E n esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocaló... more E n esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocalórico y magnetovolúmico que se ha llevado a cabo en dos familias de compuestos ricos en Fe: aleaciones R 2 Fe 17 , sintetizadas en forma policristalina, y cintas amorfas de composición FeZrBCu. Estas aleaciones presentan transiciones magnéticas de segundo orden con temperaturas críticas en torno a temperatura ambiente.
Nanostructured Fe-based alloys have softer magnetic properties, such as larger saturation polariz... more Nanostructured Fe-based alloys have softer magnetic properties, such as larger saturation polarizations and magnetic permeabilities, smaller anisotropies and coercive fields and vanishing magnetostrictions, than their precursor alloys in the amorphous state. The softest magnetic properties are obtained for the smallest nanocrystalline grain sizes (between 10 and 20 nm), and these nanostructured materials are very suitable for use as high-frequency electronic components in magnetic devices or magnetic sensors based on the magnetoimpedance (MI) effect. In this work we study the correlation between the structural, electrical and magnetic properties together with the MI effect response in some heat-treated Finemet type and FeZrB ribbons. We show that the maximum MI ratio of around 130% and a sensitivity to the applied magnetic field of 0.07% (A m −1 ) −1 is obtained in the heat-treated samples that show an optimum nanocrystalline state and exhibit softer magnetic properties.
The inspection of simplified fabrication and/or processing routes in order to produce materials w... more The inspection of simplified fabrication and/or processing routes in order to produce materials with attractive magnetocaloric properties is of paramount importance for the development of environmentally friendly magnetic cooling technology. In this work, we have made use of the melt-spinning technique to obtain directly single-phase Y 2 Fe 17 polycrystalline ribbons avoiding any high-temperature annealing for phase consolidation and homogenization. The melt-spun ribbons, with hexagonal Th 2 Ni 17-type crystal structure, exhibit a moderate maximum value of the magnetic entropy change, | DS M peak | ¼ 2.4(4.4) J kg À1 K À1 under an applied magnetic field change of 2(5) T. Although these values are similar to those for the bulk alloy, the DS M (T) curves are manifestly broader, thus giving rise to an expansion of the working temperature range and the enhancement of about 15% in the refrigerant capacity. We also show that the magnetic field dependence of |DS M peak | at T ¼ T C follows a H 2/3 power-law.
The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has... more The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened up the path for extensive research activity in the nanoscale world. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. We combined X-ray diffraction, high-resolution transmission electron microscopy, extended X-ray absorption fine structure and magnetic measurements in order to describe the influence of the microstructure and morphology on the magnetic behaviour of NiO nanoparticles (NPs) with sizes ranging from 2.5 to 9 nm. The present findings reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order, typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni 2+ magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, whereas larger NPs are formed by an uncompensated AFM core with a net magnetic moment surrounded by a SG shell. The coupling at the core-shell interface leads to an exchange bias effect manifested at low temperature as horizontal shifts of the hysteresis loop ($1 kOe) and a coercivity enhancement ($0.2 kOe). † Electronic supplementary information (ESI) available: TEM size distributions, normalized XAS spectra, variation of the coordination number due to cluster size reduction and ZFC-FC curves at 0.1 kOe. See
In this work an enhancement of the Curie temperature of the intergranular amorphous region in nan... more In this work an enhancement of the Curie temperature of the intergranular amorphous region in nanocrystalline alloys with respect to amorphous ribbons of the same composition is shown. The Curie temperature reaches a value of 125 C. The experimental results are ...
An unusual 4f -superantiferromagnetic state characterized by simultaneous antiferromagnetic and s... more An unusual 4f -superantiferromagnetic state characterized by simultaneous antiferromagnetic and spin-glass behaviors induced by particle-size reduction is revealed in metallic nanoparticles (≈9 nm) of TbCu 2 . The Néel temperature is 46 K and the glassy freezing is below ≈9 K and governed by a critical slowing down process. Neutron diffraction at 1.8 K establishes the superantiferromagnetism. The latter is settled by the nanoparticle moments and the freezing mechanism is provided by the surface spins.
Journal of Radioanalytical and Nuclear Chemistry, 2010
Abstract Neutron scattering (NS) is a ''big science'' discipline whose resear... more Abstract Neutron scattering (NS) is a ''big science'' discipline whose research spans over a wide spectrum of fields, from fundamental or basic science to technological applications. The objective of this paper is to track the evolution of Spanish research in NS from a bibliometric ...
The Joule heating is used to obtain different degrees of crystallisation in Fe 87 Zr 6 Cu 1 B 6 a... more The Joule heating is used to obtain different degrees of crystallisation in Fe 87 Zr 6 Cu 1 B 6 and Fe 91 Zr 7 B 2 . Neutron diffraction experiments have been performed on a single ribbon to follow the nanocrystallisation under the same conditions. Hysteresis loops show the evolution of the saturation magnetisation and coercive field. The size of the crystallites remains 10-15 nm, which is smaller than in samples nanocrystallised by treatment in furnaces. r (P. Gorria). 0304-8853/03/$ -see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 -8 8 5 3 ( 0 2 ) 0 0 8 5 2 -1
The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compo... more The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compositions and Curie temperatures between 230 and 300 K has been studied. Albeit the maximum entropy change is low (|!S M | max ~ 3 J kg -1 K -1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads out over a broad temperature interval (!T ~ 200 K). The estimated values for the relative cooling power are close to that of pure Gd. From the applied magnetic field dependence of the magnetic entropy change a master curve valid for these Fe-rich FeZrB amorphous alloys has been found. However, two reference temperatures of the |!S M | (T) are needed in order to get a collapsing curve for all the maximum applied magnetic field values.
Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amor... more Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amorphous ribbons of two different compositions, Fe91Zr7B2 and Fe88Zr8B4, with respective Curie temperature values of 230 and 285 K have been studied. Although the maximum magnetic entropy change is relatively moderate (|ΔSM|max ∼ 3 J K−1 kg−1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads over a broad temperature interval (∆T ∼ 200 K), giving rise to a large refrigerant capacity loss (RC ∼ 435 J kg−1) without any hysteresis. The Curie temperature can be easily tuned between 200 and 350 K by changing the boron content. Therefore, the MCE can be controlled over a wide temperature interval, thus making these amorphous alloys promising candidates for magnetic refrigeration near room temperature.
Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material... more Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material exhibits a spontaneous magnetostriction that reaches 1.6 × 10 −2 at 5 K and goes down to zero well above the Curie temperature, T C , owing to short-range magnetic correlations. Besides, Er 2 Fe 17 exhibits direct and inverse magneto-caloric effects (MCE) with moderate isothermal magnetic entropy, ∆S M , and adiabatic temperature, ∆T ad , changes (∆S M ∼ −4.7 J/kgK and ∆T ad ∼ −2.5 K near T C , and ∆S M ∼ 1.5 J/kgK and ∆T ad ∼ 0.6 K at 40 K for ∆H = 80 kOe, respectively). The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er-sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er-and Fe-sublattice. The main trends found experimentally for the temperature dependences of ∆S M and ∆T ad , as well as for the atomic magnetic moments, are qualitatively well described considering a mean field Hamiltonian that incorporates both crystalline electric field and exchange interactions. However, the discrepancies occurring in the temperature range (110 K, 250 K), where the experimental ∆S M and ∆T ad are almost zero and the theoretical curves show a unique cutoff, and the fact that the cell volume of Er 2 Fe 17 exhibits an almost constant value in the same temperature range, lead to the conclusion that the interplay between MCE and magneto-volume anomalies is fundamental to understand the physical properties of this intermetallic compound. PACS numbers: 61.05.C-, 61.05.F-, 75.10.Hk, 75.30.Kz, 75.30.Sg.
Neutron powder diffraction shows that the intermetallic Er 2 Fe 17 compound with hexagonal crysta... more Neutron powder diffraction shows that the intermetallic Er 2 Fe 17 compound with hexagonal crystal structure has a ferrimagnetic ground state (T C = 303 K). At T = 5 K the magnetic moments of Fe sublattice (µ ~ 2 µ B ) are therefore antiparalell to those of the Er one (µ ~ 9 µ B ), all of them lying on the basal plane. This compound exhibits strong magnetovolume effects up to temperatures in the vicinity of T C . Neutron thermo-diffraction experiments also show an anomalous temperature dependence of the cell volume, including a negative thermal expansion coefficient below 300 K. In addition, a positive spontaneous volume magnetostriction is observed up to T ~ 400 K, with a maximum (ω S ~ 0.02) located at T = 5 K.
Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material... more Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material exhibits a spontaneous magnetostriction that reaches 1.6 × 10 −2 at 5 K and goes down to zero well above the Curie temperature, T C , owing to short-range magnetic correlations. Besides, Er 2 Fe 17 exhibits direct and inverse magneto-caloric effects (MCE) with moderate isothermal magnetic entropy, ∆S M , and adiabatic temperature, ∆T ad , changes (∆S M ∼ −4.7 J/kgK and ∆T ad ∼ −2.5 K near T C , and ∆S M ∼ 1.5 J/kgK and ∆T ad ∼ 0.6 K at 40 K for ∆H = 80 kOe, respectively). The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er-sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er-and Fe-sublattice. The main trends found experimentally for the temperature dependences of ∆S M and ∆T ad , as well as for the atomic magnetic moments, are qualitatively well described considering a mean field Hamiltonian that incorporates both crystalline electric field and exchange interactions. However, the discrepancies occurring in the temperature range (110 K, 250 K), where the experimental ∆S M and ∆T ad are almost zero and the theoretical curves show a unique cutoff, and the fact that the cell volume of Er 2 Fe 17 exhibits an almost constant value in the same temperature range, lead to the conclusion that the interplay between MCE and magneto-volume anomalies is fundamental to understand the physical properties of this intermetallic compound. PACS numbers: 61.05.C-, 61.05.F-, 75.10.Hk, 75.30.Kz, 75.30.Sg.
The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has... more The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened up the path for extensive research activity in the nanoscale world. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. We combined X-ray diffraction, high-resolution transmission electron microscopy, extended X-ray absorption fine structure and magnetic measurements in order to describe the influence of the microstructure and morphology on the magnetic behaviour of NiO nanoparticles (NPs) with sizes ranging from 2.5 to 9 nm. The present findings reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order, typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni 2+ magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, whereas larger NPs are formed by an uncompensated AFM core with a net magnetic moment surrounded by a SG shell. The coupling at the core-shell interface leads to an exchange bias effect manifested at low temperature as horizontal shifts of the hysteresis loop ($1 kOe) and a coercivity enhancement ($0.2 kOe). † Electronic supplementary information (ESI) available: TEM size distributions, normalized XAS spectra, variation of the coordination number due to cluster size reduction and ZFC-FC curves at 0.1 kOe. See
In this paper, we analyze the behaviour of Fe-rich wires that have been submitted to a current an... more In this paper, we analyze the behaviour of Fe-rich wires that have been submitted to a current annealing, with and without an applied torsional stress. The structural relaxation and induced anisotropies achieved in the samples produce different effects in the magneto-...
A comparative study on the magnetoimpedance (MI) and torsion-impedance (TI) effects between Fe 73... more A comparative study on the magnetoimpedance (MI) and torsion-impedance (TI) effects between Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 FINEMET ribbons and wires has been performed. The samples were annealed for 1 h at temperatures below and above their primary crystallization temperature, around 520 ° C. The magnetic softening of the samples occurs during the nanocrystallization process, which enhances both the MI and TI effects up to 82% and 75%, respectively, in the wire sample, with respect to its as-quenched state. A maximum MI ratio of 50% is obtained in the nanocrystalline ribbon. The modified values of saturation magnetization, coercive field and magnetostriction, and changes in the domain structure improve the impedance response of the nanocrystalline wire for its use as a sensor element for detecting small magnetic fields, with a sensitivity of 17%/Oe and torsional stresses with a sensitivity of 700% rad/cm.
E n esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocaló... more E n esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocalórico y magnetovolúmico que se ha llevado a cabo en dos familias de compuestos ricos en Fe: aleaciones R 2 Fe 17 , sintetizadas en forma policristalina, y cintas amorfas de composición FeZrBCu. Estas aleaciones presentan transiciones magnéticas de segundo orden con temperaturas críticas en torno a temperatura ambiente.
Nanostructured Fe-based alloys have softer magnetic properties, such as larger saturation polariz... more Nanostructured Fe-based alloys have softer magnetic properties, such as larger saturation polarizations and magnetic permeabilities, smaller anisotropies and coercive fields and vanishing magnetostrictions, than their precursor alloys in the amorphous state. The softest magnetic properties are obtained for the smallest nanocrystalline grain sizes (between 10 and 20 nm), and these nanostructured materials are very suitable for use as high-frequency electronic components in magnetic devices or magnetic sensors based on the magnetoimpedance (MI) effect. In this work we study the correlation between the structural, electrical and magnetic properties together with the MI effect response in some heat-treated Finemet type and FeZrB ribbons. We show that the maximum MI ratio of around 130% and a sensitivity to the applied magnetic field of 0.07% (A m −1 ) −1 is obtained in the heat-treated samples that show an optimum nanocrystalline state and exhibit softer magnetic properties.
The inspection of simplified fabrication and/or processing routes in order to produce materials w... more The inspection of simplified fabrication and/or processing routes in order to produce materials with attractive magnetocaloric properties is of paramount importance for the development of environmentally friendly magnetic cooling technology. In this work, we have made use of the melt-spinning technique to obtain directly single-phase Y 2 Fe 17 polycrystalline ribbons avoiding any high-temperature annealing for phase consolidation and homogenization. The melt-spun ribbons, with hexagonal Th 2 Ni 17-type crystal structure, exhibit a moderate maximum value of the magnetic entropy change, | DS M peak | ¼ 2.4(4.4) J kg À1 K À1 under an applied magnetic field change of 2(5) T. Although these values are similar to those for the bulk alloy, the DS M (T) curves are manifestly broader, thus giving rise to an expansion of the working temperature range and the enhancement of about 15% in the refrigerant capacity. We also show that the magnetic field dependence of |DS M peak | at T ¼ T C follows a H 2/3 power-law.
The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has... more The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened up the path for extensive research activity in the nanoscale world. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. We combined X-ray diffraction, high-resolution transmission electron microscopy, extended X-ray absorption fine structure and magnetic measurements in order to describe the influence of the microstructure and morphology on the magnetic behaviour of NiO nanoparticles (NPs) with sizes ranging from 2.5 to 9 nm. The present findings reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order, typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni 2+ magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, whereas larger NPs are formed by an uncompensated AFM core with a net magnetic moment surrounded by a SG shell. The coupling at the core-shell interface leads to an exchange bias effect manifested at low temperature as horizontal shifts of the hysteresis loop ($1 kOe) and a coercivity enhancement ($0.2 kOe). † Electronic supplementary information (ESI) available: TEM size distributions, normalized XAS spectra, variation of the coordination number due to cluster size reduction and ZFC-FC curves at 0.1 kOe. See
In this work an enhancement of the Curie temperature of the intergranular amorphous region in nan... more In this work an enhancement of the Curie temperature of the intergranular amorphous region in nanocrystalline alloys with respect to amorphous ribbons of the same composition is shown. The Curie temperature reaches a value of 125 C. The experimental results are ...
An unusual 4f -superantiferromagnetic state characterized by simultaneous antiferromagnetic and s... more An unusual 4f -superantiferromagnetic state characterized by simultaneous antiferromagnetic and spin-glass behaviors induced by particle-size reduction is revealed in metallic nanoparticles (≈9 nm) of TbCu 2 . The Néel temperature is 46 K and the glassy freezing is below ≈9 K and governed by a critical slowing down process. Neutron diffraction at 1.8 K establishes the superantiferromagnetism. The latter is settled by the nanoparticle moments and the freezing mechanism is provided by the surface spins.
Journal of Radioanalytical and Nuclear Chemistry, 2010
Abstract Neutron scattering (NS) is a ''big science'' discipline whose resear... more Abstract Neutron scattering (NS) is a ''big science'' discipline whose research spans over a wide spectrum of fields, from fundamental or basic science to technological applications. The objective of this paper is to track the evolution of Spanish research in NS from a bibliometric ...
The Joule heating is used to obtain different degrees of crystallisation in Fe 87 Zr 6 Cu 1 B 6 a... more The Joule heating is used to obtain different degrees of crystallisation in Fe 87 Zr 6 Cu 1 B 6 and Fe 91 Zr 7 B 2 . Neutron diffraction experiments have been performed on a single ribbon to follow the nanocrystallisation under the same conditions. Hysteresis loops show the evolution of the saturation magnetisation and coercive field. The size of the crystallites remains 10-15 nm, which is smaller than in samples nanocrystallised by treatment in furnaces. r (P. Gorria). 0304-8853/03/$ -see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 -8 8 5 3 ( 0 2 ) 0 0 8 5 2 -1
The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compo... more The magneto-caloric effect exhibited by three Fe-rich FeZrB amorphous alloys with different compositions and Curie temperatures between 230 and 300 K has been studied. Albeit the maximum entropy change is low (|!S M | max ~ 3 J kg -1 K -1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads out over a broad temperature interval (!T ~ 200 K). The estimated values for the relative cooling power are close to that of pure Gd. From the applied magnetic field dependence of the magnetic entropy change a master curve valid for these Fe-rich FeZrB amorphous alloys has been found. However, two reference temperatures of the |!S M | (T) are needed in order to get a collapsing curve for all the maximum applied magnetic field values.
Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amor... more Fe-rich FeZrB metallic glasses exhibit magneto-caloric effect (MCE) around room temperature. Amorphous ribbons of two different compositions, Fe91Zr7B2 and Fe88Zr8B4, with respective Curie temperature values of 230 and 285 K have been studied. Although the maximum magnetic entropy change is relatively moderate (|ΔSM|max ∼ 3 J K−1 kg−1 under an applied magnetic field change from 0 to 50 kOe), the MCE spreads over a broad temperature interval (∆T ∼ 200 K), giving rise to a large refrigerant capacity loss (RC ∼ 435 J kg−1) without any hysteresis. The Curie temperature can be easily tuned between 200 and 350 K by changing the boron content. Therefore, the MCE can be controlled over a wide temperature interval, thus making these amorphous alloys promising candidates for magnetic refrigeration near room temperature.
Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material... more Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material exhibits a spontaneous magnetostriction that reaches 1.6 × 10 −2 at 5 K and goes down to zero well above the Curie temperature, T C , owing to short-range magnetic correlations. Besides, Er 2 Fe 17 exhibits direct and inverse magneto-caloric effects (MCE) with moderate isothermal magnetic entropy, ∆S M , and adiabatic temperature, ∆T ad , changes (∆S M ∼ −4.7 J/kgK and ∆T ad ∼ −2.5 K near T C , and ∆S M ∼ 1.5 J/kgK and ∆T ad ∼ 0.6 K at 40 K for ∆H = 80 kOe, respectively). The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er-sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er-and Fe-sublattice. The main trends found experimentally for the temperature dependences of ∆S M and ∆T ad , as well as for the atomic magnetic moments, are qualitatively well described considering a mean field Hamiltonian that incorporates both crystalline electric field and exchange interactions. However, the discrepancies occurring in the temperature range (110 K, 250 K), where the experimental ∆S M and ∆T ad are almost zero and the theoretical curves show a unique cutoff, and the fact that the cell volume of Er 2 Fe 17 exhibits an almost constant value in the same temperature range, lead to the conclusion that the interplay between MCE and magneto-volume anomalies is fundamental to understand the physical properties of this intermetallic compound. PACS numbers: 61.05.C-, 61.05.F-, 75.10.Hk, 75.30.Kz, 75.30.Sg.
Neutron powder diffraction shows that the intermetallic Er 2 Fe 17 compound with hexagonal crysta... more Neutron powder diffraction shows that the intermetallic Er 2 Fe 17 compound with hexagonal crystal structure has a ferrimagnetic ground state (T C = 303 K). At T = 5 K the magnetic moments of Fe sublattice (µ ~ 2 µ B ) are therefore antiparalell to those of the Er one (µ ~ 9 µ B ), all of them lying on the basal plane. This compound exhibits strong magnetovolume effects up to temperatures in the vicinity of T C . Neutron thermo-diffraction experiments also show an anomalous temperature dependence of the cell volume, including a negative thermal expansion coefficient below 300 K. In addition, a positive spontaneous volume magnetostriction is observed up to T ~ 400 K, with a maximum (ω S ~ 0.02) located at T = 5 K.
Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material... more Combining different experimental techniques, investigations in Er 2 Fe 17 show that this material exhibits a spontaneous magnetostriction that reaches 1.6 × 10 −2 at 5 K and goes down to zero well above the Curie temperature, T C , owing to short-range magnetic correlations. Besides, Er 2 Fe 17 exhibits direct and inverse magneto-caloric effects (MCE) with moderate isothermal magnetic entropy, ∆S M , and adiabatic temperature, ∆T ad , changes (∆S M ∼ −4.7 J/kgK and ∆T ad ∼ −2.5 K near T C , and ∆S M ∼ 1.5 J/kgK and ∆T ad ∼ 0.6 K at 40 K for ∆H = 80 kOe, respectively). The existence of an inverse MCE seems to be related to a crystalline electric field-level crossover in the Er-sublattice and the ferrimagnetic arrangement between the magnetic moments of the Er-and Fe-sublattice. The main trends found experimentally for the temperature dependences of ∆S M and ∆T ad , as well as for the atomic magnetic moments, are qualitatively well described considering a mean field Hamiltonian that incorporates both crystalline electric field and exchange interactions. However, the discrepancies occurring in the temperature range (110 K, 250 K), where the experimental ∆S M and ∆T ad are almost zero and the theoretical curves show a unique cutoff, and the fact that the cell volume of Er 2 Fe 17 exhibits an almost constant value in the same temperature range, lead to the conclusion that the interplay between MCE and magneto-volume anomalies is fundamental to understand the physical properties of this intermetallic compound. PACS numbers: 61.05.C-, 61.05.F-, 75.10.Hk, 75.30.Kz, 75.30.Sg.
The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has... more The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened up the path for extensive research activity in the nanoscale world. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size under a given threshold. We combined X-ray diffraction, high-resolution transmission electron microscopy, extended X-ray absorption fine structure and magnetic measurements in order to describe the influence of the microstructure and morphology on the magnetic behaviour of NiO nanoparticles (NPs) with sizes ranging from 2.5 to 9 nm. The present findings reveal that size effects induce surface spin frustration which competes with the expected antiferromagnetic (AFM) order, typical of bulk NiO, giving rise to a threshold size for the AFM phase to nucleate. Ni 2+ magnetic moments in 2.5 nm NPs seem to be in a spin glass (SG) state, whereas larger NPs are formed by an uncompensated AFM core with a net magnetic moment surrounded by a SG shell. The coupling at the core-shell interface leads to an exchange bias effect manifested at low temperature as horizontal shifts of the hysteresis loop ($1 kOe) and a coercivity enhancement ($0.2 kOe). † Electronic supplementary information (ESI) available: TEM size distributions, normalized XAS spectra, variation of the coordination number due to cluster size reduction and ZFC-FC curves at 0.1 kOe. See
In this paper, we analyze the behaviour of Fe-rich wires that have been submitted to a current an... more In this paper, we analyze the behaviour of Fe-rich wires that have been submitted to a current annealing, with and without an applied torsional stress. The structural relaxation and induced anisotropies achieved in the samples produce different effects in the magneto-...
A comparative study on the magnetoimpedance (MI) and torsion-impedance (TI) effects between Fe 73... more A comparative study on the magnetoimpedance (MI) and torsion-impedance (TI) effects between Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 FINEMET ribbons and wires has been performed. The samples were annealed for 1 h at temperatures below and above their primary crystallization temperature, around 520 ° C. The magnetic softening of the samples occurs during the nanocrystallization process, which enhances both the MI and TI effects up to 82% and 75%, respectively, in the wire sample, with respect to its as-quenched state. A maximum MI ratio of 50% is obtained in the nanocrystalline ribbon. The modified values of saturation magnetization, coercive field and magnetostriction, and changes in the domain structure improve the impedance response of the nanocrystalline wire for its use as a sensor element for detecting small magnetic fields, with a sensitivity of 17%/Oe and torsional stresses with a sensitivity of 700% rad/cm.
The influence of isothermal annealing (one hour at 600ºC in Ar atmosphere) on the soft magnetic p... more The influence of isothermal annealing (one hour at 600ºC in Ar atmosphere) on the soft magnetic properties and magnetoimpedance (MI) effect has been studied in ribbons of the following Nanoperm alloys: Fe 91 Zr 7 B 2 , Fe 88 Zr 8 B 4 , Fe 87 Zr 6 B 6 Cu 1 and Fe 80 Zr 10 B 10 . A maximum MI ratio of about 27 % was measured for the nanocrystalline alloy Fe 87 Zr 6 B 6 Cu 1 at a driving frequency of 0.2 MHz. The thermal annealing led to magnetic softening for this alloy, while a hardening is observed for the Fe 80 Zr 10 B 10 alloy.
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