In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

In this paper, we review some of the key concepts in ultrathin film magnetism which underpin nanomagnetism. We survey the results of recent experimental and theoretical studies of well characterized epitaxial structures based on Fe, Co and Ni to illustrate how intrinsic fundamental properties such as the magnetic exchange interactions, magnetic moment and magnetic anisotropies change markedly in ultrathin films as compared with their bulk counterparts, and to emphasize the role of atomic scale structure, strain and crystallinity in determining the magnetic properties. After introducing the key length scales in magnetism, we describe the 2D magnetic phase transition and survey studies of the thickness dependent Curie temperature and the critical exponents which characterize the paramagnetic–ferromagnetic phase transition. We next discuss recent experimental and theoretical results on the determination of the exchange constant, followed by an overview of measurements of the magnetic moment in the elemental 3d transition metal thin films in the various crystal phases that have been successfully stabilized, thereby illustrating the sensitivity of the magnetic moment to the local symmetry and to the atomic environment. Finally, we discuss briefly the magnetic anisotropies of Fe, Co and Ni in the fcc crystalline phase, to emphasize the role of structure and the details of the interface in influencing the magnetic properties. The dramatic effect that adsorbates can have on the magnetic anisotropies of thin magnetic films is also discussed. Our survey demonstrates that the fundamental properties, namely, the magnetic moment and magnetic anisotropies of ultrathin films have dramatically different behaviour compared with those of the bulk while the comparable size of the structural and magnetic contributions to the total energy of ultrathin structures results in an exquisitely sensitive dependence of the magnetic properties on the film structure.

https://iopscience.iop.org/article/10.1088/0034-4885/71/5/056501/pdf

Magnetism in ultrathin film structures

Oscillatory interlayer coupling and giant magnetoresistance in Co/Cu multilayers

In 1897 Guillaume1 discovered that face-centred cubic alloys of iron and nickel with a nickel concentration of around 35 atomic per cent exhibit anomalously low (almost zero) thermal expansion over a wide temperature range. This effect, known as the Invar effect, has since been found in various ordered and random alloys and even in amorphous materials2. Other physical properties of Invar systems, such as atomic volume, elastic modulus, heat capacity, magnetization and Curie (or Neel) temperature, also show anomalous behaviour. Invar alloys are used in instrumentation, for example as hair springs in watches. It has long been realized that the effect is related to magnetism2,3; but a full understanding is still lacking. Here we present ab initio calculations of the volume dependences of magnetic and thermodynamic properties for the most typical Invar system, a random face-centred cubic iron–nickel alloy, in which we allow for non-collinear spin alignments—that is, spins that may be canted with respect to the average magnetization direction. We find that the magnetic structure is characterized, even at zero temperature, by a continuous transition from the ferromagnetic state at high volumes to a disordered non-collinear configuration at low volumes. There is an additional, comparable contribution to the net magnetization from the changes in the amplitudes of the local magnetic moments. The non-collinearity gives rise to an anomalous volume dependence of the binding energy, and explains other peculiarities of Invar systems.

Origin of the Invar effect in iron–nickel alloys

(1993). Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions. Advances in Physics: Vol. 42, No. 5, pp. 523-639.

Ultrathin metallic magnetic films: magnetic anisotropies and exchange interactions

The {open_quotes}invar effect{close_quotes} in Fe{sub x}Ni {sub 1{minus}x} alloys occurs when the Fe content approaches 65{percent}. At this point, the magnetization falls to zero, and a martensitic structural transformation from a fcc to a bcc lattice occurs. This paper addresses the question: {open_quotes}What happens if the structural transformation is suppressed in an ultrathin alloy film?{close_quotes} We present results to this effect, showing the variation of the magnetization with changing composition in ultrathin films grown on Cu(100). We find a new low-spin, ferromagnetic phase of matter, which is a sensitive function of the atomic volume. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic Instability of Ultrathin fcc Fe x Ni 1 − x Films

Since the discovery of the photoelectric effect, photoelectron spectroscopy has evolved into the most powerful technique for studying the electronic structure of materials. Moreover, the recent combination of photoelectron experiments with attosecond light sources using high-order harmonic generation (HHG) allows direct observation of electron dynamics in real time. However, the efficiency of these experiments is greatly limited by space-charge effects at typically low repetition rates of photoexcitation. Here, we demonstrate HHG-based laboratory photoemission experiments at a photoelectron count rate of 1 ? 105 electrons/s and characterize the main features of the electronic band structure of Ag(001) within several seconds without significant degradation by the space-charge effects. The combination of a compact HHG light source at megahertz repetition rates with the efficient collection of photoelectrons using time-of-flight spectroscopy may allow rapid investigation of electronic bands in a flexible laboratory environment and pave the way for an efficient design of attosecond spectroscopy and microscopy.

https://iopscience.iop.org/article/10.1088/1367-2630/17/1/013035/pdf

Boosting laboratory photoelectron spectroscopy by megahertz high-order harmonics

We have grown on a Cu(100) substrate epitaxial Co-monolayers separated by Cu. When the thickness of the Cu-spacer is increased, we observe the magnetic coupling between the Co-monolayers to change from ferromagnetic to antiferromagnetic and back to ferromagnetic before vanishing.

Magnetic coupling between Co layers separated by Cu

Many-body effects in solids are ultimately related to the correlation among electrons, which can be probed by double photoelectron emission. We have investigated the electron pair emission from a Cu(111) surface upon photon absorption. We are able to observe for the first time the full extension and shape of a depletion zone around the fixed emission direction of one electron. It has an angular extension of approximately 1.2 rad, which is independent of the electron energy.

http://www-old.mpi-halle.mpg.de/mpi/publi/pdf/7446_07.pdf

Correlation effects in two electron photoemission.

We have grown ultrathin Co${}_{x}$Ni${}_{1\ensuremath{-}x}$ and Fe${}_{x}$Ni${}_{1\ensuremath{-}x}$ films on Cu(100) with varying stochiometry $x$. We find that these alloy films grow in a fcc phase on Cu(100). With the surface magneto-optic Kerr effect we measured the variation of the Curie temperature ${T}_{C}$ as a function of the film thickness $n$ in monolayers. Fitting an empirical scaling curve to our results we are able to extrapolate the value ${T}_{C}(n=\ensuremath{\infty})$ for samples with different stochiometry. We use this framework in order to determine ${T}_{C}(n=\ensuremath{\infty})$ for Fe${}_{x}$Ni${}_{1\ensuremath{-}x}$ alloy films, in particular for concentrations close to 65% Fe content. Bulk Fe${}_{65}$Ni${}_{35}$ shows a collapse of magnetic long-range order and a fcc-to-bcc structural transition, which is the so-called Invar effect. In ultrathin Fe${}_{65}$Ni${}_{35}$ films, we observe a ``quenching'' of the Invar effect, because growth on a Cu(100) substrate forces the film to adopt the Cu lattice spacing thereby suppressing the structural relaxation.

Frank O. Schumann

Papers

Magnetic Instability of Ultrathin fcc Fe x Ni 1 − x Films

Boosting laboratory photoelectron spectroscopy by megahertz high-order harmonics

Magnetic coupling between Co layers separated by Cu

Correlation effects in two electron photoemission.

Growth and magnetic properties of Co x Ni 1 − x and Fe x Ni 1 − x ultrathin films on Cu(100)