The scaling of the energy barrier to magnetization reversal in thin film nanomagnets with perpendicular magnetization as a function of their lateral size is of great current interest for high-density magnetic random access memory devices. Here we determine the micromagnetic states that set the energy barrier to thermally activated magnetization reversal of circular thin film nanomagnets with large perpendicular magnetic anisotropy. We find a critical length in the problem that is set by the exchange and effective perpendicular magnetic anisotropy energies, with the latter including the size dependence of the demagnetization energy. For diameters smaller than this critical length the reversal occurs by nearly coherent magnetization rotation and the energy barrier scales with the square of the diameter normalized to the critical length (for fixed film thickness), while for larger diameters the transition state has a domain wall and the energy barrier depends linearly on the normalized diameter. Simple analytic expressions are derived for these two limiting cases and verified using full micromagnetic simulations with the String Method. Further, the effect of an applied field is considered and shown to lead to a plateau in the energy barrier versus diameter dependence at large diameters.

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We show that it is possible to engineer magnetic multi-domain configurations without domain walls in a prototypical rare earth/transition metal ferrimagnet using keV He + ion bombardment. We additionally shown that these patterns display a particularly stable magnetic configuration due to a deep minimum in the free energy of the system which is caused by flux closure and the corresponding reduction of the magnetostatic part of the total free energy. This is possible because light-ion bombardment differently affects an elements relative contribution to the effective properties of the ferrimagnet. The impact of bombardment is stronger for rare earth elements. Therefore, it is possible to influence the relative contributions of the two magnetic subsystems in a controlled manner. The selection of material system and the use of light-ion bombardment open a route to engineer domain patterns in continuous magnetic films much smaller than what is currently considered possible.

Integrated magnetic sensors that allow simultaneous EPR and magnetization measurements have been developed to study single molecule magnets. A high frequency microstrip resonator has been integrated with a micro-Hall effect magnetometer. EPR spectroscopy is used to determine the energy splitting between the low lying spin-states of a Ni4 single crystal, with an S = 4 ground state, as a function of applied fields, both longitudinal and transverse to the easy axis at 0.4 K. Concurrent magnetization measurements show changes in spin-population associated with microwave absorption. Such studies enable determination of the energy relaxation time of the spin system.An understanding of decoherence and energy relaxation mechanisms in single molecule magnets (SMMs) is both of fundamental interest [1] and important for the use of SMMs in quantum computing [2]. Quantum tunneling of magnetization (QTM) has been widely studied in SMMs [3,4,5]. A recent focus is on coherent QTM in which the tunneling rates may be faster than the rate of decoherence [6,7]. Experiments on SMMs [7] and doped antiferromagnetic rings [8,9] have been reported. However, the basic relaxation mechanisms in SMMs are still under active investigation both experimentally and theoretically [10].We have developed sensors allowing simultaneous EPR and magnetization measurements at low temperatures (typically below 1 K) to study SMMs. A microstrip resonator [11] with resonance frequency between 25 and 30 GHz has been integrated on a chip with a micro-Hall effect magnetometer [12]. The high filling factor in such resonators allows measurement of photon absorption in very small crystals as well as the application of large microwave magnetic fields, needed for Rabi experiments. The fast response of the Hall sensor (> 1 MHz) also enables time-resolved measurements of the magnetization In this paper we show EPR spectroscopy of the two lowest energy levels associated with high spin states (S = 4) of a Ni 4 single crystal. We present simultaneous measurements of associated photon-induced changes in the magnetization. This represents an important advance from the experiments in refs. [7,8], in which only photon induced magnetization changes were measured, and EPR studies were not possible. In particular, it enables a direct determination of the energy relaxation time of the spin system.A schematic of our integrated sensor is presented in Fig. 1.The Hall sensor is fabricated from a GaAs/AlGaAs heterostructure 2D electron gas to form a cross of width 50 µm, chosen to optimize the coupling with a SMM crystal with lateral dimensions of about 100 µm. Our magnetometer has a Hall coefficient of SMM crystal Hall sensor microstrip H T H L resonator h ac Figure 1: Schematic of the sensor developed for this study.1780 Ω/T and a noise level of 2 µT/ √ Hz with a current of 10 µA. This permits detection of changes in the magnetization smaller than 5 × 10 −4 of the saturation magnetization of our SMM crystals (≈ 5×10 5 µm 3 ), using a lock-in detection bandwith of 300 ms. The micros...