Soft surface magnons and the first-order magnetic phase transitions in antiferromagnetic hematite

Chow, H. C.; Keffer, F.

doi:10.1103/physrevb.10.243

Cited by 29 publications

(36 citation statements)

References 20 publications

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“…49 The uniaxial anisotropy of bulk hematite has a near-zero value at room temperature due to the cancellation of the single-ion and dipolar sources of anisotropy. 6,50 For nanoparticles of <10 nm, it is well known that the Morin transition is suppressed, sometimes to below 4 K, 15 and the case is also true for mesoporous hematite solids. 18 Our films show a magnetic transition below 100 K, 23 which could be interpreted as the Morin spin-flop, and is concurrent with the onset of exchange bias.…”

Section: B Room-temperature Magnetic Propertiesmentioning

confidence: 96%

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

et al. 2012

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We investigated a hematite α-Fe2O3/permalloy Ni80Fe20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe2O3 is known to be very high (860 K), we attribute the lowtemperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μB per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains. We investigated a hematite α-Fe 2 O 3 /permalloy Ni 80 Fe 20 bilayer film where the antiferromagnetic layer consisted of small hematite grains in the 2 to 16 nm range. A pronounced exchange bias effect occurred below the blocking temperature of 40 K. The magnitude of exchange bias was enhanced relative to reports for identical compounds in large grain, epitaxial films. However, the blocking temperature was dramatically reduced. As the Néel temperature of bulk α-Fe 2 O 3 is known to be very high (860 K), we attribute the low-temperature onset of exchange bias to the well-known finite-size effect which suppresses the Morin transition for nanostructured hematite. Polarized neutron reflectometry was used to place an upper limit on the concentration and length scale of a layer of uncompensated moments at the antiferromagnetic interface. The data were found to be consistent with an induced magnetic region at the antiferromagnetic interface of 0.5-1.0 μ B per Fe atom within a depth of 1-2 nm. The field dependence of the neutron spin-flip signal and spin asymmetry was analyzed in the biased state, and the first and second magnetic reversal were found to occur by asymmetric mechanisms. For the fully trained permalloy loop, reversal occurred symmetrically at both coercive fields by an in-plane spin rotation of ferromagnetic domains.

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Section: B Room-temperature Magnetic Propertiesmentioning

confidence: 96%

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

et al. 2012

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“…Below the Néel temperature, T N ϭ955 K, ␣-Fe 2 O 3 is an antiferromagnetic ͑AF͒ insulator showing weak ferromagnetism above the Morin temperature, T M ϭ260 K, due to a slight canting of the two sublattice magnetizations. [2][3][4][5] Below T M , the direction of the magnetic moments is parallel to the ͓111͔ axis of the hexagonal unit cell. The localization of the iron 3d electrons due to the strong on-site Coulomb repulsion results in a large splitting of the d bands and a band gap of 2 eV.…”

Section: Introductionmentioning

confidence: 99%

First-principles calculation of the structure and magnetic phases of hematite

et al. 2004

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Rhombohedral ␣-Fe 2 O 3 has been studied by using density-functional theory ͑DFT͒ and the generalized gradient approximation ͑GGA͒. For the chosen supercell all possible magnetic configurations have been taken into account. We find an antiferromagnetic ground state at the experimental volume. This state is 388 meV/͑Fe atom͒ below the ferromagnetic solution. For the magnetic moments of the iron atoms we obtain 3.4 B , which is about 1.5 B below the experimentally observed value. The insulating nature of ␣-Fe 2 O 3 is reproduced, with a band gap of 0.32 eV, compared to an experimental value of about 2.0 eV. Analysis of the density of states confirms the strong hybridization between Fe 3d and O 2p states in ␣-Fe 2 O 3 . When we consider lower volumes, we observe a transition to a metallic, ferromagnetic low-spin phase, together with a structural transition at a pressure of 14 GPa, which is not seen in experiment. In order to take into account the strong on-site Coulomb interaction U present in Fe 2 O 3 we also performed DFTϩU calculations. We find that with increasing U the size of the band gap and the magnetic moments increase, while other quantities such as equilibrium volume and Fe-Fe distances do not show a monotonic behavior. The transition observed in the GGA calculations is shifted to higher pressures and eventually vanishes for high values of U. Best overall agreement, also with respect to experimental photoemission and inverse photoemission spectra of hematite, is achieved for Uϭ4 eV. The strength of the on-site interactions is sufficient to change the character of the gap from d-d to O-p-Fe-d.

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“…One possibility is that the spin reorientation to the antiferromagnetic state is similar to the Morin transition in a-Fe 2 O 3 in that the cooling field plays a role in overcoming the inherent exchange frustration of the competing antiferromagnetic and Dzyaloshinskii-Moriya exchange. 28 Fig . 2(d) is the magnetic hysteresis at 10 K. A nonsaturating magnetic hysteresis is seen with a coercivity of 0.3 T and a magnetization which has a value of %20 6 3 emu=cm 3 at 1 T. In the event that Mn and Fe order on alternate B sites to form an ordered double perovskite, a theoretical study predicts antiferromagnetic exchange between two sublattices with unequal spin, leading to a magnetic moment of approximately 1 l B per Mn-Fe pair.…”

Section: -2mentioning

confidence: 99%

The magnetic structure of an epitaxial BiMn0.5Fe0.5O3 thin film on SrTiO3 (001) studied with neutron diffraction

Cortie

Stampfl

Klose

et al. 2012

Applied Physics Letters

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High-angle neutron diffraction was used to directly reveal the atomic-scale magnetic structure of a single-crystalline BiMn0.5Fe0.5O3 thin film deposited on a SrTiO3 (001) substrate. The BiMn0.5Fe0.5O3 phase exhibits distinctive magnetic properties that differentiate it from both parent compounds: BiFeO3 and BiMnO3. A transition to long-range G-type antiferromagnetism was observed below 120 K with a (121212) propagation vector. A weak ferromagnetic behavior was measured at low temperature by superconducting quantum interference device (SQUID) magnetometry. There is no indication of the spin cycloid, known for BiFeO3, in the BiMn0.5Fe0.5O3 thin film. The neutron diffraction suggests a random distribution of Mn and Fe over perovskite B sites.

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Soft surface magnons and the first-order magnetic phase transitions in antiferromagnetic hematite

Cited by 29 publications

References 20 publications

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

Exchange bias in a nanocrystalline hematite/permalloy thin film investigated with polarized neutron reflectometry

First-principles calculation of the structure and magnetic phases of hematite

The magnetic structure of an epitaxial BiMn0.5Fe0.5O3 thin film on SrTiO3 (001) studied with neutron diffraction

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