magnetic order is retained at the
surface. Then, I will show how surface magnetization can be understood in terms of bulk
magnetoelectric effects, whereby an applied electric field induces a net magnetization. The link
between a bulk magnetoelectric response and surface magnetization is intuitively expected by the
fact that the abrupt termination of a bulk to create a surface generates also an effective electric
field at the surface. This bulk-to-boundary correspondence serves as the ground for a universal
description of surface magnetization in antiferromagnets, which has been lacking thus far [5].
Finally, I will use density functional calculations to illustrate that nominally compensated surfaces
in Cr2 O3 and centrosymmetric FeF 2 develop a finite magnetization at the surface, in agreement
with our predictions based on the corresponding bulk magnetoelectric response.
[1] K. D. Belashchenko, Phys. Rev. Lett. 105, 147204 (2010)
[2] M. S. Wornle et al., Phys. Rev. B 103, 094426 (2021)
[3] N. Hedrich et al., Nat. Phys. 17, 574 (2021)
[4] J. Nogues and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999).
[5] S. F. Weber, A. Urru, S. Bhowal, C. Ederer, and N. A. Spaldin, Phys. Rev. X 14, 021033 (2024).
Andrea Urru holds a PhD in Condensed Matter Physics from the International School of Advanced Studies (SISSA) obtained in 2020 under the supervision of prof. Andrea Dal Corso. Previously, he earned a B. Sc. and M. Sc. degrees in Physics at the University of Cagliari, under the supervision of Prof. Vincenzo Fiorentini. After the PhD, Andrea was post-doc at ETH Zürich, Switzerland where he worked with prof. Nicola Spaldin. Since 2023 Andrea is Abrahams postdoctoral fellow at Rutgers University in New Jersey, USA, where he works with profs. David Vanderbilt and Karin Rabe.
Andrea’s research activity is focused on the realistic theoretical modeling of the structural, electronic, vibrational, and magnetic properties of solid state materials, by means of ab initio techniques. He is interested in the understanding and prediction, by means of symmetry analysis and first-principles techniques (mainly Density Functional Theory and Wannier interpolation), of effects originating from the cross-coupling of magnetism with other (e.g., electric, lattice) degrees of freedom and the effects generated by the simultaneous breaking of inversion and time-reversal symmetries. Examples of such physical effects include, but are not limited to: ferroelectricity, multiferroicity, magnetoelectricity, altermagnetism, flexomagnetism, natural optical activity, non-reciprocal directional dichroism.
Due to their reduced symmetry, surfaces often host physical phenomena that are absent in the
corresponding bulk material. Of particular interest for spintronics applications is the case of
surfaces showing a non-vanishing magnetic dipole per unit area, named “surface magnetization”
[1-2], which can occur despite the corresponding bulk being a perfectly compensated
antiferromagnet with zero net magnetization. Such surface magnetization underlies intriguing
physical phenomena like interfacial magnetic coupling, and can be used as a readout method of
antiferromagnetic domains [3-4].
In this talk, I will first introduce a classification scheme based on whether the surface of interest is
magnetically compensated or uncompensated if the bulk magnetic order is retained at the
surface. Then, I will show how surface magnetization can be understood in terms of bulk
magnetoelectric effects, whereby an applied electric field induces a net magnetization. The link
between a bulk magnetoelectric response and surface magnetization is intuitively expected by the
fact that the abrupt termination of a bulk to create a surface generates also an effective electric
field at the surface. This bulk-to-boundary correspondence serves as the ground for a universal
description of surface magnetization in antiferromagnets, which has been lacking thus far [5].
Finally, I will use density functional calculations to illustrate that nominally compensated surfaces
in Cr2 O3 and centrosymmetric FeF 2 develop a finite magnetization at the surface, in agreement
with our predictions based on the corresponding bulk magnetoelectric response.
[1] K. D. Belashchenko, Phys. Rev. Lett. 105, 147204 (2010)
[2] M. S. Wornle et al., Phys. Rev. B 103, 094426 (2021)
[3] N. Hedrich et al., Nat. Phys. 17, 574 (2021)
[4] J. Nogues and I. K. Schuller, J. Magn. Magn. Mater. 192, 203 (1999).
[5] S. F. Weber, A. Urru, S. Bhowal, C. Ederer, and N. A. Spaldin, Phys. Rev. X 14, 021033 (2024).
Andrea Urru holds a PhD in Condensed Matter Physics from the International School of Advanced Studies (SISSA) obtained in 2020 under the supervision of prof. Andrea Dal Corso. Previously, he earned a B. Sc. and M. Sc. degrees in Physics at the University of Cagliari, under the supervision of Prof. Vincenzo Fiorentini. After the PhD, Andrea was post-doc at ETH Zürich, Switzerland where he worked with prof. Nicola Spaldin. Since 2023 Andrea is Abrahams postdoctoral fellow at Rutgers University in New Jersey, USA, where he works with profs. David Vanderbilt and Karin Rabe.
Andrea’s research activity is focused on the realistic theoretical modeling of the structural, electronic, vibrational, and magnetic properties of solid state materials, by means of ab initio techniques. He is interested in the understanding and prediction, by means of symmetry analysis and first-principles techniques (mainly Density Functional Theory and Wannier interpolation), of effects originating from the cross-coupling of magnetism with other (e.g., electric, lattice) degrees of freedom and the effects generated by the simultaneous breaking of inversion and time-reversal symmetries. Examples of such physical effects include, but are not limited to: ferroelectricity, multiferroicity, magnetoelectricity, altermagnetism, flexomagnetism, natural optical activity, non-reciprocal directional dichroism.
