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We found that colossal variations of resistance in La 0.225 Pr 0.40 Ca 0.375 Mn O 3 in about 100 μ s occur simultaneously with magnetic avalanches that last about 1 ms . These findings are interpreted as macroscopic fingerprints of the percolative phase separation upon application of the external magnetic field. Depending on the fraction of the ferromagnetic phase x , the colossal magnetoresistance jumps appear to be caused either by fluctuations of x , accompanied by fast propagation of the magnetic avalanches through the sample, or by a phase transition from charge ordered antiferromagnetic phase to charge delocalized ferromagnetic phase.

The magnetic ac susceptibility x of oriented Mn12Ac crystallites has been measured as a function of temperature, field, and frequency. The field has been applied at different values of the angle u with respect to the sample easy axis. For T55 K, the isothermal and adiabatic x limits have been determined as a function of field. For u50° and intermediate frequencies, Lorentzian-shaped peaks have been observed at magnetic field values Hn5nH1 with n50, 1, and 2 where H154.1 kOe. As u increases these maxima shift to higher fields, that satisfy Hncosu5const, and decrease in amplitude. The relaxation time t 1 follows Arrhenius’ law with respect to temperature and decreases sharply at H5Hn . The observed phenomenology unambiguously proves the existence of field-tuned tunneling between excited magnetic states which are thermally populated. At 5 K, the effective activation energy and the spin states involved in the tunneling process have been obtained.

We have measured the dc magnetization at low temperatures of tetragonal crystals of Mn12 acetate complex [Mn12O12[CH3COO]16[H2O]4], a material composed of a large ~Avogadro’s! number of identical magnetic molecules, each of spin 10. Exchange coupling between Mn ions within each molecule is very strong, while the interaction between molecules is negligible. A large, uniaxial anisotropy ~;60 K! gives rise to a doubly degenerate ground state corresponding to spin projections of 610 along the easy axis ~c axis!; hysteretic behavior is found below a blocking temperature Tb;3 K. Based on measurements of oriented crystallites at temperatures between 1.7 and 3.2 K, we report strong evidence for resonant tunneling of the magnetization: periodic steps in the hysteresis loop, and periodic marked increases in the magnetic relaxation rate at the magnetic fields corresponding to these steps. A total of seven increases in the relaxation rate were found within the temperature range of our experiments with a period of 0.46 T; we suggest that many more such steps would be found at lower temperatures. We attribute these observations to thermally assisted resonant tunneling of the magnetization and propose a detailed model to account for our results.

We report magnetic properties and magnetic relaxation phenomena in a sample comprised of nanocrystalline CoFe2O4 ~;97%!1g-Fe2O3 ~;3%! and polymer in potassium silicate ~as magnetic glass! nanoparticle systems in which two very different barrier distributions contributed to the relaxation behavior. We have demonstrated experimentally that only energy barrier distributions and the thermal activation process could not account for a plateau in the viscosity data at low temperatures. Quantum tunneling of magnetization is suggested below ;3 K.

The magnetic properties of BaFe12O19 and BaFe10.2Sn0.74Co0.66O19 single crystals have been investigated in the temperature range (1.8 to 320 K) with a varying field from -5 to +5 T applied parallel and perpendicular to the c axis. Low-temperature magnetic relaxation, which is ascribed to the domain-wall motion, was performed between 1.8 and 15 K. The relaxation of magnetization exhibits a linear dependence on logarithmic time. The magnetic viscosity extracted from the relaxation data, decreases linearly as temperature goes down, which may correspond to the thermal depinning of domain walls. Below 2.5 K, the viscosity begins to deviate from the linear dependence on temperature, tending to be temperature independent. The near temperature independence of viscosity suggests the existence of quantum tunneling of antiferromagnetic domain wall in this temperature range.

We have studied the static magnetic properties of nanocrystalline Co-Ti doped barium ferrite BaFe/sub 10.4/Co/sub 0.8/Ti/sub 0.8/O/sub 19/. The zero-field-cooled (ZFC) and field-cooled (FC) processes show the typical features of a small particle system. The dependence on temperature of the coercive field is discussed within the scope of an aligned and random assembly of particles. The distribution of blocking temperatures is obtained from the remanent-to-saturation magnetization ratio, leading to the mean volume of the magnetic unit (/spl ap/(6/spl plusmn/3)/spl middot/10/sup 4/ /spl Aring//sup 3/), in agreement with the mean particle volume determined by TEM (/spl ap/1.1/spl middot/10/sup 5/ /spl Aring//sup 3/). Remanence curves are used to derive the anisotropy field distribution and the interparticle magnetic interaction.<>

In this article we present experimental results on the magnetic relaxation in different systems (single domain particles, magnetic grains, and random magnets). The existence of two relaxation regimes is demonstrated. At high temperatures, the magnetic viscosity S≡1/M0∂M/∂ ln(t) is proportional to temperature in accordance with theoretical expectation for thermally activated processes. At low temperatures, the viscosity is independent of temperature, providing evidence to quantum tunneling of magnetization. Qualitative agreement between theory and experiment is found.

Low-field dc-magnetization and thermoremanent-magnetization measurements have been recorded from the rutile-related iron antimonate of composition FeSbO 4 . The zero-field-cooled susceptibility curve at low magnetic fields (H=20 Oe) shows a small peak at ∼72 K and a much broader peak at lower temperatures of ∼25 K. The temperature dependence of the remanent magnetization below ∼72 K indicates the existence of two different relaxation regimes which depend on the strength of the applied field in which the sample was cooled. The thermoremanent-magnetization measurements as a function of time show aging effects and the characteristic parameters are quite similar to those found in canonical spin glasses. The temperature dependence of the first nonlinear coefficient b 3 in the expansion of the magnetization in odd powers of ( a 1 H) shows no divergence around the peak at 72 K, which is indicative of the absence of a true spin-glass transition. The moderate degree of frustation associated with an inhibited antiferromagnetic transition at ∼72 K, is nevertheless insufficient to produce a true spin-glass transition where the magnetic correlation length is even much shorter.

Magnetic properties of the amorphous Dy-Fe-B alloy are studied in terms of the correlated-spin-glass approach of Chudnovsky et al.$-— Features predicted by the theory are clearly observed in the experiment. It is shown that the magnetization law may be presented in the form where it is determined by the dimensionless correlation function of structural disorder, C(y), only. The analysis of the magnetization curve allows one to distinguish between different models of disorder in amorphous solids. Experimental data on Dy-Fe-B are in favor of C=exp(-1/2 y 2 ).

A Reply to the Comment by M. Hanson, C. Johansson, and S. Mørup.

We report experimental studies and suggest a quantitative model of spin relaxation in Mn12 acetate in a pulsed magnetic field in the temperature range 1.95.0 K. When the field applied along the anisotropy axis is swept at 140 T/s through a nonmagnetized Mn12 acetate sample, the samples magnetization switches, within a few milliseconds, from zero to saturation at a well-defined field whose value depends on temperature but is quantized in units of 0.46 T. A quantitative explanation of the effect is given in terms of a spin-phonon avalanche combined with thermally assisted resonant spin tunneling.

[eng], [spa]

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 2018, Tutors: Javier Tejada Palacios, Jaume Calvo de la Rosa, In this work, the magnetic properties of one Soft Magnetic Material (SMM) and one Soft Magnetic Composite (SMC) are studied as a function of different particle size distribution. X-Ray Diffraction (XRD) show that both samples consist on an iron core. The basic magnetic characterization of the samples is done by measuring the isothermal magnetization M(H) curves and zero field cool (ZFC) – field cool (FC) curves. The results show that, despite being magnetic iron particles, they present an extremely narrow hysteresis loop. Moreover, it has been seen that the higher the size distribution is, the greater the magnetic saturation is achieved. In order to study them from the point of view with more technological and industrial interest, their functional properties are measured in a wide frequency range. In this regard, Impedance Spectroscopy is used to measure their magnetic permeability between 1 kHz and 13 MHz while their dielectric constant is studied from 200 MHz to 20 GHz by using a Pulse Network Analyser (PNA). Finally, reflection loss measurements are performed in order to measure directly the energy absorbed in the range between 7 GHz and 10 GHz.

[cat] En aquest treball de tesi doctoral es presenta l’estudi del magnetisme de diferents materials de naturalesa metal•lorgànica emmarcats en diferents famílies. Per un cantó, tenim molècules d’alt espí amb anisotropia magnètica uniaxial negativa. Aquesta anisotropia genera una barrera d’energia en aquestes molècules que impedeix al moment magnètic girar lliurement i l’orienta de forma preferent en una direcció de l’espai. El moment magnètic pot canviar el seu sentit d’orientació superant la barrera d’energia de forma clàssica per activació tèrmica o quànticament per efecte túnel. Aquesta propietat es pot utilitzar per aplicacions tecnològiques d’aquestes molècules en el camp de la gravació magnètica d’ultra-alta densitat o en el camp de la computació quàntica, com a qubits magnètics. En la primera part d’aquesta tesi es presenten els estudis de les propietats magnètiques d’algunes d’aquestes molècules pertanyents a la família del Mn12, en diferents entorns (des de monocristalls fins a partícules polimèriques passant per dissolucions), així com noves tècniques de caracterització magnètica d’aquestes molècules com és el dicroisme circular magnètic. Aquest mètode ens permet per primera vegada mesurar alguns fenòmens de física fonamental en aquestes molècules mitjançant caracterització magneto-òptica, com és ara l’efecte túnel del moment magnètic macroscòpic. En el segon bloc d’aquesta tesis es presenta l’estudi del magnetisme d’una nova família de materials nanoporosos altament innovadors, sintetitzats a partir de radicals trifenilmetílics perclorats (PTM), funcionalitzats amb diferents grups carboxílics. L’ús d’aquests radicals ens ha portat a l’obtenció no només de materials nanoporosos amb un alt grau de porositat i unes propietats magnètiques destacables. La cristal•lització d’aquests radicals ha permès la obtenció del primer ferromagnet nanoporós purament orgànic, amb una temperatura de transició del ordre de 125 mK. La cristal•lització de combinacions d’aquests radicals amb ions de metalls de transició en la aproximació metal-orgànica ha donat lloc a varis sistemes notables. La combinació de ions de Cu(III) amb radicals PTM ens permet obtenir la primera esponja magnètica, material nanoporós amb uns 3 nm de diàmetre i amb propietats de pèrdua i reabsorció de solvents reversibles i detectables a través de les seves propietats magnètiques, convertint-se per tant en un sensor magnètic selectiu a certs dissolvents. Per altra banda, la combinació de ions de Co(II) amb els radicals PTM ens porta a la primera estructura nanoporosa magnètica formada per sistemes helicoïdals no inter penetrats i que presenta uns fenòmens de relaxació lenta molt inusuals: aquesta relaxació presenta una forta dependència amb el camp magnètic aplicat y pot ser descrita mitjançant el model de Davidson-Cole. Així doncs, en aquesta tesi es presenten l’estudi de les propietats magnètiques de diversos materials diferents les propietats dels quals poden ser aprofitades sens dubte per aplicacions d’alt nivell tecnològic que van des de la computació quàntica a sensors magnètics de baixa temperatura, passant per la gravació magnètica d’alta densitat. Els resultats presentats han donat lloc a una sèrie de publicacions en revistes científiques d’alt nivell que es detallen a continuació: 1. Gerbier Ph., et al., Synthesis and Characterization of a [Mn12O12(O2CR)16(H2O)4] Complex Bearing Paramagnetic Carboxylate Ligands. Use of a Modified Acid Replacement Synthetic Approach, Monatshefte für Chemie 134 (2003) 265-276 2. Maspoch D.,et al., A nanoporous molecular magnet with a reversible solvent-induced mechanical and magnètic properties, Nature Materials, 2 (2003) 190-195 3. Ruiz-Molina D.,et al., Isolated Single-Molecule Magnets on the Surface of a Polymeric Thin Film, Advanced Materials, 15 (2003) 42-45 4. Domingo N., et al., Synthesis and Characterization of a new chiral nanomagnet, Polyhedron, 22 (2003) 2355-2358 5. Domingo N., et al., Magnetism of Isolated Mn12 Single-molecule Magnets Detected by Magnetic Circular Dichroism: Observation of Spin Tunneling with a Magneto-optical Technique, Physical Review B 69 (2004) 052405 6. Maspoch D., et al., Self-assembly of a Dicarboxylic Radical: a New Pure Organic Robust Paramagnetic Nanoporous Molecular Material, Journal of the American Chemical Society, 126 (2004) 730-731 7. Maspoch D., et al., A New Robust Nanoporous Pure Organic Magnet, Angewandte Chemie Int. Ed. 43 (2004) 1828-1832 8. Gerbier Ph., et al., Chiral, single-molecule nanomagnets: synthesis, magnètic characterization and natural and magnètic circular dichroism, Journal of Materials Chemistry 14 (2004) 2455-2460 9. Maspoch D., et al.,Open-shell nanoporous salts formed by the supramolecular assembly of a polycarboxylate perchlorinated triphenylmethyl radical and a Co(bpy)3]2+ cation, Crystal Engineering Communications, 6 (2004) 573 - 578 10. Maspoch D., et al., Carboxylic-substituted Polychlorotriphenylmethyl Radicals, New Organic Building-Blocks to Design Nanoporous Magnetic Molecular Materials, Comptes Rendus Chemie 8 (2005) 1213-1225, [eng] This work shows the study of magnetism of different metal-organic materials from different families. On one side, there are the high spin molecules with negative uniaxial magnetic anisotropy. This anisotropy is the origin of an energy barrier in this type of molecules that blocks the orientation of magnetic moment in a single direction of the space. The magnetic moment can change the sense of its orientation in two ways: classically, by thermal activation over the energy barrier, or in a quantic manner, by quantic tunneling effect of the macroscopic magnetic moment through the energy barrier. This property allows thinking about applications of these type of molecules for ultra-high density magnetic recording and quantum computing, as magnetic qubits. The first part of this thesis is devoted to the study of the magnetic properties of some of this molecules of the Mn12 family, in different environments (from monocrystals to polymeric thin films and dissolutions), as well as new characterization techniques for the magnetic characterization of these molecules such as magnetic circular dichroism. This technique, allowed for the first time the measurement of some fundamental physics phenomena in this molecules through magneto-optical characterization, such as the quantum tunneling of the macroscopic magnetic moment. The second part of this thesis is devoted to the study of the magnetic properties of a new family of nanoporous metal-organic and pure organic materials, synthesized on the basis of perclorated triphenylmethyl radicals (PTM), functionalized with different carboxylic groups. The use of this radicals lead to nanoporous materials with a very high degree of porosity combined with interesting magnetic properties. For example, it is shown the first pure organic nanoporous ferromagnet, with a transition temperature of the order of 125 mK. The combination of these radicals with transition metal ions in the metal-organic approximation, also lead to interesting systems. The combination with Cu(III) ions with PTM radicals lead to the first nanoporous magnetic sponge, with reversible properties of solvent absorption-desorption that can be monitorized through its magnetic properties, thus becoming a low temperature magnetic sensor with solvent selectivity. Finally, the combination of Co(II) ions with PTM radicals lead to the firms nanoporous magnetic structure with non-interpenetrated helical networks that show an unusual slow relaxation of the magnetization; this relaxation shows a strong dependence with the magnetic field and can be described with the Davidson-Cole model.

[spa] La Física Cuántica descansa sobre el principio de indeterminación que nos dice que no podemos conocer con total precisión y a la vez la posición y la velocidad de un objeto. Una consecuencia de la indeterminación en la posición es la probabilidad no nula que tiene una partícula de atravesar una región prohibida clásicamente; este es el denominado efecto túnel. El decaimiento de una partícula alfa de un núcleo, la conducción por "electron-hoping" en aislantes, las parejas de Cooper tuneleando a través de una unión de Josephson, etc, son algunos ejemplos de manifestaciones del efecto túnel. En los años 80, Caldeira y Leggett [1, 2] desarrollaron la teoría formal del efecto túnel macroscópico, que predecía que bajo determinadas circunstancias los objetos macroscópicos, podrían presentar propiedades cuánticas reservadas a los objetos microscópicos. La primera evidencia de efecto túnel macroscópico fue presentada por Clarke [3], quien mostró que, en uniones Josephson, la diferencia de fase superconductora a través de la unión puede tunelear entre el estado superconductor y el estado normal. Los físicos teóricos, guiados por el formalismo planteado por Caldeira y Legett, produjeron interesantes avances teóricos. Quiero destacar los trabajos de E. M. Chudnovsky dedicados al efecto túnel del momento magnético [4, 5]. En el ámbito experimental, hay sólidas evidencias experimentales del efecto túnel del momento magnético deducidas de los trabajos de relajación magnética a bajas temperaturas [6, 7, 8], muchas de ellas obtenidas por parte del grupo de investigación al que pertenezco. En ellas se observa la constancia de la viscosidad magnética por debajo de una determinada temperatura de "crossover", contrariamente a lo esperado clásicamente. Esto se atribuye a la inversión cuántica del momento magnético. La relajación magnética observada en la mayoría de materiales representa el promedio estadístico sobre un gran número de sucesos, con una ancha distribución de escalas temporales debido a factores aleatorios como el tamaño de las partículas o las interacciones. Por lo tanto, es difícil realizar una comparación directa entre la teoría y los experimentos. El formato elegido en esta memoria es el de compendio de artículos. Sin embargo, tal presentación ha sido completada con explicaciones para introducir, ampliar o justificar los resultados expuestos en las publicaciones. En el primer capítulo, se describe el diseño y operación de un sistema de medida de magnetización a ultra-bajas temperaturas. Su puesta a punto en el rango de muy bajas temperaturas lo ha convertido en un equipo experimental único en el mundo, lo que nos ha permitido obtener resultados muy novedosos en el panorama científico internacional. En el segundo capítulo muestro un modelo teórico que describe el comportamiento de los sistemas de partículas magnéticas a bajas temperaturas. Hago especial hincapié en la obtención de resultados para los diferentes protocolos de medida usados habitualmente en los experimentos. Analizo su utilidad para extraer los parámetros intrínsecos del material así como para llegar al esclarecimiento de los mecanismos que gobiernan su relajación. Los resultados de este modelo en comparación con resultados de experimentos en sistemas de partículas permiten una fácil interpretación de los mismos en términos del efecto túnel de la magnetización. El tercer capítulo describe tanto los resultados de los experimentos como el modelo teórico que explica los fenómenos cuánticos en clústeres moleculares. Tales compuestos han abierto la puerta para un estudio detallado de los fenómenos de relajación cuántica presentando además nuevas características asociadas al carácter discreto de su bajo momento magnético (del orden de la decena de magnetones de Bohr). Presento también un nuevo fenómeno en la Física, el efecto túnel resonante de la magnetización, basado en el hecho de que, cuando los niveles a ambos lados de la barrera dada por el hamiltoniano de espín están degenerados, la probabilidad de efecto túnel es máxima. Presento los resultados de caracterización obtenidos mediante diferentes técnicas: magnetización d.c. y a.c. al helio líquido, capacidad calorífica, experimentos bajo campos magnéticos pulsados, y, en el caso del compuesto Fe8, magnetización d.c. a ultra-bajas temperaturas obtenida en el magnetómetro descrito en el primer capítulo. El estudio de la respuesta de estos materiales bajo la acción de campos magnéticos pulsados, nos ha revelado que la magnetización puede relajarse en forma de avalanchas asociadas a la gran cantidad de energía liberada en el proceso. Las medidas a ultra-bajas temperaturas demuestran que el efecto túnel en dicho rango de temperaturas se produce por el nivel fundamental, definiendo el marco necesario para el desarrollo del experimento descrito en el capítulo 4. En el cuarto capítulo, presento los resultados de las medidas de susceptibilidad a.c. a alta frecuencia en el compuesto Fe8. Dichas medidas muestran dos picos en la dependencia de la susceptibilidad con el campo magnético y pueden ser interpretadas como una absorción de energía por los niveles que aparecen como consecuencia de la superposición cuántica de los estados a ambos lados de la barrera. Este hecho constituye la primera evidencia experimental de la coherencia cuántica mesoscópica del momento magnético.