Manipulating magnetism in otherwise nonmagnetic materials

Modulation of magnetism in graphene by a gate voltage

  • The strength of indirect exchange changes by several oreders of magnitude when moving form non-resonant to resonant coumpling with adatoms
  • Strong modulation of the ferromagnetic coupling by adjusting the Fermi level in graphene with a gate voltage is suggested.

Krainov, I. V. Rozhansky, N. S. Averkiev, E. Lähderanta,, Phys. Rev. B, 92, 155432 (2015)

Theory of resonant indirect exchange

  • A theory of indirect exchange interaction is developed with account for bound states at magnetic impurities
  • If the coupling between the magnetic impurities and 2D conducting channel is resonant the indirecdt exchange is strongly enhanced
  • For graphene with magnetic adatoms the resonant indirect exchange leads to the same magnetic order bot for AA and AB configurations of the adatoms

I.V. Rozhansky, I.V. Krainov, N.S. Averkiev, B.A. Aronzon, et. al., Appl. Phys. Lett. 106, 252402 (2015)

I.V.Rozhansky, I.V, Krainov., N.S. Averkiev, E. Lahderanta, Journal of Magnetism and Magnetic Materials 383, 34 (2015).

Magnetic ordering of dopant spins in Ge and Si

  • The low-temperature transformation from antiparallel to parallel spin orientation in a nonmagnetic compensated system Ge:As semiconductor near the metal-insulator phase transition has been experimentally observed. This effect is manifested in the temperature dependences of the dopant magnetic susceptibility.
  • The spin subsystem is ordered and the semiconductor becomes ferromagnetic in Si:P near the phase transition insulator – metal, in the insulator state, at the temperatures below 50 K.
  • The magnetization is order of 10-4 Gssm3 / g, the coercive force is 250 Oe and Curie temperature is about 50 K.
  • In the area of low temperatures transition from anti-parallel to parallel coupling of localized spins results in increased radius of electron orbit, which leads to increased impurity diamagnetic susceptibility. 

A.I. Veinger, A.G. Zabrodskii, T.L. Makarova, T.V. Tisnek, S.I. Goloshchapov,and P.V. Semenikhin, AIP Conf. Proceedings, 1610, 129-134 (2014); ibid, Semiconductors, 49 (10)1294-1301 (2015)

Magnetism in the vicinity of metal-insulator transition

  • The phenomenon of the low-temperature transition from antiferro- to ferromagnetic ordering of impurity spins in a nonmagnetic compensated n-Ge:As semiconductor near the metal-insulator phase transformation has been experimentally observed.
  • The transition from AF to FM spin coupling can be detected by rather sharp changes in the spin density and a break in the temperature dependence of the g-factor, which corresponds to the appearance of a significant internal magnetic field.
  • This break is observed when the degree of compensation is greater than 0.3. At a lower compensation, the spin density at low temperatures increases but insufficiently for the appearance of an internal magnetic field. At higher compensation, the transition temperature increases almost in proportion to the degree of compensation.

Magnetism in graphite induced by ion irradiation (closed direction)

Hydrogen chemistry is not essential for magnetic ordering.
Three factors are critical for induced magnetic ordering in nonmagnetic solids: projectile mass, projectile energy, dose of irradiation. Comparative study of magnetic properties of highly oriented pyrolytic graphite irradiated with different particles suggests that ions (H+, He+, C+) with the energy in the range of hundreds keV yield larger values of induced magnetization compared to the protons with the energy of several MeV. These values increase with ion fluence but abruptly decrease when the concentration of interstitial defects becomes so large that the graphite stacking sequence is collapsed.
Advantages of ion implantation
manipulatingA mature method.
Allows focused ion ”writing”.
Disadvantages of ion implantation
Defect based magnetism.
Decays with time.
Exists as ”magnetic islands” in a nonmagnetic matrix and is not promicing for applications.

Magnetism of graphene decorated with aromatic radicals

  • Success in functionalisation of graphene (TEGO) with Cl/Br/NO2 – anilines
  • Magnetic analysis indicated that nitroaniline decorated TEGO shows both ferro- and antiferromagnetic (AF) signal, with AF transition at 120 K.

Laser controlled magnetism in hydrogenated fullerene H:C60 films

  • Room temperature ferromagnetic-like behavior in fullerene photopolymerized films treated with monatomic hydrogen is found.
  • Excess laser irradiation destroys magnetic ordering due to structural changes, which were continuously monitored by Raman spectroscopy
  • First-principles density-functional spin-unrestricted calculations show that the excess spin from monoatomic hydrogen is delocalized within the host fullerene and the laser-induced polymerization promotes spin exchange interaction and spin alignment in the polymerized phase.

Light-induced superconductivity in K3C60

  • By exciting the metallic K3C60 (supercond. transition at T = 19 K) with ultrashort mid-infrared optical pulses (pump-probe technique) we found a non-equilibrium superconducting-like behaviour in optical properties at T far greater than in normal conditions.

Mitrano, A. Cavalleri, D. Pontiroli, M. Riccò et al, Nature 530, 461 (2016)